Solar cell and method for manufacturing the same

A solar cell is discussed. The solar cell includes a substrate of a first conductivity type, the substrate having a via hole, an emitter disposed at the substrate and having a second conductivity type opposite the first conductivity type, an anti-reflection layer disposed on a first surface of the substrate and inside the via hole, a first electrode disposed on the first surface of the substrate and in the via hole, a first electrode bus bar disposed on a second surface of the substrate that is opposite the first surface and in the via hole, and a second electrode disposed on the second surface of the substrate and connected to the substrate.

This application claims priority to and the benefit of Korean Patent Application No. 10-2010-0129715 filed in the Korean Intellectual Property Office on Dec. 17, 2010, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments relates to a solar cell and a method for manufacturing the same.

2. Description of the Related Art

Recently, as exhaustion of existing natural resources such as petroleum and coal is anticipated, interest in alternative energy is getting higher, and in this respect, solar cells producing electric power from solar energy are getting attention. Among the solar cells, a back contact type solar cell is being developed, which improves efficiency of the solar cell by increasing a light receiving area in such a way to form both of an electrode for outputting electrons to an external device and an electrode for outputting holes to the external device on the back surface of a substrate, namely on the surface located in the opposite of the surface on which light rays are incident.

SUMMARY OF THE INVENTION

Embodiments of the invention provide a solar cell and a method for manufacturing the same capable of improving efficiency of the solar cell and reducing a manufacturing time of the solar cell.

In one aspect there is a solar cell including a substrate of a first conductivity type, the substrate having a via hole, an emitter disposed at the substrate and having a second conductivity type that is opposite the first conductivity type, an anti-reflection layer disposed on a first surface of the substrate and inside the via hole, a first electrode disposed on the first surface of the substrate and in the via hole, a first electrode bus bar disposed on a second surface of the substrate that is opposite the first surface and in the via hole, and a second electrode disposed on the second surface of the substrate and connected to the substrate.

A position of an end part of the anti-reflection layer inside the via hole may be lower than a position of an end part of the first electrode extending from the first surface to the inside of the via hole.

The anti-reflection layer may include at least one layer.

The anti-reflection layer may include at least one of silicon nitride, silicon oxide, aluminum oxide, titanium oxide, and silicon oxynitride.

The anti-reflection layer may include a plurality of layers, and a number of layers of the anti-reflection layer existing inside the via hole at a position that is closer to the first surface of the substrate is greater than a number of layers of the anti-reflection layer existing inside the via hole at a position that is farther to the first surface of the substrate.

The solar cell according to the aspect may further include a second bus bar disposed on the second surface of the substrate and connected to the second electrode.

The first surface may be a light incident surface of the solar cell.

Inside the via hole, the first electrode bus bar may contact a portion of the anti-reflection layer that is inside the via hole.

The anti-reflection layer may include a first portion that is disposed on the first surface of the substrate, and a second portion that is inside the via hole, and wherein the first portion may be discontinuous from the second portion.

A portion of the first electrode bus bar that is disposed in the via hole may cover the second portion of the anti-reflection layer.

The second portion of the anti-reflection layer may contact a portion of the first electrode that is in the via hole, a portion of the first electrode bus bar that is in the via hole, and a portion of the emitter layer that is in the via hole.

In another aspect there is a method for manufacturing a solar cell including forming a via hole in a substrate of a first conductivity type, forming an emitter of a second conductivity type that is opposite the first conductivity type at the substrate, forming an anti-reflection layer on a first surface of the substrate and inside the via hole, forming a bus bar pattern on a second surface of the substrate that is opposite the first surface and inside the via hole by using a first paste, forming a first electrode pattern connected to the bus bar pattern on the first surface and inside the via hole by using a second paste, forming a second electrode pattern on the second surface by using a third paste, and by applying a heat treatment to the substrate with the bus bar pattern, the first electrode pattern and the second electrode pattern, respectively forming a first electrode connected to the emitter by penetrating the anti-reflection layer, a bus bar connected to the first electrode through the via hole, and a second electrode connected to the substrate.

The first, second and third pastes may be different from each other.

The first paste may contain a same metal component as the second paste, but a different metal component from the third paste.

The first and second pastes may contain silver (Ag) and the third paste contains aluminum (Al).

Inside the via hole, a position of an end part of the anti-reflection layer may be lower than a position of an end part of the first electrode pattern extending from the first surface of the substrate to the inside of the via hole.

The forming of the anti-reflection layer may form the anti-reflection layer with at least one layer.

The forming of the anti-reflection layer may form the anti-reflection layer including at least one of silicon nitride, silicon oxide, aluminum oxide, titanium oxide, and silicon oxynitride.

The forming of the anti-reflection layer may form the anti-reflection layer with a plurality of layers, and wherein a number of layers of the anti-reflection layer existing inside the via hole at a position that is closer to the first surface of the substrate may be greater than a number of layers of the anti-reflection layer existing inside the via hole at a position that is farther to the first surface of the substrate.

The anti-reflection layer may be formed to include a first portion that is disposed on the first surface of the substrate, and a second portion that is inside the via hole, and wherein the first portion may be discontinuous from the second portion.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The invention will be described more fully hereinafter with reference to the accompanying drawings, in which example embodiments of the inventions are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the example embodiments set forth herein.

Now, a solar cell according to an example embodiment of the invention will be described with reference toFIGS. 1 and 2.

With reference toFIGS. 1 and 2, a solar cell11according to an embodiment of the invention comprises a substrate110having a plurality of via holes181, an emitter120disposed in (at) the substrate110, an anti-reflection layer130disposed on the emitter120of a surface (hereinafter, referred to as a ‘front surface’) of the substrate110, where the front surface is a light incident surface, a plurality of front electrodes141disposed on the emitter120of the front surface of the substrate110in which the anti-reflection layer130is not disposed, a back electrode151disposed on another surface (hereinafter, referred to as a ‘back surface’) of the substrate110, where the back surface is disposed opposite to the front surface of the substrate110, on which light rays are incident, a plurality of bus bars (hereinafter, referred to as ‘a plurality of front electrode bus bars’)161for the plurality of front electrodes141, disposed in the via holes181and on the emitter120around the via holes181of the back surface of the substrate110, and connected electrically to the plurality of front electrodes141, a plurality of bus bars (hereinafter, referred to as ‘a plurality of back electrode bus bars’)162for the back electrode151disposed on the back surface of the substrate110, and connected electrically to the back electrode151, and a back surface field (BSF) unit172disposed at the back surface of the substrate110.

The substrate110is a substrate of a first conductivity type, e.g., a semiconductor substrate made of a semiconductor such as p-type silicon. In this instance, semiconductor is a crystalline semiconductor such as polycrystalline or monocrystalline silicon.

When the substrate110is of a p-type, the substrate110includes impurities of a group III element such as boron (B), gallium (Ga), and indium (In). Alternatively, the substrate110may be of an n-type, and/or be formed of materials other than silicon. When the substrate110is of the n-type, the substrate110may include impurities of a group V element such as phosphor (P), arsenic (As), and antimony (Sb).

The front surface of the substrate110is textured, having an irregular textured surface. For the sake of convenience,FIG. 1depicts only the periphery of the substrate110as being formed with a textured surface, and depicts only the periphery of the anti-reflection layer130disposed on a top thereof as being formed with an irregular surface. However, in fact, the entire surface of the substrate110has a textured surface, and due to this reason, the anti-reflection layer130disposed on the top of the front surface also has an irregular surface. In an alternative example, not only the front surface, but also the back surface of the substrate110can have a textured surface.

Due to the textured surface of the substrate110having a plurality of irregularities, the surface area of the substrate110increases, to thereby increasing an area of a light incident surface and decreasing an amount of light reflected by the substrate110, and therefore, the amount of light incident on the substrate110is increased.

The emitter120is an impurity area having a second conductivity type which is opposite to the conductivity type of the substrate110, and is, for example, an n-type. Due to this, the emitter120forms a p-n junction with the first conductivity type part of the substrate110.

Due to the built-in potential difference caused by the p-n junction between the substrate110and the emitter120, electron-hole pairs, which are electric charges generated by the light rays incident on the substrate110, are separated into electrons and holes, and the electrons move to the n-type semiconductor and the holes move to the p-type semiconductor. Therefore, when the substrate110is of the p-type and the emitter120is of the n-type, the holes move toward the substrate110and the electrons move toward the emitter120.

Since the emitter120forms the p-n junction with the substrate110, in a different embodiment of the invention, when the substrate110is of the n-type, and the emitter120is of the p-type, the electrons move to the substrate110while the holes move to the emitter120.

In this embodiment of the invention, when the emitter120is of the n-type, the emitter120can be doped with impurities of a group V element, whereas in the different embodiment of the invention, when the emitter120is of the p-type, the emitter120can be doped with impurities of a group III element.

The anti-reflection layer130disposed on the emitter120of the front surface of the substrate110is made of one from among silicon nitride (SiNx), silicon oxide (SiOx), aluminum oxide (Al2O3), titanium oxide (TiO2), and silicon oxynitride (SiOxNy).

The anti-reflection layer130reduces reflectivity of light incident on the solar cell11and increases selectivity of particular wavelengths (or bands of wavelengths) of the incident light, thereby increasing the efficiency of the solar cell11.

Also, the anti-reflection layer130performs a passivation function through hydrogen (H) that was injected when the anti-reflection layer130was being formed. In other words, as defects such as dangling bonds existing near and on the surface of the substrate110change to stable bonds due to the hydrogen (H), improvement in the passivation function of the anti-reflection layer130in reducing disappearance of charges which have moved toward the surface of the substrate110due to the defects is made possible. Therefore, since the amount of charges lost near and on the surface of the substrate110due to the defect is reduced, efficiency of a solar cell11is improved.

FIG. 1shows the anti-reflection layer130having a single layered structure, but in other embodiments, the anti-reflection layer130may have a multi-layered structure such as a double or a triple layered structure. When the anti-reflection layer130has a multi-layered structure, each layer of the anti-reflection layer130may be made of one from among silicon nitride (SiNx), silicon oxide (SiOx), aluminum oxide (Al2O3), titanium oxide (TiO2), and silicon oxynitride (SiOxNy).

In one embodiment of the invention, the anti-reflection layer130, as shown inFIG. 2, is disposed at each via hole181.

In another embodiment, as shown inFIG. 4, when the anti-reflection layer130has a triple layered structure such as first, second and third anti-reflection layers131,132and133, the number of layers formed varies according to an internal position of each via hole181. Therefore, the number of layers laminated inside each via hole181is increased as a position of the via hole181that are closer to the area adjacent to where a process gas in the form of vaporized state is sprayed to form each layer of the anti-reflection layer130, for example, the front surface of the substrate110ofFIG. 4. Also, as shown inFIG. 4, even if the number of layers laminated inside the via hole181is the same, a thickness of each layer can be varied according to a position of the layer, namely, according to distance from where the process gas is sprayed. Therefore, as the layer is formed closer to the front surface of the substrate110, namely, as spraying distance becomes short, a thickness of the layer laminated inside the via hole181is increased accordingly.

A part of the periphery of the front surface of the substrate110is exposed by an exposed part that is formed in the anti-reflection layer130and the emitter120underlying the anti-reflection layer130. Therefore, the emitter120formed in the front surface of the substrate110and the emitter120formed in the back surface of the substrate110are separated electrically from the exposed part.

The plurality of front electrodes141are disposed on the emitter120formed at the front surface of the substrate110, are connected to the emitter120electrically and physically, and extend in parallel with each other along a predetermined direction while being separated from each other.

In this instance, as shown inFIG. 1, since each front electrode141is also disposed on the via holes181formed in the substrate110, portions of the front electrodes141are disposed inside the via holes181as shown inFIG. 2.

As shown inFIG. 2, a position of an end part of the front electrode141disposed inside the via hole181is different from a position of an end part of the anti-reflection layer130disposed inside the via hole181. Thus, the end part of the anti-reflection layer130extending from the front surface of the substrate110to the inside of the via hole181is disposed much lower than the end part of the front electrode141extending from the front surface of the substrate110to the inside of the via hole181. In this instance, positions of the end parts of the anti-reflection layer130and the front electrode141may be the same within a margin of error due to differences among heights of irregularities of the textured surface formed on the front surface of the substrate110.

The plurality of front electrodes141collect charges which have moved toward the emitter120(for example, electrons) and deliver them to the plurality of front electrode bus bars161connected thereto through the via holes181.

The plurality of front electrodes141contain at least one conductive material, and the conductive material may be at least one of nickel (Ni), copper (Cu), silver (Ag), aluminum (Al), tin (Sn), zinc (Zn), indium (In), titanium (Ti), gold (Au), and a combination of the above, or may contain a different material.

The plurality of front electrode bus bars161disposed on the back surface of the substrate110are made of at least one conductive material. The front electrode bus bars161extend in parallel to each other in a direction crossing the plurality of front electrodes141disposed on the front surface of the substrate110, usually forming a stripe shape.

As shown inFIGS. 1 and 2, the plurality of via holes181are formed in the area where the plurality of front electrodes141and the plurality of front electrode bus bars161cross each other. A portion of the plurality of front electrodes141and/or the plurality of front electrode bus bars161is extended towards the front and/or back surfaces of the substrate110through the plurality of via holes181. Thus, a plurality of front electrodes141and the plurality of front electrode bus bars161disposed opposite to each other are connected to each other. Accordingly, the plurality of front electrodes141and the plurality of front electrode bus bars161are connected to each other electrically and physically through the plurality of via holes181.

The plurality of front electrode bus bars161output charges delivered from the plurality of front electrodes141connected electrically to the plurality of front electrode bus bars161to an external device.

In this embodiment of the invention, the plurality of front electrode bus bars161contain silver (Ag). On the contrary, the plurality of front electrode bus bars161may contain at least one selected from a group consisting of nickel (Ni), copper (Cu), aluminum (Al), tin (Sn), zinc (Zn), indium (In), titanium (Ti), gold (Au), and a combination of the above, or the plurality of front electrode bus bars161may contain a conductive material that is different from the above.

The back electrode151disposed on the back surface of the substrate110is disposed in such a way to be separated from the nearby plurality of front electrode bus bars161.

The back electrode151is disposed on an area of the back surface of the substrate110when the plurality of front electrode bus bars161and the plurality of back electrode bus bars162are not formed. In this instance, the back electrode151is also not disposed at the periphery of the back surface of the substrate110.

The back electrode151collects charges moving toward the substrate110, for example, holes.

The emitter120disposed at the back surface of the substrate110exposes portions of the back surface of the substrate110and includes a plurality of exposed parts183surrounding the plurality of front electrode bus bars161.

Due to the exposed parts183, electrical connection between the plurality of front electrode bus bars161collecting electrons (or holes) and the back electrode151collecting holes (or electrons) are disconnected, facilitating proper movement of electrons and holes. In other words, since the plurality of front electrode bus bars161and the back electrode151collecting different types of charges are disconnected from each other, different types of charges (e.g., electrons and holes) which have moved respectively toward the plurality of front electrode bus bars161and the back electrode151are prevented or reduced from recombination.

The back electrode151contains at least one type of conductive material such as aluminum (Al). However, in an alternative embodiment, the back electrode151may contain at least one selected from a group consisting of nickel (Ni), copper (Cu), silver (Ag), tin (Sn), zinc (Zn), indium (In), titanium (Ti), gold (Au), and a combination of the above. The back electrode151may contain a conductive material different from the above.

The plurality of back electrode bus bars162are disposed on the back surface of the substrate110and are connected to the back electrode151electrically and physically, extending in parallel with the plurality of front electrode bus bars161.

Therefore, the plurality of back electrode bus bar162collects charges delivered from the back electrode151(for example, holes) and outputs the charges to the external device.

Since the plurality of back electrode bus bars162are made of the same material as the plurality of front electrode bus bars161, the plurality of back electrode bus bars162contain conductive material such as silver (Ag). However, in an alternative example, the plurality of back electrode bus bars162may contain at least one selected from a group consisting of nickel (Ni), copper (Cu), aluminum (Al), tin (Sn), zinc (Zn), indium (In), titanium (Ti), gold (Au), and a combination of the above. The plurality of back electrode bus bars162may contain a conductive material different from the above.

Also, in this embodiment of the invention, each of the plurality of back electrode bus bars162have a stripe shape extending along a predetermined direction from each of the front electrode bus bars161, but at a distance thereof.

InFIG. 1, the number of each of the plurality of front electrodes141, the plurality of front electrode bus bars161, and the plurality of back electrode bus bars162that are disposed on the substrate110is only an example, and may be changed depending on the case.

In other words, as described above, since the plurality of front electrode bus bars161and the plurality of back electrode bus bars162are disposed in an alternate fashion with a predetermined space in the back surface of the substrate110, the back electrode151is disposed between the front electrode bus bar161and the back electrode bus bar162. In this instance, for electrical insulation of the back electrode151and the plurality of front electrode bus bars161, the exposed parts183are formed along the plurality of front electrode bus bars161, and thereby portions of the substrate110are exposed through the exposed parts183. Accordingly, the back electrode bus bars162are separated from the front electrode bus bars161that collect different kind of charges, thereby reducing a recombination rate of electrons and holes.

Different from this embodiment, portions of the back electrode151and the back electrode bus bars162may be overlapped with each other in other embodiments. For example, a portion of the periphery of the back electrode bus bar162may be disposed on the back electrode151, or, on the other hand, portions of the back electrode151may be disposed on the back electrode bus bars162. In this instance, a contact area between the back electrode151and the back electrode bus bars162increases, and thus, contact resistance between the back electrode151and the back electrode bus bars162is reduced, and charge transfer from the back electrode151to the back electrode bus bars162is reliably increased due to a stable contact therebetween.

In an alternative example, each back electrode bus bar162may have a plurality of conductors in the shape of islands disposed with a regular spacing along a predetermined direction. In this instance, each cross sectional shape of the plurality of conductors may take various forms such as rectangular, triangular, circular, or ellipsoidal shape. In this instance, too, each conductor may overlap with portions of the back electrode151.

The back surface electric field unit172is formed in an area where impurities of the same conductivity type as the substrate110are doped locally on the back surface of the substrate110with a higher concentration than that of the substrate110(for example, the back surface electric field layer172is a p+ area). Since the back surface electric field unit172is usually disposed at the back surface of the substrate110contacting the back electrode151, the back electrode151is electrically connected to the substrate110through the back surface electric field unit172.

Due to the difference of impurity density between the substrate110and the back surface electric field unit172, an electric potential barrier is established. Accordingly, while a movement of electrons is hindered toward the back surface electric field unit172which is the movement direction of holes, a movement of holes toward the back surface electric field unit172is facilitated. Therefore, the amount of charges lost due to recombination of electrons and holes at the back surface of the substrate110and its adjacent area is reduced, and a movement of required charges (e.g., holes) is accelerated, thereby increasing the amount of movement of charges toward the back electrode151and the back electrode bus bars162.

Also, since the back electrode151contacts the back surface electric field unit172that maintains a higher impurity density than that of the substrate110, contact resistance between the substrate110, namely the back surface electric field unit172, and the back electrode151is reduced and thus, efficiency of charge transfer from the substrate110to the back electrode151is improved.

An operation of the solar cell11having the structure as described above according to an embodiment of the invention will be described below.

When light rays are incident on the solar cell11and reach the semiconductor substrate110through the emitter120, electron-hole pairs are generated in the semiconductor substrate110due to light energy. In this instance, since the surface of the substrate110is a textured surface, light reflectivity is reduced at the front surface of the substrate110and the amount of light incident on the substrate110is increased. Furthermore, since reflection loss of light incident on the substrate110is reduced due to the anti-reflection layer130, the amount of light incident on the substrate is all the more increased.

The electron-hole pairs are separated from each other due to the p-n junction between the substrate110and the emitter120, and the electrons move toward the emitter120of the n-type while the holes move toward the substrate110of the p-type. In this way, the electrons, which have moved toward the emitter120, are collected by the plurality of the front electrodes141and move toward the plurality of front electrode bus bars161connected through the plurality of via holes181, and the holes, which have moved toward the substrate110, are collected by the back electrode151through the back surface electric field unit172and move to the plurality of back electrode bus bars162. When the plurality of front electrode bus bars161and the plurality of back electrode bus bars162are connected by electric wires, current flows through the electric wires and may be used by an external device as electric power.

In this embodiment, since the plurality of front electrode bus bars161are disposed at the back surface of the substrate110where light rays do not reach, an incident area of light is increased and thus, efficiency of the solar cell11is increased.

In this embodiment, inside the via hole181, the first electrode bus bar161contacts a portion of the anti-reflection layer130that is inside the via hole. Such is due to the anti-reflection layer130including a first portion that is disposed on the first surface of the substrate110, and a second portion that is inside the via hole181. The first portion of the anti-reflection layer130is discontinuous from the second portion. Additionally, a portion of the first electrode bus bar161that is disposed in the via hole181covers the second portion of the anti-reflection layer130. The second portion of the anti-reflection layer130also contacts a portion of the first electrode141that is in the via hole181, a portion of the first electrode bus bar161that is in the via hole181, and a portion of the emitter layer120that is in the via hole181.

Next, with reference toFIGS. 3A to 3H, a method for manufacturing a solar cell11according to one embodiment of the invention will be described.

As shown inFIG. 3A, first, a plurality of via holes181are formed on a substrate110made of, for example, p-type monocrystalline or polycrystalline silicon. In this instance, the via holes181are made by laser drilling formed by irradiating a laser beam, but is not limited to the above.

Next, as shown inFIG. 3B, the entire front surface of the substrate110is textured, forming a textured surface. As shown inFIG. 3B, the textured surface is not formed on the side walls of via holes181. In this instance, when the substrate110is made of monocrystalline silicon, surface of the substrate110is textured by using a sodium hydroxide solution such as KOH, NaOH, and TMAH. On the other hand, when the substrate110is made of polycrystalline silicon, the surface of the substrate110is textured by using an acid solution such as HF or HNO3. The back surface of the substrate110may also be textured, but such is not required.

Next, as shown inFIG. 3C, by applying a heat treatment to the substrate110and a material containing impurities of a group V element such as phosphorus (P), arsenic (As), and antimony (Sb) (for example, POCl3or H3PO4) at a high temperature, the impurities of the group V element are diffused into the substrate110, thereby forming an emitter120on the entire surfaces of the substrate110, namely, a front surface, a back surface, side surfaces, as well as the inside surfaces of the via holes181of the substrate. Different from this embodiment of the invention, when the conductivity type of the substrate110is an n-type in another embodiment of the invention, by applying the heat treatment to the material containing impurities of a group III element (for example, B2H6) at a high temperature, a p-type emitter120may be formed on the front surface of the substrate110. Next, phosphorous silicate glass (PSG) or boron silicate glass (BSG) that are formed as the n-type or the p-type impurities that are diffused into the inside of the substrate110, may be removed through an etching process.

Next, as shown inFIG. 3D, by using plasma enhanced chemical vapor deposition (PECVD) or chemical vapor deposition (CVD), an anti-reflection layer130is formed at the front surface of the substrate110and the inside of via holes181. In this instance, as shown inFIG. 3D, the anti-reflection layer130extends from the front surface of the substrate110to a position of some depth of the via holes181, and thus, a portion of the anti-reflection layer130occupies a part of the inside (or inside wall) of each via hole181. In other embodiments, the anti-reflection layer130may occupy the whole inside area (or inside wall) of the via hole181.

With reference toFIG. 4, the anti-reflection layer130having a multi-layer structure such as triple layers131-133may be formed by changing a process gas in the same process room (e.g., a chamber) or by using a PECVD method in separate process rooms, the first to third anti-reflection layers131-133may be formed sequentially.

In another embodiment of the invention, the anti-reflection layer130may be formed by a sol-gel method such as a spin coating method or a spray coating method.

Next, as shown inFIG. 3E, by using a screen printing method, a paste containing silver (Ag) is printed in a corresponding part on the back surface of the substrate110and is then dried, to form a bus bar pattern60on the back surface of the substrate110. The bus bar pattern60includes a plurality of front electrode bus bar patterns61and a plurality of back electrode bus bar patterns62.

In this instance, the front electrode bus bar patterns61are printed after the via holes181are formed and are printed on portions of the back surface of the substrate110, at which the via holes181are formed, and thereby the front electrode bus bar patterns61are filled into at least one part of the inside of the corresponding via hole181.

Next, as shown inFIG. 3F, by using a screen printing method, a paste containing silver (Ag) is printed on corresponding parts on the anti-reflection layer130disposed on the front surface of the substrate110, and thereby a plurality of front electrode patterns41extending in parallel to each other along a predetermined direction on the anti-reflection layer130are formed by drying the printed paste. In this instance, as the paste is also printed on the plurality of via holes181, the remaining part of each via hole181unfilled by the front electrode bus bar patterns61may be filled up, though such is not required. Accordingly, the front electrode patterns41contact the front electrode bus bar patterns61already existing inside the via holes181. In this instance, the front electrode patterns41contain silver (Ag). The length of an end part of the front electrode pattern41extending from the front surface of the substrate110to the via hole181(or to a certain position in the via hole181) is shorter than that of an end part of the anti-reflection layer130extending from the front surface of the substrate110to the via hole181(or to another position in the via hole181).

Next, as shown inFIG. 3G, by using a screen printing, a paste containing aluminum (Al) is printed on corresponding parts on the back surface of the substrate100in such a way that the paste is separated from the front electrode bus bar patterns61and extends in parallel with the front electrode bus bar patterns61, and thereby a back electrode pattern51is formed after drying the printed paste.

As described above, one or more of the pastes for the front electrode patterns41, the bus bar pattern60, and the back electrode pattern51may contain glass frit as well as metal power such as silver (Ag) or aluminum (Al). The bus bar pattern60and the back electrode pattern51are formed by using different pastes (e.g., pastes having different compositions) from each other.

For example, while the paste for front electrode patterns41has a property of penetrating a layer such as the anti-reflection layer130underlying the front electrode patterns41through a heat treatment, the paste for the bus bar pattern60and the back electrode pattern51does not have a property of penetrating a layer underlying the bus bar pattern60and the back electrode pattern51during a heat treatment. As an example, the layer penetration property of the paste may be controlled by adjusting the amount of lead (Pb) contained in the glass frit.

In this instance, a drying temperature of the patterns41,51and60may range about 120° C. to 200° C. and a formation order of the patterns41,51and60may be changed.

Next, a heat treatment process is applied to the substrate110having the front electrode patterns41, the back electrode pattern51, and the bus bar pattern60at a temperature of about 750° C. to 800° C.

Accordingly, formed are a plurality of front electrodes141connected to the emitter120, a plurality of front electrode bus bars161connected to a plurality of front electrodes141through a plurality of via holes181, a back electrode151connected electrically to the substrate110, a plurality of back electrode bus bars162connected to the substrate110and the back electrode151, and a back surface electric field unit172disposed in (at) the back surface of the substrate110contacting the back electrode151, as shown inFIG. 3H.

In other words, in the heat treatment process, by the glass frit including lead (Pb) contained in the front electrode patterns41, the front electrode patterns41form the plurality of front electrodes141contacting the emitter120by penetrating the anti-reflection layer130at a contact area thereof.

In this instance, since the anti-reflection layer130(or portions thereof) is disposed inside the plurality of via holes181, prevented or reduce is the occurrence of a shunt failure where the front electrodes141contact the substrate110by penetration of the emitter120inside the via holes181during the heat treatment process.

In other words, when the anti-reflection layer130(or portions thereof) is not disposed inside the via holes181and the penetrating operation of the anti-reflection layer130due to the heat treatment process is carried out, the front electrode patterns41disposed inside the via holes181contacts the substrate110by penetrating the emitter120formed inside the via holes181from the operation of the glass frit containing lead (Pb). Therefore, a shunt failure occurs and accordingly, parallel resistance of the solar cell11is increased and efficiency of the solar cell11is reduced along with reduction of a short circuit current (Isc).

However, as described in the example, since the anti-reflection layer130is disposed not only on the front surface of the substrate110but also inside the via holes181, when the penetrating operation of the front electrode patterns41disposed inside via holes181is carried out, the front electrode patterns41penetrate the anti-reflection layer130underlying the front electrode patterns41, even within the via holes180, and thereby contacts the emitter120in a stable manner. Therefore, since the shunt failure is prevented or reduced due to the anti-reflection layer130disposed inside the via holes181, efficiency of the solar cell11is improved.

In this instance, since a position of the end part of the anti-reflection layer130extending from the front surface of the substrate110to the inside of the via holes181is lower than that of the end part of the front electrode pattern41extending from the front surface of the substrate110to the inside of the via holes181, the emitter120is protected by the anti-reflection layer130disposed inside the via holes181, and occurrence of a leaking current due to the shunt failure caused by damage of the emitter120from the penetrating operation of the front electrode patterns41during the heat treatment process is prevented.

Also, due to the heat treatment, the back electrode pattern51form the back electrode151, and the bus bar pattern60form the plurality of front electrode bus bars161and the plurality of back electrode bus bars162. In this instance, the front electrode bus bar patterns61of the bus bar pattern60become the plurality of front electrode bus bars161while the plurality of back electrode bus bar patterns62of the bus bar pattern60become the plurality of back electrode bus bars162. In this instance, since the glass frit of the back electrode pattern51and the bus bar pattern60does not contain lead (Pb), the penetrating operation against the layer underlying the back electrode pattern51and the bus bar pattern60, namely the emitter120, is not carried out, but only a chemical combination between metal components contained in the patterns51and60and the layer (i.e., the emitter120) contacting the metal components occur. Accordingly, contact resistance against the substrate110is reduced and efficiency of charge transfer is improved, thereby increasing current flow.

Also, during the heat treatment, aluminum (Al) contained in the back electrode pattern51is diffused into the substrate110, and the back surface electric field unit172, which is an impurity area having higher impurity density than that of the substrate110, is formed inside the substrate110.

Next, by using a laser, for example, a plurality of exposed parts183are formed in the emitter120at the back surface of the substrate110and at portions of the substrate110underlying the emitter120, thereby completing the solar cell11(seeFIGS. 1 and 2). In this instance, the exposed parts183are formed around the front electrode bus bars161and thus, the back electrode151and the front electrodes141are separated electrically from each other.

Also, when the plurality of exposed parts183are formed, the emitter120formed on side surfaces of the substrate110is removed by removing parts of the side surfaces of the substrate110. Accordingly, an edge isolation is carried out whereby parts of the substrate110is exposed by forming an exposed part along the periphery of the front surface of the substrate110. Accordingly, the emitter120disposed in the front surface of the substrate110and the emitter120disposed in the back surface of the substrate110are separated electrically, thereby reducing the amount of loss due to recombination of charges.