(Invited) Operating Mechanisms of Organic Bulk-Heterojunction Solar Cells

Bulk-heterojunction solar cells contains an interpenetrated blend of donor-acceptor molecules that assists photogeneration and carrier transport. We show for thinner films in which transport or outer interface extracting limitation is not severe that carrier recombination is easily accessible from impedance analysis, and permits evaluating losses in (Fermi level) voltage. As a result reconstruction of the current-voltage characteristics in any required conditions based on a restricted set of measurements is possible. The analysis highlights carrier recombination current as the determining mechanism of organic solar cell performance. In addition the influence of the electronic density-of-states (DOS) caused by the inherent energy disorder on open-circuit voltage and fill factor is highlighted. The connection between DOS and the capacitive response is also addressed.

A discussion of impedance analysis in the much less studied case of collection-limited solar cells is discussed. Recent experimental analyses of transport processes are presented. We show how by incorporating blocking electrodes into the cell structure transport mechanisms can be understood in terms of diffusion-recombination models. Impedance spectra reveal carrier diffusion signature when blocking or extracting cathode contacts are employed. Finally contact effects are also addressed. The energy level alignment at the contacts is of primary importance in establishing the contact extracting ability and carrier selectivity. Several characterization techniques of the contact properties are explored.