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We study the effect of Gaussian energetic disorder on the organic field-effect transistors (OFETs) with surprisingly high field-effect mobility and the low contact resistance. The numerical device simulation assumes the thermally assisted hopping transport and injection in disordered semiconductors. The results are analyzed with the power-law field-effect mobility and the asymptotic power-law contact resistance model. Transistor parameters extracted by the models reveal that a higher Gaussian disorder, which leads to a lower injection barrier and a larger mobility enhancement, is the origin of the high field-effect mobility and the low contact resistance.

In this review, recent advances in compact modeling of organic field-effect transistors (OFETs) are presented. Despite the inherent strength for printed flexible electronics and the extremely aggressive research conducted over more than three decades, the OFET technology still seems to remain at a relatively low technological readiness level. Among various possible reasons for that, the lack of a standard compact model, which effectively bridges the device- and system-level development, is clearly one of the most critical issues. This article broadly discusses the essential requirements, up-to-date progresses, and imminent challenges for the OFET compact device modeling toward a universal, physically valid, and applicable description of this fast-developing technology.

We present a drain current model and a consistent parameter extraction method for organic field-effect transistors (OFETs) considering the gate voltage-dependent contact resistance and mobility by power-law. The physical origin of the asymptotic power-law dependence of contact resistance is elaborated based on the current crowding model. The parameters of a new model can be completely extracted without ambiguity by the proposed extraction method based on the linear fitting. Pentacene-based OFETs with the Au source/drain contact in a staggered configuration experimentally validate the proposed model and the extraction method, which further reveal the bulk resistivity as the major contribution to contact resistance.

Correct parameterization of the Gaussian disorder model (GDM) on spatially random sites is necessary for a complete description of charge transport in disordered materials and concomitant device characteristics. Because the GDM on spatially random sites considers both energetic and spatial disorder, it is superior to the GDM on a cubic lattice. However, analytical arguments and experimental evidence are still lacking for correct parameterization of the model over a wide range of model parameters, energetic and spatial disorder, and electric fields. We show that the model requires a set of parameters to correctly account for high mobility and its charge density dependence, and we develop such a model. The model is implemented in a numerical simulation tool for comparison with the measured device characteristics. Accurate agreement with experimental data, particularly with the high mobility values in organic field-effect transistors, is achieved throughout a wide range of temperature by adjusting both the localization length and the attempt-to-escape frequency.

Although metallic nanostructures in solar cells provide versatility in designing useful plasmonic architectures, understanding is still limited on how to exploit their multi-scale contribution as tunable performance. In this article, we suggest a characteristic model that develops into a simple and robust tool for guiding optimization of plasmonic solar devices. The model is conceptually based on the breakdown of the active region into intrinsic and plasmonic sub-circuits, by which the terminal currents are directly correlated with particle geometries and local improvement. Measurements from organic cells support the validity of our theory, and a series of simulation provides further insights into the critical trade-off between voltage and current generation, finally offering a strategy for efficiency enhancement.

Contact resistance is a major characteristic of organic transistors, and its importance has received renewed attention due to the recent revelation of mobility overestimation. In this article, we propose a method to describe the contact resistance as a closed-form compact equation of the materials, interfaces, and geometrical parameters. The proposed model allows us to quantitatively understand the correlation between charge-injection and transport properties, while providing a tool for performance prediction and optimization. This theory is applied to a set of experimentally fabricated devices to exemplify how to utilize the model in practice.

In this communication, a new theoretical model for the ac transit frequency of organic rectifier diodes is proposed. The model is built upon a full description of the drift-diffusion current in the rectifier diode for the entire forward bias regime. Such a method precisely correlates the transit frequency to a number of materials, geometrical, and operational parameters in particular the charge-Injection barriers. By systematic variation of parameters, it is found that a rectifier operating beyond 1 GHz is observable only with a highly injecting anode. Other parameters to reach that regime of operation are discussed. This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2018R1D1AB07048877), and by Materials, Components & Equipment Research Program funded by the Gyeonggi Province.

The injection barrier is a key parameter that governs the charge and current density in organic field-effect transistors (OFETs). However, the energetic disorder of organic disordered semiconductors (ODSs) hinders a clear definition of the injection barrier. We study the validity of effective injection barrier at the metal/organic disordered semiconductor by means of systematic numerical simulation of OFETs in full consideration of the Gaussian density-of-states. The contact resistance from the numerical simulation and analytical model are compared to verify this procedure. By varying the Gaussian width, the effective injection barrier is validated for low degree of disorder at which the semiconductor is under non-degenerate condition. The position of Fermi level with respect to the Gaussian width distinguishes the non-degenerate and degenerate condition of ODSs, indicating clearly the range of validity of the effective injection barrier.

Finite-element modeling for efficient organic bulk-heterojunction photovoltaics is presented. Given the complexity of the architecture, it has been hard to devise a sufficiently compact description of the physics of charge and exciton dynamics. This paper delineates the most essential set of equations that allow to reproduce the major characteristics of a current---voltage curve measured under illumination. Parameters are extracted by optimization of a fabricated polymer/fullerene cell, and by deliberately changing key parameters, further understanding is established on the phenomenological manifestation of independent physical processes.

Nonlinear transport is intensively explained through Poole-Frenkel (PF) transport mechanism in organic thin film transistors with solution-processed small molecules, which is, 6,13-bis(triisopropylsilylethynyl) (TIPS) pentacene. We outline a detailed electrical study that identifies the source to drain field dependent mobility. Devices with diverse channel lengths enable the extensive exhibition of field dependent mobility due to thermal activation of carriers among traps.

So far, the most widely addressed hopping transport model for organic disordered semiconductors (ODSs) is the so-called extended Gaussian disordered model (EGDM) [1] based on numerical simulations within the cubic lattice without spatial disorder. As known since 1973 [2] and confirmed recently [3], the EGDM is based on the parametrization of transport coefficients in the framework of the irrelevant set of parameters. Parameters responsible for the effects are not in the equations, while parameters in the equations are not responsible for the effects [3]. The deficiencies of the EGDM are particularly pronounced for systems with spatial disorder. Therefore, the study of Gaussian disordered model (GDM) with spatial disorder is necessary to provide a simulation tool for transport coefficients of charge carriers in real organic thin film transistors (OTFTs) and organic light-emitting diodes (OLEDs) based on materials with spatial disorder. In this study, we examine the GDM which contains both spatial and energetic disorders and propose a physical parametrization for OTFTs. We determine all the parameters by adjusting the localization length and the attempt-to-escape frequency to satisfy experimentally measured mobility. For the localization length, the concept of effective localization length [4] is adopted which has the order of a molecular diameter, because assumption of the very low localization length (commonly order of 10-8 cm) induces very low mobility due to the strong localization of the each state. The large effective localization length can alleviate the strong localization and so it can predict the similar scale of mobility with experimentally measured one. In addition, the proper value of the attempt-to-escape frequency is estimated from comparison between a direct calculation of Miller-Abrahams (MA) transition rate and theoretical estimations of the hopping transition rate based on MA and Marcus theories. We show that its value should be increased by more than two orders of magnitude than conventional value to simulate high mobility device. Then, we validate our parametrization of GDM through comparison between experimentally measured and numerically calculated transfer curves and charge carrier mobility of OTFTs at various temperature (Fig. 1). The staggered pentacene-based OTFT device was fabricated and measured at different temperatures T = 329, 293, 258 and 233 K. The charge carrier mobility was extracted by the transmission line method (TLM). For numerical simulation, we implemented the GDM by Baranovskii [3] into the commercial TCAD simulation tool, Silvaco ATLAS. Then, the numerical solver calculated Poisson’s, continuity and drift-diffusion equations in a self-consistent manner with characteristics of ODSs such as Gaussian density-of-states, generalized Einstein relation and GDM by Baranovskii. Numerical simulation with a single set of parameters makes an accurate fit with experimentally measured transfer curves (Fig. 1a) and charge carrier mobility (Fig. 1b) at every temperature condition. In conclusion, we propose physically based parametrization of GDM with fully spatial and energetical disorder for OTFTs. It is confirmed that the localization length and the attempt-to-escape frequency are key parameters for the practical fitting with experimental result. In addition, we can expect that the electronic design automate (EDA) industry can be promoted by providing rigorous transport model and its parametrization for OTFTs within commercial simulation software. [1] W. F. Pasveer, J. Cottaar, C. Tanase, R. Coehoorn, P. A. Bobbert, P. W. M. Blom, M. De Leeuw, and M. A. J. Michels, Phys. Rev. Lett. 94, 206601 (2005). [2] B.I. Shklovskii Sov. Phys.-Semicond. 6, 1964 (1973). [3] S. D. Baranovskii, Phys. Status Solidi Appl. Mater. Sci. 215, 1700676 (2018). [4] A. V Nenashev, J. O. Oelerich, and S. D. Baranovskii, J. Phys. Condens. Matter 27, 093201 (2015). Figure 1

The gap-type diode is a basic component of organic field-effect transistors. In the critical voltage range above 10 V, the gap-type diode works in the same way as a sandwich-type diode adapted for organic light-emitting diodes and organic photovoltaic cells. However, the gap-type diode is rarely studied, and compact modelling is not often found in previous reports that deal with this type of an organic diode. Enhanced bulk transport is exhibited using the one-pot treatment of self-assembled monolayers (SAMs) for an SiO2 surface and an Au contact metal in the gap-type organic diode. Charge-injection improvement via the SAM treatment on Au induces higher bulk transport. This phenomenon is analysed mainly through compact modelling.

We demonstrated modulation of charge carrier densities in all-solution-processed organic field-effect transistors (OFETs) by modifying the injection properties with self-assembled monolayers (SAMs). The all-solution-processed OFETs based on an n-type polymer with inkjet-printed Ag electrodes were fabricated as a test platform, and the injection properties were modified by the SAMs. Two types of SAMs with different dipole direction, thiophenol (TP) and pentafluorobenzene thiol (PFBT) were employed, modifying the work function of the inkjet-printed Ag (4.9 eV) to 4.66 eV and 5.24 eV with TP and PFBT treatments, respectively. The charge carrier densities were controlled by the SAM treatment in both dominant and non-dominant carrier-channel regimes. This work demonstrates that control of the charge carrier densities can be efficiently achieved by modifying the injection property with SAM treatment; thus, this approach can achieve polarity conversion of the OFETs.

International audience; Molecular orientation and packing motif governs charge-transport property of organic semiconductor films, especially for planar small molecules. We analyze the surface-induced orientation of copper phthalocyannine (CuPc) molecules deposited on graphene or poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) covered indium-tin-oxide (PEDOT:PSS/ITO). The CuPc films deposited on graphene are templated with preferential face-on stacking, whereas the molecules on PEDOT:PSS/ITO crystallize with edge-on ordering. Static current-voltage measurement and small-signal impedance spectroscopy are combined to elucidate the structural impact on the electrical response when those films are part of a rectifying diode. The graphene-templated diode shows enhanced out-of-plane hole conduction as compared to the diode with a PEDOT:PSS/ITO contact. Equivalent circuits describing charge injection and transport properties are proposed.

In spite of impressive improvements achieved for organic field-effect transistors (OFETs), there is still a lack of theoretical understanding of their behaviors. Furthermore, it is challenging to develop a universal model that would cover a huge variety of materials and device structures available for state-of-the-art OFETs. Nonetheless, currently there is a strong need for specific OFET compact models when device-to-system integration is an important issue. We briefly describe the most fundamental characters of organic semiconductors and OFETs, which set the bottom line dictating the requirement of an original model different from that of conventional inorganic devices. Along with an introduction to the principles of compact modeling for circuit simulation, a comparative analysis of the reported models is presented with an emphasis on their primary assumptions and applicability aspects. Critical points for advancing OFET compact models are discussed in consideration of the recent understanding of device physics.

We report an in-depth study of three-dimensional (3D) inkjet-printed flexible organic field-effect transistors (FETs) and integrated circuits (ICs), and demonstrate the necessity, the feasibility and the key aspects of modelling-driven design and analysis. In particular, we performed compact modelling of the flexible printed organic FETs to determine transistor parameters from measured electrical characteristics, and to optimize the design of transistors for a balanced inverter. We also conducted a SPICE simulation of both static and dynamic behaviours of flexible printed organic complementary inverters and ring oscillators. This study revealed design rules to guide fabrication of the 3D structure of the inverter. The study also showed that the parasitic capacitances are dominant factors that determine the transient behaviour of the ring oscillator. Our work provides insights that can guide further improvement of flexible printed organic FETs and ICs towards the realisation of increasingly complex organic ICs and unconventional electronic applications.

Planar diodes with gold electrodes and a pentacene semiconductor layer were made and electrically characterized. The gold electrodes were modified with self-assembled monolayers of biphenylthiol and biphenylthiol substituted with a terminal fluorine atom (fluorobiphenylthiol). Atomic force microscopy of the pentacene layer reveals large morphological similarities when the film is grown on gold modified with either of the two kinds of self-assembled monolayer. This is at variance with the significant increase of the current observed when the gold electrodes are treated with fluorobiphelylthiol, with a bulk mobility rising up to 1 cm2/V s, while the treatment with biphelylthiol leads to a substantial decrease of the current. These results are interpreted in terms of adjustment of the injection barrier height as a result of the interface dipole induced by the self-assembled monolayer.

Control, understanding and manipulation of the morphology and nanostructure of the active layer of bulk heterojunction solar cells are crucial for the development of polymeric solar cells. Rod–coil and all-conjugated block copolymers can self-assemble into well-ordered morphologies and provide an exciting approach to rationally design and control the nanoscale organization of these materials in organic photovoltaic devices. This article reviews the highlights of the synthesis, morphology, and properties of block copolymers for application in organic photovoltaics. Combination of the precise living polycondensation and controlled polymerization through the grafting-from or grafting-onto approaches produce various architectures of rod–coil and all conjugated block copolymers. The relationships between copolymer morphologies, physical properties and device performance are analyzed.

We have investigated effects of the solvent polarity of a polymeric insulator on field-effect mobility in a pentacene used thin-film transistor (TFT). In our work, we used a poly(4-vinylphenol) (PVP) as a gate insulator to dissolve three different solvents. As the solvent polarity of the PVP solution is increased, the field-effect mobility is dramatically increased by a factor of five (from 0.30 cm2/Vs to 1.51 cm2/Vs). We found that an increase of polar solvents in polymeric insulators results in improved electrical properties in the organic TFTs.

Here, we propose an advanced compact analytical current-voltage model for organic field-effect transistors (OFETs), which can be incorporated into SPICE-type circuit simulators. We improved the output saturation behavior by introducing a new asymptotic function that also enables more precise low-voltage current and conductance fitting. A new expression for the subthreshold current was suggested to cover all operation regimes of OFETs. All model parameters were extracted by a systematic method, and the comparison of the modeled current with the experimental data on pentacene-based OFETs confirmed the validity of the model over a wide operation range.

We propose a theoretical description of the charge distribution and the contact resistance in coplanar organic field-effect transistors (OFETs). Based on the concept that the current in organic semiconductors is only carried by injected carriers from the electrodes, an analytical formulation for the charge distribution inside the organic layer was derived. We found that the contact resistance in coplanar OFETs arises from a sharp low-carrier-density zone at the source/channel edge because the gate-induced channel carrier density is orders of magnitude higher than the source carrier density. This image is totally different from the contact resistance in staggered OFETs, in which the contact resistance mainly originates from the resistance through the semiconductor bulk. The contact resistance was calculated through charge-distribution functions, and the model could explain the effect of the gate voltage and injection barrier on the contact resistance. Experimental data on pentacene OFETs were analyzed using the transmission-line method. We finally noticed that the gate-voltage-dependent mobility is a critical factor for proper understanding of the contact resistance in real devices.

Accurate description of the electrical behavior oforganic solar cells (OSCs) becomes necessary with the advanceof OSC technology toward up-scaled modules. We propose herea compact model for OSCs, by identifying different regimespresent in the device operation in dark and under light:OFF, exponential sub-VON, power-law above-VON, and photocurrent.We also present a complete parameter extraction methodfor the model. In addition, we discuss the effect of the resistivityof transparent electrode and the device size on the OSC performance,by illustrating expected modifications on the current–voltage curve and parameter values.

Recent improvement in the performance of the n-type organic semiconductors aswell as thin gate dielectrics based on cross-linked polymers offers new opportunitiesto develop high-performance low-voltage n-type OFETs suitable for organic complementarycircuits. Using TIPS-tetracyanotriphenodioxazine (TIPS-TPDO-tetraCN)and cross-linked poly(methyl methacrylate) (c-PMMA), respectively as n-type organicsemiconductor and gate dielectric, linear regime field-effect mobility (1.8+_0.2).10−2cm2V−1s−1, small spatial standard deviation of threshold voltage (∼ 0.1 V), and operatingvoltage less than 3 V are attainable with the same device structure and contact materials used commonly for p-type OFETs. Through comparative static and dynamiccharacterizations of c-PMMA and PMMA gate dielectrics, it is shown that bothsmaller thickness and larger relative permittivity of c-PMMA contributes to reducedoperating voltage. Furthermore, negligible hysteresis brings evidence to small trapstates in the semiconductor near gate dielectric of the n-type OFETs with c-PMMA.The use of TIPS-TPDO-tetraCN and c-PMMA is fully compatible with polyethyleneterephthalate substrate, giving promise to various flexible applications.

a b s t r a c t Photogeneration process in organic materials may occur via multiple mechanisms depending on exper- imental conditions. In this work we investigate how self-assembled monolayers of two types of benzoic acid molecules (ABA and NBA) affect photocarrier generation in a sexithiophene based photovoltaic cell. Photogeneration of free carriers in organic photovoltaic cells results from dissociation of Frenkel excitons. This dissociation is either thermally assisted process and/or results from the interaction with structural defects in the bulk or with the surface states. A theoretical model is used for quantitative description of the open circuit voltage as a function of generation rates at the electrodes. The orientation and the magnitude of dipole moment of SAM molecules are the crucial factors that affect the organic photovoltaic parameters.

We report on an effective method to minimize the leakage current in an organic thin-film transistor (OTFT) by using a polymeric gate insulator, poly(vinyl phenol) (PVP). When the molecular weight (Mw) of the PVP varies, only the leakage current is affected under constant remaining electrical parameters. More importantly, through a binary mixing between two different Mw, it is found that the leakage current can be minimized. This is attributed to a reduction of the free volume in the blended PVP layer, leading to a more vigorous cross-linking reaction, as compared to a single molecular weight PVP.

We report on the equivalent circuit modeling of the relaxation behavior of an optically excited thick poly(3-hexylthiophene) (P3HT) film by means of impedance spectroscopy. Fabricated metal–semiconductor–metal devices with Au electrodes showed a nearly perfect ohmic behavior under ambient conditions. Impedance measurements on illuminated P3HT device showed a dramatical decrease of the impedance modulus under illumination and very slow relaxation to the initial state. Impedance-frequency data obtained during relaxation could not be explained by a simple parallel resistance–capacitance circuit but it could be best fitted by incorporating a constant-phase element instead of a normal capacitance. By observing the variation of the circuit parameters, it is found that the relaxation process is dominated by slow recombination (elimination) of the excess photogenerated carriers, which is confirmed by the time-varying photoconductivity of the device.

International audience; A DNA sensor based on a water-gated organic field-effect transistor is described. The semiconductor is poly [3-(5-carboxypentyl)thiophene-2,5-diyl] onto which DNA probes are covalently grafted via NHS/EDC chemistry. Clear changes in the output characteristic of the device are observed upon DNA immobilization and after DNA hybridization. Experimental data point out the importance of the electrolyte Debye length that can screen negative DNA charges and impede transduction. For this reason, deionized water was used in order to increase the Debye length up to several hundreds of nanometers. In this case, a decrease in the off current was observed upon hybridization, whereas no significant change occurred when using saline solutions.

We describe how the charge carrier injection and transport are influenced by thermal cracks in a 6,13-bis(triisopropylsilylethynyl) (TIPS) pentacene film in terms of the contact resistance and the channel resistance in a TIPS pentacene thin-film transistor (TFT). Through a post-thermal annealing (PTA) process at a certain temperature Ta, the high structural order of TIPS-pentacene molecules is produced without thermal cracks, the carrier mobility of the TIPS pentacene TFT is maximized, and both the contact resistance and the channel resistance are minimized. Our quantitative description of the relationship between the PTA treatment and the interfacial resistance behavior would provide a useful basis for understanding the thermal stability and the electrical performance of a solution-processed organic TFT.

As the performance of organic field-effect transistors improves, the limitation due to charge carrier injection at source and drain electrodes becomes crucial. This review describes the various solutions that have been developed to work around this issue. The most widespread method consists of interposing between the electrodes and the organic semi- conductor film a self-assembled monolayer made of an appropriate reactive molecule. In that case, the reduction of the contact resistance may come either from an improved morphology of the semiconductor film, or to a better energy level alignment at the interface with the electrode. The respective inference of both aspects is discussed. Alternative ways to re- duce the contact resistance by an appropriate surface treatment of the electrodes prior to the deposition of the semiconduc- tor are also presented.

The photogeneration process in organic photovoltaic cells may occur via multiple mechanisms depending on their structure and/or architecture. Nevertheless, in the case of pristine material sandwiched between two dissimilar work function electrodes, photogenerated free carriers in the bulk are most likely to result, according to Onsager’s theory, from dissociation of excitons at a given electric field E. It has been reported that one of the most limiting factors for efficiency in photovoltaic organic cells is the recombination of e–h pair which is of bimolecular type. In this work we show that bulk photogeneration is rather dominated by the initial separation of geminate e–h pair which accounts for poor I–V characteristics of organic (non-heterojunction) photovoltaic cells.

Thin-film organic field-effect transistor was made with vapor-deposited polycrystalline octithiophene on silicon oxide insulator layers. In conventional field-effect transistors, the extracted mobility does not take into account the distribution of charge carriers. However, in disordered organic field-effect transistors, the local charge carrier mobility decreases from the semiconductors/insulator interface in to the bulk, due to its dependence on the charge carrier density. It is demonstrated that the conventional field-effect mobility is a good approximation for the local mobility of the charge carriers at the interface. In this paper we present a new approach to the mobility in organic thin-film transistor (OTFTs), and with a new procedure we extract the electrical parameters of organic TFTs that possible to reproduce very well the device characteristic and mobility.

In this study, we report on the influence of the semi-conductor thickness on organic thin-film transistors (OTFTs). Devices are fabricated with a top gate/bottom contact geometry on plastic substrates. Both n-channel and p-channel OTFTs made, respectively, from a N,N-dialkylsubstituted-(1,7&1,6)-dicyanoperylene-3,4:9,10-bis(dicarboximide) derivative (Polyera ActivInk™ N1400) and from 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene) are studied and compared. Impact of the active semiconducting layer thickness on both contact resistance and mobility are uncorrelated by the use of the transfer line method (TLM). When increasing the thickness of the semi-conductor, we find an increasing contact resistance and a decreasing channel mobility for both n- and p-channel OTFTs. The loss of mobility with increased thickness is attributed to a degradation of the interface between the semi-conductors and the dielectric, and is studied by atomic force microscope (AFM).

We report on the effect of the deposition of self-assembled monolayers (SAMs) on the source and drain electrodes on the contact resistance and mobility in organic thin film transistors (OTFTs). Ultraviolet photoelectron spectroscopy (UPS) shows a variation of the work function of the electrodes depending on the SAM. OTFTs were fabricated with solution processible Polyera ActivInk™ N1400, TIPS-pentacene and TFB, giving access to both polarities of transistors and to both crystalline and amorphous materials. The transfer line method (TLM) was used to separately estimate the contact resistance and mobility. A clear correlation is found between the work function of the modified electrodes and the corresponding contact resistance. The effect on mobility is more puzzling, and tentatively attributed to morphological effects.

A series of alkyl- or alkylphenyl-1,4,5,8-naphthalenetetracarboxylic-1,4:5,8-bisimides together with the comparative series of the corresponding 3,4,9,10-perylenetetracarboxylic-3,4:9,10-bisimides have been synthesized and characterized by cyclic voltammetry. The naphthalene bisimides family shows a clear dependence of its first reduction potential – corresponding to the LUMO level – on the nature of the N-substituent. Naphthalene bisimides containing alkylphenyl groups undergo the first 1e reduction at potentials of ca. 100 mV higher than those with alkyl groups ( ca. −900 mV vs ca. −1000 mV with respect to Fc/Fc + couple). No effect of the nature of the substituent is observed for the corresponding perylene bisimide series. Due to their improved solution processibility the synthesized organic semiconductors can be used for the fabrication of all organic, flexible n-channel field effect transistors (OFETs) through spin coating and printing techniques, without the necessity of the use of vacuum deposition techniques. The best of the fabricated transistors, operating in air show the charge carriers mobility of 4 × 10 −2 cm 2 /(V s) and the ON/OFF ratio equal to 4.5 × 10 5 .

With the advent of devices based on single crystals, the performance of organic field-effect transistors has experienced a significant leap, with mobility now in excess of 10 cm2 V−1 s−1. The purpose of this review is to give an overview of the state-of-the-art of these high-performance organic transistors. The paper focuses on the problem of parameter extraction, limitations of the performance by the interfaces, which include the dielectric–semiconductor interface, and the injection and retrieval of charge carriers at the source and drain electrodes. High-performance devices also constitute tools of choice for investigating charge transport phenomena in organic materials. It is shown how the combination of field-effect measurements with other electrical characterizations helps in elucidating this still unresolved issue.

International audience; The aim of this work is to improve charge injection by interposing an appropriately oriented dipole layer between contact and semiconductor in organic thin-film transistors (OTFTs). OTFTs are fabricated with pentacene semiconductor and gold source and drain contacts. The contacts are modified with self-assembled monolayers (SAMs) made of alkane or fluorinated alkane thiols. Ultraviolet photoelectron spectroscopy (UPS) shows a respective decrease and increase of the work function of the electrodes. Consistent with these results, we observe an increase and a decrease, respectively, of the contact resistance of the OTFTs, and a further decrease when shortening the length of the fluorinated molecule.

Organic field-effect transistors (OFETs) in which the active semiconductor is made of polycrystalline octithiophene (8T) were fabricated and characterized. Several methods have been used to extract the parameters of the polycrystalline (8T) transistors as function of gate voltage at room and at low temperatures. These parameters such as the mobility, the threshold voltage, contact resistance and density of traps are extracted from the current-voltage characteristics of OFETs. The first method consists of deriving the drain current as function of gate voltage (transconductance), leading to the so-colled field effect mobility. It appears that the data must be corrected for the substantial source and drain contact resistance. In the second method, the carrier mobility has been corrected for the contact resistance. It is found to increase quasi linearly with gate voltage. Therefore, the contact resistances have been directly extracted from the experimental channel conductance. In the third method, data are interpreted according the multiple thermal trapping and release model using a power law dependence of the mobility with gate voltage together with a constant resistance. Finally, using a model where charge transport is limited by trapping and thermal release at localized states located at grain boundaries, the barrier height at grain boundaries in polycrystalline octhithiophene FETs has been estimated. It is shown that is gate bias dependent.

International audience; For organic photovoltaics (OPVs) to contribute significantly to energy generation, they need to be scaled to large areas, much like all organic electronics. Therefore, there is a need for the development of a specific model that describes the electrical properties related to the size effects and cell interconnections. We report here on the equivalent circuit models for a high-performance series-connected OPV module based on a polymer:fullerene bulkheterojunction formulation. We examine the validity of the effective single cell methodology in the conventional framework and suggest a modified model that includes the net series resistance and additional parasitic leakage conductions. The photocurrent is found to follow the diffusion-limited voltage dependence, for which an empirical treatment enables an improved reproduction of the measurement near the short-circuit point.

We physically model the injection characteristics at the metal/organic semiconductor (M/O) junctions with a Gaussian density-of-states (GDOS). By both analytical and numerical modelling, the charge carrier concentrations at the M/O junctions in an organic rectifying diode (ORD) are calculated. The results demonstrate a special attention required in the application of the Schottky–Mott rule, which defines the injection barrier (IB) for ideal metal/semiconductor junctions, to M/O junctions. By systematically changing the width of the GDOS that describes the energetic disorder in the organic semiconductor, we show that the edge of the highest-occupied molecular orbitals (HOMO) should be defined as higher rather than from the maximum of the HOMO to keep the consistency of the Schottky–Mott rule. A simple analytical expression for the IB is presented which contains the effect of the disorder in facilitating the charge carrier injection. Simulated current density-voltage characteristics of the ORDs are also presented to support the arguments.

Pentacene-based organic thin-field transistors (OTFTs) with bottom-gate, top-contact architecture were fabricated on alumina substrates. The devices were divided into two sets, depending on whether pentacene was deposited on bare alumina or on alumina modified with a fatty acid self-assembled monolayer (SAM). Previous analysis have shown that the modification of alumina with SAMs result in a substantial decrease of the size of the grains in hte polycrystalline films. Careful parameters extraction, including an original determination of the threshold voltage, and contact resistance extraction through the transfer line method (TLM), allowed us to estimate the gate-voltage dependent mobility in both series. The mobility is found to first increase at low bias, and then decrease at higher gate voltage (mobility degradation). The latter behavior is explained through an estimation of the distribution of charges across the accumulation layer, in which the pentacene film is modeled as a stack of dielectric layers, the distribution being calculated using basic equations of electrostatics and thermodynamics. A good agreement was found when the mobility in the layer next to the insulator was assumed to be negligible as compared to that in the bulk of the film. The initial rise of the mobility is interpreted in terms of multiple trap and release (MTR) with a distribution of traps located in the grain boundaries. Interestingly, the mobility corrected for both contact resistance and mobility degradation is found around 3 cm2/V s for pentacene deposited on bare alumina, and 5 cm2/V s when pentacene is deposited on SAM modified alumina. We conclude that the smaller grains grown on modified alumina are more regular, and hence less defective, than the larger grains deposited on bare alumina.

Indium tin oxide (ITO) substrates have been modified by alkanethiol and fatty acid self-assembled monolayers (SAMs). The SAMs were grown by dipping the cleaned surface into either a pure alkanethiol or a fatty acid dissolved in various solvents. They were characterized through contact angle, X-ray photoelectron (XPS) and infrared absorption-reflection spectroscopy (IRRAS). Their density and structural organization was found to greatly depend on the cleaning treatment of the ITO surface, the length of the alkyl chain, and, in the case of fatty acids, the concentration of the solution. XPS measurements brought evidence for the fact that, in the case of alkanethiols, the grafting mechanism was through the formation of ionic or covalent bonds involving thiolates. The most prominent result of this comparative study is that thiol-based SAMs are more strongly attached to the ITO substrate and better organized than fatty acids, which we attribute to the fact that the reaction of the ITO surface with fatty acids is more reversible than that with thiols.

Organic field-effect transistors were fabricated with vapor-deposited pentacene on aluminum oxide insulating layers. Several methods are used in order to extract the mobility and threshold voltage from the transfer characteristic of the devices. In all cases, the mobility is found to depend on the gate voltage. The first method consists of deriving the drain current as a function of gate voltage (transconductance), leading to the so-called field-effect mobility. In the second method, we assume a power-law dependence of the mobility with gate voltage together with a constant contact resistance. The third method is the so-called transfer line method, in which several devices with various channel length are used. It is shown that the mobility is significantly enhanced by modifying the aluminum oxide layer with carboxylic acid self-assembled monolayers prior to pentacene deposition. The methods used to extract parameters yield threshold voltages with an absolute value of less than 2 V. It is also shown that there is a shift of the threshold voltage after modification of the aluminum oxide layer. These features seem to confirm the validity of the parameter-extraction methods.

The organic thin-film transistor (OTFT) is now a mature device that has developed tremendously during the last twenty years. The aim of this paper is to update previous reviews on that matter that have been published in the past. The operating mode of OTFTs is analyzed in view of recent model development. This mainly concerns the distribution of charges in the conducting channel and problems connected with contact resistance. We also delineate what differentiates n- and p-type semiconductors, and show how this concept differs from what it covers in conventional semiconductors. In the chapter devoted to fabrication techniques, emphasis is placed on solution-based techniques and particularly printing processes. Similarly, soluble materials are given a prominent place in the section dedicated to the performance of devices. Finally, special attention is given to devices at the nanoscale level, which demonstrate a new route toward molecular electronics.