High-Efficiency Devices With Pure Solution-Processed Cu$_{\bf 2}$ ZnSn(S,Se)$_{\bf 4}$ Absorbers

Thin-film kesterite-type Cu
₂
ZnSn(S,Se)
₄
(CZTSSe) materials comprise readily available and environmentally benign elements. After reaching efficiencies in the 10% range in recent years, they have become some of the most actively studied new contenders for future solar energy production. The quest for efficiencies competitive to CdTe and CIGS has started to address multiple-challenging aspects of CZTSSe device optimization. One of the most evident difficulties is obtaining highly homogeneous material with equally uniform electronic properties-a prerequisite for advanced interface and full device optimization. While hybrid slurry ink deposition approaches have been setting the benchmark for CZTSSe performance, they often suffer from microscale deposition nonuniformities. Pure solution processes offer the potential for superior homogeneity at molecular level during synthesis. This could be advantageous to obtaining higher quality of multinary semiconductors with better uniformity at all levels. Here, we report a pure solution approach for CZTSSe based on zinc salts soluble in selenium-containing hydrazine systems, thus replacing the solid zinc hydrazinate particles used in our previous record-setting works. By this approach, we demonstrate the highest to date efficiency for a pure solution-processed CZTSSe, reaching 10.6%. We observe correlation between PL intensity and device characteristics. Macrospcopic nonuniformities were identified by this technique and addressing these is expected to yield further efficiency improvement.