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Due to their functional and processing versatility, organic semiconductors have gained much interest in recent years. Tailoring the charge transport in a polymeric semiconductor system is imperative for developing optimized devices. Factors such as selection of solvent species and annealing temperature play a significant role in deciding the morphology of the polymer semiconductor and hence the device performance. Bottom-gate bottom-contact organic field-effect transistors were fabricated with regioregular poly(3-hexylthiophene) (P3HT) as the active material. The effects of using chlorobenzene and o-dichlorobenzene as the solvents for P3HT and annealing temperatures on the device performance were evaluated. The devices made using chlorobenzene showed greater field-effect mobility compared to those fabricated with o-dichlorobenzene. This behavior can be attributed to the disentanglement of polymer chains in a solvent with better matching solubility parameters and subsequent ease of arranging in ordered structures during processing. Furthermore, annealing the o-dichlorobenzene devices at temperatures closer to the crystallization temperature of P3HT showed an increase in field-effect mobility. At temperatures closer to the crystallization temperature, a polymer film in a semidry state can allow increased ordering of the chains leading to enhanced charge transportation. [ABSTRACT FROM AUTHOR]

Improved charge carrier mobility of polymer semiconductors is often a critical factor in achieving superior performance for organic electronic devices. A novel low-temperature electric-field (LTEF) treatment on poly({4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl}{3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl}) (PTB7) films, which helps in increasing hole mobility in these films, is presented herein. The zero-field hole mobility at room temperature of the polymer film in a "hole-only" device configuration is found to increase by more than seven times from 1.1 × 10−4 to 7.8 × 10−4 cm2 V−1 s−1 following the application of an electric field strength of 2 × 105 V cm−1 across the films at low temperature (80 K). On the other hand, a room temperature electric field treatment of the devices does not appear to have any effect on the hole mobility in the PTB7 films. LTEF treatment is shown to help decrease the average barrier energy for charge carriers between adjacent molecular energy levels in the films from 302 to 266 meV, energy disorder from 53.2 to 49.3 meV, and position disorder from 2.1 to 1.7 in PTB7 films. The hole mobility improvement is attributed to the field induced enhanced order in those films by dipolar alignment, which is facilitated by higher polarization at low temperatures. [ABSTRACT FROM AUTHOR]

It is shown that the dynamic response of a thin-film transistor (TFT) comprises a polymer or organic semiconductor and can be directly correlated with charge transport parameters. Charge transport is assumed to take place in accordance with the often employed multiple trap and release model. We can simulate the non-quasistatic response of TFTs for devices in which the mobility is not constant, but depends on carrier density and other parameters. The previous model of Burns has been significantly improved and made applicable to organic and polymer transistors. Our approach works for an arbitrary density of states including both a relatively shallow exponential distribution of tail states and deeper Gaussian trap states. We set the gate voltage, VG, in our TFTs to be zero. The source voltage, VS, is varied and, thus, the value of gate-to-source voltage is varied. The predictive power of the model that is presented in this paper will be very useful for accurate device modeling and for charge transport studies. [ABSTRACT FROM AUTHOR]

Chemical versatility and compatibility with a vast array of processing techniques has led to the incorporation of organic semiconductors in various electronic and opto-electronic devices. One such device is the organic field-effect transistor (OFET). In this tutorial, we describe the structure, operation, and characterization of OFETs. Following a short historical perspective, we introduce the architectures possible for OFETs and then describe the device physics and the methods for extracting relevant device parameters. We then provide a brief overview of the myriad organic semiconductors and deposition methods that were adopted for OFETs in the past decades. Non-ideal device characteristics, including contact resistance, are then discussed along with their effects on electrical performance and on the accuracy of extracting device parameters. Finally, we highlight several measurements involving OFETs that allow access to fundamental properties of organic semiconductors and the mechanism of charge transport in these materials. [ABSTRACT FROM AUTHOR]

The number of trap states in the band gap of organic semiconductors directly influences the charge transport as well as the threshold and turn-on voltage. Direct charge transport measurements have been used until now to probe the trap states rather close to the transport level, whereas their number in the middle of the band gap has been elusive. In this study, we use PDIF-CN2, a well known n-type semiconductor, together with vanadium pentoxide electrodes to build ambipolar field-effect transistors. Employing three different methods, we study the density of trap states in the band gap of the semiconductor. These methods give consistent results, and no pool of defect states was found. Additionally, we show first evidence that the number of trap states close to the transport level is correlated with the number of traps in the middle of the band-gap, meaning that a high number of trap states close to the transport level also implies a high number of trap states in the middle of the band gap. This points to a common origin of the trap states over a wide energy range. [ABSTRACT FROM AUTHOR]

Charge carrier hopping transport is generally taken from Miller-Abrahams and Marcus transition rates. Based on the Miller-Abrahams theory and nearest-neighbour range hopping theory, Apsley and Hughes developed a concise calculation method (A-H method) to study the hopping conduction in disordered systems. Here, we improve the A-H method to investigate the charge carrier hopping transport by introducing polaron effect and electric field based on Marcus theory and variable-range hopping theory. This improved method can well describe the contribution of polaron effect, energetic disorder, carrier density, and electric field to the charge carrier transport in disordered organic semiconductor. In addition, the calculated results clearly show that the charge carrier mobility represents different polaron effect dependence with the polaron activation energy and decreases with increasing electric field strength for large fields. [ABSTRACT FROM AUTHOR]

The current vs. voltage (I-V) characteristics of single crystal rubrene Organic Field-Effect Transistors (OFETs) and polycrystalline poly(p-phenylenevinylene) (PPV) films are modeled using the polaron transport theory presented in a previous work [A. F. Basile et al., J. Appl. Phys. 115, 244505 (2014)]. The model is first applied to rubrene OFETs, where transport is two-dimensional and is confined near the interface between the insulator and the organic semiconductor. By considering the effect of image charges in the insulator and by assuming a constant intrinsic mobility, we reproduce both the positive and the negative temperature dependences of the channel mobilities measured on OFETs having a gate dielectric and an air-gap insulator, respectively. In addition, we adapt this model to the three-dimensional transport in PPV films, characterized by effective mobilities which depend on temperature, charge density, and electric field. We show that the I-V characteristics of these materials can be matched by the numerical solution of the Poisson and drift-diffusion equations assuming a constant intrinsic mobility. The polaron binding energy can account for the thermally activated behavior of the I-V characteristics and for the increase of the effective mobility at high applied voltages. Therefore, this model enables to extract the intrinsic transport parameters of organic semiconductors, independent of the device structure, and of the measurement conditions. [ABSTRACT FROM AUTHOR]

With progressively improving charge carrier mobility in organic semiconductors and miniaturization of organic field-effect transistors (OFETs), low contact resistances become ever more important. To study the capabilities of metal electrodes in OFETs and to explore the transition from contact-limited to channel-dominated transistor operation, we used flip-crystal FETs with gold electrodes having different contact resistances Rc to high-quality rubrene crystals. 4-terminal transfer and output measurements reveal that Rc decreases from 105-106 Ω cm for 15 min air exposure to 3×10³ Ωcm for at least 5 h air exposure of the gold electrodes before the flip-crystal FET is assembled. We conclude the reduction of Rc to be caused by a growing contamination layer on the gold electrodes that weakens the electrostatic coupling between rubrene crystal and gold electrode, and lowers the Schottky contact diode parameter V0. In channel-dominated (low Rc) FETs, the mobility is in the range of 10-17 cm²/(Vs); in contrast, in contact-limited (high Rc) FETs, the apparent mobility decreases significantly with increasing contact resistance. The apparent μ - Rc dependence is not intrinsic, but rather the result of incorrect assumptions of the potential and the charge carrier density in the channel region. Thus, the development of high-mobility organic semiconductors requires further efforts to improve contacts beyond traditional metal electrodes. [ABSTRACT FROM AUTHOR]

Transport states in disordered organic semiconductors are widely accepted to be Gaussian distributed with a rather narrow variance (width) of the order of roughly 100-150 meV. This is a result of intensive analysis of experimental data and theoretical work using different model assumptions on hopping transport. For this transport mechanism, the dependencies of the carrier mobility on temperature, carrier density, and field are described by analytical approximations, which can be implemented in advanced device simulation programs. However, also the carrier density itself is determined by the Gaussian density of states (GDOS) and the energy integral over the product of the GDOS and the Fermi distribution function can only be calculated numerically. But meanwhile the published [G. Paasch and S. Scheinert, J. Appl. Phys. 107, 104501 (2010)] analytical approximation for the charge carrier density of organics with GDOS is implemented in the Sentaurus Device simulator. We present here results of detailed numerical simulations of organic field-effect transistors applying this model. It turns out that an important parameter is the distance of the GDOS maximum from the band edge, which is not directly accessible from experiment since the measured DOS shows a much larger band width (the narrow transport GDOS might be the tail). Only for a special case, depending on the variance, the deviations from a non-degenerate inorganic semiconductor are rather small. Furthermore, the results are affected by the properties of the source/drain contacts (Ohmic or Schottky type). Inclusion of both the calculation of the carrier concentration assuming the GDOS and the dependence of the mobility on this concentration shows that the influence of the latter is much larger. Nevertheless, the deviations from simulations for an inorganic semiconductor show that the carrier density for the GDOS should be taken into account in device simulations. [ABSTRACT FROM AUTHOR]

In this work we study the cross-linked poly(vinyl alcohol)/poly(3-hexylthiophene) interfacial properties of an organic field effect transistor. We use cross-linked poly(vinyl alcohol) prepared with different ammonium dichromate:poly(vinyl alcohol) proportions, ranging from 0% to 35%, as insulator. Using admittance spectroscopy, we show that the interfacial properties change when the ammonium dichromate concentration is altered. The interfacial properties and the better insulation are responsible for the improvement of the device performance in these organic field effect transistors, achieving best performance in the blend with ammonium dichromate:poly(vinyl alcohol) proportion of 0.25:1. [ABSTRACT FROM AUTHOR]

The time-of-flight experiment is a transient photo-current measurement commonly used to measure the mobility of organic semiconductors. The experiment has a well established procedure to extract the average transit-time of the photo-generated carriers across the sample from the current versus time signal. In this work, we compare, for a dispersive signal, the transit-time as inferred from the experimental procedure with the statistically defined average transit-time. We show that the two quantities are not only numerically different, the experimental transit-time being always smaller than the exact one, but they also have markedly different electric field and sample length dependencies. Our results are relevant for any attempt to model an experimental time-of-flight mobility. [ABSTRACT FROM AUTHOR]

The operation of organic devices as organic field-effect transistors (OFET) depends critically on the contact between the organic layer and the material for source/drain electrodes. Small barriers for carrier injection are required for efficient operation. In order to support the understanding of organic devices, photoemission spectroscopy has been used to determine the properties of metal/organic interfaces. Values for the hole injection barrier determined in the last decade by different groups are frequently of the order of 0.5-1 eV. It is not clear whether barrier lowering due to the image charge is sufficient to make contacts with such barriers efficient for carrier injection. Indeed, no results have been reported where the preparation of the samples for the photoemission study and for the devices are the same. Here we present results of such an investigation for OFETs with gold source/drain contacts. The measured hole barrier at the gold contact of 0.6 eV results from the Au work function of 4.6 eV. Taking into account the dependencies of the mobility on the carrier concentration and on the field for the Gaussian density of states (DOS) of disordered organics, measured OFET current characteristics cannot be described well with such contacts but rather for work functions of 4.7 eV or larger. Considering the method in determining the barrier from photoemission data and the Gaussian DOS of the hopping transport states, we present a quantitative connection between the barrier as determined from photoemission and the barrier as used in the device simulation. [ABSTRACT FROM AUTHOR]

A modeling of organic field-effect transistors' (OFETs') electrical characteristics is presented. This model is based on a one-dimensional (1-D) Poisson's equation solution that solves the potential profile in the organic semiconducting film. Most importantly, it demonstrates that, due to the common open-surface configuration used in organic transistors, the conduction occurs in the film volume below threshold. This is because the potential at the free surface is not fixed to zero but rather rises also with the gate bias. The tail of carrier concentration at the free surface is therefore significantly modulated by the gate bias, which partially explains the gate-voltage dependent contact resistance. At the same time in the so-called subthreshold region, we observe a clear charge trapping from the difference between C-V and I-V measurements; hence a traps study by numerical simulation is also performed. By combining the analytical modeling and the traps analysis, the questions on the C-V and I-V characteristics are answered. Finally, the combined results obtained with traps fit well the experimental data in both pentacene and bis(triisopropylsilylethynyl)-pentacene OFETs. [ABSTRACT FROM AUTHOR]

Para-hexaphenylene (p-6P) molecules exhibit a characteristic photoinduced reaction (bleaching) resulting in a decrease in luminescence intensity upon UV light exposure, which could render the technological use of the nanofibers problematic. In order to investigate the photoinduced reaction in nanofibers, optical bleaching experiments have been performed by irradiating both pristine and coated nanofibers with UV light. Oxide coating materials (SiOx and Al2O3) were applied onto p-6P nanofibers. These treatments caused a reduction in the bleaching reaction but in addition, the nanofiber luminescence spectrum was significantly altered. It was observed that some polymer coatings [a statistical copolymer of tetrafluoroethylene and 2,2-bis-trifluoromethyl-4,5-difluoro-1,3-dioxole, P(TFE-PDD), and poly(methyl methacrylate), PMMA] do not interfere with the luminescence spectrum from the p-6P but are not effective in stopping the bleaching. Bilayer coatings with first a polymer material, which should work as a protection layer to avoid modifications of the p-6P luminescence spectrum, and second an oxide layer used as oxygen blocker were tested and it was found that a particular bilayer polymer/oxide combination results in a significant reduction in bleaching without affecting significantly the emission spectrum from the nanofibers. [ABSTRACT FROM AUTHOR]

A significant difference between the transient electric field profiles of the pentacene organic field-effect transistors (OFETs) with SiO2 and poly(methyl-methacrylate) (PMMA) insulators was found by the time-resolved microscopic optical second-harmonic generation (TRM-SHG) experiment. The profile of former device was broad and changed smoothly, while the latter one had a sharp peak. Particularly, the peak of the transient electric field in SiO2-insulated devices moved much faster than that in the PMMA-insulated one. Based on several experimental evidences and computational simulations, we proposed that these differences might arise from a higher trapped carrier density in the conductive channel on the PMMA insulator. Simple approaches were developed to evaluate the trap density and define dynamic carrier mobility in terms of the transient electric field measured by the TRM-SHG technique. This mobility quantitatively depicts that the transient hole transport in the OFET with the PMMA insulator is trap controlled. [ABSTRACT FROM AUTHOR]

We report on a quinoid heteropentacene as p-type semiconductor in organic field-effect transistors. Both single crystal and thin-film transistors were fabricated with 7,14-diphenyl-chromeno[2,3-b]xanthene (DPCX). In this small molecule organic semiconductor the field-effect mobility is as high as 0.16 cm2/V s in single-crystal devices and 0.01 cm2/V s in thin-film devices. In addition, the devices show favorable properties such as near zero onset/threshold voltages and a small current hysteresis. X-ray diffraction experiments show the molecules to be arranged in slipped stacks and to have a flat backbone in the crystals. For thin films of DPCX the situation is complicated by the coexistence of a thin-film phase with the bulk phase. However, a comparison of DPCX thin films on octadecyltrichlorosilane (OTS)-treated and bare SiO2 gate dielectrics provides clear evidence that the OTS surface treatment leads to organic thin films with a better structural order. The low-cost synthesis and purification of DPCX along with the improved processability and the good electrical characteristics suggest that quinoid heteropentacenes are promising materials for organic field-effect transistors. [ABSTRACT FROM AUTHOR]

We implement the “time-of-flight” (TOF) method for charge carrier mobility (μ) measurements in organic field-effect transistors (OFETs) by applying voltage steps, VS, to OFETs. We use the electric scheme for OFET-TOF introduced by Dunn et al. [Appl. Phys. Lett. 88, 063507 (2006)]. Our investigation of a series of low-threshold poly(triaryl amine) OFETs with different channel lengths, L, suggests that in the OFET-TOF setup, the effective voltage driving carriers across the channel, VTOF, is reduced from VS by a constant voltage that coincides with the OFET’s threshold voltage, VT: VTOF=VS-VT. Under this assumption, TOFs scale as expected from theory with both VS and channel length, L, and the extracted dynamically acquired μ agrees excellently between different samples, and experimental protocols (variation of VS / variation of L). However, “dynamic” μ is higher than the “static” μ conventionally extracted from saturated transfer characteristics, which is also less consistent between samples. Also, we observe that the TOF in OFETs switching from OFF→ON is longer than the TOF in the same OFET switching ON→OFF under the same VS. We rationalize this difference by trap filling in the populated OFET channel. [ABSTRACT FROM AUTHOR]

Organic field-effect transistors (FETs) have recently attracted attention mainly because of the low fabrication cost. Parallel-stacked structure is considered favorable for realizing a high mobility along the stacked direction in organic FETs. Among our dipyridyl-peryleneimides synthesized previously for H2 sensors (OPP, MPP, and PPP for o-, m-, and p-position of the N atom in the pyridyl ring, respectively), OPP and PPP are found to crystallize in this structure and are expected to show better FET characteristics than MPP. Therefore, the FET characteristics have been studied in the present investigation from the standpoint of the crystal structure. The FET devices based on OPP and PPP exhibit n-type characteristics, showing a mobility of about 10-5 cm2/(V s) and “on/off” ratios of about 102, whereas only poor performance was observed for the MPP-based FET. [ABSTRACT FROM AUTHOR]

The effect of atmospheric gases on the ionization energy (I) of titanyl phthalocyanine thin film was investigated by an apparatus of photoemission yield spectroscopy developed for the measurements of I for the same specimen both in vacuum and under gaseous atmosphere. It was found that the value of I is affected by the exposure to various ambient gases (i.e., air, nitrogen, and oxygen of 1 atm, and water vapor corresponding to 27% relative humidity at 300 K), and that the effect strongly depends on the gas. The ionization energies in vacuum could be determined as the onset I0 of the cube-root plot of the photoemission yield as a function of photon energy. When the sample was exposed to gases, the cube-root plot still gives an onset, but often a long tail at the low-energy side with another onset It was also observed. The first exposure to air did not affect both I0 and It much, while the following evacuation-exposure cycles caused mostly reversible decrease and increase of I0 and It by about 0.2 and 0.4 eV, respectively. Among the examined constituent gases of air, nitrogen was found to hardly affect both I0 and It except for the small temporal decrease by 0.06 eV at the first exposure. Similar trends of the change of I0 and It with those for air were found for oxygen, with the amplitudes of mostly reversible change being 0.1 eV for I0 and 0.2 eV for It. Overlapped with these changes, a slow decrease of both I0 and It, was also observed with a slope of 0.01 eV per day. For water, the trends were mostly similar with O2, except that (1) the first exposure to water vapor showed small and large changes in I0 (0.03 eV increase) and It (0.4 eV decrease) and (2) the amplitudes of the mostly reversible change of I0 (0.3 eV) and It (0.05 eV) were much larger and smaller than those for O2, respectively. A long-term decrease was also observed with a slope of 0.04 eV per day. These results indicate that the atmospheric effect by ambient air on I is mainly caused by water vapor, although oxygen also makes significant contribution. Since the observed trends are rather complex, there seem to be multiple factors affecting I, which is the energy difference between the vacuum level and the highest occupied molecular orbita. Possible microscopic mechanisms of the observed variation of I on these levels are also discussed in terms of the energy change in these levels. [ABSTRACT FROM AUTHOR]

We report on the investigations of the differences in the measured current characteristics of source/drain top contact (TOC) and bottom contact (BOC) organic field-effect transistors (OFETs). The active layer was made from poly[1,4-phenylene-(4-methylphenyl)imino-4,48-diphenylene-(4-methylphenyl)imino-1,4-phenylenevinylene-2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene-vinylene] and a highly doped silicon wafer with a thermal oxide was used for the gate and the insulator, respectively. Both transistors show a good subthreshold behavior characterized by a threshold voltage of Vth≈-5 V and an inverse subthreshold increase of S≈0.25 V/dec. The estimated mobility is in the range of 10-3 cm2/V s. The main difference between the TOC and the BOC transistors is a nonlinear increase of the drain current at small drain voltages in the output characteristics of the TOC OFETs. Numerical two-dimensional simulations show that a high concentration of donor-like traps with a Gaussian or exponential distribution are the reason for this peculiarity. The investigation of the influence of air confirms the presence of such traps. [ABSTRACT FROM AUTHOR]

The ratio between mobility and diffusion parameters (Einstein relation) in organic semiconductors has been a debating issue in the recent years. In this paper we developed an analytical model based on hopping transport theory and the Gaussian density of states. The validity of Einstein relation in organic semiconductors is discussed. It is shown that the classic Einstein relation is invalid for organic semiconductors, even for the carrier concentration for experimental purpose. [ABSTRACT FROM AUTHOR]