Wide Dynamic Range in Tunable Electrochromic Bragg Stacks from Doped Semiconductor Nanocrystals

Abstract

Electrochromic properties can be enhanced by constructing photonic architectures, in which the reflectance contributes to optical modulation along with the intrinsic dynamic absorptivity of the material. However, optimization of reflectance is challenging without a rational design approach. Here, electrochemically tunable Bragg reflectors are demonstrated that are tuned to be highly transparent in the "off" state, achieving synergistic dynamic optical modulation of absorption and reflection in the visible and near-infrared range. These Bragg stacks are composed of alternating doped semiconductor nanocrystal (NC) layers of 5 nm sized oxygen vacancy-doped WO 3-x and 15 nm sized 0.4 atomic% Sn:In 2 O 3 (ITO) NCs. Combining judicious NC selection and processing optimization with guidance from optical simulations, optimized Bragg stacks are implemented for electrochromic window applications. NCs with high absorption coefficients are essential for strong transmission modulation, though this characteristic limits the dynamic range of the Bragg reflectance. Optimal reflectance modulation including a highly transparent "off" state is confirmed with in situ reflectance and transmittance measurement. More broadly, ligand-stripped NCs can enable to fabrication of complex device architectures on low-cost flexible substrates. These results guide the design rules for accessing different types of doped semiconductor NC-based tunable Bragg stacks, an exemplary photonic structure, over a broad wavelength range.

Advances in Energy‐Efficient Plasmonic Electrochromic Smart Windows Based on Metal Oxide Nanocrystals

“…In EC applications, ITO NCs prepared by the organometallic method are often sintered at high temperatures to remove organic insulating ligands for high conductivity. [67,97] The ligand treatment has a certain effect on the plasmonic EC property of ITO NCs. However, the general thermal decomposition of these organic ligands in an inert environment will leave residual carbon impurities on the film, thus reducing the transparency of the film.…”

Effect of Ligand Treatment on the Tuning of Infrared Plasmonic Indium Tin Oxide Nanocrystal Electrochromic Devices

“…The heat radiation transmittance control was obtained in this composite by indium tin oxide (ITO) nanocrystals exhibiting a widely tunable localized surface plasmon resonance (LSPR). While metal oxide nanocrystals with plasmonic resonances represent an active field of present research, there are only a few reports on electrochromic device performance based on such materials . The device performance will depend not only on the nanocrystals plasmonic properties, but also on the morphology of the electrodes fabricated from the nanocrystals .…”

Electro-responsivity in electrolyte-free and solution processed Bragg stacks

“…The tunability of structural colours in photonic crystals can be exploited to build-up optical active components that can be utilised in a wide range of applications, such as displays, 1,2 colour-changing inks 3,4 , smart windows, 5 optical cavities, [6][7][8][9] solar cells 10 and sensors, [11][12][13][14] among others. In brief, photonic crystals (PhCs) are periodic structures in which there is a repetition of materials with different refractive indices along one, two or three spatial dimensions.…”

“…24,[35][36][37][38] However, most of the recent works in the field of electrochromic Bragg mirrors (ECBM) rely on the use of liquid electrolytes that can pose issues in terms of chemical and environmental stability. 5,[42][43][44] Among the various degrees of freedom to be exploited to engineer the PhC's optical response, electrochemical doping of plasmonic materials represents a promising approach to modulate the refractive index contrast and, hence, the structural colour without the need to replace the constituent media. 39 For this purpose, the use of heavily doped metal oxide nanoparticles with plasmonic response in the near infrared (NIR) part of the spectrum is of great interest, since the sensibly lower carrier density in these materials compared with classical bulk metals (10 21 cm -3 vs. 10 23 cm -3 ) 40,41 allows for a fine control over their dielectric function by means of electro-optical doping.…”