Efficient surface treatment based on an ionic imidazolium hexafluorophosphate for improving the efficiency and stability of perovskite solar cells

• 1-butyl-3-methylimidazolium hexafluorophosphate (BMIMPF 6 ) is introduced onto the perovskite surface with the aid of isopropanol. • Large perovskite grains and island-like Cs 0.05 (MA 0.12 FA 0.88 ) 0.95 Pb(I 0.88 Br 0.12 ) 3-x (PF 6 ) x interlayers are formed by surface reconstruction with isopropanol. • BMIMPF 6 passivates surface defect, inhibits recombination and tunes energy array. • BMIMPF 6 -treated device achieves a champion efficiency of 21.51% compared to pristine device (19.01%). Surface defects have been a primary limitation for perovskite solar cells (PSCs). Herein, the introduction of 1-butyl-3-methylimidazolium hexafluorophosphate (BMIMPF 6 ) into the perovskite surface with the help of isopropanol (IPA) solvent can passivate the surface defects. The surface reconstruction of perovskite caused by IPA makes it easier for [BMIM] + to passivate uncoordinated Pb 2+ defects, [PF] 6 − to more easily replace and compensate unstable I − /Br − ions and I − /Br − vacancies. This forms large perovskite grains and island-like Cs 0.05 (MA 0.12 FA 0.88 ) 0.95 Pb(I 0.88 Br 0.12 ) 3-x (PF 6 ) x interlayers, which significantly reduces the defect-states density and inhibits recombination, and the energy barrier for the perovskite and the hole transport layer is also lowered. As a result, the open-circuit voltage was elevated from 1.112 to 1.181 V, and the efficiency for solar cells was also increased from 19.01% to 21.51%. In addition, the surface treatment of the hydrophobic BMIMPF 6 can isolate the erosion of moisture and obtain better stability.