J. Simiyu

EFFECTS OF TiO2 FILM THICKNESS AND ELECTROLYTE CONCENTRATION ON PHOTOVOLTAIC PERFORMANCE OF DYE-SENSITIZED SOLAR CELL

Effects of film thickness and electrolyte concentration on the photovoltaic performance of TiO2-based dye-sensitized solar cell (DSSC) were studied. Nanocrystalline anatase TiO2 thin films with varying thicknesses (3.2–18.9μm) have been deposited on FTO/glass substrates by screen printing method as work electrodes for DSSC. The prepared samples were characterized by UV-Vis spectroscopy, Atomic Force Microscopy/Scanning Tunneling Microscopy (AFM/STM) and X-ray diffraction (XRD). The optimal thickness of the TiO2 photoanode is 13.5μm. Short-circuit photocurrent density (Jsc) increases with film thickness due to enlargement of surface area whereas open-circuit voltage decreases with increase in thickness due to increase in electron diffusion length to the electrode. However, the Jsc and Voc of DSSC with a film thickness of 18.9μm (7.5mA/cm2 and 0.687V) are smaller than those of DSSC with a TiO2 film thickness of 13.5μm (9.9mA/cm2 and 0.734V). This is because the increased thickness of TiO2 thin film resulted in the decrease in the transmittance of TiO2 thin films hence reducing the incident light intensity on the N719 dye. Photovoltaic performance also depends greatly on the redox couple concentration in iodide∖triiodide. Jsc decreases as the redox concentration increases as a result of increased viscosity of the solution which lowers ion mobility. Similarly, Voc decreases as the electrolyte concentration increases due to enhanced back electron transfer reaction. An optimum power conversion efficiency of 4.3% was obtained in a DSSC with the TiO2 film thickness of 13.5μm and redox concentration of 0.03mol dm−3 under AM 1.5G illumination at 100mW/cm2.

Influence of Pore Size on the Optical and Electrical Properties of Screen Printed Thin Films

Influence of pore size on the optical and electrical properties of TiO2 thin films was studied. TiO2 thin films with different weight percentages (wt%) of carbon black were deposited by screen printing method on fluorine doped tin oxide (FTO) coated on glass substrate. Carbon black decomposed on annealing and artificial pores were created in the films. All the films were 3.2 µm thick as measured by a surface profiler. UV-VIS-NIR spectrophotometer was used to study transmittance and reflectance spectra of the films in the photon wavelength of 300–900 nm while absorbance was studied in the range of 350–900 nm. Band gaps and refractive index of the films were studied using the spectra. Reflectance, absorbance, and refractive index were found to increase with concentrations of carbon black. There was no significant variation in band gaps of films with change in carbon black concentrations. Transmittance reduced as the concentration of carbon black in TiO2 increased (i.e., increase in pore size). Currents and voltages ( ) characteristics of the films were measured by a 4-point probe. Resistivity ( ) and conductivity ( ) of the films were computed from the values. It was observed that resistivity increased with carbon black concentrations while conductivity decreased as the pore size of the films increased.

Anthocyanin-sensitized nanoporous TiO2 photoelectrochemical solar cells prepared by a sol–gel process

Photoelectrochemical solar cells comprising a colloidal TiO2 photoelectrode and sensitized with anthocyanin pigments, delphinidin purple and cyanidin 3,5-diglucose extracted from Hibiscus sabdariffa and Ribes nigra plants, respectively, have been fabricated. A sunlight-to-electricity conversion efficiency of 3.16% under simulated solar light was obtained with the cell sensitized with the delphinidin purple dye. Open-circuit photovoltages of 0.2–0.3 V and short-circuit photocurrents of 15–30 mA/cm2> were obtained, which points to efficient charge-carrier injection at the semiconductor/electrolyte interface. The cells also showed a high activation energy of between 0.3–0.5 eV.