978-1-4799-0333-7/13/$31.00?2013IEEE783
AnalysingtheInfluenceofEmissionLayerThickness
andInterfaceDefectStatesonBifacialHITSolarCell
Yan-YanZhang,TongWu,Li-BinShi
CollegeofNewEnergy
BohaiUniversity
Jinzhou121013,China
Zhangyanyan729@163.com
Rui-ChenRen,Cai-XiaLi
CollegeofMining
LiaoningTechnicalUniversity
Fuxin123000,China
ruichenren@263.net
Abstract—Theinfluenceofemitterlayerthicknessand
interfacedefectstatesonthephotovoltaicperformanceofbifacial
HITsolarcellwassimulatedbytheprogramofAMPS-1D
developedbythePennsylvaniaStateUniversity.Thesimulation
resultsindicatethattheconversionefficiencyofthesolarcell
reduceswithincreasingtheemissionlayerthickness.However,
theinfluenceoftheemissionlayerthicknessoneachspecific
photovoltaiccharacteristicisdifferentbetweenthehighandlow
dopingconcentrationofemissionlayer.Theresultsalsoshow
thattheinterfacedefectstateshaveobviouseffectontheopen
circuitvoltageofbifacialHITsolarcellespeciallyontheptype
substrate.
Keywords—solarcell;thickness;interfacedefectstates;
simulation
I.INTRODUCTION
Energycrisisandenvironmentalpollutionproblems
promotetheextensiveresearchoncleanenergy.Solarenergy
isabundant,clean,andsafeasakindoftypicalrenewable
energy,thereforeitattractsalotofattention
[1]
.Thesolarcell
isthecoreelementtorealizephotoelectricconversion.
Crystallinesilicon(includingmonocrystallinesilicon
andpolycrystallinesilicon)solarcellstillholdsthe
largestshareinthephotovoltaicmarket,whichhasfinalized
structure,matureproductiontechnologyandhigher
conversionefficiency.Howeverthemanufacturingcostof
crystalsiliconsolarcellishigherandisdifficulttoreduce.
Thoughtheproductioncostofthethinfilmsolarcellof
amorphoussiliconislower,thecellhasthelightinduced
degradationphenomena,anditsconversionefficiencyisnot
high
[2]
.Atpresent,theresearchdirectionofsolarcellismainly
toimprovethephotoelectricconversionefficiencyandreduce
productioncost.
ThedevelopmentofHIT?HeterojunctionwithIntrinsic
Thin-Layer?solarcellisrapid,whichwaspreparedfirstlyby
SanyocompanyofJapanin1991
[3]
.Untilnow,theconversion
efficiencyofbifacialHITsolarcellonntypesubstratehas
alreadyreached24.7%alsopreparedbySanyocompanyvia
continuousimprovement
[4]
.BifacialHITsolarcellhasmany
advantages,suchashigheffciency,highstabilityèsaving
materialandenergy,etc.TheconversionefficiencyofHIT
solarcellonptypesubstratepreparedby
chineseacademyofscienceswas17.36%
[5]
exsistingacertain
gapwithSanyocompany.
Theemissionlayerthatisalsoknownasthewindow
layerhasanimportantimpactforthephotovoltaicproperties
ofHITsolarcell.Thereforethethicknessandinterfacedefect
statesofemitterlayerwillberesearched.B.Y.Ren
[6]
has
discussedtheeffectsofemitterlayerthicknessandinterface
defectstatesonthebifacialHITsolarcelldepositedonntype
substrate.Thispaperwilldiscusstheeffectsofthemonthe
bifacialHITsolarcellsdepositedonbothntypeandptype
substratesindetail.AMPS-1D(AnalysisofMicroelectronic
andPhotonicStructures)programdevelopedbythe
PennsylvaniaStateUniversityisusedforsimulationItcan
calculateexactlytheeffectofvariousparameters.
II.MODELLING
BifacialHITsolarcellcanbedepositedonntypeorptype
crystalsiliconsubstrate.Thethinfilmsofdopedand
intrinsicamorphoussiliconaredepositedonbothsidesof
monocrystallinesilicontoformemitterlayer,intrinsiclayer
andbacksurfacefildofthesolarcellusingthetechniqueof
plasmaenhancedchemicalvapordeposition.Becausethe
electricalconductivityofdopedamorphoussiliconthin
filmsislower,thethinfilmsoftransparentconductive
oxide(TCO)areneededtodepositedonbothendsofsolar
cell,sothatbifacialHITsolarcellisfomed
[7]
.Thespecific
structureisTCO/a-Si:H(p
+
)/a-Si:H(i)/c-Si(n)/a-Si:H(i)/a-
Si:H(n
+
)/TCOorTCO/a-Si:H(n
+
)/a-Si:H(i)/c-Si(p)/a-Si:H(i)
/a-Si:H(p
+
)/TCO,asshowninFig.1.
TheAMPSprogrammainlybasedonthespecified
boundaryconditionscalculatestherelatedparametersofsolar
cellsbysolvingone-dimensionalPoisson''sequationsuchas(1)
andthecontinuityequationssuchas(2)and(3)forelectrons
andholes.
DAtt
dd
()[()()()()()()]
dd
xqpxnxNxNxpxnx
xx
\
H
§·
?
¨?
?1
(1)
n
op
dJ1
()()
d
GxRx
qx
§·
¨?
?1
(2)
ThisstudyissupportedbythekeyProjectofEducationDepartmentin
China(No.211035).
784
p
op
dJ
1
()()
d
GxRx
qx
§·
¨?
?1
(3)
Wherethelocalvacuumlevel\,thefreeelectronn,freehole
p,trappedelectron
t
n
,trappedhole
t
p
,theionizeddonor-like
doping
D
N
andionizedacceptor-likedoping
A
N
concentrationareallfunctionsofthepositioncoordinate
x,andHisthepermittivityaswellasqisthemagnitudeof
thechargeofanelectronin(1).Where
n
J
and
p
J
arethe
electronandholecurrentdensitiesrespectively,()Rxisthenet
recombinationresultingfromdirectrecombinationandindirect
recombinationthroughgapstates,and
op
()Gx
istheoptical
generationratein(2)and(3
)[8,9]
.
AMPS-1Dsoftwarecansimulatedeviceusingtwokinds
ofsemiconductorelectronicsdescriptionmodeswhicharethe
densityofstates(DOS)modeandcarrierlifetimemode.This
paperusesDOSmode.Inthismode,thesemiconductor
electronicstatesaredividedintotheextendedstatesofvalence
andconductionband,thelocalizedstatesofvalenceand
conductionbandtails,andthegaplocalizedstates.Thebandtail
localizedstatesdescribedbyexponentialfunctionaremainly
duetothebondangledistortion,expressedas
[8]
aaoCa
()exp[(-)/]gEGEEE(4)
ddoVd
()exp[(-)/]gEGEEE(5)
where
a
Eand
d
Earethecharacteristicenergiesthatdetermine
theslopesoftheirrespectivetails,theprefactors
ao
Gand
do
Gare
thenumbersofstatespervolumeperenergyfordonor-likeand
acceptor-likerespectively,
C
Eand
V
Earetheconductionband
edgeandthevalencebandedgerespectively.Thegaplocalized
statesaremainlycausedbystructuraldefectssuchasdangling
bond
[10]
.Thegaplocalizedstatesofcrystalsiliconusethe
averagedistributionmode.Thedensityofstatesis
11
1.910u
31
cmeV
.Thegaplocalizedstatesofamorphoussiliconuse
thedoubleGaussdistributionmode,expressedas
[8]
2
pkd
a,dGa,d2
DSa,d
()
1
()exp
2
EE
gEG
W
-?ao
°°
????
??ˉ?
(6)
wheretheprefactors
Ga
G(foracceptors)or
Gd
G(fordonors)is
thenumberofstatespervolumeperenergy,
pka
Elocatesthe
centerofanacceptorGaussianwithrespecttothevalenceband
and
pkd
E
locatesthecenterofadonorGaussianwithrespectto
theconductionband,thequantity
DSa,d
W
isthestandard
deviation,shownasFig.2
[11]
.1nminterfacedefectlayerisadded
betweentheemittinglayerofamorphoussiliconandcrystal
silicon.Itisassumedthatthedefectdensityofstatesis
11
110u
cm
-2
withthedoubleGaussdistribution.Thepotentialbarrier
offrontandbackcontactbetweenTCOanda-Si:Hare1.67eV
and0.11eVrespectively.Theparametersofmaterialusedin
thesimulationaremainlyfrom[12-15]seeingTABLEIin
detail.Theabsorptioncoefficientsoflightmainlycome
from[13,14,16].TheilluminationconditionisAM1.5,
(a)Thecellonntypesubstrate
(b)Thecellonptypesubstrate
Fig.1.ThestructureschematicdiagramofbifacialHITsolarcell.
Fig.2.Thedistributiondiagramofthetailsandmidgapdefectstates.
785
100mW/cm
2
,andtheeffectivewavelengthrangeis0.3~1.1
μm
.
III.SIMULATIONRESULTSANDDISCUSSION
A.EffectofEmissionLayerThicknessonthePhotovoltaic
PropertiesofSolarCell
Theeffectsofemissionlayerthicknessonthepropertiesof
bifacialHITsolarcellsonp-typeandn-typesubstrateswere
simulatedrespectively,asshowninFig.3.FromFig.3(a)and
(b),nomatterhowmuchdopingconcentration
a
Nofthe
emissionlayeris,theconversionefficiencyofthesolarcellon
p-typeorn-typesubstratewillreducewiththeincreaseof
emissionlayerthickness.However>Fig.3isalsoshownthat
effectofemissionlayerthicknessoneachspecificperformance
ofsolarcellisdifferentbetweenhighandlowdoping
concentrationofemissionlayer.Whenthedoping
concentrationishigher(suchas
19
a
110Nu
and
20
110ucm
-3
),
withtheincreaseofemissionlayerthickness,theshortcircuit
currentofthesolarcellhasalargeattenuation,butopen-
TABLEI.PARAMETERSSETOFSOLARCELLFORSIMULATION
ParametersMaterials
a-Si??p+??a-Si??i??c-Si??n/p??a-Si??n+??
Layerthickness/nm54
5
210u
5
Mobilitybandgap/eV1.801.801.121.80
Opticalbandgap/eV1.721.721.121.72
Dopingconcentrationof
donatorsoracceptors/cm
-3
0/
19
110u
0/0
16
110u
/
16
110u
19
110u
/0
Electronaffinity/eV3.803.804.053.80
Dielectricconstant11.911.911.911.9
Effectiveconductionband
density(cm
-3
)
20
2.510u
20
2.510u
19
2.810u
20
2.510u
Effectivevalencebanddensity
(cm
-3
)
20
2.510u
20
2.510u
19
1.0410u
20
2.510u
Electronmobility
(cm
2
V
-1
S
-1
)
1020135010
Holemobility
(cm
2
V
-1
S
-1
)
124501
Bandtaildensityofstatesfor
donors/cm
-3
eV
-1
21
110u
21
110u
14
110u
21
110u
Bandtaildensityofstatesfor
acceptors/cm
-3
eV
-1
21
110u
21
110u
14
110u
21
110u
Characteristicenergyfor
donors,acceptors/eV
0.06,0.030.05,0.020.01,0.010.06,0.03
Capturecross-sectionfor
tailacceptorsstates,e,h/cm
2
15
110
u
17
110
u
15
110
u
17
110
u
30
110
u
30
110
u
15
110
u
17
110
u
Capturecross-sectionfor
taildonorsstates,e,h/cm
2
17
110
u
15
110
u
17
110
u
15
110
u
30
110
u
30
110
u
17
110
u
15
110
u
Gaussiandensityofstatesfor
donors,acceptors/cm
-3
17
510u
15
510u
é
17
510u
Gaussianpeakenergyfor
donors,acceptors/eV
0.70,1.201.12,1.02é1.20,0.80
Capturecross-sectionforgap
donorsstates,e,h/cm
2
14
110
u
15
110
u
14
110
u
15
110
u
15
110
u
17
110
u
14
110
u
15
110
u
Capturecross-sectionforgap
acceptorsstates,e,h/cm
2
15
110
u
14
110
u
15
110
u
14
110
u
17
110
u
15
110
u
15
110
u
14
110
u
Standarddeviation/eV0.220.15é0.22
Midgapdensityofstatesinc-
Si/cm
-3
eV
-1
éé
11
1.910u
é
Switch-overenergy/eVéé0.56é
786
circuitvoltageandfillfactorchangelittle,resultinginthe
reducedconversionefficiencyofsolarcellonnorptype
substrate.Whenthedopingconcentrationislower(suchas
18
a
110Nu
cm
-3
),withtheincreaseofemissionlayer
thickness,thoughtheconversionefficiencyisalsoreduced,
open-circuitvoltagehasalargeattenuationaswellasfillfactor
onbothsubstrates,andtheshortcircuitcurrentofthesolarcell
decreaseslittleonn-typesubstratecomparedwithitonp-type
substrate.
Herearetheexplanationsfortheaboveresults.
Photogeneratedcarriersinthea-Sithinfilmofemissionlayer
canbecollectedbyelectrodesmainlydependingontheelectric
fieldinthefilm,sincethedefectstatesdensityina-Sithinfilm
ishigherandcarriersdiffusionlengthisshorter.Forthehigher
dopingconcentration,thespacechargeregionintheemission
layerisverynarrow,eventheneutralzone(i.e.lowfield"dead
layer")
[17]
mayoccur,sothecarriersarenoteasytobe
collectedresultinginthereducedshortcircuitcurrent.Atthis
time,iftheemissionlayerthicknessincreases,thelowfield
regionandthephotocarrierincreasethatmakemore
photogeneratedcarrierscannotbeeffectivelycollected,
resultingintheshortcircuitcurrentdecayedrapidly.Forthe
lowerdopingconcentration,thedifferencebetweenFermi
energylevelsofpareaandnareaissmaller,resultinginthe
lowerbuilt-inpotentialofthesolarcellthatisadversedthe
improvementofopen-circuitvoltage.Inaddition,accordingto
(7)
[18]
p2
n
s
napd
()
i
D
D
Jen
LNLN
(7)
thedopingconcentrationislower,leadingtothelargerreverse
saturationcurrent,thustheopencircuitvoltageofthesolarcell
islower.Withtheincreaseoftheemissionlayerthickness,
reversesaturationcurrentfurtherincreases,thustheopen-
circuitvoltagewillfurtherreduce.Meanwhile,theshortcurrent
ofa-Si/c-Sisolarcellonntypesubstratedecayedmoreslowly
thana-Si/c-Sisolarcellonptypesubstrate.Thisismainly
becausetheminoritycarrier(hole)mobilityofa-Si(n)isless
thantheminoritycarrier(electron)mobilityofa-Si(p)as
emissionlayer.Therefore>theholesofa-Si(n)can’tbe
effectivecollectionbyn/pjunctioncomparedwiththe
electronsofa-Si(p)withthicknessincreasingofemissionlayer.
B.Effectof,nterfaceDefectStatesonthePhotovoltaic
PropertiesofSolarCell
Forbothpnjunctionandnpjunction,therearealarge
numberofinterfacedefectstatesbetweentheamorphous
siliconasemissionlayerandcrystallinesilicon.These
interfacialstatesseriouslyaffectthephotovoltaicpropertiesof
solarcells.TABLEIIliststhephotovoltaicpropertiesofbifacial
HITsolarcellunderseveralsituationswhenthedoping
concentrationandthicknessofemissionlayerare
19
a
110Nu
cm
-3
and5nmrespectively.FromTABLEII,theeffectofthe
interfacedefectstatesontheopencircuitvoltageismaximum
forbothpnjunctionandnpjunction.Thisismainlybecausethe
5101520253035
21.0
22.5
24.0
25.5
0.70
0.75
0.80
0.82
0.83
0.84
0.85
34.5
36.0
37.5
Thickness(nm)
Jsc(mA/cm
2
)
Eff(%)
N
a
=1E18cm
-3
N
a
=1E19cm
-3
N
a
=1E20cm
-3
Voc(V)
FF
5101520253035
18
20
22
24
0.700
0.725
0.750
0.835
0.840
0.845
0.850
34
36
38
Thickness(nm)
Eff(%)
Voc(V)
FF
Jsc(mA/cm
2
)
N
a
=1E18cm
-3
N
a
=1E19cm
-3
N
a
=1E20cm
-3
(a)thebifacialHITsolarcellonn-typesubstrate(b)thebifacialHITsolarcellonp-typesubstrate
Fig.3.Theeffectofthicknessofemittinglayerontheperformanceofsolarcellunderdifferentdopingconcentrations
787
interfacedefectstatesmakethecompositecurrentincrease
leadingtotheopencircuitvoltagereducingaccordingto
equation(8)
[10]
.
sc
oc
0
[ln()1]
JkT
V
qJ
TABLEIIalsoshowsthatopencircuitvoltagesoftwo
kindsofsolarcellsaresamewithoutinterfacedefectstates.
However,theopencircuitvoltageofnpjunctionislowerthan
thepnjunction’sobviouslywiththeinterfacestates.It
illustratesthattheeffectoftheinterfacedefectstatesonopen
circuitvoltageofnpjunctionismore.Thisismainlybecause
theinversionlayerofcrystalsiliconsurfaceofpnjunctionis
moresignificantwhichcansuppress
theinterfacerecombinationeffectivelyleadingtothelower
recombinationcurrentcomparedwiththenpjunction
[19]
.
IV.CONCLUSION
Thispapersimulatedtheeffectsofemitterlayerthickness
andinterfacedefectstatesonthephotovoltaicperformanceof
bifacialHITsolarcellbyAMPS.Thesimulationresultsshow
thattheconversionefficiencyreduceswithemitterlayer
thicknessincreasingforbifacialHITsolarcellsonbothntype
andptypesubstrates.However,theeffectsofemissionlayer
thicknessoneachspecificperformanceofsolarcellare
differentbetweenhighandlowdopingconcentrationof
emissionlayer.Forthesolarcellofpntype,emissionlayer
thicknesscanmainlyaffectshortcurrentofcellwithhigh
dopingofemissionlayer,butaffectopenvoltageandfillfactor
ofcellwithlowdoping.Forthesolarcellofnptype,the
differencecomparedwithpntypeisthatemissionlayer
thicknesscanalsoaffectshortcurrentobviouslywithlow
doping.Interfacedefectstatescanmaketheopencircuit
reduce,especiallyforthesolarcellofnptype.Insummary,the
thinneremissionlayerthicknessisandthelessinterfacedefect
statesare,thebettertheconversionefficiencyofbifacialHIT
solarcellis.
ACKNOWLEDGMENT
TheauthorswouldliketothankProfessorS.Fonashofthe
PennsylvaniaStateUniversityforprovidingtheAMPS-1D
programusedinthesimulations.
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JunctionsPhotovoltaicProperties
oc
V
(V)
sc
J
?mA/cm
2
?
FF
Eff
pnjunctionwithoutinterfacestates0.95037.9740.79728.763
npjunctionwithoutinterfacestates0.95037.7270.79628.533
pnjunctionwithinterfacestates0.76237.9750.85024.588
npjunctionwithinterfacestates0.73937.7290.85223.755
788
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