Noboru Ohashi

Potential mapping of pentacene thin-film transistors using purely electric atomic-force-microscope potentiometry

Potential mapping of organic thin-film transistors (TFTs) has been carried out using originally developed atomic-force-microscope potentiometry (AFMP). The technique is suitable for the accurate measurement at metal–semiconductor boundaries of working TFTs. Potential drops near metal–organic boundaries are observed for both source and drain Au top contacts of a pentacene TFT. The approximate width of the steeper potential slope is 400 nm, which is larger than the spatial resolution of AFMP. The potential drop is considered to be due to a damaged area with low carrier mobility caused by the Au evaporation, which is also reproduced by device simulation.

Analysis of barrier height at crystalline domain boundary and in-domain mobility in pentacene polycrystalline films on SiO2

Crystalline domain size and temperature dependences of the carrier mobility of commonly used pentacene polycrystalline films on SiO2 have been studied by four-point-probe field-effect transistor measurements. The mobility is found to be proportional to the crystalline domain size and thermally activated. This behavior is well explained by a polycrystalline model with the diffusion theory, and thereby the barrier height at boundary and the mobility in domain are calculated to be 150meV and 1.0cm2∕Vs, respectively. The in-domain mobility is lower than those expected in single crystals, which suggests that there exist some other limiting factors of carrier transport than the domain boundaries.

Conductivity fluctuation within a crystalline domain and its origin in pentacene thin-film transistors

Surface topography and high-resolution potential images in a thin-film transistor with a polycrystalline pentacene active layer have been measured by atomic-force-microscope potentiometry. A potential fluctuation independent of topographic features was found in large flat molecular terraces. The origin of the potential fluctuation was concluded to be the fluctuation of the top level of the highest-occupied-molecular-orbital band, which results in the variation of local carrier concentration. The full width at half maximum of the band fluctuation was estimated to be 12meV, which might reduce the mean carrier velocity in crystalline domains.

Anisotropy of electrical conductivity in a pentacene crystal grain on SiO2 evaluated by atomic-force-microscope potentiometry and electrostatic simulation

Conductivity anisotropy in a crystal grain of thin-film-phase pentacene has been estimated by a combination of atomic-force-microscope potentiometry (AFMP) and electrostatic simulation. The surface potential distribution and topography of a grain in a working pentacene thin-film transistor are simultaneously measured by AFMP. Then, the nonlinear potential profile due to the thickness variation is simulated by changing the anisotropic ratio of conductivity. The anisotropic ratio (corresponding to the anisotropy of carrier drift mobility) is estimated to be σ x : σ z = 45 : 1 , where x is the horizontal (harmonic mean of those in a - and b -axes) direction and z is the vertical ( c -axis) direction.

Epitaxial growth of atomically flat KBr(111) films via a thin film ionic liquid in a vacuum

The NaCl-type crystal (111) polar face is electrostatically so unstable that its atomically flat (111) surface has been difficult to obtain so far. In this work, based on our preliminary results on flat KBr(111) microcrystals on an α-Al2O3(0001) substrate grown via ionic liquid droplets by vacuum deposition, we have achieved atomically flat, epitaxial KBr(111) films grown all over an α-Al2O3(0001) substrate. The key point to this success is the great improvement in the wettability of the ionic liquid on the substrate by introducing an amorphous-like “wetting layer”. Furthermore, in situ growth monitoring by means of an optical microscope with a long-distance focus lens reveals the growth process in the ionic liquid: random nucleation occurs directly on the substrate, but not in the ionic liquid, and subsequently, the nucleated KBr islands grow more rapidly in the in-plane direction rather than in the out-of-plane direction to coalesce with each other, forming a continuous KBr film over the substrate. The lateral overgrowth mechanism is proposed to explain the epitaxial growth of KBr(111) films even on the amorphous-like wetting layer/α-Al2O3(0001) substrate on the basis of a cross-sectional TEM observation.

MgxC60 Fabricated by Using Mg:C60 Co-Evaporation Method for Carrier Doping

Mg:C60 co-evaporation method has been performed to obtain fulleride of MgxC60 thin films for carrier doping. Atomic concentration of the film has been examined by x-ray photoelectron spectroscopy to calibrate the composition ratio, and the results indicated that Mg concentration in the film was controlled well. Then, crystal structure of the Mg:C60 mixing film has been examined by using grazing incidence x-ray diffraction method. The diffraction patterns suggested that these films were microcrystalline MgxC60. Conductivity of the mixing films was increased as composition ratio of Mg increasing. It indicates that Mg atoms acted as carrier dopant.

Elucidation of Formation Mechanism of Bulk Heterojunction Active Layer by Real-Time UV–Visible Absorption and Grazing-Incidence Wide-Angle X-ray Scattering

The drying process of poly[3-hexylthiophene]:[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) was observed by the real-time measurement of grazing-incidence wide-angle X-ray scattering and ultraviolet–visible absorption for various drying rates. A low drying rate led to high PCBM crystallinity and low P3HT crystallinity. On the other hand, a high drying rate led to low PCBM crystallinity and high P3HT crystallinity. These crystallinities directly affected photovoltaic performance. From these findings, we clarified the fundamental process of the formation of bulk heterojunctions and that drying rate control is an effective method for optimizing the morphology and performance.

Mobility Limiting Factors in Practical Polycrystalline Organic Thin Films

Pentacene is one of the most extensively studied semiconducting materials for organic thin-film transistors (OTFTs) and has been still a benchmark material in this research field. Although precise studies on the structure and electronic properties of pentacene single crystals give us exact knowledge about this material, there exists a large gap between the fundamental physics of single crystal and device characteristics brought by practical polycrystalline films. Understanding the bottlenecks of carrier transport in pentacene OTFT is therefore important to maximize the performance of OTFTs utilizing not only this benchmark material but also any semiconducting small molecules. In this chapter, the reality of the carrier transport band in practical polycrystalline organic thin films is explained by making pentacene into a representative case, mainly based on our efforts of 10-years. The major topics included are as follows: grain morphology and crystal structure of pentacene thin films, extrinsic factors that are more-or-less introduced and disturb the carrier transport in OTFTs, intrinsic structure and properties of pentacene polycrystalline thin films, equations to express the overall carrier mobility in polycrystalline films, and influence of surface chemical modification on the crystallographic and electronic structures.