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

Abstract

Wurtzite-derived copper–zinc–tin\nsulfide nanoparticle\nfilms were observed to undergo a phase transformation to a kesterite\nphase when exposed to Se vapor at 500 °C. The resulting dense\nand selenized Cu₂ZnSn­(S,Se)₄ (CZTSSe) films\nwere found to have the same bilayer kesterite structure as absorber\nlayers derived directly from kesterite Cu₂ZnSnS₄ (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\ntop layer was fully sintered into micrometer size grains, while the\nbottom unsintered layer consisted of small, nanometer size kesterite\ngrains. When compared to films formed from kesterite CZTS nanoparticles,\nsolar cells fabricated from the wurtzite-derived CZTS nanoparticles\nwere found to have lower power conversion efficiencies (PCE). Surprisingly,\nfor those CZTSSe films that were formed from wurtzite-derived nanoparticles,\nit was found that extensive selenization leads to the disappearance\nof the bottom unsintered layer and the formation of a thin film composed\nof only micrometer-sized grains. These results have significant importance\nfor the improvement of the performance of CZTSSe solar materials.\nSolar cells fabricated from kesterite nanoparticles have delivered\na PCE of 9%despite the presence of an unsintered layer. These\nresults indicate that the use of wurtzite-derived CZTS nanoparticles\nhas the potential to remove the unsintered layer in kesterite CZTSSe\nsolar 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).