AU614644B2 - Sculpted solar cell structures - Google Patents

Description

il

I

COMMONWEALTH OF AUSTRALIA Patent Act 195P 4 6 4 COMPLETE S P E C I F I C A T I N

(ORIGINAL)

Class Int. Class Application Number :PI9362 Lodged :18 July 1988 Complete Specification Lodged Accepted Published 4 Priority: helated Art S Name of ApplT'bant Address of Applicant s Actual Inventor 1 «1 UNISEARCH LIMITED 221-227jAnzac Parade, Kensington, New South Wales, Commonwealth of Australia.

GREEN, Martin Andrew WENHAM, Stuart Ross NARAYANAN, Srinivasarmhan, Address for Service F.B. RICE CO., Patent Attorneys, 28A Montague Street, BALMAIN. 2041.

Complete Specification for the invention entitled: "SCULPTED SOLAR CELL STRUCTURES" The following statement is a full description of this invention including the best method of performing it known to Us:- S0089098 18/0 1 /O7/ 89 11_111_1.1--- -2 The present invention relates to methods of shaping the surfaces of substrates which are to be used to form solar cells, and to solar cell surfaces so shaped.

Solar cells having substantially flat surfaces suffer from a number of disadvantages. For example, substantial surface reflection tends to occur so that a large part of the incident light is never absorbed by the cell.

Further, light that is absorbed tends to travel in a direction substantially perpendicular to the cell surfaca, deep into the cell and away from the regions of high collection probability near the p/n junctions, such that much of the light energy is not utilized to provide current.

1, tSome attempt has been made to overcome these 15 problems. For example, reference is made to the effot applicant's own Australian Patent No. 565214, entitled "Laser Grooved Solar Cell", where grooves are made in the ,fit surface of a solar cell substrate and dopant is then diffused into the surface to form a p/n junction which follows the contours of the grooved surface. Because the p/n junctions follow the grooves and therefore lie deeper ;t or I in the substrate, light passing deep into the substrate may still pass near a junction and may therefore act to produce current. The grooved surface also provides some 25 improvement in the reflection properties of the surface, as light incident on the groove may be reflected onto another wall of the groove rather than away from the solar cell altogether, so that there is a better chance of more light energy being absorbed.

However, these prior art shaping techniques have depended on the crystal structure of the solar cell substrate (usually silicon) to govern the final surface shape. With silicon, chemical etches are available which quickly etch the silicon 100 planes but only slowly etch the 111 planes. Previous techniques have relieu on Xz -3revealing the 111 planes by chemical etching (although in AU 565214 a laser is used to provide the initial groove and an etch is then used to reveal the 111 planes, to give a shaped grooved or inverted pyramid shaped holed surface structure, the sides of the or pyramid being angled at the normal 111 plane angle of 550.

We have found that better, more efficient, surface shapes of a solar cell can be produced irrespective of the use of the silicon 111 planes by "sculpting" the solar cell surfaces to give the desired shape. We have also found a number of highly advantageous and efficient surface shapes.

The present invention provides a method of pricessing a silicon solar cell substrate, comprising the steps of sculpting the solar cell surface by scribing the surface of the substrate to form a pattern of grooves and subsequently chemically etching the patterned surface to give a specifically desired cross-sectional shape to the grooves over substantially the entire surface, the sculpting being such that the cross-sectional shape of the grooves do- not follow crystal planes of the silicon substrate, and the surface shape of the substrate is such as to substantially maximise antireflection properties and predominantly cause light transmitted into the silicon to 25 be focussed or steered so as to cause it to be multiply totally internally reflected from the silicon surface prior to reaching the rear surface of the cell, and diffusing a dopant into the surface to form a p-n semiconductor junction.

The present invention further provides a method of processing a silicon solar cell substrate, comprising the steps of sculpting the solar cell surface by scribing the surface of the substrate to form a plurality of recesses and subsequently chemically etching the patterned surface, to give a plurality of discrete upstanding structures of oo 06 o omr oc o oo oo o co 0400 00 0 0 o 00l O 00 o o

I

004 0 0 0 eVra 0 0a o o o ;08 ua 08 0 0 o 0 0*

I

4.

specifically desired shape, over substantially the entire surface of the substrate, the shape of each of the structures being a predetermined shape whose form does not depend on crystal planes of the silicon substrate, the predetermined shape being such as to substantially maximise antireflection properties and predominately cause light transmitted into the silicon to be focused or steered so as to cause it to be miiltiply totally internally reflected from the silicon surface prior to reaching the rear surface of the cell, and diffusing a P.-n dopant into the surface to form a semiconductor junction Two major types of sculpted surface are envisaged in this invention. In the first, a series of grooves are created in the surface, the grooves being sculpted with a predetermined desirable cross section. In the second the scriber is used to create a cross-hatched type pattern on the substrate surface, which is then etched to form a plurality of adjacent upstanding structures of predetermined shape.

The processing in accordance with both the above aspects of the present invention is preferably such as to o a0 give a spacing between adjacent grooves or discrete oO0.. upstanding structures which is comparable with the o minority carrier diffusion length of the substrate .25 material to maximise the path length of light within the silicon and within a diffusion length of the p-n junction formed in close proximity to the sculpted surface, whereby 0 to maximise carrier collection probabilities.

The concept of "sculpting" the solar cell surface means that advantageous surface properties can be produced 4 and maximised.

In particular it can provide: o 0 0 S(i) reduced surface reflection by enstring that the o r majority of incident light is reflected at least three times prior to escaping the surface; 0o~ 0 Ik/ 0r (ii) regions of higher collection probability for generated charge carriers through the location of rectifying junctions in close proximity to where the majority of light passes and is absorbed; (iii) steering or focusing of the light into the regions of high collection probability by obliquely coupling it into the silicon; (iv) large increases in the effective diffusion length by incorporating light trapping in the vicinity of the top surface to repeatedly totally internally reflect the light through the regions of high collection probability.

A number of particularly advantageous surface shapes have been determined.

The present invention further provides a solar cell 8 0 substrate comprising a silicon substrate having a sculpted *a 8 surface shape, comprising a plurality of parallel grooves, °the cross-sectional shape of the grooves being such that the angle between a plane running across the tops of the 20 grooves and any point on the cross-sectional profile of *Q any groove wall is 600 or greater, the silicon substrate having a p-n semiconductor junction diffused into its surface.

The grooves preferably have a substantially wedge- .*88 25 shaped cross-section, opposite walls of the grooves being o. doped with different polarity dopants, and the solar cell surface being metal plated to electrically connect one polarity area of one groove to a different polarity area *888 of the adjacent groove, whereby to give a plurality of 44 30 series connected solar cell devices on the same substrate.

The present invention yet further provides a solar cell substrate comprising a silicon substrate having a sculpted surface shape, comprising a plurality of parallel grooves which are angled into the substrate to give a plurality of adjacent "leaning" structures, the silicon NT U

I"

-6 substrate having a p-n semiconductor junction diffused into its surface.

The present invention yet further provides a silicon solar cell substrate having a sculpted surface shape, comprising a plurality of regularly shaped and spaced adjacent contoured upstanding finger-like structures contoured so that their slope is steeper towards the base o1 the structure and gradually gets less steep towards the top of the structure, and a p-n semiconductor junction diffused into the surface of the substrate.

The present invention yet further provides a silicon solar cell substrate comprising a semiconductor substrate having a sculpted surface shape, comprising a plurality of adjacent upstanding inverted substantially pyramid shaped structures, and a p-n semiconductor junction diffused into the surface of the substrate, the angle between a plane running across the tops of the pyramids and any of the 4t 4 It pyramid sides being 600 or greater.

411* These above advantageous structures may be produced in accordance with the method of the present invention, using appropriate scribing and etching techniques.

Creating upstanding structures which are adjacent to each other and have walls sloping at 600 or over, and which are close enough to each other such that incident 25 light is reflected at least three times before escaping 4 4 the surface advantageously means that very little light is lost.

L :0 The present invention can be extremely advantageous when applied with: polycrystalline substrates, mainly for reflection 4 ~properties, since conventional forms of texture etching to expose the 111 silicon planes are not applicable; (ii) low quality low lifetime substrates, where effective diffusion length boosts provide marked ,,iWAL/ I IVt Tcs

F.

-6a improvements in efficiency; (iii) space cells, where improved radiation damage tolerance is provided by the machined surface; 0 0 00a4 0 0 00 0 0 0 0e0 00 0 *0444444 o 44 04440 o 044 044 44 4444 00 44 440440 044 00 0 0044 44, 44 0 444 4 444 "1 r o r t n I i 7 (iv) high efficiency cells in general, where reduced reflection, light trapping and effective diffusion length increases are all of primary importance.

Features and advantages of the present invention will become apparent from the following description of embodiments thereof, by way of example only, with reference to the accompanying drawings, in which: Fig. i shows a schematic cross-sectional view through a silicon solar cell substrate surface shaped in accordance with an embodiment of the present invention; Fig. 2 shows a computer model of a cross-sectional shape having advantageous properties for the surface of a silicon -ell substrate; Fig. 3 shows a further schematic cross-sectional view through a silicon solar cell substrate surface shaped in accordance with an embodiment of the present invention; Fig. 4 shows a schematic cross-sectional view through a silicon solar cell substrate surface, illustrating a groove in the surface which has been cut at an angle; Fig. 5 shows a schematic cross-sectional view through a silicon solar cell substrate surface shaped in accordance with an embodiment of the present invention; Fig. 6 shows a schematic cross-sectional view through a silicon solar cell substrate surface to illustrate a step in the manufacture of the surface of Fig. Fig. 7 shows a surface according to Fig. illustrating the path light may take which is incident on that surface; Figs. 8 and 9 show the surface of Fig. 5 in more detail; and Fig. 10 shows a cross-sectional schematic diagram through a silicon solar cell substrate surface shaped in accordance with a further embodiment of the present invention.

t I

~II

I(

I I II i ii II Ii 14 1 1- If Ir 8 The laser in conjunction with appropriate selective and non-selective etches has proven capable of sculpting the silicon surface to produce virtually any desired shapes or structures.

It is of particular interest to note that if textured silicon surfaces had pyramid faces at 60 degrees to the horizontal rather than 55 degrees, then all normal incident light would be reflected three times. This alone without any anti-reflection coating would reduce the surface reflection to about 4% for the solar spectrum, which in the presence of an appropriate thickness surface oxide layer would further reduce to less than Where practical it is therefore advantageous to have all laser and otherwise machined surfaces with slopes in excess of 60 degrees to the horizontal.

To provide an increase in the effective diffusion length, the aim is to steer the light into regions of high collection probability for the longest practical distance. A 100 silicon surface chemically textured to expose the 111 silicon planes, acts to increase the effective diffusion lengths by approximately 35% due to the oblique coupling of light into the substrate. These improvements in diffusion length however can be greatly enhanced in accordance with the present invention through 2it 25 deliberate shaping of the surface to couple or steer the t I I ,light into the silicon at more desirable angles. Hence a far more attractive proposition than relying on the j natural structures formed through exposure of the 111 planes, is to create structures with steeper faces such as in Figure 1. Figure 1 shows a cross-section through a silicon surface having upstanding structures 1 which may siltbe walls of grooves 2 or adjacent finger-like structures tt r surrounded by a "moat" 2. Reference numeral 3 illustrates a semiconductor junction created by diffusing dopant into S 35 the sculpted substrate surface. The junction 3 follows a d *1 EIC I.

9 the contours of the surface. Note that in the following drawings it should be assumed that semiconductor junctions which follow the surface contours are present, even though they are not actually shown in the drawings.

The steep faces 4 of the Fig. 1 structure, which are preferably 600 and over, act to greatly reduce surface reflection. Incident light will be reflected on this surface at least three times before escaping the surface, greatly reducing the amount of light which is lost to the solar cell. This is illustrated by broken line 5, which represents an incident light ray. This particular ray is reflected four times from the structures 1 before escaping the surface. Each time reflection occurs some of the light will be coupled into the surface of the solar cells. The more reflections that occur, the less light that is lost. The steeper faces not only provide for greatly reduced surface reflection, but couple the light into the silicon in a manner whereby multiple total internal reflections take place within the protruding structure before the light passes into the main bulk region as illustrated by light rays 6. Appropriate sizing of such structures relative to the minority carrier diffusion length of the bulk material results in extremely high collection probabilities for carriers generated within the protruding region due to the almost total enclosure by the diffused layers 3. Line 7 shows the approximate active volume boundary for this structure. In addition the multiple total internal reflections can cause virtually all light to be absorbed within these regions of high collection probability hence potentially giving high Hsc values, even for substrates with quite modest diffusion lengths. For such structures or grooves, we have found it advantageous that the groove walls are spaced some 1 1/2 to 1 1/4 times the diffusion length of 35 the particular substrate material, and similarly for the spacing between the adjacent upstanding structures.

R4 A computer model was established to simulate the performance of various structures. A piecewise linear approximation of the surface shape of a structure 1 was used such as that shown in Fig. 2. The computer simulation for this particular structure predicted an effective diffusion length increase by a factor of four when the dimensions of the protruding structure were appropriately matched to the true bulk minority carrier diffusion lengths.

The predicted results through computer modelling hold particular interest for future generations of solar grade substrates produced from feedstock of lower quality than the currently used semi-conductor grade. Such substrates could realistically be expected to have diffusion lengths in the vicinity of 30-40 microns, hence making machined surfaces with their accompanying diffusion length boosts and reduced reflection particularly attractive.

Even for high lifetime substrates, micro-machining of the surface can be used as a means for coupling light into the substrate in a path direction almost parallel to the cell surfaces. The benefits of this, although not as marked as fur low lifetime substrates, are still quite significant.

-Such techniques are also capable of providing j 25 improved resistance to radiation damage for a cell's electrical output. The dimensions of the relevant structure are designed on the basis of matching them to the expected end of life diffusion length for the 4 substrate. in this way, the short circuit current density for the solar cell would have a lower than optimal value during the early years of its life, but with relatively small reductions subsequently as the substrate diffusion t lengths fall.

(I

i TR r jA.

0 p. ri~l; 11 Machining of the silicon surface in accordance with the present invention is conveniently accomplished through the use of a laser scriber in conjunction with appropriate etching solutions. The laser provides the sites for nucleation of etching with the solution type, concentration, temperative and etching duration being the primary variables in determining the final formation.

Without relying on the silicon 111 planes for shape determination, the etching conditions can be varied to form virtually any shape. For instance the use of low concentration NaOH results in depletion towards the bases of grooves, hence leading to reduced reaction rates in these regions and the formation of shaped structures. At the other extreme, the use of high concentration NaOH 20%) at moderate temperatures (60-80 degrees celcius) produces uniform etch rates as a function of depth within the laser groove thus resulting in "U" shaped formations. Variations between these extremes enables the production of virtually any desired slope on groove walls. -n addition, high temperatures coupled with a high NaOH concentration can be used to greatly accelerate the reaction in the surface regions but not deep in the groove where the rapid evolution of gases ret-rds replenishment of the etching solution. Fig. 3 shows the typical shape formed from such etching conditions where a distinct curvature of the groove walls can be created. It should be noted however that although this shape bears considerable resemblance to the structure modelled in Fig. 2, a relatively flat base region to the hole exists. Light striking this region will not receive the benefits of oblique coupling. However due to the facts that such holes are typically 150 microns deep and the width of the flat region is only about 10 microns, then unless light is within 4 degrees of normal incidence, none will strike the flat region. A range of 30-200 e (I 1 L i" i t f( t II 1 i f L I I I (E Itt SIf

'IIII~

V

ii: z i~iOf 19. A sola; cell substrate comprising a silicon substrate having a sculpted surface shape, comprising a plurality of parallel grooves, the cross-sectional shape of the grooves being such that the angle between a plane running across ,/2 12 microns is usual with substrates of usual carrier diffusion length, for the depth of grooves.

With all such structures, the location, periodicity and depth of structures are determined by the laser scriber. Once a certain depth hole is drilled by the laser, subsequent etching has virtually no effect on the depth or location of that hole base. The depth of a hole may however offectively be reduced by appropriate etching away of i-i silicon surface to bring it into close proximi ith the base.

T' irmation of some sculptured surface shapes benefit greatly from the use of an isotropic etch such as the acid etch comprised of nitric acid and Hydrofluoric Acid. In particular, with polycrystalline structures the isotropic properties are useful for providing uniformity across a sculptured surface. Unfortunately such etches are not as effective at removing the silicon redeposited in the grooves during laser scribing and hence must be u.ed in conjunction with an etch such as NaOH.

With the described techniques for machining the solar .V cell surfaces, structures of virtually any shape can be rr formed.

There are three basic ways in which the surface can be machined in accordance with the present invention. A series of parallel grooves may be formed in the surface of 44, 1 the substrate, the profile of the grooves subsequently ri:, being shaped by chemical etch, to give cross-sectional shaped grooves such as in Figure 1 (ref numeral 3 (ref numeral 30), 5 (ref numeral 60). Alternatively, the surface could be initially scribed in a cross hatched pattern, the cross hatches subsequently being etched to leave a plurality of adjacent upstanding 3 dimensional structures, as with cross-sections such as shown in Figure 1 (ref numeral 3 (ref numeral 31) and 5 (ref numeral 61).

i i -13 A further variation in the formation techniques of structures involves the use of the laser scriber (or equivalent) to scribe grooves at an angle to the normal of the top surface (see Fig. 4).

The use of angular scribes makes it feasible to form structures such as in Fig. 5 with relative ease. one possible sequence for formation is depicted in Fig. 6. By separately scribing the set of grooves labelled from the set of grooves it is feasible to have different polarity diffusion types in the respective groove wall regions. Hence, structures 14 typified by Fig. 8 wherein opposed walls 10, 11 of the triangular shaped grooves 12 have opposite polarity diffusion 'types can be formed making practical, on high resistivity substrates, the series connection of devices on the same substrate via appropriate metallization. Examples of such inter connection techniques are shown in Figs. 9a and 9b. The same reference numerals are used to represent the same com~ponents as in Fig. 8. In Figure 9a, the nlO and p11 type regions on adjacent devices 14 are plated with metal plate 13, gradually filling the hollow region between devices with plated metal until each n-type region 10 is $4 electrically interconnected to the p-type 11 region of the adjacent device 14. If it is necessary to prevent large areas of metal coverage, all surfaces can be protected by a thermally grown oxide 15. An appropriate chemical etch can then be used to prematurely attack the SiO 2 15 layer at the points (:orners), hence exposing very small region of n-type 10 and p-type 11 silicon. Subsequent plating 13, such as with electroless nickel followed by electroless copper, leads again to the series ft interconnection of the devices on the same substrate, with the cross-section indicated by Fig. 9b.

14 Figure 7 shows a cross-sectional view of upstanding structure 61 of Figure 5 illustrating the path light may take which is incident on the structure, showing that there are a number of internal reflections increasing absorbance efficiency of the cell.

In addition to the potential for interconnecting cells on the same substrate, structures such as Fig. with the appropriate selection of Angles A and A' by angular scribes, can provide exceptionally long path lengths for light through the silicon. This results as the light 20 undergoes multiple total internal reflections 21 in the protruding regions. Furthermore, the active volume for the substrate can be greatly reduced, very high collection probabilities for all generated carriers can be achieved, and metal shading losses can be kept to a minimum.

It will be appreciated that variations in the scribing angles and spacings between scribes, in conjunction with appropriate selective or non-selective etches, can produce structures of virtually any desired shape and size.

S C V C S c eC c Ar r

Claims (33)

1. A method of processing a silicon solar cell substrate, comprising the steps of sculpting the solar cell surface by scribing the surface of the substrate to form a pattern of grooves and subsequently chemically etching the patterned surface to give a specifically desired cross-sectional shape to the grooves over substantially the entire surface, the sculpting being such that the cross-sectional shape of the grooves does not follow crystal planes of the silicon substrate, and the surface shape of the substrate is such as to substantially maximise antireflection properties and predominantly cause light transmitted into the silicon to be focussed or steered so as to cause it to be multiply totally internally reflected from the silicon surface prior to reaching the rear surface of the cell, and diffusing a 0 o dopant into the surface to form a p-n semiconductor junction. "o

2. The method of claim 1, the processing being such as So0 to give a spacing between adjacent grooves which is o" oa comparable with the minority carrier diffusion length of the substrate material to maximise the path length of light within the silicon and within a diffusion length of the p-n junction whereby to maximise carrier collection °probabilities. QI

3. The method of claim 2, wherein the minority carrier diffusion length is the expected end of life diffusion j^o" length, whereby to maximise carrier collection probabilities over the lifetime of the solar cell.

4. The method of claims 1, 2 or 3, the chemical etch evo conditions being chosen to give depletion at the bases of the grooves to produce a shaped cross-section groove structure such that the angle between opposed sides of the grooves at the apex of the is 600 or less.

The method of claim 4, the chemical etchant being NaOH of a concentration of 5% or less. r IL 7 S16

6. The method of claims i, 2 or 3, the chemical etch conditions being chosen so as to accelerate the etching reaction proximate to the surface of the substrate as compared with relatively deeper in the grooves, whereby to give a contoured cross-section to the groove walls, the contour being such that the walls get progressively steeper away from the mouth and towards the base of the grooves.

7. The method of claim 6, the chemical etch conditions being chosen so that the contour is such that the angle between a plane running parallel to the tops of the grooves and any tangent at any point on the cross sectional profile of any groove wall is 600 or greater.

8. The method of ciaims i, 2 or 3 wherein the surface of the substrate prior to sculpting is substantially planar ind the step of scribing is performed with a scriber directed at an angle other than 900 to the surface of the substrate to form grooves which are "angled" with respect to the surface.

9. The method of claim 8 wherein adjacent grooves extending at opposed angles to each other are formed, and wherein the step of chemical etching is applied to remove the substrate between the opposed grooves, whereby to give a plurality of adjacent grooves of substantially wedge- shaped cross-sections.

10. The method of claim 9, wherein, during the doping step, different polarity dopants are used for each groove in an opposed pair of grooves, so that when the substrate between the grooves is removed opposite walls of the wedge-shaped groove are of different polarity, and compris-ng the further step of metal plating the substrate to electrically connect one polarity area of one groove to a different polarity area of an adjacent groove, whereby to produce a series connection of solar cell devices in the same substrate. Cr I I S 00 #5I o 55 Goa 0 00 a 00 ,4 -0 1/ r 'I 17

11. The method of claim 8, wherein parallel grooves are angled at the same angle into the substrate, giving a plurality of adjacent "leaning" structures.

12. A method of processing a silicon solar cell substrate, comprising the steps of sculpting the solar cell surface by scribing the surface of the substrate to form a plurality of recesses and subsequently chemically etching the patterned surface, to give a plurality of discrete upstanding structures of specifically desired shape, over substantially the entire surface of the substrate, the shape of each of the structures being a predetermined shape whose form does not depend on crystal planes of the silicon substrate, the predetermined shape LI r, being such as to substantially maximise antireflection II, properties and predominately cause light transmitted into the silicon to be focused or steered so as to cause it to be multiply totally internally reflected from the silicon surface prior to reaching the rear surface of the cell, and diffusing a dopant into the surface to form a p-n semiconductor junction.

13. The method of claim 12, the processing being such as to give dimensions for the structures and the spacing zrt¢ c between the structures which are comparable with the minority carrier diffusion length of the substrate material to minimise the path length of light within the silicon and within a diffusion length of the p-n junction whereby to maximise carrier collection probability.

14. The method of claim 13, wherein the minority carrier diffusion length is the expected end of life diffusion length, whereby to maximise carrier collection probabilities over the lifetime of the solar cell.

The method of claims 12 or 13 wherein, in the step of scribing a cross hatched pattern of grooves is formed, and wherein the chemical etch conditions are chosen to give depletion at the bases of the recesses to produce a i r 18 plurality upstanding pyramid structures, the conditions being chosen such that the angle between any side of any pyramid and a plane through the apexes of the pyramid is 600 or greater.

16. The method of claims 12, 13 or 14, wherein in the step of scribing, a cross hatched pattern of grooves is formed, and the subsequent chemical etch conditions are chosen so as to accelerate the etching reaction proximate to the surface of the substrate as compared with progressively deeper in the recesses, whereby to give a plurality of adjacent upstanding finger-like structures having contoured walls, the contouring being such that the walls get progressively steeper from the apex of the finger-like structures towards their bases.

17. The method of claim 16, wherein the chemical etch conditions are chosen such that the angle between a tangent at any point on the contour profile of any STI finger-like structure and a plane through the apexes of St the structures is 600 or greater.

18. The method of claims 12, 13 or 14, wherein a laser is used to scribe a cross hatched pattern of grooves on the surface of the substrate, the grooves being angled such %rt that pairs of parallel grooves are angled away from each other into the substrate, and wherein the step of chemical .I etching is applied to remove the substrate between the r pairs of parallel grooves, whereby to give a plurality of upstanding inverted substantially pyramid shaped r structures.

19. A solar cell substrate comprising a silicon substrate having a sculpted surface shape, comprising a plurality of parallel grooves, the cross-sectional shape of the grooves being such that the angle between a plane running across the tops of the grooves and any point on the cross-sectional profile of any groove wall is 600 or greater, the silicon substrate having a p-n semiconductor 4T 19 junction diffuseJ into its surface.

A solar cell substrate in accordance with claim 19, wherein the grooves are shaped in cross-section.

21. A solar cell substrate in accordance with claim 19, wherein the walls of the grooves are contoured so that their slope is steeper toward the base of the grooves and gradually gets less steep towards the top of the grooves.

22. A solar cell substrate in accordance with claim 19, wherein the grooves are substantially wedge shaped in cross-section.

23. A solar cell substrate in accordance with claim 22, wherein opposite walls of the grooves are doped with different polarity dopants, and the solar cell surface is metal plated to electrically connect one polarity area of a groove to a like polarity area of an adjacent groove, Swhereby there are a plurality of series connected solar ET cell devices on the same substrate.

24. A solar cell substrate comprising a silicon substrate having a sculpted surface shape, comprising a plurality of parallel grooves which are angled into the substrate to give a plurality of adjacent "leaning" structures, the Ssilicon substrate having a p-n semiconductor junction diffused into its surface. t t It

25. A solar cell substrate in accordance with any of claims 19 to 24, wherdin the spacing between adjacent grooves is comparable with the minority carrier diffusion length of the substrate material, to maximise the path I length of light within the silicon and within a diffusion length of the p-n junction whereby to maximise carrier l collection probabilities.

26. A solar cell substrate in accordance with claim wherein the minority carrier diffusion length is the expected end of life diffusion length, whereoy to maximise carrier collection probabilities over the lifetime of the cell. SO 0 _Ij-l) r: a: 20 B* o e *r S I Ir is S r I I It 1111

27. A silicon solar cell substrate having a sculpted surface shape, comprising a plurality of regularly shaped and spaced adjacent contoured upstanding finger-like structures contoured'so that their slope is steeper towards the base of the structure and gradually gets less steep towards the top of the structure, and a p-n semiconductor junction diffused into the surface of the substrate.

28. A solar cell substrate in accordance with claim 27, wherein the structures are such that the angle between a tangent at any point on the contour profile of any finger-like structure and a plane through the apexes if the structures is 600 or greater.

29. A silicon solar cell substrate comprising a p-n semiconductor substrate having a sculpted surface shape, comprising a plurality of adjacent upstanding inverted substantially pyramid shaped structures and a p-n semiconductor junction diffused into the surface of the substrate, the angle between a plane running across the tops of the pyramids and any of the pyramid sides being 600 or greater.

A solar cell substrate in accordance with any of claims 28 to 30, wherein the dimensions of the structures and the space in between the structures are comparable with the minority carrier diffusion length of the substrate material to minimise the path length of light within the silicon and within a diffusion length of the p-n junction whereby to maximise carrier collection probability.

31. A solar cell substrate in accordance with claim wherein the minority carrier diffusion length is the expected end of life diffusion length whereby to maximise carrier collection probabilities over the lifetime of the solar cell. St t I IC -21

32. A method of processing a silicon solar cell substrate, substantially as herein described with reference to Figures 1, 2 and 3 of the drawings, or Figures 4 and 10 of the drawings, or Figures 5 to 9 of the drawings.

33. A silicon solar cell substrate, substantially as herein described with reference to Figures 1, 2 and 3 of the drawings, or Figures 4 and 10 of the drawings, or Figures 5 to 9 of the drawings. DATED this 20 day of February 1991 UNISEARCH LIMITED Patent Attorneys for the SApplicant: F.B. RICE CO. t it t 44 ,srp.I'~ r~-rttfl-V~C ~~t~VWVt~ a i-