Hydrazine-Processed Ge-Substituted CZTSe Solar Cells

The p-type Cu2ZnSn(SxSe1-x)(4) (with x approximate to 0; CZTSe) thin-film solar cell absorber, made from earth-abundant elements, was substituted with Ge using a hydrazine-based mixed particle-solution approach for the film deposition. The crystallographic unit cell parameters of the absorber layer decrease with gradual incorporation of Ge. A solar cell fabricated from a 40% Ge-substituted absorber showed a 9.1% power conversion efficiency, a higher open-circuit voltage, and a wider band gap compared with the device based on the unsubstituted absorber layer. This result shows the possibility of substituting, using the hydrazine-processing approach, the metal site of CZTSe with Ge for further device optimization. One area for further improvement in the substituted absorber layer devices includes reduction of a ZnSe secondary phase, which was apparent in the higher-Ge-content films.