Well-characterized F8T2 polyfluorene (Dow Chemical) has been prepared with weight average molecular weights (Mw) ranging from about 20,000 to 120,000. This semiconducting polymer has been used by Plastic Logic to fabricate arrays of 4,800 thin film transistors (TFTs) with 50 dpi, to be used as backplanes for active matrix displays. In this paper, the effects that molecular weight and thermal treatment have on the electrical characteristics of F8T2-based TFTs are reported. First, transistor performance improves with increasing molecular weight, with maximum values of TFT mobility approaching 1x 10^-2 cm² /V-s. Consistently higher mobilities are obtained when the F8T2 semiconductor makes contact with PEDOT/PSS versus gold electrodes. Alignment of F8T2 on a rubbed polyimide substrate is maintained after quenching, as determined by measurement of the dichroic ratios. Early-stage results on the development of inks based on F8T2 polyfluorene are also reported.

We present results on the electrical characterization of gate-planarized organic polymer thin-film transistors (OP-TFTs). We investigated the time dependence of the OP-TFT current. Over a relatively short time range (several 100ms), we observed a decrease of the OP-TFT current corresponding to the establishment of the steady-state regime, and is slower when the transistor is in the weak accumulation regime or in the OFF-state. We believe that this is associated with carrier thermalization in the organic semiconductor. Over longer time scales, the decrease of the OP-TFT current is due to device aging and can be associated with a threshold voltage shift, up to 20V after an electrical stress at V_GS=-30V for 30min at room temperature. This shift is fully reversible once the gate polarization is removed and might be associated with charge trapping in the semiconductor.

The phase behaviour of poly(9,9-dioctylfluorene-co-bithiophene) semiconducting polymer, (F8T2) in top gate thin film transistor device structures fabricated using inkjet printing is investigated. The source, drain and gate electrodes are patterned by inkjet printing from a solution of a conducting polymer, poly(3,4-ethylene dioxythiophene) (PEDOT) doped with poly(styrene sulfonic acid) (H. C. Starck), and a polymer layer is used as the dielectric. At room temperature, the as-spun semiconductor films exhibit an isotropic, amorphous phase. Field effect mobilities of more than 4 x 10^-3 cm² / Vs, and on / off current ratios greater than 10⁵ are observed. Upon annealing at elevated temperatures, crystalline, and liquid crystalline phases are exhibited. The crystalline domains are identified by polarised optical and atomic force microscopy. We investigate the crystallinity as a function of the annealing temperature. The order in the material is found to correlate well to the field effect mobility in the TFT device structure. The results of TFTs fabricated using inkjet printing to deposit the semiconductor film are also shown.

Polymer field-effect-transistors (FETs) have been proposed for use in display driver circuitry, information storage and processing, and identity tags. To maximize performance in polymer FETs it is important to have a high carrier mobility. A major issue in this area is how the FET mobility, as measured by analysis of source-drain current-voltage characteristics, relates to the bulk mobility, as measured by a technique such as the time-of-flight (TOF) photocurrent or space-charge-limited current (SCLC) methods. Here we report comparative FET, TOF and SCLC measurements of polyfluorene copolymer devices. Poly (9,9-dioctylfluorene-co-bithiophene) (F8T2) and poly (9,9-dioctylfluorene-co-bis-N, N’-(4 butylphenyl)-bis-N,N’-phenyl-1,4-phenylenediamine) (PFB) were used as the active material. Polymer FETs were fabricated in two different structures. The first involved a silicon substrate with a thermally grown oxide dielectric onto which Au source-drain electrodes were deposited. The polymer was then spin-coated on top and annealed to improve the chain packing. The second involved spin-coating the polymer onto a glass substrate followed by the thermal evaporation of top Au source-drain contacts. A polymer insulator was then spin-coated followed by a top Au gate electrode. TOF and SCLC measurements were conducted on diode structures consisting of a polymer layer (of order 1 μm and 100nm respectively) sandwiched between indium-tin-oxide (ITO) or ITO coated with poly (ethylenedioxythiophene)/polystyrene sulphonic acid (PEDOT/PSS) on glass bottom electrode and a Au or Al top electrode. Trapping effects, and the difference between bulk and surface packing can describe the differences of mobilities derived from the three methods. It is interesting to note that they are all within a similar order.

We have used thermal treatment and rubbed polyimide alignment layers to produce large domains of poly(9,9-dioctylfluorene-co-bithiophene) alternating copolymer (F8T2). The direction of rubbing on the polyimide surface determines the orientation of these domains, allowing us to create thin-film transistors with channel lengths parallel and perpendicular to the liquid crystal polymer director. We showed that thermal annealing at temperatures ranging from 150 to 350°C modifies the polymer structure from an amorphous to ordered phase as observed by X-ray diffraction. Polarized light optical microscopy showed that this ordered phase is associated with very large ordered domains and corresponds to a thermotropic, nematic liquid-crystal phase. We investigated thermal annealing effects on both F8T2 structural ordering and the associated electrical properties of the thin film transistors (TFTs). Enhanced mobility of holes is observed with ordering. Field-effect mobility parallel to the polymer backbone is as much as 6.5 times greater than the perpendicular configuration.

High field-effect hole mobility of (formula available in paper)and threshold voltage is -3.2 V) in organic-inorganic layered perovskite film (formula available in paper)prepared by a vapor phase deposition technique have been demonstrated through the octadecyltrichlorosilane treatment of substrate. Previously, the (formula available in paper)films prepared on the octadecyltrichlorosilane-covered substrates using a vapor evaporation showed not only intense exciton absorption and photoluminescence in the optical spectroscopy but also excellent crystallinity and large grain structure in X-ray and atomic force microscopic studies. Especially, the (formula available in paper)structure in the region below few nm closed to the surface of octadecyltrichlorosilane monolayer was drastically improved in comparison with that on the non-covered substrate. Though our initial (formula available in paper)films via a same sequence of preparation of (formula available in paper)and octadecyltrichlorosilane monolayer did not show the field-effect properties because of a lack of spectral, structural, and morphological features. The unformation of favorable (formula available in paper)structure in the very thin region, that is very important for the field-effect transistors to transport electrons or holes, closed to the surface of non-covered (formula available in paper)dielectric layer was also one of the problems for no observation of them. By adding further optimization and development, such as deposition rate of perovskite, substrate heating during deposition, and tuning device architecture, with hydrophobic treatment, the vacuum-deposited (formula available in paper)have achieved above-described high performance in organic-inorganic hybrid transistors.

Recent progress in the field of organic electronics is due to a fruitful combination of both innovative molecular design and promising low-cost material/device assembly. Targeting the first strategy, we present here the general synthesis of fluoroarene-containing thiophene-based semiconductors and the study of their properties with respect to the corresponding fluorine-free hole-transporting analogues. The new compounds have been characterized by elemental analysis, mass spectrometry, and 1H- and 19F NMR. The dramatic influence of fluorine substitution and molecular architecture has been investigated by solution/film optical absorption, fluorescence emission, and cyclic voltammetry. Single crystal data for all of the oligomers have been obtained and will be presented. Film microstructure and morphology of this new class of materials have been studied by XRD and SEM. Particular emphasis will be posed on the solution-processable oligomers and polymers.

Raman spectroscopy is capable of distinguishing neutral and charged states of organic molecules due to its sensitivity for charge induced changes in the molecular geometry and the bond strength.Combined with in situ electrical measurements it provides a powerful tool for characterizing charged molecules in the channel of organic field effect transistors OFETs. The active 3nm C₆₀ layer in an OFET structure with bottom gate con figuration was characterized by in situ Raman spectroscopy using the 514.5 nm (2.41 eV)Ar⁺ laser line as a function of the drain source (V_ds) voltage. The Raman spectra show pronounced changes upon application of a drain source V_ds voltage. The experimental findings are compared to the vibrational spectra calculated for molecules under external bias fields. Complementary to the Raman characterization,the I_d-V_ds and I_d-V_gs characteristics were recorded and the mobility value of 0.1 cm²/V s was derived from these measurements.

The electrical bistable behavior of two organic materials, 2-amino-4, 5-imidazole-dicarbonitrile (AIDCN) and 2-(4-Diphenylamino-benzylidene)-malononitrile (DBMN) were investigated. Samples were prepared in a single layer structure, namely Al electrode/ organic material layer/ Al electrode. The Al electrodes and the AIDCN layer were formed with vacuum evaporation, and the DBMN layer was formed with spin coating. The specimens were initially electrical insulators (off-state) at low applied voltage. Beyond a critical value of voltage, the current suddenly increased by several orders. They remained in the low impedance state (on-state) even when the voltage was lowered below the critical value. The results of the surface observations with an atomic force microscope and the structure analysis with X-ray diffraction indicated that the AIDCN layer was composed of multi-crystalline grains, whereas the DBMN layer was amorphous. The on-state current was proportional to the square of the applied voltage, which suggested it was limited by space charge in the organic layer. The on/off transition behaviors and the surface morphology suggested that the transition of AIDCN would be due to tunneling charge injection caused by the charge accumulation at the interface between the organic layer and the metal electrode. The transition of DBMN would be the one between the states of almost equal energy levels, such as a change of polarization direction.

The hole transport in the amorphous poly(2-methoxy-5-(3’,7’-dimethyloctyloxy)-p-phenylene vinylene) (OC₁C₁₀-PPV) and in the more ordered poly[2,5-bis(3’,7’-dimethyloctyloxy)-p-phenylene vinylene] (OC10C10-PPV) has been investigated both in field-effect transistors (FETs) and light-emitting diodes (LEDs) as function of temperature and applied voltage. From J-V measurements on LEDs a difference of 15x has been found in the hole mobility between OC₁C₁₀-PPV and OC₁₀C₁₀-PPV. In FETs the dependence of the field-effect mobility on the carrier density is much stronger in OC₁C₁₀-PPV than in OC₁₀C₁₀-PPV. These differences in the mobility in both FETs and LEDs are determined by the difference in microscopic transport parameters between the two materials, which results from a different ordering in the polymeric film of the PPV derivatives. Due to their specific chemical composition OC₁C₁₀-PPV is an amorphous polymer and the transport is the same in all directions, while OC₁₀C₁₀-PPV is more ordered and the transport shows anisotropy between sandwich and in-plane devices.

We have investigated the electrical properties of organic thin-film transistor by using two-dimensional drift-diffusion simulations. The dependence of electrical haracteristics on the mobility model and on the barrier height of the contacts is carried out. We found that the field dependence of the carrier mobility is responsible for non-linearity of the drain current. This non-linear behavior is mainly related to the field-dependence of the mobility and to the barrier height of the contacts. The simulation allow us to clear understand the differences in the mobility derived by the analysis of I-V curve (as done experimentally by using standard MOSFET theory) and the intrinsic mobility of the organic layer. The effects of the interface traps has also be considered. The dependence of the threshold voltage on the density, energy level and model of the traps has been outlined. Results of the simulations have been compared with experimental data. The comparison between experimental data and simulation allow us to clearly identify the physical mechanism responsible for the measured characteristics. Finally we also consider the effect of the device bending on the electrical characteristic of all-plastic OTFT.

A high-performance organic diode is demonstrated by using C₆₀ sandwiched between a cathode and an anode using metals with different diffusivity and donor ability. In this manuscript,copper (Cu)and aluminum (Al)are selected as the cathode and anode, respectively. C₆₀ is used as the organic electron-acceptor for its high stability and high carrier mobility. The as-prepared diode shows poor performance.However,after heat treatment, the Cu/C₆₀ interface becomes an Ohmic contact through Cu diffusion and charge-transfer processes,allowing highly efficient electron injection from the Cu electrode. On the other hand, a rectified C₆₀/Al contact is formed, prohibiting efficient electron injection from the Al electrode into C₆₀. Hence,a high-performance organic diode is formed through a heat treatment process, not by the selection of metals with different work functions. Due to the high mobility of C₆₀, the device shows megahertz frequency response, and it can also handle rather high current density (363 A/cm² at 2.4V). This opens the way for the formation of high-performance organic electronic devices.

We report on light emission on organic thin film transistors of tetracene and polyfluorene (Poly(9,9-di(ethylhexyl)fluorene) (PF2/6)). The utilized transistor structure is a bottom gate configuration with interdigitated source and drain electrodes on a Si/SiO₂ substrate with a channel length of 5 μm. Light emission occurs above a source drain voltage of 30V even if the gate voltage is higher than the drain voltage. The light output can be controlled by the gate voltage. The light emission occurs close to the drain electrode as observed by light microscope images of operating transistors. In order to understand the functional principle of a light emitting transistor a resistor capacitor equivalent circuit model has been utilized to describe charge carrier transport, carrier distribution and the electrical potential distribution in such a device. The model extends the common thin film transistor theory for unipolar charge transport to ambipolar charge transport. Analytical expressions for output and transfer characteristics as well as for the potential and charge carrier distributions are obtained. Further, the effect of contact resistors on the output and transfer characteristics are simulated. The model is used to explain the underlying mechanisms of the present devices. Imperfections on the contact electrodes, most probably due to under-etching of the electrodes are seen as the main reason for the electron injection.

Due to its outstanding carrier transport capabilities the aromatic hydrocarbon pentacene is still one of the most promising out of all organic semiconducting materials investigated so far. Pentacene appears in several polymorphic structures that significantly differ with respect to the d(001) spacing. It is shown, that precise control of the epitaxial growth process of thin films enables not only to adjust the formation of the polymorphic phases, but also to influence grain size and shape. The relative volume fraction of the pentacene polymorphs is determined by several parameters which are substrate material, deposition rate, film thickness and substrate temperature. A comparison of X-ray diffraction and Raman measurements reveals that the phase with the smaller layer-by-layer spacing grows on top of the other]. Moreover, there is a strict correlation between evaporation rate and maximum grain size. In addition to structural we also investigated the electrical properties of pentacene thin films focussing on polymorphism and its influence on the transport properties. Apart from the fact that the charge carrier mobility is strongly influenced by the grain size it turned out that the bulk phase is related to a lower intrinisic mobility than the thin film phase.

Face-to-face interactions of aromatic molecules in the crystalline state may prove to be the most relevant for materials destined to be used in thin-film transistor or photovoltaic applications. We have designed functionalized pentacene derivatives that maximize these interactions in the solid state. We present here a description of a number of pi-stacked crystalline motifs that we can access, along with a discussion of the dependence of resistivity and band gap on the nature and amount of pi-overlap in the crystal.

In order to develop a printable organic thin film transistor with high performance, it is required to develop not only printable materials and a printing process but also a suitable device structure for it. We have newly designed an organic field effect transistor with a diagonal configuration of source and drain electrode, named as the Top and bottom contact (TBC) configuration. It has several advantages due to its unique structure. For example, it can be prepared by a simple stacking process without any micro-machining process or related photolithography procedures. This is thus suitable for applying the simple printing technique such as a screen-printing. In the proposed structure, source and drain electrodes are arranged diagonally across the active layer. Therefore, the channel length can be controlled by the deposited active layer thickness. In this study, we have prepared the pentacene transistor with the TBC configuration. By inserting the extra insulator layer, leakage current between the top and bottom electrode was remarkably reduced. The output current density was about two orders larger than the conventional organic transistor with a top contact configuration. These high performances are mainly due to the improvement of the carrier injection efficiency owing to the short channel length (ca.0.5μm).

The field effect devices prepared using active channels fabricated from doped conducting polymers, such as PEDOT/PSS (poly-(3,4-ethyldioxythiophene/poly(styrene sulfonic acid)), polypyrrole/Cl-, and polyaniline/Cl- with various dopants are reported. Normally in the "on" state, the devices have a sharp switch off at a small gate voltage. The current ratio I_on/I_off can exceed 10⁴ at room temperature. The temperature dependence of the dc conductivity of the PEDOT/PSS follows the variable range hopping law both before and after application of the gate voltage. The activation energy, T₀, increases even for on/off ratios as small as 1.07 demonstrating that the electric field effect has changed the bulk charge transport in the active channel despite the expected screening due to mobile charge carriers. Based on these transport and optical studies we propose that the conducting polymer is near the metal-insulator transition and that the field effect in the transistor is related with electric field modulating this transition in the region underneath of gate through field induced ion motion. The time dynamics of current with the gate modulation strongly supports our proposal. Application of the Doped Polymer Field Effect Devices (DPFEDs) to form circuit components has been demonstrated.

New findings are presented relating to the optimal choice of gate insulators in organic field effect transistors (OFET). It was recently found that some organic semiconductors operate better when low-k materials are used in the gate. This is quite contrary to the conventional trend to use high permittivity dielectrics for low voltage operation. Interaction between the insulator and the semiconductor materials plays an important role in carrier transport. On one hand, the insulator is often responsible for the morphology of the semiconductor layer, but on the other hand it can also change the distribution of states by local polarisation effects. Carrier localisation is enhanced by insulators with large permittivities, due to the random dipole field present at the interface. We have investigated this effect on a number of disordered organic semiconductor materials and show here that the use of low-k materials may lead to improvements in mobility, reduced temperature activation and hysteresis. In particular, the behaviour of the threshold voltage is interesting. The differences in the underlying physics compared to the case of FETs based on band-like semiconductors, is also discussed.

The electrical properties of polymeric thin film transisitors (P-TFTs) based on poly(9,9-dioctylfluorene-co-bithiophene) alternating copolymer (F8T2) have been studied. Device performance was compared for amorphous silicon nitride deposited by LPCVD and PECVD techniques, aluminum oxide deposited by sputtering, titanium oxide deposited by sputtering, and thermal silicon oxide gate dielectrics. A heavily n-type doped crystalline silicon wafer coated with the desired gate dielectric was used. Photolithographic patterning of source/drain electrodes directly on top of the F8T2 layer is also discussed. The main conclusion from this work is that traps within the F8T2 define the conduction process within the device.

Organic thin-film transistors have seen a dramatic improvement of their performance in the last decade. They have been also proposed as gas sensors. This paper deals with the interesting new aspects that polycrystalline based conducting polymer transistors present when operated as chemical sensors. Such devices are capable to deliver multi-parameter responses that are also extremely repeatable and fast at room temperature. Interesting are also the perspectives for their use as chemically selective devices in array type sensing systems.

In this work,a field effect transistor based on deoxyguanosine derivatives (a DNA basis)is demonstrated by means of systematic transport experiments. Our nanodevices were fabricated starting from a deoxyguanosine derivative (dG(C₁₀)₂) layer interconnecting planar nano-electrodes,with separation in the 20-40nm range. The three terminal devices exhibit a maximum voltage gain of 0.76. Though the quick aging and the reproducibility of the devices have to be improved, the realization of a transistor-like device represents a starting point towards the development of planar solid-state bio-molecular electronic devices.

New polymerisable liquid crystalline organic semiconductors based on small molecule "reactive mesogens" are reported. These molecules, comprising p-conjugated cores with reactive endgroups, were designed, synthesised then solution processed into thin films. The mesogenic morphology was attained thermally and fixed through a post fabrication photopolymerisation reaction. Thermal, optical and electrical properties of these thin films were characterised to reveal details of morphology and molecular orientation. Both bulk (time-of-flight) and surface (field-effect) charge carrier mobilities were measured. The relationship between molecular structure, corresponding macrostructure processability and charge mobility is discussed. Fabrication and characterisation of field-effect transistors based on reactive mesogens are presented and discussed.

We have investigated the effects of illumination, both broadband and monochromatic, on the electrical performance of organic polymer thin-film transistors (OP-TFTs). In each case, providing the illumination is sufficiently absorbed by the organic polymer, the drain current of a device biased in the OFF-state is significantly increased. We have observed increases in the OFF-state drain current as large as several orders of magnitude depending on the intensity of the incident illumination. Whereas, the drain current of a device biased in the strong accumulation regime is relatively unaffected by the incident illumination. The illumination also serves to decrease the threshold voltage and increase the subthreshold slope, but has little effect on the field-effect mobility of the charge carriers. We explain these effects in terms of the photo-carrier generation in the channel region of the device due to the incident illumination. We have also studied how our OP-TFTs respond to the turn-on and turn-off of gate bias under illumination and to the turn-on and turn-off of illumination at certain gate biases.

A flexible insulator film would be one of the most important elements of flexible organic field-effect transistors (OFETs). It should be produced from a soft organic material rather than a stiff inorganic material. Many polymeric materials were spin-coated from the solution and the resulted films have to be baked or cured to obtain a good insulator. Since those procedures impose a restriction on the OFETs, a fabrication process without using a solvent has been desired. Poly-p-xylylene derivatives have been made into an insulator film by a non-solvent procedure, chemical vapor deposition (CVD). The insulator film has additional advantages, pinhole-free, resistance to many solvents and no thermal stress to a material beneath. We have fabricated and characterized OFETs with the inverted staggered geometry, substrate/ gate electrode/ poly-p-xylylene derivatives/ organic semiconductor/ source-drain electrodes. And the CVD enables to form an insulator film even above the organic semiconductor. So we fabricated the staggered type configuration, substrate/ source-drain electrodes/ organic semiconductor/ poly-chloro-p-xylylene/ gate electrode. The device performance of a staggered type transistor indicated that the molecular arrangement of organic semiconductor at the insulator interface is more dominant than the damage or chemical deterioration due to the attack of the radicals during the CVD procedure.

The understanding of metal/organic interfaces is fundamental for achieving high quality contacts in organic based field effect transistors,OFETs. Here the formation of In and Ag interfaces on 3,4,9,10 perylene tetracarboxylic dianhydride (PTCDA)and N,N dimethy l3,4,9,10 perylene tetracarboximide (DiMe-PTCDI)is investigated by in situ Raman spectroscopy. Upon metal deposition a significant enhancement of the Raman signals arising from internal vibrational modes is observed for these two perylene derivatives,clearly indicating the presence of surface enhanced Raman scattering (SERS). Molecules in direct contact with the metal are involved in a weak dynamical charge transfer in the electronic ground state, resulting in a breakdown of the Raman selection rules for the free molecule. Both metals show very weak interactions with the organic thin films. This finding is opposite to previous suggestions of covalent bond formation between In and O atoms in PTCDA. The Raman scattering of molecules close to the metal interface is enhanced by coupling to the plasmon resonance of small metal clusters,indicating a high roughness of the metallic film. The enhancement factors can be used to extract information on diffusion of the metal atoms into the organic film and on the morphology of the contact layer.