Sputtering Deposited and Energy Band Matched ZnSnN₂ Buffer Layers for Highly Efficient Cd‐Free Cu₂ZnSnS₄ Solar Cells

Abstract Kesterite Cu₂ZnSnS₄ (CZTS) solar cells with CdS buffer layers have the problem of toxicity and cliff‐like energy band diagram disfavoring higher photoelectric conversion efficiency (PCE (%)). The preparation method for ZnSnO buffer layers which enable Cd‐free CZTS solar cells to reach the certified PCE (%) of 11.4 is limited to atomic layer deposition and sputtering deposition severely reduces PCE (%). Here, it is showed that sputtering deposited ZnSnN₂ is an efficient buffer layer for Cd‐free CZTS solar cell, and without antireflection coating or additional passivation layers its champion PCE (%) has reached 10.00 which is comparable to the certified value. No buried junction and current blocking behavior are observed in the CZTS\ZnSnN₂ junctions. Elemental inter‐diffusion is observed at the interface between CZTS and ZnSnN₂. Most importantly, the energy band of ZnSnN₂ is well‐matched with that of CZTS. The former straddles the latter. The conduction band offset is spike‐like with the conduction band of ZnSnN₂ higher than that of CZTS, which effectively suppresses interface recombination and results in PCE (%) comparable to the certified value.