Controlled Templated Growth of Low-Dimensional Perovskite for Improved Interfacial Electronic Coupling and Passivation in Wide Bandgap Perovskite Solar Cells

High-efficiency inverted (<i>p-i-n</i>) wide-bandgap (WBG ≥ 1.67 eV) perovskite solar cells (PSCs) are essential for tandem photovoltaics but remain constrained by surface defects and interfacial recombination at the perovskite/electron transport layer (ETL) interface. This study introduces an interfacial engineering approach involving the formation of a phase-pure quasi-2D perovskite layer (<i>n</i> = 2; PEA₂FAPb₂X₇, X = I/Br) atop a WBG perovskite (1.67 eV). By optimizing the 2D precursor concentration, we enable a templated epitaxial-like growth of low-dimensional perovskite along the (110) plane of the 3D perovskite absorber layer, enhancing interfacial electronic coupling and defect passivation. Density functional theory (DFT) calculations reveal that phenylethylammonium (PEA⁺) cations preferentially adsorb at Pb²⁺ and I^- vacancies, effectively passivating surface defects, suppressing nonradiative recombination, and improving interfacial band alignment. This interfacial modification results in a 100 mV increase in open-circuit voltage (<i>V</i>_OC) and 17% improvement in power conversion efficiency (PCE). The optimized 3D/2D PSCs demonstrate a champion PCE of 21.22% (0.06 cm²), 20.58% (0.175 cm²), and 18.72% (0.805 cm²), and an improved <i>V</i>_OC of 1.23 V. Additionally, unencapsulated devices retain over 80% of their initial PCE after 4000 h of storage in N₂ environment, along with excellent thermal and photostability. These findings underscore the potential of templated quasi-2D perovskite interlayers in improving both the efficiency and long-term stability of WBG perovskite top cells for tandem photovoltaics.