Patent Application
20110174373
The present invention relates to a photoelectric conversion cell for generating electric power by light absorption, and a photoelectric conversion module with a plurality of the photoelectric conversion cells.
In a known photoelectric conversion module such as a solar cell, a constituent unit is a photoelectric conversion cell with a linear collector electrode formed on a light receiving surface, and a plurality of the photoelectric conversion cells are connected in series on a substrate made of glass and the like. In the solar cell disclosed in Japanese Patent Application Laid-Open No. 2002-373995, a plurality of collector electrodes and auxiliary electrodes crossing the collector electrodes are provided on a light receiving surface. These collector electrodes and auxiliary electrodes are formed by collectively printing a metal paste into a desired pattern by screen printing.
However, the structure of the solar cell of the Japanese Patent Application Laid-Open No. 2002-373995 raises temperature as a result of irradiation with sunlight and the like. Thus, stress is generated in the auxiliary electrodes made of the metal paste, causing the auxiliary electrodes to fall off easily. This reduces electrical connection reliability, so photoelectric conversion efficiency is likely to decrease.
Thus, a photoelectric conversion cell and a photoelectric conversion module exhibiting excellence in connection reliability, and which are capable of maintaining photoelectric conversion efficiency at a high level have been desired.
A photoelectric conversion cell according to one aspect of the present invention includes: first and second electrode layers spaced apart from each other; a first semiconductor layer of a first conductivity type provided on the first electrode layer; a second semiconductor layer of a second conductivity type provided on the first semiconductor layer, the second semiconductor layer forming a pn junction with the first semiconductor layer; a connecting portion for electrically connecting the second semiconductor layer and the second electrode layer; and a plurality of collector electrodes each with a linear portion and a projecting portion, the linear portion extending on the second semiconductor layer from a position above the connecting portion toward an end of the second semiconductor layer, the projecting portion overlapping at least partially the connecting portion in top perspective view, while projecting from at least one of opposite ends of the linear portion in its shorter side direction. In two adjacent ones of the plurality of collector electrodes, the projecting portions of the adjacent collector electrodes are spaced apart from each other.
A photoelectric conversion module includes a plurality of the photoelectric conversion cells as recited above. The plurality of photoelectric conversion cells include first and second photoelectric conversion cells. In the first and second photoelectric conversion cells, respective ones of the first and second electrode layers face in the same direction, and the second electrode layer of the first photoelectric conversion cell and the first electrode layer of the second photoelectric conversion cell are electrically connected.
Embodiments of the present invention are described in detail below by referring to the drawings.
A photoelectric conversion cell and a photoelectric conversion module of a first embodiment according to the present invention are described first.
The photoelectric conversion module 21 includes a plurality of lined up photoelectric conversion cells 20. The second electrode layer 8 of one of adjacent photoelectric conversion cells 20 is electrically connected to the first electrode layer 2 of the other one of the adjacent photoelectric conversion cells 20. This structure easily makes series connection between the adjacent photoelectric conversion cells 20. In one photoelectric conversion cell 20, the connecting portion 7 is provided to divide the photoelectric conversion layer PV. This means that, in
The substrate 1 supports the photoelectric conversion cells 20. Examples of the material of the substrate 1 include glass, ceramics, resin, and metal. In order to form the photoelectric conversion module 21, two or more substrates 1 on which the corresponding photoelectric conversion cells 20 are provided may be arranged in series. Or, a plurality of photoelectric conversion cells 20 may be provided on one substrate 1. Providing a plurality of photoelectric conversion cells 20 on one substrate 1 makes the formation of the photoelectric conversion module 21 easy.
The first and second electrode layers 2 and 8 are made of a conductive material such as Mo, Al, Ti, and Au, and are formed by sputtering or vapor deposition on the substrate 1. In the adjacent photoelectric conversion cells 20 shown in
The photoelectric conversion layer PV includes the first and second semiconductor layers 3 and 4. The photoelectric conversion laye PV is capable of absorbing light and converting the absorbed light to electric power, and is made of a semiconductor material such as a silicon based semiconductor material and a compound semiconductor. Examples of the silicon based semiconductor material include single-crystalline silicon, polycrystalline silicon, and amorphous silicon. Examples of the compound semiconductor include single-crystalline compound semiconductors and polycrystalline semiconductors such as III-V compound semiconductors, II-VI compound semiconductors, and chalcopyrite (also called CIS) compound semiconductors.
The photoelectric conversion layer PV using the chalcopyrite compound semiconductor, that is one of the thin-film photoelectric conversion layers PV listed above, exhibits high conversion efficiency. So, this photoelectric conversion layer PV achieves photoelectric conversion efficiency despite its thinness that is substantially the same as that of conventional single-crystalline silicon, and is especially preferable accordingly. The chalcopyrite compound semiconductor is also preferable as it does not contain harmful cadmium. Examples of the chalcopyrite compound semiconductor include Cu (In, Ga) Se2 (also called CIGS), Cu (In, Ga) (Se, S)2 (also called CIGSS), and CuInS2 (also called CIS). The photoelectric conversion layer PV is formed for example by sputtering, vapor deposition and coating if formed into a thin film. Cu (In, Ga) Se2 is a compound consisting mainly of Cu, In, Ga, and Se. Cu (In, Ga) (Se, S)2 is a compound consisting mainly of Cu, In, Ga, Se, and S.
The first and second semiconductor layers 3 and 4 have different conductivity types; one is n-type and the other is p-type that form a pn junction. The first semiconductor layer 3 may be of the p-type and the second semiconductor layer 4 may be of the n-type. Or, the relationship may be reversed. The pn junction between the first and second semiconductor layers 3 and 4 is not necessary formed by direct junction between the first and second semiconductor layers 3 and 4. As an example, a different semiconductor layer of the same conductivity type as the first or second semiconductor layer 3 or 4 may be placed between the first and second semiconductor layers 3 and 4. Or, an i-type semiconductor layer may be placed between the first and second semiconductor layers 3 and 4 to form a pin junction.
The first and second semiconductor layers 3 and 4 may be connected to form a homo junction or a hetero junction. In order to form the hetero junction, if the first semiconductor layer 3 is a chalcopyrite compound semiconductor such as CIGS (what is generally called a light absorption layer), examples of the second semiconductor layer 4 include CdS, ZnS, ZnO, In2Se3, In (OH, 5), (Zn, In) (Se, OH), and (Zn, Mg) O (what is generally called a buffer layer). In this case, the first and second semiconductor layers 3 and 4 are formed for example by chemical bath deposition (CBD). In (OH, S) is a compound consisting mainly of In, OH, and S. (Zn, In) (Se, OH) is a compound consisting mainly of Zn, In, Se, and OH. (Zn, Mg) O is a compound consisting mainly of Zn, Mg, and O.
The conductive layer 5 may be provided on the second semiconductor layer 4 as shown in
A plurality of photoelectric conversion cells 20 may be lined up, and these photoelectric conversion cells 20 may be electrically connected to form the photoelectric conversion module 21. In order to easily form series connection between adjacent photoelectric conversion cells 20, the second semiconductor layer 4 and the second electrode layer 8 are electrically connected by the connecting portion 7 in the photoelectric conversion cell 20 as shown in
Preferably, the connecting portion 7 is formed together with the conductive layer 5, and is integrated with the conductive layer 5. In other words, the conductive layer 5 has an extending portion that extends from the second semiconductor layer 4 toward the second electrode layer 8, and the extending portion is connected to the second electrode layer 8. This means that the conductive layer 5 constitutes the connecting portion 7, so that process steps are simplified and electrical connection reliability is enhanced.
One of ends of the photoelectric conversion layer PV may be defined as a first end A, and the other end thereof opposite the first end A may be defined as a second end B. If the linear portions 6 of the collector electrodes extend from the first end A toward the second end B, the connecting portion 7 is preferably nearer the second end B than the first end A of the photoelectric conversion layer PV in plan view, namely is preferably near the second end B. More preferably, in plan view, the connecting portion 7 extends linearly in a direction substantially parallel to the second end B of the photoelectric conversion layer PV. This structure makes the connecting portion 7 get nearer the second end B to reduce an area between the connecting portion 7 and the second end B that becomes a dead space making no contribution to power generation, thereby enhancing photoelectric conversion efficiency.
The linear portions 6 of the collector electrodes are formed on the second semiconductor layer 4, and are intended to reduce electrical resistance in order to extract charges well generated in the second semiconductor layer 4. The linear portions 6 are arranged like railway tracks in order to suppress shield of light to be transmitted to the photoelectric conversion layer PV. Preferably, the linear portions 6 arranged like railway tracks extend to reach the first end A of the second semiconductor layer 4, namely to reach the first end A of the photoelectric conversion layer PV in plan view. This structure makes the linear portions 6 protect the outer periphery of the photoelectric conversion layer PV. Thus, generation of a chip at the outer periphery of the photoelectric conversion layer PV is suppressed, so that photoelectric conversion is performed well even at the outer periphery of the photoelectric conversion layer PV. Further, current generated at the outer periphery of the photoelectric conversion layer PV can be taken out efficiently by the linear portions 6 extending to the first end A, so that power generation efficiency is enhanced.
As described, the linear portions 6 extending to the first end A can protect the outer periphery of the photoelectric conversion layer PV, so that the total thickness of the members placed between the first electrode layer 2 and the linear portions 6 can be reduced. This allows reduction of the members and shortening of a time required to prepare the members. The total thickness of the members placed between the first electrode layer 2 and the linear portions 6 (in the example of
It is also preferable that end sections of the linear portions 6, of the conductive layer 5, and of the photoelectric conversion layer PV are on the same plane, preferably at the first end A of the photoelectric conversion layer PV to which the linear portions 6 extend. This produces a result that current generated at the first end A of the photoelectric conversion layer PV as a result of photoelectric conversion can be taken out well.
The extension of the linear portions 6 to the first end A of the photoelectric conversion layer PV preferably means that the linear portions 6 extend as far as to the outermost first end A of the photoelectric conversion layer PV completely, to which the present invention is not limited. More specifically, in order to effectively suppress development of a crack from the first end A of the photoelectric conversion layer PV to suppress generation of a chip, the outermost first end A of the photoelectric conversion layer PV and ends of the linear portions 6 may be spaced by a distance of 1000 μm or less. In order for the linear portions 6 to collect power more efficiently at the first end A, a distance between the first end A and the ends of the linear portions 6 is preferably 500 μm or less.
In order to suppress shield of light to be transmitted to the photoelectric conversion layer PV and to suppress generation of a chip at the outer periphery of the photoelectric conversion layer PV, the linear portions 6 preferably have a width of 50 to 400 μm. The linear portions 6 may have a plurality of separate branches.
In upper perspective view, the projecting portions 9 of the collector electrodes overlap the connecting portion 7, and project from at least one of opposite ends of the linear portions 6 in its shorter side direction in
The projecting portions 9 of adjacent collector electrodes are separated. More specifically, a tip end of the projecting portion 9 of one of the collector electrodes is an open end that is not connected to both the linear portion 6 and the projecting portion 9 of the other one of the collector electrodes. Thus, the projecting portions 9 do not extend continuously like a line, but they are broken between adjacent collector electrodes. So, according to this structure, stress generated in the projecting portions 9 is relaxed at the broken part, so that the projecting portions 9 are prevented from falling off even if the photoelectric conversion cells 20 are placed under high temperature during use. As a result, electrical connection reliability is enhanced to maintain photoelectric conversion efficiency at a high level. The provision of the projecting portions 9 realizes favorable conduction between the linear portions 6 and the second electrode layer 8. This also enhances photoelectric conversion efficiency.
The aforementioned projecting portions 9 enhance manufacturing yield when the linear portions 6 and the projecting portions 9 are formed by screen printing with a metal paste. The reason therefor is as follows. Screen printing is a process where a screen is rubbed with a metal paste with a rubber spatula that is what is called a squeegee to print the metal paste. Openings of a desired pattern at which the metal paste is squeezed out are defined on the screen. The squeegee may collide with the edge portions of the openings when the squeegee moves from the openings to the screen, thereby damaging the screen easily. In the conventional structure such as that disclosed in Japanese Patent Application Laid-Open No. 2002-373995, long linear auxiliary electrodes that continuously extend are arranged to cross a plurality of linear portions. When the squeegee is caused to move in this structure, the squeegee gives an impact across the edge portions of the openings on the screen for forming the auxiliary electrodes, thereby damaging the screen easily. Thus, the structure of Japanese Patent Application Laid-Open No. 2002-373995 fails to perform accurate screen printing, thereby reducing manufacturing yield. In contrast, the aforementioned projecting portions 9 of the photoelectric conversion cell 20 effectively lessens the impact of the squeegee to be applied to the edge portions of the openings on the screen for forming the projecting portions 9 even when the squeegee is caused to move. This suppresses a damage of the squeegee to enhance manufacturing yield.
At this time, by making the direction in which the linear portions 6 extend like railway tracks parallel to the printing direction of screen printing, the linear portions 6 are allowed to have a linewidth that is made small stably. This makes light shield by the linear portions 6 less likely, so that photoelectric conversion efficiency is enhanced. That is, the squeegee preferably moves in a direction substantially the same as the direction in which the linear portions 6 extend like railway tracks.
As shown in
As shown in
In order to allow the metal paste to get under the screen well during screen printing, and to form a desired pattern favorably, the depth of the recess 10 is preferably from 1% to 10% of the width of the recess 10 (from 1 μm to 10 μm if the width of the recess 10 is 100 μm, for example).
The recess 10 may be formed during simultaneous formation of the conductive layer 5 and the connecting portion 7 by a thin film forming technique. In this case, for forming the recess 10, a condition of thin film formation may be controlled to finish the formation of the conductive layer 5 with a surface of the conductive layer 5 recessed. Or, the recess 10 may be formed by removing a surface of the conductive layer 5 by etching and the like. The bottom part of the recess 10 is preferably deeper than the upper surface of the photoelectric conversion layer PV. In this case, the projecting portions 9 getting into the recess 10 get nearer the second electrode layer 8. This reduces electrical resistance, so that current generated as a result of photoelectric conversion can be taken out well.
In order to achieve favorable conduction between the linear portions 6 and the second electrode layer 8, a length L of the projecting portion 9 defined in a direction along the connecting portion 7 is preferably 5 μm or more, and more preferably, 400 μm or more. In order to prevent the projecting portions 9 from falling off by relaxing stress, a distance M between the opposite projecting portions 9 is preferably 50 μm or more. In
The length L is preferably from 0.15 to 0.40 times a distance N between the linear portions 6. This structure provides favorable electrical connection between the linear portions 6 and the second electrode layer 8, maintains favorable printing performance, and lessens the impact of the squeegee. Specifically, the distance N is preferably from 1.5 to 2.5 mm, and the length L is preferably from 225 to 1000 μm.
As an example, the linear portions 6 and the projecting portions 9 may be formed by printing a paste of metal such as Ag (mixture of metal particles and resin), and by curing the metal paste. That is, the linear portions 6 preferably contain metal particles and resin, as this enhances tolerance to bending stress of the linear portions 6 after curing.
The linear portions 6 and the projecting portions 9 preferably contain solder, as this increases tolerance to bending stress and reduces resistance further, More preferably, the linear portions 6 and the projecting portions 9 contain metals of two types or more of different melting points. In this case, the linear portions 6 and the projecting portions 9 are preferably heated and cured at a temperature at which at least one of these metals melts, and at least one of the remaining metals does not melt. Thus, the metal of a lower melting point is caused to melt to thereby densify the linear portions 6 and the projecting portions 9. As a result, resistance is reduced, and spread of the melted metal during heating and curing is prevented with the metal of a higher melting point.
The photoelectric conversion cell 20 may be manufactured as follows.
First, as shown in
Next, as shown in
As shown in
Then, as shown in
In a final step, as shown in
A photoelectric conversion cell and a photoelectric conversion module of a second embodiment of the present invention are described below.
The photoelectric conversion cell 120 and the photoelectric conversion module 121 of the second embodiment are different from the photoelectric conversion cell 20 and the photoelectric conversion module 21 of the first embodiment in that, the projecting portions 9 extend toward the second electrode layer 8 to be connected to the second electrode layer 8, so that connecting portions 17 are constructed of the projecting portions 9. The connection of the projecting portions 9 to the second electrode layer 8 to form the connecting portions 17 achieves more reliable connection, thereby enhancing connection reliability.
The photoelectric conversion cell 120 and the photoelectric conversion module 121 are formed as follows. First, as shown in
As described, the first and second semiconductor layers 3 and 4, and the conductive layer 5 are successively formed. Then, the penetrating trenches P2 for defining the connecting portions 17, and the penetrating trench P3 for defining a space between the photoelectric conversion cells 120, are formed at the same time. This simplifies a step of trench process, thereby enhancing productivity.
A photoelectric conversion cell and a photoelectric conversion module of a third embodiment of the present invention are described next.
In the photoelectric conversion cell 220 and the photoelectric conversion module 221 of the third embodiment, the recess 10 of the connecting portion 7 is formed to reach a level deeper than the upper main surface of the second semiconductor layer 4. Projecting portions 29 are provided to get into the recess 10, and front end surfaces S of the projecting portions 29 (the front end surfaces S are shown by bold lines in
The present invention is not limited to the embodiments described above. Numerous modifications can be devised without departing from the scope of the present invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2009-018411 | Jan 2009 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2010/051095 | 1/28/2010 | WO | 00 | 3/21/2011 |
