Space charge and active-layer capacitance of bulk heterojunction-based phototransistors

Abstract

In phototransistors with bulk heterojunctions, long conductive channels with tens of microns length supply the discontinuous phase morphology and unbalanced charge transport, result in an obvious storage of photoinduced charges; however, the traditional theory of transistors only considers the geometric capacitance of gate dielectrics. In this study, we built an experimental model and an equivalent circuit of bulk heterojunction-based phototransistor, and developed an experimental method for the monitoring the space charge and apparent capacitance of phototransistors by the carrier lifetime and gain. In PDPPBTT:PC₆₁BM-based devices with a gain of up to 3.2 K, the apparent capacitance became tens times the geometric capacitance of gate dielectrics. The solvent additive and PC₆₁BM content can effectively control the carrier lifetime, film morphology and the corresponding apparent capacitance. In particular, as the only parameter with a contrary change to gain, the regulation of capacitance plays a crucial role in reconciling the contradiction between gain and response speed and realizing high gain and fast response. These results indicate the important role of space charge and the potential applications of bulk heterojunctions in long carrier channel-based devices.