Transparent conducting oxide films for thin film silicon photovoltaics

The requirements for applications of transparent conducting oxide (TCO) films in thin film silicon solar cells are reviewed with a focus on sputtered Al doped zinc oxide and fluorine doped tin oxide films. TCO films are employed as a front contact and as part of a highly reflective back contact so that the silicon absorber layer is embedded by TCO films. The optoelectronic properties of TCO layers and their influence on the solar cell performance are discussed. In addition, recent results on the stability of such films in solar cells and solar modules are presented. While zinc and tin oxides have similar optical and electrical properties, the chemistry is quite different. SnO2 is highly stable against environmental substances as well as acids and bases but rather unstable against hydrogen plasma. ZnO is little affected by H plasma but quite unstable in acids. This latter property bears the advantage that an easy post-deposition texturing is possible by wet chemical etching. For solar cells, damp heat tests for more than 1000 h revealed the high stability of even non-encapsulated solar modules consisting of a ZnO:Al front contact, microcrystalline silicon cell and ZnO/Ag back reflector.