Application of sandwich-type polyoxometalates modified TiO<sub>2</sub> in dye-sensitized solar cells

Abstract

<p indent=0mm>Nowadays, the rapid consumption of fossil energy has led to global energy shortage. At present, more than 80% of the energy consumed globally is derived from non-renewable fossil fuels such as coal, oil and natural gas. The burning of fossil fuels will also bring about a series of problems such as the greenhouse effect, the falling quality of global air, the formation of acid rain and the pollution of water bodies. Therefore, whether new energy sources that can replace fossil energy can be found has become an increasingly popular concern problem. Dye-sensitized solar cells (DSSCs) as a new type of renewable energy source have attracted wide attention in recent years, because of their simple fabrication process, low production costs, relatively high photoelectric conversion efficiency (PCE), and being environmentally safe. But the recombinations of photogenerated electrons holes are seriously affected the photoelectric conversion efficiency of DSSCs. Polyoxometalates (POMs) are a class of metal oxide clusters with the highest oxidation state of tungsten, molybdenum or vanadium. Due to their structural diversity and unique physicochemical properties, they have been applied in catalysis, photochemistry, biology and medicine. However, due to POMs have excellent photoelectrochemical properties, which can be used as an excellent electronic extractant in the photoanode of DSSCs. Here, the sandwich-type polyoxometalates compound K₁₅{K₃[(A-α-PW₉O₃₄)₂Fe₂(C₂O₄)₂]}·29H₂O with the intramolecular electron transfer properties is composited with TiO₂ by sol-gel method to prepare a POM@TiO₂ composite photoanode. The energy band structure of the polyoxometalate is studied by optical and electrochemical measurements. The result shows that the LUMO energy level <sc>(-0.06 V)</sc> of the sandwich-type polyoxometalates is lower than that of TiO₂ and the optical band gap is <sc>2.28 eV.</sc> The performance test shows that the DSSC efficiency of POM@TiO₂ composite photoanode production reached 6.33%, which is 15% higher than that of battery whose anode is pure TiO₂ (5.52%). Electrochemical impedance spectroscopy (EIS), dark current tests and voltage decay curve tests demonstrate that sandwich-type POMs effectively suppress electron-hole recombinations and increase electron lifetime. The incident monochromatic photon-electron conversion efficiency (IPCE) test further demonstrates that the introduction of the POMs increases the monochromatic conversion efficiency (from 35% to 53%).