Detrimental Effects of Oxygen Vacancies in Electrochromic Molybdenum Oxide

Using the small polaron model, we show clearly that oxygen vacancy defects are detrimental to the electrochromic responses of molybdenum oxide (MoO 3 ). The observed phenomenon is explained by studying the oxidation states of the molybdenum (Mo) metal, which is central to the site specific small polaron model. First, we used the small polaron pair to explain the red-shift between the absorption peak induced by the vacancy defects and the inserted lithium (Li). Next, we show that any Mo 5+ defects results in either a poor cathodic optical pair, or forms a site that is inactive for Li insertion. The main reason for the latter is due to the inability to generate Mo 4+ in the reported optimal potential range. Finally, if the oxide starts with any intrinsic Mo 4+ defect, we provided evidence to show that Mo 6+ –Mo 4+ remains the only possible site for Li insertion, thereby greatly reducing available active sites. This optical modulation from Li insertion into this aforementioned pair is also low when compared with the Mo 6+ –Mo 6+ pair. We therefore conclude that the most effective modulation is achieved by the Mo 6+ –Mo 6+ pair and defects creation will be detrimental to the electrochromic performance of MoO 3 .