Charge transport in organic semiconductors: influence of processing and doping

Resume : Efficient electrical doping of organic semiconductors relies on identifying appropriate molecular dopants that are capable of ionizing semiconductor molecules with a high yield, thereby creating mobile charges. Two fundamental doping mechanisms, i.e., charge transfer complex (CTC) and ion pair (IPA) formation, have been identified, but their occurrence depending on molecular structure, energy levels, and structure of thin films is elusive. To explore the relevant relationships we investigated crystalline and non-crystalline organic semiconductors in combination with a range of dopants in solution and in the solid state. We find that molecular redox-potentials determined by cyclic voltammetry are proper guides to predict IPA formation efficiency in solution, and to some degree in non-crystalline solids. For semiconductor/dopant material pairs that form co-crystals in the solid, CTC formation dominates, even when IPA formation prevails in solution. Yet, molecular redox-potentials correctly capture the ionic character of CTCs, which impacts the efficiency of mobile charge carrier generation from such bound states.

Resume : Control over structures on a molecular through nanoscopic lengthscales is vital to optimize polymeric devices for energy generation. Tuning intermolecular interactions is critical to controlling the crystallinity, self-assembly, and packing motif, factors which in turn crucially impact charge transport properties such as carrier mobility in organic electronic devices. The inherent amphiphilicity of rigid donor-acceptor copolymers can be leveraged to allow the formation of highly ordered lyotropic mesophases. These lyotropic mesophases consist of chain extended polymers exhibiting close, ordered π-stacking. Crystallinity developed in solution can be transferred to the solid state, and thin films of donor-acceptor copolymers cast from lyotropic solutions exhibit improved crystalline order in both the alkyl and π-stacking directions and significant improvement in carrier mobility compared to those cast from isotropic solution. Further, higher degree of liquid crystalline attraction correlates to higher degrees of alignment after shearing and further improvements in mobility compared to isotropically aligned materials. This approach of rational side chain design bridges the gap from solution structure to solid state structure, and is a promising and general approach to allow the expression of lyotropic mesophases in rigid conjugated polymers.

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. Is it possible to go above what seems to be an intrinsic limit? [1] Charge carrier mobility is limited by low dimensionality, modest transfer integrals, and thermal agitation that causes energetic disorder. Searching a molecular solution to a material problem, I will discuss how clever design of structure could lead to a novel generation of organic semiconductors with unprecedented mobility values, over large time- and length-scales. Encouraging results at short time- and length-scales [2] or at low temperature [3] indicates that µ > 100 cm2/V.s could be reached in devices. [1] Isr. J. Chem. 2014, 54, 595 [2] Adv. Mater. 2016, 28, 7106 [3] Synth. Met. 2003, 133, 649

Resume : Doping organic semiconductors (OSCs) for enabling new functionality and improving opto-electronic device performance is done by adding strong molecular acceptors as p-dopants to the OSC host. I will discuss the broad range of phenomena observed upon molecularly p-doping conjugated polymers (CPs) and molecules (COMs), where two different competing scenarios emerge: (i) integer-charge transfer between OSC and dopant forming ion pairs (IPAs), and (ii), partial charge transfer with the emergence of OSC/dopant ground-state charge transfer complexes (CPXs). As prototypical representatives for these two scenarios, IPA formation in F4TCNQ-doped poly(3-hexylthiophene) (P3HT) is juxtaposed to CPX formation in is parent oligomer, and the respective doping-induced modification of the density of states (DOS) is both generally discussed and experimentally assessed by photoelectron spectroscopy. I will further focus on the underlying nanostructure of a number of oligo- and polymers, either p-doped with the Lewis acid BCF or with TCNQ derivatives of different doping strength. Correlating grazing-incidence X-ray diffraction with infrared spectroscopy for thermally annealed films reveals finally insight into the complex growth behavior of p-doped P3HT.

Resume : Many organic electronic devices contain interfaces between electron donating (D) and electron accepting (A) materials. The new, intermolecular electronic states in which an electron is transferred from D to A, are termed charge-transfer (CT) states. Ground-state CT in host-dopant systems results in an increase in conductivity, while D-A blends where the CT state is an excited state are used in organic solar cells and so-called “exciplex” OLEDs. In both ground- and excited-state CT complexes, the generation of free charge carriers contributing to (photo-) conductivity involves CT, and subsequent CT state dissociation. In this talk, we will explore these processes in more detail, and investigate the influence of the relative energy level offsets between various donors and fullerenes or their highly fluorinated versions. We further explore the use of D-A CT complexes in charge injection and extraction layers for organic solar cells and OLEDs, as well as their application in organic near-infrared detectors with peak detection wavelengths up to 1700 nm.

Resume : The measurement of transient space charge limited current (TSCLC) is a common method to determine the mobility of charge carriers for organic semiconductor thin films. This method is preferable over e.g. Hall effect or field effect measurements, as the analyzed current direction is in line with typical device architectures. For the TSCLC method, a voltage step is applied and the transit time of injected charge carriers is determined using displacement currents. Consequently, the difficulty of this method is the use of an adequate RC time constant for the sample charging, as it needs to be much shorter than the transit time. This parameter generally limits the application of TSCLC strongly, in terms of obtainable charge carrier mobility or minimum required film thickness. Here, we demonstrate a measurement circuit with a low RC time constant, which works in a wide current range (1µA to 0.5A) and thus allows for significant flexibility in terms of minimum film thickness or detectable charge carrier mobility. The circuit is fast enough to measure e.g. charge carrier mobilities of up to 3,1e-4cm²/Vs for 270nm thick P3HT, without using limited bridge circuitry. For this purpose, capacitor coupled fast transistor switches generate a voltage step to avoid voltage oscillations and a fast operational amplifier is used for amplification of the voltage over a variable measurement resistor. We demonstrate the circuit working principle by measuring benchmarked P3HT and MTDATA diodes.

Resume : Understanding charge dynamics in semiconductors for solar cells is critical to their identification, development and potential commercial implementation. Optical spectroscopic techniques such as photoluminescence spectroscopy, transient absorption spectroscopy, time-resolved microwave conductivity and terahertz spectroscopy are traditionally employed to elucidate optoelectronic properties in these systems. Yet many such techniques involve high-power optical sources and photo-generated carrier densities many orders of magnitude higher than present under typical solar cell operating conditions. An alternative approach is to study carrier dynamics using steady-state illumination rather than with high-fluence pulsed optical sources. In this presentation I describe a simple and versatile, contactless technique to study the photoconductivity of a semiconductor, as function of incident optical power-density, at optical power densities ≤ 1 sun. By employing thin solution-processed films of the highly-studied, high-performance hybrid-halide perovskite methylammonium lead iodide ((MA)PbI3) as a test system, I evaluate a proxy for mobility-lifetime product, with a strong dependence on incident optical power densities even below 1 mW/cm2.

Resume : We report on different series of fluorene- and carbazole-based derivatives that are functionalized with siloxane chains to become non-volatile liquid at room-temperature (RT). Some of these materials are strongly emissive and exhibit Thermally Activated Delayed Fluorescence (TADF) property. As RT neat liquids, these materials exhibit excellent performances in terms of PLQY, Amplified Stimulated Emission (ASE) threshold and charge-carrier mobilities. In particular, unprecedented ambipolar charge transport with hole and electron mobilities exceeding 10-4 cm2/V.s could be observed in the RT liquid state. Finally, these materials led to the first demonstration of an organic distributed feedback (DFB) laser based on a monolithic liquid molecular semiconductor.

Resume : Combining a molecular-level theoretical description with experimental measurements of specially tailored organic solar cells, we provide concise relationships between the open circuit voltage, photovoltaic gap, charge transfer state energy, and interfacial morphology of the donor-acceptor interface. In particular, we show how an electrostatic bias – generated across the interface - helps to split interfacial charge transfer states, but reduces the photovoltaic gap. As suggested by the theoretical model and underpinned experimentally, this negative influence on the gap and open circuit voltage can, however, be cured by adjusting the roughness of the donor-acceptor interface.

Resume : The collection efficiency of photogenerated charges in polymer solar cells (PSCs) is strongly influenced by the built-in field (Ein) that develops across the photoactive materials. Here, by investigating the Ein-development regimes in PSCs by introducing two types of interlayers, electric dipole layers (EDLs) and charge transport layers (CTLs), we optimized the device architecture to result in a larger Ein. By incorporating a pair of EDLs on both sides of the photoactive layer, we modulated the Ein by shifting the vacuum energy at each metal-semiconductor interface, providing a larger Ein than that in conventional PSCs using typical CTLs, such as metal oxides and/or conducting polymers. Our devices using paired EDLs exhibit an average PCE of 9.8%, which far surpasses the average PCE of ~ 8.5% for paired CTLs.

Resume : The power conversion efficiency of organic solar cells using non-fullerene (small molecule) acceptors have dramatically increased up to 13% in the last couple of years.1,2 The photo-current in these blends are maximized via complementary absorption of donor and acceptor molecules which has been a generic approach for the OPV devices in the last decades. Even though the efficiencies are among the highest, such polymer:NFA devices have typically show EQEs not more than 80%, due to un-avoidable geminate recombination losses.3,4 These losses are likely due to small driving energetic offsets or non-optimal nano-morphology where amorphous phases ending up too mixed domains or crystalline donor phases resulting in too separated pure donor or acceptor domains. Thus, the ability to reduce these losses whilst maintaining an optimal morphology and rather high voltage (>1V) for EQEs > 80 % has therefore become imperative to further improve the efficiencies in polymer:NFA systems. Here we report highly efficient small molecule acceptor solar cells (~12%) enabled by using a commonly known unstable donor PBDTTT-EFT (or PCE10) with quantum efficiencies exceeding 90%. This is achieved with an initial overlapping absorption bands of donor and acceptor which increases the photon absorption strength in the wavelength range from about 570 nm to 680 nm and, thus, almost all incident photons are absorbed in the active layer. The excitons harvested upon absorption in this region are efficiently and rapidly dissociated into charges with >90% PLQ in <300 fs. The charges generated are found to dissociate with near unity efficiency, with negligible geminate recombination losses, resulting in a short-circuit current density Jsc ≈ 20 mA/cm2. This is combined with an open-circuit voltage Voc > 1V compared to the ~1.6 eV band gap. Most importantly, the unique nano-morphology of PBDTTT-EFT:EHIDTBR blend results in a robust microstructure leading to a substantially improved device stability >1000h which is surprising for a donor material (PCE10) known with its instability. Understanding the highly efficient charge separation in non-fullerene acceptors can pave the way to robust and recombination free organic solar cells further boost the EQE values > 90%.

Resume : Twenty-three years after introduction of the organic bulk heterojunction (BHJ) solar cell, there are still fundamental questions to be answered. An important one is how the nanoscale morphology of the BHJ blend governs the nongeminate recombination behavior. In this contribution, we present new data on an old material: P3HT:PCBM. By tailoring the morphology and combining electron tomography with steady-state and transient electrical measurements, we observed a novel kind of structure-processing-property relationship in P3HT:PCBM devices. In particular, we found that the size of the phase-separated domains affects the strength and carrier concentration dependence of the recombination more severely than predicted by current theoretical models. We discuss whether these findings provide new avenues for understanding the role of phase separation and interface states in the nongeminate recombination dynamics.

Resume : Organic solar cells using halogenated solvents recorded high efficiencies over 10%. However, halogenated solvents are very toxic to human health and environment. These solvents must be replaced with environmentally friendly non-chlorinated solvents. In this regards, we have investigated all-polymer solar cells employing PTB7-Th and PNDI2OD-T2 polymers to find the dependence of photovoltaic properties on the processing solvent. We find that toluene and xylene are better processing solvents than halogenated solvents for the photovoltaic performance of PTB7-Th:PNDI2OD-T2 based devices. 3D TEM demonstrated that more ideal blend morphology in toluene and xylene-used active layer films was responsible for the solvent-property relationship.

Resume : Organic field-effect transistors (OFETs) have gained attentions as a next generation organic semiconductor-based technology because of their features: flexibility, solution-processability, and so on. To broaden its application, many efforts have been done to improve performance of OFETs, especially, charge carrier mobility (mobility). Recently, our group reported that dinaphtho[1,2-b:2',1'-d]thiophene (DNT–W) has same phase of the HOMO spreading along the molecular long axis and shows the relatively high hole mobility of 1.6 cm2/Vs [1]. Due to the unique HOMO, the effective intermolecular orbital overlap maintains against displacement. However, DNT–W OFETs showed large threshold voltage due to energetic misalignment between its HOMO level and Fermi level of gold electrodes. In this study, we focused on a more pi-extended molecule, chryseno[2,1-b:8,7-b']dithiophene (ChDT)[2], taking over the same feature of the HOMO as DNT–W. At first, ChDT skeleton was synthesized in 5 steps from the commercially available 1,5-dihydroxynaphthalene. After bromination at alpha-positions of ChDT, C10–ChDT and C10–Th–ChDT were prepared by cross coupling reactions. As expected, π-extended ChDTs have shallower HOMO levels and small threshold voltages. The hole mobility of C10–Th–ChDT reached as high as 10 cm2/Vs, while that of C10–ChDT was 2.6 cm2/Vs. In the presentation, we will show their fundamental properties and discuss relationship between the high mobility transport and their crystal structures. [1] T. Okamoto and J. Takeya et al., Chem. Mater. 2013, 25, 3952. [2] T. Okamoto et al., Adv. Sci. 2018, 5, 1700317.

Resume : The transport of charge carriers in organic semiconductor diodes and organic bulk heterojunction solar cells has a major impact on the device performance. The charge carrier mobility is influenced strongly by the morphology and the energetic disorder of the semiconductor layer. We investigate intrinsic or extrinsic defect states by electrooptical measurements and defect spectroscopy, and discuss their impact on the charge transport. Recently, we have shown by kinetic Monte Carlo simulations that the charge transport through a bulk heterojunction film depends on the shape of the domains, which can be expressed in terms of the tortuosity [1]. The tortuosity describes how twisted the pathway of charge carriers towards the electrodes is, and also depends on the anisotropy of the domain structure. It has a strong impact on the magnitude of the charge carrier mobility and its electric field dependence. This relation opens the, as of yet theoretical, possibility to tune the morphology in a deliberate fashion in order to optimise the transport and extraction of charge carriers in organic solar cells. [1] M. C. Heiber et al. Phys. Rev. Appl. 8, 054043 (2017)

Resume : Crystalline order determines the optoelectronic and transport properties of organic semiconducting materials, determines their functionality, and dictates their performance in devices. Yet, all polycrystalline materials form through a process of stochastic nucleation of crystallites from the melt, a vapor source or a drying ink. Consequently, the exact location and orientation of individual crystalline domains cannot be determined or controlled. This can lead to non-uniformity of microstructure and undesirable heterogeneity of semiconductor properties at different locations of a film. This issue is particularly problematic in organic electronic devices and circuits, where severe anisotropy of transport properties can cause large variations of transport properties in different parts of a circuit. In addition, the microstructure of different parts of films cannot be locally engineered to meet the requirements of a wide range of circuit components at the same time. We have developed a new method which can program the spatial microstructure and texture of a uniformly coated semiconductor film. The method relies on controlling the temporal sequence of nucleation and direction and geometry of crystallization in different parts of a film, simply by patterning the local thickness prior to initiating crystallization [1]. Using this approach we demonstrate fully programmable crystallization of organic thin film transistors (OTFTs) and achieve superior transport properties as well as dramatically improved device to device reproducibility. We go on to show that this can be applied in situ during solution-coating, making it compatible with scalable meniscus-guided techniques as well as R2R manufacturing. [1] L. Yu, M. R. Niazi, G. O. Ngongang Ndjawa, R. Li, A. R. Kirmani, R. Munir, A. H. Balawi, F. Laquai, A. Amassian “Programmable and coherent crystallization of semiconductors” Sci. Adv. 3, e1602462, 2017.

Resume : Organic semiconductors are attracting a great deal of interest for use in flexible electronic applications such as solar cells, logic circuits and displays. While these technologies have already demonstrated good performances, flexible organic-based memories are yet to deliver on all their promise in terms of volatility, operational voltage, write/erase speed, as well as the number of distinct attainable levels. Here, we report a multilevel non-volatile flexible optical memory thin-film transistor based on a blend of a reference polymer semiconductor, namely poly(3-hexylthiophene), and a photochromic diarylethene, switched with ultraviolet and green light irradiation. A three-terminal device featuring over 256 (8 bit storage) distinct current levels was fabricated, the memory states of which could be switched with 3-ns laser pulses. In addition, our memory features robustness over 70 write–erase cycles and non-volatility exceeding 500 days. The device was implemented on a flexible polyethylene terephthalate substrate, validating the concept for integration into flexible electronics and smart devices.

Resume : We have investigated the stability of fully printed Organic Thin Film Transistors (OTFTs). The p-channel devices have been fabricated on a polyethylene naphtalate (PEN) foil (100 μm thick), with staggered top gate configuration. The contacts, 160÷200 nm thick, have been inkjet printed using a silver nanoparticles based ink, while the organic polymeric semiconductor and the fluoropolymer-based dielectric, 50 nm and 400 nm thick respectively, have been gravure printed. Device channel length and width range from 30 to 400 μm and from 100 to 900 μm, respectively. The OTFTs have shown very low leakage current and small parasitic contact resistance, field-effect mobility up to 0.24 cm^2/(Vs) and ON/OFF ratio around 10^3. The electrical performance of devices have been analysed under different conditions, in particular high gate biases, environmental conditions and mechanical strain. It has been observed that gate bias stress and light soaking shifted the transfer characteristics, both in the subthreshold and OFF-region. Hence, the complete relaxation dynamics have been monitored in devices without polarisation and in dark. Under substrate bending, the subthreshold voltage increased of about 20% for a 2% of strain, fully recovered after removing the mechanical stress. The analysis of the collected experimental data has been the first step towards the comprehension of the mechanisms causing the observed instabilities, in order to improve our printed organic transistor reliability.

Resume : As OLED technology gain ground in optoelectronic market, an urging demand for low cost, green and conformable light sources is rising. Already, commercial RGB & Yellow emitting polymers have been used for flexible, lightweight applications. Variations from those primary emission colors can be achieved by blending those polymers with the proper weight ratios. In this way, can be produced tones between RYGB colors and more importantly white. Furthermore, by blending materials with different energy bandgap Eg and proper molecular orbital levels (HOMO-LUMO) the External Quantum Efficiency (EQE) of the devices can rise significantly. In the present work we investigate several polymer blends with a range of weight ratios. For each case it will be investigated the range of EQE and CIE coordinates versus the variation of polymer propositions. NIR-Vis-far UV Spectroscopic Ellipsometry, Photoluminescence, Electroluminescence and nanostructural, by Atomic Force Microscopy, characterization provide valuable information towards the optimization and functionalization of the final WOLED and other tailored color devices in terms of CIE color coordinates, operational voltages, power efficiency and brightness. Finally, the technology is transferred from small scale rigid substrate to large scale flexible with printing techniques. The produced devices are compared to those prepared on rigid substrates with the same tools described previously.

Resume : Utilizing organic light emitting diodes (OLEDs) for lighting requires a homogeneous operation across large areas. This poses the challenge to suppress local variations in temperature and current density. While OLED research mostly focuses on leveling electric properties, also the thermal properties are expected to be highly relevant. The temperature distribution in the organic layers crucially affects the electric properties due to the peculiar coupling between thermal and charge transport. Here we investigate the impact of two distinct heat transport parameters on the temperature distribution and current in OLEDs, namely (i) the thermal conductivity of the organic layers and (ii) the heat transfer coefficient between the device surface and the environment. Both parameters depend on the layer composition and morphology and cannot improve upon molecular doping. We monitor the heat dissipation with a 3D drift-diffusion model that self-consistently couples charge and heat transport. We establish that the heat transfer coefficient is able to compensate unintentional device heating much more efficiently than the thermal conductivity of the organic layers. Intentionally elevated operating temperatures, that may improve the OLEDs electric performance, are not necessarily beneficial, as any increase in operating temperature decreases the device stability. We propose analytic expressions that aid the interpretation of the heat transport in terms of the two heat transport parameters.

Resume : In the past decade, the understanding of charge transport and recombination in organic light-emitting diodes (OLEDs) has greatly increased. It is now possible to predict the electrical characteristics of unipolar devices using multi-scale simulation combining first-principles quantum-chemical calculations with a 3D Master Equation (ME) simulation technique [1]. It is also possible to mechanistically simulate electroluminescence in a multilayer white OLED using 3D kinetic Monte Carlo (KMC) simulation [2]. 3D-KMC simulation has difficulty in convergence when the applied voltage is close to or below the built-in voltage, while 3D-ME simulation is orders of magnitude faster in this voltage regime. However, as a mean-field approach, 3D-ME simulation neglects the short-range Coulomb correlation between the individual charge carriers which can be well taken into account in KMC simulations [3]. To alleviate this problem, we propose a modified double-carrier 3D-ME model which is systematically benchmarked against 3D-KMC simulations while maintaining good simulation efficiency. We show that a mechanistic 3D simulation of prototypical OLED devices at low voltages is now made possible. [1] A. Massé et al., Phys. Rev. B 93, 195209 (2016), F. Liu et al., App. Phys. Lett. 109, 243301 (2016). [2] M. Mesta et al., Nat. Mater. 12, 652 (2013). [3] F. Liu et al., Phys. Rev. B 96, 205203 (2017).

Resume : The ability to control charge density by doping is fundamentally important in any semiconductor technology. Our current understanding of the doping process in organic semiconductors, which occurs by charge transfer to a different molecule, is still incomplete as the doping efficiency and its effect on charge transport is strongly coupled to the materials’ microstructure. I will start by showing that even very light doping (~10^-5 mol.) leads to a decrease in mobility by forcing charges through more pathways in the low-mobility amorphous fraction of the film. At higher doping levels, I will compare a recently-reported doping process, called sequential processing, where the polymer film is doped post-deposition, to the traditional solution doping. By using IR spectroscopy coupled with theory I will show that the induced charges are strongly localized on single chains and over at most 3 thiophene rings. The observed increase in mobility observed in sequential processing as compared to traditional solution processing is ascribed to a better connectivity of the crystallites through the amorphous regions of the films associated to a straightening of the polymer chains. Overall, our work shows the complexity of the dopant/matrix interaction and the need for multiple characterization techniques in order to unravel the origins of the conductivity changes in doped polymers.

Resume : In this work we show for the first time that the dynamics of morphological changes that take place during the annealing of a film of two semiconducting materials used for organic solar cells, poly(3-hexyl-thiophene 2.5-diyl) (P3HT) and [6,6] phenyl C61 Butyric acid methyl ester (PCBM) can be captured and analyzed by measuring and fitting the relative permittivity to an analytical model. The fitting reveals information that cannot be easily extracted by just examining the drawn spectrum like absorption peaks positions, it give a clear indication that the nature of the P3HT related excitons is two dimensional in the pristine film which is in excellent correlation with the expected lamella structure of the film. The dimensionality of the excitons decreases to one dimension upon blending with PCBM but recovers during annealing. This recovery is accompanied by nanoscale interactions between the blend's constituents that can be tracked precisely by the model parameters. We further found that the blend's phase separation progression as indicated by the x-ray diffraction pattern is correlated with the excitonic nature, which fundamentally explains why charge transport in the blend is enhanced upon annealing. Our results indicate to the immense potential of relative permittivity to relate electronic as well as optical properties in organic optoelectronic devices to the device morphology.

Resume : Oriented conducting polymer films are prepared by doping rubbed thin films of PBTTT or P3HT with a solution of F4TCNQ in acetonitrile. The resulting charge transport (conductivity σ) and thermoelectric properties (Seebeck coefficient S) are enhanced in the direction of polymer chains and this results in improved power factors σS^2. In this contribution, we present a study of the doping mechanism and its kinetics as a function of the alkyl side chain length and structure (interdigitated versus non interdigitated). The changes in crystal lattice have been followed in situ during doping by grazing incidence X-ray diffraction. The kinetics of doping was followed in parallel by UV-vis spectroscopy. Our results indicate that i) the kinetics of the structural changes coincides with that of polaron and F4TCNQ- anion generation and ii) the doping kinetics is determined by the structure and length of alkyl side chains. The doping kinetics is slower when alkyl side chains are long (C18) and/or interdigitated. Moreover, the longer the side chains, the lower the final doping level achieved in the films. This last effect is attributed to the reduced charge transfer efficiency between the polymer and F4TCNQ when the distance between dopant and polymer chain is increased. A. Hamidi-Sakr et al. Adv. Funct. Mat. 2017, 27, 1700173

Resume : Laser-based nanostructuring of soft materials offers a compromise between spatial resolution, throughput and hardware availability compared with alternative patterning techniques. Particularly in the field of plastic electronics, nanostructuring of semiconducting polymer thin films represents a challenging frontier for the development of next-generation devices. Here we present an innovative nanostructuring strategy for semiconducting polymer films that uses three principal steps. First, a sacrificial spin-on multilayer structure is fabricated bottom-up, comprising a semipermeable interlayer protecting the pristine semiconductor film, followed by a layer of functional small-molecules, e.g. crystallisable solvents or dopants. Second, a scanning laser is used as a local source of heat in order to diffuse the small-molecular component into the polymer film. Finally, the multilayer stack is cleaved off to recover the starting planar film format. Using well-known polymers such as poly(3-hexylthiophene) and poly(9,9-dioctylfluorene) as examples, we demonstrate micrometre-scale-resolution spatial patterning of: (i) chain orientation, enabling polarised light emission/detection and anisotropic phonon transport, (ii) refractive index, yielding metamaterials-inspired photonic structures, and (iii) local doping, i.e. material composition. The reported findings are envisaged to provide a versatile means for fabricating next-generation nanostructured photonic and optoelectronic devices.

Resume : In this contribution, I will review our recent theoretical works addressing the structural packing of widely used conjugated polymer chains in the crystalline, disordered, and amorphous states by means of molecular dynamics (MD) simulations and the resulting electronic properties at a quantum-chemical level. We will focus on four different polymers, namely the hole transporting IDTBT, PBTTT, and CDTBT and electron transporting NDITT chains. The molecular dynamics simulations performed with properly parameterized force fields are able to reproduce all key structural features measured experimentally. Our results highlight the unique characteristics of IDTBT associated to the fact that the torsion angles between the units along the chain are preserved when going from one phase to another; this implies that charge transport is less affected by interfaces between different regions. IDTBT also features a small and similar degree of energetic disorder in the three phases in contrast to the other polymers. In the amorphous phase, the size and nature (branched versus linear) of the alkyl chains also dictate the degree of spatial overlap between adjacent chains, and hence the efficiency of intermolecular charge transport.

Resume : Organic semiconductors offer many advantages for energy conversion, saving and storage applications. However, the poor electrical properties, particularly low carrier mobilities, trapping and recombination phenomena, are a critical limitation for real applications. Molecular doping is an interesting strategy to tune electrical transport in organic films. But dopant-host interactions are still not well-understood, making design strategies for high performance applications challenging. New experimental approaches and insights are needed to correlate chemical structure, film morphology and electrical transport in doped organic films. In this talk I will present our results on combining Raman and impedance spectroscopies to elucidate microscopic interactions between dopants, both molecular and chemical, and conjugated polymers. By comparing doped polymer films to electrically charged ones, we are able to distinguish between the influence of the ionized dopant and the signature of free charge on the vibrational spectrum of the donor-acceptor polymer PCPDTBT. With this approach we are able to identify fabrication parameters for to maximize electrical interactions between dopant and host, while minimizing unwanted changes to polymer morphology.

Resume : In contrast to inorganic semiconductors, doping mechanisms in amorphous organic semiconductors are much more complicated due to internal complexity of both host and dopant materials and stronger Coulomb interactions. Several theories were put forward to explain different aspects of the doping process for certain material systems. However, comprehensive and congruent picture of the doping process in organic semiconductors is still missing and requires additional systematic research efforts. Using kinetic Monte Carlos simulations we show here that both intrinsic disorder and dopant-induced disorder stimulate ionization of dopants even if dopant’s EA and host’s IP difference is larger than the Coulomb attraction between the ionized dopant and the generated polaron. We show that the charge mobility can either increase or decrease depending on the intrinsic disorder of the material. Using device simulations we model the position and shape of the energy level distribution as a function of the doping concentration and the resulting changes in the injection process. This allows us to explain the experimentally observed change of the Fermi level position without need for empirical models (e.g. trap states). Furthermore, we demonstrate that doping improves the conductivity by the combined effect of the improved injection to the doped material and enhanced bulk conductivity.

Resume : Owing to their outstanding mechanical properties and to their excellent printability, semiconducting polymers are of particular interest for the cost-efficient fabrication of organic optoelectronic devices, such as transistors, solar cells or thermoelectric generators. For the optimization of such devices the control over the doping and over the nanostructure is crucial. However, many polymer:dopant complexes suffer from poor miscibility and poor solubility in organic solvents, which leads to inefficient doping and a sub-optimal nanostructure and results in a low electrical conductivity. We explore the doping of polar polythiophene derivatives, such as the oligo ethylene glycol side chain bearing p(g42T-T), with various acids. Due to the improved solubility of p(g42T-T) in organic solvents the polymer and the dopants can be efficiently co-processed in solution at room temperature, resulting in a conductivity of up to 120 S/cm for acid doped p(g42T-T). In addition, the miscibility and thermal stability of the polymer:dopant systems was substantially enhanced, such that p(g42T-T) doped with the non-toxic and inexpensive propanedisulfonic acid retained its conductivity for 17 hours upon annealing at 120°C in air.

Resume : Organic semi-conductive polymers have become more and more attractive by their price and low processability requirements. Dispersed in aqueous solutions, these polymers can be deposited with a wide range of processes to create thin, flexible and transparent films. Such films can be used for instance as transparent electrodes in organic light emitting diodes or organic photovoltaic cells. Poly(3,4-ethylene dioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is currently the only PEDOT:polyanion commercially available. However other counterions exist such as poly(4-styrene triuoromethyl(bissulfonylimide)) (PSTFSI) which stabilize PEDOT as well as PSS and give comparable conductive properties and transparency to the films. Further, the rheological properties of both aqueous systems are quite different. PEDOT:PSTFSI inks can indeed form a physical gel even at low concentrations making it attractive for processing purposes while PEDOT:PSS shows only shear thinning behavior for concentrations as high as 1%wt. This is due to structural differences between the polyanions: PSTFSI establishing more easily hydrogen bonding than PSS. Here, we present a study of the link between rheological properties and film properties observed in both systems PEDOT:PSS and PEDOT:PSTFSI and using various deposition processes such as doctor blade, screen-printing, inkjet, and soft blade deposition.

Resume : Thermoelectric (TE) materials are important for sustainable development because they can be used to harvest waste heat into electricity. Compared with the inorganic TE materials, organic polymers have unique advantages of the low cost, high abundance, high mechanical flexibility and low or no toxicity. But the thermoelectric performance of polymers must be significantly improved for practical application. In this work, we demonstrate that the power factor of poly(3,4-ethylenedio-xythiophene):poly(styrenesulfonate) (PEDOT:PSS) can be improved by coating a layer that can lower its work function. The highest Seebeck coefficient can increase to 49.9 μV K-1 with a conductivity of 1174 S cm-1. The corresponding power factor is 292.3 μW (m-1 k-2). The enhancement in the power factor is ascribed to two effects of these chemicals. One is related to the decrease in the doping level of PEDOT:PSS, and the other is the chemical-induced dipole moment in the surface of PEDOT:PSS film, increasing the gap between Fermi level (EF) and hopping formalism level (Etr).

Resume : Thin films of polymers have been widely explored as active layer in the development of organic devices [1,2]. Langmuir-Schaefer technique is useful for fabrication of electric devices based on thin films of polythiophene, because it offers the ability to control of the organized structures [3]. In this work, active layers were fabricated by LS films from solutions of neat poly(3-hexylthiophene) (P3HT) and mixed with stearic acid (SA), to study the influence of SA on the optical, morphological and electrical properties in ITO/active layer/Al devices. The growth and morphology of active layers fabricated were studied through spectroscopic measurements (UV-visible) and atomic force microscopy (AFM). In devices, direct current measurements were carried out using a Keithley source and Conwell and Arkhipov model was used to calculate the height of the injection barrier between the polymer and electrode. In the ac characterizations were performed employing a Solartron 1260A impedance analyzer and a theoretical model for impedance was proposed using equivalent electrical circuit. The junction of the experimental and theoretical spectra allows a study of the properties as interface effects, resistance, capacitance and conductivity of the P3HT and the influence of SA. We acknowledge support from INEO-CNPq and FAPESP. [1] G. LI et al. Nature Photonics, 6, 153-161 (2012). [2] H. ZHENG. et al. Nature communications, 4 (2013). [3] V. V. ARSLANOV, Russian Chemical Reviews, 10, 883-898 (2000).

Resume : Recently, Diketopyrrolopyrrole (DPP)-based donor–acceptor (D-A) type copolymers are receiving a lot of interests because of their high field-effect mobility as well as ambipolar characteristics wherein both p- and n-type charge transport can occur. From a circuit design point of view, the ambipolar semiconducting polymer is preferable for facilitating fabrication of such circuits using complementary-like inverters with high gain. In this presentation, the study on a newly-synthesized novel dithienyl-DPP based polymer obtained by copolymerization with the weakly electron-deficient benzotriazole (BTZ) monomers will be discussed. In particular, unlike the conventional doping method in which an interstitial small-molecule based dopants were added by thermal deposition, a method of introducing a side-alkyl chain containing fluorine, which is an electrophilic element, into a DPP-BTZ backbone will be introduced. For this purpose, we prepared four kinds of DPP-BTZ semiconducting copolymers with different ratios of alkyl side-chain to fluoroalkyl side-chain (i.e. alkyl chaina : fluoroalkyl chains = 3:7, 5:5, 7:3 and 10:0, respectively) and investigated the effects of fluorine in the side-chains on the charge transports via DPP-BTZ semiconducting copolymer films.

Resume : The external quantum efficiency of an organic light emitting diode (OLED) is limited by out-coupling efficiency of emissive guest–host systems. It is known that a molecule emits the triplet intensity, which is perpendicular to the transition dipole moment. Consequently out-coupling efficiency is influenced by molecular orientation in emissive systems. It is required to differentiate between meridional and facial isomers while using homoleptic Ir-complexes as phosphorescent material in OLED devices. We show that usage of isomerically pure blue emitting homoleptic Ir-complexes in an emissive guest-host system can lead to a different molecular orientation and further orientation factor Θ. The orientation factor describes the ratio of perpendicular orientated transition dipole moment vectors (TDVs) to the total amount of TDVs.[1] Whereas the facial isomer of the used blue emitting Ir-complex shows a preferential vertical orientation of the TDVs, the meridional isomer is predominantly isotropic aligned. This tendency will be shown in a selection of several guest molecules. [1] Tobias D. Schmidt, Thomas Lampe, Daniel Sylvinson M.R., Peter I. Djurovisch, Mark E. Thompson, Wolfgang Brütting, Phys. Rev. Applied 2017, 8, 037001.

Resume : Charges can be viewed to migrate through amorphous materials like disordered organic semiconductors due to hopping between localised states. The charge motion is governed by a complex interplay of energetic disorder, electric field, interactions, temperature, and other parameters. To interpret the motion, it is highly desirable to predict steady state charge carrier densities (SSCCD) in the presence of electric fields with a Fermi-Dirac-like distribution (FDD). A first attempt empirically modified the FDD with field-dependent effective temperatures.[1] This concept correctly predicts SSCCD for low fields and low charge carrier densities, but implies qualitatively wrong mobilities. Here, we provide the exact analytical SSCCD for arbitrary field strengths and low charge carrier densities. The key is to modify the FDD with a field-dependent local correction energy that is reminiscent to the local chemical potential observed by Cottaar et al..[2] We will elucidate the nature of this correction energy and demonstrate its effect on charge transport in intrinsic and doped systems for distinctly different electric field strengths and charge carrier densities. [1] B. I. Shklovskii et al. in Transport, Correlation and Structural Defects, by H. Fritzsche, World Scientific, Singapore (1990), p. 161 [2] J. Cottaar et al., Phys. Rev. B, 82, 205203 (2010)

Resume : In this work, we perform a joint experimental and computational study on synthesis, structure and mixed ion/electronic transports in thin films of a new π-conjugated oligothiophene-based liquid crystal 4T/PEG4. In excellent agreement with molecular dynamics simulation, structural characterization using X-ray diffraction and optical spectroscopy indicate that the neat 4T/PEG4 self-assembles to form the typically observed smectic morphology for liquid crystals. Interestingly, upon blending with LiTFSI salt, the 4T/PEG4 thin film structure drastically changes and becomes more ordered. This rather simple blending method results in more than an order increase in electronic conductivity of 4T/PEG4 after exposing to F4TCNQ vapor. Moreover, impedance spectroscopy measurement indicates that F4TCNQ-doped 4T/PEG4-LiTFSI sample exhibits mixed ionic/electronic conduction characteristics. At room temperature, the ionic conductivity of 4T/PEG4-LiTFSI at blending concentration r = 0.1=[Li+]/[EO] is on the order of 10-5S/cm. Upon exposing the sample to F4TCNQ vapor, the electronic conductivity as high as 2 x 10-3 S/cm can be achieved without sacrificing ionic conductivity. UV-vis absorption spectroscopy measurement indicates that charge transfer complex is the dominant F4TCNQ doping mechanism, suggesting the formation of co-crystal between F4TCNQ and 4T/PEG4. Our results show that 4T/PEG4 is a promising candidate to study the mixed ionic/electronic conductivities in a single materials system.

Resume : Symmetrical Pt complexes with different 8-oxyquinolinol derivatives, namely, unsubstituted 8-oxyquinolinol and 2-methyl-8-oxyquinolinol, have been synthesized and purified to better than 99.998 wt%. Using a developed technique based on simultaneous measurements of photoluminescence and reflection spectra of a condense phase under controlled ligand partial pressure and temperature the corresponding P(ligand)-T diagrams have been plotted in the temperature range from 350 K to the maximum melting temperatures. Depending on synthesis conditions crystalline preparations with different concentrations of point defects (nonstoichiometry) have been prepared within the homogeneity ranges. The influence of nonstoichiometry on photoluminescence, cell parameters, and chemical activities of the complexes was studied. OLED structures, prepared on the base of the same complexes as emitting materials, demonstrated significant variation of EL parameters (intensity in several times, energy efficiency within 40 rel%) with nonstoichiometry. The research was supported by the Russian Science Foundation by grant 14-13-01074П.

Resume : This paper focuses on lanthanum cobaltite – ceria composite cathode materials for NOx removal in solid state electrochemical reactor (SSER). A porous Ce0.8Gd0.2O1.9 (GDC) skeleton was prepared using glycine nitrate method, into which nano-level lanthanum cobaltite (LSCo) was impregnated. Phase structure and surface morphology of the cathode material were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. An anode-supported SSER with the configuration of YSZ-NiO anode, YSZ electrolyte, GDC-LSC cathode was fabricated. Air and 120ppm NO balanced by N2 were respectively fed into cathode for performances comparison. The results show that the open circuit voltage (OCV) and the maximum power density (MPD) of SSER reaches 1.1V and 500mW cm-2 at 700oC when air was fed into cathode and thus SSER worked in SOFC mode. For comparison, OCV decreases down to 0.15 V at 700oC when cathode atmosphere was switched into 120ppm NO, and meanwhile NOx removal efficiency can reach 93.3%.

Resume : Fermi line in twisted bilayer graphene at zero-energy splits into two seperated points positionned along the transverse direction of the reciprocal space. Here we focus on charge transport through an intrinsic twisted bilayer graphene (TBG) sheet. We found taht the minimal conductivity and the shot noise of TBG are sensitive to the twist defect since they exhibit an anisotropic beahavior and the minimum conductivity could be suppressed for somme specific value of twist defect while the shot noise takes the unit value.

Resume : Seebeck effect is one of the carrier transport phenomenon related with thermal energy. Seebeck coefficient, which is generated potential by temperature gradient, is a useful tool for elucidating the contribution of charged carriers in materials. Surveying Seebeck coefficient as a function of energy state can provide a physical property of materials. Here, we report energy level dependent Seebeck coefficient of thermally reduced graphene oxide (TrGO) was measured, using field effect transistor with micro heater. Impurity such as boron or nitrogen doped TrGO, which has different energy state, also was prepared, and energy level dependent Seebeck coefficient was compared. As a function of energy state, the major charged carrier was different with TrGO, but boron or nitrogen doped TrGO had hole transport or electron transport, respectively, in most of controlled energy level. Investigation of conductance as a function of ambient temperature showed the relative bandgap opening by doping effect. Using Seebeck coefficient and relative bandgap with TrGO, boron or nitrogen doped TrGO, the relative electronic structure of TrGO was estimated. The results provide the Seebeck coefficient can be used in not just only energy harvesting area but also understanding the electronic structure of materials.

Resume : An uniaxial strain on the graphene induces a moving of Dirac points and the Dirac cones become anisotropic. We theoretically study the effect of the uniaxial strain on the electronic transport in the graphene. We focus on the effects of the uniaxial strain on the transmission across a potential barrier, on the conductivity as well as on the shot noise. We find that the conductivity along the strain axis increases with the strain modulus and decreases in the other direction. Particularly for clean and undoped graphene, the transport properties are similar to those of a diffusive metal with a Fano factor (the ratio of shot noise power and current) F=1/3. This transport regime in graphene, which is called pseudo-diffusive [1], is not restricted at the neutrality point but extends over a finite energy range around zero energy. We show that the strain leaves the Fano factor unchanged but affects the energy range which show pseudo-diffusive transport. [1] J. Tworzydlo, B. Trauzettel, M. Titov, A. Rycerz, and C. W. J.Beenakker, Phys. Rev. Lett. 96, 246802 (2006).

Resume : Many examples of semiconductor organogels and supramolecular structures with applications in organic electronics can be found in literature.1,2 Most efforts done so far to improve device efficiency have been focused on refining the optoelectronic properties of semiconductors while device morphology has been much less studied. Supramolecular chemistry strategies are very attractive in this case, using mainly p-p stacking interactions to build semiconducting domains. The use of hydrogen-bonded materials is still scarce, despite the incorporation of such interactions in semiconductors have resulted in 50% more efficient photovoltaic devices than when analogue molecules without hydrogen-bonds were used. Here we present a family of small semiconductors based on diketopyrrolopyrrole (DPP) containing hydrogen-bonding units with different strength and position within the molecular structure. Control on the aggregation of such molecules strongly influences the optoelectronic properties and the morphology of such semiconductors. Such simple electroactive unit presents excellent optoelectronic properties compared to much more complex organic semiconductors thanks to the incorporation of hydrogen-bonding units. Robust supramolecular structures and gels have been achieved, showing in many cases absorption reaching the infrared region and appropriate energy levels for future device fabrication. The synthesis, characterization and charge transport measurements will be discussed using differents aggregation states. References [1] A. Ajayaghosh et al, Angew. Chem. Int. Ed.. 51, 1766 (2012). [2] S. I. Stupp et al, Chem. Mater. 27, 1201(2015)

Resume : The synthesis and characterization of solution-processable dithieno[3,2-b:2′,3′-d]thiophene (DTT) derivatives, 2,6-bis(phenylethynyl)dithieno[3,2-b:2′,3′-d]thiophene (BP-Et-DTT) and 2,6-bis(thiophen-2-ylethynyl)dithieno[3,2-b:2′,3′-d]thiophene (BT-Et-DTT) have been studied for organic thin-film transistors. Thermal, optical, and electrochemical properties of the DTT-based semiconductors were investigated and the solution-sheared thin films exhibited p-channel characteristics with the highest hole mobility of 0.32 cm2 V−1s−1 for BP-Et-DTT. The thin films exhibited micrometer-sized crystalline fiber structures along the shearing direction, resulting in fiber-alignment-induced charge-transport anisotropy. Furthermore, bulk heterojunction (BHJ) ambipolar transistors were fabricated with an optimized blending ratio of BP-Et-DTT and the representative n-channel semiconductor, PDIFCN2. Complementary-like inverters were fabricated based on the two identical ambipolar transistors, resulting in moderate voltage gains of up to 16.

Resume : Organic field-effect transistor (OFET) consisting of organic or polymer materials is quite promising platform for flexible wearable devices with tremendous advantages in low-cost, flexibility, large-area compatible solution processing. Recently, organic semiconductors blended with polymeric binders have been the topic of intensive investigations as a promising approach to improve the performance of the OFETs. In particular, controlled phase separation in organic semiconductor:polymer blend provides new perspective in organic semiconductor electronics. For example, bi-layers of the semiconductor and the gate-dielectric or semiconductor and the top-passivation layer in the OFET device can be simply achieved by a vertically separated bilayer structure of organic semiconductor:insulating polymer blend through an one-step film deposition process. In this work, we demonstrated unique OFET devices with 3-dimensional configuration based on self-organization of organic semiconductor: insulating polymer blends. Organic semiconductor molecules were vertically segregated on top of a polymer phase and simultaneously crystallized at the center of the printed line pattern immediately after solvent evaporation without the use of an additive. In this presentation, three-dimensional self-organization of organic semiconductors in a printed microstructure and its application to OFETs and pressure sensors will be discussed.

Resume : Efficient molecular doping in polymeric semiconductors [1] has been attracting much attention for applications such as thermoelectric generators. Here, we demonstrate a new concept, anion exchange doping, to improve efficiency and thermal stability of molecular doping. Thin films of PBTTT were doped by exposing them to F4TCNQ acceptor dopant solution. In the presence of an ionic compound, EMIM-TFSI, F4TCNQ radical anions intercalated in the PBTTT thin films are replaced to TFSI anions with a high efficiency (>97.5%), which is confirmed by UV-Vis-NIR, FT-IR and ESR spectroscopies. Doping and anion exchange efficiencies are improved by controlling the chemical hardness in both anions and countercations of ionic compounds. Use of hard countercations (metal ions) increases the conductivity up to 620 S cm^-1, with the doping level up to 1.4 × 10^21 cm^-3 (one hole per one monomer unit). We also show that the conductivity is further improved by uniaxial alignment [2] of PBTTT films. Metallic signatures, i.e., Pauli paramagnetism and weak localization, in highly doped films are confirmed by ESR and magnetotransport measurements. In addition, it is found that an introduction of stable anion significantly improves the thermal stability. Almost infinite selection of ionic compounds will help further improvements in doping efficiencies and stabilities. [1] S. Kang, S. Watanabe et al., Nature Mater. 15, 896 (2016). [2] Y. Yamashita, J. Takeya et al., Chem. Mater. 28, 420 (2016).

Resume : Polymer field-effect transistors (PFETs) have been developed for flexible display, e-skin, and other wearable devices. There are some pathways to achieve superior stretchability of semiconducting polymer layers as active channel components: 1) physically blending semiconductor/insulator polymers and 2) chemically introducing non-conjugated, stretchable segments on the conjugated backbone. Here, we synthesize a series of novel diketopyrrolopyrrole (DPP)-based polymer derivatives containing non-conjugated linear alkyl backbone segments. A moderate introduction of the non-conjugated segments produces excellent hole stretchability of the resulting polymer films on deformable gate dielectrics, without any severe degradation in hole conductivity. Process-controlled structures of the PFETs are systematically investigated atomic force microscopy, synchrotron-based X-ray, etc. In particular, the effects of the non-conjugated alkyl segments on the strain-dependent device performance of the PFETs are carefully investigated.

Resume : Polythiophenes are materials that can be employed to manufacture sensors for volatile organic compounds (VOCs) [1], these types of devices have a high appeal for several purposed such as environment control and homeland security. Langmuir-Blodgett (LB) was used to produce the VOCs sensor since it is a technique that allows the fabrication of highly ordered thin films, which properties can be monitored at molecular level [2]. Herein we have mixed amphiphilic molecules (stearic acid - SA) with the poly(3-alkylthiophenes) (P3ATs) derivatives, in order to improve the polymers deposition via LB technique. The solutions of P3ATs/SA were fabricated with 43 molecular percentage of stearic acid that was chosen, among other films with distinct molecular percentages, due to the formation of quite homogeneous films and higher conductivity obtained. To characterize the LB films were performed optical, morphological and electrical measurements. In order to analyse the sensing aptitude, it was measured the current versus time with a fixed applied voltage on the presence of the VOCs: toluene, dichloromethane and tetrahydrofuran in dynamic flow, dragged by nitrogen. In I vs. t curves it is possible to observe the polymer films sensibility to the VOCs, with downward trend in the current. We acknowledge support from FAPESP, CNPQ/INEO and LNNano. [1] Shokuhi Rad, Ali. Journal of Molecular Modeling, 21.11 (2015): 2. [2] Galanti, Agostino et al. Chemistry - A European Journal, 22.28 (2016): 9709.

Resume : Organic thin film transistors (OTFTs) are considered basic components within organic electronics, for instance in electronic papers, sensors and memory devices. Although their performances are able to compete with amorphous silicon, most of them do not present the required stability for practical applications. For this reason, the research of new air stable organic semiconductors is still an important issue in organic electronics. In this context, triindole-based compounds appear to be promising building block as semiconductor materials, which have been explored as hole transporting layers for optoelectronic devices. Here, we present a series of triindole derivatives where four different donor moieties are attached on the 3, 8 and 13 positions of the aromatic core. Their optical and electrochemical properties, as well as their semiconducting characteristic as active layer in OTFTs are presented. The molecular order within the final devices is also studied by means of X- ray analysis. All the transistors based on these materials show excellent air stability performances during long period of time.

Resume : Semiconducting polymer field-effect transistors (FETs) are being intensively developed for next-generation flexible electronics. Blends comprising a small amount of semiconducting polymer mixed into a deformable insulating polymer matrix can yield superior stretchability and environmental stability in FETs, while their electrical properties tend to be degraded, in comparison to the neat semiconductor. Here, we demonstrate that ultrasound-assisted polymer blends including two conjugated polymers with the same backbone segment but distinct regioregularity, such as regioregular and regiorandom poly(3-hexyl thiophene). The resulting polymer blend films show highly-conjugated semiconducting fibrils embedded into the poorly-conjugated polymer matrix, yielding excellent charge-carrier transport and stretchability in deformed OFETs. In addition, a moderate doping of the blends leads to high mobility and on/off current ratio. Structural investigations show that these doped polymer films contain highly-connected semiconductor fibrils, which have a vertical distribution gradient. This particular morphology enables a quasi three-dimensional spatial distribution of charge-carrier pathways within the less-ordered conducting matrix, in which charge accumulation and depletion via a gate bias is substantially different from neat semiconductor, and where high on-current and low off-current are simultaneously realized in the stable doped state.

Resume : According to band transport theory, suppressing molecular vibrations is crucial for developing high mobility organic semiconductor [1]. To suppress molecular vibrations, our group reported sulfur-bridged V-shaped binaphthalene (dinaphtho[2,3-b;2’,3’-d]thiophene, DNT–V) as a conceptually new organic semiconducting π-core and alkylated DNT–V showed high mobility over 6.5 cm2/Vs and stabilized crystal phase up to 150 °C [2]. It is suggested that the reduced molecular vibrations stem from elongated rotational radius. In this study, to further reduce molecular vibrations in DNT–V series, phenyl groups (phenyl and decyl-phenyl) were introduced. The C–H•••π interaction of the phenyl groups is expected to bring fine-tuning of assembled structure and reduce molecular vibrations effectively, leading to enhancement of mobility and additional stabilization of crystal phase of materials. From the result of single crystal structure analysis and thermal analysis, the suppression of molecular vibrations assisted by the phenyl groups interaction was suggested. Furthermore, mobility up to 8.1 cm2/Vs was realized in solution grown single crystalline film of decyl-phenyl substituted DNT–V. Thus, introduction of phenyl groups effectively suppresses molecular vibrations and is a good molecular design strategy for high mobility organic semiconductors. [1] J. Takeya et al., Nat. Commun. 2016, 7, 11156. [2] T. Okamoto and J. Takeya et al., Adv. Mater. 2013, 25, 6392.

Resume : Organic light emitting diodes have emerged as the main stream low power and high resolution display technology. OLEDs command various competitive advantages due to the superior optical properties and proven reliability for the successful commercialization. Consequently extensive investigations have been carried out, regarding the injection and transport mechanisms of charge carriers, the exciton dynamics and the radiative recombination of electrons and holes, the optical properties of illumination layer, etc. The key factors involved in the efficient light generation are the emission processes and the high input currents of electrons and holes as the source of the light generation. This paper examines the optimal conditions for attaining the maximal input currents, given the layer properties and applied voltage. Specifically the injection of electrons and holes from the respective electrodes and their subsequent transport are investigated both experimentally and theoretically. The carrier injection due to the thermionic emission has thus far been highlighted and examined in detail in the literature. However this paper points out that the carrier injection via the field emission can be equally important. In particular, the currents due to the phonon assisted Fowler-Nordheim tunneling are shown to constitute an important component when the applied voltage is increased. Thus the thermally assisted tunneling is analyzed in this paper as a function of the potential barrier height at the interface and other parameters. The results obtained are compared with those due to the thermionic emission. Also investigated in this paper is the carrier transport through the transport layer. Such mechanisms constitute the fundamental issue. The hopping of electrons and holes from a molecule to another has been proposed and examined in detail. Also the usual transport through the conduction band has been proposed. Because of the overlap of  electron wave functions in the molecule the LUMO levels are taken to form the conduction band, although narrow. In view of the basic issues yet to be resolved the carrier mobilities are extracted in this work by fabricating simple test structures with differing layer thicknesses and comparing the differences of the corresponding I-V behavior. In conclusion, the carrier injection from the electrodes is systematically examined and the scheme for extracting the carrier mobilities is also presented. These results are useful for devising the optimal conditions for attaining the maximal input current.

Resume : Organic light-emitting diodes (OLEDs) are currently prime candidates in display and solid-state lighting (SSL) in the global market. Therefore, it is high-priority to design and fabricate high efficiency and energy effective OLEDs. To fulfill the current requirement, it is vital important to understand the fundamental process that governs the power efficiency of the OLED devices. In the present study, we have investigated the effect of electron charge carrier mobility on recombination zone in the emissive layer by using commercialized electrical simulation package SETFOS. The device consists of 4,4’-Bis(N-carbazolyl)-1,1’-biphenyl (CBP) as a host, bathophenanthro- line (Bphen), 2,2',2"-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi), and 1,3,5-Tri(m-pyridin-3-ylphen- lyl)benzene (TmPyPb) as ETMs, and 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) as hole-transporting material (HTM). Simulation outcome reveals that the total recombination rate density within the EML is significantly decreasing from 3.7 x106 to 2.9 x103 cm-3s-1 as the electron transporting material changed from Bphen to TmPyPb, a decrement of 99.92%. By further changing to TPBi, recombination rate density decreased to 99 cm-3s-1. The decrease in recombination rate density in TPBi based device is attributed to the low electron mobility as compare to Bphen and TmPyPb. It is noteworthy to say that besides electron mobility of ETLs, the alinement of energy levels between ETL and EML play a vital role to enhance recombination rate density into the emissive layer. We have validated the simulation results with experimental data to evaluate the effective role of electron carrier mobility on LED device performance. The Bphen based device exhibited a maximum power efficiency of 29.3 lm/W at 100 cd/m2, an EQE of 8.7%, and a low operation voltage, due to its efficient device structure. Keywords: organic light-emitting diode, charge carrier mobility, recombination rate density, electron transporting material.

Resume : Multi-component bulk heterojunctions (BHJs) are an effective way to broaden light absorption spectrum and modify film morphology of organic solar cells (OSCs). Here, we demonstrate the quaternary BHJ OSC composed of two conjugated polymer donors, one fullerene derivative acceptor, and one non-fullerene acceptor. We enhanced the device performance by optimizing the ratio between the components and the concentration of solvent additive, 1-Chloronaphtalene (CN). As a consequence, the quaternary OSC exhibited 2% higher efficiency over that of the conventional binary references.

Resume : Organic photovoltaic (OPV) is a relatively new and fast-growing field from both scientific and industrial points of views. One of the major obstacles that limit further development of the technology is the use of a transparent conductive anode due to the low mobility of charge carriers within the organic material. Transparency, however, impairs the conductivity and leads to power loss in the anode. The solution is to embed a highly conductive metal grid on top of the anode. It is most desired to implement a tool for grid shape optimization instead of performing it experimentally. Recently, we reported on a 2D simulation showing drastic power loss in OPV due to the finite conductivity of the anode. We have extended the simulation and implemented a metal grid on top of the transparent contact along with a novel approach for simulating the delicate role of the interplay between the electrodes and the active-layer. The common approach is to treat the electrode as a one-electron problem using either Ohmic or the Schottky based approach rather known as Scott-Malliaras. However, at high-polarized organic materials and for contacts with finite conductivity, the assumptions of the single electron’s picture do not hold, and one should account for the interaction of a single moving charge with its neighbors. This is accounted by using mean-field approach derived from the Thomas-Fermi theory of screening. This simulation allows us to anticipate an optimal position and width of grid lines.

Resume : Semiconductor materials are essential for the advancement of polymer solar cell technology. The design of novel materials has brought more attention to bulk heterojunction polymer:fullerene (PF) and polymer:non-fullenere solar cells in the past decade. A typical example is the synthesis of the benzodithiophene-co-thienothiophene (BDT-TT) polymers and the structural changes in their TT-units through reduction and addition (e.g. fluorination) reactions leading to power conversion efficiency (PCE) of over 10%. This work examines the effect of UV light on the photostability of PF solar cells. First, it studies the effect of the chemical structure of the polymer on photostability of PF cells made from two classes of BDT-TT: 1D polymers and 2D polymers. The polymers differ only by the substituted side chains on the BDT-unit with alkoxy groups on the 1D and alkylthienyl groups on the 2D polymers. Through combined experimental techniques, the relationship between the polymer chemical structure and the UV-stability of the solar cells is explored on the one hand, and on the other hand, the effect of diiodooctane on their UV-stability. Based on the polymer chemical structure, solar cells of the 1D polymers are found to be more stable (10-18% loss in PCE) than those of the 2D ones (35-48% loss in PCE). Second, it examines the effect of the changes in the TT substituents of the BDT-TT polymers on photostability of the solar cells. The solar cells based on the reduced TT-unit are found more stable than the fluorinated TT ones. Finally, we postulated the mechanisms behind the observed relation between UV-degradation and DIO within the films. These findings pave the way for new materials and additives that yield efficient as well as stable organic solar cells.

Resume : Squaraine (SQ) dyes receive increasing attention as donor materials in organic photovoltaics since they offer a unique combination of high absorption in the deep-red with a general environmental stability and non-toxicity. Together with common fullerene acceptors, SQ-based bulk-heterojunction devices have been demonstrated to deliver a high open-circuit voltage, but suffer from a comparatively low fill factor [1]. However, little effort has been made to develop a better understanding of the underlying loss mechanisms. In this work, we combine steady-state with transient optoelectronic characterization methods to study the loss mechanisms in photovoltaic devices of a benchmark squaraine (SQIB) blended with a fullerene (PC60BM). These devices show a gradual decrease of the fill factor when increasing the active layer thickness and incident light intensity. We show that the low fill factor is a consequence of slow charge carrier collection competing with bimolecular recombination. [1] Scheunemann et al., Appl. Phys. Lett. 111 (2017) 183502.

Resume : Non-fullerene acceptors have attracted much interest recently in the effort to further improve the efficiency of organic solar cells. Here we report recent results concerning the performance of non-fullerene acceptors containing alkyl solubilising groups rather than the more conventional arylalkyl groups. We show that alkylation results in a reduction in the optical band gap and an improvement in the solar cell device efficiency. The performance of a variety of different donor polymers is examined and the utilisation of low band-gap polymers affords blends with weak absorption in the visible region. Such blends are utilised to demonstrate semi-transparent devices via the modification of the top electrode thickness.

Resume : Doping is an important process to control the electrical behavior of organic semiconductors. Doping of organic materials increases the number of charge carriers filling trap states and introducing free carriers. In the emerging application as thermoelectrics, the carrier concentration dictates both the electrical conductivity and the thermopower. We will discuss our efforts to elucidate structure-property correlations between the electrical conductivity and thermopower of semiconducting polymers using model polymer systems including poly(3-hexylthiophene) and the thienothiophene-based polymer, PBTTT. We find that changes in processing conditions can increase the electrical conductivity by > 50 times at the same apparent carrier concentration. The differences in performance can be understood by the nanoscale connectivity between ordered domains and quantitated using synchrotron-based X-ray scattering methods and temperature-dependent thermopower and transport measurements. The role of the miscibility of dopants and polymers will also be discussed as a critical factor in controlling their electrical conductivity.

Resume : Efficient chemical doping of the organic semiconductors using organic molecules with high electron affinity is one promising strategy to improve the electrical performance of organic electronic devices and extend the range of achievable functionality. However uncontrolled diffusion and/or intercalation of the dopant molecules in the crystal structure of the OSC films have unwanted consequences in the charge transfer and charge mobility of the semiconductor. A recent interest has emerged for new strong p-dopants such as fluorinated fullerene (C60F48) with advantage of low volatility, bulkier shape and higher thermal stability than other used dopants. In this work we investigate the evaporation of C60F48 on top of didodecyl[1]benzothieno[3,2- b ][1]benzothiophene (C8-BTBT) from the early stages of the growth. We combine Grazing incidence x-ray diffraction, atomic force microscopy and Kelvin probe force microscopy (KPFM) to determine the structural and electronic properties of the C60F48/C8-BTBT system. We show that the BTBT-C8 layer templates the growth of C60F48 and enhances the thermal stability of the underlying C8-BTBT layer. Finally, the role of C60F48 as interfacial dopant is investigated by KPFM and by electrical characterization of C8-BTBT field effect transistors.

Resume : The standard technique to separately and simultaneously determine the carrier concentration per unit volume (N, cm-3) and the mobility (μ) of doped inorganic single crystals is to measure the Hall effect. However, this technique has not been reported for bulk-doped organic single crystals. Here, we measure the Hall effect in bulk-doped single-crystal organic semiconductors [1]. A key feature of this work is the ultra-slow co-deposition technique, which reaches as low as 10-9 nm s-1 and enables us to dope homoepitaxial organic single crystals with acceptors at extremely low concentrations of 1 ppm. Both the hole concentration per unit volume (N, cm-3) and the Hall mobility (μH) of bulk-doped rubrene single crystals, which have a band-like nature, are systematically observed. We find that these rubrene single crystals have (i) a high ionization rate and (ii) scattering effects because of lattice disturbances, which are peculiar to this organic single crystal. 1. C. Ohashi, M. Hiramoto et al., Adv. Mater., 1605619, (2017)

Resume : Crystal engineering, the possibility of designing solids exploiting intermolecular interactions is a well established research field often associated with pharmaceutical and drug design. Some of these concepts could be applied for shaping intermolecular interactions in organic semiconductors. Here, by preparing cocrystals of the organic semiconductor perylene with tetracyanoquinodimethane (TCNQ) and its fluorinated derivatives, we show how crystal structures can be modified [1]. We focus our study on how stoichiometry of the molecular pair and structure impacts on the optical gap and degree of charge transfer and show how polymorphism and degree of charge transfer can be tuned.

Resume : Synthesis of conjugated aromatic polymers typically involves carbon coupling polymerisations utilising transition metal catalysts and metal containing monomers. This polymerisation chemistry creates polymers where the aromatic repeat units are linked by single carbon-carbon bonds along the backbone. In order to reduce potential conformational, and subsequently energetic, disorder due to rotation around these single bonds, an aldol condensation reaction was explored, in which a bisisatin monomer reacts with a bisoxindole monomer to create an isoindigo repeat unit that is fully fused along the polymer backbone. This aldol polymerization requires neither metal containing monomers or transition-metal catalysts, opening up new synthetic possibilities for conjugated aromatic polymer design, particularly where both monomers are electron deficient. The condensation reaction locks the repeat units together with a carbon-carbon double bond link, eliminating free rotation of the repeat units and thus rigidifying the polymer conformation. Polymers with very large electron affinities can be synthesised by this method, resulting in air stable electron transport, demonstrated in solution processed organic thin film transistors. The rigid, planar nature of the backbone also facilitates extended delocalisation of both frontier molecular orbitals and a subsequently low bandgap. We present an electrical, optical and morphology characterisation of polymer thin films, illustrating structure-property relationships for this new class of polymers

Resume : We demonstrate a universal method to obtain record-high electronic Seebeck coefficients while preserving reasonable conductivities in doped blends of organic semiconductors through rational design of the density of states (DOS). A polymer semiconductor with a shallow HOMO level was mixed with materials with a deeper HOMO to form binary blends of the type Ax:B1-x (0 ≤ x ≤ 1) that were p-type doped by F4TCNQ or n-type doped by N-DMBI. We achieve Seebeck coefficients S up to 2000 µV/K. Surprisingly, we find that this methodology to increase the Seebeck coefficient only works for strongly phase separated systems. In well-mixed systems we find a much lower and more constant S, despite the energy levels being (virtually) identical in both cases. The results are quantitatively interpreted in terms of a variable range hopping (VRH) model where a peak in S (and a minimum in conductivity) arise when the percolation pathway contains both host and guest sites, in which the latter acts as energetic trap. For well-mixed blends of the investigated compositions, VRH enables percolation pathways that only involve isolated guest sites, whereas the large distance between guest clusters in phase separated blends enforces (energetically unfavorable) hops via the host. The experimentally observed trends are in good agreement with the results of kinetic Monte Carlo simulations accounting for the differences in morphology.

Resume : The recent development of semicrystalline polymer semiconductors with field-effect mobilities comparable to and sometimes exceeding those of amorphous silicon has exposed the limitations of commonly used disorder-based charge transport models which were developed originally for more disordered, lower mobility materials. I will present our recent study on the structural, electric and thermoelectric characterisation of semicrystalline polymers for which we have shown a correlation between charge density dependence of mobility and the size of the crystalline domains in the polymer film.[1] Our results contradict the charge transport models based on energetic and positional disorder and provide direct experimental evidence for narrow band, disorder free conduction at the charge densities typically observed in thin film transistors. To explain these results, we propose an alternative charge transport model that explicitly accounts for the presence of interdispersed crystalline and amorphous regions within the polymer film and for local electron-electron interactions. This approach captures all the unique features observed experimentally, and provides a rationale for further improvement of both electric and thermoelectric performance in this class of materials. [1] Di Pietro, R. et al. (2016), Coulomb Enhanced Charge Transport in Semicrystalline Polymer Semiconductors. Adv. Funct. Mater., 26, 8011.

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

Resume : In organic photovoltaics (OPVs), the photoconversion efficiency is determined by the dissociation of excitons at the interface between donor and acceptor. The short exciton diffusion length (LD) is one of the factors that limit the amount of excitons that can reach the interface. Several attempts have been made to enhance LD; however enhancing exciton diffusion and correlating it to intermolecular interactions, morphology and crystalline order is not so well explored. In order to establish structure-property relations for exciton diffusion in organic semiconductors, it is instructive to control intermolecular interactions, film morphology and crystallinity. Several processing methods including annealing (thermal or solvent)1, addition of solvent additive2 and crystal nucleating agents3 etc. have been suggested for controlling the structural order and crystallinity within the film. Here we controlled the degree of crystallinity of the film by polymer processing. We then investigated the important problem of how crystallinity affects exciton diffusion in these organic solar cell materials. For this, we used a random copolymer and investigated the role of processing (both thermal and solvent vapour annealing) on polymer crystallinity and exciton diffusion. We found enhancement of the relative degree of crystallinity and exciton diffusion in processed films. The exciton diffusion coefficient is increased by more than a factor of 3 when thin films are annealed with CS2 solvent vapour and becomes double upon melt annealing at 200 °C. The corresponding films show about 50% enhancement in the degree of crystallinity. Hence we show that polymer ordering in the film is very important for exciton diffusion and can be controlled by polymer processing. 1. a) C. Sinturel, et al., Macromolecules, 2013, 46, 5399; b) M. Sim, et al., J. Phys. Chem. Lett. 2014, 118, 760-766; c) M. Li et al., Adv. Mater., 2015, 27, 6296-6302. 2. a) F. Liu, et al., Prog. Polymer Science, 2013, 38, 1990–2052; b) G. J. Hedley, et al., Nat. Comm., 2013, 4, 2867. 3. N. D. Treat, et al., Nat. Mater., 2013, 12, 628–633

Resume : Over recent years several new classes of conjugated polymers have shown promise as materials for polymer field-effect transistors with high field-effect mobilities. Many of the recently discovered high mobility polymers, in particular donor-acceptor copolymers, owe their excellent charge transport properties to a low degree of energetic disorder associated with a well-defined backbone conformation with small variations in torsion angles. In this presentation we will present our current understanding of the transport physics of these materials and focus in particular on the relationship between molecular structure, thin film processing and charge transport and thermoelectric properties of these materials.

Resume : Decoupling thermal and electronic transport has been a decades long dream for scientists and engineers in the fields of thermoelectrics, electrical insulation and heat management. For this dream to come true, different fundamental mechanisms should exist for thermal and electrical conductivity, or, alternatively, the material system should offer orthogonal channels for heat and electronic carrier transport. To be able to understand these issues, I will start my talk presenting the techniques we have recently developed to measure thermal conductivity in thin films. For out of plane transport, we have adapted the standard 3-omega electrical measurements, by incorporating a transferrable insulating layer that ensures ideal heating in the resistor when conductive (and potentially porous) organic materials are investigated. Secondly, we have developed Raman thermometry on free standing films, as a powerful tool for the determination of in-plane thermal conductivity. With these tools in hand, we have explored two systems currently being studied for thermoelectric applications. For the case of conjugated polymers, we have determined the anisotropy in the thermal conductivity of thin films, as well as studied the role of crystallinity. Moreover, by preparing samples with gradients of doping, we have been able to correlate processing conditions with doping efficiency, leading to a tunable electronic transport. The interplay between morphology and doping will be discussed also in terms of thermal conductivity. Finally, we have looked at the thermal and electrical properties of composites of polymers and carbon nanotubes. In this case, electrons travel through percolating pathways while heat transport appears to occur through an effective medium of parallel and series connected regions. To understand this, we have looked at different polymers (both semiconducting and insulating), different types of carbon nanotubes (in terms of chirality, length, etc.) and several processing methods that result in variations in CNT degree of dispersion.

Resume : Conjugated polymer poly(3,4-dioxyethylenthiophene) (PEDOT) has gained attention for room-temperature thermoelectric applications. Still, low thermoelectric efficiencies of polymers might be more easily increased, were a model for its transport properties available. We validated Kang-Snyder model transport model using the concept of transport energy. To this aim, PEDOT and PEDOT-based nanocomposites embedding CuO nanoplatelets were prepared by in situ EDOT polymerization using Fe(III) tosylate. The mixture was filmed by blade coating and annealed in air at 70 °C. Films were then submitted to cyclic voltammetry followed by a slow potential linear scan up to the final potential and then left in open circuit voltage conditions to stability. We found that Kang-Snyder model adequately fits the trends observed in pure PEDOT and in its nanocomposites. Transport and Fermi energy were verified to depend on the polymer oxidation level only, while the transport coefficient was found to be sensitive to PEDOT stacking and was modulated by the introduction of CuO nanoplatelets. The temperature dependence of the electrical conductivity and the effect of the polymer oxidation level on both the electrical conductivity and the Seebeck coefficient confirmed that PEDOT set apart from all other conductive polymers, reporting a transport exponent of 1 (instead of 3). Micromorphological factors leading to this peculiar transport regime will be commented upon.

Resume : Careful control of the doping level is required to capitalize on the unique benefits of organic semiconductors. Most studies in this field focus on the influence of backbone structures on the thermoelectric properties while the effects of side chains are less explored. In this contribution, for the first time, the polarity of fullerene derivatives is tailored by modifying the side chain. This enhances the miscibility of the host and dopant molecules. A fullerene derivative with a hydrophilic triethylene glycol type side chain ((PTEG-1)) is used as the host and (4-(1,3-dimethyl-2,3-dihydro-1H benzoimidazol-2-yl)phenyl)dimethylamine (n-DMBI) as the dopant [1]. It is found that PTEG-1 molecules readily form layered structures parallel to the substrate after solution processing. The fullerene cage plane is alternated by the triethylene glycol side chain plane; the n-DMBI dopants are mainly incorporated in the side chain plane without disturbing the π-π packing of PTEG-1. Since the polar side chains offer the space for accommodating dopant molecules and influence the molecular order, their length also plays a key role in the doping process. We systematically examine the effects of the side chains of a series of fullerene derivatives by varying their polarity and length. By optimizing the side chains, we find an electrical conductivity of 2.3 S/cm with a power factor of 23.1 μW/mK2 which leads to an estimated ZT value of 0.1-0.2. This represents one of the best results for solution-processed n-type organic thermoelectrics. Our work offers insights into the roles of side chains in n-type organic thermoelectrics. [1] J. Liu, L. Qiu, G. Portale, M. Koopmans, G. ten Brink, J.C. Hummelen, and L.J.A. Koster, N-type organic thermoelectrics: Improved power factor by tailoring host-dopant miscibility, Adv. Mater. 1701641 (2017).

Resume : The major limitations to the development of solution-processed and efficient polymer thermoelectric generators (TEGs) are ascribable to n-type conductors, for which air stability remains a challenge. Here we exploit chemical tailoring of the imide unit in a well-known naphthalene diimide copolymer to reduce the energy level of lowest-unoccupied molecular orbital (LUMO), while retaining good electronic properties. As a result, we demonstrate that the modified co-polymer can achieve a drastically improved air stability with respect to the parent system, and a better conductivity, which we assign to a better miscibility with the same benzimidazole derivative dopant. Noticeably, the electrical conductivity decreases of less than a factor of 2 over 16 hours of direct air exposure of the doped films, as opposed to the parent copolymer which loses several orders of magnitude within the same timeframe. Such result clearly highlights the effectiveness of chemical tuning in improving air stability of solution-processable polymer conductors and opens the possibility of their ambient processing, through scalable techniques such as printing, for future cost-effective polymer TEGs.

Resume : Oriented conducting polymer films are prepared using a simple two steps method based on i) P3HT alignment using high-temperature rubbing and ii) doping from solution of F4TCNQ in acetonitrile. The rubbing temperature TR controls the semi-crystalline morphology of the films : films rubbed at TR≤130°C consist of a smectic-like oriented phase whereas films prepared for TR≥150°C show a periodic lamellar structure whose periodicity depends on the undercooling. [1] After doping, the resulting charge transport (conductivity σ) and thermoelectric properties (Seebeck coefficient S) are enhanced in the direction of polymer chains and this results in improved power factors σS^2.[2] Correlations between the initial film morphology in rubbed films and the anisotropy of S and σ are determined versus TR. An optimum rubbing temperature is determined that corresponds to the highest anisotropy in charge transport and thermoelectric properties and a strongly enhanced power factor. [1] A. Hamidi-Sakr et al. Adv. Funct. Mater. 2016, 26, 408. [2] A. Hamidi-Sakr et al. Adv. Funct. Mat. 2017, 27, 1700173.