Ti 3 C 2 T X -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 Cs 2 AgBiBr 6 (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 Ti 3 C 2 T X -MXene. MXene layers were selectively introduced at the FTO/TiO 2 ( FMT ) and TiO 2 /CABB ( FTM ) interfaces within FTO/TiO 2 /CABB/Carbon DPSC stacks. Physicochemical analysis confirms that MXene underlayer integration ( FMT ) produces the most uniform ETL, as reflected in lower roughness parameters ( R a: 8.98 nm, R q: 11.46 nm ) and lower asperities ( R z: 79.97 nm ) compared to FT and FTM . EIS analysis reveals that FMT delivers the most favorable charge-transport characteristics, with the lowest R S and R Ct, the highest R Rec, and superior carrier lifetime and diffusion metrics (τ e = 0.376 s, L n = 63.8 μm, D e = 1.08 × 10 4 μm 2 s –1, η cc = 99.5%). Henceforth, the FMT ETL-based DPSC achieves a champion PCE of 6.92% ( V OC = 0.98 V, J SC = 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-TiO 2 band alignment and reduced defect densities.