Controlling molecular orientations in non-crystalline thin films fabricated from small molecules by varying the substrate temperature during the vapor deposition, influences of molecular orientation in organic light-emitting diodes were investigated. With the randomly oriented host matrix in a light-emitting layer, efficiency roll-off was suppressed by 30% at a current density of 100 mA cm−2, while the external electroluminescence quantum efficiency was greatly enhanced by 24% with the horizontal orientation of a linear-shaped fluorescent dopant.
New method of vacuum deposition is reviewed for the control of phase-separated structure of crystalline donors and acceptors in organic photovoltaic cells. This method utilizes a liquid as a non-sticking co-evaporant of small molecular organic semiconductors and crystallizes their single and blend films. Blend films based on H2Pc and C60 with much improved crystallinity have been produced by this method and confirmed by analysis using UV-Vis, XRD and FESEM. Used in organic photovoltaic cells, a variety of blend films, such as blends of fullerenes (acceptors) and H2Pc, PbPc, AlPcCl, and rubrene (donors), have been produced by this method and have achieved striking enhancement of short-circuit current density. In this manuscript, we also present the principle of this new method, co-evaporant induced crystallization. Since it drastically improve the principle of vacuum deposition of organic semiconductors, this method would not be limited for organic photovoltaic cells but be useful for other organic devices.
Two-dimensional grazing incidence X-ray diffraction (2D-GIXD) is one of the powerful methods to analyze crystal growth and structure of organic thin films. In this report, we show some experimental examples of 2D-GIXD measurements on organic semiconductor thin-films performed at SPring-8. First, polymorphic transformation of pentacene depending on film thickness observed by means of in-situ real-time 2D-GIXD is shown. Secondly, real-time observation of change in structure during thin-film growth of oligothiophenes by means of 2D-GIXD is shown. Finally, a result of crystal structure analysis from 2D-GIXD data of polycrystalline an oligothiophene thin film is reported.
Controlling in-plane orientation and obtaining single-crystalline thin films are among the important factors to improve the carrier transport in organic thin film transistors (TFTs). Since the surface of the substrate is often amorphous in organic TFTs, epitaxial growth technique is not applicable to control the orientation of the thin films. This article focuses on graphoepitaxy as the technique to control the in-plane orientation of organic thin films and improve the carrier transport. While graphoepitaxy has been studied nearly forty years in inorganic materials, graphoepitaxy of organic materials shows novel and peculiar characteristics which are not observed in inorganic systems. Organic graphoepitaxy is not a mere diversion of old technique but a phenomenon which derives new aspects and possibility from organic materials.
Structures and properties of molecular conductors based on axially-ligated phthalocyanine complexes are presented. Electrochemical oxidationof [MIII(Pc)(CN)2]−, where Pc is phthalocyaninato and M = Co and Fe, gives partially oxidized salt crystals inwhich the π-π stacking structure depends on the cationic species. For the Co salts, charge disproportionation leads to thermally activated conduction in the one-dimensional π-π stacking conductors, while in the two-dimensional π-π stacking conductors, localization of charges is suppressed. For the Fe salts, carrier locarization always occurs due to enhanced charge disproportionation by the interaction between π-electrons (charge carriers) and localized d-spins. However, application of external magnetic fields induces remarkable reduction of the electrical resistivity, leading to giant negative magnetoresistance.
Electron spin resonance (ESR) is an exceptionally sensitive tool to investigate unpaired electrons such as charge carriers in organic semiconductors. Here we review the ESR analyses of charge carrier dynamics in high-mobility organic transistors on the basis of motional narrowing effect: the Brownian motion of carriers makes the ESR spectrum narrower. Particularly, the evaluation of the charge transfer rate between trap sites and/or crystalline domains enables us to understand the charge transport mechanism from a microscopic perspective. We also compare the temperature dependence of the charge transfer rate and field-effect mobility in order to discuss what limits the performance of organic transistors.
Electronic structures of π-conjugated organic semiconducting materials are fundamental factors for functionalities that organic electronic devices exhibit. Single crystals, which have less defects or impurities, are favorable benchmark samples to access the intrinsic properties of each material itself. Beside lively studies about the electric characteristics on the organic single crystals, electronic investigations are far limited mainly because of a technical problem of sample charging. In the present article, we introduce sample charging-durable methodologies, which allow us to demonstrate the valence band of organic single crystals, and a couple of works on rubrene and pentacene single crystals as examples.