24.8%-Efficient Planar Perovskite Solar Cells via Ligand-Engineered TiO <sub>2</sub> Deposition

Planar perovskite solar cells (PSCs) have been extensively researched as a promising photovoltaic technology, wherein the electron extraction capability plays a crucial role in the power conversion efficiency (PCE). Here, we proposed a ligand-engineered deposition (LD) strategy based on the coordination ability of ligands (such as tartaric acid) to regulate TiO2 film and interfacial electron transport. The LD strategy can effectively inhibit particle aggregation of TiO2 film through the steric hindrance of assembled ligands. Furthermore, the decreased interfacial contact impedance and enhanced electron extraction are achieved due to the smooth topography, and cross-linked structure formed by tartaric acid that bonds with Ti and Pb atoms. Accordingly, an impressive PCE of 24.8% with fill factor exceeding 0.83 is successfully obtained, which is the highest PCE among the TiO2-based planar PSCs reported so far. In addition, unencapsulated PSCs can maintain ~95% of their initial efficiency upon exposure to ambient air for 2000 hours.