CdTe cells and modules: Prospects for higher efficiency

Thin-film cadmium telluride is an attractive solar-cell material for several reasons. Its band gap is an excellent match to the solar spectrum, its absorption coefficient is high, it can be deposited by a variety of straightforward methods, and most importantly, it has been proven to work in both the laboratory and the commercial world. On the other hand, its photovoltaic performance has been limited by compensating electronic levels that result in low carrier density and excessive recombination at the internal grain boundaries. Of practical concern, the record CdTe efficiency has been unchanged for ten years. Two types of efficiency difference are explored and broken into individual components. One is the difference between laboratory CdTe efficiency and that of crystalline GaAs of similar band gap (about 10% difference in absolute efficiency), and the other is the difference between industrial and laboratory CdTe (about 5%). In each case, the specific losses can be reasonably accurately quantified. Most of the laboratory limitation is due to the low lifetime and carrier density of thin-film CdTe, which substantially reduce both voltage and fill-factor compared to a crystalline reference. The additional industrial reductions are due to a variety of factors that primarily affect current (CdS and TCO optical losses; interconnect-area) and fill-factor (lateral series resistance). There are specific strategies for reducing each of the individual performance losses.