Below is the ultraviolet visible light absorption spectrum of WO3 thin film under different temperatures. pH value of precursory sol is 2.8, PEG content is 50%, heat treatment time is 3h. From the spectra, light absorption range of samples is almost the same which is below 4710nm. With the increase of heat treatment temperature, light absorption rage of thin film wave length ranges from 300~450nm increases. This is because the crystalline degree of sample increases with temperature which results in the improvement of light absorption efficiency.
Below is the ultraviolet visible light absorption spectrum of sample with different adding amount of citric acid. Compared to the last graph, the absorption of all the samples is below 470nm, it is in accordance with WO3 theory band gap 2.7eV. With the increase of citric acid adding amount, light absorption of thin film increases within wave length 300~450nm, it is mainly because grain size and roughness increase on the surface of thin film. Nano crystalline scattering effect increases the spread distance of photon in film, it increases light absorption rate of thin film, it is benefit for light absorption efficiency.