Laser Processing of TiO2 films on ITO-glass for Dye-sensitized Solar Cells

UoM administered thesis: Phd

  • Authors:
  • Aseel Hadi

Abstract

Mesoporous TiO2 thin film has been considered as a benchmark material in the applications of dye sensitised solar cells (DSSCs) due to a combination of the physical properties that are inherent to the metal oxide and its particular structuring, in addition to its chemical stability and commercial availability. For DSSCs, a more important functionality of mesoporous TiO2 thin films is their extremely high surface and internal surface areas, resulting in high adsorption of dye molecules. However, a major drawback of fabrication of mesoporous TiO2 thin films is its high-temperature furnace sintering at 450C-500C for 30 min. The high-temperature process prevents the possibility of integrating different electro-optical devices on the same substrate, and the sintering time required would be a hurdle for potentially rapid manufacturing of mesoporous metal oxide thin films for DSSCs. This thesis demonstrates for the first time the use of a fibre laser with a wavelength of 1070 nm and a pulse width of milliseconds for generation of 1) mesoporous nanocrystalline (nc) TiO2 thin films on ITO coated glass, and 2) compact TiO2 layer and mesoporous TiO2 film on ITO coated glass. The first one was achieved by complete vaporisation of organic binder and inter-connection of TiO2 nanoparticles; and the second one was achieved by full crystallisation of TiO2 precursor to form the compact TiO2 layer and the same sintering process described above. Both processes were one-step, and achieved by stationary laser beam irradiation of 1 minute, compared with 30 min for furnace-sintering to form a mesoporous TiO2 film, and 2 h for two-step furnace treatment to form compact layer and mesoporous film on ITO glass. No thermally damaging of the ITO layers and the glass substrates was observed. The DSSC with the laser-sintered TiO2 photoanode at the optimised laser processing condition of 85 W/cm2 and 100 ms/50 ms pulse mode reached higher power conversion efficiency (PCE) of 3.20% for the TiO2 film thickness of 6 m compared with 2.99% for the furnace-sintered; the DSSC with the laser-treated compact TiO2 layer and mesoporous TiO2 film on ITO glass at the optimised laser treatment condition of 85 W/cm2 and 125 ms/25 ms, reached 5.76% compared to 4.83% with the furnace-treated. Electrochemical impedance spectroscopy (EIS) studies revealed that the laser sintering effectively decreased charge transfer resistance and increased electron lifetime of the TiO2 thin films. It is believed that the use of the fibre laser with over 40% wall-plug efficiency offers an economically-feasible, industrial viable solution to the major challenge of rapid fabrication of large scale, mass production of mesoporous metal oxide thin film based solar energy systems, potentially for perovskite and monolithic tandem solar cells, in the future. Another part of the thesis presents a detailed investigation on the improvement of photovoltaic performance of furnace-sintered TiO2 films on ITO-coated glass using an excimer laser with a wavelength of 248 nm and possesses a rectangular beam profile and has a full width at half maximum (FWHM) pulse duration of 13-20 ns. This was achieved by modifying the surface of the furnace-sintered TiO2 films to increase the roughness, which led to increased optical absorbance via light-trapping. The laser process was carried out with variation of laser fluence and number of pulses per unit area. Under the optimised laser fluence of 34 mJ/cm2 and number of pulses of 50, the DSSC with the laser-modified TiO2 photoanode showed a high power conversion efficiency of 2.99% than 2.10% without the laser treatment. EIS studies showed that the films modified under the optimised laser parameter effectively decreased charge transfer resistance and increased electron lifetime of the TiO2 thin films.

Details

Original languageEnglish
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Award date1 Aug 2018