Assessment of the long-term performance of partly covered photoelectrochromic devices under insolation and in storage

This work comprises an experimental study of the long-term performance and stability of partly covered photoelectrochromic (PEC) devices under various operating conditions. The most durable PEC device was tested for nearly one year in open circuit storage in the dark exhibiting a 26% reduction in the power conversion efficiency of the photovoltaic (PV) unit. As regards optical performance, it was found to gradually converge to a contrast ratio of about 1.5:1, having passed through a transitional period of about 80 days, that caused a decrease in the coloration depth, without affecting reversibility. This gradual loss of coloration depth was attributed to the reaction of photoelectrons with trliodide ions at the electrochromic WO3 film-electrolyte interface, that takes place prior to the formation of the colored LixWO3 tungsten bronze, and is enhanced while the liquid electrolyte wets the WO3 film. This phenomenon was also observed in redox type electrochromic devices, verifying the proposed loss mechanism. PEC devices were also exposed to insolation in short (SC) and open (OC) circuit conditions. The PEC device tested in SC operation for more than four months, exhibited a contrast ratio comparable to that of the devices kept in storage, although an 80% reduction in the power conversion efficiency of the PV element has been observed. On the other hand, OC operation of 3 months did not affect the performance of the PV element, but, resulted in irreversible coloration of the device tested, due to trapping of Li ions within the WO3 matrix. Having identified the causes for the observed adverse phenomena, remedies can be sought, such as thin film barrier layers to prevent the loss of photoelectrons to the electrolyte, modified electrolytes to enhance the PV element stability, avoidance of prolonged OC operation to prevent irreversible coloration. Indeed, a 60 nm thick ZnS barrier deposited on top of the WO3 film was found to effectively suppress the loss current reactions, doubling the contrast ratio of the stabilized PEC devices to 2.8:1, at the expense of coloration speed.