Lead-Free Perovskite Tandem Solar Cells with Wide Bandgap Tin Perovskite and CIGS

Wide-bandgap (WBG) tin perovskite solar cells offer a promising solution for the development of multijunction and nontoxic photovoltaic devices. However, rapid crystallization of the perovskite film always results in numerous defects within it, creating nonradiative recombination sites that impair both the efficiency and the stability of the devices. In this work, a ferrocene-based additive, 1,1-bis(diphenylphosphino)ferrocene-palladium(II) dichloride dichloromethane complex (Pd(dppf)Cl2), was introduced to enable the fabrication of high-quality perovskite films and high-efficiency perovskite devices. The functional groups in Pd(dppf)Cl2 (e.g., benzene rings, Cl, etc.) interact with the perovskite to enhance the perovskite film quality. Additionally, the reducibility of ferrocene offers an effective approach to improve film properties, as ferrocene can efficiently reduce Sn4+ to Sn2+ in both the solution and the film. Finally, the single junction device with a Pd(dppf)Cl2 additive achieved a power conversion efficiency (PCE) of 11.52% and demonstrated improved stability. Furthermore, a fully lead-free tandem device based on WBG Sn PSCs was fabricated, achieving a remarkable PCE of 14.51%.