AA-CVD and ALD of Vanadium Pentoxide Thin Films for Electrochromic Applications

Despite the high potential impact of vanadium based electrochromic devices, there are still many issues to be resolved before widespread market implementation can be realised. The current limitations arise from cost, the slow electrochemical kinetics and the poor cycling stability of vanadium pentoxide.

  In this work we initially employed aerosol assisted chemical vapor deposition (AACVD)  at 400°C in combination with a 600°C  anneal in air to grow films on fluorine doped tin dioxide (FTO) coated glass. The aerosol was generated from a methanolic solution of vanadyl (IV) acetlyacetonate using an ultrasonic humidifier and a nitrogen carrier gas. All annealed films were found to be monoclinic vanadium pentoxide but only the thicker films demonstrated a viable electrochemical response. The lack of response from the thinner films was proposed to be due to either the formation of an unstable interfacial oxide or non-uniform growth due to the aerosol method.  To determine the reasons for this limitation two experiments were performed, firstly the addition of a sub-nm alumina barrier layer by atomic layer deposition between the FTO and the V₂O₅ to inhibit the formation of any interfacial mixed oxide; Secondly, the low temperature growth of V₂O₅via atomic layer deposition using vanadyl triisopropoxide and oxygen as reagents.

  Initial indications are that the inclusion of an alumina barrier layer does not improve the uniformity of the thinner AACVD films. The ALD grown V₂O₅ films however demonstrated significant improvements in uniformity even at very high optical transparencies. These observations are discussed with reference to electron microscopy, UV-visible spectroscopy, XRD and the samples’ electrochemical responses.