Band-gap engineering of Cu2ZnSn1-xGe xS4 single crystals and influence of the surface properties

Abstract Thin film solar cells based on Cu 2 ZnSn(S,Se) 4 are very promising, because they contain earth-abundant elements and show high absorptivity. However, the performance of these solar cells needs to be improved in order to reach efficiencies as high as that reported for Cu(In,Ga)Se 2 -based devices. This study investigates the potential of band-gap engineering of Cu 2 ZnSn 1− x Ge x S 4 single crystals grown by chemical vapour transport as a function of the [Ge]/([Sn] + [Ge]) atomic ratio. The fundamental band gap E 0 is found to change from 1.59 to 1.94 eV when the Ge content is increased from x  = 0.1 to x  = 0.5, as determined from spectroscopic ellipsometry measurements. This knowledge opens a route to enhancing the performance of kesterite-based photovoltaic devices by a Ge-graded absorber layer. Furthermore, the formation of GeO 2 on the surface of the as-grown samples was detected by X-ray photoelectron spectroscopy, having an important impact on the effective optical response of the material. This should be also taken into account when designing photovoltaic solar cells.