o Ref 1: Solar Vapor Generation Enabled by Nanoparticles. Day, Jared; Halas, Naomi; Lal

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1 Introduction Solar Energy A.Uses of Solar Energy o Ref 1: Solar Vapor Generation Enabled by Nanoparticles. Day, Jared; Halas, Naomi; Lal Surbhi; Neumann, Oara; Nordlander, Peter; and Urban, Alexander. ACS Nano. Vol. 7 No. 1 November Pages Panels of pipes are heated by the sun Hot water is stored for use throughout the day Subwavelength metallic and carbon based nanoparticles increase efficiency particles absorb optical radiation and releases heat to the surroundings o Ref 2: Concentrating on Solar Electricity and Fuels. Müller-Steinhagen, Hans; Roeb, Martin. Science. Vol. 329 No August Pages Concentrating Solar Power (CSP) uses mirrors to focus solar radiation onto an absorber CSP generates temperatures from degrees Celsius This heat can be used to boil water to run steam turbines that generate electricity There are 630 CSP plants in the world today o Ref 3: Solar Photovoltaic Cells. Mickey, Charles D. Journal of Chemical Education. Vol. 58 No. 5 May Pages A direct conversion of solar radiation into electricity Excitation of electrons into the conduction band Separation forces electrons to return to ground state through a wire Directional flow of electrons through a wire produces electricity Solar power has many applications from electricity for the home to powering electronics o Ref 4: Photocatalytic Hydrogen Production by Direct Sunlight: A Laboratory Experiment. Koca, Atif; Sahin, Musa. Journal of Chemical Education. Vol. 80. No. 11 November Pages Photocatalysis of water produces hydrogen gas Hydrogen can be burned as a fuel or converted into electricity Hydrogen cars are currently too dangerous for mass production o Ref 5: Releasing Stored Solar energy within Pond Scum: Biodiesel from Algal Lipids. Blatti, Jillian L; Burkart, Micheal D. Journal of Chemical Education Vol.89. P Biodiesel is derived from the lipids and oils of microalgae 1

2 Biodiesel produces less pollution than petroleum, and the microalgae absorb CO 2. Unlike other biofuel, microalgae can live in a wide range of environments Unlike hydrogen fuel, current gas stations can be easily converted to biodiesel B. General Types of Solar Cells o Ref 3: Band gap is important when choosing a semiconductor. 1.5eV is ideal. Charge carriers can be controlled by doping N-type doping creates extra negative (electron) charge carries. Ex. Si-P P-type doping creates extra holes. Ex. Si-B P-n junction creates excess holes next to excess electrons to increase conduction Homojunction is the addition of small amounts of dopant to a pure material Heterojunction is the joining of two different semiconductors o Ref 6: Efficient Light-to-Electrical Energy Conversion: Nanocrystalline TiO2 Films Modified with Inorganic Sensitizers. Meyer, Gerald. Journal of Chemical Education. Vol. 74 No. 6 June Pages Sensitizer donates electrons to the semiconductor The sensitizer is regenerated by electrons from the electrolyte Electric current can be generated at lower energies than the bandgap Electron transfer happens much more rapidly than recombination Ru(II)polypyridyl coordination compounds are stable and efficient sensitizers LHE (light-harvesting efficiency) is 99% o Ref 7: The Molecular Engineering of Organic Sensitizers for Solar-Cell Applications. Delcamp, Jared; Grätzel, Michael; Holcombe, Thomas W.; Nazeeruddin, Mohammad K.; and Yella, Aswani. Angew. Chem. Int. Ed No. 52. Pages Organic molecules would decrease cost, be lightweight, efficient, and flexible Work in the same way as inorganic sensitized solar cells Donor-pi-bridge-acceptor structure is most promising Both electron donating and accepting Multiple substitution sites for the creation of analogs The JD21 analog shows 8.4% power conversion efficiency o Ref 8: Significant Improvement of Polymer Solar Cell Stability. Krebs, Frederik C, Spanggaard, Holger. Chemistry of Materials. Vol. 17(21) P Polymer-based solar cells could become a cheap source of renewable energy 2

3 Many of the most efficient systems have a conjugated polymer such as polythiophene This polymer has alkyl side chains to stabilize it The widespread application of organic solar cells depends on the resolution of several factors One factor is the lifespan of the organic polymer, which is currently less than 1000 hours Another factor is cell efficiency, which is currently between 2% and 4% for cells C. Statement of Need and Outline of Approach Materials and Methods Results Discussion Conclusion References 3

4 Solar Energy Introduction A. Uses of Solar Energy As the world begins to use up all of its oilfields and coal mines, a new power supply is in increasing demand. Converting solar radiation into a renewable power source is a promising solution. There are five main types of solar power: heating, concentrating solar power (CSP), solar cells, photocatalysis, and biomass fuels. The first type of solar power is heating large panels of water pipes. The hot water can be stored for use throughout the day. Nanoparticles are being developed to increase the efficiency of heating. 1 The sub-wavelength metallic and carbon nanoparticles absorb solar radiation and dissipate the energy as heat to the surrounding fluid. 1 Heating has practical applications for homes and businesses which receive large amounts of sunlight during the day. The second type of solar power is CSP. CSP plants use thousands of mirrors to concentrate solar radiation on an absorber which can reach temperatures from 200 to 1000 degrees Celsius. 2 The heat can boil water used to power steam turbines which generate electricity. There are already 630 CSP plants in the world. 2 The third type of solar power is solar cells. Solar cells directly convert solar radiation to electricity. Electrons absorb solar radiation and are excited into the conduction band. 3 The electrons are forced to return to ground state through a wire generating electrical current. 3 Most solar power research is in the field of solar cells. The application of solar cells ranges from powering entire houses in the daytime to powering personal electronic devices. The fourth type of solar energy is photocatalysis, which uses sunlight to create hydrogen from water. 4 Hydrogen can be burned as fuel or used to generate electricity. 4 Hydrogen was initially of great interest as an alternative fuel for cars, but due to the high combustibility of hydrogen and difficulty with storage, interest is waning. The fifth form of solar power is biodiesel synthesized from the oils and lipids of microalgae. 5 Microalgae can be grown in a wide range of environments, even in contaminated water that is unsuitable for human and animal consumption. The biodiesel produces less greenhouse gas than petroleum, and gas that is produced would be consumed by the next generation of microalgae. 5 Also, biodiesel would not require a large change to the current infrastructure because existing gas stations can be converted to biodiesel. B. General Types of Solar Cells There are four main types of solar cells: semiconductor, sensitized inorganic, organic dye, and organic polymer solar cells. Semiconductor solar cells absorb solar radiation and excite electrons into the conduction band. 3 Electrons that fall back to ground state are energetically useless, but electrons that are forced through a wire generate an electric current. Conductivity of 4

5 the semiconductor material depends on the number of charge carriers (electrons) and the band gap. 3 Materials with a band gap of 1.5 ev are ideal. 3 Silicon, for example, has a band gap of 1.8 ev which is a little higher than ideal. 3 The number of charge carriers is more easily controlled by doping. There are two types of doping. P-type doping increase the number of holes, or positive charges. 3 For example, silicon can be doped with boron, which can be satisfied with three bonds, leading to the creation of a hole. N-type doping increases the number of electrons, or negative charge carriers. 3 Silicon can be n-type doped with phosphorus, which can hold an fifth electron pair. To further increase conduction, p-type doped material can be placed next to n-type doped material to form a p-n junction. 3 Junctions can be either homojunctions where small amounts of dopants are added to pure material or heterojunctions in which two different semiconductor materials are used. 3 The second type of solar cells, sensitized inorganic solar cells, work much the same as semiconductor cells but use a sensitizer to push electrons into the semiconductor material. 6 The sensitizer is regenerated by electrons in the electrolyte. 6 This method allows electric currents to be generated at lower energies than the bandgap because electron transfer happens much more quickly than recombination. 6 Ru(II)polypyridyl coordination compounds have been found to be stable and efficient sensitizers and have lightharvesting efficiencies (LHE) of 99%. 6 The third type of solar cells, organic dye sensitized solar cells, work the same way as sensitized inorganic solar cells but use organic materials which would decrease production cost and weight and increase efficiency and flexibility. 7 Compounds containing a donor-pi-bridge-acceptor structure are the most promising partly because they are both electron donating and accepting. 7 These compounds also contain multiple substitution sites for analog engineering. 7 Grätzel was able to construct the JD21 analog which showed 8.4% power conversion efficiency. 7 The fourth type of solar cell is organic polymer solar cells, or plastic solar cell. Organic polymer cells are a cheaper alternative to inorganic cells. 8 Organic polymer cells have a conjugated polymer with side chains to stabilize it. 8 A few issues with organic polymer cells still need to be addressed. One of these issues is the efficiency of the cells, which currently is still below those of inorganic cells. 8 Another issue is the durability of the organic polymers, which start break down after around 1000 hours. 8 While this is enough for some applications, powering houses or other large scale applications will require more durable cells. C. Statement of Need and Outline of Approach Materials and Methods Results Discussion 5

6 Conclusion References (1) Solar Vapor Generation Enabled by Nanoparticles. Day, J.; Halas, N.; Lal, S.; Neumann, O.; Nordlander, P.; Urban, A. ACS Nano. 2012, 7, (2) Concentrating on Solar Electricity and Fuels. Müller-Steinhagen, H.; Roeb, M. Science 2012, 329, (3) Solar Photovoltaic Cells. Mickey, C. D. J. Chem. Educ. 1981, 58, (4) Photocatalytic Hydrogen Production by Direct Sunlight: A Laboratory Experiment. Koca, A.; Sahin, M. J. Chem. Educ. 2003, 80, (5) Releasing Stored Solar energy within Pond Scum: Biodiesel from Algal Lipids. Blatti, J. L.; Burkart, M. D. J. Chem. Educ. 2012, 89, (6) Efficient Light-to-Electrical Energy Conversion: Nanocrystalline TiO 2 Films Modified with Inorganic Sensitizers. Meyer, G. J. Chem. Educ. 1997, 74, (7) The Molecular Engineering of Organic Sensitizers for Solar-Cell Applications. Delcamp, J.; Grätzel, M.; Holcombe, T. W.; Nazeeruddin, M. K.; Yella, A. Angew. Chem. Int. Ed. 2013, 52, (8) Significant Improvement of Polymer Solar Cell Stability. Krebs, F. C.; Spanggaard, H. Chem. Mater. 2005, 17,