Photovoltaic yield from exciton dissociation in organic dye layers

Abstract

Researchers working on organic photovoltaic cells are confronted with the problem of finding the optimal thickness of the organic layers. Obviously, there is a competition between the absorption length of the incident light and the exciton diffusion length, leading to optimal film thicknesses lying somewhere between these two mean path lengths (D. Wöhrle, L. Kreienhoop and D. Schlettwein, PhthalocyaninesandRelatedMacrocyclesinOrganicPhotovoltaicJunctions, ed. C. C. Leznoff and A. B. P. Lever, VCH, New York, 1996). In this paper we investigate the general conditions for optimization of the thickness of the organic layer for a maximum photovoltaic yield. At normal absorption, the optimal layer thickness is typically 1.5 times the exciton diffusion length. As an application we consider the zinc phthalocyanine layer in an organic p–n cell, for which an exciton diffusion length of 30±10 nm is determined from simulations. Using this value, a good correlation between simulations and experiments is found with respect to the thickness-dependent photocurrent yield of the solar cells.