Enhanced Wide-Bandgap Perovskite Solar Cells via Kinetically Optimized C₆₀ Electron-Transport Layers

High-efficiency tandem solar cells require wide-bandgap (WBG) perovskites as the top absorber, yet such devices often suffer severe nonradiative recombination, voltage losses, and halide segregation. This work demonstrates that carefully controlling the deposition kinetics of the fullerene electron-transport layer (ETL) offers an elegant route to overcome these issues without complex passivation strategies. WBG perovskite solar cells using a FA_{0 .8}Cs_{0 .2}Pb(I_{0 .8}Br_{0 .2})₃ absorber were fabricated in a p-i-n architecture with C₆₀ ETLs deposited at three different evaporation rates. When the C₆₀ deposition rate was slowed to 0.1 Å s^-1, our devices achieve a 20.4% PCE with a relatively low Voc deficit (~0.48 eV) without complex molecular passivation, 2D/3D heterostructures, or multistep surface reconstruction. The improvement originates from suppressed nonradiative recombination and reduced shunt leakage: The slow-deposited C₆₀ film yields a higher open-circuit voltage (~1.17 V), increased fill factor (80%), and reduced saturation current density and trap-state density compared with faster deposition. Photoluminescence, impedance spectroscopy, and transient photovoltage analyses reveal that slower deposition produces a compact and well-ordered C₆₀ layer which minimizes trap-assisted recombination, decreases Urbach energy (16.68 meV), and lowers the ideality factor (n ≈ 1.33). Structural characterizations confirm improved C₆₀ molecular interface and smoother morphology at slow deposition rates. This work provides a simple processing guideline for high-performance WBG perovskite solar cells and offers valuable insights for scalable tandem cell fabrication.