Solar cell and method of manufacturing the same

A solar cell and a method of manufacturing the same are disclosed. The solar cell includes a substrate of a first conductive type; an emitter layer of a second conductive type opposite the first conductive type; at least one first electrode on the emitter layer and electrically connected to the emitter layer; a passivation layer on the substrate, the passivation layer including a plurality of exposing portions to expose respective portions of the substrate; and an electrode conductive layer on the passivation layer, the electrode conductive layer including a plurality of second electrodes electrically connected to the respective plurality of exposing portions, wherein in each of the plurality of exposing portions, an area of an exposed surface of the substrate is greater than an area of a virtual interface that is coplanar with an interface between the substrate and the passivation layer and which is located over the exposed surface.

This application claims priority to and the benefit of Korean Patent Application No. 10-2009-0058182 filed in the Korean Intellectual Property Office on Jun. 29, 2009, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments relate to a solar cell and a method of manufacturing the same.

2. Description of the Related Art

Recently, as existing energy sources such as petroleum and coal are expected to be depleted, interests in alternative energy sources for replacing the existing energy sources are increasing. Among the alternative energy sources, solar cells have been particularly spotlighted because, as cells for generating electric energy from solar energy, the solar cells are able to draw energy from an abundant source and do not cause environmental pollution.

A general solar cell includes a substrate and an emitter layer, formed of a semiconductor, each having a different conductive type such as a p-type and an n-type, and electrodes respectively formed on the substrate and the emitter layer. The general solar cell also includes a p-n junction formed at an interface between the substrate and the emitter layer.

When light is incident on the solar cell, a plurality of electron-hole pairs are generated in the semiconductor. Each of the electron-hole pairs is separated into electrons and holes by the photovoltaic effect. Thus, the separated electrons move to the n-type semiconductor (e.g., the emitter layer) and the separated holes move to the p-type semiconductor (e.g., the substrate), and then the electrons and holes are collected by the electrodes electrically connected to the emitter layer and the substrate, respectively. The electrodes are connected to each other using electric wires to thereby obtain an electric power.

SUMMARY OF THE INVENTION

Embodiments provide a solar cell and a method of manufacturing the same capable of improving an operation efficiency of the solar cell.

In one aspect, there is a solar cell comprising a substrate of a first conductive type, an emitter layer of a second conductive type opposite the first conductive type on the substrate, at least one first electrode electrically connected to the emitter layer, a passivation layer on the substrate, the passivation layer including a plurality of exposing portions exposing portions of the substrate, and an electrode conductive layer on the passivation layer, the electrode conductive layer including a plurality of second electrodes electrically connected to the exposed portions of the substrate exposed by the exposing portions, wherein in each exposed portion, a size of an exposed surface of the substrate is greater than a size of a virtual interface between the substrate and the passivation layer.

Each of the plurality of exposing portions may include an over-etching portion having the exposed surface of the substrate.

Each of the plurality of over-etching portions may have a height ranging from the virtual interface to the substrate, and the heights of the plurality of over-etching portions may be equal to one another.

The height may be approximately 1 μm to 40 μm.

Angles between the plurality of exposing portions and the passivation layer may vary depending on a formation location of the passivation layer.

The plurality of exposing portions may include at least one exposing portion substantially perpendicular to the passivation layer and at least one exposing portion inclined to the passivation layer. The at least one exposing portion substantially perpendicular to the passivation layer may be positioned in the substantial center of the substrate, and the least one exposing portion inclined to the passivation layer may be positioned at an edge of the substrate. An inclined angle between the at least one exposing portion inclined to the passivation layer and the passivation layer may decrease as the at least one exposing portion approaches the edge of the substrate.

A plurality of exposing portions formed in the passivation layer along a first direction may have the same angle with respect to the passivation layer, and a plurality of exposing portions formed in the passivation layer along a second direction different from the first direction each may have different angles with respect to the passivation layer.

The plurality of exposing portions formed along the second direction may be inclined to the passivation layer. Inclined angles between the plurality of exposing portions formed along the second direction and the passivation layer may increase as the exposing portions formed along the second direction approach an edge of the substrate.

An angle between the at least one exposing portion positioned in the substantial center of the substrate and the least one exposing portion positioned at the edge of the substrate may be within about 45°.

A width of each of the exposing portions may be approximately 10 μm to 100 μm.

The passivation layer and the first electrode may be positioned on the substrate to be opposite to each other.

A number of layers constituting the passivation layer may be equal to or greater than 2.

In another aspect, there is a solar cell comprising a substrate of a first conductive type, an emitter layer of a second conductive type opposite the first conductive type on the substrate, at least one first electrode electrically connected to the emitter layer, a passivation layer on the substrate, the passivation layer including a plurality of exposing portions exposing portions of the substrate, each of the plurality of exposing portions including an over-etching portion having a predetermined height in a direction from a virtual interface between the substrate and the passivation layer to the substrate, and an electrode conductive layer on the passivation layer, the electrode conductive layer including a plurality of second electrodes electrically connected to the portions of the substrate exposed by the exposing portions.

A vertical cross-sectional shape of each of the exposing portions may include a circle, an oval, a polygon, or a stripe. Each of the over-etching portions may have a hemispherical shape, a conic shape, or a polygon pyramid shape.

The exposing portions may have substantially the same vertical cross-sectional shape as the over-etching portions.

The predetermined height of each of the over-etching portions may vary depending on a measured location of the predetermined height in each of the over-etching portions.

The plurality of exposing portions may include a plurality of exposing portions inclined to the passivation layer. A measured location of a maximum height of each of the over-etching portions may vary depending on an inclined direction of the exposing portions inclined to the passivation layer.

The predetermined height of each of the over-etching portions may vary depending on a measured location of the predetermined height in each of the over-etching portions. A measured location of a maximum height of each of the over-etching portions may vary depending on a formation location of the exposing portions.

The maximum height of the over-etching portion may be approximately 1 μm to 40 μm.

The plurality of over-etching portions may include at leas two over-etching portions each having a different shape.

In another aspect, there is a method of manufacturing a solar cell comprising forming an emitter layer of a second conductive type opposite a first conductive type on one surface of a substrate of the first conductive type, forming an anti-reflection layer on the emitter layer, stacking a passivation layer on another surface of the substrate, removing portions of the passivation layer and a portions of the substrate to form a plurality of exposing portions exposing portions of the substrate, forming a first electrode electrically connected to the emitter layer, and forming a plurality of second electrodes electrically connected to the portions of the substrate exposed by the plurality of exposing portions.

The forming of the first electrode may comprise forming a first electrode pattern on the anti-reflection layer and then performing a thermal process on the first electrode pattern to form the first electrode electrically connected to the emitter layer.

The thermal process may be simultaneously performed in the forming of the first electrode and the forming of the second electrodes.

The method may further comprise forming a back surface field (BSF) layers between the second electrodes and the substrate.

The BSF layers may be formed through the thermal process.

The forming of the BSF layers may comprise forming the plurality of exposing portions on the passivation layer and injecting impurities into the plurality of exposing portions using the passivation layer as a mask.

The forming of the plurality of exposing portions may comprise irradiating a laser beam on portions of the passivation layer.

The laser beam may have a wavelength of about 355 nm and a pulse width of about 1 μm.

The forming of the plurality of exposing portions may comprise removing the passivation layer and portions of the substrate to a depth of about 1 μm to 40 μm from a surface of the substrate abutting on the passivation layer.

In another aspect, there is a solar cell including a substrate of a first conductive type; an emitter layer of a second conductive type opposite the first conductive type on the substrate; at least one first electrode on the emitter layer and electrically connected to the emitter layer; a passivation layer on the substrate, the passivation layer including a plurality of exposing portions to expose respective portions of the substrate; and an electrode conductive layer on the passivation layer, the electrode conductive layer including a plurality of second electrodes electrically connected to the respective plurality of exposing portions of the substrate, wherein in each of the plurality of exposing portions, an area of an exposed surface of the substrate is greater than an area of a virtual interface that is coplanar with an interface between the substrate and the passivation layer and which is located over the exposed surface of the substrate.

In another aspect, there is a solar cell including a substrate of a first conductive type; an emitter layer of a second conductive type opposite the first conductive type on the substrate; at least one first electrode electrically connected to the emitter layer; a passivation layer on the substrate, the passivation layer including a plurality of exposing portions to expose respective portions of the substrate, each of the plurality of exposing portions including an indented portion in the substrate having a predetermined height in a direction from a virtual interface to the substrate, the virtual interface being coplanar with an interface between the substrate and the passivation layer and being located over the indented portion of the substrate; and an electrode conductive layer on the passivation layer, the electrode conductive layer including a plurality of second electrodes electrically connected to the respective portions of the substrate exposed by the exposing portions.

In another aspect, there is a method of manufacturing a solar cell including forming an emitter layer of a second conductive type opposite a first conductive type on one surface of a substrate of the first conductive type; forming an anti-reflection layer on the emitter layer; forming a passivation layer on another surface of the substrate; forming a plurality of exposing portions that exposes portions of the substrate; forming a first electrode electrically connected to the emitter layer; and forming a plurality of second electrodes to be electrically connected to the exposed portions of the substrate.

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 embodiments set forth herein.

FIG. 1is a partial perspective view of a solar cell according to an example embodiment.FIG. 2is a cross-sectional view taken along line II-II ofFIG. 1.FIG. 3illustrates various examples of an angle and a horizontal cross-sectional shape of an exposing portion depending on an irradiation location of a laser beam and a moving direction of a substrate.FIG. 4illustrates various examples of an angle and a horizontal cross-sectional shape of an exposing portion depending on an initial location of an exposing portion formation device.FIG. 5illustrates an angle and a horizontal cross-sectional shape of a plurality of exposing portions depending on a location of a substrate when the plurality of exposing portions are formed using two exposing portion formation devices.FIG. 6illustrates changes in an over-etching portion depending on an angle of an exposing portion with respect to a passivation layer.FIG. 7illustrates an amount of rear electrode existing in an inner space of an exposing portion.

As shown inFIG. 1, a solar cell1according to an embodiment includes a substrate110, an emitter layer120on an incident surface (hereinafter, referred to as “a front surface”) of the substrate110, on which light is incident, an anti-reflection layer130on the emitter layer120, a passivation layer190on a rear surface of the substrate110opposite the front surface of the substrate110, a plurality of front electrodes141electrically connected to the emitter layer120, a plurality of front electrode current collectors142, a rear electrode conductive layer155, and a plurality of back surface field (BSF) layers170. The plurality of front electrode current collectors142are connected to the plurality of front electrodes141and extend in a cross direction of the front electrode current collectors142and the front electrodes141. The rear electrode conductive layer155is positioned on the passivation layer190and includes a plurality of rear electrodes151electrically connected to the substrate110. The plurality of BSF layers170are positioned between the substrate110and the plurality of rear electrodes151.

In the example embodiment, the substrate110may be formed of silicon doped with impurities of a first conductive type, for example, a p-type, though not required. Examples of silicon include single crystal silicon, polycrystalline silicon, and amorphous silicon. When the substrate110is of a p-type, the substrate110contains 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 made of other materials than silicon. When the substrate110is of the n-type, the substrate110may contain impurities of a group V element such as phosphor (P), arsenic (As), and antimony (Sb).

Unlike the configuration illustrated inFIGS. 1 and 2, the surface of the substrate110may be textured to form a textured surface corresponding to an uneven surface.

The emitter layer120is an impurity portion having a second conductive type (for example, an n-type) opposite the first conductive type of the substrate110. The emitter layer120and the substrate110form a p-n junction.

A plurality of electron-hole pairs produced by light incident on the substrate110are separated into electrons and holes by a built-in potential difference resulting from the p-n junction. Then, the separated electrons move toward the n-type semiconductor, and the separated holes move toward the p-type semiconductor. Thus, when the substrate110is of the p-type and the emitter layer120is of the n-type, the separated holes and the separated electrons move to the substrate110and the emitter layer120, respectively. Accordingly, the holes in the substrate110and the electrons in the emitter layer120become major carriers.

Because the substrate110and the emitter layer120form the p-n junction, the emitter layer120may be of the p-type when the substrate110is of the n-type unlike the embodiment described above. In this case, the separated electrons and the separated holes move to the substrate110and the emitter layer120, respectively.

Returning to the embodiment when the emitter layer120is of the n-type, the emitter layer120may be formed by doping the substrate110with impurities of a group V element such as P, As, and Sb. On the contrary, when the emitter layer120is of the p-type, the emitter layer120may be formed by doping the substrate110with impurities of a group III element such as B, Ga, and In.

The anti-reflection layer130formed of silicon nitride (SiNx) and/or silicon oxide (SiOx) is positioned on the emitter layer120. The anti-reflection layer130reduces a reflectance of light incident on the substrate110and increases a selectivity of a predetermined wavelength band, thereby increasing the efficiency of the solar cell1. The anti-reflection layer130may be omitted, if desired.

The passivation layer190is positioned on the rear surface of the substrate110to reduce a recombination of charges around the surface of the substrate110and to increase an inner reflectance of light passing through the substrate110. Hence, a re-incidence of the light passing through the substrate110can increase. The passivation layer190has a single-layered structure or a multi-layered structure.

The passivation layer190includes a plurality of exposing portions181to expose portions of the substrate110.

The plurality of exposing portions181are positioned to be spaced apart from one another at a constant distance of about 0.5 mm to 1 mm, for example. In the embodiment, a vertical cross-sectional shape of each of the exposing portions181obtained when the exposing portion181is cut in a direction perpendicular to a central axis of the exposing portion181is a circle, but is not limited thereto. For example, the vertical cross-sectional shape of the exposing portion181may have various shapes, such as an oval or a polygon.

An angle between each of the plurality of exposing portions181and the surface of the passivation layer190varies depending on a formation location of each of the exposing portions181. More specifically, the plurality of exposing portions181include at least one exposing portion181formed substantially perpendicular to the surface of the passivation layer190and at least one exposing portion181inclined to the surface of the passivation layer190. In this case, an inclined angle and an inclined direction of the exposing portion181with respect to the surface of the passivation layer190vary depending on a formation location of the exposing portion181.

Further, a horizontal cross-sectional shape of the exposing portion181obtained when the exposing portion181is cut in a direction parallel to the surface of the passivation layer190varies depending on the angle between the exposing portion181and the surface of the passivation layer190. Hence, a width d of each of the exposing portions181varies. In the embodiment, the width d of each of the exposing portions181is approximately 10 μm to 100 μm depending on the angle between the exposing portion181and the surface of the passivation layer190. Thus, each of the exposing portions181has a cross-sectional area of about 100 μm2(=10 μm×10 μm) to 10,000 μm2(=100 μm×100 μm). When the cross-sectional area of the exposing portion181is less than about 100 μm2, a contact strength between the rear electrodes151and the substrate110is reduced because of a small area of the exposed portion of the substrate110exposed by the exposing portions181. Further, when the cross-sectional area of the exposing portion181is larger than about 10,000 μm2, a function of the passivation layer190is reduced because of a reduction in an area of the passivation layer190.

The cross-sectional area of the exposing portion181increases as the angle between the exposing portion181and the surface of the passivation layer190decreases, and the cross-sectional area of the exposing portion181decreases as the angle between the exposing portion181and the surface of the passivation layer190increases. For example, if a desired cross-sectional area of the exposing portion181is 100 μm2, the exposing portion181may have a cross-sectional area close to about 100 μm2when the angle between the exposing portion181and the surface of the passivation layer190is about 90°. As the angle between the exposing portion181and the surface of the passivation layer190is close to about 0°, the cross-sectional area of the exposing portion181increases. Accordingly, as the number of the exposing portions181having the inclined angle less than about 90° increases, the number of the exposing portions181having a cross-sectional area larger than a desired cross-sectional area increases. In the embodiment, the inclined angle of the exposing portion181inclined to the surface of the passivation layer190may be an acute angle.

Accordingly, in the embodiment, the angle between each of the exposing portions181and the surface of the passivation layer190is approximately 45° to 90°, so that contact characteristic of the exposing portions181and operation characteristic of the passivation layer190are uniformized by reducing a deviation between the cross-sectional areas of the exposing portions181, i.e., a deviation of contact areas between the substrate110and the rear electrodes151. Therefore, among the angles between the exposing portions181and the surface of the passivation layer190, a largest difference between a maximum angle and a minimum angle is about 45°.

As shown inFIG. 3, the angle between the exposing portion181and the surface of the passivation layer190varies depending on an irradiation location of an exposing portion formation device20, for example, a laser beam irradiation device.

For example, the exposing portion181formed in a portion of the passivation layer190closest to the irradiation location of the laser beam irradiation device20(i.e., a portion of the passivation layer190on which a laser beam output from the laser beam irradiation device20is incident at an angle of about 90°) has an angle close to 90° with respect to the surface of the passivation layer190. On the contrary, the exposing portion181formed in a portion of the passivation layer190farthest away from the irradiation location of the laser beam irradiation device20(i.e., a portion of the passivation layer190on which the laser beam is incident at an angle of about 45°) has an angle close to 45° with respect to the surface of the passivation layer190.

As shown inFIG. 3, when the plurality of exposing portions181are formed in the passivation layer190, an angle, a horizontal cross-sectional shape, a cross-sectional area, etc. of each of the exposing portions181vary depending on a location relationship between the substrate110and the laser beam irradiation device20, an irradiation (operation) direction of the substrate110or the laser beam irradiation device20, etc.

In other words, as shown inFIG. 3, the plurality of exposing portions181may be formed while moving both the substrate110, on which the passivation layer190is formed, and the laser beam irradiation device20.

InFIG. 3, (a) and (b) show the exposing portions181formed in the passivation layer190when the substrate110moves in the direction of an arrow “A” or “B” (approximately an X-axis direction) and the laser beam irradiation device20irradiates a laser beam while moving up and down. As shown in (a) and (b) ofFIG. 3, the exposing portions181foamed in the same row have substantially the same angle with respect to the surface of the passivation layer190, and the exposing portions181formed in different rows have different angles with respect to the surface of the passivation layer190. The angles of the exposing portion181decreases as the exposing portion181moves to the lower side (refer to (a) ofFIG. 3) or the upper side (refer to (b) ofFIG. 3) of the substrate110, and thus a width of the horizontal cross-sectional shape of the exposing portion181increases. Namely, the horizontal cross-sectional shape of the exposing portion181becomes close to a circle as the exposing portion181moves to the lower side (refer to (a) ofFIG. 3) or the upper side (refer to (b) ofFIG. 3) of the substrate110. Hence, the cross-sectional area of the exposing portion181may be close to a desired cross-sectional area (at this time, the angle of the exposing portion181is about 90°).

InFIG. 3, (c) and (d) show the exposing portions181formed in the passivation layer190when the substrate110moves in the direction of an arrow “C” or “D” (approximately, a Y-axis direction) and the laser beam irradiation device20irradiates a laser beam while moving left and right. As shown in (c) and (d) ofFIG. 3, the exposing portions181formed in the same column have substantially the same angle with respect to the surface of the passivation layer190, and the exposing portions181formed in different columns have different angles with respect to the surface of the passivation layer190. Accordingly, the angle of the exposing portion181decreases as the exposing portion181moves to the left side of the substrate110. The horizontal cross-sectional shape of the exposing portion181becomes close to a circle as the exposing portion181moves to the right side of the substrate110. Hence, the cross-sectional area of the exposing portion181may be close to a desired cross-sectional area.

As described above, when the plurality of exposing portions181are formed while moving both the substrate110and the laser beam irradiation device20in a predetermined direction, the angles of the exposing portions181vary depending on their positions, and so the horizontal cross-sectional shape and the cross-sectional area of each exposing portion181also vary depending on their positions. Further, at least two exposing portions181having the same angle are formed in a moving direction of the substrate110and an irradiation direction of the laser beam irradiation device20. In particular, there exists a row or a column consisting of exposing portions181having the same angle according to an arrangement form of the exposing portions181(for example, in a case where the predetermined number of exposing portions181are arranged in a matrix form in the row and column directions of the substrate110).

Consequently, the angle (and the horizontal cross-sectional shape and the cross-sectional area) of each exposing portion181varies depending on an irradiation distance between the laser beam irradiation device20and the passivation layer190varying depending on a location of the passivation layer190. The angle of the exposing portion181decreases as the irradiation distance increases. Further, the exposing portions181formed using laser beams having different irradiation distances have different angles (and different horizontal cross-sectional shapes and different cross-sectional areas), and the exposing portions181formed using laser beams having the same irradiation distance have substantially the same angle (and the same horizontal cross-sectional shape and the same cross-sectional area). Because an inclined direction of each exposing portion181is affected by the irradiation direction of the laser beam, the exposing portions181having the same inclined angle (and the same horizontal cross-sectional shape and the same cross-sectional area) may be inclined in different directions.

Moreover, the laser beam irradiation device20may irradiate a laser beam in various directions other than the irradiation directions shown inFIG. 3, and the substrate110may move in various directions other than the moving directions of the substrate110shown inFIG. 3. Even in this case, the angle, the horizontal cross-sectional shape, and the cross-sectional area of each exposing portion181vary depending on the irradiation distance of the laser beam.

Unlike the examples illustrated inFIG. 3, in some examples illustrated inFIG. 4, the plurality of exposing portions181may be formed in the substrate110by fixing the substrate110having the passivation layer190and then changing only the irradiation location of the laser beam irradiation device20.

FIG. 4illustrates various examples of an angle and a horizontal cross-sectional shape of an exposing portion depending on an initial location of an exposing portion formation device according to an example embodiment.

The substrate110is positioned at an initial location corresponding to a location of the laser beam irradiation device20for forming the exposing portions181, and then the exposing portions181are formed at corresponding locations of the passivation layer190while changing an irradiation direction of the laser beam irradiation device20. In the embodiment, a location of the laser beam irradiation device20corresponding to the substrate110when the substrate110moves to the initial location is referred to as an initial location of the laser beam irradiation device20. Consequently, the laser beam irradiation device20forms the exposing portions181while changing the irradiation direction of the laser beam at its initial location.

InFIG. 4, (a) illustrates an example where the initial location of the laser beam irradiation device20corresponds to a substantial center of the substrate110; (b) illustrates an example where the initial location of the laser beam irradiation device20is an upper left corner of the substrate110; (c) illustrates an example where the initial location of the laser beam irradiation device20is an upper right corner of the substrate110; (d) illustrates an example where the initial location of the laser beam irradiation device20is a lower left corner of the substrate110; and (e) illustrates an example where the initial location of the laser beam irradiation device20is a lower right corner of the substrate110.

As shown inFIG. 4, an irradiation angle of a laser beam becomes small as the laser beam irradiation device20becomes distant from its initial location in which an angle of the laser beam irradiation device20with respect to the substrate110, i.e., an irradiation angle of the laser beam is kept at about 90° (i.e., as an irradiation distance of the laser beam irradiation device20becomes long). Accordingly, the angles of the exposing portions181decreases and the cross-sectional areas of the exposing portions181increase as the irradiation distance of the laser beam irradiation device20becomes long. That is to say, when the vertical cross-sectional shape of the exposing portion181is a circle, the horizontal cross-sectional shape of the exposing portion181becomes close to a circle as the irradiation distance of the laser beam irradiation device20from its initial location becomes shorter.

As shown inFIG. 4, because the inclined directions of the exposing portions181are related to the irradiation direction of the laser beam, the plurality of inclined exposing portions181are inclined toward an exposing portion181having a maximum angle. That is, the plurality of inclined exposing portions181is inclined toward the initial location of the laser beam irradiation device20.

As described above with reference toFIG. 3, inFIG. 4, the horizontal cross-sectional shape, the cross-sectional area, and the angle of each of the exposing portions181formed in the passivation layer190vary depending on the irradiation distance of the laser beam varying depending on the location of the passivation layer190. Accordingly, the exposing portions181formed at the same irradiation distance from the initial location have the same angle with respect to the passivation layer190, and the exposing portions181formed at different irradiation distances from the initial location have different angles with respect to the passivation layer190. Consequently, the angles (and the horizontal cross-sectional shapes and the cross-sectional areas) of the exposing portions181formed at the same distance from an exposing portion181having a maximum angle are equal to one another.

FIGS. 3 and 4illustrate the examples where the exposing portions181are formed in corresponding portions of the passivation layer190using one exposing portion formation device20. However, in some examples, the plurality of exposing portions181may be formed using at least two exposing portion formation devices.

For example, as shown inFIG. 5, the plurality of exposing portions181may be formed using two exposing portion formation devices21and22. In this case, angles of the exposing portions181formed by the exposing portion formation device21may be equal to angles of the exposing portions181formed by the exposing portion formation device22, and thus horizontal cross-sectional shapes and cross-sectional areas may be equal to one another. That is to say, the number of exposing portions181each having an angle may be equal to the number of exposing portion formation devices.

Further, as shown inFIGS. 3 to 5, because the inclined direction of the exposing portion181is related to the irradiation direction of the laser beam, the inclined exposing portions181are inclined toward the exposing portion181having the maximum angle.

The above-described exposing portions181may be formed in a predetermined area of the substrate110over the surface of the substrate110as well as the passivation layer190. More specifically, each of the exposing portions181includes a portion occupying an area ranging from a contact surface (i.e., a virtual interface S1) between a virtual surface of the substrate110and the passivation layer190to a predetermined portion of the substrate110, and the portion is referred to as an over-etching portion183(or an indented portion183). In other words, each of the exposing portions181includes the over-etching portion183that passes through the passivation layer190and projects from the exposing portion181. The over-etching portion183is described below with reference toFIG. 6.

FIG. 6illustrates changes in an over-etching portion depending on an angle of the exposing portion181with respect to the passivation layer190.

Because each of the plurality of over-etching portions183has a maximum width substantially equal to a width of the corresponding exposing portion181, the maximum width of each of the over-etching portions183is approximately 10 μm to 100 μm.

Further, each of the over-etching portions183has a predetermined height H1starting from the virtual interface S1between the passivation layer190and the substrate110.

In the embodiment, it is preferable that the heights H1of the plurality of over-etching portions183corresponding to the plurality of exposing portions181are substantially equal to one another. For example, each of the over-etching portions183has the maximum height H1of about 1 μm to 40 μm, and may be about 2 μm to 20 μm. Because a vertical cross-sectional shape of each of the over-etching portions183is substantially the same as a vertical cross-sectional shape of the exposing portion181, the vertical cross-sectional shape of the over-etching portions183includes a circle, an oval, or a polygon.

Accordingly, in the exposing portion181including the over-etching portion183, a contact area between the substrate110and each rear electrode151increases because of the over-etching portion183, and thus a sum of the contact areas of the rear electrodes151each contacting the substrate110increases as compared with the exposing portion not including the over-etching portion183.

As described above with reference toFIGS. 3 to 5, because the angle of the exposing portion181varies depending on the location of the passivation layer190, the shape of the over-etching portion183varies depending on changes in the angle of the exposing portion181.

In the embodiment, an intensity of the laser beam irradiated onto the passivation layer190has a Gaussian distribution. Thus, a portion P1of the over-etching portion183corresponding to a middle portion of the laser beam is positioned more deeply toward the substrate110than the over-etching portion183corresponding to an edge of the laser beam. Further, each of the over-etching portions183has a height ranging from the virtual interface S1to an end of the over-etching portion183in a vertical direction, and the height of each over-etching portions183varies depending on a location of each over-etching portions183. In other words, a height H1ranging from the virtual interface S1to the portion P1of the over-etching portion183corresponding to the middle portion of the laser beam is a maximum height of the over-etching portion183. In the embodiment, the portion P1is referred to as a maximum height point.

As described above, because each of the over-etching portions183has a predetermined height extending from the virtual interface S1and extends from the virtual interface S1, a size of a surface S2(also referred to as an exposed surface) of the substrate110exposed by each over-etching portion183is greater than a size of the virtual interface S1. Because the height of the over-etching portion183varies depending on its location, the over-etching portion183may have a hemispherical shape or a conic shape when the vertical cross-sectional shape of the exposing portion181is a circle. Further, when the vertical cross-sectional shape of the exposing portion181is a polygon, the over-etching portion183may have a polygon pyramid shape. Accordingly, the exposed surface S2may be smooth, may be irregular, or have sharp corners.

As described above, the angle of the exposing portion181varies depending on the location of the passivation layer190. Thus, as the angle of the exposing portion181decreases, a cross-sectional area of the virtual interface S1increases and the maximum height point P1extending from a middle point P2(hereinafter, referred to as a ‘virtual middle point’) of the virtual interface S1moves in the inclined direction of the exposing portion181.

In other words, the maximum height point P1exists on an extension line of the virtual middle point P2. Thus, in the case of the exposing portion181having the angle of about 90°, the maximum height point P1and the virtual middle point P2exists on the same vertical line. However, in the case of the exposing portion181having the inclined angle less than about 90°, a vertical line passing through the virtual middle point P2is different from a vertical line passing through the maximum height point P1. Here, the vertical line refers to a line perpendicular to the virtual interface S1.

A difference between the vertical line passing through the virtual middle point P2and the vertical line passing through the maximum height point P1, hereinafter referred to as a shift of the maximum height point P1, becomes greater as the inclined angles becomes lesser. For example, the shift of the maximum height point P1when an inclined angle is less than about 45° is greater than the degree of shift of the maximum height point P1for when the inclined angle is greater than about 45°. The shift of the maximum height point P1is greater for those exposing portions181that are further from the exposing portion formation device20, or those at a location on the substrate110that is further from an exposing portion whose maximum height point P1is not shifted, or very slightly shifted.

Accordingly, as shown inFIG. 6, as the angle of the exposing portion181decreases, the maximum height point P1gradually moves in the inclined direction from the virtual middle point P2.

The plurality of front electrodes141are positioned on the emitter layer120, are electrically connected to the emitter layer120, and extend in a fixed direction to be spaced apart from one another. The front electrodes141collect charges (for example, electrons) moving to the emitter layer120and transfer the collected charges to the front electrode current collectors142.

The plurality of front electrode current collectors142are positioned on the same level layer as the first electrodes141on the emitter layer120and extend in a cross direction of the front electrode current collectors142and the front electrodes141. The front electrode current collectors142collect the charges transferred from the front electrodes141and output the charges to an external device.

The front electrodes141and the front electrode current collectors142are formed of at least one conductive material. More specifically, the front electrodes141and the front electrode current collectors142may be formed of at least one selected from the group consisting of nickel (Ni), copper (Cu), silver (Ag), aluminum (Al), tin (Sn), zinc (Zn), indium (In), titanium (Ti), gold (Au), and a combination thereof. Other conductive materials may be used.

The rear electrode conductive layer155is formed of a conductive material and is positioned on the passivation layer190.

The rear electrode conductive layer155includes the plurality of rear electrodes151electrically connected to exposed portions of the substrate110exposed through the plurality of exposing portions181. Accordingly, when the angle of each of the exposing portions181is less than about 45°, as shown inFIG. 7, a collecting operation of the rear electrodes151is distributed by a side wall of the exposing portion181, and thus the rear electrodes151may exist in only a predetermined space of the exposing portion181. In this case, because a normal contact between the rear electrodes151and the exposed substrate110is not achieved, an electrical connection between the rear electrodes151and the exposed substrate110may not be achieved. Accordingly, it is preferable that the angle of each of the exposing portions181is kept at about 45° to 90°, so as to easily bring the substrate110into contact with the rear electrodes151through the exposing portions181.

In some examples, each of the rear electrodes151may have the same stripe shape as the front electrodes141and thus may extend in one direction while the rear electrodes151are electrically connected to the substrate110. In this case, a vertical cross-sectional shape of the exposing portions181on the passivation190may be a stripe shape. As described above, a location of the maximum height point P1of the over-etching portion183projecting from the exposing portion181varies depending on the angle of the exposing portion181. Further, the number of rear electrodes151having the stripe shape is much less than the number of rear electrodes151having the circle, oval, or polygon shape.

The rear electrodes151collect charges (for example, holes) moving to the substrate110and transfer the collected charges to the rear electrode conductive layer155. The rear electrode conductive layer155collect the charges transferred from the rear electrodes151and output the charges to an external device.

The plurality of BSF layers170are positioned between the plurality of rear electrodes151and the substrate110. The BSF layers170are an area (for example, a p+-type area) that is more heavily doped with impurities of the same conductive type as the substrate110than the substrate110.

The smooth movement of electrons to the rear surface of the substrate110is disturbed by a potential barrier resulting from a difference between impurity doping concentrations of the substrate110and the BSF layers170. Accordingly, the BSF layers170prevent or reduce a recombination and/or a disappearance of the electrons and holes in an interface of the substrate110and the rear electrodes151.

As described above, because the exposing portion181includes the over-etching portion183removing a portion of the substrate110as well as the passivation layer190, the size of the substrate surface (or the exposed surface) S2exposed by the exposing portion181increases by the size of the over-etching portion183. Hence, the contact area between the substrate110and the rear electrodes151increases. In addition, a formation area of the BSF layer170increases by the size of the over-etching portion183, and thus the recombination and/or the disappearance of the charges resulting from the BSF layer170are further reduced.

In the solar cell1according to the embodiment having the above-described structure, the passivation layer190is positioned on the rear surface of the substrate110to reduce the recombination and/or the disappearance of the charges resulting from unstable bonds existing in the surface of the substrate110. An operation of the solar cell1will be below described in detail.

When light irradiated to the solar cell1is incident on the substrate110through the anti-reflection layer130and the emitter layer120, a plurality of electron-hole pairs are generated in the substrate110by light energy based on the incident light. Hence, a reflection loss of light incident on the substrate110is reduced by the anti-reflection layer130, and thus an amount of light incident on the substrate110further increases.

The electron-hole pairs are separated by the p-n junction of the substrate110and the emitter layer120, and the separated electrons move to the n-type emitter layer120and the separated holes move to the p-type substrate110. The electrons moving to the n-type emitter layer120are collected by the front electrodes141and then are transferred to the front electrode current collectors142. The holes moving to the p-type substrate110are collected by the rear electrodes151and then are transferred to the rear electrode conductive layer155. When the front electrode current collectors142are connected to the rear electrode conductive layer155using electric wires (not shown), current flows therein to thereby enable use of the current for electric power.

In the embodiment, because the passivation layer190having the single-layered structure or the multi-layered structure is positioned between the substrate110and the rear electrode conductive layer155, the recombination and/or the disappearance of the charges resulting from unstable bonds existing in the surface of the substrate110are reduced and thus the operation efficiency of the solar cell1is improved.

The contact area between the substrate110and the rear electrodes151increases because of the over-etching portion183of each exposing portion181, and a contact resistance between the substrate110and each rear electrode151decreases because of an increase in the contact area. Thus, a charge transfer efficiency is improved. Further, a formation area of the BSF layer170formed between the substrate110and the rear electrodes151increases because of the over-etching portion183of each exposing portion181, and thus the recombination and/or the disappearance of electron and holes around the rear surface of the substrate110is further reduced. Hence, the operation efficiency of the solar cell1according to the embodiment is improved. Because the inclined angles of the exposing portion181is about 45° to 90°, the deviation between the cross-sectional areas of the exposing portions181and the deviation of the contact areas between the substrate110and the rear electrodes151are reduced. Hence, the contact characteristic of the exposing portions181inside the substrate110and the operation characteristic of the passivation layer190can be uniformized, and the operation efficiency of the solar cell1can be further improved. Further, because the electrical connection between the rear electrodes151and the substrate110is reliably achieved, a defective rate of the solar cell1can be reduced, and the operation efficiency of the solar cell1can be further improved.

Therefore, among the angles between the exposing portions181and the surface of the passivation layer190, a largest difference between a maximum angle and a minimum angle is about 45°.

FIGS. 8A to 8Gare cross-sectional views sequentially illustrating each of stages in a method of manufacturing a solar cell according to an example embodiment.

First, as shown inFIG. 8A, a high temperature thermal process of a material (for example, POCl3or H3PO4) containing impurities of a group V element such as P, As, and Sb is performed on a substrate110formed of p-type single crystal silicon or p-type polycrystalline silicon to distribute the group V element impurities on the substrate110. Hence, an emitter layer120is formed on the entire surface of the substrate110including a front surface, a rear surface, and a side surface. Unlike the embodiment, when the substrate110is of an n-type, a high temperature thermal process of a material (for example, B2H6) containing group III element impurities is performed on the substrate110or the material containing the group III element impurities is stacked on the substrate110to form the p-type emitter layer120on the entire surface of the substrate110. Subsequently, phosphorous silicate glass (PSG) containing phosphor (P) or boron silicate glass (BSG) containing boron (B) produced when p-type impurities or n-type impurities are distributed inside the substrate110is removed through an etching process.

If necessary, before the emitter layer120is formed, a texturing process may be performed on the entire surface of the substrate110to form a textured surface of the substrate110. When the substrate110is formed of single crystal silicon, the texturing process may be performed using a basic solution such as KOH and NaOH. When the substrate110is formed of polycrystalline silicon, the texturing process may be performed using an acid solution such as HF and HNO3.

As shown inFIG. 8B, an anti-reflection layer130is formed on the substrate110using a chemical vapor deposition (CVD) method such as a plasma enhanced chemical vapor deposition (PECVD) method. In addition, the anti-reflection layer130may be formed inside the exposing portions181.

As shown inFIG. 8C, a portion of the rear surface of the substrate110is removed using a wet or dry etching method, and thus a portion of the emitter layer120on the rear surface of the substrate110is removed. Hence, the emitter layer120is completed.

As shown inFIG. 8D, a passivation layer190is formed on the rear surface of the substrate110using the CVD method such as the PECVD method, a sputtering method, etc. The passivation layer190may have a single-layered structure including a silicon oxide (SiOx) layer or a multi-layered structure including a silicon oxide (SiOx) layer and a silicon nitride (SiNx) layer.

As shown inFIG. 8E, a laser beam is irradiated onto a corresponding portion to remove the passivation190and the substrate110. Hence, a plurality of exposing portions181to expose portions of the substrate110is formed.

In this case, because angles of the plurality of exposing portions181formed depending on an irradiation distance of the laser beam vary, the exposing portions181perpendicular to the surface of the passivation layer190and the exposing portions181inclined to the surface of the passivation layer190exist, and also a width d of each exposing portion181varies depending on the angles of the exposing portions181. In the embodiment, the width d of each exposing portion181is approximately 10 μm to 100 μm.

Each of the exposing portions181includes an over-etching portion183that occupies an area ranging from the surface of the substrate110to a predetermined portion of the substrate110and exposes a predetermined surface (the exposed surface) S2of the substrate110. A maximum height H1of the over-etching portion183starting from a virtual interface S1between the passivation layer190and the substrate110is approximately 1 μm to 40 μm, and may be approximately 2 μm to 20 μm. The maximum height H1of the over-etching portion183may vary depending on a kind of laser beam, an irradiation intensity of laser beam, the number of irradiation operations of the laser beam, etc. The laser beam used in the embodiment has a wavelength of 355 nm and a pulse width equal to or less than about 1 μm, but the intensity and the wavelength of the laser beam used in the embodiment vary depending on a material or a thickness of the passivation layer190. In the embodiment, it is preferable that the maximum heights H1of the plurality of over-etching portions183on the substrate110are substantially equal to one another.

In some examples, when the plurality of rear electrodes151have a stripe shape, the exposing portions181have a stripe shape extending in a fixed direction. Thus, the over-etching portions183, whose a location of a maximum height point P1varies depending on the angles of the exposing portions181, projecting from the exposing portions181have a stripe shape.

Next, as shown inFIG. 8F, a paste containing Ag is coated on a corresponding portion of the anti-reflection layer130using a screen printing method and then is dried at about 120° C. to 200° C. to form a front electrode and front electrode current collector pattern140. The front electrode and front electrode current collector pattern140includes a front electrode pattern and a front electrode current collector pattern that cross each other and extend in a cross direction thereof. In the embodiment, a width of the front electrode current collector pattern may be greater than a width of the front electrode pattern. Other width relationships between the front electrode current collector pattern and the front electrode pattern may be used.

Next, as shown inFIG. 8G, a paste containing Al is coated on a corresponding portion of the passivation layer190using the screen printing method and then is dried at about 120° C. to 200° C. to form a rear electrode conductive layer pattern150on the passivation layer190and on an exposed portion of the substrate110exposed by the exposing portions181.

In the embodiment, a formation order of the patterns140and150may vary so that the patterns140and150may be formed together or separately.

Afterwards, a firing process is performed on the substrate110, on which the front electrode and front electrode current collector pattern140and the rear electrode conductive layer pattern150are formed, at a temperature of about 750° C. to 800° C. t to form a plurality of front electrodes141, a plurality of front electrode current collectors142, a rear electrode conductive layer155including a plurality of rear electrodes151electrically connected to the substrate110through the exposing portions181, and a plurality of BSF layers170. As a result, the solar cell1shown inFIGS. 1 and 2is completed.

More specifically, when a thermal process is performed, the plurality of front electrodes141and the plurality of front electrode current collectors142, that pass through the anti-reflection layer130of a contact portion and contact the emitter layer120, are formed due to an element such as Pb contained in the front electrode and front electrode current collector pattern140. Furthermore, metal components contained in each of the patterns140and150chemically couples with the layers120and110, and thus a contact resistance is reduced. Hence, a current flow is improved.

When the maximum height H1of the over-etching portion183is less than about 1 μm, a contact strength between the substrate110and the rear electrode151decreases because of the very small size of the exposed portion of the substrate110through the exposing portion181, and thus a formation effect of the over-etching portion183cannot be efficiently obtained.

On the other hand, when the maximum height H1of the over-etching portion183is greater than about 40 μm, the irradiation intensity of the laser beam increases. Hence, atom bonds in an irradiation area of the laser beam and the substrate110around the irradiation area are damaged because of heat resulting from the laser beam irradiated onto the passivation layer190. Namely, a portion of the substrate110may be damaged, and a charge transfer efficiency may be reduced.

Further, during the thermal process, Al contained in the rear electrodes151is distributed to the substrate110contacting the rear electrodes151to form a plurality of impurity layers doped with impurities of the same conductive type as the substrate110, for example, p-type impurities. The plurality of impurity layers form the plurality of BSF layers170. An impurity doping concentration of the BSF layers170is greater than an impurity doping concentration of the substrate110, and thus the BSF layers170are a p+-type area.

In the embodiment, the plurality of BSF layers170are formed in a contact area between the rear electrodes151and the substrate110during a thermal process on the substrate110without performing a separate process. Alternatively, a plurality of impurity layers, that are doped with impurities of the same conductive type as the substrate110but more heavily than the substrate110, may be formed on the rear surface of the substrate110using a separate process. Here, the impurity layers may serve as the BSF layers170. The plurality of impurity layers may be formed through the following process. For example, as shown inFIG. 8E, after a plurality of exposing portions181are formed on the passivation layer190, impurities (for example, p-type impurities) of the same conductive type as the substrate110are injected into the rear surface of the substrate110using the passivation layer190as a mask through the CVD method to form the plurality of impurity layers. An impurity doping concentration of each of the plurality of impurity layers may be greater than an impurity doping concentration of the substrate110, and thus the plurality of impurity layers may be a P+-type layer. Afterwards, a front electrode and front electrode current collector pattern, a rear electrode conductive layer pattern, etc., may be formed, and a firing process may be performed on the substrate110, on which the front electrode and front electrode current collector pattern, the rear electrode conductive layer pattern, etc., are formed. As a result, a solar cell may be completed.

Exposing portions or exposed portions refer to a portion of the substrate that is exposed by the passivation layer portion, regardless of a technique used to form the exposing portion through the passivasion layer.

In embodiments of the invention, reference to front or back, with respect to electrode, a surface of the substrate, or others is not limiting. For example, such a reference is for convenience of description since front or back is easily understood as examples of first or second of the electrode, the surface of the substrate or others.