Surface In Situ Growth of Two-Dimensional/Three-Dimensional Heterojunction Perovskite Film for Achieving High-Performance Flexible Perovskite Solar Cells

Organic-inorganic hybrid flexible perovskite solar cells (F-PSCs) have garnered considerable interest owing to their exceptional power conversion efficiency (PCE) and stable operational characteristics. However, F-PSCs continue to exhibit significantly lower PCE than their rigid counterparts. Herein, we employed 3-chloro-4-methoxybenzylamine hydrochloride (CMBACl) treatment to grow in situ two-dimensional (2D) perovskite layers on three-dimensional (3D) perovskite films. Through comprehensive physicochemical characterization, including X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and photoluminescence (PL) mapping, we demonstrated that CMBACl treatment enabled the in situ growth of two-dimensional (2D) perovskite layers on three-dimensional (3D) perovskite films via chemical interactions between CMBA⁺ cations and undercoordinated Pb²⁺ sites. The organic cation (CMBA⁺) bound to uncoordinated Pb²⁺ ions and residual PbI₂, while the chlorine anion (Cl^-) filled iodine vacancies in the perovskite lattice, thereby forming a high-quality 2D/3D heterojunction structure. The CMBACl treatment effectively passivated surface defects in the perovskite films, prolonged charge carrier lifetimes, and enhanced the operational stability of the photovoltaic devices. Additionally, the hybrid 2D/3D architecture also improved energy band matching, thereby boosting charge transfer performance. The optimized flexible devices demonstrated a PCE of 23.15%, while retaining over 82% of their initial efficiency after enduring 5000 bending cycles under a 5 mm curvature radius (R = 5 mm). The unpackaged devices retained 94% of their initial efficiency after 1000 h under ambient conditions with a relative humidity (RH) of 45 ± 5%. This strategy offers practical guidelines for selecting interface passivation materials to enhance the efficiency and stability of F-PSCs.