EFFICIENT ORGANIC PHOTOVOLTAIC CELLS - THE ROLE OF EXCITONIC LIGHT COLLECTION, EXCITON DIFFUSION TO INTERFACES, INTERNAL FIELDS FOR CHARGE SEPARATION, AND HIGH CHARGE-CARRIER MOBILITIES

Organic photovoltaic thin film structures made by vacuum vapor deposit ion have been studied, From the spectral response as a function of the absorption coefficient we conclude that electron-hole generation take s place at the interface between the organic thin film and one or both of the semitransparent cover electrodes. The observed short circuit c urrents, however, are too large to be explained on the basis of a dire ct light-induced charge transfer at the organic thin film/ electrode i nterface, Rather, a contribution of the bulk-absorbed photons is neces sary to account for the observed quantum yield, Transfer of the energy to the interface sites can be explained by diffusional migration of e xcitons. The efficiency of charge separation can be improved by combin ing donor and acceptor type partners in organic double layers. The eff iciency of power conversion, however, not only depends on a suitable c hoice of the absorption spectra and of the ionic energy levels of the materials employed, but also on the internal cell resistance, a fact t hat calls for high charge carrier mobilities, and hence for using mate rials with crystal structures that allow strong re-electron interactio ns, and for deposition in high chemical purity and structural perfecti on.