On the Germanium Incorporation in Cu2ZnSnSe4 Kesterite Solar Cells Boosting Their Efficiency

The presence of Ge during the synthesis of thin film kesterite Cu2ZnSnSe4 (CZTSe) solar cell absorbers boosts their power conversion efficiency, especially due to an improved open circuit voltage. The mechanism underlying this beneficial effect of Ge is still under debate. We gained deep insights into the role of Ge by applying advanced synchrotron nanoprobe-based X-ray fluorescence spectroscopy and X-ray absorption near-edge structure spectroscopy to cross-sectional lamellas taken from high efficiency devices. We observe that Ge remains in the CZTSe absorber layer after the synthesis process with a specific heterogeneous distribution. Different grains contain different Ge concentrations. Moreover, Ge depletion exists at random grain boundaries but not at symmetric Σ3-boundaries, leading to different band alignments. The incorporated Ge occupies Sn lattice sites in the CZTSe crystal structure; however, the concentration is only 0.1 to 0.5 at %. Also Ge aggregates in nanoscale inclusions, which we could identify to be GeO2 that likely lessen the beneficial effect of Ge on the photovoltaic performance.