Solar Cell FundamentalsSolar Cell FundamentalsSJSU Short CourseD. W. ParentD. W. Parent1Topics• History• Solar Cell Basics– Diode– Major parts of a Si Solar CellHow does it work–How does it work• Absorption• LossesEl i l Ch i i–Electrical Characteristics• VOC, ISC, RS, RSH, Max Power Point–Advantages/Disadvantages2ggHistory• 1839 Becquerel– Observed that there was a light dependant voltage between two electrode immersed in an electrolyte.• 1876 The same effect was demonstrated in liselenium• 1941 First silicon based solar cell demonstratedii f d l ll h• 1954 Beginning of modern solar cell research.3History of Cell Efficiency• M.A. Green, “Very High Efficiency Solar Cells-Science and Technology”, IEEE Transactions on Electron Devices, Vol. 46, No. 10, pp. 1940-1947, (1999).•Notice: This trend is not like “Moore’s Law”•Notice: This trend is not like “Moore’s Law”.• Price per installed Watt is probably a better figure of merit to track.4• Deregulation (Blame anyone you like.) made power generated by solar or wind economically viable!Wh t i S l C ll?What is a Solar Cell?• A structure that converts solar energy directly to DC electric energy.– It supplies a voltage and a current to a resistive load (light, battery, motor).Power Current x Voltage Current2x R Voltage2/R–Power = Current x Voltage=Current2x R= Voltage2/R• It is like a battery because it supplies DC power.It i t lik b tt b th lt li d•It is not like a battery because the voltage supplied by the cell changes with changes in the resistance of the load5of the load.Uses for Solar Energy• It is fun to watch PG&E’s meter run backward.• Renewable power•Power for remote locations•Power for remote locations6Basic Physics of Solar Cells• Silicon (Si) is from group 4 of the period table. When many Si atoms are in close yproximity, the energy states form bands of forbidden energy states.gy• One of these bands is called the band gap(Eg) and the absorption of light in Si is agap(Eg) and the absorption of light in Si is a strong function of Eg.7Basic Physics of Solar Cells• Si is covalently bonded: It shares electrons.– When a Si atom is replaced with a group 3 (Al, B) it forms a positive particle called a hole that can moveforms a positive particle called a hole that can move around the crystal through diffusion or drift (electric field).–When a Si atom is replaced with a group 5 (As, P) it forms an electron that can move around the crystal.–By selectively doping the Si Crystal when can change yypgy gthe resistivity and which type of carrier transfers charge (carries current). Because we can selectively dope a Si crystal it is called a semiconductor.8yBasic Physics of Solar Cells• A solar cell is a very large diode.– When Si that is doped p-type is next to a region of Si doped ntype the holes from the ptype side diffuse todoped n-type, the holes from the p-type side diffuse to the n-type side. The electrons diffuse to the p-type side.–This creates an electric field. – This electric field makes it easy for current to flow in one direction, but hard to flow in the other.,– This electric field also separates electrons and holes that have been created by the absorption of sun light. When the electrons and holes are separated electric power can9the electrons and holes are separated electric power can be extracted from the circuit.Cross section of a Sl CllSolar Cell10Solar Cells are modeled as diodes with an fixed current sourcewith an fixed current sourceThe shut resistance is present in all diodes (like the 1N914), but can usually be neglected in the case Rshunt is a major problem in solar), y gof signal processing diodes because there are is so small (Rshunt is proportional to 1/Area.)problem in solar cells given that they are large diarea devices.RS limitsRS limits power as well.11This is simplified. Usually a two diode model is required.VOC, ISC, and the maximum power point are important metrics for solar cells.Voc is the open circuit voltageMaximum power point. (Yes it looks like alooks like a minimum.)Isc is the short circuit current.12A large series resistance lowers the maximum power point.Voc is not affected.Maximum power point. Rs=10Rs=1Rs 113Isc is reduced.A small shunt resistance series resistance lowers the maximum power point, VOC and ISC.Voc is affected.Maximum power point. Rshunt=100Rshunt100Rs=1Meg14Isc is reduced.A commercial cell can put out 1 Amp of currentcurrent.1Ω reduces Isc by half!Even .1Ω makes a difference.15It looks like Rshunt can by 1k and not affect the performancepRshunt has been hard to solve for large area cells in100Ω1kΩ1MegΩlarge area cells in our lab.1Meg Ω16Test Set up for Solar cells17Solar cell under