# Titanium Dioxide in Chromogenic Devices: Synthesis, Toxicological Issues, and Fabrication Methods
## 5 TiO\({}_{2}\) in Photoelectrochemical (PEC) and Photovoltachromic Devices  
In contrast to EC devices, PEC devices [178] are capable of changing their transmittance properties upon light absorption and do not require any external voltage to produce such effects. The activating stimulus, for PECs, is the impinging light (as in photochromic films) but, in this case, the device absorbs light and generates a sufficient driving force capable of activating the transmittance modulation in an EC material, as described hereafter. Unlike photochromic molecules [150; 179; 180], which are widely used in solar control lenses where one material hosts light detection and coloration at the same time, the two processes require distinct materials within PEC devices. The first coupling of a TiO\({}_{2}\)-based ruthenium polypiridine-sensitized nanocrystalline electrode (acting as a photoanode) with a WO\({}_{3}\) EC counter electrode was proposed by [PERSON] et al. in 1996 [181]. In this device architecture, the dye-sensitized TiO\({}_{2}\) played the same role as it does in Gratzel cells [182; 183; 184], producing the suitable photovoltage required to color the EC cathodic material deposited on the counter electrode. In fact, TiO\({}_{2}\) indeed plays a different role in PECs, compared to EC devices. In this case, it is of fundamental importance for it to harvest solar energy and convert it into electricity in order to guarantee the operation of a PEC device. Because TiO\({}_{2}\) plays the role of a photoanode, devoted to the photovoltaic generation of electric charge, coloration is generally due to other EC materials, such as WO\({}_{3}\) or poly-(3_A_-ethylenedioxythiophene) (PEDOT), which require relatively low photovoltage to undergo full transmittance modulation. This first PEC device, if exposed to sunlight in short-circuit conditions, undergoes a sensible modulation of transmittance because photogenerated electrons are free to move towards the counter electrode via an external circuit. The latter electrode, negatively charged, activates intercalation of small cations from a liquid electrolyte. This phenomenon is then very similar to the intercalation process typically observed in EC devices, resulting in a reversible coloration of WO\({}_{3}\). The physical separation between photogeneration of electrons (on the TiO\({}_{2}\)-based photoanode) and electron/ion insertion (on the EC counter electrode) is the reason why [PERSON]'s device is considered a \"separated\" architecture for PEC devices (Figure 8, left side). The two electrode substrates are effectively sealed using a Surlyn thermoplastic, leaving an empty space to be filled by means of a liquid electrolyte; generally, this is a solution of iodine ions in propylene carbonate. The ions are used to reduce the dye after its oxidation due to electron generation. Furthermore, a study conducted by [PERSON] [185], dealing with the same \"separated architecture\" for PECs, explained that the I\({}_{2}\) typically used in the electrolytes of dye-sensitized cells is decreased in order to maximize both the photovoltage and the transparency of devices. Subsequently, [PERSON] et al. [186] used a polyaniline counter electrode instead of the cathodic WO\({}_{3}\). Because the former material shows an anodic behavior, the device was colored in the initial conditions and underwent bleaching if irradiated, in short-circuit conditions. [PERSON] et al. [187] utilized an organic dye for a TiO\({}_{2}\) photo-electrode; in that case, the EC material adopted was poly(3,3-diethyl-3.4-dihydro-2_H_-thieno-[3,4-b][1,4]dioxepine) (PProDot-Et\({}_{2}\)). An abrupt change in PEC architectures was proposed by a research group from the German Fraunhofer Institute, who deposited, for the first time, both TiO\({}_{2}\) and the EC layer on the same electrode in a \"combined architecture\" where PV and EC materials shared the same substrate, which had some interesting effects on the device operation [188; 189; 190] (Figure 8 right side).