Co-thermally evaporated Ag-Au thin films as composition-tuned back electrodes for CsEuCl₃-based lead-free perovskite solar cells

Lead-free perovskite solar cells are promising candidates for sustainable photovoltaic technologies, but their performance and practical use are still limited by the choice of stable, efficient, and affordable back electrodes. Silver (Ag) is highly conductive and low-cost, but it can suffer from corrosion and ion diffusion, while gold (Au) provides better stability but is expensive. In this study, Ag–Au thin-film back electrodes with different compositions, Ag95Au5, Ag50Au50, and Ag5Au95, were prepared by co-thermal evaporation and integrated into glass/FTO/ZnO/CsEuCl₃/NiO/Ag–Au lead-free perovskite solar cells. Structural and morphological analyses confirmed that the electrode composition strongly influenced film crystallinity, surface uniformity, and interfacial quality. Among the tested electrodes, the Ag-rich Ag95Au5 contact showed the best photovoltaic performance, achieving a power conversion efficiency of 9.78%, with Jsc = 20.9 mA·cm⁻², Voc = 0.88 V, FF = 0.532, and a reduced series resistance of 15.7 Ω·cm². This improvement is attributed to the favorable balance between the high electrical conductivity of Ag and the interfacial stability provided by a small amount of Au. These findings demonstrate that low-Au Ag–Au back electrodes can reduce material cost while improving charge collection, providing a practical and scalable route for developing efficient lead-free perovskite solar cells.