(NH4)2S-induced improvement of interfacial wettability for high-quality heterojunctions to boost the chloride-assembled CZTSSe solar cells

Concernin the crucial interfacial issues in multi-layered kesterite Cu2ZnSn(S,Se)4 (CZTSSe) solar cells, (NH4)2S treatment has been proven to be effective in eliminating surface secondary phases. While for the CZTSSe absorbers without impurity phases, what can the low-temperature (NH4)2S treatment do to the absorbers, thus to the device performance? Herein, the chloride-fabricated CZTSSe absorbers are surface treated with the (NH4)2S solution at room temperature, and its influence on the device performance is investigated in detail. Surprisingly, such treatment can make the absorbers' surface become nearly super-hydrophilicity, greatly decreasing the surface wetting angle from 63.1° ± 3.4° to 7.3° ± 0.6° after 50 min-treating, and thus lead to marked differences in the interfacial properties of the CdS/CZTSSe heterojunctions deposited in a chemical bath. Consequently, for the best-performing CZTSSe cells, combining the passivated interfacial defects, increased carrier concentration, reduced carrier recombination, and prolonged minority lifetime, the efficiency is improved from 6.54% to 9.88%, together with the 37 mV and 7.9% increase in VOC and FF, respectively. This study confirms the newfound results that the (NH4)2S treatment can effectively adjust the wettability of the absorbers to form high-quality heterojunctions to boost the device efficiency, which would be valuable for an in-depth understanding of the intrinsic mechanisms of interfacial processing.