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Organic semiconductors with distinct molecular properties and large carrier mobilities are constantly developed in attempt to produce highly‐efficient electronic materials. Recently, designer molecules with unique structural modifications have been expressly developed to suppress molecular motions in the solid state that arise from low‐energy phonon modes, which uniquely limit carrier mobilities through electron–phonon coupling. However, such low‐frequency vibrational dynamics often involve complex molecular dynamics, making comprehension of the underlying electronic origins of electron–phonon coupling difficult. In this study, first a mode‐resolved picture of electron–phonon coupling in a series of materials that are specifically designed to suppress detrimental vibrational effects, is generated. From this foundation, a method is developed based on the crystalline orbital Hamiltonian population (COHP) analyses to resolve the origins—down to the single atomic‐orbital scale—of surprisingly large electron–phonon coupling constants of particular vibrations, explicitly detailing the manner in which the intermolecular wavefunction overlap is perturbed. Overall, this approach provides a comprehensive explanation into the unexpected effects of less‐commonly studied molecular vibrations, revealing new aspects of molecular design that should be considered for creating improved organic semiconducting materials. [ABSTRACT FROM AUTHOR]

Halide double perovskites have gained significant attention, owing to their composition of low‐toxicity elements, stability in air, and recent demonstrations of long charge‐carrier lifetimes that can exceed 1 µs. In particular, Cs2AgBiBr6 is the subject of many investigations in photovoltaic devices. However, the efficiencies of solar cells based on this double perovskite are still far from the theoretical efficiency limit of the material. Here, the role of grain size on the optoelectronic properties of Cs2AgBiBr6 thin films is investigated. It is shown through cathodoluminescence measurements that grain boundaries are the dominant nonradiative recombination sites. It also demonstrates through field‐effect transistor and temperature‐dependent transient current measurements that grain boundaries act as the main channels for ion transport. Interestingly, a positive correlation between carrier mobility and temperature is found, which resembles the hopping mechanism often seen in organic semiconductors. These findings explain the discrepancy between the long diffusion lengths >1 µm found in Cs2AgBiBr6 single crystals versus the limited performance achieved in their thin film counterparts. This work shows that mitigating the impact of grain boundaries will be critical for these double perovskite thin films to reach the performance achievable based on their intrinsic single‐crystal properties. [ABSTRACT FROM AUTHOR]

The cornerstones of emerging high-performance organic photovoltaic devices are bulk heterojunctions, which usually contain both structure disorders and bicontinuous interpenetrating grain boundaries with interfacial defects. This feature complicates fundamental understanding of their working mechanism. Highly-ordered crystalline organic p–n heterojunctions with well-defined interface and tailored layer thickness, are highly desirable to understand the nature of organic heterojunctions. However, direct growth of such a crystalline organic p–n heterojunction remains a huge challenge. In this work, we report a design rationale to fabricate monolayer molecular crystals based p–n heterojunctions. In an organic field-effect transistor configuration, we achieved a well-balanced ambipolar charge transport, comparable to single component monolayer molecular crystals devices, demonstrating the high-quality interface in the heterojunctions. In an organic solar cell device based on the p–n junction, we show the device exhibits gate-tunable open-circuit voltage up to 1.04 V, a record-high value in organic single crystalline photovoltaics. Realizing organic p–n junctions based on ordered crystalline materials with dimensions comparable to the exciton diffusion length of most organic semiconductors remains a challenge. Here, the authors report a strategy to form molecular monolayer crystal-based p–n junctions with thickness below 5 nm. [ABSTRACT FROM AUTHOR]

A novel trap mitigation mechanism using molecular additives, which relieves a characteristic early turn‐on voltage in a high‐mobility p‐type acene‐based small‐molecule organic semiconductor, when processed from hydrous solvents, is reported. The early turn‐on voltage is attributed to photo‐induced trapping, and additive incorporation is found to be very effective in suppressing this effect. Remarkably, the molecular additive does not disturb the charge transport properties of the small‐molecule semiconductor, but rather intercalates in the crystal structure. This novel technique allows for the solution‐processing of small molecular semiconductors from hydrous solvents, greatly simplifying manufacturing processes for large‐area electronics. Along with various electric and spectroscopic characterization techniques, simulations have given a deeper insight into the trap mitigation effect induced by the additive. [ABSTRACT FROM AUTHOR]

In organic device applications, a high contact resistance between metal electrodes and organic semiconductors prevents an efficient charge injection and extraction, which fundamentally limits the device performance. Recently, various contact doping methods have been reported as an effective way to resolve the contact resistance problem. However, the contact doping has not been explored extensively in organic field effect transistors (OFETs) due to dopant diffusion problem, which significantly degrades the device stability by damaging the ON/OFF switching performance. Here, the stability of a contact doping method is improved by incorporating "dopant‐blockade molecules" in the poly(2,5‐bis(3‐hexadecylthiophen‐2‐yl)thieno[3,2‐b]thiophene) (PBTTT) film in order to suppress the diffusion of the dopant molecules. By carefully selecting the dopant‐blockade molecules for effectively blocking the dopant diffusion paths, the ON/OFF ratio of PBTTT OFETs can be maintained over 2 months. This work will maximize the potential of OFETs by employing the contact doping method as a promising route toward resolving the contact resistance problem. [ABSTRACT FROM AUTHOR]

Understanding the electronic structure of organic-organic heterointerfaces is crucial for many device applications of organic semiconductors. Here we have developed a simple analytical model to describe the effect of static dipolar disorder in a polymer dielectric on the density of states of an adjacent organic semiconductor. The degree of energetic disorder varies strongly with distance from the interface. Using a simple mobility model, we have been able to explain quantitatively both the magnitude as well as the gate voltage dependence of the field-effect mobility for polymer gate dielectrics with different dielectric constants. [ABSTRACT FROM AUTHOR]

Abstract: The molecular structure and electrical properties of two n‐type organic small molecules which have the same core but different alkyl chains are compared. The effect of the length of alkyl chains on charge transport along the alkyl‐chain stacking direction (out‐of‐plane) is investigated by a combination of techniques. The results show that a longer alkyl chain corresponds to lower out‐of‐plane mobility (µout‐of‐plane). More importantly, it is found that this effect not only plays a significant role in affecting the performance of diode‐structure devices such as organic light‐emitting diodes (OLEDs) or organic photovoltaics (OPVs), but also has profound influence on the performance of organic field‐effect transistors (OFETs), especially for OFETs which are in staggered configuration. [ABSTRACT FROM AUTHOR]

The Seebeck coefficient of both electrons and holes in a family of ambipolar diketopyrrolopyrrole copolymers is investigated using field-effect gated measurements. It is found that at high carrier concentrations the magnitude of the Seebeck coefficients is not strongly dependent on the molecular structure of the backbone within this family of polymers. Additionally, the Seebeck coefficients of electrons and holes have very similar magnitudes showing that electrons and holes experience a similar energetic landscape. Subtle differences in the carrier concentration dependence between the polymers are observed, which are interpreted in terms of different degrees of energetic disorder on the basis of molecular dynamics and electronic structure simulations. [ABSTRACT FROM AUTHOR]

The channel temperature (Tch) of solution‐processed 6,13‐bis(triisopropylsilylethynyl)‐pentacene (TIPS pentacene) thin film transistors (TFTs) is closely monitored in real time during current–voltage (I–V) measurements carried out in a He exchange gas cryostat at various base temperatures (Tb) between 300 K and 20 K. This is done using a platinum (Pt) resistance temperature sensor embedded within the transistor channel. Under large gate (Vg) and source‐drain (Vds) voltage biases, an increase in Tch is observed, the magnitude of which depends on the thermal conductivity of the substrate. The increase in Tch is associated with a simultaneous increase in the transistor drain current (Id) and becomes particularly pronounced at cryogenic Tb. These experimental observations are rationalized using a 1D theoretical model and are attributed to current‐induced Joule heating. However, even though the heating of the channel unquestionably plays an important role, the corresponding amount of increase in Id at cryogenic Tb and large voltage biases cannot be fully accounted for unless at low temperatures μTIPS is enhanced in the presence of strong electrical fields. Therefore it is concluded that the I–V characteristics of TIPS pentacene TFTs at low Tb and large voltage biases are a result of a complex interplay between current‐induced Joule heating and electrical field effects. [ABSTRACT FROM AUTHOR]

Polymer semiconductors provide unique possibilities and flexibility in tailoring their optoelectronic properties to match specific application demands. The recent development of semicrystalline polymers with strongly improved charge transport properties forces a review of the current understanding of the charge transport mechanisms and how they relate to the polymer's chemical and structural properties. Here, the charge density dependence of field effect mobility in semicrystalline polymer semiconductors is studied. A simultaneous increase in mobility and its charge density dependence, directly correlated to the increase in average crystallite size of the polymer film, is observed. Further evidence from charge accumulation spectroscopy shows that charges accumulate in the crystalline regions of the polymer film and that the increase in crystallite size affects the average electronic orbitals delocalization. These results clearly point to an effect that is not caused by energetic disorder. It is instead shown that the inclusion of short range coulomb repulsion between charge carriers on nanoscale crystalline domains allows describing the observed mobility dependence in agreement with the structural and optical characterization. The conclusions that are extracted extend beyond pure transistor characterization and can provide new insights into charge carrier transport for regimes and timescales that are relevant to other optoelectronic devices. [ABSTRACT FROM AUTHOR]

Wide-bandgap conjugated polymers with a linear naphthacenodithiophene (NDT) donor unit are herein reported along with their performance in both transistor and solar cell devices. The monomer is synthesized starting from 2,6-dihydroxynaphthalene with a double Fries rearrangement as the key step. By copolymerization with 2,1,3-benzothiadiazole (BT) via a palladium-catalyzed Suzuki coupling reaction, NDT-BT co-polymers with high molecular weights and narrow polydispersities are afforded. These novel wide-bandgap polymers are evaluated as the semiconducting polymer in both organic field effect transistor and organic photovoltaic applications. The synthesized polymers reveal an optical bandgap in the range of 1.8 eV with an electron affinity of 3.6 eV which provides sufficient energy offset for electron transfer to PC70BM acceptors. In organic field effect transistors, the synthesized polymers demonstrate high hole mobilities of around 0.4 cm2 V-1 s-1. By using a blend of NDT-BT with PC70BM as absorber layer in organic bulk heterojunction solar cells, power conversion efficiencies of 7.5% are obtained. This value is among the highest obtained for polymers with a wider bandgap (larger than 1.7 eV), making this polymer also interesting for application in tandem or multijunction solar cells. [ABSTRACT FROM AUTHOR]

Organic thermoelectric materials, which can transform heat flow into electricity, have great potential for flexible, ultra-low-cost and large-area thermoelectric applications. Despite rapid developments of organic thermoelectric materials, exploration and investigation of promising organic thermoelectric semiconductors still remain as a challenge. Here, the thermoelectric properties of several p- and n-type organic semiconductors are investigated and studied, in particular, how the electric field modulations of the Seebeck coefficient in organic field-effect transistors (OFETs) compare with the Seebeck coefficient in chemically doped films. The extracted relationship between the Seebeck coefficient ( S) and electrical conductivity ( σ) from the field-effect transistor (FET) geometry is in good agreement with that of chemically doped films, enabling the investigation of the trade-off relationship among σ, S, carrier concentration, and charging level. The results make OFETs an effective candidate for the thermoelectric studies of organic semiconductors. [ABSTRACT FROM AUTHOR]

Injection and extraction of charges through ohmic contacts are required for efficient operation of semiconductor devices. Treatment using polar non-solvents switches polar anode surfaces, including PEDOT:PSS and ITO, from barrier-limited hole injection and extraction to ohmic behaviour. This is caused by an in-situ modification of the anode surface that is buried under a layer of organic semiconductor. The exposure to methanol removes polar hydroxyl groups from the buried anode interface, and permanently increases the work function by 0.2-0.3 eV. In the case of ITO/PEDOT:PSS/PBDTTT-CT:PC71BM/Al photovoltaic devices, the higher work function promotes charge transfer, leading to p-doping of the organic semiconductor at the interface. This results in a two-fold increase in hole extraction rates which raises both the fill factor and the open-circuit voltage, leading to high power conversion efficiency of 7.4%. In ITO/PEDOT:PSS/F8BT/Al polymer light-emitting diodes, where the organic semiconductor's HOMO level lies deeper than the anode Fermi level, the increased work function enhances hole injection efficiency and luminance intensity by 3 orders of magnitude. In particular, hole injection rates from PEDOT:PSS anodes are equivalent to those achievable using MoO3. These findings exemplify the importance of work function control as a tool for improved electrode design, and open new routes to device interfacial optimization using facile solvent processing techniques. Such simple, persistent, treatments pave the way towards low cost manufacturing of efficient organic optoelectronic devices. [ABSTRACT FROM AUTHOR]

In spintronics, pure spin currents play a key role in transmitting, processing and storing information. A pure spin current is a flow of electron spin angular momentum without a simultaneous flow of charge current. It can be carried by conduction electrons or magnons and has been studied in many inorganic metals, semiconductors and insulators, but not yet in organic semiconductors. Charge carriers in π-conjugated organic materials are localized spin-1/2 polarons which move by hopping, but the mechanisms of their spin transport and relaxation are not well understood. Here we use ferromagnetic resonance spin pumping in a ferromagnet/conjugated polymer/nonmagnetic spin-sink trilayer to demonstrate the ability of polarons to carry pure spin currents over hundreds of nanometres with long spin relaxation times of up to a millisecond and to exhibit Hanle precession. By systematically comparing charge and spin transport on the same trilayer we show that spin-orbit coupling mediates spin relaxation at room temperature. [ABSTRACT FROM AUTHOR]

Many of the remarkable electrical and optical properties of organic semiconductors are governed by the interaction of electronic excitations with intra- and intermolecular vibrational modes. However, in specific systems this interaction is not understood in detail at a molecular level and this has been due, at least in part, to the lack of easy-to-use and widely available experimental probes of the structural dynamics. Here we demonstrate that thermal diffuse scattering in electron diffraction patterns from organic semiconductors, such as 6,13-bistriisopropyl-silylethynyl pentacene, allows the dominant lattice vibrational modes to be probed directly. The amplitude and direction of the dominant molecular motions were determined by comparison of the diffuse scattering with simulations and molecular dynamics calculations. Our widely applicable approach enables a much deeper understanding of the structural dynamics in a broad range of organic semiconductors. [ABSTRACT FROM AUTHOR]

This review is focused on understanding of the charge-transport physics of high-mobility organic semiconductors at a molecular level. We review recent high-mobility small-molecule and conjugated polymer materials with a focus on crystalline materials that have been able to exceed mobilities of 0.5-1 cm2/V s. We discuss some of the main, competing factors that govern charge transport in these materials and present theoretical approaches that have been developed to describe systems in which moderately strong intermolecular electronic interactions and strong electron-phonon interactions are present. Finally, we review recent experimental results that have aimed to address the important question of whether at room-temperature charge carriers in these high-mobility organic semiconductors are in fact simply extended Bloch electrons that undergo occasional scattering processes or are localized on individual molecules and move by hopping. [ABSTRACT FROM AUTHOR]

Polymer/organic optoelectronic devices have drawn the attention of both the academic and industrial research communities due to the potential for a low-cost, large-area, solution-processable technology alternative to conventional inorganic optoelectronics. Issues related to the stability and degradation of the organic/polymer-based optoelectronics are hampering the progress in the field. The use of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT : PSS) as the anode and reactive metals as the cathode, as well as the degradation of organic semiconductors in ambient atmosphere, are some of the stability issues to be addressed. To resolve these issues, in the past decade, there has been a growing interest in research of hybrid optoelectronic devices which employ metal oxides as air-stable charge injecting/extracting layers that sandwich the photo-responsive organic active layer and protect it from the ambient oxygen and moisture and prevent photo-oxidation by absorbing UV light. Herein, we review the recent advances made in hybrid optoelectronics and discuss the tremendous potential of these devices. [ABSTRACT FROM AUTHOR]

The mobility μ of solution-processed organic semiconductorshas improved markedly to room-temperature values of 1–5 cm2 V−1 s−1. In spite of their growing technological importance, the fundamental open question remains whether charges are localized onto individual molecules or exhibit extended-state band conduction like those in inorganic semiconductors. The high bulk mobility of 100 cm2 V−1 s−1 at 10 K of some molecular single crystals provides clear evidence that extended-state conduction is possible in van-der-Waals-bonded solids at low temperatures. However, the nature of conduction at room temperature with mobilities close to the Ioffe–Regel limit remains controversial. Here we investigate the origin of an apparent ‘band-like’, negative temperature coefficient of the mobility (dμ/dT<0) in spin-coated films of 6,13-bis(triisopropylsilylethynyl)-pentacene. We use optical spectroscopy of gate-induced charge carriers to show that, at low temperature and small lateral electric field, charges become localized onto individual molecules in shallow trap states, but that a moderate lateral electric field is able to detrap them resulting in highly nonlinear, low-temperature transport. The negative temperature coefficient of the mobility at high fields is not due to extended-state conduction but to localized transport limited by thermal lattice fluctuations. [ABSTRACT FROM AUTHOR]

Abstract: Most conjugated polymer field-effect transistors rely on charge accumulation between an organic semiconductor and a gate dielectric. Much less use has been made of devices based on heterojunctions between two semiconducting polymers. Here we have realized ambipolar polymer field-effect transistors based on bilayers of two polyfluorene-based semiconducting polymers, poly(9,9-dioctylfluorene-alt-N-(4-butylphenyl)diphenylamine) and poly(9,9-dioctylfluorene-alt-benzothiadiazole). We show that the energy offset between the lowest unoccupied molecular orbitals of the two polymers of 1–1.5eV is sufficient to confine electrons at the bi-layer interface for n-type operation. However, the transfer current–voltage characteristics exhibit an unusual plateau. This is shown to be due to hole charge trapping in donor-like states in the TFB layer during p-type operation. [Copyright &y& Elsevier]

Organic field-effect transistors (FETs) are presently attracting significant academic research and industrial development interests as they offer performance capabilities comparable to those of thin-film amorphous silicon transistors but at the same time are compatible with low-temperature solution/printing-based manufacturing on flexible plastic substrates. In this paper, we review recent materials advances to improve the field-effect mobility of solution-processed organic semiconductors, discuss recent insight into the physics that determines the electronic structure at the semiconductor-gate dielectric interface in these devices, and provide an overview over some of the near- and medium-term applications. [ABSTRACT FROM AUTHOR]

The article provides an overview on the working principles of organic field-effect transistors and describe how both hole and electron transport take place in these devices. The authors first introduce the basics of field-effect transistors and define the parameters affecting the performance of the device in general. They then demonstrate the prerequisites for fabricating n-channel transistors. Lastly, they discuss both unipolar and ambipolar light-emitting transistors.

Organic semiconductors show efficient electroluminescence which has led to their commercialization in light-emitting diodes, but has also raised fundamental questions about their recombination and emission physics. Organic ambipolar field-effect transistors can support both hole and electron transport at the semiconductor–dielectric interface. Using their ability to emit light owing to charge recombination within the transistor channel should enable new ways to study the recombination physics and realize new electrooptical devices. Here we demonstrate ambipolar light-emitting transistors based on a semiconducting polymer with both efficient electron and hole transport and good photoluminescence efficiency. In our device configuration, electrons and holes injected from separate calcium and gold electrodes recombine radiatively within the channel. We can move the recombination zone with the applied gate and source–drain bias to any position within the channel. This provides a direct visualization and proof of coexisting electron and hole accumulation layers in an ambipolar transport regime. [ABSTRACT FROM AUTHOR]

Organic semiconductors have been the subject of active research for over a decade now, with applications emerging in light-emitting displays and printable electronic circuits. One characteristic feature of these materials is the strong trapping of electrons but not holes: organic field-effect transistors (FETs) typically show p-type, but not n-type, conduction even with the appropriate low-work-function electrodes, except for a few special high-electron-affinity or low-bandgap organic semiconductors. Here we demonstrate that the use of an appropriate hydroxyl-free gate dielectric-such as a divinyltetramethylsiloxane-bis(benzocyclobutene) derivative (BCB; ref. 6)-can yield n-channel FET conduction in most conjugated polymers. The FET electron mobilities thus obtained reveal that electrons are considerably more mobile in these materials than previously thought. Electron mobilities of the order of 10-3 to 10-2?cm2?V-1?s-1 have been measured in a number of polyfluorene copolymers and in a dialkyl-substituted poly(p-phenylenevinylene), all in the unaligned state. We further show that the reason why n-type behaviour has previously been so elusive is the trapping of electrons at the semiconductor-dielectric interface by hydroxyl groups, present in the form of silanols in the case of the commonly used SiO2 dielectric. These findings should therefore open up new opportunities for organic complementary metal-oxide semiconductor (CMOS) circuits, in which both p-type and n-type behaviours are harnessed. [ABSTRACT FROM AUTHOR]

We have investigated the dependence of bias-stress induced degradation in organic field-effect transistors on the device configuration. We show that separation of contact and channel effects is essential for understanding bias-stress instabilities. In coplanar device configurations, an increase in source contact resistance during current flow is primarily responsible for a rapid device degradation. In contrast, in staggered device configurations, the significantly slower reduction in current is primarily due to charge trapping in the channel leading to an increase in threshold voltage, while the contacts themselves do not exhibit significant degradation. [ABSTRACT FROM AUTHOR]

The article "Theo Murphy International Scientific Meeting Between the UK and China on the Chemistry and Physics of Functional Materials" by Wenping Hu et al. is cited.

Wehave investigated the degradation effects of ozone exposureon organic field-effect transistors based on 2,8-difluoro-5,11-bis(triethylsilylethynyl)anthradithiopheneas the organic semiconducting channel layer, as well as on thin filmsof this widely used, high-mobility, small molecule semiconductor.Electrical I–Vmeasurementsshowed a loss of transistor characteristic behavior. We present 1H Nuclear Magnetic Resonance (NMR) spectroscopy results aswell as X-ray Photoemission Spectroscopy (XPS) and Fourier TransformInfrared (FTIR) spectroscopy measurements showing the oxidation ofthe parent molecule, from which we suggest various possible reactionpaths. [ABSTRACT FROM AUTHOR]

Substituted side chains are fundamental units in solution processable organic semiconductors in order to achieve a balance of close intermolecular stacking, high crystallinity, and good compatibility with different wet techniques. Based on four air-stable solution-processed naphthalene diimides fused with 2-(1,3-dithiol-2-ylidene)malononitrile groups (NDI-DTYM2) that bear branched alkyl chains with varied side-chain length and different branching position, we have carried out systematic studies on the relationship between film microstructure and charge transport in their organic thin-film transistors (OTFTs). In particular synchrotron measurements (grazing incidence X-ray diffraction and near-edge X-ray absorption fine structure) are combined with device optimization studies to probe the interplay between molecular structure, molecular packing, and OTFT mobility. It is found that the side-chain length has a moderate influence on thin-film microstructure but leads to only limited changes in OTFT performance. In contrast, the position of branching point results in subtle, yet critical changes in molecular packing and leads to dramatic differences in electron mobility ranging from 0.001 to >3.0 cm2 V-1 s-1. Incorporating a NDI-DTYM2 core with three-branched N-alkyl substituents of C11,6 results in a dense in-plane molecular packing with an unit cell area of 127 Å2, larger domain sizes of up to 1000 x 3000 nm2, and an electron mobility of up to 3.50 cm2 V-1 s-1, which is an unprecedented value for ambient stable n-channel solution-processed OTFTs reported to date. These results demonstrate that variation of the alkyl chain branching point is a powerful strategy for tuning of molecular packing to enable high charge transport mobilities. [ABSTRACT FROM AUTHOR]