High efficiency n-type Si solar cells on Al2O3-passivated boron emitters Jan Benick,1,a͒Bram Hoex,2M.C.M.van de Sanden,2W.M.M.Kesls,2Oliver Schultz,1
and Stefan W.Glunz1
1Fraunhofer Institute for Solar Energy Systems(ISE),Heidenhofstras2,D-79110Freiburg,Germany
2Department of Applied Physics,Eindhoven University of Technology,P.O.Box513,5600MB Eindhoven,
The Netherlands
͑Received1May2008;accepted9May2008;published online25June2008͒
In order to utilize the full potential of solar cells fabricated on n-type silicon,it is necessary to
achieve an excellent passivation on B-doped emitters.Experimental studies on test structures and
theoretical considerations have shown that a negatively charged dielectric layer would be ideally
suited for this purpo.Thus,in this work the negative-charge dielectric Al2O3was applied as
surface passivation layer on high-efficiency n-type silicon solar cells.With this front surface
passivation layer,a confirmed conversion efficiency of23.2%was achieved.For the open-circuit
voltage V oc of703.6mV,the upper limit for the emitter saturation current density J0e,including the
metalized area,has been evaluated to be29fA/cm2.This clearly shows that an excellent passivation
of highly doped p-type c-Si can be obtained at the device level by applying Al2O3.©2008
American Institute of Physics.͓DOI:10.1063/1.2945287͔
n-type silicon has an enormous potential for widescale application in the photovoltaics industry.Its relative toler-ance to common impurities͑e.g.Fe͒1potentially results in higher minority carrier diffusion lengths compared to p-type c-Si substrates with a similar impurity concentration.Fur-thermore n-type c-Si does not suffer from the boron-oxygen related light-induced degradation͑LID͒,which is known to cau the LID for c-Si solar cells bad on p-type Czochral-ski c-Si.2
In order to benefit from the advantages of the c-Si bulk material,a technology for adequate passivation of the B-doped emitters is esntial.However,at the device level the excellent passivation
quality as achieved for highly doped n-type emitters has not been realized so far for highly B-doped p-type c-Si.SiO2,the most effective passivation for highly doped n-type surfaces,3does not show the same per-formance on highly B-doped surfaces.4–7The high boron solubility8combined with the prence of a smallfixed posi-tive charge density9contribute to this gap in performance. a-SiN x:H,the cond standard passivation layer for n+-doped surfaces,does not passivate highly doped p-type surfaces effectively due to the high concentration of built-in positive charges.6,10,11Nevertheless,Chen et al.have shown a-SiN x:H passivation on highly doped p-type surfaces with J0e values below10fA/cm2for sheet resistivities above 100⍀/sq.12However,no n-type cells have been fabricated using this approach which would demonstrate the potential of this technology at the device level.Alternative passivation layers under investigation for highly doped p-type surfaces are a-Si:H and a-SiC x:H.With a-Si:H J0e values below 30fA/cm2have been reached for sheet resistivities above 100⍀/sq.6,13a-SiC x:H shows only poor passivation proper-ties so far,with J0eϾ400fA/cm2on highly doped p-type surfaces͑R sheet=100⍀/sq͒.14Apart from SiO2,all other lay-ers,especially tho rich in Si,show a considerable absorp-tion for photons with a wavelengthϽ600nm which is unde-sirable for the application as antireflection coating.
For passivation of highly doped p-type c-Si,a dielectric
containing afixed negative-charge density without any ab-
sorption in the visible part of the solar spectrum would be
ideal.One dielectric layer meeting the specifications is the
negative-charge dielectric Al2O3,which can be fabricated in
a low temperature process.
Hoex asured emitter saturation currents below
10fA/cm2on highly doped p-type c-Si surfaces of unmet-
alized lifetime samples coated with Al2O3synthesized by
atomic layer deposition͑ALD͒.15The high density offixed
negative charges͑up toϳ1013cm−2͒within this layer pro-vides an effectivefield effect passivation on highly p-type
doped surfaces.16The excellent passivation of lightly doped
p-type c-Si by Al2O3has already been demonstrated at the
rear of a diffud emitter p-type c-Si solar cell.17In this
paper,it will be proven that the excellent surface passivation
of highly doped p-type c-Si by Al2O3can be accomplished at
the device level by achieving very high energy conversion
efficiencies.
The effect of built-in charges on the passivation quality
yiyofor highly doped p-and n-type surfaces is shown in Fig.1.
For this experiment,symmetrical p+/n/p+and n+/p/n+life-time samples͑1⍀cm n-or p-type c-Si͒were passivated by a105nm thick thermal SiO2and subquently a charge den-sity in the range between−4and4ϫ1012cm−2was applied on both sides of the samples by means of corona charging.9 The quasi-steady-state photoconductance͑QSSPC͒method18 is ud to measure effective lifetimeeff.The implied V oc was extracted from the QSSPC data as propod by Sinton:19
implied V oc=
kT
中韩词典
q
the suburbs͑⌬n+N dop͒⌬n
n i2
,͑1͒
where⌬n is the excess carrier density,k the Boltzmann con-stant,T the temperature,q the elementary charge,N dop the bulk doping concentration,and n i the intrinsic carrier density.
The obrved detrimental effect of positive charge on the passivation of highly doped p-type surfaces can be explained by the surface depletion of the majority carriers͑i.e.,the holes͒induced by the positive charges.The surface deple-
a͒Electronic mail:jan.benick@i.fraunhofer.de.
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APPLIED PHYSICS LETTERS92,253504͑2008͒
0003-6951/2008/92͑25͒/253504/3/$23.00©2008American Institute of Physics
92,253504-1
tion enhances the minority carrier ͑i.e.,the electron ͒concen-tration at the surface,leading to an enhanced surface recom-bination.The opposite effect occurs when a negative-charge density is applied.In this ca,an accumulation layer is in-duced,providing an effective field effect passivation at the p -type surface.By applying a negative-charge density of −4ϫ1012cm −2,the implied V oc is incread from below 650mV ͑without surface charging ͒to approximately 690mV.An analogous effect,but with opposite polarities,can be obrved for highly n -type doped surfaces.
In order to investigate the excellent level of surface pas-sivation of highly doped p -type c -Si surfaces by Al 2O 3at the device level,n -type passivated emitter with rear locally diffud 3͑PERL ͒solar cells ͑as shown in Fig.2͒were fab-ricated on ͗100͘1⍀cm,FZ,n -type c -Si wafers with a thick-ness of 250m.The cells ͑A =4cm 2͒feature a front sur-face with inverted pyramids and evaporated Al /Ti /Pd /Ag front contacts which are thickened by electroplating.The rear surface exhibits a local P diffusion ͑R sheet Ϸ20⍀/sq ͒and is covered with a 100nm thick thermally grown SiO 2and a 2m thick
aluminum layer.BBr 3diffusion at 890°C
followed by a drive-in oxidation at 1050°C result in a ho-mogeneous B emitter with a sheet resistance of 140⍀/sq ͑6ϫ1018cm −3surface doping concentration,1.5m depth ͒.This front side B emitter is passivated by a stack consisting of a 30nm Al 2O 3film followed by a 40nm thick SiN x .The deposition of the Al 2O 3was performed by plasma-assisted ALD ͑on an Oxford Instruments FlexAL™tup ͒at a tem-perature of 200°C.16The plasma-assisted chemical vapor deposition SiN x was deposited at 400°C ͑SINA XS,Roth &Rau AG ͒.
The one-sun parameters of the PERL solar cells featur-ing the Al 2O 3front side passivation are summarized in Table I .The best cell exhibits a V oc of 703.6mV,a J sc of 41.2mA /cm 2,and a FF of 80.2%resulting in an indepen-dently certified solar cell efficiency of 23.2%͑aperture area measurement ͒.The exceptional high values for V oc ,despite the lack of a two-step emitter,prove the outstanding ability of Al 2O 3for the passivation of highly doped p -type surfaces in the solar cell devices.
To gain a deeper insight into the front surface passiva-tion,an upper limit of the emitter saturation current J 0e can be determined from the open-circuit voltage V oc and the satu-ration current density J 0=J 0b +J 0e by employing the one-diode equation:
V oc =
贵阳会计网kT q ln ͩJ sc J 0b +J 0e
+1ͪ
.͑2͒
The V oc is determined by the saturation current densities of
both the emitter J 0e and the ba J 0b .Thus,to obtain an upper limit for J 0e ,a reasonable J 0b has to be derived.The satura-tion density of the ba,which also includes recombination in the bulk and at the rear side,can be calculated by
J 0b =qn i 2D p LN D ·
S rear,eff cosh ͑W /L ͒+D p /L sinh ͑W /L ͒D p /L cosh ͑W /L ͒+S rear,eff sinh ͑W /L ͒
.͑3͒
The effective surface recombination velocity ͑SRV ͒of a point contacted rear is given by 20
S rear,eff =
D p
nobler
W ͫ
p 2W ͱf arctan
ͩ2W p ͱf ͪ−exp ͩ−W
p
ͪ
+
D p fWS cont
ͬ
−1
+
S pass
1−f
,͑4͒
where D p =11.6cm 2/s is the hole diffusion coefficient,W =250m the wafer thickness,p =135m the contact pitch,f =5%the metallization fraction,and S cont and S pass the SRVs of the metalized and the passivated ctions of the rear side,respectively.S cont has been calculated by numerical modeling in PC1D ͑Ref.21͒on an idealized cell structure with intrin-sic bulk lifetime,assuming S front =0cm /s.A strong P back surface field is prent beneath the contacts.In this ca,S cont is independent of the actual SRV of the metal-Si interface,
TABLE I.Results of n -type PERL solar cells passivated by Al 2O 3͑AM1.5G,100mW /cm 2,25°C ͒.
V oc ͑mV ͒
moonyj sc
音标下载͑
mA /cm 2͒FF ͑%͒
͑%͒Average ͑28cells ͒696.9Ϯ5.640.9Ϯ0.378.8Ϯ1.822.5Ϯ0.7Best
703.6
41.2
80.2
七年级英语上册23.2a
a
Independently confirmed by Fraunhofer ISE CalLab.tears
FIG.1.͑Color online ͒The effect of surface charge density on surface pas-sivation quality.Both the B and P emitters have comparable sheet resistiv-ities
of approximately 140⍀/sq with surface doping concentrations of 6ϫ1018cm −3for the B and 8ϫ1018cm −3for the P emitter.Both emitters are passivated by a 105nm thick thermal SiO 2.
FIG.2.͑Color online ͒PERL solar cell structure on n -type silicon.Note that this structure has a homogeneous emitter in contrast to the two-step emitter in the original PERL structure.
leading to S cont ϳ55cm /s.Applying Eqs.͑2͒and ͑3͒,the upper limit for the total dark emitter saturation currents J 0e ,total are 45fA /cm 2for S pass =0cm /s ͑J 0b =10fA /cm 2͒and 29fA /cm 2for a more realistic but still very good S pass =5cm /s ͑J 0b =25fA /cm 2͒,including the recombination in the contacted and passivated areas of the emitter.To estimate the impact of the contacted area on J 0e ,total ,using PC1D and a S cont of 106cm /s,we have calculated the dark saturation current in the contacted region,J 0e ,cont ,to be 1800fA /cm 2.This results in an area-weighted dark saturation current for this region,f cont ϫJ 0e ,cont ,of 20.3fA /cm 2͑contacted area f cont =1.1%͒.The area-weighted value for the passivated re-gion has been calculated,͑1−f cont ͒ϫJ 0e ,pass =9.9fA /cm 2,using the J 0e value of ϳ10fA /cm 2extracted by Hoex nonmetalized lifetime test structures with a comparable B emitter.15This leads to a J 0e ,total of 30.2fA /cm 2which is in good agreement to our previous calculation of 29fA /cm 2.A V oc of 702mV agreeing very well with the measured V oc of the cells has been obtained,taking into account a J 0b of 25fA /cm 2͑S pass =5cm /s ͒from Eq.͑3͒.Thi
s calculation shows that about 66%of the recombination in the emitter is due to the contacted area.
The high internal quantum efficiency ͑IQE ͒in Fig.3also shows the effective front side passivation.The very high IQE values of ϳ100%in the 300–600nm range clearly demonstrate that the negative-charge dielectric Al 2O 3is an excellent front surface passivation layer on B-doped emit-ters.Not only an excellent passivation quality has been reached on highly p -doped c -Si by Al 2O 3resulting in a V oc of 703.6mV but moreover no additional detrimental effects such as optical absorption or inversion channel shunting are prent,which would result in a poor performance at the device level.
In summary,an exceptionally high conversion efficiency of 23.2%for an n -type PERL solar cell with a front side B-doped emitter has been reported in this work.To date the highest reported efficiencies on n -type material were 22.7%͑681mV ͒on a backside-contact solar cell 22and also 22.7%͑702mV ͒on a rear emitter PERT solar cell.23This study demonstrates the excellent performance of our n -type solar cells and the superior passivation of highly B-doped surfaces by the negative-charge dielectric Al 2O 3.The passivation of highly doped p -type c -Si has been obtained at the device level achieving the required technology for high-efficiency diffud emitter solar cells on n -type c -Si.
1
D.Macdonald and L.J.Geerligs,Appl.Phys.Lett.85,4061͑2004͒.2S.W.Glunz,S.Rein,J.Y .Lee,and W.Warta,J.Appl.Phys.90,2397͑2001͒.3
J.Zhao,A.Wang,P.P.Altermatt,S.R.Wenham,and M.A.Green,Sol.Energy Mater.Sol.Cells 41–42,87͑1996͒.4
R.R.King and R.M.Swanson,IEEE Trans.Electron Devices 38,1399͑1991͒.5
A.Cuevas,M.Stuckings,J.Lau,and M.Petravic,Proceedings of the 14th EC PVSEC,Barcelona,Spain ͑H.S.Stevens,Bedford,U.K.,1997͒,2416.6
P.P.Altermatt,H.Plagwitz,R.Bock,J.Schmidt,R.Brendel,M.J.Kerr,and A.Cuevas,Proceedings of the 21st EU PVSEC,Dresden,Germany,2006͑unpublished ͒.7
J.Zhao,J.Schmidt,A.Wang,G.Zhang,B.S.Richards,and M.A.Green,Proceedings of the Third WCPEC,Osaka,Japan,2003͑unpublished ͒.8
E.H.Nicollian and J.R.Brews,MOS Physics and Technology ͑Wiley,New York,1982͒.9
S.W.Glunz,D.Biro,S.Rein,and W.Warta,J.Appl.Phys.86,683͑1999͒.10
M.J.Kerr,disrtation,Australian National University,2002.11
J.Libal,R.Petres,T.Buck,R.Kopecek,G.Hahn,R.Ferre,M.Vetter,I.Martín,K.Wambach,I.Roever,and P.Fath,Proceedings of the 20th EU PVSEC Barcelona,Spain,2005͑unpublished ͒.12
F.Chen,I.Romijn,A.Weeber,J.Tan,B.Hallam,and J.Cotter,Proceed-ings of the 22nd EU PVSEC,Milan,Italy,2007͑unpublished ͒.13
H.Plagwitz,Y .Takahashi,B.Terheiden,and R.Brendel,Proceedings of the 21st EU PVSEC,Dresden,Germany,2006͑unpublished ͒.14
M.Vetter,R.Ferre,I.Martín,P.Ortega,R.Alcubilla,R.Petres,J.Libal,and R.Kopecek,Proceedings of the Fourth WCPEC,Waikoloa,HI,2006͑unpublished ͒.15
B.Hoex,J.Schmidt,R.Bock,P.P.Altermatt,M.
C.M.van de Sanden,and W.M.M.Kesls,Appl.Phys.Lett.91,112107͑2007͒.16
B.Hoex,S.B.S.Heil,E.Langereis,M.
C.M.van de Sanden,and W.M.M.Kesls,Appl.Phys.Lett.89,042112͑2006͒.17
J.Schmidt,A.Merkle,R.Brendel,B.Hoex,M.C.M.van de Sanden,and W.M.M.Kesls,Prog.Photovoltaics ͑2008͒.18
R.A.Sinton,A.Cuevas,and M.Stuckings,Proceedings of the 25th IEEE PVSC,Washington,DC ͑IEEE,New York,1996͒,457.19
R.A.Sinton and A.Cuevas,Appl.Phys.Lett.69,2510͑1996͒.20
B.Fischer,disrtation,Universität Konstanz,2003.21
P.A.Basore and D.A.Clugston,Proceedings of the 25th IEEE PVSC,Washington,DC,͑IEEE,New York,1996͒,377.22
P.J.Verlinden,R.A.Sinton,K.Wickham,R.A.Crane,and R.M.Swanson,Proceedings of the 14th EC PVSEC,Barcelona,Spain ͑H.S.Stevens,Bedford,U.K.,1997͒,96.23
J.Zhao and A.Wang,Proceedings of the Fourth WCPEC,Waikoloa,HI,2006͑unpublished ͒
.
FIG.3.͑Color online ͒EQE,IQE,and reflection of an Al 2O 3-passivated PERL solar cell.
