Room-Temperature-Processed Fullerene/TiO₂ Nanocomposite Electron Transporting Layer for High-Efficiency Rigid and Flexible Planar Perovskite Solar Cells

Room-temperature-processed TiO₂ (R-Lt-TiO₂) electron transporting layers (ETLs) possess low conductivity and connectivity, resulting in poor photovoltaic performance. Herein, an ethanol (EtOH)-soluble, highly conducting fullerene derivative, C₆₀RT₆, was used as an additive for Lt-TiO₂ ETLs. Room-temperature processed nanocomposite ETL (R-Fu/Lt-TiO₂) is prepared simply by spin coating a C₆₀RT₆ and G-TiO₂ NPs (TiO₂ nanoparticle prepared by grinding the bulk TiO₂ powder) mixture. R-Fu/Lt-TiO₂ has better aligned with the frontier orbitals of the FA_xMA_1−xPbI₃, better continuity, conductivity, flatness, and higher surface hydrophilicity compared to Lt-TiO₂ ETL. Perovskite films spin coated on R-Fu/Lt-TiO₂ ETLs also have slightly larger grains and thickness compared to those deposited on Lt-TiO₂. Perovskite solar cells (PSCs) based on a R-Fu/Lt-TiO₂ ETL possess higher power conversion efficiency (PCE, up to 20% on glass substrate), less (negligible) current hysteresis, and better long-term stability compared to those using R-Lt-TiO₂ as an ETL. The flexible PSC (used indium tin oxide/polyethylene terephthalate (ITO/PET) as a substrate) with a R-Fu/Lt-TiO₂ ETL achieves a PCE of 18.06% and retains 90% of the initial PCE after 500 bending cycles with a bending radius of 6 mm. The PCE of the flexible cell with a Lt-TiO₂ ETL is only 8.2%, and loses 60% of the initial value after 500 bending cycles.