Mitigating Surface and Grain Boundary Defects in Perovskite Solar Cells Through Guanidinium Halide Passivation

Guanidinium salt treatments provide a simple yet effective approach to suppress ion migration and stabilize grain boundaries in perovskite solar cells (PSCs). This study investigates the effects of guanidinium halide (GuaX, where X = I, Br, or Cl) surface treatments on PSC performance and stability, addressing challenges related to ion migration and grain boundary instability. Low-energy electron microscopy reveals that GuaX treatments modulate the work function, reducing it from ~5.44 eV in untreated films to ~4.96 eV in GuaI-treated films, a change attributed to differences in electronegativity and ionic size. Conductive atomic force microscopy demonstrates improved and uniformed current distribution, particularly in GuaCl-treated films, owing to GuaCl's ability to mitigate surface and grain boundary defects. Current–voltage mapping highlights GuaCl's role in stabilizing charge transport at grain boundaries. Optimized GuaX treatments substantially enhance photovoltaic performance, with GuaCl-treated PSCs achieving a power conversion efficiency of 21.10%, an open-circuit voltage of 1.15 V, and a fill factor of 80.16%. Surface photovoltage analysis further confirms a significant reduction in trap-state density (from 29–16 meV), while density functional theory calculations indicate that GuaCl exhibits the highest adsorption energy (−2.58 eV), indicating strong interaction with the perovskite. Moreover, stability tests under ambient conditions demonstrate exceptional durability, with GuaCl-treated PSCs retaining over 95% of their initial efficiency after 60 days.