Vacancies substitution induced interfacial dipole formation and defect passivation for highly stable perovskite solar cells

Although an efficient charge transport is essential to high-performance perovskite solar cells (PSCs), the serious charge trapping in perovskite films is still a barrier to improve the efficiency of PSCs. To overcome this issue, we efficiently suppress the charge trapping by using polar compound materials to reduce defects and improve the match of work functions in PSCs. 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM]BF4) is used to form an interfacial dipole layer and triphenylphosphine oxide (TPPO) is employed to passivate defects. The interfacial dipole layer not only reduces the surface work function of electron transport layers (ETLs), but also substitutes organic/Cs cation vacancies. Oxygen atoms in TPPO molecules fill anion vacancies on perovskite crystal surfaces. As a result, the power conversion efficiency (PCE) of the champion PSCs has been improved to 21.1% from 18.7%. The target PSCs retained 98.3% of its initial PCE after 214 days in dry air condition (relative humidity about 22% at 25 °C) due to the reduced defect density in perovskite.