<p>Zinc sulfide, copper sulfide, and their mixed ZnS-CuS nanoparticles are synthesized by the co-precipitation method, analyzed, and used as an interfacial layer which improved the power conversion efficiency of organometallic perovskite solar cells from 12.98 to 15.21% by improved interfacial charge transport and better electrical contact between spiro-OMeTAD and the silver electrode. The better performance compared to the reference device is due to the facilitated hole extraction/transport of holes with better alignment of E_HOMO levels of interfacial layer (metal sulfide), also confirmed by the computational analysis. This strategy demonstrates the potential of mixed metal sulfide interfacial layers for improving charge transport and photovoltaic performance in perovskite solar cells.</p> Graphical Abstract <p></p>