Improvement of Open-Circuit Voltage Deficit via Pre-Treated NH₄⁺ Ion Modification of Interface between SnO₂ and Perovskite Solar Cells

Passivation is a popular method to increase power conversion efficiency (PCE), reduce hysteresis related to surface traps and defects, and adjust mismatched energy levels. In this paper, an approach is reported using ammonium chloride (AC) to enhance passivation effects by controlling chlorine (Cl) and ammonium ions (NH₄⁺) on the front and back side of tin oxides (SnO₂). AC pre-treatment is applied to indium tin-oxide (ITO) prior to SnO₂ deposition to advance the passivation approaches and compare the completely separated NH₄⁺ and Cl passivation effects, and sole NH₄⁺ is successfully isolated on the SnO₂ surface, the counterpart of AC-post-treatment, generating ammonia (NH₃) and Cl. It is demonstrated that multifunctional healing effects of NH₄⁺ are ascribed from AC-pre-treatment being the basis of SnO₂ crystallization and adjusting bifacial interface energy levels at ITO/SnO₂ and SnO₂/perovskite to enhance photo-carrier transport. As calculated by density functional theory, how the change of the passivation agent from Cl to NH₄⁺ more effectively suppresses non-radiative recombination ascribed to hydrated SnO₂ surface defects is explained. Consequently, enhancement of photo-carrier transport significantly improves a superior open-circuit voltage of 1.180 V and suppresses the hysteresis, which leads to the PCE of 22.25% in an AC-pre-treated device 3.000% higher than AC-post-treated devices.