Kesterite Cu₂ZnSn(S,Se)₄ Absorbers Converted from Metastable, Wurtzite-Derived Cu₂ZnSnS₄ Nanoparticles

Wurtzite-derived copper–zinc–tin sulfide nanoparticle films were observed to undergo a phase transformation to a kesterite phase when exposed to Se vapor at 500 °C. The resulting dense and selenized Cu 2 ZnSn(S,Se) 4 (CZTSSe) films were found to have the same bilayer kesterite structure as absorber layers derived directly from kesterite Cu 2 ZnSnS 4 (CZTS) nanoparticles ( Guo, Q.; Ford, G. M.; Yang, W.-C.; Walker, B. C.; Stach, E. A.; Hillhouse, H. W.; Agrawal, R. J. Am. Chem. Soc. 2010, 132, 17384−17386). The top layer was fully sintered into micrometer size grains, while the bottom unsintered layer consisted of small, nanometer size kesterite grains. When compared to films formed from kesterite CZTS nanoparticles, solar cells fabricated from the wurtzite-derived CZTS nanoparticles were found to have lower power conversion efficiencies (PCE). Surprisingly, for those CZTSSe films that were formed from wurtzite-derived nanoparticles, it was found that extensive selenization leads to the disappearance of the bottom unsintered layer and the formation of a thin film composed of only micrometer-sized grains. These results have significant importance for the improvement of the performance of CZTSSe solar materials. Solar cells fabricated from kesterite nanoparticles have delivered a PCE of 9%—despite the presence of an unsintered layer. These results indicate that the use of wurtzite-derived CZTS nanoparticles has the potential to remove the unsintered layer in kesterite CZTSSe solar cells ( Miskin, C. K.; Yang, W.-C.; Hages, C. J.; Carter, N. J.; Joglekar, C. S.; Stach, E. A.; Agrawal, R. Prog. Photovoltaics: Res. Appl. 2014, DOI: 10.1002/pip.2472).