Investigations on the effect of na ca and alum on tio2 nanorods for natural dye sensitized solar cells

Abstract

Dye sensitized solar cell (DSSC) is a photoelectrochemical cell. Unlike conventional solar cells it does not have a p-n junction rather a photoelectrode, electrolyte and counter electrode assembled in a sandwich structure. The photoelectrode is a mesoporous wide band gap semiconductor sensitized with dye. The electrolyte is a redox species that undergoes rapid oxidation and reduction. An inert, high catalytic metallic film is used as counter electrode. In general, the DSSC are prepared using synthetic dyes as sensitizer and TiO2 nanoparticle film as photoelectrode, which exhibits certain drawbacks. Efficiency and stability in longer run decides its significance in the market. They can be enhanced by improvising the characteristics and properties of the components in DSSC. Hence, in the present work, we have focused more on to the optimization of the structure of photoelectrode and stability of natural dye which may lead to better efficiency at lower outlay. The iodide /triiodide liquid electrolyte and platinum films were used as electrolyte and counter electrode respectively. Anthocyanins from natural dyes such as Caesalpinnia Pulcherrima (C.Pulcherrima) and Rose (Rosa) flowers are extracted and used as dyes for DSSC. The carboxyl groups present in anthocyanins shows a good anchoring property that enhances sensitization with the OH groups of TiO2 molecules. Rutile TiO2 nanorods prepared over FTO substrates by hydrothermal method is used as photoelectrode in DSSC. From the absorbance studies it was inferred that the TiO2 film shows a maximum peak at UV region with a band gap extracted from Tauc plot to be 3.1eV. The XRD spectrum of TiO2 film confirms the presence of pure rutile phase with good crystallinity. The EDAX studies shows the composition of Ti:O to be 55.27:36.07. FESEM shows the morphology of TiO2 nanorods of length ~2.4 µm arranged uniformly with an inclined angle shift. Each rod is made of small rods that combine to form a thicker one which was observed from the HRTEM images and the SAED pattern shows the lattice fringes with interplanar spacing d = 0.246 nm. Moreover the rod morphology persists at higher temperature and the properties of nanorods can be tuned by adding buffer layer, doping and etching. Polar solvents such as ethanol, acetone and methanol are used to extract anthocyanins from flowers. The pH of anthocyanins is stable at low pH due to the release of flavylium ions. This was attained by the addition of HCl, which degrades the dye after few minutes. Hence a bio solvent such as citric acid was used to extract the anthocyanins, which increased the anthocyanin content, color intensity and optical absorbance of natural dye. The citric acid inhibits the production of phenylpropoxidase and lessens dye degradation. The DSSC fabricated with citric acid as solvent shows higher Incident photon conversion efficiency compared to other polar solvents and this enhance the photovoltaic efficiency of 0.99% and 0.83% for Rosa and C.Pulcherrima respectively. The performance of photoelectrode for DSSC can be enhanced by surface modification. It can be done by buffer layer, etching and doping. Buffer layer was added to photoelectrode to increase the dye concentration in the film. More dye molecules can be attached to the photoelectrode by the introduction of functional groups that bonds with the TiO2 molecules. Two algae namely sodium alginate extracted from the cell walls of brown algae and spirulina extracted from Spirulina.Platensis was used as buffer layers. The natural dye sensitized TiO2 film with algal buffer layer shows higher absorbance in the visible region. The FTIR spectra confirms the presence of C=O, N-H, Ti-O and OH functional groups. Higher dye concentration of 7.86 x 10-8 mol cm-2 was measured for Rosa sensitized TiO2 films with algal buffer layer. The DSSC fabricated with algal buffer layers and citric acid as solvent for dye extraction showed a conversion efficiency of 1.47% and 1.81% for Rosa and C.Pulcherrima respectively. The dye adsorption in the TiO2 film is enhanced by etching the surface that creates high roughness. The alum consists of potassium aluminium sulphate, where the sulphate ions must have etched the nanorods to have a cactus shape as observed from FESEM and the length of nanorods decreased from 2.4% to 1.8% on increasing the alum concentration. The XRD and absorbance spectra of 1%, 2% and 3% alum treated TiO2 nanorods shows no change in the peak, which infers absence of foreign atoms in the crystal structure of TiO2. Highest photovoltaic performance efficiency of 1.82% and 2.01% was measured for 3% alum treated films sensitized with Rosa and C.Pulcherrima respectively. The dyeing hours determine the effect of sensitization in TiO2 films. An experiment determining the color strength of dye sensitized TiO2 films at different hours were observed for 3% alum treated TiO2 film and an enhanced efficiency of 2.12% and 2.33% was measured for Rosa and C.Pulcherrima respectively. Doping of foreign atoms in TiO2 lattice induce changes in the electronic levels of the crystal structure. The doping of sodium ions (2%, 4% and 6%) in the crystal structure of TiO2 improves the dye concentration and increase the light absorbance in the visible region. The light shift of peaks in the XRD spectra shows the effective substitution of Na atom that induce strain in the TiO2 lattice. The bandgap reduced from 3.1 eV to 2.75 eV on increasing dopant concentration. The 6% Na doped nanorods shows appearance of nanoflowers above nanorods that increase the multiple light scattering and hence more light absorbance. The HRTEM shows the crystallinity of film with dots that represents the Na atoms on the surface and lattice fringes have interplanar spacing of d210=0.25nm. The DSSC fabricated with 6% sodium doped nanorods (inclusive of buffer layer and citric acid as solvent) showed an enhanced efficiency of 1.97% and 2.15% for Rosa and C.Pulcherrima respectively. The passivation of Photocatalytic activity was attained by Ca doping. 1%, 2%, 3% and 4% Ca doped TiO2 nanorods were prepared by Hydrothermal method. XRD peak shift was observed only for 4% Ca doping. The morphology of nanorods changed to nanospike (2% and 3%), which started to break for 4% Ca concentration. The XPS spectra of 3% Ca doped TiO2 films confirms the Ca doping with +2 oxidation state. The HRTEM of 3% Ca doped TiO2 film shows nanorods with sharp edges at all directions and interplanar spacing of d200 = 0.326 nm. Also the 3% Ca doped TiO2 film showed lower dye degradation that enhanced the DSSC photovoltaic efficiency of 2.32% and 2.52% for Rosa and C.Pulcherrima respectively. The photodegradation of Rosa dye against Ultra Violet (UV) rays was protected by the cosensitization of Rosa dye with astaxanthin pigments extracted from Haematococcus Pluvialis (H.Pluvialis). Lower dye degradation was observed for cosensitized dye with ratio ((Rosa : Astaxanthin) – 60:40). The DSSC was fabricated with surface modified photoelectrode sensitized with cosensitized dye ((Rosa : Astaxanthin) – 70:30). The 6 % Na doped, 3 % alum treated and 3 % Ca doped DSSC shows efficiency of 2.20 %, 2.56 % and 3.10 % respectively. From the perspective of commercialization, natural dyes are showing promising results in the aspects of cost, manufacturing and material availability. On the other hand, the efficiency is not much pronounced comparable to the synthetic sensitizers. The progress of research in the field of natural DSSC to improve its efficiency and stability will lead to compete with the synthetic dyes based DSSC in the near future. The present work is such an attempt to improve the efficiency of natural DSSC from 0.54 % to 3.10 % for Rosa and 0.43% to 2.52% for C.Pulcherrima.