Solar cell and method for manufacturing the same, and method for forming impurity region

Disclosed is a method for manufacturing a solar cell. The method includes forming an impurity layer on a substrate of a first conductive type, the impurity layer having impurities of a second conductive type opposite the first conductive type; forming a first emitter portion having a first impurity concentration in the substrate using the impurity layer by heating the substrate with the impurity layer; forming a second emitter portion having a second impurity concentration at the first emitter portion using the impurity layer by irradiating laser beams on a region of the impurity layer, the second impurity concentration being greater than the first impurity concentration; and forming a first electrode connected to the second emitter portion and a second electrode connected to the substrate.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of Korean Patent Application No. 10-2009-0020533 filed in the Korean Intellectual Property Office on Mar. 11, 2009, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

(a) Field of the Invention

Embodiments of the invention relate to a solar cell and a method for manufacturing the same, and a method for forming an impurity region of a solar cell.

(b) 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 generating electric energy from solar energy have been particularly spotlighted. A silicon solar cell generally includes a substrate and an emitter region, each of which is formed of a semiconductor, and a plurality of electrodes respectively formed on the substrate and the emitter region. The semiconductors forming the substrate and the emitter region have different conductive types, such as a p-type and an n-type. A p-n junction is formed at an interface between the substrate and the emitter region.

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

SUMMARY OF THE INVENTION

According to an aspect of the present invention, a method for forming an impurity region of a solar cell may include forming an impurity layer containing impurities on a substrate of a first conductive type; and diffusing the impurities into the substrate by irradiating laser beams on the impurity layer.

The forming of the impurity layer may form the impurity layer by using a screen printing method.

The forming of the impurity layer may form the impurity layer by using a sputtering method, an ink jetting method, a spin-on coating method, or a spraying method.

According to another aspect of the present invention, a method for manufacturing a solar cell may include forming an impurity layer on a substrate of a first conductive type, the impurity layer having impurities of a second conductive type opposite the first conductive type; forming a first emitter portion having a first impurity concentration in the substrate using the impurity layer by heating the substrate with the impurity layer; forming a second emitter portion having a second impurity concentration at the first emitter portion using the impurity layer by irradiating laser beams on a region of the impurity layer, the second impurity concentration being greater than the first impurity concentration; and forming a first electrode connected to the second emitter portion and a second electrode connected to the substrate.

An impurity doped depth of the second emitter portion may be different from an impurity doped depth of the first emitter portion.

The second emitter portion may be a line type or a dot type.

The forming of the impurity layer may form the impurity layer by using a screen printing method.

The forming of the impurity layer may form the impurity layer by using a sputtering method, an ink jetting method, a spin-on coating method, or a spraying method.

The impurity layer may be formed on an entire surface of the substrate.

The impurity layer may be formed on a portion of the substrate.

The method may further include removing the impurity layer after the forming of the second emitter portion.

According to another aspect of the present invention, a method for manufacturing a solar cell may include forming a first impurity layer and a second impurity layer on a substrate of a first conductive type, the first impurity layer containing impurities of a second conductive type opposite the first conductive type and the second impurity layer containing impurities of the first conductive type; forming a first impurity portion in a portion of the substrate on which the first impurity layer is formed and a second impurity portion in a portion of the substrate on which the second impurity layer is formed, by heating the substrate; forming a first high-doped impurity portion at the first impurity portion and having an impurity concentration higher than an impurity concentration of the first impurity portion by irradiating laser beams on a portion of the first impurity layer and a second high-doped impurity portion at the second impurity portion and having an impurity concentration higher than an impurity concentration of the second impurity portion by irradiating laser beams on a portion of the second impurity layer; and forming a first electrode connected to the first high-doped impurity portion and a second electrode connected to the second high-doped impurity portion.

The first impurity portion and the second impurity portion may be simultaneously formed.

The first high-doped impurity portion and the second high-doped impurity portion may be simultaneously formed.

According to another aspect of the present invention, a method for manufacturing a solar cell may include forming an impurity portion in a substrate of a first conductive type, the impurity portion having a second conductive type opposite the first conductive type; forming an impurity layer containing impurities of the second conductive type on a portion of the impurity portion; forming a high-doped impurity portion by injecting the impurities of the second conductive type into a region of the impurity portion, by irradiating laser beams on the impurity layer; and forming a first electrode connected to the high-doped impurity portion and a second electrode connected to the substrate.

The forming of the impurity layer may form the impurity layer by using a screen printing method.

The forming of the impurity layer may form the impurity layer by using a sputtering method, an ink jetting method, a spin-on coating method, or a spraying method.

According to another aspect of the present invention, a solar cell may include a substrate of a first conductive type; a first emitter portion positioned in the substrate and containing impurities of a second conductive type opposite the first conductive type; a second emitter portion positioned at the first emitter portion and containing impurities of the second conductive type, the second emitter portion having an impurity concentration higher than an impurity concentration of the first emitter portion; a first electrode connected to the second emitter portion; and a second electrode connected to the substrate, wherein the second emitter portion has a thickness equal to or less than a thickness of the first emitter portion.

According to another aspect of the present invention, a solar cell may include a substrate of a first conductive type; a first back surface field portion positioned in the substrate and containing impurities of the first conductive type; a second back surface field portion positioned at the first back surface field layer and containing impurities of the first conductive type, the second back surface field portion having an impurity concentration higher than an impurity concentration of the first back surface field portion; an emitter region positioned in the substrate and containing impurities of a second conductive type opposite the first conductive type; a first electrode connected to the emitter region; and a second electrode connected to the second back surface field portion.

According to another aspect of the present invention, a substrate of a first conductive type; a first back surface field portion positioned in the substrate and containing impurities of the first conductive type; a second back surface field portion positioned at the first back surface field portion and containing impurities of the first conductive type, the second back surface field portion having an impurity concentration higher than an impurity concentration of the first back surface field portion; a first emitter layer positioned in the substrate and having a second conductive type opposite the first conductive type; a second emitter portion positioned at the first emitter layer and containing impurities of the second conductive type, the second emitter portion having an impurity concentration higher than an impurity concentration of the first emitter portion; a first electrode connected to the second emitter portion; and a second electrode connected to the second back surface field 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 only the embodiments set forth herein.

In the drawings, the thickness of layers, films, panels, regions, etc., are exaggerated for clarity. Like reference numerals designate like elements throughout the specification. It will be understood that when an element such as a layer, film, region, or substrate is referred to as being “on” another element, it can be directly on the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly on” another element, there are no intervening elements present. Further, it will be understood that when an element such as a layer, film, region, or substrate is referred to as being “entirely” on another element, it may be on the entire surface of the other element and may not be on a portion of an edge of the other element.

Reference will now be made in detail to embodiments of the invention, examples of which are illustrated in the accompanying drawings.

First, a solar cell according to an example embodiment of the present invention will be described in reference toFIG. 1.

FIG. 1is a partial cross-sectional view of a solar cell according to an example embodiment of the invention.

Referring toFIG. 1, a solar cell10according to an example embodiment of the invention includes a substrate120, an emitter region130positioned in (at) a surface (hereinafter, referred to as ‘a front surface’) of the substrate120on which light is incident, an anti-reflection layer160on the emitter region130, a plurality of front electrodes (first electrodes)150positioned on the front surface of the substrate120and connected to the emitter region130, a rear electrode (second electrode)180positioned on a surface (a rear surface) of the substrate120, opposite the front surface of the substrate120, on which the light is not incident and connected to the substrate120, and a back surface field (BSF) region170positioned between the substrate120and the rear electrode180.

The substrate120is a semiconductor substrate formed of first conductive type silicon, for example, p-type silicon, though not required. Examples of silicon include crystalline silicon, such as single crystal silicon and polycrystalline silicon, and amorphous silicon. If the substrate120is of the p-type, the substrate120may contain impurities of a group III element such as boron (B), gallium (Ga), and indium (In). Alternatively, the substrate120may be of an n-type. If the substrate120is of the n-type, the substrate120may contain impurities of a group IV element such as phosphorus (P), arsenic (As), and antimony (Sb).

The front surface of the substrate120may be textured to form a textured surface corresponding to an uneven surface. Hence, a surface area of the substrate120increases and a light reflectance of the front surface of the substrate120is reduced. Hence, alight absorption increases and the efficiency of the solar cell1is improved.

The emitter region130is an impurity region with impurities (e.g., n-type impurities) of a second conductive type opposite the first conductive type of the substrate120. The emitter region130is substantially positioned in (at) the entire surface, that is, the entire front surface of the substrate120, on which light is incident. Thus, in this embodiment, the front surface functions as a light receiving surface.

The emitter region130includes a first emitter portion131and a plurality of second emitter portions133. The first emitter portion131and the second emitter portions133have different impurity concentration from each other. In this embodiment, an impurity concentration (that is, an impurity doped concentration) of the second emitter portions133is heavier than that of the first emitter portion131. In addition, an impurity doped depth of each second emitter portion133is less than that of the first emitter portion131, and thereby a thickness of each second emitter portion133is less than that of first emitter portion131. However, alternatively, the impurity doped depth of each second emitter portion133may be equal to or larger than that of the first emitter portion131, and thereby a thickness of each second emitter portion133may be equal to or larger than that of first emitter portion131. Each second emitter portion133may have a thickness of about 0.2 μm to about 2.0 μm from the surface of the substrate120.

Since the impurity concentration of the second emitter portions133is heavier than that of the first emitter portions131, the sheet resistance of the second emitter portion133is less than that of the first emitter portion131.

The emitter region130forms a p-n junction with the substrate120.

By a built-in potential difference generated due to the p-n junction, a plurality of electron-hole pairs, which are generated by incident light onto the semiconductor substrate120, are separated into electrons and holes, respectively, and the separated electrons move toward the n-type semiconductor and the separated holes move toward the p-type semiconductor. Thus, when the substrate120is of the p-type and the emitter region130is of the n-type, the separated holes move toward the substrate120and the separated electrons move toward the emitter region130.

Because the emitter region130forms the p-n junction with the substrate120, when the substrate120is of the n-type, then the emitter region130is of the p-type, in contrast to the embodiment discussed above, and the separated electrons move toward the substrate120and the separated holes move toward the emitter region130.

Returning to the embodiment, when the emitter region130is of the n-type, the emitter region130may be formed by doping the substrate120with impurities of the group V element such as P, As, Sb, etc., while when the emitter region130is of the p-type, the emitter region130may be formed by doping the substrate120with impurities of the group III element such as B, Ga, In, etc.

In reference toFIG. 1, the anti-reflection layer160positioned on the emitter region130is preferably made of silicon nitride (SiNx), etc. The anti-reflection layer160reduces reflectance of light incident onto the substrate120and increases selectivity of a specific wavelength band, thereby increasing efficiency of the solar cell10. In this embodiment, the anti-reflection layer160has a single-layered structure, but the anti-reflection layer160may have a multi-layered structure such as a double-layered structure. The anti-reflection layer160may be omitted, if desired.

The plurality of front electrodes150are spaced apart from each other by a predetermined distance to be parallel to each other and extend in a direction on the second emitter portions133of the emitter region130. The front electrodes150collect charges, for example, electrons, moving toward the emitter region130.

At this time, a width of each of the front electrodes150is equal to or less than that of each of the underlying second emitter portions133.

As described above, each second emitter portion133is in contact with the overlying front electrode150, and thereby functions as an ohmic contact reducing the contact resistance with the front electrode150.

The front electrodes150are preferably made of at least one conductive metal material. Examples of the conductive metal material may be 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 metal materials may be used.

The rear electrode180is positioned on the entire rear surface of the substrate120which is opposite to the light receiving surface, and is electrically connected to the substrate120. The rear electrode180collects charges, for example, holes, moving toward the substrate120.

The rear electrode180is preferably made of a conductive metal material. Examples of the conductive metal material may be at least one selected from the group consisting of Ni, Cu, Ag, Al, Sn, Zn, In, Ti, Au, and a combination thereof. Other conductive metal materials may be used.

The back surface field region170positioned between the rear electrode180and the substrate120. The back surface field region170is an area heavily doped by impurities of the same conductive type as the substrate120, and thereby, in this embodiment, the back surface field region170is an area of a p+-type. A potential barrier is formed by an impurity concentration difference between the substrate120and the back surface field region170, thereby distributing the movement of charges (for example, electrons) to a rear portion of the substrate120. Accordingly, the back surface field region170prevents or reduces the recombination and/or the disappearance of the separated electrons and holes in the rear surface of the substrate120.

An operation of the solar cell10of the structure will be described in detail.

When light irradiated to the solar cell10is incident on the substrate120of the semiconductor through the anti-reflection layer160and the emitter region130, a plurality of electron-hole pairs are generated in the substrate120by light energy based on the incident light. At this time, since a reflection loss of light incident onto the substrate120is reduced by the anti-reflection layer160, an amount of the incident light on the substrate120increases.

The electron-hole pairs are separated by the p-n junction of the substrate120and the emitter region130, and the separated electrons move toward the emitter region130of the n-type and the separated holes move toward the substrate120of the p-type. The electrons that move toward the emitter region130are collected by the front electrodes150in contact with the second emitter portions133, while the holes that move toward the substrate120are collected by the rear electrode180. When the front electrodes150and the rear electrode180are connected with electric wires, current flows therein to thereby enable use of the current for electric power.

At this time, since the front electrodes150are directly in contact with the second emitter portions133heavily doped by the impurities of the n-type, for example, the contact power between the second emitter portions133and the front electrodes150improves, and thereby transmission efficiency of the charges (e.g., electrons) increases to improve efficiency of the solar cell10.

Next, referring toFIGS. 2A to 2G, a method for manufacturing the solar cell10according to an example embodiment of the present invention.

FIGS. 2A to 2Gare sectional views sequentially showing processes for manufacturing a solar cell according to an example embodiment of the present invention.

As described above, the first conductivity type is a p-type or n-type but, in this embodiment, it is assumed that the substrate120is a semiconductor substrate containing impurities of the p-type.

As shown inFIG. 2A, a paste110for doping (hereinafter, referred to as “a doping paste”) is applied on the substantially entire surface of the light receiving surface of the semiconductor substrate120in which the p-type impurities are doped, and then dried.

In this embodiment, the doping paste110contains P as the impurities (dopants), but may contain another impurities of a group V element such as As and Sn. In an alternative embodiment, when the substrate120is of the n-type, the doping paste110contains impurities of the p-type. In this case, the doping paste110contains impurities of a group III element such B etc.

In the embodiment, an impurity layer for doping such as the doping paste110is applied by a screen printing. However, alternatively, the impurity layer may be formed on the substrate120by using various manners such as a sputtering method, an ink jetting method, a spin-on coating method, a spraying method etc.

Before the printing of the doping paste110on the substrate120, a saw damage removal process or a cleaning process etc., may be performed on the surface of the substrate120to remove damage portions that have occurred on the surface of the substrate120in slicing silicon for the substrate120and to thereby improve a surface state of the substrate120. In addition, a texturing process may be performed to increase the amount of light incident onto the substrate120.

Next, as shown inFIG. 2B, by performing a heating process on the substrate120with the doping paste110, the impurities of the n-type in the doping paste110are doped into the substrate120to form an emitter region130in (at) the front surface of the substrate120.

That is, by heating the substrate120with the doping paste110in a furnace, the impurities of the n-type contained in the doping paste110are driven into the substrate120to form the emitter region130in (at) the substrate120.

At this time, by adjusting the impurity concentration of the doping paste110and printing dimensions, the impurity concentration and dimensions of the emitter region130are controlled. Accordingly, the emitter region130having a predetermined impurity doped thickness is formed into the substrate120by using the doping paste110and the heat diffusion process described above.

In first performing the heating process diffusing the impurities of the n-type into the substrate120, an oxidized substance containing the impurities such as P, for example, PSG (phosphorus silicate glass), may be grown on the surface of the substrate120. Thus, after the heat diffusion process, the grown oxidized substance may be removed by etching using HF etc., to prevent or reduce an efficiency decrease of the solar cell10due to the oxidized substance.

Next, referring toFIG. 2C, the doping paste110is secondly heated by irradiating laser beams on portions of the doping paste110. At this time, the irradiation positions of the laser beams correspond to front electrode formation portions.

Thereby, regions of the impurity layer, that is, the doping paste110, corresponding portions on which the laser beams are irradiated are heated, and thereby the n-type impurities of the heated regions are further driven into the substrate120. Thus, portions Lo of the emitter region130corresponding to the laser beam irradiation regions are chanced into portions with the impurity concentration heavier than that of a portion Lx of the emitter region130formed through the first heating process. The portion Lx corresponds to a portion on which the laser beams are not irradiated. Accordingly, the emitter region130is divided into a first emitter portion131and a plurality of second emitter portions133having the different impurity concentration from each other, to complete the emitter region130with the first emitter portion131and the plurality of second emitter portions133. As described above, the portions Lo corresponding to the laser beam irradiation regions is changed into the plurality of second emitter portions133. In addition, since the impurity doping process is twice performed on the portions Lo, the plurality of second emitter portions133have the impurity concentration higher than that of the first emitter portion131corresponding to the portion Lx of the emitter region130.

By the usage of the laser beams, since the doping paste110is partially or selectively heated to be capable of performing a selective thermal diffusion process, the impurity high-doping is realized only on desired portions, that is, the front electrode formation portions.

At this time, a shape of the second emitter portions133is defined by a laser beam irradiation shape. Thus, when the laser beams are irradiated on each region of the doping paste110in a continuous stripe shape, each of the emitter portions133formed in each of the front electrode formation portions has a continuous stripe shape. In this case, the number of second emitter portion133formed in each of the front electrode formation portions is singular. When the laser beams are discontinuously irradiated on each region of the doping paste110in a dotted shape, each of the emitter portions133formed in each of the front electrode formation portion has a dotted shape. In this case, the number of second emitter portions133formed in each the front electrode formation portion is plural.

Referring toFIG. 2C, an impurity doped depth of the first emitter portion131from the surface of the substrate120is greater than that of the second emitter portions133. However, by controlling process conditions such as the irradiation intensity or the irradiation time of the laser beams, the impurity doped depth of the first emitter portion131is equal to or less than that of the second emitter portions133.

As described, when forming the first and second emitter portions131and133using the thermal diffusion process and the laser beam irradiation, characteristics of the first and second emitter portions131and133such as the impurity doped depth and the sheet resistance are varied based on the process conditions such as the resistance of the substrate120, the impurity concentration contained in the doping paste110, the irradiation intensity, the irradiation time and so on.

For example, by adjusting the impurity concentration of the doping paste110and the temperature applied to the doping paste110, the impurity concentration of the first and second emitter portions131and133may be controlled to be an optimum state.

When the impurity doped depth of the second emitter portions133is too shallow, the contact resistance with overlying first electrodes increases such that efficiency of precipitates, for example, the improvement of conductivity, is reduced. The precipitates are mainly formed in (at) the surface of the substrate120by the impurities of the doping paste110and silicon of the substrate120.

The movement speed of charges is in inverse proportion to the impurity concentration. Thus, when the impurity doped depth of the second emitter portions133is too deep, the movement speed of charges is reduced. Thereby, time until the charges (e.g., electrons) moving toward the second emitter portions133to reach the front electrodes is prolonged so that they do not move to the front electrodes through the surface of the substrate120, and the recombination and/or the disappearance of the electrons and holes increase. Thus, a collection probability of the charges is reduced to decrease the efficiency of the solar cell10. Thereby, the emitter portions133may have a depth of about 0.2 μm to 2.0 μm.

Meanwhile, the impurity doped depth and the impurity concentration of the second emitter portions133may be defined by the sheet resistance, etc. The sheet resistance of the second emitter portions133may be varied by the process conditions. The sheet resistance of the second emitter portions133is less than that of the first emitter portion131.

When the sheet resistance is too small, the surface concentration of P ions increases. Thus, there is a problem that the high-doped portions function as recombination sites, and thereby FSRV (front surface recombination velocity) increases and an open circuit voltage (Voc) decreases, to thereby reduce conversion efficiency of the solar cell10. Meanwhile, when the sheet resistance is too large, the contact resistance with the front electrodes150increases and a fill factor (FF) decreases. Thereby, there is a problem that the efficiency of the solar cell10reduces. In considering the problems, in this embodiment, each second emitter portion133may have the sheet resistance of about 20 Ω/cm2to 100 Ω/cm2.

Thereby, since when the plurality of second emitter portions133are formed by using the laser beams, the controlling of the dimensions of the laser irradiation portions Lo is easy, the controlling of the dimensions of the plurality of emitter portions133become also easy to form only as desired. Further, the temperature controlling of heated portions is realized by varying the output of the laser beams, and thereby the controlling of doped characteristics such as the impurity concentration and a profile etc., of the second emitter portion133becomes good or is improved.

Next, referring toFIG. 2D, the doping paste110is removed. At this time, the doping paste110may be removed by an etching process, which does not influence the substrate120with the emitter region130.

The manner of forming high-doped portions in (at) the emitter regions130by using the doping paste110and the laser beams is applicable to another example. For example, after forming the emitter region130in/on a substrate by a thermal diffusion manner using POCl3or B2H6gas, doping paste or impurity layer may be formed only on desired portions for the emitter region by using a screen printing or various manners, and laser beams may be irradiated on the doping paste or impurity layer. Thereby, high-doped emitter portions (that is, the second emitter portion133) may be formed in (at) the desired portions of the substrate120.

Next, referring toFIG. 2E, an anti-reflection layer160is formed on the emitter region130. The anti-refection layer160reduces reflectance of incident light. The anti-reflection layer160may be preferably made of silicon nitride (SiNx), etc. The anti-refection layer160may be formed by a plasma enhanced chemical vapor deposition (PECVD) method, a chemical vapor deposition (CVD) method, or a sputtering method.

Next, referring toFIG. 2F, a front electrode paste is printed and dried on the anti-reflection layer160by using screen printing to form a front electrode pattern151. At this time, the front electrode paste is formed on positions corresponding to the second emitter portions133of the emitter region130. The front electrode paste preferably includes Ag and glass frits, though not required, and the glass frits include Pb, etc.

Next, referring toFIG. 2G, a rear electrode paste is printed and dried on a rear surface of the substrate120to form a rear electrode pattern181.

At this time, the printing order of the front and rear electrode patterns may be changed.

Next, a heating process is performed on the substrate120with the front and rear electrode patterns to form a plurality of front electrodes150contacting with the second emitter portions133of the emitter region130, a rear electrode180contacting with the substrate120, and a back surface field (BSF) region170. Accordingly, a solar cell10is completed shown inFIG. 1.

That is, by the heating process, the front electrode pattern151comes into contact with the second emitter portions133of the emitter region130by penetrating the anti-reflection layer160and the rear electrode pattern181is in contact with the substrate120.

In addition, by the heating process, since Al included in the rear electrode paste is doped into the substrate120, the back surface field region170is formed. The back surface field region170has an impurity concentration (p+) heavier than that of the substrate120. As described above, the back surface field region170prevents or reduces the recombination of the electrons and the holes due to an impurity concentration difference between the back surface field region170and the substrate120and helps the movement of the holes toward the rear electrode180.

Since the plurality of front electrodes150are in contact with only the plurality of emitter portions133having the higher impurity concentration and less sheet resistance than the first emitter portion131, the contact characteristics between the front electrodes150and the second emitter portions133are improved. In addition, since the plurality of emitter portions133are formed on portions of the first emitter portion131on the surface of the substrate120, charge loss due to life time shortening by highly doped impurities as well as contact resistance both decrease to improve the efficiency of the solar cell10.

That is, when the second emitter portions133were further formed in (at) portions on which the first electrodes150are not positioned, impurities that are doped highly in (at) the surface of the substrate120exist in excessive amounts in the silicon of the substrate120, and thereby precipitates were formed. Accordingly, the life time of the charges (carriers) was reduced to decrease the efficiency of the solar cell, due to the precipitates.

To solve the problems, the second emitter portions133, that is, portions contacting with the first electrodes150, have the impurity concentration heavier than the first emitter portion131, that is, the remaining portion of the emitter region130, and thereby the life time of the charges are prolonged.

In solar cell10of a selective-emitter structure obtained by partially and highly doping selective portions of the emitter region130, the contact resistance between the first electrodes150and the emitter region130decreases, and disappearance of carriers, in particular, minority carriers is reduced.

In addition, since the front electrodes150include Ag, the front electrodes150have good electric conductivity, and since the rear electrode180includes Al having good affinity with silicon, the rear electrode180has good contact with the substrate120as well as good electric conductivity.

Next, referring toFIG. 3, a solar cell according to another example embodiment of the present invention will be described.

FIG. 3is a partial cross-sectional view of a solar cell according to another example embodiment of the invention.

As compared withFIG. 1, the elements performing the same operations are indicated as the same reference numerals, and the detailed description thereof is omitted.

Referring toFIG. 3, a structure of a solar cell10aaccording to this embodiment are almost the same as that shown inFIG. 1

That is, the solar cell10ashown inFIG. 3includes an anti-reflection layer160positioned on a front surface of a substrate120, a plurality of first impurity portions130ain (at) a rear surface of the substrate120, a plurality of second impurity portions170apositioned in (at) the rear surface of the substrate120and spaced apart from the first impurity portions130a, a plurality of first electrodes185in contact with the plurality of first impurity portions130aand a plurality of second electrodes187in contact with the plurality of second impurity portions170a. The plurality of first impurity portions130afunction as emitter regions and the plurality of second impurity portions170afunction as back surface field regions.

Unlike the solar cell10ofFIG. 1, the emitter regions130aare partially or selectively positioned in (at) the rear surface of the substrate120, instead of substantially the entire front surface of the substrate120. Thereby, the number of emitter regions130ais plural and extend in parallel in a predetermined direction in (at) the rear surface of the substrate120. As described above, the plurality of emitter regions130aare impurity portions doped with impurities of a conductive type opposite a conductive type of the substrate120. Thus, the plurality of emitter regions130aforms p-n junctions with the substrate120.

As described in reference toFIG. 1, each of the emitter regions130aincludes a first emitter portion131aand a second emitter portion133ahaving different impurity concentrations, doped thicknesses and sheet resistances from each other. Like the solar cell10ofFIG. 1, the impurity concentration of each second emitter portion133ais more than that of the first emitter portions131a, and the impurity doped thickness and the sheet resistance of the second emitter portion133aare less than those of the first emitter portion131a.

In addition, unlike the solar cell10ofFIG. 1, the back surface field regions170aare partially or selectively positioned in (at) the rear surface of the substrate120, instead of substantially the entire rear surface of the substrate120, and thereby, the number of back surface field regions170ais plural. Further, the back surface field regions170aare spaced apart from each other and extend in parallel to the emitter regions130ain (at) the rear surface of the substrate120. Hence, the first impurity regions130aand the second impurity regions170aare alternately positioned on the back surface of the substrate120.

As described above, the plurality of back surface field regions170aare impurity portions doped with impurities of a conductive type that is the same as a conductive type of the substrate120. At this time, unlike the solar cell10ofFIG. 1but similar to each emitter region130a, each back surface field region170aincludes a first back surface field portion171and a second back surface field portion173having the different impurity concentrations, doped thicknesses and sheet resistances from each other.

The impurity concentration of the second back surface field portion173is more than that of the first back surface field portion171, and the impurity doped thickness and the sheet resistance of the second back surface field portion173are less than that of the first back surface field portion171.

Similar to the solar cell10, each second emitter portion133aand each second back surface field portion173of the solar cell10ahave a continuous line shape or a dot shape, respectively.

In addition, at least one of the each second emitter portion133aand the each back surface field portion173may have a thickness of about 0.2 μm to about 2.0 μm from the surface of the substrate120.

Thereby, since all of the plurality of emitter regions130aand the plurality of back surface field regions170aare positioned in (at) the rear surface of the substrate120, the plurality of first electrodes185connected to the plurality of emitter regions130aand the plurality of second electrodes187connected to the plurality of back surface field regions170aare also positioned on the rear surface of the substrate120.

That is, the plurality of first electrodes185is positioned on the plurality of emitter regions130a, to contact the plurality of emitter regions130a, respectively. The plurality of second electrodes187are positioned on the plurality of back surface field regions170a, to contact the plurality of back surface field regions170a, respectively.

At this time, each of the first electrodes185is connected to the second emitter portion133aof each of the emitter regions130acorresponding to the each first electrode185, and each of the second electrodes187is connected to the second back surface field portion173of each of the back surface field regions170acorresponding to the each first second electrode187. Accordingly, the plurality of second emitter portions133aand the plurality of second back surface field portions173function as ohmic contacts reducing the contact resistance with the overlying front electrodes185and the overlying second electrodes187, respectively.

In the embodiment of the present invention, because the first electrodes185which would prevent light from being incident on the substrate120are positioned in (at) the rear surface of the substrate120, an amount of light incident on the substrate120increases to improve efficiency of the solar cell10a.

In addition, as described in reference toFIG. 1, since the first and second electrodes185and187are directly in contact with the second emitter portions133aand the second back surface field portions173, respectively, the contact between the second emitter and back surface field portions133aand173and the first and second electrodes185and187increases to improve transmission efficiency of the charges to the first and second electrode185and187.

In an alternative example, the solar cell10amay further include a passivation layer positioned on at least one of the front and rear surface of the substrate120. At this time, the passivation layer converts defects, such as a dangling bond, existing around the surfaces of the substrate120into stable bonds to thereby prevent or reduce a recombination and/or a disappearance of charges moving to the surfaces of the substrate120, and thereby the efficiency of the solar cell10afurther improves.

Next, referring toFIGS. 4A to 4Gas well asFIGS. 2A to 2G, a method for manufacturing the solar cell10aaccording to another example embodiment of the present invention is described.

FIGS. 4A to 4Gare sectional views sequentially showing processes for manufacturing a solar cell according to another example embodiment of the present invention.

Next, a first doping paste211and a second doping paste212are formed in a predetermined distance on a rear surface, on which light is not incident, of the substrate120(FIG. 4B). The forming order of the first and second doping paste211may be changed.

The first and second doping pastes211and212have impurities of different conductive types from each other, respectively. In this embodiment, the first doping paste211contains impurities of a conductive type opposite that of the substrate120and the second doping paste212contains impurities of a conductive type that is the same to that of the substrate120.

InFIG. 4B, the first and second doping pastes211and212are partially or selectively formed across the substrate120in parallel and are spaced apart from each other, but may partially or selectively be formed across the substrate120contacting with each other.

In the embodiment, the first and second doping pastes211and211are printed by screen printing using a screen mask with a predetermined pattern. As described above, in an alternative example, an impurity layer such as the first and second doping pastes211and212may be formed on the rear surface of the substrate120by using various methods such as a sputtering method, an ink jetting method, a spin-on coating method, or a spraying method.

In similar to prior described embodiment, before the formation of the first and second doping pastes211and212, a saw damage removal process, a cleaning process, or a texturing process etc., may be performed on the substrate120to improve the efficiency of the solar cell10a.

Next, likeFIG. 2B, by performing a heating process on the substrate120with the first and second doping pastes211and212, the impurities of the second conductive type contained in the first doping paste211are doped into the underlying regions of the substrate120and the impurities of the first conductive type contained in the second doping paste212are doped into the underlying regions of the substrate120, to thereby form a plurality of emitter regions130aand a plurality of back surface field regions170a. At this time, the plurality of emitter regions130aand the plurality of back surface field regions170aare simultaneously formed (FIG. 4C). The formation positions of the plurality of emitter regions130acorrespond to the position of the first doping paste211and the formation positions of the back surface field regions170acorrespond to the position of the second doping paste212.

At time, by adjusting the impurity concentration of the first and second doping pastes211and212and printing dimensions, the impurity concentration and dimensions of the plurality of emitter regions130aand the plurality of back surface field regions170aare controlled, respectively. In this case, an oxidized substance grown in the heating process for the first and second doping pastes211and212may be removed,

Next, likeFIG. 2C, by irradiating laser beams on portions of the first doping paste211and portions of the second doping paste212, the portions of the first and second doping pastes211and212are secondarily heated. Thereby, a portion of each emitter region130ais formed into a high-doped portion in which the impurity concentration is increased and a portion of each back surface field region170ais formed into a high-doped portion in which the impurity concentration is increased.

Accordingly, the plurality of emitter regions130ainclude a plurality of first emitter portions131aand a plurality of second emitter portions133awith the different impurity concentrations to complete the plurality of emitter regions130a, and the plurality of back surface field regions170ainclude a plurality of first back surface field portions171and a plurality of second back surface field portions173with the different impurity concentrations to complete the plurality of back surface field regions170a(FIG. 4D). Each second emitter portion133amay have the sheet resistance of about 20 Ω/cm2to 100 Ω/cm2.

At this time, the positions of the plurality of first emitter portions131aand the plurality of first back surface field portions171correspond to portions Lx on which the laser beams are not irradiated, while the positions of the plurality of second emitter portions133aand the plurality of second back surface field portions173correspond to portions Lo on which the laser beams are irradiated. In addition, the portions Lo correspond to first electrode formation positions and second electrode formation positions.

When the laser beams are simultaneously irradiated on the portions of the first and second doping pastes211and212, the plurality of second emitter portions133aand the plurality of second back surface field portions173are also simultaneously formed. At this time, shapes of each second emitter portion133aand each second back surface field portion173are defined by an irradiation shape of the laser beams.

In the embodiment described above, by controlling the pattern formation of the doping pastes and the laser beam irradiation, the first and second emitter portions131aand133aand the plurality of first and second back surface field portions171and173, which have the different impurity concentrations, respectively are formed.

However, by using the control of the pattern formation of the doping pastes and the laser beam irradiation according to this embodiment, emitter regions having substantially regular impurity concentration irrelative to positions and without the second emitter portions133amay be formed, and back surface field regions having substantially regular impurity concentration irrelative to positions without the second back surface field portions173may be formed.

In this case, doping pastes are patterned and formed on the substrate, and at least one portion of the pattern of the doping paste is heated by laser beams, to form the emitter regions and the back surface field regions on the rear surface of the substrate120. A shape of the doping paste is defined by a shape of the desired emitter region and/or the back surface field regions. Thereby, each emitter region has a substantially regular impurity concentration irrelative to positions and each back surface field region has a substantially regular impurity concentration irrelative to positions.

In addition, in case an emitter region is formed in (at) a front surface of a substrate and back surface field regions are partially or selectively formed in (at) a rear surface of the substrate, the pattern formation of the doping pastes and the laser beam irradiation may be adapted to the formation of the back surface field regions. For example, doping paste is patterned and formed on portions of the substrate, on which the formation portions of the back surface field regions correspond to, and then laser beams are irradiated on the doping paste to selectively form the back surface field regions on the rear surface of the substrate.

Similar to the description above, when forming high-doped portions in (at) the emitter regions130aand the back surface field regions170aby using the doping pastes211and212and the laser beams, after forming the emitter regions130aand the back surface field layers170ain/on a substrate by a thermal diffusion manner using POCl3or B2H6gas, doping pastes or impurity layers may be formed only on desired portions for the emitter regions and the back surface field regions by using screen printing or various methods, and laser beams may be irradiated on the doping pastes or impurity layers. Thereby, high-doped back surface field portions (that is, the second back surface field portions173) and high-doped emitter portions (that is, the second emitter portions133a) may be formed in (at) the desired portions of the substrate120.

The formation manner and the effect of the second emitter portions133aand the second back surface field portions173are the same as the second emitter portions133described referring toFIG. 2C, and thereby the detailed description thereof is omitted.

As described referring toFIG. 2G, the control for an irradiation region or processing conditions of the laser beams is easy. Thus, when forming the second emitter portions133aand the second back surface field portions173, the second emitter portions133aand the second back surface field portions173are formed only on desired regions and the control of doped characteristics such as the impurity concentration and a profile etc., of the second emitter portions133aand the second back surface field portions173becomes good or improved.

Next, referring toFIG. 4E, the first and second doping pastes211and212on the substrate120are removed.

Next, likeFIGS. 2F and 2G, a first electrode pattern185ais formed on the second emitter portions133a(FIG. 4F) and a second electrode pattern187ais formed on the second back surface field portions173(FIG. 4G). The first electrode pattern185amay not include Pb, and the first and second electrode pattern185aand187amay include Ag. The first electrode pattern185aand the second electrode pattern187aare formed only on the second emitter portions133aand the second back surface field portions173, respectively. The forming order of the first and second electrode patterns185aand187amay be changed.

Next, a heating process is performed on the substrate120to form a plurality of first electrodes185contacting with the plurality of second emitter portions133aand a plurality of first electrodes187contacting with the plurality of second back surface field portions187. Accordingly, a solar cell10ais completed as shown inFIG. 3.

Since the second emitter portions133ain which the impurities are highly doped as compared to the first emitter portions131aare electrically connected to the first electrodes185and the second back surface field portions173in which the impurities are highly doped as compared to the first back surface field portions171are electrically connected to the second electrodes187, the contact characteristics of the first and second electrodes185and187are improved to realize an ohmic contact. In addition, since only portions of the emitter regions130aand the back surface field regions170aare highly doped to form the high-doped portions133aand173, the lift time of carriers (charges) increases. Accordingly, efficiency of the solar cell10aimproves.