TiO2/halide perovskite interface: The impact of surface state passivation on energy alignment and photovoltaic performance of perovskite solar cells

In perovskite solar cells (PSCs) trap-assisted recombination is the dominant mechanism limiting the cell performance. Here to overcome this challenge we apply the band gap tuning and surface passivation strategy for reducing the recombination losses at the interface between the TiO2 layer and perovskite absorber. The TiO2 surface was modified by SO42- anions and Cd2+ cations using a low-cost chemical solution technique. X-ray photoelectron spectroscopy analysis revealed that the chemical modification of the TiO2 surface leads to the downward shift of the valence band maximum pointing out the increased conduction electron density and resulting in minimizing the barrier losses at the interface. Electrochemical impedance measurements proved that the modification of the TiO2 surface significantly decreases the charge transfer resistance at the interface with perovskite light absorber. As a result, open circuit voltage and fill factor parameters of the PSCs were enhanced and the hysteresis is decreased. Moreover, the surface passivation significantly improves the air stability of PSCs indicating that the stability of the whole device critically depends on the TiO2/perovskite interface structure. Thus, our study provides valuable guideline toward the designing of hetero-interfaces to enhance both the power conversion efficiency and stability of PSCs.