The role of interface between LiPON solid electrolyte and electrode in inorganic monolithic electrochromic devices

Abstract

Transparent lithium phosphorus oxynitride (LiPON) thin films were deposited by RF sputtering at room temperature for electrochromic (EC) applications. We carried out a basic study from single layer, double layers, and eventually to devices. The interactions between LiPON and bottom EC electrode during sputtering were highlighted. The effects of N2 pressure on structural, morphological, chemical, and electrical properties of the LiPON films were investigated. The Li ionic conductivity of the films increased with decreasing N2 pressure. The results of Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy indicated a probable increase of nitrogen content in the films with decreasing N2 pressure. However, low pressure sputtering process induced the formation of a resistive layer between LiPON and bottom electrode. The electrochemical impedance measurements were conducted to evaluate the charge transfer processes across the interfaces. In the case of WO3 electrode, the charge transfer resistance at the WO3/LiPON interface could be considerably high up to 5100 Ω cm2. For the NiO/LiPON interface, the highest value was 1400 Ω cm2. The high charge transfer resistance significantly hindered the ion transport, thus leading to irreversible Li insertion/extraction processes and lowered charge capacity in the electrode layers. The cycling reversibility and optical contrast for the complete EC device were compromised accordingly. The device using a 130 nm-thick LiPON ion conductor showed an optical modulation of 40% at 550 nm driven by −1.5 V (coloration) and 1 V (bleaching) with switching time of 30 s.