Purpose

1. Diatom-CdS Nanostructures as a Method to Enhance the Efficiency of a Dye-Sensitized Solar Cell UV-Vis Analysis of CdS Frustules Introduction UV-Vis reflectance and transmission spectroscopy was used to evaluate the effectiveness of CdS coated diatom frustules as a method to decrease the reflective nature of a Dye-Sensitized Solar Cell. Using a PerkinElmer reflectance spectrometer and the dual beam 150 mm integrating sphere below, optical glass coated with CdS was compared to identically coated optical glass containing a thin layer of CdS coated frustules. The difference in reflectance The Dye-Sensitized Solar Cell (DSSC) provides an inexpensive alternative to standard solar cells and presents the possibility of a solution to modern energy crises. This type of solar cell is greatly inefficient in its production of electricity, as most cells reflect over 70% of the incident light. The integration of light-trapping materials into a DSSC may decrease the amount of light reflected and allow for increased efficiency. Certain algae known as diatoms are characterized by the presence of an internal matrix nanostructure used to trap light for photosynthesis. These structures, biosilica frustules, are composed of naturally produced SiO2 and contain nanoscale complexities not replicable by synthetic preparation. Literature has indicated that it is possible by complex methods to extract these frustules from the interior of a diatom and preserve their structure in a salt. The development of a simple preparation method of Cadmium sulfide coated biosilica frustules would indicate that such frustules can be easily and cost effectively integrated into a DSSC. The characterization of frustules coated in this manner using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) will shed light on the feasibility and success of such a method. UV-Vis analysis of a CdS coated frustules in comparison to an equivalent CdS coating without frustule incorporation will demonstrate whether such enhancement of a solar cell allows for the increased capture of light. Positive results in these studies would allow for increased use of the inexpensive DSSC as an efficient, primary source of alternative energy. CdS Coating of Biosilica Frustules A chemical bath deposition method was used to coat the frustules with CdS, taking advantage of the structural properties of CdS that allow it to coat solids with a thin film during a precipitation reaction. A bath solution containing CdCl2 as a source of Cadmium ions, thiourea (NH2)2CS, as a source of sulfide ions, and NH4Cl was prepared and maintained at 80°C to increase the reaction rate. The frustule solid suspended in 10 ml of water was added to this solution. Shortly after, NH4OH was added to initiate the reaction. The addition of this basic solution elevated the pH and lowered the solubility product of CdS to 8.0x10-29, below the product of the existing Cd2+ and S2- ion concentrations. CdS was deposited, coating the frustules and their intricate structures with a thin semiconducting layer of CdS. The density differences between coated frustules and excess salt allowed for the extraction of the desired material from the bath solution. and transmission of these samples demonstrate the impact that CdS coated frustules will have when integrated into a DSSC among imbedded semiconductor material. Dual Beam 150 mm Integrating Sphere Reference Beam Reflectance Port at 6o Transmission Port Sample Beam Figure 2. Completed CdS precipitation reaction. Figure 8. Integrating sphere used for reflectance/transmission data Specular Trap SEM and EDS Analysis of Diatom Frustules I chose to view the structures I had created using scanning electron microscopy (SEM), and analyzed their composition using energy dispersive spectroscopy (EDS), in order to evaluate my methods in terms of my first hypothesis. Three separate microscope stages were prepared containing natural state Pinnularia sp., pure frustule solids, and CdS coated frustules, respectively. The difference in the processing stage of these samples was evident by their colors: green, white, and orange. These samples were imaged and analyzed by the author using a Hitachi Scanning Electron Microscope located at the Mt. Sinai School of Medicine. Figure 10. Total reflectance spectrum of CdS Coated Glass (Red) and CdS/Frustule Coated Glass (Blue). The incorporation of coated frustules decreases the amount of light reflected by the coated glass. Figure 9. Total transmission spectrum of CdS Coated Glass (Red) and CdS/Frustule Coated Glass (Blue). The incorporation of coated frustules increases the amount of light transmitted. Reflectance and transmission spectroscopy data demonstrate that the integration of CdS coated frustules into an existing semiconducting layer, like that of a DSSC, allows more light to pass through and simultaneously reduces the light reflecting backward before interaction with cell materials. Throughout the visible spectrum, comparisons between the two samples show that the decrease in reflectance of the frustule-incorporated sample was greater than the increase in transmission of the same sample. This relationship indicates that the CdS coated frustules effectively trapped light in their complex matrix structure by as much as 5% of the total light exposure. This trapped light, which would ordinarily be reflected, will remain in a DSSC for the increased production of electricity. Purpose • To develop, use, and determine the effectiveness of a simple method for CdS coating of biosilica frustules. • To determine if the integration of CdS coated frustules can increase the amount of light captured by a DSSC. Hypothesis Figure 3. Pinnularia sp. Algae Figures 4a & 4b. Intact biosilica frustules Figure 3. shows an SEM image of Pinnularia sp. algae in its natural state. These algae cells retain their organic cell mass, but what is more clearly seen is the internal matrix structure. This complex nanostructure traps and retains light after it enters the algae cell, allowing for more efficient photosynthesis. The structures in figures 4a and 4b appear nearly identical to those in figure 3., though figures 4a and 4b are SEM images of the biosilica frustules after having been chemically extracted from the algae. The white color of this sample when viewed by the naked eye demonstrated that the organic cell matter had been totally removed. In these images the important matrix nanostructure remains entirely intact, as evidenced by the continuous matrix pattern or small boxes seen in the images. The isolated biosilica frustules are still fully capable of trapping enormous amounts of light. My method will effectively produce intact CdS coated biosilica frustules and such frustules will reduce the reflective nature of a Dye-Sensitized Solar Cell. DSSC Creation & Output Measurement Culture and Frustule Isolation (a) (b) Figure 5. shows an SEM image of extracted biosilica frustules after CdS coating. The characteristic form of the frustules is preserved in the coating process. The complex matrix nanostructure needed for the retention of light remains intact and the box-like structures remain open, though it is clear that CdS particles now adhere to the silica walls of the frustules. (c) (d) Figures 11a-d. Creation of a blackberry-based DSSC and measure of cell output DSSC Data Two otherwise identical DSSC’s were prepared. The test DSSC contained CdS coated frustules incorporated into the TiO2 layer. As expected, increased light capture in the test cell allowed more photons to interact with the dye, producing more electricity and improving efficiency. Figure 5. CdS coated frustules Figure 6. CdS Coated Frustules Figure 1. illustrates the process of isolating intact biosilica frustules from the Pinnularia sp. so as to exploit their naturally occurring optical characteristics. Pinnularia sp. was chosen due to its traditional diatom structure and its relative ease of growth. The desired biosilica frustules were contained within significant organic cell matter that did not contribute to light-trapping qualities. This organic matter was removed, so that only the inorganic nanostructure capable of harnessing light remained. Following extraction, the frustules were centrifuged and separated from the SDS solution to ensure purity. The collected frustule solid was rinsed and suspended in water in preparation for coating with Cadmium sulfide. This semiconductor coating was necessary to preserve the light-trapping capabilities of biosilica frustules while allowing for the flow of electric current needed when integrated into a DSSC. Figure 6. shows the same image on a smaller scale, allowing for the exact identification of the CdS particles. The once smooth silica walls of the frustules are coated with the spherical CdS particles, giving the structure its lumpy look. This provides excellent proof that the CdS particles were effectively deposited on the frustules in a thin layer. Figure 7. shows an energy dispersive spectroscopy (EDS) spectrum of the portion of the sample shown in figure 5, further confirming the success of my deposition method. This technique of determining the composition of a sample shows the strong presence of Cadmium and Sulfur as expected, along with significant Silicon from the initial biosilica structure. The isolated frustules have been coated with the semiconductor CdS while maintaining their crucial structure. The new semiconducting nature of this structure will allow it to be incorporated directly into a solar cell to trap additional light without interrupting the electrical flow and functionality of the cell. Figure 12. Standard DSSC Voltage and Frustule-Enhanced DSSC Voltage as a function of light exposure CdS coated biosilica frustules Conclusion Experimental data supports both of my original hypotheses. SEM and EDS data confirms that intact biosilica frustules were successfully isolated from Pinnularia sp. algae, and these frustules were coated with a thin layer of Cadmium sulfide without deterioration of the important matrix nanostructure. UV-Vis data supports that CdS coated biosilica frustules trap significant light and decrease the amount of light reflected by a surface very similar to a Dye-Sensitized Solar Cell, increasing photon-dye interaction. Experiments comparing a standard DSSC to one containing CdS coated biosilica frustules indicate that this improvement effectively increases the electrical output efficiency of the solar cell. Figure 7. EDS Spectrum of CdS coated frustules All images and spectra shown were taken by the author