Effects Of Tio2 Compact Layer On Photovoltaic Characteristics Of TiO2/Nb2O5 Dye Sensitized Solar Cells

Abstract

Titanium dioxide has been widely used in dye sensitized solar cells as a semiconductor, however, the recombination rate of photo generated hole-electron pairs on TiO2 is very high which reduces its electron transport efficiency. On the other hand, Niobium pentoxide is an n-type transition metal oxide semiconductor with good chemical stability and a conduction band potential that is 0.2 – 0.4 eV higher than that of TiO2. This potential difference can form an energy barrier at the electrode-electrolyte interface thereby reducing the rate of recombination of photo-injected electrons. Composite metal oxide semiconductor electrodes possess promising properties could enhance the conversion efficiency of dye sensitized solar cells. However, charge recombination at the interfaces still affects the performance of solar cells fabricated from composites. In this work we introduce TiO2 blocking layer on TiO2/Nb2O5 composite photoelectrode to assess its potential application in dye sensitized solar cells. TiO2 compact layers were deposited on FTO glass substrate by spray pyrolysis technique which was subsequently followed by deposition of nanoporous TiO2/Nb2O5 composite layer by screen printing technique. The structural properties of the deposited films were studied by Raman spectroscopy and X-ray diffraction. Optical properties of the films were evaluated using UV-VIS spectrophotometer in wavelength range of 300-1100 nm. SCOUT software was used to determine thickness and to retrieve the optical constants; refractive index nf and extinction coefficient kλ of TiO2 compact layer. This was done based on the interactive curve fitting using O‟Leary-Johnson-Lim (OJL) interband–transition dielectric model together with Drude model. The photovoltaic parameters of the prepared dye sensitized solar cells were evaluated using current density-voltage (J-V) characterization from which, the best conversion efficiency was attained with 60 nm thick TiO2 compact layer with current density Jsc equal to 8.16 mAcm-2, open circuit voltage Voc of 0.79 V, fill factor FF of 0.578 and efficiency  of 3.39 %. Further, interfacial charge transfer in the cell was studied using electrochemical impedance spectroscopy. Nyquist plots of the device were measured at a DC bias of 0.6 V. For the best performing cell, recombination resistance Rrec was 899.36 Ohm.cm2, chemical capacitance Cμ=4.3×10-5 Fcm-2, transport resistance Rtr 4.53×102 Ohm.cm2, geometrical capacitance Cg=8.82×10-6 Fcm-2 and recombination resistance210 .sOhmRcm. Charge carrier life time of the solar cells were also determined with the best performing solar cell having charge carrier lifetime of 3.86×10-2 s while charge carrier lifetime for solar cell showing lowest efficiency to be 2.50×10-4 s.