Organic semiconductors attract considerable attention due to promising applications in organic light emitting diodes, field effect transistors, and organic solar cells. Moreover, solubility of some organic semiconductors in organic solvents favors them for printed large area OLED displays and inexpensive printed microelectronics. However, low mobility of carriers in organic semiconductors limits usability of organic semiconductors in integrated circuits and need to be overcome. For this reason, the knowledge of intrinsic properties achievable in very pure and perfect crystals is important. Therefore, we have carried out a program to grow high quality single crystals of organics. Solution growth, melt growth, solvothermal method and vapor transport crystal growth have been applied and will be reported. For research purpose, using a gas phase transport method, we have produced millimeter - sized crystals of numerous organic semiconductors with higher quality and purity. Structure quality has been evaluated by x-ray topography methods. Field effect transistors have been prepared on surfaces of single crystals. Some of organic semiconductors like rubrene, pentacene, copper phthalocyanine exhibit carrier mobilities comparable or even higher than amorphous silicon. However, characterization of starting materials, crystals, thin films and resulting devices remains the crucial issue. The relation between organic semiconductor properties, used device fabrication technologies and resulting device characteristics is the object of presented here studies.

Control over supramolecular organization of electronically active π-conjugated organic molecules provides great opportunities to fine-tune and optimize their electrical properties for applications in organic field-effect transistors and sensors. Here we report high-quality single-crystal organic semiconductors with conjugated organic molecules via facile solution processing. We show a well-faceted, high-quality 1D single-crystal microwire using self-organized poly (3-hexylthiophene), P3HT with unprecedented electrical characteristics such as a low resistance (0.5 MΩ), a channel current as high as 25 μA, and a well-resolved gate modulation via solution growth. Furthermore, we report the fabrication of high-quality 1D triisopropylsilylethynyl pentacene (TIPS_PEN) microribbons via a simple solution process with well-defined facets and remarkable electrical characteristics, such as field-effect mobility as high as 1.4 cm²/V.s. We found that 1D single-crystal microwires and microribbons are formed spontaneously through facile self-assembly of individual conjugated molecules. Our findings indicate that π-conjugated organic single-crystals are capable of very efficient charge transport.

We present an electric force microscope and transport study of the degradation of the contact between Au and TPD, a triarylamine widely employed as a hole transporting layer in light emitting diodes. TPD was dispersed into a polystyrene (PS) binder and spin casted onto a quartz substrate with coplanar gold electrodes. Electric force microscopy was used to map the electrostatic potential drop in the device channel while a voltage was applied and the current was measured. Two contact degradation mechanisms were observed. When the TPD-PS film was allowed to age in high vacuum, the TPD crystallized out of solution. We show that the observed loss of current is the result of both a decrease in bulk mobility and a decrease in injection efficiency. The operating temperature of a freshly prepared device was then varied from 296 K to 330 K to simulate heating that might occur during light emitting diode operation. While the current increased in an apparently smooth way as the temperature was raised, electric force microscopy revealed that the underlying injection efficiency had undergone a dramatic change. Above a temperature of 330 K, running current through the device led to a dramatic decrease in injection efficiency which we found was associated with the creation of a dipole layer at the injecting contact. Upon decreasing temperature, we found that a measurable charge remained in the device channel when the applied voltage was switched to zero. The decay of the associated electrostatic potential, which appears to be governed primarily by charge-charge repulsion and not diffusion, provides an estimate the zero-field mobility of the holes in the film.

We have investigated crystalline structures in organic semiconducting thin films with regioregular poly(3-alkyl thiophene)s, pentacene, and oligofluorene-thiophene derivatives, using grazing-incidence X-ray diffraction and atomic force microscopy. We found that crystalline morphologies and molecular orientation in these thin films strongly affect electric characteristics of these film-based organic thin-thin transistors (OTFTs). Specifically, in solution- and vacuumdeposited films, preferentially perpendicular orientation of π-conjugated crystalline planes with respect to dielectric substrates require results in high-performance charge mobility in the OTFT devices.

The performance of polymer field-effect transistors is highly dependent on their processing history. For instance, thermal processing plays a role in micro-structure development and consequently in device performance. A transport model was developed based on the semiconductor micro-structure where highly mobile states are located in the crystalline areas and defects and disordered regions correspond to areas where carriers are trapped. By applying this model to electrical characterization data of PQT-12 (a regio-regular polythiophene), it is found that annealing tightens the energetic distribution of the traps. Films quenched from the melt performed worse than annealed films due to an increased trap density and broader energy distribution of the traps. X-ray diffraction in grazing and specular geometry was carried out at the Stanford Synchrotron Radiation Laboratory on PQT-12 thin films to reconcile the predictions of the transport model with the micro-structure of the PQT-12 thin films. In all cases the polymer crystallites are textured with the π-stacking direction in the plane of charge transport and the rocking curves indicate the existence of a population of highly oriented crystallites. Annealing the as-spun films improves the crystallinity and texture, in agreement with the transport model. Quenching produces defects in the films, which are likely to produce traps, thereby lowering the carrier mobility.

In this paper, pentacene and metalphthalocyanine was deposited on polycarbonate (PC) thin film using vacuum deposition and their morphologies and molecular orientations were investigated using X-ray Diffraction (XRD) spectrometer and UV/Vis spectrometer. From these results, pentacene and metalphthalocyanine molecules were concluded to be perpendicular on the PC film. The organic field effect transistor (OFET) with pentacene thin film or copperphthalocyanine thin film as an active layer and PC thin film as a gate dielectric film was fabricated on the polyethylene naphtalate (PEN) film (substrate). The drain current versus drain voltage characteristics of the OFET were measured.

This paper reports on two sets of data. In the first one, organic thin-film transistors (OTFTs) where fabricated by vapor depositing a pentacene layer on a gate dielectric made of alumina. The devices divide in two sets, depending on whether the alumina surface was or was not modified with a self-assembled monolayer (SAM) of eicosanoic acid. Atomic force microscopy (AFM) images show that the presence of the SAM strongly reduces the size of the crystal grains. A careful analysis of the current-voltage characteristics of the devices, which includes the use of the Transfer Line Method (TLM), allowed the extraction of the gate voltage (V_G) dependent mobility corrected for contact resistance. A remarkable feature is that the mobility decreases with V_G at high gate bias, which is interpreted by assuming that the mobility in the layer close to the insulator-semiconductor interface is substantially lower than that in the bulk of the film. Modeling the charge distribution in the conducting channel allowed us to extract the bulk mobility, which is found to saturate at around 5 cm²/Vs in SAM-modified devices and 3 cm²/Vs in devices with bare alumina. The difference is interpreted in terms of more or less defective grains. The second set of results deals with a transistor, in which the active element is reduced to a single monomolecular layer. This was achieved by using a SAM of a bifunctional molecule comprising a quaterthiophene moiety linked to a short alkyl chain. AFM images of the SAM after various adsorption times show that the size of the well-organized domains is around 25 ± 5 nm. Working transistors could only be realized by reducing the channel length L down to a magnitude comparable to that of the domain size, which was achieved with the help of e-beam lithography. In one occasion, well-defined current-voltage characteristics was recorded, which allowed to extract a gate-voltage independent mobility of 0.0035 cm²/Vs.

Memory effects are commonly seen in organic thin-film transistor (OTFT) characteristics. In the absence of memory effects associated with the gate dielectric, the hysteresis in p-channel pentacene-based OTFTs, as measured in air and under illumination, was found to be dominated by trapped electrons, rather than trapped holes, in the semiconductor. The responsible acceptor type traps have very long lifetime. The immobile, previously stored negative charge requires extra holes to balance it, resulting in early establishment of the channel and extra drain current. This model is unique in that it discusses the majority carrier population influenced by trapped charge opposite in sign to the majority carriers in a simple electrostatic manner, to explain history dependence. The model was supported by drain current transient decay data. This memory effect is ambient and illumination sensitive. We studied the presence or absence of this effect under various ambient and illumination conditions, and found the responsible acceptor type traps mostly extrinsic and their formation reversible. Efforts were taken in the quantitative analysis to exclude the bias stress effect from the memory effect due to the charged acceptors.

The following report presents innovative technique for surface modification and device construction of top-contact pentacene-based thin film transistors (TFTs) with saturation mobility about 2.0 cm²/Vs. In the experiment we have utilized PSPI as a modification layer presenting a non-polar interface on which the semiconductor, pentacene, could grow. The surface of the modification layers was exposed to a polarized ultraviolet light with dose ranging from 0.2 J to 8 J. Ultraviolet light was applied to achieve a non-polar surface on which high performance TFTs have been subsequently fabricated. The experimental results showed that the parasitic contact resistances of silver electrodes could be extracted by gated-transfer length method, and the corrected field-effect mobility of pentacene TFTs for linear region was as high as 2.25 cm²/Vs. In this study, we were able to control the surface energy of polymer-based gate dielectric layers and the surface energy of the PSPI layer increasing the energy from about 38 to 42 mJ/m² by differentiating doses of polarized ultraviolet light. When the PSPI film was exposed to 1 J of polarized ultraviolet light, the surface energy of PSPI, measured by the contact angle method, was about 38 mJ/m². The measured energy matched the theoretically calculated surface energy of a pentacene crystal. Hence, the higher mobility OTFTs with low surface energy gate dielectric were obtained by spin-coating the PSPI as a modifier.

In order to realize the high-performance solution-processed transistors (OTFTs) on plastic substrate, it is essential to have a solution-processible organic gate insulator which can give high field-effect mobility and on/off ratio in the devices and should endure sever photolithography process. Our crosslinked gate insulator film has a good chemical resistance to electrode etchants. However, the etchant exposure of the gate insulators resulted in an increase of the off-current while keeping the on-current the same. We also demonstrate solution processed n-type OTFTs with high mobility based on the soluble derivatives of fullerene (C60) as n-type channel materials. We obtained high electron mobilities of 0.02-0.1 cm²/V.s depending on the workfunction of the source and drain metals, demonstrating that the electron injection is contact-limited. Furthermore, we fabricated n-type OTFTs by all solution deposition process including source and drain metals as well as gate insulators and organic semiconductors. These types of OTFTs can be well suited for a wide range of existing and future flexible circuits and display applications which require a simplified process and low-weight and low-cost products.

Contact resistances often contribute significantly to the overall device resistance in organic field-effect transistors (OFETs). Understanding charge injection at the metal-organic interface is critical to optimizing OFET device performance. We have performed a series of experiments using bottom-contact poly(3-hexylthiophene) (P3HT) OFETs in the shallow channel limit to examine the injection process. When contacts are ohmic we find that contact resistivity is inversely proportional to carrier mobility, consistent with diffusion-limited injection. However, data from devices with other electrode materials indicate that this simple picture is inadequate to describe contacts with significant barriers. A generalized transmission line method allows the analysis of nonohmic contacts, and we find reasonable agreement with a model for injection that accounts for the hopping nature of conduction in the polymer. Variation of the (unintentional) dopant concentration in the P3HT can significantly alter the injection process via changes in metal-organic band alignment. At very low doping levels, transport suggests the formation of a barrier at the Au/P3HT interface, while Pt/P3HT contacts remain ohmic with comparatively low resistance. We recently observed that self-assembled monolayers on the metal source/drain electrodes can significantly decrease contact resistance and maintain ohmic conduction under conditions that would result in nonohmic, high resistance contacts to untreated electrodes. Finally, we discuss measurements on extremely short channel devices, in the initial steps toward examining transport through individual polymer chains.

The high frequency mobility of charge carriers on isolated ladder-type polymer chains with lengths ranging from 13 to 54 monomer units was measured. Experiments were performed on isolated chains in dilute solution and on bulk solid samples. The ac mobility of charge carriers measured at a microwave frequency near 30 GHz is strongly dependent on the chain length. The intra-chain motion of charge carriers can be described by one-dimensional diffusion between infinitely high reflecting barriers, representing the chain ends. Theoretical analysis of the experimental data yields an intra-chain mobility of charges on isolated ladder-type polymers in dilute solution near 600 cm²/Vs. For bulk solid samples the intra-chain mobility has a lower value of 30 cm²/Vs, which is attributed to energetic disorder in the bulk due to interactions between different polymer chains. With the high intra-chain mobility the ladder-type polymer is a promising candidate for future use as an interconnecting wire in molecular-sized electronics.

In the present paper a new concept towards O-CMOS technology is presented substantiating the importance of the semiconductor/dielectric interface for charge carrier transport in organic semiconductors. It will be demonstrated that by controlling the interface properties of either SiO₂ or PMMA, unipolar p- and n-type OFETs can be realized using a single organic semiconductor and even a single metal for source and drain contacts. Two dielectric/semiconductor interface modifications are considered for the realization of complementary OFETs on the basis of pentacene, otherwise known for its exclusive hole transporting properties. Selective modification of the SiO₂ dielectric interface with traces of vacuum deposited Ca, allows for electron transport in pentacene and the realization of complementary pentacene OFETs on a single substrate. By this technique electron traps are removed due to a reaction of atomic Ca with oxygen from available hydroxide groups, resulting in the formation of an oxidized Ca layer. In a second approach, it is demonstrated that by selective UV treatment of a PMMA dielectric surface, unipolar n-type pentacene OFETs can be converted to unipolar p-type by the introduction of electron traps in the form of -OH and -COOH groups at the PMMA interface. Both methods allow for the realization of CMOS organic inverter stages with decent electrical properties.

Polymer field-effect transistors with a field-effect mobility of μ ≈0.3 cm²/V^.s have been demonstrated using regioregular poly(3-hexylthiophene) (rr-P3HT). Devices were fabricated by dip-coating the semiconducting polymer followed by annealing at 150°C for 10 minutes. The heat annealed devices exhibit an increased field-effect mobility compared with the as-prepared devices. Morphology studies and analysis of the channel resistance demonstrate that the annealing process increases the crystallinity of rr-P3HT and improves the contact between the electrodes and the P3HT films, thereby increasing the field effect mobility of the films. Based on the results obtained from unipolar FETs using rr- P3HT, we have also applied postproduction heat treatment to ambipolar polymer FETs fabricated with rr-P3HT and C₆₁- butyric acid methyl ester (PCBM). Devices were fabricated using aluminum (Al) source and drain electrodes to achieve an equivalent injection for the both holes and electrons. As the case of P3HT unipolar FETs, the thermal annealing method also improves the film morphology, crystallinity, and the contact properties between Al and active layer, thereby resulting in excellent ambipolar characteristics with the hole mobility of 1.7×10^-3 cm²/V^.s and the electron mobility of 2.0×10^-3 cm²/V^.s.

Based on the achievement of having fabricated the first integrated circuit solely by means of conventional mass-printing methods, we report on the challenges and perspectives of printed electronics as far as fast, continuous printing processes are taken into account. We have identified a number of issues that have to be addressed when conventional printing is adapted in order to print electronic devices and integrated circuits thereof from solutions of functional organic materials. We report on our results of evaluation of various printing methods, i.e. offset, gravure and flexographic printing, and their suitability for the individual layers. The choice of the combination of printing techniques and the adjustment of the process parameters largely influence the performance of printed electronic devices and integrated circuits. We finally comment on the conclusions that can be drawn from the results of our printing experiments about the future potential of printed electronics.

A common strategy to improve the electrical performance of organic field effect transistors is to optimize the charge carrier mobility of the semiconducting thin film. Polymer semiconductor transport properties have shown a dependence on the chain length, due principally to the strong influence of molecular weight on the thin film microstructure. In this work, we report on a study of the influence of increasing molecular weight of poly(2,5-bis(3-docecylthiophen-2-yl)thieno[3,2-b]thiophenes) (pBTTT-C12) on the polymer bulk thermal properties, thin film microstructure and the electrical performance of thin film field effect transistor devices. Clear differences can be observed within a number average molecular weight range of 8,000 - 18,000 Dalton. A Liquid crystalline phase was only observed at the highest molecular weight, different thin film morphology was observed within the molecular weight range, and the field effect mobility was shown to increase with increasing molecular weight.

Regioregular poly(3-hexylthiophene) (RR-P3HT) has been a benchmark for organic field effect transistors containing a conjugated polymer. Here, we focus on a new class of materials that we have developed: block copolymers of RRP3HT and of a non-conjugated polymer. By varying the physical properties of the non-conjugated segment, it is possible to fine-tune the physical and electrical properties of the copolymer for various device applications. Here, we report the synthesis, UV-Vis, and conductivity of a series of block copolymers of RR-P3HT containing poly(methyl acrylate) (PMA) as the second segment. We also present preliminary results of these polymers in FET devices. We find that despite the presence of the insulating segment, these polymers have relatively high mobilities, especially when the substrate is treated with octyltrichlorosilane. For example, a mobility of 0.03 cm²V^-1s^-1 is reported for the block copolymer P3HT-b-PMA containing 41 mole % PMA, for a film drop-cast from a chloroform solution under ambient conditions. This work demonstrates that using block copolymers is a good approach for improving the material's properties while maintaining a high solid-state order that is critical for good OFET performance.

In this report, various organic Alq3 amorphous layers are prepared by vacuum deposition at different substrate temperatures T_sub (from 30 to 180°C). The surface morphology, structural information, electrical and optical properties of these as-deposited layers are investigated by atomic force microscopy, X-ray diffraction, J-E curves, and photoluminescence studies, respectively. Furthermore, a temperature dependence of dark electrical conductivity σ(T) deduced from J-E curves of these organic amorphous layers is presented. Finally, effects from T_sub on the physical properties of these organic Alq3 amorphous layers are discussed and a model based on a thermal interconversion between Alq3 isomers is proposed to explain these experimental results.

The electronic transport properties of polycrystalline pentacene-based thin film transistors (TFTs) were investigated at the microscopic level using microRaman spectroscopy. All the pentacene film, which were thermally evaporated as a layer with thickness of 70 nm, featured polycrystalline structure with only "thin film" phase polymorph and grain morphology as verified by x-ray diffraction (XRD) measurements. We have investigated the molecular vibrational modes of pentacene in the active channel during operations the organic TFT devices using in-situ Raman spectroscopy. Extra vibrational modes resulting from vibrational coupling effect in pentacene film were studied. The interlayer and intralayer intermolecular vibrational coupling energy was calculated from the Davydov splitting using a simple coupled-oscillator model. The results suggest that the C-H in-plane bending vibrational coupling energy of pentacene molecules in solid film is affected by operating device. Additionally, the aromatic C-C stretching vibrational modes also were investigated. However, it is rather difficult to obtain the variations of lattice parameters of pentacene film in a very small active channel by using electron diffraction and XRD. At the same time, MicroRaman technique provides the capability to explore the intermolecular coupling and molecular structure modifications.

Ambipolar conduction in organic field-effect transistor is very important feature to achieve organic CMOS circuitry. We fabricated an ambipolar pentacene field-effect transistors consisted of gold source-drain electrodes and double-layered PMMA (Polymethylmethacrylate) / PVA (Polyvinyl Alcohol) organic insulator on the ITO(Indium-tin-oxide)-patterned glass substrate. These top-contact geometry field-effect transistors were fabricated in the vacuum of 10^-6 Torr and minimally exposed to atmosphere before its measurement and characterized in the vacuum condition. Our device showed reasonable p-type characteristics of field-effect hole mobility of 0.2-0.9 cm²/Vs and the current ON/OFF ratio of about 10⁶ compared to prior reports with similar configurations. For the n-type characteristics, field-effect electron mobility of 0.004-0.008 cm²/Vs and the current ON/OFF ratio of about 10³ were measured, which is relatively high performance for the n-type conduction of pentacene field-effect transistors. We attributed these ambipolar properties mainly to the hydroxyl-free PMMA insulator interface with the pentacene active layer. In addition, an increased insulator capacitance due to double-layer insulator structure with high-k PVA layer also helped us to observe relatively good n-type characteristics.

We have already reported on the pentacene photo FET having a photo-sensitive gate dielectric layer (poly(Nvinylcarbazole): PVK). This photo FET showed excellent photo-switching properties upon illuminating the gate dielectric layer, because the gate capacitor was rapidly charged up by providing a large amount of photo-generated charges in the dielectric layer to the gate capacitor. When the gate bias and photo-illumination were turned off, accumulated charges diffuse from the channel region to the source electrode and the channel immediately becomes OFF state. Therefore, this photo FET can switch ON-OFF states rapidly, however, memory property is poor. In this study, we have tried to give a memory function to the photo FET. For keeping the accumulated charges at the channel region even after turning off the gate bias and photo-illumination, we intentionally formed Schottky barriers at the interface between the semiconductor and the DS electrodes by using aluminum as a low work function metal instead of gold. As a result, the photo FET did not show any gate voltage modulations of drain currents under dark condition. On the other hand, the photo FET showed the drastic increase in drain current upon illuminating the gate dielectric layer. Further, this high-drain current state was kept even after turning off the gate bias and photo-illumination (written state), probably because the accumulated charges at the channel region could not escape from the source electrode owing to the Schottky barrier at the semiconductor/DS electrodes interface. From these results, we have concluded that we could develop the novel organic rewritable optical memory.

We fabricated a novel type photo-FET using poly(N-vinylcarbazole) (PVK) as a photosensitive gate dielectric. For the photo-FET, photo-illumination to the PVK insulator layer makes the field-effect mobility μ_FET two orders of magnitude higher than dark condition. In particular, under blue-light illumination condition the on-off ratio was also a few ten times higher than dark condition. Furthermore, by introducing blocking layer between semiconductor layer and insulator layer lead. We concluded that the improvement of the transistor properties resulted from effective charge accumulation at the conductive channel by photo illuminations.

In this paper, we have demonstrated the current increase with repeated measurements of I_d-V_ds curves with different V_g values which results from the non-uniform carrier accumulation in the channel region of a pentacene-based thin film transistor (TFT). The mobility of our device reaches 0.07 cm²/Vs even the substrate was not heated during pentacene deposition. Besides, the devices show good air-stable properties. The magnitude of I_d decreased less than 30% after exposure in air for 2 weeks. By repeating the I_d-V_ds measurements from 0 to -50 V with the V_g values of 0, -10, -20, -30, -40, and -50 V for 10 minutes, we observed a four times current increase from -0.75 to -2.8 μA at V_g = -50V and V_ds = -50V. The current increase comes from the holes accumulation near the drain. When the source and drain were exchanged, the current decreases to the 0.08 μA. After another 10 minutes operation, the current will recover back to the original values. Such a process is reversible and shows the potential of the memory device base on this pentacene transistor.

Many researches report that the mobility in organic material is dependent on not only the gate field but also the grain size. There is also some evidence to prove that the gate length is strongly related to the carrier mobility. We construct both the analytical model of organic thin film transistor and the large signal circuit model designed by T-CAD to fit the measured I _DS - V _DS curves. We first apply basic I _DS - V _DS equations in both triode and saturation regions with mobility μ best fitted to measured I-V curves. The "best-fitted" μ increases with the gate length, and is related to the increase of total channel resistance due to the presence of small grains size of pentacene next to source/drain electrodes. We then use the Advanced Design System software to design the large signal circuit model. Similar to the MOSFET, we add the additional parameters to fit the I _DS - V _DS curves, ex: Rgd, Rgs, and Rp. Here, Rgd. With the circuit simulation, we find that Rgd presents the leakage current from gate to source, and it affects the slope of curves in the saturation region in the I _DS - V _DS curves. The equivalent circuit can fit the I _DS - V _DS curves very well with the proper parameter set.