Transition-Metal Oxides for Kesterite Solar Cells Developed on Transparent Substrates

Fabrication on transparent soda-lime glass/fluorine-doped tin oxide (FTO) substrates opens the way to advanced applications for kesterite solar cells such as semitransparent, bifacial, and tandem devices, which are key to the future of the PV market. However, the complex behavior of the p-kesterite/n-FTO back-interface potentially limits the power conversion efficiency of such devices. Overcoming this issue requires careful interface engineering. This work empirically explores the use of transition-metal oxides (TMOs) and Mo-based nanolayers to improve the back-interface of Cu₂ZnSnSe₄, Cu₂ZnSnS₄, and Cu₂ZnSn(S,Se)₄ solar cells fabricated on transparent glass/FTO substrates. Although the use of TMOs alone is found to be highly detrimental to the devices inducing complex current-blocking behaviors, the use of Mo:Na nanolayers and their combination with n-type TMOs TiO₂ and V₂O₅ are shown to be a very promising strategy to improve the limited performance of kesterite devices fabricated on transparent substrates. The optoelectronic, morphological, structural, and in-depth compositional characterization performed on the devices suggests that the improvements observed are related to a combination of shunt insulation and recombination reduction. This way, record efficiencies of 6.1, 6.2, and 7.9% are obtained for Cu₂ZnSnSe₄, Cu₂ZnSnS₄, and Cu₂ZnSn(S,Se)₄ devices, respectively, giving proof of the potential of TMOs for the development of kesterite solar cells on transparent substrates.