Durability improvement of electrochromic WO₃ thin films by deposition of an ultra-thin Al₂O₃ layer via atomic layer deposition

Introduction of an ultra-thin Al₂O₃ protective layer via atomic layer deposition (ALD) significantly enhances the durability and performance of amorphous tungsten trioxide (a-WO₃) thin films for electrochromic devices. The Al₂O₃ layer, despite being less than 4 nm thick, effectively suppresses the dissolution of WO₃ that typically occurs through hydration reactions forming WO₃·H₂O and tungstic acid (H₂WO₄). XRD and XPS analyses confirmed the presence of Al₂O₃ and its minimal impact on the amorphous structure of WO₃ thin films. Optical transmittance measurements showed that the Al₂O₃-protected a-WO₃ thin films maintained a higher transmittance modulation (ΔT) compared to bare a-WO₃ thin films, with the ALD30 sample retaining 87.9 % of its initial transmittance modulation after 1000 cycles, in contrast to the rapid degradation observed in unprotected films. Microstructural analysis revealed significantly fewer surface cracks and reduced thickness loss in Al₂O₃-coated films after 100 cycles. Additionally, ICP-MS analysis indicated a much lower W concentration in the electrolyte for Al₂O₃-protected samples, confirming reduced dissolution.