EP2401772A2

EP2401772A2 - Mwt-semiconductor solar cell comprising a plurality of narrow conductive fingers that have predetermined lengths, contact the semiconductor material and run parallel to each other

Info

Publication number: EP2401772A2
Application number: EP10701388A
Authority: EP
Inventors: Hans-Joachim Krokoszinski
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 2009-02-24
Filing date: 2010-01-29
Publication date: 2012-01-04
Also published as: WO2010097268A3; WO2010097268A2; DE102009047778A1

Abstract

The invention relates to a MWT-semiconductor solar cell comprising a plurality of narrow conductive fingers that have predetermined lengths, contact the semiconductor material and run in parallel to each other, which are located at the front side of the cell facing the light, and comprising plated through-holes for electrically connecting the conductive fingers to bus bars that are provided at the back of the cell and insulated against the surrounding, wherein the conductive fingers are arranged on the cell in a strip shape. According to the invention, the lengths of the fingers are reduced for each finger strip by an increased strip number per cell, wherein electrically interconnected finger groups, comprising five to ten parallel fingers, are formed by a transverse conductive bus piece and each bus piece is connected to at least one plated through-hole.

Description

MWT semiconductor solar cell with a plurality of the semiconducting material contacting, parallel to each other narrow conductive

Fingers of predetermined length

description

The invention relates to a MWT semiconductor solar cell having a plurality of semiconductive material contacting, parallel to each other narrow conductive fingers of predetermined length, which are located on the light-facing front of the cell, and via holes for electrically connecting the conductive fingers with provided on the back of the cell, with respect to their environment isolated busbars, wherein the conductive fingers are arranged in stripes distributed on the cell, according to the preamble of patent claim 1.

The subject matter of the invention focuses on structures of a crystalline silicon solar cell with a front-side emitter and rear-side emitter solder contacts, that is, with a so-called MWT structure (metal wrap-through).

Silicon solar cells on a crystalline silicon wafer with a so-called silver fingergrid on a front emitter and back emitter busbars which are connected via metallized via holes to the front metal contact pads form the prior art discussed above.

The Fig. FIG. 1 illustrates by way of example a silicon wafer 1 with the silver finger grid 2, wherein the rear side emitter bus bars 4 run parallel to one another. The electrical connection is realized via the mentioned metallized through-holes (vias) 3.

Known is the coupling of three front-side fingers to three via holes, d. H. As a rule, there are as many vias as there are fingers on the front side, shown in FIG. 1B. The rows of holes and thus the backside busbars just below them divide the wafers into three strips of width 2s, where s is one-sixth the length of the side Structure area (= wafer edge length - 2 * edge width) is. To do this to Florian, et al., "Industrially feasible mc-Si metal wrap-through (MWT) solar cells with high emitter sheet resistance exceeding 16% efficiency", 23rd European Photovoltaic Solar Energy Conference and Exhibition, Valencia, Spain, 2008.

At 156mm wafer edge length and 1mm margin width, the side length of the texture area is 154mm. It results s as follows: s = 154mm / 6 = 25,666mm. On both sides of the Emitterbusbars, which connect the metallized vias of a series, the surfaces with z. B. screen-printed aluminum 6 metallized. In this case, small areas 5 are printed in the aluminum surfaces with solderable material, usually silver thick film paste, so that the overlapping aluminum can be contacted by soldering.

After sintering the thick-film pastes, the insulation between the silver emitter busbars 4 and the flat aluminum-BSF metal surfaces 6 in the form of a laser trench 7 is produced with a laser.

The hole or via distance corresponds to a typical variant of the prior art with three busbars and the coupling of three fingers to a hole about three times the finger spacing, ie lying in the range of = 6mm. The number, the shape and the positions of the base soldering points 5 in the aluminum surface 6 can be varied as desired.

From WO 0031803 Al a further contacting design is known that manages with only a few, matrix-like distributed through-hole, between which front side tracks are arranged, which were designed with a computer-aided optimization method similar to a spider web.

The following disadvantages arise from the prior art.

An effective increase in the efficiency of a front side fingergrid solar cell is to increase the current generation per cm ^{2 of} area by reducing the shading of the front surface due to the metallization there. In addition to the relocation of the relatively wide busbars to the back in a MWT cell, it is possible to form the front side fingers on the selective emitter in fine line printing. That is, a reduction of the finger width is made to well below 100μm. However, this reduction in the finger width leads to an increase in the finger resistance per cm length and thus the series resistance, which adversely enters into the so-called fill factor.

A major disadvantage of the proposed MWT structure are the elongated, continuous emitter paths on the back, which connect the metallized rows of holes together. The need to provide effective isolation between these tracks and the surrounding aluminum surfaces 6 by forming a circumferential laser trench 7 around the emitter bus bars 4 results in severe mechanical weakening and damage to the wafers. The trenches, which run practically from one wafer edge to the opposite edge, in this sense represent predetermined breaking points for the manufactured solar cells.

Also to be considered is the maximum current density, which is present in the region of the via holes and which can of course only be limited.

From the foregoing, it is therefore an object of the invention to provide a further developed semiconductor solar cell with a plurality of the semiconducting material contacting, parallel to each other extending narrow conductive fingers of predetermined length, the semiconductor cell based on the so-called MWT principle. With the novel cell arrangement to be created, and in particular the means for electrical contacting, a simple, low-resistance structure of the metallization of the solar cell front side should be accompanied by an improved arrangement of the vias, which avoids the long, linear isolation trenches known from the prior art. On the other hand, it is necessary to reduce the maximum distance of any point on one of the conductive fingers of the front to the next via hole (VIA) so far that a significantly reduced finger width does not lead to an increase in resistance of the front side metallization. The solution must be the last one Make room for a sufficient number of through holes, so that the current density to be supported relative to the respective hole remains low.

The object of the invention is achieved by a semiconductor solar cell according to the combination of features of claim 1, wherein the dependent claims represent at least expedient refinements and developments.

Starting from the generic MWT solar cell according to the invention, the length of the fingers per finger strip by an increased number of strips each

Cell reduced, each electrically interconnected finger groups, consisting of five to ten parallel fingers, each formed by a transverse conductive track piece.

Each track piece is at least one via hole in

Connection.

Ausgestaltend the respective conductive track piece can run at an angle deviating from 90 ° to the longitudinal direction of the parallel fingers.

In a further refinement, the conductive track pieces can contact neighboring finger groups in the strip direction.

The number of fingers per finger group is at least as large as the number of stripes per cell.

The areas around the via holes on the cell back side are surrounded with a solderable metallization, wherein the maximum extent of the solderable metallization from the center of the respective through hole is not greater than the average distance of the conductive fingers on the cell front side.

The conductive metallizations on the back of the cell are spaced from each other without contact.

Further ausgestaltend the via holes and the areas with conductive metallization are arranged like a matrix in rows and columns. The matrix here preferably forms a dot matrix with at least five and a maximum of ten columns.

The regions of solderable metallization on the back of the cell are surrounded by another conductive metallic surface applied with a different polarity, with a conductive layer between the conductive metallization and the solderable metallic surface, ultimately only local, d. H. is formed in self-contained isolation trench.

The large-scale metallization of the back has, analogously to the prior art, solder pads.

The solder pads, in turn, are arranged in rows that are parallel to and between the gaps of the via holes.

The presented semiconductor solar cell is hereinafter specified to the case of the MWT structure on a p-wafer with front emitter and backside aluminum BSF. An embodiment of the n-type material with rear aluminum emitter and front phosphorus-doped front surface field (FSF) is analogous. The explanations were also based on virtually full-square wafers. Of course, the structures according to the invention can be transferred to any other wafer shape, such as pseudo-square, hexagonal, octagonal or round wafers. Furthermore, in the present MWT cell structure, there is no mention of possibilities of realizing a selective emitter structure or, in the case of a backside emitter, a selective front surface field. However, this does not mean that the teaching according to the invention of the contacting structure can not be applied to such a development; on the contrary .

The invention will be explained in more detail below with reference to embodiments and figures.

Hereby show: 1A and 1B illustrate the design of a standard MWT solar cell for three passing busbars in a pseudo-square wafer shape (A cross-sectional view, B top view);

2A to D typical strip widths starting from the prior art (FIG. A) and three strip widths reduced according to the invention (FIGS. B to D);

Figure 3A shows typical prior art MWT contact structures with three strips of width 2s, with the via holes in the center of the strips indicated (no true-to-scale mapping in the x and y directions);

FIG. 3B shows an exemplary structure according to the invention with seven strips of width 2y corresponding to FIG. 2C, wherein for simplicity the vias are omitted in the cross-sectional view and provided with dark hatching, a current collection region for a via hole in comparison to the dark-shaded representation of FIG. 3A;

FIG. 3C is an exemplary structure of nine strips of the nibs 2z with nine via-hole rows as shown in FIG. 2D, with dark hatching also indicating the typical area of a via hole for a via hole; FIG.

Fig. 4 possible advantageous embodiments of the conductive

Track pieces as connecting tracks of a finger group for a contact structure with seven Durchkontaktierungslöcher- rows and seven tracks per track piece and in the circled detail a connection between adjacent groups of fingers for redundancy improvement in the sense of an extension of the piece of track according to figurative representation down to the next finger to make the connection to the adjacent finger group;

5 shows exemplary numbers and distances of the emitter

Contact points on the back for a MWT cell with front emitter in an off-scale image with respect to x, y and z direction, where EL represents emitter solder points and BL base solder points, and

6 shows a possible embodiment of the connecting element structure using the example of a contact arrangement with seven emitter pad rows and six base pad rows and an exemplary dimension starting from a standard MWT cell.

According to FIG. 2A, the division of a MWT cell according to the prior art is shown, with all details of the cell structure omitted for simplicity, and only the geometry is shown schematically. The cell is divided by imaginary dividing lines into three areas of width 2s = 154 / 6mm = 51.333mm, the center of which is the position of the vias for the connections from the front to the back.

The Fig. 2B to 2D correspond to variants of this starting structure according to the invention, which lead to the solution of the problem set.

When the number of stripes is increased from three to five (FIG. 2B), the strip width decreases from 2s to 2x, where, for the example of a 6 "* 6" wafer, x = 154 / 10mm, ie 15.4 mm.

As the number of cell strips increases from three to seven (Example Structure 2, Fig. 2C), the width of the individual strips will decrease to 2y, where y = 154/14 mm = 11 mm.

If the number of cell strips is increased from three to even nine (example structure 3, FIG. 4D), the width of the individual strips will decrease to 2z, where y = 154/18 mm = 8.555 mm. The numbers of the vias and the total number of fingers can in principle be chosen independently of each other. An advantageous embodiment, however, if these are approximately equal, z. For example:

3 columns of 25 holes = 75 vias (3 fingers / via, i.e. 75 fingers)

5 columns of 15 holes = 75 vias (5 fingers / via, i.e. 75 fingers)

7 columns, 11 holes = 77 vias (7 fingers / via, ie 77 fingers)

9 columns a 9 holes = 81 vias (9 fingers / via, ie 81 fingers)

If fewer fingers are needed (eg, exactly 60) to achieve optimal series resistances, different numbers of vias and fingers, depending on the number of fingers per via, will result. For example:

3 rows of 20 holes = 60 vias (3 fingers / via, ie 60 fingers)

5 columns - 20 holes = 100 vias (3 fingers / via, ie 60 fingers)

7 columns - 20 holes = 140 vias (3 fingers / via, i.e. 60 fingers) or z. B.

4 columns of 15 holes = 60 vias (4 fingers / via, ie 60 fingers)

6 columns of 15 holes = 90 vias (4 fingers / via, i.e. 60 fingers)

8 columns = 15 holes = 120 vias (4 fingers / via, i.e. 60 fingers) or z. B.

5 columns a 12 holes = 60 vias (5 fingers / via, ie 60 fingers)

6 columns, 12 holes = 72 vias (5 fingers / via, ie 60 fingers)

7 columns a 12 holes = 84 vias (5 fingers / via, ie 60 fingers)

As shown in FIG. 3A, the area of the current drainage area of a via hole in a prior art structure (three stripes) is: 3d * 2s = 6mm * 51.333mm = 308mm ²

Here, the maximum distance of a point on one of the fingers to the next hole at a track pitch of d = 2mm (the slope of the vertical track is neglected): s + d = (25.666 + 2) mm = 27.666 mm In the exemplary structure 1, 2B according to the invention, with five strips of width 2x, the current collection area is the same size as in the prior art:

5d + 2x = 10mm * 30.8mm = 308mm ²

The maximum distance of a point on one of the fingers to the next via at a track-to-track pitch of d = 2mm (the slope of the vertical track is neglected) is x + 2d = (15.4 + 4) mm = 19.4 mm

According to FIG. 3B, in the example structure 2 with seven strips of the width 2y according to the invention, the current drainage area is likewise the same size as in the prior art:

7d + 2y = 14mm * 22mm = 308mm ²

The maximum distance of a point on one of the fingers to the next via at a track-to-track distance (pitch) of d = 2mm (the slope of the vertical track is neglected):

y + 3d = (11 + 6) mm = 17 mm

According to FIG. 3C, in the example structure 3 according to the invention with nine strips of width 2z, the current collection area is likewise the same size as in the prior art:

9d + 2z = 18mm * 17, 111mm = 308mm ²

The maximum distance of a point on one of the fingers to the next via at a track-to-track pitch of d = 2mm (the slope of the vertical track is neglected) is: z + 4d = (8.555 + 8) mm = 16.555 mm

Thus it has become clear that the areas of the river basins are not restructured from 3 to 5 and 7 to 9 strips and the connection of 5 or 7 or 9 lanes instead of 3 lanes to one via to change. This means that each via is loaded with the same current as in the prior art, but that nevertheless the maximum distances of arbitrary points on the fingers of the next hole with the number of strips are significantly reduced: Longest path to the hole (slope of the vertical Railway neglected)

State of the art s + l * d = (25.666 + 2) mm = 27.666mm

5 rows of holes x + 2 * d = (15.4 + 4) mm = 19.4 mm

7 rows of holes y + 3 * d = (11.0 + 6) mm = 17.0 mm

9 rows of holes z + 4 * d = (8,555 + 8) mm = 16,555 mm

So with nine stripes, the maximum track length drops to 16,555 / 27,666 = 60%. Thus, the series resistance of the two exemplary structures according to the invention will be significantly lower than that of the standard structure.

Thus, according to the invention, the web width of the fingers can be reduced by up to 40% (eg from 100 μm to 60 μm) before the series resistance of the standard three-strip structure is again achieved.

The division into five or seven or nine stripes are only examples. It could also be four, six, eight or ten stripes. Also in all of these cases (with roughly the same total number of vias from 75 to 81), the area of the current drainage area of a single vias would be the same as in the illustrated example structures with five, seven and nine stripes.

A possible advantageous embodiment of the connecting tracks is shown in FIG. 4 shown. It provides to extend the tie lines beyond the own finger group to the first trace of the adjacent (either above or below) finger group to improve an improvement of the redundancy of the connection to the nearest via hole (via).

Referring to Figure 5, a silver-based (e.g., circular) solder pad (Pd) is printed on the cell back around the metallized vias. In the example structure 3 with nine stripes, the distance between two emitter vias becomes or solder spots 18mm (Center Center). With a diameter of the pad of 4 mm, the clear distance between the emitter solder pads is 14 mm. The lateral spacing of the nine emitter pad rows is 2z = (154/9) mm = 17, 111mm.

In the areas between the emitter solder pad rows, the base solder pads (solder pads) are printed for and overlapping the aluminum pads. In the example of the structure with 7 emitter solder pad rows (FIG. 6), they form six rows. If these have to be connected to the emitter pad rows of the (eg underlying) neighboring cell, this results in a fork structure with six fingers, which open into a transverse path, the z. B. on the (as seen from above) first row of emitter pads, from which branch off the seven fingers for all lying in seven rows emitter pads (Figure 6).

The metallization structures according to the invention of MWT cells are based on the known linear-parallel finger structure of today's standard cells and the MWT cell structure derived therefrom. They take over the large number of vias from the known design, so typically between 60 and 80 vias with 60 to 80 parallel fingers. But they leave the paradigm that the vias must be arranged in just two or three rows, as dictated by the usual solid two or three emitter busbars of standard cells. Distributing the same number of vias over a larger number of stripes increases the distance between each vias across the fingers and reduces their spacing along the fingers. This results in decisive advantages with regard to increasing the efficiency of MWT cells:

1.) The reduced distance of the vias along the fingers reduces the maximum distance of a point on the fingers to the next via. A transition from 3 to 7 stripes with a finger distance of 2 mm reduces them to 16.555 / 27.666, ie 60%, equivalent to a reduction of - 40%. This significantly reduces the series resistance contribution of the finger system of the front. 2.) The resolution of three continuous emitter busbars of (eg) 3 mm width, which in the structure according to the prior art connect the (for example) 77 fingers on the front side into (e.g. For example, nine rows of nine (e.g., circular) single pads reduces the area of the back emitter area (area of the emitter doped area busbars) of FIG

3 * [154mm * 3mm] = 3 * 462mm ² = 1386mm ²

81 * (π * 2 ² [mm ² ]) = 81 * 12.57 mm ² = 1018 mm ²

This means that the singular emitter solder pads in 9 rows of 9 pads only need 1018/1386, ie 73.5% of the area previously covered by silver. That saves 26.5% of the backside silver paste amount.

3.) The resolution of three 3 mm wide emitter busbars connecting, in the prior art structure, the (eg) 77 fingers on the front side into (for example) nine rows with nine each

(For example, circular) emitter solder pads changed the length of the path to be engraved by the laser for the isolation of emitter pads to base surfaces

(eg)

3 tracks of 154 mm length, 3 mm width d. H. 3 * 308 mm + 6 * 3 mm = 942 mm on (eg)

81 pads with 4 mm diameter, d. H. to 81 * 4 n [mm] = 81 * 12.57 mm =

1018 mm; thus causes a negligible 8% increase in the

Laser path.

But it is crucial that these laser trenches in the form of 81 circular or other self-contained structures, eg. As square or rectangular contours, and not, as in the prior art as a border of two or three busbars, which represent the highly sensitive as desired breaking points of the wafer as continuous linear trenches from wafer edge to laser edge.

4) The resolution of three continuous emitter busbars of 3 mm width, which in the structure according to the prior art, the (eg) 77 fingers of Connect the front side on the back with each other in (for example) nine rows of nine (eg circular) emitter solder pads changed the back interdigital structure of the most copper-based fasteners of a fork, which in the prior art connects three continuous emitter busbars to the four base pad rows of the neighboring cell, into a fork that connects the nine emitter busbars shown as an example to eight base pad rows of the neighboring cell.

Claims

claims

A MWT semiconductor solar cell having a plurality of the semiconductive material contacting, parallel to each other narrow conductive fingers of predetermined length, which are located on the light-facing front of the cell, and via holes for electrically connecting the conductive fingers with on the back of the Cell provided, isolated from their environment busbars, the conductive fingers are arranged distributed in stripes on the cell, characterized in that the length of the fingers per finger strip is reduced by an increased number of strips per cell, each electrically interconnected finger groups consisting of at least five to ten parallel fingers, each formed by a transverse conductive sheet, and each sheet communicates with at least one via hole.

2. Cell according to claim 1, characterized in that the conductive track pieces extend at an angle of approximately 90 ° to the longitudinal direction of the parallel fingers.

3. Cell according to one of the preceding claims, characterized in that the number of fingers per finger group substantially corresponds to the number of strips per cell.

4. Cell according to one of the preceding claims, characterized in that the lying around the through-holes on the cell back areas are surrounded with a solderable metallization, wherein the maximum extent of the solderable metallization from the center of the respective passage hole is not greater than the average distance of the conductive Finger on the cell front.

5. Cell according to claim 4, characterized in that the solderable metallizations on the back of the cell are non-contact with each other and spaced from the surrounding polarity different rear side metallization.

6. Cell according to claim 4 or 5, characterized in that the through-holes and the areas with solderable metallization are arranged in a matrix-like manner in rows and columns.

7. Cell according to claim 6, characterized in that the matrix has a dot grid with greater than or equal to 5 columns and rows.

8. A cell according to any one of claims 4 to 7, characterized in that the areas are provided with solderable metallization on the cell back side of another, acted upon by a different polarity conductive metallic surface, wherein between the conductive metallization and the conductive metallic surface, an isolation region , In particular, an isolation trench is formed.

9. Cell according to claim 8, characterized in that the conductive metallic surface has solder pads.

10. Cell according to claim 9, characterized in that the solder pads are arranged parallel to and between the columns of the via holes.

11. Cell according to one of the preceding claims, characterized in that the finger groups of the front side, the metallization of the via holes and the backside metallization surfaces over and around the exit region of the via holes consist of a silver-containing paste.

12. Cell according to one of the preceding claims, characterized in that the two-dimensional metallization on the back of the cell made of aluminum or an aluminum-containing material.

13. Cell according to claim 12, characterized in that the large-area aluminum or aluminum-containing metallization on the back of the semiconductor material made of aluminum paste or a thin film produced by sputtering or vapor deposition.

14. A cell according to any one of the preceding claims, characterized in that the solderable metallization or the solderable metal surfaces on the cell back around the via holes without contact from each other and from the surrounding back side metallization are spaced so that closed gaps between the solderable metal surfaces on the Cell back around the via holes and the surrounding backside metallization are formed.

15. Cell according to claim 14, characterized in that the self-contained gaps between the solderable metal surfaces on the cell back represent the isolation between the polarities of the cell.

16. A cell according to claim 15, characterized in that the insulation between the polarities of the cells in the self-contained gaps between the solderable metal surfaces on the cell back around the via holes and the surrounding large-area backside metallization is formed by laser bombardment or by Ätzpastendruck with subsequent Etching is realized.