Rongbin Ye

Air-stable ambipolar organic thin-film transistors based on an organic homostructure

Ambipolar organic thin-film transistors (TFTs) based on a fluorinated copper phthalocyanine (F16CuPc)∕copper phthalocyanine (CuPc) homostructure layer were fabricated and characterized. The homostructure TFTs showed typical air-stable ambipolar characteristics, with hole and electron mobilities of 1.44×10−3 and 9.97×10−4cm2∕Vs, respectively, which are comparable to unipolar mobilities in these single-layer devices. X-ray diffraction analysis suggests highly ordered F16CuPc and CuPc polycrystalline thin films could be continuously grown via an intermediate-phase layer in the homostructure.

Effect of Thermal Annealing on Morphology of Pentacene Thin Films

In this study we investigate the effect of thermal annealing on pentacene thin films grown at room temperature. The morphology and structure of the films was examined by atomic force microscopy and X-ray diffraction techniques. Only the thin film phase was observed with a vertical periodicity of 15.0 A. The thermal annealing significantly improves the pentacene molecular ordering although the grain size only slightly increased. The largest intensity ratio and the smallest full-width at half maximum of the 001 diffraction lines were gained at the thermal annealing temperature of 60°C. We also found that the RMS roughness is increased due to recrystallization with increasing thermal annealing temperature, which closely follows an Arrhenius behavior as σ∝exp (-EBar/kBT) with the activation energy EBar=0.056 eV. Furthermore, the re-evaporation of the pentacene films from the substrate occurred at the thermal annealing temperature of over 95°C.

High-Performance Air-Stable Ambipolar Organic Thin-Film Transistor Based on F16CuPc and CuPc

We report on the fabrication of organic thin-film transistors based on fluorinated copper phthalocyanine (F16CuPc) and copper phthalocyanine (CuPc) layers. The thin-film transistors showed high-performance air-stable ambipolar characteristics with a field-effect hole mobility of 3.26 ×10-3 cm2/Vs and a field-effect electron mobility of 3.34 ×10-3 cm2/Vs, which are comparable to the unipolar field-effect mobilities of single-layer thin-film transistors. These outstanding ambipolar characteristics, along with the highly stable chemical nature of F16CuPc and CuPc, make this transistor an attractive candidate for organic complementary metal oxide semiconductor (CMOS) circuits.

Dependence of ambipolar transport on first active layer thickness in organic homostructure transistors

We investigated the dependence of ambipolar transport on the first active layer thickness in organic thin-film transistors based on an F16CuPc/CuPc homostructure. Ambipolar transport is observed at the first layer (F16CuPc) film thickness between 4 and 12 nm. By increasing the first layer thickness, n-channel operating characteristics are enhanced, whereas p-channel operating characteristics are suppressed. As a result, a balanced transport behavior is shown at the first layer thickness of 5 nm. When the first film thickness is larger than 12 nm, only the n-type operating characteristics are observed. The electron field-effect mobility is enhanced up to 2.6×10-2 cm2 V-1 s-1 and the threshold voltage is suppressed to -7.4 V. This result gives us a guide for improving the device performance of n-channel OTFTs employing an organic pn junction.

On the Correlation between Morphology and Electronic Properties of Fluorinated Copper Phthalocyanine (F16CuPc) Thin Films

We report on the correlation between morphology and electronic properties of fluorinated copper phthalocyanine (F16CuPc) thin films deposited on SiO2/Si substrates at different substrate temperatures. Highly ordered films with the 200 plane spacing of d200 = 14.3 Å is observed. Increasing the substrate temperature significantly improves the molecular ordering of F16CuPc, and the smallest FWHM was gained at a substrate temperature of 120°C. The mobility is strongly dependent on the substrate temperature. Increasing in grain size at higher substrate temperatures improves the mobility of F16CuPc TFTs, but gaps generated between grains degrade the performance of F16CuPc TFTs at a substrate temperature higher than 110°C. When deposition of F16CuPc is done at a substrate temperature of 100°C, the maximum mobility of 4.25 × 10−3 cm2/Vs. can be obtained.

Growth conditions effects on morphology and transport properties of pentacene thin films

Pentacene thin films were prepared by vacuum evaporation on SiO2/Si (100) and glass substrates with various growth conditions. X-ray diffraction, atomic force microscopy, and electrical conductivity measurements were employed to characterize the various properties of the films. Two distinct crystalline phases were observed with the 001 spacing of 15.0 Å (thin film phase) and 14.0 Å (single crystal phase), respectively. The single crystal phase appeared over a critical thickness of the thin film phase, and the critical thickness was strongly dependent on growth conditions: increasing the substrate temperature decreased the critical thickness. The substrate temperature enhanced the electrical conductivity of the films parallel to the substrate surface, although the substrate temperature induces the single crystal phase. On the other hand, gaps generated between grains degraded conductivity at a too high substrate temperature. The maximum conductivity of 2.0×10−7S/cm was gained at a substrate temperature of 40°C.

High Performance Pentacene Thin Film Transistors with a PVA Gate Dielectric

Pentacene thin film transistors were fabricated and characterized with PVA thin films as a gate dielectric. The maximum process temperature was 70°C, which corresponds to a baking temperature of the spin-coated polymeric dielectric. Glass and flexible PET foils were used as substrates. Theses devices showed high performance electrical characteristics and worked at a low operating voltage of − 5 V. The highest field effect mobility of 2.6 cm2/Vs and the lowest threshold voltage of − 1.7 V were obtained on a flexible substrate.

Improved Organic Thin Film Transistor Performance Utilizing a DH-α6T Submonolayer

In this paper, we have reported on improved titanyl phthalocyanine (TiOPc) thin film transistor (TFT) performance utilizing an α,α′-dihexylsexithiophene (DH-α6T) submonolayer. By the detailed XRD and AFM analysis, highly ordering α-TiOPc thin films could be deposited on SiO2/Si substrates modified by a DH-α6T submonolayer. Organic TFT with a TiOPc/ DH-α6T bilayer as active layers realized high mobility of 2.45 × 10−1 cm2 /V s, which is about 100 times greater than that of TiOPc single layer device.

Formation of an S2- Molecular Ion and Its Luminescence Characteristics in KCl:MnS Crystals

It was found that when a KCl crystal doubly doped with a trace of MnS was irradiated with UV light, the S2- molecular ions were effectively produced through a photochemical reaction. These S2- molecular ions exhibited characteristic vibronic luminescence composed of zero-phonon lines and phonon-side bands below 160 K. Such a vibronic structure related to zero-phonon lines could also be observed for the first time in the luminescence excitation band. From the analysis of the spectral positions of zero-phonon lines in the emission and excitation spectra, the average vibrational frequencies of the S2- molecular ion in the ground state and excited state were determined to be 1.78×1013 s-1 and 1.13×1013 s-1 respectively.

Effects of thermal annealing on structure, morphology and optoelectronic properties of TiOPc ultrathin films

This article reports the effects of thermal annealing on structure, morphology and optoelectronic properties of titanyl phthalocyanine (TiOPc) ultrathin films grown at room temperature. When the thermal annealing temperature (TA) is over 160 °C, a peak at 2θ =7.5° with d010 = 1.194 nm was observed, which suggests that TiOPc molecules were restructured and crystalline α-form films were formed. By the detailed x-ray diffraction analysis, the optimum thermal annealing conditions are as follows: TA is 160 °C, and the thermal annealing time is 4 h. The root-mean-square roughness and grain size are also increased due to recrystallization with increasing TA. Similarly, these absorption spectra of TiOPc ultrathin films reveal the gradual formation of the crystalline α-phase, as evidenced by the development of a red-shifted absorption band, peaking 845 nm, and the corresponding reduction of the 720 nm amorphous phase peak when TA was increased from 120 °C to 180 °C. Furthermore, device performance of TiOPc thin film transistors could be improved and mobility was enhanced by thermal annealing, which originated from amorphous TiOPc films transformed into crystalline α-TiOPc films and increasing of grain size. At TA = 160 °C, the device achieved the highest performance with field-effect mobility of 6.34 x 10 -3 cm 2 V -1 s -1 .