能量转换与存储原理教学资料designofcsisolarcells.ppt
Design of Silicon Solar CellsHighly recommended readingJenny Nelson,The Physics of Solar Cells,Chapter 7,Monocrystalline Solar CellsWhy Silicon?Si has a band gap of 1.1 eV,which is not far from the optimal value of 1.4 eV for a single junction cell Si is great for MOSFETs because its surface is easily passivated by thermal oxidation.Massive amounts of research have been done on silicon.The solar cell research community borrowed all of the methods for making wafers,doping,patterning and making electrical contacts.Si is perhaps the best understood material in the world.Si is very stable.Si is the second most abundant element in the earths crust.Design of a basic silicon solar cellNelson,p.187 Why are n+p Si junctions preferred over p+n junctions?Why is there a thin heavily doped layer and a thick lightly doped layer?Why those thicknesses?How thick should the wafer be?Issues to considerAbsorptionThe diffusion lengthWill the wafer break?This is usually most important for wafers.determines where the light is absorbed Near the band edge,not all of the light is absorbed and some minority carriers are lost due to back surface recombination.At h=3.5 eV,the light is absorbed right at the surface.E(eV)1/(m)1.251002.513.50.01Nelson p.182transmission=e-axAbsorption depth in SiSurface texturingNelson p.190Surface texturing scatters light so that it travels more horizontally though the cell.This helps absorb photons with energy just above the band gap.It reduces reflection also.Ln must be long enough to allow the carriers to reach the junctionnnnDLIf p 1018 cm3,then will be greater than 1 ms and primarily determines by the density of recombination centers.Modern electron grade Si has a lifetime above 1 ms.The diffusivity depends on the doping densityImpurity concentration(cm-3)Mobility(cm2/Vs)Streetman p.99 The Einstein Relationship(Streetman p.129)tells us that The electron mobility is higher than the hole mobility The mobility drops substantially when n or p exceeds 1017 cm-3qkTDmDesign of a basic silicon solar cellNelson,p.189 The base should be p-type so that the minority carriers will be electrons,which diffuse faster than holes.The base should be lightly doped so that recombination will be slower and D will be higher.Why dont Si solar cells work well at l 1100 nm?400600800100012000.10.20.30.40.50.60.70.8IPCEWavelength(nm)Eg=1.1 eVl=1240 nm-eV/EBack surface fieldHeavily doping the Si near the back contact creates a barrier for electrons and reduces surface recombination.This becomes very important for harvesting the low energy photons as the wafer gets thin.Nelson p.190What should the doping level of the substrate be?The minority carrier diffusion length is L=(Dnn)1/2 and should be as long as possible.The doping level must be 1018 cm-3 to avoid rapid Auger recombination and should be as low as possible.On the other hand,to get a high Voc,we need a low Io.)()DqA(I22n0DpipAninpnpnpNLnDNLnDqALpDLnA high doping density is needed to minimize I0.(Note that Dn and Ln vary with doping,but NA is still the dominant factor.)A doping level of 1016 cm-3 ends up being a good compromise between the two.The top of the cell is heavily doped to minimize series resistanceThe resistivity of the n-type layer is The sheet resistance is DeNqm1tNqDeSm1The power loss due to the series resistance isThe fraction of power loss is For a typical commercial Si cell,s=40/Jmpp=30 mA/cm2 Vmpp=450 mVS must be 4mm to keep the power loss below 4%.3223SbJPSlossmppmppSMaxPowerPolossVJSPP12p2intGreen,Solar Cells p.146Passivated Emitter Solar Cell(PESC)Martin Green et al.invented the concept of passivating most of the top surface with a thin oxide to prevent surface recombination.The PESC took the efficiency to 20%in 1985.Nelson p.196Rear point contact cellIntroduced by Dick Swanson at Stanford in 1992 with an efficiency of 22%.Front surface is textured and passivated.This technology is being commercialized quite successfully by SunPower.Since there is no metal on top,there are no shadowing losses.These cells look great because very little light is reflected from them.Nelson p.196Passivated emitter,rear locally diffused(PERL)cellDemonstrated by Martin Green in 1994.Holds the world record for Si with =24%.Rear point contacts minimize the area of metal and rear surface recombination.Nelson p.197Solar Cell Efficiency Table(v31)Lab Efficiencies M.Green,Progress in Photovoltaics(2008)Minority Carrier Lifetimes affect Dark Current DpipAninNLnqDNLnqDJ220Typical Silicon PV Doping281920162019010*04.11000194.1075.3100061.105.3710*06.1cmmAJMobilities are affected by doping.Pretty low.right?sVscmVscmcmNcmNpnnpAD6316319101500150101022mmDark Current affects the Voc VmVJJqkTVSCOC55.010*15.42ln*025.0ln8028010*04.1cmmAJWorld Record Si Device has short-circuit current densityOur estimated J0 27.42cmmAJSCWorld Record Si PV device has a Voc=0.7V.There must be very,very low recombination rates in these devices!
