Theory on the electrical characteristics of III-type organic semiconductor heterojunctions

Heterojunctions are the fundamental unit to construct functional semiconductor devices. The charge transport properties of type-I and -II heterojunctions with a depletion zone have been well investigated. In contrast, the type-III heterojunction is rarely realized in traditional inorganic semiconductors, so there is a lack of systematic theory to elucidate the electrical properties. Here, owing to the abundant energy levels in organic semiconductors, type-III heterojunction has been realized with two organic molecules, HAT-CN and TAPC. This new heterojunction with accumulation carriers offers the opportunity to theoretically and experimentally study the electrical properties. The current density–voltage (J–V) measurements show that the heterojunction outputs almost symmetry currents at various temperatures under both forward and reverse biases, completely different from those diode properties in type-I and -II heterojunctions. The theoretical model suggests that the interfacial charge-transfer (CT) state, or called as polaron pair, which is spontaneously formed between the heterojunction, plays an important role in generating the symmetry J–V characteristics. Specifically, the J–V characteristics is determined by the CT-assisted recombination at forward bias and by the polaron-assisted tunneling at reverse bias. Therefore, our proposed model introduces a dissociation probability Pdiss(E, T) to describe the CT excitons unique in organic type-III heterojunctions, which is temperature- and electric field-dependent. The modified model can well reproduce the J–V curves at all bias and temperatures. It is believed that these results enrich the understanding of organic type-III heterojunction and extend its implementation in optoelectronic devices.