Photovoltaic heterojunctions of inorganic semiconductors in the defective limit

A large amount of current research in thin film photovoltaics based on inorganic semiconductors aims to deposit and process these active layers using inexpensive and low temperature processing methods resulting in high densities of electronic states in the bandgap associated with structural and point defects. We define semiconductors in which the Fermi level is pinned throughout the bulk as being at the defective limit and explore the consequences for heterojunctions designed to separate photocarriers. Using this novel concept of the defective limit, heterojunction band offsets can be predicted for less-common materials such as the earth-abundant materials also under intense investigation. Because of the Fermi level pinning in the bulk, only Type II heterojunctions will separate charges but the high defect concentrations will also result in small minority carrier diffusion lengths. These limitations require the use of nanostructured device architectures for cells of such materials with appreciable power conversion efficiency.