(Invited) (NH₄)₂WS₄precursors for Electrochromic Devices

In this research, we investigate the effect of metal doping on the electrochromic (EC) performance of tungsten trioxide (WO 3 ) films. These films were prepared by a novel method involving solution processing and thermal annealing. In this procedure, ammonium tetrathiotungstate ((NH 4 ) 2 WS 4 ) was dissolved in dimethylformamide at a high concentration ratio (200 mg/mL) to obtain a homogeneous solution and then spin-coated onto the indium thin oxide (ITO) substrate for use as a working electrode. Subsequently, the film was annealed at the different temperatures (200, 300, 400, and 500 o C) to form a crystal structure of WO 3 . X-ray diffraction, Raman, and X-ray photoelectron spectroscopic results confirm the crystal formation of WO 3 . Moreover, in order to improve the electrochromic performance, different concentrations (10, 20, 30, and 40 mM) of different metal chlorides such as PtCl 4 , PdCl 2 , AuCl 3 , AgCl, CuCl 2 , NiCl 2 , NaCl, and KCl were added into the (NH 4 ) 2 WS 4 precursor and the mixture was coated on the ITO substrate and annealed at 500 o C. The results indicate that the performance of the Au-doped WO 3 film was better than those with other dopants, likely as a result of the plasmonic effect. Furthermore, nanocrystal quantum dots (QDs) provide tunable optoelectronic properties on the basis of their dimension. CdSe QDs, which are size-dependent colloidal nanocrystals, are used for efficient electrochromic devices owing to their unique properties in modulating quantum confinement, resulting in enhanced electron insertion during the electrochromic process. Incorporating a well-known metal oxide electrochromic material such as WO 3 into CdSe QDs enhances the redox process. Herein, we propose a facile method for producing and optimizing CdSe QDs doped in WO 3 . The fabrication of the electrochromic film involves a solution and annealing process. Moreover, the effect of the QD size to optimize the electrochromic layer is studied. As a result, the coloration efficiency of WO 3 and optimized CdSe QD-WO 3 are obtained as 68.6 and 112.3 cm 2 /C, respectively. Thus, size-tunable nanocrystal QDs combined with a metal oxide yield high-performance electrochromic devices and are promising candidates for producing smart windows. Acknowledgement This research was supported in part by a Creative Materials Discovery Program through the NRF funded by Ministry of Science and ICT (2017M3D1A1039379) and in part by the Basic Research Laboratory of the NRF funded by the Korean government (2018R1A4A1022647).