WO3 Applied in Dye-Sensitized Solar Cells (DSSCs)

Dye-sensitized solar cell (DSSC) is one of the most popular renewable energy equipment, which has been studied for its light weight, simple manufacturing process, low cost, transparency, good plasticity, and environmental friendliness. The photoanode is one of the main factors affecting the energy conversion efficiency of DSSCs. The efficiency of DSSC-based titanium dioxide (TiO2) is significantly limited by the high interface photogenerated electron-hole recombination rate and the low electron mobility of the TiO2 photoanode.

In order to reduce the recombination rate in DSSC, surface modification of TiO2 with wide band gap semiconductors has proven to be an effective method. Tungsten trioxide (WO3) is an n-type semiconductor with a small band gap of 2.6 eV, with unique thermal, optical, physical and chemical and electrical properties. These characteristics make WO3 a promising TiO2 coupling agent.Thus, WO3 has been applied in dye-sensitized solar cells to enhance its photocatalytic performance. The synthesis process of PT/WO3-incorporated TiO2 film is as follows:

H3PW12O40 doped TiO2 (PT) was prepared by the sol-gel method. In short, the H3PW12O40 aqueous solution was added dropwise to the isopropanol solution of titanium tetraisopropoxide. The resulting mixture was acidified with hydrochloric acid and stirred at room temperature. Then, the mixture is heated to form a uniform H3PW12O40/TiO2 hydrogel. Transfer the hydrogel to an autoclave and place it in an oven at 200°C for 1 hour. Finally, the hydrogel is dried and washed to obtain a white product. To synthesize WO3 nanoparticles, 2.0 g of polyvinylpyrrolidone (PVP K-30) was dissolved in 20 mL of deionized water. Then, mix 10 mL of 0.5 M ammonium metatungstate ((NH4)6W7O24·6H2O) solution into it, and then stir for 1 hour. After sonicating for 30 minutes, 5.0 g polyethylene glycol (PEG 1000) was added. The mixture was stirred for another 2 hours at room temperature and then dried in an oven at 60°C. Finally, the resulting solid was annealed in air at 500 °C for 1 hour at a heating rate of 2 °C min-1.

The PTA-TiO2 composite material is a mixture of PT and pure P25 in a mass ratio of 1:5. WO3 nanoparticles are added to the paste in mass ratios of 1:50, 1:70, and 1:100 (WO3:TiO2 powder). The PTA and WO3-incorporated TiO2 pastes were prepared by the planetary milling of 0.35 g PTA–TiO2 composite, a specific amount of WO3, 0.7 mL DI water, 0.5 mL acetyl acetone, 0.35 mL acetic acid, 0.15 g PEG 20,000-, and 0.35-mL Triton X-100 for 5 h. In each step, FTO glass is thoroughly sonicated in acetone, isopropanol, methanol, and deionized water for 15 minutes to remove contaminants. The TiO2 paste doped with PTA and WO3 was deposited on the FTO glass by using 3 M scotch tape as the coating guide and the doctor blade of the glass rod. Then, the film was calcined at 450°C for 30 minutes at a heating rate of 2°C min-1.

The sensitizer N719 is dissolved in pure ethanol at a concentration of 20 mg dye per 100 ml alcohol solution. After cooling the TiO2 electrode to 80°C, immerse it in the dye solution at room temperature for 24 hours. After sensitization, clean the electrode with absolute alcohol and dry it with a hair dryer. The Pt-covered counter electrode was placed on the TiO2 electrode and sealed with a 25-micron thick hot-melt gasket made of ionomer Surlyn 1702 (Dupont). A drop of electrolyte, that is, a solution of 0.3 M DMPII, 0.05 M I2, 0.5 M LiI, 0.5 M 4-tert-butylpyridine in acetonitrile, is dropped into the hole on the back of the counter electrode. The active unit area is about 0.25 cm2.

Dye-sensitized solar cells (DSSCs) based on phosphotungstic acid–TiO2 composites (PT) and WO3 nanoparticle incorporated TiO2 photoanodes were successfully constructed by doctor blading TiO2 paste containing PT and various amounts of WO3. The introduction of WO3 (WO3/TiO2 = 1/70) into the PT/P25 films could further favor electron transfer by depressing possible recombination, resulting in a higher short-circuit current density of 14.76 mAcm−2 and an energy conversion efficiencies of 4.94%.

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