Development of high band-gap thin films related to CZTSSe, for use as top absorber material in thin film tandem solar cells.

Today, record single-junction solar cell efficiencies are approaching their theoretical limit of 30 % under 1-sun illumination. One method to increase efficiency even further is the development of tandem cells. A tandem solar cell consists of two cells: a high band-gap cell (>1.5 eV) harvests the high-energy photons and a lower band-gap cell (<1.5 eV) harvests the low-energy photons. This approach leads to theoretical efficiencies of 44 % under 1-sun illumination. However, tandem solar cell development has been mainly focused on costly materials; and is not yet feasible at low-cost as there is no stable and abundant high band-gap top cell alternative. ThINThIN aims to study high band-gap thin films related to CZTSSe, for use as top absorber material in thin film tandem solar cells. CZTSSe solar cells are stable and have a relatively high record efficiency. Even more, adding Ge or Si to CZTSSe has potential to increase the band-gap to 2.0 eV. Therefore, CZTSSe, and high band-gap CZTGeSSe and CZTSiSSe materials will be developed, studied, and implemented in single-junction solar cells and as top absorber materials in tandem cells. This CZTSiSSe absorber material will be completely novel and has potential to be low-cost, as it contains no rare or expensive elements. Scientific understanding of CZTSSe related materials will be enhanced; and novel high band-gap single-junction solar cell technologies and a low-cost proof-ofprinciple tandem solar cell will be developed.