|Title of Invention||
A BACK CONTACT SOLAR CELL AND A METHOD OF MANUFACTURING THE SAME
|Abstract||An improved solar cell design and method of fabrication that primarily uses two materials, n-type doped silicon and aluminum to form a p-n alloy junction back contact solar cell. The .aluminum alloy junctions are placed on the back (unilluminated) side of the cell, thereby combining the desirable features of aluminum (as a dopant, contact metal and light reflector), with the advantages of a back contact cell. The cell design and method of fabrication includes such features as surface texturing, front and back surface field minority carrier mirrors, surface passivation using oxidation layers, use of Al contacts as light reflectors, intrinsic protection against reverse bias due to contiguous n+ and p+ regions, and an improved bus bar contact design suitable for interconnecting cells using a surface mount technology. An improved method of ohmic contact formation uses a self-alignment technique for forming the ohmic contacts.|
not efficiency but lowering the ousts per unit area of a solar cell. A profiling candidate for this task is silicon solar cells, especially those cells fabricated from thin (-100 μ m) silicon substrates where high-quality silicon is effectively utilize* The challenge at present is to decrease the unit costs for these solar cell* so that they may be competitive with traditional fossil fuel power supplies at present energy prices* One way to do this is through improved fabrication techniques.
In addition to fabrication techniques, certain design structures offer advantages over other designs. One such auperlor design seems to be back contact solar cells, in particular employing thin silicon substrates*
Homojunction si:ollicon polar cells have a p-n junction for separatists pftp'fconaneratefi electrons from photogenerated holes. For the solar cell to function properly, electrons must be directed toward the contact for the n-type material and holes must be directed toward the contact for the p-type material. Light intensity in a aamiconductor decreases monotonically yith depth, thus the p-r junction is preferably close to the illuminated surface, to reduce recombination of holes and electrons, prior to their being separated by the p-n junction. In thin silicon solar cells, though the thickness of a cell is smaller than in conventional silicon soJ.ar cells (- 300 μ m) , and ,the probability of a photon facing Converted into an ! electron-hole or charge-carrier pair is less, the average lifetime of a photogenerated electron-hole pair can be such that thf photogenerated electron-pair will survive being awept to their r^spectiv4 contacts. That is to say, in a thin silicon solar
cell the minority carrier diffusion length can be relatively large compared to the thickness of the cell so performance of the cell is not unduly compromised. In the present invention the minority carrier diffusion length is equal to the thickness of the cell or greater.
Further, conventional (front-contact) j silicon solar cells have a structure in which a large p-n junction is fomed over the entire substrate on the illuminated side of the cell. This conventional design has the virtue of simplicity; in that; no patterning is required for the emitter (typically the p-type layer in a p-n junction cell) since it covers the entire front surface. However,
layer interface will be rectifying (like a Schottky diode) rather than be ohttic, with a corresponding power loss associated with t**e turn-on voltage of the diode. But the higher the dopant concentration, the greater the recombination of electrons and holes in the emitter layer, which i« deleterious and typically occurs greatest near the surface where incoming light shines. Finally, texturing of the front surface to increase light trapping means contact lines have to run over a rough surface without loss of continuity, which can be difficult to achieve. In addition, some texturing methods,j such as the porous silicon method, will make creating an emitter diffusion layer of acceptable uniformity more difficult.
For this reason and others, for a conventional cell structure a balance must be sought between the desirability for a heavily-doped surface to promote ohmic contact formation and reduced shadowing and the desirability of a lightly-dopedj surface for reduced carrier recombination and effective surface passivation. Constraints due to texturing and shadowing art also a problem. An alternative approach is to place the p-n junction; on the back (non-illuminated side) of the cell. In such a back-contact solar cell the requirements for texturing and passivatinfl the front surface are
cell generally employs interdigitated contacts, nearly half the back surface area is covered with; positive contact metal and the other half is covered with negative contact metal. Because the p-n junction is on the back of the cell, however, the minority carrier diffusion length in the starting material (base) must exated the cell thickness in order to obtain satisfactory enetgy conversion efficiency. The best results for this approach are from a Stanford University gjoup, which has reported efficiencies of 21.3% at cine sun (100 mW/cm2)
SUMMARY OF THE INVENTION
The present invention reduces the
fabrication cost of silicon solar cells while
preserving a relatively high volar conversion ,
efficiency by employing a back omitact silicon soljar
cell that uses a single materia) •*- preferably j
aluminum (Al) — as both the ji-type dopant material
and the ohmic contact materia) ih a bulk layer of n-type silicon (Si). In addition, a novel
fabrication for the back pontftpt ffid lines is
employed that in a preferred fl^wUment uses a
relatively inexpensive •oreeft jfJ*"inied, se|f-aligned
contact system. A novel feature of this contact
system is that it is self-Aligned by applying anodic
oxidation to one set of the contacts to insulate
this set from the other set of contacts, thereby
eliminating any need for precine alignment of
successive mask sets to achieve the grid line
In a preferred embodiment several other, beneficial features are designed into the silicon; back contact solar cell of the present invention, including but not limited to: surface texturing (formed both during crystal growth and chemically), front and back surface field ittinority carrier mirrors, passivation of surfaces using silicon oxide layers, use of antirefleotive coatings, use of the ohmic contacts as a back surface light reflector/ intrinsic protection against damage from a reverse-bias condition due to heavily^doped contiguous n* and p* regions, and improved negative and positive contact bus bars that allow a 'surface mount technology1 design when connecting cells in series.
Fig. 1 illustrates a oross-section of 3 dendritic web silicon bl&ftk of the preferred embodiment of the invent|bh;
Fig. 2 illustrates j:ft* aross-s£ction of the web at the aluminum dttpcwiHmi stage for the positive contact;
Fig. 3 illustrates the cross-section of the web at the heat treatment stage;
Fig. 4A illustrates the cross-section of the web during the insujqtioft of the first metal stage to form aluminum o^ide;
• Fig. 4B illustrates a close-up vi£w of the back surface portion of the Web of Fig. 4A;
Fig. 5A illustrates the cross-section of the web during the removal of silicon oxide from the back surface;
Fig. 5B illustrates a close-up view of the back surface of the web of Fig. 5A;
Fig. 6A illustrates the cross-section of the web during the deposition of metal for the negative contact;
Fig. 6B illustrates a close-up view of| the back surface of Fig. 6A;
Fig. 7 is a bottom, back view of the
finished cell; j
Fig. 8 is a plan view of the back sid€j surface of a substrate having 8 cells;
Fig. 9 is an enlarged view of the rear
surface of one of the cells on the substrate of Fig.
Fig. 10 is a sectional view taken along lines 10-10 of Fig. 9;
Fig. 11 is an enlarged detail view illustrating a portion of a pair of positive electrodes and the region therebetween; and
Fig. 12 is an flHilartf»4 detail view illustrating a corner po»t Loft at the top of the F£g. 9 cell.
DETAILED DESCRIPTION OF THE PHIiFEKKED EMBODIMENTS]
Turning attentitifi IJPV tP the figures, j there is disclosed a preffi|rr While Figs. 1-7 ihoW dendritic web silicon, it is understood that the invention is meant to apply to any form of silicon, including I float zone silicon, Czochnraltfkl silicon, magnetic Czocharalski silicon, o«$t silicon, and sheet silicon, provided the minority (Barrier diffusion j length under operating cell conditions exceeds the cell thickness.
The starting material for the back contact silicon (Si) solar cell of the present invention is any n-type silicon starting material, shown as layer. 10. Common n-type dopants for Si include the atoms from group V of the periodic table, and include such elements as Li, Sb, P, and As. However, it should also be understood that the disclosed cell structure would also function for starting silicon material that is p-type, or even undoped, since layer 10 | functions primarily as an absorber of light.
In the preferred embodiment illustrated in the figures, the starting material (for the bulk layer 10 in Fig. 1) is dendritic veb silicon doped n-type with antimony (Sb). fhfc dfcndritic web silicon ribbon is grown using a process similar to that used to produce Cxoohralski (CZ) silicon. ! However, silicon grown by othtr methods may alsojbe used in addition to dendritic web silicon, such is float zone silicon, CZ sllidon, cast silicon, and
sheet silicon. The Si dendritic veb is typically grown at a thickness of 100 microns, though other thicknesses may be used* At this thickness the minority carrier diffusion length is usually more than the thickness of the cell, often two or three times the thickness. On both the top and bottom surfaces of the Sb-doped 6i ribbon is placed a shallow n* layer, 20, diffused into both surfaces while the web is still in the growth furnace. If the n* surface layers 20 are not introduced during web growth, they can be incorporated at the beginning of the procean by ifty proven method known in the art, including • iihtilt Jkfttous front and back diffusion from a liquid dopant aource using rapid thermal processing. The n* layers create a "surface field11 that drives hole* aw i
open-circuit voltage to enhance the solar conversion
efficiency. Further, the back n* layer promotes]
ohmic contact to the nagative oontact metal there,
as described below.
In addition, surface texturing of^both top
and bottom surfaces is provided in order to! trap
more incident light. Such surface texturing, shown
in the form of a saw-tooth pattern 30, may be grown-
in, introduced by anodic etching to create a porous
j Si layer (as per the method outlined by Y.S. Tsuo et
al., "Potential Applications of Porous Silicon jjn
Photovoltaics", Conf. Record 23rd IEEE Photovoltaic
Specialists Conf. (Louieville, KV)(1993), i
incorporated by reference herein)/ or introduced mechanically by sawing or optically such as by iaser
etching. Though in the preferred embodiment texturing and doped surfaces are shown, their use lis optional in the general case. Further the texturing of the bottom surface of the cell is not shown in0, Figs. 2-6 for clarity.
Figs. 1-6 illustrate important steps in the fabrication of the stilar e*H* including the use of aluminum as both dopant and ohmic contact material as well as its maskititf liping a self-aligning anodic oxidation technique. Aluminum or aluminum material herein is defined as either pur6 Al or an Al-Si alloy in which the silicon concentration is less than eutectic composition (88.7% Al and 11.3% Si, by weight). This aluminum is deposited over approximately half the total back surface area in stripes spaced edge to edge about] 100 /m (microns) apart and each 100 /im wide, as shown in Fig. 2, at spacing 4tK The spacing 40
between stripes 50 shoulcj be |t«s than a minority;
carrier (hole) diffusion length f0r efficient I
1 carrier collection. The J.in* width and spacing thus
could be reduced to below 100 /iBi to some advantage.
The useful upper limit t0\: 1.1 he width value is about
2000 jim; while the useful ratios ftir edge-*to-edge ;
spacing 40 is from about fct> Mtii ici About 300 jim.
Thus the parallel stripei bf iluminum each form
separate regions of dopant source material for the
p-n junction solar cell, and, as explained further
herein, are joined (contiguous) at their base to
form a bus bar region for th« poeitive contact bus
In a preferred method for depositing
aluminum it is deposited by screen printing the ?
aluminum, a process known per se in the art, in in
aluminum paste. The stripes of aluminum are shown
in Fig. 2, where aluminlitp iM d•posited, as stripes 50, running into the piffle ul ill* paper. However, methods other than screw)) printing for depositing the aluminum are within the scope of the invention, such as electron beam «vapov«ti.oh or sputtering, although these method* flwy J-H^Ulr© more?costly patterning by photolithography «ftd so are less desirable.
Generally speaking, the Al layer of p-type material is relatively thin when compared to the n-type bulk layer, about 2 to 20 /xm thick for a bulk layer with thickntas of fttmUt 100 pm.
Note that in the £r©ffcrred embodiment of the present invention the choice of alumimim serves at least three purposes simultaneously: it acts as a p-type dopant source, it acts as a positive contact metal, and it acts as a partial back surface light reflector over that back surface area it covers, which is approximately 50% of the back area.
Turning attention now to Fig. 3, which illustrates heat treatment, there is shown a figure depicting the effects of heat treatment of the ! deposited Al layer in ah approximately 850'C oxygen-rich environment. In this step the p-n junction is formed by alloying the screen-print deposited Al with the Si. It is further believed that maintaining a temperature of 850*C for an extended period of time, as for example 30 minutes or mo'te, is beneficial to forming a satisfactory alloy. The range in temperatures Can be from 577*c, the aluminum-silicon euteotic temperature, to I420'
nitrogen, or chemically active such as with oxygei^
or hydrogen. Mixtures of ambient gases are also j
possible. Times at elevated temperatures can range
i from 30 seconds to several hours. In the preferred
embodiment it is expected that the use of an oxygin-rich environment at this temperature will enable j oxide (Sio2) to grow on any exposed Si, which will! passivate the surface and decrease harmful recombination effects.
The temperature is then lowered in the Si-Al alloy, and Si regrows by liquid phase epitaxy ! until, the eutectic temperature (577*C) is reached!* As a result, the regrown 6i is now doped p-type with Al (about 1018 cm"3), as denoted by the p* layer composition 60 in Fig. 3. The required p-n junction is formed as the Al concentration exceeds the donor concentration in the starting Si, and the eutectic alloy (about 88.7% Al and 11,31 Ai, by weight) remains on the surface to serve fee a contact to the p-type silicon. It should be noted that the p-n junction may be quite deep (1 to 20 microns from the surface) but since the junction is at the back of the cell, where very little light is absorbed, the junction depth is only of secondary importance as compared to a conventional front oontact solar cell. The depth of the alloy junction can be controlled by using an Al-Si mixture as the screen-printed material deposited instead of J)Ur« Al. This is because as the Si concenlj'fttjffft i* increased toward the eutectic composition, th
increasing the alloying temperature, in accordance with the aluminum-siliooh ph&B& diagram.
Further, the bulk lifetime of minority carriers will probably increase at the approximately 850'C temperature due to the property of dendritic web silicon (the preferred type of Si used in the present invention) to have any quenched-in defects, such as Si vacancies and self-interstitials, anneal ; out. Cooling at a controlled rate of 10*C per minute versus a more rapid cooling will also allow quenched-in si defects to anneal out, reducing harmful recombination sites.
The foregoing heat treatment may be effected using a belt furnace process in which the samples are loaded on a belt and the belt is slowly pulled through stable hot zone* in a furnace. In the alternative to heating the Si/Al mixture at 850*C for about 30 minutes in such a furnace, one may employ a variety of other techniques to form the Si/Al eutectic, such as the use of a rapid thermal processing unit that employs by way of example quartz lamps to heat the 6i to 1000*C and maintain that temperature for 30 seconds, which would increase throughput in $ commercial setting, or by a conventional quartz tub* futlMOt,
Having such • J>+ jpyjan (region 60 from Fig. 3) immediately adj^oeni to the n* region at the back surface layer (the fraoR surface layer 20 ftfom Fig. 1) also has the unanticipated benefit of protecting the solar om\\ fI utn overheating when jit is reverse biased, as fpjr etnH»j4» by shidowing in a module. A "module" is § grtiUfi u|! interconnected cells which are protected by glass or other cover material, and which produce ft significant amount of power, typically 10 to 100 watts, when illuminated.i
This p*n* design builds in promotion against reverse bias, eliminating any n*#4 tQ H*V« protection diodes, commonly referred to 4D "bypass diodes" to guard against reverse biaa. Tilt p'n* junction acts as a Zener diode which breaks down under modest reverse biases with only a small voltage and consequently only a small amount of power is dissipated in the cell, tftus protecting the cell.
Turning attention now to Figs. 4A and 4B, another step in the preferred embodiment of the present invention is illustrated. This step' provides a unique "self-aligning" feature for aligning the negative contact (to the n-type region) with respect to the positive contact (to the p-type region). In order to insulate the p-type layer contact (positive contact) ftoto the n-type layer contact (negative contact), th© present invention does not require complicated masking techniques that have been used in the past, but rather, employs the use of insulation by forming an oxidation layer to coat the first (positive) set of Al contacts and electrically isolate this set from the second (negative) set of contacts. As shown in Figs. 4A and 4B, this is done by insulating the Al-Si p* layer composition and the exposed Al stripes 70 that are on the outside of the web blank 10, with an oxide layer 80 (insulator), which forms naturally on exposed Si, Si-Al and Al materials, in the form of Al^Os, SiOa, or some variation thereof, in the presence of oxygen. The oxic(t layer should be grown to cover the Al stripes 70 to a thickness of approximately 0.1 |in to 1 fin. As shown in Figs. .4A and 4B, at this stage the oxide layer 80 also covers the n-layer surface regions 90 in between the Al ' stripes 70. As described more fully below, the
oxide layer on the surface regions 90 is subsequently removed (refer to the step illustrated in Fig. 5B below) in order to enable ohroic contact with the cathode (n-type Si) of the solar cell diode.
The preferred method of forming the oxidation layer in Figs. 4A and 4B is by anodic oxidation, in which the layer surface of the developing cell is immeraed in e weak electrolyte (such as borates, phosphite* or carbonates) and subjected to an applied voltage. Current flows as a result of a voltage applied between an inert electrode and the contact metal (the Al-Si eutectic)* The thickness of the anodic oxide can reach 1 micron if the voltage which drives the ' anodization current reaches 700 V (14 A/V or 1.4 nm/V)• Such oxides should be compact and free from pinholes. Because ohroic contact must be made to a positive contact bus bar (at region 110, shown in Fig. 7) when the solar cell is finished in order to make contact with the exposed Al stripes 70, the growth of anodic oxide must be inhibited in the bus bar region (and this bus bar region must be shielded throughout the process). One way to do this is to
use a compressive yet conductive medium to contact the area to be occupied by the bus bar, such as a carbon-impregnated closed-call sponge. A closed-cell sponge is preferable an it will not absorb the electrolyte.
Besides anodiu ox Mat ion, any other method for insulating the aluufnum Or aluminum-silicon eutectic layer is also *nvi§iori#d by the present invention, such as oxidising aluminum in an oxygen-containing plasma.
After the oxidation layer 80 has been added by anodic oxidation or fchy other suitable method, the n-type Si surface covered by the oxidation layer in interstitial surface regions 90 must be exposed in order to permit the negative contact metal layer of Al to be deposited thereon. Thus, referring to Figs. 5A and 5B, there is illustrated how the oxidation layer is removed from the Si layer on the back surface, but not the oxidation covering the Al stripes 70. In a preferred method of doing this, hydrofluoric acid is used to selectively etch Away and remove the interstitial Sio2 (silicon dioxide) 20 because hydrofluoric acid does not react and remove AI2O3 (aluminum oxide). Consequently, the interstitial Si02 is removed while the AliDj ijisulative layer remains covering the stripe contacts 70 (see Fig^ 5B). Other chemicals having similar effects may be used, or other oxide removal techniques may be employed, such as by light sandblasting of the silicon dioxide layer, which ilso provides the beneficial effect of lightly damaging the exposed silicon surface, which promotes ohmic contact to(the n-type base. Use of sShdblluting eliminates th4 need for an n+ diffused 1»ye» on the back of the cell, which is usually |i|r0vj«jflicl (primarily to promote an ohmic contact. Reactive ton etching (R1E) may also be used to remove BiOz While leaving Al203 undisturbed. Ion milling m4y also be used to lightly damage the sutlfitofe W prfejhote ohmic contact in a manner analogous to |Slliltto J flying.
Figs. 6A and l» i|lUstiSte the next step in the process of fabricating the solar pell of the present invention, viz. the application of a second metal layer to form the self"aligned negative ohmic
contact metal (for the n-type Si layer contact;. j
This second metal may be any suitable contact met|l, including aluminum and silver. Again, screen printing is the preferred method for depositing tkis • second metal, but other methods such as electron beam evaporation or sputtering Are also acceptable. This second metal layer, designated as metal layer 100, covers nearly the entire back of the cell. This layer is insulated from the first metal contact stripes 70 by the anodic oxide 80 while making ohmici contact to the n* regions 90, found in between th^ metal stripes 70 formed from the first aluminum j layer deposited. The second metal layer also helps form a back surface light reflector to aid in reflecting any light not absorbed by the silicon ; material in a first past back into the silicon material.
Fig. 7 shows the completed solar cell as viewed from the back, where tfte Oell is covered with the second metal (aluminum or other ohmic metal) or aluminum-silicon eutectid. The solar cell of the present invention has an unobstructed front surface, which is a significant advantage over conventional solar cells. At the back* both metal contacts j (ohmic metal contacts 70 and 100) serve as partial light reflectors in addition to being ohmic j
contacts. Furthermore, the bUi~bar design allows
for the simplified interwmniiuiion of solar cells of
the present design in sftries, aM the present design
eliminates the need for utunbataoinia external front-
to-back inter-cell contacts tp be manufactured, but
uses more of a "surface-'ftounl0 technology design]
that dispenses with suet) contftiaia* 1
Thus referring FQ fi^l* ' there is showft bus bar region 110, whidH i* thi tth-oxid:.zed are!
where the bus bar contact leading to the first positive contact metal stripes 70 is located, as explained above. The bus bar region 110 as can b4 seen is smaller in area thfcn th« area covered by ^he negative contact metal, layer 100, but both the j positive and negative metal contacts lend themselves nicely to a modular surface mount design. The Al-Si eutectic fingers 70 emanate vertically , upwardly from the bus bar }10 of Fig. 7 but are npt
visible in the figure due to the overlying second'
metal layer 100. j
If desired, other ohmic contact metals j
than Al or Ag may be deposited to form the positive and negative ohmic contacts c}te»tiribed herein, such as by way of example usintf a \Ifclftium/palladium/ copper sandwich or screan-printed silver as contacts.
Regarding the U»e cit thti-reflective (AR) coatings, a coating layer or |«y«re would ordinarily appear on the outermost front JUujtiinatedj surface, but they have been omitt*4 $*Wft figs. 1-7 for clarity. It is possible that in AR coating is optional with the present deaign, since texturing, or perhaps texturing in combination with a passivating oxide, as explained above, may be sufficiently effective §0 as to preclude the need for an AR coating. Hovtyer, *n Aft coating such as silicon nitride applied by plasma-enhanced chemical vapor deposition (PECVD) or titanium dioxide applied by atmospheric pressure chemical vapor deposition (APCVD) can be used. Hydrogen ion implantation (to improve minority carrief diffusion length) could also be introduced prior to ah Aft coating deposition, provided the Surface oxide was absent or perhaps very thin.
REDUCTION TO PRACTICE With reference to Pig. 8, complete aluminum alloy junction iht«rdigitated back contact (IBC) solar cells 120 w#jrfc fabricated using Czochralski silicon waffle as starting substrates. These solar cells serve to cjemonstrate the use of aluminum alloy junctions in a back contact configuration. The aluminum was deposited by electron beam evaporation, rather than by screen printing. The negative electrodes were not self-aligned using anodic oxidation or some other technique, but rather were manually aligned with respect to the aluminum^silicoh eutectic positive electrodes with the aid Of $ contact aligner as used for fabricating integrated circuits.
The silicon substjat* wafers were single-side polished, 3 inches in diameter, 13-17 mils thick, phosphorus-doped to 3-20 n-cm, and with a (111) surface. Two wafers (designated CZ-7 and CZ-8) were processed with test structures and solar cells on the polished side. Fig. 8 shows the back side of one such wafer. The lapped side (hidden from view in Fig. 8) haa a phosphorus diffusion and an antireflective (AR) ooatilW 95 (see Figs. 10 and 12). Each of the eight pol||* dills is 1.00 cm square, ignoring the 2 tytty vl*l« bus bar 112 for the negative electrodes. They lire referenced below in Tables 2 and 3 by the ntyhbet qf n* fingers forming the negative contact (4, 8, IB And 25) and by their location (interior (I) pjr puflphferal (Pj)). Thej four interior cells have only tlt# tUttctic alloy contacting the p* region* while the peripheral dells have the second metal also deposited on! the eutectic. Better results vart obtained for waf^r
CZ-8, so only the process and test results for this wafer will be described.
The process usecj in fabricating the IBC cells for wafer CZ-8 is summarized in the table below. A noteworthy feature of this process is that the back aluminum alloy junction and the phosphorus-doped n+ layers, formed across the front surface and at the exposed back surface between aluminum electrodes, were created in a single high-temperature step. Photolithography was employed to pattern the evaporated aluminum and to define the second metal which serves as the negative electrode. A back side view of the aluminum alloy junction IBQ solar cell is shown in Fig. 9, while cross-sectional views are shown in Figs. 10-12.
Some comments reyardiny the CZ-8 process: Alloy/n* diffusion prices* includes a stepped
slow-cool in the rapid thetinal bfriiotssing (RTP) unit
(*50#C/min from 100CTC to I2b'(3)|
There was no apparent harm in painting liquid phosphorus dopant over the aluminum before the 1000*C RTP process which simultaneously formed the p+ and n* regions;
Depth of p* region determined to be 5 /im by
cross-sectional scanning electron microscopy for
1000'C, 30 sec RTP process;
Sheet resistance of front n+ surface measured 25 n/D after 1000'C, 30 »ec RTP diffusion;
Mask 2 was skipped becaue* it was only needed for some test patterns, Wot for the IBC cells;
The Ti/Al contact to n* surface is limited to a thickness of 0.55 /an by the lift-off process;
Without etching n* nilicon between positive electrode (eutectic) and Negative electrode (Ti/Al), the p-n junction was severely shunted.
Note that functioning back contact solar cells were obtained, thereby demonstrating the viability of the disclosed structure. Light-to-electrical energy conversion efficiencies up to 9.0% have been measured. Refinements in substrate material and processing t*chTllqpl«« are expected to raise the efficiency to 4bUb.al§ ijitt demonstrated to date.
While the above (provides a full and complete disclosure of the piffurred embodiment of the invention, various Iftfi^f iftttfcitUl* alternate constructions and equivalents my be employed. For example, while the preferred embodiment has been
described with reference to Aluminum for forming the p-type diffusion and th# 0hmi& pafttacts, other group III metals such as galliuin and indium may be employed for this purpose. A suitable group III element is one which will dissolve the silicon and remain behind as a trace Amount to serve as a dopant when the silicon solidifies. In addition, while the*
preferred embodiment has been described with reference to an n-typ« bulk silicon layer 10, p-type bulk silicon may be us©4 io fabricate the back-contact solar cell, When A p-type bulk silicon layer is used, a thin p4 layer ie formed on the top surface as layer 20, but an h* layer is formed on the bottom of the bulk layer 10. As will be appreciated! by those skilled in the art, in the p-type embodiment, the minority carriers are electrons. Therefore, the above description and illustrations should not be construed as limiting the scope of the invention, Which is defined by the appended claims.
WHAT IS CLAIMED IS:
1. A back-contact solar: cell apprising:
a semiconductor bulk layer of a first
conductivity type having u front surface and a back surface;
a plurality of spaced doped semiconductor regions of opposite conductivity type formed in said bulk layer near said back surface and forming a plurality of semiconductor junctions therewith;
a first set of spaced ohmic contacts connec to said plurality of spaced doped semiconductor regions and located along said back surface;
a second set of ohmic contacts connected to said back surface of said) bulk layer in the spaces between said first set of ohmic contacts; and
insulator means for electrically isolating said first set of spaced ohmic contacts from said second set of ohmic contacts.
2. The solar cell according to claim 1, wherein said first set of ohmic contacts is in the form of substantially mutually parallel conductive stripes.
3. The solar cell according to claim 2, wherein said conductive stripes are joined at one end to form a bus bar contact.
4. The solar cell according to claim 1, whereiri
said semiconductor bulk layer has a thickness no greater than the diffusion length of the minority carriers of said first conductivity type.
5. The solar cell according to claim 1, wherein said bulk layer is formed of n-type silicon.
6. The solar cell accoording to claim 5 wherein said n-type silicon is dendritio web silicon.
7. The solar cell according to claim 1 wherein said first set of spaced ohmic contacts comprise an
alloy of said bulk layer semiconductor material and a Group III metal comprising the acceptor dopant for said plurality of spaced doped semiconductor regions.
8. The solar cell acqording to claim 7 wherein said Group III metal is selected from the group consisting of aluminum, gallium and indium.
9. The solar cell according to claim 1 wherein said insulator means comprises an insulative layer covering said first set of ohmic contacts.
10. The solar cell according to claim 1, further
including an anti-reflective coating on said front
11. The solar cell according to claim 1, wherein
said bulk layer is formed of n-type material, and
said front and back surfaces are initially doped]n*.12. The solar cell according to claim 1 wherein
said second set of ohmic contacts are comprised of
ohmic metals selected froom group consisting pf
silver, aluminum, copper, titnium and palladium!.
13. The solar cell according to claim 1 wherein at
least one of said front and back surfaces is
textured to increase light trapping in said bulk!
14. A method of manufacturing a back-contact solar
cell wtih self-aligning ohmic contacts, said method
comprising the steps of: ij
i (a) providing a semiconductor bulk layer of a
■* first conductivity type, said bulk layer having |a
front surface and a back surface;
(b) forming a plurality of semiconductor diffusion regions of opposite conductivity type in said bulk layer near said back surface;
(c) forming a first set of spaced ohmic contacts for said diffusion regions on said back surface using an ohmic contact metal material;
(d) electrically insulating said first set of ohmic contacts from the spaces therebetween; and
(e) forming a second set of ohmic contacts on said back surface in said spaces using an ohmic contact metal material, said second set of ohmic contacts being electrically insulated from said first set of ohmic contacts.
15. The method of manufacturing a back contact solar cell according to claim 14 wherein said step (a) of providing is performed with n-type silicon.
16. The method of manufacturing a back contact solar cell according to claim 14 wherein said step (a) of providing is performed with n-type silicon having an n* surface diffusion layer on the.front surface thereof.
17. The method of manufacturing a back contact solar cell according to claim 14 wherein said step
(a) of providing is performed with n-type silicon
having an n* surface diffusion layer on the front and
back surfaces thereof.
18. The method of manufacturing a back contact
solar cell according to claim 14 wherein said steps
(b) and (c) are concurrently performed.
19. The method of manufacturing a back contact solar cell according to claim 14 wherein said steps (b) and (c) are concurrently performed by applying a patterned layer containing a Group III metal to the back surface of said bulk layer, heating at least the back surface and adjacent interior regions of said bulk layer so that the bulk layer material in said interior regions and said patterned layer form an alloy, and allowing the alloy to cool so that said diffusion regions are formed using the Group III metal as a an acceptor and the first set of contacts are formed from the cooled alloy remaining at the back surface.
20. The method of manufacturing a back contact solar cell according to claim 19 wherein said patterned layer comprise a mixturre of Group III metal and the bulk layer material
21. The method of manufacituring a back contact solar cell according to claim 20 wherein said mixture comprises aluminum and sillcon.
22. The method of manufacturing a back contact solar cell according to claim 19 wherein said patterned layer comprises a plurality of individual stripes.
23. The method of manufacturing a back contact solar cell according to cluim 22 wherein said stripes are substantially mutually parallel.
24. The method of manufacturing a back contact solar cell according to claim 19 wherein said patterned layer is applied by screen printing.
25. The method of manufacturing * back contact solar cell according to alaim 14 Wherein said step (d) of electrically insulating is performed by forming an insulative layer over said first set of ohmic contacts and spaces therebetween, and selectively removing portion* Of said insulative layer overlying said space* from said back surface of said bulk layer, so thit Mid insulative layer
covers substantially only said first set of ohmic contacts and the spaces the ohposed.
26. The method of manufacturing a back Contact solar cell according to claim ab wherein said step of selectively removing comprises the step of etching the portions of said insulative layer overlying said spaces.
27. The method of manufacturing a back contact solar cell according to claim 26 wherein said step of etching is chemical,
28. The method of manufacturing a back contact solar cell according to claim 26 wherein said step of etching is reactive ion etching.
29. The method of manufacturing ft back contact solar cell according to claim 25 wherein said step of selectively removing comprises the step of sandblasting the portions of said insulative layer overlying said spaces*
30. The method of manufacturing a back contact solar cell according to claim 25 wherein said step of selectively removing comprises the step of ion
milling the portions of said insulative layer overlying said spaces*
31. The method of manufacturing a back contact
solar cell according to claim 14, further comprising i
the steps of texturing at least one of said front
and back surfaces of said bulk layer.
32. The method of manufacturing a back contact cell
according to claim 14 further comprising the step of
applying an anti-reflective coating on said front
33 • A back-contact solar cell substantially as herein described with reference to the accompanying drawings.
34* A method of manufacturing a back-contact solar cell with self-aligning ohmic contacts substantially as herein described with reference to the accompanying drawings.
|Indian Patent Application Number||1317/MAS/1996|
|PG Journal Number||26/2007|
|Date of Filing||25-Jul-1996|
|Name of Patentee||M/S. EBARA CORPORATION|
|Applicant Address||11-1, HANEDA ASAHI-CHO, OHTA-KU, TOKYO 144-8510.|
|PCT International Classification Number||H01L031/00|
|PCT International Application Number||N/A|
|PCT International Filing date|