Elucidating Operating Modes of Bulk-Heterojunction Solar Cells from Impedance Spectroscopy Analysis.

We discuss the progress and challenges in the application of impedance spectroscopy analysis to determine key processes and parameters in organic bulk-heterojunction solar cells. When carrier transport or outer interface extraction do not severely influence the solar cell performance, a simple method to quantify the open-circuit voltage loss caused by the kinetics of charge carrier recombination is provided, based on the determination of chemical capacitance and recombination resistance. This easily allows distinguishing between energetic and kinetic effects on photovoltage, and establishes a benchmark for the performance comparison of a set of different cells. A brief discussion of impedance analysis in the much less studied case of collection-limited solar cells is introduced.