Strategic defect passivation through halide anion incorporation from ionic liquids in MAPbI3 perovskites

Perovskite materials are widely used as promising light absorption materials for perovskite solar cells. Poor film morphology, with crystal defects and small grain sizes, significantly impacts their performance and stability, causing charge recombination and suboptimal optoelectronic properties. To address these challenges, we incorporate five ionic liquids (ILs) for defect passivation in methylammonium lead iodide (MAPbI₃) perovskite compositions. All ILs share the same imidazolium cation but vary in halide anions. Considering the electronegativity trend of halide anions (F > Cl > Br > I), we expect that ILs could interact with the perovskite material, enhancing its morphology and crystal stability. Furthermore, the imidazolium of ILs is expected to form coordinate covalent bonds with under-coordinated Pb. This study systematically investigates the influence of ionic liquid (IL) incorporation on the crystallinity, morphology, and optoelectronic properties of perovskite films using a range of techniques, thereby achieving defect passivation. Notably, the most exceptional results are obtained by adding 10 % 1-Butyl-3-methylimidazolium iodide (BMIM[I]), which demonstrates substantial grain growth (885 nm), maximum film coverage (98.74 %), a smoother morphology, maximum crystallite size (69.97 nm) and the highest UV absorption retention after 50 days under dark ambient conditions (75 % relative humidity). FTIR confirms the interaction between IL halide-based anions and MAPbI₃. XRD analysis indicates overall higher crystallinity and complete conversion of PbI₂ to perovskite. UV–Vis results show enhanced absorption with a little change in the bandgap for all ILs. Photoluminescence (PL) analysis shows that BMIM[I]-treated films exhibit the highest PL intensity, indicating reduced defect-assisted recombination.