Improving Performance of Dye-Sensitized Solar Cells (DSSC)

Introduction
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.

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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.
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 was fully burnt to white ash at around 700ËšC in a muffle furnace and the Rice Ash Husk
Extraction of Silica
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
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
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.

Impurities

% in RHA after burning

% in RHA after reflux with HCl

Calcium

0.926

0.402

Magnesium

0.537

0.198

Manganese

Not detected

Not detected

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.

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 (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 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 %.
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.

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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