Oxygen‐Deficient TiO2‐Based Dual‐Functional Electrochromic Smart Windows: Achieving High Coloration Efficiency and Energy Storage Through Oxygen Defect Engineering

Abstract Electrochromic smart windows (ECSWs) have yet to achieve widespread adoption, primarily due to the high cost associated with the exorbitant material used presently. In this study, we explore titanium dioxide (TiO 2 ), a more abundant and cost‐effective alternative, as an electrochromic (EC) material with potential for enhanced performance. We address the issue of TiO 2 ’s low coloration efficiency ( CE ) by introducing oxygen defects. Using reactive sputtering, we varied the oxygen flow rate during TiO 2 deposition, creating oxygen defect concentrations between 29.9% and 41% and interaction of film with different electrolytes, EC, and energy performance was studied. This led to significant improvements in opacity and EC performance, with only 338 nm thick film, the device shows 55%, 47%, and 44% modulation in solar, luminous, and NIR transmittance and the film shows high CE (22.79 and 26.99 cm 2 C −1 at 550 and 632 nm, respectively). The film also exhibited excellent dual‐functionality, with an areal capacitance ( C a ) of 34 mF cm −2 , demonstrating its energy storage capability. The scalability of the process was confirmed by powering a timer display for 11 minutes with a large‐area (25 cm 2 ) ECSW. This work paves the way for affordable, dual‐functional ECSWs, offering a sustainable solution for modern infrastructure applications.