Limit of Voc in polymeric bulk heterojunction solar cells predicted by a double-junction model

In this paper, we will describe the limit of the open-circuit voltage (Voc) in polymer bulk heterojunction (BHJ) solar cells using a double-junction model. In addition to the often-discussed donor/acceptor junction, we also consider the role of a Schottky junction formed between the cathode and the donor material. Such Schottky junction exists in many BHJ solar cells as a consequence of the undesirable vertical phase separation. The double-junction model was investigated using the impedance spectroscopy and transient photovoltage measurements. By studying different BHJ systems and comparing the characteristics of devices with and without an interlayer between the cathode and the BHJ layer, we found that the Voc is determined by the junction with the smaller negative capacitance onset and that the cathode modification can tune the Voc maximum from the Schottky junction limited regime to the donor/acceptor junction limited regime.