Self-Powered, Inkjet Printed Electrochromic Films on Flexible and Stretchable Substrate for Wearable Electronics Applications

Abstract

Electrochromic films have been used as a non-emissive material for display applications. Such materials have already been integrated in antiglare rearview mirrors for passenger vehicles as well as smart windows intended for energy savings for buildings. However, most electrochromic materials are deposited on rigid substrates, which prevent its use in flexible and stretchable electronic applications, where low temperature deposition techniques are desired. Additionally, electrochormics require an external power source to drive the underlying reduction/oxidation reaction. In this work, electrochromic materials inkjet-printed onto flexible and stretchable substrates have been explored. These devices are "self-powered" by organic solar cells also fabricated on flexible and stretchable substrate such as PDMS and PET. A set of inks based on a combination of synthesized and commercially obtained WO_3 nanoparticles, W-TiO_2 and TiO_2 nanoparticles were evaluated. The microstructure of the nanoparticles used in this study were examined under scanning electron microscopy for examining nanoparticle morphology, x-ray diffraction for chemical and structural characterization, and dynamic light scattering for particle size determination. Electrochromic layers were then ink-jet printed on flexible and stretchable PDMS substrates, using synthesized Ag nanowires as conductive, yet highly transparent electrodes. The stretchable printed electrochromic devices under various stress conditions and electrochromic performances were evaluated and demonstrated clear switching behavior under external bias, with 7 second coloration time, 8 second bleaching time, and 0.36-0.75 optical modulation at ?=525 nm. Cyclic voltammetry and galvanostatic charge/discharge measurements demonstrated high areal capacitance, with limited stability upon cycled operation. The electrochromic devices were then integrated in an Internet of Things (IoT)-enabled switching configuration, self-powered by PCDTBT:PC_70BM organic photovoltaics. The bulk heterojunction devices were evaluated with varying hole-transport layers and substrates, and exhibited the strongest performance of PCE? 3%, V_oc=0.9V and J_sc ? 10-15 mA/cm^2. The described self-powered, IoT-enabled, ink-jet printed electrochromic devices, fabricated on flexible substrates, are demonstrative of potential applications for wearable electronics.