Study of accumulation channel thickness determined by heterojunction of donor and acceptor organic semiconductors with similar and dissimilar molecular structure

In organic field effect transistors (OFETs), the accumulation channel for charge transport occurs within few layers on the top of gate dielectric. However this is the most poorly known parameter in OFETs, as it is extremely difficult to determine the charge accumulation thickness experimentally in unipolar OFETs. With the main objective to determine the thickness of accumulation channel, we have fabricated heterojunction OFETs using donor and acceptor organic semiconductors with similar (CuPc & F16CuPc) and dissimilar (pentacene & F16CuPc) molecular structure. The transistor characteristics of OFETs based on the organic heterojunction of similar and dissimilar molecular structure changed from unipolar transport (normally off) to normally on or unipolar to ambipolar transport by varying the bottom layer thickness and deposition order of organic molecules in the heterojunction. In normally off transistors, no conducting channel exist at zero gate bias whereas in normally on transistors, a conducting channel exist at heterojunction interface at zero gate bias. In ambipolar OFETs, the ambipolar charge transport occurs when the bottom layer thickness is >= 3 nm and in normally on OFETs, when the bottom layer is >= 10 nm the normally on transistor becomes normally off due to the shifting of the field effect channel farther from the heterojunction channel. Therefore, from the transistor characteristics of ambipolar and normally on OFETs based on heterojunction of semiconductors with similar and dissimilar molecular structure, the charge accumulation thickness at organic/gate dielectric interface has been found to be similar to 3 nm and the charge accumulation thickness at heterojunction interface is similar to 10 nm.