Development of ZnO-InP heterojunction solar cells for thin film photovoltaics

As a photovoltaic material, InP is nearly ideal for a single junction solar cell in terms of its band gap and absorption coefficient. However, its widespread use is limited due to the expense of substrates and depositing monocrystalline material over large areas. As a route towards reducing the cost, there is interest in examining polycrystalline III-Vs, where film thicknesses ~1 μm could be employed, assuming a reflective back surface. This work examines one aspect towards delivering high efficiency polycrystalline InP-based solar cells by focusing on the use of low-cost, earth-abundant ZnO as the emitter layer. Due to surface Fermi-level pinning in InP, ZnO is an ideal choice for this purpose. We have studied the effects of using aluminum-doped zinc oxide, as well as thin i-ZnO interlayers, deposited onto p-type InP substrates. So far, a maximum power conversion efficiency of 7.3% has been realized, which is a record for this type of heterojunction cell structure. We further discuss the impact of the i-ZnO interlayer on enhancing short wavelength response in the cell.