Synthesis and optical property of Titania nanotubes for solar cells

Nanotubes of TiO2 were synthesized by DC Anodization in an electrolyte solution of ethylene glycol (EG), ammonium fluoride (0.3% wt NH4F) and deionized water (2% vol H2O). Two sets of N-doped TiO2 nanotubes were prepared using ammonia (NH3) as Nitrogen source. In Set 1, the amount of ammonia was varied while in Set 2, the calcination temperature was changed. The samples were analyzed using Scanning Electron Microscope (SEM), X-ray diffraction (XRD), UV-Visible Spectrometry (UVS), Energy dispersive X-ray Spectroscopy (EDX), and X-ray Photoelectron Spectroscopy (XPS). The performance of each sample was tested in a dye-sensitized solar cell (DSSC).
In both sets, the optical band gap was successfully reduced from 3.15 eV to 2.85 eV in Set 1, and from 3.46 eV to 2.99 eV in Set 2. A band gap of 2.85 eV corresponds to a red shift in light absorption into the visible region with wavelength of 435 nm. XPS results confirm the successful doping of the samples with Nitrogen in both Sets 1 and 2. Overall, Set 1 samples performed better in the DSSC than Set 2 samples. The maximum power conversion efficiency (n) of DSSCsin Set 1 was 7.89%, obtained for the sample doped with 6% vol NH3. A lower the maximum power conversion efficiency of 4.62% was obtained for the sample calcinated at 125 C in Set 2. The efficiencies 7.89% and 4.62% respectively correspond to approximately 50% efficiency improvement in Set 1 and 42% in Set 2.