Suppressing Deep Ion Trapping in TiO2 Electrochromics via Interfacial Engineering of WO3–Al2O3/VOx Heterostructures

Single-component metal oxide electrochromic films suffer poor charge utilization and deep-site ion trapping, giving non-uniform charge injection. Targeting heterostructure engineering rather than new material discovery, bare TiO2, TiO2/WO3, TiO2/WO3-Al2O3 and TiO2/WO3-Al2O3/VOx films were electrodeposited at room temperature. A WO3-rich underlayer beneath the mesoporous TiO2 scaffold enlarges the electroactive interface, and conformal WO3–Al2O3 and VOx form a three-dimensional interpenetrating heterostructure. X-ray photoelectron spectroscopy revealed mixed-valence states and oxygen vacancies promoting polaron hopping and ion accessibility. Aluminum stabilizes W5+ and suppresses deep ion trapping, homogenizing response and raising coloration efficiency 6.9-fold over bare TiO2. VOx adds multivalent redox activity, spatially distributing charge storage, reducing stress and retaining 92% optical modulation after 1000 cycles. The optimized film gives 10.24 s coloration, 87.10% optical modulation and 43.72 cm2·C−1 coloration efficiency at ±2 V. Performance thus reflects interfacial electronic interactions and defect-mediated ion transport rather than added electroactive mass, establishing architectural and interfacial design as key to fast, efficient electrochromic smart windows. Poor charge utilization and ion trapping limit the electrochromic performance of single-component metal oxide films, motivating heterostructure engineering. Here, the authors demonstrate that TiO₂/WO₃-Al₂O₃/VOₓ heterostructures improve electrochromic performance via enhanced charge utilization and ion transport.