Junction capacitance and donor-acceptor interface of organic photovoltaics

To obtain quantitative information of the bicontinuous interface between the donors and acceptors (D/A), the junction capacitance of various organic photovoltaic (OPV) structures was measured by direct current biased impedance spectroscopy, without breaking the vacuum after device fabrication. A correlation between the extracted interfacial area and power conversion efficiency (PCE) was established for bilayer and bulk heterojunction OPVs. The latter was analyzed by two opposite models, one of which follows the same trend as the bilayer, such that the PCE increases with the enlarged interface. However, the increase is much less than expected, which may be caused by the loose D/A domains, “wasted” due to the poor electrical connection. The finding provides not only a better understanding of the device structure but also an opportunity for the further research of the structure-performance relationship of OPVs.