Nanophotonic enhancement and improved electron extraction in perovskite solar cells using near-horizontally aligned TiO₂ nanorods

While vertically oriented metal oxide nanowires have been intensely researched for use as electron transport layers (ETLs) in halide perovskite solar cells (HPSCs), horizontal nanowires (oriented roughly parallel to the substrate) have received much less attention despite their higher photonic strength due to overlapping electric and magnetic dipolar Mie resonance modes. Herein, we demonstrate the fabrication of an assembly of horizontally aligned TiO₂ nanorods (HATNRs) on FTO substrates via a facile hydrothermal route. The HATNRs are employed as the ETL to achieve 15.03% power conversion efficiency (PCE) in HPSCs which is higher than the PCE of compact TiO₂ based devices (10.12%) by a factor of nearly 1.5. A mixed halide, mixed cation organometal perovskite FA_0.83MA_0.17Pb(Br_0.17I_0.83)₃ with optimized composition is used as the active layer. The excellent refractive index matching between the perovskite and TiO₂, coupled with strong Mie scattering in the nanorod geometry results in broadband near-zero backscattering and high forward scattering, upon coating of HATNRs with perovskite. The maximum suppression of backscattering is found at ∼600 nm. The HATNRs ETL also improves the extraction of electrons from the perovskite layer and results in superior blocking of carrier recombination at the perovskite layer/FTO interface.