NL2006933C2

NL2006933C2 - Photo-voltaic cell.

Info

Publication number: NL2006933C2
Application number: NL2006933A
Authority: NL
Inventors: Johannes Adrianus Maria Roosmalen; Evert Eugene Bende; Ilkay Cesar
Original assignee: Stichting Energie
Priority date: 2011-06-14
Filing date: 2011-06-14
Publication date: 2012-12-17
Also published as: TW201304165A; WO2012173474A3; WO2012173474A2; CN103733347A; EP2721641A2

Description

P94836NL00

Title: Photo-voltaic cell

Field of the invention

The invention relates to a photo-voltaic cell and a method of manufacturing such a cell.

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Background

For various reasons it is desirable to provide terminals for both poles of a photo-voltaic cell on the back surface of the cell. The photo-voltaic cell 10 comprises a semi-conductor body wherein pairs of movable charge carriers of mutually opposite charge are excited. The terminals collect charge carriers of mutually opposite charge from the semi-conductor body via a semi-conductor junction and without intervening junction, via an emitter region or regions and a base region or base regions respectively. Various ways of realizing such a cell 15 with back surface terminals to the emitter and base regions are known.

One class of solutions involves vias (conductor connections through holes in the cell), to feed current collected from the front surface to the terminal on the back surface. A grid of metal electrodes on the front surface may be used to conduct collected current to a via, which conducts the current 20 from one polarity of charge carriers to a terminal for one pole on the back surface. The opposite pole is connected to conductors that collect current from the opposite polarity of charge carriers from the back surface. Examples of cells of this type are MWT cells (Metal Wrap Through cells) and EWT cells (Emitter Wrap Through cells). With such cells it is possible to cover a 25 relatively large fraction of the cell surface with the emitter region or regions, which is advantageous for efficiency.

Another class of solutions eliminates the need for vias by collecting the currents for both poles from emitter and base regions on the back surface. Examples of cells of this type are called IBC cells (Interdigitated Back Contact 2 cells). An IBC cell has two conductor grids on the back surface, to contact the emitter and base regions, each with a series of parallel fingers connected to a central conductor (called the bus bar) transverse to the fingers. The fingers of both grids extend parallel to each other, fingers of one grid alternating with 5 those of the other grid. In an IBC cell each finger of a grid collects charge carriers from the volume of the bulk of the cell directly adjacent the finger (“above” the finger, by reference to a situation wherein when the cell is oriented with the back surface horizontally at the lowest side) as well as charge carriers that arrive laterally from the volume of the bulk adjacent 10 fingers of the other grid. The distance between successive fingers of the grid is typically no more than a few millimeters, to limit losses in this lateral current: with increasing distance increasingly more charge carriers will recombine before reaching the finger. The width of the emitter regions under the fingers is preferably larger than that of the base regions, in order to improve 15 efficiency. In conventional IBC cells the fraction of the back cell surface that is covered with emitter regions may be about eighty percent for example. IBC cells have the problem that they require complex junction patterns and conductor grids, with small features of adjacent regions with opposite polarities and many narrow fingers, which increases resistive and 20 recombination losses. Moreover, current loss may occur underneath the bus bars.

Summary 25 Among others, it is an object to provide for a photo-voltaic cell wherein currents of both polarities can be collected efficiently from the back surface with wider and/or less complicated back surface structures than in IBC cells.

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In an alternative aspect, it is an object to provide for a photo-voltaic cell wherein currents of both polarities can be collected from the back surface with less loss underneath a bus bar.

5 According to one aspect a photovoltaic cell is provided comprising a semi-conductor body having a first and second surface facing each other. In use, the first and second surface will be the back surface and the front surface of the photo-voltaic cell that will be turned away from the light energy source (e.g. the sun) and toward it respectively. Typically the semi-conductor body is a 10 thin body, with a much larger diameter (width an/or length) than thickness, the first and second surface being separated by the thickness. The photovoltaic cell has - first charge carrier collection regions on the first surface, coupled to a bulk of the semi-conductor body via a semi-conductor junction in or on the 15 semi-conductor body; - second charge carrier collection regions on the first surface, coupled to a bulk of the semi-conductor body without intervening semi-conductor junction; - conductors on the second surface, electrically coupled to the first 20 and second charge carrier collection regions by the semi-conductor body, at least substantially without any further current path from the conductors to the first and second charge carrier collection regions electrically in parallel with the semi-conductor body, the conductors extending from first areas on the second surface opposite the first charge carrier collection regions to second 25 areas on the second surface opposite the second charge carrier collection regions. As used herein, “opposite” means at, or including, the same two dimensional position on the first and surfaces, when viewed along a direction perpendicular to the surfaces. Similarly, positions will be said to be “underneath” or “over” positions on the surface if they have the same two 30 dimensional position as the position on the surface, when viewed along a 4 direction perpendicular to the surface, as it would be if the back surface was oriented horizontally at the lower side of the photo-voltaic cell.

Thus a photo-voltaic cell is provided with both base and emitter contacts on the back surface that collect light excited charge carriers from the 5 semi-conductor body. The conductors on the front surface provide for lateral transport of light excited majority charge carriers from the bulk of the semiconductor body at positions over the first charge carrier collection regions (e.g. emitter regions and/or regions containing a bus bar) to the bulk of the semiconductor body at positions over the second charge carrier collection regions.

10 This reduces recombination loss, and it removes a limit on the width of the first charge carrier collection regions beyond which the collectable current saturates. This makes it possible to realize improvements in efficiency by using wider first charge carrier collection regions (e.g. emitter regions and/or regions containing a bus bar). An emitter fraction (fraction of the back surface 15 covered by emitter regions) of ninety six percent may be reached.

In an embodiment the first charge carrier collection regions are emitter regions and the second charge carrier collection regions are base regions, the semi-conductor junction being provided on or in the first surface in a pattern wherein the semi-conductor junction is provided in regions 20 corresponding to the emitter regions and wherein the semi-conductor junction is absent in regions corresponding to the base regions. In this way it is possible to obtain higher efficiency with wider emitter regions.

In an embodiment, the base regions comprise conductor material along a set of parallel base lines and the emitter regions lie between successive 25 pairs of the parallel base lines, the conductors on the second surface extending transversely from the parallel base lines to the emitter regions. This provides for a simple layout with high efficiency.

In an embodiment, a distance between at least one successive pair of the parallel base lines is at least ten millimeter. From distances in the middle 5 between such parallel base lines hardly any majority charge carriers would reach the base conductors without the conductors.

In an embodiment, the conductors comprise parallel conductor lines extending transverse to parallel base lines. This provides for efficient 5 transport. In an embodiment, a distance between at least one successive pair of the conductor lines is less than five millimeter. At this distance at least a significant part of the majority charge carriers will be able to reach the conductors. This may be realized for example by means of parallel conductor lines extending from opposite at least one successive pair of the parallel base 10 lines to opposite the emitter region between the pair of the parallel base lines, continuously between the pair of the parallel base lines. In other embodiments, a gap of less than five millimeter between the parallel conductor lines extending from respective ones of the pair of the parallel base lines may be left. In other embodiments a gap may be left between the parallel conductor 15 lines extending from a first one of the pair of the parallel base lines to a second one of the pair of the parallel base lines.

More generally, the conductors may be provided in any pattern wherein a distance from any point on the second surface opposite to the emitter regions to a nearest one of the conductors is no more than five 20 millimeter.

In an embodiment the first charge carrier collection regions on the first surface, comprise a bus bar regions and emitter finger regions branching from the bus bar region, the second charge carrier collection regions on the first surface comprising base regions lying between successive pairs of the 25 emitter finger regions, extending on the second surface opposite the base regions. In this way efficiency loss due to recombination over a bus bar can be reduced.

Brief description of the drawing 30 6

These and other objects and advantageous aspects will become apparent from a description of exemplary embodiments, using the following figures.

Figure 1 shows a back surface 10 of a photo-voltaic cell 5 Figure 2 shows a front surface 20 of the photo-voltaic cell

Figure 3 shows a cross section through the photo-voltaic cell

Figure 4 shows a back surface

Figure 5 shows a front surface

Figure 6 shows a graph of current versus emitter width 10

Detailed description of exemplary embodiments

Figure 1 schematically shows a back surface 10 of a photo-voltaic cell comprising a first and second set of parallel conductor lines 12, 14, forming 15 emitter regions and base regions respectively. In operation the output current of the photo-voltaic cell flows through first and second set of parallel conductor lines 12, 14. Although only a first and second set of parallel conductor lines 12, 14 are shown, it should be appreciated that additionally other conductor structures, such as one or more bus bars and/or fingers extending from them 20 may be present Although a series of conductor lines is shown that begins and ends with conductor lines 12 of the same set, it should be appreciated that alternatively a series may be used that begins and ends conductor lines 12, 14 of different set, the number of conductor lines in the first and second set being equal to each other. It is emphasized that the figure is schematic: the cell may 25 have al large diameter (width an/or length) that allows for many more repeated conductor lines. The thickness of the cell is much smaller than this diameter, the first and second surface being separated by the thickness. The edges of at the perimeter of the cell will not be referred to as surfaces of the cell.

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Successive parallel conductor lines 14 may be located at mutual distances of more than ten millimeters and for example several tens of millimeters. Parallel conductor lines 12 of the first subset are provided alternating with parallel conductor lines 14 of the second subset in an 5 interdigitated manner. Conductor lines 12 of the first subset cover a much wider part of the distance between successive pairs of parallel conductor lines 14 of the second subset than the width of the conductor lines 14 of the second subset. By way of example, a photo-voltaic cell is shown wherein conductor lines 12 of the first subset are present substantially over the entire area 10 between successive pairs of parallel conductor lines 14, i.e. over the entire area between spacing strips without conductor material adjacent and in parallel with parallel conductor lines 14. However, it should be appreciated that other layouts may be used: for example conductor lines 12 of the first subset may be realized as a grid or network of conductor lines in the area between successive 15 pairs of parallel conductor lines 14.

Figure 2 shows a front surface 20 of the photo-voltaic cell comprising a third set of parallel conductor lines 22. The position of parallel conductor lines 12, 14 of the back surface is shown by dashed lines, although of course these conductor lines are not present on front surface 20. In the embodiment 20 that is shown in the figure the conductor lines 22 of the third set run perpendicularly to the conductor lines 12, 14 of the first and second set. More generally the conductor lines 22 of the third set may run transverse to these conductor lines 12, 14, i.e. non parallel and preferably at an angle between thirty and a hundred and fifty degrees. The conductor lines 22 of the third set 25 need not be straight, or isolated from one another: they may be bent, branch out or form a network. More generally, the conductor lines 22 run at the front surface from an area or areas covered on the back surface by the first set of conductor lines 12 to an area or areas on the back surface covered by the second set of conductor lines 14. Preferably, the distance between successive 8 conductor lines 22 at the front surface is less than five millimeter and preferably less than two millimeter.

Figure 3 shows a cross section through the photo-voltaic cell, comprising a semi-conductor body 30. The semi-conductor body 30 has a back 5 surface 10 on which conductor lines 12, 14 of the first and second set are shown in cross section (the plane of cross-section being transverse to the length direction of these conductor lines 12, 14) and a front surface 20 on which a conductor line 22 of the third set of is shown (the length direction of this conductor line 22 lying in the plane of cross section). Semi-conductor body 10 30 may have a thickness of a hundred and sixty micrometer between the front and back surface 10, 20, and preferably a thickness in a range of ten to five hundred micrometer.

Semi-conductor body 30 may have a first conductivity type (n-type for example), due to intrinsic doping. The conductor lines 12 of the first set 15 may be coupled to the bulk of semi-conductor body 30 via emitter regions 32. The transitions between the emitter regions and the bulk of semi-conductor body 30 form semi-conductor junctions (transitions between volumes of mutually opposite conductivity type).

By way of example, the emitter regions 32 are shown as local regions 20 in semi-conductor body 30 wherein doping that provides for a second conductivity type is present (e.g. p-type if the semi-conductor body 30 is n-type or vice versa). Such regions may be created for example by diffusion or implantation of doping material through a mask that exposes the emitter regions, or by creation of added doped emitter regions through a mask on the 25 back surface (e.g. by deposition) or by diffusion or implantation of doping material in a surface layer or creation of an added surface layer followed by selective removal of this layer outside the emitter regions. In another example, the emitter regions 32 are deposited on top of semi-conductor body 30, forming a crystalline silicon / amorphous silicon heterojunction. The conductor lines 12 30 of the first set may be as wide as the emitter regions 32 or narrower. Although 9 conductor lines 12 are shown that form a continuous coveragem it should be appreciated that alternatively each or part of conductor lines 12 may be patterned for example as a central finger with side fingers extending from it, in a zigzag or meander pattern etc.

5 The conductor lines 14 of the second set on the back surface 10 may be coupled to the hulk of semi-conductor body 30 without intervening semiconductor junction. The conductor lines 14 of the second set may be provided in further regions (not shown) in or on the back surface 10 with enhanced doping of the same conductivity type as the bulk of semi-conductor body 30 or with 10 deposited amorphous silicon (called BSF=Back Surface Field areas).

The conductor lines 22 of the third set on the front surface 20 “float” relative to the conductor lines 12, 14 on the back surface 10 in the sense that they have no other electric connections to the conductor lines 12, 14 on the back surface 10 than by means of the bulk of the semi-conductor body 30, or at 15 least no connections that are electrically significant. The conductor lines 22 of the third set are not electrically connected to any of the conductor lines 12, 14 of the first and second set on the back surface 10 by conductor material in vias through semi-conductor body. Vias from the front surface 20 to the back surface 10 through semi-conductor body 30 may be absent altogether.

20 Furthermore, different conductor lines 22 of the third set may “float” relative to each other in the sense that they have no other mutual electric connections than via the bulk of the semi-conductor body 30, or at least no connections that are electrically significant. Surface 20 may be provided with a front surface field to enhance surface passivation and lateral conductivity. Surface 20 may 25 be doped underneath the conductor lines 22 to reduce the contact resistance. Conventional structures such as texture and antireflection and/or surface passivation coatings may be present on surface 20, but they are not shown for the sake of clarity.

In operation, light excites free charge carriers in semi-conductor 30 body 30 and net currents of change carriers of mutually opposite conductivity 10 types flow to the conductor lines 12, 14 of the first and second set on the back surface 10 respectively. The operation will be described in terms of an orientation of the cell wherein back surface 10 is horizontal at the lowest side of the cell, but it should be appreciated that this orientation is used for the 5 purpose of explanation, even though in operation the cell may be oriented differently. Above (i.e. adjacent) the conductor lines 12, 14 of the first and second set the charge carriers flow to the conductor lines 12, 14 directly. Conductor lines 22 of the third set assist to provide lateral transport majority charge carriers from the regions above (i.e. adjacent) the emitter regions in 10 semi-conductor body 30 to locations on the front surface 20 opposite the conductor lines 14 of the second set.

Absent conductor lines 22 of the third set significant numbers of excited majority charge carriers would reach the conductor lines 14 of the second set only from a few millimeters lateral distance to the position of the 15 conductor lines 14 of the second set. Majority charge carriers that are excited above the conductor lines 12 of the first set (the emitter) have to travel laterally towards the conductor lines 14 of the second set (the base). This leads to additional polarization over the emitter. Effectively the laterally moving charge carriers represent a lateral current that encounters a resistance formed 20 by the semi-conductor body, which is relatively high because the semiconductor body is quite thin and has low conductivity. As a result the junction is increasingly forward biased as a function of lateral position, which results in increased loss due to recombination without contribution to the net current from the photo-voltaic cell.

25 Figure 6 shows a simulation of useful current to the conductor lines 14 of the second set (the base) as a function of the emitter width for a number of cell thicknesses without conductor lines 22. The results were obtained for a wafer with a square resistance of lohm.cm and thicknesses that vary between 50 and 1000 micrometer. The cell is characterised by: an open circuit voltage 30 (Voc) of 643mV, a short circuit current density Jsc of of 39.9mA/cm2, a diode 11 current density constant (Io) of 376 fA/cm2, a maximum power point current density (ImPP) of 37.7mA/cm2, a maximum power point voltage (Vmpp ) of 563 mV and external series resistance of le-6 ohm. The dashed line indicates the length scale limit at which 5% of the Impp is lost due the effect of lateral 5 current.

As can he seen, for most practical thicknesses the maximum power point current density more than halves when the emitter has a width of more than 5 millimeter. If a loss of 5% is taken as the maximum acceptable loss, the emitter has a width of more than 4 millimeter is unacceptable even for very 10 thick semi-conductor bodies.

With conductor lines 22 of the third set significant numbers of excited majority charge carriers reach the conductor lines 14 of the second set from a much larger lateral distance. Effectively, conductor lines 22 of the third set shunt the semi-conductor body over the emitter, lowering the resistance for 15 the lateral current and reducing the forward bias of the junction. This makes it possible to use much wider emitter areas without a large loss of efficiency.

Processes for the manufacture of photo-voltaic cell with base and emitter contacts are known per se. Such processes comprise creation of local emitter regions at the back surface 10 of a semi-conductor substrate, separated 20 by further regions without emitter and applying tracks of conductor material on the local emitter regions and the further regions.

The emitter regions may be created for example by adding doping in the substrate, using doping that provides for a second conductivity type opposite to the first conductivity type of the semi-conductor substrate, or by 25 adding semi-conducting material of the second conductivity type on the substrate. In an embodiment the further regions are preferably provided with back surface field areas, i.e. areas enhanced doping of the first conductivity type. The tracks may be applied by printing in a pattern corresponding to the first and a second set of conductor lines for example, optionally using a firing 12 through paste after depositing a dielectric layer on the back surface and followed by a firing step.

Contrary to conventional processes the conductor lines 14 of the second set, that is, the conductor lines 14 that collect the net current of 5 majority charge carriers excited in the bulk of the semi-conductor substrate may be applied at mutual distances of more than ten millimeter, which would make current collection inefficient if the third set of conductor lines 22 was not used.

The process of manufacturing the current photo-voltaic cell 10 additionally comprises creating a set of conductor lines on the front surface 20 of the substrate that extend transverse to the direction of conductor lines 12, 14 of the first and second type. Processes for creating a set of conductor lines on the front surface 20 are also known per se. The conductor lines may be applied by printing in a pattern corresponding to the third set of conductor 15 lines 22 for example, optionally using a firing through paste after depositing a dielectric layer on the front surface and followed by a firing step. A printable paste containing silver grains may be used for example. Doping of the first conductivity type may be provided in or on the semi-conductor body below (i.e. adjacent) conductor lines 22 to provide for low contact resistance.

20 Although a layout with straight parallel conductor lines on the back surface of the photovoltaic cell has been shown, it should be appreciated that alternatively bent conductor lines may be used, or lines made of a plurality of line sections at non-zero angles to each other or interrupted or non continuous lines. Also in this case a third set conductor lines 22 may be provided at the 25 front surface, running from an area or areas covered by the first set of conductor lines 12 to an area or areas covered by the second set of conductor lines 14.

Figure 4 shows a back surface 10 of another embodiment of a photovoltaic cell. Compared to the cell of figure 1, the conductor lines 14 of the 30 second subset are much more closely spaced, preferably successive parallel 13 conductor lines 14 of the second set may be located at mutual distances of less than five millimeters. As a result a high efficiency can be reached without conductor lines 22 of the third set transverse to the parallel conductor lines 12, 14 of the first and second set. First and second bus bars 16, 18 are provided on 5 the back surface, coupled to the conductor lines 12, 14 first and second set respectively.

Figure 5 shows a front surface of this embodiment of a photovoltaic cell. The positions of one bus bars 16 coupled to the first set of conductor lines is shown by dashed lines, but of course it should be understood that this 10 conductor is not located on the front surface. On the front surface an auxiliary set of conductor lines 50 is provided, that extend from the positions where the conductor lines 14 (not shown) of the second set are located on the back surface to an area at a position where the first bus bar 16 (connected to the conductor lines 12 of the first set) is located on the back surface.

15 In operation, conductor lines 50 of the auxiliary set transport majority charge carriers excited in the semi-conductor body adjacent the first bus bar 16 (i.e. above first bus bar 16, when back surface 10 is seen as a lowest horizontal surface) in the semi-conductor body to locations above the conductor lines 14 of the second set. Absent conductor lines 50 of the auxiliary set 20 significant numbers of majority charge carriers excited above the first bus bar 16 would not reach the conductor lines 14 of the second set. The remaining charge carriers would recombine without contributing to the net current from the photo-voltaic cell. With conductor lines 50 of the auxiliary set more excited majority charge carriers reach the conductor lines 14 of the second set from 25 locations above first bus bar 16. This increases the efficiency of the photovoltaic cells. The manufacturing process of the cells of this embodiment is similar to that of the cells of the embodiment of figures 1-3, except that the conductor lines 50 of the auxiliary set are applied at a different position compared to the conductor lines 22 of the third set. The bus bars may be 30 applied on the cell in the same step as the conductor lines 12, 14 of the first 14 and second set, or in a different step for example isolated from the semiconductor by a dielectric layer.

In an embodiment the photo-voltaic cells of the type of figures 1-3 may have a bus bar connected to be provided with conductor lines 50 of the 5 auxiliary set as well. In this case both a conductor lines 22 of the third set and conductor lines 50 of the auxiliary set are provided on the front surface, the latter running transverse to the former from locations opposite a bus bar 16 connected to the conductor lines 12, 14 of the first and second set. In one embodiment, a plurality of conductor lines 50 of the auxiliary set may branch 10 off the conductor line 22 of the third set closest to the bus bar 16.

Claims

A photovoltaic cell, comprising - a semiconductor body with a first and second surface facing each other; - first charge carrier collection areas on the first surface, coupled to a bulk of the semiconductor body via a semiconductor junction in or on the semiconductor body; - second charge carrier collection areas on the first surface coupled to a bulk of the semiconductor body without intermediate semiconductor junction; 10. conductors on the second surface electrically coupled to the first and second charge carrier collection areas via the semiconductor body at least substantially without any further current path from the conductors to the first and second charge carrier collection areas electrically parallel to the semiconductor body, the conductors extending from first regions on the second surface opposite the first charge carrier collection regions to second regions on the second surface opposite the second charge carrier collection regions.

2. A photovoltaic cell according to claim 1, wherein the first charge carrier collection areas are emitter areas and the second charge carrier collection areas are base areas, the semiconductor junction being provided on or in the first surface with a pattern wherein the semiconductor junction is provided in areas corresponding to the emitter regions and in which the semiconductor junction is absent in the regions corresponding to the base regions.

A photovoltaic cell according to claim 2, wherein the base regions contain semiconductor material along a set of parallel baselines and the emitter regions lie in successive pairs of the parallel baselines, the conductors on the second surface extending transversely from the parallel baselines to the emitter regions.

A photovoltaic cell according to claim 3, wherein a distance between at least one consecutive pair of the parallel baselines is at least 10 millimeters.

A photovoltaic cell according to claims 3 to 4, wherein the conductors on the second surface comprise parallel conductor lines extending transversely to the baselines.

A photovoltaic cell according to claim 5, wherein the distance between at least one consecutive pair of the conductor lines is less than five millimeters.

7. A photovoltaic cell according to claim 5, wherein the parallel conductor lines extend from at least one consecutive pair of the parallel baselines to opposite the emitter region between the pair of the parallel baselines, continuous between the pair of the parallel baselines or a gap over of less than 5 millimeters between the parallel conductor lines extending from respective of the pairs of parallel baselines, or between the parallel conductor lines extending from a first of the pair of the parallel baselines 20 to a second of the pair of the parallel baselines .

A photovoltaic cell according to claim 2, wherein the conductors are provided in a pattern in which a distance from any point on the second surface opposite the emitter regions to a nearest one of the conductors is no more than five millimeters.

A photovoltaic cell according to claim 1, wherein the first charge carrier collection areas on the first surface comprise a bus bar area and emitter finger areas that branch off from the bus bar area, wherein the second charge carriers comprise collection areas on the first surface base areas that are between successive pairs of the emitter finger regions lie and extend on the second surface opposite the base regions.

A method for manufacturing a photovoltaic cell, comprising: 5. providing a body of semiconductor material with a first and second surface facing each other; - creating a pattern of regions with and without semiconductor junction at the first surface on or in the semiconductor body; depositing a pattern of first charge carrier collection areas on the first surface coupled to a bulk of the semiconductor body via the areas with the semiconductor junction; depositing a pattern of second charge carrier collection regions on the first surface coupled to a bulk of the semiconductor material through the regions without the semiconductor junction; 15. depositing a pattern of conductors on the second surface, at least substantially without any current path from the conductors to the first and second charge carriers collecting regions electrically in parallel with the semiconductor body, the conductors extending from first regions on the second surface opposite the first charge carrier 20 collection areas to second areas on the second surface opposite the second charge carrier collection areas.