MICROWAVE ASSISTED SYNTHESIS OF ZNO AND TIO2 PHOTO-ELECTRODE FOR DYE SENSITIZED SOLAR CELL
ABSTRACT
Dye sensitized solar cells (DSSCs) have been under extensive research. Since the color of the device can be easily varied by choosing different dyes and cells on flexible substrates, DSSCs are especially attractive for Building Integrated Photovoltaics (BIPV). The cell concept is believed to reduce the production costs and energy payback time to a large extent compared to standard silicon cells or other thin film cells. Dye sensitizer absorbs the incident sunlight and exploits the light energy to induce vectorial electron transfer reaction. DSSCs differ from conventional semiconductor devices in that they separate the function of light absorption from charge carrier transport. Thus DSSCs have the following advantages compared with the silicon based photovoltaics. (1) It is not sensitive to the defects in semiconductors such as defects in Si. (2) The semiconductor-electrolyte interface (SEI) is easy to form and also it is cost effective for production. (3) It is possible to realize the direct energy transfer from photons to chemical energy. Nanocrystalline dyed sensitized solar cells are photoelectrochemical cells based on principles similar to the processes in natural photosynthesis. Both use organic dye to absorb the incoming light and produce excited electrons. If a semiconductor such as zinc oxides (ZnO) or titanium dioxide (TiO2) is irradiated with light greater than the band gap energy, excited electron–hole pairs are generated that can be utilized in many applications such as solar cells to generate electricity, chemical processes to create or degrade specific compounds, or in producing self cleaning surfaces using ZnO and TiO2 as photo-electrode to fabricate dye sensitized solar cell (DSSC). ZnO and TiO2 nanocrystalline filmsr were synthesized on FTO glass substrates from Zn(NO3)2.6H2O complex and TiCl4 respectively using microwave assisted chemical bath deposition method at various time intervals 0.5 min, 1.0 min, 1.5 min and 2.0 min. DSSC were fabricated using synthesized ZnO and TiO2 photo-anode as photo-electrode respectively, graphite as a counter, agar and galatin dye prepared by sol gel method as electrolyte/dye. Characterization techniques such as X-ray diffraction (XRD), UV-vis-infrared spectroscopy, scanning electron microscopy (SEM), and solar simulation were utilized for structural, morphological, optical and I-V characteristic studies of the coated nanocrystalline films and the DSSCs developed.
The morphology of the films shows that they were homogeneous and porous. The average crystalline size estimated from the most significant and intense peaks are 11.8Å and 15.25Å for ZnO oxide and TiO2 oxide film respectively. The transmittance (T) of the films was carried out using a UV-vis-NIR spectrophotometer in the wavelength range from 300 to 1,100 nm. From the transmittance of ZnO and TiO2 nanocrystalline films, we noticed a very useful transparency of the film in a large wavelength domain from 300 to 1,100 nm. The transmittance averagely increased within the visible region for both transparent conducting oxide materials and converged near 95% for TiO2 nanofilms. In any case, the values of transmittance (T) above average in the visible domain, which finds applicability in DSSCs, since it was used as the photoelectrode. Both films show a very low reflectance <25% within the visible region. The estimated optical band gap obtained lies in the range 4.0 – 4.25eV for ZnO and TiO2 films. This is as a result heat treatment as well as quantum size effect of the crystallite size was a major contributing factor for the increase in band gap. This is evident since the crystal size according to XRD deduction was of a few order of Angstroms. The photoelectric conversion efficiency obtain for ZnO gelatin is 0.001% and 0.15% for ZnO agar, while that of TiO2 agar and TiO2 galatin did not yield a good result due to photo-degradation of the electrode material.
Since a highest photoelectric conversion efficiency of 0.15% was obtained using ZnO with agar dye, it was observed that ZnO has photoanode advantage in dye-sensitized solar cells (DSSCs). Hence, the photovoltaic performance of ZnO agar is better than that of ZnO gelatin. Finally, DSSCs have a promising future for the development of efficient and flexible optoelectronics. Even if DSSCs have lower light to electricity conversion efficiency, they are considerably cheaper to be fabricated.