Record 1.1 V Open‐Circuit Voltage for Cu₂ZnGeS₄‐Based Thin‐Film Solar Cells Using Atomic Layer Deposition Zn1‐xSnxOy Buffer Layers

The Cu 2 ZnGe X Sn 1‐ X S 4 (CZGTS) thin‐film solar cells have a limited open‐circuit voltage ( V OC ) due to bulk and interface recombination. Since the standard CdS buffer layer gives a significant cliff‐like conduction band offset to CZGTS, alternative buffer layers are needed to reduce the interface recombination. This work compares the performance of wide bandgap Cu 2 ZnGeS 4 (CZGS) solar cells fabricated with nontoxic Zn x Sn 1– x O y (ZTO) buffer layers grown by atomic layer deposition under different conditions. The V OC of the CZGS solar cell improved significantly to over 1 V by substituting CdS with ZTO. However, V OC is relatively insensitive to ZTO bandgap variations. The short‐circuit current is generally low but is improved with KCN etching of the CZGS absorber before deposition of the ZTO buffer layer. A possible explanation for the device behavior is the presence of an oxide interlayer for nonetched devices.