It is well known of regioregular poly(3-hexylthiophene) (P3HT) to self-assemble on hydrophobic surfaces, because the regular arrangement of its side chains allows an efficient π-stacking of the conjugated backbones. This should be the reason for the very high reported field effect mobilities (0.2 cm²/Vs)¹. We present an alternative approach to increase the field effect mobility and the transistor stability by introducing a strong acceptor dopant in the main chain of P3HT while preserving the regioregularity of the 3-hexylthiophene segments in the polymer chain. P3HTs with different contents of acceptor molecules which are fixed linked in the main chain of the polymer, were synthesized using the McCullough Grignard metathesis method. As acceptor unit has been integrated 9-dicyanomethane-fluorene. The introduced dopant amount has been varied in order to obtain an optimum between the processability of the polymers and the resultant transistor performance. The utilized organic field effect transistor (OFET) substrates with SiO₂ gate dielectric were always pre-treated with a silylating agent (HMDS) to facilitate the self-organization properties of the polymers by hydrophobization of the SiO₂ surface. The optimized doped structures showed higher field effect mobility by one order of magnitude compared to the conventional P3HT, (μ_FE~9 x 10^-3 cm²/V.s for the doped compound vs. μ_FE~5 x 10^-4 cm²/V.s for P3HT), combined with a marked current modulation with ON/OFF ratio of ~ 7 x 10³ and a better operational stability of the resultant OFET-devices.

Electrical bistable device with high switching voltage is developed. The device structure is Al electrode layer/ metal dispersed layer/ organic material layer/ Au electrode layer, whose thickness is 60/30/80/60nm, respectively. The organic material is a type of bisquinomethane, 2, 5-bis {(3’, 5’-di(tert-butyl)-4’-oxocyclohexa-2’, 5’-dienylidene)-(4"-methoxyphenyl) methyl} thiophene (DODMT). The metal dispersed layer is formed by co-evaporation of DODMT and Al. With this structure, the switching voltage around 18V is attained. The device works even without the metal dispersed layer, in that case, with switching voltage of around 5V. The origin of bistability is estimated to be tunneling charge injection caused by charge accumulation at the Au/organic interface and subsequent enhancement of electrical field. Charge accumulation has also been observed in the metal dispersed layer. It would retard charge injection from Al electrode to DODMT layer, and consequently, to the Au/organic interface, and make the switching voltage high.

This paper presents electrical characteristics for high-performance pentacene-based organic field-effect transistors (OFETs) manufactured on polymer substrates. The mobilities as high as 2.13 cm²/V-s are reported for devices with a bottom-contact configuration and solution cast dielectric layers. The influence of the dielectric choice on pentacene structure and carrier mobility as well as a method for the improvement of current injection is discussed.

Sensitivity is a key issue in designing high performance organic based chemical and biological sensors. Several strategies have been adopted in the past to improve this important figure of merit. In this paper, a pentacene based organic thin film transistor is operated as an alcohol sensor and it is demonstrated that the sensitivity is enhanced when the device works in the accumulation mode.

An organic field effect transistor (FET) device based on a solution-processible tetrabenzoporphyrin (BP) has been developed. BP is derived from its precursor that is soluble in some organic solvents and gives an amorphous film of high quality by spin coating. A polycrystalline film of BP is obtained by thermal conversion of the precursor at about 200 degree C. The FET characteristics are found to largely depend on the purity, device structure, and fabrication process. The device performance was: mobility of 1.7 X 10^-2 cm²/Vs and on/off ratio of 10⁵. We have also analyzed the crystal structure of BP and characterized its electronic and morphological properties.

Organic Thin Film Transistors (OTFTs) have been fabricated, in a standard bottom gate configuration, with Langmuir-Schafer (LS) or cast thin films of regioregular poly[1,4-(2,5-dioctyloxyphenylene)-2,5-thiophene], synthesized via an organometallic protocol, as active layers. The transistors electrical characterization has evidenced that LS based devices exhibit better performance level than cast film ones. Appealing perspectives for newly substituted conjugated polymers in OTFT sensing devices are discussed.

Organic strain gauge and other sensors require high-gain, precision dc amplification to process their low-level output signals. Ideally, amplifiers would be fabricated using organic thin-film field-effect transistors (OTFT's) adjacent to the sensors. However, OTFT amplifiers exhibit low gain and high input-referred dc offsets that must be effectively managed. This paper presents a four-stage, cascaded differential OTFT amplifier utilizing switched capacitor auto-zeroing. Each stage provides a nominal voltage gain of four through a differential pair driving low-impedance active loads, which provide common-mode output voltage control. p-type pentacene OTFT's are used for the amplifier devices and auto-zero switches. Simulations indicate the amplifier provides a nominal voltage gain of 280 V/V and effectively amplifies a 1 mV dc signal in the presence of 500 mV amplifier input-referred dc offset voltages. Future work could include the addition of digital gain calibration and offset correction of residual offsets associated with charge injection imbalance in the differential circuits.

In this study we report on new concepts to generate light emission in organic thin film transistors. The initial physical understanding of light emission from tetracene based field-effect transistors was proposed to be originated from a strong underetching of the drain and source electrodes. This underetched electrodes in combination with the evaporated tetracene is thereby believed to generate a virtual OLED at the drain electrode. Accumulated holes have to leave the gate oxide interface to reach the drain electrode by crossing the bulk of the organic semiconductor. Light then occurs by injection of electrons in a large electric field in the bulk. Today's transistors do not show the underetching anymore but are still emitting light only at the drain electrode, again supporting the initial interpretation of a defect state at the edge of the drain electrode. In this context the question how electrons can overcome a potential barrier of 2.7 eV is still open. Therefore an investigation of the gold tetracene interface by UPS and XPS techniques has been started and preliminary data indicate the unexpected result that the barrier for electrons is comparable to that for holes. In a further step the generation of an ambipolar transistor by interface doping with calcium was tried and an n-type pentacene transistor could be fabricated but the strategy failed for tetracene. Finally an electrochemical interface doping was performed by the application of Lithium triflate in PEO to a thin interface layer between gate oxide and tetracene. This leads to light emission but unfortunately also to the loss of the gate voltage influence. Based on these results a possible strategy will be presented.

We report on transistors and light-emitting diodes using a conjugated polymer consisting of alternated segments of fluorene units and low-band gap donor-acceptor-donor (D-A-D) units. The D-A-D segment includes two electron-donating thiophene rings combined with a thiadiazolo-quinoxaline unit, which is electron withdrawing to its nature. The resulting polymer is conjugated and has a band gap of around 1.27 eV. Here we present the corresponding electro- and photoluminescence spectra, which both peak at approximately 1 micrometer. Single layer light-emitting diodes demonstrated external quantum efficiencies from 0.03% to 0.05%. The polymer was employed as active material in thin film transistors, a field-effect mobility of 0.003 cm²/Vs and current on/off ratio of 104 were achieved at ambient atmosphere.

We report on an empirically based physical model developed for small-molecule organic thin film transistors (OTFTs). The model is an extension of an adapted MOSFET model for hydrogenated amorphous silicon TFTs accounting for an arbitrary energy distribution of mobile and trap states and allows the extraction of the parameters from the measured device characteristics. Ideally all parameters can be derived from the material properties of the organic semiconductor, but often those are masked by extrinsic effects. To provide model input and validation data sets we fabricated top contact pentacene TFTs on heavily doped and thermally oxidized wafers. The device structure allows the systematic study of the influence of the source and drain contacts and the properties of the semiconductor/metal interface on the device characteristics by varying the contact metal, deposition parameters, or the silane coupling agent used to treat the gate dielectric prior to deposition. From this a functional dependence of the contact and interface effects is incorporated into the model. The subthreshold regime is mainly used to test the charge trapping and charge-configuration model because charge-configuration related effects are usually more exposed in this regime. Currently the model routinely exhibits > 90% accuracy for most devices. Further insight into the actual physical mechanisms is expected from comparing the extracted trap state distributions with those extracted with other methods.

Nanoscale thin film transistors with conjugated polymers as the active layer were fabricated to investigate their chemical sensing properties. Guarding electrodes as close as 20 nm to the two sides of the channel were employed to eliminate undesirable spreading currents and ensure that the sensor active area is truly nanoscale. The response of drain current exhibited opposite directions in nanoscale sensors compared to large scale devices, for the same analyte-semiconductor combination. The transition in response behavior was observed occurring in a certain interval of channel length. The chemical sensing mechanisms in both microscale and nanoscale transistors are briefly discussed.

We studied on the effect of nanoparticle dispersion on electrical properties of polymer field effect transistor (FET). Various kinds of powder materials were prepared and dispersed into P3HT films. In order to determine the electronic structures of particle-dispersed P3HT films, photoelectron emission spectra were measured. Using these spectra ionization potentials (I_Ps) of these films were determined. The shifts of IP originated from the particle dispersions were very small (from 4.69 to 4.72eV). Similarly, any differences were not observed between the electron absorption spectra of the particle-dispersed P3HT films and that of the pristine P3HT films. According to these results, no chemical reactions would occur between P3HT matrices and dispersed particles. On the other hand, Fermi levels of particle-dispersed P3HT films were obviously shifted from that of the pristine P3HT film. The shift was well correlated to the difference between the workfunction of P3HT and that of the dispersed material. Namely, the dispersion of the particles with the lower workfunction contributed to the decrease of workfunction of the P3HT film. Further, the decrease of workfunction of P3HT film resulted in the decrease of the off-current values of the P3HT-FET. This would be because the electrons transferred from the dispersed particles neutralized the excess holes in the P3HT film. Actually, the Ag particle dispersion remarkably improved on-off ratio of the P3HT-FET.

This study first demonstrated the feasibility of using the photoalignment method to adequately control the structural anisotropy of pentacene films, which are active semiconducting layers, in thin-film transistors (TFTs) with conspicuous anisotropic electrical characteristics. The photoaligned pentacene films were characterized with respect to structure and morphology using x-ray diffraction, atomic force microscopy and Raman scattering. Compared to the uncontrolled pentacene films, a maximum 25-times increase in field-effect mobility (up to 0.82 cm²/Vs) is achieved in the photoaligned pentacene-based TFTs by aligning pentacene orientation parallel to the current flow direction using a photoaligned polyimide layer. Mobility anisotropic ratios ranging between 2.7-8.3 for the current flow parallel and perpendicular to the alignment of the photoaligned pentacene films have been observed for photoaligned pentacene-based TFTs.