Band-gap engineering of Cu2ZnSn1-xGexS4 single crystals and influence of the surface properties

Thin film solar cells based on Cu2ZnSn(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 Cu2ZnSn1-xGeS4 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 GeO2 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. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.