Dual-Size TiO2 Nanoparticle-Engineered Interfaces for Broadband Light Scattering and Efficiency Enhancement in Perovskite Solar Cells

While power conversion efficiencies (PCEs) of perovskite solar cells (PSCs) can be improved by incorporating sophisticated nanostructures into the electron transport layer (ETL), conventional approaches in this regard are often limited by complex fabrication processes and poor reproducibility. In this study, we propose a simple and efficient strategy involving a mixed ETL composed of two types of TiO₂ nanoparticles (NPs) with different average sizes (15 and 250 nm) to improve the photovoltaic performance of PSCs. The 15 nm TiO₂ NPs primarily function as efficient electron transporters, facilitating the extraction and transfer of photogenerated electrons from the perovskite layer. The larger 250 nm TiO₂ NPs serve as effective light-scattering centers that enhance light-harvesting efficiency by increasing the optical path length within the active layer. The scattering effects are examined using the principles of Rayleigh and Mie scattering theories. The 250 nm TiO₂ NPs, with particle sizes comparable to or larger than the incident light wavelength, follow Mie scattering, leading to significant forward and backward scattering that enhances light trapping in the active layer. Conversely, the 15 nm TiO₂ NPs, being considerably smaller than the incident wavelength, predominantly follow Rayleigh scattering, as characterized by wavelength-dependent scattering intensity, thus contributing minimally to light redirection while ensuring high electron conductivity. Overall, we achieve a synergistic effect between enhanced light scattering and efficient charge transport by optimizing the ratio of the TiO₂ NPs, thereby improving the overall PCE of PSCs.