Improving Performance of Dye-Sensitized Solar Cells (DSSC)

Suppression of recombination channels of Dye-sensitized solar cells made of SnO₂ using core shell structure of SiO₂ extracted from rice husk

• N. F. Ajward, D.L.N. Jayathilaka, J.C.N. Rajendra and V.P.S.Perera

Introduction

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Dye sensitized solar cells (DSC) are one of the most promising types of solar cells for next generation of solar cell technology that has power conversion efficiency as high as 12% (Nazeeruddin et al., 2011). Compared with conventional silicon photovoltaics, DSSCs offer the cost savings in the materials and a range of solution deposition methods for device manufacture. However, there are still many challenges to be met before DSCs can truly compete with current 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 lot of work has been done to improve efficiency of DSSCs taking different avenues, which includes increasing the surface area of the metal oxide, developing new dyes with broad absorption spectra, suppressing the recombination channels and introducing light-scattering materials in the film.

Utilization of mesoporous film made of nano particles of titania for DSSC is the imperative 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 other 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 improvement is principally accepted as the suppression of recombination of germinated charge carriers due to passivation of trap states and charge carrier confinement.

Materials such as Al₂O₃, MgO, and ZrO₂ have been used previously as barrier layers in DSSCs, but no record available for the use of SiO₂ for the same purpose (Kay and Grätzel, 2002). But SiO₂ particles have been used to scatter light in TiO₂ films of DSScs.

In this research work we improved the performance of DSSCs by introducing thin barrier layer of SiO₂ surrounding the SnO₂ crystallite to prevent recombination of charge carriers in the diffusion assisted transportation. Here the thin barrier of insulating material enhance the lifetime of germinated charge carriers of DSSC so as to improve the efficiency.

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Methodology

Rice hHusks (RH) of BG 300 rice variety was collected and initially washed with tap water to remove soils and dirt. It was further washed with distilled water and dried at 120 ËšC. The dried RH will bewas fully burnt to white ash at around 700ËšC in a muffle furnace and the Rrice husk Husk ash Ash (RHA) was collected. which is white in colour.

Extraction of Silica

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Aforementioned dried RHA was refluxed with 2M HCl and thoroughly 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 room 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 room temperature for 3.5 hours. The precipitated SiO₂ was separated by filtration and thoroughly washed with distilled water. The silica obtained was oven dried at 120 ⁰C for 12 hours and cool down to room temperature.

Preparation of SnO₂ Particles

Tin (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 structures

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Tin (IV) Oxide particles were coated with ultra thin layer of silica by the following method. 0.5g of SnO₂ particles were weighted 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 drop wise to that mixture. Sodium silicate around the SnO₂ particles suppose to turn into SiO₂ in the process of acidification.

Fabrication of DSSC with SiO₂/SiO₂ composite

The paste as prepared was used to coat films on Cconducting Ttin Ooxide (CTO) glass plates by the doctor blade method that cut 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 ultrasonic agitation. CTO plates coated with SnO₂/SiO₂ films were dried on a hot plate heated up to 120 ï‚°C for 5 minutes. Then the films were sintered at 450 ï‚°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 platinum sputtered conducting glass substrates using clips. The capillary space in between the two plates of cells were filled with electrolyte containing 0.5M potasium iodide, 0.05M iodine in a mixture of acetonitrile and ethylene carbonate 1:4 by volume.

Characterization Techniques

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I-V characteristics of the cells were measured under the illumination of 100 mWcm⁻² simulated light source and computer controlled setup consisting of potentiostat/galvanostat. Elemental analysis of RHA was done using Atomic Absorption Spectroscopy. X-ray diffraction (XRD) patterns and SEM images were also obtained for SnO₂/SiO₂ composite films.

Results and discussion

According to the literature reports, silica extracted from RH is in naorange with least impurity levels. Elements that present as impurities in RHA of BG 300 rice variety were analyzed with atomic absorption spectroscopy. Percentages of impurities in RHA after burning and refluxing with HCl are given in table 1.

Table 1: Percentages of impurities in RHA after burning and after refluxing in HCl.

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Impurities	% in RHA after burning	% in RHA after reflux with HCl
Calcium Who’s Writing My College Essays? Our writers are top-tier—university grads, many with Master’s degrees, who’ve passed tough tests to join us. They’re ready for any essay, working with you to hit your deadlines and grading standards with ease and professionalism.	0.926	0.402
Magnesium	0.537 Are Your Papers Original? Always! We start from scratch—no copying, no AI—just pure, human-written work with solid research and citations. You can even get a plagiarism report to confirm it’s 95%+ unique, ready for worry-free submission.	0.198
Manganese	Not detected	Not detected Can You Match Any Citation Style? You bet! From APA to IEEE, our writers nail every style with precision. Give us your guidelines, and we’ll craft a paper that fits your academic standards perfectly, no sweat.
Ferrous	0.269	0.060

It 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.

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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 increase the efficiency of DSSCs. Figure 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 (V_oc) and short-circuit photocurrent (I_sc) 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 reason 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 suppression of recombination of injected electrons by the photo excitation 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 may 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 afterward is not significant as in Isc after reaching the maximum. Anyway further increment of the thickness of the ultra thin layer of SiO₂ happens to decrease both Isc and Voc.

The amount of dye adsorbed on the semiconductor film is also a detrimental factor on the performance of DSSCs. We have noticed 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 %.

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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 affect 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 consequence of low dye adsorption on SnO₂ films. The adsorption of dye on SnO₂ films decrease with the increment of SiO₂ % 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 resolution 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 amorphous 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.

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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. Insertion is the XRD obtain only for SiO₂ powder.

Conclusions

The 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 uniform 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.

References

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1. Bergeron B.V. , Marton A., Gerko Oskam G., and Meyer G.J.; (2005) Dye-Sensitized SnO₂ Electrodes with Iodide and Pseudohalide Redox Mediators; J. Phys. Chem. B, , 109 (2), 937–943.

2. Kay A. and Grätzel M.; (2002) Dye-Sensitized Core−Shell Nanocrystals:  Improved Efficiency of Mesoporous Tin Oxide Electrodes Coated with a Thin Layer of an Insulating Oxide; Chem. Mater., 14 (7), 2930–2935.

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 1–3, 57–64.

4. Nazeeruddin M. K., Baranoff E, Gratzel M., (2011) Dye-sensitized solar cells: A brief overview; Solar Energy 85 1172–1178.

5. Niinobe D. , Makari Y., Kitamura T., Wada Y., and Yanagida; S.; (2005) Origin of Enhancement in Open-Circuit Voltage by Adding ZnO to Nanocrystalline SnO₂ in Dye-Sensitized Solar Cells; J. Phys. Chem. B, , 109 (38), 17892–17900.

6. Regan B O and Gratzel M; (1991) A low cost high efficient solar cell based on dye sensitized colloidal TiO2 films; Nature 353 737.

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