Horng Long Cheng

Effect of surface free energy in gate dielectric in pentacene thin-film transistors

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.

Influence of molecular structure and microstructure on device performance of polycrystalline pentacene thin-film transistors

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.

Electric field-induced structural changes in pentacene-based organic thin-film transistors studied by in situ micro-Raman spectroscopy

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.

Effects of solvents and vacancies on the electrical hysteresis characteristics in regioregular poly(3-hexylthiophene) organic thin-film transistors

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.

Nanoimprinting-induced efficiency enhancement in organic solar cells

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.

Electron transport properties in fluorinated copper–phthalocyanine films: importance of vibrational reorganization energy and molecular microstructure

Electron transport (ET) properties of a series of fluorinated copper-phthalocyanine (F(16)CuPc) thin films, which were deposited at different substrate temperatures (T(sub)) ranging from 30 to 150 degrees C, have been investigated by quantum mechanical calculations of the reorganization energy (lambda(reorg)), X-ray diffraction (XRD), atomic force microscopy (AFM), and microRaman spectroscopy. Density functional theory calculations were used to predict the vibrational frequencies, normal mode displacement vectors, and electron-vibrational lambda(reorg) for the F(16)CuPc molecule. The electron mobilities (mu(e)) of F(16)CuPc thin films are strongly dependent on the T(sub), and the value of mu(e) increases with increasing T(sub) from 30 to 120 degrees C, at which point it reaches its maximum value. The importance of electron-vibrational coupling and molecular microstructures for ET properties in F(16)CuPc thin films are discussed on the basis of theoretical vibrational lambda(reorg) calculations and experimental observations of resonance Raman spectra. We observed a good correlation between mu(e) and the full-width-at-half-maximum of the vibrational bands, which greatly contributed to lambda(reorg) and/or which reflects the molecular microstructural quality of the active channel. In contrast, the crystal size analysis by XRD and surface grain morphology by AFM did not reveal a clear correlation with the ET behaviours for these different F(16)CuPc thin films. Therefore, we suggest that for organic films with weak intermolecular interactions, such as F(16)CuPc, optimized microscopic molecular-scale parameters are highly important for efficient long-range charge transport in the macroscopic devices.

Spontaneous Formation of an Ideal-Like Field-Effect Channel for Decay-Free Polymeric Thin-Film Transistors by Multiple-Scale Phase Separation

We demonstrate semiconducting polymer-based thin-film transistors (PTFTs) with fast switching performance and an uncommon nondecaying feature. These PTFTs based on widely studied poly(3-hexylthiophene) are developed by incorporating the insulating polymer into the active channel and subjecting the compound to specific, spontaneous multiple-scale phase separation (MSPS). An in-depth study is conducted on the interfacial and phase-separated microstructure of the semiconducting/insulating blending active layer and its effect on the electrical characteristics of PTFTs. The polyblends exhibit a confined crystallization behavior with continuously semiconducting crystalline domains between scattered insulator-rich domains. The insulator-rich domains can block leakage current and strengthen the gate control of the channel. A small amount of the insulating polymer penetrates the bottom of the active channel, resulting in effective interface modification. We show specific MSPS morphology of the present blending films to reduce charge trapping effects, enhance charge accumulation, and create a high-seed switching channel. The findings enable us to develop the required morphological conceptual model of the ideal-like field-effect-modulated polymer-based active channel. The polyblend-based PTFTs with MSPS morphology also have promising sensing functions. This study offers an effective approach for overcoming the major drawbacks (instability and poor switching) of PTFTs, thus allowing such transistors to have potential applications.

Light sensing in photosensitive, flexible n-type organic thin-film transistors

Control of the operating voltage in organic thin-film transistor (OTFT) based photosensors is a very important issue, which can effectively enhance photosensitivity by reducing the contribution of the field-effect current to the output current under darkness. In this study, we show a highly sensitive flexible organic photosensor, which is made by the use of cross-linked poly(4-vinylphenol) as a polymer dielectric layer and N,N′-ditridecyl-3,4,9,10-perylenetetracarboxylic diimide (PTCDI-C13H27) as an n-type active layer on a transparent polyethersulfone (PES) substrate, by tuning both source–drain and source–gate voltages to around the threshold voltage (Vt = 3.0 V). Interestingly, a maximum photocurrent/dark current ratio was obtained when the operating voltage was reduced to around Vt. The time-response characteristics and sensitivity of the PTCDI-C13H27-based photosensor were investigated. Considerable interest has been focused on developing a flexible in-cell remote touch screen that should comprise photosensitive OTFTs and switch OTFTs simultaneously. In this work, both switch-OTFTs and photo-OTFTs can be formed on the flexible PES substrate by use of the same fabrication process. The electrical characteristics of switch-OTFTs under bending states are discussed in terms of photoluminescence and time-resolved photoluminescence measurements, as well as quantum theory calculations.

Reformation of conjugated polymer chains toward maximum effective conjugation lengths by quasi-swelling and recrystallization approach

A combined quasi-swelling and recrystallization (QSRC) approach is developed to alter distorted segments of conjugated polymer into highly ordered crystalline domains with super-long effective conjugation lengths (>100mer). These highly extended conjugated chains, at their maximum, are expected to have important implications for photonic and electronic applications and theoretical studies.

Excimer laser irradiation induced suppression of off-state leakage current in organic transistors

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.