| dc.description.abstract | TiO2 dye sensitized electrochemical solar cells employing naturally occurring anthocyanin dye pigment have been fabricated. The films were coated by screen-printing method from P25
Degussa as the starting material. Absorption characteristics of the dye pigment was studied as well as current – voltage (I-V) and incident photon to current conversion efficiency (IPCE) on complete solar cells sensitized with the anthocyanin pigment. Electron transport and
recombination studies were carried out on complete solar cells sensitized with ruthenium
complex dye employing time resolved intensity modulated photocurrent (IMPS) and
photovoltage spectroscopy (IMVS). Structural characteristics have shown that the films were crystalline composing mainly of anatase TiO2 with particle sizes ranging between 15 to 30 nm in diameter. XPS analysis on the films showed that Ti was completely oxidized in TiO2 with Ti 2p state exhibiting spin-orbit splitting that resulted in peaks at 461.5 eV and 467.0 eV of binding energy. Absorption characteristics of anthocyanin pigment showed that the pigment lost the absorption maximum at 550 nm upon adsorption on TiO2 surface. However, this was regained after the pH was adjusted to 2.00. The I-V characteristics of the solar cells showed that the overall efficiency depended strongly on the pH of the dye pigment with the sample at pH 2.00 showing the best performance both in terms of current-voltage (I-V) and incident photon to current conversion efficiency (IPCE) characteristics. The photocell recorded Jsc of 3.17 mA/cm2, Voc of 0.49 V, FF = 66 % giving the overall efficiency of 1.04 %. The results were for TiO2 film thickness 6.5 µm and exposed area of 0.48 cm2. The same cell at pH 2.00 exhibited the highest overall IPCE compared with the other pH levels. The results show that anthocyanin dyes are able to convert light to electricity with quantum efficiencies of about 4 % maximum in the visible spectrum. compared with ruthenium N719 sensitized solar cell of similar film thicknesses which recordedJscof 6.16 mA/cm2,Voc of 0.83 V,FF of 65 % giving the overall efficiency of 3.29 %. The anthocyanin-sensitized solar cell recorded anIPCEmax of 4% in the visible compared to 27% for ruthenium sensitized solar cell. The difference in IPCE maximum between the anthocyanin pigment and ruthenium dye complex is due to their kind of transition with the anthocyanin being ligand to ligand charge transition while ruthenium complex being metal to ligand charge transfer (MLCT) transition.
Electron transport and recombination studies were carried out on complete solar cells
sensitized with ruthenium complex dye at film thickness 3.0, 6.0 μm, 12.8 μm, 23.5 μm and 25.3 μm. The 3.0 μm thick film exhibited the fastest photovoltage decay while the 12.8 μm thick film had the slowest photovoltage decay. This was attributed to the effect of electron recombination via redox electrolyte affecting the thinnest films and a balance between charge transport and recombination being attained at film thickness 12.8 μm. The samples, in general, exhibited linear and non-linear photovoltage decay profiles with the non-linear decay profile being observed at the onset of the decay while the linear at longer time scales of the decay. The non-linear profile at the onset of decay was attributed to the reorganization and switching effects the cells are in and the linear profile at the longer time scale being the steady state. Normalized current transients showed that there was a fast current decay at shorter time scale for thick films when illuminated from the backside. However, a single exponential decay was observed at longer time scales under same illumination mode. Charge accumulation in the films was observed to increase with film thickness, which was attributed to the rise in trap states with increase in film thickness. | en |
