Organic semiconductors: hybrid interfaces and charge transport

Resume : Charge transport in organic solids is intimately dependent on the precise order between aromatic cores in the solid state. We developed a simple, reliable model for tuning this solid-state order (silylethyne functionalization) which has realized impressive charge transport performance across a wide array of aromatic and hetero-aromatic cores. However, this approach does not work for all systems, either due to difficulties in synthesis of appropriate precursors, or due to low stability or solubility of the resulting product. Here, I will present a very recently developed "core" system that allows precise solid-state tuning of the order of a very wide array of aromatic chromophores. Using simple, mild coupling techniques, these cores can be tuned to induce a wide array of pi-stacked orders, arising from a simple library of tunable core molecules. Stability in these systems is dramatically enhanced compared to e.g. acenes, and hole mobilities higher than 4 cm2/Vs have been observed. The scope, assessment of disorder, and thin-film structural properties of these new chromophores will also be discussed.

Resume : In organic semiconductors, band-like transport is peculiar to charge carriers that are delocalized over more than a single molecular unit and it is associated with an increase in carrier mobility upon decreasing of temperature. Hitherto, how such type of transport is related to molecular design is not fully understood and, generally, a comprehensive picture of its origin is lacking[1]. In this work, we used two dicyanoperylenecarboxydiimide derivatives (PDI) molecules with different lateral chains that were either fluorinated, PDIF-CN2 or alkylated, PDI8-CN2. The former derivative would exhibit band-like transport whilst hopping transport was identified as the main conduction mechanism for the latter. Both molecules were exposed to comparable extrinsic disorder that is considered to stem from the substrate1. A comparative study that includes multiple characterization techniques such as inelastic neutron scattering, THz and PE spectroscopy, GIWAXS, electrical characterization but also modeling and simulations allows us to unravel the origin of band-like transport mechanism by looking at the molecular fluctuations and their effect on the phonon modes of both compounds. [1] N. A. Minder, S. Ono, Z. Chen, A. Facchetti, A. F. Morpurgo, Adv. Mater. 2012, 24, 503.

Resume : The organic field-effect transistors (OFETs) usually comprise electronic materials of wide conductivity spectrum covered from conductor to insulator and semiconductor. Although the OFETs represent field-effect transistors using “organic semiconductors” as an active layer, the performance of OFETs depends not only on the properties of the organic semiconductors, but is also affected by the properties of the gate dielectrics which contact directly with the active layer. Therefore, controlling the interface between the organic semiconductor and the gate dielectrics and the surface of the dielectric forming the interface is critical to optimize the OFETs. In this presentation, the study on the top contact bottom gate (TCBG) structural OFETs consisting of a diketopyrrolopyrrole(DPP)-based Donor(D)−Acceptor(A)-type copolymer as an active layer and a newly-synthesized methyl-methacrylate-based triblock-copolymer as a gate dielectric will be discussed. In particular, three kinds of the triblock-copolymer gate dielectrics, where benzene-contacting block in the triblock have no fluorine (F), one F, and five Fs, respectively, were prepared to investigate the effect of the amount of F in the gate dielectric layer on the charge transports through the DPP-based semiconducting copolymer film. From the preliminary results, it was revealed that the more F atoms in the gate dielectric enhanced the electron-transporting characteristics in the DPP based semiconducting copolymer.

Resume : Over the past few decades, developments in high mobility donor–acceptor copolymers based field effect transistors (OFET) have favoured the enhancement of performances in printed and flexible polymer electronics. However, structure-transport property relationship is still intensively investigated, with the aim of providing a rationale for further development of the field. One of the most extensively studied copolymers is Poly{[N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)} (P(NDI2OD-T2)), an n type pi-conjugated copolymer system that stands out yielding mobility values (>1 cm^2/Vs), while displaying a considerable amount of disorder. So far, the studies have suggested how intra-, inter- and supra-molecular features, like chain elongation, alignment or entanglement can have drastic effects on charge transport. In order to elucidate how such different aspects determine the formation of efficient carrier percolation pathways, in this work we have exploited the pre-aggregation properties of P(NDI2OD-T2) for controlling the formation of sub-monolayer sparse networks of polymer fibrils with a different degree of substrate coverage, up to a uniform thin film. By combining morphological, spectroscopic and OFET electrical analysis of such networks, we determine the percolation onset and its dependence on processing conditions. We observe a good field effect behaviour in the sub-monolayer networks with a coverage as low as 11% from a very diluted solution of 0.025g/L in toluene which could be the onset of percolation. Our results are important to clarify the interplay and relative weight of intra- and inter-molecular charge transport in sizing carrier mobility in donor-acceptor copolymers.

Resume : Biomaterials for organic devices have been attracting considerable interest. The advantages of biomaterials which are isolated from the nature are biocompatible, environmental friendly, and biodegradable. In addition, these biomaterials are well dissolved in various organic solvents that can be utilized for low-cost and large area solution process. Therefore, there have been emerging efforts to serve biomaterials as building block of biocompatible and transient electronics. For instance, natural biopolymers such as DNA, gelatin, chitosan, silk fibroin, and albumin have been reported as a dielectric in organic field-effect transistors (OFETs). Especially, biopolymers with high densities of aromatic and polar groups showed high capacitance values and achieved high-performance OEFTs with low voltage operation. Human hair keratin has robust physical property and biocompatibility as an autologous protein with abundant supply. However, most of research with extracted human hair keratin tend to focus on bio-application rather than device application. In this study, we report human hair keratin as a high-capacitance dielectric layer for OFETs using solution process. Due to its hygroscopic property to generate mobile ions and form electrical double layers, the keratin dielectric layer showed high capacitance value of ~1.27 μF/cm2 at 20 Hz. As a result, 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) FETs with keratin dielectric exhibited high electrical property with a saturation field-effect mobility of 0.35 cm2/V·s at a low voltages (gate voltage from 1 to -2V, drain voltage of -1 V) under ambient conditions. The possibility of neuromorphic system was explored with PCDTPT/keratin transistors. The PCDTPT/keratin transistors exhibited synaptic memory effects via proton conduction in keratin dielectric. In addition, flexible PCDTPT/keratin transistors were successfully demonstrated on polyarylate substrates. To demonstrate the transient OFETs, the PCDTPT/keratin transistors were also fabricated on keratin film as a substrate. The dissolution test of PCDTPT/keratin transistors on keratin substrate was performed in alkaline solution and the keratin substrate and keratin dielectric layer had completely dissolved after 7 days. We believe that this human hair keratins have a potential as a high-performance and biocompatible building block for eco-friendly electronic devices.

Resume : It is still an important task to explore high-performance n-channel organic semiconductors with ambient stability. To this end, we have attempted two different approaches using (1) characteristic acceptor molecules based on birhodanines and (2) mixed-stack charge-transfer crystals. For the first approach, birhodanines (5,5'-bithiazolidinylidene-2,2'-dione-4,4'-dithiones) are prepared by a several-minute one-step reaction at room temperature from commercially available reagents [1]. The tetrathione analogues, 5,5'-bithiazolidinylidene-2,4,2',4'-tetrathiones, are strong acceptors with the LUMO level of −4.2 eV, where the thin-film transistors are operated in air, and not degraded after several-month storage in air. The birhodanines are slightly weaker acceptors with the LUMO level of −3.8 eV, but the transistors show reasonable air stability. Many kinds of chemical modifications are possible by replacing the 3-substituents, by which we can change the solubility and structures. For the second approach, mixed-stack charge-transfer crystals such as (BTBT)(TCNQ) exhibit n-channel transistor properties with ambient stability [2]. Since D-to-D hole hopping and A-to-A electron hopping are both represented by the same transfer, it is somewhat mysterious that many TCNQ complexes show only electron transport. However, the dominance of electron transport is explained by the large contribution of the donor HOMO−1 to the electron transport owing to the symmetry matching. [1] K. Iijima et al, J. Mater. Chem. C 2017, 5, 9121. [2] R. Sato et al, J. Phys. Chem. C, 2017, 121, 6561.

Resume : Organic semiconductors (OSCs) have attracted much attention for low-cost, flexible and human-friendly optoelectronics. However, achieving high electron injection efficiency is difficult from air-stable electrodes and cannot be equivalent to that of holes. Here, we present a novel concept of electrode composed of a bilayer of tetratetracontane (TTC) and polycrystalline organic semiconductors (pc-OSC) covered by a metal (M) thin film layer. Field-effect transistors of single crystal organic semiconductors (sc-OSCs) with the new electrodes of M/pc-OSC/TTC (M: Ca or Au) show both highly-efficient electron and hole injection. Contact resistances of electron and hole injection from Ca/pc-OSC/TTC and Au/pc-OSC/TTC are fascinatingly lower than those from pure Ca for electrons and Au for holes, respectively. Equivalent high field-effect mobilities of holes (22 cm2V–1s–1) and electrons (5.0 cm2V–1s–1) can be observed in a rubrene single crystal, which are the highest among field-effect transistors so far proposed. Higly efficient light-emission will be demonstrated via amipolar carrier injection.

Resume : The knowledge of the thermal conductivity of organic thin films is crucial for the design of novel thermoelectric materials as well as the modelling of heat dissipation in organic semiconductor devices such as OLEDs, OTFTs and organic solar cells. While the accurate determination of the electrical conductivity in organic thin films is well established, measuring the thermal conductivity still remains quite challenging. Time-dependent as well as steady-state methodologies such as fluorescence thermometry, time-domain thermoreflectance or the 3ω-method require either complex preparation procedures, advanced setups or complex mathematical models to fit the experimental data. In this talk we exploit a novel approach to one laser Raman thermometry that enables the facile determination of the in-plane thermal conductivity of organic thin films. The experiment is performed on free-standing architectures realized entirely from solution in a two-step process. The film is warmed up within the dimensions of the laser spot according to the intensity of the probing laser. The shift of the Raman-active modes as a function of temperature enables the determination of the local temperature raise depending on the absorbed laser power. Finally, the steady-state heat equation is solved in COMSOL Multiphysics and the in-plane thermal conductivity is determined by fitting the model to the experimental data. The method is demonstrated to be robust with solution processed thin (and often rough/porous) organic films. We also performed 3ω measurements on organic thin films supported on glass in order to determine the cross-plane thermal conductivity as well as the thermal boundary resistance of the film-glass interface. Our results show that the thermal conductivity in organic semiconductors such as P(NDI2OD-T2), PCDTBT and P3HT is anisotropic, while the thermal boundary resistance of solution processed films deposited on glass corresponds to van der Waals type of adhesion.

Resume : Due to their exceptionally long spin lifetimes, organic semiconductors (OSCs) could also have an important impact on spintronics, where carrier spins play a key role in transmitting, processing and storing information.[1] However, spin transport properties in OSCs have yet to be fully understood. Our group has recently demonstrated spin pumping by ferromagnetic resonance (FMR) in a ferromagnet (FM)/conjugated polymer/nonmagnetic (NM) spin-sink trilayer to prove the ability of polarons to carry pure spin currents over hundreds of nanometers with long spin relaxation times of up to a millisecond.[2] Following these results, we started to investigate the impact of molecular design on the spin pumping ability in small molecule OSCs and polymers by electron spin resonance spectroscopy (ESR). Our findings highlight that alkylated side chains suppress the spin injection and decrease the spin diffusion length for vertical spin pumping in benzothieno[3,2-b][1]benzothiophene (BTBT) and dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT) derivatives. Other class of materials will also be covered: brickwall OSCs and polymers. Combined with quantum-chemical calculations, our results try to provide a theoretical background, to elucidate the spin transport mechanism taking place in these carbon-based materials and how molecular design impacts spin injection. [1] G. Szulczewski, Nature Materials 2009, 8, 693. [2] S. Watanabe, Nature Physics 2014, 10, 308.

Resume : In this presentation, we report a single-step interface engineering methodology which can be employed on both contact-electrodes and gate dielectric simultaneously for all-inkjet-printed, flexible, transparent organic thin-film transistors (OTFTs) and inverters. Dimethylchlorosilane-terminated polystyrene (PS) introduced via also inkjet-printing as a surface modifier onto inkjet-printed poly(4-vinylphenol) dielectric and poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) electrodes before forming 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene) semiconductor layers. The functionalized PS delivered much better interface properties yielding a significant decrease in interface trap density between the gate dielectric and the semiconductor layer, and improved the ordering of the TIPS-pentacene layer. The electrical properties of the PS-treated OTFTs were significantly improved, as shown by a field-effect mobility (μFET) of 0.27 cm2 /V s and an on/off current ratio (Ion/Ioff) of greater than 106, and transmittance of 70.0% (at 550 nm). The inkjet-printed inverters have shown full-swing switching performance with a high voltage gain of up to 7.17 V/V at VDD = -20 V, while maintaining their properties under 1000-cycle mechanical bending stress with a speed and bending radius of 10 mm/s and 5 mm, respectively. These results provide an attractive pathway toward the realization of high-performance flexible electronics with a low-temperature and low-cost printing process.

Resume : Organic semiconductors have potential to enable low cost, large scale, flexible electronics. The major challenge to achieve commercial applications with organic semiconductors is obtaining stable thin films with good electrical properties. Organic semiconductors exhibiting liquid crystalline (LC) mesophases have shown promising results for the fabrication of stable, ordered films for organic electronics. In particular, soluble small molecules that form crystalline films at room temperature, but demonstrate ordered LC phases at higher temperatures, combine the advantages of good chemical purity, molecular self-assembly, high carrier mobility, and excellent structural and thermal stability. In this talk I will present our recent work on LC dyes. The soluble small molecule dyes form crystalline films with good structural integrity. We observed that annealing and cooling the films through LC mesophases at higher temperatures leads to increased crystal domain size, but also local structural defects in the crystalline film. These defects were correlated with increased π – π interactions between molecular neighbours, leading to an increase in both hole and electron mobility. We show how this effect can be exploited to create stable organic thin films demonstrating balanced ambipolar transport. [1] Tchamba et al, ACS Appl. Mater. Interfaces, 2017, 9, 6228–6236

Resume : A competitive market entry for organic electronics requires the implementation of a simple, cheap and scalable solution technique. One of the major advances in solution processing comes from the idea of blending a small molecule semiconductor with an amorphous insulating polymer, to take advantage of the high carrier mobility of the crystalline component and the processability enhancement provided by the polymer. By employing this strategy, the Bar-Assisted Meniscus Shearing (BAMS) technique has already succeeded in fabricating high performance Organic Field Effect Transistors (OFETs). [1] Although the superior performance of these two-component OFETs is attributed to the vertical phase separation of the two materials, the details of this structuration are not always understood or studied enough. This question is addressed here [2] by combining AFM and FFM techniques. Furthermore, to stablish a correlation between this stratification and the electrical properties, we have performed KPFM measurements in operating OFETs. In parallel, the influence of the semiconductor morphology on the OFET response under mechanical stress has been investigated.[3] A detailed AFM and SKPM analysis of the devices before and after bending showed that the change in current observed upon deformation can be justified by a variation in contact resistance together with the formation of nanocracks in the semiconductor film. REFS: [1] I. Temiño, et al. Adv. Mater. Technol. 2016, 1, 1600090. [2] A. Pérez-Rodríguez, I. Temiño, et al. ACS Applied Materials & Interfaces, submitted. [3] S. Lai+, I. Temiño+, et al. Adv. Electron. Mater. 2017, 1700271. (+These authors contributed equally to the work)

Resume : Solution processable semiconducting polymers with excellent film forming capacity and mechanical flexibility are emerging as alternatives to conventional inorganic semiconductors. However, the required optimization of the charge transport within conjugated polymers can be obtained only through a full control of the molecular assembly at the different length scales. Here, we present a general strategy, based on Langmuir-Schäfer deposition, to control the polymer organization to form uniaxially aligned architectures at the water-air interface prior to their transfer onto a flat substrate. In particular, we demonstrate that the hole mobility of isoindigo-based conjugated polymer, [1] in our films processed in air is comparable with those previously reported for films prepared under N2 atmosphere. Interestingly, the transport is anisotropic: structural (GIXD) and morphological (AFM) characterization revealed that the mobility is higher in the direction perpendicular to the polymer backbone. This method, based on the (macro)molecular pre-assembly at the water-air interface, can be applied to grow layer-by-layer structures of polymers possessing different electronic properties. [1] S. Bonacchi, M. Gobbi, L. Ferlauto, M.-A. Stoeckel, F. Liscio, S. Milita, E. Orgiu, P. Samorì, ACS Nano, 2017, 11, 2000–2007

Resume : The extensive research in the field or organic electronics has led to remarkable enhancements of field-effect transistors (OFETs) charge carrier mobilities, which exceed now the benchmark value of 10 cm2 V-1 s-1. OFETs showing such high efficiency are typically designed with relatively large channel lengths (tens of microns) in order to reduce the detrimental effect of contact resistance, which ultimately hinders the real potential of this technology. As a result, a downscaling of the device size leads typically to a large reduction of the extracted field-effect mobility and other device performance. Molecular doping could offer an effective tool to resolve some of these issues, however it has been proven challenging to implement this approach without affecting structural properties of the materials and operational functionality of devices. Here we present a doping approach involving the employment of multi-layered active layers in bottom-gate top-contact field-effect transistors. An optimal injection from the contacts is obtained by confining the dopant in the bottom layer, thus exempting the accumulated channel from doping and therefore preserving the typical charge transport of the pristine polymer. In order to achieve such device architecture, solution based polymer blend systems have been employed; this approach has been proved valid for different type of semiconducting polymers, both p- and n- type.

Resume : Non-volatile organic memories can be fabricated by careful blending of semiconducting and ferroelectric polymers. Here, we present a class of materials that has the same functionality in a single material, by combining different functional sub-units whose strong electronic coupling leads to new and exciting properties. We have synthesized disc-like semiconducting organic molecules that are functionalized with strong dipolar side groups. These materials self-assemble into supramolecular polymers, which provides long-range polar order that supports collective ferroelectric behavior of the side groups, as well as charge transport through the stacked semiconducting cores. We find that the ferroelectric polarization couples to the charge transport and leads to a bulk conductivity that is both switchable and rectifying. When sweeping the applied electric field, the conductivity will switch from a high to low state at exactly the ferroelectric coercive field. Detailed analysis of the current-voltage curves shows that the current is a combination of Ohmic and space-charge-limited currents. This demonstrates that it truly is the bulk conductivity that is modulated by the ferroelectric polarization. A simple quasi-1D hopping model is developed to investigate the effect of the asymmetric potential caused by the polarization. This model reproduces the experimental on/off ratio using reasonable parameters.

Resume : Organic electron-transporting materials are essential for the fabrication of organic p-n junctions, photovoltaic cells, n-channel field-effect transistors, thermoelectric generator and complementary logic circuits. Rylene diimides are promising n-type candidates because they are robust, versatile and they have excellent thermal and oxidative stability, high electron affinities, and, in many cases, high electron mobilities. Our recent results on perylene and naphthalene-diimide-based small molecules are summarized and analyzed. We performed a multiscale structural analysis by combining different X-ray scattering methods (powder X-ray Diffraction, Grazing Incidence X-ray Diffraction, GIXD, specular XRD and X-Ray Reflectivity, XRR), to achieve a deep knowledge of the molecular organization in the bulk, in the film, and in the interface region, crucial for any technological development. In-situ and real time X-ray scattering measurements have proved to be extremely powerful for describing at nano-scale the structure of films during the first stages of formation, as well as on the evolution of the molecular arrangement and of the surface morphology during the film growth. Moreover, in-situ measurements give significant information on the polymorphs stability, the segregation and reversibility of solid state transformations, strongly related to the thermoelectric and transport properties of this materials.

Resume : Modern technological devices based on organic semiconductors strongly rely on the energy position of the molecular orbitals and the associated energy gap (1). So far there is not any reliable technique that can determine the transport energy gap in the bulk of an organic semiconductor (2). This often results in the adoption of inadequate substitutes, such as the fundamental, optical or interface gaps (2). In this work, we demonstrate for the first time a non-destructive in-device approach based on in-device molecular spectroscopy (i-MOS) (3, 4, 5) technique that allows both mapping the relative energy of molecular orbitals and extracting the transport energy gap of a molecular semiconductor (6). Moreover, through the injection of energetic electrons into the organic semiconductor, we can access and manipulate unprecedented electronic transport regimes, such as negative differential resistance (NDR) (7) arising from a Marcus inversion (8) phenomenon (6). Our results demonstrate that i-MOS is extremely powerful spectroscopic technique for organic electronic community as well as revolutionary tool for exploring new charge injection physics in organic semiconductor with novel NDR possibilities for electronic circuiting (6). References 1. V. Shrotriya, Y. Shao, D. Sievers, Band structure engineering in organic semiconductors. Science. 352, 1446–1449 (2016). 2. J. L. Bredas, Mind the gap! Mater. Horiz. 1, 17–19 (2014). 3. M. Gobbi et al., Determination of energy level alignment at metal/molecule interfaces by in-device electrical spectroscopy. Nat. Commun. 5, 4161 (2014). 4. A. Atxabal et al., Energy Level Alignment at Metal / Solution-Processed Organic Semiconductor Interfaces. Adv. Mater. 10, 1606901–6 (2017). 5. T. Arnold et al., Hot Electrons and Hot Spins at Metal-Organic Interfaces. Acepted in Adv. Funct. Matt. (2017). 6. R. Yan et al., Esaki Diodes in van der Waals Heterojunctions with Broken-Gap Energy Band Alignment. Nano Lett. 15, 5791–5798 (2015). 7. R. A. Marcus, On the Theory of Oxidation-Reduction Reactions Involving Electron Transfer. I. J. Chem. Phys. 24, 966–978 (1956). 8. A. Atxabal et al., Molecular spectroscopy and Marcus inversion regime in a solid-state device. In-preparation.

Resume : Thermoelectric devices are promising and environmentally friendly systems to recover energy from industrial waste heat or natural heat. Organic semiconductors represent promising candidates as thermoelectric materials due to their peculiar features: low thermal conductivity, high flexibility and low cost. It is however recognized that the ultimate performances are strongly dependent on the morphological aspects and consequently on the synthesis procedure. The causal relationships synthesis/morphology and morphology/properties are still under debate. In this scenario atomistic simulations provide a fundamental tool to get insights into this problem by obtaining a microscopic scale description of the polymerization process which is presently not experimentally available. In this work, we present a novel computational tool to model polymerization by means of a suitable combination of first principles DFT simulations and classical molecular dynamics. The present tool allows to generate realistic polymer samples starting from the basic monomers by simulating the actual experimental synthesis. The system dimensions are in the order of several tenths of nm corresponding to about 10^6 atoms. We present the results of the application of the above procedure to the case of poly(3,4-ethylenedioxythiophene)-Tosylate (PEDOT-Tos) under several experimental conditions i.e. different solvents, oxidants, and temperatures. The overall effect on the morphological and thermoelectric properties will be discussed.

Resume : We fabricated ZnO nanorod arrays decorated with Au nanoparticles for use as the electron transport layers in perovskite solar cells. We show with current-voltage measurements, impedance analysis and photoluminescence spectroscopy that the Au nanoparticles reduce recombination losses at the ZnO-perovskite interface. We compare the performance of double cation-mixed halide and triple cation-mixed halide perovskites in this architecture. The use of Au nanoparticles in the solar cells resulted in an increase in the open circuit voltage (Voc) and fill factor, leading to an increase in the maximum power conversion efficiency from 11.52 % to 12.62 % (double cation) and from 11.9 % to 12.66 % (triple cation). We discuss these results in terms of trap filling and surface passivation by the Au nanoparticles at the ZnO-perovskite interface.

Resume : Low-cost, portable, and real-time monitoring, electronic devices based on π-conjugated species，wherein small organic molecules, oligomers and polymers, and low-dimensional carbon materials, worked as the active materials, should be generally concerned as the next generation of sensing devices. On one hand, the inherent characteristics of π-conjugated systems, such as their susceptible to noncovalent interaction including hydrogen bonds, charge transfer, dipole-dipole interactions, photoexcitation and reversible transformations, enabling effective detection of various stimuli. On the other hand, their light-weight, mechanical flexibility and their generally good solution peocessability as well as good compatibility with large-area and flexible solid supports, endow them manufacturing of sensing devices using a wide range of desired or arbitrary solid supports as substrates, such as glass, paper, plastic, and others more. More importantly, diverse assembly processible methods, such as interfacial assembly, casting, spin coating, layer-by-layer assembly, evaporation, and roll-to-roll protocols (inkjet printing, screen printing) etc. could be used to achieve new materials with new functions, where the tedious synthesis work for π-conjugated systems bearing specific sensing groups could be avoided. With a special emphasis on the state-of-the-art works recently, the main aim of this report is to provide a general overview about the importance of supramolecular assembly in the progress of sensing field. First, we briefly introduce the concept of sensors, and then we will put emphasis on the impactful protocols of how to improve the performance of the sensor through nanoassembly of π-conjugated systems. Finally, we put forward an outlook for future directions in sensing field. We hope that the discussions presented herein will be beneficial to the design of future sensing devices with high performance.

Resume : This works focus on an enhancement of solar cell performance by incorporating of Dawson-type POM-based compounds ,organogermyl and organosilyl complexes including organic chains having terminal -COOH and allyl groups, that is, two organogermyl complexes. Multifunctional Polyoxometalate (POM)-based inorganic-organic hybrid compounds, [α2-P2W17O61{(RGe)}]7- and [α2-P2W17O61{(RSi)2O}]6- into monolacunary site of Dawson polyoxoanion [α2-P2W17O61]10-. In this paper, we report full details of the synthesis and characterization of these complexes and their molecular structures [1]. References [1] Nomiya, K., Togashi, Y. , Kasahara, Y. , Aoki, S. , Seki, H. , Noguchi, M. , Yoshida, S. , Inorg. Chem. 2011, 50, 9606–9619.

Resume : In this study, we designed and prepared a varieties of anthracene-based organic small molecules for OSCs application. A variety of novel anthracene based donor-acceptor (D-A) type molecules, namely, 2-((5-(10-(thiophen-2-yl)anthracen-9-yl)thiophen-2-yl)methylene)malononitrile (Mono-ThAnt-E1A), (Z)-2-(3-oxo-2-((5-(10-(thiophen-2-yl)anthracen-9-yl)thiophen-2-yl)methylene)-2,3-dihydro-1H-inden-1-ylidene)malononitrile (Mono-ThAnt-E2A), 2-(1-(5-(10-(thiophen-2-yl)anthracen-9-yl)thiophen-2-yl)ethylidene)malononitrile (Mono-ThAnt-E1B), and 2-(1-(5-(10-(thiophen-2-yl)anthracen-9-yl)thiophen-2-yl)ethylidene)malononitrile (Mono-ThAnt-E1C) were synthesized. Electrochemical and electrooptical properties was calculated before fabrication for OSCs. We observed that the results for 7% doping gave best value for all materials, and Mono-ThAnt-E1C was best candidate for applications (Efficiency = 4.39). From this study, we concluded the strong electron withdrawing groups and acetyl groups significantly influenced the properties of antrahcene based structures and this study may be useful for the design of novel anthracene based organic compounds for future OSCs application.

Resume : Naphthalene diimide (NDI) derivatives have been attracted considerable attention for small molecule-based n-type organic semiconductors and investigated extensively due to their high electron mobility, low-cost, solution processability, and high thermal and environmental stability. We have recently investigated a series of N-substituted NDI small molecules by incorporating various N-substituted groups for high performance perovskite solar cells (PSCs) and field effect transistors (FETs). Both NDI-1 and NDI-2 consisting of different N-substituted groups were synthesized by simple one-step reaction and exhibit high thermal stability. NDI derivatives exhibit high electron mobility in FET measurements. With single crystal structure analysis, the intermolecular interactions (e.g. hydrogen bonds, face-to-face and edge to face pi-pi interactions) and space-filling characteristics of NDI derivatives are investigated. NDI-based PSCs exhibit very high power conversion efficiency (PCE), which is higher than that of fullerene-based PSCs.

Resume : For large-area, printable, and flexible electronic applications using advanced semiconductors, novel dielectric materials with excellent capacitance, insulating property, thermal stability and mechanical flexibility need to be developed to achieve high-performance, ultralow-voltage operation of thin-film transistors (TFTs). Herein, we report on the facile fabrication of multifunctional hybrid multilayer gate dielectrics with tunable surface energy via a low-temperature solution-process to produce ultralow-voltage organic and amorphous oxide TFTs. The hybrid multilayer dielectric materials are constructed by iteratively stacking bifunctional phosphonic acid-based self-assembled monolayers combined with ultrathin high-k oxide layers. The nanoscopic thickness-controllable hybrid dielectrics exhibit the superior capacitance (up to 970 nF/cm2), insulating property (leakage current densities < 10-7 A/cm2), and thermal stability (up to 300 °C) as well as smooth surfaces (root-mean-square roughness < 0.35 nm). In addition, the surface energy of the hybrid multilayer dielectrics are easily changed by switching between mono- and bifunctional phosphonic acid-based self-assembled monolayers for compatible fabrication with both organic and amorphous oxide semiconductors. Consequently, the hybrid multilayer dielectrics integrated into TFTs reveal their excellent dielectric functions to achieve high-performance, ultralow-voltage operation (< ±2 V) for both organic and amorphous oxide TFTs.

Resume : A new technique for analysis of "Ligand partial pressure - Temperature" diagram have been developed for the metal-organic phosphor tris(8-hydroxyquinoline) M ( M=Al, Ga, In). The detection of mono- and bivariant equilibriums within the p(8-Hq)-T diagram have been done using simultaneous measurements of photoluminescence and reflection spectra of a condense phase under controlled p(8-Hq) and T. Using preparations with the purity better 99.999 wt% the corresponding p(8-Hq)-T diagrams were plotted in the temperature range from 350 K to the maximum melting temperatures. The homogeneity ranges of different polymorphs vs nonstoichiometrical composition have been established. For the alfa-phases it were demonstrated that changing the synthesis conditions within the homogeneity range of the phases resulted to the drastically changes of the nonstoichiometry, cell volumes, chemical activities and electroluminescent properties of the OLED structures, prepared on the base of alfa-Mq3. The research was supported by the Russian Foundation for Basic Research by grant 16-32-60035.

Resume : Thin films (50-100 nm) of inorganic glasses and hybrid materials (HM’s) based on inorganic glass matrices and various organic phosphors were prepared by layer-by-layer vacuum sputtering and glass or metal coated glass substrates. HM-based films with locally controlled PL and glass films with locally controlled crystalline areas were fabricated by Pharos SP laser (wavelength 1030 nm, pulse duration from 180 to 5000 fs, frequency repetition from 1 to 1000 kHz, maximum power 6 W). The HM-based films demonstrated the shift in PL wavelength due to processing of the exchange reaction between organic complexes and glass matrices. Local crystallization of glass and local formation of HM's thin films by femtosecond laser beam allowed creating 2D integrated optics elements. The research was supported by the Russian Science Foundation by grant 14-13-01074П and 16-03-00541.

Resume : To enhance the performance of inverted organic solar cells (OPVs), we have designed and synthesized novel donor-acceptor type interfacial modifiers (IMs) with high dipole moments about 10 debye, Our IMs are able to provide better surface properties for the ZnO based cathode buffer layer and achieved via a strategy that combines dual-functional self-assembled monolayer (SAM) modification of ZnO cathode buffer layer. As a result, the PTB7-Th:PC70BM based inverted OPVs treated with IMs on ZnO layer showed enhanced PCE (about 115%) compared to conventional ZnO based OPVs. These results are obtained very reproducibly regardless of the type of polymer donor material in OPVs. Also, our IMs incorporated OPVs showed enhanced thermal stability over 80% after 1,000 hr at 85 oC/85% RH condition which is much higher than conventional ZnO based OPVs (below 40%). Furthermore, we found our IMs can be used as a cathode buffer layer for high-efficiency, low-temperature ZnO based planar perovskite solar cells. The incorporation of IMs improves the quality of PbI2 layers and final perovskite layers during sequential deposition, while charge extraction is enhanced via electric dipole effects. Leveraged by IMs modification, our low-temperature ZnO based PSCs achieve an unprecedentedly high power conversion efficiency of 18.82% with a Voc of 1.13 V, a Jsc of 21.72 mA cm−2, and a FF of 0.76. Finally, we conclude that our strategy used IMs can be used to produce additional performance enhancements of OPVs and PSCs.

Resume : Organic field-effect transistors (OFETs) show great potential for the fabrication of low cost, flexible, and high-performance electronic devices in the past decade1,2. Among OFET devices, ambipolar OFETs based on organic semiconducting materials able to transport both types of charge carriers (hole and electron) have a great interest for optoelectronic applications such as solar cells and ultrathin CMOS logic circuits.3. In order to fabricate high performance ambipolar OFETs, the design and synthesize of efficient narrow-bandgap molecules is still needed. In this work, a narrow bandgap pi-conjugated molecule based on a naphthalenediimide core was successfully synthesized via the incorporation of additional π-donor (thiophene) and acceptor (benzothiadiazole) units to achieve an overall bandgap of 1.8eV, as determined by cyclic voltammetry and by DFT calculations. The ambipolar mobilities of this derivative were evaluated in field effect transistor configuration (OFET) and have been found to be significantly improved by thermal deposition. Charge carrier mobility of 0.021 and 0.032 cm2 V-1 s-1 for hole and electron respectively were obtained.

Resume : Multi-dimensional halide perovskites have emerge as a new class of optoelectonic materials. For the first time, we explore the fabrication of dimensionality-engineering organometallic halide perovskite via low-pressure vapor-assisted solution process (LP-VASP). ). It was found that 2D/3D mixed halide perovskite can be formed by LP-VASP with larger A-site organic cation. The phenyl ethyl-ammonium iodide (PEAI)-doped lead iodide (PbI2) is firstly coated onto the substrate and then reacts with methyl-ammonium iodide (MAI) vapor in the low pressure heating oven. The doping ratio of PEAI in MAI-vapor treated perovskite has significant impact on the crystalline structure, surface morphology, grain size, optical properties (UV-Vis absorption and photoluminescence spectra), and the resultant device performance. The dimensionality of the as-fabricated perovskite film reveals an evolution from 2D, hybrid 2D/3D to 3D structure when the doping level of PEAI/PbI2 changes from 2 to 0. The scanning electron microscope (SEM) images and Kelvin probe force microscopy (KPFM) mapping show that the PEAI-containing perovskite grain is formed around MAPbI3 perovskite grain to benefit MAPbI3 grain growth. The device employing perovskite with PEAI/PbI2 = 0.05 achieves a champion power conversion efficiency (PCE) of 19.10 % with a open-circuit voltage (VOC) of 1.08 V, a short-circuit current density (Jsc ) of 21.91 mA/cm2 and a remarkable fill factor (FF) of 80.36%.

Resume : We report high-performance top-gate bottom-contact flexible field-effect polymer transistors (FETs) fabricated by blade-coating of diketopyrrolopyrrole (DPP)-based and naphthalene diimide (NDI)-based polymers (P(DPP2DT-T2), P(DPP2DT-TT), P(DPP2DT-DTT), P(NDI2OD-T2), P(NDI2OD-F2T2), and P(NDI2OD-Se2)) as the semiconducting channel materials. All the polymers displayed good FET characteristics. The highest hole mobility of 1.51 cm2V-1s-1 and the highest electron mobility of 0.85 cm2V-1s-1 were obtained from P(DPP2DT-T2) and P(NDI2OD-Se2) polymer FETs, respectively. The on/off ratios exceeded 107 for both the cases. Note that, to the best of our knowledge, both values are record carrier mobilities among solution-processed flexible polymer FETs. The impacts of polymer structures on the FET performance are well explained by the features of crystallinity, edge-on orientation tendency, and interconnectivity of polymer fibrils in the film state. Additionally, we demonstrated that all the polymer-based flexible FETs were highly resistent to tensile stress with negligible change of carrier mobilities and on/off ratios in a bending test. Conclusively, we present high-performance flexible durable FETs, demonstrating high potentials of semiconducting conjugated polymers for use in the flexible electronic applications.

Resume : Mixed-cation lead mixed-halide perovskites based on FA0.83Cs0.17Pb(I0.6Br0.4)3 is currently of particular interest due to its suitable band gap allowing its immediate application on silicon solar cells by a tandem strategy. So far, solar cells based on FA0.83Cs0.17Pb(I0.6Br0.4)3 involve the use of two layers of electron transporting materials. Despite the high photovoltaic performance achieved, more simplified device structure is desirable towards large-scale and low-cost fabrication. In this work, by controlling the concentration of the perovskite precursor we achieve thickness-tunable compact FA0.83Cs0.17Pb(I0.6Br0.4)3 perovskite thin films on FTO substrates with a large grain size up to 12 microns. They are then employed to fabricate solar cells with a simplified planar structure without the use of electron-transport (ETL) layers. Functional ETL-free FA0.83Cs0.17Pb(I0.6Br0.4)3 perovskite solar cells are obtained exhibiting a PCE of 13.79% and prolonged stability, which is highly encouraging for the future large-scale fabrication of FA0.83Cs0.17Pb(I0.6Br0.4)3-based solar cells.

Resume : An organic-inorganic hybrid photodiode structure has been realized using tin naphthalocyanine dichloride (SnNcCl2) as a single organic component electron donor material on p-Si substrates. SnNcCl2 films were grown by thermal sublimation under high vacuum condition (at 10-7 mbar) using molecular beam deposition (OMBD) technique. The growth mechanism of SnNcCl2 films has been studied as a function of substrate temperature using atomic force microscopy (AFM) and x-ray reflectivity (XRR). The bias dependent external quantum efficiency (EQE) measurement of a typical device exhibited two broad peaks extending from 400 nm to 700 nm and 900 to 1100 nm with maximum responsivities of 0.21 A/W at 560 nm and 0.15 A/W at 980 nm at –6 V bias. The observed results demonstrated the capability of the devices to be used as a broadband photodetector in visible to NIR wavelength range. All the combined results of optical studies such as ground state absorption spectroscopy, photo luminescence (PL) spectroscopy and ultrafast transient absorption spectroscopy (UTAS) confirmed the generation of triplet excitons. Moreover, It is established that the free charge carriers generated due to dissociation of triplet excitons are responsible in achieving high EQE in mid-visible range around 550 nm. Finally, a broad photo-response from 300 nm to 1100 nm with E.Q.E and detectivity greater than 50 % and 1012 jones, respectively, throughout the region has been achieved at –4 V bias by co-evaporating PTCDI-C8 with SnNcCl2 on textured p-Si substrates. The peak responsivity of the ultra-broadband device showed a typical value of 0.6 A/W at around 980 nm which is better than earlier reports.

Resume : Conjugated polymers are widely used in material sciences, from organic-field effect transistors (OFETs) to polymer solar cells (PSCs) and organic light-emitting diodes. This is possible due to the ability to fine-tune the optoelectronic properties of semi-conducting polymers. It has been shown that the incorporation of a fluorine atom in the polymer backbone is an efficient way to stabilize the HOMO and LUMO levels and reduce the bandgap of conjugated polymers. Furthermore, due to non-covalent interactions of the fluorine moiety, we observe increased of the π - π stacking of the material, which generally leads to greater charge mobility. Indeed, fluorinated polymers are among the best for PSCs, with power conversion efficiencies exceeding 10%. In this study, we developed a new series of copolymers and terpolymers based on fluorinated dithienyldiketopyrrolopyrrole (fDPP) units for applications in PSCs and OFETs.[1] Moreover, we synthesized the nitrile homologue, where the nitrile replaces the fluorine moiety, to compare the effect of the electron-withdrawing functional group on the optoelectronic properties of the resulting material. Results show polymers with deeper conductive and valence band, which lead to a higher open-circuit voltage in photovoltaic devices. These results help to better understand the role of fluorination on the properties and performances of conjugated polymers in organic electronic applications. [1] Bura, T. et al., Macromolecules 2017, 50, 7080

Resume : The design simplicity, lager areal integration to the integrated circuit and amplification the capability of inferior signal make organic field effect transistors (OFETs) are more engaging for physical, chemical or biological sensing applications, where active and dielectric materials are chosen in such a way that one of them or both are sensitive to a specific stimulus and the immediate effect of it can be extracted easily. In this work, we are going to present flexible OFETs based temperature sensor suitable for wearable medical applications. Here we have used Alumina (Al 2 O 3 ) and strontium titanate (STO) to make a suitable bilayer dielectric system where strontium titanate (STO) film used as temperature sensitive materials. Low-cost solution process has been used to prepare STO based dielectric ink. This ink can be spin coated or screen printed on top of Al 2 O 3 layer to make suitable bilayer dielectric combination. We have fabricated OFETs on top of ultra-thin (10 m) flexible substrate which is working around 1.2V with the highest observed mobility of 1.4 cm 2 /Vs. The temperature sensing properties in these devices have been attributed to the variation of capacitance of our bilayer dielectric system with temperature. As temperature increases, the capacitance of the dielectric system increases. As a result, the charge accumulation at the dielectric/semiconductor interfaces enhances due to the applied gate field and also enhances the sensitivity of temperature measurement. We have found the response and recovery time of our device 400 ms and 580 ms, respectively. The devices were tested to monitor the changes in body temperature of mice under anaesthesia doses. The sensitivity measured in these devices is about 5 milli-Kelvin.

Resume : Nowadays, there is much effort focused on the improvement of organic field effect transistors (OFETs) performance due to their high potential as new devices for low-cost and flexible electronics. The improvement of OFETs performance and stability have been widely discussed and it remains a current challenge.[1] Recently, the blending of the organic semiconductor (OSC) with an insulating polymer has been shown to be a promising route to improve the OFETs performance.[2] However, it is crucial to understand the physical phenomena behind it better so we can optimize the fabrication processes. In the present work, a p-type (DPTTA)[3] and a n-type (PDI8CN2)[4] OSCs have been blended with an insulating polymer (PS) and integrated in OFETs. The devices were fully characterized morphologically and electrically, including the calculation of the density of traps. In both cases, it was observed that a lower density of traps was achieved when the PS matrix was employed due to a vertical phase separation of the two materials during the crystallization process. This was in agreement also with the results obtained from a bias stress stability study and the extraction of the density-of-states (DoS). The study concludes hence that the OSC/dielectric interface is much better in the blended materials, obtaining better subthreshold swing and threshold voltage, due to the fact that the PS layer prevents the direct contact of the OSC with the SiO2. References: [1] H. Sirringhaus, Adv. Mater. 2009, 21, 3859. [2] K. Zhao et al. Adv. Funct. Mater. 2016, 26, 1737. [3] A. Campos et al. Adv. Electron. Mater. 2017, 1700349.. [4] B. A. Jones et al. J. Am. Chem. Soc. 2007, 129, 15259.

Resume : Two-dimensional (2D) transition metal dichalcogenides (TMDs) have attracted great attention for potential applications in electronics, optoelectronics, sensing, etc. Among them, monolayer MoS2 is one of the most investigated materials and many approaches have been explored for tuning its electrical and optical properties. In our work, we have studied the effect of a series of benzene derivatives on both mechanically exfoliated and CVD-grown monolayer MoS2. Our photoluminescence (PL) spectroscopy studies revealed that physisorbed benzene derivatives influence the fraction of charged (trion) and neutral exciton by surface charge transfer depending on their functional groups and dipoles. DFT calculations confirmed that fluorinated aromatic molecules lead to p-doping in monolayer MoS2 whereas methyl-substituted ones induce n-doping, which is in accordance to experimental data. We have observed in the corresponding field-effect transistor (FET) devices that as-investigated aromatic molecules have produced changes in charge carrier density, charge carrier mobility, threshold voltage, on/off ratio and transfer hysteresis. Our study has demonstrated versatile routes to tune the optical and electrical properties of TMDs, opening intriguing perspectives for various applications in opto-electronics.

Resume : Scientists have shown improved interesting in semiconductor due to its functional properties in last several decades. Semiconductor can be used in various industries including solar cell, photovoltaic, computer chips and others important applications. hence organic semiconductor becomes an important field of research due to its potential1. Compared to inorganic semiconductor, organic semiconductor has better light harvesting ability2, in the other word, photovoltaic device can be manufactured in thinner form1. What is more, organic semiconductor is more flexible compared to the inorganic semiconductor. Narrow bandgap material is desired for photovoltaic device which will discuss more in detail in the following part) and the narrow bandgap can be achieved by synthesis conjugated polymer/molecules3. People thus have improving interesting in researching organic semiconductor. Charge transport in organic semiconductor is different from the inorganic semiconductor. Organic semiconductor is basically comprised by series conjugated polymers/molecules (which contained carbon and hydrogen atoms). Such Polymers/molecules are made up by linear series of overlapping pz orbitals with sp2 or sp hybridization, then conjugated chain of delocalized electrons are created4. Charge carriers move freely in the backbones; however, it is hard for the charged transport between adjacent polymer since it was precluded by the van de waal force1,5. Organic semiconductor is thus having lower mobility than the inorganic semiconductor; Low mobility may cause combination of electrons and hole which make the photovoltaic device inefficient6. What is more, organic semiconductor charge carriers transport by hopping between adjacent polymer. The energy gap in conjugated molecules is typically 1.5-3.0 eV7, charge transport happens when the active energy is above the energy gap. Furthermore, smaller band gap allows carrier transport easier in organic semiconductor; Because it is easier for charge carrier jump from HOMO to LUMO. Thereby low bandgap polymer for organic semiconductor is an attractive field to be researched. ? Energy gap tuning Theoretically, bandgap can be narrowed by increasing the length of conjugated polymer or fusing more aromatic rings in the polymer backbone1. However, such methods become ineffective once the length of polymer exceed certain point. There are two chemical ways that are mainly used to shrink the bandgap (a?stabilizing the quinoid resonance structure and (b) utilizing donor?acceptor interactions8. There are two resonance structures including quinoid and aromatic. Bandgap can be shrunk by both structure; however, quinoid can shrink the bandgap more than aromatic, meanwhile, quinoid structure is relatedly unstable compared to the aromatic1. But research has revealed that quinoid structure can be stabilized by fuse aromatic rings into the polymers9. In case of utilizing donor and acceptor interaction approach, the mechanism of such approach. When the alternative acceptor and donor was synthesized, two LUMO are interacted each other and new LUMO is generated; New HOMO is generated in the same way. New bandgap is then created and smaller than the old bandgap. ? Organic heterojunction Organic heterojunction is where the absorption layers, consisted of donor and acceptor material. Such multiple layers determine act of the excitons (electrons-hole pair). In the organic semiconductor device, photons are absorbed within heterojunction, creating excitons. Excitons are separated into charge carriers within he heterojunction. Hence, combination of donor and acceptor polymer have significant effect on the conversion coefficient of photon into photoelectrons. In this case, fine nanostructure of the heterojunction is required. Interfacial charge recombination happen within the heterojunction and it can be reduced by larger domains. Larger domains enable more ordered molecular packing as well, hence development of charge carrier mobilities10. ? Hybrid interface Hybrid interface attract researcher attention as well due to its mechanical flexibility and photosensitivity. Hybrid interface can be used to improve the light absorption efficiency and the electron conductivity, which may has significant effect on the semiconductor performance. ZnO is one of the common material that used in semiconductor interface. Nano-ridge patterning of ZnO in an inverted polymer solar cell was shown to improve the power conversion efficiency by almost 25% compared to planar ZnO nanoparticle based films11. Such a patterning strategy increases the interfacial area for electron collection. However, ZnO may cause electron inhomogeneity, which often mitigated by modifying the surface or apply buffer layer between the ZnO film and active layer of semiconductor12. Reference 1. Letian Dou, Yongsheng Liu, Ziruo Hong, Gang Li, and Yang Yang. (2015) Low-Bandgap Near-IR Conjugated Polymers/Molecules for Organic,Electronics. Chemical reviews. 115(23), 12633-12655. Available from http://pubs.acs.org.iclibezp1.cc.ic.ac.uk/doi/abs/10.1021/acs.chemrev.5b00165 [Accessed in 01/12/2017] 2. Brabec, J.; Sariciftci, N. S.; Hummelen, J. C. Plastic Solar Cells. Adv. Funct. Mater. 2001, 11, 15?26 3. Sun, S.-S.; Dalton, L. R. Introduction to Organic Electronic and Optoelectronic Materials and Devices; CRC Press: New York, 2008. 4. Pope, M.; Swenberg, C. E, (1999). Electronic Processes in Organic Crystals and Polymers; Oxford Univ.: UK. 5. Köhler; Bässler (2012). "Charge Transport in Organic Semiconductors". Topics in Current Chemistry. 312: 1. 6. Kok Haw Ong, (2010). LOW BAND-GAP DONOR POLYMERS FOR ORGANIC SOLAR CELLS. Dept. of chemistry, Imperial College. 7. Wood, Sebastian, (2014). Directly Probing Thin Film Morphology - Optoelectronic Property Relationships in Organic and Hybrid Solar Cells. Dept. of physics, Imperial College. 8. Havinga, E. E.; Hoeve, W.; Wynberg, H. Alternate donoracceptor small-band-gap semiconducting polymers; Polysquaraines and polycroconaines. Synth. Met. 1993, 55, 299?306. 9. Wudl, F.; Kobayashi, M.; Heeger, A. J. Poly(isothianaphthene). J. Org. Chem. 1984, 49, 3382?3384. 10. Ji-Seon Kim and Martin Heeney, (2015). High-Performance Organic Near-IR Photodetectors via Advanced Molecular Structure Control and Analysis 11. N. Sekine, C.-H. Chou, W.L. Kwan, Y. Yang, ZnO nano-ridge structure and its application in inverted polymer solar cell, Org. Electron., 10 (2009), pp. 1473-1477 12. R. Shivanna, S. Rajaram, K.S. NarayanInterface engineering for efficient fullerene-free organic solar cells, Appl. Phys. Lett., 106 (2015), p. 123301

Resume : The degradation of organic light emitting diodes(OLEDs) at the elevated temperatures or with temperature changes have been reported. However, in the real operation, these experiments cannot explain the degree of degradation of the devices accurately, because, the local temperatures of each cell of OLED display panels change with irregular periods. For simulating the real changes of the operating conditions, the degree and the pattern of degradation of OLED devices were analyzed by changing the period of temperature with irregularities. We induced irregular local temperatures by applying the the irregularly defined current traces to the OLED cells. Based on the analyzed data, we predicted the life time and the reliability of the performance of the devices with machine-learning algorithm.

Resume : Solar cells have obtained more and more attention over time, with many studies focusing on increasing the power conversion efficiency of the materials used, among other factors. Interest has also turned towards minimising the degradation of the solar cells to increase their potential use. More recent studies looking at perovskite solar cells have shown that there is formation of reactive oxygen species (ROS) when the material is exposed to light and oxygen. This is in large part responsible for the degradation observed in perovskite materials and solar cells [1,2]. However, the generation and role of ROS in the potential degradation of organic materials such as semiconducting polymers has not been addressed. This study will focus on the ROS superoxide, and its generation from various materials used within organic solar cells. Moreover, we will explore the effect of energetics and film morphology on the yield of superoxide generation. The goal is to study to potential correlation between the structure of the polymer material, the superoxide yield and the polymer semiconductor film’s overall stability. The results from this study are intended to facilitate and advance the use of such materials in a range of applications (e.g. optoelectronic, solar cells, and medicinal). 1.N. Aristidou, C. Eames, I. Sanchez-Molina, X. Bu, J. Kosco, M. Islam and S. Haque, Nature Communications, 2017, 8, 15218. 2.N. Aristidou, I. Sanchez-Molina, T. Chotchuangchutchaval, M. Brown, L. Martinez, T. Rath and S. Haque, Angewandte Chemie International Edition, 2015, 54, 8208-8212.

Resume : Organo-metallic hybrid perovskite materials are unique materials that feature extremely high power conversion efficiency, therefore holding great promise for low-cost and up-scalable device applications in opto-electronics.[1] However, the charge injection mechanism in such material is not fully understood and a comprehensive picture of its origin is lacking.[2,3] Field-effect devices based on methylammonium lead iodide (MAPbI3) were fabricated in which the source and drain electrodes are treated with different self-assembled monolayers. The different SAMs chemisorbed on the electrodes allow us to explore the dependence of the electrical performances from chemical composition, morphology and energy levels at the interface. The electrical performance of the perovskite was found to be strongly correlated with the wettability of the sample and the deposition technique employed. [1] X. Li, D. Bi, C. Yi, J.-D. Décoppet, J. Luo, S. M. Zakeeruddin, A. Hagfeldt, M. Grätzel, Science 2016, 58. [2] S. P. Senanayak, B. Yang, T. H. Thomas, N. Giesbrecht, W. Huang, E. Gann, B. Nair, K. Goedel, S. Guha, X. Moya, C. R. McNeill, P. Docampo, A. Sadhanala, R. H. Friend, H. Sirringhaus, Sci. Adv. 2017, 3, e1601935. [3] S. Olthof, K. Meerholz, Sci. Rep. 2017, 7, 40267.

Resume : Nano-heterojunction wires composed of organic conjugated polymers and inorganic quantum dots can be applied to the optoelectronic devices such as organic light-emitting diodes, thin-film transistors, and photovoltaics. The charge separation and transport dynamics have been investigated as a function of the shape and size of the quantum nanoparticles including dots, rods, and tetrapods. However, the location and further orientation of quantum nanoparticles along to the nanowire longitudinal axis has not been rarely reported. In this study, the CdSe quantum nanorods confined in crystallization-driven self-assembled nanowires was controlled by changing the solution processing. The gradually diffusion of poor solvent at top layer into good solvent containing conjugated polymers and quantum rods at bottom layer showed the different orientations of nanorod as well as the lengths of hybrid nanowires, which was confirmed by transmission electron microscopy tomography, and correlated grazing incidence wide-angle X-ray scattering. This research might provide a useful strategy for the fabrication of hybrid nano-heterojunction wires with customized performance in optoelectronic device.

Resume : To enhance electron injection in n-type organic field-effect transistors (OFETs), nonconjugated polyelectrolyte (NPE) layers are interposed between a [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) layer and Au electrodes. A series of NPEs based on an ethoxylated polyethylenimine backbone with various counterions, including Cl−, Br−, and I−, improve electron mobilities up to ≈10−2 cm2 V−1 s−1 and yield on–off ratios (Ion/Ioff) of 105 in PCBM OFETs. Ultraviolet photoelectron spectroscopy reveals that all of the NPEs lead to reduced electron injection barriers (φe) at the NPE/metal interface; this reduction in φe is consistent with dipole formation or n-type doping at the electrode interface. Absorption measurements of PCBM films treated with NPEs are consistent with n-doping of the PCBM. Regardless of the type of anion, thick NPE layers lead to high conductivity in the films independent of gate bias, whereas thin NPE layers lead to dramatically improved electron injection and performance. These results demonstrate that thin polyelectrolyte layers can be used to achieve controlled interfacial doping in organic semiconductors. Furthermore, this study provides valuable information about the function of NPEs, which may be exploited to improve device performance and to design new materials for future use in optoelectronic devices.

Resume : Solar water oxidation is a promising but challenging technology. Albeit its conceptual simplicity, there remain many problems for its practical application originating from intrinsic limitations of candidate materials. While inorganic materials generally have better electrical/ electrochemical properties than organic materials, they also suffer from issues of low extinction coefficient (EC) and fast recombination of excitons. In contrast, organic materials have a much higher EC in the visible light region and more functional diversity. Conventionally, however, most efforts have been devoted to the utilization of inorganic materials for solar water oxidation. In this study, we report the fabrication of efficient photoanode for solar water oxidation by hybridization of organic and inorganic materials. More specifically, we could fabricate an organic/inorganic heterojunction photoanode by deposition of semiconducting melanin doped with polyoxometalate on an n-type inorganic semiconductor. The resultant hybrid photoanode exhibited an excellent photoelectrochemical performance due to the improved light-harvesting, charge separation, and catalytic charge transfer efficiencies through hybridization. More importantly, our approach was found to be applicable to various inorganic semiconductors, such as TiO2, Fe2O3, and BiVO4. We believe that this study can provide inspiration for the design and fabrication of novel organic/inorganic hybrid devices.

Resume : Polyaniline (PANI) is one of the most investigated conjugated polymers because of its high environmental stability and unique electronic properties. The excellent properties of π-electron delocalization at the backbone of such conjugate polymer can be an initiator of charge transfer mechanism with carbon nanoparticles (CNPs) resulting oxidation-resistance. Thus, the unique combination of these two has ability to expand a class of novel composite materials having synergic charge transfer effect as well as tunibility of their properties through rational chemical synthesis. Our experimental findings impart a new possibility for ‘writing-reading’ of memory bits using electrical bias towards the application in polymeric data storage devices. CNPs have been deployed into the polymeric matrix as charge trapping sites, which has been functioned to the concept of non-destructive “electro-typing” nanoscopic data sheet. The core idea of this device is to make an electrical image through charge trapping mechanism, which can be ‘read’ further by the subsequent electrical mapping. The density of stored charges at the carbon-polyaniline layer, near the metal/polymer interface, has been found to depend on the amount of injected charge carriers. Work has been demonstrated through an understanding and specific investigation on the dynamics of localized electronic transport mechanism of CNPs-PANI composite layer using atomic force microscopy integrated with different kinds of electrical mode.

Resume : Recently, p-n junctions based on organic and 2D materials have been recognized as the easiest way to fabricate hybrid 2D van der Waals heterojunction devices for electronic and optoelectronic applications. Depositing organic materials on 2D materials is typically demonstrated by thermal evaporation that high voltage and vacuum systems are needed. In the present work, we present a simple way to fabricate p-organic/n-2D heterostructure, where Pedot:PSS was chosen to be the p-organic material due to its high conductivity, excellent film forming ability and good stability, while WS2 was chosen as the n-2D material due to its well-known properties. By systematically studying the gate dependence and temperature dependence of I-V characteristics of the Pedot:PSS/WS2 junction, it was found that the device showed a diode-like behavior with rectification ratio of ~5 and a turn on voltage of ~1V at room temperature. Furthermore, the rectification ratio of the junction reached up to ~103 at Vg=20V with Vds ranging from -4 to 4V. The maximum rectification ratio decreased with reducing temperature. On the basis of our results, we demonstrate that the simple technique introduced in our fabrication of organic/2D van der Waals heterojunction could extend to include other organics and 2D materials.

Resume : The electronic properties of the interface formed between Au and organometallic triiodide perovskite (CH3NH3PbI3) were investigated using ultraviolet photoelectron spectroscopy (UPS) and X-ray photoemission spectroscopy (XPS). The CH3NH3PbI3 films were prepared onto the Au surfaces by spin casting with various solution concentrations to control the film thickness and their morphology was examined using atomic force microscopy (AFM). The CH3NH3PbI3 film exhibited a band gap of 1.70 eV and the maximum valence band edge of 6.31 eV. The energy levels shift downward by 0.26 eV with a perovskite coverage of 50 wt% upon it, indicating the band bending at the interface. The observed energy level shift means the presence of the interface dipole exists at the Au/CH3NH3PbI3 interface. These findings are important for understanding how the perovskite materials function in electronic devices, and the design of new materials for use in perovskite-based optoelectronic devices.

Resume : Morphology stabilization of polymer donor acceptor solar cells is key to achieving high reproducibility and reliability for fundamental studies and technological applications. Morphology-stabilized crosslinked1 polymer donor network cells infiltrated with fullerene acceptors has been reported to give the desired donor-acceptor morphology with built-in phase contiguity.2 For the poly(3-hexylthiophene-2,5-diyl):phenyl-C61-butyric acid methyl ester donor-acceptor model system, this enables full mapping of the PCE, FF, Jsc, Voc vs composition – thickness landscape, and first direct experimental determination of cell built-in potential.3 Improvement in power conversion efficiency over the standard remixed biblend devices has also been achieved. However, questions about generality of this approach to current state-of-the-art photoactive layers remains unanswered. In this talk, we will discuss our recent work on rational design for acceptor infiltration, applicability to non-fullerene acceptors, and systematic in-situ investigations of crosslinked polymer donor – acceptor – solvent interactions. 1. R.Q. Png, P.J. Chia, J.C. Tang, B. Liu, S. Sivaramakrishnan, M. Zhou, S.H. Khong, H.S.O. Chan, J.H. Burroughes, L.L. Chua, R.H. Friend, P.K.H. Ho, “High-performance polymer semiconducting heterostructure devices by nitrene-mediated photocrosslinking of alkyl side chains”, Nature Materials 9, 152 (2010). 2. B. Liu, R.Q. Png, L.H. Zhao, L.L. Chua, R.H. Friend, P.K.H. Ho, “High internal quantum efficiency in fullerene solar cells based on crosslinked polymer donor networks”, Nature Communications 3, 1321 (2012). 3. B. Liu, R.Q. Png, J.K. Tan, P.K.H. Ho, “Evaluation of built-in potential and loss mechanisms at contacts in organic solar cells: device model parameterization, validation and prediction”, Advanced Energy Materials 4, 1200972 (2014).

Resume : Silicon nanocrystals (SiNCs) have gained great interest due to their outstanding optoelectronic properties, compared to their bulk counterpart. However, on the way to electrical device applications, it is necessary to thoroughly understand the electronic charge transport in thin films of SiNCs. Here we describe the fabrication and electrical characterization of Si nanogap electrode devices to study the electronic properties of hexyl functionalized SiNCs of diameter 3 nm. Planar, highly doped Si electrodes with contact separation down to 30 nm were fabricated from silicon-on-insulator (SOI) substrates, by electron beam lithography and reactive ion etching. These nanogaps were filled by mechanically pressing the SiNCs from solution via a pressure-transducing PDMS membrane. This novel approach enables the formation of homogeneous SiNC thin films with precise control of the thickness, without voids or cracks in-between the Si electrodes, leading to stable and reproducible device performance. Conductance was significantly enhanced for devices featuring thin films of functionalized SiNCs in 100 nm nanogaps, compared to an untreated device. Importantly, the measured conductance showed the anticipated scaling with nanogap width, ranging from 5 to 50 µm in size. We propose our device scheme as prototype for charge transport investigations of novel hybrid nanomaterials at the nanoscale, involving all-silicon contact electrodes.

Resume : Highly efficient perovskite devices have been demonstrated based on the mixed-cation lead mixed-halide perovskite system (FAPbI3) 1-x (MABr)x , as it allows for band gap tuning and optimisation of optoelectronic properties and stability [1][2]. However there is still much to elucidate regarding the microscopic properties of these materials that contribute to their elevated photovoltaic performance compared to single-cation perovskite systems. The mixed cation perovskite (FAPbI3) 0.85 (MABr)0.15 has been characterized through steady-state and time-resolved photoluminescence and through streak camera measurements, over a temperature range from 4 K to 330 K, in order to investigate phase behaviour and charge carrier dynamics in this material. Previous work on the widely studied perovskite MAPbI3 has shown that the monomolecular decay rate, which is attributed to trap-assisted recombination in such materials, decreases as the temperature is reduced due to the passivation of trap states, whereas the higher order bimolecular and Auger recombination rates increase [3]. Preliminary results show similar trends in the mixed-cation mixed-halide perovskite, but with some key differences in the dynamics within each structural phase. The data also provides experimental evidence for the phase transition temperatures of this mixed-cation perovskite system, with the tetragonal to cubic phase transition occurring below room temperature. [1] Jeon et al. Nature, 2015, 517, 476 [2] Baena et al. Energy Environ. Sci., 2015, 8, 2928 [3] Milot et al. Adv. Func. Mater., 2015, 25, 6218

Resume : Coumarin are an class of natural products with diverse biological activities. They have anti-inflammatory, antioxidant, anticoagulant, antibiotic and antimicrobial.1 The use of coumarin in the area of new materials has arisen the interest of research groups, the possibility of its use as sensitizing dyes in solar cells. Coumarins absorb strongly in the visible region, so they have a great chance of being good sensitizers for semiconductors with wide band gap, another factor is due to these compounds possess high quantum yield fluorescence.2 Among the methods of coumarin synthesis described in the literature we can cite the reaction of Knoevenagel, Pechmann and Perkin.3 Our research group developed a method of synthesis of coumarin derivatives by multicomponent reaction (MCR).4 In this work, we described the synthesis of dyes by MCR between 4-aminocoumarin, aldehyde derivatives and ethyl benzoylacetate, catalyzed by Lewis acids. The reaction occurs in the presence of NbCl5, AlCl3, POCl3, FeCl3 or ZrCl4, in N2, under reflux, in DCM, DCE, CAN, MeOH or THF for 5, 24 or 48h. The results show that by using NbCl5, for 24h and DCE, it was possible to obtain coumarin derivative with moderate yields. [1] J-C. Jung, O-S. Park, Molecules, 2009, 14, 4790 [2] X. Liu, J. M. Cole, P. G. Waddell, T. C. Lin, J. Radia, A. Zeidler, J. Phys.Chem.A, 2012, 116, 727 [3] H. Valizadeh, A. Shockravi, Tetrahedron Lett., 2005, 46, 3501 [4] W. H. dos Santos, L. C. da Silva-Filho, Synthesis, 2012, 44, 3361

Resume : Organometal halide perovskites have recently emerged as one of the most promising technologies for efficient and cheap solar cells.[1] The research in this type of solar cells suffered an inflexion point when the liquid electrolyte was replaced by solid state hole conductors (SSHC) which increased significantly the PCE and stability of the devices. [2] From this moment on, most of the research on perovskite based solar cells makes use of SSHC which the first and most used material is the organic molecule spiro-OMeTAD. While the use of Spiro-OMeTAD is widely spread, this material is reported to have a low intrinsic hole mobility and conductivity, that is partially solved by means of dopants or molecular modifications. In this work we present solution process perovskite based solar cells, in which the hole conductor material (Spiro-OMeTAD) has been deposited by vacuum sublimation in absence of any dopant. Surprisingly, the control in the deposition parameters of pristine Spiro-OMeTAD permits to significantly enhance the solar cell efficiency, reaching values up to 8.5% of PCE, a remarkable high value for such an undoped SSHC. 1 Ansari, M.I. et al. J. Photochem. Photobiol., C, 35 (2018) 1 2 Kim, H.-S. et al. Sci. Rep. 2, (2012) 591 3 Leijtens, T. et al. Adv. Mater. 25 (2013) 3227. 4 Nguyen, W.H. et al. J. Am. Chem. Soc. 136 (2014) 10996

Resume : Perovskite solar cells have become a very promising technology, with power conversion efficiencies exceeding 20%.1 The most common hole transport material (HTM) used in perovskite devices, spiro-O-MeTAD, has been described as a bottleneck to develop inexpensive and stable cells, due its multi-step synthesis and instability to atmospheric oxygen.2 Thus, in this work we have synthesized donor-π-acceptor anchoring groups covalently bonded to reduced graphene oxide for application as HTMs. Thiophene derivatives are used as anchoring ligands on graphene due to their relatively high hole mobility, simple synthesis, and ability to impart solubility. In addition to the good solubility and intrinsic electronic features for the ligand, the π–π interaction between the anchoring groups and graphene could potentially tailor the band gap of graphene. Here, we use ligands with a maximum of 4 thiophene units, in order to avoid aggregation and to guide electron injection.3 These new materials were fully characterized – thermal and optoelectronic properties. The effect of the substituent group according with its electronic properties is studied and electrically characterized. The reaction steps begin with graphene oxide, synthesized by graphite oxidation as an adaptation of Hummers method.4 The formed carboxylic acid groups on the graphene are esterified using diazomethane, followed by the reduction to the corresponding alcohol using LiAlH4. Subsequently, the alcohols are oxidized to aldehyde groups in the presence of DDQ. The aldehyde groups are then converted to Schiff bases promoting the conjugation of the ligand anchoring group with the graphene π system. 1Yang, S., et al, J. Mater. Chem. A, 2017, 5, 11462-11482. 2 Shi, D., et al., Science Advances, 2016. 2(4) e1501491. 3 Mishra, A., et al, Angewandte Chem, 2009. 48(14), p. 2474-2499. 4 Hummers, W.S. and R.E. Offeman, J. Am. Chem. Soc., 1958. 80(6), 1339-1339.

Resume : Given the rapid development of perovskite-based photovoltaic devices, the main challenge is the toxicity of conventional lead perovskites and the stability of the materials and devices. As what have been proved, low-dimensional lead perovskites exhibit enhanced stability which is favorable from commercial aspect. The removal of toxic lead is by the replacement with tin generally which has more severe sensitivity to oxidative attack and ambient conditions. Combining both the improvements, low-dimensional hybrid tin halide perovskites are investigated for solar cell and light-emitting diodes (LED) applications. However, the spatial confinement and poor film morphology reduce the efficiency of tin perovskite photovoltaic devices and require further study. 　 Here I will introduce background information about 2D hybrid tin halide perovskites and their application for solar cells and LEDs. Latest work on additional addictive modulated perovskite morphology will also be represented. Moreover current work on alignment of perovskite layers to facilitate charge transport will be included in specific examples.

Resume : In this paper we report the fabrication and electrical and photo-electrical characterization of polymeric and chlorophyll-a (Chl-a) thin films based photovoltaic cells (PVs). Exploiting the advantage that Chl-a has p-type semiconductor features and can be used as constitutive of both active layer and buffer layer, and poly(3-hexylthiophene-2.5-diyl) (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) have physical properties suitable for photovoltaic applications, customized architectures were prepared by spin-coating. The active layer was tailored as P3HT:PCBM + Chl-a (1:1:1) physical mixture, while in order to improve the hole collection to anode the hole selective layer (HSL) was build either as blend between poly(3.4-ethylenedioxythiophene)-poly(stryrenesulfonate) (PEDOT:PSS) and Chl-a in 1:1 ratio or as bi-layer structure Chl-a/PEDOT:PSS. Lithium fluoride (LiF) was preferred as electron selective layer (ESL), while indium thin oxide (ITO) and aluminum (Al) were used as front contact and back contact, respectively. The obtained devices were electrical and photo-electrical characterized, and their performances were discussed in terms of ITO/PEDOT:PSS/P3HT:PCBM (1:1)/LiF/Al conventional structures (CS). Parameters characterizing PVs were calculated and discussed, and the results indicate that a customized PEDOT:PSS + Chl-a (4:1) HSL improved with more than 50% the external quantum efficiency (EQE) of fabricated photovoltaic cells, compared with the results of CS. Keywords: chlorophyll-a, P3HT, PCBM, photovoltaic cells

Resume : The world energy demands for renewable and cheap resources of solar energy have generated a large interest in sensitized solar cell technology due to high power conversion efficiency with low cost of production[1]. Quantum dots (QDs) sensitized solar cell as one type of sensitized solar cell is promising for future energy conversion technology. Most materials used in QDs sensitized solar cell were based on cadmium (Cd) or other toxic materials and heavy metals such as Pb and Sn[2-4]. Here, in this work, we employ Cd-free and low toxic colloidal binary InP and InP passivated with ZnS as an alternative for future eco-friendly quantum dots sensitized solar cells (QDSSCs). These QDs were synthesized by hot injection technique under nitrogen flow to avoid oxygen contamination. This is the first time that colloidal InP-Zns QDs are successfully employed as sensitizer in QDSSCs. It is found that InP passivated by ZnS shows down-shift in band gap energy, which is responsible for broader absorption. Higher power conversion efficiency of InP-ZnS QDSSCs compared to that of single core InP is attributed to broader absorption range and faster electron injection efficiency. Under light illumination, photogenerated electron in QDs is directly injected to conduction band of TiO2 substrate since the thickness of ZnS shell is constructed to be thin as much as possible to allow the electron injected to TiO2. Photogenerated hole is transferred to electrolyte to allow the QDs reset. Reference: 1. Oregan, B. and M. Gratzel, A Low-Cost, High-Efficiency Solar-Cell Based On Dye-Sensitized Colloidal Tio2 Films. Nature, 1991. 353(6346): p. 737-740. 2. Jamshidi, A., et al., Efficiency Enhanced Colloidal Mn-Doped Type II Core/Shell ZnSe/CdS Quantum Dot Sensitized Hybrid Solar Cells. Journal of Nanomaterials, 2015. 3. Yuan, C.Z., et al., Improving the Photocurrent in Quantum-Dot-Sensitized Solar Cells by Employing Alloy PbxCd1-xS Quantum Dots as Photosensitizers. Nanomaterials, 2016. 6(6). 4. Yang, S.L., et al., InP and Sn:InP based quantum dot sensitized solar cells. Journal of Materials Chemistry A, 2015. 3(43): p. 21922-21929.

Resume : Perovskite solar cells (PSCs) with over 22% of power conversion efficiency (PCE) are attracting extensive interest in renewable energy to deliver low-cost electricity. These cells are composed of the transparent conducting layer (TCO), the electron transport material (ETM), organo-halide-perovskite as an absorber, the hole transport material (HTM), and a metal a back contact. In this work, we explored the possibility to develop efficient perovskite solar cells using inorganic n-type caesium carbonate (Cs2CO3) as ETM, which can be synthesized at low temperature (< 200 C) [1,2]. The work function of Cs2CO3 can be modified from 3.45 to 3.06 eV by an annealing treatment below 200 °C. The affinity for such material is around 2.8 eV. Also, the valance band level (Ev= 6.2 eV) could act as hole blocking layer (HBL) because of the perovskite Ev is 5.4eV. Aforementioned material properties make Cs2CO3 suitable to replace the currently used ETM which requires a very high-temperature processing. Also, both substrate and superstrate structures are possible to be prepared due to the low processing temperature of Cs2CO3. The optimum thickness of Cs2CO3 could be between1-5 nm. Solution process techniques were used to fabricate the devices. The films were structurally and morphologically characterized by X-ray diffraction and scanning electron microscope (SEM). XRD analysis showed the preferential growth of Cs2CO3 and perovskite layers. SEM results showed a full coverage of the films with fewer pinholes for perovskite layer only. Continues wavelength photoluminescence (PL) measurements, of perovskite films on Cs2CO3, showed a decrease in the PL intensity peak at 785 nm in comparison to the perovskite film on glass. This decrease may indicate that the charges created in perovskite films are injected in the Cs2CO3 layer. The I-V characteristics of the cells were carried out under 1-sun illumination. Promising results have been achieved for the cells Cs2CO3 (6 wt%)/Perovskite/Spiro with an efficiency of 8.14% (Voc= 1.023V, Jsc= 15.40mA/cm2, and FF= 51.7%), whereas, Without Cs2CO3 the efficiency did not exceed 5.25%. Very high open circuit voltage around 1.023V indicates the high quality of interfaces. Hence, it is possible to fabricate perovskite solar cells using Cs2CO3 as an alternative ETM. References: 1. Liao, H.H.; Chen, L.M.; Xu, Z.; Li, G.; Yang, Y. Highly efficient inverted polymer solar cell by low temperature annealing of Cs2CO3 interlayer. Appl. Phys. Lett. 2008, 92, 173303. 2. Xin, Y.; Wang, Z.; Xu, L.; Xu, X.; Liu, Y.; Zhang, F. UV-Ozone treatment on Cs2CO3 interfacial layer for the improvement of inverted polymer solar cells. J. Nanomater. 2013, 2013, doi:10.1155/2013/104825.

Resume : We investigate the mechanism for molecular doping in organic semiconductors with a combination of state-of-the-art many-body techniques (GW and Bethe-Salpeter formalisms coupled to atomistic polarizable models)[1,2] and a carefully parametrized model Hamiltonian. Our accurate calculations show that in typical molecular p-type semiconductors (e.g, pentacene or alpha-NPD) the dopant acceptor levels lies very deep into the gap, also for strongly electron withdrawing dopants such as F4TCNQ or F6TCNNQ. Nevertheless, the inclusion of the electron-hole interaction strongly stabilizes dopant-semiconductor charge transfer states and, together with structural relaxation (polaronic) effects, rationalize the possibility for room-temperature dopant ionization.[3] We further show that the electron affinity of dopant molecules depends on the host semiconductor, with differences up to 1 eV arising from intermolecular interactions in the solid state. Our findings reconcile available experimental data and question the relevance of the standard model implying shallow impurity levels, highlighting instead the importance of excitonic and polaronic effects for the ionization of dopant molecules. 1. Li, D'Avino, Duchemin, Beljonne, Blase, J. Phys. Chem. Lett. 7, 2814 (2018) 2. Li, D'Avino, Duchemin, Beljonne, Blase, Phys. Rev. B 97, 035108 (2018) 3. Li, D'Avino, Pershin, Jacquemin, Duchemin, Beljonne, Blase, Phys. Rev. Mater. 1, 025602 (2017)

Resume : Charge carrier mobility, µ (cm2/V.s), is commonly used to benchmark organic semiconductors. Reproducible room temperature mobility values in the range of 10 to 20 cm2/V.s have been measured, in OFETs fabricated with single crystals of best performing molecular semiconductors. Importantly, mobility values increases when lowering temperature. This is viewed as an evidence of band-like transport. However, the charge transport mechanism operating in single-crystals is still under debate. I will report on recent results, obtained in close collaboration with G. Schweicher, J. Cornil, David Beljonne, P. Samori & S. Seki. We have observed that the 2,7-isomer of BTBT exhibits a peculiar behavior as compared to the corresponding 1,4-, 3,8-, and 4,9-isomers. In a nutshell, the 2,7-isomer has an ionization potential 5.3 eV, whereas the one of the other isomers is around 5.8-5.9 eV. This is rationalized by electrostatic effects but also by a much larger delocalization of charges in the 2,7 isomer. Charge carrier mobility measured by FI-TRMC ranges from 0.1 to 0.5 cm2/V.s for 1,4-, 3,8-, and 4,9 isomers. Surprisingly, the 2,7-isomer exhibits a mobility of 170 cm2/V.s. It must be stressed that FI-TRMC measures mobility at the semiconductor dielectric interface, like in a transistor, and that charges travel back and forth, under the influence of the microwave oscillating field, over distances on the order of 1-100 nm. FI-TRMC is a promising new method that allows to probe locally the intrinsic charge transport. Energetic disorder and dimensionality of electronic interactions appear as key concepts to understand the results. Experimental results have been rationalized by quantum calculations using band-like model.

Resume : Polymer semiconductors with steadily improved electronic properties are being synthesized, achieving charge mobility in excess of 5 cm2/Vs for electrons and holes. Such performances are sufficient for a large range of applications of printed, light-weight and mechanically robust circuits, in diverse fields such as wearable electronics, smart packaging, and bio-electronics. Yet, charge transport properties in high mobility donor-acceptor polymer films is still under debate. In this presentation I will show the useful information that can be gathered on the nexus between film microstructure and electronic properties, and on the nature of charge carriers, in particular by combining electrical and morphological data with charge modulation spectroscopy (CMS) and microscopy (CMM), techniques which can selectively probe and map carriers at the buried semiconductor-dielectric interface in a working field-effect transistor (FET). In particular, I will show that CMS allows to reveal a neat two-dimensional charge transport regime in molecularly ordered, nanometers thin, Langmuir-Schäfer monolayers of an electron transporting polymer. In aligned polymer thin films, I will show how the interplay between transport in the ordered and more amorphous phases can determine charge transport regime, from purely temperature activated transport to temperature independent transport close to room temperature. As another example, I will show how local charge-modulation spectra recorded with a focused probe in different points of a polymer FET channel allow to distinguish genuine charge-induced spectral features from field-dependent spurious effects related to non-optimal charge injection, thus allowing a proper spectral assignment and a correct assessment of the nature of carriers. This allows to safely map the charge distribution of carriers in polymer FETs under different bias conditions, from unipolar to ambipolar accumulation regime. CMM also enables the investigation of the specific spatial distribution of transition dipole moments associated with holes and electrons accumulation regime over the same film microstructure.

Resume : One of the most important factors limiting electrical mobility in organic semiconductors (OSC) is strong electron-phonon coupling, which makes charge transport very sensitive to inter- and intramolecular vibrations. Specifically, low-frequency vibrations were predicted to control coherent (band-like) charge transport in high mobility OSC [1,2]. In this work, we applied Raman spectroscopy to study the impact of electron-phonon coupling on charge transport in high mobility OCS. We derived a relation between Raman intensity and the contribution of vibrational modes to local and non-local electron-phonon coupling. As a result, we formulated a figure-of-merit (FOM) for the search of OSC with weak non-local electron-phonon coupling and hence having potentially high carrier mobility. We measured FOM for a series of high-mobility OSС and found that it clearly correlates with the charge intrinsic mobility. Moreover, the FOM temperature dependence also correlates with that of charge mobility. We anticipate that the proposed approach enables an easy and practical way for fast search of high mobility OSC prior their studies in electronic devices. The authors acknowledge funding from Russian Foundation for Basic Research (projects #16-32-60204 mol_a_dk, #17-02-00841). [1] I.Y. Chernyshov et al., J. Phys. Chem. Lett., 8 (2017) 2875-2880. [2] S. Illig et al., Nat. Commun., 7 (2016) 10.

Resume : Typically semicrystalline conjugated polymers exhibit much higher mobility than their amorphous counterpart. The archetypal example is regio-random P3HT, with mobilities on the order of 10-6cm2/V.s as compared to regio-regular P3HT with mobilities on the order of 0.1 cm2/V.s. The oft-cited reason for these differences is the fact that in polymer crystallites charges delocalize and take a partial 2D character, enabling higher mobility. The amount of delocalization and how it depends on structure and processing is however difficult to measure. I will show that charge modulation spectroscopy in the IR with model materials allows to determine the delocalization of the polaron in P3HT when aided by theory. In order to extract systematic trends we will use 100% regio-regular P3HT of well-defined molecular weights (PDI~1.1). I will show how delocalization depends on molecular weight and substrate interface. Finally, I will show how polymers with the same backbone but different degree of delocalization due to side-chain chemistry give rise to different polaron signature and different carrier mobilities. Hence we are able to distinguish the effect of backbone straightening and delocalization induced by crystallinity.

Resume : Barrier-free (ohmic) contacts are a key requirement for efficient organic optoelectronic devices, such as organic light-emitting diodes, solar cells, and field-effect transistors. Here, we propose a simple and robust way of forming an ohmic hole contact on organic semiconductors with a high ionization energy (IE). The injected hole current from high work function metal-oxide electrodes is improved by more than an order of magnitude by using an interlayer for which the sole requirement is that it has a higher IE than the organic semiconductor. Insertion of the interlayer results in electrostatic decoupling of the electrode from the semiconductor and realignment of the Fermi level with the IE of the organic semiconductor. The ohmic-contact formation is illustrated for a number of material combinations and solves the problem of hole injection into organic semiconductors with a high IE of up to 6 eV.

Resume : We show how inclusion of mesoscale order resolves the controversy between experimental and theoretical results for the energy-level profile and alignment in a variety of photovoltaic systems, with direct experimental validation [1,2]. We explain how this order and interfacial roughness generate electrostatic forces that drive charge separation and prevent carrier trapping across a donor-acceptor interface [2]. Comparing several of small-molecule donor-fullerene combinations, we illustrate how tuning of molecular orientation and interfacial mixing leads to a trade-off between photovoltaic gap and charge-splitting and detrapping forces, with consequences for the design of efficient photovoltaic devices. By accounting for long-range mesoscale fields, we obtain the ionization energies in both crystalline [3] and mesoscopically amorphous systems with high accuracy [4]. [1] C. Poelking, M. Tietze, C. Elschner, S. Olthof, D. Hertel, B. Baumeier, F. Wuerthner, K. Meerholz, K. Leo, D. Andrienko, Nature Materials, 14, 434, 2015 [2] C. Poelking, D. Andrienko, J. Am. Chem. Soc., 137, 6320, 2015 [3] M. Schwarze, W. Tress, B. Beyer, F. Gao, R. Scholz, C. Poelking, K. Ortstein, A. A. Guenther, D. Kasemann, D. Andrienko, K. Leo, Science, 352, 1446, 2016 [4] C. Poelking, D. Andrienko J. Chem. Theory Comput., 12, 4516, 2016

Resume : We will review some of our recent theoretical works evidencing pinning effects at interfaces made of organic semiconductors covalently bonded to metals or oxides. Pinning implies that the HOMO or LUMO levels of related molecules (for instance oligomers of growing size or a given conjugated backbone with different electroactive substituents) are aligned in a very similar way with respect to the Fermi level of the substrate although the energies of these levels strongly vary in the isolated systems. We will show that the efficiency of pinning is strongly connected to the strength of the hybridization between the orbitals of the atoms forming the substrate and the orbitals of the atoms forming the anchoring group of the molecular part.

Resume : Organic semiconductors combine efficient charge transport with low-temperature deposition and large-area processing on flexible substrates, making them a promising class of materials for the new generation of ionizing radiation detectors. However, high-energy photon absorption is challenging as organic materials are constituted of atoms with low atomic numbers. Two approaches are here reported to increase the X-ray sensitivity. One is the development of an organic thin-film based, fully flexible, direct X-ray detectors, with sensitivity values up to several hundreds of nC/Gy at ultra-low bias of 0.2 V[1,2]. Such large sensitivity values in organic semiconductors are possible due to a photoconductive gain effect, that increases the amount of charge measured by a factor of up to 5 x 104 for each photogenerated charge carrier. We propose bulk doping by X-ray generated trapped electronic charges as the microscopic origin of the photoconductive gain. The second approach is to blend organic material with high-Z nanoparticles (NPs) to obtain a high X-ray stopping power and sensitivity[3]. The combination of both strategies is showed in the photoconversion processes in direct X-ray radiation detectors based on the semiconducting polymer poly(9,9-dioctyfluorene) blended with Bi2O3 NPs[4]. When operated in reverse bias, the detectors reach the state of the art sensitivity of 24 μC/Gy/cm2, providing a fast photoresponse. In forward operation, a slower detection dynamics but improved sensitivity (up to 450±150 nC/Gy) due to photoconductive gain is observed. [1] L. Basiricò et al., Nat. Comm. 7, 13063, 2016. [2] S. Lai et al., Adv. Electron. Mater., 1600409, 2017. [3] C.A. Mills et al., J. Phys. Appl. Phys., 46, 275102, 2013. [4] A. Ciavatti et al. App.Phys.Lett., 111, 183301, 2017.

Resume : π-Conjugated functional organic materials are envisioned as essential components of next-generation optoelectronic devices such as flexible displays, low-cost solar panels, electronic papers, printable RFID tags, and molecular sensors. These new technologies are expected to revolutionize the role of electronics in everyday life and complement current inorganic-based optoelectronic devices that have greatly impacted our society starting from the second half of the 20th century. To this end, the theoretical design and synthetic tailoring of π-conjugated architectures have been very crucial to optimize the physicochemical and optoelectronic properties of organic materials for a particular application. Herein, we demonstrate that thin-films prepared from properly designed organic semiconductors form favorable nanostructures, which can be used for a variety of organic optoelectronic applications ranging from surface-enhanced Raman spectroscopy (SERS)(1) platforms to bulk-heterojunction organic photovoltaics (OPVs)(2). 1. M. Yilmaz, E. Babur, M. Özdemir, R. L. Gieseking, Y. Dede, U. Tamer, G. C. Schatz,* A. Facchetti,* H. Usta,* G. Demirel* Nature Materials, 2017, 16, 918–924. 2. M. Ozdemir, S. W. Kim, H. Kim, M.-G. Kim, B. J. Kim*, C. Kim*, H. Usta* Advanced Electronic Materials, 2017, Online Early View (DOI: 10.1002/aelm.201700354).

Resume : Achieving ohmic contacts in organic solar cell is key to maximising the power conversion efficiency and operational stability. One approach is to employ electrodes with Fermi levels well-matched to the respective band edges of the adjacent photoactive layer. PEDT:PSSH and low work function metal have been important work-horses for hole and electron collection respectively but the work function of PEDT, in particular, is not sufficiently deep for many of the state-of-the-art donor polymers with ionisation energies approaching 5 eV or deeper. Inverted cells using evaporated MoOx and solution-processed metal oxides for hole and electron collection, respectively, have often been used to overcome this limitation. However, the work function of MoOx quickly degrades in ambient and thus limiting the use of MoOx to top contact. Here, we show both standard and inverted devices with ultrahigh work function (>5.2eV) self-compensated p-doped polymers as HCL1 and in-situ activated ultralow work function (<3.5eV) self-compensated n-doped polymers at ECL. All solution-processing in ambient has been demonstrated. The devices further revealed that work function control is key to optimisation of the cell parameters. 1. C.G. Tang, M.C.Y. Ang, K.K. Choo, V. Keerthi, J.K. Tan, M. Nur Syafiqah, T. Kugler, J.H. Burroughes, R.Q. Png, L.L. Chua, P.K.H. Ho, “Doped polymer semiconductors with ultrahigh and ultralow work functions for ohmic contacts”, Nature 539, 536 (2016).

Resume : We will present the application of different types of photochromic materials (spirooxazines, spiropyranes, dihetarylethenes) for designing photo-switchable organic field-effect transistors (OFETs). A top-contact bottom-gate OFET architecture incorporating an interfacial light sensitive (photochromic) layer sandwiched between the semiconductor and dielectric layers will be highlighted. Switching of the designed devices in electrooptical (light+gate bias) mode between two or multiple distinct electrical states will be discussed. Some of the established fundamental “material structure – device behavior” relationships will be presented. We will also demonstrate multibit optical memory elements with advanced electrical characteristics: short programming times (<1-3 ms), wide optical memory windows (10 V for OFETs operating at 1.5 V), high switching coefficients (Ksw = IDS(state-1)/IDS(state-2) >10000), good write-read-erase cycling stability and small current fading (<10% after 107 s for the best devices). The effect of the photochromic and semiconductor materials on the device performance will be considered. The experimental evidences for the trap-assisted device operation mechanism will be presented. Finally, we will show that the proposed device concept paves a route towards the development of efficient photodetectors with high current multiplication rates, which might be utilized in the emerging wearable personal healthcare stations and other exciting applications.

Resume : Self-assembled monolayers (SAMs) are a well-used tool to modify the work function of metal electrodes in organic field-effect transistors (OFETs) in order to minimise injection barriers to organic semiconductors. There is a huge range of SAM-forming molecules available to either increase the electrode work function (for p-type devices) or decrease it (for n-type devices). However, interest in ambipolar materials for light-emitting OFETs, planar light-emitting diodes or for logic circuits necessitates injection of both holes and electrons at different electrodes on the same chip. Here we present a method to assemble SAMs of different molecules on adjacent electrodes by a process of chemisorption followed by selective electrochemical desorption of SAMs from one electrode, allowing a different SAM to be deposited on this electrode subsequently. With SAMs of decanethiol and pentafluorothiophenol (PFBT) we achieve work function differences between the differently functionalised electrodes of 1.4 eV. We follow this process by Kelvin probe force microscopy and UV and x-ray photoelectron spectroscopies and incorporate these electrodes into OFETs. Our technique is further extended to photoswitchable SAMs which allow optical modulation of the electrode’s properties. By making asymmetric pairs of electrodes with SAMs of different photoswitchable molecules we show a method to introduce multi-wavelength response to electrodes for OFETs.

Resume : We report p-type charge transport in pentacene organic thin film transistors (OTFTs) using graphene oxide (GO) as the gate dielectric material. The devices were fabricated on ITO-coated glass substrate with two different metal contacts, viz. gold and silver palladium, as source and drain contact. Fabricated devices were controlled by the applied source, drain, and gate biases. Transport mechanisms of two devices were studied from output characteristics. The output characteristics in both the devices were obtained by sweeping the drain-source voltage (Vds) from 0 to -3 V in steps of 0.01 while varying the gate voltage from 0 to -3 V in steps of 0.5 V. The current-voltage data with different gate voltages were analyzed and fitted with Fowler–Nordheim (FN) tunnelling model. Multistep hopping is found to be dominant conduction mechanism with low drain-source voltage (Vds) and at very high voltage region, Fowler-Nordheim (FN) tunnelling mechanism is observed. In FN plots the slope has been calculated to find out the dependence between field enhancement factor and applied electric field. The breakdown electric field with GO as a gate dielectric is found to be 1×10^8 V/m. Field effect mobility and ON/OFF ratio of both the devices were determined from transfer characteristics. Our study suggests the suitability of GO as a gate dielectric for pentacene TFT and also provides new insights in charge transport phenomena for organic thin film transistors.

Resume : We investigated the influence of the emitter (amorphous-Si, a-Si, or polythiophene derivatives: poly(3-hexylthiophene), P3HT, and poly(3-[3,6-dioxaheptyl]-thiophene), P3DOT) and the interface passivation (intrinsic a-Si or SiOX and methyl groups or SiOX) on the c-Si based 1 × 1 cm2 planar heterojunction solar cell parameters. We observed higher short circuit currents for the P3HT or P3DOT/c-Si solar cells than those obtained for a-Si/c-Si devices, independent of the interface passivation. The cause is investigated using EQE and UV/Vis measurements. The maximum power conversion efficiency, PCE, was 11% for the P3DOT/SiOX/c-Si heterojunction solar cell. The best performing hybrid heterojunction solar cells reached an open circuit voltage (VOC) of 659 mV with the layer combination P3DOT/SiOX/c-Si due to improved wetting on SiOX of the hydrophilic 3,6-dioxaheptyl side chains of the polymer. This value is among the highest reported for c-Si/polythiophene devices. The reason for this surprisingly high VOC was investigated using wafer lifetime in combination with surface photovoltage measurements, which reveal a field effect passivation in the wafer induced by the polymer emitter. Additionally, the influence of the contact layers is examined and discussed.

Resume : Singlet fission is the process whereby one photon creates two triplet excited states. If both triplet states could be harvested by a single-junction solar cell, the solar cell efficiency would increase by up to 1/3. There has been much academic and industrial interest in developing new materials for singlet fission, but to date no material has proved ideal. Carotenoids are the most widespread of the natural pigments, important for photosynthesis, vision, human health and industry (market value $1.2bn). Work on astaxanthin [1,2] (the pigment which colours lobsters) shows that carotenoids are good candidates for singlet fission sensitizers for solar cells: they have strong absorption and fast (<100fs) singlet fission, independent of energetic driving force. There are hundreds of naturally occurring carotenoids and each of them can form a range of different dimer or aggregate structure (eg H- or J-aggregates), making energetic matching to a high efficiency solar cell possible. To determine how carotenoid structure affects singlet fission, and how to exploit carotenoids as singlet fission sensitizes, we use model systems to create identical dimer structures of a range of carotenoids. The model systems are made of synthetic ‘maquette’ proteins that hold the carotenoids in a specific dimer geometry. I will show that singlet fission is surprisingly robust in carotenoids and does not depend on either the aggregate or intramolecular structure. I will discuss the pros and cons of using carotenoids as singlet fission sensitizers. [1] Musser et al., Journal of the American Chemical Society, 137, 5130 (2015) [2] Musser et al., Journal of the American Chemical Society, under review (2018)

Resume : The ability to de-novo design peptides (short proteins segments) to self-assemble into functional structures has attracted an extensive attention in recent years, especially towards their implementation in biomedical and biotechnological applications. Many of the self-assembling peptide sequences include charged amino acids, making the resulting nanostructures amenable to proton conduction. Motivated by the opportunity to employ this phenomenon in novel bioelectronic applications, the aim of the work I will present was to discover the structural factors that govern proton transport processes in these biomimetic structures. I will demonstrate that proton transport is enhanced significantly by the introduction of a single charged amino acid into the sequence of a heptapeptide that self-assembled into fibrils.* Moreover, I will show that acidic residues are more effective than basic ones in promoting proton-conduction of the peptide fibrils due to two orders of magnitude larger doping effect, and a threefold higher charge carrier mobility value. I will further demonstrate that the structural motif of the monomeric peptides has a critical impact on proton conductivity of their assemblies, as well. I will show that D,L a-cyclic peptide assemblies exhibit superior conductivity and better thermal stability than assemblies of linear peptides with a similar sequence. *Silberbush, O., Amit, M., Roy, S., Ashkenasy, N., Adv. Funct. Mater. 2017, 27 (8).

Resume : The knowledge of the electromechanical response of organic semiconductors to external stresses is not only interesting from a fundamental point of view, but also necessary for the development of real world applications, ranging from mechanical sensors to wearable or biocompatible devices. The common interpretation of the electrical response of organic semiconductor crystals to mechanical stress relies on the assumptions that (i) deformation affects charge mobility mostly along the strained direction, and (ii) compressive strain increases mobility via a reduction of intermolecular distances and the associated increase of electronic overlap, with tensile stress producing the opposite effect. Here [M. Matta et al., Mater. Horiz. 5 40 (2018)] we demonstrate how this interpretation is oversimplified, by means of multiscale modeling predictions and experimental measurements. For the latter, an original configuration is adopted, where a single crystal field effect transistor is assembled on top of a flexible polymeric cantilever. In particular, our simulations reveal that uniaxial strain conditions can give rise to unusual responses, namely mobility hardly changing or even decreasing while compressing. Moreover, both calculations and experiments show that the electro-mechanical responses along the directions exhibiting higher mobility and closer packing are strongly coupled: if strain is applied along one axis, mobility strongly varies also along the other axis.

Resume : Peptide based molecules have shown to interact with charged molecules, like DNA and various polar molecules. However, charge transfer through peptide thin films is not favourable. As a result, fabrication of OFET using peptide molecules are not explored. We have attempted to fabricated peptide based organic field-effect transistors combining Pentacene molecules as one of the semiconductor channel materials. In this work, we fabricated ArTAA_Py-Py Pentapeptide - pentacene based bilayer organic thin film transistor for volatile organic compound (VOC) vapours sensor with selectivity and enhanced sensitivity. Ultra-thin ArTAA_Py-Py Pentapeptide molecules of different thickness were grown using spin coating on pentacene semiconducting films. The device designs are optimized to enhance the detection performance. We have studied the morphology of the films using atomic force microscopy (AFM) and also optical properties of the films were studied using UV-Vis spectroscopy. The gas sensing performances of different thickness devices were tested for different volatile organic compound (VOC) vapours relevant to environmental monitoring, such as, ethanol, 2 propanol and acetone at room temperature (RT). It was observed that sensitivity of OFET increased when reduced the thickness of peptide film. From literature we have seen that these VOCs are generally detected at higher temperature but it is indeed interesting to emphasize that we could able to sense those gases efficiently at RT. Additionally, the OFET based devices exhibit higher selectivity, enhanced sensitivity with comparably fast response time (~ 3sec) and recovery time (~4sec). Gas sensing results confirmed that ArTAA_Py-Py Pentapeptide based OFETs show excellent response towards ethanol gas at room temperature.

Resume : Photo(electro)catalytic hydrogen evolution reaction (HER) together with oxygen reduction reaction (ORR) have the potential to produce H2O2; a simple case for the creation of environmentally friendly fuels in high demand for today’s energy industry. It is therefore of great interest to create cheap and photo(electro)catalysts on large scales that can be applied without causing harmful pollution. Here we introduce small molecule organic semiconductors based on hydrogen-bonded pigments (epindolidione EPI and quinacridone QNC) as efficient photo-catalysts. Using thin films of either material as an electrode in a photo-electrochemical reaction led to H2O2 evolution with efficiency that outperformed all other previously described photocatalytic materials*. These results encouraged us to expanded the research: (i) Combining HER and ORR H2O2 evolution was achieved using colloidal nanocrystals with improved H2O2 generation rate as a result of increased surface:volume ratio (ii) Photo(electro)catalytic HER on thin film electrodes fabricated from EPI. Similarly, improving film fabrication to achieve higher surface area improved incident photon to hydrogen conversion rates for in this case (HER while applying 0V overpotential vs Ag/AgCl). Our research shows that conventional, nontoxic organic pigments common in our daily lives can be applied to efficiently create renewable energy via the reduction of O2 to H2O2 or H+ to H2. 1. M. Jakešová, et al. Adv. Funct. Mater. 2016, 26, 5248–5254

Resume : Despite the recent success of optoelectronic organic devices such as photovoltaic cells, the fundamental understanding of the involved photo-physical processes is still incomplete. This is mainly due to the complex microstructure of blends utilized in customary devices that hampers precise microscopic interface studies. Polycyclic aromatic molecules such as oligoacenes are versatile building blocks which form crystalline films and can also be modified chemically, e.g. by fluorination which turns the p-type pentacene (PEN) into the n-type per¬fluoro¬pentacene (PFP). By tuning growth kinetics and employing template effects, crystalline molecular films of defined molecular orientation and even selective polymorphs can be prepared. They allow for detailed excitonic studies of the various phases [1,2], while temperature dependent measurements indicate further a notable strain at the interface with inorganic supports [3]. For the case of PFP on alkali halides, it is possible to prepare hetero-epitaxial organic films, which allow for polarization and directional resolved optical measurements on individual crystalline molecular domains [4]. This enables in particular an experimental investigation of the correlation between molecular packing motifs and singlet-exciton fission processes. Using time- and polarization-resolved pump-probe experiments a pronounced exciton fission efficiency is only found along the crystalline direction where -conjugated molecules are slip-stacked [5]. In the last part, structural and optical properties of molecular acceptor/donor hetero-systems are discussed at the examples of PEN/PFP and PEN/C60. Using the concept of templated film growth again affords the realization of well defined molecular interfaces with different relative molecular orientations. While this yields highly oriented PEN/PFP hetero-stacks, either in standing or lying molecular orientation [6], evidence for the formation of Diels-Alder adducts between C60 and pentacene is found [7]. Time resolved luminescence and linear absorption spectroscopy are performed to explore the energetics and dynamics of charge transfer excitons at the interfaces [8]. [1] I. Meyenburg et al. Phys. Chem. Chem. Phys. 18, 3825 (2016). [2] T. Rangel et al. PNAS 115, 284 (2018). [3] L. von Helden, T. Breuer and G. Witte, Appl. Phys. Lett. 110, 141904 (2017). [4] T. Breuer and G. Witte, Phys. Rev. B 83, 155428 (2011). [5] K. Kolata et al. ACS Nano 8, 7377 (2014). [6] T. Breuer and G. Witte, ACS Appl. Mater. & Interfaces 7, 20485 (2015). [7] T. Breuer, A. Karthäuser, G. Witte, Advanced Material Interfaces 3, 1500452 (2016). [8] A. Rinn et al. ACS Appl. Mater. & Interfaces 9, 42020 (2017).

Resume : Organic photovoltaics (OPVs) are a promising solar technology because of their unique potential to give lightweight, flexible and low-cost solar cells. Devices with power conversion efficiency (PCE) of more than 10% are being regularly reported using bulk heterojunction (BHJ) device architectures.[1-3] Device efficiency depends on exciton harvesting (leading to charge generation) and charge extraction, and both these processes are very sensitive to BHJ morphology. Charge generation occurs at the interface between the electron donor and acceptor, and for efficient exciton harvesting the donor and acceptor domain sizes should be smaller than the exciton diffusion length (LD) which is typically rather short, about 5 nm.[4] However, small domains have a large donor-acceptor interface area which enhances the encounter probability of non-geminate charge pairs and charge recombination.[5] This means that for a given exciton diffusion length, as domain size is varied there is a trade-off between exciton harvesting and charge extraction. Developing a way of increasing exciton diffusion length would overcome this trade-off by enabling efficient light harvesting from large domains, which would then also give improved charge extraction. Here we controlled morphology, structural order and crystallinity of the active layer of two-dimensional small molecules named SMPV1 and DR3TBDT by solvent vapor annealing. We found an enhancement in both the exciton diffusion length and also in the domain size. This engineered increase of the exciton diffusion length in combination with the larger domain size leads to efficient light harvesting and charge extraction, and hence a substantial (20%) increase in device efficiency. In both molecules, carbon disulfide (CS2) shows the most promising results, with more than three-fold enhancement in exciton diffusion coefficient (D) and two-fold enhancement in exciton diffusion length. The optimized CS2 annealed devices consistently show a PCE between 7.0 and 7.7%, and a charge extraction efficiency above 80% at short circuit conditions which is enabled by a large average domain size of about 30 nm. Large domain size would normally reduce exciton harvesting but in our case higher device efficiency is obtained because of the increase in exciton diffusion length. Our results suggest that larger domains are preferential in photovoltaic blends provided that sufficiently long exciton diffusion is achieved. Hence we show that control of processing conditions can enhance both exciton diffusion length and domain size in organic semiconductors blends. [1] S. Zhang, L. Ye, J. Hou, Adv. Energy Mater. 2016, 6, 1502529. [2] J. Zhao, Y. Li, G. Yang, K. Jiang, H. Lin, H. Ade, W. Ma, H. Yan, Nat. Energy 2016, 1, 15027. [3] L. Lu, T. Zheng, Q. Wu, A. M. Schneider, D. Zhao, L. Yu, Chem. Rev. 2015, 115, 12666. [4] O. V. Mikhnenko, P. W. Blom, T.-Q. Nguyen, Energy & Environmental Science 2015, 8, 1867. [5] G. J. Hedley, A. Ruseckas, I. D. W. Samuel, Chem. Rev. 2017, 117, 796.

Resume : Solution-processable all-small-molecule organic solar cells (OSCs) have shown dramatic progress in improving stability and photovoltaic efficiency. However, knowledge of photoexcitation dynamics in this novel class of materials is very limited. To fully exploit the design capacities inherent in small molecule chemistry, the elementary processes and branching yields must be known in detail. Here, we present a combined computational–experimental study of photoexcitation dynamics of a prototypical all-small-molecule photovoltaic blend, p-DTS(FBTTh2)2 as a donor and NIDCS-MO as an acceptor. Femtosecond spectroscopy data show that excitonic coupling is small and that the charge transfer states are localized, at first glance contradicting the high internal quantum efficiency (IQE) and open circuit voltage (VOC) of this material. A target analysis of the femtosecond spectra yields exciton dissociation rates of 1/(25 ps) and 1/(100 ps) for the as-deposited and annealed blend, respectively. These rates are far slower than in typical polymer based organic solar cells. Still, internal quantum yields are high because parasitic quenching processes are found to be even slower. In the framework of semi-classical Marcus theory, we demonstrate that our system shows near-optimum energy conversion and charge separation yields, due to negligible activation energy for charge generation but high activation energy for charge recombination, allowing enough time to separate localized charge transfer states. We thus justify both the high internal quantum yields and the high open circuit voltage found in this system. Finally, we predict that highly efficient and stable low-optical bandgap systems can be realized by reducing the electronic coupling between the donor and acceptor.[1] [1] J. Shi et al., Energy Environ. Sci., 2018, 11, 211

Resume : Poor performance in organic photovoltaics has often been accredited to the low dielectric function of organic materials impacting on poor charge separation. We have grown organic co-crystals of perylene-TBPA which undergo a phase change at 250K; while the low temperature phase has a typically low dielectric permittivity of 6, the value of the permittivity in the high temperature phase is substantially higher at 20 [1]. By single crystal x-ray diffraction, we show that perylene-TBPA exhibits a molecular reorientation of the acceptor molecule, TBPA, in the high temperature phase. The high dielectric permittivity originates from the large dipole of TBPA and its reorientation with the electric field, which disappears in the low temperature phase. Such a high permittivity is promising for applications in organic electronics and by analysing the material in both high temperature and low temperature phases it is possible for us to directly assess the relationship between the TBPA dipole reorientation and charge transfer dynamics in the material. Using optical reflectivity and time resolved spectroscopy at different temperatures, we show how the large change in dielectric permittivity is influencing the photophysics of this system, which is of potential interest for photovoltaics and organic electronics as a whole. 1. Harada, J., et al. Journal of the American Chemical Society, 2015. 137(13): p. 4477-4486.

Resume : Two small push-pull π-conjugated organic molecules have been studied in the context of their use as donors in bulk heterojunction (BHJ) organic solar devices. In vacuum-evaporated solar cells SA321 showed a better power conversion efficiency (4.75%) with respect to TV38 (2.2%). Along with their electron-acceptor counterpart, i.e. a soluble PCBM fullerene, the SA321 molecule based photovoltaic cells have higher efficiency, irrespective of whether the active layer is solution-processed or vacuum-processed. It has been suggested that this improvement arises from improved exciton diffusion. With this study we want to depict the multilevel computational protocol designed to provide a theoretical rationalization for the SA321 and TV38 electronic and excitonic properties at the solid-state. In order to gain access to the morphology of these materials, we performed classical Force Field (FF) Molecular Dynamics (MD) simulations on the pristine SA321 and TV38 molecules. On these morphologies, we carried out Time Dependent Density Functional Theory (TD-DFT) and MicroEletrostatic calculations to evaluate, respectively the conformational and the electrostatic component to the first singlet excited state energetic disorder. Then, we evaluated the internal and external reorganization energies as well as the excitonic couplings between neighboring molecules that were injected into the energy transfer rate expression taken from the semiclassical Marcus theory. Based on these rates, Kinetic Monte Carlo (KMC) simulations were performed which yielded the diffusion coefficient and the exciton diffusion length.

Resume : The current–voltage characteristics of semiconductor diodes and solar cells can be described by the Shockley equation. For real devices, the dominant bulk charge carrier loss mechanism can lead to a voltage dependent saturation current density. The latter can alternatively be replaced by a constant prefactor in combination with the introduction of an ideality factor ≠ 1 in the voltage dependent exponent [1]. However, real devices can also show shunt and series resistance, as well as extraction limitations due to low conductivities [2], which all influence the current-voltage characteristics and, indeed, the apparent ideality factor. It is clear that the ideality factor can only be a reliable figure-of-merit for devices if the contributions of the dominant recombination mechanism and other limitations can be distinguished. Here, we discuss under which conditions the ideality factor does contain information on the bulk recombination and to which extent the description of the ideality factor can be generalised to include extraction limitations. [1] K. Tvingstedt and C. Deibel. Adv. Ener. Mater. 6, 1502230 (2016) [2] Würfel, Neher, Spies, Albrecht, Nature Communications 6, 6951 (2015)

Resume : In organic solar cells continuous donor and acceptor networks are considered necessary for charge extraction, whereas discontinuous neat phases and molecularly mixed donor-acceptor phases are generally regarded as detrimental. However, the impact of different levels of domain continuity, purity and donor-acceptor mixing on charge transport remain only semi quantitatively described. Here, we study co-sublimed donor-acceptor mixtures, where the distance between the donor sites is varied in a controlled manner from homogeneously diluted donor sites to a continuous donor network. Using transient measurements, spanning from sub picoseconds to microseconds we measure photo-generated charge motion in complete photovoltaic devices, to show that even highly diluted donor sites (5.7-10% molar) in a buckminsterfullerene matrix enable hole transport. Hopping between isolated donor sites occurs by long range hole tunneling through several buckminsterfullerene molecules (~4 nm). In addition, we find that at larger electric fields, a significant fraction of holes is extracted directly via the fullerene phase. Hence, these results question the relevance of ‘pristine’ phases and whether a continuous interpenetrating donor-acceptor network is the ideal morphology for charge transport.

Resume : Deoxyribonucleic acid (DNA) was successfully incorporated as a nano-layer at the interface between the indium tin oxide (ITO) transparent electrode and the photoabsorbing polymer blend film in organic solar cells. An initial strong improvement in open circuit voltage (from 0.39V to 0.73V) and power conversion efficiencies (PCE from 2% to 5%) was obtained by spin casting a 1-6 nm thick DNA electron extracting layer (EEL) [1]. Subsequently, we reached state of the art efficiencies of 8.3% by developing metal oxide/DNA composites as EELs leading to solar cells that are better than those with only metal oxide (i.e. ZnO) or DNA interlayers alone, and even with PEIE analogues [2]. We also succeeded in transferring this device structure on plastic substrates (PET/ITO), with similar results, as well as in perovskite solar cells. Via Kelvin probe, atomic force and scanning tunneling microscopy, we show that the DNA nanolayer improves the rectifying ratio and shunt resistance of the cells, as well as lowering the work function of the electron-collecting contact via the formation of an interfacial dipole. DNA layers were deposited in benign water/alcohol solvents at room temperature. These results show that natural materials, in this case DNA, can be incorporated in bio-hybrid organic semiconductor devices delivering uncompromising and even improved levels of performance. [1] J. Dagar et al. ACS Energy Lett.1, 510 (2016). [2] J. Dagar et al., Nanoscale, 9, 19031 (2017).

Resume : The working mechanism of organic semiconductor devices crucially depends on the precise energy level alignment of different organic layers. The transport levels in such devices directly correlate with molecular energies, namely the ionization energy (IE) and the electron affinity (EA). However, a prediction of solid-state IE and EA values from molecular structures is challenging because they are sensitive to film morphology and composition. In a comprehensive photoelectron spectroscopy study on various small molecule semiconductors, we observe that changes in IE as a function of molecular orientation or mixing ratio in thin films are proportional to the π-π-stacking component of the molecular quadrupole tensor (Qπ), calculated within density functional theory. The results demonstrate how the underlying electrostatic interactions enable a precise energy level alignment at organic heterojunctions by tuning a single molecular parameter. We prove the relevance for device application by exploiting this effect in solar cells consisting of ternary mixtures of two donor molecules (ZnPc, F4ZnPc) and one acceptor molecule (C60). The open-circuit voltage can be continuously tuned with the mixing ratio of the two donor molecules [1]. Furthermore, the adjustment of Qπ of the donor induces an electrostatic gradient at the interface and alters the dissociation barrier of charge transfer states, reflecting in a change of the short-circuit current. [1] Schwarze et al., Science 352, 1446 (2016)

Resume : For organic electronic applications, π-conjugated polymers have many advantages over their inorganics counterpart, such as easily modulated optoelectronic properties, solubility in organic solvents and mechanical properties of plastics. This synergy enables the possibility to develop efficient, flexible and lightweight devices using low-cost and large-area roll-to-roll printing. The fabrication of organic electronic devices is effectively cheaper and more environment friendly than of inorganic ones. However, some drawbacks remain in such devices due to synthetic procedures used to obtain many of the polymers used. To reach the low-cost and eco-friendly potential of organic electronics, the direct (hetero)arylation polymerization (DHAP) is a solution. Not only does this technique reduce the number of synthetic steps via the formation of C-C bonds between (hetero)arenes and (hetero)aryl halides without organometallics intermediates, it also has the capacity to achieve high molecular weight and high-performing materials and with optimized conditions defect-free polymers are achievable. Here, we present a high-molecular weight PNDIOD-T2 via a DHAP which had demonstrated better performances than analog from Stille coupling, the commercial polymer N2200. It is shown that with air-aging of the active layer (PBDB-T:PNDIOD-T2 ) of inverted all-polymer solar cells, a PCE of 7,3% have been achieved without the utilization of additives which is encouraging for large-scale production.

Resume : The power conversion efficiencies of organic and perovskite photovoltaic devices continue to rise, but efforts to scale-up and commercialise have been frustrated by the twin challenges of defects and degradation, which limit their performance and lifetime. The characterisation of defects is a key experimental challenge, which we have addressed by developing a suite of complementary spatially-resolved techniques to correlate optical appearance, physical structure, chemical composition, and local electrical properties. This includes the novel technique of transient photovoltage/photocurrent mapping that provides unique insight into the natures of defects in terms of local charge carrier dynamics with ~50 ?m resolution. In the case of solution deposited organic solar cells we use this method to distinguish light scattering defects, from defects inhibiting efficient charge extraction. Applying spatially resolved characterisation techniques in situ to operational devices enables us to monitor degradation processes, and by accurately controlling the stresses applied (environmental composition, temperature, light exposure), we are able to distinguish different degradation mechanisms. Mapping the photocurrent generation of organic and perovskite solar cells under stress enables us to correlate the overall loss in performance with specific local degradation sites and hence identify ways to improve the stability.

Resume : Recent studies [1,2,3] have challenged the view that free-charge generation in all-organic blends predominantly occurs on ultrafast time scales via delocalized hot charge transfer states. However, the mechanisms behind very efficient and weakly electric field-dependent charge separation on longer time scales remain highly debated. We investigate field-dependent charge separation in a one-dimensional lattice model of an all-organic bilayer. Charge separation is considered as a sequence of phonon-assisted transitions among eigenstates of an electron-hole pair supported by the interface. We find that the yield of charge separation starting from the strongly bound charge transfer state is weakly field-dependent and above 50% even at zero electric field. Moderate diagonal static disorder and good charge delocalization increase the yield by strengthening the transitions from the initial state toward long-lived states with increased electron-hole separation, from which further separation proceeds without kinetic obstacles. We also observe almost field-independent (at low to moderate fields) charge separation starting from a donor exciton state, while the yield varies from around 40% (from the lowest-energy donor state) to around 80% (from states energetically well above the lowest-energy donor state). [1] Vandewal et al., Nat. Mater. 13, 63 (2013). [2] Jankovic and Vukmirovic, Phys. Rev. B 95, 075308 (2017). [3] Grupp et al., J. Phys. Chem. Lett. 8, 4858 (2017).

Resume : Two interface materials based on functionalized fullerene derivatives were synthesized and applied in Organic Photovoltaics (OPVs). An amphiphilic diblock fullerene derivative C60-4TPB containing both hydrophilic ionic groups and hydrophobic fullerene group was spin-coated and a vertical rearrangement will be formed in the ultrathin layer after solvent vapor annealing. That is the C60 part next to the active layer and the hydrophilic groups attacted to ZnO, leading to enhancement of electronic collection and increased power conversion efficiency (PCE). Amination-functionalized fullerene derivatives named PCBDCU has also been applied as an additive in inverted OPVs with a structure of ITO/PTB7-Th: PC71BM: x% PCBDCU/MoO3/Ag. The device with 10% doping with PCBDCU can reach a PCE of 6.37% which is twice of binary devices without PCBDCU (3.20%). Solid alcohol 1-Pyrenemethanol (PyM) was introduced to modify the zinc oxide (ZnO) layer in the inverted OPVs as a cathode buffer layer (CBL). The PyM can modify the surface defects and improve the electron mobility of ZnO CBL due to the self-assembled of PyM on the ZnO surface for its hydrogen bonds and the conjugated structure in PyM. With a blend of PTB7:PC71BM as active layer, the device with ZnO/PyM CBL exhibited a notable power conversion efficiency (PCE) of 8.26%, which is better than that control devices based on bare ZnO CBL (7.26%). After the addition of PyM, the device based on PTB7-Th: PC71BM exhibited a higher PCE of 9.10%, obviously improved from 7.79% in control devices.