Improving Performance of Dye-Sensitized Solar Cells (DSSC)

Improving Performance of Dye-Sensitized Solar Cells (DSSC) forbiddance of recombination channels of Dye-sensitized solar cells made of SnO2 using hollow shell structure of SiO2 extracted from rice beatN. F. Ajward, D.L.N. Jayathilaka, J.C.N. Rajendra and V.P.S.PereraIntroductionDye sensitized solar cells (DSC) atomic number 18 one of the most promising types of solar cells for next generation of solar cell technology that has power conversion efficiency as spicy as 12% (Nazeeruddin et al., 2011). Comp atomic number 18d with conventional silicon photovoltaics, DSSCs offer the cost savings in the clobbers and a range of solution deposition methods for dodge manufacture. However, there are pipe down many challenges to be met before DSCs can truly compete with authoritative silicon solar cell technology. Device efficiency, stability and lifetimes and scalable methods for device fabrication are the key issues in this field of research. A fix of work has been done to improve effi ciency of DSSCs taking different avenues, which includes growing the surface area of the metal oxide, developing new colors with broad engrossment spectra, suppressing the recombination channels and introducing light- dusting materials in the film.Utilization of mesoporous film made of nano particles of titania for DSSC is the unequivocal innovation made by Gratzel and co-workers in 1991 to achieve high efficiencies (Regan B O and Gratzel M., 1991). After that it was realized the possibility to achieving high efficiencies even with otherwise high band gap semiconductors such as SnO2 and ZnO made in nano range (Bergeron et al., 205, Keis et al., 2002). However DSSCs of high efficiencies comparable to that made of TiO2 films has been achieved with other high band gap semiconductor films made in the form of composites (Niinobe et al., 2005. The proceeds is principally accepted as the suppression of recombination of germinated posture carriers payable to passivation of gin stat es and charge carrier confinement.Materials such as Al2O3, MgO, and ZrO2 have been used antecedently as hindrance layers in DSSCs, but no record purchasable for the use of SiO2 for the same purpose (Kay and Grtzel, 2002). But SiO2 particles have been used to scatter light in TiO2 films of DSScs.In this research work we improved the surgical operation of DSSCs by introducing tenuous barrier layer of SiO2 surrounding the SnO2 crystallite to prevent recombination of charge carriers in the diffusion assisted transportation. Here the slue barrier of insulating material enhance the lifetime of germinated charge carriers of DSSC so as to improve the efficiency. methodologyRice hHusks (RH) of BG ccc rice variety was collected and ab initio piddle-washed with tap water to remove soils and dirt. It was further washed with distilled water and dehydrated at 120 C. The dried RH will bewas fully burned to white ash at around 700C in a muffle furnace and the Rrice husk Husk ash Ash (RHA) was collected. which is white in colour.Extraction of SilicaAforementioned dried RHA was refluxed with 2M HCl and exhaustively washed with distilled water and dried. 10 g of the sample was stirred in 80 ml of 2.5 N sodium hydroxide solution. It was then boiled in a covered 250 ml Erlenmeyer flask for 3 hours and the solution was filtered using a Whatman No. 41 filter paper. Filtrate was allowed to cool down to style temperature and added 5 N H2SO4 until it reaches pH 2. Then NH4OH was added to the suspension until it reaches pH 8.5 and allowed to be at get on temperature for 3.5 hours. The precipitated SiO2 was separated by filtration and thoroughly washed with distilled water. The silicon oxide obtained was oven dried at 120 0C for 12 hours and cool down to room temperature.Preparation of SnO2 ParticlesTin (ivIV) chloride was dissolved in distilled water to obtain 0.5 M solution and ammonia was added stirring the solution to obtain fine particles of SnO2. The SnO2 particles are thoroughly washed with distilled water to remove chlorine ions. Then the particles are suspended in diluted ammonium solution for stabilization.Preparation of SnO2 and SiO2 core shell structuresTin (IV) Oxide particles were come outed with basal sheer layer of silicon dioxide by the following method. 0.5g of SnO2 particles were freighted and grinded in an agate mortar with 2 ml of ethanol. Then measured volumes of 0.5M sodium silicate which was prepared by dissolving extracted silica in NaOH was added at a time to different SnO2 samples that has been prepared as described above. After that 1 ml of acetic acid was added dangle wise to that mixture. Sodium silicate around the SnO2 particles suppose to turn into SiO2 in the surgery of acidification.Fabrication of DSSC with SiO2/SiO2 compositeThe paste as prepared was used to coat films on Cconducting Ttin Ooxide (CTO) glass plates by the doctor blade method that fill out into the size of 1.5 x 1 cm2. Prior to coating the films on the CTO glass, they were thoroughly cleaned by detergent, distilled water and acetone with immoderatesonic agitation. CTO plates coated with SnO2/SiO2 films were dried on a hot plate heated up to 120 C for 5 minutes. Then the films were sintered at 450 C in a furnace for 30 minutes. When the films cooled down to the room temperature they were immersed in Ru-bipyridyl N-719 dye solution (0.5 mM in ethanol) for 12 h. After the dye adsorption, films were rinsed with ethanol and sandwich with atomic number 78 sputtered conducting glass substrates using clips. The capillary space in between the devil plates of cells were filled with electrolyte containing 0.5M potasium iodide, 0.05M iodine in a mixture of acetonitrile and ethylene carbonate 14 by volume.Characterization TechniquesI-V characteristics of the cells were measured under the toy of 100 mWcm2 simulated light source and computer controlled frame-up consisting of potentiostat/galvanostat. Elemental analysis of RHA was done using Atomic Absorption Spectroscopy. roentgenogram diffraction (XRD) patterns and SEM images were withal obtained for SnO2/SiO2 composite films.Results and discussionAccording to the literature reports, silica extracted from RH is in naorange with least slag take aims. Elements that show up as impurities in RHA of BG 300 rice variety were examined with atomic absorption spectroscopy. Percentages of impurities in RHA aft(prenominal) burning and refluxing with HCl are given over in table 1.Table 1 Percentages of impurities in RHA later burning and after refluxing in HCl.Impurities% in RHA after burning% in RHA after reflux with HClCalcium0.9260.402Magnesium0.5370.198atomic number 25Not detectedNot detectedFerrous0.2690.060It is inferred from these results that the impurity level of RHA is low and can be reduced further by refluxing with HCl. That is because these impurities present in the RHA as oxides can be removed easily by acid wash. In this study we have investigated the possibility of using SiO2 thin barrier around the SnO2 particles to impede leakage of electrons for recombination processes which is one approach to gain the efficiency of DSSCs. embodiment 1(a) shows the measured open-circuit photo-voltage (Voc) and short-circuit photocurrent (Isc) of DSSCs with different SiO2% by weight in the SnO2/SiO2 films.Figure 1 (a) Open-circuit photovoltage (Voc) and short-circuit photocurrent (Isc) of DSSCs with different SiO2 % in SnO2 films (b) Suppression of recombination of injected electrons in the conduction band of SnO2 by SiO2 shell.Initial increment of SiO2 % in the film gradually covers the SnO2 particles as an ultra thin layer and beyond certain limit of SiO2 contributes to the growth of the SiO2 layer around the SnO2 particles increasing the thickness. This is the motive why both the Isc as well as the Voc increase initially with the increment of SiO2 % in the SnO2 films of DSSCs. The increment of Isc and Voc is attributed to the suppressio n of recombination of injected electrons by the photo temper of the dye in the conduction band of SnO2 due to the development of ultra thin layer of SiO2 around SnO2 particles (Figure 1b). The highest photocurrent of DSSCs with the addition of 2.5 % of SiO2 whitethorn have been achieved due to the perfect coverage of SnO2 particles with ultra thin layer of SiO2. But Voc continues to increase further up to 4% of SiO2 in SnO2 films. It is noticeable that the decrement of Voc afterwards is not significant as in Isc after reaching the maximum. leastways further increment of the thickness of the ultra thin layer of SiO2 happens to ebb both Isc and Voc. The amount of dye adsorbed on the semiconductor film is as well as a detrimental factor on the performance of DSSCs. We have detect that the dye absorbed on SnO2 films decreased with the increment of SiO2%. To quantitatively analyze it, we have desorbed the dye adsorbed on SnO2 films with different SiO2 %. This was done by allowing the films to adsorb dye for determined period and completely desorbing the dye by immersing the dye adsorbed SnO2 films in known volume of 0.5 M KOH solution. The concentration of the dye in the KOH solution was estimated spectroscopically at the wave length of 550 nm. Figure 2 given bellow shows the deviation of dye adsorbed on SnO2 films for different SiO2 %.Figure 2 (a) Variation of dye adsorbed on SnO2 films for different SiO2 % and (b) structure of the N-719 dye.It is evident from the Figure 2 that the dye adsorption on SnO2 films decrease with the increment of SiO2 %. This may rival adversely on photocurrent of DSSCs. Although dye aggregations on semiconductor films also results to decrease photocurrent there should be sufficient amount of dye adsorbed on SnO2 crystallites for efficient operation of DSSCs. The decrement of Isc at higher SiO2 percentages is main issuance of low dye adsorption on SnO2 films. The adsorption of dye on SnO2 films decrease with the increment of Si O2 % because of the acidity of SiO2 which prevent chelation of N-719 dye on SnO2 films by the carboxylic groups.XRD and SEM analysis was also carried out to characterize the SiO2 ultra thin layer coated on SnO2 particles. Figure 3 shows the SEM of SnO2 film with 4.5% of SiO2. The occlusion of the SEM images was not sufficient to identify the SiO2 thin layer. But it can be seen that the SnO2 particles are distributed in wide range of particle sizes which also affect adversely on the performance of DSSCs. The XRD pattern of the SnO2 film with 4.5 % of SiO2 is given in Figure 3(b). There was no any peaks appeared for SiO2 in the XRD pattern of the SnO2 films as well. The insertion in the Figure 3(b) is the XRD obtain for SiO2 powder obtained by acidification of Na2SiO3 with acetic acid and sintering at 450 C for 30 minutes. It is found to be in uncrystallised form and most probably the SiO2 around the SnO2 is also amorphous. Because of the amorphous nature of SiO2 and low percentage might produce significant peaks for SiO2 in the XRD pattern. Figure 3 (a) SEM image of SnO2 film with 4.5% of SiO2 (b) XRD pattern of the SnO2 film with 4.5 % of SiO2. foundation is the XRD obtain only for SiO2 powder.ConclusionsThe silica extracted from rice husk is with low impurity levels suitable for coating ultra thin layers of SiO2 arround SnO2 to fabricate DSSCs. Deposition of ultra thin layer of SiO2 on SnO2 particles improved the performance of DSSCs. The reason for decrement of cell performance with higher percentages of SiO2 is not only due to the barrier thickness, but also due to the low dye adsorption. It was observed by the SEM images that the particle size of SnO2 is widely diverse because of particle aggregation. It is recommended to use resembling size of SnO2 particles for better performance of DSSCs. some chemical treatment also required to enhance the adsorption of dye on SiO2 ultra thin layer on SnO2 particles.References1. Bergeron B.V. , Marton A., Gerko Osk am G., and Meyer G.J. (2005) Dye-Sensitized SnO2 Electrodes with Iodide and Pseudohalide Redox Mediators J. Phys. Chem. B, , 109 (2), 937943.2. Kay A. and Grtzel M. (2002) Dye-Sensitized CoreShell Nanocrystals Improved susceptibility of Mesoporous Tin Oxide Electrodes Coated with a Thin Layer of an Insulating Oxide Chem. Mater., 14 (7), 29302935.3. Keis K., Bauer C., Boschloo G., Hagfeldt A., Westermark K., Rensmo H., Siegbahn H. (2002) Nanostructured ZnO electrodes for dye-sensitized solar cell applications Journal of Photochemistry and Photobiology A Chemistry, 148, issue 13, 5764.4. Nazeeruddin M. 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