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

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 1S e level filling was observed, while the smallest studied QDs (2.8 nm) exhibited only a partial 1S e level population. In addition, 4.5 and 6.3 nm QDs also showed partial 1P e level filling with electrons. The data from both cyclic voltammetry measurements and electrochemically driven spectral bleaching enabled determining the electrochemically derived 1S e 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.