Higher efficiency tandem solar cells through composite-cell current matching.

We expand in detail on a new approach to current matching in double junction solar cells that increases the theoretical maximum efficiencies attainable for many bandgap pairs. In this approach, either or both cell types are repeated one or more times, which provides for improved current matching and 2-terminal operation for a wide variety of bandgap pairs, opening up the opportunity to utilize materials not previously considered. While a multijunction design in which the bandgap of every cell is fully optimized will have higher efficiency, this approach achieves simplicity and potential cost savings by using only two cell types. Of particular interest are tandem cells with silicon as the base cell, where significant improvements in efficiency can be achieved with composite-cell current matching. This is illustrated for a 2.19 eV/Si(3) device with a theoretical maximum efficiency of 42.9%, well in excess of the 27.7% achievable for a 2.19 eV/Si device. The benefits of utilizing composite-cell stacks in Si-based triple-junction devices are also discussed.