Boosting lead-free solar efficiency: Gradient band alignment coupled with ETL engineering in chalcogenide perovskite solar cells

Perovskite solar cells (PSCs) have demonstrated outstanding power conversion efficiencies (PCEs), but their reliance on toxic lead (Pb) raises environmental and health concerns. To address this, we explore the lead-free chalcogenide perovskite (CP) BaZrS[Formula: see text]Se x as a promising alternative, owing to its excellent thermal stability, strong light absorption and environmental benignity. This study introduces a linear band gap grading (LBGG) strategy by tuning the sulfur-to-selenium ratio ([Formula: see text] to 3) to tailor the absorber’s optoelectronic properties. Three device architectures (Au/HTL/BaZrS[Formula: see text]Se x /CdS/FTO (fluorine-doped titanium oxide)) are investigated using Cu 2 O, Spiro-OMeTAD and CuI as hole transport layers (HTLs). Key parameters, including absorber thickness, shallow acceptor density ([Formula: see text], total defect density ([Formula: see text], temperature and interface defect density (IDD) are systematically optimized. The Cu 2 O-based device achieves the highest PCE of 28.25%, followed closely by 27.99% with Spiro-OMeTAD and 27.73% with CuI. The best device also yields a short-circuit current density ([Formula: see text] of 28.54[Formula: see text]mA[Formula: see text] [Formula: see text] [Formula: see text]cm[Formula: see text], open-circuit voltage ([Formula: see text] of 1.1659[Formula: see text]V, and fill factor (FF) of 84.89%, supported by J–V and external quantum efficiency (EQE) analyses. The results confirm the effectiveness of the LBGG approach in enhancing charge transport and minimizing recombination. This work highlights BaZrS[Formula: see text]Se x as a high-performance lead-free absorber and provides a scalable route toward environmentally sustainable perovskite photovoltaics.