A synthetic N
Dye-sensitized solar cells (DSSCs) are unique and attractive devices for the conversion of solar energy into electricity based on sensitization of wide band gap semiconductors [
LiI (Aldrich), iodine (CDH, India) were used as redox couple. Ethanol (Merck, India) was dried following the method of Lund and Bjerrum [
X-ray diffraction pattern of ZnO powder was obtained with X-ray diffractometer (Model 3000, SEIFERT, Germany) using CuK
Electrochemical measurements were made with the help of a bipotentiostat (model no. AFRDE4E, Pine Instrument Company, USA) and a computer controlled e-corder (model no. 201, e-DAQ, Australia). A 150 W xenon arc lamp with lamp housing (model no. 66057) and power supply (model no. 68752), all from Oriel Corporation, USA, was used as a light source. The semiconductor electrode was illuminated after passing the collimated light beam through a 6 inch long water column (to filter IR radiation) and condensing it with the help of fused silica lenses (Oriel Corporation, USA). This IR-filtered light is referred to as “white light” in the text. Whenever required, the UV part of the white light was cut off by using a long pass filter (model no. 51280, Oriel Corporation, USA) and the resultant light (
This water soluble metal complex was prepared by mixing aqueous solutions (0.01 M) nickel (II), and the ligand, 3,30-bis[N,N-di(carboxymethyl)-aminomethyl]-o-cresol sulphonphthalein (Na4H2L) (0.01 M), in 1 : 1 ratio. The pH of the mixtures was raised to 11 by addition of NaOH solution and the mixture was refluxed on a water bath for 2 h. After cooling the solution, Me2CO (ca 50 cm3) in 100 cm3 aqueous solution was added until shining microcrystals were formed. The metal (II) complex with Na4H2L was analyzed for M, C, H, N, and S content. Detailed process of the synthesis, characterization, spectral and properties of metal complex have been given somewhere else [
Thin films of ZnO of nanometer-sized colloidal ZnO were prepared following the method of Spanhel and Anderson [
For studying the performance of test dye on ZnO thin-film electrode sandwich type cells were fabricated. For this purpose, the thin-film electrode was dipped into 0.1 mM aqueous solution (deionized water) of test dye for overnight. The dye-coated film was air dried. This was used as photoelectrode in the cell. To provide space for filling cell electrolyte and to assemble the cell, the hot-melt sheet (SX1170-60, 50
The crystalline structure of the ZnO particles was analyzed through X-ray diffraction (XRD). The XRD of ZnO powder annealed at 450°C is shown in Figure
XRD of ZnO powder prepared by Spenhel and Anderson method and sintered at 450°C.
Figure
SEM image of the nanocrystalline thin film prepared by using zinc oxide colloidal sol (annealing conditions: at 450°C in air for 1.5 h).
Ligand Na4H2L (Scheme
Structure of ligand Na4H2L.
Scheme
Structure of nickel (II) metal complex with 3,30-bis[N,N-di(carboxymethyl)-aminomethyl]-o-cresol sulphonphthalein ligand as proposed by Pandey and Narang [
Schematic presentation of preparation of ZnO sol and its thin film on conducting glass substrate.
The cyclic voltammograms of xylenol nickel (II) complex in aqueous solution containing 0.1 M NaClO4 obtained in the potential range of −1.5 V to 1.0 V at different scan rates (
(a) Cyclic voltammogram in aqueous medium containing 0.1 mM test dye + 0.1 M NaClO4 supporting electrolyte at different scan rates I-20, II-50, III-60, and IV-100 mV/s. (b) Cyclic voltammogram in aqueous medium containing 0.1 mM test dye + 0.1 M NaClO4 supporting electrolyte at scan rate of 100 mV/s (10 cycles).
Since the optical property of a compound, to be used as a sensitizer, plays the significant role in mediating the photoelectrochemical process at the semiconductor electrode (photosensitization) in a photoelectrochemical cell, the absorption spectrum of the test dye of 0.1 mM concentration in aqueous medium was determined and the same is shown in Figure
Absorption spectra of (a) aqueous solution of Ni(II) xylenol complex (0.1 mM) and (b) ZnO/Ni(II) xylenol complex thin film electrode.
In order to study that the photo induced electron injection by dye molecules into the conduction band of ZnO electrode and the dye regeneration processes are thermodynamically feasible or not, an energy level diagram has been constructed on electrochemical scale. The flat band potential of the ZnO electrode (−506 mV) has been obtained from the current-potential plot (discussed later) which provided the position of the Fermi level at the surface of the semiconductor electrode under flat band condition. Assuming the conduction band edge at 0.1 eV above the Fermi level, we get
Schematic energy level diagram for injection of electrons from an adsorbed dye molecule in excited state (
In order to assess the sensitizing ability of the dye, current-potential (
Keeping the semiconductor electrode under at short circuit condition, it was illuminated with white light of different intensities using neutral density filters and the responded currents were measured and the result
Short circuit photocurrent as a function of intensity of light for nanocrystalline ZnO/dye-sensitized solar cell.
The transient current-time profiles were recorded to know the sustainability of the photocurrent observed initially on illumination of the semiconductor electrode. For such an assessment, the time dependent response of dye-sensitized ZnO electrode to light was determined in terms of short circuit photocurrent and the results are shown in Figure
Transient current-time (
Incident photon-to-current conversion efficiency (IPCE) was evaluated for each wavelength of monochromatic light. From the value of short circuit photocurrent
The IPCE as a function of the wavelength of incident monochromatic light
for sandwich type nanocrystalline ZnO-dye-sensitized solar cell.
For the determination of fill factor of the cell, current was recorded as a faction of cell voltage keeping the sandwich cell illuminated with desired light. The maximum power
Based on the findings of these investigations it can be concluded that xylenol Ni(II) metal complex can extend the spectral response of high band-gap semiconductor electrodes to visible light (up to about 600 nm wavelength). The dye used in this study was unable to harvest the majority of the energy from the solar spectrum since this dye has significant light absorption only up to
A new transition metal complex has been studied for its photosensitizing properties by using it in dye-sensitized solar cell. Based on the findings of the present investigations, it can be concluded that Ni(II) xylenol metal complex acts as a donor-type sensitizer, and it is capable of extending the spectral response of ZnO electrode up to
Financial support from the Ministry of New and Renewable Energy (MNRE) and University Grant Commission, New Delhi, to S. Kushwaha is gratefully acknowledged. Author (S. Kushwaha) is also thankful to Metallurgical and Ceramic Engineering Departments of the Institute of Technology, Banaras Hindu University for providing the SEM and XRD facilities.