Solar cell and method of manufacturing the same

This application is a continuation application of International Application No. PCT/JP2011/051479, filed on Jan. 26, 2011, entitled “SOLAR CELL AND METHOD FOR PRODUCING SAME,” which claims priority based on Article 4 of Patent Cooperation Treaty from prior Japanese Patent Applications No. 2010-014597, filed on Jan. 26, 2010, the entire contents of which are incorporated herein by reference.

1. Field of the Invention

The invention relates to a back junction solar cell in which an n-type semiconductor layer and a p-type semiconductor layer are formed on a rear surface of a semiconductor substrate.

2. Description of Related Art

Solar cells directly convert sunlight energy, which is clean and unlimitedly supplied, into electric energy, and, thus, are expected as a new energy source.

Conventionally, a solar cell in which an n-type semiconductor layer and a p-type semiconductor layer are formed on a rear surface of a semiconductor substrate has been known, which is so-called a back junction solar cell (e.g., Patent Document 1). The back junction solar cell receives light through a light-receiving surface to generate carriers.

FIG. 1A is a cross-sectional view of a conventional back junction solar cell 100 . As shown in FIG. 1( a ) , solar cell 100 includes n-type semiconductor layer 120 and p-type semiconductor layer 130 , which are amorphous semiconductor layers, formed on rear surface 112 of semiconductor substrate 110 . N-type semiconductor layer 120 and p-type semiconductor layer 130 are alternately arranged.

Patent Document 1: Published Japanese Translation of PCT International Application No. 2009-524916

In a back junction solar cell, carriers are taken only from a rear surface. Accordingly, a current density in the back junction solar cell becomes larger than that in a solar cell in which carriers are taken from a light-receiving surface and rear surface of a semiconductor substrate.

An aspect of the invention provides a solar cell that comprises a semiconductor substrate having a light-receiving surface and a rear surface; a first semiconductor layer having a first conductivity type; a second semiconductor layer having a second conductivity type, the first semiconductor layer and the second semiconductor layer being formed on the rear surface, and a trench formed in the rear surface, wherein the first semiconductor layer is formed on the rear surface in which the trench is not formed, and the second semiconductor layer is formed on a side surface of the trench in an arrangement direction in which the first semiconductor layer and the second semiconductor layer are alternately arranged and on a bottom surface of the trench.

In addition, a solar cell according to the invention includes: a semiconductor substrate; a first semiconductor layer formed on the semiconductor substrate; an insulation layer formed on the first semiconductor layer and formed to include an aperture exposing the first semiconductor layer; a second semiconductor layer arranged alternately with the first semiconductor layer on the semiconductor substrate and formed to cover the insulation layer; an underlying electrode formed on the first semiconductor layer and the second semiconductor layer and formed to include an isolation trench on the insulation layer, the isolation trench electrically isolating the first semiconductor layer and the second semiconductor layer from each other; and a collection electrode formed on the underlying electrode.

In addition, a method of manufacturing a solar cell according to the invention, includes the steps of: preparing a semiconductor substrate in which a first semiconductor layer and a second semiconductor layer arranged alternately with the first semiconductor layer are formed; forming an underlying electrode such that the underlying electrode covers the first semiconductor layer and the second semiconductor layer; forming an isolation trench separating the underlying electrode into a first portion connected to the first semiconductor layer and a second portion connected to the second semiconductor layer; and forming a collection electrode on each of the first portion and second portion of the underlying electrode by using a plating method.

FIG. 1A is a cross-sectional view of conventional back junction solar cell 100 . FIG. 1B is a graph showing a current density in solar cell 100 .

FIG. 2 is a plan view seen from rear surface 12 side of solar cell 1 A according to an embodiment.

FIG. 3 is a cross-sectional view taken along the A-A′ line in FIG. 2 .

FIG. 4 is a cross sectional view taken along vertical direction z vertical to arrangement direction x and longitudinal direction y and arrangement direction x of solar cell 1 B according to the embodiment.

FIG. 5 is a cross-sectional view taken along vertical direction z vertical to arrangement direction x and longitudinal direction y and arrangement direction x of solar cell 1 C according to the embodiment.

FIG. 6 is a flowchart for illustrating a method of manufacturing solar cell 1 A according to the embodiment.

FIG. 7 is a drawing for illustrating the method of manufacturing solar cell 1 A according to the embodiment.

FIG. 8 is a drawing for illustrating the method of manufacturing solar cell 1 A according to the embodiment.

FIG. 9 is a drawing for illustrating the method of manufacturing solar cell 1 A according to the embodiment.

FIG. 10 is a drawing for illustrating the method of manufacturing solar cell 1 A according to the embodiment.

FIG. 11 is a drawing for illustrating the method of manufacturing solar cell 1 A according to the embodiment.

FIG. 12 is a drawing for illustrating the method of manufacturing solar cell 1 A according to the embodiment.

FIG. 13 is a drawing for illustrating the method of manufacturing solar cell 1 A according to the embodiment.

FIG. 14A is a cross-sectional view taken along vertical direction z vertical to arrangement direction x and longitudinal direction y and arrangement direction x of a solar cell according to an example in a computational model. FIG. 14B is a cross-sectional view taken along vertical direction z vertical to arrangement direction x and longitudinal direction y and arrangement direction x of a solar cell according to a comparative example in the computational model.

FIG. 15 is a graph showing a relationship between a relative current density and a distance from origin 0 .

First of all, in order to determine a magnitude of a current density of conventional back junction solar cell 100 shown in FIG. 1A , a current density is calculated by using a device simulator. Specifically, current densities of n-type semiconductor layer 120 and p-type semiconductor layer 130 and a current density of a region r from rear surface 112 of semiconductor substrate 110 to a depth of 0.05 μm in solar cell 100 are measured. It is assumed that the vertical axis is a current density and the horizontal axis is a length along arrangement direction x in which n-type semiconductor layer 120 and p-type semiconductor layer 130 are alternately arranged. As shown in FIG. 1A , an end portion of p-type semiconductor layer 130 is set to be positioned at 500 nm in arrangement direction x. An end portion of n-type semiconductor layer 120 , the end portion adjacent to the end portion of p-type semiconductor layer 130 , is set to be positioned at 550 nm in arrangement direction x. FIG. 1B shows the result.

As shown in FIG. 1B , it can be seen that the current density becomes larger in end region 120 a of n-type semiconductor layer 120 and end region 130 a of p-type semiconductor layer 130 . Holes being carriers move to p-type semiconductor layer 130 . The holes generated near n-type semiconductor layer 120 are concentrated to the end portion of p-type semiconductor layer 130 on rear surface 112 . This likely causes the current density to be larger.

The temperature at the end portion of the semiconductor layer with a higher current density increases along with generated electricity as compared with other portions. This temperature increase results in deterioration in the semiconductor layer and time-related deterioration such that an end portion of the semiconductor layer comes off from a semiconductor substrate.

Subsequently, one example of solar cell 1 according to an embodiment is described by referring to the drawings. In the following description of the drawings, same or similar reference numerals are given to denote same or similar portions. Note that the drawings are merely schematically shown and proportions of sizes and the like are different from actual ones. Thus, specific sizes and the like should be judged by referring to the description below. In addition, there are of course included portions where relationships or percentages of sizes of the drawings are different with respect to one another.

(1) Schematic Configuration of Solar Cell 1 A

A schematic configuration of solar cell 1 A according to an embodiment is described by referring to FIGS. 2 and 3 . FIG. 2 is a plan view seen from rear surface 12 side of solar cell 1 A according to an embodiment. FIG. 3 is a cross-sectional view seen from the A-A′ line in FIG. 2 .

As shown in FIGS. 2 and 3 , solar cell 1 A includes semiconductor substrate 10 n , first semiconductor layer 20 n , second semiconductor layer 30 p , insulation layer 40 , first electrode 50 n , second electrode 50 p , connection electrode 60 n , and connection electrode 60 p.

Semiconductor substrate 10 n has light-receiving surface 11 receiving light and rear surface 12 provided on the opposite side from light-receiving surface 11 . Semiconductor substrate 10 n generates carriers by receiving light on light-receiving surface 11 . The carriers mean holes and electrons which are generated by absorption of light into semiconductor substrate 10 n.

As shown in FIG. 3 , trench 13 is formed in rear surface 12 of semiconductor substrate 10 n . Trench 13 has side surface 17 and bottom surface 19 . In solar cell 1 A, side surface 17 and bottom surface 19 are continuous to form an arc shape.

Semiconductor substrate 10 n is a wafer shaped substrate capable of being made of a general semiconductor material including a crystalline semiconductor material such as a single crystal Si or polycrystalline Si having an n-type or p-type conductivity type or a compound semiconductor material such as GaAs or InP. Fine uneven portions may be formed on light-receiving surface 11 and rear surface 12 of semiconductor substrate 10 n . Although not illustrated in the drawings, a structure (e.g., an electrode) blocking light from entering is not formed on light-receiving surface 11 of semiconductor substrate 10 n . For this reason, semiconductor substrate 10 n can receive light on entire light-receiving surface 11 . Light-receiving surface 11 may be covered with a passivation layer. The passivation layer has a passivation property to inhibit re-coupling of the carriers. The passivation layer is a substantially intrinsic amorphous semiconductor layer which is formed in such a manner that, for example, a dopant is not added or a minute amount of dopant is added. In addition to this, an oxide film or a nitride film may be used. The description is made on assumption that semiconductor substrate 10 n is an n-type single crystal silicon substrate in solar cell 1 A. Accordingly, a conductivity type of semiconductor substrate 10 n is an n-type. Thus, minority carriers are holes.

As shown in FIG. 3 , first semiconductor layer 20 n is formed on rear surface 12 of semiconductor substrate 10 n . Specifically, first semiconductor layer 20 n is formed on rear surface 12 in which trench 13 is not formed. Also, first semiconductor layer 20 n is formed to have a longitudinal direction. This longitudinal direction is set as longitudinal direction y. First semiconductor layer 20 n has a first conductivity type. In solar cell 1 A, first semiconductor layer 20 n has an n-type conductivity type.

As shown in FIG. 3 , first semiconductor layer 20 n includes i-type amorphous semiconductor layer 22 i and n-type amorphous semiconductor layer 25 n . I-type amorphous semiconductor layer 22 i is formed on rear surface 12 of semiconductor substrate 10 n . N-type amorphous semiconductor layer 25 n is formed on i-type amorphous semiconductor layer 22 i . With such configuration of n-type semiconductor substrate 10 n , i-type amorphous semiconductor layer 22 i , and n-type amorphous semiconductor layer 25 n (so-called, BSF structure), the re-coupling of the minority carriers on the rear surface of semiconductor substrate 10 n can be inhibited.

As shown in FIG. 3 , second semiconductor layer 30 p is formed on rear surface 12 of semiconductor substrate 10 n . Specifically, second semiconductor layer 30 p is formed on side surface 17 of trench 13 and bottom surface 19 of trench 13 in arrangement direction x. In solar cell 1 A, second semiconductor layer 30 p is also formed on insulation layer 40 . Second semiconductor layer 30 p is formed to cover the end portion of first semiconductor layer 20 n . Also, second semiconductor layer 30 p is formed along longitudinal direction y. Second semiconductor layer 30 p has a second conductivity type. In solar cell 1 A, second semiconductor layer 30 p has a p-type conductivity type. Accordingly, semiconductor substrate 10 n and second semiconductor layer 30 p form a p-n junction.

As shown in FIG. 3 , second semiconductor layer 30 p includes i-type amorphous semiconductor layer 32 i and p-type amorphous semiconductor layer 35 p . I-type amorphous semiconductor layer 32 i is formed on rear surface 12 of semiconductor substrate 10 n . Accordingly, the junction between second semiconductor layer 30 p and side surface 17 and the junction between second semiconductor layer 30 p and bottom surface 19 are hetero-junctions. P-type amorphous semiconductor layer 35 p is formed on i-type amorphous semiconductor layer 32 i . With the configuration of n-type semiconductor substrate 10 n , i-type amorphous semiconductor layer 32 i , and p-type amorphous semiconductor layer 35 p , the property of the p-n junction can be improved.

As shown in FIG. 3 , first semiconductor layer 20 n and second semiconductor layer 30 p are alternately arranged along arrangement direction x. In solar cell 1 A, width L 2 of second semiconductor layer 30 p formed on rear surface 12 in arrangement direction x is preferably longer than width L 1 of first semiconductor layer 20 n formed on rear surface 12 in arrangement direction x. With this, a series resistance can be lowered and thus a fill factor of the solar cell can be further reduced. Note that arrangement direction x and longitudinal direction y are perpendicular to each other.

First semiconductor layer 20 n and second semiconductor layer 30 p are in contact with each other on rear surface 12 . Specifically, first semiconductor layer 20 n and second semiconductor layer 30 p are in contact with each other on a boundary between rear surface 12 in which trench 13 is not formed and trench 13 . With this, a junction area between the semiconductor substrate and the semiconductor layer can be maximized, so that the rear surface of semiconductor substrate 10 n can be effectively passivated. As a result, effectiveness of generating electricity can be improved.

Each of i-type amorphous semiconductor layer 22 i , i-type amorphous semiconductor layer 32 i , n-type amorphous semiconductor layer 25 n , and p-type amorphous semiconductor layer 35 p can include an amorphous semiconductor containing hydrogen and silicon. An example of such amorphous semiconductor includes an amorphous silicon, amorphous silicon carbide, or amorphous silicon germanium. In addition to this, other amorphous semiconductors may be used. Each of i-type amorphous semiconductor layer 22 i , i-type amorphous semiconductor layer 32 i , n-type amorphous semiconductor layer 25 n , and p-type amorphous semiconductor layer 35 p may include one kind of amorphous semiconductor. Each of i-type amorphous semiconductor layer 22 i , i-type amorphous semiconductor layer 32 i , n-type amorphous semiconductor layer 25 n , and p-type amorphous semiconductor layer 35 p may include a combination of two or more kinds of amorphous semiconductors.

Insulation layer 40 has an insulation property. Insulation layer 40 is formed on first semiconductor layer 20 n . As insulation layer 40 , aluminum nitride, silicon nitride, or silicon oxide may be used.

First electrode 50 n is electrically connected to first semiconductor layer 20 n . As shown in FIG. 2 , first electrode 50 n is formed along longitudinal direction y. First electrode 50 n preferably includes transparent electrode layer 52 n and collection electrode 55 n . Transparent electrode layer 52 n is formed on first semiconductor layer 20 n . Specifically, transparent electrode layer 52 n is formed in a state of being held between insulation layers 40 in arrangement direction x. Also, transparent electrode layer 52 n is formed on second semiconductor layer 30 p formed on insulation layer 40 . Transparent electrode layer 52 n is formed of a conductive material having translucency. As transparent electrode layer 52 n , indium oxide tin, tin oxide, or zinc oxide may be used. Collection electrode 55 n is formed on transparent electrode layer 52 n . Collection electrode 55 n may be formed using a resin-type conductive paste having a resin material as binder and conductive particles such as silver particles as fillers or using silver for the sputtering method. Besides, silver, nickel, copper or the like may be formed as an underlying metal by using a sputtering method, and thereafter copper or the like may be formed by the plating method.

Second electrode 50 p is electrically connected to second semiconductor layer 30 p . As shown in FIG. 2 , second electrode 50 p is formed along longitudinal direction y. Second electrode 50 p preferably includes transparent electrode layer 52 p and collection electrode 55 p . Transparent electrode layer 52 p is formed on second semiconductor layer 30 p . Collection electrode 55 p is formed on transparent electrode layer 52 p . Transparent electrode layer 52 p and collection electrode 55 p may respectively use materials same as those of transparent electrode layer 52 n and collection electrode 55 n.

First electrode 50 n and second electrode 50 p collect carriers generated by receiving light. First electrode 50 n and second electrode 50 p are isolated by isolation trench 70 from each other in order to prevent a short circuit. Isolation trench 70 is provided in transparent electrode 52 . Isolation trench 70 is provided in transparent electrode 52 formed on second semiconductor layer 30 p formed on insulation layer 40 . Accordingly, the bottom of isolation trench 70 is second semiconductor layer 30 p . Second semiconductor layer 30 p protects insulation layer 40 at the bottom of isolation trench 70 . With this, second semiconductor layer 30 p protects the junction between first semiconductor layer 20 n and second semiconductor substrate 10 n . Isolation trench 70 is formed along longitudinal direction y. Note that second semiconductor layer 30 p has a p-type, which means that it has a low conductivity. For this reason, a leak between first electrode 50 n and second electrode 50 p via second semiconductor layer 30 p is extremely small.

Connection electrode 60 n is electrically connected to first electrodes 50 n . Connection electrode 60 p is electrically connected to second electrodes 50 p . Connection electrode 60 n and connection electrode 60 p further collect photosynthesized carriers collected by first electrodes 50 n and second electrodes 50 p.

(2) Schematic Configuration of Solar Cell 1 B

A schematic configuration of solar cell 1 B according to the embodiment is described by referring to FIG. 4 . The description duplicated with that of solar cell 1 A is omitted below. In other words, portions different from those of solar cell 1 A are mainly described. FIG. 4 is a cross-sectional view taken along vertical direction z vertical to arrangement direction x and longitudinal direction y and arrangement direction x of solar cell 1 B according to the embodiment.

As shown in FIG. 4 , first semiconductor layer 20 p is formed on rear surface 12 of semiconductor substrate 10 n . Specifically, first semiconductor layer 20 p is formed on rear surface 12 in which trench 13 is not formed. Also, first semiconductor layer 20 p is formed along longitudinal direction y. In solar cell 1 B, first semiconductor layer 20 p has a p-type conductivity type. Accordingly, the conductivity type of semiconductor substrate 10 n is different from that of first semiconductor layer 20 p . Accordingly, semiconductor substrate 10 n and first semiconductor layer 20 p form a p-n junction.

As shown in FIG. 4 , first semiconductor layer 20 p includes i-type amorphous semiconductor layer 22 i and p-type amorphous semiconductor layer 25 p . I-type amorphous semiconductor layer 22 i is formed on rear surface 12 of semiconductor substrate 10 n . The p-type amorphous semiconductor layer 25 p is formed on i-type amorphous semiconductor layer 22 i.

As shown in FIG. 4 , second semiconductor layer 30 n is formed on rear surface 12 of semiconductor substrate 10 n . Specifically, second semiconductor layer 30 n is formed on side surface 17 of trench 13 and bottom surface 19 of trench 13 in arrangement direction x. In solar cell 1 B, second semiconductor layer 30 n is also formed on insulation layer 40 . Second semiconductor layer 30 n is formed to cover an end portion of semiconductor layer 20 p . Also, second semiconductor layer 30 n is formed along longitudinal direction y. In solar cell 1 B, second semiconductor layer 30 n has an n-type conductivity type. Accordingly, the conductivity type of semiconductor substrate 10 n is same as that of second semiconductor layer 30 n.

As shown in FIG. 4 , second semiconductor layer 30 n includes i-type amorphous semiconductor layer 32 i and n-type amorphous semiconductor layer 35 n . I-type amorphous semiconductor layer 32 i is formed on rear surface 12 of semiconductor substrate 10 n . The n-type amorphous semiconductor layer 35 n is formed on i-type amorphous semiconductor layer 32 i.

As shown in FIG. 4 , in solar cell 1 B, width L 1 of first semiconductor layer 20 p formed on rear surface 12 in arrangement direction x is longer than width L 2 of second semiconductor layer 30 n formed on rear surface 12 in arrangement direction x. In other words, width L 2 is shorter than width L 1 .

(3) Schematic Configuration of Solar Cell 1 C

A schematic configuration of solar cell 1 C is described by referring to FIG. 5 . The description duplicated with that of solar cell 1 A is omitted below. In other words, portions different from those of solar cell 1 A are mainly described. FIG. 5 is a cross-sectional view taken along vertical direction z vertical to arrangement direction x and longitudinal direction y and arrangement direction x of solar cell 1 C according to the embodiment.

As shown in FIG. 5 , trench 13 a and trench 13 b are formed in rear surface 12 of semiconductor substrate 10 n . Trench 13 a has side surface 17 and bottom surface 19 . Side surface 17 is included to be continuous with bottom surface 19 . In solar cell 1 C, side surface 17 and bottom surface 19 are continuous to form an angle therebetween. However, side surface 17 and bottom surface 19 may be continuous to form an arc shape.

Trench 13 b has side surface 17 and bottom surface 19 . Side surface 17 includes side surface 17 a and side surface 17 b . Side surface 17 a and side surface 17 b are continuous to form an angle therebetween. However, side surface 17 a and side surface 17 b may be continuous to form an arc shape. Side surface 17 b and bottom surface 19 are continuous to form an angle therebetween. However, side surface 17 b and bottom surface 19 may be continuous to from an arc shape.

In solar cell 1 C, trench 13 a and trench 13 b are formed in rear surface 12 . However, only trench 13 a may be formed in rear surface 12 . Or, only trench 13 b may be formed in rear surface 12 .

(4) Method of Manufacturing Solar Cell 1 A

A method of manufacturing solar cell 1 A according to the embodiment is described by referring to FIGS. 6 to 13 . FIG. 6 is a flowchart for illustrating a method of manufacturing solar cell 1 A, according to the embodiment. FIGS. 7 to 13 are views, each for illustrating a method of manufacturing solar cell 1 A according to the embodiment.

As shown in FIG. 6 , the method of manufacturing solar cell 1 A includes steps S 1 to S 5 .

Step S 1 is a step of forming first semiconductor layer 20 n having a first conductivity type on rear surface 12 of semiconductor substrate 10 n . Firstly, semiconductor substrate 10 n is prepared. To remove particles on the surface of semiconductor substrate 10 n , semiconductor substrate 10 n was subjected to etching with an acid or alkali solution. On rear surface 12 of prepared semiconductor substrate 10 n , i-type amorphous semiconductor layer 22 i is formed. On formed i-type amorphous semiconductor layer 22 i , n-type amorphous semiconductor layer 25 n is formed. I-type amorphous semiconductor layer 22 i and n-type amorphous semiconductor layer 25 n are formed by, for example, a chemical vapor deposition method (CVD method). With this step S 1 , first semiconductor layer 20 n is formed on rear surface 12 .

Step S 2 is a step of forming insulation layer 40 having an insulation property. With step S 2 , insulation layer 40 is formed on formed first semiconductor layer 20 n . Specifically, as shown in FIG. 7 , insulation layer 40 is formed on n-type amorphous semiconductor layer 25 n . Insulation layer 40 is formed by, for example, the CVD method.

Step S 3 is a step of forming trench 13 in rear surface of semiconductor substrate 10 n . Step S 3 includes steps S 31 and S 32 .

Step S 31 is a step of removing insulation layer 40 formed on first semiconductor layer 20 n . An etching paste is applied onto insulation layer 40 by using the screen printing method. Seeing rear surface 12 from vertical direction z, the etching paste is applied onto an appropriate portion of insulation layer 40 on which second semiconductor layer 30 p is formed. For this reason, width L 1 and width L 2 are determined depending on a width of the etching paste in arrangement direction x and a gap of the etching paste in arrangement direction x.

After that, annealing is performed at 200° for approximately 4 minutes, and, as shown in FIG. 8 , the portion of insulation layer 40 onto which the etching paste was applied is removed. Accordingly, first semiconductor layer 20 n is exposed. In some processing condition, one portion of first semiconductor layer 20 n is also removed.

Step S 32 is a step of removing exposed first semiconductor layer 20 n and forming trench 13 in rear surface 12 of semiconductor substrate 10 n . Alkali cleaning is performed on exposed first semiconductor layer 20 n . Accordingly, as shown in FIG. 9 , first semiconductor substrate 20 n is removed and trench 13 is formed in rear surface 12 of semiconductor substrate 10 n . A depth of trench 13 is properly adjusted depending on the processing condition. At step S 32 , insulation layer 40 which is left residual without being removed serves as a protection layer to protect first semiconductor layer 20 n . Note that trench 13 may be formed by other methods. For example, trench 13 may be formed by cutting.

Step S 4 is a step of forming second semiconductor layer 30 p in trench 13 formed in rear surface 12 of semiconductor substrate 10 n . On rear surface 12 of semiconductor substrate 10 n , i-type amorphous semiconductor layer 32 i is formed. On formed i-type amorphous semiconductor layer 32 i , p-type amorphous semiconductor layer 35 p is formed. I-type amorphous semiconductor layer 32 i and p-type amorphous semiconductor layer 35 p are formed by, for example, the CVD method. With this step S 4 , second semiconductor layer 30 p is formed on rear surface 12 . In other words, second semiconductor layer 30 p is formed on side surface 17 of trench 13 and bottom surface 19 of trench 13 in arrangement direction x. As shown in FIG. 10 , in solar cell 1 A, second semiconductor layer 30 p is formed entirely. Accordingly, second semiconductor layer 30 p is formed not only on rear surface 12 but also on insulation layer 40 . Second semiconductor layer 30 p coves an end portion of insulation layer 40 and an end portion of first semiconductor layer 20 n . First semiconductor layer 20 n and second semiconductor layer 30 p are in contact with each other on a boundary between rear surface 12 in which trench 13 is not formed and trench 13 . By performing step S 32 after step S 3 so as not to form trench 13 , first semiconductor layer 20 n and second semiconductor layer 30 p are in contact with each other on rear surface 12 other than the boundary.

Step S 5 is a step of forming first electrode 50 n and second electrode 50 p . Step S 5 includes steps S 51 to S 54 .

Step S 51 is a step of removing second semiconductor layer 30 p and insulation layer 40 . An etching paste is applied onto second semiconductor layer 30 p formed on insulation layer 40 by using the screen printing method. After that, annealing is performed at approximately 70° for 5 minutes or so, and, then, as shown in FIG. 11 , the portions of second semiconductor layer 30 p and insulation layer 40 onto which the etching paste was applied are removed. If insulation layer 40 is left without being completely removed, insulation layer 40 is cleaned using hydrogen fluoride (HF). With this, first semiconductor layer 20 n is exposed thereby forming an aperture. Second semiconductor layer 30 p and insulation layer 40 may be removed separately, not simultaneously.

Step S 52 is a step of forming transparent electrode layer 52 . As shown in FIG. 12 , transparent electrode layer 52 is formed on first semiconductor layer 20 n and second semiconductor layer 30 p by using a physical vapor deposition method (PVD method). After that, an underlying metal layer (not shown) for plating to be an underlying layer of collection electrode 55 is formed by the PVD method. For the underlying metal layer, for example, Ni and Cu are used.

Step S 53 is a step of forming isolation trench 70 for preventing a short circuit. Isolation trench 70 is provided in transparent electrode 52 formed on second semiconductor layer 30 p formed on insulation layer 40 . An etching paste is applied onto the underlying metal layer by using the screen printing method. Annealing is performed at approximately 200° for 4 minutes or so, and, then, as shown in FIG. 13 , the portion of the underlying metal layer onto which the etching paste was applied and the corresponding portion of transparent electrode 52 are removed. Accordingly, isolation trench 70 is formed. Note that, the patterning using a photo resist may be performed without using the etching paste in the step of forming isolation trench 70 .

Step S 54 is a step of forming collection electrode 55 . Collection electrode 55 is formed in the underlying metal by using the plating method. Accordingly, first electrode 50 n and second electrode 50 p are formed. An end portion of first electrode 50 n is connected to connection electrode 60 n . An end portion of second electrode 50 p is connected to connection electrode 60 p . Consequently, solar cell 1 A shown in FIG. 3 is formed.

Note that in a case where collection electrode 55 is formed in the underlying metal by using the plating method without forming isolation trench 70 in the underlying metal portion and transparent electrode 52 , a step of separating collection electrode 55 into first electrode 50 n and second electrode 50 p . Since collection electrode 55 is thickly formed by the plating method, this method requires a longer time for separating collection electrode 55 . Also, a material to be cut is increased, and thus a manufacturing cost is also increased. On the other hand, in the embodiment, isolation trench 70 is formed in the underlying metal and transparent electrode 52 at step S 59 before collection electrode 55 is formed by using the plating method. Thus, first electrode 50 n and second electrode 50 p are isolated from each other by isolation trench 70 . With this method, collection electrode 55 can be separated into first electrode 50 n and second electrode 50 p only by forming isolation trench 70 in the underlying metal thinner than collection electrode 55 . Accordingly, a time required for the manufacturing step can be shortened.

(5) Comparative Evaluation

To ascertain the effects of the invention, a current density is evaluated by a computational model. Specifically, calculation is made on a current density of holes 80 moving to p-type second semiconductor layer 30 p . Conditions for the computational model are as follows.

P-type second semiconductor layer 30 p is formed on rear surface 12 of n-type semiconductor substrate 10 n with a thickness of 200 μm. As shown in FIGS. 14A and 14B , it is assumed that an end portion of second semiconductor layer 30 p on rear surface 12 in arrangement direction x is set as origin O. In a region up to distance x 1 from origin O toward first semiconductor layer 20 n adjacent to the end portion of second semiconductor layer 30 p , holes 80 generated in semiconductor substrate 10 n by receiving light with light-receiving surface 11 are targeted. Distance x 1 is 350 μm. It is assumed that a length in longitudinal direction y of semiconductor substrate 10 n and second semiconductor layer 30 p is infinite. Holes 80 are evenly generated within semiconductor substrate 10 n . Holes 80 move to second semiconductor layer 30 p via the shortest route. Holes 80 having arrived at second semiconductor layer 30 p are collected at once.

FIG. 14A is a cross-sectional view taken along vertical direction z vertical to arrangement direction x and longitudinal direction y and arrangement direction x of a solar cell according to an example in the computational model. FIG. 14B is a cross-sectional view taken along vertical direction z vertical to arrangement direction x and longitudinal direction y and arrangement direction x of a solar cell according to a comparative example in the computational model. As shown in FIG. 14A , in the solar cell according to the example, a trench with depth h is formed in semiconductor substrate 10 n . The trench is formed to have an arc shape with radius h from a side surface of the trench to the bottom surface of the trench in arrangement direction x. In example 1, depth h is 1 μm. In example 2, depth h is 5 μm. In example 3, depth h is 10 μm. In example 4, depth h is 20 μm. As shown in FIG. 14B , a trench is not formed in the solar cell according to the comparative example. The results of calculating the current densities using these solar cells are shown in FIG. 15 .

FIG. 15 is a graph showing a relationship between a relative current density and a distance from origin O. In FIG. 15 , the vertical axis is a relative current density and the horizontal axis is a distance (μm) from origin O along direction x 2 . Direction x 2 is a direction along rear surface 12 in arrangement direction x.

As shown in FIG. 15 , in the comparative example, the relative current density is 770 at origin O. In a portion positioned 1 μm away from origin O, the relative current density is 0.2. This means that holes 80 are concentrated on the end portion of the rear surface of second semiconductor layer 30 p.

On the other hand, as compared with the solar cell according to the comparative example, the current densities in origin O are greatly decreased in each of the solar cells according to the examples. Specifically, in example 1, the relative current density is 70 at origin O. In example 2, the relative current density is 18 at origin O. In example 3, the relative current density is 9 at origin O. In example 4, the relative current density is 4 at origin O. Also, it can be seen that holes 80 move to a more distant position as depth h is deeper.

It can be seen from these results that the current density can be further decreased as a junction surface between semiconductor substrate 10 n and second semiconductor layer 30 p is deeper.

(6) Operation and Effects

In a solar cell according to the embodiment, in solar cell 1 A, second semiconductor layer 30 p is formed on side surface 17 of trench 13 and bottom surface 19 of trench 13 in arrangement direction x. Accordingly, among carriers formed near first semiconductor layer 20 n inside semiconductor substrate 10 n , generated are carriers closer to second semiconductor layer 30 p formed on side surface 17 and bottom surface 19 rather than the end portion of second semiconductor layer 30 p on rear surface 12 . For this reason, the carriers do not move in a concentrated manner to the end portion of second semiconductor layer 30 n on rear surface 12 but also move to second semiconductor layer 30 p formed on side surface 17 and bottom surface 19 in a dispersed manner. Accordingly, the current density is considered to be decreased. This can inhibit the time-related deterioration.

Furthermore, second semiconductor layer 30 p is formed on side surface 17 of trench 13 and bottom surface 19 of trench 13 , so that an area of the junction between semiconductor substrate 10 n and second semiconductor layer 30 p becomes wider. This can lower the series resistance. Thus, the fill factor of the solar cell can be improved.

In solar cell 1 according to the embodiment, side surface 17 is inclined to be continuous with bottom surface 19 . Also, in solar cell 1 according to the embodiment, side surface 17 and bottom surface 19 may be continuous to form an arc shape. With this, among carries generated near first semiconductor layer 20 n , further generated are carriers closer to second semiconductor layer 30 p formed on side surface 17 and bottom surface 19 , rather than the end portion of second semiconductor layer 30 n on rear surface 12 . As a result, the current density can be decreased, which can inhibit the time-related deterioration.

In solar cell 1 according to the embodiment, the junction between second semiconductor layer 30 p and side surface 17 and the junction between second semiconductor layer 30 p and bottom surface 19 are hetero-junctions. Different from a solar cell in which a junction is formed by dispersing dopant into a semiconductor substrate, the solar cell with the hetero-junction has a clear junction boundary. For this reason, the solar cell with the hetero-junction tends to have a larger current density. In solar cell 1 , a portion in which the hetero-junction is formed is set to be on side surface 17 and bottom surface 19 , so that the current density can be decreased.

In solar cell 1 according to the embodiment, semiconductor substrate 10 n has a first conductivity type different from second semiconductor layer 30 p having a second conductivity type. Accordingly, minority carriers move to second semiconductor layer 30 p formed in trench 13 . By forming trench 13 , minority carriers moving to second semiconductor layer 30 p via a shorter distance are generated. For this reason, the minority carriers to be re-coupled can be reduced.

In solar cell 1 A according to the embodiment, width L 2 of second semiconductor layer 30 p formed on rear surface 12 in arrangement direction x is preferably longer than width L 1 of first semiconductor layer 20 n formed on rear surface 12 in arrangement direction x. This can lower the series resistance. Thus, the fill factor of the solar cell can be made smaller.

In solar cell 1 B according to the embodiment, semiconductor substrate 10 n has a conductivity type same as that of second semiconductor layer 30 n . Also, width L 2 of second semiconductor layer 30 p formed on rear surface 12 in arrangement direction x is shorter than width L 1 of first semiconductor layer 20 n formed on rear surface 12 in arrangement direction x. With this, since width L 2 is shorter, the moving distance of the minority carriers generated near bottom surface 19 can be shortened.

In solar cell 1 according to the embodiment, first semiconductor layer 20 n and second semiconductor layer 30 p are in contact with each other on rear surface 12 . Also, first semiconductor layer 20 n and second semiconductor layer 30 p are in contact with each other on a boundary between rear surface 12 in which trench 13 is not formed and trench 13 . With this, an area of the junction between the semiconductor substrate and the semiconductor layer can be maximized. Thus, the efficiency of generating electricity can be improved.

In this way, the solar cells of the embodiment decrease a current density and inhibit time-related deterioration in a back junction solar cell.

As described above, the contents of the invention are disclosed through the embodiment. However, it should not be understood that the description and drawings, which constitute one part of the invention, are to limit the invention. The invention includes various embodiments which are not described herein. Accordingly, the invention is only limited by the scope of claims and matters specifying the invention, which are appropriate from this disclosure.