SnO₂ Modification with Tetrapropylammonium Hydroxide to Enhance Bottom Contact for High-Performance Perovskite Solar Cells

The bottom interface in n-i-p structured perovskite solar cells plays a pivotal role in determining both device efficiency and long-term operational stability. Surface modification of colloidal SnO2 nanoparticles at an early stage presents a promising strategy to tailor their structural and electronic characteristics, thereby influencing the crystallization behavior of the subsequently deposited perovskite layer. In this work, we employ tetrapropylammonium hydroxide (TPAOH) to functionalize SnO2 surfaces, effectively passivating the intrinsic defects such as tin interstitials (Sni) and oxygen vacancies (VO). This modification not only optimizes the optoelectronic properties of SnO2 but also refines film morphology. Moreover, the introduction of TPA+ ions contributes to the stabilization of the perovskite bottom interface by passivating iodine interstitials (Ii) and formamidinium vacancies (VFA), thereby mitigating ion migration. As a result, devices employing TPA+-modified SnO2 (denoted as SnO2-TPA) achieve a power conversion efficiency (PCE) of 24.32% when paired with a perovskite film of 1.53 eV bandgap─surpassing the performance of devices using commercial SnO2 (denoted as SnO2-c), which yields a PCE of 22.76%. Notably, flexible perovskite solar cells incorporating SnO2-TPA demonstrate a PCE of 19.60%, representing an approximate 20% improvement over those using SnO2-c. Furthermore, SnO2-TPA-based devices exhibit enhanced stability under various stress conditions, including continuous ultraviolet (395 nm) irradiation for ∼90 h, thermal annealing at 40 °C for 700 h, and ambient humidity (15% relative humidity) for 1200 h.