Creation of oxygen vacancies to activate WO ₃ for higher efficiency dye-sensitized solar cells

In this work, oxygen vacancies were created to activate tungsten trioxide (WO ₃ ) as a highly efficient counter electrode (CE) in dye-sensitized solar cells (DSSCs). The levels of oxygen vacancies (OVs) in WO ₃ were finely formed and tuned by doping with different weight percentages (3, 5, and 9 wt%) of urea and annealing in a N ₂ environment at 470 °C. The urea doped WO ₃ significantly improved the electrocatalytic behaviour in the iodide-triiodide electrolyte. The effects of OVs on the catalytic performance of WO ₃ CEs were fully studied and understood. This improvement was attributed to the introduction of OVs into WO ₃ , which acted as surface shallow states to facilitate electron transfer from the WO ₃ counter electrode to the electrolyte. At a high temperature of 470 °C, urea decomposes into the reactive H ₂ gas that can remove oxygen atoms from the WO ₃ surface and create OVs. 5 wt% urea was found to be the optimal urea concentration that led to the highest catalytic performance as evidenced by the cyclic voltammetry measurements. The 5 wt% urea doped WO ₃ CE-based device achieved a power conversion efficiency (PCE) of ∼10.5%, which was improved from the reference Pt CE-based device at ∼9.3% and non-active WO ₃ CE based cell at 3.32%. This has led to the optimal number of OVs that facilitate the charge transfer at the CE/electrolyte interface.