High-efficiency Cu2ZnSn(S,Se)4 solar cells are reported by applying In2S3/CdS double emitters. This new structure offers a high doping concentration within the Cu2ZnSn(S,Se)4 solar cells, resulting in a substantial enhancement in open-circuit voltage. The 12.4% device is obtained with a record open-circuit voltage deficit of 593 mV.

The heterojunctions formed between Cu2ZnSn(SxSe1−x)4 (CZTSSe) and three Cd-free n-type buffers, ZnS, ZnO, and In2S3, were studied using femtosecond ultraviolet photoemission and photovoltage spectroscopy. The electronic properties including the Fermi level location at the interface, band bending in the CZTSSe substrate, and valence and conduction band offsets were determined and correlated with device properties. We also describe a method for determining the band bending in the buffer layer and demonstrate this for the In2S3/CZTSSe system. The chemical bath deposited In2S3 buffer is found to have near optimal conduction band offset (0.15 eV), enabling the demonstration of Cd-free In2S3/CZTSSe solar cells with 7.6% power conversion efficiency.

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Photoluminescence (PL) lifetime of 36 ns on Cu 2 ZnSnS 4 absorber was achieved by optimizing absorber formation. The high quality absorber enabled us to achieve 9.2% efficiency Cu 2 ZnSnS 4 submodule. Comparison between the PL lifetime and atomic composition showed lower Cu/Sn ratio resulted in longer lifetime. The lower Cu/Sn ratio made bandgap higher and voltage deficit lower. The PL intensity mapping accelerated the quality clarification of the various absorbers and contributed to the achievement of high efficiency Cu 2 ZnSnS 4 submodules.

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Previously, open circuit voltage of 960mV was reported on Se-free Cu(In,Ga)S 2 solar cell with CdS buffer layer. In this paper, we report our latest progress toward 1000mV on Se-free Cu(In,Ga)S 2 solar cell with Cd-free buffer layer. Highest open circuit voltage of 973mV was demonstrated by rapid thermal annealing and Zn 1-x Mg x O buffer layer application. Index Terms-Cu(In,Ga)S 2 , High open circuit voltage, Rapid thermal annealing, Zn 1-x Mg x O buffer layer.

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