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1、Solar Energy Materials&Solar Cells 74(2002)5155Rapid thermal technologies for high-efficiencysilicon solar cellsA.Ebonga,*,Y.H.Choa,M.Hilalia,A.Rohatgia,D.RubybaUniversity Center of Excellence for Photovoltaic Research and Education,School of Electrical andComputer Engineering,Georgia Institute of T
2、echnology,Atlanta,GA 30332-250,USAbSandia National Laboratory,Albuquerque,MexicoAbstractThis paper shows that rapidly formed emitters in less than 6min in the hot zone of aconveyor belt furnace or in 3min in an rapid thermal processing(RTP)system,in conjunctionwith a screen-printed(SP)RTP Al-BSF and
3、 passivating oxide formed simultaneously in 2mincan produce very simple high-efficiency n+-p-p+cells with no surface texturing,pointcontacts,or selective emitter.It is shown for the first time that an 80O/&emitter and SP Al-back surface field(BSF)formed in a high throughput belt furnace produced 19%
4、FZ cells andgreater than 17%CZ cells with photolithography(PL)contacts.Using PL contacts,we alsoachieved 19%efficient cells on FZ,18%on MCZ,and B17%boron-doped CZ by emitterand SP Al-BSF formation in o10min in a single wafer RTP system.Finally,manufacturablecells with 45O/&emitter and SP Al-BSF and
5、Ag contacts formed in the conveyor belt furnacegave 17%efficient cells on FZ silicon.Compared to the PL cells,the SP cell gave B2%lowerefficiency along with a decrease in Jscand fill factor.This loss in performance is attributed to acombination of the poor blue response,higher series resistance and
6、higher contact shading inthe SP devices r 2002 Elsevier Science B.V.All rights reserved.Keywords:Rapid thermal processing;Silicon;Solar cell;Screen-printing1.IntroductionLow cost and high efficiency are the keys to large-scale acceptability ofphotovoltaic(PV)systems.The cost break down of current Si
7、 PV modules revealsthat wafer,cell processing,and module assembly account for approximately 45%,25%and 30%of the module cost,respectively 1.The cost of silicon wafer can be*Corresponding author.E-mail address:(A.Ebong).0927-0248/02/$-see front matter r 2002 Elsevier Science B.V.All rights reserved.P
8、II:S092 7-0248(0 2)00047-8reduced by low-cost solar grade polysilicon feedstock material,increased wafer size,reduced kerf losses during slicing,and thinner substrates.However,the singlecrystalline silicon grown with CZ method and cast multi-crystalline silicon accountsfor more than 75%of the PV cel
9、ls fabricated today.The lower efficiency realizedfrom CZ substrates compared to FZ wafers has been partly attributed to defects andlight-induced degradation due to the presence of Biand Oi2.Glunz et al.3 haveshown that cell processing involving prolonged heat treatments for CZ substratescan reduce t
10、he lifetime degradation and produce efficiency improvement of around1%absolute.However,conventional furnace processing(CFP)can take more than1h at 8509001C for phosphorus diffusion alone.This could limit the throughput ofa manufacturing line.The purpose of this study is to develop and demonstrate ra
11、pidtechnologies for emitters without sacrificing cell efficiency on single crystal siliconmaterials,including FZ,CZ,and magnetic CZ silicon.2.Device fabricationOur approach towards rapid thermal technologies for high-efficiency cellsinvolves:(1)rapid emitter formation by diffusion under tungsten hal
12、ogen lampsby belt furnace processing(BFP)and rapid thermal processing(RTP),instead ofconventional infrared furnace processing,and(2)use of screen-printed(SP)aluminum followed by 2min RTP for simultaneous back surface field(BSF)andin situ oxide formation.The belt emitter was formed at 9251C in 6min,R
13、TP emitterwas formed at 8801C in 3min in a single wafer RTP system and a conventionalfurnace emitter was formed at 8651C in about 1h.After the emitter formation,Alwas SP on the back and formed in an RTP system at 8501C for 2min in oxygenambient.This resulted in simultaneous formation of excellent Al
14、 BSF and frontoxide passivation.Cells were fabricated with SP as well as photolithography(PL)contacts.The PL cells had only one masking step involving lift-off.No surfacetexturing,point contacts or selective emitter to keep the cell design simple.In SP cellsAg contacts were fired through a single la
15、yer PECVD SiN AR coating.3.Results and discussionThe emitter profiles for BFP and the RTP for 8090O/&are shown in Fig.1.Asrevealed by Fig.1,the junction formed in RTP is deeper(0.33mm with 1E20surface concentration)than that formed by the BFP(0.23mm with o1E20 surfaceconcentration).This is attribute
16、d to the lower wafer temperature compared to the settemperature in the belt furnace.Table 1 shows the light IV data for RTP,BFP andCFP cells fabricated on FZ silicon.It is quite clear that RTP technologies using beltfurnace or RTP system for emitter formation produce essentially same efficiency(19%)
17、as the CFP cells with lengthy emitter formation.Table 1 shows a 1.1mA/cm2difference in Jscand 5mV difference in Vocbetween the BFP and RTP cells on FZsilicon.A.Ebong et al./Solar Energy Materials&Solar Cells 74(2002)515552However,the efficiencies are 19%for both the cells(independently confirmed byS
18、andia National Laboratories).IQE and reflectance curves in Fig.2 reveal that thedifference in the short circuit current is due to slightly higher front surfacereflectance.This is attributed to higher phosphorus surface concentration of the RTPemitter,which led to a slightly thicker passivating oxide
19、,degrading the reflectance.Table 1 shows that the FZ cell with 45O/&belt emitter with SP Ag contacts gavean efficiency of 17%.This 2%absolute reduction in efficiency compared to thecounterpart PL cells is associated with 7mV decrease in Voc;2.4mA lower Jsc;and0.028 reduction in FF.Detailed modeling
20、4 has shown that the 2%efficiency lossfor SP cells can be attributed to poor metal conductivity,high contact resistance1.E+141.E+151.E+161.E+171.E+181.E+191.E+201.E+2100.10.20.30.40.50.60.70.80.9Junction depth(micron)Doping concentration(cm-3)BLPRTPFig.1.Emitter profiles for 8090O/&belt line and rap
21、id thermal processing.Table 1Electrical output parameters Of CFP,RTP and BFP solar cellsCell IDVoc(mV)Jsc(mA/cm2)FF(%)Z(%)BFP-PL-FZ63637.380.219.0RTP-PL-FZ64136.281.919.0CFP-PL-FZ63437.180.518.9BFP-SP-FZ62934.977.417.0BFP-PL B-doped CZ61036.179.517.5RTP-PL B-doped CZ60835.179.016.9BFP-MCZ-PL63635.68
22、1.318.4A.Ebong et al./Solar Energy Materials&Solar Cells 74(2002)515553(0.2%),high surface recombination(0.4%),sheet loss(0.1%),emitter doping(0.3%),AR coating and absorption in SiN(0.3%)and higher grid shading(0.5%).Table 1 also shows that PV grade CZ material gave efficiencies of B17%for bothBFP a
23、nd RTP cells with PL contacts.This 2%lower efficiency of CZ cells comparedto FZ cell is attributed to lower bulk lifetime.In order to overcome this deficiency,crucible-grown magnetic CZ was used.This material gave 18.4%efficient cells withPL contacts or a 1.4%higher efficiency compared to CZ silicon
24、.A combination ofrapid technologies and magnetic CZ silicon can produce lower cost,high throughputand high-efficiency cells.4.ConclusionsThis paper shows that rapidly formed 80O/&emitters in o6min in the hot zoneof a conveyor belt furnace or in 3min in an RTP system can produce 19%efficientcells wit
25、h no surface texturing,point contacts,or selective emitter.We also achieved19%efficient cells on FZ by emitter and SP Al-BSF formation in o10min in theRTP system.Finally,SP manufacturable cells with 45O/&emitter and SP Al-BSFformed in the conveyor belt furnace gave 17%efficient cells on FZ silicon.T
26、he SPcell gave B2%lower efficiency along with a decrease in Jscand fill factor(FF).Thisis attributed to the poor blue response,higher series resistance and higher contactshading in the SP devices.01020304050607080901003505507509501150Wavelength(nm)IQE(%)SP-17%BFP-PL-19%RTP-PL-19%IQEReflectanceFig.2.
27、IQE and hemispherical reflectance of 19%efficient BFP-PL and RTP-PL and 17%SP cells.A.Ebong et al./Solar Energy Materials&Solar Cells 74(2002)515554References1 A.Rohatgi,P.Doshi,T.Krygowski,Pushing the frontiers of silicon PV technologies:novelapproaches to high-efficiency,manufacturable silicon cel
28、ls,AIP Proceedings of NREL/SNLPhotovoltaics Program Review,1997,pp.109115.2 R.L.Crab,Photon induced degradation of electron irradiated silicon solar cells,Proceedings of theNinth IEEE Photovoltaic Specialists Conference,Silver Springs,1972,pp.329330.3 S.W.Glunz,S.Rein,W.Warta,J.Knobloch,W.Wettling,C
29、omparison of lifetime degradation inboron,gallium doped p-type CZ-Si.Technical Digest,11th PVSEC,Saporo,Hokkaido,Japan,1999,pp.549552.4 P.Doshi,J.Mejia,K.Tate,A.Rohatgi,Modeling and characterization of high-efficiency silicon solarcells fabricated by rapid thermal processing,screen printing,and plasma-enhanced chemical vapordeposition,IEEE Trans.Electron Devices 44(9)(1997)14171424.A.Ebong et al./Solar Energy Materials&Solar Cells 74(2002)515555
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