# Titanium Dioxide in Chromogenic Devices: Synthesis, Toxicological Issues, and Fabrication Methods
## 5 TiO\({}_{2}\) in Photoelectrochemical (PEC) and Photovoltachromic Devices
Moreover, a thin transparent platinum layer was deposited on the counter electrode, acting as a catalyzer. Such device colored in open-circuit conditions and bleached in the dark, if short-circuited. A modulation of 41% was observed in the full irradiation of devices (1 sun), which took place in a couple of minutes. Another PEC with a \"separated architecture\" was proposed by [PERSON] et al. [191], using poly-(3,4-ethylenedioxythiophene) (PEDOT) as a cathodic EC material; the authors observed a CE of 280 cm\({}^{2}\)/C and a transmittance modulation of 22% at 630 nm. [PERSON] et al. [192] scaled up their PECs (with combined architecture) to dimensions of (\(10\times 10\)) cm\({}^{2}\). A solid, ornolyte-based lithium ion conductor was used, combining the electrolytic properties of organic polymers to a strong, inorganic structure; the modulations of visible transmittance reached 63% (Figure 5, right side). A significant step forward in the design of TiO\({}_{2}\)-based PECs was proposed by [PERSON] et al. [193], who proposed the first photovoltachromic device, embodying a dye-sensitized frame-type TiO\({}_{2}\) photoanode. The center of the device was transparent and acted as a smart window, changing color depending on the available external irradiance. A photovoltachromic device not only undergoes smart transmittance modulation, like PEC devices, but also provides photovoltaic conversion of solar energy. The fast coloration time observed (as low as 4 s) was accelerated by the platinum catalyst deposited on the counter electrode. This device also allowed for energy harvesting to be delivered to the electric grid. A significant boost in photovoltaic conversion efficiency was reported in 2011 [194], with a photovoltachromic device that reached a fair conversion efficiency (6.55%); the counter electrode was specially designed to allow for a separation of the device features (photovoltaic and PEC), with many circuitries devoted to smart coloration and electricity generation, respectively. Other works appeared, with the same approach, to demonstrate not only the electro-optical figures of merit of such devices, but also the consequent benefits in terms of energy yield and visual comfort [195; 196]. A systematic work, dealing with the main figures of merit of PEC devices containing dye-sensitized TiO\({}_{2}\) photoelectrodes, has been reported by the research group of [PERSON] et al. [197; 198]. They also investigated the properties of devices containing poly(methyl methacrylate)-based gel electrolytes [199] and the long-term performance of partly covered devices [200]. A new class of solar cells, based on mixed organic-inorganic halide perovskites [201] has held some significance since 2012. The self-assembly of CH\({}_{3}\)NH\({}_{3}\)PbBr\({}_{3}\) perovskite, within the nanoporous TiO\({}_{2}\) typically used in dye-sensitized cells, had already been studied by [PERSON] et al. [202], who reported a conversion efficiency of 2.19%. In these devices, perovskite acts as a light harvester, replacing the role of dyes in Gratzel cells. Later, it was observed that organic-inorganic halide perovskites could conduct both electrons and holes; this point led to further evolution of perovskite-based PVs towards more simple, planar perovskite solar cells [203; 204]. The role  
Figure 8: Cross-section of a typical photoelectrochromic (PEC) device with “separated architecture” (**left**). Coloration in open circuit (**upper part**) and bleaching in short circuit (**lower part**) of PEC device with “combined architecture” (**right**). of mesoporous TiO\({}_{2}\), as reported by [PERSON] et al. (2015), was to assists in the collection of charge carriers, as demonstrated by the efficiency of these cells, reaching 22% in 2016. On the other hand, it was 15.6% in planar cells. Worldwide, perovskite-based technologies for PVs have attracted the attention of researchers and manufacturers, with the aim of a fast increase of the figures of merit and the commercialization of stable, large-area solar modules without relevant forms of hysteresis.