Fabrication And Characterization Of Tio2/Nb2O5 Composite Photo-Electrodes Deposited Using Electrophoretic Technique For Application In Dye-Sensitized Solar Cells
Nguu, John N
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This study has investigated a rapid, simple and cost effective electrophoretic deposition (EPD) technique for fabrication of nanoparticulate TiO2/Nb2O5 composite electrode films for use in dye-sensitized solar cells (DSSCs). The EPD technique parameters were categorized into suspension-related and process-related parameters. The suspension-related parameters were maintained constant by fixing the solvent (2-propanol), charging system (Mg(NO3)2.6H2O), pH (≈ 4.9) of suspension and EPD apparatus. The optimized process-related parameters of EPD technique obtained were: particle concentration (0.25 g/L), DC applied voltage (35V), and deposition time (90s). The optical properties like light absorption, transmission, extinction coefficient, Urbach energy, refractive index, and optical band gap energy of the composite thin films were investigated in this study. Band gap energies ( gE ) for pure TiO2, (3.932 eV), pure Nb2O5 (3.858 eV) and TiO2/Nb2O5 composite films (3.884eV) were obtained based on the plot of the Tauc equation. The theoretical g E of TiO2 is =3.25 eV and g E =3.49 eV for Nb2O5. The difference between theoretical and experimental values of g E for both pure TiO2 and Nb2O5 films was explained in terms of development of sub-band gap states lying deep in the tail of density of states in the TiO2/Nb2O5 composite films. The band gap decreased slightly from 3.8911eV for films annealed at 450°C for 1 hour to 3.7965 eV for similar films annealed at same temperature for 6 hours. The narrowing of band gap was attributed to the decrease in transitions due to reduction of the oxygen defect width as a result of air annealing which fills up the vacant oxygen levels. Light is absorbed in a material when extinction coefficient ( k ) is greater than zero i.e., k > 0, while light travels straight through the material when k = 0. In this study, a value of 0.0354 at 400 ≤ λ ≤ 700nm was obtained for extinction coefficient which means there was some absorption of light by the TiO2/Nb2O5 composite electrode thin films due to their large band gap energy. The results were consistent with the knowledge about high band gap semiconductors that must require sensitization with a dye to function as photoelectrodes in a dye sensitized solar cells.. The relatively small extinction coefficient was attributed to low absorption of light by the TiO2/Nb2O5 composite electrode thin film due to its large energy band gap. The calculations of Urbach energy ( UE ) yielded a value equal to 454 meV, which vi indicated substantial introduction of tail states at the band edges that influence the electron transport. The average refractive index (n) of 1.643 was obtained for wavelengths between 1800nm and 2800nm. The majority carriers in the TiO2/Nb2O5 composite films were found to be electrons (n-type) which could be caused by caused by incorporation of Nb+5, and O-2 ions in the composite. Charge carrier density ( dn ) value for TiO2/Nb2O5 composite films were 6.14 x 1018 cm-3 which was lower than the value (1.00 x 1019 cm-3) reported in existing research. This was attributed to electrons trapping in surface states of the high surface area composite film while reduction of carrier mobility could have resulted from increased grain boundaries between the Nb2O5 and TiO2 nanoparticles in a composite film. The SEM images showed the TiO2/Nb2O5 composite films were devoid of cracks and essential for dye adsorption. Both TiO2 and Nb2O5 nanoparticles were shown, by XRD graphs, to be presented in the composite films in the ratio 1.78:1 (or 1:0.56). The XRD graphs confirmed that surface of both TiO2 and Nb2O5 particles suspended in 2-propanol became charged and therefore the charged particles were moved and deposited on oppositely charged electrode by the applied electric field. The XRD revealed dominant peak of TiO2 at 2θ = 25.5° and that of Nb2O5 at 2θ = 26.5°. Applying Scherer’s equation, the average crystallite sizes at these peak positions were 15.36 nm and 15.49 nm and hence the crystallite dimensions were confirmed to the nanoscale. The solar cell produced using the composite thin film had the following parameters: VOC = 0.66V, JSC = 5.25 mA/cm2, FF = 0.57, and = 2. The low conversion efficiency observed was accredited to factors such as composition of the composite, morphological defects, poor adhesion between film and FTO, or the type of dye used. Nonetheless, efficiencies in these types of devices where the electrode is deposited by EPD has remained low. Internal resistances were investigated by EIS measurements to explain the low cell efficiencies. All the cells exhibited the Gerischer impedance (RD) characterized by Nyquist plot having a single, main arc in which impedance was associated with the electron recombination with electrolyte species being faster than the transport of electron within the TiO2 film. The faster back reaction could have been caused by the relatively poor adhesion of the film on to FTO glass slides. The poor adhesion introduced high transport resistance and low interfacial (recombination) resistances. Blocking the electron leak by structural modification that includes developing a more compact composite structure and improved film/FTO glass vii adherence that enhances electron conduction across the interface may be a possible way to obtain high JSC s and efficiencies in DSSCs. Key words: Dye-sensitized solar cell, electrophoretic deposition, niobium (v) oxide, TiO2/Nb2O5 composite electrode thin film, electrochemical impedance spectroscopy.
University of Nairobi
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