Electrophoretically-Deposited CdSe Quantum Dot Films for Electrochromic Displays and Smart Windows

Abstract

Electrophoretically deposited (EPD) quantum dots (QDs) can be charged electrochemically via electron injection from a conducting substrate, leading to pronounced changes in their electrical and optical properties. The 180–550 nm thick EPD films composed of CdSe QDs with different diameters (2.8–6.3 nm) demonstrate a strong and reversible electrochromic response due to bleaching of excitonic transitions. The number of injected electrons was found to increase with QD size from 1.3 (QD diameter of 2.8 nm) to 6 (QD diameter of 6.3 nm) electrons per nanoparticle. As a result for 3.4 nm, 4.5 nm, and 6.3 nm QDs a complete 1Se level filling was observed, while the smallest studied QDs (2.8 nm) exhibited only a partial 1Se level population. In addition, 4.5 and 6.3 nm QDs also showed partial 1Pe level filling with electrons. The data from both cyclic voltammetry measurements and electrochemically driven spectral bleaching enabled determining the electrochemically derived 1Se level energies. Additionally, we demonstrate fast charging–discharging kinetics for EPD CdSe QD films with complete absorption bleaching and recovery in a sub-100 ms time scale, which opens prospects for utilizing such films in various applications such as electrochromic displays, smart windows or tunable color filters for photography.

Waveguide-Based Spectroelectrochemical Characterization of Band Edge Energies in Submonolayers of CdSe Quantum Dots Tethered to Indium–Tin Oxide Electrodes

“…This estimated range of electrons injected per QD is consistent with the estimates of Houtepen and coworkers for low coverage QD films, and more recently, the electrochromic studies of Aniskevich et al, but lower than studies by Houtepen and coworkers for multilayer QD films where five electrons or more appear to be injected per QD, limited by void size in the QD film . Other studies have reported that up to 50–150 electrons per QD can be injected by either chemical or electrochemical reduction in the solution. ,,,, If 50–150 electrons per QD were transferred during voltammetric reduction, however, due to charge injection into surface traps or injection leading to absorbance bleaching, the Faradaic peak currents and the charge density in Figure S8 would have been correspondingly 50–150× larger.…”

“…200 meV of the difference in E CB between the two measurement approaches arises from the stabilization of the one-electron reduced state of the CdSe QD film by the Li + counter ion (see Figure and associated discussion below); and (ii) the remaining difference is attributed to stabilization of the reduced CdSe QD by the near-surface environment, specifically the ODiPA/ITO interfacial region (see discussion in the text). In addition, plotted in (A) and (B) are E CB estimates obtained from spectroelectrochemical studies of adsorbed or electrophoretically deposited monolayer/multilayers of CdSe QDs (Spittel et alref ; Aniskevich et alref ) and CdSe QD solutions (Ashokan and Mulvaneyref ), demonstrating the range over which estimated E CB values can vary.…”

“…19,30,79,80,90 Interestingly, the spectroelectrochemical data published by other research groups, plotted in Figure 5, show substantial variability and E CB values closer to vacuum compared to our data, but farther from vacuum relative to the E CB values estimated from our UPS studies. 12,24,26 Some of these discrepancies may arise from how their energy axes were corrected to vacuum, which are somewhat different than our procedure for calibrating the pseudo-reference electrode and converting to the vacuum scale. 75 In the studies by Ashokan and Mulvaney, 26 the electrochemistry/spectroelectrochemistry of suspended QDs was probed [in a low dielectric constant solvent (tetrahydrofuran) with a large cation supporting electrolyte, tetrabutylammonium hexafluorophosphate (TBAPF 6 ) at 0.1 M concentration], which suggests that the differences in their E CB values relative to those estimated from UPS represent the effect of capping ligands and possibly the solvent/supporting electrolyte, that is, there was no effect from immobilization at an electrode surface.…”

“…Assuming that the vacuum levels for both the UPS and spectroelectrochemistry experiments are the same, we infer that (i) up to ca. 200 meV of the difference in E CB between the two measurement approaches arises from the stabilization of the one-electron reduced state of the CdSe QD film by the Li + counter ion (see Figure 6 electrophoretically deposited multilayer QD films by Aniskevich et al 12 These data are incorporated into our assessments of different experimental conditions on estimated E CB values discussed below.…”

“…Semiconductor nanocrystals are increasingly used as prototypical photocatalysts for the production of fuels (e.g., H 2 ) from sunlight, 1−3 as a component of the active layers in thin film photovoltaics 4−8 and in light emitting diodes, 9,10 as components of the semiconducting layer in thin film fieldeffect transistors, 11 and more recently as the active layer in electrochromic "smart windows". 12 Understanding how the band edge energetics of these materials are controlled by tethering the nanocrystal at charge injecting/charge harvesting interfaces is critical to optimizing their performance in these platforms. Quantum dots (QDs) based on semiconductor materials such as CdS, CdSe, PbS, and PbSe are among the most synthetically accessible of these nanocrystalline materials and enable composition-and size-dependent tunability of electronic properties and absorption and emission from the UV through the mid-IR region of the electromagnetic spectrum.…”