Systematically Optimized Bilayered Electron Transport Layer for Highly Efficient Planar Perovskite Solar Cells (η = 21.1%)

Understanding and controlling interfacial charge transfer at the heterojunction of optoelectronic devices is currently receiving extensive interest. Here, we study the parameters that can influence the electron extraction in planar perovskite solar cells (P-PSCs) using spin-coated SnO 2 and TiO 2, anodized-TiO 2 (a-TiO 2 ), and bilayered electron transport layers (ETL) composed of SnO 2 and TiO 2 or SnO 2 on a-TiO 2 (SnO 2 @a-TiO 2 ). These are the varied free energy difference (Δ G ) values between the ETL and perovskites, electron mobility (μ e ) of the ETL, and quality of physical contact between the ETL and fluorine-doped tin oxide (FTO). Among the various ETLs, the bilayered ETL (SnO 2 @a-TiO 2 ) gives a large Δ G as well as defect-free physical contact. The resulting P-PSC exhibits a PCE of 21.1% and stabilized efficiency of 20.2% with reduced hysteresis. This result emphasizes that a large free energy difference (Δ G ) value plays an important role in electron extraction. More importantly, the defect-free physical contact is also crucial for achieving improved electron extraction.