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Enhanced Efficiency of Cd-Free Cu(In,Ga)(Se,S)2 Minimodule Via (Zn,Mg)O Second Buffer Layer and Alkali Metal Post-Treatment

New record efficiencies have been achieved on Cd-free Cu(In,Ga)(Se,S)2 thin-film photovoltaic submodules prepared by a two-step sulfurization after selenization process. Aperture area efficiencies of 19.2% and 19.8% were independently confirmed on a 30 cm × 30 cm submodule (841 cm2) and on a 7 cm × 5 cm minimodule (24.2 cm2), respectively. These achievements were brought about by transferring several key techniques, especially atomic layer deposited (Zn,Mg)O second buffer layer and K treatment of the absorber surface, from the fundamental study of small-area cell development. The former technique was applied to the submodule and both techniques were implemented into the minimodule. The (Zn,Mg)O second buffer layer increases transmittance of the window layer and improves junction quality resulting in the reduced interface recombination. The K treatment, which was developed by reference to the postdeposition treatment widely used in the co-evaporation process, significantly enhances open-circuit voltage and fill factor. Several material and device characterizations performed to illuminate the effects of the K treatment showed that increased free carrier concentration and reduced carrier recombination throughout the whole absorber film contributed to the improved performance. Contrary to the conventional postdeposition treatment in the co-evaporation process, significant depletion of Cu at the absorber surface was not observed, which can be attributed to S-rich circumstances of our absorber surface. The achievement of nearly 20% efficiency on the minimodule having identical structure to the production modules ensures further performance improvements in industrial Cu(In,Ga)(Se,S)2 modules in the near future.