Organic heterostructures in organic field-effect transistors

A heterojunction is an interface between two semiconductor materials of differing energy gap, and inorganic heterojunctions have been studied as the basis of electronic devices for over half a century. Organic heterojunctions have also been successfully applied in organic electronic devices over the past two decades. However, a theoretical understanding comparable to that of inorganic heterojunctions has yet to be developed for organic heterojunctions. Organic heterojunctions have been drawing increasing attention following the discovery of high conductivity in organic heterojunction transistors constructed with active layers of p-type and n-type thin crystalline films. In contrast with the depletion layers that form in inorganic heterojunctions, electron- and hole-accumulation layers have been observed on both sides of organic heterojunction interfaces. Heterojunction films with high conductivity have been used as charge injection buffer layers and as a connecting unit for tandem diodes. Ambipolar transistors and light-emitting transistors have also been realized using organic heterojunction films as active layers. This review highlights the organic heterojunction effect and the application of organic heterostructures in organic field-effect transistors. The category of heterojunction, including organic and inorganic semiconductor heterojunctions, is given on the basis of the work function of the constituent materials, and it is shown that a rich variety of organic heterojunctions are possible based on the formation of molecular pairs.