LIMITING EFFICIENCY OF MULTIPLE BAND SOLAR CELLS: AN OVERVIEW

摘要

Previous work has shown that introducing additional bands as impurity bands into a solar cell offers, in principle, scope for higher performance. In this paper we compare the limiting efficiency obtainable for the 3 and 4 band multiple band solar cell with that obtainable from the unconstrained and two terminal current constrained tandem stacks. It is found that the many interleaved circuit elements in the multiple band solar cell result in a limiting efficiency which is slightly lower than that of the tandem stacks. However by preventing some of the corresponding transitions from occurring, it is possible to remove some of the redundancy in the equivalent circuit diagram and obtain different efficiencies. Limiting efficiencies are calculated for the three band and four band solar cell where some transitions are forbidden such as by parity rules. By extending this idea, it is shown that it is possible in principle, for a multiple band solar cell to achieve the same efficiency as the two terminal tandem solar cell involving N-1 energy transitions (where N is the number of bands). Prom this result, it can be concluded that as the number of bands in a multiple band solar cell becomes very large, the limiting efficiency will approach the same efficiency as an infinite two terminal tandem solar cell which is 86.8%.