PHOTOVOLTAIC ELEMENT AND FABRICATION METHOD THEREOF

Abstract

A photovoltaic element is obtained which allows use of a low-melting solder and can suppress reduction in reliability and output thereof due to mechanical stress or water intrusion.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view which shows an embodiment of a photovoltaic element in accordance with the present invention;

FIG. 2 is a sectional view taken along the line 30-30 of FIG. 1;

FIG. 3 is a plan view which shows a photovoltaic element in its condition subsequent to formation of a resin coating layer;

FIG. 4 is a schematic view which shows plural photovoltaic elements connected in series by tab electrodes;

FIG. 5 is a sectional view which shows a construction of a photovoltaic module including photovoltaic elements in accordance with an embodiment of the present invention; and

FIG. 6 is a schematic sectional view which shows an interface between a thermosetting resin layer and a resin coating layer and its vicinity in an enlarged fashion.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a plan view illustrating an embodiment of a photovoltaic element in accordance with the present invention and FIG. 2 is a sectional view taken along the line 30-30 of FIG. 1.

As shown in FIG. 2, an embodiment of a photovoltaic element 11 in accordance with the present invention includes an n-type single crystal silicon substrate 1 having a thickness of about 140 μm-about 300 μm, a substantially intrinsic i-type amorphous silicon layer 2 formed on a top surface (light-receiving surface) of the silicon substrate 1 and having a thickness of about 5 nm-about 20 nm, and a p-type amorphous silicon layer 3 formed on the i-type amorphous silicon layer 2 and having a thickness of about 5 nm-about 20 nm. Formed on the p-type amorphous silicon layer 3 is an ITO translucent conductive film 4 having a thickness of about 30 nm-about 150 nm.

A collector electrode 5 is formed on a predetermined region of the translucent conductive film 4 by thermosetting a silver (Ag) paste. As shown in FIG. 1, this collector electrode 5 comprises plural finger electrodes Sa spaced apart at regular intervals in the X direction and a bus bar electrode 5b configured to extend in the X direction. Those finger electrodes 5a extend in the Y direction in parallel to each other. The bus bar electrode 5b further collects the current collected by the finger electrodes 5a.

The finger electrode 5a and the bus bar electrode 5b have a thickness of about 10 μm-about 100 μm. Also, the bus bar electrode 5b has a width of about 1 nm-about 3 nm (2 nm, for example). The surface side (A side in FIG. 2) on which the collector electrode 5 is formed is a light incident side of the photovoltaic element 11.

As also shown in FIG. 2, a substantially intrinsic i-type amorphous silicon layer 6 having a thickness of about 5 nm-about 20 nm and an n-type amorphous silicon layer 7 having a thickness of about 5 nm-about 20 nm are sequentially formed on a bottom surface of then-type single crystal silicon substrate 1. A translucent conductive film 8 comprised of an ITO film having a thickness of about 30 nm-about 150 nm is formed on the n-type amorphous silicon layer 7. Similar to the top collector electrode 5, a back collector electrode 9 is formed on the translucent conductive film 8. The back collector electrode 9 also comprises finger electrodes and a bus bar electrode, as similar to the top collector electrode 5.

Both the top and bottom surfaces of the n-type single crystal silicon substrate 1 have a texture structure (irregular profile). Accordingly, the similar irregular surface profile is succeeded to the i-type amorphous silicon layers 2 and 6, p-type amorphous silicon layer 3, n-type amorphous silicon layer 7 and translucent conductive films 4 and 8, respectively formed on the top and bottom surfaces.

In this embodiment, the photoelectric conversion layer consists of the n-type single crystal silicon substrate 1, i-type amorphous silicon layers 2 and 6, p-type amorphous silicon layer 3 and n-type amorphous silicon layer 7.

The i-type amorphous silicon layers 2 and 6, p-type amorphous silicon layer 3 and n-type amorphous silicon layer 7 are formed by an RF plasma CVD process.

The translucent conductive films 4 and 8 are formed by a magnetron sputtering process.

The collector electrodes 5 and 9 are formed by a screen printing process using an Ag paste.

As shown in FIG. 2, a resin coating film 10 is formed on the translucent conductive film 4 of the light incident side. The resin coating film 10 is formed from an acrylic resin containing silicon oxide as an additive.

FIG. 3 is a plan view which shows the photovoltaic element in condition subsequent to formation of the resin coating film 10. As shown in FIGS. 2 and 3, the resin coating film 10 is not in contact with a lateral surface of each side of the bus bar electrode 5b and spaced a distance L₁ (about 1 nm-about 2 nm) from the lateral surface of the bus bar electrode 5. This leaves a region 24 on a surface of the translucent conductive film 4 outside the lateral surface of the bus bar electrode 5, where the resin coating film 10 is not deposited. Accordingly, a width of the region 24 is equal to the distance L₁. As shown in FIG. 3, the resin coating layer 10 is deposited such that it covers regions over the finger electrodes 5a.

The resin coating film 10 has a function to prevent wetting and spreading of a thermosetting resin layer 23a which is formed when a solder paste layer deposited via application of the below-described thermosetting conductive solder paste is thermoset. Another function thereof is to protect a surface of the translucent conductive film 4 from damage. A further function thereof is to stop water or the like from contacting with a surface of the translucent conductive film 4.

The resin coating film 10 is formed by applying an acrylic resin containing silicon oxide as an additive such as by an offset rotary printing process or a spray process using a mask, and then heating the applied resin at about 150° C. for 150 seconds to harden it.

As shown in FIG. 2, a collector electrode covering layer 23 is formed on a surface of the bus bar electrode 5b. This collector electrode covering layer 23 is constituted by a solder layer 23b formed on a top surface of the bus bar electrode 5b and a thermosetting resin layer 23a formed in a region on a lateral surface of the bus bar electrode 5b. One end of the tab electrode 12 is positioned, through the collector electrode covering layer 23, on the top surface of the bus bar electrode 5b. The end of the tab electrode 12 is electrically connected to the bus bar electrode 5b through the solder layer 23b. The other end of the tab electrode 12 is connected to a back electrode of the neighboring photovoltaic element 11, as shown in FIG. 4, whereby plural photovoltaic elements 11 are connected in series.

The tab electrode 12 has a width portion that extends outwardly from each lateral surface of the bus bar electrode 5b a distance of L₂. Preferably, this distance L₂ is made about comparable to or longer than the distance L₁ so that the tab electrode can cover the thermosetting resin layer 23a. As shown in FIG. 6, in the case where the thermosetting resin layer 23a extends beyond the distance L₁, which is the width of the region 24, to have a width of a distance L₃, the distance L₂, which is a width of a projecting portion of the tab electrode 12, is preferably made about comparable to or longer than the distance L₃. Dimensioning the end of the tab electrode 12 to have a width enough to cover the thermosetting resin layer 23a prevents deterioration and discoloration of the thermosetting resin layer 23a by an ultraviolet light or the like. In view of accuracy and others of a fabrication process, the distance L₂ is preferably set larger than the distance L₃.

In FIG. 6, the thermosetting resin layer is shown to extend over the end of the resin coating film 10. This case is considered to occur when the thermosetting resin is large in amount, for example. However, within the variation of amount in the fabrication, the provision of the coating film having a nature of repelling the thermosetting resin minimizes this extension and enables it to stop at the end of the coating film.

The configuration described in the present invention as “the thermosetting resin is positioned between the collector electrode and the end of the coating film” encompasses the case shown in FIG. 6.

On the back side of the photovoltaic element 11, the other end of the tab electrode 12 is connected, through a solder layer 25b, to a bus bar electrode 9b of the back side, as shown in FIG. 2. Also on the back side, a collector electrode covering layer 25 is constituted by the solder layer 25b formed in a region on the bus bar electrode 9b and a thermosetting resin layer 25a formed in a region on a lateral surface of the bus bar electrode 9b.

Due to the presence of the resin coating film 10, the thermosetting resin layer 23a of the top side is restrained from wetting and spreading and accordingly confined between the bus bar electrode 5b and the end of the resin coating film 10, as shown in FIG. 2. On the other hand, the thermosetting resin layer 25a of the back side is flared because of the absence of the resin coating film 10.

The bus bar electrode 9b is electrically connected, through the solder layer 25b, to the other end of the tab electrode 12.

The solder layers 23b and 25b have respective lateral sides integrally formed with the thermosetting resin layers 23a and 25a. Accordingly, they are bonded to the bus bar electrodes 5b and 9b, respectively, with high bonding strength. This allows use of a low-melting solder component, such as an Sn—Bi solder, for formation of the solder layers 23b and 25b.

Also, the thermosetting resin layer 23a can enhance the bonding strength between the tab electrode 12 and the bus bar electrode 5b.

Also, the thermosetting resin layer 23a can prevent water intrusion into an interface between the translucent conductive film 4 and the bus bar electrode 5b, because it covers an entire lateral surface of the bus bar electrode 5b. Likewise, the thermosetting resin layer 25a can prevent water intrusion into an interface between the translucent conductive film 8 and the bus bar electrode 9b, because it covers an entire lateral surface of the bus bar electrode 9b. Accordingly, such photovoltaic elements, when installed outdoors in the form of a photovoltaic element, can prevent reduction in reliability and output of the module due to water intrusion and mechanical stress that occurs as the temperature rises and drops.

The collector electrode covering layer 23 consisting of the solder layer 23b and the thermosetting resin layer 23a, as well as the collector electrode covering layer 25 consisting of the solder layer 25b and the thermosetting resin layer 25a, can be formed in the manner as described below.

That is, a thermosetting conductive solder paste (product of Tamura Kaken Corp., product name “TCAP-5401-11”) is applied onto the bus bar electrodes Sb or 9b as by a dispenser, screen printing or offset printing process. The thermosetting conductive solder paste contains Sn—Bi solder particles, a thermosetting resin (epoxy resin), a hardener, a flux and others. The solder particles are contained in the amount of about 78% by weight and the epoxy resin is contained in the amount of about 18% by weight.

By the application of the thermosetting conductive solder paste onto the bus bar electrodes 5b or 9b, as described above, a solder paste layer is formed. Subsequently, a tab electrode comprised of a copper foil is placed on the solder paste layer. With this arrangement, they are heated at 160° C. for 360 seconds. Since a melting point of the Sn—Bi solder in the solder paste layer is 139° C., such heating causes the Sn—Bi solder to melt. The melted solder component coalesces in a region between the copper foil tab electrode 12 and the bus bar electrode 5b or 9b made of an Ag paste to form the solder layer 23b or 25b. Concurrently, the thermosetting resin contained in the solder paste layer 10 bleeds out over a region on a lateral surface of the bus bar electrode 5b or 9b to form the thermosetting resin layer 23a or 25a. In the solder layer 23b or 25b, the Sn—Bi solder is alloyed with the bus bar electrode 5b or 9b as well as with the copper foil tab electrode 12 by the action of the flux contained in the solder paste layer to create electrical connection therebetween.

By the above-outlined process, the photovoltaic elements 11 can be connected in series by the tab electrodes 12, as shown in FIG. 4.

In the above-described process, heating may be effected to such a degree that the solder paste layer is provisionally hardened to a strength sufficient to endure transport to the subsequent sealing step. After provisional hardening, it can be finally hardened in the sealing process (laminate curing). For example, the solder paste layer may be provisionally hardened by heating at 160° C.-200° C. for an approximate period of several seconds—30 seconds and then finally hardened in the sealing process (laminate curing) by heating at 140° C.-160° C. for an approximate period of 15 minutes-40 minutes. In this case, because final hardening can be effected by heating in the sealing process (laminate curing), the time required for provisional hardening can be shortened. Accordingly, an overall process time can be shortened.

FIG. 5 is a sectional view which shows a construction of a photovoltaic module including plural photovoltaic elements 11 connected in series by the tab electrodes 12. As shown in FIG. 5, in the photovoltaic module, the tab electrodes 12 connect plural photovoltaic elements 11 to each other. Such photovoltaic elements 11 connected to each other by the tab electrodes 12 are sealed by a filler 13 composed of an EVA (ethylene vinyl acetate) resin. Also, a surface protective member 14 comprising a transparent glass is provided on a top surface (on the light incident side) of the filler 13. Also, a PET (polyethylene terephthalate) film 15, an aluminum foil 16 and a PET film 17 are sequentially placed on a bottom surface of the filler 13 that seals the plural photovoltaic elements 11.

In the embodiment of the photovoltaic element according to the present invention, the thermosetting resin layer is formed integrally with the solder layer to cover the bus bar electrode serving as the collector electrode, as described above. Accordingly, in the case where a photovoltaic module incorporating such photovoltaic elements is installed outdoors, even if any photovoltaic element is subjected to a mechanical stress due to rise and drop of temperature, its construction in the vicinity of the collector electrode can be prevented from destruction. This suppresses reduction in reliability and output of the module. Also, water intrusion into the vicinity of the collector electrode can be prevented. Accordingly, the output decline of the module due thereto can also be suppressed.

In the above embodiment, the n-type single crystal silicon substrate is used as a substrate for the photovoltaic element. However, a p-type single crystal silicon substrate can be used alternatively. In this case, an i-type amorphous silicon layer and an n-type amorphous silicon layer are formed on the top side, while an i-type amorphous silicon layer and a p-type amorphous silicon layer are formed on the back side. Further, an n-type or p-type crystalline semiconductor substrate may be doped at its surface with a p-type or n-type dopant to form an pn junction, resulting in the provision of the photoelectric conversion layer. The present invention can also be applied to other types of photovoltaic elements.

Claims

1. A photovoltaic element which has a photoelectric conversion layer and a collector electrode provided on at least one surface of the photoelectric conversion layer, said photovoltaic element characterized in that a collector electrode covering layer is provided for covering a surface of the collector electrode provided on said at least one surface of the photoelectric conversion layer; a tab electrode for electrical connection to an exterior electrode is provided, through said collector electrode covering layer, on a top surface of the collector electrode; a portion of the collector electrode covering layer that lies between said tab electrode and the collector electrode comprises a solder layer; and another portion of the collector electrode covering layer that covers a lateral surface of the collector electrode comprises a thermosetting resin layer.

2. The photovoltaic element as recited in claim 1, characterized in that a coating film is provided on said photoelectric conversion layer, and said thermosetting resin layer lies between the collector electrode and an end of said coating film.

3. The photovoltaic element as recited in claim 2, characterized in that a translucent conductive film is provided on the photoelectric conversion layer, and the coating film and the collector electrode are provided on said translucent conductive film.

4. The photovoltaic element as recited in claim 2, characterized in that the coating film is a resin coating film.

5. The photovoltaic element as recited in claim 1, characterized in that the tab electrode has a width sufficient to cover the thermosetting resin layer located outwardly of said lateral side of the collector electrode.

6. The photovoltaic element as recited in claim 1, characterized in that the collector electrode is a bus bar electrode.

7. The photovoltaic element as recited in claim 1, characterized in that the solder layer is formed from an SnBi alloy.

8. A method for fabrication of the photovoltaic element recited in claim 1, characterized in that it comprises the steps of: applying a thermosetting conductive solder paste onto the collector electrode to form a solder paste layer;providing the tab electrode on said solder paste layer; andheating the solder paste layer while the tab electrode is placed thereon so that a solder component contained in the solder paste layer melts to form the solder layer between the tab electrode and the collector electrode and a thermosetting resin contained in the solder paste layer forms the thermosetting resin layer in a region on the lateral surface of the collector electrode, whereby the solder layer and the thermosetting resin layer constitutes the collector electrode covering layer on the surface of the collector electrode.

9. The method for fabrication of the photovoltaic element as recited in claim 8, characterized in that it further comprises the step of forming the coating film on the photoelectric conversion layer, whereby when the solder paste layer is heated to form the thermosetting resin layer, the coating film restrains the thermosetting resin from wetting and spreading so that the thermosetting resin layer lies between the collector electrode and the end of the coating film.

10. The method for fabrication of the photovoltaic element as recited in claim 9, characterized in that it further comprises the step of forming the translucent conductive film on the photoelectric conversion layer, and that the step of forming the coating film is the step of forming the coating film on the translucent conductive film.

11. A solar cell module characterized in that it includes a plurality of the photovoltaic elements recited in claim 1 which are connected to each other by the tab electrode.