Metal cluster enhanced organic solar cells

An enhancement of the photovoltaic conversion efficiency of an organic solar cell by incorporation of small metal clusters has been reported recently [1]. The enhancement is explained in terms of resonant light absorption in the metal cluster which is accompanied by a strengthened electric field in the vicinity of the particle. It is therefore assumed to be based on an enhanced absorption of the organic dye film. In contrast we will show here that an excited plasmon in a metal cluster is also capable to emit an electron directly in a preferential direction if the particles are placed inside an oriented electrical field like the one existing in the depletion layer of a Schottky junction. Thereby a primary photocurrent is observed in a spectral region without any direct absorption in the organic film. We will present results obtained at a Schottky junction formed at the interface of ITO and zinc phthalocyanine. In order to study the influence of the metal particles we evaporated a thin silver film on top of the ITO substrate and tempered the system in a vacuum, thereby forming small separated silver clusters. We investigated the influence of the silver clusters on the optical extinction spectra and on the short circuit photocurrent spectra of such constructed organic solar cells. The experimental data will be discussed using a qualitative energy diagram.