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Lab#5Current/VoltageCurvesandEfficiencyMeasurements
October22,2013PHYS4580,6280,and7280
R.J.EllingsonandM.J.Heben
Simplesolarcellstructure
TheDiodeEquaRon
IdealDiodeLaw I=netcurrent;I0=ʺdarksaturationcurrentʺ,akadiodeleakagecurrent,akareversesaturationcurrent; V=appliedvoltageacrosstheterminalsofthediode;q=electroncharge;k=Bol;mannʹsconstant;T=absolutetemperature(K). I0isapropertyofthejunctioninterfacesandrecombinationinthedevice.
hSp://en.wikipedia.org/wiki/Diode
LightGeneratedCurrentisOppositeDirecRonofForwardDarkCurrent
+
‐ JForward
hν
JLight
LInkTqVII −⎥
⎦⎤
⎢⎣⎡ −⎟
⎠⎞⎜
⎝⎛= 1exp0
PrincipleofSuperposi;on
Wherenisthediodequalityfactor,reflectstypeofrecombina;oninthedevice
p‐typeemiSer(window)n‐typebase(absorber)
n‐typeemiSer(window)p‐typebase(absorber)
Vbi
+ ‐ +‐
+Junderforwardbias
HomojuncRonsolarcell(e.g.,Silicon)
+Junderforwardbias
+‐
J/V?
+
‐
J/V?
Beforecontact
Atequilibrium
+J
+V
hνJL JL
JL
JL
LightGeneratedCurrentisOppositeDirecRonofForwardDarkCurrent
TypicalSideviceconfigura;on
GotoAnimaRon
IVCurve
IL=lightgeneratedcurrent.
LInkTqVII −⎥
⎦⎤
⎢⎣⎡ −⎟
⎠⎞⎜
⎝⎛= 1exp0
• TheIVcurveofasolarcellisthesuperposiRonoftheIVcurveinthedarkwithaconstantlight‐generatedcurrent(IL).
• ThelightcurrentshietheJ/VcurvedownsoVOCoccursnearthe“turn‐onknee”inthediodecurve.
Lindholm,F.A.,J.G.Fossum,andE.L.Burgess,"ApplicaRonofthesuperposiRonprincipletosolar‐cellanalysis",IEEETransac;onsonElectronDevices,vol.26,no.3,pp.165–171,1979.
Shortcircuitphotocurrent
Theshort‑circuitcurrent(ISC)isthecurrentthroughthesolarcellwhenthevoltageacrossthesolarcelliszero(i.e.,whenthesolarcellisshortcircuited).UsuallywriMenasISC,theshort‑circuitcurrentisshownontheIVcurvebelow.
ISCisduetothegenerationandcollectionoflight‑generatedcarriers.ForanidealPVcellwithmoderateresistiveloss,ISCandthelight‑generatedcurrentareidentical(ISCisthelargestcurrentwhichmaybedrawnfromthesolarcell).
Shortcircuit(photo)current(ISC)orcurrentdensity(JSC)
ISC(orJSC)isthecurrent(orcurrentdensity)whenthedeviceleadsare“shorted”(i.e.,connectedelectricallytooneanother)
ISCdependsonanumberoffactors,asfollows:
theareaofthesolarcell.Toremovethedependenceofthesolarcellarea,itismorecommontolisttheshort‑circuitcurrentdensity(JscinmA/cm2)ratherthantheshort‑circuitcurrent;
thenumberofphotons(i.e.,thepoweroftheincidentlightsource).Iscfromasolarcellisdirectlydependantonthelightintensity;
thespectrumoftheincidentlight.Formostsolarcellmeasurement,thespectrumisstandardisedtotheAM1.5spectrum;
theopticalproperties(absorptionandreflection)ofthesolarcell(discussedinOpticalLosses);and
thecollectionprobabilityofthesolarcell,whichdependschieflyonthesurfacepassivationandtheminoritycarrierlifetimeintheabsorber.
Opencircuitphotovoltage(VOC)
Theopen‑circuitvoltage,Voc,isthemaximumvoltageavailablefromasolarcell,andthisoccursatzerocurrent.Theopen‑circuitvoltagecorrespondstotheamountofforwardbiasonthesolarcellduetothebiasofthesolarcelljunctionwiththelight‑generatedcurrent.Theopen‑circuitvoltageisshownontheIVcurvebelow.
AnequaRonforVocisfoundbysekngthetotalcurrentequaltozero,giving:
⎟⎟⎠
⎞⎜⎜⎝
⎛+= 1ln
o
LOC I
IqnkTV
Solarcellfillfactor(FF)andMaxPowerPoint(mpp)
Graphofcelloutputcurrent(redline)andpower(blueline)asfunctionofvoltage.Alsoshownarethecellshort‑circuitcurrent(Isc)andopen‑circuitvoltage(Voc)points,aswellasthemaximumpowerpoint(Vmp,Imp).ClickonthegraphtoseehowthecurvechangesforacellwithlowFF.
AtbothoftheoperatingpointscorrespondingtoISCandVOC,thepowerfromthesolarcelliszero.Theʺfillfactor“(FF)istheparameterwhich,inconjunctionwithVocandIsc,determinesthemaximumpowerfromasolarcell.TheFFisdefinedastheratioofthemaximumpowerfromthesolarcelltotheproductofVocandIsc.Graphically,theFFisameasureoftheʺsquarenessʺofthesolarcellandisalsotheareaofthelargestrectanglewhichwillfitintheIVcurve.TheFFisillustratedbelow:
Pout=I*VdP/dV=0
Solarcellefficiency
Theefficiencyofasolarcell(sometimesknownasthepowerconversionefficiency,orPCE,andalsooftenabbreviatedη)representstheratiowheretheoutputelectricalpoweratthemaximumpowerpointontheIVcurveisdividedbytheincidentlightpower–typicallyusingastandardAM1.5Gsimulatedsolarspectrum.
Theefficiencyofasolarcellisdeterminedasthefractionofincidentpowerwhichisconvertedtoelectricityandisdefinedas:
FFIVP SCOC=max
whereVocistheopen‑circuitvoltage;whereIscistheshort‑circuitcurrent;andwhereFFisthefillfactorwhereηistheefficiency. PowerinAM1.5Gspectrumis1kW/m2,or100mW/cm2
Fora10x10cm2cell,theinputpower(AM1.5G)is100mW/cm2x100cm2=10W.
inc
SCOC
PFFIV=η
ImpactofElectricalLossDuetoHighSeriesResistance(RS)PVcells
Curren
tden
sity(m
A/cm
2 )
Volts(V)
DiodeequaRonwithRSandRSH:
RSH=10,000