Efficient and Stable Tin Perovskite Solar Cells Enabled by Graded Heterostructure of Light-Absorbing Layer

Lead-free tin perovskite solar cells (TPSCs) have attracted widespread attention in recent years due to their low toxicity, suitable bandgap, and high carrier mobility. However, the photovoltage and efficiency of TPSCs are still much lower than those of the lead counterparts because of the high trap density and unfavorable band structure in tin perovskite films. To overcome these issues, efficient and stable TPSCs with a graded heterostructure of light-absorbing layer are reported, in which the narrow-bandgap tin perovskite dominates at the bulk, whereas the wide-bandgap tin perovskite is distributed with a gradient from bulk to surface. This heterostructure can selectively extract the photogenerated charge carriers at the perovskite/electron transport layer interface, reduce the density of trap states, and impede the oxidation process of Sn(2+)to Sn(4+)in air. As a consequence, this graded heterostructure of tin perovskite layer contributes to an increase of 120 mV in the open-circuit voltage and a maximum power conversion efficiency of 11% for TPSCs with longer operational stability.