Depth-sensitive atomic ratio between iodine/lead and nitrogen/lead in lead halide perovskite film during octylammonium iodide-based surface modification

Atomic ratios of iodine/lead (I/Pb) and nitrogen/lead (N/Pb) are carefully examined during octylammonium iodide (OAI) based surface modification on lead halide perovskite (PVSK). X-ray photoemission spectroscopy (XPS) study showed that modification at moderate concentration (20 mM modification) increases the “I/Pb” ratio from 2.08 to 2.72. Angle-resolved XPS study is performed by varying the takeoff angle from 150° to 90°. OAI treatment increases the I/Pb ratio at each takeoff angle, showing the coordination of Pb (II) has been fully improved. Besides, the atomic ratio between N2 (from OAI) and Pb (from PVSK) decreases from 1.13 to 0.86 (20 mM modification), showing that OAI prefers to react with the surface layer of PVSK. Due to the improved coordination environment of Pb (II), the work function of PVSK increases by approximately 0.3 eV as revealed by Kelvin probe force microscopy study, the built-in potential of perovskite solar cell (PSC) increases from 1.0 to 1.05 V. Accordingly, carrier extraction is accelerated, while carrier lifetime is prolonged, and the power conversion efficiency of PSCs is upgraded from 20.47% to 24.00%, with the fill factor rising from 77.32% to 82.79% and the open-circuit voltage from 1.11 to 1.16 V. The study helps understand the close relationship between two-dimensional (2D) precursor molecule modification and coordination behavior of Pb (II) in PVSK.