Ti ₃ C ₂ T _<i>X</i> -MXene-Assisted Dual-Interfacial Passivation for HTL-Free Cs ₂ AgBiBr ₆ Double Perovskite Solar Cells

The demand for sustainable and lead-free photovoltaic technologies has intensified interest in Cs2AgBiBr6 (CABB) double perovskites. In this work, we advance HTL-free CABB double-perovskite solar cells (DPSCs) by engineering a bifacial interfacial passivation strategy using two-dimensional Ti3C2TX-MXene. MXene layers were selectively introduced at the FTO/TiO2 (FMT) and TiO2/CABB (FTM) interfaces within FTO/TiO2/CABB/Carbon DPSC stacks. Physicochemical analysis confirms that MXene underlayer integration (FMT) produces the most uniform ETL, as reflected in lower roughness parameters (Ra: 8.98 nm, Rq: 11.46 nm) and lower asperities (Rz: 79.97 nm) compared to FT and FTM. EIS analysis reveals that FMT delivers the most favorable charge-transport characteristics, with the lowest RS and RCt, the highest RRec, and superior carrier lifetime and diffusion metrics (τe = 0.376 s, Ln = 63.8 μm, De = 1.08 × 104 μm2 s–1, ηcc = 99.5%). Henceforth, the FMT ETL-based DPSC achieves a champion PCE of 6.92% (VOC = 0.98 V, JSC = 16.19 mA cm–2, FF = 0.57), surpassing both bare FT (PCE = 3.74%) and FTM-based DPSCs (PCE = 4.93%). The improvement stems from more efficient electron extraction from the CABB absorber and pronounced suppression of interfacial recombination, enabled by favorable MXene-TiO2 band alignment and reduced defect densities.