Molecular orientation and interface compatibility for high performance organic thin film transistor based on vanadyl phthalocyanine

Organic thin film field-effect transistors (OTFTs) with mobility up to 1.0 cm2 V(-1) s(-1) and on/off ratio of 10(6)-10(8) as well as good environmental stability were demonstrated by using vanadyl phthalocyanine (VOPc), a pyramid-like compound with an ultra closely pi-stacked structure. The high performance, remarkable stability, low price, easy availability and nontoxicity of VOPc enabled it to be a promising candidate for OTFTs. Furthermore, we found that the mobility of the devices on OTS-modified Si/SiO2 substrates was 2 orders of magnitude higher than that of devices on Si/SiO2 substrates. Significantly, the relationship between field effect property and insulator surface property was explained from two new aspects of distribution of molecular orientation and interface compatibility, which might provide not only a useful model to explain why the surface modification with OTS could largely improve the field-effect performance but also a guide for rational optimization of device structure for higher performance. In addition, the field effect property of VOPc devices under vacuum, i.e., the oxygen doping effect on the VOPc devices, was measured. We found that the hole mobility decreased by several orders of magnitude with decreasing pressure. At a pressure below 10(-2) Pa, the device on OTS-modified substrates exhibited ambipolar conduction. These results indicated that the oxygen doping exerted essential effect on the field-effect property of VOPc, which was clearly distinct from that observed for pentacene-based OFETs.