Interfacial Engineering of Triple Cation Perovskite Solar Cells Using Graphitic Carbon Nitride-Modified Hematite Electron Transport Layer for Enhanced Photovoltaic Performance

Organic–inorganic halide perovskite solar cells (PSCs) demonstrate impressive power conversion efficiencies (PCEs), yet they encounter significant issues concerning interfacial defects and stability. This work mitigates these constraints by implementing a dual interfacial passivation approach utilizing graphitic carbon nitride (g-C₃N₄) at the interfaces of the Fe₂O₃ electron transport layer (ETL)/CsFAMA perovskite (PVK) and PVK/hole transport layer (HTL). The Fe₂O₃ ETL, despite its chemical stability and cost-effectiveness, is hindered by surface roughness and trap states that impede efficient charge extraction. Through the incorporation of g-C₃N₄, a nitrogen-rich 2D semiconductor, we attained defect passivation through coordination with undercoordinated Pb²⁺ ions and halide vacancies, thereby inhibiting ion migration and improving interfacial energy alignment. Structural characterization (XRD, Raman, scanning electron microscope [SEM]) confirms the layered morphology of g-C₃N₄ and its compatibility with the PVK matrix, while optical analysis reveals enhanced light absorption (400–550 nm) and retained transparency (~80%). The dual-modified devices achieved a champion PCE of 15.97% (12.89% for the control) and a low hysteresis index (HI) of 0.01 (0.06 for the control) with a high V_OC = 1.12 V, J_SC = 19.49 mA cm⁻², and FF 73.19%. Electrochemical impedance spectroscopy and photoluminescence studies demonstrate reduced charge recombination and improved carrier extraction. Critically, the modified devices retain approximately 87% of their initial PCE after 500 h under continuous illumination, highlighting exceptional operational stability. This work establishes dual interfacial engineering with g-C₃N₄ as a robust strategy for advancing efficient, hysteresis-free, and durable PVK photovoltaics, bridging the gap toward commercial viability.