Differential coloration efficiency of electrochromic amorphous tungsten oxide as a function of intercalation level : Comparison between theory and experiment

• Optical absorption in amorphous tungsten oxide (aWO3), for photon energies below that of the bandgap, can be rationalized in terms of electronic transitions between localized states. For the study of this phenomenon, we employed the differential coloration efficiency concept, defined as the derivative of the optical density with respect to the inserted charge. We also made use of its extension to a complex quantity in the context of frequency-resolved studies. Combined in situ electrochemical and optical experiments were performed on electrochromic aWO3 thin films for a wide lithium intercalation range using an optical wavelength of 810 nm (1.53 eV). Quasi-equilibrium measurements were made by chronopotentiometry (CP). Dynamic frequency-dependent measurements were carried out by simultaneous electrochemical and color impedance spectroscopy (SECIS). The differential coloration efficiency obtained from CP changes sign at a critical intercalation level. Its response exhibits an excellent agreement with a theoretical model that considers electronic transitions between W4+, W5+, and W6+ sites. For the SECIS experiment, the low-frequency limit of the differential coloration efficiency shows a general trend similar to that from CP. However, it does not change sign at a critical ion insertion level. This discrepancy could be due to degradation effects occurring in the films at high Li+ insertion levels. The methodology and results presented in this work can be of great interest both for the study of optical absorption in disordered materials and for applications in electrochromism.

Ämnesord

TEKNIK OCH TEKNOLOGIER  -- Materialteknik -- Annan materialteknik (hsv//swe)

ENGINEERING AND TECHNOLOGY  -- Materials Engineering -- Other Materials Engineering (hsv//eng)

NATURVETENSKAP  -- Fysik -- Atom- och molekylfysik och optik (hsv//swe)

NATURAL SCIENCES  -- Physical Sciences -- Atom and Molecular Physics and Optics (hsv//eng)

Nyckelord

Localized states

Hopping-conductivity models

Transition metal oxides

Electrochemical impedance spectroscopy

Polarons

Disordered solids

Sputter deposition

Optical absorption

Thin films

Electrooptical effects

Teknisk fysik med inriktning mot materialvetenskap

Engineering Science with specialization in Materials Science

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