The Role of Excitonic Light Collection, Exciton Diffusion to Interfaces, Internal Fields for Charge Separation, and High Charge Carrier Mobilities

Abstract

Organic photovoltaic thin film structures made by vacuum vapor deposition have been studied. From the spectral response as a function of the absorption coefficient we conclude that electron — hole generation takes place at the interface between the organic thin film and one or both of the semitransparent cover electrodes. The observed short circuit currents, however, are too large to be explained on the basis of a direct light-induced charge transfer at the organic thin film/electrode interface. Rather, a contribution of the bulk-absorbed photons is necessary to account for the observed quantum yield. Transfer of the energy to the interface sites can be explained by diffusional migration of excitons. The efficiency of charge separation can be improved by combining donor and acceptor type partners in organic double layers. The efficiency of power conversion, however, not only depends on a suitable choice of the absorption spectra and of the ionic energy levels of the materials employed, but also on the internal cell resistance, a fact that calls for high charge carrier mobilities, and hence for using materials with crystal structures that allow strong π-electron interactions, and for deposition in high chemical purity and structural perfection.

Key Words:

• Photovoltaic
• electron donor
• electron acceptor
• electron-hole generation
• p-type
• n-type
• exciton diffusion
• mobilities
• charge separation