Strategic Optimization of Gradient-Structured HTL-free Lead-free Double-Perovskite Solar Cells for Enhanced Photovoltaic Performance

Lead-free perovskite solar cells (PSCs) based on Cs₂TiBr₆ are promising for environmentally sustainable photovoltaics, but their efficiency is often constrained by poor band alignment and recombination losses. Here, we use SCAPS-1D simulations to investigate a hole transport layer (HTL)-free Cs₂TiBr₆ PSC employing an optimized gradient doping (G_d) strategy to overcome these limitations. The absorber layer doping concentration was exponentially graded with a gradient factor G = 300, identified after varying G from 1 to 1000, and paired with reduced bulk (1 × 10¹⁶ cm^–3) and interfacial (1 × 10¹⁴ cm^–3) defect densities. This combination enhances the band alignment, strengthens the internal electric field, and suppresses trap-assisted recombination. The resulting device achieves a PCE of 21.0% (Voc = 1.374 V, Jsc = 18.25 mA cm^–2, FF = 83.69%), outperforming uniformly doped devices (10.85%) and previously reported Cs₂TiBr₆ PSCs (≤19.30%). These findings establish gradient doping as a practical design pathway for scalable, stable, and high-efficiency lead-free PSCs.