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 approximate to 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.