Search

We describe an unusual phenomenon of time-dependent current growth in organic transistors, particularly n-type transistors. For an organic transistor based on N,N-ditridecyl-3,4,9,10-perylene tetracarboxylic diimide with a polyimide dielectric layer, the time-dependent increase in the drain current and an approximately hysteresis-free electricity were obtained under dc-bias stress. These phenomena could be attributed to (a) reduction in the trap-state density located at the interface between polyimide and semiconductor, (b) gate field effect enhanced by electric dipoles within polyimide, and (c) a low interface trap lifetime. This study reveals that polymer dielectrics with moderate polar groups are suitable for application in stable organic devices.

Electrical characteristics of pentacene-based ambipolar organic field-effect transistors (OFETs) are examined and are shown to significantly differ from those of unipolar OFETs. The electrical and hysteresis characteristics of the ambipolar OFETs depend on the applied source-drain bias and sweeping gate voltage range. The ambipolar OFET characteristics, such as charge mobility, subthreshold swing, threshold voltage, and off-current level, are controlled by the dual-carrier recombination and release process, through which opposite-sign charges can capture and release majority charges. This study contributes to advancing the development of more applications based on ambipolar OFETs.

The performance of bilayer heterojunction, pentacene and perylene tetracarboxylic diimide organic solar cells (OSC) is dramatically enhanced by performing nanoimprinting on the hole transport layer in which poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) nanogratings are built. A standard OSC embeded with PEDOT:PSS gratings enables photocarriers to move toward electrodes. In particular, the visualization of morphologies for these heterojunction layers reveals pillar-like grains that are induced by the geometric effect of PEDOT:PSS gratings. Toghther with analyzing absorption and time-resolved photoluminescence spectra, a consistent correlation between OSC performance, photophysical data, and pillar-like morphology has been obtained for the device with imprinted PEDOT:PSS nanogratings.

Polymorphism control of organic semiconductor films greatly affects optoelectronic device performance. The thin-film phase is always the dominant phase in the early growth of pentacene films; however, the performance of the bulk phase is superior to that of the thin-film phase. In the present study, a nanoimprinted polyimide layer is used to induce the growth of the bulk phase in the early stage. The bulk phase appears in the initial few layers and its dominance of the polymorphisms in the pentacene film increases with increasing thickness. The mechanism of the phase transformation is explained.

The charge carrier transport efficiency and issues of patterning in organic semiconductors limit the potential range of microelectronic and optoelectronic applications of organic devices in nanoscale. We demonstrate high-performance organic field-effect transistors (OFETs) with a mobility of approximately 2.5 cm2/V s using nanogroove gate-dielectrics formed by nanoimprinting. The preferred flow of charge carriers in OFETs parallel to the nanogrooves yields a high mobility anisotropic ratio (above 220), providing a built-in autopattern organic semiconductor function with nanoscale resolution. This nanostructure embedded device has great potential for use in the manufacture and lithography-free patterning of organic semiconductor films in integrated circuits.

The role of residual solvents and vacancies within poly(3-hexylthiophene) (P3HT) active layers, which are made from different boiling point (bp) solvents, on the electrical hysteresis characteristics of P3HT-based transistors was investigated. The improved electrical performance and reduced hysteresis of P3HT films, which are spin coated by high bp solvents, can be interpreted by superior crystalline quality and homogeneity and low vacancies. The hysteresis is dominated by the vacancy-related charge traps in the semiconductor created during film solidification and subsequence solvent evaporation. Furthermore, residual solvents, which initially occupied the vacancies, can contribute to conductivity of regioregular P3HT, thus altering electrical properties and smaller hysteresis.

The authors report the suppression of the off-state leakage current and subthreshold swing (SS) in inkjet-printed poly(3-hexylthiophene) thin-film transistors with asymmetric work function source and drain electrodes. Indium tin oxide (ITO) material was used as source/drain electrodes and the source electrode was irradiated by KrF excimer laser. The dominant mechanisms for the suppressive Ioff could be attributed to the increase in the work function of ITO source irradiated by the excimer laser. Lower trap state density formed on the laser irradiated source electrode. Holes could be easily injected into the channel at small lateral electric field resulting in smaller threshold voltage and SS.

The authors have fabricated the pentacene thin films on polymethylmethacrylate (PMMA) and on silicon dioxide dielectric surfaces featuring similar surface energy and surface roughness. On both surfaces the pentacene films displayed high crystal quality from x-ray diffraction scans, although the film on PMMA had significantly smaller grain size. The pentacene transistors with PMMA exhibited excellent electrical characteristics, including high mobility of above 1.1cm2∕Vs, on/off ratio above 106, and sharp subthreshold slope below 1V∕decade. The analysis of molecular microstructure of the pentacene films provided a reasonable explanation for the high performance using resonance micro-Raman spectroscopy.

The surface free energy of a dielectric has a strong influence on the performance of pentacene thin-film transistors. Research shows that by matching surface free energy in the interface of the dielectric and the orthorhombic thin-film phase of pentacene film, the field-effect mobility of transistors is enhanced reaching above 2.0cm2∕Vs. The authors suggested that a more complete first monolayer of pentacene was formed upon the gate dielectric surface with almost identical surface free energy, benefiting carrier transportation. The research also discusses the mechanism of surface free energy effects on the crystalline size and structural disorder in pentacene film.

The following research describes the process of fabrication of pentacene films with submicron thickness, deposited by thermal evaporation in high vacuum. The films were fabricated with the aforementioned conditions and their characteristics were analyzed using x-ray diffraction, scanning electron microscopy, polarized Raman spectroscopy, and photoluminescence. Organic thin-film transistors (OTFTs) were fabricated on an indium tin oxide coated glass substrate, using an active layer of ordered pentacene molecules, which were grown at room temperature. Pentacene film was aligned using the ion-beam aligned method, which is typically employed to align liquid crystals. Electrical measurements taken on a thin-film transistor indicated an increase in the saturation current by a factor of 15. Pentacene-based OTFTs with argon ion-beam-processed gate dielectric layers of silicon dioxide, in which the direction of the ion beam was perpendicular to the current flow, exhibited a mobility that was up to an order of magn...

We have investigated the electric field-induced microscopic structural changes in polycrystalline pentacene-based organic transistors by using in situ micro-Raman spectroscopy. Extra vibrational modes resulting from molecular coupling effect in pentacene film were studied. The herringbone packing of pentacene molecules in solid film is affected by external field and the process is proven to be partially irreversible. In the meantime, in-phase coupling of the C-H bending mode was found to be highly related to the carrier transport of pentacene film. Obtained results suggest that optimal intermolecular π-orbital overlap of pentacene molecules is still a critical factor impacting the carrier transportation for pentacene film featuring polycrystalline morphology.