Photochromic coloration of WO3 with visible light

Abstract

Thin amorphous films of tungsten oxide (WO 3 ) 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 WO 3 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 ͑CdS͒ 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 WO 3 . Apart from a more detailed understanding of the photochromic effect this may have also relevance for technical applications. © 1996 American Institute of Physics. ͓S0003-6951͑96͒01420-9͔Due to the high potential of large area optical devices capable to be switched between a transparent and a strong absorptive state there is a large research activity within the last two decades regarding transition metal oxides. 1-3 One of the most promising candidates is tungsten oxide, often discussed in the context of ''smart windows'' in order to control the solar input of buildings or with respect to large area displays. 2,4 For most of these purposes the so-called electrochromic effect is used, where coloration and bleaching of tungsten oxide is caused by an electrochemical reaction. Much less attention has been paid so far to the light-induced coloration of WO 3 , i.e., photochromism, where an identical absorption band is formed upon irradiating bare tungsten oxide thin films with light. 1,5 Taking into account that the photochromic coloration can be made completely reversible by exposing the sample to oxygen gas, 6 the basic requirements for technical applications like erasable optical storage devices are met by the photochromic effect. However, since the formation of color centers in WO 3 requires irradiation in the near-ultraviolet range 1,6 where no compact light sources such as laser diodes are available at the moment, this may impede the development of integrated storage devices based on WO 3 considerably.In this letter we will demonstrate that the spectral sensitivity of photochromism in tungsten oxide can be shifted from the near-UV into the visible range by the use of a thin cadmium sulfide interlayer between the substrate and the WO 3 film. In addition to a more detailed understanding of the coloration mechanism, this result may also have relevance for the above-mentioned applications.It has already been demonstrated that the photochromic effect in tungsten oxide is intimately connected to optically excited electron (e Ϫ ) hole ͑h ϩ ) pairs which can decompose water being incorporated in WO 3 to a considerable amount. 6 The light-induced decomposition of H 2 O can be written as: 7,8 H 2 Oϩ2h ϩ ⇔Oϩ2H ϩ , ͑1͒which describes the creation of protons (H ϩ ) and metastable oxygen radicals ͑O͒. The protons together with the optically e...

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