Films of directionally oriented carbon nanotubes as counter electrodes for electrochromic devices

Abstract

Connected displays are the salient nature of our e-connected society requiring high-quality electronic conductive materials integrated in smart devices. Herein, we highlight the use of carbon nanotubes (CNTs) not only as substitutes for ITO (In2O3:Sn) but mainly as a counter electrode for electrochromic devices (ECDs). Using a dry spinning process, conductive CNT films with directionally oriented tubes were successfully laid on polyethylene terephthalate (PET) flexible substrates. The CNTs were synthesized as vertically aligned spinnable arrays by chemical vapor deposition. The transparency and optical absorption properties were tuned by adjusting the number of CNT layers (L) within the film from light gray for 1L to dark black above 10L. The CNT films, showing pseudo-capacitive to capacitive behavior in lithium-based electrolytes, were further integrated in ECDs based on poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) active layers. The fabricated PET/ITO/PEDOT:PSS/electrolyte/CNT/PET device exhibited reproducible and clear color switching from light to dark blue with a coloration efficiency of 88 cm2 C−1. This study highlights novel architecture of ECDs using CNT-based films as conductive counter electrode.