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

Introduction of an ultra-thin Al ₂O ₃ protective layer via atomic layer deposition (ALD) significantly enhances the durability and performance of amorphous tungsten trioxide (a-WO ₃) thin films for electrochromic devices. The Al ₂O ₃ layer, despite being less than 4 nm thick, effectively suppresses the dissolution of WO ₃ that typically occurs through hydration reactions forming WO ₃·H ₂O and tungstic acid (H ₂WO ₄). XRD and XPS analyses confirmed the presence of Al ₂O ₃ and its minimal impact on the amorphous structure of WO ₃ thin films. Optical transmittance measurements showed that the Al ₂O ₃-protected a-WO ₃ thin films maintained a higher transmittance modulation ( ΔT) compared to bare a-WO ₃ 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 Al ₂O ₃-coated films after 100 cycles. Additionally, ICP-MS analysis indicated a much lower W concentration in the electrolyte for Al ₂O ₃-protected samples, confirming reduced dissolution.