Photochromic coloration of WO3 with visible light

Abstract

Thin amorphous films of tungsten oxide (WO3) are well known to change their optical absorption behavior upon light exposure, usually referred to as photochromic effect. Since the formation of color centers is closely related to the optical creation of electron-hole pairs the sensitivity of the photochromic effect in WO3 is limited to energies above its band-gap energy of 3.25 eV, corresponding to the near-ultraviolet range. We will demonstrate that in case of a thin cadmium sulfide (CdS) layer underneath the tungsten oxide film the sensitivity of photochromism is shifted into the visible range. This result is interpreted in terms of charge carrier injection from the CdS into the WO3. Apart from a more detailed understanding of the photochromic effect this may have also relevance for technical applications.