Interfacial Electron Transfer Barrier at Compact TiO₂/CH₃NH₃PbI₃ Heterojunction

Low-temperature solution-processed CH₃NH₃PbI₃ interfaced with TiO₂ has recently been demonstrated as a highly successful type-II light harvesting heterojunction with ≈20% efficiency. Therefore, an efficient ultrafast photoexcited electron transfer from CH₃NH₃PbI₃ to TiO₂ is expected. However, by probing the photoexcited charge carrier dynamics in CH₃NH₃PbI₃/quartz, CH₃NH₃PbI₃/TiO₂ (compact), and CH₃NH₃PbI₃/PCBM in a comparative study, an electron transfer potential barrier between CH₃NH₃PbI₃ and the compact TiO₂ (prepared with the spray pyrolysis method) formed by surface states is uncovered. Consequently, the CH₃NH₃PbI₃ photoluminescence intensity and lifetime is enhanced when interfaced to compact TiO₂. The electron accumulation within CH₃NH₃PbI₃ needed to overcome this interfacial potential barrier results in the undesirable large current-voltage hysteresis observed for CH₃NH₃PbI₃/TiO₂ planar heterojunctions. The findings in this study indicate that careful surface engineering to reduce this potential barrier is key to pushing perovskite solar cell efficiencies toward the theoretical limit. An electron transfer potential barrier between CH₃NH₃PbI₃ and TiO₂ is unveiled through transient spectroscopy, which is also responsible for the strong current-voltage hysteresis observed for this heterojunction.