Organic semiconductors for energy and electronics: from fundamental properties to devices

Resume : The donor-acceptor interface, where charge separation and recombination occurs, is the most important part of organic bulk heterojunctions.1 Therefore, any change in the relative orientation of the interfacial D&A components can dramatically influence solar cell efficiency.2 Typical polymer blends offer limited opportunity to structurally control these interfaces leading to numerous D-A configurations, which are very complex and limit our understanding.1 Therefore, designing efficient OPVs remains extremely challenging. Recently, we demonstrated that macrocycles are great for controlling the properties of conjugated materials.3,4 Furthermore, research suggests that improved understanding, control and manipulation of interfacial D-A interactions may be key to further advance OPV materials.2 Herein, we demonstrate that through synthetic control the molecular orientation of the D&A components can be precisely engineered, providing a clear picture of the exact interfacial structure. Furthermore, we show that subtle changes in the molecular orientation can dramatically influence the properties of the resulting charge-transfer states and thus overall device performance. Therefore, having great control over the D-A interface can provide lots of fundamental understanding and hence help pave the way towards next-generation OPV materials. 1. Nat Photonics 8, 385 (2014). 2. J Am Chem Soc 136, 9608–9618 (2014). 3. J Am Chem Soc 140, 1622–1626 (2018). 4. J Org Chem 85, 207–214 (2020).

Resume : Since the discovery of giant magnetoresistance in 1988, the field of spintronics has developed rapidly, underpinning applications for memory storage and spin-based quantum computing. One of the challenges in the field is spin injection, which has been achieved optically in inorganic crystalline semiconductors, but not yet in organic semiconductors. Here, we apply group theory and computational methods to design molecular materials in which spin can be injected optically via circularly polarized light (CPL), in analogy to GaAs. Such an approach has not been reported elsewhere. When designing molecules for their optical excitation properties, additional design rules can be defined by considering the relationship between the symmetry of the molecule and its excited state properties using group theory. Group theory is a powerful tool to understand spectroscopic properties, explain optical phenomena and define materials’ structures in crystals and molecular compounds. The use of group theory to identify eligible families of materials prior to detailed calculation can save time by first identifying any structures that fail some criterion on grounds of symmetry. Our procedure is to first identify candidate point groups, then determine the symmetry of the excited state using group theory based on symmetries of HOMO and LUMO. Then we proceed to full calculation of excited states using relativistic quantum chemistry methods, in order to find circularly polarized doubly degenerate triplet states corresponding to ms=±1. We generalize this procedure and prepare a list of requirements for potential optical spin injection materials. The theory reveals that molecules with C3h symmetry are good candidates. A series of molecules with C3h symmetry was synthesized, some for the first time, thus we are able to present preliminary experimental validation of our approach with spectroscopic studies of a new family of molecules. The research shows how symmetry can be used in molecular design for spintronics applications.

Resume : Organic (opto)electronics has played a major role in academic and industrial research in the last 30 years, eventually reaching market maturity and holding promises for sizeable further growth. Materials are constantly developed possessing better performances and higher stability. Comparatively less efforts are dedicated to the development of sustainable synthetic strategies for their preparation. Protocols are tortuous, inefficient and heavily relying in flammable and toxic organic solvents. According to the Green Chemistry prescriptions, processes should be developed minimizing the use of solvents other than water, the use of heavy metal catalysts, the number of steps and the amount of wastes produced. As the vast majority of organic semiconductors are water insoluble, the development of sustainable synthetic routes for their preparation requires dedicated efforts. The use of specifically devised surfactants along with water and water insoluble reagents, leads to the formation of a range of interface rich systems the like of micellar solutions, emulsions, microemulsions and dispersions. Within such microheterogenous environment, a wide variety of C-C and C-N forming reactions become accessible at room temperature, low catalyst loading and at high yield with no need for inert atmosphere. In this contribution we will show how this kind of “benchtop” chemistry enables the efficient preparation of notable small molecule as well as polymeric organic semiconductors.

Resume : Molecular intermixing between electron donating and accepting units in bulk heterojunction (BHJ) organic solar cells (OSCs) is a well-known phenomenon, whose impact on the device function is, however, not fully understood. This is, in part, because the actual degree of intermixing in most of real OSCs is so far unknown. Herein a simple calorimetric method that provides the average composition of intermixed domains in BHJs is presented. The method exploits the fact that the glass transition of a homogeneous blend, such as the vitrified intermixed domains in OSCs, depends in a coherent manner of the composition. The application of this methodology to two model system BHJs with different donor:acceptor (D:A) miscibility evidences that, as expected, the composition of intermixed domains is highly dependent on the D:A miscibility. However, strongly and slightly intermixed BHJs exhibit a similar capability to generate charged species, as deduced from transient absorption spectroscopy, indicating that, at least for the materials systems studied here, an intermixing as little as 10 % is sufficient for charge generation. The analysis of the effective charge carrier mobility in the dark reveals that the more intermixed BHJ exhibits better charge transport characteristics than the slightly intermixed one, which suggests that the higher miscibility leads to a phase morphology that is more efficient for charge transport. The full compositional characterization of OSCs will contribute to advancing quantitative morphology-function models for high-performing non-fullerene acceptor OSCs that allow the rational design of these devices

Resume : Ternary organic solar cells (OSC) are among the best-performing organic photovoltaic devices to date, largely due to the recent developments of non-fullerene acceptors (NFA). However, the impact of energy transfer and back hole transfer from the electron acceptor to the electron donor not well understood. In this work, we study the photophysics of several NFA based ternary OSC blend composed of a polymer donor PM6 by using ultrafast spectroscopy. Surprisingly, we find that after exclusive excitation of the donor, singlet energy transfer to the lowest bandgap acceptor efficiently competes with electron transfer. Subsequently, singlets on the lowest bandgap material undergo hole transfer to the donor, resulting in free charge generation. We demonstrate that it is the ionization energy (IE) offset, which controls both the exciton quenching and charge separation efficiency. This implies, while designing the ternary OSCs, the IE offset between energy donor and acceptor cannot be reduced to zero to maximize the Voc, which will result in reduced charge separation efficiencies and in turn, to lower device photocurrent and performance.

Resume : Significant breakthroughs with unprecedented power conversion efficiencies (PCEs) exceeding 16% have been achieved for polymer:NFA organic solar cell (OSC) blends recently. Among systems that are able to achieve remarkable efficiencies is the PM6:Y6 bulk-heterojunction (BHJ) blend, which can successfully reach PCEs of up to 15.7%. The highly efficient PM6:Y6 system can achieve high open circuit voltages (VOC) while maintaining exceptional fill-factor (FF) and short-circuit current (JSC) values. With a low energetic offset, the blend system was found to exhibit radiative and non-radiative recombination losses that are among the lower reported values in the literature. Recombination and extraction dynamic studies revealed that the device shows moderate non-geminate recombination coupled with exceptional extraction throughout the relevant operating conditions. Several surface and bulk characterization techniques including GIWAXS, photo-conductive atomic force microscopy (pc-AFM), and NMR spectroscopy were employed to understand the phase separation, long-range ordering, as well as donor:acceptor (D:A) inter- and intramolecular interactions at an atomic-level resolution. The synergy of multifaceted characterization and device physics was used to uncover key insights on the structure-property relationships of this high performing BHJ blend. Detailed information about atomically resolved D:A interactions and packing revealed that the high performance of over 15% efficiency in this blend can be correlated to a beneficial morphology that allows high JSC and FF to be retained despite the low energetic offset.

Resume : A significant breakthrough in the efficiency of organic solar cells (OSCs) has been achieved due to the synthesis of new materials offering a plethora of donor:acceptor combinations. However, the device performance is interlinked in a complex way not only with the properties of the individual components but also its strong dependence on the blend’s microstructure and phase morphology. Consequently, identifying high-performing donor:acceptor combinations has so far been an intricate process nearly uniquely relying on tedious and time-consuming trial-and-error materials selection methods. To overcome this unsustainable approach we developed a methodology that rapidly elucidates how blend composition and processing conditions affect the final blend morphology and microstructure. We demonstrate that transient absorption spectroscopy (TAS), vapor phase infiltration (VPI) and differential scanning calorimetry (DSC) measurements can be jointly harnessed to fast-screen OSC blends. VPI infuses inorganic materials into an organic matrix by exposure to gaseous precursors that diffuse into the film and in-situ convert to an inorganic product. In BHJ films, the diffusion process proceeds selectively through domains with high free-volume leading to inorganic deposition selectively along the diffusion paths. The diffusion network is easily visualized with electron microscopy. Using this labelling approach to spatially map OSC BHJ is in concept similar to the staining approach used to image low contrast biological. The spatially mapping of the phase morphology of organic solar cells via VPI giving insights into the size, shape, distribution and connectivity of specific domains, resolved through fast and straightforward HRSEM characterization, the information obtained on the general phase behavior as well as the phase purity (or at least degree of order) via DSC, and the indirect correlations on the fine structure of, e.g., the intermixed phases and phase-pure domains, provided by the charge separation dynamics from TAS, can, when combined, be used to identify best working compositions and deliver understanding why specific deposition methodologies do not lead to well-performing devices.

Resume : Despite the advantages provided by transient photovoltage (TPV) and photocurrent (TPC) techniques in obtaining valuable information regarding device performance and charge-carrier dynamics on organic solar cells, they are limited to the extreme points of the J-V curve under illumination, i. e. in the open circuit situation where the internal electric field is zero and at the short-circuit point where the field is determined by the built-in voltage. In this study, we present a variant technique that allows getting relaxation times in the microsecond scale in the whole J-V fourth quadrant of solar cells. The analysis of the measurements uses an equivalent circuit by considering a capacitor in parallel with the shunt resistance, in which the increment of charge generated at the electrodes by the light flash is monitored by a variable external load resistance RL. Transient measurements were then obtained on two distinct bulk heterojunction structures: ITO/PEDOT:PSS/P3HT:PC(61)BM/Ca-Al and ITO/PEDOT:PSS/P7B7-Th:PC(71)BM/Ca-Al. This theoretical-experimental study allowed a better knowledge of the relaxation effects of the photocarriers, as well as a detailed analysis of the competition between recombination and charge extraction under the effect of the internal electric field.

Resume : The introduction of non-fullerene acceptors has provided several recent record efficiencies in organic photovoltaic (OPV) cells, reaching now above 16% for single-junction devices. While these developments have provided a strong boost to the OPV field, more efforts have to be devoted to the up-scaling of such high performance OPV devices, which includes scalability of the active layers as well as the employed electrodes and interlayers. In terms of interlayers, metal oxide thin films have been widely used in OPV devices where they act as contact layers selective to either hole or electron transport, and thus support efficient carrier extraction from the cells. Well-known examples are titanium and molybdenum oxides, used in both organic and perovskite solar cells, with new variations appearing as the technologies develop further. In recent work, we have demonstrated that sputtered metal oxides thin films may act as interlayers in organic photovoltaic (OPV) devices, where they can act as both efficient and stable carrier extraction layers. Here, recent progress made on reactively sputtered metal oxide hole1 and electron2 contact layers is presented. In both systems, a strong correlation between initial material composition and annealing condition to the microstructure of the films is given, leading to a pronounced improvement in their carrier extraction capabilities. Supported by a variety of surface science characterization studies, the importance of the energy band alignment, work function, microstructure, oxygen vacancies, optical and electrical properties and intrinsic stability on their performance as contact layers in OPV devices is discussed. Importantly, a new crystalline MoOx system employed for efficient hole extraction is shown to lead to a significantly prolonged OPV device lifetimes, and a new crystalline TiOx layer is shown to lead to an efficient electron extraction with s-shape free current-voltage characteristics, in striking difference to established TiOx interlayers. In order to meet the requirements on scalable OPV development, the up-scaling of these new metal oxide interlayer systems is discussed, considering recent results on industrially relevant OPV device development3. This includes Sheet-to-Sheet (S2S) and Roll-to-Roll (R2R) processing of OPV devices and modules, using combined solution and vacuum based techniques. 1 M. Ahmadpour, A. L. F. Cauduro, C. Méthivier, B. Kunert, C. Labanti, R. Resel, V Turkovic, H.-G. Rubahn, N. Witkowski, A. K. Schmid and M. Madsen, Crystalline molybdenum oxide layers as efficient and stable hole contacts in organic photovoltaic devices, ACS Appl. Energy Mater. 2, 420 (2019) 2 M Mirsafaei, P B. Jensen, M. Ahmadpour, H. Lakhotiya, J. L. Hansen, B. Julsgaard, H.-G. Rubahn, R. Lazzari, N. Witkowski, P. Balling and M. Madsen, Sputter deposited titanium oxide layers as efficient electron selective contacts in organic photovoltaic devices, ACS Applied Energy Mater. acsaem.9b01454 (2019) 3 E. Destouesse, M. Top, J. Lamminaho, H.-G. Rubahn, J. Fahlteich and M. Madsen, Slot-die processing and encapsulation of non-fullerene based ITO-free organic solar cells and modules, Flex. Print. Electron. 4, 045004 (2019)

Resume : The field of organic photovoltaics (OPV) recently demonstrated tremendous increase in power conversion efficiency (PCE). The latest generation of donor materials and non fullerene acceptors (NFAs) exhibit PCEs up to 17%, making the technology mature for scaling up and commercialization. One of the current challenges to achieve high throughput fabrication and widespread commercialisation is to develop hole transport layers (HTLs) that are suited for this new class of active layers and compatible with industrial roll-to-roll manufacturing processes. To this end, the effects of various HTLs on the performance and stability of NFA-based OPVs were studied in this work. First, the widely used PEDOT:PSS is investigated as a solution processed HTL. Various formulations of PEDOT:PSS were tested and integrated into OPV devices. Our results show that the use of PEDOT:PSS can dramatically decrease the initial efficiency of NFA-based OPVs as compared to OPVs containing MoO3 as the HTL. These losses are critically dependent on the NFA family being used in the active layer, making specific families of NFAs incompatible with solution process manufacturing. Then, photo-degradation and thermal degradation tests pointed out the crucial role of the choice of interlayer on the photo-stability of OPVs, as observed in earlier studies on fullerene-based-OPV devices. Finally, electrical characterizations and structural analysis were performed on PEDOT:PSS layers based on different formulations in order to determine the origins of degradation. Due to the severe drawbacks demonstrated by PEDOT:PSS when used as a HTL in an OPV structure, several alternatives were tested as a replacement, which will be discussed in this presentation.

Resume : In the view of a rapid increase in efficiency of organic solar cells, reaching their long-term operational stability represents one of the main challenges to be addressed on the way toward commercialization of this photovoltaic technology. However, intrinsic degradation pathways occurring in organic solar cells under realistic operational conditions remain poorly understood. The light-induced dimerization of fullerene-based acceptor materials discovered recently is considered to be one of the main causes for burn-in degradation of organic solar cells. In this work, we reveal the mechanism of the light-induced dimerization of the fullerene derivatives and establish important correlations with their molecular structure and electronic properties. We also show that conjugated polymers and small molecules undergo similar light-induced crosslinking regardless of their chemical composition and structure. In case of conjugated polymers, crosslinking leads to a rapid increase in their molecular weight and consequent loss of solubility, which can be revealed in a straightforward way by gel permeation chromatography analysis via a reduction/loss of signal and/or smaller retention times. Our results, thus, shift the paradigm of research in the field toward designing a new generation of organic absorbers with enhanced intrinsic photochemical stability in order to reach practically useful operation lifetimes required for successful commercialization of organic photovoltaics.

Resume : The use of abundant and clean solar power to generate electrical energy has extremely high potential in the field of renewable energies. Organic photovoltaics (OPVs) are expected to play a major role in future power generation because of their capability to deposit solar cells on flexible substrates by printing from solution or spray-coating. Single junction solar cells fabricated using bulk heterojunction geometry have already achieved more than 15% efficiency. Further improvement of devices requires optimization of every step of operation from photon absorption to charge extraction. The first step in the operation of OPVs is the absorption of light which generates the bound electron-hole pairs called excitons. These excitons have to travel to an interface between the electron donor and acceptor materials where they can split into free charges, and then these charges have to be extracted by the electrodes to give the photocurrent. Excitons travel to interface through either Fӧrster resonance energy transfer (FRET) or diffusion. FRET depends on the overlap between the emission of donor and the absorption of the acceptor, therefore only a fraction of excitons is transported to interface through FRET and most through diffusion. However, the distance excitons travel during their lifetime, quantified as exciton diffusion length, is much shorter than the light absorption depth. Therefore, most of the excitons created in planar heterojunction OPVs are lost. We investigated the exciton diffusion in highly efficient non-fullerene acceptors (NFAs) and found that exciton diffusion coefficient is strongly dependent on the structure of the materials. Furthermore, we found that in all materials investigated, exciton diffusion coefficient is more than order of magnitude higher than fullerene. Although the development of NFAs reduces the distance which excitons have to travel due to diffusion of NFAs into the donor layer, however, enhanced three-dimensional exciton diffusion length (> 35 nm) in NFAs increases the device efficiency due to efficient charge generation. Furthermore, the large exciton diffusion length enables bulk heterojunction with large domains to be fabricated, thus enabling the efficient charge extraction and reducing the charge recombination problems.

Resume : One key advantage of solution-processable organic semiconductors is the opportunity of blending different materials in order to attain novel material properties and applications. The concept of ternary blend organic solar cells makes use of exactly that idea: three (or more) organic chromphores are combined to better match the solar irradiance spectrum and thus increase the amount of light absorbed, which in turn will increase the power output of the solar. Ternary organic solar cells consisting of a polymer donor and nonfullerene acceptors are delivering power conversion efficiencies in the excess of 17%. Now, further improvement needs to be directed towards enhancement of the operational lifetime of organic photovoltaics. Here, we selected three NFAs with different electron affinities and structural properties and found that the most crystalline third component, O-IDTBR, is selectively miscible within the acceptor phase. This reduced trap-assisted recombination and delivered a power conversion efficiency (PCE) of 16.6% and a fill factor of 0.76, compared to binary devices of PM6:Y6 (15.2% PCE). The charge transport and recombination analyses revealed that the third component acts as a charge relay for improved charge transfer of both donor and acceptor materials leading to a more ordered transport in ternary devices. Further, we find that minimizing trap formation in ternary devices deactivates light-induced traps upon full sun illumination (AM1.5G), compared to binary cells. As a result, under constant illumination, ternary devices do not show any loss in PCE for more than 225h, in comparison to binary cells which lose more than 60% of their initial performances.

Resume : During the last years, new material developments have led to constant improvement in the power conversion efficiency (PCE) of solution-processed organic photovoltaics (OPV) to nowadays record values above 17 % on small lab cells. In this work, we show the developments and results of a successful upscaling of such highly efficient OPV systems to the module level on large areas, which yielded two new certified world record efficiencies, namely 12.6 % on a module area of 26 cm² and 11.7 % on a module area of 204 cm². The decisive developments leading to this achievement include the optimization of the module layout as well as the high-resolution short-pulse laser structuring processes involved in the manufacturing of such modules. By minimizing the inactive areas within the total module area that are used for interconnecting the individual solar cells of the module in series, geometric fill factors of over 95 % have been achieved. A production yield of 100 % working modules during the manufacturing of these modules and an extremely narrow distribution of the final PCE values underline the excellent process control and reproducibility of the results. Challenges during this development are discussed along with the requirements material systems need to fulfill in order to be upscalable and eventually be used in an industrial production of fully printed OPV modules.

Resume : Electron acceptors with a chemical structure of A-DA’D-A (where A denotes an acceptor moiety and D a donor moiety) are rapidly gaining prominence in organic solar cells (OSCs). In OSCs containing these acceptors, record power conversion efficiencies (PCEs) exceeding 16% are now widely reported. Despite advances in new material design and PCEs, the fundamental interplay between molecular structures and device performance are still falling far behind. Here, we choose two model A-DA’D-A type acceptors Y3 and Y18 that have almost identical structures and examine how the presence of two extra alkyl chains (attached to the terminals of the DA’D core) in Y18 impact on the OSC device performance and its solid state properties. These properties include (i) charge transport, (ii) heat transfer, and (iii) electronic disorder. We found that bulk-heterojunction (BHJ) OSCs that use Y3 and Y18 have markedly different PCEs of ~13 and 16 %, respectively. Correspondingly, the BHJ with Y18 possesses significantly improved electron mobility, thermal diffusivity, and Urbach energy (EU). Among these properties, the extremely low EU value of 23 meV stands out because it is even under the thermal energy (~26 meV) which sets the electronic disorder limit at room temperature. All of these remarkable properties can be rationalized with a simple model in which the extra alkyl chains in Y18 help to suppress the formation of rotamers, unlike the case of Y3 in which rotamers are relatively easy to exist. The resulting disorder-free molecular conformations are more planar, leading to improved electron transport, enhanced phonon transfer, and suppressed electronic disorder, and ultimately much enhanced PCEs in excess of 16%. This work provides direct guidance for the future material design of non-fullerene fused ring acceptors using the A-DA’D-A type structures.

Resume : Perovskite solar cells (PSCs) have attracted considerable research interest over the past decade due to the repaid growth of power conversion efficiency (PCE) and low-cost fabrication process. In this work, we designed and synthesized conjugated polyelectrolytes (CPEs) to replace the benchmark PEDOT:PSS as the hole transporting material (HTM) of inverted PSCs and investigated the influence of the type of ionic moieties on the device performance. Herein, three CPEs with ammonium (NH3+), trimethylammonium (NM3+) and sulfonate (SO3-) moieties, namely PFNH3BT、PFNM3BT and PFSO3BT, respectively, were synthesized using fluorene (FL) and benzothiadiazole (BT) as building blocks. FL is a useful monomer to introduce various functional groups into polymer main chain by simply grafting the 9-position of the ring with two functionalized alkyl chains. The electron-deficient BT monomer and two thiophenes were applied to lower the HOMO level and reduce the twist angles between FLs and BTs. As expected, the HOMOs of these CPEs are deeper than that of PEDOT:PSS, effectively minimizing the energy loss for extracting holes from the perovskite film and, therefore, increasing the open-circuit voltage. More importantly, the type of ionic groups significantly influences the growth behavior of perovskites and interfacial charge recombination process. The SEM top view, X-ray diffraction patterns, and time-resolved photoluminescence of perovskite films confirmed the perovskites can grow into bigger crystals/grains with less defects on these CPEs than on the PEDOT:PSS surface. Moreover, the charge recombination resistance on the perovskite/HTM interface decreases in the order PFSO3BT > PFNH3BT > PFNM3BT > PEDOT. As a result, replacing PEDOT:PSS with PFNH3BT, PFNM3BT and PFSO3BT noticeably increases the PCE from 12.9 % to 17.7%, 14.9 % and 17.1 %, respectively.

Resume : A fast and flexible all-organic electrochromic device, fabricated using polythiophene and PCBM as active materials has been reported here. The device shows quantifiable improvement in electrochromic performance using parameters like switching speed, coloration efficiency, color contrast, and cycle life. Spectroscopic investigations have been carried out using Raman and UV-vis to establish a bias induced redox switching based mechanism for reported improvement in the performance. An enhanced stability for duration of longer than 2500 s and 250 cycles has been reported with an ultrafast response of few hundred milliseconds. A very high coloration efficiency of 321 cm2/C is achieved, making the proposed device one of the best reported P3HT-based electrochromic devices.When device is unbiased, the “initial” state (red curve), maximum absorption is observed in the vicinity of 550 nm indicating the absorption of green color (appearing magenta) in the transmitted light responsible for the magenta appearance to device. At 1 V bias (black curve), the absorbance decreases and all the wavelengths very poorly gets absorbed almost equally making the device appear transparent to the visible spectrum. On bias reversal (-1V) the magenta color gets reinstalled as evident from the increase in the absorption back in the green region mimicking the same spectrum as that of the unbiased device. The device shows more than 50 % color contrast corresponding to 550 nm when switched between magenta and transparent states.

Resume : Abstract: Small molecules have been recently highlighted as active materials owing to facile synthesizing methods, well-defined molecular structure, and highly reproducible performance. The design strategies of small molecule donors for high efficiency organic solar cell need further improvement .Halogenation is considered to be one of the effective way to modulate the electronic properties of conjugated small molecules . Herein, we report a series of benzodithiophene (BDT) based active materials with various halogen atoms substituted at the end-group, and how that halogen atoms affect the morphology of BHJ architectures through microstructure analyses. The material with chlorine atoms shows well-mixed morphology and interpenetrating networks when blended with PC71BM, facilitating efficient charge transportation. Morphological analyses revealed that chlorination has helped in the formation of fine interpenetrating network effectively .The controlled morphology helps to achieve high performance with power conversion efficiency (PCE) of 10.5 % and the highest fill factor of 78.0% without additives. In addition, it is applied to two-terminal (2T) tandem solar cell, attaining the outstanding PCE up to 15.1% with complementary absorption in the field of the 2T-tandem solar cells introducing the SM-SCs. These results indicate that tailoring interaction with halogen atoms is an effective way to control BHJ architectures, thereby achieving the remarkable performances in SM-SCs. Keywords: dithienobenzodithiophene (DTBDT), halogen-free processing, small molecule-based organic solar cells (SMOSCs), molecular engineering, pinhole-free SMOSCs

Resume : In this study we investigate charge carriers transport properties of PBDTTPD layers. Different type of structures were made with P type polymer as active layer. Structures with transparent ITO electrode and single layer of PBDTTPD with electrode on top were investigated by photo-CELIV and TOF methods to examine photogenerated holes’ mobility in the layer. Field effect transistor structures with SiO2 dielectric layer and PBDTTPD active layer were investigated by current transients method to examine holes’ mobility in parallel to the substrate direction where holes travel near dielectric layer and polymer layer interface. Holes’ mobility dependence on temperature were measured to estimate the disorder of the spincoated PBDTTPD material. Energetic disorder parameter σ was calculated according to Basslers hopping model.

Resume : In recent years, quantum dots (QDs) have attracted substantial attention for the fabrication of high-brightness, high-efficiency, pure-color quantum dot light emitting diodes (QLEDs). To achieve high performance, devices must be designed to obtained more and more electron-hole pairs recombine inside QDs. In this paper, high performance red quantum dot light-emitting diodes based on small molecule Tris(4-carbazoyl-9-ylphenyl)amine (TCTA) doped hybrid polymeric Poly(N,N'-bis-4-butylphenyl-N,N'-bisphenyl)benzidine: Poly(9-vinylcarbazole) (poly-TPD:PVK) hole transporting layer (HTL) have successfully fabricated. Efficient hybrid HTL was attributed to the high mobility of TCTA which made efficient hole injection from anode, leading to the balance of charge carriers inside our devices. Consequently, QLEDs device based on a 20-nm-hybrid-HTL presented a maximum luminance of 254,041 cd/m2 and a maximum current efficiency of 14.6 cd/A, corresponding to over 200% enhancement in comparison with reference devices base on poly-TPD or PVK only.

Resume : Organic solar cells (OSCs) possess the advantages of low-cost, lightweight, and intrinsic flexibility of active layers. Especially, the flexible active layer is the most distinguished characteristic of the OSCs which makes it most suitable to be fabricated into flexible devices. In addition, the optimized photoactive layer thickness of the OSCs is very thin (typically around 100–200 nm) with partial light transparency, and the color of the active layer can be tuned by modulating energy bandgap of the donor and acceptor materials, which renders the OSCs also suitable for semitransparent solar cells. Therefore, flexible and semitransparent OSCs have attracted tremendous attentions in recent years for their promising applications in wearable energy resources and building-integrated photovoltaics (BIPV).

Resume : Organic solar cells (OSCs) are attracting dut to thier colorful, transparant, flexible and affordable mass producible capabilities. Recently, the efficiency of OSCs has risen sharply to more than 16%, raising expectations for commercialization. However, from the point of view of commercialization, it is most important to be able to maintain stable long-term efficiency rather than initial efficiency. Unfortunately, it is generally known that organic solar cells have significantly lower stability, especially photostability, than other inorganic solar cells. Initially, this weak stability was thought to be due to the decomposition of organic matter through photoreaction. However, even if moisture and oxygen are controlled and the organic material itself is not decomposed under the condition that the stability of the device is found to be inferior in stability, it may be due to other causes. The effect on stability will be presented from changes in the structure of photoactive materials and the application of new interlayer materials.

Resume : Poly(3,4-ethylenedioxythiophene) (PEDOT) is an ubiquitous conductive material used in numerous optoelectronic applications such as photovoltaics, light emitting diodes, smart windows, transparent heaters (TH) and more. Its high conductivity, good flexibility, high transparency in thin film and easy processing make it an excellent alternative to Indium Tin Oxide (ITO). Indeed, owing to the intrinsic brittleness of ITO and Indium scarcity, alternative materials have to be developed to cope with the increasing demand for flexible devices and greener materials. Recently, we developed highly conductive PEDOT based materials with very high electrical conductivity, up to 5400 S.cm-1, through structure and dopant engineering.[1] These thin films exhibit low sheet resistance (<60 Ω.sq-1) associated with good transparencies (>87%) and a very low haze factor (<1%). We demonstrated for the first time the efficient use of thin films of PEDOT for transparent heater applications, on rigid and flexible substrates.[2] PEDOT materials are known to lack chemical stability overtime or under harsh conditions, making the device integration complex. We extensively studied ageing factors to better understand degradation mechanisms. Impacts on doping level, structure and electrical transport of PEDOT after ageing under different environmental stresses (light, temperature, humidity, oxygen) were analyzed through UV-vis-NIR, XPS, GIWAXS and temperature dependence of conductivity measurements. On the strength of this knowledge and the identification of the main damaging factors, we studied different encapsulations which allowed us to stabilize the electrical performance by significantly lowering the degradation rate. [1] M. Gueye et Al, Chem. Mater., 28, 3462-3568 (2016) [2] M. Gueye et Al, ACS Appl. Mater. Interfaces, 9, 27250-27256 (2017)

Resume : We unveil the loss mechanisms of photogenerated carriers in highly efficient bulk heterojunction solar cells (BHJSC) containing fullerene and non-fullerene acceptors by coupling photocurrent and quasi-steady-state photoinduced absorption (PCPIA) spectroscopy [1]. By obtaining the absorption cross section of free carriers, we directly evaluate the concentration of photocarriers existing in a full device. From the modulation-frequency-dependent and excitation-fluence-dependent data, we reveal the predominant recombination processes occurring in the studied BHJSC. The bimolecular recombination rate of photocarriers in BHJSC achieved from our PCPIA measurements is consistent with that assessed from commonly used charge extraction measurements. [1] L. Q. Phuong et al., J. Phys. Chem. C 123, 27417 (2019).

Resume : Substantial development has been made in non-fullerene small molecule acceptors (NFSMAs) that resulted to significant increase in the power conversion efficiency of non-fullerene-based polymer solar cells. In order to achieve the better compatibility with narrow bandgap non-fullerene small molecule acceptors, it is important to design the conjugated polymers with wide bandgap with suitable molecular orbital energy levels. Here we have designed and synthesized two D-A conjugated copolymers with same thienyl-substituted benzodithiophene and different acceptors, i.e. DTBIA (P1) and TDTBTA (P2) (and investigated their optical and electrochemical properties. Both P1 and P2 exhibit similar deeper highest occupied molecular orbital energy level and different lowest unoccupied molecular orbital energy level. Both the copolymers have complementary absorption with well-known non-fullerene acceptor ITIC-F. When blended with a narrow bandgap acceptor ITIC-F, the PSCs based on P1 shows a power conversion efficiency of 11.18 % with a large open circuit voltage of 0.96 V, Jsc of 16.89 mA/cm2 and FF of 0.69, which is larger than that for P2 counterpart (PCE=9.32 %, Jsc=15.88 mA/cm2, Voc =0.91 V and FF=0.645). Moreover, the energy loss for the PSCs based on P1 and P2 is 0.49 eV, and 0.63 eV respectively. Compared to P2, the P1 based PSCs showed high values of IPCE in the shorter wavelength region (absorption of donor copolymer), more balanced hole and electron mobilities and weaker bimolecular recombination.

Resume : Two D-A conjugated polymers based on same 8,10-bis (2-octyldodecyl)-8,10-dihydro-9H-bisthieno [2`,3`:7,8; 3”,2”:5,6]naphtho [2,3-d]imidazole-9-one donor and different acceptor units, i.e. benzothiadiazole (P104) and fluorinated benzothiadiazole (P105) were synthesized and their optical and electrochemical properties were investigated. The effect of incorporation of fluorine atoms into the benzothiadiazole BT acceptor unit on the photovoltaic performance when combined with the narrow bandgap non-fullerene acceptor ITIC-F was investigated. The overall power conversion efficiency of P105 :ITIC-F showed higher PCE (10.65 %) as compared to P104 :ITIC-F counterpart (8.32 %) is resulted from the improved values of Jsc, Voc and FF. High value of Voc is related to the deeper HOMO energy level of P105 and the larger values of both Jsc and FF are attributed to the efficient exciton dissociation and charge transfer due to the increased value of dielectric constant and reduced value of exciton dissociation and energy loss and more balanced charge transport. The intra/interchain interaction can be modulated by F atom substitution in the BT unit, resulting reduction in - stacking distance and increase in the crystal coherence length, benefiting the charge transport in the active layer. These results provide a simple effective strategy to fine-tune the optical and electrochemical properties and therefore improve the overall photovoltaic response. Acknowledgments This work was supported by RFBR (№18-53-80066, №18-53-45028, №18-29-23004, №18-53-53031).

Resume : The introduction of non-fullerene acceptors (NFAs) in organic solar cells (OSCs) recently allowed to reach efficiencies as high as 16% [1], unmatched by fullerene acceptors. Such impressive performance can be attributed to the NFA’s better light-harvesting capability, combined with reduced losses in non-radiative energy and charge-carrier recombination. Among the hundreds of NFAs designs, the NFA halogenation has proved to boost the photovoltaic performance of the resulting OSC due to improved electronegativity, increased crystallinity and higher charge mobilities, compared with the unsubstituted NFA [2]. While a big effort is being devoted to find the combination enabling the best OSC efficiency, little work is, however, being carried out in studying stability/degradation patterns of halogenated NFAs and polymeric donors. In addition, the existing literature seems to disagree on the actual influence of NFA presence on the OSC stability [3]. Here, two of the most promising acceptors within the NFA family - namely ITIC and ITIC-4F, will be analyzed. Photostability at different excitation wavelengths of the non-halogenated acceptor vs. its halogenated counterpart will be investigated via Raman, photoluminescence and absorption spectroscopy. Moreover, the materials’ thermal stability will be discussed by looking at how halogenation affects their temperature dependent optical, electrical and morphological features. [1] Nature communications 10.1 (2019): 2515 [2] Advanced Energy Materials 8.15 (2018): 1702870 [3] Joule, 2019, 3, 1, 215-226 and https://www.nanoge.org/proceedings/HOPV19/5c531d980044c8385969db89

Resume : Polymer solar cells have a great industrial interest thanks to their improved stability and power conversion efficiencies for both single and tandem architectures. This first results from the optimization of the active layer with performant donor/acceptor heterojunction. The sandwiching interfacial layers (Electron Transport Layer ETL and Hole Transport Layer HTL) also largely control the final properties. In this study, a novel organometallic material was synthetized and used as an ETL material for performant and stable PV devices [1]. This compound was characterized by X-ray diffraction, UV-vis optical absorption, and Fourier transform infrared (FTIR) and Raman microscopies. The functional properties were further investigated with the hybrid material integrated in ETL of PV regular device. The active layer of the cells is a heterojunction combining a low bandgap polymer acceptor (PTB7-Th) and a PC70BM fullerene as donor. The energy band gap of this new material is close to that commonly used in classical interfacial layers. The combined optimization of optical, electrical, and morphological properties, lead to a sound and reproducible conversion efficiency close to 10%. The remarkable value also results from the surface roughness of the hybrid layer that favored the hole-blocking behavior and thereby increased the fill factor up to 73 %. Last but not least, the hybrid ETL strongly improves the device stability in air compared to that based on ZnO as ETL. [1] D. Fredj and al. New Antimony-Based Organic–Inorganic Hybrid Material as Electron Extraction Layer for Efficient and Stable Polymer Solar Cells ACS Appl. Mater. Interfaces 2019, 11, 47, 44820.

Resume : The power conversion efficiency of single junction Organic photovoltaics (OPVs), recently surpassed 17%, attracting the scientific community to search the answer of renewable energy demands in organic materials [1]. Their ability to get processed on large area flexible substrates using low cost solution casting techniques; making them suitable for different applications, such as building integrated and indoor energy generation applications, in addition to their outdoor deployment. The non-fullerene acceptors (NFAs) has emerged as one of the essential parts of these high performing plastic cells [2, 3]. The higher performance of these devices can be attributed to the comparatively broader absorption range spreading up to the near infrared region of solar spectrum (NIR), and also possible non-radiative energy transfer from donor to acceptor molecules. The phenomenon of this energy transfer is termed as Forster resonance energy transfer, where the excitation energy absorbed from a donor molecule is transferred to the acceptor molecule of suitable band gap, without undergoing to the relaxation process. The photoluminescence quenching is considered as one of the signatures of FRET to occur. The FRET can further be transferred between first acceptor molecule to the second one, resulting in increased extent of charge generation and hence the performance improvement of ternary devices. In our research, we have achieved an optimal 12% PCE for NFA containing binary devices based on PTB7-Th: COi8DFIC photoactive system with 0.64 cm2 area. Both static and dynamic photoluminescence studies proven the involvement of FRET in better performance of these cells. More than 90% PL quenching showed the consumption of most of the absorbed photons in charge generation process. We are focusing on performance enhancement of these devices, adding another acceptor molecule with suitable energy gap, to achieve highly efficient ternary system. Based on this model, we have fabricated ternary PV devices using fullerene based (PC70BM) second acceptor molecule, and reached up to 10.8% PCE for these devices [4]. We have also investigated the energy transfer mechanism in both the binary and ternary blends. The area of these devices is larger than ordinary laboratory scale devices promising their scalability without compromising the performance. The suitability of other non-fullerene acceptors, to enable optimal energy transfer (FRET) between acceptor molecules to enhance the performance of these ternary devices is underway. The talk will also cover the energy transfer mechanism involved in an efficient charge transportation and collection to achieve optimal performance. References: 1. https://www.nrel.gov/pv/cell-efficiency.html 2. Wei, L. et al. Molecular order control of non-fullerene acceptors for high efficiency polymer solar cells, Joule, 3, 2019, 819. 3. Cui, Y. et al. Over 16% efficiency organic photovoltaic cells enabled by a chlorinated acceptor with increased open-circuit voltages, Nature Communications, 2019, 10: 2515. 4. Misra, Ravi K., Jayawardena, Imalka; Silva, S. Ravi P. ‘Investigating the effects of second acceptor on the performance of non-fullerene acceptor based organic solar cells’ accepted for presentation in Next Generation Materials for Solar Photovoltaics 2020, London, UK.

Resume : Time-resolved spectroscopy is an unrivalled tool to study photophysical phenomena, involving both neutral and charged excited species, for instance, the processes involved in photocurrent generation and those limiting the device efficiency. However, it is not straightforward to extrapolate (transient) spectroscopy results acquired after pulsed laser excitation with high photon density to devices operating under steady-state one sun illumination conditions. In order to bridge that gap, we quantified the rates of the different photophysical processes, which we identified by transient absorption spectroscopy, and used them to simulate operating solar cell devices. The simulations indeed reproduce the experimentally-measured performances and thus explain the losses during photocurrent generation. Finally, our simulations enable us to gain information which are experimentally not accessible, such as the density of neutral excited states, namely triplet excitons, in an operating organic solar cell. This paves the way to a better understanding of the impact of those states, their formation and recombination processes on the device performance.

Resume : Spectral upconversion luminophors have the significant potential in the photovoltaic systems, because they provide a route to absorb the solar photons with energy below the bandgap of photoactive materials. However, obtaining the high performance of ultrathin polymer solar cells (PSCs) by introducing upconversion luminophors has been greatly challenging. In order to allow the photonic interaction between photoactive layer and upconversion luminophors placed underneath the cell, a back-side reflector electrode that exists in typical PSCs for increasing the photon traveling length has to be removed, which indicates that an absolute level of PSC performance becomes degraded unexpectedly. In this regard, here we suggest a facile solution concept that can overcome this issue by exploiting a wavelength-selectively reflective electrode consisting of metal/dielectric multilayer. The proposed electrode transmits the photons only at excitation and emission wavelengths of the upconversion luminophors and reflects them otherwise, thereby enabling the effect of photocurrent enhancement without compromising absolute performance levels. To maximize the benefit from the upconversion luminophors, they are separately reinforced by plasmonic metal nanostructures. Details of optical properties and photovoltaic performance in both experiments and numerical modeling offer the optimal design process for high-performance PSCs incorporated with upconversion luminophors.

Resume : In bulk heterojunction (BHJ) organic solar cells, the energy offset at the donor-acceptor interface is the driving force for charge separation. In principle, both electron affinity (EA) and ionization energy (IE) offsets equally control the charge separation efficiency. For low-bandgap non-fullerene acceptor (NFA) blends, however, we find that the EA offset is practically unimportant, and photocurrent is controlled by the IE offset only because ultrafast energy transfer from the donor to the lower bandgap NFA takes place before the electron transfer. Our observations suggest that the energy transfer rate and sizable IE offsets are key quantities for the design of efficient NFA-based BHJ, in which donors and lower bandgap acceptors are combined to achieve high photocurrent densities.

Resume : With the recent rise in awareness of energy and environmental issues in modern society, it has been important to find more efficient and cheaper alternative energy sources than fossil fuels. Dye-sensitized solar cells (DSSCs), generally composed of a photoelectrode, a liquid electrolyte, and a Pt counter electrode, are considered to be a promising candidate as a large-area and low-cost renewable energy source, due to their various advantages, such as reasonable photovoltaic efficiency, low production cost, brief fabrication process, semi-transparency, and flexibility. Despite the fact that considerable progress has been made in the field of DSSCs, the risk of leakage of volatile liquid electrolyte limits significantly the commercialization of DSSCs. Significant efforts have been made on solid-state dye-sensitized solar cells (ssDSSCs) to replace former liquid electrolyte system, as well as the development of hole transporting materials (HTMs) with low cost, facile synthesis and high charge-carrier mobility has been of high priority. Many different HTMs such as TPD, NPB, spiro-OMeTAD, PEDOT, and P3HT have been used in ssDSSCs. Carbazole-containing materials have attracted much attention due to their good charge transport properties, synthetic versatility of the carbazole reactive sites with a wide variety of functional groups, and fine-tuning of optical and electrical properties, which have been exploited as new types of HTMs in organic light-emitting diodes and as electron donor group in organic sensitizers of D-π-A-type in DSSCs. Carbazole-based materials were also investigated as HTMs for efficient ssDSSCs and perovskite solar cells. In this study, we report on synthesis and characterization of four carbazole-based alternating copolymers, poly{[N-(2-ethylhexyl)-3,6-carbazole]-alt-aniline] (P-I) and poly{bis[6-N-(2-ethylhexyl)-carbazole-3-yl]-alt-aniline} (P-II), poly{bis[6-N-(2-ethylhexyloxylphenyl)-carbazole-3-yl]-alt-aniline} (P-III), and poly{[N-(2-ethylhexyloxylphenyl)-3,6-carbazole]-alt-[N-(4-aminophenyl)carbazole]} (P-IV), for use as polymeric HTMs in ssDSSCs. The structural integrity of four polymers was identified by 1H and 13C NMR spectroscopy. From photophysical and electrochemical characterization, HOMO, LUMO, and bandgap energy levels of compounds were measured. The feasibility as carbazole-based polymeric HTMs in ssDSSCs was investigated in this study.

Resume : Non-fullerene acceptors have attracted much attention for their great potential to achieve high power conversion efficiencies in organic photovoltaic devices [1]. However, the energetics at organic/metal or organic/organic interfaces in such non-fullerene-based devices are not fully understood. Thereinto, investigation of the energy level alignment at these interfaces and its depth-dependent variation (energy level bending) is of great importance [2]. The fabrication of homogenous ultrathin organic semiconductor films is crucial for the study of energetics at (hybrid) organic interfaces [3], but such films are difficult to achieve by solution-process due to pin-hole formation and the similar solubility parameters between conjugated molecules or polymers. Here, Langmuir–Schäfer (LS) method is utilized to fabricate homogenous mono- and multilayers of non-fullerene acceptors on top of metals, or high efficiency donor polymers on top of non-fullerene acceptors. Precise depth-dependent energy level bending at the organic-metal and organic-organic interfaces is obtained by photoelectron spectroscopy (XPS and UPS), providing insight into the properties of the corresponding organic photovoltaic devices. [1] Jianhui Hou, Et al, Nature Materials, 17(2018) 119–128. [2] Qinye Bao, et al, Advanced Materials Interfaces, 6 (2019) 1800897. [3] Qinye Bao, et al, Advanced Functional Materials, 26 (2016) 1077.

Resume : Copper thiocyanate (CuSCN) has recently attracted great attention as a low-cost solution processable hole transport layer (HTL) in both highly efficient perovskite and organic solar cells (Arora et al. Science, 2018, Yaacobi-Gross et al. Adv. Mat., 2015). We have recently reported a surprising new use for CuSCN as a photoactive component in hybrid inorganic/organic solar cells together with the methanofullerenes PC61BM and PC71BM (Eisner et al., Adv. Sci., 2018). Efficient charge transfer at the inorganic/organic heterointerface results in open circuit voltages approaching 1V and power conversion efficiencies (PCEs) of over 1% in simple planar heterojunction solar cells. Interestingly, when CuSCN and the methanofullerene are blended together in solution a mesostructured CuSCN-nanowire:fullerene heterointerface is created spontaneously, resulting in greatly increased charge generation leading to short circuit currents of over 8 mAcm-2 and PCEs of over 5% in solar cells. Here, we present an in-depth investigation into the nature of charge generation in this unique organic/inorganic heterointerface through the use of luminescence and highly-sensitive EQE measurements. We first identify the formation of a charge-transfer (CT)-like state between the CuSCN and the fullerene in the planar heterojunction devices, in analogy to CT states observed at all-organic heterointerfaces. Additionally, in the presence of the nanowire:fullerene mesostructure a completely new CT-like state is observed, which is strongly redshifted compared to the original one. Both the relative intensities of the states, and the energetic position of the nanowire CT-like state can be tuned by the density of nanowire formation. These results raise intriguing questions on the nature of charge transfer at hybrid organic/inorganic heterointerfaces. We consider the implications of tuneable charge-transfer states at nano-structured interfaces on photovoltaic device performance and for potential new device applications.

Resume : Matched energy level alignment is a key requirement for efficient organic devices such as organic light-emitting diodes, photovoltaics, and field-effect transistors. The effect of thermal stress/annealing on energy level alignment and related properties of the devices are less discussed compared to the extensively explored effect on morphology and corresponding device performance. Here all polymer solar cells (all-PSCs) are employed to study thermal annealing effects on energy level alignment and the corresponding effect on the device properties of the all-PSCs. It is found that optimized energy level alignment can be achieved by thermal annealing. An interface dipole layer at the donor/acceptor interface is introduced by energy level realignment that assists charge generation by reducing geminate recombination so that the voltage loss is dramatically reduced, improving the performance of the all-PSCs.

Resume : Despite relatively high performance of perovskite solar cells, there is still a long list of things to do before requirements for commercialization are met. A possible weak link is dopant-containing hole transporting materials, comprising considerable amounts of oxidized hole transporting material (HTM). It was found that investigated oxidized HTMs can react with 4-tert-butylpyridine forming pyridinated derivatives. During thermal stability testing of the films kept at 100 °C it was noticed that the oxidized HTMs start to degrade and partly revert to the original unoxidized material and partly react with tBP, if it is present in the film. Recently, we have synthesized hole-selective phosphonic acid derivatives capable of forming self-assembled monolayers and used them for the modification of the ITO surface. Formation of the monolayer was proved by means of several techniques, including contact angle measurements, FTIR, SFG etc. Developed system allowed for construction of perovskite solar cells in p-i-n configuration which routinely demonstrated over 20% power conversion efficiency, with the highest achieved result of 21.2%. The developed materials function without doping, can be deposited using diluted solutions, from environmentally friendly solvents (such as alcohols) utilizing various solution deposition techniques. We believe that by further optimization of the molecular structure even better result can be achieved, without compromising price and stability.

Resume : Nowadays, organic photovoltaic structures (OPVs) have received great attention in the scientific community due to their potential of becoming cheaper alternatives to conventional silicon solar cells. The design of an efficient organic photovoltaic cell consists of donor material and an acceptor one, stack between two electrodes, either as bulk-heterojunction or layer-by-layer configuration. Frequently, as donor material is used poly(3-hexylthiophene) – P3HT, a conjugated polymer, while the acceptor is a fullerene derivative, i.e. phenyl-C71-butyric acid methyl ester – PC71BM. Despite their relatively good power conversion efficiency (PCE) in a controlled atmosphere, OPVs time stability in ambient conditions without encapsulation is relatively poor. Our study is focused on the fabrication and characterization of a customized active layer by spin-coating, involving tin oxide nanoparticles (SnO2 np), P3HT:PC71BM:SnO2 np (1:1:0.5) in order to improve both the PCE and time stability. The optical and morphological analysis of prepared glass/P3HT:PC71BM:SnO2 np (1:1:0.5) active layer showed a high absorption coefficient and relatively uniform distribution of SnO2 nanoparticles into the P3HT:PC71BM network. The specific parameters of OPVs were determined and were discussed in terms of P3HT:PC71BM (1:1) polymeric blend. Keywords: P3HT, PC71BM, SnO2 nanoparticles Acknowledgments: The Romanian National Authority for Scientific Research, UEFISCDI, under the project 40PCCDI/2018.

Resume : Polyvinylpyrrolidone (PVP) has been used as the electrode interfacial layer (EIL) in various of organic solar cells (OSCs) for work function modification . However, detailed insight into the effect of PVP interlayer on the physicochemical properties of the ITO electrode in the inverted OSCs is still absent. In this work, we investigate the ITO/PVP interface by photoelectron spectroscopy and establish the mechanisms for the energy level alignment on different substrate. The results indicate that the dipole formation is not only driven by the intrinsic molecular dipole moments associated with the γ-lactam of PVP, but also the energetic stabilization by its image charges in the contacting (semi-)conductor. In addition, high-performance I-OSCs were achieved by introducing a self-assembled ultrathin PVP layer with simple immersion method. This work provides enhanced understanding of the PVP-based EIL and demonstrates its great potential in inverted OSCs fabrication, which can pave the way to simplified manufacturing of low-cost and large-area organic electronic technologies.

Resume : Imbalanced charge transport is a common issue in organic solar cells. Here, we provide insight into the interplay of bulk and surface recombination in the presence of mismatched electron and hole mobilities. As a model system, we use an organic blend system based on a squaraine dye and a PCBM acceptor with a two orders of magnitude higher electron than hole mobility. We show that small variations in the work function of the anode have a strong effect on the light intensity dependence of the open circuit voltage (Voc). Combining experiments and a well calibrated numerical model, we demonstrate that the ratio between electron and hole mobility critically determines whether Voc is dominated by surface recombination or bimolecular recombination in the bulk. With the help of our numerical model we generalize these findings and determine under which circumstances the effect of contacts is stronger or weaker compared to the idealized case of balanced mobilities. Finally, we provide analytical expressions for Voc that take into account the pile-up of space charge due to highly imbalanced mobilities.

Resume : Space charge effects can significantly degrade charge collection in organic photovoltaics (OPVs), especially in thick‐film devices. The two main causes of space charge are doping and imbalanced transport. Although these are completely different phenomena, they lead to the same voltage dependence of the photocurrent, making them difficult to distinguish. Herein, a method is introduced on how the build‐up of space charge due to imbalanced transport can be monitored in a real operating organic solar cell [1]. The method is based on the reconstruction of quantum efficiency spectra and requires only optical input parameters that are straightforward to measure. This makes it suitable for the screening of new OPV materials. Furthermore, numerical and analytical means are derived to predict the impact of imbalanced transport on charge collection. It is shown that when charge recombination is sufficiently reduced, balanced transport is not a necessary condition for efficient thick‐film OPVs. [1] Wilken, S., Sandberg, O.J., Scheunemann, D. and Österbacka, R. (2020), Sol. RRL. doi:10.1002/solr.201900505

Resume : Charge carrier mobility is a central property that determines the performance of organic semiconductors in many devices and has been shown to be highly dependent on molecular structure and the related thin-film nanostructure [1]. Planar molecules are expected to enhance intermolecular interactions and facilitate charge transport along with the stacking directions. However, it often remains difficult to design highly planar molecules that are both soluble and have the desired opto-electronic properties. In this work, we investigate a donor-acceptor-donor small-molecule based on a TAT-TPD-TAT structure, where TAT represents a planar, highly soluble, functionalized electron donor triazatruxene moiety and TPD an electron acceptor thiophene-thienopyrroledione-thiophene unit. These molecules led recently to promising results in solar cells [2], which were partly attributed to a relatively high SCLC hole mobility of 2.10-3 cm2/V*s in as-deposited films. Here, an in-depth investigation of the SCLC mobility as a function of annealing treatment is presented. It is shown that the mobility increases by two orders of magnitude when changing from the as-deposited to the crystalline state. A remarkably high SCLC mobility of 0.2 cm2/V*s could be achieved in 30 µm thick devices. [1] Y. Tsutsui, Y. H. Geerts et al. Adv. Mater., 2016, 28(33): 7106-7114. [2] T. Han, T. Heiser et al. J. Mater. Chem. C, 2017, 5(41), 10794-10800.

Resume : Interface phenomena are one of the primary factors determining the performance of thin film opto-electronic devices. However, examining and understanding the physics at these interfaces is one of the most challenging aspects in semiconductor characterization. To address this, we investigated the interfaces between organic semiconductors evaporated under vacuum using in-situ spectroscopic ellipsometry, and developed a novel analysis method to examine, firstly, the growth of pristine organic semiconductor films and, secondly, the interfaces between different organic semiconductor systems. The novel analysis method derives – at every time point/thickness and without any model fitting – dynamic data representative of the refractive index and absorption coefficient which is typically obtained from model fitting to the ellipsometry data. Comparing the growth of the pristine films on quartz/silicon wafers (e.g. C60) with that deposited on a different organic semiconductor base (e.g. C60 on SubPc), we notice distinctive anomalous features in the probed energies (0.7-5.9 eV/ 210-1690 nm) at specific energies above the absorption edge starting from the interface with the underlying organic semiconductor. We will present data for different planar heterojunctions where the energies at which the anomalies occur change with the underlying layer. Understanding what these anomalies mean could provide insights into the physics of charge generation in organic semiconductor systems.

Resume : Organic photovoltaics (OPV) have recently reached certified power conversion efficiencies (PCE) of more than 17% for single junction solar cells. This progress is mostly owed to the development of novel photoactive materials including fluorinated polymers and non-fullerene acceptors. Besides high efficiencies, scaling up of OPV devices to large area modules free from rare materials and fabricated without the use of toxic solvents and/or additives need to be demonstrated in order to approach the market. We recently reported ITO-free organic solar cells based on a fluorinated copolymer with low synthetic complexity processed from an o-Xylene/p-Anisaldehyde mixture, yielding a PCE of 7.8%. After scale-up of the polymer synthesis, blade coated OPV modules with an active area of 66 cm² and PCE above 6% were fabricated. In the present study, large area OPV modules (total area: 44 x 32 cm²) with 14 solar cells monolithically interconnected in series were fabricated by slot-die coating of solution-processable layers and rotary screen printing of the top grid electrode. In comparison to small area test solar cells, no voltage losses have been observed, which could have been caused, e.g., by reduced parallel resistance due to local coating defects. Luminescence and thermographic imaging techniques have been applied to identify local losses in the modules, giving insights into further optimization possibilities for the module fabrication.

Resume : In this work we study the thermoelectric and optical properties of PEDOT:PSS thin films. We mainly focus on the influence of the thickness of the films on their thermal and optical properties. We address the problem in a comprehensive manner based on the concept of thickness gradients, i.e. we study a single sample deposited through blade coating on a commercial glass microscope slide, where the thickness of the PEDOT:PSS thin film changes along the deposition direction. The thickness at each position on the samples was determined using spectroscopic ellipsometry and ranges between 10 nm and 250 nm. The key advantages of this approach are: (i) the high throughput of experimental results gathered within one sample, and (ii) the obtained results provide and excellent relative determination of the influence of the thickness on investigated particular property (thermal conductivity and Raman in this case), since a single fabrication step is used making each measurement consistently comparable. In particular, we have found a two-fold increase for the thermal conductivity as the thickness decreases. In order to understand this behavior, we have studied in detail the Raman spectra for each thin film thickness. Our observations suggest that the increase in the thermal conductivity originates from a morphology change of the PEDOT:PSS which is induced varying the thickness of the films.

Resume : Hybrids structures present great potential for application in optoelectronics, photovoltaics, light-emitting diodes and photodetectors. They allow combining the versatility of the chemical structure of the organic materials with the well-established properties of the inorganic compounds [1]. A PN diode-based in the heterojunction of zinc oxide (ZnO) and polyhexylthiophene (P3HT) using gold (Au) and zinc doped aluminum (AZO) as electrodes, with AZO/ZnO/P3HT/Au architecture was studied. The AZO transparent electrode was obtained during the deposition of zinc acetate dihydrate onto a 10nm thickness Al layer by spray pyrolysis at 450oC. The Al atoms diffuse upward while the ZnO is deposited doping it in the first layers, after that the further ZnO layers are not doped by Al. Then, AZO film forming the transparent electrode is deposited in situ and at the same step that the active layer of ZnO. The P3HT layer was deposited by spin coating and finally, was deposited a gold electrode by vacuum metallization. The diode was characterized by measurements of I x V curves and also by capacitance in the function of frequency and voltage. The values of rectification ratio, barrier height (ΦB), series resistance (RS), density of donors (Nd) and acceptors (Na) were ~ 103; 0.7 eV; 4800 Ω; 4.0 x 1018 cm-3 and 5.8 x 1018 cm-3, respectively. The discussion of the effects of interface states by capacitance measurements is done matching with the different mechanisms of conduction observed in logarithmic of current curves. The hybrid PN diode, coupled with the use of the transparent AZO electrode present quite relevant results for applications in printed photodetectors.

Resume : Inverted organic photovoltaics (OPVs) have been studied due to the low stability in conventional structure. For the inverted OPVs, insertion of electron transport layer (ETL) between the photoactive layer and cathode is essential to improve the power conversion efficiency, and the metal oxide (e.g. ZnO) and conjugated polyelectrolyte have been widely used. In most studies, optimization and mechanism analysis have been focused under AM 1.5 sun condition. However, the working mechanism of OPVs varies depending on the light intensity, and thereby further understanding on the function of ETL needs to optimize OPVs performance under low-intensity light including indoor conditions. In this study, we demonstrate that ZnO provides better performance as well as reproducibility compared with a conjugated polyelectrolyte ETL regardless of the light intensity. In addition, appropriate ETL material should afford excellent charge selectivity and effective coverage of the bottom ITO surface to prevent shunt site formation, which enables the development of highly efficient OPVs under low-intensity light including indoor conditions.

Resume : Direct Arylation Polycondensation (DArP) has been demonstrated as a versatile and more environmentally friendly method for the synthesis of conjugated polymers. Conventional C-C couplings in emulsion have been reported to give conjugated polymer dispersions with narrow particle distributions and potential scalability. In this work, the application of DArP in an emulsion polymerisation was investigated. This entailed overcoming the well-known challenges of the DarP procedure but also considering reaction parameters that favour the emulsion stability and partition of reagents into either the aqueous or organic phase. Hence, judicious parameter screening concluded in the preparation of stable emulsions of polymers containing various electron deficient moieties e.g. diketopyrrolopyrroles (DPP) and naphthalene diimides (NDI). The nanoparticle dispersions obtained were fully characterised showing that this method results in defect-free polymers with molecular weights (Mn > 10 kg mol-1) comparable to conventional solution polymerisations and reported values. The well-defined aqueous CPNs with narrow particle distribution were used to fabricate OFETs directly from the aqueous emulsion and the thin films exhibit mobilities in top-gate-bottom-contact field effect transistors comparable to the literature (~ 0.2 cm2 V-1 s-1 for poly(DPP-co-tetrafluorobenzene)). In conclusion, aqueous dispersion of CPNs synthesised by DarP and subsequent device fabrication can be performed using the champion amongst environmentally friendly solvents: water.

Resume : Organic photovoltaics (OPVs) have been considered as one of the keys for energy harvesting. The reason is that the OPVs have various possibilities to be used as applications such as low power consumption electronic devices or the Internet of Things. To realize these applications, the OPVs need to have high power conversion efficiency under any irradiation circumstances. Here, we exhibited highly efficient semi-transparent OPVs (ST OPVs) based on quaternary blends. The optimized quaternary OPVs (Q-OPVs) work more effectively with a broadened absorption spectrum and enhanced charge transportability by suppressing recombination than the binary OPVs in this article. Besides, the ST Q-OPVs operate efficiently in both sunlight and indoor light conditions and have color codability. These characteristics of the ST Q-OPVs can provide the chance for applicability to a variety of applications.

Resume : The earth receives more energy from sun in an hour (4.3x1020J) than its annual energy demand (4.1x1020J), indicating a large gap between its present utilization and a huge unused potential. Solar photovoltaics (PVs) is one of techniques directly converting solar energy to electricity. The market of solar PVs is dominated by the well-established Si cell technology so far, reaching to its saturation point with the power conversion efficiency (PCE) of 26.1% for single crystal, and 22.3% for the multi-crystalline based solar cells. The field of organic photovoltaics (OPVs), on the other hand, started to attract attention again due to demonstration of highly efficient single junction OPVs with up to 17.4% PCE. OPVs showed huge potential due to their ability to get processed on large area flexible substrates using low cost solution casting techniques; making them suitable for different applications, such as building integrated and indoor energy generation applications, in addition to their outdoor deployment. The introduction of non-fullerene acceptors (NFAs) in organic active layer resulted in such a high performance; , due to their absorption up to the near infrared region of solar spectrum (NIR). Ternary systems with more than one donor or acceptor in organic active layer, provides even higher efficiency due to the improved photon collection and charge generation in larger part of solar spectrum. In our recent work, we fabricated binary and ternary NFA based PV devices using PTB7-Th: COi8DFIC and PTB7-Th: COi8DFIC: PC70BM respectively. The PCE achieved for these binary and ternary cells was 12% and 10.8% for 0.64 cm2 cells. The emission spectrum of donor and absorption of both the acceptor molecules in complimentary range of solar spectrum, enhanced the possibility of nonradiative energy transfer from donor to acceptor molecules. The photoluminescence quenching, φq of up to 90% showed the conversion of most of the absorbed solar photons to the charge generation (manuscript under preparation). Our current research is now focused on upscaling of these devices and achieving highly efficient large area modules of 64 cm2 area based on these binary blends using slot-die coating technique. The process optimisation parameters, investigation on optimal connections of pixels and minimizing resistance losses during upscaling are the scope of this study and will be the part of the talk.

Resume : Ternary blend in Organic Photovoltaics (OPVs) have been known as a way to increase performance in both low light intensity and AM 1.5G conditions. We have conducted a comparative study on 2 Donors:1 Acceptor (2D:1A) blends and 1 Donor:2 Acceptors (1D:2A) blends; 1D:2A blends have the higher performance and thermal stability than 2D:1A blends. In particular, non-fullerene acceptor (NFA) used for the 1D:2A blends have a good effect on packing property and crystallinity. The 1D:2A with optimized morphology reduced charge recombination on various irradiation conditions while the efficiency of 2D:1A devices depends on the fluorescence resonance energy transfer mechanism. The synergistic effect of 1D:2A blend with NFAs secures better efficiency and sustainable photoelectric properties under both indoor and outdoor operation conditions. Our analysis of the experiments reveals that NFA optimizes the arrangement of molecules to build more efficient ternary bulk heterojunctions in 1D:2A blends. Finally, the optimized ternary OPVs show effectively high power conversion efficiency of over 25% in both 200 lux and 1000 lux light–emitting diode conditions and present new opportunities in various practical applications.

Resume : The latest efficiency achievements in organic solar cells, 17.3% power conversion efficiency for multijunction and 17.0% for single-junction solar cells, combined with their appealing mechanical properties, which allow for easy scalability, and therefore, low price, are the characteristics that define OPV as a highly appealing energy generation technology of the future. In working conditions, oxygen and light cause structural changes in the organic materials that the organic solar cells are composed of, causing a decrease in their performance over time. The utilization of stabilizing additives in the active layer can counteract singlet oxygen-mediated processes1 and radical chain oxidation2, resulting in solar cells with a drastically extended lifetimes. Additionally, bendable and stretchable devices experience mechanical crack formation and delamination that also decrease cells’ lifetimes. The presented work reports on the implementation of a naturally-occurring additive, -carotene, for stabilization of bulk heterojunction solar cells with differing dominating degradation mechanisms, and explains the stabilization mechanism responsible for the tremendous lifetime improvements.1 As an outlook, combined photochemical and mechanical stabilization of the cells is explored, setting out a promising direction for highly flexible and stable OPV devices.

Resume : Organic Solar Cells (OSCs) have attracted the attention of the scientific community due to the various advantages that make them suitable to replace the expensive silicon in conventional photovoltaic: they are lightweight, flexible, and have a low production cost. Tin oxide films (SnO2) are commonly used as cathode buffer layer in OSCs. Although these films present a high visible light transmittance, near-infrared light reflectivity and excellent electrical properties, pure SnO2 has a low conductivity. The aim of this work is to improve the latter with doping. The undoped and Sb doped tin oxide films (SnO2:Sb) have been prepared by sol gel technique. The effects of Sb doping on the structure and photoelectric properties films were investigated using different characterization techniques such as, X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), UV-Visible spectroscopy (UV) and Hall Effect measurement. From the Hall Effect measurement, it was clearly observed that the electrical conductivity of the Sb-doped SnO2 is strongly enhanced, compared to the undoped SnO2. We then studied the inverted bulk heterojunction (BHJ) solar cells based using poly(3-hexylthiophene-2,5-diyl) : [6,6]-phenyl-C61-butryric acid methyl ester (P3HT:PC61BM) as active layer, where the doped and undoped SnO2 films are used as a cathode buffer layer. The device’s PV properties will be investigated through the measurement of the I/V characteristic and external quantum efficiency (EQE).

Resume : The inclusion of non-fullerene acceptors (NFAs) in the fabrication of organic solar cell (OSC) devices in recent years have paved the way to achieve power conversion efficiencies (PCE) of well over 10%, with the recent record being in excess of 17% for a solution processed OSC. These developments are critical for the OSC field, as they take the technology many steps closer to successful large scale commercialisation. The replacement of fullerenes with NFAs has been one of these crucial developments which have enabled higher PCE OSCs, however there are few studies at this moment on vacuum evaporated NFA OSCs. One of the levers to improve the PCEs of OSCs is the morphology of the thin films. In this work we attempt to study the morphology of vacuum evaporated NFA films using techniques such as GIWAXS, XRD, and AFM, and combine these results with optical measurements such as Spectroscopic Ellipsometry and UV-vis Spectroscopy to see how the morphology affects the optical properties of the thin films. The goal is to subsequently draw a correlation to the device performance of NFA OSCs. We would then compare the results with the current theory as well as solution processed OSCs in literature. It is our hope that this work will contribute to the current understanding of the behaviour and role of NFAs in OSCs as well as allow for the fabrication of more efficient evaporated OSCs.

Resume : As organic semiconductors attract increasing attention to application in diverse fields such as bioelectronics and artificial photosynthesis, understanding and improving their robust operation in a variety of challenging environments is a critical task. In particular, the application of bulk-heterojunctions to artificial photosynthesis via photoelectrochemical water splitting is an emerging topic, but previous work[1] has established the limitation of poor stability in these systems that require the use of oxide (e.g. TiO2) protecting layers. Herein, we identify the key factors that govern long-term operational stability of bulk heterojunction photocathodes with a direct organic/water interface using sacrificial electron acceptors and impedance spectroscopy. These insights lead to a significant improvement in the performance of stability and a new benchmark in the direct H2 production using organic semiconductors. Aspects of catalysis and charge-carrier separation/transport are also discussed. [1] L. Yao, A. Rahmanudin, N. Guijarro, K. Sivula, Adv. Energy Mater. 2018, 1802585.

Resume : Potassium-ion batteries (PIBs) have recently attracted a particular attention since there are abundant resources of potassium in both Earth’s crust and oceans, which make this element almost an order of magnitude cheaper than lithium. Moreover, KIBs usually show higher discharge voltages in comparison with lithium-ion and, especially, sodium-ion batteries thus featuring a potential for reaching high energy densities. However, the current progress in the development of PIBs is strongly limited by scarce number of suitable inorganic cathodes, which can deliver decent capacities and charge-discharge cycling stabilities. Organic redox-active materials represent a promising alternative since they are based on abundant elements, can be cheap and environment friendly. In this work, we present potassium-ion storage properties of a series of new polymeric materials derived from triquinoyl as a precursor. Using 2.2 M KPF6 in diglyme as electrolyte, the best materials demonstrated impressive specific discharge capacity of 396 mAh/g at the current density of 0.5 A/g in combination with good operation stability for hundreds or even thousands of cycles. One of the polymers showed also good rate performance while delivering the capacity of 205 mAh/g at 12 A/g. The obtained results are among the best reported so far for both organic and inorganic electrode materials utilized in potassium-ion batteries.

Resume : Associated with the growth of the internet of things the number of intelligent devices such as sensors, actuators or radio transmitters will raise strongly. Micro energy harvesting with organic photovoltaics is a very promising solution to achieve a sustainable powering of these devices without using/exchanging batteries or laying cables. Typical bandgaps of absorber materials in organic solar cells (OSC) are perfectly suited for typical indoor light spectra which are tailored to the visible range. Since a modern and energy efficient indoor light source has a very narrow spectral distribution, thermalization losses and non-absorption of sub-bandgap energy photons are extremely reduced compared to sunlight. Thus theoretical power-conversion efficiencies of more than 50 % are possible. Besides the narrow spectrum the indoor illumination intensity is much lower, typically by a factor of 300-2000 compared to the 1000 W/m2 of the AM1.5G spectrum. At such low light intensities good device performance can only be achieved with very large values for the parallel resistance (Rp) which is a challenge, especially for larger areas (modules). Our approach is to use a novel electrode system which ensures a sufficiently large Rp even in the presence of coating defects due to a self-passivation effect. This way we could achieve promising results both for small area cells and larger area modules on flexible substrates at an illumination intensity corresponding to 200 lux.

Resume : Water-based organic nanoparticle dispersions (solar paint) enable the control of the nanoscale architecture of the active layer whilst eliminating the need for hazardous organic solvents during device fabrication, thus providing a route to completely green manufacturing of printed solar cells. However, the structure-function of these nano-colloidal systems is complex and requires characterisation techniques that can map chemical structure in three dimensions on the nanoscale. This paper will describe the structure-function relationships of organic electronic nanoparticulate thin films used in organic photovoltaics, from small scale devices all the way up to large scale printed solar cells. In particular, the role that synchrotron X-ray techniques can play in characterising these materials will be discussed including recent work using laminography to determine the 3D chemical structure of the colloidal nanoparticles. Finally, the future prospects and economics for large scale manufacture of solar cells based on printing will be explored.

Resume : For industrial application, high stability, low cost and decent efficiency are prerequisites for organic solar cells (OSCs). Single-component materials (SCMs) with covalently-bonded donor and acceptor in one molecule, present intrinsic high morphological stability and low synthetic complexity. Herein, for the first time, a series of SCMs are systematically researched. They exhibit the highest stability so far for OSCs, with either 95% PCE remaining after aging accelerated under concentrated light (equal to 10 years under 1 sun), or 100% PCE remaining after aging at 160 oC for over 400 hrs, or 100% PCE remaining after aging at 90 oC and 1 sun bi-pressure for 300 hrs. The ultra-high stability evidences SCMs as extremely promising OSC frontier. However, the PCE is around 2-3% due to insufficient phase separation and the consequent severe recombination loss, and their development is limited by the blur understanding of photophysics. Encouragingly, beneficial from enhanced thermal-driven phase separation, 6.3% PCE and 65% FF are achieved for the first time, and herein complete charge carrier dynamics including charge generation, separation, transport and recombination are systematically researched. High crystalline and high charge carrier mobility are demonstrated. From transient and small perturbation measurements indicate efficient charge dissociation and low recombination loss. SCMs show significant potential for 10% efficiency with 20 years lifetime for practical application.

Resume : In organic heterojunction devices, current generation results from the sequence of photon absorption, charge separation, and charge collection in competition with recombination. To understand and design organic PV devices, we need models of these processes that incoporate both the device architecture and the molecular nature of the materials. Device models work fairly well in describing charge collection and recombination, and resulting curent-voltage curves, but usually with some empirical form for the charge generation efficiency and recombination coefficients. A full description of microscopic processes such as interfacial charge transfer requires molecular scale models. For design purposes, we would like to be able to predict device behaviour from the properties of the molecular components, but it is challenging to combine these aspects in a single model. In this talk we will discuss the degree to which molecular level models and time-resolved device models can explain measurements both of charge carrier dynamics, and of overall device behaviour. We will then address the challenges in bringing the two approaches together into a single framework.

Resume : Organic solar cells that have achieved high open-circuit voltage with the use the non-fullerene acceptors has been demonstrated very low voltage losses which are comparable to metal halide perovskite solar cell with a similar optical gap. However, their short circuit current is much lower, largely due to the presence of the wide distribution of sub-band tail states that limit the collection of photo generate charge carriers. Here we study a wide range of material combinations. Our results suggest an intrinsic correlation between the distribution of tail states and photocurrent collection efficiency of organic polymer:acceptor system.

Resume : Organic field effect transistors (OFET) have so far not achieved major commercial impact, despite their many attractive properties such as low-cost, low-temperature processing, and flexibility. In this talk, we will discuss recent work which addresses some of the shortcomings of the OFET. In particular, we discuss vertical transistor structures which have very short channel length without micropatterning. These structues allow much higher current densities than the lateral OFET despite rather simple processing technology. These devices are e.g. well suited to drive organic light emitting diodes (OLED), allowing all-organic flexible OLED displays. Recently, we have achieved current densities as high as kA/cm2 /1/ which will allow to use these devices for applications where higher power is required, such as bright OLED displays. These devices have also shown record operating frequencies for organic transistors, above 40MHz /2/. They also show very interesting nonlinear properties such as negative differential resistance (NDR) due to the heating-induced improvement of mobility /3/. Recent studies prove excellent stability, sufficient for broad applications /4/ 1. M. P. Klinger et al., Sci. Rep. 7, 4471 (2017) 2. B. Boroujeni et al., Sci. Rep. 8, 7643 (2018) 3. M. Klinger et al., Scient. Rep. 8, 9806 (2018) 4. F. Dollinger et al., Adv. Electron.Mater.5, 190057 (2019)

Resume : The photothermal effect, a conversion process between photons and heat, of plasmonic nanoparticles is a promising concept leading to new applications in solar energy harvesting and photodetection.[1][2] In this work, a solution-processed flexible photo-thermoelectric energy conversion device based on colloidal plasmonic gold nanorods and PEDOT:Tos/Carbon has been fabricated for solar energy conversion. The gold nanorod photothermal layer was carefully designed to exhibit a suitable optical absorption to cover the entire visible spectrum. By integrating the gold nanorods onto one of the electrodes of a PEDOT:Tos/Carbon thermoelectric (TE) device, the hybrid device can produce electrical output under simulated solar illumination. Based on a coupling between the photothermal effect from gold nanorods and the Seebeck effect provided by the organic TE device, under AM1.5G simulated illumination (100 mW cm-2), a temperature difference can be generated across the device, thus providing an electrical power output of 63.57 nW. Combining several advantages in terms of flexibility, solution-processability, light weight, and the absence of toxic heavy metal elements, the current hybrid nanorod/organic TE devices are promising for alternative solar energy conversion strategies. [1] Minghui He,et al. Nano Energy 49(2018)588-595. [2] Hengyang Xiang,et al. Small 2018,14(16) ,1704013.

Resume : Unlike LCD, OLED display can realize true deep black and has the excellent characteristics that display should be equipped as a key information communication tool in the 4th industrial revolution with fast response speed, high contrast ratio, wide viewing angle and high color gamut. In addition, it is possible to expand the scope of application to not only the best display that can realize form factor free, but also a transparent display and a mirror display with top emission. However, IR drop phenomenon is the most critical issue in realizing such a large top emitting OLED display. Since IR drop phenomenon causes luminance non-uniformity, the compensation method must be applied. At present, since the IR drop compensation method is difficult to implement, the bottom emission method is applied to the commercially available OLED TV. However, the bottom emission method causes a reduction in the lifetime of the OLED, which is a self-luminous device, because the aperture ratio is restricted due to the interference of the TFT backplane metal line. Therefore, when applied as a top emitting method that can maximize the aperture ratio without interfering with the lower TFT backplane line, it is possible to improve the life of the OLED by lowering the current density, and furthermore, it can be applied to the transparent display or the mirror display. In this paper, we report on the results of a new compensation method that can compensate for IR Drop in such a large top emission OLED display to secure luminance uniformity. This new IR Drop Compensation method can increase the luminance uniformity to more than 85%, and has been successfully applied in 55-inch transparent display, 55-inch mirror display and 65-inch 4k, 8k top-emitting QD(Quantum Dot) -OLED display.

Resume : Organic electronics is nowadays gaining increasing attention from both the scientific and industrial communities due to the wide range of applications that can be explored. For example, among these applications, huge efforts are spent in the improvement of Organic Light Emitting Diodes (OLED), Organic Photovoltaic (OPV), Organic Field-Effect Transistors (OFET). Nevertheless, there exist a large family of applications, including high resolution flexible displays, wireless sensors and radio frequency identification systems (RFID), that require a high operational speed of the main electronic components and, in particular, of the transistors. The widely adopted figure of merit employed to characterize the performances of the transistors in terms of operational speed is the transition frequency (fT) and is defined as the frequency at which the current gain of a transistor, in short circuit condition, is equal to 1. Our aim is to fabricate organic field effect transistors by solution-processable techniques, which are compatible to large-area fabrication methods, improving their operational speed. In this landscape, top-gate bottom-contacts (TGBC) OFETs have been fabricated, mainly on flexible and very thin plastic substrates by a combination of printing, coating and laser-sintering techniques. Both p-type and n-type organic semiconductors have been used and finely processed reaching fT = 20 MHz which corresponds to the highest frequency achieved by OFET on plastic substrate.

Resume : The present paper deals with the new dielectric materials susceptible to satisfy Nanoelectronics device needs for both single and integrated devices. High k materials especially hafnium oxides have been widely used in device applications such transistors and sensors. For nanoelectronics applications, nanosheets are rather suitable than other materials. A wide investigation is carried out in this work for studying nanosheets that can be easily grown on carbon materials. This motivation comes from the fact that graphene (carbon based material) is thought to be the silicon successor for large scale electronics. The 2D nanosheets supposed to have large area and very thin layers are mostly produced using delamination techniques. Various materials including graphene, metal oxides, boron nitride, and metal disulfides have been studied. The present paper deals with the study, synthesis and characterization of delaminated nanosheets used as high k gate materials for nanotransistors on graphene and silicon based substrates. Carbon, titanium oxides and perovskite based nanosheets are described as they show good insulating properties not different from that of corresponding large area dielectrics.

Resume : Artificial neural networks (ANNs) based on synaptic devices, which can simultaneously perform processing and storage of data, have superior computing performance compared to conventional von Neumann architectures. Here, we present a ferroelectric-coupled artificial synaptic device with reliable weight update and storage properties for ANNs. The artificial synaptic device, which is based on a ferroelectric polymer capacitively coupled with an oxide dielectric via an electric-field-permeable semiconducting single-walled carbon nanotube (SWCNT) channel, is successfully fabricated by inkjet printing. By controlling the ferroelectric polarization, synaptic dynamics, such as excitatory and inhibitory postsynaptic currents and long-term potentiation/depression characteristics, are successfully implemented in the artificial synaptic device. Furthermore, the constructed ANN, which is designed in consideration of the device-to-device variation within the synaptic array, efficiently executes the tasks of learning and recognition of Modified National Institute of Standards and Technology (MNIST) numerical patterns.

Resume : The Light Emitting Transistors (LETs) will allow widespread device applications, such as nano / micro scaled light sources, displays, and wearable electronics. Because of these applications, LETs are being studied actively. However, LET has the disadvantage of being limited in the light-emitting semiconductor material and low efficient. Electrochemiluminescent (ECL) is a light emission process induced by between the reduced and oxidized luminophores that have been generated by electrochemical charge transfer reactions. The Ru(bpy)3Cl2 luminophore was added to the normal ion gel as a material with luminescent properties. Configured ion gel emits red-orange light under AC (2.5 VPP) and DC (2.5V) bias. The application of light emitting ion gel gate insulator to the thin film transistors releases the constraints of the semiconductor material and exhibits luminescence in both p- and n-channels as well as ambipolar. Here we demonstrate inorganic p- and n-channel light emitting electrolyte gated transistors (EGTs) operating at low voltage AC and DC. The P3HT, ZnO and Graphene were employed as a semiconductor layer of LETs and light emitting ECL ion gel was used as a gate insulator. All the fabrication processes were solution processed except for ITO electrodes. As a result, when the thin film transistor is turned on, the ion gel gate dielectric emits 616 nm of red-orange light and the P3HT LETs has a maximum luminance of 0.34 Lv at -2.5V DC bias.

Resume : The development of high performance p-channel thin film transistors (TFTs) has been delayed due to the charge carrier accumulation and device optimization issues. Tremendous efforts have been made to improve the device performance, but the field effect mobility of p-channel TFTs still remained 0.01 ~ 5 cm2/Vs. Recently, Electrolyte gated transistors (EGTs) have attracted attention for switching devices due to their low operating voltage and great device performance. The electrolyte gating could realize electrical double layer (EDL) formation and a three-dimensional carrier transport channel and thus substantially increased charge accumulation in the channel region. Using this EGT structure, we successfully demonstrated p-channel TFTs with ultra-high mobility (>90 cm2/Vs) and low operating voltage (0.5 V). As a semiconductor, copper iodide (CuI) was employed due to its small effective hole mass and high Hall mobility. For electrolyte gating, solid-state electrolyte was used as a gate insulator. We also studied the vacancy engineering for tuning the electrical properties of CuI-EGT. The vacancy species and concentration in CuI film can be controlled by annealing ambient and additional dopant. As a result, the deficiency and abundance of iodine vacancy (VI) induced operation voltage shifts positive and negative side, respectively. The TFT mobility was also affected by the VI concentration. All the fabrication processes were conducted by solution process, low temperature.

Resume : Carbon atomic wires (CAWs) are chains of sp-hybridized carbon atoms with outstanding mechanical, optical and electronic properties, as predicted by many theoretical works. Researchers have focused on the synthesis and stabilization of CAWs and on the study of their properties at single molecule or monolayer level. However, very limited investigations addressed the thin film electrical properties of these materials. We report about the first field-effect transistors (FETs) employing sp-carbon molecules as semiconducting material. A major improvement in the devices’ performance is achieved upon blending the semiconducting molecules with an insulating polymer. This allows to obtain field-effect mobility >10-2 cm2/Vs, improved subthreshold swing (<0.5V/dec) and high reproducibility. We analyse the relations between the FETs performance and the morphology of the active layer. The degree of alignment of the crystallites and the semiconducting material - insulating polymer phase separation are key aspects that are addressed by polarized optical microscopy and microstructural investigations, comprising AFM and GIWAXS. Furthermore, Charge Modulation Spectroscopy and Microscopy (CMS - CMM) studies give insights on the carriers’ behaviour and on the charge localisation. Our investigation demonstrates the potential of CAWs in solution-processable, large-area, flexible electronics and aims at shedding light on the peculiar transport features of sp-based systems.

Resume : Colloidal semiconductor nanoplatelets (NPLs), owing to their efficient and colour pure luminescence, are considered as frontier materials in light-emitting diode (LED) technology. The performance of electrically driven NPL-LEDs are strongly bound to the design of electrodes interfaces to ensure a proper charge injection balancing into the emitting layer. A family of conjugated polar polymers, with the same pendant functionalities, but different backbones, are synthesized, characterized and incorporated as cathode modifier in NPL-LED direct architectures. The polymers, synthesized by Suzuki coupling containing fluorene (P1), benzothiadiazole (P2) or cyclopentadithiophene monomers (P3), are endowed with two terminal phosphonated pendant groups on the lateral alkyl chains. The rational chemical design of the backbone enables the selective tuning of HOMO or LUMO energy levels. Moreover, the phosphonated group, and its affinity with Aluminum cathode, imparts an interfacial dipole to the metal with a consequent reduction of the metal work function. We show that NPL-LEDs exhibit external quantum efficiency and lifetime that can be correlated to the electronic properties of the polar polymers. In particular, the interface achieved with P1-2, leads to an enhancement of the LEDs performance accompanied by a very low turn on voltage. On the contrary, the further lowering of the energy gap of P3 activates undesired non-radiative pathways at the organic/inorganic interface.

Resume : Flexibility is particularly important for future electronic applications such as scalable and wearable electronics. Organic-based flexible resistive random access memory (RRAM) devices have many advantages such as a low-temperature manufacturing process and excellent flexibility. Gelatin, widely used in the food industry, has many fascinating properties include skin-attach ability, transparency, film-forming ability, solution ability, and high-degree flexibility. The effect of process temperature on flexibility of gelatin memory devices has been investigated. The modulus of gelatin via low process temperature (below 80 °C), at the displacement between 20 and 30 nm, was around 16-18 GPa. Their mechanical property brings out the benefit for flexible electronics. The flexible gelatin memory device exhibits the ON/OFF ratio of 103-104, low set voltage, and uniform current distribution (coefficient of variation < 30 %). The ON/OFF ratio could be maintained ~104 after bending 1000 cycles. This indicates that the device performance was reliable under mechanical bending stress. The proposed device is an attractive candidate for the future generation of bio-inspired flexible memory devices, with eco-friendly solution based processing for large area printable electronic devices.

Resume : Organic field-effect transistors (OFETs) represent a very promising sensor platform for detection of gaseous analytes. OFET-based sensors showed a high sensitivity, while achieving a decent selectivity is a significant challenge for these devices. To solve this issue, some analyte recognition element should be introduced in OFET structure. Since the majority of charge carriers in operating OFET flow in a few molecular layers of organic semiconductor adjacent to dielectric, a modification of this interface with receptor molecules appears to be the most promising. We have applied aromatic carboxylic acids with different functional groups in various positions as receptor components responsible for interactions with the analyte in OFETs comprising naphthalene diimide with perfluorinated side chains (F-NDI) as semiconductor. The designed OFETs with receptor interlayers showed strong and reversible response to a range of analytes, in particular amine-type and sulfur-containing compounds. Moreover, it was revealed that the sensor selectivity profiles strongly depend on the structural features of the used receptor materials. We have shown that molecular modification of the interface between semiconductor and dielectric layers in OFETs provides a perspective approach to tune the sensitivity and selectivity of gas sensors. This study makes one more step towards the creation of electronic nose based on OFETs for detection of vapor biomarkers or food spoilage components.

Resume : Adequate ultraviolet (UV) radiation exposure is an important issue for people's health. It is necessary for the synthesis of vitamin D, to prevent rickets and also kills germs. However, elevated doses become dangerous and are associated with the development of diseases such as skin cancer [1]. A simple and low-cost UV-sensor is imperative for ensuring people's protection. In this sense, printed electronics offers riches possibility for low cost and recyclable sensors that can assist with public wellbeing. Zinc oxide (ZnO) is a promising material for this proposal, it’s considerably cheap, non-toxic and has a wide bandgap of about 3.4 eV, which results in a high optical absorbance at UVA wavelengths. Furthermore, it can be obtained by spray pyrolysis deposition of a precursor solution (zinc acetate dehydrate), what is an excellent alternative to sophisticated deposition processes like RF sputtering or pulsed-laser deposition. In this work, zinc oxide (ZnO) film has been obtained by spray pyrolysis deposition and used as an active layer for a UV sensor based on a transparent electrolyte-gated thin film transistor. As the source, drain and gate were used ITO electrodes and as dielectric was used a cellulose-based electrolyte [2], being all layers transparent. The transistor architecture stands out among the other devices that can be applied as a UV sensor due to the multiple electrical parameters that can be analyzed under irradiation. When it was varied the power density of the incident light over the transistor (355 nm), it was possible to move the device curve in the reverse voltage direction, resulting in changes of 0.7 V in the threshold voltage value (from +0.16 to -0.54 V). The electron mobility increased from 0.17 cm²/Vs to 0.42 cm²/Vs, the I_UV/I_dark ratio increased three orders of magnitude at V_GS=0V and on/off ratio also decreased three orders of magnitude. All those parameters: threshold voltage, electron mobility, I_UV/I_dark ratio and on/off ratio present linear behavior when plotted versus irradiance. Those results confirm the potential of our ZnO-based electrolyte-gated thin film transistor as a good UV sensor.

Resume : Doping of organic semiconductors is key to enabling the use of these flexible, highly processable and cheap materials in various emerging energy applications. However, there is limited knowledge of the interplay between doping mechanism and the resulting performance stability, microstructure and thermal transport. Here we focus on the dopant/host system poly(2,5-bis(3-alkylthiophen-2-yl)-thieno[3,2-b]-thiophene) : 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (PBTTT:F4TCNQ) representing the state-of-the-art in the fields of thermoelectrics and transistors. In particular, we show an improved vapour-phase doping method which enables judicious selection of the doping mechanism. With this, we are able to introduce desired fractions of integer charge transfer (ICT) or charge transfer complex (CTC) species. Electrical characterization shows that up to 33% of CTC fractions do not affect thermoelectric properties while substantially improves their thermal stability upon prolonged exposure to elevated temperatures. Spectroscopic and structural analysis reveals that the CTC species are preferentially ‘locked’ in the crystalline domains of the polymer network and, therewith, are less prone to thermal de-doping. Finally, thermal characterization shows that moderate doping levels the thermal conductivity can be reduced via the dopant acting as a phonon scattering site. Hence, we outline strategies for material selection and improving performance of organic thermoelectric devices.

Resume : Thin film organic field effect transistors (TF-OFETs) are known to be very sensitive to polar gases because major charge transport occurs in the thin layer close to the environment and sorption of polar analytes strongly affects the charge distribution [1,2] while having poor selectivity. Covering the sensor with receptor layer can provide semi-selective response keeping high sensitivity and but an arrangement of sensor array is necessary for complex gas mixtures analysis. We have used amphiphilic tetramethyldisiloxane derivatives of benzothieno-bithiophene (BTBT)-containing organic semiconductor to fabricate TF-OFETs based gas sensors. molecule structure and film morphology. Addition of the receptor layer induced selectivity to different analytes due effect on gas sorption kinetics. In this work we have compared sensing properties of TF-OFETs with different metal-porphyrins receptor layers and combined them into array. TiO-containing receptor layer have improved selectivity towards NH3 while supress response to H2S. The array of 5 different receptors and machine learning methods on their response parameters have led to efficient differentiation of NH3, H2S, EtSH and NO2 at as low as 100 ppb concentration. Considered system is very flexible to the desired application and can be easily tuned to required target gases by choice of receptor layer. It can be suited to environmental monitoring and medical breath analysis. This work was supported by RSF (project №19-73-30028) and performed in the framework of leading science school NSh-5698.2018.3. References [1] M. Andringa et al., Organic Electronics, 2010, 11(5) [2] X. Wang et al, Synthetic Metals, 244

Resume : Polymeric thin film deposition using ICVD (Initiated Chemical Vapor Deposition) is a versatile method to form various forms of functional surfaces. This freedom in functional polymer selection is due to the fact that polymerization starts on the surface of substrate by controlling variables such as the monomer flow rate, substrate temperature and type of initiators. In this study, thin films of ferroelectric poly(vinyl difluoride) were deposited using Vinyl difluoride as the monomer, Tert-Butyl peroxide and Triethylamine as the initiators at different substrate temperatures. The chemical characterization of the films was done using Fourier transform infrared spectroscopy, whereas structural characterizations were performed using Scanning Electron Microscopy, Atomic Force Microscopy and Electrostatic Force Microscopy methods. Effects of initiator type and substrate temperature on the morphology and the ferroelectric behavior of the thin films were investigated. It was observed that surface morphology strongly depended on the initiator type; island-like structures formed when TBPO was used as the initiator. Systematic EFM studies showed that these islands display ferroelectric behavior under external bias which can be related to the deposition kinetics. The spontaneous formation of these ferroelectric nanostructures and the ability to tune their size via process parameters, make these structures promising for potential applications in organic electronics or energy storage.

Resume : In recent years, several reports indicated that metallic nanostructures can act as a tool to efficiently increase the emission properties of organic emitters [2,3] by modifying their spontaneous decay rates. However, many questions are still under investigation and need to be thoroughly studied. For instance, the interaction range of plasmonic nanoparticles and their influence on the injection, transport and recombination mechanisms within a specific environment such as organic light emitting diode (OLED) hetero-structure. In this context, we report the investigation of the role of Ag nanoparticle gratings on the emission properties of OLED devices. Thereby, we have performed a study of the behaviour of Ag arrays related to their periodicity, which emphasizes the existence of two coupling regimes dominated by near or far field effects. Thereafter, we studied the influence of these regimes on the emission directivity of nearby emitters. Finally, the benefit of Ag nanoparticle arrays on the electrical and optical performances of the OLED has been, carefully, studied. This is particularly crucial to develop efficient organic light emitting diodes (OLED) which may pave the way to new organic laser diode. The studied structures are regular square arrays of silver nano-cylinders of 100nm diameter fabricated on glass/ITO substrate and covered by 100nm of organic material. Various Ag arrays periods (p) ranging from 180nm to 480nm by a step of 20nm, fabricated by using electron beam lithography technique, have been simulated and experimentally studied. Depending on the grating type and period, the obtained results emphasize the existence of two coupling regimes: localized surface plasmon resonance (LSPR) regime [4], and surface lattice resonance (SLR) regime [5]. The former is connected to the so-called short period gratings of p<280nm, for which a single wide resonance with a high damping rate is obtained. In this case, each metallic nanoparticle of the grating presents intense confined (LSP) modes. The second regime is related to inter-particle distance p≥280nm, for which a set of narrow resonances appear on the extinction spectra revealing the excitation of collective surface lattice modes (SLR). In this case, the coupling between nanoparticles is mediated via delocalized photonic modes dominated by far-field dipolar effects; and in which, the excited sharp resonances closely follow the dispersion of characteristic wavelength known as Rayleigh anomalies[6]. In addition to that, we also performed fluorescence lifetime measurements by using a time correlated single photon counting technique (TCSPC). The obtained results show significant radiative lifetime reductions in the case of the short period regime; revealing the cavity feature of the LSP modes. Besides, only slight reductions have been obtained in the SLR regime showing a dominant diffraction effect. This part of our work emphasizes that, by tuning the nanoparticle grating features, it is possible to select specific optical responses of emitters from the near to the far field zone [7]. Based on the previous study, we investigate the plasmon-excited states coupling processes in OLED devices. Specially, two devices containing gratings of p=180nm and p=480nm have been fabricated and studied. The typical OLED structure consists of metallic nanoparticle array on top of glass/ITO substrate, followed by a stack of injection, transport and emissive organic layers covered by a metallic cathode. IVL measurements and angular radiative patterns have been measured and compared to a reference device realized without nanoparticles. Our experimental results indicate important enhancement of the IVL properties of the plasmonic OLEDs. They also indicate that the emission pattern of the OLED containing the short-period grating (p = 180nm) is modified to a more directional one, fully contained in a cone aperture of 30 degrees, as compared to the well-known Lambertien pattern of the reference OLED. In the case of the OLED containing a long-period array (p = 480nm), the obtained pattern shows a maximum emission in preferred directions, at ± 30° from the normal to the substrate. This effect is attributed to the excitation of hybrid-delocalized modes; the metallic nanoparticles produce constructive interferences at well-defined directions, by interacting via a dipole-dipole coupling, leading to a significant redirection of the emitted light into diffractive modes. To summarize, we report a thorough study of plasmonic effect of Ag nanoparticle arrays indicating the existence of two interaction regimes: near and far field depending on the grating period. We demonstrate that plasmonic structures can be used as an integrated nano-optical compound to control and shape the emission of OLED devices in terms of intensity, efficiency and directionality. It can open a new way to possible applications in the lightning sources domain to model the spatial and spectral emission features. References: [1] B. Geffroy, P. le Roy, and C. Prat, “Organic light-emitting diode (OLED) technology: materials, devices and display technologies,” Polymer International, 55(6), 572–582 (2006). [2] T. D. Neal, K. Okamoto, and A. Scherer, “Surface plasmon enhanced emission from dye doped polymer layers,” Optics Express, 13(14), 5522-5527 (2005). [3] G. Vecchi, V. Giannini, and J. Gómez Rivas, “Shaping the Fluorescent Emission by Lattice Resonances in Plasmonic Crystals of Nanoantennas,” Physical Review Letters, 102(14), 146807-146810 (2009). [4] M. A. Garcia, “Surface plasmon in metallic nanoparticles: fundamentals and applications,” Journal of Physics D: Applied Physics, 44(28), 283001-283043(2011). [5] D. Khlopin, F. Laux, W. P.Wardley, J. Martin, G. A.Wurtz, J. Plain, N. Bonod, A. V. Zayats, W. Dickson and D. Gerard, “Lattice modes and plasmonic linewidth engineering in gold and aluminum nanoparticle arrays,” JOSA B, 34 (3), 691-700 (2017). [6] A. Maradudin, I. Simonsen, J. Polanco and R. M. Fitzgerald, “Rayleigh and Wood anomalies in the diffraction of light from a perfectly conducting reflection grating,” Journal of Optics, 18(02), 024004-024013 (2016). [7] A. G. Nikitin, A. V. Kabashin, and H. Dallaporta, “Plasmonic resonances in diffractive arrays of gold nanoantennas: near and far field effects,” Optics Express, 20 (25), 27941-27952 (2012).

Resume : Organic light-emitting transistors (OLETs) can have several advantages over the more traditional organic light-emitting diodes (OLEDs) such as higher brightness and efficient light outcoupling. However, the main difficulty in creation of high-performing OLET is the combination of efficient luminescence and high charge mobility in one material. Also, optical waveguiding effect in OLET active layer strongly decreases light emission from its top surface degrading the OLET performance. To avoid this, 2D films thinner than 100 nm would be beneficial. In this work, we report on OLETs based on solution-processed 2D films of BTBT-based molecules, 2,7-bis(4-decylphenyl)[1]benzothieno[3,2-b][1]benzothiophene (DPBTBT), which uniquely combine high charge mobility reaching 7.5 cm2/Vs and pronounced electroluminescence. DPBTBT molecules are self-organized in large-area ultrathin films consisting of one or a few molecular layers, which are single crystal according to optical, atomic-force and photoluminescence microscopy. The 2D DPBTBT OLETs demonstrated very good shelf-life stability in the ambient air under normal laboratory conditions. The charge-carrier mobility changes by about 15% after 3 months of storage. High charge mobility, good shelf-life stability and good luminescence properties suggest that 2D BTBT-based materials are a promising platform for OLETs. This work is supported by Russian Science Foundation (project № 18-12-00499).

Resume : The piezoelectric properties of poly (vinylidene fluoride) - PVDF can be improved by inducing the crystalline phase in the polymeric chain, microscopic effect, and also with the distribution of crystallites in a film, macroscopic effect. The formation of films through PVDF composites and structures with a high dipole moment induces a distribution in the orientation of the crystallites and in the formation of the PVDF preferably in the β phase. For this, it is necessary to study: i) the ideal proportion of the structures in the PVDF, ii) the best processing method to favor the preferred orientation to improve the piezoelectric effect in these systems. This work will present the results obtained in the production of PVDF films with different structures with a high dipole moment and also a study in the different ways of obtaining the films, such as annealing and poling. Differential Scanning Calorimetry (DSC) and DC electrical measurements will be used when the PVDF film has been exposed to mechanical stress. FAPESP, INEO/INCT, LNNano-LMF. This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoa de Nível Superior-Brasil (CAPES)- Finance Code 001.

Resume : Substantial improvement in device performance of organic thin film transistor (OTFT) has emerged exciting challenges toward analog circuit applications such as low frequency amplifier for sensors, backplane circuits for electronic-ink display, image sensors etc. Thus understanding of science and technology have become equally important to cope with the required demand. Organic metal-insulator-semiconductor capacitor (OMISCAP) has been proved to be a model system to study the operation of OTFTs. Organic semiconductors are typically intrinsic in nature with high band gap, resulting extremely low intrinsic carrier concentration. In principle, organic semiconductors form Schottky type contacts with metal electrodes, which eventually inject charge carriers into the organic semiconductor by thermionic emission process. In case of OMISCAP, the injected charge carriers, also known as space charges, influence the electric field, potential and the carrier concentration and thereby the capacitance characteristics. Moreover in this device, the metal is the source of charge and the applied gate field modifies the magnitude of charge within the semiconductor to yield capacitance variation with the applied gate bias. Traditional thought of doped semiconductor based device physics misinterpret the characteristic behavior of the devices In this presentation, I would like to share our recent results on OMISCAP as a test structure to determine the quality of metal semiconductor junction and its impact on OTFT performance.

Resume : Organic electrochemical transistors (OECTs) have garnered strong interest due to ability to efficiently transduce biological signals into electronic ones thanks to the mixed ion and electron conduction of the semiconducting film in the channel. However, the lack of understanding of these mechanisms during the operation of OECTs hinders the use of OECT-based bioelectronic technologies in circuits. Here we investigate how polymer hydration in aqueous biological media affects mixed conduction properties of n-type (electron transporting) semiconductor transistors. We use electrochemical quartz crystal microbalance with dissipation monitoring (eQCM-D) coupled with UV-Vis and impedance spectroscopy to observe the hydration of n-type polymers with varying lengths of ethylene glycol side-chains attached to NDI-T2 backbone. By varying the hydrophilic content of the n-type polymer, we modulate the hydration which in turn governs the in situ (dynamic) morphology of the film and hence the electronic and ionic mobilities. Our study suggests that hydration is not de facto beneficial for the n-type OECT performance, as has recently been observed for the p-type counterparts. [1] [1] Savva et al Adv. Funct. Mater. 2020 DOI: 10.1002/adfm.201907657

Resume : Thermally activated delayed fluorescent (TADF) emitters are extremely attractive due to their potential to harvest all triplet excitons via reverse intersystem crossing (rISC) process into the singlet manifold thereby ensuring 100% internal quantum efficiency. [1] However, due to pronounced charge-transfer character of TADF compounds, there are difficulties in achieving deep blue emission. Additionally, TADF-OLEDs suffer from early efficiency roll-off associated with high long-lived triplet exciton population. Therefore, TADF emitters with large rISC rate facilitating triplet up-conversion are required. To this end, we designed new TADF emitters based on 1,8-naphthyridine acceptor and differently substituted carbazole donor groups. Photophysical characterization of the compounds revealed high photoluminescence quantum yield (up to 86%) in mCP host with large rISC rates (up to 1.1×106 s-1). We fabricated vacuum and solution processed TADF-OLEDs employing 7% naphthyridine-doped emissive layer. Devices exhibited deep blue emission with CIE color coordinates (0.14, 0.16), external quantum efficiency of up to 17.6% and high brightness (up to 23000 cd/m2). Most importantly, due to the large rISC rates TADF OLEDs demonstrated weak efficiency roll-off. Obtained results indicate that 1,8-naphthyridine based emitters are promising for TADF-OLED application. [1] H. Uoyama, K. Goushi, K. Shizu, H. Nomura, and C. Adachi, Nature 492, 234. (2012).

Resume : Semiconductor photocatalysts absorb light and convert it to photogenerated charges that can drive redox reactions on their surface. Organic semiconductors are increasingly being employed for photocatalytic applications, however, photocatalysts formed from a single organic semiconductor typically suffer from inefficient intrinsic charge generation, which leads to low photocatalytic activities. We demonstrate that fabricating organic nanoparticle (NP) photocatalysts that contain a heterojunction between a donor polymer (PTB7-Th) and non-fullerene acceptor (EH-IDTBR) can result in hydrogen evolution photocatalysts with greatly enhanced photocatalytic activity. Control of the nanomorphology of these NPs was critical to achieving optimum hydrogen evolution, and was achieved by varying the stabilizing surfactant employed during NP fabrication. Converting the morphology from a core-shell structure to an intermixed donor/acceptor blend increased photocatalytic activity by an order of magnitude, resulting in hydrogen evolution photocatalysts that are among the most active reported to date, with a hydrogen evolution rate of over 60,000 µmolh-1g-1 under 350 to 800 nm illumination and external quantum efficiencies over 6% in the region of maximum solar photon flux. DOI: 10.1038/s41563-019-0591-1 Nat. Mater. In Press

Resume : For increasing light intensity and/or decreasing temperature the open-circuit voltage (Voc) of organic solar cells is expected to increase which is usually also observed experimentally. At higher light intensities and/or lower temperatures Voc often tends to saturate which can consistently be explained to originate from surface recombination. In some cases however, Voc even decreases with increasing light intensity and/or decreasing temperature which we refer to as the ‘reverse Voc-effect’. This interesting phenomenon will be analyzed in detail by means of intensity and temperature dependent steady-state and transient Voc-measurements in com-bination with photo- and electroluminescence as well as numerical device simulations. Note that in contrast to a study published by Ullbrich et al. we can exclude the reverse Voc-effect to be due to heating of the devices by the light source as we have carried out the measurements with great care using pulsed illumination. Based on the complete analysis we can conclude that the reverse Voc-effect originates from a strong variation of the charge carrier selectivity of one of the contacts upon an increase of the quasi Fermi level splitting in the absorber (due to higher light intensity and/or lower temperature). This can be due to e.g. an inappropriate electrode work function in combination with a majority doping of the absorber in the vicinity of that electrode.

Resume : Solution-processed organic solar cells have demonstrated a drastic increase in power conversion efficiency (PCE) over the recent years thanks to a wide variety of novel donor and acceptor materials [1]. This communication will present a new promising ternary blend considering industrial constraints and its mechanisms through advanced characterization tools. This in-depth study of this particular ternary blend will also help to design new ternary blends for OPV in the future. For this work, we have selected the polymer PTQ10 due to its relatively low-cost synthesis, its deep HOMO level and its high reported PCE with several non-fullerene acceptors (NFA) [2], [3]. Blended with the NFA 4TIC-4F [4], the solar cells fabrication process meets most of the criteria for large-scale fabrication: non-chlorinated solvent processing, thickness tolerant performances, no post annealing treatment and reproducible PCE over several batches. In these semi-industrial conditions with the inverted architecture ITO/ZnO/PTQ10:4TIC-4F/MoO3/Ag, we obtained a promising experimental PCE over 8% with a high short-circuit current (JSC) over 19 mA.cm-² confirmed by external quantum efficiency analyzes. Notably, the relatively low fill factor (FF) of 0.55 limit the performances of this binary blend. We found out that adding PC61BM as a ternary component enables to reach a PCE close to 10% by increasing the open circuit voltage (VOC) from 0.77 V to 0.79 V and the FF from 0.55 to 0.64 without sacrificing the JSC. The increased VOC can be attributed to the higher LUMO level of PC61BM (-4.00 eV) than that of 4TIC-4F (-4.21 eV). Following a charge carrier mobility study by the space charge limited current (SCLC) method, we found out that adding PC61BM improves the hole mobility (µh). Additionally, the optimal FF is obtained where µh/µe ~1. Grazing-Incidence Wide-Angle X-ray Scattering (GIWAXS), Atomic Force Microscopy (AFM) and Contact angle analyzes were also carried out in order to understand the effect of PC61BM on the bulk-heterojunction morphology. Besides, the ternary blend shows a better photostability than its binary counterparts, which is encouraging for industrial applications: after 500 hours of exposure at an AM 1.5 light at 50 °C, the binary blend device maintains about 60% of its initial PCE (i.e. stabilized PCE~4.7%), while the ternary blend device maintains about 70% of its initial PCE (i.e. stabilized PCE~6.5%). [1] Y. Xu et al, Chin. J. Chem. 2019, 37, 207 [2] C. Sun et al, Nature Communications 2018, 9, 743 [3] Y. Wu et al, Science China Chemistry 2019 [4] X. Shi et al, Adv. Funct. Mater. 2018, 28, 1802324

Resume : In the last few years a significant evolution in the field of organic photovoltaics (OPVs) is achieved thanks to the synthesis of new generation donor-acceptor polymers with reported power conversion efficiencies (PCE) above 10%. This study is focusing on one of those polymers known as PCE11 (poly[(5,6-difluoro-2,1,3-benzothiadiazol-4,7-diyl)-alt-(3,3´´´-di(2-octyldodecyl) 2,2´;5´,2´´;5´´,2´´´-quaterthiophen-5,5´´´-diyl)]). PCE11 is a polymer that shows highly tunable optical properties under various processing conditions and has a reported PCE of 11.5%.[1] We employ temperature dependent Resonance Raman Spectroscopy (RRS) to develop a basic understanding on how temperature affects the polymer conformation and thus its optoelectronic properties. Moreover, by combining experimental data obtained by steady state RRS and Absorption it is possible to extract additional information regarding the excited-state structure and dynamics. In particular, the shape of the absorption spectra can be determined by the displacements between the ground and excited state potential energy surface minima in each mode, which are directly related to the intensities of RR bands. In the present study, we use a computational method called Resonance Raman Intensity Analysis (RRIA) that provides access to a quantitative picture of the excited state, such as the change in the molecular geometry, the initial excited-state dynamics, as well as the nature of that state, gaining thus insights that will aid in the interpretation of the photophysical behaviour of this donor-acceptor polymer. [1] N. Li et al., Nat. Commun., 8, 14541 (2017)

Resume : Organic solar cells have gained renewed interest due to recent progress reducing the efficiency gap towards inorganic and hybrid photovoltaic technologies. The separation of photogenerated excitons into free charge carriers is considered to be most efficient from charge transfer (CT) states, which emerge at heterojunctions between donor and acceptor materials. Currently discussed models describe CT states as molecular states incorporating reorganization and energetic disorder, whose respective impact on device parameters is yet to be understood in detail. CT states can be investigated in solar cell devices with sensitive electro-optical absorption and emission spectroscopy in the sub-bandgap region, both of which are connected by the optoelectronic reciprocity relations. Based on temperature dependent variations of the spectral linewidths of CT absorption and emission, we were able to separate the contributions of molecular reorganization and energetic disorder to the CT state ensemble. In this study, we focused on solution processed polymer-fullerene solar cells in which we found energetic disorder to play a large to dominant role in the formation of CT states. We discuss if current models might be limited in their ability to describe these systems, including their optoelectronic reciprocity and radiative limit of the dark saturation current.

Resume : We will discuss our recent work to better understand the role of charge transfer (CT) states in organic solar cell function. For one, we have been exploring organic semiconductor-based thin films that feature crystalline grains of up to 1 mm in extent, termed microcrystalline films. We have found that CT states incorporating these long-range-ordered films can be highly delocalized, contributing to noticeably lower energy losses. In another system, we are studying donor-acceptor pairs that feature very high optical gaps (>3 eV) but relatively small frontier orbital energy offset (<1 eV). Such interfaces present multiple CT states that reveal new insight about photocurrent generation and nonradiative recombination at donor-acceptor interfaces.

Resume : Organic solar cells (OSCs) incorporating non-fullerene acceptors (NFAs)[1] have reached a remarkable efficiency of 17%.[2] However, many of the reported materials and devices may not have demonstrated their full potential yet, because optimization of device processing is too often conducted using a “combinatorial” approach rather than knowledge-based strategies. The design of rational processing protocols of OSCs requires first to identify the relevant interrelationships between processing, structure and properties (i.e. the PSP relationship) for the individual components of OSCs, which is so far insufficiently studied for most NFAs. In this talk, the PSP relationship of a family of high-performing NFAs –namely ITIC[3], ITIC-M, ITIC-2F and ITIC-Th— is reported. First, a detailed structural analysis of the crystalline and glassy phases of ITIC-based NFAs was conducted, by employing a combination of fast scanning calorimetry (FSC), grazing incidence X-ray scattering (GIWAXS) and polarized optical microscopy-spectroscopy (POM-S). The study evidences that NFAs may exhibit a complex phase behavior, which can include various crystalline polymorphs. For example, up to three different crystalline forms were found for ITIC, which nature was rationalized with thermodynamic arguments. Moreover, experimental conditions for stabilizing the different phases at room temperature were identified e.g. casting solvent, thermal treatments and additives. Second, the optical and electronic properties of these phases were characterized by means of UV-vis, photoluminescence spectroscopy and field-effect transistor measurements, which allowed us to establish PSP relationships for these compounds. For example, one of our intriguing key findings was the identification of the so-called metastable ITIC phase II polymorph. It exhibits better electron transport than the regularly used as-cast polymorph (μ = 0.05 cm2/V s vs 0.016 cm2/V) and thus might be exploited to enhance the performance of OSCs. These results highlight the necessity of conducting detailed structural investigations on NFAs that allow the design of customized processing protocols and will eventually result in a rational optimization of OSC devices. [1] G. Zhang, J. Zhao, P. C. Y. Chow, K. Jiang, J. Zhang, Z. Zhu, J. Zhang, F. Huang, H. Yan, Chem. Rev. 2018, 118, 3447. [2] Y. Lin, B. Adilbekova, Y. Firdaus, E. Yengel, H. Faber, M. Sajjad, X. Zheng, E. Yarali, A. Seitkhan, O. M. Bakr, A. El-Labban, U. Schwingenschlögl, V. Tung, I. McCulloch, F. Laquai, T. D. Anthopoulos, Adv. Mater. 2019, 1902965. [3] F. Gao, Joule 2019, 3, 908.

Resume : In the last few years, non-fullerene acceptors (NFA) have dominated organic solar cells. Whilst in thin films, exceptional fill factor (FF) can be obtained, in thicker junctions however, FF is usually affected. Reduced FF is the manifestation of voltage-dependent charge photogeneration and/ or inefficient free charge extraction. In this study, we compare two NFAs, named Y6 and LM11, when blended with PM6. Our results show that although PM6:LM11 device has a higher open-circuit voltage (Voc) than PM6:Y6, lower FF limits the efficiency. We employed time-delayed collection field (TDCF) measurements to reveal the reason behind the poorer FF in PM6:LM11 device by studying charge generation and recombination in both systems.

Resume : The performance of solar cells based on molecular electronic materials has historically been limited by relatively high non-radiative voltage losses. With the advent of non-fullerene acceptors, non-radiative voltage losses as low as 0.2V have been achieved by reducing the energetic offset between the initial photo-excited singlet state and charge-transfer state in the bulk-heterojunction. However, the devices with the lowest voltage losses appears to be limited instead by a low Fill Factor (FF), for reasons which are not yet fully understood. In this work we consider devices based on the high performing C8-ITIC acceptor blended with PBDB-T donors with different level of fluorination that modulates the energy offset. We have previously reported that the non-radiative voltage losses in the blend with the lowest offset was as low as 0.23V, however the FF of these devices was limited to 55% whereas the devices based on the largest offset system had FF around 70%. To understand the relation between the reduced voltage losses and the limited FF, we first use time resolved spectroscopy to study the impact of the energy offset on the early time processes such as: charge transfer dissociation and recombination. We find that the charge transfer dissociation into free charges is slower for the low offset system. We then measured the free charge carrier lifetime using a newly developed optoelectronic technique, and show that the device with lowest offset has a smaller charge carrier lifetime. Using a device model that accounts for the reformation of the charge transfer state from the free charge carriers, we were able to explain the experimental observation. These results may serve as guidelines for developing new efficient organic solar cells with both a low voltage loss and a high FF.

Resume : The topic of Non-Fullerene Acceptors (NFAs) in the field of Organic Photovoltaics (OPV) has become tremendously important to industrial and academic research, as the rapid development of these materials has pushed the device power conversion efficiency over the 15% threshold [1]. However, the commercialisation of solution-processed photovoltaic modules requires processing of thick active-layer films (150-300 nm) and therefore materials with high charge mobilities [2]. NFAs typically possess a highly anisotropic and two-dimensional conjugated structure which is critical to their organisation in the solid-state, affecting in turn their electronic functions (e.g. carrier mobility) [1]. With our work we have shown first insights into the crystallisation of the NFAs and its effect on the charge transport [3]. To this end, we compared a large body of newly identified non-fullerene single crystals and previously reported structures, including novel non-fullerene molecules. We identified how the structural design of the NFAs facilitates their organisational motifs in the solid-state, exploring the importance of the crystal packing and topological connectivity on the charge transport. The influence of the solid-state organisation on the charge carrier mobility and performance of organic solar cells is the focus of our investigations that are currently underway. References: [1] J. Yuan, et al. Joule 2019, 3, 1140-1151. [2] J. Zhang, et al. Nature Energy, 2018, 3, 720-731. [3] P. Mondelli et al. Materials Horizons 2020, DOI: 10.1039/C9MH01439J

Resume : Solution‐processed polymer bulk heterojunction organic photovoltaic (BHJ-OPV) devices have gained serious attention during the last decade. They are one of the leading next generation photovoltaic technologies for low cost power production (1). The active layer of a polymer solar cell consists of a thin solid film of an electron donor blended with an electron acceptor. The morphology of the active layer is one of the important factors for the solar cell performance. To control the morphology, one needs to understand the morphology formation on a molecular level. It is known that molecular interactions govern morphology formation and purity of mixed domains of conjugated polymers and small-molecule (2). Therefore, understanding these interactions relations would give use insight to the challenges of coating OPV from green solvent to achieve the required nanostructure interpenetrating network of donor and acceptor domains based on a rational choice of solvent (3). In this work, we first calculate the Hansen solubility parameters (HSP) (3),(4) of the donor and acceptor materials, followed by careful choice of solvents with selective relative solubilities for the two materials. After the theoretical screening procedure, solubility tests were performed to determine the HSP parameters relevant for the systems. Then finally, thin films were prepared by spin-coating from the solvent screened using this method. Our results show that the blend film morphology prepared in this way is similar to those prepared rom the commonly used halogenated solvents. References: 1. Brabec, C. J. et al. Polymer-Fullerene Bulk-Heterojunction Solar Cells. Adv. Mater. 22, 3839–3856 (2010). 2. Ye, L. et al. Quantitative relations between interaction parameter, miscibility and function in organic solar cells. Nat. Mater. 17, 253–260 (2018). 3. Holmes, N. P. et al. Unravelling donor-acceptor film morphology formation for environmentally-friendly OPV ink formulations. Green Chem. 21, (2019). 4. Jalan, I., Lundin, L. & van Stam, J. Using Solubility Parameters to Model More Environmentally Friendly Solvent Blends for Organic Solar Cell Active Layers. Materials (Basel). 12, (2019).

Resume : Going beyond the traditional concept of electronic devices, we convey the idea of making electronics edible. This unconventional approach exploits the electronic properties of natural and food-based materials for developing ingestible functional devices. Critical biomedical, pharmaceutical, and food industry applications are targeted by the proposed field. In this framework, we explore the potential of cost-effective and edible substance, honey, to be used as electrolytic gate viscous dielectric. Honey-gated organic field effect transistors (OFETs) based on both n & p type semiconductors are fabricated. A distinctive feature of these transistors is their long-term stability, reproducibility and low voltage < 1V operation in air. Devices exhibit forward-looking electronic performances, notably, electron and hole mobility–capacitance product of 3.5 × 10–3 μF/Vs and 23 × 10–3 μF/Vs, respectively, surpassing ones of the previously reported water-gated OFETs. Furthermore, the observed devices responsivity to humidity provides promising opportunities for sensing applications. We then demonstrate, for the first time, the implementation of honey-based integrated circuits: inverting logic gate and ring oscillator.

Resume : Photodetection in the short-wave infrared (SWIR) wavelength window represents one of the core technologies allowing for many applications. Most current photodetectors suffer from high cost due to the epitaxial growth requirements and the ecological issue due to the use of highly toxic heavy-metal elements. Toward an alternative SWIR photodetection strategies, in this work, high-performance heavy-metal-free flexible photodetectors sensitive to 𝜆 = 1.5 µm photons are presented based on the formation of a solution-processed hybrid composing of a conjugated diketopyrrolopyrrole-base polymer/PC70BM bulk heterojunction organic host together with inorganic guest NaYF4:15%Er3+ upconversion nanoparticles (UCNPs). Under the illumination of 𝜆 = 1.5 µm photons, optimized hybrid BHJ/UCNP photodetectors exhibit a photoresponsivity of 0.73 mA/W and 0.44 mA/W respectively for devices built on rigid ITO/glass and flexible ITO/polyethylene terephthalate (PET) substrates. These hybrid photodetectors are capable of performing SWIR photodetection with a fast operation speed, characterized by a short photocurrent rise time down to 80 µs, together with an excellent mechanical robustness for flexible applications.

Resume : Bulk-heterojunction photodiode detectors containing a circular dichroic donor compound have been show to work well as detectors for circular polarized light. Within the green spectral range the photodiodes convert circular polarized light into a handedness-dependent photocurrent with a maximum dissymmetry factor of 5% polarization discrimination [1]. However, the bulk-heterojunction architecture limits the dissymmetry factor: High temperature annealing is required to induce aggregation of the chiral donor to show giant circular dichroism with a dissymmetry factor approaching 40% boosted by excitonic coupling [2]. This causes phase separation of the donor-acceptor blend, and the devices fail to function. To overcome this issue, we fabricate bilayer devices to boost the polarization discrimination efficiency up to 40%. The devices consist of a solution-processed and subsequently thermally annealed donor layer to maximize the optical dissymmetry, and are capped by a vapor deposited fullerene acceptor and spacer layer. With that, we provide a novel roadmap to integrated platforms for future chiroptical imaging and sensing purposes. [1] Schulz, Balzer, Scheunemann, Arteaga, Lützen, Meskers, Schiek. Adv. Funct. Mater. 29 (2019) 1900684. [2] Schulz, Zablocki, Abdullaeva, Brück, Balzer, Lützen, Arteaga, Schiek. Nat. Commun. 9 (2018) 2413.

Resume : Future light-weight, flexible and wearable electronics will employ visible-lightcommunication schemes to interact within indoor environments. Organic photodiodes are particularly well suited for such technologies as they enable chemically tailored optoelectronic performance and fabrication by printing techniques on thin and flexible substrates. However, previous methods have failed to address versatile functionality regarding wavelength selectivity without increasing fabrication complexity. This work presents a filterless concept for inkjet-printed color-selective OPDs which exploits the selective absorption of a novel BHJ system comprised of a transparent wide-bandgap polymer donor and non-fullerene acceptors. In contrast to previous work, our approach simultaneously provides spectral selectivity, high performance and solves fabrication challenges. Thus, it successfully decouples the ink rheological properties and the active layer optical absorbance. Thereby, the device spectral response solely depends on the choice of the non-fullerene acceptor while the polymer donor enables constant printing parameters. A thorough morphological and spectroscopic investigation by transmission electron microscopy and transient absorption finds excellent charge-carrier dynamics enabling state-of-the-art responsivities >102mAW-1 and cutoff-frequencies >1.5MHz. Finally, the color selectivity and high performance is demonstrated in a filterless visible-light-communication system capable of demultiplexing intermixed optical signals.

Resume : Aromatic organic compounds hold great promise for becoming the next generation of battery electrode materials and, particularly, for advancing the development of Na-ion batteries (SIBs). However, so far, the small group of organic compounds found to be suitable for application as anode material in SIBs – the anode is the main bottleneck in the development of SIBs – largely consists of aromatic sodium carboxylates. These compounds usually exhibit a high reduction potential (limiting the voltage of the full cell) and often show significant degradation issues when tested under faster charge/discharge conditions. Applying fundamental principles of organic chemistry, we were able to design a macrocyclic conjugated hydrocarbon for application as SIB anode material with excellent redox behaviour at low potential. No functional groups such as carboxylates, which would shift the reduction potential to higher values, were introduced; the excellent redox properties rely on the compound’s capability to switch between a locally aromatic neutral state and globally aromatic doubly reduced state. The compound, which can be synthesized in a single step from low-cost starting materials and purified by scalable methods, exhibited a specific capacity of 148 mAh g-1 at a current density of 100 mA g-1. As a result of the molecular design, extraordinarily stable cycling performance without any capacity fading at all for 500 cycles and outstanding performance and retention of capacity at higher current densities were observed.

Resume : The development of ionizing radiation detection system over large areas is a crucial task in different areas such as nuclear waste management, radiotherapy or personal protection devices. Despite the excellent detecting performance exhibited by the inorganic materials (e.g. a-Se, CZT), the increasing quest for flexible, portable, low cost and low power consumption sensors pushed the scientific community to look for alternative materials and technologies able to fulfill these new kinds of requirement. Electronic devices based on organic materials have already demonstrated to be a promising alternative to achieve a novel class of direct, flexible, portable and low-cost ionizing radiation detectors. In particular, the excellent direct X-ray detection performance exhibited by solution-processed flexible organic thin film transistors based on 100 nm thick microcrystalline (6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene)) active layer has been recently reported [1] [2]. This class of detectors exhibits a higher sensitivity to the ionizing radiation than the other reported the polymeric devices thanks to a photoconductive gain mechanism. However, in such devices high-energy photon absorption is challenging because of the small cross section of interaction between radiation and organic materials. In fact, these are typically constituted by low-Z elements. Here, we propose a possible approach to enhance the sensitivity of the organic detectors by increasing the radiation capture cross section by means of the addition of high-Z atoms into the basic molecular structure of the material. In detail, by synthesizing new solution-processable organic molecules derived from TIPS-pentacene and 2,8-difluoro-5,11-bis(triethylsilylethynyl)anthradithiophene, with Ge-substitution in place of the Si atoms to increase the material atomic number, we demonstrate how boosting the X-ray detection performance with respect to TIPS-pentacene-based detectors, reaching sensitivity values up to 9 · 105 µC Gy-1 cm-3 [3]. [1] L.Basiricò et al., Nat. Commun., 7, 13063 (2016) [2] S. Lai et al., Adv. Electron. Mater., 3, 1600409 (2017) [3] A.Ciavatti et al., Adv. Funct. Mater., 1806119 (2018).

Resume : Acenes are planar polyaromatic hydrocarbons consisting of linearly fused benzene units and can be considered as the narrowest zig-zag graphene nanoribbons. The small HOMO-LUMO gap and partial open‐shell character imprint these compounds with interesting electronic and magnetic properties. They have been predicted for use as molecular wires in single molecule electronics, semi-conductors, for solar-cells applications, organic field-effect transistors, organic light emitting diodes etc. Although they have been known for decades and can be potentially used in many applications, the absence of simple and practical synthesis hinders them to wider use. The preparation in bulk of acenes beyond pentacene has been described only recently. Hexacene has been prepared in 2012 by T.J. Chow et al. by cheletropic thermal decarbonylation of the carbonyl-bridged precursor and heptacene by H.F. Bettinger et al. in 2017 by thermal cleavage of diheptacene in the solid state, more than 70 years after the first attempted synthesis. We developed new improved general methodology for preparation of acenes longer than pentacene: The concept is based on the preparation of “carbon monoxide protected” precursors formed by cycloaddition reaction between carbonyl-masked benzdiene and arynes. The CO bridged aromatic compounds are stable and aromatize quantitatively upon heating under vacuum in the solid state, or during sublimation. The generated compounds are pure enough to be used directly in device fabrication. We extend our methodology towards the synthesis of nonacene and new family of angularly fused acenes.

Resume : Scintillators could convert radiations (e.g., high-energy X-rays) into low-energy visible light, playing a decisive role in X-ray imaging, computed tomography and other fields. However, today’s inorganic scintillators are normally need to be synthesized at ultrahigh temperature (1700 °C) and must include heavy metals. Here, we show organic single crystal scintillators, which could be prepared at very low temperature and exclude heavy metals completely. For example, 9,10-diphenylanthracene (9,10-DPA), a carbon-hydrogen compound, its single crystals are prepared at 65°C by solution process, showing intense X-ray radioluminescence with the lowest dose rate for X-ray detection of only 14.3 nGy s−1, which is 380 times lower than the dose used for today’s commercial X-ray diagnostics based on inorganic scintillators. Moreover, the scintillators exhibit very fast response with decay time of 1.63 ns, which is 3 orders of magnitude shorter than that of the conventional inorganic scintillator (e.g., CsI:Tl with decay time ∼1000 ns), allowing significantly shortening the deadtime for radio-detection, eliminating afterglow interference, achieving perfectly instantaneous X-ray imaging. In addition, the organic single crystal scintillators show high cycle performance under X-ray irradiation as well as excellent environmental stability. When for X-ray imaging, they display resolution exceeding 20.00 lp mm−1 for both biological and industrial samples, which is the first demonstration of organic crystals for X-ray imaging. These results indicate the potential that organic single crystal scintillators could open the door for many significant fields, from medical diagnostics, homeland security, advanced manufacturing to nuclear technology and environmental monitoring, and bring a revolution for low-cost, high-sensitivity and low radiation new X-ray imaging systems. References [1] Du Z., Zhang X., Guo Z., Xie J., Dong X., Zhu S., et al. X-Ray-Controlled Generation of Peroxynitrite Based on Nanosized LiLuF4:Ce3+ Scintillatorsand their Applications for Radiosensitization. Adv. Mater. 30, 1804046, 2018. [2] Hajagos T. J., Liu C., Cherepy N. J., Pei Q., High-Z Sensitized Plastic Scintillators: A Review. Adv. Mater. 30, e1706956, 2018. [3] Chen, Q. Wu J., Ou X., Huang B., Almutlaq J., Zhumekenov A. A., et al. All-inorganic perovskite nanocrystal scintillators. Nature 561, 88-93, 2018. [4] Wang, Y. Emergence of Uranium as a Distinct Metal Center for Building Intrinsic X-ray Scintillators. Angew. Chem. Int. Ed. 57, 7883-7887, 2018. [5] Wei, H. DeSantis D., Wei W., Deng Y., Guo D., Savenije T. J., et al. Dopant compensation in alloyed CH3NH3PbBr3−xClx perovskite single crystals for gamma-ray spectroscopy. Nat. Mater. 16, 826-833, 2017.

Resume : Colloidal nanoparticles of organic semiconductors have attracted increasing interest in recent years due to the eco-friendly fabrication of printed electronics afforded by colloidal ink technology [1], including solar modules. Utilising the colloidal nanoparticle approach, it is now possible to engineer the microstructure of the light absorbing/charge generating layer of organic photovoltaics; decoupling film morphology formation from film deposition [2]. This seminar presents recent work on core-shell nanoparticle synthesis and single-component nanoparticle synthesis for printed organic electronics applications, via a miniemulsion synthesis route [2,3]. Synchrotron-based X-ray spectromicroscopy and electron microscopy were used to characterise donor–acceptor domain size and domain composition. Donor-acceptor material systems studied span traditional polymer donor:fullerene acceptor systems and new polymer donor:non-fullerene acceptor systems, including P3HT and TQ1 donor materials and PC61BM and N2200 acceptor materials. We report for the first time the ability to synthesise inverted core-shell donor-acceptor nanoparticles from non-fullerene acceptors due to the varied surface energy of this new suite of acceptor materials. This seminar demonstrates the high level of control over film morphology enabled by customising nanoparticulate colloidal inks, where both two-phase and three-phase film architectures can be designed and engineered for new age printed electronics [4]. 1. Chen Xie, et al. Nat. Commun. 9, 5335. (2018). 2. Melissa Marks, Natalie P. Holmes, et al. Phys. Chem. Chem. Phys. 21, 5705−5715. (2019). 3. Natalie P. Holmes, et al. Chem. Mater. 30, 6521−6531. (2018). 4. Natalie P. Holmes, et al. Nano Energy 19, 495−510. (2016).

Resume : A donor-acceptor or push-pull polymer with a repeating unit of phenanthrocarbazole-thiophene-diketopyrrolopyrrole-thiophene (PCZ-T-DPP-T) has exhibited highly red-selective absorption, a so-called dual-band suppression, which is a prerequisite for color-selective thin-film organic photodiodes and full-color image sensors. However, the origin of the significant suppression of the high-energy band II ab-sorption (400-500 nm) compared to the lowest-energy red-selective band I absorption (625-800 nm) is still unclear. We herein investigate the structure and the electronic structure of the PCZ-T-DPP-T oligomer models by (time-dependent) density functional theory calculations to unravel the origin of such feature. Contrary to typical donor-acceptor polymers used for photovoltaic applications, this PCZ-T-DPP-T copolymer in its monomer model shows the frontier molecular orbitals (MO’s), HOMO and LUMO, both coming from the low-band-gap T-DPP-T units and the next frontier MO’s, HOMO-1 and LUMO+1, both coming from the slightly-larger-band-gap PCZ units. Severe torsion (~50) between the two planar units, PCZ and T-DPP-T, prevents mixing (or hybridization or delocalization) between these frontier MO’s. Therefore, the HOMO-to-LUMO lowest-energy transition localized on the T-DPP-T units is much stronger than the next lowest-energy transitions corresponding to the crossover between different non-hybridized units, PCZ and T-DPP-T, achieving the dual-band sup-pression. Based on this, we propose a principle applicable for designing highly red-selective copolymers: (1) minimized donor-acceptor hybridization (e.g., by introducing severe torsion between six-membered rings and five/six-membered rings) and (2) HOMO/LUMO from one unit with extremely low band gap (e.g., T-DPP-T) and HOMO-1/LUMO+1 from the other unit with slightly larger band gap.

Resume : Arsenic contamination in ground water has become a global problem that requires an immediate solution. In this project, the photocatalytic oxidation of As (III) to As (V) and its subsequent removal from groundwater was studied using a P3HT: IDFBR polymer blend as the photocatalyst. In the first stage, a 1:1 blend solution of P3HT: IDFBR was prepared before 30μl aliquots of the solution were spin coated onto a glass substrate to produce blend films. The polymer blends generated superoxide species by means of absorbing light, a phenomenon which was controlled using a superoxide detection system. Acidified molybdate, ammonium vanadate and rhodamine B dye were used to trace the concentration of As (V) in the course of the reaction. In this case, the absorbance of the complex formed by interaction of the dye, As (V) and the chemicals was measured using the UV−Visible Spectrometer at 590nm. This was quantified using a calibration curve. This study indicated that the thickness and surface area of the P3HT: IDFBR polymer blend influenced the production of superoxide species and hence the oxidation of arsenite. It was also necessary to maintain an optimum level of superoxide production to avoid the degradation of the polymer by excess superoxide species. The results portrayed an efficient oxidation of arsenite using the P3HT: IDFBR polymer blend. The pre-oxidized arsenic was later removed via polymer-enhanced ultrafiltration.

Resume : Authentication and access control against malicious attacks become critical for edge devices to secure the connectivity and the functionality in Internet-of-Thing network, where the system mostly consists of sensors, communication devices, and simple-task processors but high-level computing resource is not available. Implementing the authentication within the edge devices can be realized by physical unclonable function (PUF) devices. The authentication requires the predetermined challenges to the PUFs replying the unique responses correspondingly. Among these PUFs, circularly polarized light (CPL) detectable devices can significantly enhance the security due to inherent randomness resulting from unpredictable mixture patterns of chiroptical-conjugated polymers. At the same time, each irregular pattern naturally delivers the unique arrangement resulting in the characteristic responses. In this study, the CPL devices’ responses can instinctively generate the security key strings of ‘0’s and ‘1’s corresponding to the direction of the CPL. Additionally, the more challenge-response pairs (CRPs) lead to the more reliable security keys. With the chiroptical-conjugated polymer devices, the number of the CRPs extensively increases by additional combination of the CPL directions. The actual security keys generated by the CPL detectable devices are investigated to check the randomness and the uniqueness for the PUF performance.

Resume : In recent years, photodetectors based on organic-inorganic lead halide perovskites have been studied extensively. However, the inclusion of lead in those materials can cause severe human health and environmental problems, which is undesirable for practical applications. Here, we report high-performance photodetectors with a tin-based perovskite/PEDOT:PSS vertical heterojunction. The device demonstrates broadband photo-response from NIR to UV. The maximum responsivity and gain are up to 2.6 × 10^6 A/W and 4.7 × 10^6, respectively. Moreover, a much shorter response time and higher detectivity can be achieved by reducing the thickness of PEDOT:PSS. The outstanding performance is due to the excellent optoelectronic properties of the perovskite and the photo-gating effect originated from the heterojunction. Furthermore, devices fabricated on flexible substrates can demonstrate not only high sensitivity but also excellent bending stability. This work opens up the opportunity of using lead-free perovskite in combination with organic semiconductor as highly sensitive photodetectors.

Resume : We report original all in-situ Piezoresponse Force Microscopy studies in ultra-high vacuum on 10’s of nanometer thick films of Croconic acid (CA), an organic ferroelectric, grown on ferromagnetic Cobalt films. Croconic acid, showing a value of polarization that is the highest among organic ferroelectrics and comparable to some inorganic ferroelectric materials, could be the most suitable candidate for development of multifunctional hybrid composite multiferroics. Furthermore, fundamentally intriguing properties may arise at ferromagnet/organic ferroelectric interface. On one side, organic multiferroics are scarce and on the other side, detailed studies on ferroelectric properties of CA are lacking. To investigate the ferroelectric properties, we switch the ferroelectric polarization of individual grains of CA on Cobalt and study the switching properties such as coercive field, its dependence on frequency of probing signal, switching current etc. To explore the multiferroic aspect, we have been successful in electrically writing macroscopic areas of CA film on Cobalt following which we attempt to perform in-situ characterization of the magnetic properties of the Cobalt film underneath to study the magnetoelectric nature of the multiferroic. Such a multiferroic system is expected to have unbounded potential for flexible, non-toxic inexpensive and low power consuming technological applications starting from ultra-energy efficient memory to ultra-sensitive magnetoelectric sensors.

Resume : The presence of noncovalent interactions (H-bonding, p-p stacking, metalophillic interactions) in organic semiconductors has been demonstrated to be beneficial in several applications, resulting in the enhancement of charge transport and device efficiency.1,2 Particularly, the incorporation of H-bonding in organic semiconductors has been proven to increase solar cell efficiency by 50%.3 Nevertheless, the race for achieving efficiency records, has hampered research focused on solving other fundamental issues. Regarding H-bonding, no comparative studies have been performed, finding scattered examples in literature with different semiconductors, H-bonding units and lacking complete studies including both, the optoelectronic and self-assembly properties. Here we show a comparative study using diketopyrrolopyrrole (DPP) as a model electroactive segment. Different families of H-bonded DPPs displaying different H-bonding functions (urea and amide) and placed in different positions of the DPP core have been synthesized. The influence of H-bonding on the optoelectronic properties will be discussed, as well as the gelation mechanism of some of the derivatives and the effect of supramolecular chirality provided by H-bonding. Photoconductivity studies have been performed as well to shed light onto the electrical properties. With these results, we expect to provide handles to the organic electronics community to implement H-bonds into state-of-the-art materials and obtain superior devices 1. Hydrogen-bonded diketopyrrolopyrrole derivatives for energy-related applications. J. Mater. Chem. A. 7 (2019), 23451-23475. 2. Charge transport enhancement in supramolecular oligothiophene assemblies using Pt(II) centers as a guide. J. Mater. Chem. A. 7 (2019), 16777-16784. 3. T. Aytun , L. Barreda, A. Ruiz-Carretero, A. J. Lehrman, S. Stupp. Chem. Mater. 4, (215), 1201-1209.

Resume : In this work [1], we experimentally demonstrate a novel device concept that converts standard monochromatic organic light-emitting diodes (OLEDs) to high-quality white OLEDs (WOLEDs). This breakthrough was accomplished by depositing a dielectric distributed Bragg reflector (DBR) on a top-emitting, single emissive layer OLED. The key to our color-conversion design is the introduction of the DBR Fabry-Pérot modes (Bragg modes) for selecting the desired wavelengths and for concentrating the electric field in the emissive layer, something that has never been considered before. We show that the Bragg modes, although primarily located inside the DBR stack, can significantly overlap with the emissive layer. When Bragg mode resonances are designed to be positioned at red, green and blue colors, outcoupling of photons from the entire visible spectrum is achieved and broadband white electroluminescence is obtained. The Bragg modes and the stopband can be tuned independently by variation of the DBR total thickness and period, respectively, offering great versatility in the optimization of white-light emission spectra. The internal quantum efficiency and color temperature can, therefore, be optimized independently; thus, our device offers prospects for utilizing highly efficient and stable blue thermally activated delay fluorescent (TADF) materials. Our WOLED devices showed a 20% increase of external quantum efficiency and a 30-fold increase of on-shelf lifetime, as compared with the unconverted blue OLED. Importantly, the Bragg modes do not require precise layer thicknesses which means low-quality, low-cost DBRs such as all-plastic ones can be used, paving the way for the fabrication of large scale solution-processed WOLEDs. [1] K. S. Daskalakis, F. Freire-Fernández, A. J. Moilanen, S. van Dijken and P. Törmä, Converting an organic light-emitting diode from blue to white with Bragg modes, ACS Photonics, DOI: 10.1021/acsphotonics.9b01206 (2019).

Resume : The study of the chemical structure of fullerene has been growing a lot, the attractive for its study is due to its exceptional affinity to collect electrons. This property makes this material of great importance for use in areas of optoelectronic devices such as solar cells. In the area of organic electronics the most widely used and highly soluble form is PCBM. In addition to the application of this material in the area of electronic devices, it presents exceptional characteristics in the anti-cancer and anti-viral area, promising candidates for cytoprotection, DNA photocleavage and enzymatic inhibition. In this work, amphiphilic Poly-Fulerenes derivatives were used and studied, which have advantages over C60 fullerene due to the fact that some applications of these materials have been difficult to exploit due to their hydrophobic nature. A more consistent approach is required to incorporate fullerene into polymers to develop structures in which fullerene is arranged in nano and mesoscale structures with controlled sizes to understand and amplify the properties of fullerene for technological applications. Within this context, in this work were manufactured and characterized in the form of Langmuir, Langmuir-Blodgett and Langmuir-Schaeffer films derived from poly-fullerenes. The formation of Langmuir films was studied through pressure isotherms and Brewster angle microscopy, where information about the homogeneity, phase behavior and film morphology were analyzed. These characterizations gave information about the quality, uniformity and morphology of the films. We acknowledge support from CAPES-PRINT-UNESP, INEO-CNPq and FAPESP.

Resume : In this study, highly stable p-type and n-type water-gated field-effect transistors (WGFET) were developed by solution processing technique and they were exploited to realize a robust inverter working in an aqueous environment. The p-type WGFET was fabricated by ink-jet printing of a monochiral dispersion of semiconducting single wall carbon nanotubes (SWCNTs) as the active material on interdigitated gold electrodes with high reproducibility. The developed p-type SWCNTs based WGFET was very stable under hours of bias stress condition or repeated transfer interrogations with no sign of deterioration and minimal current or threshold voltage shifts. The n-type WGFET was also developed by standard Dimatix printing technique using a solution of a newly synthesized n-type polymer semiconductor based on a naphthalene diamine (NDA) core. This n-type WGFET showed extensive operating stability and reproducibility in an aqueous environment also under long bias stress condition. Finally, an inverter operating in an aqueous environment was realized and characterized by long-term stability measurements. The tests suggested a promising cycling robustness despite of working in an environment that typically considered deteriorating to electron-transporting organic molecules. This study will set the steps toward further advanced circuitry designs that can operate in aqueous environment.

Resume : Luminescent Solar Concentrators (LSCs) are devices capable to concentrate the light thanks to waveguide effect. They are gaining attention particularly for application in building integrated photovoltaic, as they potentially allow to collect sunlight over large area windows and concentrate it on small area solar cells. Dedicated literature groups LSCs in two categories: full-spectrum and colorless devices. The first class features LSCs capable to harvest a large portion of the solar spectrum and re-emit the collected light with high efficiency. They are strongly absorbing and colored devices, and involved fluorophores usually display small separation between absorption and emission spectra (Stokes shift). Colorless devices, on the other side, are mainly based on UV absorbing luminophores (typically lanthanide chelates) displaying large Stokes shift and nearly no reabsorption. Although their efficiencies are lower than those obtainable with full-spectrum devices, they offer easier integration into buildings due to their negligible distortion of the transmitted light and marginal efficiency loss for increasing device size. We here present two classes of organic luminophores, cycloparaphenylenes and [1]benzothieno[3,2-b][1]benzothiophene S,S,S’,S’-tetraoxides, which display large Stokes shift and high thermal and photochemical stability. These features makes the presented derivatives suitable luminophores in large area colorless LSCs.

Resume : Solution processability, a key feature of some organic semiconductors, allows the application of high-throughput and low-cost fabrication techniques to plastic electronics. Nonetheless, this very same feature becomes a severe hurdle whenever different organic materials need to be layered on top of each other: even partial dissolution of the underlying layer leads to detrimental intermixing. Amongst the different strategies explored to tackle this issue is the introduction of solubilizing chains that can be removed via thermal treatment after deposition. This so-called "latent pigment approach" leads to the removal of the insulating functionalizing chains from the active layer, often resulting in improvements of charge transport properties. This concept was also extended to n-type naphthalene diimide (NDI) containing polymers through functionalization with a cleavable ester functionality leading to promising results in terms of solubility modulation. In this contribution we describe the preparation of an alternating naphthalene dianhydride bithiophene copolymer (PNDAT2) via thermal treatment of films of a soluble polymeric precursor obtained by direct arylation polycondensation (DAP). The latter combines very good solution processability, high molecular weight and low defectivity. Conversely, PNDAT2, the first example of a conjugated polymer containing the naphthalene dianhydride unit, is completely insoluble and shows features similar to those of the NDI based analogue.

Resume : Polymetallaynes are a special class of strongly conjugated Wolf type-III metallopolymers allowing direct access to the properties inferred by the metal center, e.g. stable redox chemistry [1]. However, polymetallaynes with redox-active transition metals are rare, and so far lack the solubility needed for processing. Yet studies on oligo-nuclear analogons demonstrated high conductivity [2], indicating the promise of these structural layouts for electronic applications. We developed a copper-free dehydrohalogenation polymerization to obtain a Ru-containing polymetallayne P[Ru(dppe)2-DDBT] [3]. The polymer was characterized using NMR, IR and UV-Vis spectroscopy, MALDI MS, GPC and CV. Despite a degree of polymerization >32, P[Ru(dppe)2-DDBT] is fully solution-processable. In CV studies in solution, two separated metal-centered redox processes are observed, indicating charge delocalization and that a mixed-valence species exists. In thin-film, a single oxidation wave with higher peak current intensity is found, consistent with a two-electron process and localized charge. First OFET devices confirmed semiconducting behaviour with low hole mobility. The developed synthetic route is easily adaptable, allowing to access a new class of soluble and redox-active Wolf-III metallopolymers for organic electronics applications. [1] a T. Swager, Macromolecules 2017, 50, 4867, b M. Wolf, J. Inorg. Organomet. Polym. Mater. 2006, 16, 189 [2] F. Schwarz, G. Kastlunger, F. Lissel, H. Riel, K. Venkatesan, H. Berke, R. Stadler and E. Lörtscher, Nano Lett. 2014, 14, 5932 [3] P.-Y. Ho, H. Komber, K. Horatz, T. Tsuda, S. Mannsfeld, E. Dmitrieva, O. Blacque, U. Kraft, H. Sirringhaus, F. Lissel, Polym. Chem. 2020, 11, 472 (Special issue: Polymer Chemistry Emerging Investigators)

Resume : Semi-conducting polymers are promising for the development of low-cost flexible optoelectronic devices. These technologies are not yet mature and several limitations emerge. Synthetic routes rely on complex protocols using costly and hardly removable catalysts. Residual metal traces affect performances, involving numerous further purification steps.[1] Therefore, it becomes necessary to develop versatile, economically viable and “green” approaches for large-scale syntheses. Recently, we investigated new synthetic pathways for efficient polymers, by taking care of purity, variety of targeted structures and versatility of synthetic processes. Based on previous work on carbazole-based polyazomethines, [2] new polymers have been targeted, with the possibility to tune the energy gap, photo/electro luminescence properties. More particularly, bio-based -conjugated polyazomethines [3] or polythiazolothiazoles have been successfully synthesized from divanillin monomer, through metal-free condensation reactions. Divanillin monomer was obtained via enzymatic dimerization of vanillin. All polymers were characterized, and some of them exhibit interesting optical properties, such as a strong absorption and good emitting properties. References. 1. Usluer, O., Abbas, M., Wantz, G., Vignau, L., Hirsch, L., Grana, E., Brochon, C., Cloutet, E., Hadziioannou, G ACS Macro Letters 2014, pp. 1134-1138. 2. Oriou, J., Ng, F., Hadziioannou, G., Brochon, C., Cloutet, E. Journal of Polymer Science, Part A: Polymer Chemistry, 2015, pp. 2059-2068. 3. Guillaume Garbay, Lauriane Giraud,# Sai Manoj Gali, Georges Hadziioannou, Etienne Grau, Stéphane Grelier, Eric Cloutet, Henri Cramail, Cyril Brochon ACS OMEGA, 2020, submitted

Resume : Conjugated polymers with polar side chains are attracting increasing interest for bio-electronic and electrochemical applications, such as organic electrochemical transistors and energy storage devices [1]. The appeal arises from the ability to tune the redox and electrical properties, which depend on the polymer backbone, separately from the ion transporting properties, controlled by the side chain. Side chain engineering thus offers a route to tune the electronic and ion-transport properties of the organic material for a specific application. In the case of anodes for electrochemical storage devices, we seek reversible charging behaviour and stability in aqueous environments as well as high capacity. Here, we present a strategy, based on side chain engineering, to improve the stability and reversibility of deep chargeable n-type organic electrodes operating in aqueous solutions. We select a conjugated polymer backbone with a large electron affinity to achieve electrochemical reduction in water electrolytes, and attach polar side chains to introduce ionic conductivity. Whilst increasing density of polar side chains improves ionic conductivity [2], fully glycolated polymers show irreversible charging behaviour. To understand this behaviour and achieve reversible deep charging we study the electrochemical, spectroscopic, electrical and mechanical properties of polymer films with mixed side chains as a function of side chain type and ratio. We isolate different mechanisms that influence stable operation of the electrodes. Finally we propose a design rule for maximising the reversible deep charging of the polymer and show how these materials can be exploited in charge storage devices. [1] Moia, D.; Giovannitti, A.; Szumska, A. A.; Maria, I. P.; Rezasoltani, E.; Sachs, M.; Schnurr, M.; Barnes, P. R. F.; Mcculloch, I.; Nelson, J. Energy Environ. Sci 2019, 12, 1349. [2] Giovannitti, A.; et al. Chem. Mater. 2018, 30 (9), 2945–2953.

Resume : Electron transport materials (ETMs) are widely used as interlayers to lower the cathode electrode work function in organic solar cells and organic light-emitting diodes, for example. The usual interpretation for their operating principle is a chemical interaction between the ETM and the electrode, inducing partial or integer charge transfer or collectively an intrinsic dipole moment caused by preferential molecular orientation. Herein, we systematically explore the commonly used ETM bathophenanthroline (BPhen) deposited on a series of conducting substrates. The energetics at the BPhen interface follows the typical integer charge transfer (ICT) model with an extra displacement of the vacuum level by up to 1.4 eV. The extra displacement is ascribed to the ‘‘double dipole step’’ formed by the positive and negative charged species and their induced image charges when they are close to the surface of substrates. After n-type doping the displacement is further increased to 1.8 eV, yielding a larger work function modification than obtained using typical electrolytes and zwitterions as cathode interlayer.

Resume : Wearable electronic devices and on-skin biosensors might play an important role in the development of preventive and personalized medicine. However, these devices require specific semiconductor materials, which should have simultaneously good charge-transport properties, mechanical flexibility, low cost, and biocompatibility. Herein, we present the results of our systematic study of vat dyes, which are widely used for textile dying, as low-toxic organic semiconductor materials potentially suitable for wearable electronics applications. Some of the vat dyes were recently applied as semiconductor materials in organic field-effect transistors (OFETs). We significantly extended the range of known vat dyes with semiconductor properties and show that their OFET performance is strongly dependent on the nature of the used dielectrics thus yielding decent electrical characteristics for proper material combinations. We have also performed a thorough biocompatibility assessment for vat dyes and a number of reference semiconductors (pentacene, P3HT, C60, etc.) using human embryonic lung fibroblasts model. Cell culture was grown on thin films of the studied materials and biological effects (apoptosis, oxidative stress, etc.) were monitored by a set of complementary techniques. The obtained data allowed us to identify the safest materials in terms of their action on human cells and feature the potential of vat dyes are promising organic semiconductors for wearable electronics applications.

Resume : The tremendous scientific effort devoted to achieving full control over the correlation between structure and function in organic and polymer electronics, has prompted the use of supramolecular interactions to drive the formation of highly ordered functional assemblies, which have been integrated into real devices. Self-assembly of organic semiconducting materials constitutes a most powerful tool to generate low-dimensional and crystalline functional architectures. These include 1D nanostructures (nanoribbons, nanotubes, and nanowires) and 2D molecular crystals with tunable and unique optical, electronic and mechanical properties. However, integration of supramolecular nanostructures into real devices currently represents a significant challenge to the advancement of (opto)electronics. Here, for the first time, we reported a novel procedure enabling the use photolithography to directly integrate multiple organic crystals simultaneously into micro-/nano-devices. Electrodes could be patterned with an arbitrary shape and position with a high degree of precision, the spatial resolution below 300 nm. This photolithography technique provides a valuable tool for investigating inherent photonic properties of one-dimensional semiconducting materials. As proof-of-concept, organic crystalline donor-acceptor heterojunction photovoltaic devices based on planar asymmetric Au-Ag electrodes have also been developed.

Resume : Conventional methods to obtain conjugated polymer (CP) thin-films for high performance organic electronics devices involves the use of large quantities of volatile organic solvents for synthesis, purification, and deposition. Unfortunately, these solvents are environmentally detrimental particularly halogenated solvents (e.g. chloroform, dichlorobenzene) which are problematic for scale-up due to their toxicity. The deposition CP thin-films from colloidal dispersions in water has emerged as an environmentally benign approach for organic transistors and solar cells.[1] These aqueous inks are usually obtained by an emulsification process where the CP is dissolved in a good solvent, dispersed into a continuous phase with water using a surfactant, and evaporation of the organic solvent generates a nanoparticle dispersion. Unfortunately, halogenated solvents is required to ensure good solubility for uniform dispersion, and prior polymer synthesis is needed before emulsification. To overcome this, we demonstrate an elegant approach to obtain aqueous CP inks where readily soluble starting conjugated monomers and reagents are emulsified for direct polymerization to form colloidal dispersions of the CP in water. [2] Inks based on indacenodithiophene and diketopyrrolopyrrole CP semiconductors will be discussed where organic transistors with comparable mobilities to conventional solution processing methods are described. [1] J. Mater. Chem. C, 2017,5, 2745-2757; [2] Manuscript Submitted (2020).

Resume : The primary block to achieving higher efficiencies in bulk-heterojunction organic solar cells (BHJ-OSCs) is energy losses through non-radiative pathways due to the decay of a charge-transfer state (CTS) to the ground state via energy transfer to vibrational modes. It has previously been suggested that the open circuit voltage (Voc) in OSCs is largely determined by the energy of the donor-acceptor CT state (Benduhn et al., Nat. Energy. 2017) and thus a large number of recent studies have focussed on increasing the energy of CT states by minimizing the energy offset between the donor and acceptor. This relationship can be rationalized by understanding that a higher overlap of the vibrational modes of the CT and ground states increases the rate of non-radiative recombination. However, more recent studies have found that increasing the CT state energy does not always result in a reduction in the non-radiative voltage losses, which suggests that other properties of the CT state to ground state transition appear to affect the trend (Azzouzi et al., Phy. Rev. X, 2018). We have recently investigated the effect that the CT state properties have on the voltage losses of BHJ-OSCs by using a series of increasingly fluorinated PBDB-T donors, in conjunction with a variety of different fullerene and non-fullerene acceptors (Eisner et al., JACS, 2019). In this work, we showed that the hybridization of charge transfer and local excited states occurs when the CT state and first excited state lie close in energy, and that this hybridization can lead to an increase in the CT to ground state oscillator strength due to intensity borrowing. By using a three-state molecular model, we further showed that hybridization leads to the suppression of the non-radiative voltage losses exhibited by such blends, especially and when the oscillator strength of the local excited state to ground state transition of the lower band-gap component is large. Here, we extend the use of our model to look at more novel systems, specifically investigating how hybridization of CT-state and local excited states affects the voltage losses and charge generation efficiency of recently published high efficiency (>14% PCE) systems. We combine low-temperature luminescence and charge-extraction measurements, to explore why recent high-efficiency organic blends can simultaneously achieve low non-radiative voltage losses and high fill factors. Finally, we use our model to predict the maximum PCE that can be achieved with organic semiconductors, taking into account hybridization and our experimental results.

Resume : Organic Light-Emitting Diodes (OLEDs) able to directly emit circularly polarized (CP) electroluminescence (EL) are gaining much interest, due to their possible applications in anti-glaring screens, 3D-displays and in medical diagnosis with a simplified device architecture. In this view, we have recently demonstrated that chiral lanthanide complexes can be employed in solution processed CP OLEDs, obtaining highly CP emission [1]. Typically, Europium complexes featuring beta-diketonate ligands exhibit the strongest photoluminescence quantum yield (PLQY) and have been largely employed in OLED technology. The introduction of a proper chiral ligand provides high level of polarization (gPL), but at the expenses of its PLQY [1]. Here we present a new Eu-complex with at the same time very good PLQY (60%) and gPL (0.6). Moreover, once it is incorporated in a suitable solution processed OLED architecture, remarkable EQE up to 0.3% and gEL of about 0.5 are observed. The development of devices embedding other classes of chiral lanthanides emitters can be envisaged to possibly bring the organic chiral photonic technology to the next readiness level. [1] Zinna et al. Adv. Funct. Mater. 2017, 27, 1603719. This work was carried out with the financial support of Ministero dell'Istruzione, dell'Università e della Ricerca, with the project “Towards a CHeap and portable InstRument for bioAnaLysis based on enAntiospecific luminescence and aBsorption essays (CHIRALAB)” prot. 20172M3K5N.

Resume : Anthracene derivatives are classic and unique luminophores which combine high photoluminescence quantum yields (PLQY), large exchange energies (ΔEST) and narrow Stokes shifts. This makes them ubiquitous in many applications, particularly organic light emitting diodes and triplet-triplet annihilation upconversion (TTAUC). Anthracenes should, therefore, be highly valuable building blocks for luminescent conjugated polymers, particularly if TTA is desired. Unfortunately this has been thwarted by the extended π system of anthracene which imparts a strong tendency for it to aggregate. This firstly suppresses materials solubility, restricting molecular weight and processability. Secondly, aggregation prompts photodimerisation,1 aggregation caused quenching (ACQ)2 and the formation of excimers in the solid state.3 These constitute PL quenching pathways and their exact extent is hard to predict and must be controlled. We have surmounted aggregation through molecular encapsulation to develop true conjugated anthracene polymers with high molecular weights and minimal ACQ which do not form excimers. These materials provide a new lease of life for an established luminophore and are ideal annihilators towards single material solid state TTAUC films. Results for this unprecedented application will be presented. 1 Zhu, L. et al., J. Am. Chem. Soc. 2011, 133, 12569–12575. 2 Egbe, D. A. M. et al., Macromolecules 2010, 43, 1261–1269. 3 Freeman, D. M. E. et al., Polym. Chem. 2016, 7, 722–730.

Resume : Among various reported bulk heterojunction active materials, the blend of poly(3-hexylthiophene (P3HT) and [6,6] phenyl C71 butyric acid methyl ester (PC71BM) have potential to achieve higher power efficiency in organic solar cells [1]. In this context, Langmuir-Schaefer (LS) technique is useful for fabrication of electric devices based on thin films, because it offers the ability to control of the organized structures [2]. In this work, active layers were fabricated by LS films from solutions of neat P3HT and mixed with PC71BM in different molecular weight ratios to study the influence of PC71BM on the optical, morphological and electrical properties in LS films deposited onto interdigitated electrodes. A Langmuir trough KSV 5000 was used to fabricate the thin films and to realize studies of pressure isotherms (π-A). The growth and morphology of active layers fabricated were studied through spectroscopic measurements (UV-visible, BAM and Raman). In interdigitated electrodes, direct current measurements were carried out using a Keithley source under illumination using the solar simulator Oriel Verasol (100 mW/cm2 - AM 1.5). In the alternating current characterizations were performed employing a Solartron 1260A impedance analyzer. The junction of the different characterizations allows a study of the properties as orientation of molecules, morphology, UV-Vis absorption, electrical resistance of materials and photoconductivity of the active layers. We acknowledge support from CAPES, INEO-CNPq and FAPESP. [1] SINGH, A. et al. Organic Electronics, 51, 428-434, (2017). [2] BAO, Q. et al, Advanced Functional Materials, 26, 1077-1084 (2016).

Resume : This study explores light-assisted charge propagation in self-assembled and self-aligned networks of quasi one-dimensional needle-like crystallites of an oligoacene derivate dihydrotetraazaheptacene (DHTA7) [1]. The crystallites were epitaxially grown on insulating hexagonal boron nitride, acting as van der Waals dielectric support. Electrostatic force microscopy was employed to demonstrate that upon external illumination the conductivity of organic crystallites can be enhanced by more than two orders of magnitude. Further, by exploiting the highly anisotropic optical properties of these organic nanostructures, a selective charge propagation along the crystallites was triggered that matches the orientation of the molecular backbones with the incident light’s polarization direction. Our results demonstrate the possibility to realize a "light-gate" for switching on the conductivity of organic nanostructures and even guiding the charge propagation along desired directions in self-assembled crystallite networks. [1] A. Matković, J. Genser, M. Kratzer, D. Lüftner, Z. Chen, O. Siri, P. Puschnig, C. Becker, and C. Teichert, Advanced Functional Materials 29, 1970300, (2019).

Resume : Squaraines are small molecular quadrupolar donor-acceptor-donor (DA-D) chromophores absorbing in the red spectral range and are considered for application as, e.g., photovoltaic materials [1,2]. A prototypical anilino squaraine with branched alkyl side chains (SQIB) is known to crystallize into two polymorphs with differing optical properties. In spin-casted thin films these two phases emerge with a strongly preferred out-of-plane and rather random in-plane orientation upon thermal post-annealing directed by the annealing temperature [3]. Here, we investigate the surface induced growth of vapor deposited SQIB on varying dielectric and conductive substrates and correlate the observed polymorphs to the substrate properties. We approach a complete picture of morphology and molecular orientation relative to the substrate geometry by X-ray diffraction, scanning probe microscopy, and polarized light microscopy. [1] D. Scheunemann, A. Lützen, M. Schiek et al., Appl. Phys. Lett. 111 (2017) 183502. [2] O.S. Abdullaeva, F. Balzer, K. Dedek, M. Schiek et al., Langmuir 32 (2016) 8533. [3] F. Balzer, A. Lützen, M. Schiek et al., Cryst. Growth Des. 17 (2017) 6455.

Resume : Singlet exciton fission is a carrier multiplication process in organic semiconductors that generates two electron-hole pairs for one photon absorbed, affording quantum efficiencies up to 200%. Recent advancements in singlet fission have been materials-limited due to the rarity of molecules that meet the energetic requirement for the process, that the energy of the lowest triplet excited state E(T1) be approximately half the energy of the lowest singlet excited state E(S1). Also important is to ensure the chemical stability of the candidate compounds that would broaden their application prospect. A better understanding of the intrinsic mechanism and reasonable design strategy are highly desired. We exploited excited-state aromaticity views toward novel singlet fission candidates.[1] Based on the theoretical and experimental study, we found that one type of dyes that have been accidentally synthesized by H. von Pechmann in 1882 would be suitable for stable singlet fission dyes[2]. Theoretical models have been set up to further analyze the special excited state behaviors, e.g. big energy gaps between the low-lying singlet and triplet states, low E(T1)s and dramatic change within the transformable isomerides, aiming at a novel strategy for manipulating the excited state energy and stability of semiconductors with the aromaticity view. 1. Brostein, H. et al., J. Am. Chem. Soc. 2019, 141, 13867. 2. H. von Pechmann，Ber. 1882, 15, 881.

Resume : The most efficient perovskite solar cells (PSCs) employ an n–i–p device architecture that uses a 2,2′,7,7′‐tetrakis(N,N‐di‐p‐methoxyphenyl‐amine)‐9,9′‐spirobifluorene (Spiro‐OMeTAD) for hole transporting material (HTM). However, the Spiro-OMeTAD is susceptible to heat and easily oxidized with atmospheric oxygen. Moreover, Lithium bis(trifluoromethane)sulfonimide (LiTFSI) is hygroscopic and accelerates the rapid deterioration of perovskite structure, leading to the lack of long-term stability of PSCs. In this work, we apply negative dipole self-assembled molecules to modify the zinc oxide nanoparticles (m-ZnO NPs) for replacing the Spiro-OMeTAD HTM. Based on the molecules dipole moment, the m-ZnO NPs energy alignment has been tuning to ohmic contact, which assist charges tunneling to the electrode. The controllable surface dipole can achieve better charges equilibrium further enhance the Fill Factor and Current density. Since m-ZnO NP is a well-protected layer for preventing perovskite from contacting the high-humidity environment, the stability of the devices is greatly improved.

Resume : Materials exhibiting thermally activated delayed fluorescence (TADF) are heralded as next generation emitters for organic light-emitting devices (OLEDs). Their narrow singlet-triplet energy gaps (ΔEST) are typically achieved through intramolecular charge transfer (ICT) and enable triplet harvesting. The crucial problem facing these materials is obtaining a high photoluminescence quantum yield (PLQY) alongside rapid reverse intersystem crossing (rISC) in the same material.[1] A high PLQY is vital for good efficiency; and has been enhanced to 100% in numerous emitters. However, this is always to the detriment of fast rISC,[2] which is essential for OLED stability at both high brightness and in the long term. We recently introduced a new structural class of 3D molecule capable of ICT, based upon an electron poor ring-fused triptycene core.[3] The three fused benzene ‘‘fins’’ of triptycene are able to communicate through the spatial overlap of their molecular orbitals, termed homoconjugation. We showed that homoconjugation facilitates a great increase in ICT oscillator strength and a narrowing of ΔEST compared to a single fin, without a red shift in PL. We have now extended this concept to ICT TADF triptycenes. We will present results showing that homoconjugation enables an increase in ICT oscillator strength alongside enhanced rISC to tackle the greatest problem facing TADF. 1 JMCC, 2017, 5, 8622–8653. 2 Adv. Sci., 2018, 1700989. 3 S. Montanaro et al., JMCC, 2019, 7, 12886–12890.

Resume : Flexible plastic substrates have received attention as components in next-generation optoelectronic devices such as organic light-emitting diodes (OLEDs) and organic solar cells because of lightweight, inexpensive, and enable to roll-to-roll mass production. To improve the performance of the devices based on flexible substrates, nano-structuring technology has become indispensable. The planar substrate causes unwanted surface reflection and internal reflection, whereas the nanostructures induce scattering of the light and remove the reflections. Recently, surface modulation of the plastic film has been widely used to enhance the optical scattering of the film such as micro-meshed surface, micro-lens array attached surface, nanostructured surface, and nano-imprinted surface. However, these methods involve multiple processes, low throughput, high cost. And these various kinds of nanostructural patterns involved harsh processing conditions to produce a number of defects. In this work, we report the simple and low-cost method to produce buckling nanostructures by different thermal expansion coefficients of poly(dimethylsiloxane) (PDMS) substrate and silicon dioxide (SiO2) layer. Liquid PDMS was cured on a petri dish with thickness as 500 um and then heated up to 85℃ in a vacuum chamber. The SiO2 was deposited on heated PDMS by plasma-enhanced chemical vapor deposition (PECVD) method. During cooling down of film, the compressive stress occurred at SiO2/PDMS interface due to the difference in thermal expansion coefficient (α_PDMS=9.0 x 10-4 K-1, α_(〖SiO〗_2 )=0.6 x 10-6 K-1). To release the compressive stress, the buckling structures were spontaneously formed on the PDMS substrate. The size of buckling structures simply controlled by the thickness of the PDMS substrate and SiO2 layer. The average total transmittance was achieved of 90.1 % and the average haze was achieved of 60 % in the visible wavelength region (400 nm < λ < 700 nm) with pattern period as 7 um and height as 2 um. The wrinkle patterned PDMS diffracts light at the interfaces between the air and the buckling structures, confirmed by electromagnetic simulation.

Resume : Unpaired electrons residing on fullerenes are of fundamental importance for spintronic and energy-related applications. However, materials carrying free electrons are typically reactive, difficult to stabilize and transient or short-lived. A protecting environment is required for shielding and assembling stable fullerene radicals. We have exploited a supramolecular design to protect the paramagnetic azafullerenyl C59N• radical inside a ring-shaped molecule [10]cycloparaphenylene ([10]CPP). This can encircle a stable dimer bisazafullerene (C59N)2 via concave-convex  interactions, and the fullerene-fullerene bond can then be broken with laser irradiation. The generation and lifetime of the resultant paramagnetic C59N• was monitored by electron paramagnetic resonance (EPR) spectroscopy, showing a characteristic triplet signal. Without the [10]CPP the C59N• radical has a sub-second lifetime, but when encircled by [10]CPP the triplet signal remained intense and was observable even after several weeks. [10]CPP effectively blocks the recombination process and increases the lifetime of the radical by a factor >108 paving the way for the exploitation of fullerene radicals.

Resume : A significant challenge for many semiconducting polymer applications is the relative difficulty in patterning these materials with high resolution. Doping induced solubility control (DISC) patterning[1,2,3] is a recently developed technique which uses the change in polymer solubility upon doping, along with an optical dedoping process,[4] to achieve high resolution optical patterning. In a previous study, it was observed that the resolution exceeded the diffraction limit, however it was unclear what mechanism enabled this.[2] Here, we use diffraction to spatially modulate the light intensity and determine the dissolution rate, revealing a second order dependence on light intensity. This rate law is independent of wavelength, indicating that the previously identified photochemical reaction of the dopant molecule does not affect the resolution; instead, it is entirely controlled by thermal dedoping. We develop a simple model of film thermal transport which can accurately predict feature size and resolution. Our model reveals that the combination of low film thermal conductivity and high interfacial conductivity, along with a temperature dependent solubility, results in the observed non-linear dissolution rate law. These properties are general to nearly all conductive polymers, including undoped materials, indicating it should be possible directly pattern undoped polymers at high resolution. We demonstrate this by patterning sub-400nm feature sizes in undoped Pff-BT4T-2OD. 1. Jacobs, I. E. et al. ACS Nano 2015, 9, 1905. 2. Jacobs, I. E. et al. Adv. Mater. 2017, 29, 1603221. 3. Jacobs, I. E. et al. Adv. Mater. 2017, 29, 1703063 4. Fuzell* J., and Jacobs* I. E. et al. J. Phys. Chem. Lett. 2016, 7, 4297.

Resume : π-conjugated polymers are being used in several electronic devices, including organic photovoltaics (OPV). Due to their generally disordered nature, especially regarding chain length and conformation, curiosity on their behaviour has pushed research in this direction. Benzodithiophene (BDT) has been one of the widely used building blocks for the synthesis of OPVs due to their high power conversion efficiencies.[1] Its rigid and planar structure facilitates tuning the molecular energy levels with distinct band gaps being achieved. However, when in the solid state (thus high concentrations) they show unpredictable behaviour. Designing a more controlled self-assembly would reduce the non-radiative decay processes, specifically charge recombination due to formation of aggregates. Encapsulating alkyl chains would prevent interpolymer interactions, thus lowering the formation of aggregates and π-π stacking.[2] By controlling the polymer chains’ arrangement and conformation, structurally ordered and defect-free backbones can be synthesised. This talk will encompass the synthesis and characterisation of novel encapsulated BDT monomers and a series of encapsulated polymers, showing how they prevent the packing and aggregation of the polymers, therefore affecting the structure-property relationships, efficiencies and thus the performance of the newly formed devices. 1. Zhang, S. et al., Adv. Mater., 2018, 30, 1800868. 2. Leventis, A. et al., J. Am. Chem. Soc., 2018, 140, 1622-1626.

Resume : Use of singlet fission (SF) materials is a promising strategy to overcome Shockley-Queisser efficiency limit in single junction solar cells(1). SF is the process in which two triplet excitons are generated from one singlet excited state. One of the main requirements for SF to occur is for the energy of singlet excited state to be at least twice as high as the energy of triplet state. Pyridinium phenoxides (PyPh) are zwitterionic compounds similar to betaine dyes that are known to exhibit a variety of optic properties such as solvatochromism and thermochromism(2). Simple structure of PyPh makes it an attractive model for theoretical studies. It also permits to synthesise a variety of its derivatives on a relatively large scale using commercially available inexpensive building blocks. Computational screening of a range of PyPh isomers has demonstrated that nearly all of them have potential to exhibit SF due to sufficiently large gap between their triplet and singlet energy levels. Here, together with theoretical findings, we report synthesis and photophysical characterisation of several pyridinium phenoxide (PyPh) derivatives that demonstrate SF properties. To our knowledge it is the first time SF is reported for this family of materials. (1) Shockley, W.; Queisser, H. J. Detailed Balance Limit of Efficiency of P‐n Junction Solar Cells. Journal of Applied Physics 1961, 32 (3), 510–519. https://doi.org/10.1063/1.1736034. (2) Reichardt, C. Solvatochromism, Thermochromism, Piezochromism, Halochromism, and Chiro-Solvatochromism of Pyridinium N-Phenoxide Betaine Dyes. Chem. Soc. Rev. 1992, 21 (3), 147–153. https://doi.org/10.1039/CS9922100147.

Resume : This work presents a study of the morphology and electrical properties of polystyrene (PS) films in different degrees of sulfonation for application as an ammonium hydroxide gas sensor. Ionomers are polymers made up of low polarity chains, which have a relatively low concentration> 15%) of ionic groups attached to them [1]. The ionomers were prepared by copolymerizing styrene and methyl methacrylate via free radical and the sulfonation reaction was carried out using the Makowski method [2,3]. Ionomer films were prepared using the casting technique. This technique, although simple, has been shown to be very efficient for use as an ammonia gas sensor, when deposited on electrodes of interdigitated circuits (IDE). Conductivity measurements were carried out in DC regime to verify the electrical behavior of the deposited polymer, then tests were performed for the detection of ammonia gas. The results showed that it is possible to detect ammonia gas cycle after cycle with excellent reproducibility. FAPESP, INEO/CNPq, LNNano-LMF This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoa de Nível Superior-Brasil (CAPES)- Finance Code 001. [1] Lundberg, RD.; Encyclopedia of Po/ymer Science and Engineering, vol 8, 393, 28 ed., John Wiley & Sons, Inc., N.Y. 1987. [2] OLIVATI, Clarissa de Almeida. Estudo das propriedades elétricas e ópticas de dispositivos eletroluminescentes poliméricos. 2004. Tese de Doutorado. Universidade de São Paulo. [3] MAKOWSKI, H. S.; LUNDBERG, R. D.; SINGHAL, G. H. US Patent: 3,870,841. EXXON Research and Engineering Company, 1975.

Resume : In recent years, solid state mechanical responsive emittive materials have drawn attention due to its vast applicability in OLEDs, optical storage, biological probes and optical sensors. Mechanochromism (MC) is the mechanical-responsive luminescence alteration with reversible emission property in response to mechanical stimuli. Five AIE bearing mechanochromic materials based on cyano-triphenylethylene building block have been synthesized and characterized. The dyes consist phenanthroimidazole or N-phenylcarazole as additional chromophore (AM24, AM25 and AM26) on pyrene core and simple pyrene unit as donor (AM23 and AM27) varying the position of para and meta tethering group on cyano-triphenylethylene. In UV- visible spectra, all the dyes exhibited predominant π-π* absorption. In emission, all the dyes showed positive solvatochromism. Compounds displayed enhanced emission in the solid state when compared to dilute solution due to restriction of intramolecular rotation in the aggregated phase as obtained from photoluminiscence quantum yield and aggregation induced emission study. Dyes AM23 and AM24 displayed red shifted piezochromism while AM25 displayed blue shifted mechanochromism. In conclusion, these compounds display substantial AIE characteristics and piezo-stimuli responsive fluorescence change in aggregation state, which makes them promising candidates as novel mechanochromic, sensor and display materials.

Resume : Conjugated polymers are of increasing interest as photocatalysts on account of their abundance, low cost, tuneable optical and electronic properties, and the prospect of designing higher performance systems. Efficient computational methods are needed in order to sufficiently sample the chemical phase space for this materials exploration while incorporating the effects of chemical structure, polymer microstructure and the local environment. In order to explore the materials computationally we first need to understand the mechanism of photocatalytic hydrogen evolution in the materials. Using experimental results from a variety of polymer systems we perform an in-depth computational study, using DFT, of the possible reaction pathways in different liquid environments. We focus on aqueous dispersions of conjugated polymers using triethylamine as a sacrificial electron donor. We examine whether the identified reaction pathway, when combined with solvation data obtained using molecular dynamics, can explain differential hydrogen evolution in these systems. Having identified the rate limiting step in the reaction pathway, we proceed to apply a computationally more efficient approach to explain the relative performance of different polymer photocatalysts. We investigate the extent to which this method could be utilised as screening tool to guide materials design and experimental investigations in the large chemical phase space.

Resume : Recent years have witnessed great progress of both conjugated polymers in terms of structural diversity, processing, morphology analysis and application in all sub-areas of organic electronics. However, with step-growth polycondensation still being the dominant reaction mechanism, molecular weight (MW) control remains a major drawback, which complicates comparability of individual studies. Another issue pertains to the role of prevalent chemical defects that eventually form during direct arylation polycondensation (DAP) of aromatic monomers. Here we present examples of high performance copolymers that can be made via DAP with a minimum of reaction steps and controlled MW weight to address the influence of main chain defects and MW on properties usually observed for short chain systems. Selected examples include copolymers based on naphthalene diimide, diketopyrrolopyrrole and benzothiadiazole copolymers, which are used for applications in all-polymer organic photovoltaics, n-channel organic field-effect transistors and organic thermoelectrics.

Resume : Recent advances in polymeric Organic Photovoltaic (OPV) cells have boosted the quest for scalable and sustainable methodologies for the synthesis of -extended p-type donor materials and n-type acceptor materials (either oligomeric or polymeric) for the active layer. In fact, the state of art, highly performing active layers make use of complex molecular architectures which are not scalable at industrial level.1 Sustainability and scalability of the synthetic process will play a fundamental role for definitive consecration of OPVs, and generally it depends from: (a) the number of synthetic steps; (b) the cost of the materials; (c) yields; (d) the number of steps of purification that require column chromatography. In this contribution we introduce our innovative monomeric and polymeric systems, built through a synthetic strategy that combine direct arylation (DAr) with an intramolecular cross aldol condensation in a one-pot component cascade reaction.3 We will present the versatility of the strategy for the rapid construction of several thiophene-based scaffolds such as benzodithiophenes and anthradithiophenes, using starting materials commercially available at low cost or obtained with easy and scalable synthetic sequences.4,5,6 Selected compounds was incorporated in oligomers and polymers used for fabrication of devices with over 10% efficiencies and their properties will be discussed.

Resume : The rational design of phase purity for two-phase conjugated polymer systems is challenging but crucial for organic/printed electronics. In this presentation we report a ‘mixed-flow design’ concept for printing two-phase conjugated polymer systems promoting phase purity for application in both bulk-heterojunction solar cells and thin-film transistors. The key aspect of this work lies in the mixed-flow design concept with the integration of both laminar and extensional flows using a unique designed microstructured shear blade. The fluid simulation is utilized as a tool for the flow pattern design to induce the shear, stretch and push-out effects to achieve optimal polymer chain conformation for phase purity. Experimentally, this mixed-flow strategy enhances semiconductor blend thin film crystallinity and increases phase purity with proper percolation. The improved morphology leads to higher short-circuit currents, enhanced fill factors, and significantly improved power conversion efficiency (PCE, enhanced by ~50% compared with conventional blade coating method) for printed all-polymer solar cells. In addition, this printing technique also enhances the performance of all-semiconductor polymer ambipolar transistors (mobility = + ~70%) as well as unipolar semiconductor polymer-insulating polymer transistors (mobility = + ~ 100%) , suggesting the versatility of this methodology for various two-phase conjugated polymer systems.

Resume : Conjugated polymers have shown great promise for applications in bioelectronics because of their processability, biocompatibility and chemical tunability. Their ability to conduct both ionic and electronic charge is utilized in organic electrochemical transistors (OECTs). In particular, n-type (electron transporting) polymers are used in electrolyte gated OECTs to detect metabolites with high efficiencies, [1] and to build complementary logic circuits. [2] However, the low performance of n-type polymers halts the practical use of these devices at the biological interface. In order to understand what governs the performance of n-type OECTs, we investigate the transport properties, thin film morphology and water/ion/polymer interactions of two top OECT performers: the ladder type polymer poly(benzimidazobenzophenanthroline) (BBL) and a donor-acceptor type copolymer (P-90). We find that BBL OECTs exhibit tens of times higher transconductance and faster switching speeds than P-90 devices. Electrochemical UV-Vis spectroscopy, impedance spectroscopy and quartz crystal microbalance with dissipation monitoring (eQCM-D) reveal that despite the absence of a hydrophilic component in its chemical structure, BBL exhibits higher doping efficiency and volumetric capacitance than P-90. Electrochemical atomic force microscopy suggests that BBL undergoes a drastic and permanent change in morphology upon electrochemical doping which is associated with a large uptake of ions and water. This high performance of BBL in OECTs is unexpected since previous studies suggest that OECT operation requires the polymer to have a hydrophilic component to hydrate and enable ion penetration. Our results suggest that the particular packing of BBL promotes cation-to-electron coupling in a way that is not hindered by water uptake, while hydration leads to more disorder in P-90. We suggest that mixed transport and ion/electron coupling can be maximized by designing a highly crystalline polymer film containing a small content of hydrophilic component to bring the performance of n-type OECTs closer to the bar set by the p-type counterparts. References 1. Ohayon, D., et al., Biofuel powered glucose detection in bodily fluids with an n-type conjugated polymer. Nature Materials, 2019. 2. Sun, H., et al., Complementary Logic Circuits Based on High-Performance n-Type Organic Electrochemical Transistors. Advanced Materials, 2018. 30(9): p. 1704916.

Resume : Edible electronics is an emerging technology that exploits the use of bioinspired and natural materials to manufacture devices that can be safely ingested or disposed in the environment. By overcoming the safety constraints associated with standard ingestible electronics, this technology potentially enables a large scale of applications in medicine and food industry, including biosensing within the gastrointestinal tract, tracking and quality control of foodstuff. Thinking through such a technology designed for distributed applications and compatible with safe operation in the gastrointestinal tract one of the challenges is represented by lower as much as possible the voltage operation range of the electronics components. In this framework, we are presenting a low-voltage n-type Organic Field Effect Transistor gated by a chitosan-based solid and edible electrolyte, a cheap and abundant biopolymer. The semiconductor used guarantee a good water and air stability of the device and the high double-layer capacitance of the electrolyte (~7uF/cm2) allow its operation below 700mV gate voltage. Through impedance analysis the behaviour of the device under different humidity condition and the lack of ionic charge penetration in the channel is also discussed. In an attempt to move towards a totally edible and green electronics, in which the presence of residual solvents can be relevant, we are also presenting a device in which both active and electrolyte layers are processed in water.