Dye-Sensitized Solar Cells: Gratzel Cells - Coursework Example

Dye-Sensitized Solar Cells (Gratzel Cells) Description Dye sensitized solar cells were invented in early 1990s by Michael Gratzel. These organic cells have proved to be inefficient even though they are low priced. Gratzel cells operate on principle similar to plant photosynthesis (Kalyanasundaram, 2009). However, Gratzel has recently established a solution for the cells to make them more efficient and cheaper (Agrawal and Tiwari, 2011). Organic dyes are used to absorb sunlight in Gratzel’s cells. In these cells, the sunlight hits the titanium dioxide nano-particles located below the dyes to release electrons. Dyes that were initially used to produce these cells contained ruthenium atoms that constitute an expensive metal. Gratzel and his team found an alternative to the expensive ruthenium (Kalyanasundaram, 2009). The team employed complex three part molecules that consist of a group of molecules that readily loses electrons, a group of molecules that readily accepts the released electrons and a bridging unit that contains a light absorbing group of molecules that are related to that found in plant chlorophyll (Agrawal and Tiwari, 2011). These changes are expected to result in more efficient organic solar cells that can compete effectively with silicon solar cells (Armaroli and Balzani, 2011). It is argued that the recent improvement on Gratzel cells published by Gratzel and his team improves efficiency by 12.3% and as such, its performance is presently comparable to silicon solar panels that are widely found in the market (Kalyanasundaram, 2009). These improvements have also resulted in cost reduction of the cells. It is expected that improvement in Gratzel cells that can enhance their lifetime by use of materials that can withstand harsh sunny conditions will enable the cells to be greatly adopted (Gevorkian, 2011). The new improved Gratzel cells have a greenish tint that increases the efficiency of converting light energy into electricity (Fonash, 2010). The greenish tint allows the cells to absorb spectrum colours with the highest energies while at the same time rejecting the rest. Construction The Gratzel cell has three parts. The top part of the cell is the anode that is made of fluoride doped tin dioxide. The backside of the anode has titanium dioxide that forms a highly porous structure (Kalyanasundaram, 2009). The plate is then immersed in a mixture of ruthenium dye and solvent to result in a thin layer of the dye that covalently bonded to the surface of the titanium oxide (Agrawal and Tiwari, 2011). A separate plate made of thin layer iodide electrolyte is then spread over conductive sheet made of platinum (Armaroli and Balzani, 2011). The plates are joined and sealed together to prevent leakage of the electrolyte. Operation Dye sensitized cell has a mesoporous oxide layer that is made of nanometre sized particles that are sintered together to allow electronic conduction (Gevorkian, 2011). Titanium oxide is the material of choice although zinc oxide and Nb2O5 can also be used. The cell also has a monolayer of charge transfer dye that is attached to nano-crystalline film. It is this monolayer that is photo excited to result in injection of an electron into the conduction band of the oxide (Agrawal and Tiwari, 2011). Electron donation from the electrolyte that is often an organic solvent that contains a redox system restores the original state of the dye (Kalyanasundaram, 2009). The redox system may be iodide/triiodide couple. Iodide regenerates the sensitizer to intercept the recapture of the conduction band electron by the oxidized dye (Armaroli and Balzani, 2011). The reduction of the triiodide regenerates the iodide at the counter electrode and the circuit is completed via electron migration via the external load. The generated voltage corresponds to the differences that exist between the Fermi level of the electron in the solid state and the redox potential of the electrolyte (Agrawal and Tiwari, 2011). Thus, Gratzel cells produce electric power from solar energy without suffering any permanent chemical transformation. Efficiency Solar cells are characterized by various measures. One of these measures is the amount of electrical power produced when a specified amount of solar energy is illuminated on the cell (Fonash, 2010). This is what is referred to as solar conversion efficiency and is expressed as a percentage (Agrawal and Tiwari, 2011). The electrical conversion efficiency of the new improved Gratzel cell is 30% as compared to 26% for silicon cells. Dye-sensitized solar cells are extremely efficient in terms of quantum efficiency (Kalyanasundaram, 2009). Loses that are often small result from conduction losses in the titanium oxide and the clear electrode or optical losses in the front electrode (Gevorkian, 2011). It is estimated that quantum efficiency for green light is 90%. The optimum voltage generated by Dye-sensitized solar cells is the difference between the Fermi level of the titanium oxide and the redox potential of the electrolyte (Armaroli and Balzani, 2011). This is estimated to be about 0.7 V. it should be noted that only photons absorbed by the dye produce current. Degradation Dye-sensitized solar cells are degraded when they are exposed to UV radiation (Kalyanasundaram, 2009). The efficiency and protection of the cell may be attained by use of barrier layer that may be made of UV stabilizers and luminescent chromospheres that absorb UV in addition to antioxidants (Armaroli and Balzani, 2011). Advantages Dye-sensitized solar cells are advantageous since they portray similar performance under real life working condition (Agrawal and Tiwari, 2011). Moreover, they are the most efficient form of solar technology. With increased efficiency published recently, the Dye-sensitized solar cells are likely to be deployed where higher efficiency cells are more viable. In addition, Dye-sensitized solar cells are advantageous because they are capable of capturing power even in low light or even rainy conditions (Armaroli and Balzani, 2011). Furthermore, Dye-sensitized solar cells are the only solar cells that can be constructed truly transparent with their colour depended on the sensitizer used (Kalyanasundaram, 2009). The cost of Dye-sensitized solar cells is relatively cheap and can work on broad scale. The Gratzel cells do not require complex setup to be constructed and as such, they are less expensive in comparison to silicon-based solar panels. Unlike other solar panels, Dye-sensitized solar cells are mechanically robust and as such can be engineered into flexible sheets (Gevorkian, 2011). Moreover, the cells do not require special protection from minor elements such as hailstones and tree strikes. Dye-sensitized solar cells can also be constructed on various substrates such as metal sheets or plastic foils without reducing its efficiency. Disadvantages Dye-sensitized solar cells use liquid electrolyte that is not stable to varying temperatures (Fonash, 2010). Due to this instability, the electrolyte might freeze at low temperatures interrupting power supply and can potentially cause physical damage (Agrawal and Tiwari, 2011). Moreover, higher temperatures may cause the liquid to expand making it almost impossible to seal the panels together. Previously, Gratzel cells employed ruthenium that is very expensive but this problem has since been resolved by new improvement in the cell (Kalyanasundaram, 2009). Another disadvantage of Dye-sensitized solar cells is that the electrolyte has volatile organic compounds that might be hazardous to human health (Armaroli and Balzani, 2011). Development Early experimentation of Dye-sensitized solar cells deployed dyes, which were only sensitive to high frequency end of the solar spectrum. Newer dyes respond to a wide range of frequency. These newer dyes have a higher chance of photon conversion into electron that has an overall efficiency of 90% (Agrawal and Tiwari, 2011). The lost power is attributed to optical losses in top electrode. A good solar cell needs to be able to produce electricity for at least 20 years. However, newer dyes are known to breakdown in high light situations (Kalyanasundaram, 2009). Various studies have been undertaken in the last decade to address this problem and as such new alternative dyes have emerged, which is temperature stable and light in nature (Gevorkian, 2011). These dyes also have a higher efficiency conversion as compared to earlier versions. Although the development of Dye-sensitized solar cells has been around for the last two decades, they are still at the start of their development. Recent studies have indicated efficiency gains in these cells. References Agrawal, B., and Tiwari, G. 2011. Building Integrated Photovoltaic Thermal Systems: For Sustainable Developments. London: Royal Society of Chemistry Armaroli, N., and Balzani, V. 2011. Energy for a Sustainable World: From the Oil Age to a Sun-Powered Future. London: John Wiley & Sons Fonash, S. 2010. Solar Cell Device Physics, 2nd Ed. London: Academic Press Gevorkian, P. 2011. Large-Scale Solar Power System Design (GreenSource): An Engineering Guide for Grid-Connected Solar Power Generation. New York: McGraw-Hill Prof Med/Tech. Kalyanasundaram, K. 2009. Dye-sensitized Solar Cells. Jakarta: EPFL Press Read More