Materials perspectives for next-generation low-cost tandem solar cells

Recent progress in commercial single-junction photovoltaic (PV) technologies has brought device efficiency closer to their practical limits. Module prices are dropping rapidly and solar energy has already reached grid parity in many areas. Solar module costs now constitute a small fraction of the total systems costs and to continue system cost reduction, increasing efficiency becomes even more critical. Multi-junction solar cells are the most logical path to increase PV module performance beyond 25%, but the varieties available today are still are out-performed in efficiency or cost by the main stream single-junction technologies: crystalline Si, CdTe and CIGS. Finding a solution for this problem is attracting a growing research interest. We review the latest developments in the field with particular focus on thin-film high band gap candidates such as perovskites on commercial bottom cells including silicon, CdTe and CIGS. We also review the current state of all-chalcopyrite tandems, and promising concepts using CdTe-alloys as a top cell on silicon.