Correlation between Interfacial Structures and Device Performance: The Double‐Edged Sword Effect of Lead Iodide in Perovskite Solar Cells

Abstract The interfacial electronic structure of perovskite layers and transport layers is critical for the performance and stability of perovskite solar cells (PSCs). The device performance of PSCs can generally be improved by adding a slight excess of lead iodide (PbI 2 ) to the precursor solution. However, its underlying working mechanism is controversial. Here, we performed a comprehensive study of the electronic structures at the interface between CH 3 NH 3 PbI 3 and C 60 with and without the modification of PbI 2 using in situ photoemission spectroscopy measurements. The correlation between the interfacial structures and the device performance was explored based on performance and stability tests. We found that there is an interfacial dipole reversal, and the downward band bending is larger at the CH 3 NH 3 PbI 3 /C 60 interface with the modification of PbI 2 as compared to that without PbI 2 . Therefore, PSCs with PbI 2 modification exhibit faster charge carrier transport and slower carrier recombination. Nevertheless, the modification of PbI 2 undermines the device stability due to aggravated iodide migration. Our findings provide a fundamental understanding of the CH 3 NH 3 PbI 3 /C 60 interfacial structure from the perspective of the atomic layer and insight into the double‐edged sword effect of PbI 2 as an additive.