X-ray nanoprobe and electron beam investigations of kesterite and chalcogenide thin film solar cells

Abstract

As an alternative for Si-based solar cells Cu2ZnSn(S,Se)4 (CZTSSe) and Cu(In,Ga)Se2 (CIGS) thin film solar cells have recently received great attention as their efficiency has been steadily increased over the past decades. Nevertheless, the efficiency is still below the theoretically predicted upper limit, which is mainly related to inhomogeneities in the absorber composition, structural defects, and the formation of secondary phases. This thesis provides insights into efficiency-limiting properties of highly efficient Ge-doped CZTSe, CZTSSe, and CIGS thin film solar cells by accessing local compositional, structural, microstructural, and functional properties using a combinatorial approach of electron- and advanced synchrotron-based methods at the nanoscale: First, it has been demonstrated that an exceptionally homogeneous element distribution in the CZTSe absorber accompanied by the absence of harmful Cu-, Zn- or Sn-based secondary phases is achieved by introducing small amounts of Ge during the synthesis. Within the absorber, Ge is heterogeneously incorporated into the CZTSe matrix, but also forms nanoscale GeO2 secondary phases. Second, the interplay of performancelimiting nanoscale features in CZTSSe solar cells were revealed. Investigations of the local composition indicates the formation of harmful ZnS(Se) secondary phases whose presence, number, and dimension increases strongly with the reduction of Cu and the increase of Zn content. Furthermore, the local composition of the CZTSSe phase shows strong variations within the absorber, which significantly reduces the efficiency. Third, a CIGS solar cell that underwent a post-deposition treatment with RbF was investigated in a combinatorial approach, which simultaneously monitors the local composition and device functionality. This work demonstrates that Rb-enriched grain boundaries have a negligible influence on the collection efficiency, indicating the passivation of detrimental defects.