Surface analysis of chalcopyrite materials for photovoltaics

Chalcopyrite-structure semiconductors are leading candidates for photovoltaic applications. The nature of the surface of these materials is critical to determining the properties of the heterojunctions that collect current in the devices and can determine whether or not carrier recombination at the heterojunction limits device performance. This paper reviews the author's group work on surface microchemical and microstructural analysis of Cu(In _1-yGa _y)Se ₂ chalcopyrite alloys. This includes various scanning probe and electron microscopies, photoelectron spectroscopy, and other techniques. Results demonstrate mechanisms of growth of the material along with the nanostructure of surfaces. The close-packed {112} type surfaces have the lowest surface energies. When the materials are grown using a multistage deposition process, the grain boundaries include significant void volume. The chemical bath deposition method used to form the junction then fills the grain boundaries with CdS and dopes all surfaces of the grains, including much of the grain boundaries, with Cd. The grain boundaries are also anion deficient in the final material. It is concluded that the final structure is effectively a self-assembled radial-junction solar cell and that this is the mechanism by which grain boundaries are rendered not only harmless but actually helpful to device performance.