Dye Sensitized Solar Cells Based on Highly Porous Tio2 Films

Abstract

Light harvesting efficiency in dye sensitized solar cell is currently enhanced by the employment of an additional TiO2 scattering layer among other means. However, this approach raises the overall photoelectrode thickness hence imposes limitations on effective charge transport especially in dense electrolyte media. The additional film layer further reduces light intensity on the adsorbed dye consequently decreasing photocurrent generation. In light of this, there is still the challenge of light scattering optimization versus charge transport and photocurrent generation. In addition, though TiO2 is a relatively cheap material, the addition of TiO2 layer raises the production cost of the dye sensitized solar cell effectively and rendering it not cost effective. In this study, TiO2 thin films of thicknesses; 3.21, 8.23, 13.52 and 18.93 μm were coated onto FTO/glass by screen printing method, annealed at 500 oC in a tube furnace for 30 minutes in air. Films with same thickness, 3.21 μm were coated with the incorporation of carbon black at varying weight percentages; 0.5, 1.0, 1.5, 2.0 and 3.0 wt % which decomposed upon annealing to create artificial pores in the films. Transmittance and reflectance spectra were measured using a double beam Shimadzu UV-Vis-NIR spectrophotometer in the photon wavelength range of 300-900 nm. The results showed the films to be generally highly transparent in the visible with an average value of 68 % in the wavelength range of 500 and 700 nm. However, transmittance dropped from 79 % at 3.21 μm to 59 % at 18.93 μm. On the other hand, at 400 nm, reflectance rose from 16 % at 3.21 μm to its peak of 27 % at 18.93 μm. Absorbance spectra of the films studied by the UV-Vis-NIR spectrophotometer in the wavelength range of 300-900 nm were relatively low though observed to directly depend on film thickness. At 400 nm, the estimated optical band gap and refractive index rose from 3.49 eV and 1.38 with thickness, 3.21 μm to 3.81 eV and 1.68 with thickness 18.93 respectively. With the incorporation of carbon black, average transmittance for film of 0 wt % was 79 % and drastically dropped to an average of 10 % at 3 wt %. The measured absorbance spectra was found to be above the quantity absorbed by bare TiO2 and the absorption increased all through as the concentration of carbon was increased. It was observed that the presence of artificial pores had no effect on TiO2 band gap. In another case, refractive index was affected by the porosity of the films. The refractive index of the films increased from 1.35 to 1.60 at 400 nm with carbon black concentrations of 0 wt% and 3 wt% respectively. Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM) studies revealed a dependence of increasing porosity with increase in the concentrations of carbon black. X-ray diffraction spectroscopy showed the film with and without carbon black to be crystalline and anatase phase. The effects of porosity and film thickness on the electrical properties of TiO2 thin films were studied. The films were found to be generally conductive and the conductivity depended on the porosity and film thickness. The conductivity ranges from 4.4 to 384.61 Semiens/cm with film thickness ranging from 3.21 to 18.93 μm. Conductivity was observed to decrease as the porosity of the films increased. Resistivity increased from 2.6 x 10-3 to 22.63 x 10-2 Ω.cm as film thick rose to 18.93 μm. Dye Sensitized Solar Cells Sensitized (DSSCs) fabricated with N719 dye and TiO2 thicknesses; 3.21, 8.23, 13.52 and 18.93 μm were characterized using a solar simulator at 100 mW/cm2 solar irradiance. The active area of the cells was 0.48 cm2. Current densities, Jsc of 5.0, 6.8, 8.2 and 7.5 mA/cm2 and open circuit voltage, Voc of 0.731, 0.708, 0.711 and 0.687 V respectively were recorded. The results showed current density to increase as the films became thicker due to increased dye adsorption but decreased at 18.9 μm as a result of reduced light intensity. Therefore the optimum film thickness obtained for DSSCs was 13.52 μm with the maximum conversion efficiency of 3.5 %. A simple experiment to study the effect of reflection on the photovoltaic properties was carried out. Another dye sensitized solar cell was characterized with and without a mirror beneath the cell. Current density of cell improved from 3.65 mA/cm2 to 4.81 mA/cm2 by placing a mirror behind the platinum (Pt) counter electrode. I-V characteristics of DSSCs with varying electrolyte concentrations were also recorded: cells with Redox concentrations of 0.03, 0.1 and 0.15 mol/dm3 had Jsc of 8.2, 7.8 and 6.6 mA/cm2; and Voc of 0.734, 0.681 and 0.679 V, respectively. 0.03 mol/dm3 was found to be the optimum electrolyte concentration. The influence of porosity on the photovoltaic performance of DSSC was studied. DSSCs with 0 and 1.5 wt % carbon black were fabricated. The cells had electrode thickness of 13.5 μm, sensitized with N719 ruthenium complex and triiodide electrolyte solution of 0.03 mol/dm3. There was a significant improvement in the photovoltaic performance of the 1.5 wt% DSSC. The cell with 0 wt % of carbon black had Jsc and Voc of 8.2 and 0.711 V respectively. In the case of 1.5 wt % based DSSC, Jsc and Voc were found to be 9.9 mA/cm2 and 0.711 V. The overall cell conversion efficiencies of 0 wt% and 1.5 wt% based DSSCs were 3.5 and 4.3 %, respectively. This represents an increase of 22.8 % in efficiency due to increased photocurrent generation by the artificial pores created by decomposition of carbon black. Finally, the electrochemical impedance spectroscopy of TiO2 based DSSCs with electrode thicknesses 3.2, 8.2, 13.5 and 18.9 μm were studied. The interfacial reactance at the FTO/TiO2 interface (RFT) were found to rise as the electrode became thicker due to higher trap sites.