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