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

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:PC 61 BM-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 PC 61 BM 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. This study demonstrated an experimental method for monitoring the space charge and apparent capacitance of phototransistors using the carrier lifetime and gain, simultaneously obtained high gain and fast response rates in the optimized OPTs.