New materials for organic electronics: from synthesis to processing, characterization and device physics

Resume : Organic photovoltaics based on semiconducting polymers offer a promising route towards low cost, printable and flexible technologies for solar energy conversion. Significant advances have been made in recent years towards improving the power conversion efficiency (PCE) of these devices up to 10-12%, now approaching those required for commercial applications. However, the successful implementation of this technology remains limited by operational lifetime, which is largely due to the unstable nature of the organic semiconductors themselves. Many of the best performing polymers in terms of efficiency are in fact highly unstable, including the widely studied benzodithionene polymer PTB7 and its derivatives. Currently the mechanism causing such polymers degrade is poorly understood, which hinders the development of more stable derivatives with competitive operating lifetimes. In this work, we investigate the photooxidative stability of polythieno[3,4-b]-thiophene-co-benzodithiophene (PTB7) and its analogue PTB7-Th with alkylthienyl side chains on the central benzodithiophene unit. We reveal how these side chains play a critical role in the rate of photochemical degradation, and how this impacts device operating lifetimes. In addition, we discuss the important role of high boiling point solvent additives such as 1,8-diiodooctane on polymer reactivity, with the rate of degradation seen to be critically dependent on the extent of drying of the thin film during device fabrication. These insights can in turn help in the design of more air- and light-stable polymers for organic photovoltaics that maintain both high efficiencies and competitive operating lifetimes.

Resume : Novel design of polymers has brought more attention to bulk heterojunction polymer:fullerene solar cells (PSCs) in the past ten years. A typical example is the synthesis, through chemical structure engineering, of the benzodithiophene-co-thieno[3,4-b]thiophene (BDT-TT) polymers leading to power conversion efficiency (PCE) of over 10%. In this work, we study both device PCE and stability for a set of PBDT-TT polymers and the effect of an additive on the solar cell performances. Our results show an enhancement in efficiency (0-56 % rise in PCE) and an accelerated UV-degradation for devices processed with additive. Additionally, we conducted a systematic UV-degradation study on the solar cells. We found that based on the polymer chemical structure, PSCs of polymers with alkoxy side chains are more stable (~20 % loss in PCE) than those with alkylthienyl side chains (~40% loss in PCE). These findings pave the way for new materials that yield efficient as well as stable organic solar cells.

Resume : -Preferable a contributed talk. Organic photovoltaics have surpassed 11% power conversion efficiency through the careful design of low bandgap polymer donors to be used with fullerene acceptors. Despite this impressive progress, most of these high performance devices exhibit poor stability. Additionally, P3HT remains the only cost-effective donor polymer and the limitations of using fullerenes in photovoltaic devices continue to persist. We have recently shown that the efficiencies of P3HT devices can reach as high as 6.4% by replacing PCBM with a non-fullerene small molecule acceptor.1 The use of these acceptors in multi-component heterojunctions can further increase the efficiencies achieved by the P3HT devices. We have developed a novel approach towards high efficiency organic photovoltaic devices; where P3HT is combined with two non-fullerene acceptor molecules in a ternary blend. Ternary blends that make use of two polymer materials have been explored in the past, however the lack of entropic driving force and the potential for intermolecular interactions between polymer chains renders this approach problematic.2 The introduction of small molecule acceptors into the P3HT binary devices leads to an increase in efficiency up to 7.7% and importantly a vast improvement in device stability. Tuning of the second acceptor’s LUMO allows a higher VOC to be achieved, and through careful selection of the acceptors and their ratios a favourable blend morphology can be achieved, thereby reducing recombination in the blends. This is the highest efficiency value reported for P3HT based solar cells, to date. The universality of the method is proved with a commercially available low band gap polymer PCE10 and a power conversion efficiency up to 11.0 ± 0.4% have been achieved with a high open-circuit voltage of 1.03 ± 0.01 V. 1. S. Holliday et al. High-efficiency and air-stable P3HT-based polymer solar cells with a new non-fullerene acceptor, Nat. Commun., 2016, 7, 11585 2. C. B. Nielsen et al. Non-fullerene electron acceptors for use in organic solar cells, Acc. Chem. Res., 2015, 48, 2803-2812

Resume : We present the first investigation of the use of N-heteroacenes as acceptors in bulk heterojunction solar cells. The optical and electronic properties of tetraazapentacene (TIPS-TAP), triptycene-tetraazapentacene (TIPS-TAP-1T) and bistriptycenyl-tetraazapentacene (TIPS-TAP-2T) compounds are characterised by means of optical and electronic spectroscopies and correlated with photovoltaic performance. The cells with TIPS-TAP-2T significantly outperform those with other acceptors, achieving power conversion efficiency of 2.5% without the use of additives. We investigate the photophysics of charge separation at the donor/acceptor interface and find that it is fundamentally different from ‘conventional’ polymer-fullerene systems. For example, in the blends with the tetraazapentacene derivatives, exciton dissociation is relatively slow and charge separation is strongly field dependent. In agreement with photovoltaic device characterisation, we observe that charge generation is improved in TIPS-TAP-2T and TIPS-TAP-1T compared to TIPS-TAP, and that geminate recombination is significantly reduced in the case of TIPS-TAP-2T as compared to the other derivatives. The combination of improved film microstructure and more efficient charge separation result in the observed enhancement in the photovoltaic performance.

Resume : Compared with inorganic or perovskite photovoltaics, the key limiting factor for organic solar cells (OSCs) is large voltage loss, which is usually over 0.7 V. A significant contribution of the large voltage loss in OSCs is due to strong non-radiative recombination, which causes voltage loss of more than 0.35 V in most cases. The origin of the strong non-radiative recombination in OSCs has been puzzling the community for almost one decade, limiting rational design of materials to overcome this critical issue. In this study, we systematically investigate several exceptional OSC systems (including fullerene and nonfullerene electron acceptors), where the voltage loss is reduced to <= 0.7 V. We find that the quantum efficiency of electroluminescence in these systems have been significantly increased. Our work would pave the way to rational design of novel OSC materials for small voltage loss and high efficiency.

Resume : Organic semiconductors offer very promising and unique properties towards light sensing applications e.g. tunable absorption spectra, solution processability, mechanical flexibility and high internal quantum efficiencies. For that reason, optical sensors using these materials fabricated by industrial relevant printing techniques will become more and more relevant for many applications in e.g. sensing in wearables and medical diagnostics, environmental monitoring, or automotive industry. In this work we present highly efficient multi-layer organic photodiodes based on the polymer-fullerene blend PTB7:PC70BM, an AZO electron transport layer, and PEDOT:PSS electrodes, which were entirely aerosol jet printed on flexible PET substrates. Furthermore, we demonstrate a new direct-printed patterning technique comprising aerosol jet and inkjet printing, which allows for very precise, reproducible and highly registration accurate deposition of a multi-layer devices with feature sizes down to a few micrometers. We present a comprehensive electrical and optical characterization of the printed layers and devices in dependency of the active layer thicknesses, surface topography and transparency. The devices exhibited specific detectivities of >1E12 Jones over a broad wavelength range (400-750 nm) and maximum responsivities of 0.25 A/W.

Resume : Over the past few years, great improvement has been made in organic devices performances even if problems with their stabilities still persist, constraining the use of costly encapsulations. In operating conditions, organic photodiodes (OPD) are exposed to many stresses (light, water, oxygen, temperature, electrical bias) which induce degradations and performances losses. In this work we aim to understand and uncorrelate the role of each stress in the device degradation by investigating the impact of environment and light on electrical characteristics of operating semi-transparent OPD. For this purpose, different ageing have been performed in dark, under light and in different atmospheres such as inert gas, dry air, and ambient air. Scenarios are proposed to explain the degradation based on the analysis of the device figures of merit and numerical simulations. Finally, complementary electrical and chemical characterizations have been performed in order to highlight the degradation mechanisms. For instance, we show that oxygen induces acceptor traps in the bandgap of the active layer that reduces photocurrent and increases charges injection and capacitance in the quasi-static regime in the dark. An asymmetry of the external quantum efficiency depending on the illumination side is also found. This highlights the irreversible impact of oxygen on the reliability of optoelectronics devices even in the absence of light.

Resume : Organic bulk heterojunction solar cells contain a complex morphology of two organic semiconductors. The energetic distribution of localised states depends strongly on both, the properties of the neat materials and the blend morphology. Therefore, energetics and morphology play a major tole for the physical processes involved in photocurrent generation, for instance the recombination of charge carriers. This nongeminate recombination occurs across the donor-acceptor interface through charge transfer states. In disordered materials with low charge carrier mobilities it is usually described by the so called reduced Langevin rate. The latter is often orders of magnitude smaller than predicted by the Langevin rate, which is proportional to the sum of electron and hole mobility. Based on kinetic Monte Carlo simulations, we find that for typical phase dimensions, the nongeminate recombination is governed rather by the geometric mean of mobilities [1,2]. Another property of nongeminate recombination in organic blend systems is that the recombination order is often, particularly at low temperatures, increased above the expected order of two as two particles are involved. We present transient absorption data of organic semiconductor blends and show how the order or recombination relates to both, the diode ideality factor [3] and the energetic disorder. We will discuss what is required to bring these different perspectives into one unified picture for recombination in organic solar cells. [1] M. C. Heiber et al. Phys. Rev. Lett. 114, 136602, 2015 [2] M. C. Heiber et al. Phys. Rev. B 93, 205204, 2016 [3] K. Tvingstedt and C. Deibel. Adv. Ener. Mater. 6, 1502230, 2016

Resume : The tuning of the electronic properties of a conjugated polymer post-polymerisation is a potentially interesting approach to materials development. Provided functionalization chemistries can be found with close to quantitative yields, then the role of the substituent on the polymer properties can be investigated within the same batch of polymer. This eliminates the problems associated with batch-to-batch variations or differences in polymer chain length. If the functionalization can be performed under mild conditions such an approach can also be used to incorporate sensitive functionalities onto the polymer backbone. In this talk I will present out recent work towards developing functionalization approaches. I will discuss the incorporation of reactive endgroups and co-monomers which can be easily modified under mild conditions. I will discuss the role of the substituent on polymer properties and demonstrate how these approaches can be used to introduce reactive groups which can be cross-linked under very mild conditions.

Resume : Interfacial engineering has been recently identified as a fundamental strategy for maximizing efficiency and stability of organic electronic devices. To this aim, a new class of water/alcohol soluble materials comprising a π-conjugated backbone with pendant polar or ionic groups has been recently developed. The advantage of polar conjugated polymers is found in the combination of desirable properties: facile solution processability, chemical tunability, lightness and flexibility with the growing demand for eco-friendly materials. Moreover, the solubility in water and alcohols orthogonal to common solvents used for active layer deposition opens the way to all-solution-processed organic multilayer devices. In this view, we design, synthesize and test a whole series of polar conjugated polymers featuring a fluorene-based backbone with pendant phosphonate and/or amine groups [1]. We highlight their role as the cathode interfacial layer in performance enhancement of three types of common optoelectronic devices, i.e. polymer-based OLED [2], colloidal quantum dot LEDs [1] and organic solar cells. [1] A. Castelli, F. Meinardi, M. Pasini, F. Galeotti, V. Pinchetti, M. Lorenzon, ... & S. Brovelli, Nano Letters, 2015, 15(8), 5455. [2] W. Mróz, R. Ragni, F. Galeotti, E. Mesto, C. Botta, L. De Cola, G. M. Farinola, U. Giovanella. J. Mater. Chem. C, 2015, 3, 7506

Resume : Securing the world’s population energy supply in a renewable way is one of the biggest challenges of our generation. Solar energy is a great alternative to solve fossil fuel exhaustion and climate change. Organic solar cells are attractive due to their light weight, flexibility and scalability, but several issues need to be solved to make this technology competitive. One of the main challenges is achieving the optimal active layer morphology. So far, none of the common materials used (pi-conjugated polymers or small molecules) form the desired structures to obtain devices with correct working mechanism: connection among semiconductors to form percolating domains and correct orientation (face-on with respect to the electrodes) of such domains. Therefore, the search of new materials is necessary. Here we introduce supramolecular strategies in organic photovoltaics and present a library of hydrogen-bonded materials to address the issue of achieving strong percolating domains not disrupted when acceptor materials, such as fullerenes or non-fullerene based molecules are added. Recently, the use of hydrogen-bonded semiconductors has shown 50% improvement in device efficiency. However, this is just the tip of the iceberg because only one hydrogen-bonding unit was used. A study of the hydrogen-bonding strength (amide, urea or semicarbazone), number (1, 2 or 4 hydrogen bonds) and position within the molecular structure is proposed to explore the role of self-assembly in achieving percolating domains and morphology improvement.

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 purification steps. It is necessary to develop versatile, economically viable and “green” approaches for large-scale syntheses. In this contribution, we develop new synthetic tools for efficient polymers, by taking care of purity; variety of targeted structures and versatility of synthetic processes. Original polymers with aryl-vinyl repeating units are targeted, with the possibility to tune (minimize) the energy gap, photo/electro luminescence properties. More particularly, original alternated squaraine-based -conjugated polymers and/or -conjugated polyazomethines have been successfully synthesized, through metal-free condensation reactions. They exhibit interesting optical properties, such as a strong absorption and good emitting properties. Finally, the most promising polymers have been integrated in OLED as active materials.

Resume : The past decades have witnessed a remarkable increase in the performance of organic electronic devices such as thin-film transistors, light-emitting diodes, solar cells and sensors, but the patterning techniques that are compatible with these materials are still very few. In this work we report on a new approach to directly pattern functional organic polymers with photolithographic techniques and a minimal loss in terms of performances. This result is achieved by blending the chosen organic polymer with a new molecule, consisting in an aromatic core, functionalized with triple boron-fluorine groups (3BNF2), as cross-link agent to allow a facile and direct patterning. 3BNF2 can undergo cross-link upon irradiation with white and UV light sources commonly used in commercial photolithography setups. Thanks to its particular design, 3BNF2 can react just with itself, without affecting the structure and the properties of any other molecule in the systems. Furthermore, this molecule is extremely soluble in chlorinated solvents and in toluene, like the most common and performing organic functional polymers currently available on the market. To prove the effectiveness of our approach we used 3BNF2 in blend with a commercial N-type semiconducting polymer to make photo-curable Organic Thin-Film Transistors (OTFTs) with arbitrary shapes and high mobility.

Resume : Donor-acceptor or push-pull type conjugated polymers have become a dominating class of active materials in the field of organic electronics. Their adjustable light-harvesting, charge transfer and charge transport characteristics have been beneficially applied in organic photovoltaics, photodetectors and thin-film transistors. The conventional synthetic approach towards these push-pull polymers is based on Suzuki or (mostly) Stille cross-coupling of complementary functionalized heterocyclic precursors. In the ideal world, this should give rise to a perfect alternation of the employed building blocks throughout the polymer backbone and this alternation of electron rich (donor/push) and electron deficient (acceptor/pull) moieties leads to a substantial decrease of the bandgap. In recent years, however, it has become increasingly clear that the ‘real’ structure of the resulting alternating copolymers is often quite different from the projected one. Structural imperfections can for instance result from homocoupling of two identical building blocks. Furthermore, the end groups of these donor-acceptor copolymers are often also not those expected or targeted. In this contribution, recent results from our group providing insights on the impact of homocoupling ‘defects’ on the device characteristics of organic solar cells will be presented. On the other hand, end group analysis of different types of low bandgap copolymers via MALDI-TOF mass spectrometry showed some surprising results.

Resume : The self-organization of pi-conjugated organic materials forming highly ordered supramolecular architectures has been extensively investigated in the last two decades in view of optoelectronic applications. Indeed, the control of both the mesoscopic and nanoscale organization within thin semiconducting films is the key issue for the improvement of charge transport properties and achievement of high charge carrier mobilities. These well-ordered materials are currently either self-organized semiconducting polymers or liquid crystals. In this context, we endeavored to investigate the self-organization of semiconducting liquid crystalline materials incorporating different kind of pi-conjugated systems in unique molecular or macromolecular architectures. Here we describe the design and synthesis of (i) dyads and triads combining discotic or calamitic pi-conjugated mesogens, and (ii) side-chain liquid crystal semiconducting polymers where the backbone is a pi-conjugated polymer and the side groups are pi-conjugated discotic mesogens. In this work, we will give the details on the synthesis, structural characterization and morphology studied by Polarized-light Optical Microscopy (POM), Differential Scanning Calorimetry (DSC), Temperature-dependent small-angle X-ray diffraction, Grazing-incidence X-ray scattering (GIXS) and Atomic Force Microscopy (AFM). Moreover, their charge transport properties studied in OFET configuration will also be depicted.

Resume : Backbone coplanarity of donor-acceptor conjugated polymers is an important factor that determines all properties from the single chain to aggregates and thin films, and hence is of importance for the performance of such materials in opto-electronic devices. Here we report the synthesis, characterization and charge transport performance of novel copolymers PNDIFu2 made from alternating naphthalene diimide (NDI) and bifuran (Fu2) units is reported. Usage of potentially biomass-derived Fu2 as alternating repeat unit enables quasi-flat polymer backbones due to reduced steric interactions between the imide oxygens and Fu2 units. Backbone planarization induces stronger aggregation in solution with respect to PNDIT2, which eases directional alignment of polymer chains within aggregates using off-center spin-coating. As a consequence, optimized PNDIFu2 films show a markedly stronger transport anisotropy when tested in FETs, achieving a ratio of 18.3 for PNDIFu2-C20 for backbones aligned perpendicular and parallel to the electrodes. Overall, optimized FETs made from PNDIFu2 show a maximum electron field-effect mobility of 0.21 cm2/Vs, which is approximately three-times lower compared to PNDIT2. Thus, the initially thought beneficially greater backbone planarity, closer π-π stacking distance and stronger aggregation leading to enhanced anisotropy for PNDIFu2 do not result in superior transport properties than in PNDIT2. A likely reason for this unexpected behavior is a higher intra-molecular reorganization energy than in PNDIT2, counterbalancing the otherwise beneficial properties of PNDIFu2 owing to stronger polaron localization.

Resume : Pentacene is one of the most investigated molecules in the field of organic field effect transistors (OFETs). Incorporation of nitrogen atoms into the aromatic core of an acene increases its electron affinity and stabilizes the frontier molecular orbitals. Thus, azaacenes have become widely studied materials due to their potential application as n-type semiconductors in OFETs [1]. Additionally, azaacenes and their derivatives are currently the object of study in the fields of organic light emitting diodes (OLEDs) and hole transporting materials in organic photovoltaics. This significant interest to azaacenes has led to the development of synthetic pathways for these molecules. Currently, a variety of azaacenes, especially tetraazaacenes containing two pyrazine rings, are readily available for structural studies and device fabrication due to well-developed synthetic procedures [2]. However, some nitrogen doped acenes, which should be important for understanding of various structure-property relations in the field of OFRETs, including 5,7-diazapentacene (quinolino[3,2-b]acridine) and its derivatives, have never been synthesized, and, thus, studied. Here we report our progress towards synthesis of 5,7-diazapentacene derivatives using the Friedlander condensation as a key step for building a pentacyclic framework. This involves a novel approach to synthesis of aromatic diamino diketones, via Sonogashira coupling followed by the one-pot reduction of nitro groups and the hydrolysis of triple bonds using SnCl2 in ethanol. The synthetic route allows attachment of various functional groups onto the framework of 6,8-diazapentacene. A variety of 5,7-diazapentacene precursors bearing alkyl chains have been synthesized in order to improve the solubility. The precursors with additional benzene rings were synthesized as well in order to improve stability of the final acene aromatic system as per “Clar’s rule”. All the precursors demonstrate strong fluorescence, which makes them interesting materials for studies in the field of OLEDs. Friedlander condensation of 2-aminobenzophenone with various diketones (1,2-, 1,3-, 1,4-cyclohexanediones and 2,3,7,8-tetrahydroacridine-4,5(1H,6H)-dione) followed by dehydrogenation gives a variety of nonlinear azaacenes. The optoelectronic properties of these materials are being studied to study their suitability for use in devices such as OFETs or OLEDs. 1. Anthony, J.E., Chemical Reviews, 2006. 106(12): p. 5028-5048. 2. Bunz, U.H.F., Accounts of Chemical Research, 2015. 48(6): p. 1676-1686.

Resume : The exploration of strong acceptors to develop excellently organic semiconductors with high carrier mobility and excellent stability is an intensive research topic in the community of organic electronics. Naphthalenediimide (NDI), benzothiadiazole (BT) and N-heteroacene (NHT) are the most frequently used electron-deficient units to design high performance n-type and ambipolar organic semiconductors. Herein, strong acceptors, benzothiadiazole-fused naphthalenediimides (BT-f-NDI), that combine naphthalenediimide (NDI), benzothiadiazole (BT) and N-heteroacene (NHT) in the same molecule are firstly reported. A series of BT-f-NDIs were synthesized by the condensation of tetrabromo-NDI and benzothiadiazole diamine. The strong acceptors with LUMO energy levels of ~4.5 eV were obtained. Organic field-effect transistors (OFETs) based on the BT-f-NDI were fabricated by solution process, showing good n-chanel field-effect character of high electron mobility and stability under ambient conditions.

Resume : We demonstrate an ionic polyfluorene (PF) derivative containing trimethylammonium hexafluorophosphate groups –(CH3)3N+PF6–, namely P1-PF6, into [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as the electron transporting layer (ETL) for the fabrication of inverted perovskite solar cells (PSCs). The P1-PF6-doped PCBM layer was characterized with miscellaneous techniques, including SEM, AFM, UPS, PL quenching, and 4-probe conductivity measurements. The incorporation of the ionic polymer P1-PF6 helped to smooth the PCBM layer and to achieve better contact between PCBM and top electrode. Moreover, the results from PL quenching, UPS, and conductivity measurements indicated that the introduction of P1-PF6 enhanced the electron transporting and charge extraction of the doped PCBM film. Inverted PSCs with the configuration of ITO/PEDOT:PSS/CH3NH3PbI3/doped PCBM/Ag were fabricated and evaluated. The devices were characterized under the standard AM1.5G illumination condition in the ambient environment without encapsulation. The enhanced JSC of 17.32 mA/cm2 and PCE of 12.55% were observed from the PCBM:P1-PF6-based device compared with the un-doped one (JSC = 15.68 mA/cm2, PCE = 10.67%). In conclusion, a conjugated polyelectrolyte P1-PF6 was successfully incorporated into PCBM as the ETL to improve photovoltaic performance of inverted planar heterojunction PSCs for the first time.

Resume : Metal-Organic Frameworks (MOFs) are a versatile class of crystalline materials showing great promise in a wide range of applications (e.g. gas sensing and storage, luminescence, pressure sensors, and catalysis ). They consist of inorganic nodes, often metal-oxides, which are linked through organic molecules leading to highly porous crystal structures. As a result, their very structure puts them at several cross-sections: classical solids and molecules, surfaces and bulk,. . . Furthermore, since they combine properties either intrinsic to solids or molecules, these materials also provide ample opportunities to investigate fundamental materials properties from both physical and chemical perspective.[1] In short, they are a dream-come-true playground for the material scientist. By changing the metal-oxide or modifying the organic linker molecules, the electronic structure of these materials can be modified at will. In this work, we show how, based on DFT calculations, linker functionalization modifies the global electronic structure of MOFs.[2,3] We compare the results for the luminescent UiO-66 and flexible MIL-47 MOFs revealing the role of the metal-oxide nodes and propose other functional groups to further reduce the band gap size.[3] [1] Beilstein J. Nanotechnol. 5, 1738-1748 (2014) [2] Inorg. Chem. 54(22), 10701-10710 (2015), [3]“Linker functionalization in MIL-47(V)-R Metal-Organic Frameworks: Understanding the electronic structure”, D.E.P. Vanpoucke, submitted (2016)

Resume : Development of compact, cheap and fast memory elements is one of the most rapidly growing areas of the modern electronics. Since the number of possible memory device applications is very high, it is important to consider alternative approaches for the memory technologies. In particular, the Resistive Random Access Memory (RRAM) is non-volatile, fast and endurable[1]. Active elements may be inorganic, organic as well as composite materials[1]. Advantages of the organic-based RRAM technology are low cost, easy processing and good scaling opportunities. The operation principles of resistive memory presume an ability of the system to switch between at least two states “ON” and “OFF” with different resistances (resistive switches RS). By variation of working conditions multiple stable states can be achieved. RS speed can be less than 15 ns, switches are non-volatile, on/off ratio over 2 orders of magnitude, switching electric field below breakdown values, retention time 3.5 months, number of rewriting cycles 105 or higher[2,3]. Samples can be fully fabricated by printing techniques, and we demonstrated that there are no significant differences in their performance compared to samples on hard substrates. As it was mentioned, it is important that organic-based memory devices are technologically adaptive and flexible. Therefore we have chosen widely spread commercially available polymers polystyrene (PS), polyvinylchloride (PVC) and polycarbonate (PC) as base materials for our experiments. These polymers are electrically isolating in pristine conditions, soluble in organic solvents and very cheap. To modify the RSE features organic photoactive semiconductor particles were added to the polymer matrix[4]. These particles of various sizes (1.3-2.3 nm in length and 0.3-0.8 nm height) allowed us to control switching voltage using external light source[4]. Introduction of inorganic nanoparticles (NP) can lead to increase in photoresponse in composite structures due to charge transport between NP and organic media. In this work we observed RS in composite structures consisting of polystyrene matrix with incorporation of colloidal CdSe nanoplates, tert-butyl-substituted lutetium diphthalocyanine, hexadecachloro-substituted lutetium triphthalocyanine and other dyes. By varying current compliance (by device settings or by series resistance) we obtained RS not only between ON and OFF states, but also between several intermediate states. For each state we performed impedance spectroscopy and observed typical single semicircles. Such type of impedance spectra is attributed to the equivalent scheme of parallel resistance Rc and capacitance C. Shift of the semicircles from zero at Z` axes is described by a series resistance R0, which is contributed to the contact resistance. For each impedance spectrum we performed theoretical fitting and calculated parameters Rtotal=Rc+Ro, R0, C. In the intermediate states with Rtotal 12.4 kOhm, 10.15 kOhm, 7.4 kOhm contact resistance R0 and capacitance C stay quite stable. After the switch to the highest conducting state with Rtotal=2.3kOhm the value of R0 is reduced in 2 times and C increases in more than 3 times. Drastic change in the capacitance is Maxwell-Wagner (M-W) effect and is usually explained by appearance of structural heterogeneities. This concept is well explained by filament formation mechanism of RS: metal or carbon filaments are built. In intermediate states only single filaments appear and in the highest conducting state a system of interwoven filaments arises. Single filaments in the intermediate state do not influence the sample capacitance, but in the case of multiple filaments the M-W effect takes place. Thus by in situ impedance spectroscopy we observed the formation of filamentary structure. We demonstrated that incorporation of CdSe nanoplates into organic matrix provides a promising material for construction of high density and efficient memory. Application of impedance spectroscopy is a powerful method for investigation RS filament formation mechanism. This work was supported by RFBR (project № 16-07-00961, 15-03-05890, 16-33-60005). [1] F. Pan, S. Gao, C. Chen, C. Song, F.Zeng; Materials Science and Engineering R, 83, 2014, 1 [2] M.S. Kotova, M.A. Dronov, A.V. Rzhevskiy, S.V. Amitonov, T.V. Dubinina, V.E.Pushkarev, L.I. Ryabova, D.R. Khokhlov; Organic Photonics and Photovoltaics, 4, 1(2016), 17-23 [3] M. Dronov, I. Belogorokhov, D. Khokhlov, MRS Proceedings, 2011, 1337 [4] M. Dronov, I. Belogorokhov, M. Kotova, MRS Proceedings, 2015, 1729

Resume : Optical microcavities play an important role for the next-generation light technology. Recently, we succeeded in fabricating spherical microcavities from π-conjugated polymers (CPs) by simple self-assembly process.[1,2] We found that the microcavities show whispering gallery mode (WGM) resonant photoluminescence (PL) upon focused laser excitation, where PL generated inside the sphere is confined via total internal reflection at the polymer/air interface.[3–9] The resonance occurs when the wavelength of the light is an integer multiple of the circumference of the microsphere. The CP-based microcavities have benefits to the conventional microcavities in the following points: [1] simple and low-energy fabrication process to obtain well-defined microspheres, [2] the microcavities function as both cavity and emitter, [3] the microcavities have high refractive index and photoabsorptivity, and [4] potent use for electrically-driven WGM and laser oscillation. In this seminar, recent results on the fundamentals of the self-assembly of the CPs, resonant PL from the CP microspheres, intra- and intersphere light energy conversion, and the future prospects to realize light-, electrically-, and chemically-driven WGM and lasing will be presented. References 1) T. Adachi, et al., J. Am. Chem. Soc. 2013, 135, 870−876. 2) L. Tong, et al., Polym. Chem. 2014, 5, 3583−3587. 3) K. Tabata, et al., Sci. Rep. 2014, 4, 5902/1−5. 4) S. Kushida, et al., Macromolecules 2015, 48, 3928−3933. 5) S. Kushida, et al., ACS Nano 2016, 10, 5543–5549. 6) D. Braam, et al., Sci. Rep. 2016, 6, 19635/1–6. 7) S. Kushida, et al., RSC. Adv. 2016, 6, 52854–52857. 8) Y. Aikyo, et al., Chem. Lett. 2016, 45, 1024–1026. 9) Y. Yamamoto, Polym. J. 2016, 48, 1045–1050.

Resume : Organic semiconductors have been attracting lots of attention because they are applied especially to organic thin-film transistors (OTFTs). It is well known that OTFT device performance is influenced by the molecular packing structure and the surface morphology of the organic layer. Here we present the crystal structure and surface morphology of the Dph-DBTTT organic semiconductor thin film grown on the Si substrate investigated by grazing incidence wide angle x-ray scattering (GI-WAXS) and atomic force microscopy (AFM). Our results show that Dph-DBTTT forms a typical herringbone crystal structure. However, it is observed that the orientations of Dph-DBTTT molecules gradually changes as the distance from the interface near the substrate to the film surface increases, which may degrade OTFT performance. Further details on the crystal structures and the film thickness dependence of the morphology will be discussed in this presentation.

Resume : In this study, the hybrid organic/inorganic white-light emitting heterostructure is produced by combining the organic conjugated polymer F8T2 with the inorganic GaN-based epi-layers. The emitted white light consists of the green/yellow light and the blue light of GaN-based LED. Furthermore, the International Commission on Illumination (CIE) coordinate of the white light emission of the present device is at (0.28, 0.30), which is close to the standard white light (0.33, 0.33). The most important things in this study is that (1) organic/inorganic interface can produce light efficiently; (2) the main mechanism of this emitting organic/inorganic interface is electroluminescence; (3) this interface could be polarized, which might be the key to provide efficient recombination at the region near the heterojunction interface. These results are proved by the experiment of insetting the SiO2 layer into hybrid interface and the ultraviolet photoelectron spectroscopy (UPS) measurement. Then, to observe the transportation and the recombination of carriers in the F8T2/GaN-based LED, time-resolved photoluminescence (TRPL) and time-resolved electroluminescence (TREL) are used. By separating the green/yellow light and the blue light in TRPL and TREL measurement, the characteristic of the recombination carrier of the green/yellow light and the blue light can be defined, respectively. Also, it is worth noting that the electron-hole recombination time at the hybrid interface is much shorter than that of the GaN-based LED due to the polarized interface. Our work explores the applicability of polymer/GaN epi-layers emitting diodes for electrical and optical characteristics. The more detail would be discussed during this talk as well.

Resume : Direct printing allows solution processing of functional materials and opens a new route to fabricate low-cost electronic devices. One crucial parameter that influences the wettability of inks in all printing techniques is the surface free energy (SFE) of the substrate. Siloxanes, with their huge variety of sidechains, and their ability to form self-assembled monolayers, offer the possibility to control the SFE of substrates from e.g. hydrophilic to hydrophobic. This is a suitable approach to adjust the substrate conditions to the used ink, instead of the ink to a substrate. In this work the influence of different fluorinated and non-fluorinated siloxanes on the SFE of glass substrates is examined. Several different siloxanes were mixed and a fine tuning of the surface energy is demonstrated. The polar and disperse components of the SFE are determined with the OWRK-method. Furthermore, the influence of the SFE on the pinning of droplets and wet films is determined with dynamic contact angle measurements. The obtained results were used to directly influence the resolution of inkjet printed silver structures. A nano-particulate silver ink was used for printing single drops, squares and source-drain structures for transistors. These structures are examined in terms of diameter, edge quality, thickness homogeneity and functionality. We show that by adjusting the SFE of the substrate, the printed resolution could be increased by up to 70 % by decreasing the printed drop size.

Resume : Low molecular weight amorphous organic materials, have been receiving growing attention, exhibiting interesting optical, electrical, photoelectrical, magnetic and lithographic properties, opening up the road to the development of organic devices in the nanoscale with new functions and substantially enhanced performance. Herein, a design strategy for the synthesis of solution-processable anthracene-based semiconducting materials with well-defined molecular structures is presented. Multi-ring aromatic moieties are attached to a polyfunctionalized planar or tetrahedral core, also incorporating other groups, such as esters or cycloaliphatic moieties. Combination of groups is performed in such a way that provides control of physicochemical properties, such as solubility, thermal stability, and Tg. Depending on the targeting application, the crystallization of the films formed by these molecules can be also controlled. Selected compounds bearing suitable non-planar groups provide amorphous homogeneous films stable at temperatures well-above 100 °C, reaching up to 150 °C. These molecules provide frontier orbital and emission spectrum tuning capabilities and have been evaluated as main components in the active area of OLEDs, providing very promising results. Additional incorporation of suitable acid sensitive functional groups and sulfonium salts, as photoacid generators, allows self-patterning, where the UV exposed part of the film is removed. A flexible, efficient and cheap methodology, suitable for the preparation of these anthracene-based molecules in large scale will be also described.

Resume : Amorphous layers prepared by means of physical vapour deposition of small organic molecules are widely used in organic light-emitting diodes (OLEDs). While most of the research is mainly focused on the development of new materials and device architectures, little attention has been paid to the properties of each glassy layer. For many organic small molecule glass-formers it has been shown that the kinetic and thermodynamic stability, the density, or orientation of the molecules can be improved by properly setting the substrate temperature during deposition. Here we use a simple phosphorescent OLED stack, consisting only on two organic layers, to check the influence of the deposition temperature on the OLED performance. OLEDs comprising a green emitter achieve external quantum efficiencies of 19.4 % and 23.9 % at a current density of 1 mA/cm2 for deposition temperatures of 300 K and 343 K, respectively, reflecting a significant enhancement of 23 %. Other emitters (red and blue) were tested at these two temperatures using the same OLED stack achieving also significant improvements of 18 % and 103 %, respectively. The improvement of thermodynamic and kinetic stability of the emission and electron transport layer as a function deposition temperature is presented as a possible cause for this significant enhancement. This simple possibility to optimize the deposition conditions of the active layers provides a new strategy to achieve high efficiency OLEDs.

Resume : Single crystals of thiophene-phenylene co-oligomers (TPCO) have demonstrated high potential for organic optoelectronics due to unique combination of high charge carriers mobility and excellent photoluminescence (PL) efficiency [1]. Despite a considerable amount of experimental research, the detailed understanding of high PL in TPCO single crystals is still lacking. Time-resolved, polarization-sensitive PL parameters can provide important insights for establishing the relationship between the molecular structure and optical properties of TPCO. In this contribution, we analyze time-resolved PL anisotropy in a TPCO crystal based on the 1,4-bis{5-[4-(trimethylsilyl)phenyl]thiophen-2-yl}benzene molecule. X-ray diffraction experiments indicated the herringbone packing motif as have been reported for crystals based on analogous TPCO with the same terminal group [1]. Regardless of the excitation polarization, PL is mainly polarized along the molecule axis. However, quite surprisingly, PL induced by the excitation orthogonally polarized with respect to the molecular axis exhibits much higher intensity as compared to the parallel one. Such an unexpected PL anisotropy geometry is discussed in relation to the J-aggregates crystal packing and the impact of a luminescent impurities. This work was partially supported by Russian Science Foundation (grant 15-12-30031). [1] S.Hotta et al.,JMCC, 2(6),965(2014); L. Kudryashova et al., ACS Appl. Mater. Interfaces, 8(16),10088(2016)

Resume : In this study the sensing La2O2CO3 nanomaterials were coated on Au/SiO2/Si substrate by electrostatic spray method. The objective being to observe the effects of temperature and concentration of polyvinylpyrrolidone (PVP) on the nanopartiles for sensing the impact of carbon dioxide performance. The results showed that high-resolution transmission electron microscope (TEM) image and energy-dispersive x-ray spectroscopy (EDS) analysis of the La2O2CO3 structure had a polycrystalline structure, and when the polyvinylpyrrolidone polymer concentration was 6 wt%, the sample had a preferable characteristic. When the carbon dioxide at 2000 ppm, the response was close to 18% at 400 ° C. The increase in collection time can make it more responsive to carbon dioxide gas, which improved the gas sensing ability. We expect these results to open up the way towards high performance La2O2CO3 nanomaterials for gas sensors.

Resume : We will present a simple, cost effective and precisely controllable methods to fabricate high-quality and controllable band gap graphene oxide (GO) films as a hole injection layer (HIL) for high efficiency polymer light-emitting diodes (PLEDs). The electrophoretic deposition (EPD) method and the electrical reduction method to tune the GO to the reduced graphene oxide (RGO) thin films show better luminance and current density for making high efficiency PLEDs by comparing the performance of PLEDs with RGO film as HIL with GO and (poly (3,4-ethylenedioxythiophene) polystyrene sulfonate) (AI4083). Each layer of PLEDs is the non-alkali or non-alkaline earth metals, it facilitates us to fabricate PLEDs in the ambient environment. The thickness of the GO films could be controlled from 80 to 350 A with the EPD system. These deposited GO films were reduced by the electrical method. The GO films with various thicknesses and the RGO films with a different reduction degree were prepared to optimize the film for HIL layer. It has been observed that GO with 80 A thickness followed by 10 s reduction is the best candidate as HIL of PLEDs. The optimum condition could be confirmed by Raman spectroscopy, XPS, UPS and UV–Vis spectroscopy. Finally, we fabricated high efficiency PLEDs using RGO as HIL and compared the performance of the device with AI4083 HIL device. The maximum luminance was 12830 (RGO HIL) and 3,958 cd/m2 (AI4083 HIL). The EPD process and the electrical reduction method are expected to be useful as an industrial approach in fabrication of high efficiency PLEDs.

Resume : The fabrication of organic light-emitting diodes (OLEDs) by high-throughput printing techniques requires the development of solution-processable electron injection layers (EILs). Over the last few years, amine-based EIL materials like Polyethylenimine (PEI) have received a lot of attention as they can be used in combination with a multitude of electrode materials in normal as well as inverted device architectures.1–3 In this work, we fabricate solution-processed OLEDs, which use PEI as EIL, and we investigate their performance and operational stability. We show that the cathode metal that is in contact with the EIL plays a crucial role for the operational lifetime of the devices. In case of Al, the primary degradation mechanism during electrical driving is caused by excitons which reach and subsequently degrade the emitter/PEI interface. At a current density of 20 mA cm-2, an LT50 lifetime of ~ 200 h is achieved. In contrast, in case of Ag, an additional mechanism degrades the OLED performance; during operation holes accumulate at the emitter/PEI interface and as a result the emitter quantum yield drops. Consequently, the operational lifetime of such OLEDs is significantly shorter with an LT50 value of < 10 h. Finally, we show that the degradation by excitons can be significantly slowed down by the use of a PEI:ZnO composite EIL. As a result, the LT50 lifetime of OLEDs with an Al cathode is increased by a factor of five, without adversely affecting OLED performance. 1: Zhou et al., Science 2012, 336 (6079), 327–332. 2: Stolz et al., ACS Appl. Mater. Interfaces 2014, 6 (9), 6616–6622. 3: Stolz et al., ACS Appl. Mater. Interfaces 2016, 8, 12959–12967.

Resume : The new method of nonstoichiometry investigation in the metal-organic (MO) crystalline materials has been developed. Using a quasi-closed ampoule system the method allowed monitoring the changes in fluorescent characteristics of metal–organic samples with temperature, annealing time and partial pressure of ligand-forming substance (pL) in situ. When combined with X-ray analysis the method made it possible to find the conditions for the synthesis of single-phase metal-organic crystalline samples with a certain disordering in the crystal lattice determined by atomic defects similar to the nonstoichiometry phenomenon in classic semiconductors. The influence of pL and temperature on structural and luminescent properties of MO crystalline samples (99.9985 wt%) has been also studied for the samples quenched after annealing’s. It was found that direct changes of pL at the fixed annealing temperature caused the systematic changes of photoluminescent spectra maximum, decay kinetics and structural characteristics. The OLED devices were fabricated by vacuum thermal evaporation using single-phase MO samples with different nonstoichiometry, as emitting materials. It was found that OLED-devices based on nonstoichiometric MO demonstrated improved stability versus OLED based on stoichiometric MO. The research was financially supported by the Russian Foundation for Basic Research (Contract 16-32-60035).

Resume : Single-crystal organic filed effect transistors (OFETs) can provide electrical performance exceeding that of a-Si devices. Solution-processed devices could be more advantageous enabling low production costs. For industrial applications of organic single crystals, their high morphological homogeneity and thickness uniformity on large areas are needed. As charge transport in OFETs occurs within a few monolayers of organic semiconductor, ultrathin devices are beneficial from the viewpoint of low material consumption, mechanical flexibility and optical transparency. Herein, we report on solution-processed oligothiophene-based monolayer single crystals with sizes up to a few mm and study their performance as OFET active layers. As materials we used oligothiophenes and oligothiophene-phenylenes with five conjugated rings and different terminal linear alkyl substituents, which were proved to be crucial for 2D crystallization in monolayers. The monolayer single crystals were grown on various surfaces, both with high and low surface energies, and studied by optical polarization microscopy, atomic-force microscopy, spectroscopic ellipsometry, and x-ray diffraction. Our in situ studies of crystal growth allowed us to suggest a possible growth mechanism. The OFET mobility was increased tenfold with use of oligomers with longer alkyl substituents. The best devices showed the charge mobility exceeding by an order of magnitude the highest reported mobilities for oligothiophene-based monolayer OFETs [1]. Thus, large-area solution-processed monolayer single crystals can be a promising platform for ultrathin organic electronic field-effect devices. 1. Smits E.C.P., et al., Nature, 2008, v.455 p.956.

Resume : The development of new materials with tunable optoelectronic properties is key to the continued evolution of organic photovoltaics (OPV). In this work, we present a novel push-pull molecule composed of acceptor (A) moieties bonded to each side of a donor (D) core (A-D-A). By making subtle changes to the position of octyl groups on the alkylthiophene-substituted benzodithiophene (BDT) D core, the overall D or A character of the molecule can be changed. Here we compare two regioisomers, oBDT and mBDT, which were individually blended with a well-studied D or A; namely the polymer PBDTBDD (P141) or the fullerene PC71BM, respectively. The four “D:A” pairings (P141:oBDT, P141:mBDT, oBDT:PC71BM and mBDT:PC71BM) were integrated into bulk heterojunction OPV devices, and their photophysics was studied using photoluminescence (PL), transient absorption and pump-push photocurrent techniques. Of the four devices, the highest short-circuit current and power conversion efficiencies were obtained from the oBDT:PC71BM and P141:mBDT blends. In correlation with device performance, we observe efficient charge separation in these two blends in contrast to mBDT:PC71BM and P141:oBDT. These results indicate that oBDT has more D character, while mBDT has more A character. We speculate that the position of the bulky octyl groups sterically influences the degree of ‘twisting’ between the D and A units, thereby changing the extent of conjugation, and hence photophysical behaviour, of the A-D-A molecule.

Resume : We present an in-depth study of the correlation between molecular weight, processing conditions and charge transport in high molecular weight diketopyrrolopyrrole copolymers. We observe a rapid increase in aggregation in solution with increasing molecular weight which strongly limits solubility and processability beyond MW = 200 kg mol-1, causing severe limitation in the charge transport properties. We also isolate the presence of bulk electronic defects manifest itself as an apparent charge density dependence of the mobility. High molecular weight polymer chains are already obtained after as little as 15 minutes of reaction time and by limiting aggregation they allow the formation of highly textured films. Defects are passivated by exposure to ambient atmosphere or by chemical doping using 2,2-(perfluoronaphthalene-2,6-diylidene)dimalononitrile, which leads to a filling of the trap states.[1] By combining fast reaction times without further washing steps (partially responsible for the formation of trap sites) we are able to achieve high and balanced hole and electron transport, among the best reported for an isotropic, spin coated DPP polymers. [1] Di Pietro, R. et al. (2016), The impact of molecular weight, air exposure and molecular doping on the charge transport properties and electronic defects in dithienyl-diketopyrrolopyrrole-thieno[3,2-b]thiophene copolymers. J. Mater. Chem. C, 4, 10827 doi: 10.1039/c6tc03545k

Resume : The organic electronics originated from low cost, lightweight, and flexible electronics is developing through the printed electronics, stretchable electronics, and recently into wearable electronics for the applications to flexible displays, touch panels, soft sensors and actuators. Here, wet-processable, stretchable, and highly conducting polymer is one of the promising candidates for the key materials of the organic electronics. Poly(3,4-ethylenedioxythiophene) doped with poly(4-styrenesulfonic acid) (PEDOT:PSS) is one of the most successful conducting polymers which can be applied to the electrical and optical devices such as antistatic coatings, aluminum solid capacitors, organic light-emitting diodes, and solar cells. However, there are still technical issues for the practical applications: The PEDOT:PSS is available in the form of an aqueous dispersion of colloidal particles, which is disadvantageous to form an ultra-thin layer in the molecular order and to fill into an etching pit of the aluminum solid capacitors. Moreover, the “solvent effect” is necessary to improve the electrical conductivity by a few orders of magnitude, the mechanism of which can be explained in terms of the crystallization of the PEDOT molecules and the removal of an excess PSS from the surface of the colloidal particles preferable to the transport of charge carriers. This study deals with synthesis and characterization of novel self-doped highly conducting polymers. The oxidative polymerization of the derivatized EDOT monomer, in which the ethylenedioxy ring is substituted with an alkoxy sulfonate group, was carried out under vigorous stirring at room temperature for 24 h. The dark blue aqueous solution of the self-doped PEDOT (SD-PEDOT) was obtained with concentration, viscosity, and pH of 1.0 wt%, 22.4 mPas, and 1.9, respectively. It is seen that the dynamic light scattering measurement clearly indicates the narrow particle size distribution of the SD-PEDOT with the average diameter (median diameter: D50) of 2.5 nm. We should emphasize here the D50 of the SD-PEDOT is much smaller compared to the PEDOT:PSS colloidal gel particles (D50 = 16 nm), suggesting that the SD-PEDOT is fully dissolved in water. Surprisingly, the electrical conductivity of the SD-PEDOT film was found to be 875 S/cm, which is comparable to the most conductive PEDOT:PSS (Clevios PH1000, Heraeus). Thus, we have succeeded in synthesizing the water-soluble self-doped highly conducting polymers.

Resume : In this work, we study Chemical Vapor Deposition (CVD) as an alternate route for polymer film deposition in order to achieve non-solution based organic optoelectronic devices. Polymer semiconductors are generally deposited by spin coating from a solution. Spin coating is not feasible for up-scaling. The conventional route for up-scaling, which uses a printing/coating technology, yields films with inferior electronic properties compared to spin coated films. Poly (p-phenylene vinylene) (PPV) is a polymer with high absorption in the blue to green region of the electromagnetic spectrum and hence has the potential to be used as an active material in solar cells. CVD allows good control over the deposition of the film. The film grown is a pre-polymer and can be polymerized by annealing or by exposure to UV light. Since the deposition is at room temperature and polymerization post growth can be done by UV treatment, it is possible to deposit the film on flexible substrates. We study the electrical properties of the CVD grown PPV using field effect transistors. CVD grown PPV shows good transistor characteristics. Realization of organic field effect transistors based on CVD of PPV is being demonstrated for the first time. The electrical characteristics of the film with various post growth treatments are studied. We investigate the gate bias dependent mobility of the PPV films. We model the carrier dependent mobility to understand charge carrier transport in CVD grown PPV.

Resume : Rylene diimides form a unique and interesting class of organic semiconductors showing for instance n-type behavior in OFETs or high luminescence of interest for sensor applications. The electronic and optical properties of rylene diimide molecules can be tuned by molecular engineering and adequate choice of substituents. But, in the solid state, their collective properties depend on the molecular packing. For instance, they can form different polymorphs made of either J- or H-type aggregates, as observed for perylene bisimide derivatives*. In this case, a competition between H-bonding and π - π stacking interactions determines the polymorphism, hence the electronic properties in the solid state. In the present study, we have studied a naphthalene-bisimide based organogelator named NDIC8. We have correlated the structure, especially polymorphism, with optical and electronic properties in the solid state. Three polymorphs were prepared: gel, liquid crystalline and crystalline phases and their molecular packing was investigated by X-ray and electron diffractions. The UV-vis and Infra-red spectra of the NDIC8 polymorphs uncover the nature of intermolecular H-bonding between amides. Temperature-dependent TEM, UV-vis and FTIR help understand the molecular reorganization upon transformation between the different polymorphs. Finally, a global phase diagram has been obtained. * A. Sarbu, L. Biniek, J.M. Guenet, P.J. Mesini, M. Brinkmann, J. Mater. Chem. C., 2015, 3, 1235-1242

Resume : Organic electronics is gaining more and more attention from scientific public and from industries because of his unique properties such as flexibility, transparency, and solution-processability. High throughput manufacturing methods such as roll-to-roll coating and inkjet printing allow organic electronics to be suitable for new and diverse applications and to be an attractive viable way to make low cost electronics. Before employing this technology for the production of complex circuits, the performance of the elementary block, i.e. the transistor, has to be optimized. This task is nowadays favored by the recent improvements in polymers charge carrier mobility. The willingness to fabricate transistors capable of high frequency operation is spurred by possible applications like high-resolution flexible displays or devices able to communicate via wireless. Nevertheless, a record frequency of transition of 27 MHz is achieved for transistors with lithographic contacts and evaporated semiconductor, while of 3 MHz for transistors fabricated without using masks in the production flow. In order to reach an high frequency operation preserving the advantages of the low cost manufacturing methods discussed before, not only the chemistry of the polymers but also the device architecture should be optimized. The latter problem is the subject of my thesis work. The channel length was downscaled in maskless approach by means of laser sintering technique, down to a minimum of 1 um. The semiconductor layer was deposited with a large-area, roll-to-roll compatible coating technique which optimizes charge transport and the effects of contacts treatments and of semiconductor doping were investigated. The overlap capacitance, another critical factor in limiting the frequency of transition, was minimized by reducing the finger widths of the contacts again relying on a laser sintering technique. Mobilities of 0.07 cm^2/Vs for holes and 0.3 cm^2/Vs for electrons were extracted for a downscaled transistors channel of 1 um, and a frequency of transition of 3 MHz and 10 MHz was found for p-type and n-type OFETs. Such frequencies are among the highest ever achieved for solution-processed polymer FET fabricated with a completely maskless approach.

Resume : PVDF-TRFE based terpolymers such as PVDF-TRFE-CTFE (VDF: vinylidene fluoride, TRFE: trifluoroethylene, CTFE: chlorotrifluoroethylene) present an important electromechanical response, making of them interesting candidates for actuation based devices. Before considering industrial applications, the deformation of the material while submitted to an activation input signal has to be determined. However for frequencies below a few dozens of Hertz, the electrostriction equation linking polarization to the mechanical deformation doesn’t describe the physical reality correctly. The device behavior is modified by leakage currents, ions movement and viscoelastic response. Based on the electrical and mechanical response in a static regime, a mathematical model of polarization response in dynamic regime is proposed. The quantification of the different components of the electrical device response permits afterwards a correct prediction of the low frequency mechanical deformation of the polymer based devices. Applying experimentally different test signals to a cantilever and measuring the device response, allows then the comparison of the measured device response to the modeled material deformation.

Resume : In this study, polyanilines (PANI)/single wall carbon nanotubes (SWCNT), PANI/ multi wall carbon nanotubes (MWCNT), and PANI/reduced graphene oxide (RGO) nanocomposites are synthesized via in situ polymerization. The core-shell structures are evidenced by SEM and TEM images whereas XRD and Raman spectroscopy are employed to investigate structural properties. In addition, electrical, thermal and thermoelectric (TE) transport properties are investigated as a function of nano-fillers content. The power factor and figure of merit (ZT) of the composites are deduced from measurements of the electrical conductivity (σ), Seebeck coefficient (S) and thermal conductivity (k). The electrical transport properties are explained by using mixing rule and percolation model. Effective mass approximation (EMA) model is used to explain the thermal properties. It is shown that inclusion of the three different kinds of fillers into polymer matrix exhibits three different behaviors which are explained by an existing physical model. Compared to that of pure PANI, the TE performance of the composites exhibits a ZT enhancement of few orders of magnitude.

Resume : The organic spinterface describes the spin-polarized properties that develop, due to spin-dependent hybridization and charge transfer, at the interface between a ferromagnetic (FM) metal and the molecules of an organic semiconductor [1,2]. The organic spinterfaces were also indirectly observed by the strong coupling that may exist between paramagnetic molecules and the FM substrate [3-6]. However, the study of spinterfaces was so far limited to sublimable molecules deposited in ultra-high vacuum conditions in order to prevent the oxidation of the FM substrate. Alternatively, the strong interaction of the molecules with the FM layer may alter molecular properties, such as spin-crossover [7]. While intercalating a graphene layer [8] is one solution that introduces strong constraints on the FM selection, we investigated the possibility to use interlayer exchange coupling as the mediator of the molecule/FM magnetic coupling. Using X-ray magnetic circular dichroism (XMCD), we studied the magnetic coupling between manganese phthalocyanine (MnPc) molecules and a Cu(001)/Co FM substrate separated by a wedge-shaped Cu spacer. The XMCD data show that the Mn ion within MnPc molecules can be magnetically coupled to the Co substrate at room temperature when separated by up to 4 ML of Cu. The XMCD intensity evolves in an oscillatory manner with increasing Cu thickness, in agreement with ab initio calculations. By decreasing the temperature, we could observe stronger oscillations in the magnetic coupling and this over a much larger Cu thickness range (up to 12 ML). The phase and the periods of the oscillatory coupling is found to be the same than that of the prototypical Cu(001)/Co/Cu/Co system. Finally, we theoretically considered the spintronic performance of a Co/Cu(3ML)/MnPc stack. The calculations reveal a spin-polarization of the density of states in the vicinity of the Fermi level that reaches +74% suggesting thus promising spintronic performance [9]. [1] Methfessel et al., Physical Review B 84, 224403 (2011) [2] Djeghloul et al., Scientific Reports 3, 1272 (2013) [3] Scheybal et al., Chemical Physics Letters 411, 214 (2005) [4] Wende et al., Nature Materials 6, 516 (2007) [5] Javaid et al., Physical Review Letters 105, 077201 (2010) [6] Iacovita et al., Physical Review Letters 101, 116602 (2008) [7] Miyamachi et al., Nature Communications 3; 938 (2012) [8] Hermanns et al., Advanced Materials 25, 3473 (2013) [9] Gruber et al., Nano Lett. 15, 7921 (2015)

Resume : Organic conducting polymers are of increasing scientific interest and are promising candidates for various applications [1]. Nowadays, one the most important challenges faced by the community concerns their processability along with their long term stability. In this context, herein, I'd like to present one approach consisting in the preparation of photoactive materials stable as inks in environment-friendly phases through miniemulsification or nanoprecipitation techniques [2-3]. For instance, composite particles made of the low band-gap PCDTBT polymer together with the n-type PC71BM have been prepared in aqueous media. The materials ratio was varied and the properties of composite particles were studied along with their integration. (1) Skotheim, T. A.; Reynolds, J. R. Handbook of Conducting Polymers, CRC Press, NY, 2007, 1680 pages. (2) Cloutet, E. and coll. Macromol. Rapid Commun. 2015, 36(20), 1816-1821. (3) Cloutet, E. and coll. Langmuir 2017 under revision.

Resume : For at least the past 10 years, printed electronics has promised to revolutionize our daily life by making cost-effective electronic circuits and sensors available through mass production techniques, for their ubiquitous applications in wearable components, rollable and conformable devices, and point-of-care applications. While passive components, such as conductors, resistors and capacitors, had been already fabricated by printing techniques at industrial scale, despite the great potentiality, printing processes have been struggling to meet the requirements for mass-produced electronics and optoelectronics applications. In the case of logic integrated circuits (ICs), the main limitations have been represented by the need of suitable functional inks, mainly high-mobility printable semiconductors and low sintering temperature conducting inks, and evoluted printing tools capable of higher resolution, registration and uniformity than needed in the conventional graphic arts printing sector. In this presentation, I will give a talk on the recent progressive of my group on development of printed organic integrated circuits. I will mainly talk about on development of high performance inkjet printed unipolar and ambipolar polymer field-effect transistors (FETs), and applications to elementary organic complementary inverter, ring oscillators and various logic circuits. Several issues should be addressed including method to improve charge injection properties, reducing operating voltage, and deposition over a large area by graphic art printing processes. By development of organic semiconductors and gate dielectrics, printing processes, contact resistance, and mobility of organic semiconductors, we have obtained high field-effect mobility more than 6 cm2/Vs for both of p-channel and n-channel FETs with less than 5 V operating voltage, and ambipolar OFETs and the CMOS polymer ring oscillator showed very high operating frequency of 50 KHz. In addition, various logic circuits were demonstrated.

Resume : π-conjugated electron-deficient molecules have demonstrated great potential as n-channel semiconductors for the realization of cost-effective and flexible printed optoelectronic devices. Over the past few decades, considerable research efforts have focused on the theoretical design and synthetic development of various families of electron-transporting molecular semiconductors with a wide range of electrical and optical properties. To this end, 4,4-Difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) stays as a relatively unexplored π-core for use in semiconductor structures, despite it has been heavily studied in the past decades as highly fluorescent functional dyes for chemosensors, fluorescent switches and biochemical labels. Here, we describe the molecular design, synthesis, full characterization, and OTFT device performance of novel solution-proccessable BODIPY-based small molecules for organic thin-film transistors (OTFTs). Solution-processed top-contact/bottom-gate OTFTs exhibited electron mobilities as high as 0.01 cm2/V·s and extremely high current on/off ratios of >108. Film microstructural and morphological characterizations indicate the formation of relatively long (~0.1 mm) and micron-sized (1-2 μm) crystalline fibers and ribbons along the shearing direction. To the best of our knowledge, these OTFT performances are the highest reported to date (~10-1000× improvement) for BODIPY-based molecular semiconductors, and our findings demonstrate that BODIPY core holds great promise to become a key player for enabling solution-processed, electron-transporting semiconductor films.* * M. Ozdemir, D. Choi, G. Kwon, Y. Zorlu, B. Cosut, H. Kim, A. Facchetti*, C. Kim*, H. Usta* "Solution-Processable BODIPY-Based Small Molecules for Semiconducting Microfibers in Organic Thin-Film Transistors" ACS Applied Materials & Interfaces, 2016, 8, 14077–14087. Acknowledgements: This work is supported by the Scientific and Technological Research Council of Turkey (TUBITAK Grant-114M226)

Resume : We present a study on charge transport on two widely used semiconducting polymers, poly{[N,N′-bis(2-octyl- dodecyl)-1,4,5,8-naphthalenedicarboximide-2,6-diyl]-alt-5,5′-(2,2′- bithiophene)} (P(NDI2OD-T2)) and poly(3-hexylthiophene) (P3HT). [1] The electrical characterisation of field effect transistors reveals an increase in charge density dependence of the mobility connected to an overall improvement of device performance. We unambiguously tie such observation to the increase of the average crystallite size of the polymer film by combining the results of optical spectroscopy and X-ray characterization. The picture that emerges from the experimental evidence demonstrates that charge transport in semicrystalline polymers cannot be described using any disorder based model such as for example multiple trap and release or variable range hopping. To explain the experimental observation we propose a mechanism that explicitly accounts for the presence of interdispersed crystalline and amorphous regions within the polymer film and for the Coulomb interaction between charge carriers accumulated within the same crystallite. With this approach we are able to provide a coherent picture of charge transport that can capture all the unique features we observed experimentally. [1] Di Pietro, R. et al. (2016), Coulomb Enhanced Charge Transport in Semicrystalline Polymer Semiconductors. Adv. Funct. Mater., 26, 8011, doi:10.1002/adfm.201602080.

Resume : Recent advances in gravure printing allow realization of highly-scaled fully printed transistors with channel lengths in the single micron range. The ability to realize such transistors requires a complete re-working of the transistor structure and process, including development of layouts that exploit the improved resolution while dealing with substrate distortion induced overlay limitations, as well as realization of aggressive scaling of all printed layers to improve electrostatics in such highly scaled OTFTs. Here, I review our work on scaling gravure printing resolution, and the accompanying changes in device structure and material choices to enable realization of high-performance fully-printed organic thin film transistors based on leading edge gravure printing technology

Resume : Polymer field effect transistors (PFETs) have potential applications in large-area flexible displays, sensors, radiofrequency identification tags, and logic circuits with low-cost processability. However, they are still inadequate for commercialization and practical implementation due to their operational instability. We investigated the mechanism of the electrical instability of high performing PFETs that show a certain degree of ambipolarity and unraveled the effect of electron conduction on the stability of hole current and the double-slope behavior. The model polymer used in this study is regioregular D-A copolymer poly[4-(4,4-dihexadecyl-4H-cyclopenta[1,2-b:5,4-b']dithiophen-2-yl)-alt-[1,2,5]thiadiazolo[3,4-c]pyridine] (PCDTPT). While performing current-voltage measurement, we found that consecutive sweeping of gate voltage changed the shape of the transfer curves. We conducted a thorough study of bias-stress of the system and found that electron trapping at the gate dielectric/polymer interface greatly alters the device characteristics including the occurrence of the double-slope. We provide several solutions to improve the device operational stability.

Resume : Conjugated polymers recently made great progress in building organic electronics. Nonetheless, the conventionally processed polymer transistors face challenges to develop high-performance devices and our understanding of them remains very limited. Here we present planar-processed polymer transistors using the promising donor-acceptor (D-A) copolymers, which are enabled by a planar ohmic contact engineering like that used for silicon metal-oxide-semiconductor field-effect transistors (MOSFETs). These novel polymer transistors exhibit ideal p-FET characteristics with unparalleled performance. In particular, the lowest subthreshold slope observed of 85 mV/dec approaches the theoretical limit of 60 mV/dec at room temperature given simple solution-based processes. Our results shed light on the ambiguous operating principle of ambipolar OFETs and demonstrate the versatility in doping D-A copolymers, thereby paving a clear path toward the practical application of polymer transistors.

Resume : Blending small conjugated molecules as organic semiconductor with an amorphous insulating polymer has been demonstrated to improve the organic field effect transistors (OFETs) processability, reproducibility and stability.[1-2] The key at the basis of the superior performance of OFETs fabricated with blended films seems to be the vertical phase separation of the two components. Thus, deciphering the vertical stratification of the blends and how it changes as a function of the processing parameters is of fundamental importance. This issue is addressed here for blends of 2,7-Dioctyl[1]benzothieno[3,2–b][1]benzothio-phene (C8-BTBT), a promising organic semiconductor with some of the highest mobilities reported to date, and polystyrene (PS) processed by a solution-shearing technique.[2] We show that Friction Force Microscopy can be used to discriminate pure C8-BTBT and PS regions due to their different frictional behavior, allowing us to obtain a nanoscale characterization of their lateral and vertical distribution. The deciphered vertical structure has been correlated with the macroscopic electrical performance of the OFETs for three ratios of C8-BTBT and PS. In addition, Kelvin Probe Force Microscopy measurements help quantifying the electrical potential along the channel during operation of the OFETs and contacts-related effects. [1] M. R. Niazi et al. Nat. Commun. 6, 8598 (2015). [2] I. Temiño et al. Adv. Mater. Technol. 1, 1600090 (2016).

Resume : Organic thin-film transistors (TFTs) can typically be fabricated at temperatures below 150 °C and thus not only on glass, but also on unconventional substrates, such as plastics, paper and textiles, which makes them potentially useful for flexible, large-area electronics applications, such as rollable or foldable displays and sensors. In some of the more advanced applications envisioned for organic TFTs, such as the integrated row and column drivers of flexible active-matrix displays, the TFTs have to be able to control electrical signals of a few volts at frequencies of several megahertz. The first requirement for achieving high switching frequencies is efficient charge transport in the semiconductor. This requirement can be met by choosing small-molecule organic semiconductors that provide good molecular ordering and large carrier mobilities even when processed at low temperatures. The second requirement is a small channel length. To meet this requirement, we have developed a process in which the TFTs are patterned using high-resolution silicon stencil masks, which makes it possible to fabricate bottom-gate, top-contact organic TFTs with a channel length of 1 µm. For 11-stage complementary and unipolar ring oscillators based on TFTs with a channel length of 1 µm, signal propagation delays per stage as short as 6.6 µs and 420 ns have been measured at a supply voltage of 3 V.

Resume : A fundamental disadvantage of semiconducting polymers is that their charge transport is unbalanced because electron transport is hindered by traps. This electron trapping seems to be universal in organic semiconductors and is dominated by a trap located at ~3.6 eV below vacuum. This universal defect also quenches the excitons in conjugated polymers. However by blending poly(p-phenylene vinylene) (PPV) derivatives with wide band gap polymers the electron traps are deactivated. PLEDs made from such a blend exhibit a balanced transport and enhanced efficiency due to the strong reduction of non-radiative trap-assisted recombination. An important question is whether the strong electron trapping is specific for polymers or is also present in films from evaporated small molecules. To this end electron transport in a series of small molecules has been studied. It is found that also for a range of small molecules the electron transport is severely limited by traps with a concentration of ~1024 m-3. This indicates that electron trapping is a generic property of organic semiconductors, ranging from vacuum-deposited small-molecules to solution-processed conjugated polymers.

Resume : The research interest on alternative materials for innovative ionizing radiation detection is rapidly growing, in particular to envisage the need of large-area conformable sensor flat panels for applications that span from cultural heritage preservation to the security of public buildings. Organic materials have a strong potential for such an application, thanks to their mechanical flexibility and the possibility of deposition over large and bendable substrates by means of low-cost wet-technologies as printing techniques. Therefore, these feature permits to overcome the constraint of traditional inorganic materials, i. e. expensive or complex growth techniques and stiff mechanical properties. Recently, the employment of solution-grown organic materials as reliable direct X-ray detectors, operating at room temperature, have been demonstrated [1,2]. These studies opens the way to the development of a new class of fully flexible organic-based direct detectors with higher performances. In this work, we will report about results on organic thin-films based, fully bendable, devices as direct X-ray detectors, obtaining sensitivity values up to several hundreds of nC/Gy at ultra-low bias of 0.2 V. An analytical model accounting the signal amplitude and sensitivity values achieved and describing the mechanisms of collection and transport of the X-ray generated charges have been also developed. Finally, we assessed the possibility to use the detector under mechanical strain and gave the first demonstration of a 2×2 pixelated matrix organic detector. [1] A. Ciavatti, E. Capria, A. Fraleoni-Morgera, G. Tromba, D. Dreossi, P.J. Sellin,P.Cosseddu, A.Bonfiglio, B. Fraboni : Advanced Materials 27 7123 (2015) [2] L.Basiricò, A.Ciavatti, T.Cramer, P.Cosseddu, A.Bonfiglio, B.Fraboni : Nature Communications 7,13063 (2016)

Resume : Recently, the emitting dipoles with perfect horizontal orientation parallel to the substrate result in the theoretical external quantum efficiency (EQE) limit over 45% without any extra light extraction structures compared to 25~30% for randomly oriented emitting dipoles. Large effort has been poured to increase the horizontal emitting dipole ratio (Θ) to get 70~82%. However, it will be difficult to obtain an even higher Θ due to the amorphous nature of typical emitting layers. In this regard, organic crystals would be the better emitters because of the orientational and positional ordering. However, organic crystals have rarely been used for OLEDs due to low stability and efficiency. In this work, we realized a crystal OLED with unprecedented high EQE of 38.8% using the perfectly oriented Pt based thin film emitting layer possessing PLQY of 96% and Θ of 93%. We investigated the emitting dipole orientation of the thin films fabricated using Pt complexes and discussed the structural relationship between X-ray structural analyses and structures in thin films based on quantum chemical calculations. The emitting dipole orientation of the crystal emitting layers was largely affected not only by the crystallinity of the emitting layer but also by the molecular arrangement in the crystal. (K.-H. Kim, J.-J. Kim et al. Adv. Mater. 2016, 28, 2526)

Resume : Organic photodiodes (OPDs) have been under major investigation for the past years, as they enable low cost fabrication of sensor systems for imaging, medical or industrial applications. While the device performance regarding responsivity and detectivity is continuously improving, the increase in detection speed still lacks behind. One of the major limiting factors can be attributed to trapping and detrapping effects. Especially for low light intensities, the bandwidth is strongly influenced by the lower generation of free charges. Interface and interlayer engineering can be used to reduce these effects. Unfortunately, this burdens the fabrication by printing processes since additional steps and layers are necessary. In this work, we are following an approach of blending insulating polymers into a P3HT:PCBM-system, which has already shown beneficial effects in OFETs or OSCs. The device stack corresponds to the inverted structure and is fabricated solely based on materials allowing for fully printed OPDs. Starting from a basic morphological study using PMMA with different molecular weights, we examined both the steady-state and dynamic characteristics of the OPDs. Transient photocurrent measurements show that the inclusion of PMMA results in an increase of the 3dB-bandwidth without adversely affecting steady-state performance under illumination or in the dark. This increase is attributed to a reduced trap density in higher ordered P3HT when PMMA is present.

Resume : Organic Light-Emitting diodes (OLEDs) able to directly emit circularly polarized (CP) electroluminescence (CP-OLEDs) are gaining much interest, due to their possible applications in anti glaring screens, 3D-displays and in medical diagnosis. We have recently demonstrated that chiral lanthanide complexes can be employed in solution processed CP OLEDs, obtaining highly CP emission [1]. Here, we present a strategy to improve devices efficiency both in terms of external quantum efficiency and degree of polarization of emitted photons, based on the optimization of the active layer formulation and devices architecture [2]. In this way, 75 % of emitted photons at 595 nm are circularly polarized with an EQE enhanced by one order of magnitude with respect to our earlier proof-of-concept prototype. Moreover, by comparing experimental data and modeling the main factors affecting polarization inside such devices are identified: the position of the recombination zone allied with the reflection on the cathode plays a major role on the polarization outcomes. The development of devices embedding other classes of chiral emitters can be envisaged to possibly bring the organic chiral photonic technology to the next readiness level. [1] F. Zinna, U. Giovanella, L. Di Bari, Adv. Mater. 2015, 27, 1791. [2] F. Zinna, M. Pasini, F. Galeotti, C. Botta, L. Di Bari, U. Giovanella, Adv. Funct. Mater. 2017, 27, 1603719.

Resume : Printed electronics is emerging as a new, large scale and cost effective technology that will be disruptive in fields such as energy harvesting, consumer electronics and medical sensors. The performance of printed organic electronic devices relies principally on the carrier mobility and molecular packing of the polymer semiconductor material. Unfortunately, the analysis of such materials is generally performed with destructive techniques, which are hard to make compatible with in situ measurements, and pose a great obstacle for the mass production of printed electronics devices. A rapid, in situ, non-destructive and low-cost testing method is needed. In this study, we demonstrate that nonlinear optical microscopy is a promising technique to achieve this goal. Using ultrashort laser pulses we stimulate two-photon absorption in a roll coated polymer semiconductor and map the resulting two-photon induced photoluminescence (TPPL) and second harmonic response. We anticipate that this non-linear optical method will substantially contribute to the understanding of printed electronic devices and demonstrate it as a promising novel tool for non-destructive and facile testing of materials during printing of the device and at any moment during its lifespan. This will help the production and development of high quality printed technologies where the semiconductor material can be accessed by infrared light, such as solar cells, displays and sensors.

Resume : Organic Field-Effect Transistor (OFET)-based memory devices are the most promising candidate for the flash memory function of organic electronics because of its nondestructive read out, single transistor realization, and ease of the integration of circuit. But the repetitive bias stress applied to the OFET-based memory device during electrical programming/erasing process impedes the reproducible, and long-term operation of memory functionality. In this study, optically programmable memory device based on plasmonics effect is suggested. Thermally deposited noble metal nanoparticles (NPs) on the semiconducting dinaphtho[2,3-b:2’,3’-f]thieno[3,2-b]thiophene (DNTT) layer of OFET provide enhanced light-absorption characteristics introduced by the localized surface plasmon resonance (LSPR) of noble metal NPs. Direct correlation between enhanced light-absorption and photogenerated current was confirmed by short-term transient photocurrent measurement. Also, the charge trapping and retention characteristics of noble metal NPs was confirmed by measuring long-term transient photocurrent measurement. The detailed mechanism of enhanced light-absorption and the charge trapping and retention properties induced by noble metal NPs was described. This novel device is expected to be a promising candidate for optically programmable organic memory devices with stable and reproducible operation.

Resume : The concept of polaron in nowadays high charge mobility (i.e. > 1 cm2/Vs) p-conjugated polymers will be reinvestigated. I will survey and compare the physico-chemical properties of polarons (i.e. localization length, electronic transitions, spin density) in state-of-the-art homo-polymers and donor-acceptor polymers, relevant for photovoltaic and thermoelectric applications. In particular, I will discuss three case studies: 1) photoinduced infra-red active vibrational features (IRAV) of crystalline and amorphous polymers (e.g. P3HT and PCPDTBT) [1,2]; 2) polaron localization vs. delocalization phenomena in donor-acceptor systems (e.g. P(NDI2OD-T2)) [3]; 3) polaron transfer mechanisms [4] and their connection to macroscopic thermoelectric properties (e.g. Seebeck coefficient) in polymers. For each case study joint theoretical and experimental insights will be discussed. [1] J. Yin, Z. Wang, D. Fazzi, Z. Shen, and C. Soci, J. Phys. Chem. C, 2016, 120, 1994–2001 [2] S. Kahmann, D. Fazzi, G. Matt, W.Thiel, M.A. Loi, C. J. Brabec J. Phys. Chem. Lett. 2016, 7, 4438–4444 [3] S. Wang, H. Sun, U. Ail, M. Vagin, P. O. Å. Persson, J. W. Andreasen, W. Thiel, M. Berggren, X. Crispin, D. Fazzi, S. Fabiano Adv. Mater., 2016, DOI: 10.1002/adma.201603731 [4] D. Fazzi and M. Caironi Phys. Chem. Chem. Phys., 2015, 17, 8573-8590.

Resume : Charge mobility in organic semiconductors is modest, due mainly to the soft, amorphous nature of these solution processed materials. This limits the efficiency and technological utility of organic electronics. Disorder makes calculating the underlying electronic structure and simulating charge transport challenging. Here we study fullerene adducts, solution processable derivatives of C60. We generate assemblies (1000 molecules) of different fullerene adducts using a coarse-grained molecular dynamics. 1 We build a tight binding model of the electronic structure of these assemblies, with parameters from quantum chemical calculations, and calculate electronic densities of states for different fullerene adducts. Organic materials are soft, and deform in the presence of a free charge to form a localised charge (a polaron). We have developed a model for polaron formation by self-consistently solving the tight-binding Hamiltonian for the system with the site energies perturbed by the dielectric response of the lattice to the charge density. This allows us to study the spatial delocalisation of the polaron state and polaron transport in these large assemblies, as a function of fullerene structure and composition. In order to study the dynamic response of the coupled lattice and electronic system, we build a simplified 1D system and directly propagate the joint equations of motion. 1 F.Steiner et al, Materials Horizons, 2014, 2, 113-119.

Resume : Chemical doping of organic semiconductors is known to increase conductivity, as well as carrier mobility at low concentration due to trap filling. At higher concentration, doping is used to enhance charge injection through effective barrier lowering at electrode/organic interfaces. However, the impact of molecular dopants on the electronic structure of the host, in particular with regards to the formation of new gap states, is not well understood, and a deeper understanding of this issue is necessary to push the boundaries of this new technology. In this work, we investigate the doping process at different regimes for poly[(4,8-bis-(2-ethylhexyloxy)-benzo(1,2-b:4,5-b′)dithiophene)-2,6-diyl-alt-(4-(2-ethylhexanoyl)-thieno[3,4-b]thiophene-)-2-6-diyl)] (PBDTTT-c) p-doped with the complex Mo(tfd-COCF3)3. Admittance spectroscopy measurements performed at temperatures ranging from 100 to 300K highlight the presence of two gap states. The first energy level is situated ca. 280 meV above the polymer HOMO and is intrinsic to the semiconductor. With doping, a second peak is observed suggesting the formation of a gap state ca. 450 meV above the polymer HOMO. This is consistent with the appearance of sub-gap absorption peaks by UV-visible spectroscopy and photoluminescence quenching. Such result highlights the interplay between doping and deep gap state creation, and represent a step towards organic technology optimization for a more efficient doping process.

Resume : Thiophene-phenelyne co-oligomers (TPCO) single crystals are promising materials for organic light emitting transistors and injection lasers as they combine high luminescence yield with efficient charge transport. However, the nature of excellent luminescent properties of TPCO single crystals have not yet been totally understood. In this work, we demonstrate that luminescent properties of TPCO can be dominated by impurities that are longer TPCO appeared during the synthesis. We have studied TPCO with the conjugated cores PTTP and PTPTP (P and T are phenylene and thiophene rings, respectively) with various end groups by using different synthetic routes. The TPCO crystals were grown from the vapor phase and solution. We have found that the photoluminescence (PL) of the TPCO single crystals can be dominated by the unintentional dopants, which act as an excitation energy funnel significantly increasing the PL efficiency. Furthermore, we show that intentional doping of TPCO crystals with longer TPCO can be used to control the energy transfer and maximize the PL quantum yield in single crystals. Doping level was controlled down to 100 ppm by using sensitive photodeflection and photoluminescence methods. Doping of TPCO by longer co-oligomers is a promising route to create highly efficient optoelectronic and light emitting materials and to tune their luminescence properties. This work was supported by Russian Science Foundation (project № 15-12-30031).

Resume : Conversion efficiency of organic thin-film solar cell reached 12%. In 1991, I proposed pin junction incorporating co-deposited i-interlayer consisting of donor/acceptor organic semiconductors (so-called bulkheterojunction), which is an indispensable for present organic solar cells [1,2]. In this paper, effects of impurity doping at ppm level in photovoltaic organic semiconductors, including (i) ‘seven-nines’ purification of organic semiconductors [3], (ii) pn-control of single and co-deposited organic semiconductors by impurity doping [4-9], (iii) Ionization sensitization of doping showing the doping efficiency of 100% [10], (vi) ppm-doping effects in the simplest n+p-homojunction organic photovoltaic cells [11], and (v) ppm-doping effects in the organic single crystals, will be presented. 1. M. Hiramoto et al., Appl. Phys. Lett., 58, 1062 (1991). 2. M. Hiramoto et al., J. Appl. Phys., 72, 3781 (1992). 3. M. Hiramoto, M. Kubo, Y. Shinmura et al., Electronics, 3, 351 (2014). 4. M. Kubo et al., Appl. Phys. Lett., 98, 073311 (2011). 5. M. Kubo et al., AIP Advances, 1, 032177 (2011). 6. Y. Shinmura et al., AIP Advances, 2, 032145 (2012). 7. Y. Shinmura et al., Appl. Phys. Express, 7, 071601 (2014). 8. N. Ishiyama et al., Appl. Phys. Lett., 99, 133301 (2011). 9. N. Ishiyama et al., Org. Electron, 14, 1793 (2013). 10. Y. Shinmura et al., Appl. Phys. Lett., 105, 183306 (2014). 11. C. Ohashi et al., Org. Electron., 27, 151-154 (2015).

Resume : Raman spectroscopy is a powerful tool to characterize electron-phonon coupling in various semiconductors. Here, we investigate the polaronic nature of two well-known high-mobility, thiophene-based polymers (PBTTT and DPPT-TT) by Raman spectroscopy combined with Density Functional Theory (DFT) calculations. Chemical and electrochemical hole doping of these polymers leads to characteristics changes in the intensity ratios of prominent Raman-active modes but no significant frequency shifts. The data suggest a localization of positive polarons on the electron-rich thienothiophene cores that are present in both polymers. DFT calculations also show that the mode intensity ratio variations are most likely caused by the local electric field that originates from negatively charged dopant molecules or electrolyte anions and the positive polaron on the polymer chain. The characteristic changes of the Raman mode intensities with the degree of doping also enable in-situ mapping of charge carrier concentration in the channel of electrolyte-gated polymer transistors with high spatial resolution.

Resume : Organic conducting polymers are of increasing scientific interest and are promising candidates for various applications [1]. Nowadays, one the most important challenges faced by the community deals with their processability along with their long term stability. In this context, herein, I'd like to present one approach consisting in the search for an alternative to polystyrenesulfonate (PSS) in the stabilization process of poly(3,4-ethylenedioxythiophene) (PEDOT) inks. A solution is for instance based on (trifluoromethanebis(sulfonyl)imide) (TSFI) side groups attached to a polystyrene backbone. This PEDOT:polyelectrolyte system showed interesting features as compared to PEDOT:PSS, for instance its rheological behavior, its doping, opto-electronic performance and long term stability. The influence of the synthesis parameters and the composition on the named properties were studied. The new PEDOT:polyelectrolyte system was finally successfully integrated as transparent electrode in OLED and OPV devices [2] and as channel material in organic electrochemical transistors [3]. (1) Skotheim, T. A.; Reynolds, J. R. Handbook of Conducting Polymers, CRC Press, NY, 2007, 1680 pages. (2) Cloutet, E. and coll. Angew. Chem. Int. Ed. 2015, 54 (29), 8506–8510. (3) Hadziioannou, G.; Malliaras, G.G. and coll. J. Polym. Sci.: Part B, Polym. Phys. 2016, 54(2), 147-151.

Resume : Charge-exciton interaction is the main dominating mechanisms limiting the efficiency in Organic Light-emitting Transistors (OLETs). Thus, a comprehensive physical understanding of this detrimental process in charge accumulation devices is mandatory in order to unlock the technological potentiality of OLETs in real-setting applications. We have recently introduced a novel confocal microscopy method for mapping the charge density spatial distribution in OFETs based on the photoluminescence electro-modulation (PLEM) [1]. Here, we implement time-resolved PLEM spectroscopy on the picosecond timescale to investigate throughout the charge-exciton interaction in organic transistors [2]. The results show that the injected charges reduce the exciton radiative recombination in two ways: (i) charges may prevent the generation of excitons and (ii) charges activate a further non-radiative channel for the exciton decay. Moreover, the measurements clearly reveal that not only trapped charges, as it was already reported, but rather the entire injected charge density contributes to the quenching of excitons. Based on these insights we suggest possible measures to achieve high exciton density in OLETs, which might foster the development of high brightness OLETs. [1] Koopman W., Toffanin S., Natali M., Troisi S., Capelli R., Biondo V., Stefani A., Muccini M. Nano Lett. 2014, 14, 4. [2] Koopman W., Natali M., Donati G. P., Muccini M., Toffanin S. ACS Photonics DOI: 10.1021/acsphotonics.6b00573.

Resume : Conducting polymers are an emerging class of materials for large-area solid-state energy conversion and storage applications. These materials enable new paths toward more sustainable energy-related technologies without the need of expensive, or even toxic metal-based compounds. Unlike their p-type counterparts, n-doped conducting polymers typically suffer from a low electrical conductivity (< 0.01 S/cm). Despite continuous efforts to understand charge transport mechanism in these materials and how it affects the device performance, the interplay between chemical structure, polaron delocalization lenght, and conductivity remains unclear. Here we show that n-doped polymers do not necessarily have to follow the typical design rules of semiconducting polymers for field-effect transistors. In contrast to undoped polymers where in fact regioregularity of the backbone and crystallinity are pursued for their beneficial effect on charge carrier mobility, polymers used in their doped state should be designed to have long polaron delocalization lengths in order to reach high conductivity. In particular, we will show that linear torsion-free polymer backbones enable a delocalized anion to form upon doping, leading to high conductivity as compared to distorted n-doped donor-acceptor polymers. Understanding these principles will guide the design of next-generation high-conductivity polymers that can find applications in batteries, supercapacitors, and thermoelectrics.

Resume : The continuous demand for organic thin-film transistors (OTFTs) with improved performance has been the driving force behind the tremendous progressed witnessed during the past decade in the field of printed electronics. The most common approach towards this goal has been the development of new semiconductors with enhanced charge transport characteristics accompanied by in-depth understanding of the material’s structure-property relationship. In this presentation I will discuss an alternative strategy to materials, and ultimately OTFT and integrated circuits, development based on the use of molecular dopants/additives in combination with binary semiconducting small-molecule/polymer blends. I will describe how the incorporation of different types of dopants can lead to semiconducting systems and devices that combine highly attractive features such as solution processability and high carrier mobility with significantly improved OTFT bias stability. The role of the dopant and the underlying mechanism responsible for the performance enhancement observed in several different material systems will also be discussed.

Resume : Due to their ease of processing, organic semiconductors are promising candidates for applications in high performance flexible displays and fast organic electronic circuitry. Recently, a lot of advances have been made on organic semiconductors exhibiting surprisingly high performance and carrier mobilities exceeding those of amorphous silicon1. However, there remain significant concerns about their operational and environmental stability. Here, we report a novel technique for dramatically improving the operational stability, performance and uniformity of high mobility polymer devices such as field-effect transistors and diodes by the addition of specific small molecule additives to the polymer semiconductor film. We demonstrate for the first time polymer FETs that exhibit stable threshold voltages with threshold voltage shifts of less than 1V when subjected to a constant current operational stress for 1 day under conditions that are representative for applications in OLED active matrix displays. The approach constitutes in our view a technological breakthrough; it also makes the device characteristics independent of the atmosphere in which it is operated, causes a significant reduction in contact resistance and bulk trap density and significantly improves device uniformity2. We will discuss in detail the microscopic mechanism by which the molecular additives lead to this significant improvement in device performance and stability. [1] D. Venkateshvaran*, M. Nikolka* et al., Nature, 515, 384−388 (2014) [2] M. Nikolka*, I. Nasrallah* et al., Nature Materials, In Press

Resume : Electrodes based on silver nanowires have been demonstrated using techniques such as dip-coating, spin-coating, and roll to roll combined with different processing steps. This applicability is interesting for solar cells, light emitting diodes, and vertical organic field effect transistors (VOFETs). A VOFET is essentially a semiconductor diode stacked on capacitor separated by a common source electrode, so when the capacitor is charge up with gate voltage the channel region is affected due to the electrical transparency required from source electrode. Intermediate metal electrodes deposited by thermal evaporation are most heavily used, although its limited transparency resulting in devices operating at high voltages. In this work, we report a VOFET with silver nanowires deposited by Meyer rod coating as source electrode using cross-linked polyvinyl alcohol as gate insulator and C60 fullerene as n-type semiconductor. Silver nanowires electrodes were characterized previously on dielectric surface showing sheet resistance of 22.7 ± 1.55 Ω/sq and 85.2 ± 20.5 nm of roughness surface. Devices presented high current density (mA/cm²) and ON/OFF ratio of ~700 with supply voltages of 2V. A frequency operation of 13 MHz was recorded operating at 1.5V drain and gate voltages, in agreement with theoretical cutoff calculated. Such results are mainly due to the low capacitance by area of dielectric (~20nF/cm²) and the high transparency to the DC electric fields of source electrode.

Resume : To make high-performance semiconductor devices, a good ohmic contact between the electrode and the semiconductor layer is required to inject the maximum current density across the contact. Achieving ohmic contacts requires electrodes with high and low work functions to inject holes and electrons respectively. However, it is challenging to produce electrically conducting films with sufficiently high or low work functions, especially for solution-processed semiconductor devices. Hole-doped polymer organic semiconductors are available in a limited work-function range, but hole-doped materials with ultrahigh work functions and, especially, electron-doped materials with low to ultralow work functions are not yet available. The key challenges are stabilizing the thin films against de-doping and suppressing dopant migration. Here we report a general strategy to overcome these limitations and achieve solution-processed doped films over a wide range of work functions (3.0–5.8 eV), by charge-doping of conjugated polyelectrolytes and then internal ion-exchange to give self-compensated heavily doped polymers. Mobile carriers on the polymer backbone in these materials are compensated by covalently bonded counter-ions. Although our self-compensated doped polymers superficially resemble self-doped polymers, they are generated by separate charge-carrier doping and compensation steps, which enables the use of strong dopants to access extreme work functions. We demonstrate solution-processed ohmic contacts for high-performance organic light-emitting diodes, solar cells, photodiodes and transistors, including ohmic injection of both carrier types into polyfluorene—the benchmark wide-bandgap blue-light-emitting polymer organic semiconductor. We also show that metal electrodes can be transformed into highly efficient hole- and electron-injection contacts via the self-assembly of these doped polyelectrolytes. This consequently allows ambipolar field effect transistors to be transformed into high-performance p- and n-channel transistors. Our strategy provides a method for producing ohmic contacts not only for organic semiconductors, but potentially for other advanced semiconductors as well, including perovskites, quantum dots, nanotubes and two-dimensional materials.

Resume : The efficiency of bulk-heterojunction polymer:fullerene solar cells critically depends on the dominant length scale of the morphology formed during drying of the photoactive film. Time-resolved studies that allow studying the kinetics of film and morphology formation have been performed on a blend of a diketopyrrolopyrrole-based polymer with a fullerene derivative, deposited from solvents with and without co-solvent. Using in-situ time-resolved optical and diffraction techniques, combined with ex-situ transmission electron microscopy, we are able to determine the parameters that influence the dominant length scales in the phase separated films. Without co-solvents, large fullerene domains are formed by liquid-liquid phase separation, whose size is governed by the drying rate when normalized to the final thickness. With co-solvents, on other hand, the dominant length scale is not determined by liquid-liquid demixing, but by polymer aggregation. The width of the polymer fibers that are formed by the aggregation can be controlled by adjusting the nature of the co-solvent mixture and by several other factors such as molecular weight of the polymer, introducing branching in the polymer, and the temperature at which the layers are processed from solution. With these studies we have established some guide lines that assist to find the optimal morphology via rational steps.