CA1164734A

CA1164734A - Method for applying an anti-reflection coating to a solar cell

Info

Publication number: CA1164734A
Application number: CA000375170A
Authority: CA
Inventors: Michael Schneider
Original assignee: Exxon Research and Engineering Co
Current assignee: ExxonMobil Technology and Engineering Co
Priority date: 1980-07-11
Filing date: 1981-04-10
Publication date: 1984-04-03
Also published as: JPS5749279A; FR2486718A1; GB2079537B; FR2486718B1; AU542589B2; ES503827A0; GB2079537A; BE889579A; IT8122882A0; AU7275481A; DE3127156A1; ES8301556A1; IT1137610B

Abstract

ABSTRACT OF THE DISCLOSURE

The present invention teaches a method for applying an anti-reflection coating 14 to solar cells shown in Figure 3 whereby the coating is interdispersed among the cell's electrodes 20 on the cell's junction surface. The method includes coating the solar cell's surface 12 with an anti-reflection layer 14, selectively etching a pattern through the anti-reflection layer, which corresponds to the desired electrode pattern 18 and electrolessly plating a layer of nickel 18 onto the cell's surface.

Description

`i 1 6'173~1

2 The present invention relates to solar cells and

3 in particular to solar cells having an anti-reflection

4 coating which is applied to the solar cell prior to elec-troding.
6 Photovoltaic devices such as silicon solar 7 cells promise a viable alternative to non-replenishable 8 fossil fuel energy generation. Light energy (photons) 9 incident on a solar cell's surface must enter and be ab-sorbed within the cell to be converted to electrical 11 energy. The efficiency of the solar cell is directly pro-12 portional to the amount of light absorbed by the cell.
13 Depending upon the particular cell fabrication 14 process, the surface of the solar cell may be substa~-tially reflective of light energy, reducing the solar 16 cell's efficiency. Polished silicon, for example, may 17 have a reflectivity of 40~ in the spectral region of 0.35 18 and 1.2 microns. The problem is well recognized in the 19 art and numerous solutions have been employed. A known effective technique for reducing the unwanted reflection 21 is an anti-reflection layer in contact with the solar 22 cell's surface. The anti-reflection (hereinafter A-R) 23 layer is selected to coordinate its index of refraction, 24 thickness and transmittance characteristics to surface re-flection characteristics and the spectral region of in-26 terest. For silicon solar cells having a principal spec-27 tral absorbance between 0.35 and 1.2 microns, metal 28 oxides such as tin oxide, titanium dioxide and magnesium 29 fluoride are known anti-reflective coatings. Typically these oxide layers are applied to the cell's surface 31 after the cell fabrication has been completed, coating 32 virtually the entire cell surface including the cell's 33 electrodes. Applying the A-R coating after the comple-34 tion of the cell restricts the temperature at which the ~. ~

" i 1 6~7.~

1 A-R coating may be applied or treated. In contrast to 2 the typical coating sequence, the present invention 3 teaches an A-R coating method where an A-R la~er precedes 4 the electroding of the solar cell, avoiding the tempera-ture limitations of the aforedescribed p~ior art process.
6 The process is of further advantage in permitting elec-7 trical contact to the cell's electrode after the A-R
8 coating, facilitating automated production of solar cells 9 and solar panel assembly.
The art has generally recognized the advantage 11 of exposing at least a portion of the solar cell's elec-12 trodes through the A-R coating. In U.S. Patent 3,949,463, 13 for example, Lindmayer et al teach a method for applying 14 an A-R coating ~o a silicon solar cell where the A-R
coating does not overcoat the cell's current collecting 16 electrode. The technique is further exemplified in U.S.
17 Patent 3,904,453 where Revesz et al use photolithographic 18 techniques in the formation of solar cell electrodes which 19 are not overcoated with the cell's A-R coating.
SUr~A~Y OF THE IN~ENTION
21 The present invention teaches an improved solar 22 cell having an A-~ coating which is applied to the cell 23 prior to constructing the cell's electrodes. Junction 24 diffused silicon wafers are coated with an A-R layer by either spin-on coating or spray deposition, both tech-26 niques employing a heating of the cell at about 200C to 27 about 300C. An electrode pattern is masked onto the 28 A-R coa~ed surface and the oxide is removed from the un-29 masked regions by chemical etching means to expose the solar cell's surface. The etched region is contacted with 31 an electroless nickel sensitizer and plated with a layer 32 of electroless nickel plating. The electrode pattern 33 mask is then removed and the nickel plated electrodes are 34 contacted with solder flux and molten solder which forms a layered nic]cel/solder electrode interdispersed through 36 the A-R coating. The process taught herein permits heat ~. ,~

I 3 6473~

1 treatment/heat processing of the A-R coating and facili-2 tates electrical connections to the A-R coated cell.

4 In the drawing, where like components are com-monly designated, Figures 1 through 3 illustrate cross-6 sectional views of the solar cell during successive steps 7 in the fabrication tectmique of the present invention.

9 In Figure 1, a silicon wafer 8 having a first type conductivity region 10 which may comprise P-type or 11 N-type silicon,is diffused to form a region 12 of con-12 ductivity type opposite to that of region 10, forming a 13 semiconductor junction otherw;se termed a P-N (or N-P) 14 junction in the region of the interface between regions 10 and 12. The diffusion and junction forming processes 16 are well known in the art. Furthermore, the present in- `
17 ventive A-R coating is operable for either N on P or P
18 on N type cells. In a preferred embodiment, diffused 19 layer 12 extends a relatively short distance into the wafer to form a shallow junction, less than about 0.5 21 microns in depth.
22 The diffused cell is then optionally etched in 23 a buffered solution of hydrofluoric acid to remove sur-24 face oxidation and possible adsorbed contaminants. The cell is then coated with an A-R layer 14 comprising a 26 metal oxide selected from the group consisting of titan-27 ium dioxide (TioX), silicon dioxide (SiOx), magnesium 28 fluoride, and silicon nitride. As recognized by those 29 of the art, the precise oxidation state of the A-R
layer 14 may vary dependent upon the metal oxide used and 31 its preparation conditions; however, oxidation state 32 variations are compensated for in adjusting the thickness 33 of the layer to provide the desired optical properties 34 of the A-R layer.
The formation of A-R layer 14 is, itself, known 36 in the art, and may comprise the alternate techniques ~..~

i ~ fi~73~1 1 described hereinbelow. The thickr.ess and refractive in-2 dex of the particular A-R coating utilized are approxi-3 mated by the theoretical relationship for constructive 4 interference in the A-R coating; d = ~/4n where d is the approximate thickness of the A-R layer, ,~ is wave 6 length, and n is the refractive index of the A-R coating 7 material.
8 Spin-On Coating: ~ayer 14 may be formed by 9 applying an amount of spin-on coating material such as a solution of titanium and silica in alcohol based solvent, ll commercially available from Emulsiton Corporation under 12 the trade name of "Titanium Silica Film C". The coating 13 is applied by the known technique of contacting the cell 14 surface with the coating li~uid and spinning the cell in commercial photoresist spinner means. Layer thickness 16 is controlled by regulating the amount of liquid applied 17 to the surface and the spinning speed which controls the 18 centrifugal liquid soreading force. ~pproximate regula-l9 tion of the layer thickness may be provided by monitor-ing the interference color of the layer, whereby a deep 21 blue reflection color is indicia of an appropriate thick-22 ness for the A-R coating. The soin-on coated layer is 23 then densified by heat treating the cell at a temperature 24 ranging from about 200C to about 400C for a time rang-ing from about 15 min. to about 30 min.
26 Spray Deposition: The A-~ layer may be fabri-27 cated by thermal decomposition of a solution containing 28 a metal solute wnich will decompose to the desired metal 29 oxide upon heating. Spraying (or misting) the solution onto a heated cell is a conventional deposition tech-31 nique which provides a controlled growth of the metal 32 oxide A-R layer. Particular spray solution compositions, 33 spray rates and decomposition temperatures are known in 34 the art and, as is the case with each of the A-R layer compositions recited herein, constitute no part of the 36 present invention.

~ I 6473~
..~

1 Vacuum Deposition: Several alternate vacuum 2 deposition techniques are known for depositing metal 3 oxide for forming the A-R coating including sputtering, 4 electron beam and ion beam deposition. Although general-ly more costly, vacuum deposition techniques generally 6 facilitate accurate control over layer thickness.
7 Each A-R layer construction technique taught 8 herein re~uires elevated temperature processing which, 9 for cells having electrodes previously constructed onto the cell's surface, may cause tem?erature damage to the 11 electrode itself or migration of the electrode material 12 into and through the junction barrier, particularly when 13 the junction is of the shallow type where junction 14 regions are relatively near the electrode surface.
The A-R coated surface is then masked to expose 16 only that area of the cell sur_ace which corresponds to 17 the desired electrode pattern. Accordingly a layer 16 18 may alternatively comprise a silk screened asphalt based 19 ink mask patterned onto A-R layer 14 or a photoresist material which has been patterned by photolithographic 21 means to provide a masking layer exposing a region of 22 A-R layer 14 which corresponds to the desired electrode 23 pattern. The masked surface is then contacted with an 24 etchant chosen to provide selective etching of the pa~ti-cular A-R coating composition employed. For example, 26 where A-R coating 14 comprises a layer of TiOX, fabri-27 cated by heat treated spin-on deposition, an etching solu-28 tion of buffered hydrofluoric acid is used to selective-29 ly remove the A-R coating in the unmasked regions of the cell surface. Referring to Figure 2, it is to be recog-31 nized that the etching solution is chosen to selectively 32 etch the A-R coating in preference to the underlying sili-33 con layer 12. Furthermore, the solution concentration 34 of the etchant is selected to minimize etchant under-cutting of the mask. Accordingly, the etching solution 36 removes the unmasked regions of A-R layer 14 which ... .... . .

` ~ Jfi~734 1 correspond to the desired electrode pattern, exposing the 2 underlying silicon surface 12'.
3 The cell is then prepared for plating by con-4 tacting at least the exposed silicon regions 12' with an electroless nickel sensiti~er such as gold cyanide or 6 gold chloride, for example. The cell is then subjected 7 to electroless nickel plating which comprises contacting 8 at least the sensitized exposed silicon regions with an 9 electroless nickel plating solution to electrolessly plate a layer of nickel 18 onto at least the sensitized 11 exposed silicon surface 12'. Nickel layer 18 in the re-12 gion contacting the formerly exposed silicon surface is 13 about 0.1 microns in thickness.
14 Subsequent to the electroless nickel deposition, masr~ing layer16 is removed using a suitable solvent. The 16 removal of masking layer 16 further removes stray nickel 17 deposition which may at least partially overcoat masking 18 layer 16. Referring to Figure 3, the removal of masking 19 layer 16 produces a cell having an A-R coating 14 and an inter-dispersed electrode 18. In a preferred embodiment, 21 the cell is heat treated at a temperature ranging from 22 about 250C to about 350C to improve the adherence of 23 the electroless nickel layer 18 to the underlying sili-24 con 12.
Electrolessly deposited nickel layer 18 is in-26 sufficiently conductive to serve as a suitable current 27 carrying electrode for most solar cell applications.
28 Accordingly, a conduction supportive electrode layer 20 29 comprising a relatively high electroconductivity metal may be formed by solder dipping, electroplating or the 31 like. In a preferred embodiment, the surface area of the 32 cell comprising at least the nickel electrode 18 is con-33 tacted first with a solder flux agent and then with 34 molten solder to form layer 20 comprising solder. In an alternate embodiment prior to the aforedescribed addition 36 of a conduction supporting electrode layer, a generally , . . .

- `' ' `I 1 64734 ... .

1 circular outer circumferential region of both surfaces 2 of the silicon cell is etched using an etchant comprising 3 a mixture of hydrofluoric and nitric acids. This pro-4 cess of edge region etching reduces the probability of edge region leakage currents and is well known in the art.
6 A suitable masking pattern, generally circular and 7 slightly less is diameter than the generally circular 8 solar cell may be used to confine the etching to the 9 outer circumferential edge regions of the cell.
An ohmic electrode generally designated 22 may 11 be formed concurrent to the formation of layer~ 18 and 12 20 comprising a first region 18' layer of electroless 13 nickel and a second region layer 20' comprising solder 14 for example. As taught with respect to layer 18, layer 22 may preferably be heat treated to increase its ad-16 herence to silicon layer 10.
17 As readily recognized from the teaching of the 18 present specification, this invention is of advantage in 19 permitting heat treatment of the cell after application of the A-R coating and of further advantage in facili-21 tating electrical connection to the A-R coated cell. To 22 assist one skilled in the art, the following examples 23 detail a particular embodiment of the present invention.
24 Example 1 A 5.6 cm diameter N-type single crystal sili-26 con wafer was diffused to form a P-N junction having a 27 relatively shallow junction depth of about 0.3 microns.
28 The diffused wafer was then cleaned in a solution of hydro-29 fluoric acid to remove surface oxidation, rinsed in dis-tilled deionized water, and blown dry. The cleaned cell 31 was then placed on a photoresist spinner and, using an 32 eye dropper, an amount of a solution of titanium/silica, 33 commercially available from the Emulsitone Corporation 34 located in Whippany, New Jersey, was applied to the cell's junction surface. The solution was spin coated at approxi-36 mately 3,500 rpm for about 10 seconds. The cell was then ~,~

..... .

" i 1 6~73~

1 heat treated at a temperature of about 200C to cure and 2 densify the A-R coating. After cooling, tlle cell's coat-3 ing was visually observed to be deep blue in color having 4 a relatively hard, mar-resistant surface. The A-R coated

5 c211 was then masked by silk screening an asphalt based

6 ink, patterned to expose the A-~ layer only in a region

7 to correspond to the desired electrode pattern. The mask

8 pattern was baked at about 100C to cure the asphalt based

9 ink and assure the adherence of the mask during the sub-sequent processing steps. The desired electrode pattern 11 was a central, tapered wldth bus, tapering from about 4 mm 12 near one circumference of the cell to about 2 mm near the 13 diametrically opposed circumference of the cell. Eighteen 14 grid line electrodes, generally perpendicular to the bus 15 and approximately equally spaced, were about 0.3 mm in 16 width. The masked surface was then immersed in a dilute, 17 buffered hydrofluoric acid etch comprising a well known 18 etchant mixture of 15 parts by weight of ammonium fluoride 19 and eight parts water, this being mixed in a ratio of 4 to 1 with hydrofluoric acid. The etchant removed the TiOX
21 A-R layer in the exposed (unmasked) areas,exposing the 22 underlying silicon layer. Visual observance of the sur-23 face revealed virtually no undercutting or lifting of the 24 mask. The cell was then immersed in an electroless gold sensitizer comprising 50 ml hydrofluoric acid, 50 ml of 26 0.5% w/v of gold chloride and 900 ml of water which solu-27 tion coated both the etched surface and the back of the 28 cell (the N-type surface). The sensitized cell was then 29 immersed in an electroless nickel plating solution com-prising 65 gm ammonium acetate, 50 gm of ammonium chloride, 31 30 ml of nickel chloride and 10 gm of sodium hypophos-32 phate. Electroless plating continued for about 10 min-33 utes, whereafter the cell was rinsed in distilled de-34 ionized water and blo~m dry. The masking layer was then removed from the cell in a chloronated solvent such as 36 I~hibisol, a trade ~ of the Pentone Corporation of `-`` i 16~73~
_ 9 _ 1 Tenafly, ~ew Jersey. The removal of the mask also re-2 moved any stray nickel deposit, leaving essentially the 3 desired grid pattern on the cell's top surface and an 4 approximately full surface coverage of the bottom (N-type) surface. The outer circumferential region of each 6 surface was then ground to minimize leakage current paths 7 through the cell's edge. The cell was then heat treated 8 at about 300C for about 15 minutes to improve the ad-9 herence of the nickel to the silicon and alleviate edge grinding work damage. The cell was then immersed in 11 solder flux and dipped into a molten solder bath which 12 adhered to the nickel plated electrode areas of both sur-13 faces, completing the fabrication of the cell.
14 The cell's reflectivity characteristics were determined using a Beckman Model ',DX-la spectrometer fitted 16 with an integrating sphere for measuring total spectral 17 and diffuse reflection. The cell having an A-R coating 18 in accordance with the present invention had an average 19 reflectance of about 3~ in the spectral region of about.4 microns to about 1.1 microns as compared to a typical re-21 flectance of 30% for conventionally prepared cells.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of applying an anti-reflective coating and an electrode pattern to a solar cell sur-face, said method comprising:
coating at least said surface with an anti-reflection layer of a metal oxide;
masking a portion of said coated surface to form an electrode pattern in at least a portion of said surface which remains unmasked;
etching said coating in the unmasked portion of said surface to remove said oxide coating;
sensitizing at least said etched region of said solar cell surface;
electrolessly plating a layer comprising nickel onto at least said sensitized region of said solar cell surface;
removing said masking from said solar cell surface whereby said nickel layer remains only in said electrode pattern;
coating said nickel layer with a layer of conduction supporting metal.

2. The method of claim 1 wherein said anti-reflection coating comprises an oxide of titanium/
silica.

3. The method of claim 2 wherein said anti-reflection oxide is formed by spin-on coating said surface with a solution of titanium/silica in an alcohol based solvent.

4. The method of claim 3 wherein said spin-on coating is subsequently heat treated at a temperature ranging from about 200°C to about 400° C for a period ranging from about 15 minutes to about 30 minutes.

5. The method of claim 2 wherein said oxide coating is formed by spraying a solution of titanium/
silica in an alcohol based solvent onto a heated cell surface.

6. The method of claim 1 wherein said mask-ing comprises screening an asphalt based ink coating a portion of at least one surface of said solar cell, patterned to expose an area of said surface which corresponds to an electrode pattern for said solar cell.

7. The method of claim 1 wherein said elec-troless nickel plating comprises contacting at least said etched surface and a bottom surface of said solar cell with an electroless plating solution comprising ammonium acetate, ammonium chloride, nickel chloride and sodium hypophosphate.

8. The method of claim 7 wherein said plat-ing produces a layer of nickel ranging in thickness from about 0.08 microns to about 1.2 microns.

9. The method set forth in claim 8 wherein said nickel layer is heat treated at a temperature ranging from about 250°C to about 350°C for a time period ranging from about 10 minutes to about 30 minutes.

10. The method of claim 1 wherein said conduc-tion supporting metal comprises solder.