Solar Cells 3 generations of solar cells: Starting with 1st, photovoltaics Bulk technologies – wafer-based, typically 8cm in diameter, 3mm in thickness usually cut from a crystalline silicon Monocrystalline- absolutely pure

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: Photons pass through- energy of photon is less then bandgap energy of silicon semiconductor Photons are reflected- photon energy is greater then the band gap energy of silicon Photon is absorbed- photon energy is within the bandgap energy of silicon

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 Over 95% of all solar cells produced worldwide are silicon semiconductors Silicon semiconductors can be doped on one side to produce p-material (boron doped silicon) or the other side can be doped to produce more n-material (phosphorous doped silicon)

These valence electrons escape their orbits leaving holes 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 Np-junctions are really made by diffusing an n-type dopant into one side of a p-type wafer (or vice versa) A diffusion of electrons occurs from the n-type side of the junction in to the p-type side of the junction. Electrons diffuse across the p-n junction and recombine with holes on the p-type side. Forming depletion zone. This separation of positive and negative charge is an electric field, allowing current to flow only one way (like a diode).

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: 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 Any number of modules can be connected together to give the desired electrical output

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) Higher cell temps => lower output => lower efficiency Efficiency- % of power converted (from absorbed light to electrical energy) and collected The 1st generation of solar cells, (photovoltaic) are comprised of one layer of p-n junction Starting at 6% efficiency – 12% Cannot convert more than 25% of the solar energy into electricity because IR radiation does not have enough energy to separate positive and negative charges in the material Efficiency is improved in 2nd generation photovoltaics by layering p-n junctions. Each layer designed to absorb a successively longer wavelength of light (lower energy) – absorbing more ot the solar spectrum – increasing amount of electrical energy produced At least 30% efficiency 3rd generation- semiconductor that does not rely of p-n junctions (dye sensitive cells, organic polymer cells, quantum dot solar cells)

2nd gen. – each layer is designed to absorb longer wavelengths 2nd gen. – each layer is designed to absorb longer wavelengths. Absorbing more of the solar spectrum. More power output.

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 People love the technology but they don’t want it on their house because it is “ugly” Shingles are highly absorptive can actually add to the buildings thermal load Thus, shingles need to be thermally isolated from the building so they don’t heat it

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 Spray on solar power cells Solar cell window coating- power home appliances with the absorbed 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 Not all energy is used immediatly Batteries good for small amounts of energy, but batteries don’t have long life span and large batteries are very expensive

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.phy-astr.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.phy-astr.gsu.edu/hbase/mod1.html>.