Annealing‐Free Gradient Doping Strategy to Build Amorphous Nb:TiO _X Electron Transport Layer for Efficient Perovskite Solar Cells

A low-temperature, annealing-free strategy is reported for fabricating Nb-doped TiO₂ (NTO) electron transport layers (ETLs) via dynamic co-sputtering with controlled power modulation. This method produces a vertically graded composition (Ti-rich at the bottom and Nb-rich at the top) -without requiring any post-deposition thermal treatment. The resulting NTO films exhibit a smooth, amorphous morphology and a chemically homogeneous interface. The engineered doping gradient enables deliberate modulation of the NTO band structure, optimizing energy level alignment with the perovskite absorber, mitigating interfacial energy barriers, and facilitating more efficient charge extraction. Despite the absence of annealing, the film exhibits low defect density and smooth interfaces, thereby minimizing recombination losses and improving open-circuit voltage and fill factor. Perovskite solar cells (PSCs) incorporating the compositionally graded NTO ETLs achieve a power conversion efficiency of 19.9% and an open-circuit voltage of 1.12 V, outperforming devices employing single-target sputtered and conventional co-sputtered ETLs. This work establishes a scalable and thermally benign pathway toward high-performance, flexible, and cost-effective PSCs.