Solar Cells Based on CuInSe2 and Related Compounds: Material and Device Properties and Processing

Abstract

This paper summarizes recent material and device results obtained at the Institute of Physical Electronics at Stuttgart University (IPE). Properties of the material system Cu(In, Ga)(S, Se)₂ were analysed and wherever possible a correlation between the material properties and the device characteristics is made. Different high‐vacuum techniques of absorber preparation are presented and compared. The formation of different alloys of the family Cu(In, Ga)(S, Se)₂ is possible for the co‐evaporation and at least one of the sequential evaporation techniques. The model for Cu‐rich growth of CuInSe₂ known from the co‐evaporation process can also be used for the Cu‐rich growth in the sequential evaporation processes. The surface composition of slightly (In, Ga)‐rich bulk compositions is always determined to be the defect chalcopyrite Cu(In, Ga)₃Se₅. Solar cells prepared with different processes and therefore different morphologies yielded similar device performance. An exponential decay of the density of states from the valence and conduction bands was obtained. Improved cell performance is achieved using absorber layers with higher carrier concentrations. The carrier concentration can be increased by using Na‐containing substrates or by utilizing a new Cd‐free buffer layer. Device efficiencies in the range of 15% were achieved using the Cd‐free buffer layer.