Carrier-induced absorption as a mechanism for electrochromism in tungsten trioxide
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Research Letter
Carrier-induced absorption as a mechanism for electrochromism in tungsten trioxide Wennie Wang, and Hartwin Peelaers, Materials Department, University of California, Santa Barbara, California 93106-5050, USA Jimmy-Xuan Shen, Department of Physics, University of California, Santa Barbara, California 93106-9530, USA Chris G. Van de Walle, Materials Department, University of California, Santa Barbara, California 93106-5050, USA Address all correspondence to Wennie Wang at [email protected] (Received 28 April 2018; accepted 15 June 2018)
Abstract We present a first-principles investigation on the optical absorption of tungsten trioxide, an electrochromic material. Using state-of-the-art techniques, the absorption spectra are calculated for the cubic, monoclinic, and amorphous phases. For both crystalline and disordered structures, doping induces strong absorption in the infrared. Absorption in the visible range increases with the degree of structural distortion; the absorption coefficient in the blue exceeds 103 cm−1 at doping levels above 1020 cm−3 in the monoclinic phase. Increased disorder in disordered structures significantly enhances the visible-range absorption. We identify the microscopic mechanism as optical absorption originating at conduction-band-derived states that are filled by doping.
Introduction Tungsten trioxide (WO3) is one of the most widely studied electrochromic materials[1]; it has been used in smart windows, driving mirrors, displays, automobile glazing, and as a photocatalyst.[1,2] The introduction of dopants to the system (typically monovalent dopants such as H, Li, or Na[3,4]) leads to a strong decrease in transmittance at infrared and visible wavelengths, and typically blue coloration. Surprisingly, the mechanism of electrochromism is still poorly understood. A number of mechanisms have been proposed to explain electrochromism in WO3, including absorption by deep defect levels[5] or by small polarons.[6,7] Here we propose an alternative explanation, which is consistent with all the experimental observations: the absorption of light in momentumconserving (i.e., direct) transitions of electrons filling conductionband states. Indeed, the intercalation of dopants leads to the introduction of carriers in the conduction band,[8,9] as evidenced by the observation of higher electrical conductivities.[10,11] It was noted[4,8,12] that these carriers can lead to an increase in IR reflectivity; however, to our knowledge, the impact of carrier absorption on absorption at visible wavelengths has not been explored to date. Carriers introduced by doping can absorb photons and be excited to higher lying conduction-band states. In this paper, we demonstrate that carrier-induced absorption significantly contributes to the optical absorption not only at infrared but also at visible wavelengths. We analyze how this absorption depends on doping level and demonstrate the correlation between the absorption coefficient and the structure of the material. Our proposal for the role of carrier-indu
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