Characterization of Surface and Heterointerface of Cu₂ZnSn1–xGexSe₄ for Solar Cell Applications

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This study analyzes the electronic structure of Cu 2 ZnSn 1– x Ge x Se 4 (CZTGSe) surface and band alignments of CdS and CZTGSe using the Ge/(Sn + Ge) composition ratios of x = 0–1 based on inversed, ultraviolet, and X‐ray photoemission spectroscopy. The conduction band offsets of the CdS/CZTGSe interface linearly decrease with increasing x , which are so‐called spike conduction band connections, whereas the valence band offsets are observed to be independent of x . Moreover, the hole density near the CZTGSe surface increases with an increase in x ; this corresponds to the increase in the measured built‐in‐potential with x . However, the open circuit voltage ( V OC ) saturates to ≈0.5 V for x over 0.2 in spite of the favorable band alignment and larger built‐in‐potential of x at the interface. External‐quantum‐efficiency spectral analysis shows that the carrier recombination in CZTGSe bulk is suppressed with an increase in x , whereas the recombination near the CZTGSe surface is enhanced. The relationship between these recombination centers, electronic structure, and saturation of V OC is discussed.