Low-Temperature Microwave Processed TiO₂ as an Electron Transport Layer for Enhanced Performance and Atmospheric Stability in Planar Perovskite Solar Cells

Low-temperature (∼150 °C) solution processed planar perovskite solar cells (PSCs) using TiO2 as an electron transport layer (ETL) offer promise for a simple fabrication methodology and compatibility with polymeric substrates and perovskite-based tandem devices. However, the amorphous nature and presence of trap states on the low-temperature TiO2 surface hinder the effective carrier transport. Further, perovskite crystallization on ETL develops lattice strain resulting in the creation of unwanted defect centers. Herein, a low-temperature microwave processed compact TiO2 (MW-TiO2) film is reported that possesses lower surface oxygen vacancy defects and enhanced conductivity and promotes efficient electron extraction owing to the enhanced built-in potential (Vbi) at the MW-TiO2/perovskite interface. The suppressed heterogeneous nucleation of MAPbI3 crystals on the less defective MW-TiO2 surface relieves the interfacial strain, thereby making it a superior template for the growth of strain relaxed, high-quality perovskite films with a more n-type character having larger grains, resulting in suppressed interfacial/surface and bulk trap density. Further, MW-TiO2 mitigates the interfacial energetic disorder and Urbach energy owing to reduced strain, thereby boosting the open-circuit voltage (Voc) by 40 mV, while improved optoelectronic properties of MW-TiO2, lower interfacial charge transfer resistance, and high-quality perovskite films simultaneously improve the short-circuit current density (Jsc) and fill factor (FF) by 6.82 and 9.37%, respectively, over HT-TiO2 based devices. Compared to high-temperature (500 °C) annealed TiO2 based MAPbI3 planar PSCs, MW-TiO2 based devices exhibited a substantial performance enhancement of 22%, leading to the best efficiency of ∼18% and superior atmospheric stability (25 °C, 55% relative humidity) while maintaining 80% of its initial value after 2500 h. Experimental results are validated by device simulation studies with model accounting for trap-assisted interfacial and bulk recombination. Finally, MW processed flexible devices maintained over 80% of their initial power conversion efficiency (PCE) after 1000 bending cycles, thereby exhibiting excellent mechanical robustness. These results elucidate the critical role of MW-TiO2 in rendering improved performance, flexibility, and stability in low-temperature PSCs.