Capturing ion trapping and detrapping dynamics in electrochromic thin films.

Area of Science:

• Materials Science
• Electrochemistry
• Solid-State Physics

Background:

• Ion trapping causes performance degradation in electrochromic oxide thin films.
• Detrapping procedures can rejuvenate degraded films, but dynamics are poorly understood.
• Coloration mechanisms in electrochromic oxides require further clarification.

Purpose of the Study:

• To visualize ion trapping and detrapping dynamics in amorphous tungsten oxide (WO3).
• To elucidate the origins of shallow, deep, and irreversible traps.
• To clarify coloration mechanisms involving polaron hopping.

Main Methods:

• In-situ visualization of ion trapping and detrapping dynamics.
• Analysis of trap formation during long-term cycling.
• Identification of hopping mechanisms (small and bipolaron) responsible for optical absorption.

Main Results:

• Shallow traps originate from orthorhombic Li2WO4 formation.
• Deep traps involve mixed W4+-Li2WO4, amorphous Li2WO4, and W4+-Li2O.
• Irreversible traps stem from non-decomposable W4+-Li2WO4 couples.
• Bipolaron hopping contributes to short-wavelength optical absorption.

Conclusions:

• A comprehensive picture of electrochromism based on polaron hopping is presented.
• Ion trapping and detrapping are prevalent in other cathodic electrochromic oxides.
• This research paves the way for developing more durable electrochromic devices.