Kenta Sueyoshi 1262997 CHEM 317 AA 6/3/15 Dye-Sensitized Soar Cells: The Grätzel Cell In the past decade, the world has especially been worried about the energy crisis. With the human population using 15TW a year, 85% of the energy coming from fossil fuels, and the energy causing pollution, scientists need to find a new source of renewable energy (Balzani, 2008). Some possible sources being investigated are wind, water, wave, and solar. Solar energy is found to be the most promising, since more than 10000 times the amount of energy humans use hits the surface of the earth every day from the sun. However, efficiently using the large amount of energy available is a large obstacle. One way this energy can be used is through a dyesensitized solar cell (DSSC). A dye-sensitized solar cell is a simple cell using a dye. In the case of a Chemistry 317 class at the University of Washington, the students used pomegranate juice and blackberry extract as their dye. In these cells, there is a layer of dye, electrolyte solution, and a titanium dioxide layer between two plates of conductive glass. Titanium dioxide is a white substance commonly found sunblock. This white material is a semiconductor, making it more conductive than an insulator, but less than that of a conductor. In the case of titanium dioxide, it can absorb and transfer energy at the UV light spectrum. However, by adding the pomegranate and blackberry dyes, this absorption band can be shifted to the visible spectrum. The electrolyte solution in the middle finishes the cell by allowing a flow of electrons in a circuit like way.
By using the red-ish pomegranate dye and the dark red blackberry dye, the dyes absorb light corresponding to their color, producing a current in the solar cell. The students in Chemistry 317 found that on average, a solar cell made with the pomegranate dye produced about 0.236V/cm2, while the blackberry dye produced 0.156V/cm2. At first glance, it may seem like the pomegranate dye does a better job at converting sunlight into electricity, but many other factors effect the efficiency of these cells. For one, the preparation of the pomegranate cells may have been done better than the blackberry cells. This was, after all, a classroom made solar cell, so the product of what is made will not be perfectly the same every time. Another source of differencing conversion efficiency could result in the concentration of the dye used to make each cell. Since the pomegranate dye was already a solution, while the blackberry dye was an extract from blackberries, the solution could have been different. If the concentration is different, the amount of dye molecules inside the cell will differ. The more dye there is in a given area, the more like can be converted, causing a higher voltage per area. Both 0.236V/cm 2 and 0.15V/cm2 are not very large values; they are a significant amount for a cell that was made in an undergraduate lab course. Along with the inconsistent, small voltage values, each of the dye molecules can’t be reused in the solar cells, so the life span of this small cell is very short. By using a DSSC, the sunlight can readily be converted into electricity, but means of storing the electricity becomes a problem. One method of storage being researched is copying plants; through photosynthesis. By using light from the sun, some water, and carbon dioxide in air, photosynthesis would allow storage of solar energy in carbohydrates (Balzani, 2008). This is a great idea because making one carbohydrate requires six carbon dioxides, so the more photosynthesis happens, the less carbon dioxide there will be in the atmosphere. Another method being researched right now to store the produced solar energy is water splitting. Since water is
made of hydrogen and oxygen, which can be burned as a fuel later, water splitting turns out to be an effective way to store energy. However, since it’s not as easy of a process as it sounds, scientists like Hong et al. have to discover the right metal to be mixed in to the reaction to help split water (2014). Pollution-wise, water splitting is the best method of burning fuel because no matter how much hydrogen gas you burn, only water is produced, so no pollution happens.
References: Balzani, V.; Credi, A.; Venturi, M.; Photochemical Conversion of Solar Energy. ChemSusChem 2008, 1, 26-58. Hong, E.; Kim D.; Kim, J. H.; Heterostructured metal sulfide (ZnS-CuS-CdS) photocatalyst for high electron utilization in hydrogen production from solar water splitting, Journal of Industrial and Engineering Chemistry, 2014, 20, 3869-3874.
