Beyond 11% efficient Cu2ZnSn(Se,S)4 thin film solar cells by cadmium alloying

•Cd-alloying CZTSSe (sulfur 1%) film was prepared by selenizing CZTS precursor/CdS.•Cd alloying affected microstructure, device performance and defects of absorber.•A highest efficiency of 11.2% for CZTSSe solar cells was achieved.

Cd-alloying CZTSSe (sulfur about 1%) film was prepared by selenizing CZTS precursor with CdS on the top. The XRD and Raman spectra indicated that Cd alloyed into CZTSSe lattice and the calculated Cd/(Cd+Zn) ratio was 0.13. Some small grains with increased Sn/Cu ratio existed near back contact. Grain growth enhanced after Cd alloying, resulting in a more homogenous Sn/Cu ratio along thickness direction. SIMS profile of solar cell confirmed Cd incorporation in bulk while also a higher Cd content near absorber surface. Corresponding to microstructure, electrical properties were also modified by Cd alloying. In the absorber bulk, the trap energy levels and density of Cd-alloying sample were 38 meV and 1.47 ×1016 cm−3, while 118 meV and 6.98 ×1015 cm−3 for reference sample. The dominant recombination was in bulk, instead of at interface as in reference sample. Back contact was improved owing to a smaller series resistance and a smaller rise of Rs at low temperature. J-V curve observed an improvement of Voc and Jsc. The EQE curve indicated sharper absorption edges and band gap reducing from 1.00eV to 0.95 eV. As a consequence, a highest efficiency of 11.2% for CZTSSe solar cells was achieved.