Transcript Solar Cells
Solar Cells Photoelectric Effect This effect is in direct contradiction to the laws of classical physics and without which solar cells would not exist. Here light travels in the form of photons with energy described by: E hf Not all photons are reflected some are absorbed or transmitted When a photon is absorbed, the energy of the photon is transferred to an electron in the crystal lattice This is known as the photovoltaic effect Solar cells are made of semiconducting material, traditionally Silicon Energy of the photon is transferred to the valence electrons in the n-type layer These valence electrons escape their orbits leaving holes (electrons are majority carriers, holes are minority carriers) This creates mobile electron-hole pairs In the p-type layer electrons are the minority carriers and holes are the majority carriers When p-type and n-type layers are placed together a p-n junction is formed and current begins to flow When electrons from the n layer move into the p layer a depletion zone is formed This zone separates the positive and negative charges and prevents a flow between them – an electric field is created The flow of electrons is a DC current (I) The electric field of the cell is a voltage (V) Here power (P) is given by: P VI The diode created by the electric field allows current to flow in only one direction across the junction Connecting the sides of the cell externally will cause electrons to flow to their original p side to meet with holes Many of these cells are connected to in order to create a solar panel By connecting the cells in series a higher voltage is obtained By connecting the cells in parallel a higher current is obtained Efficiency ( ) Maximum power (Pm) in W Irradiance of input light (E) measured in W/m2 Surface area or solar cell (Ac) in m2 Fill factor (FF) Open circuit voltage (Voc) Short circuit current (Isc) Pm E Ac Pm Ac E FF Voc I sc Voc I sc Solar cells 1st generation- large area single layer n-p junction Crystalline silicon Monocrystalline 3rd generationSemiconducting device which does not rely on p-n junctions 2nd generationmulti. Layer p-n junctions Usually silicon Polycrystalline Ribbon silicon Thin Films deposited on supporting substrates Dye sensitized cells Organic polymer cells Quantum dot cellsElectron-confined nanoparticles Thin-Films Less material needed to create solar cell but less energy conversion efficiency However, multi-layer thin films may have higher efficiencies then silicon wafers Use flexible resin film substrates instead of glass sheet substrates Solar Shingles Thin film PVs can be used as shingles Cells shown here are triple junction Each shingle has a pair of wires coming off its back so the system can be wired inside the attic Conductive Polymers Built from thin films of organic semiconductors The high-efficiency cells made from GaAs where used in Deep Space 1 Dye-sensitive solar cells Absorption occurs in dye molecules Electrons are passed on to the n-type TiO2, holes are passed to an electrolyte on the other side of the dye Heat and UV light cause cells to degrade Low-cost production Moderate efficiency (less then 10%) Quantum Dots Semiconductor quantum dots can slow the cooling of hot electrons This could enhance efficiencies up to 66% Combination of quantum dots with polymers creates cells that can absorb IR radiation Energy Storage Energy produced by solar cells can be stored in batteries For domestic systems it is most effective to run the meter backward- in effect selling the electricity to the grid When electricity is needed and the solar cells are not producing the electricity can be bought back from the network References Dye-Sensitized Solar Cells. European Institute for Energy Research. 2005. 23 April 2006 <http://www.eifer.uni-karlsruhe.de>. Green, Martin A. Solar Cells. New Jersey:Prentice-Hall, 1982. Highlights of the 2003 NCPV and Solar Program Review Meeting. NREL. 2003. 23 April 2006 <http://www.nrel.gov>. Lovgre, Stefan. Spray-On Solar-Power Cells Are True Breakthrough. National Geographic News. 14 Jan. 2005. 18 April 2006 <http://news.nationalgeographic.com>. Photodetectors. HyperPhysics. 20 April 2006 <http://hyperphysics.phyastr.gsu.edu/hbase/ligdet.html>. PV Cells. Specmat.com. 23 April 2006 <http://www.specmat.com>. Solar Cells. 23 April 2006 <http://www.corrosion-doctors.org/Solar/cells.html>. Solar Cell. Wikipedia. 25 April 2006 <http://en.wikipedia.org/wiki/Solar_cell>. Solar Electricity. The Electricity Forum. 23 April 2006 <http://electricityforum.com/solar-electricity.html>. U.S. Department of Energy. Roofing. 10 Dec. 2004. 23 April 2006 <http://www.eere.energy.gov>. U.S. Department of Energy. Solar Shingles. 5 Jan. 2006. 23 April 2006 <http://www.eere.energy.gov>. Wave-Particle Duality. HyperPhysics. 25 April 2006 <http://hyperphysics.phyastr.gsu.edu/hbase/mod1.html>.