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

High-efficiency inverted () 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 = 2; PEAFAPbX, 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 Pb 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 () and 17% improvement in power conversion efficiency (PCE). The optimized 3D/2D PSCs demonstrate a champion PCE of 21.22% (0.06 cm), 20.58% (0.175 cm), and 18.72% (0.805 cm), and an improved of 1.23 V. Additionally, unencapsulated devices retain over 80% of their initial PCE after 4000 h of storage in N 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.