WO3-based photochromic composite films with various film thickness were fabricated using peroxo-iso-polytungstic acid as filler and transparent urethane resin as matrix, and the thickness of the composite films were 180, 250, 620, and 1300 µm. All the composite films showed reversible photochromic properties. The coloring reaction rate constant of the composite films with the thickness of 180, 250, 620, and 1300 µm were 3.91 × 10−1, 3.80 × 10−1, 3.63 × 10−1, and 2.69 × 10−2 min−1, respectively. The bleaching half-life period of the composite films with the thickness of 180, 250, 620, and 1300 µm were 45.4, 67.8, 138, and 200 min, respectively. The coloring speed increased with decrease of the film thickness, and the bleaching speed decreased considerably with decrease of the film thickness.

Phosphorus tungstic heteropolyacid was synthesized by mixing P2O5 and peroxoisopolytungstic acid (W-IPA) in methanol solution. WO3-based composite films fabricated from phosphorus tungstic heteropolyacid and transparent urethane resin showed reversible photochromic properties. The coloring reaction rate constants of the films with P/W ratios of 0, 0.05, 0.083, and 0.20 were 1.55 × 10−2, 1.29 × 10−2, 1.88 × 10−2, and 1.76 × 10−2 min−1, respectively, while the bleaching half-life periods τ were 21.5, 14.9, 12.6, and 10.2 h, respectively; thus, the bleaching speed of the composite films was accelerated by phosphorus addition.

WO 3 -based composite films were fabricated from peroxo-iso-poly tungstic acid and a transparent urethane resin, and the photochromic properties of the films were evaluated under sunlight from three seasons: spring, summer, and winter. All composite films exhibited photochromism under solar irradiation, and the coloring degree of the films varied with the sunlight intensity for each season. We estimate the energy efficiency of the colored composite films for various seasons.

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