Method of fabricating solar cells

Heretofore a common method of fabrlcatina ־m SOI״ cells has included the steps of / dlff Phosphine into P-type silicon her .. . ^yp silicon ribbon so as to shallow !unction, (2) forming ,:. .:. ribbon a thin coating of silicon nitride, (3j Photolith^tr°^{de ΡΒ}“^{βΓη 1}״ ^{th9 Sill־On nitride}“:r״«״-־ “.”“׳ .;ד*: ־ ד״ ,״

׳. .^״־.¹;.:.ה.״״ ״ ».

as to alloy the aluminum and silicon, (0 ־ilicon on both sides of the ribbon with sintering the nickel to form . .... plating additional metal(s) onto the

-. more detailed description of

- S. Patent 4451969. a grid electrode by

Portions of the silicon, a________ ^SUChTh^ae^P₈^rt^OCeSS Γ θ ^f°^Und י*.״ י ^Patet 4451969. ' ־ ohotnH+h ^6Ρ8 °^{f fOrming a grid} electrode by Photolithography, which necessariTv ו ^Ύresidual resist . * ^{ly lnv01ves} removal of resist, is extremely effective. However, :

Photolithography involves several time consuming steps and : ^^e overall c^f forming solar cells.

Illustrating the state of the prior art are D.s Patents Nos. 4375007, 4388346 references cited therein, r silicon solar cell with alloy or°A1°⁰ln^ta_D^Cr; 7 ״^{lth M}־^Sb °ϊ ° , ׳ ^{P fon}׳״ ^{to for}״ ^a thick-film

- -ס ricaל⁸⁸” ״ ^{USefU1 f}°^{r 10}״ ^reSlstaaca coated with⁷־!“ ~ ^{tO 3 S1HC0} — ™

U.S. r ^{4 3} ־state devices, screen-printing a dispersion of base metal particles dis^i_dincluding a vaporizable binder polyme ______

-- On firing the screen-printed device.

and 4640001, and the

O.S. Patent 4375007 discloses a

Patent 43Θ8346 discloses electrodes for solid The electrodes are formed by 10wer⁹2^Y^^^^tegite?te5%^k °°^ntalnln5 lower raeitma. _ooated a liquid vehicle ^!r and a fluorocarbon z the coated without being oxidized to form a anti-reflection coating is then polymer, r ־־ metal particles sinter contact electrode. An applied to the cell.

More recently, U.S.

diי ־ ^{Patent 46}«»«I (Koiwai et al) .^! ״״־ * ״״. ד־'־“^גמ.עה ״״.«.״.» _w .׳ temperature between 9ςη λα ^{F on} at a between 250 degrees C and 600 degrees C Th״ «111»®*’ ^{the surfaoe of the si}“״°־־ TherefoVe'-VK־־'^⁸ ’ ״ ־Printed electrodes.

־3insure that the electrodes win bondto the silicon wafer. At the tine this invention was _{״ was} ״־la^{e y}ce⁹־״^ni־eH “³י ^WldeSPrMd '-^'־tovoltalc fabr- ״ dependent upon the developnent of h cell “th¹⁹’'״⁶⁰‘־'? °^aPable °^{f Pr0dU01n}’ iSliablesolar °^{f 12}‘

-c— ---־־׳ ——, . The cost of solar cells i.Va semiconductor devices denena־ . ' ^Uk other ' materials ״ ״.־ ^P °°^St °^{f the} starting materials and the cost of converting the starting ־liicon 1ה7ה״ ^{fi,>1Shed Pr}°^dUet- ^ΤΗβ ~^si° “¹״־־־into at least 12, efficient silicon solar cells at cost cannot be accomplished without a suitable

1־Χν*’^{111Μ״οη teohnique tor}

OBJECTS OF THE INVENTION

0־«^ΑρΖ־־־ί־׳־״«^{β prtaary object iB to prOTlde} ״־ג <׳ ־!!-י- in w ’ ^{tHe fabrica״}™ of solar

11S in which a metal paste is nitride layer to form j trough a silicon layer to form a conductive contact.

Another object of this invention <_eimproved low cost method of making photovoltaic semiconductor junction devices in which ד fired through a s_{illcon nltride} ‘ “ conductive contact ״Uh .. . ^{9 f a} surrounding silicon nitride L ‘Γ״’ layer and also as an a '4 ^{an ant}i~^reflection substrate ״״¹״־mental protector for the ϊ™7^0Τ07”^Ρ״^ί3η‘ °^{bJeCt i־ tO} “ — ־ ieyer of ״‘ ^“^1־ metal 0001^°. °^Μ״η9 ° ^{each cel1}׳ ^and formation of a

Xr ^a־C°״^{PHShed thr}°“^{9h the} *“‘״־ס nitride

Yet another object of the present invention is to a paste silicon to provide a silicon solar --1 achieves the metal the °^{f η3}“^η5 “¹“ ^{Cells in} ־»״h nit ?d ? ⁸¹¹״θ' ^{18 fired thr}°^u^ ־ tride layer to form a conductive contact.

An additional object of this invention is an Improved method of fabricating ־. with a silicon nitride coating which nitride ^a ”“״״ ^{ln the} y in preparation for application of a r ־°ο^Ο־־״°ν^ηά t^b’ <״־־ Performance 1״“ course of contact formation.

Another object of this invention is to provide improved method of makina ־Ju״ . Provide an less expensive but “ ίΖ^״״ ״ ^ ־^{β 18} forming nickel or silver contacts. °^f

The foregoing objects are achieved by abncation procedure of the character involves the following steps־) ־. substrate which has a P-N ju״_{otlOT &״}. nitride on its front surface adjacent 1 selectively coating the silicon nitride ־־leoted metal paste so that the layer forms contact pattern on the silicon nitride, , ^e substrate to rapidly and efficiently

X ^nltride -

- ״ον־! method— cell which already has cell previous a solar cell described which (1) providing a silicon ^{1 and a} layer of silicon : the junction, (2) layer with a a selected and (3) heating cause the metal layer and form a silicon solar a back ohmic contact or the latter the front silver or substrate it for contact may be formed after the front surface contact has been formed. ^{c nas}

A preferred embodiment involves, inter alia, the ha a Γ^{6Ρ<!! Pr}°^{Vidin9 a S״icon} ־«״־trate which front ׳׳־Λ״⁰““־ ^a ״־־״1־ nitride ־״ ״־ front surface, (2) coating the back side of the silicon - I״: o’: r־ ^Ρ^³' ׳’ ---eX״ng

We of the silicon substrate with a seiected nickel paste and (4) heating the silicon to rapidly and efficiently cause (a) the aluminum to form an adherent conductive back side <. 1 (b) the metal and frit components in the nasto u penetrate the silicon nitride layer and r ״־־ductive front side contact. XT״‘ -y be further processed, e.g., to p^e connection to electrical circuits.

The metal paste that is f_ired into the have the following characteristic. First capable of rapidly penetrating the ס־!־^ at relatively low temperature! ־ integrity is not destroyed, i e ^aif£u־e - ־־ thus degrade cell performance ThtY’״ “Τ’׳״ :!!¹::⁰:¹⁶¹ד^{ayer t0} ״״* --- x:r^causeability oi t_h:^U: U: T⁰‘ ^Slgnl״cantly alter the ״־tl-reflectio״ elXt ^״de ״ and ״ilTl״^Obi^er^Ct^tS־p^O:ea^theh^lnVr^tlOn '״ ^P״t^A חג part appear hereinafter. The invp״>-4״ ־־־־rdingly comprises the several steps 21?

seeps and the relation substrate must it must be nitride layer s at which the solar cell --., temperatures below about _oth which ““יי ^{reSP60t t0 eaCh} °^f others which are exemplified _{ln the f0110wi}¹ΓΓ;. ‘^{Ηβ ־COPe} °^{£ the} W“־״tio״ of _whloh will be indicated in the claims.

BRIEF DESCRIPTION OF THE DRAWTWro of th“^{1 3 fU״er Underatand1}״* of the nature and objects of the present invention, reference should be had to th־ following detailed descrlnti״״ w . together with the » ^{P hi h 18 to be} considered number of the steosTT⁷¹’ ^άΓβ1״^η9 ״^{hlCh 1״UBt}«te a r Of the steps involved in making solar cells according to a preferred form of the invention

Throughout the drawing lik״ to Similar structure. reference numbers refer ־־vera/coa⁶“/!״»״״ “^{iCkheS־es and de}Pth3 of the nor shown exactly i_n accord⁸^{either Shown to scale}proportions T “⁰«dance with their relative ions, for convenience of illustration.

detailed description of the invention of the^inventlon^elates to’tte’p T ⁶״»⁰«¹״ent ' -s enbodrment, thsWprovided as a start! ״ Partially fi״_{ished cell}2 _ג ’ ^Ρ °® ^aa substrate 2, preferablv’ ^{״ Y ״nished} “111 has conductivity silicon rlbb ⁰¹”^{Prl־ing a len}9th 0f/pT_typ^y__ front side) o“Z־h shallow junction 4 (e g _a Γ” ״^{lth a} relatively

-- ^a״^{d ab}- :

״ .״״״, ,»»״:ter rear a P+ a layer of an aluminum a volatile organic _ _ . monobutyl that can be removed by de0 ׳ II ; ^{ThS 0}“״ ^SUe ^^hereinafter the yer 01 al ₄⁸‘^β '1״ י»“׳״״» ״th

Ι־־Ι6״ί4 T ^all°^yed ‘⁸ ״“^{b8trate Md} ״ ״־-;I:׳ ״.־ . - ׳“־.'״:״״ X“::׳״ ״. “‘:¹”־ junction 4 and N+ conductivity r־ >״״ instance, F-type si Hr־ ״״ κ ° ^{UCtivity re}«i°n 6 may be formed in a type silicon substrate 2 bv t-ho , layer 12 and P+ r»< ./ diffusio״ of phosphorus, «an side^a01 paste comprising aluminum powder in a ־ ־ vehicle, such as terpineol, diethylene glycol ether acetate or ethylcellulose, ׳' tXTnTX⁸.^׳ Τ^β”^{heatin9 the BUbStrate}־f the paste a T ° Γ Γ7' °^° ~‘⁸and form the P. JL » ’ ”¹״״״ ^{the s}״b־trate sub־t״l ’ . ^H°״^ever׳ °^ther i°«s of οΤΧ;^Τ;::״Γ^{Γ eleotrode}׳^and — — Partially finished cell 1/ ®”^{ployed t0} P^rovide

Of course, the not limited to the XT“’ ¹⁸ :׳.:ג .::::ם;—- χ\::1״:° : i^{ioon on} “‘־*»* ¹“¹’״¹¹י chemical or physical v« ° ¹‘⁷“יי ^by~ש״מ: ~Ft --;τ״ ϊλτ שד -χ

Also, the invention nay be applied to substrates. Thus, ״? “,היב ^r1bb°^ns ״^sim״ar «־ ׳*>־*־ *־.g., x:2^a::::2s:_t;:״:^trates—״־ ׳־rawin־ the’Ant 1'־ iXATwATa^tV A'“^״ ״θ ־i״Lⁿ־X:^e״:^Ca^te? ^CdT^״Ve ״^et׳^1־ boiling !Lid ? borosilicate gl^7^ _a low monoethyl ether or terpineol^n^ethyl“^{e glyc01}of the paste. Preferably, foAoTand other ׳ ‘ considerations, the metal is nickel or s״ ״ is applied according to a nrprt 4 ^**י ^paste

--ע., i״ the form of a grid ! ΓΑ‘ ^‘׳״ Plurality of narrow ״״gers att^ed' .

, *״yeij» attached at one er4 4״.« - v bar or runner ״״ת r ״ ^{na t0 a} bus illuminated in V S Pat” 17^״״^{βΜβ 9rld} -Strode is applied u־i״, any ;/״A ³⁸⁸⁶°³⁶- ^P“^t6 “ ־&lt;» 7•״ _PAm , ^{y the} *^n0Wn techniques which are commonly used i״ the electronics industry Lr/ printing and direct wv-44. ^Screen ־־tisfactory. ” °^Μ־ »״^״׳־־‘ ⁵״^״re to _teⁿ״^er!r^te_The^S 7^״ oxygen a^־;_ph־re. i e⁹fAn°^rt^UCted ״P in ״־y case it shouldA^ -⁵¹^^ ^but I the organics and melt the^lasArit ''^S~^S °^{Γ Pi}׳^r01V²® / through the silicon nitride layer h ״' T‘ furnace win vary with th» r&lt; &lt; ' i^-ilfflSjJUthe tho^^ITn^e 7^j-־־r--—׳!S-tKB®rature, generally I ’0 ״inutes, incl din thAr⁶ and

-*״ ..»־ ׳it .1 ה ™ x^{10n of};

The . ¹V^{8 rea£}ty ® firing _1־ conducted in a fuWc'e^t? ίη Fl ργα !Λ. , ל.—---an &lt;

Preferably, however, the paste consists of 20 to 70 wt « metal, 5 to 30 wt. » glass frit, and 1 to 12 wt » Organics. Most preferably, the paste has only about 5 wt glass frit and between 1 wt. « to 2.0 wt. » ethylene g ycol monoethyl ether or between 1.0 wt. t to 10 wt « terpineol. The glass frit comprises 5-80 wt. » lead'and UP to 40 wt. % silicon oxide in the case of a lead ³⁸°־⁵ ל־* ^lead1 ׳^{to 40} ’ ^sil1» ־»־־־ ־nd -30» boron oxide in the case of a lead borosilicate fi ^{are 8}”*table for .^e P^ast^j5t^emperature_0TIbOTt^OODJegress c I ^Thua’ ^for ®*ample, a furnace containing '1 ^e״t air heated to 800 degrees c has been found ¹־t the^flrino ^r0dUCe/^{he de־i}«^a ״־״“« i15-60 ״ seconds silver ״ ? _tP־«tes whicTT^J^----nick ” ⁸⁸ “⁸י^Belected conductive metal, if am^ t &lt; ? ^{Sele0ted contalned} 1» Paste 16, the ent air is preferably substantially replaced with nitrogen, e.g., 90-100» nitrogen and 0-10» oxygen ס״־ Hre⁹־th^i״nw ⁴יי¹ י⁸’¹’ י ^{PaSte 16 beCOMS}־nd fires through the silicon nitride layer 10 to depressions 17 and ohmic contact 18. Th r ״ s la⁸ : !XT! V“^״° ״^and - ~e Ltai and S rit bond to the front surface of the substrate forming ohmic contact 18. ^,

In an alternate preferred embodiment, the rear side ״f ^a ~־te has been coated with an aluminum past 1״ i icoTnl id‘⁶י^,' ׳^flrln9 °^f ״^{etal PaSta} through ־imult ״ ־״:it״ Γη⁰ ““ ¹⁸ —PHshed aneously with fl״ng of the aluminum paste to alloy aluminum to the rear side of the silicon substrate.

In the preferred embodiment of the invention, the conductive metal is silver or nickel.

Paste 16 may contain other metals In addition to or in ° silver and nickel. These other metals are palladium, platinum and rhodium, bike nickel and silver at⁶־ r־latl^Pai^le,°^f¹⁰“^,0 ״״ °°^ntaOt־ P«^f־«bly t a relatively low temperature.

of orXr,¹⁰⁹^״^eXa^{Ple iUu־trates th}־ preferred mode Of practicing the invention.

EXAMPLE

A partially finished cell of P-type EFG-arnwn «־.n 4 , . ^ype “¹»-growi silicon ribbon 1. _{provlded hav}1_ng ״ _{conductlvlty Qf}הב τ °^{e11 has a sha11}°״ ^p־^{n junct1}°^{n ab}°^ut moo angstroms deep, and a silicon nitride layer of about 800 angstroms on its front surface The ״״״ .Γ ־ u , * me rear side of the cel 1 has a layer of aluminum alloyed to the silicon

A layer of silver-containing paste Is selectively ־reen printed onto the silicon nitride layer. The layer ^id electrode having _{the (orm} illustrated ״

The paste contains silver metal, a lead borosilicate glass f t and terpineol as the liquid vehicle. The silver lead 5-308 T״ °^{ontalns 58}־^M lead, 5 308 boron oxide, and 5 to 408 silicon oxide The the paste. **־ * ^resP^ectively of

Once the paste has dried (which may but need not involve heating the paste to evaporate any volatile vehicle), the silicon substrate is passed into an air , furnace and heated_inair to about BOO degrees c Tt . '!

Then emove from the furnace and allowed to coo; a room temperature. The overall time in th» 7 about 2 minutes. The firin״ ״s ^ls⁵efficient to cause any organics”!X X ‘° a״a burn and the lead boroLTat g sufficiently fluid to fir״ . ^{f t0 become}״״ך_? a״d^ybo״d ^οη'Χ^οΧ:!^^⁶XL XT ^fT — - ־-erX and substrate ⁸ °^° ^־°^ntact the ־c־ordi״:^UTthT ^־°^lar °^{ell־ haVi}׳” “^nta־t־ formed foregoing «.״φΙ.Γη’ΧΤ’Χ and ^8Χ6^{Ρ1״ίε&lt;! by the}hTXX- χΓ r rdlT XTXo״ circuit voltage 5 לס« mv * ⁰?״־

Of 30.2 mA per square ce λ ^{ClrCUit cur}r־־nt density ______^{P Square cen}timeter; _{a flll fa}__nr. _Λ^y pleasant results.

and an efficiency of 13.8 percent ThTf ״ '״^ith ™״It־ obtained with cX! ^⁸ techniques tofo™ ״ ^and h s to form ohmic contacts.

received a t^§O*0 20” ^£r0” ^c°tacts which t¹. ^xeT_ UO-20 second«?) ha?», 4 containing inn T ^O“ from 8 cells subjected to th» κ ״ ® ^average ״alues foil־־״־ The aveX ^h *“ ^calc״l«ed as ־«־rt circult _curre⁹״: ™itage was 588 mV, the fill f_actor ״ . _ ^per ־^,joare centimeter;

was 0.75; and the efficiency was 12.9 percent.

finis^T ?^{ltrlde remainln9} °^{n the} side of the finished devices serves as an coatinc t״ ״., effective anti-reflective _Drp , , connection it is to be noted, as previously mentioned, that the silicon nitride coati״״ the^־hⁿ°t1^{CraCR dUr1}^{9 the thermal} treatment Involved in the heating step.

or silver.instead of aluminum then the front contact 1, -de of silver. The bach contact _{may b&lt;־} J™**“ ‘״ immediately after application of the paste defining the ont contact has been applied and dried.

״ '^ince ‘T* ^{and Other cl,an}9« w be made in the above processes without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description or shown in the ccompanylng drawing shall be interpreted as illustrative The present invention is indicated by the broad gene a expressed^ the terms 1״ which the appended cl.ij are