TiO₂-Assisted Halide Ion Segregation in Mixed Halide Perovskite Films

In metal halide perovskite solar cells, electron transport layers (ETLs) such as TiO₂ dictate the overall photovoltaic performance. However, the same electron capture property of ETL indirectly impacts halide ion mobility as evident from the TiO₂-assisted halide ion segregation in mixed halide perovskite (MHP) films under pulsed laser excitation (387 nm, 500 Hz). This segregation is only observed when deposited on an ETL such as TiO₂ but not on insulating ZrO₂ substrate. Injection of electrons from excited MHP into the ETL (k_et = 10¹¹ s^-1) followed by scavenging of electrons by O₂ causes hole accumulation in the MHP film. Localization of holes on the iodide site in the MHP induces instability causing iodide from the lattice to move away toward grain boundaries. Suppression of segregation occurs when holes are extracted by a hole transport layer (spiro-OMeTAD) deposited on the MHP, thus avoiding hole build-up. These results provide further insight into the role of holes in the phase segregation of MHPs and hole mobility in perovskite solar cells.