Losses in CuInSe2-based thin film monolithic tandem solar cells

Two critical aspects for the development of tandem solar cells using CuInSe₂-alloy materials are addressed. First a structure for monolithic tandem solar cells using wide and narrow bandgap Cu(InGa)Se₂-alloys in the top and bottom cells is demonstrated. This structure uses an ITO layer as both back contact to the wide bandgap Cu(InGa)Se₂ and interconnect to the bottom cell. Three different options for the emitter in the bottom cell are compared. Second, progress on the wide bandgap cell using the Cu(InGa)(SeS)₂ system is presented, and different loss mechanisms in Cu(InGa)Se₂ and CuInS₂ based devices are identified. Efficiency with Cu(InGa)Se₂ is reduced by lower optical absorption and poor current collection while with CuInS₂, it is reduced by interface recombination.