# Titanium Dioxide in Chromogenic Devices: Synthesis, Toxicological Issues, and Fabrication Methods
## 5 TiO\({}_{2}\) in Photoelectrochemical (PEC) and Photovoltachromic Devices
In this roadmap, [PERSON] et al. (2016) demonstrated that perovskite solar cells could be fabricated at room temperature with a significant reduction in the fabrication steps. They also demonstrated that a customized de-wetting of CH\({}_{3}\)NH\({}_{3}\)PbI\({}_{3\times}\)Cl\({}_{x}\) perovskite after spin-coating may lead to the formation of micro-islands, partially covering the surface to increase the device transparency. In their planar device, a compact TiO\({}_{2}\) layer and a p-type spiro-OMeTAD hole transporter were embodied (206,207). Such devices allowed a further step forward in the design of photovoltaic architectures. A multifunctional device, including a semi-transparent perovskite-based PV film, was reported in 2015. It was coupled with an EC material to generate a complex device (Figure 9). The first electrode hosted the semi-transparent PV film (compact TiO\({}_{2}\) was obtained by spin-coating a mildly acidic solution of titanium isopropoxide in ethanol) on one side but was also coated with indium tin oxide (ITO) on the other. The latter acted as one of the electrodes of the EC part of this architecture. The second electrode hosted the cathodic EC material and was \"glued\" to the first one using a specially designed quasi-solid polymer electrolyte (PEG-plasticized PEO containing lithium iodide) (208). The PV film supplied electric power, which could drive transmittance modulation until the transient coloration process was complete. Two external circuits connected the photo-anode of the solar cell to the EC electrode and the PV cathode to the secondary electrode of the EC device. [PERSON] et al. (2019) scaled up a solid-state device to 30 \(\times\) 30 cm\({}^{2}\), which included a dye-sensitized TiO\({}_{2}\) layer, WO\({}_{3}\) as the EC material, and a solid ornolyte electrolyte based on omrosilane within a PEC, thus demonstrating a \"combined architecture\". All the layers were fabricated by the sol-gel technique and deposited by dip-coating. The T\({}_{\text{vis}}\) modulated spontaneously from 76% to 35% in 15 min under 0.75 sun illumination (750 W/m\({}^{2}\)). [PERSON] et al. (2019) investigated the applicability of salicylic acid derivatives as suitable dyes for TiO\({}_{2}\) films in PEC devices instead of the typical ruthenium-based sensitizers. They obtained the best results using 5-methylsalicylic acid, observing an optical modulation of 40.8% at 550 nm (Table 2). Figure 9: Axonometric view of a perovskite-based photovoltaic device. Transparent conductive oxides (TCO), solid-state polymer electrolyte (SPE) (**a**). Pictures of the device in bleached (**b**) and colored (**c**) states. Reproduced by permission of The Royal Society of Chemistry (208). \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Reference** & **Fabrication Techniques for TiO2** & **Role of TiO2** & **Main Figures of Merit** \\ \hline [PERSON] et al. [159] & Hydrothermal synthesis assisted by microwave irradiation. & Electrochromic material & AR of 57\%, 9\% and 22\% between colored and bleached states, at 250, 550 and 850 nm \\ \hline \multirow{2}{*}{[PERSON] et al. [166]} & Two step process by combining chemical & & \\  & bath deposition and nebulized & Electrochromic material & \(\Delta\)T of 78\%, CE of 128.3 cm\({}^{2}\)/C \\  & spray deposition & & \\ \hline \multirow{2}{*}{[PERSON] et al. [168]} & Spin-coated nanocrystalline films modified with the viologen from an aqueous solution & Electrochromic material & The absorbance difference at 608 nm between the bleached and colored state of meso-nc TiO2 EC cell was about 0.43. \\ \hline [PERSON] et al. [170] & Surfactant-assisted nonaqueous sol–gel route & Electrochromic material & CE of 130 cm\({}^{2}\)/C \\  & & sol–gel route & at 800 nm; coloration time of 5 s \\ \hline [PERSON] et al. [171] & One dimensional brookite TiO2 & & \\  & nanoneedles coated on glass substrates & Electrochromic material & \(\Delta\)T of 67\% and \(\Delta\)OD of 0.85 \\ \hline [PERSON] et al.