Cu-doped mesoporous TiO ₂ interlayer for highly efficient and stable hybrid perovskite solar cells

Titanium dioxide (TiO₂)-based electron transport layers (ETLs) are widely used in perovskite solar cells (PSCs) but suffer from low electron mobility, deep-level defects, and detrimental interfacial recombination, which lower open-circuit voltage (<i>V</i>_OC) and short-circuit current density (<i>J</i>_SC) and ultimately limit power conversion efficiency (PCE). In this study, Cu-doped mesoporous TiO₂ (Cu-m-TiO₂) was developed, integrating the inherent structural advantages of conventional mesoporous TiO₂ (m-TiO₂) with interlayer engineering functionalities to overcome these limitations. In contrast to conventional m-TiO₂, Cu-m-TiO₂ provides optimized electronic structure, surface chemistry, and interfacial energetics through strategic Cu incorporation <i>via</i> photodeposition. This approach enables synergistic improvements in electron transport, defect passivation, and energy level alignment while preserving the intrinsic benefits of m-TiO₂, including efficient charge extraction and interfacial carrier separation. Consequently, PSCs incorporating Cu-m-TiO₂ demonstrated a <i>V</i>_OC of 1.162 V, <i>J</i>_SC of 26.19 mA cm^-2, and fill factor (FF) of 84.4%, achieving a champion PCE of 25.68%, significantly higher than the 23.12% obtained with m-TiO₂. Beyond efficiency enhancement, Cu-m-TiO₂ substantially improved device longevity, retaining 94.6% of initial PCE after 2450 h under dry-room conditions without encapsulation. This multi-functional approach demonstrates transitioning from conventional single-function layers to multi-functional integrated layers for enhanced device performance.