Charge Carrier Transport and Generation via Trap-Mediated Optical Release in Organic Semiconductor Devices

The impact of intermixed donor-acceptor domains in organic bulk heterojunction (BHJ) solar cells, using low-donor-content devices as model systems, is clarified. At low donor contents, the devices are found to exhibit anomalously high open-circuit voltages independent of the donor-acceptor energetics. These observations can be consistently explained by a theoretical model based on optical release of trapped holes, assuming the donors behave as trap sites in the gap of the acceptor. Our findings provide guidelines for reducing the large open-circuit voltage losses in organic solar cells and avoiding morphology-induced losses in state-of-the-art BHJ solar cells and photodetectors.