Oxygen-vacancy mediated charge dynamics in nano-engineered Ba/Co-doped MTiO₃ electron transport layers for enhanced organic solar cells

Engineering oxide electron-transport layers (ETLs) through defect and interfacial electronic-state control is an effective strategy for reducing charge-collection losses in inverted organic solar cells (OSCs). Herein, a hierarchical TiO₂/CoTiO₃ (CTO) bilayer ETL was developed by integrating oxygen-defect chemistry, transition-metal redox states, and nanoscale architecture. H₂Ti₂O₅·H₂O (HTO), BaTiO₃ (BTO), and CoTiO₃ (CTO) nanoparticle–nanorod architectures were synthesized through sequential hydrothermal/topochemical conversion and introduced as functional interlayers on TiO₂. Structural and spectroscopic analyses confirmed orthorhombic HTO, tetragonal BTO, and rhombohedral CTO while retaining interconnected 0D/1D hierarchical morphologies. XPS revealed increasing oxygen-vacancy-associated surface states from HTO (20.11%) and BTO (22.55%) to CTO (33.19%), together with enhanced Ti³ ⁺ formation and Co²⁺/Co³ ⁺ redox activity. These modifications improved electron selectivity and suppressed interfacial charge losses. CTO also showed broadened visible–near-infrared absorption and a reduced apparent optical transition energy, attributed mainly to defect- and Co-derived sub-band-gap states rather than the intrinsic band gap. In FTO/TiO₂/CTO/PTB7Th:PC71BM/MoO₃/Ag inverted OSCs, the optimized CTO interlayer delivered an average PCE of 11.47 ± 0.63%, JSC of 24.25 ± 0.59 mA cm⁻², VOC of 0.83 ± 0.01 V, and FF of 0.57 ± 0.01 across six devices. The best device achieved 12.01% PCE, 25.26 mA cm⁻² JSC, and 0.82 V VOC. PL and EIS confirmed reduced recombination, 91.10% charge collection, and a 65.17 ms electron-transport lifetime. Overall, Co-induced electronic modulation coupled with optimized oxygen-defect states, rather than oxygen vacancies alone, governs efficient electron extraction, establishing TiO₂/CTO as a promising defect-engineered ETL platform for defect-engineered oxide interfaces in high-performance OSCs.