Dual-Band Electrochromic Devices Utilizing Niobium Oxide Nanocrystal

Electrochromic window coatings that can modulate visible and near-infrared (NIR) light independently are important to the development of smart windows which can control solar heating and lighting to improve energy efficiency of buildings. Our work utilizes the electrochromism of monoclinic Nb₁₂O₂₉ nanocrystals to develop devices that can modulate the transmittance of NIR and visible light separately. The devices incorporate a secondary ion storage layer that allows for the charge and discharge of the electrochromic material, leading to reversible coloration. The configuration of the device consists of a working electrode composed the niobium oxide nanocrystals coated on a transparent conductive electrode, a non-aqueous gel electrolyte, and a counter electrode. The overall transmittance of the device can be modulated by applying different electrochemical potentials between the two electrodes. The dual-band electrochromic behavior of the niobium oxide nanocrystal allows for selective modulation of the NIR transmittance with low device potentials, along with significant visible modulation at higher potentials. In this work we study the performance trade-offs of varying film thicknesses, transparent conductive electrode materials, and ion storage layers. For optimum transmittance modulation, charge balance between the two electrodes is key. We compare the charge storage capacity and transmittance of NiO nanocrystals, CeO₂ nanocrystals, and amorphous VO_x thin films to determine optimum conditions for charge balance, allowing for full modulation of the electrochromic niobium oxide. We also show that these materials can be solution processed, without high temperature annealing steps, onto both rigid glass and flexible plastic substrates using blade coating. Optimal parameters for device assembly and overall performance will be discussed.