KR20120070313A - Solar cell - Google Patents

Abstract

PURPOSE: A solar cell is provided to improve the efficiency of a solar cell by changing an energy band of a semiconductor to be favorable for the movement of the electric charge. CONSTITUTION: A substrate(110) is formed into a crystal semiconductor having a P or N conductive type. An impurity part is formed into a non crystal semiconductor having the P or N conductive type. Buffer electrodes(161, 162) are located on the impurity part and made out of a conductive material. The buffer electrode has a work function equal to or larger than impurity if the impurity part is a P-type. The buffer electrode has the work function equal to or smaller than the impurity if the impurity part is an N-type.

Description

Solar cell {SOLAR CELL}

The present invention relates to a solar cell.

Recently, as energy resources such as oil and coal are expected to be depleted, interest in alternative energy to replace them is increasing, and solar cells that produce electric energy from solar energy are attracting attention.

Typical solar cells have a semiconductor portion that forms a p-n junction by different conductive types, such as p-type and n-type, and electrodes connected to semiconductor portions of different conductivity types, respectively.

When light is incident on the solar cell, a plurality of electron-hole pairs are generated in the semiconductor, and the generated electron-hole pairs are separated into electrons and holes charged by the photovoltaic effect, respectively, and the electrons are n-type. It moves toward the semiconductor portion and holes move toward the p-type semiconductor portion. The transferred electrons and holes are collected by the different electrodes connected to the p-type semiconductor portion and the n-type semiconductor portion, respectively, and the electrodes are connected by a wire to obtain electric power.

The technical problem to be achieved by the present invention is to improve the efficiency of the solar cell.

A solar cell according to an aspect of the present invention is a substrate having a p-type or n-type conductivity type and made of a crystalline semiconductor, at least one impurity portion located on the substrate and having a p-type or n-type conductivity type and made of an amorphous semiconductor And at least one buffer electrode disposed on the at least one impurity portion and made of a conductive material, and wherein the at least one impurity portion is p-type, the at least one buffer electrode is the same as or greater than the at least one impurity. Has a function and the at least one impurity portion is n-type, the at least one buffer electrode has a work function equal to or less than the impurity.

When the at least one impurity portion is p-type, the at least one buffer electrode may have a work function of 5.0 eV to 6.4 eV.

When the at least one impurity portion is p-type, when the at least one buffer electrode is positioned on the incident surface of the substrate to which light is incident, copper (Cu) oxide, nickel (Ni) oxide, or tungsten (W) oxide It may be made of.

When the at least one impurity portion is p-type, when the at least one buffer electrode is located on the surface of the substrate opposite to the incident surface of the substrate to which light is incident, gold (Au), nickel (Ni), At least one selected from the group consisting of platinum (Pt), chromium (Cr), copper (Cu), tungsten (W), and oxides thereof.

When the at least one impurity portion is n-type, the at least one buffer electrode may have a work function of 3.7 eV to 4.3 eV.

When the at least one impurity portion is n-type, the at least one buffer electrode may be formed of magnesium (Mg) oxide or titanium (Ti) oxide when positioned on the incident surface of the substrate to which light is incident.

When the at least one impurity portion is n-type, when the at least one buffer electrode is positioned on the surface of the substrate opposite to the incident surface of the substrate on which light is incident, the at least one buffer electrode is magnesium (Mg), At least one selected from the group consisting of silver (Ag), aluminum (Al), titanium (Ti), and oxides thereof.

According to another aspect of the present invention, a solar cell is a substrate having a first conductivity type and formed of a semiconductor, located on a first surface of the substrate, and having an second conductivity type opposite to the first conductivity type, and formed of a semiconductor. A terminator, an electric field portion located on the first surface of the substrate and having the first conductivity type and consisting of a semiconductor, a first buffer electrode disposed on the emitter portion and made of a first conductor, and a second conductor positioned on the electric field portion. And a second buffer electrode, wherein the first conductor and the second conductor have different work functions.

When the first conductivity type is n-type and the second conductivity type is p-type, the first conductor may be made of a material that reduces the Fermi level of the emitter portion, and the second conductor may have a Fermi level of the electric field portion. It is good to be made of a material to increase the.

When the first conductivity type is p-type and the second conductivity type is n-type, the first conductor is preferably made of a material that increases the Fermi level of the emitter portion, and the second conductor is a Fermi of the electric field portion. It is preferred to be made of a material which reduces the level.

 When the first conductivity type is n-type and the second conductivity type is p-type, the first buffer electrode has a work function equal to or greater than the work function of the emitter part, and the second buffer electrode is formed in the electric field part. When the first conductivity type is p-type and the second conductivity type is n-type, the first buffer electrode has a work function that is less than or equal to the work function of the emitter portion. The second buffer electrode may have a work function equal to or greater than the work function of the electric field part.

 When the first conductivity type is n-type and the second conductivity type is p-type, the first buffer electrode has a work function of 5.0 eV to 6.4 eV, and the second buffer electrode is one of 3.7 eV to 4.3 eV. When the first conductivity type is p-type, the second conductivity type is n-type, the first buffer electrode has a work function of 3.7eV to 4.3eV, the second buffer electrode is 5.0 It may have a work function of eV to 6.4eV.

When the first conductivity type is n-type and the second conductivity type is p-type, the first buffer electrode may be formed of copper (Cu) oxide, nickel (Ni) oxide, or tungsten (W) oxide. The second buffer electrode may be formed of at least one selected from the group consisting of magnesium (Mg), silver (Ag), aluminum (Al), titanium (Ti), and oxides thereof.

When the first conductivity type is p-type and the second conductivity type is n-type, the first buffer electrode may be formed of magnesium (Mg) oxide or titanium (Ti) oxide, and the second buffer electrode may be formed of gold (Au). ), Nickel (Ni), platinum (Pt), chromium (Cr), copper (Cu), tungsten (W) and at least one selected from the group consisting of oxides thereof.

The solar cell according to the above feature may further include a plurality of first electrodes electrically connected to the first buffer electrode and a second electrode electrically connected to the second buffer electrode.

The solar cell according to the above feature may further include an anti-reflective portion positioned on a second surface of the substrate, which is located opposite to the first surface of the substrate.

The anti-reflection portion may be made of a transparent conductive material.

Preferably, the substrate is made of a crystalline semiconductor, and the emitter portion and the electric field portion are made of an amorphous semiconductor.

The first surface of the substrate may be located opposite to the incident surface to which light is incident.

According to still another aspect of the present invention, a solar cell includes a substrate having a first conductivity type, a substrate formed of a semiconductor, and positioned on a surface of the substrate, and having a second conductivity type opposite to the first conductivity type, and formed of a semiconductor. An emitter portion, a plurality of electric field portions disposed on the surface of the substrate and having the first conductivity type and consisting of a semiconductor, a plurality of first buffer electrodes disposed on the plurality of emitter portions and composed of a first conductor, and the plurality of electric fields And a plurality of second buffer electrodes positioned on the step portion and formed of a second conductor different from the first conductor.

When the first conductivity type is n-type and the second conductivity type is p-type, each of the plurality of first buffer electrodes has a work function equal to or greater than a work function of each of the plurality of emitter portions, and the plurality of Each of the second buffer electrodes has a work function equal to or smaller than the work function of each of the plurality of electric fields, and the first conductive type is p-type and the second conductive type is n-type. Each of the buffer electrodes may have a work function equal to or less than the work function of each of the plurality of emitter parts, and each of the plurality of second buffer electrodes may have a work function equal to or greater than the work function of each of the plurality of electric field parts. .

 When the first conductivity type is n-type and the second conductivity type is p-type, the first buffer electrode has a work function of 5.0 eV to 6.4 eV, and the second buffer electrode is one of 3.7 eV to 4.3 eV. Having a function, wherein the first conductivity type is p-type and the second conductivity type is n-type, the first buffer electrode has a work function of 3.7 eV to 4.3 eV, and the second buffer electrode is 5.0 eV to It can have a work function of 6.4 eV.

When the first conductivity type is n type and the second conductivity type is p type, the plurality of first buffer electrodes may include gold (Au), nickel (Ni), platinum (Pt), chromium (Cr), and copper ( Cu), tungsten (W), and at least one selected from the group consisting of oxides, and the plurality of second buffer electrodes include magnesium (Mg), silver (Ag), aluminum (Al), titanium (Ti), and At least one selected from the group consisting of oxides thereof.

When the first conductivity type is p-type and the second conductivity type is n-type, the plurality of first buffer electrodes may include magnesium (Mg), silver (Ag), aluminum (Al), titanium (Ti), and the like. At least one selected from the group consisting of oxides, wherein the plurality of second buffer electrodes are gold (Au), nickel (Ni), platinum (Pt), chromium (Cr), copper (Cu), tungsten (W) and At least one selected from the group consisting of oxides thereof.

The solar cell according to the above feature may further include a plurality of first electrodes positioned on the plurality of first buffer electrodes and a plurality of second electrodes positioned on the plurality of second buffer electrodes.

Preferably, the substrate is made of a crystalline semiconductor, and the plurality of emitter portions and the plurality of electric field portions are made of an amorphous semiconductor.

The surface of the substrate is preferably located opposite to the surface of the substrate to which light is incident.

According to a feature of the invention, by using the work function of the conductor in contact with the semiconductor, the amount of charge transfer from the semiconductor to the electrode by changing the energy band of the semiconductor to favor the transfer of charge according to the conductivity type of the semiconductor in contact This increases, and the efficiency of the solar cell is improved.

1 is a partial perspective view of a solar cell according to an embodiment of the present invention.
FIG. 2 is a cross-sectional view of the solar cell shown in FIG. 1 taken along line II-II.
3A and 3B are diagrams schematically illustrating a case where energy distortion occurs in the p-type impurity portion and the n-type impurity portion, respectively.
4A and 4B are diagrams schematically illustrating a case in which the energy bands of the p-type impurity portion and the n-type impurity portion according to the embodiment of the present invention are changed to favor the movement of electric charges.
5 is a graph illustrating a change in efficiency of a solar cell according to a work function change in a p-type impurity part according to an exemplary embodiment of the present invention.
6 is a graph illustrating a change in efficiency of a solar cell according to a work function change in an n-type impurity part according to an exemplary embodiment of the present invention.
7A through 7F are diagrams sequentially illustrating an example of a method of manufacturing a solar cell according to an embodiment of the present invention.
8 is a partial perspective view of a solar cell according to another embodiment of the present invention.
FIG. 9 is a cross-sectional view of the solar cell illustrated in FIG. 8 taken along the line IX-IX. FIG.
10A to 10O are diagrams sequentially illustrating an example of a method of manufacturing a solar cell according to another embodiment of the present invention.

DETAILED DESCRIPTION Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings so that those skilled in the art may easily implement the present invention. The present invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. In the drawings, parts irrelevant to the description are omitted in order to clearly describe the present invention, and like reference numerals designate like parts throughout the specification.

In the drawings, the thickness of layers, films, panels, regions, etc., are exaggerated for clarity. When a layer, film, region, plate, or the like is referred to as being "on" another portion, it includes not only the case directly above another portion but also the case where there is another portion in between. On the contrary, when a part is "just above" another part, there is no other part in the middle. Also, when a part is formed as "whole" on the other part, it means not only that it is formed on the entire surface (or the front surface) of the other part but also not on the edge part.

First, a solar cell according to an exemplary embodiment of the present invention will be described in detail with reference to FIGS. 1 and 2.

1 and 2, the solar cell 11 according to the exemplary embodiment of the present invention has an incident surface (hereinafter, referred to as a 'front surface') that is a surface of the substrate 110 and the substrate 110 to which light is incident. 'On' the front protection unit 191, the emitter region (emitter region) 121 located on the front protection unit 191, the front buffer electrode (161) located on the emitter portion 121 The antireflection unit 130 positioned on the front buffer electrode 161, the front electrode unit 140 positioned on the antireflection unit 130, and the surface of the substrate 110 that is opposite to the incident surface [hereinafter, referred to as “rear ( rear surface), a rear protection unit 192 positioned on the rear protection unit 192, a rear electric field unit 172 located on the rear protection unit 192, a rear buffer electrode 162 positioned on the rear electric field unit 172, and And a rear electrode 151 positioned on the rear buffer electrode 162.

The substrate 110 is a semiconductor substrate made of a semiconductor such as silicon of a first conductivity type, for example, an n-type conductivity. At this time, the semiconductor is a crystalline semiconductor such as monocrystalline silicon or polycrystalline silicon.

 When the substrate 110 has an n-type conductivity type, impurities of pentavalent elements, such as phosphorus (P), arsenic (As), and antimony (Sb), are doped to the substrate 110.

Alternatively, the substrate 110 may be of a p-type conductivity type and may be made of a semiconductor material other than silicon. When the substrate 110 has a p-type conductivity type, the substrate 110 is doped with impurities of trivalent elements such as boron (B), gallium (Ga), indium (In), and the like.

This substrate 110 has a textured surface that is an uneven surface on the front surface. For convenience, in FIG. 1, only the edge portion of the substrate 110 is shown as an uneven surface, and the front protection part 191, the emitter part 121, the front buffer electrode 161, and the anti-reflection part 130 are also positioned thereon. Only the edge part is shown by the uneven surface. However, substantially the entire front surface of the substrate 110 has a concave-convex surface, which causes the front protective part 191, the emitter portion 121, the front buffer electrode 161, and the anti-reflection to be disposed on the front surface of the substrate 110. The part 130 also has an uneven surface as a whole. In an alternative example, the substrate 110 may have a texturing surface on the front as well as on the back.

The front protection part 191 positioned on the front surface of the substrate 110 is made of an amorphous semiconductor. In this case, the front protection part 191 may be located entirely on the front surface of the substrate 110 or on the front surface of the substrate 110 excluding the edge portion.

The front protection part 191 shifts a defect such as a dangling bond mainly existing on and near the surface of the substrate 110 to a stable bond and moves toward the surface of the substrate 110 by the defect. A passivation function is performed to reduce the disappearance of one charge to reduce the amount of charge lost on and near the surface of the substrate 110 by the defect.

The front protection part 191 may have a thickness of about 1 nm to 10 nm, and may be made of amorphous amorphous silicon (a-Si).

If the thickness of the front protective part 191 is greater than or equal to about 1 nm, the front protective part 191 is uniformly applied to the entire surface of the substrate 110, and thus, the passivation function may be satisfactorily performed. The charge transfer to the emitter unit 121 positioned at may be performed more smoothly, and the amount of light absorbed in the front protection unit 191 may be reduced to increase the amount of light incident into the substrate 110. The front protection part 191 may be omitted as necessary.

The emitter portion 121 disposed on the front protective portion 191 is an impurity portion having a second conductivity type (eg, p-type conductivity type), and is formed of an amorphous semiconductor, for example, an amorphous semiconductor, which is different from the substrate 110. It is made of silicone. Accordingly, the emitter part 121 forms a hetero junction as well as a p-n junction with the substrate 110.

Due to the built-in potential difference due to the pn junction formed between the substrate 110 and the emitter section 121, the electron-hole pairs, which are charges generated by the light incident on the substrate 110, So that the electrons move toward the n-type and the holes move toward the p-type. Accordingly, when the substrate 110 is n-type and the emitter portion 121 is p-type, the separated holes move toward the emitter portion 121 and the separated electrons move toward the rear surface of the substrate 110.

Since the emitter portion 121 forms a p-n junction with the substrate 110, in contrast, when the substrate 110 has a p-type conductivity type, the emitter portion 121 has an n-type conductivity type. In this case, the separated holes move toward the rear surface of the substrate 110 and the separated electrons move toward the emitter portion 121.

When the emitter portion 121 has a p-type conductivity type, impurities of the trivalent element may be doped into the emitter portion 121, and conversely, when the emitter portion 121 has an n-type conductivity type, the emitter portion ( 121) may be doped with impurities of the pentavalent element.

The front buffer electrode 161 positioned on the emitter portion 121 is made of a transparent conductive material and has a work function that is the same as or different from that of the emitter portion 121.

The front buffer electrode 161 transfers the charge (major carrier) transferred to the emitter unit 121, which is a semiconductor, to the anti-reflection unit 130, which is a conductor. In this case, the front buffer electrode 161 may use the work function of the emitter portion 121 to favor the movement of the multiple carriers (holes or electrons) of the emitter portion 121 moving toward the anti-reflection portion 130. Change the energy band. To this end, the front buffer electrode 161 has a different work function according to the conductivity type of the emitter portion 121, and the work function depends on the material, so that the front buffer electrode ( The material of 161 may also vary.

Next, referring to FIGS. 3A and 3B, the change in energy bands of the p-type and n-type impurity portions in contact with the conductor according to the work function will be described.

In general, the energy band of the impurity portion (e.g., emitter portion 121) made of a semiconductor is controlled by band distortion (i.e., energy barrier) according to the work function of the conductor (e.g., the front buffer electrode 161) in contact with each other. The degree of occurrence varies, and the amount of charge transferred from the impurity portion to the conductor varies according to the degree of band distortion. In this example, the work function refers to the energy difference between the vacuum level and the Fermi level in the conductor or semiconductor crystal.

Therefore, when the Fermi level of the conductor is changed and the work function of the conductor is changed, the impurity portion of the semiconductor in contact with the conductor is raised or lowered in order to match the Fermi level with the conductor.

For example, when the emitter portion 121, which is an impurity portion, is p-type, an energy band is formed between the p-type emitter portion 121 and the conductor as shown in FIG. 3A. At this time, when the material forming the conductor changes in the direction in which the Fermi level (E _f ) of the conductor increases, that is, when the work function is reduced, the Fermi level (E _f ) of the p-type emitter part 121 is also in contact. The same potential is maintained by moving in the direction of movement of the Fermi level (E _f ) of the conductor.

As described above, as the Fermi level E _{f of the} emitter portion 121 increases and the work function of the p-type emitter portion 121 decreases, the energy band of the p-type emitter portion 121 also rises to generate energy distortion. For this reason, as shown in FIG. 3A, the energy distortion portion between the p-type emitter portion 121 and the conductor acts as an energy barrier. Therefore, at the interface between the p-type emitter portion 121 and the conductor, the difference between the Fermi level (E _f ) and the household appliance band level (Ev) increases, thereby increasing the number of carriers of the p-type emitter portion 121 and the household appliance conductor (Ev). Hole (h +) moving to a conductor adjacent to each other is depleted at the interface between the p-type emitter portion 121 and the conductor, and thus, the hole (h +) of the hole (h +) moving from the p-type emitter portion 121 to the conductor The amount is reduced, and as shown by the arrow ①, the movement of the holes h + to the conductor is not smoothly performed by the energy barrier.

On the contrary, when the emitter portion 121 is n-type, an energy band is formed between the n-type emitter portion 121 and the conductor as shown in FIG. 3B. At this time, when the Fermi level (E _f ) of the conductor moves in the downward direction, that is, when the work function increases, the Fermi level (E _f ) of the n-type emitter portion 121 is also in contact with the Fermi level (E _f ). _f ) to move in the direction of movement to maintain the same potential.

For this reason, similar to FIG. 3A, the energy band of the p-type emitter part 121 is also lowered in the direction in which the Fermi level E _{f of the} emitter part 121 descends, thereby causing the p-type emitter part 121 to be lowered. Distortion occurs in the energy band of. Accordingly, as shown in FIG. 3B, the difference between the Fermi level E _f and the conduction band Ec increases at the interface between the n-type emitter portion 121 and the conductor, thereby increasing the number of carriers and conduction bands of the n-type emitter portion 121. Electrons (e-) moving to the conductors in contact with (Ec) are depleted at the interface between the n-type emitter portion 121 and the conductor, and electrons (e-) move from the n-type emitter portion 121 to the conductor. As shown by the arrow (2), the amount of 줄어들 decreases, and the movement of electrons (e-) to the conductor is not smoothly caused by the energy barrier caused by the energy distortion.

Therefore, based on the description with reference to FIGS. 3A and 3B, when the impurity portion of the semiconductor is p-type, considering the movement of holes h + rising to the conductors along the household appliance Ev, FIG. 4A. As can be seen that the Fermi level of the p-type impurity portion is lowered, the energy band of the p-type impurity portion is advantageous in the movement of the hole (h +) moving in the upward direction as it moves in the downward direction. Considering the movement of the electron (e-) descending to the conductor adjacent to the conduction band (Ec), the energy band of the n-type impurity portion moves in the rising direction due to the increase in the Fermi level of the n-type impurity portion as shown in FIG. 4B. It can be seen that it is advantageous for the electron (e-) movement.

When the emitter portion 121 made of amorphous silicon has a work function of about 4.9 eV to 5.9 eV when the p-type emitter is formed in the p-type emitter, the front buffer electrode ( The 161 may have a work function equal to or greater than that of the p-type emitter part 121. For example, the front buffer electrode 161 may have a work function of about 5.0 eV to 6.4 eV. Therefore, when the emitter portion 121 is p-type, the front buffer electrode 161 has this work function (about 5.0 eV to 6.4 eV) and should be made of a transparent material. For example, the front buffer electrode 161 may be made of a transparent conductive oxide such as copper (Cu) oxide, nickel (Ni) oxide, or tungsten (W) oxide.

On the other hand, when the emitter portion 121 is n-type, since it has a work function of about 4.1 eV to 4.6 eV, the front buffer electrode 161 is moved in order to move the energy band of the n-type emitter portion 121 in a rising direction. Has a work function equal to or smaller than the n-type emitter portion 121, and may have, for example, 3.7 eV to 4.3 eV. Therefore, when the emitter portion 121 is n-type, the front buffer electrode 161 has such a work function (about 3.7 eV to 4.3 eV) and is a transparent material, for example, magnesium (Mg) oxide or titanium (Ti) oxide. It may be made of a transparent conductive oxide such as.

Referring to FIG. 5, the n-type substrate 110 includes an n-type substrate 110, a p-type emitter portion 121 having a work function of 5.0 eV, and a n-type back region electric field portion 172 having a work function of 4.3 eV. In the heterojunction solar cell, the efficiency of the solar cell according to the work function of the transparent conductor, that is, the front buffer electrode 161 in contact with the p-type emitter part 121 will be described.

As shown in FIG. 5, as the work function of the front buffer electrode 161 increases, the efficiency of the solar cell increases. At this time, the efficiency increase rate of the solar cell with respect to the work function change of the front buffer electrode 161 is increased. The work function of 161 rises steeply until it reaches about 5.0 eV, and when the work function of the front buffer electrode 161 reaches about 5.0 eV, the efficiency increase rate of the solar cell is significantly lower than that when the work function of the front buffer electrode 161 is less than about 5.0 eV. The efficiency of maintains a stable state.

Therefore, when the emitter portion is p-type, when the work function of the front buffer electrode 161 is about 5.0.eV or more, the efficiency of the solar cell 11 is increased more stably due to the increase of the work function, and the front buffer electrode ( When the work function of the 161 is about 6.4 eV or less, the front buffer electrode 161 having the work function having a desired size more easily and stably is more easily formed.

6, the p-type substrate 110, the n-type emitter unit 121 having a work function of 4.3 eV, and the p-type substrate 110 having a work function of 5.0 eV are described. In the heterojunction solar cell provided, the efficiency of the solar cell according to the change of the work function of the transparent conductor, that is, the front buffer electrode 161, which is in contact with the n-type emitter part 121 will be described.

As shown in FIG. 6, as the work function of the front buffer electrode 161 decreases, the efficiency of the solar cell increases. At this time, the efficiency increase rate of the solar cell with respect to the work function change of the front buffer electrode 161 is increased. When the work function of 161 decreases rapidly to about 4.3 eV, and the work function of the front buffer electrode 161 reaches about 4.3 eV, the efficiency increase rate of the solar cell is significantly lower than that of about 4.3 eV. The efficiency of the solar cell remains stable. That is, when the work function of the front buffer electrode 161 is about 4.3 eV or less, the efficiency change of the solar cell 11 is stabilized.

Therefore, when the emitter portion is n-type, when the work function of the front buffer electrode 161 is about 4.3.eV or less, the efficiency of the solar cell 11 is increased more stably due to the decrease in the work function. When the work function of the front buffer electrode 161 is less than about 3.7 eV, there is little change in efficiency of the solar cell 11 due to the decrease in the work function of the n-type impurity portion, and it is difficult to use a material having a work function of 3.7 eV or less. This happens. Therefore, when the work function of the buffer electrode 161 is about 3.7 eV or more, the front buffer electrode 161 is more easily formed while improving the efficiency of the solar cell 11.

As described above, since the front buffer electrode 161 is positioned on the front surface of the substrate 110 to which light is incident, the front buffer electrode 161 may not interfere with the light incident to the substrate 110. To this end, the front buffer electrode 161 may be about 1 nm to 10 nm. When the front buffer electrode 161 is about 1 nm or more, the front buffer electrode 161 is formed more stably and uniformly on the emitter portion 121, and when the front buffer electrode 161 is about 10 nm or less, the front buffer electrode The amount of light absorbed in the autonomous region is further reduced or prevented to increase the amount of light incident into the substrate 110.

The emitter part 121 performs a passivation function together with the front protection part 191. Accordingly, the amount of charge dissipated at and near the surface of the substrate 110 together with the front protective portion 191 is reduced.

The anti-reflection unit 130 disposed on the front buffer electrode 161 reduces the reflectance of light incident on the solar cell 11 and increases the selectivity of a specific wavelength region, thereby increasing the efficiency of the solar cell 11.

The anti-reflection unit 130 is made of a transparent material having a low resistivity value and good conductivity. For example, the anti-reflection unit 130 is made of a transparent conductive material such as transparent conductive oxide (TCO) such as ITO.

Since the anti-reflection portion 130 is made of a conductive material, the anti-reflection portion 130 is electrically connected to the emitter portion 121 through the front buffer electrode 161 which is a conductor.

In this case, the work function of the front buffer electrode 161 is determined according to the conductivity type of the emitter unit 121, and thus the occurrence of energy distortion of the emitter unit 121 is reduced or prevented, so that the emitter unit ( The amount of charge that travels from 121 to the anti-reflection portion 130 increases.

The front electrode unit 140 includes a plurality of front electrodes 141 and a plurality of front bus bars 142 connected to the plurality of front electrodes 141.

The front electrodes 141 are positioned on the anti-reflection portion 130 and are spaced apart from each other and extend in parallel in a predetermined direction. The plurality of front electrodes 141 collects charges, for example, holes moved toward the emitter unit 121 through the anti-reflective unit 130 connected thereto.

The plurality of front bus bars 142 are positioned on the anti-reflection portion 130 and extend side by side in a direction crossing the plurality of front electrodes 141.

In this case, the plurality of front bus bars 142 are positioned on the same layer as the plurality of front electrodes 141 and are electrically and physically connected to the corresponding front electrodes 141 at points crossing the front electrodes 141.

Accordingly, as shown in FIG. 1, the plurality of front electrodes 141 have a stripe shape extending in the horizontal or vertical direction, and the plurality of front busbars 142 have a stripe shape extending in the vertical or horizontal direction. The front electrode 140 is positioned in a lattice shape on the entire surface of the substrate 110.

The plurality of front busbars 142 may move to the emitter part 121 to be collected and moved by the plurality of front electrodes 141 as well as moved charges (eg, holes) transferred through the anti-reflection part 130. Collect charges (eg holes).

Since each front busbar 142 must collect charges collected by the plurality of front electrodes 141 that cross each other and move in a desired direction, the width of each front busbar 142 is greater than the width of each front electrode 141. Big.

The plurality of front busbars 142 are connected to an external device and output the collected charges (eg, holes) to the external device.

The front electrode part 140 including the plurality of front electrodes 141 and the plurality of front bus bars 142 is made of at least one conductive material such as silver (Ag). However, in alternative embodiments, the conductive material may be nickel (Ni), copper (Cu), aluminum (Al), tin (Sn), zinc (Zn), indium (In), titanium (Ti), gold (Au) And at least one selected from the group consisting of a combination thereof, but may be made of other conductive materials other than.

In FIG. 1, the number of front electrodes 141 and front busbars 142 disposed on the substrate 110 is only one example, and may be changed in some cases.

As the conductivity between the emitter portion 121 and the anti-reflective portion 130 that is the conductor is improved by the front buffer electrode 161, the amount of electric charge moving from the emitter portion 121 to the anti-reflective portion 130 increases, The amount of charge collected by the front electrode portion 140 also increases. Therefore, the series resistance of the solar cell 11 is reduced, and the efficiency of the solar cell 11 is improved.

Similar to the anti-reflection portion 130, the front buffer electrode 161 contains a metal material and is electrically connected to the front electrode portion 140, and thus, unlike the present example, the anti-reflection portion 130 may be necessary. May be omitted.

When the anti-reflective unit 130 is omitted, the manufacturing cost and manufacturing time of the solar cell 11 is shortened. When the anti-reflective unit 130 is present, the conductivity is improved, so that the front electrode unit (from the emitter unit 121) is improved. 140, the amount of charge transfer increases, and the front buffer electrode 161 may be protected from oxygen or contaminants.

The rear protection unit 192 positioned on the rear surface of the substrate 110 performs the passivation function in the same manner as the front protection unit 191, thereby reducing the dissipation of charges transferred to the rear surface of the substrate 110 by defects. In an alternative example, the backside protection 192 may not be located at the edge of the backside of the substrate 110.

The back protector 192 may be made of an amorphous semiconductor in the same manner as the front protector 191, and may be made of the same material as the front protector 1921. Therefore, the rear protection unit 192 may be made of intrinsic amorphous silicon.

The rear protective part 192 performs a passivation function in the same manner as the front protective part 191, thereby reducing the dissipation of charges transferred to the rear surface of the substrate 110 by defects.

The rear protection unit 192 has a thickness such that charges moved toward the rear surface of the substrate 110 may move to the rear electric field unit 172 through the rear protection unit 192. In this example, one example of the thickness of the back protection 192 may be about 1 nm to 10 nm.

If the thickness of the rear protective part 192 is about 1 nm or more, the rear protective part 192 is uniformly applied to the rear surface of the substrate 110, so that the passivation effect may be more stably obtained. The charge transfer to the rear electric field 172 is more easily performed, and the light passing through the substrate 110 is reduced in the amount of light absorbed in the rear protection 192, thereby re-incident into the substrate 110. The amount of can be further increased. The rear protection unit 192 may be omitted as necessary.

The back surface field part 172 is an impurity doped at a higher concentration than the substrate 110 with impurities of the same conductivity type as the substrate 110, and may be, for example, an impurity portion of n +. Similar to the emitter portion 121, when the rear electric field 172 has an n-type conductivity type, impurities in the pentavalent element may be doped into the rear electric field 172, and the rear electric field 172 may be doped. In the case of the p-type conductive type, the rear electric field part 172 may be doped with impurities of trivalent elements.

Similar to the emitter portion 121, the backside electric field portion 172 is made of an amorphous semiconductor such as amorphous silicon to form a heterojunction with the substrate 110.

Due to the impurity concentration difference between the backside electric field 172 and the substrate 110, a potential barrier is formed, and the backside electric field 172 which is a moving direction of one of electrons and holes (eg, electrons) is formed by the potential barrier. Interfering with the movement of other charges (e.g., holes) toward the side, while facilitating the movement of the corresponding charges (e.g., electrons) toward the backside electric field 172. Thus, the amount of charge lost due to the recombination of electrons and holes in the back field 172 and its vicinity and the movement of electrons from the substrate 110 to the back field 172 is accelerated to the back field 172. Increase the amount of electron transfer

The back field 172 may have a thickness of about 10 nm to 25 nm. When the thickness of the rear electric field 172 is about 10 nm or more, a potential barrier that prevents hole movement may be better formed, thereby further reducing charge loss, and the thickness of the rear electric field 172 may be about 25 nm or less. The amount of light absorbed in the rear electric field part 172 may be further reduced to further increase the amount of light re-incident into the substrate 110.

The rear buffer electrode 162 positioned on the rear electric field 172 is made of a conductive material and has the same or different work function as the rear electric field 172.

The rear buffer electrode 162 transfers charges transferred to the rear electric field unit 172 to the rear electrode 151. In addition, similarly to the front buffer electrode 161, the rear buffer electrode 162 uses its work function to transfer charges (electrons or holes) to the rear electrode 151 according to the conductivity type of the rear electric field unit 172. The energy band of the back field 172 is changed to favor the movement of.

Therefore, as described with reference to FIGS. 3A and 3B, when the rear electric field unit 172 is p-type, the energy band of the p-type rear electric field unit 172 moves in a downward direction to move the p-type rear electric field unit 172. As the work function of) increases, it is advantageous to the movement of the hole (h +), and when the rear electric field unit 172 is n-type, the energy band of the n-type rear electric field unit 172 moves in an upward direction to move the n-type rear electric field. As the work function of the system unit 172 decreases, it is advantageous to move the electrons e-.

Therefore, like the emitter part 121, when the back electric field part 172 that is an n-type or p-type impurity is p-type, the back buffer electrode 162 is equal to or larger than the p-type back electric field part 172. In addition, when the rear electric field unit 172 is n-type, the rear buffer electrode 162 has a work function equal to or smaller than that of the n-type rear electric field unit 172. In this case, as an example, the back buffer electrode 162 may have a work function of about 5.0 eV to 6.4 eV in the case of the p-type back electric field 172, and the back buffer in the case of the n-type back electric field 172. The electrode 162 may have a work function of 3.7 eV to 4.3 eV.

Compared to the front buffer electrode 161 disposed on the front surface of the substrate 110 to which light is incident, the rear buffer electrode 162 is positioned on the rear surface of the substrate 110 regardless of whether light is incident, and thus the front buffer electrode 161. Unlike the (), it does not need to be transparent and may have a thickness equal to or thicker than that of the front buffer electrode 161.

Therefore, in the case of the p-type rear electric field 172, the rear buffer electrode 162 may be formed of gold (Au), nickel (Ni), platinum (Pt), chromium (Cr), copper (Cu), and tungsten (W). And at least one selected from the group consisting of oxides thereof, and in the case of the n-type rear electric field 172, the rear buffer electrode 162 may include magnesium (Mg), silver (Ag), aluminum (Al), and titanium ( Ti) and oxides thereof.

As described above, since the rear buffer electrode 162 is made of a metal material, light passing through the substrate 110 is reflected toward the substrate 110.

In addition, the back buffer electrode 162 may have a thickness of about 1 nm to 100 nm. When the thickness of the rear buffer electrode 162 is about 1 nm or more, the back buffer electrode 162 may be formed more stably and uniformly on the rear electric field part 172, and when the thickness of the rear buffer electrode 162 is about 100 nm or less. To avoid unnecessary material cost increase and manufacturing time increase.

In the present example, the front buffer electrode 161 and the rear buffer electrode 162 may be formed of impurity portions of different conductivity types (for example, the p-type emitter portion 121a and the n-type rear electric field portion 172a or the n-type emitter portion ( 121a) and the p-type rear electric field part 172a], the front buffer electrode 161 and the rear buffer electrode 162 have different work functions and are made of different materials.

The rear electrode 151 positioned on the rear buffer electrode 162 is made of at least one conductive material such as aluminum (Al). However, in alternative embodiments, the conductive material may be nickel (Ni), copper (Cu), tin (Sn), zinc (Zn), indium (In), titanium (Ti), silver (Ag), gold (Au) And at least one selected from the group consisting of a combination thereof, but may be made of other conductive materials other than.

As such, since the rear electrode 151 is made of a metal material, similarly to the rear buffer electrode 162, light passing through the substrate 110 is reflected toward the substrate 110.

Unlike the present embodiment, the solar cell 11 may further include a plurality of rear busbars on the rear electrode 151 or on the rear buffer electrode 162 where the rear electrode 151 is not located. The plurality of rear busbars are disposed to correspond to the plurality of front busbars 142 and are electrically and physically connected to the rear electrodes 151. Since the plurality of rear busbars are connected to the external device in the same manner as the plurality of front busbars 142, the plurality of rear busbars transfer charges transferred from adjacent rear electrode 151 portions to the external device. Similar to the plurality of front busbars 142, since the charges collected at the rear electrode 151 must be transferred to an external device, the plurality of rear busbars have a better conductivity than the rear electrode 151 such as silver (Ag). It may be made of.

The rear buffer electrode 162 also contains a metal material similar to the rear electrode 151 and collects the charges from the rear electric field 172 and outputs them to an external device. The electrode 151 can be omitted.

When the rear electrode 151 is omitted, the manufacturing cost and manufacturing time of the solar cell 11 are shortened. When the rear electrode 151 is present, the conductivity is improved to be output from the rear electric field unit 172 to the external device. The amount of charge is increased and the back buffer electrode 162 may be protected from oxygen or contaminants.

The solar cell 11 according to the present example having such a structure is a heterojunction solar cell in which the substrate 110 and the emitter portion 121 are made of different kinds of semiconductors, and the operation thereof is as follows.

When light is irradiated onto the solar cell 11 and sequentially passes through the anti-reflection unit 130, the front buffer electrode 161, the emitter unit 121, and the front protection unit 191, the light is incident to the substrate 110. The energy generates electron-hole pairs in the substrate 110. In this case, since the surface of the substrate 110 is a textured surface having an uneven surface, the incident area of the substrate 110 increases and the light reflectivity decreases, thereby increasing the amount of light incident on the substrate 110, thereby increasing the efficiency of the solar cell 11. This is improved. In addition, the reflection loss of light incident on the substrate 110 by the anti-reflection unit 130 is reduced, so that the amount of light incident on the substrate 110 is further increased.

These electron-hole pairs are separated from each other by the pn junction of the substrate 110 and the emitter portion 121 so that the holes move toward an impurity portion (eg, the emitter portion 121) having a p-type conductivity type and the electrons It moves toward an impurity portion (e.g., rear electric field portion 172) having an n-type conductivity type, and the moved holes and electrons are moved to corresponding electrodes (e.g., front electrode portion 140 and back electrode 151). Each is delivered and collected. When the plurality of bus bars 142 and the rear electrode 151 of the front electrode unit 140 are connected with a conductive wire, a current flows, and this is used as power from the outside.

In this case, since the protection parts 191 and 192 are positioned on the front and rear surfaces of the substrate 110, the amount of charge loss due to defects existing on and near the front and rear surfaces of the substrate 110 may be reduced. The efficiency is improved. At this time, since the protection parts 191 and 192 are directly in contact with the surface of the substrate 110 having a high frequency of defects, the passivation effect is further improved.

In addition, the amount of charge loss is reduced due to the back side electric field 172 disposed on the back side of the substrate 110, so that the efficiency of the solar cell 11 is further improved.

By heterogeneous bonding between the substrate 110 and the emitter portion 121, a high open voltage Voc is obtained due to band gap energy between the substrate 110 and the emitter portion 121. For this reason, the solar cell 11 obtains higher efficiency than the solar cell using the same type of junction.

In addition, buffer electrodes 161 and 162 are positioned between the emitter portion 121, which is a semiconductor, and the anti-reflective portion 130, which is a semiconductor, and the rear electric field 172, which is a semiconductor, and the rear electrode 151, which is a semiconductor, respectively. . As a result, the energy bands of the emitter portion 121 and the rear electric field portion 172 along the buffer electrodes 161 and 162 are advantageous for the movement of the corresponding charges moving to the front electrode portion 140 and the rear electrode 151, respectively. The charge transfer amount to the front electrode 140 and the rear electrode 151 increases. For this reason, the short-circuit current of the solar cell 11 increases, and the efficiency of the solar cell 11 further improves.

Next, an example of a manufacturing method of the solar cell 11 will be described with reference to FIGS. 7A to 7F.

As shown in FIG. 7A, an etch stop layer 60 is formed on one surface of the substrate 110 made of, for example, n-type single crystal silicon, and the etch stop layer 60 is used as a mask. Etching a surface (eg, incident surface) of the substrate 110 on which no 60 is formed, forming a textured surface having a plurality of protrusions on the front surface of the substrate 110, which is the incident surface, and then etching The prevention film 60 is removed.

In an alternative example, the surface of the substrate 110 to be etched without forming the etch stop layer 60 is exposed to an etchant or an etching gas to form a texturing surface on at least one of the front and rear surfaces of the substrate 110. can do.

Then, as shown in FIG. 7B, the front protection portion 191 and the back protection of the semiconductor material are formed by using a film formation method such as plasma enhanced vapor deposition (PECVD) on the front and rear surfaces of the substrate 110. The portion 192 is formed. In this case, the semiconductor material may be intrinsic amorphous silicon.

When the protection parts 191 and 192 are formed on the front and rear surfaces of the substrate 110, the protection part 191 or 192 is disposed on the front or rear surface of the substrate 110 by exposing the front or rear surface of the substrate 110 to a process chamber. ), And then, the protection unit 192 or 191 is formed on the rear or front side of the substrate 110 by exposing the rear or front side of the substrate 110 to the same process chamber. In this case, the forming order of the protection parts 191 and 192 formed on the front and rear surfaces of the substrate 110 may be changed.

 Then, as shown in FIG. 7C, a second conductive type of impurity, for example, a trivalent element such as boron (B), is contained on the front protective portion 191 disposed on the front surface of the substrate 110. An amorphous silicon film is formed to form the emitter portion 121, and an amorphous silicon film containing an impurity of a pentavalent element such as phosphorus (P) is formed on the back surface of the substrate 110, for example, phosphorus (P). To form a rear electric field portion 172. In this case, the order in which the emitter unit 121 and the rear electric field unit 172 are formed may be changed, and the emitter unit 121 and the rear electric field unit 172 may be formed by changing impurities injected into the process chamber in the same process chamber. It can be formed in a separate process chamber.

Next, referring to FIG. 7D, the front buffer electrode 161 is formed on the emitter unit 121 by using physical vapor deposition such as sputtering or evaporation. The back buffer electrode 162 is formed on the electric field part 172.

In this case, the front buffer electrode 161 and the rear buffer electrode 162 may have the same thickness, but the rear buffer electrode 162 located on the rear side of the substrate 110 that is not related to the incident light is the front buffer electrode ( Thicker than 161). As an example, the front buffer electrode 161 may have a thickness of about 1 nm to 10 nm, and the rear buffer electrode 162 may have a thickness of about 1 nm to 100 nm.

The front buffer electrode 161 and the rear buffer electrode 162 are made of a material having a work function that is determined according to the conductivity type of the semiconductor, ie, the emitter part 121 and the rear electric field part 172, which are in contact with each other. In addition, the front buffer electrode 161 is made of a transparent material to transmit incident light to the substrate 110.

When the emitter unit 121 or the rear electric field unit 172 is p-type, the front buffer electrode 161 or the rear buffer electrode 162 is equal to or larger than the work function of the emitter unit 121 or the rear electric field unit 172. Work function, for example, 5.0 eV to 6.4 eV. To this end, the front buffer electrode 161 may be formed of copper (Cu) oxide, nickel (Ni) oxide, or tungsten (W) oxide, and the rear buffer electrode 162 may be formed of gold (Au) and nickel (Ni). , Platinum (Pt), chromium (Cr), copper (Cu), tungsten (W) and at least one selected from the group consisting of oxides thereof.

On the other hand, when the emitter unit 121 or the rear electric field unit 172 is n-type, the front buffer electrode 161 or the rear buffer electrode 162 is equal to the work function of the emitter unit 121 or the rear electric field unit 172. Or a small work function, for example, a work function of 3.7 eV to 4.3 eV. To this end, the front buffer electrode 161 may be made of magnesium (Mg) oxide or titanium (Ti) oxide, and the rear buffer electrode 162 may be magnesium (Mg), silver (Ag), aluminum (Al), or titanium ( Ti) and oxides thereof.

Next, as shown in FIG. 7E, a transparent conductive film such as ITO or IZO is formed on the front buffer electrode 161 by PECVD to form the antireflection portion 130.

Next, as shown in FIG. 7F, the front electrode paste 40, which is a conductive paste containing silver (Ag), is coated on the antireflection portion 130 using a screen printing method, and then dried and The back electrode paste 50, which is a conductive paste containing aluminum (Al) or silver (Ag), is coated on the buffer electrode 162 and then dried, and the plurality of front electrodes 141 positioned on the anti-reflection portion 130. ) And a front electrode 140 having a plurality of front busbars 142 and a rear electrode 151 positioned on the rear buffer electrode 162, thereby completing the solar cell 11 (FIG. 1 and 2).

The front electrode paste 40 is provided with a front electrode portion 41 and a front bus bar portion 42, as shown in FIG. 7F. The electrode portion 41 is a plurality of front electrodes 141, and the front bus bar portion 42 is a plurality of front bus bars 142.

Next, referring to FIGS. 8 and 9, a solar cell 12 according to another embodiment of the present invention will be described. Compared with the solar cell 11 shown in Figs. 1 and 2, the components that perform the same function are given the same reference numerals and detailed description thereof will be omitted.

The solar cell 12 shown in FIGS. 8 and 9 is a rear heterojunction solar cell in which not only the rear electric field part but also the emitter part and the parts connected to the emitter part are located on the rear side of the substrate rather than the incident surface. It has a structure similar to the solar cell 11 shown in FIG. 1 and FIG.

That is, the solar cell 12 is disposed on the substrate 110, the front protective part 191 positioned at the front of the substrate 110, the front electric field part 171 located at the front protective part 191, and the front electric field part 171. Located above the anti-reflection unit 130, the rear protection unit 192a located on the rear side of the substrate 110, the plurality of emitter units 121a located on the rear protection unit 192a, and the rear protection unit 192a. A plurality of first buffer electrodes 161a respectively positioned on the plurality of rear electric field parts 172a and the plurality of emitter parts 121a, and a plurality of second buffer electrodes respectively positioned on the plurality of rear electric field parts 172a ( 162a, a plurality of first electrodes 141a respectively positioned on the plurality of first buffer electrodes 161a, and a plurality of second electrodes 151a respectively positioned on the plurality of second buffer electrodes 162a.

The substrate 110 is made of an crystalline semiconductor of n-type or p-type, and the front surface of the substrate 110 has an uneven surface having an irregular surface.

Substrate 110 may have a texturing surface on the back as well as on the front.

As shown in Figures 8 and 9, the backside of the substrate 110 of this example has a flat surface instead of a texturing surface. As a result, the components disposed on the rear surface of the substrate 110 may be formed to be in close contact with the rear surface of the substrate 110 more uniformly and stably, and thus, between the components disposed on the rear surface of the substrate 110 and the substrate 110. Contact resistance is reduced.

The front protection portion 191 is made of intrinsic amorphous silicon and performs a passivation function to reduce the amount of charge lost on and near the surface of the substrate 110 by the defect.

The front electric field part 171 is made of amorphous silicon, and is an impurity part in which impurities of the same conductivity type (eg, n-type) as the substrate 110 are contained at a higher concentration than the substrate 110.

Compared with the rear electric field 172 of FIGS. 1 and 2, the front electric field unit 171 performs the same function except for the formation position.

Therefore, a front electric field function that prevents charge (eg, hole) movement toward the front surface of the substrate 110 due to a potential barrier due to a difference in impurity concentration between the substrate 110 and the front surface electric field unit 171. Therefore, the front field effect that the holes moving toward the front surface of the substrate 110 is returned to the rear surface of the substrate 110 by the potential barrier is obtained, thereby increasing the output amount of the electric charge output to the external device and The amount of charge lost by recombination or defects at the front of 110 is reduced.

The front electric field unit 171 performs the passivation function together with the front protection unit 191 as well as the front electric field function.

The anti-reflection unit 130 positioned on the front electric field unit 171 reduces the reflectivity of light incident on the solar cell 12 and increases the selectivity of a specific wavelength region, thereby increasing the efficiency of the solar cell 12.

At least one of the front protection part 191 and the anti-reflection part 130 disposed on the front surface of the substrate 110 may be omitted as necessary.

The rear protective part 192a disposed at the rear of the substrate 110 includes a plurality of first rear protective parts 1921 and a plurality of second rear protective parts 1922 that are spaced apart from each other. The first rear protective portion 1921 and the second rear protective portion 1922 are alternately positioned on the substrate 110 and extend in a predetermined direction in parallel with each other.

The rear protector 192a is made of the same material as the front protector 191. Thus, the back protection 192a is made of intrinsic amorphous silicon.

Since the back protection part 192a is identical to the back protection part 192 of FIGS. 1 and 2 except for the number formed, the back protection part 192a performs a passivation function, so that charges transferred to the back surface of the substrate 110 may be caused by defects. Reduce extinction

The plurality of rear electric field parts 172a performs the same function except for the number of formations with the rear electric field parts 172 of FIGS. 1 and 2. Accordingly, the plurality of backside electric fields 172a are present on the second backside protection portion 1922, and n-type or p-type impurities in which the same conductivity type impurities as the substrate 110 are doped at a higher concentration than the substrate 110. It is wealth.

Each rear electric field portion 172a extends in a predetermined direction along the second rear protective portion 1922 over each second rear protective portion 1922.

The rear electric field 172a uses the potential barrier due to the difference in the impurity concentration between the substrate 110 and the rear electric field 172a to reduce the charges lost due to the recombination of electrons and holes in the rear electric field 172a and its vicinity. The amount is reduced and the electron movement to the back field 172a is accelerated to increase the electron movement to the back field 172a.

In addition, the rear electric field unit 172a performs a passivation function together with the first rear protective portion 1921. Thus, the amount of charge dissipated at and near the surface of the substrate 110 with the second backside protective portion 1922 is reduced.

The plurality of emitter portions 121a also perform the same function except for the position and number of formation of the emitter portion 121 of FIGS. 1 and 2.

Accordingly, the plurality of emitter portions 121a are positioned above the plurality of first rear protection portions 1921 of the rear protection portion 192a, and thus the first rear protection portions 1921 on the plurality of first rear protection portions 1921. It extends along the direction.

Thus, as shown in FIGS. 8 and 9, the emitter portion 121a and the rear electric field portion 172a are alternately positioned at the rear surface of the substrate 110.

Each emitter portion 121a is a p-type or n-type impurity portion and is made of amorphous silicon.

The plurality of emitter portions 121a also perform a passivation function together with the first rear protective portion 1921, so that the amount of electric charges dissipated at the rear surface of the substrate 110 due to the defect is reduced, so that the solar cell 12 The efficiency is improved.

A plurality of first buffer electrodes 161a and a plurality of second buffer electrodes 162a positioned on the plurality of emitter portions 121a and on the plurality of rear electric field portions 172a are respectively front buffers shown in FIGS. 1 and 2. Compared with the electrode 161 and the rear buffer electrode 162, the same except for the formation position and the number of formation.

That is, the plurality of first buffer electrodes 161a collects the corresponding charges from the plurality of emitter units 121a and transfers them to the plurality of first electrodes 141a positioned thereon, and the plurality of second buffer electrodes 162a. Collects the corresponding charges from the plurality of backside electric fields 172a and transfers the corresponding charges to the plurality of second electrodes 151a positioned thereon.

At this time, the first buffer electrode 161a and the second buffer electrode 162a, which are conductors, have a corresponding charge according to the conductivity type of the semiconductor, that is, the emitter portion 121a and the rear electric field portion 172a, as described above. In order to facilitate the movement of one of the holes and the charge, the work function is used to change the energy band of the semiconductor

Therefore, as described above, when the semiconductor is in contact with the p-type impurity portion (emitter portion 121a or rear electric field portion 172a) (work function: about 4.9 eV to 5.9 eV), the conductor (first buffer electrode 161a) or The lower the energy band of the p-type impurity portion in contact with the second buffer electrode 162a, the easier the hole movement to the contacting conductor (the first buffer electrode 161a or the second buffer electrode 162a). 161a or the second buffer electrode 162a is equal to or larger than the work function of the p-type impurity portion.

In addition, when the semiconductor is in contact with the n-type impurity portion (rear electric field portion 172a or emitter portion 121a) (work function: about 4.1 eV to 4.6 eV), the conductor (second buffer electrode 162a or first) is used. As the energy band of the n-type impurity portion in contact with the buffer electrode 161a increases, electron movement to the contacting conductor (the second buffer electrode 162a or the first buffer electrode 161a) is easier, and thus the second buffer electrode 162a ) And the first buffer electrode 161a are equal to or smaller than the work function of the n-type impurity portion.

Therefore, when the p-type impurity portion is in contact, the first and second buffer electrodes 161a and 162a may be about 5.0 eV to 6.4 eV, and gold (Au), nickel (Ni), platinum (Pt), and chromium may be used. (Cr), copper (Cu), tungsten (W), and at least one selected from the group consisting of oxides thereof.

In addition, when the n-type impurity portion is in contact with each other, the first and second buffer electrodes 161a and 162a may be about 3.7 eV to 4.3 eV, and may include magnesium (Mg), silver (Ag), aluminum (Al), and titanium. (Ti) and oxides thereof.

In this case, since the thicknesses of the first and second buffer electrodes 161a and 162a are located on the rear surface of the substrate 110, they may be 1 nm to 100 nm.

In this example, the first buffer electrode 161a and the second buffer electrode 162a are formed of impurity portions of different conductivity types (eg, the p-type emitter portion 121a and the n-type rear electric field portion 172a or the n-type emi). The contact portion 121a and the p-type backside electric field portion 172a]. In this example, the first buffer electrode 161a and the second buffer electrode 162a have different work functions and are made of different materials. have.

By the plurality of first buffer electrodes 161a and the plurality of second buffer electrodes 162a, the plurality of emitter portions 121a, the plurality of rear electric field portions 172a, and the plurality of first buffer electrodes 161a, respectively. Ohmic contact is formed between the second buffer electrode 162a and the plurality of second buffer electrodes 162a, thereby improving conductivity between the plurality of emitter portions 121a and the plurality of rear electric field portions 172a. The amount of charge transfer to the first buffer electrode 161a and the plurality of second buffer electrodes 162a is improved.

Since the first and second buffer electrodes 161a and 162a contain a metal, light passing through the substrate 110 is reflected toward the substrate 110.

The plurality of first electrodes 141a positioned on the plurality of emitter portions 121a and the plurality of second electrodes 151a positioned on the plurality of rear electric field portions 172a differ only in the formation position and the number thereof. 2 performs the same function as the front electrode 140 and the rear electrode 151.

Accordingly, the plurality of first electrodes 141a positioned on the plurality of emitter portions 121a extend along each of the emitter portions 121a and the plurality of second electrodes 151a positioned on the plurality of rear electric field portions 172a. Extends along each rear electric field 172a.

Accordingly, the plurality of first and second electrodes 141a and 151a may be connected to the plurality of emitter portions 121a and the plurality of rear electric field portions 172a through the plurality of first and second buffer electrodes 161a and 162a, respectively. Electrically connected.

As a result, each of the first electrodes 141a moves toward the corresponding emitter portion 121a to collect charges transferred through the first buffer electrode 161a, for example, holes, and each second electrode 151a The charge moves toward the rear electric field 172a and collects charges transferred through the second buffer electrode 162a, for example, electrons.

The plurality of first and second electrodes 141a and 151a contain a metallic material such as aluminum (Al) or silver (Ag), but nickel (Ni), copper (Cu), tin (Sn), and zinc (Zn). ), At least one conductive metal material selected from the group consisting of indium (In), titanium (Ti), gold (Au), and combinations thereof or other conductive metal materials.

As described above, since the plurality of first and second electrodes 141a and 151a are made of a metal material, the plurality of first and second electrodes 141a and 151a respectively form the first and second buffer electrodes 161a and 162a. Reflected light is reflected toward the substrate 110.

Since the first and second buffer electrodes 161a and 162a similarly contain a metal material and can collect charges from the emitter portion 121a and the rear electric field portion 172a and output them to an external device. Alternatively, the first and second electrodes 141a and 151a may be omitted as necessary.

When the first and second electrodes 141a and 151a are omitted, the manufacturing cost and manufacturing time of the solar cell 12 are shortened, and when the first and second electrodes 141a and 151a are present, the conductivity is improved. The amount of charge output from the emitter unit 121a and the rear electric field unit 172a to the external device increases, respectively, and the first and first buffer electrodes 161a and 162a may be protected from oxygen or contaminants.

8 and 9, a plurality of first buffer electrodes 161a, a plurality of first electrodes 141a, and a plurality of backside electric fields 172a positioned on the plurality of emitter portions 121a, respectively. The two buffer electrodes 162a and the plurality of second electrodes 151a coincide with portions of the plurality of emitter portions 121a and the plurality of rear electric field portions 172a, respectively. Accordingly, at least one of each of the first buffer electrode 161a and each of the first electrodes 141a may be disposed on the entire upper surface of each emitter portion 121a disposed below and have the same planar shape as the corresponding emitter portion 121a. Can be.

In addition, at least one of each of the second buffer electrodes 162a and each of the second electrodes 151a may also be disposed on the entire upper surface of each of the rear electric field parts 172a located at the bottom, and may have the same planar shape as the corresponding rear electric field parts 172a. Can have

In addition, each of the first buffer electrodes 161a, each of the first electrodes 141a, and each of the second buffer electrodes 162a and each of the second electrodes 151a may have the same planar shape.

In this case, since the contact area is reduced by increasing the contact area between the lower layer and the upper layer, the conductivity between the lower layer and the upper layer is improved.

8 and 9, the widths of the emitter unit 121a and the rear electric field unit 172a are the same, but may be different from each other.

For example, when the width of the p-type impurity portion, which is one of the emitter portion 121a and the rear electric field portion 172a, is large, the collection amount of holes having a slower moving speed than the electrons increases, resulting in a difference in the moving speed of the electrons and the holes. The problem caused is reduced or prevented.

In the solar cell 12 having such a structure, in addition to the effect of the solar cell 11 illustrated in FIGS. 1 and 2, since the electrode is not disposed on the front surface of the substrate 110 to which light is incident, the solar cell 12 is provided. The incident area of is increased than that of the solar cell 11 shown in Figs. For this reason, since the amount of light incident on the substrate 110 increases, the efficiency of the solar cell 12 is further improved.

In addition, similar to the solar cell 11 of FIGS. 1 and 2, the first and second buffer electrodes 161a and 162a of the conductor contacting the emitter portion 121a and the rear electric field portion 172a, which are semiconductors, Since the amount of charge that moves from the emitter portion 121a and the backside electric field portion 172a to the first and second buffer electrodes 161a and 162a increases, the efficiency of the solar cell 12 is further improved.

Next, an example of a method of manufacturing the solar cell 12 according to another embodiment of the present invention will be described with reference to FIGS. 10A to 10O. In this case, the same reference numerals are assigned to the same elements as compared with FIGS. 7A to 7F, and detailed description thereof will be omitted.

Referring to FIG. 10A, as shown in FIG. 7A, an etch stop layer 60 made of silicon oxide film (SiOx) or the like is stacked on the rear surface of the substrate 110, and then etched using the etch stop layer 60 as a mask. The entire surface of the substrate 110 on which the barrier layer 60 is not formed is etched to form a texturing surface, and then the etch barrier layer 60 is removed.

Next, as shown in FIG. 10B, a front protective part 191 made of intrinsic amorphous silicon and a first rear protective film 922 are formed on the front surface of the substrate 110 and the rear surface of the substrate 110.

Next, as shown in FIG. 10C, an impurity higher than the substrate 110 using impurity (dopant) of a pentavalent element on the front protective part 191 and the first rear protective film 922 using PECVD or the like. An amorphous silicon layer having a doping concentration is formed to form the front electric field part 171 and the back electric field part 172a. At this time, the formation order of the front electric field unit 171 and the rear electric field unit 172a can be changed.

Next, as shown in FIG. 10D, the etch stop layer 61 is formed on the rear field part 172a, and then the etch stop layer 61 is selectively patterned to form an etch stop layer on a desired portion of the back field part 172a. After the 61 is positioned, as shown in FIG. 10E, the back surface field portion 172a exposed under the etch stop layer 61 as a mask using a wet etching method, a dry etching method, or the like is positioned below it. 1 Remove the rear protective film 922 in order. As a result, a plurality of second backside protection portions 1922 are formed, and a plurality of backside electric fields 172a positioned thereon is completed. In this case, in order to protect the films formed on the entire surface of the substrate 110, an etch stop layer made of silicon oxide (SiOx) may be formed on the front field 171 of the substrate 110.

Next, as shown in FIG. 10F, the same material as that of the first rear passivation layer 922 is formed on the etch stop layer 61 on the rear surface of the substrate 110 exposed by PECVD or the like and on the plurality of rear field portions 172a. A second backside protective film 921 is formed of intrinsic uncut silicon, and in turn an emitter portion 121a made of amorphous silicon and containing impurities of trivalent elements is formed.

Next, referring to FIG. 10G, the etch stop layer 62 is formed on the emitter unit 121a and then selectively patterned to place the etch stop layer 62 at a desired position. Then, as shown in FIG. 10H, the portion of the emitter portion 121a exposed by using the etch stop layer 62 as a mask using a wet etching method, a dry etching method, or the like, and the second rear passivation layer 921 positioned below it. ), The etch stop layers 61 and 62 are also removed. In this case, the method of removing the emitter unit 121a and the second rear protective layer 921 and the method of removing the etch barrier layers 61 and 62 may be the same or different. When the emitter unit 121a, the second rear passivation layer 921 and the etch stop layers 61 and 62 are removed using the same method, only the desired portion may be etched by adjusting etching conditions such as the used etching material or etching time. To be done. This completes the plurality of emitter portions 121a and the plurality of first rear protective portions 1921 located below them.

By using such a method, the plurality of emitter portions 121a and the plurality of first rear protective portions 1921 and the plurality of rear electric field portions 172a and the plurality of second rear protective portions 1922 below them. Is formed, the plurality of emitter portions 121a and the plurality of first rear protective portions 1921 and lower portions of the rear electric field portions 172a and the plurality of second rear protective portions 1922 thereunder. The order of formation can be changed.

Next, as shown in FIG. 10I, a film formation prevention layer 63 made of silicon oxide (SiOx) or the like is formed on the exposed rear surface of the substrate 110, the plurality of emitter portions 121a, and the plurality of rear electric field portions 172a. Thereafter, as shown in FIG. 10J, at least a portion of each emitter portion 121a is exposed by selectively removing the anti-film formation film 63 using photolithography or the like.

Next, referring to FIG. 10K, after the first buffer electrode film 160 is formed using a sputtering method or a vapor deposition method, the deposition preventing film 63 and a portion of the first buffer electrode film 160 disposed thereon are removed. As a result, the plurality of first buffer electrodes 161a are formed on the plurality of emitter portions 121a.

Next, as shown in FIGS. 10I to 10K, after the deposition prevention film 64 is formed on the rear surface of the substrate 110, a part of the deposition prevention film 64 is removed to remove the portion of the plurality of rear electric field parts 172a. After exposing at least a portion, a second buffer electrode film (not shown) is formed, and then, the deposition prevention film 64 and the lower second buffer electrode film are removed to remove the plurality of second electrodes on the plurality of rear electric field parts 172a. The buffer electrode 162a is formed (FIGS. 10M to 10N).

In this example, the first and second buffer electrodes 161a and 162a may have a thickness of about 1 nm to 100 nm.

As described above, the first and second front buffer electrodes 161a and 162a are made of a material having a work function determined according to the conductivity type of the semiconductor, ie, the emitter portion 121a and the rear electric field portion 172a, which are in contact with each other.

Therefore, when the emitter unit 121a or the rear electric field unit 172a is p-type, the first buffer electrode 161a or the second buffer electrode 162a may be larger than the work function of the emitter unit 121a or the rear electric field unit 172a. Have a work function equal to or greater than, for example, 5.0 eV to 6.4 eV. To this end, the first and second buffer electrodes 161a and 162a may be formed of gold (Au), nickel (Ni), platinum (Pt), chromium (Cr), copper (Cu), tungsten (W), and oxides thereof. It may be made of at least one selected from the group consisting of.

On the other hand, when the emitter unit 121a or the rear electric field unit 172a is n-type, the first and second buffer electrodes 161a and 162a are equal to or larger than the work function of the emitter unit 121a or the rear electric field unit 172a. It has a small work function, for example, a work function of 3.7 eV to 4.3 eV. To this end, the first and second buffer electrodes 161a and 162a may be formed of at least one selected from the group consisting of magnesium (Mg), silver (Ag), aluminum (Al), titanium (Ti), and oxides thereof.

As described above, the first and second buffer electrodes 161a and 162a, which are conductors, are in contact with semiconductors of different conductivity types (that is, the emitter portion 121a and the rear electric field portion 172a). The buffer electrodes 161a and 162 have different work functions and are made of different conductors (materials).

Next, as shown in FIG. 10O, an electrode paste is applied on the plurality of first buffer electrodes 161a and the plurality of second buffer electrodes 162a by using screen printing, and then heat treated. As a result, a plurality of first electrodes 141a extending along the plurality of first buffer electrodes 161a and a plurality of second electrodes 151a extending along the plurality of second buffer electrodes 162a are formed. At this time, the electrode paste contains a conductive material such as silver (Ag) or aluminum (Al).

Then, the anti-reflective portion 130 is formed on the front surface electric field portion 171 positioned on the front surface of the substrate 110 by using PECVD or sputtering, to complete the solar cell 12 (FIGS. 8 and 9). ).

In another example, when the etch stop layer, which is an insulating layer, is disposed on the front surface field part 171 of the substrate 110, the anti-reflective part 130 is formed after removing the etch stop layer. As such, when positioned in the insulating film on the front surface of the substrate 110, the front protective portion 191 and the front surface of the front surface of the substrate 110 from the processes performed to form the components located on the rear surface of the substrate 110 Since the electric field unit 171 is protected, deterioration of the front electric field unit 171 and the front protection unit 191 is prevented, and thus, the film characteristics of the front electric field unit 171 and the front sign unit 191 are prevented. Advantageously maintained.

Unlike the present example, the anti-reflection unit 130 can be changed in order of formation.

Although the preferred embodiments of the present invention have been described in detail above, the scope of the present invention is not limited thereto, and various modifications and improvements of those skilled in the art using the basic concepts of the present invention defined in the following claims are also provided. It belongs to the scope of rights.

110: substrate 121, 121a: emitter portion
130: antireflection portion 140: front electrode portion,
142: front busbars 141, 141a, 151, 151a: electrodes
161, 162, 161a, and 162a: buffer electrodes 171, 172, and 172a: electric field part
191: front guard 192, 192a: rear guard
1921, 1922: rear protective part

Claims (27)

a substrate having a conductivity type of p-type or n-type and composed of a crystalline semiconductor,
At least one impurity portion disposed on the substrate and having a p-type or n-type conductivity type and composed of an amorphous semiconductor, and
At least one buffer electrode disposed on the at least one impurity portion and made of a conductive material
Including,
When the at least one impurity portion is p-type, the at least one buffer electrode has a work function equal to or greater than the at least one impurity, and when the at least one impurity portion is n-type, the at least one buffer electrode is the impurity Has a work function equal to or less than
Solar cells. In claim 1,
When the at least one impurity portion is p-type, the at least one buffer electrode has a work function of 5.0eV to 6.4eV. The method of claim 1 or 2,
When the at least one impurity portion is p-type, when the at least one buffer electrode is positioned on the incident surface of the substrate to which light is incident, copper (Cu) oxide, nickel (Ni) oxide, or tungsten (W) oxide Consisting of solar cells. The method of claim 1 or 2,
When the at least one impurity portion is p-type, when the at least one buffer electrode is located on the surface of the substrate opposite to the incident surface of the substrate to which light is incident, gold (Au), nickel (Ni), A solar cell comprising at least one selected from the group consisting of platinum (Pt), chromium (Cr), copper (Cu), tungsten (W) and oxides thereof. In claim 1,
When the at least one impurity portion is n-type, the at least one buffer electrode has a work function of 3.7eV to 4.3eV. The method of claim 1 or 5,
When the at least one impurity portion is n-type, when the at least one buffer electrode is located on the incident surface of the substrate to which light is incident, the solar cell made of magnesium (Mg) oxide or titanium (Ti) oxide. The method of claim 1 or 5,
When the at least one impurity portion is n-type, when the at least one buffer electrode is positioned on the surface of the substrate opposite to the incident surface of the substrate on which light is incident, the at least one buffer electrode is magnesium (Mg), At least one solar cell selected from the group consisting of silver (Ag), aluminum (Al), titanium (Ti) and oxides thereof. A substrate having a first conductivity type and composed of a semiconductor,
An emitter portion disposed on a first surface of the substrate and having a second conductivity type opposite to the first conductivity type and composed of a semiconductor;
An electric field unit located on the first surface of the substrate and having the first conductivity type and made of a semiconductor;
A first buffer electrode positioned on the emitter portion and formed of a first conductor, and
A second buffer electrode on the electric field part and formed of a second conductor
Including,
The first conductor and the second conductor have different work functions
Solar cells. 9. The method of claim 8,
When the first conductivity type is n-type and the second conductivity type is p-type, the first conductor is made of a material for reducing the Fermi level of the emitter portion,
And the second conductor is made of a material that increases the Fermi level of the electric field. 9. The method of claim 8,
When the first conductivity type is p-type and the second conductivity type is n-type, the first conductor is made of a material that increases the Fermi level of the emitter portion, and the second conductor is a Fermi level of the electric field portion. Solar cell consisting of reducing material. 9. The method of claim 8,
When the first conductivity type is n-type and the second conductivity type is p-type, the first buffer electrode has a work function equal to or greater than the work function of the emitter part, and the second buffer electrode is formed in the electric field part. Have a work function equal to or less than the work function,
When the first conductivity type is p-type and the second conductivity type is n-type, the first buffer electrode has a work function equal to or less than the work function of the emitter part, and the second buffer electrode is formed in the electric field part. Have a work function equal to or greater than the work function
Solar cells. In claim 11,
When the first conductivity type is n-type and the second conductivity type is p-type, the first buffer electrode has a work function of 5.0 eV to 6.4 eV, and the second buffer electrode is one of 3.7 eV to 4.3 eV. Has a function,
When the first conductivity type is p-type and the second conductivity type is n-type, the first buffer electrode has a work function of 3.7 eV to 4.3 eV, and the second buffer electrode has a work of 5.0 eV to 6.4 eV. Having a function
Solar cells. In claim 11 or 12
When the first conductivity type is n-type and the second conductivity type is p-type, the first buffer electrode is made of copper (Cu) oxide, nickel (Ni) oxide, or tungsten (W) oxide, and The second buffer electrode is at least one solar cell selected from the group consisting of magnesium (Mg), silver (Ag), aluminum (Al), titanium (Ti) and oxides thereof. In claim 11 or 12
When the first conductivity type is p-type and the second conductivity type is n-type, the first buffer electrode is made of magnesium (Mg) oxide or titanium (Ti) oxide, and the second buffer electrode is gold (Au). At least one selected from the group consisting of nickel (Ni), platinum (Pt), chromium (Cr), copper (Cu), tungsten (W) and oxides thereof. 9. The method of claim 8,
The solar cell of claim 1, further comprising a plurality of first electrodes electrically connected to the first buffer electrode and a second electrode electrically connected to the second buffer electrode. 16. The method of claim 15,
The solar cell of claim 1, further comprising an anti-reflective portion positioned on the second surface of the substrate opposite to the first surface of the substrate. 17. The method of claim 16,
The anti-reflection portion is a solar cell made of a transparent conductive material. 9. The method of claim 8,
The substrate is a crystalline semiconductor, the emitter portion and the electric field portion is a solar cell of the amorphous semiconductor. 9. The method of claim 8,
And the first surface of the substrate is opposite to the incident surface to which light is incident. A substrate having a first conductivity type and composed of a semiconductor,
A plurality of emitter portions disposed on a surface of the substrate and having a second conductivity type opposite to the first conductivity type and composed of a semiconductor;
A plurality of electric field parts disposed on the surface of the substrate and having the first conductivity type and made of a semiconductor;
A plurality of first buffer electrodes disposed on the plurality of emitter portions and formed of a first conductor, and
A plurality of second buffer electrodes disposed on the plurality of electric fields and formed of a second conductor different from the first conductor;
Containing
Solar cells. 20. The method of claim 20,
When the first conductivity type is n-type and the second conductivity type is p-type, each of the plurality of first buffer electrodes has a work function equal to or greater than a work function of each of the plurality of emitter portions, and the plurality of Each of the second buffer electrodes has a work function equal to or less than a work function of each of the plurality of electric fields,
When the first conductivity type is p-type and the second conductivity type is n-type, each of the plurality of first buffer electrodes has a work function equal to or less than a work function of each of the plurality of emitter portions, and the plurality of Each of the second buffer electrodes has a work function equal to or greater than a work function of each of the plurality of electric fields.
Solar cells. 22. The method of claim 21,
When the first conductivity type is n-type and the second conductivity type is p-type, the first buffer electrode has a work function of 5.0 eV to 6.4 eV, and the second buffer electrode is one of 3.7 eV to 4.3 eV. Has a function,
When the first conductivity type is p-type and the second conductivity type is n-type, the first buffer electrode has a work function of 3.7 eV to 4.3 eV, and the second buffer electrode has a work of 5.0 eV to 6.4 eV. Having a function
Solar cells. In claim 21 or 22
When the first conductivity type is n type and the second conductivity type is p type, the plurality of first buffer electrodes may include gold (Au), nickel (Ni), platinum (Pt), chromium (Cr), and copper ( Cu), tungsten (W), and at least one selected from the group consisting of oxides, and the plurality of second buffer electrodes include magnesium (Mg), silver (Ag), aluminum (Al), titanium (Ti), and At least one solar cell selected from the group consisting of oxides thereof. The method of claim 21 or 22,
When the first conductivity type is p-type and the second conductivity type is n-type, the plurality of first buffer electrodes may include magnesium (Mg), silver (Ag), aluminum (Al), titanium (Ti), and the like. At least one selected from the group consisting of oxides, wherein the plurality of second buffer electrodes are gold (Au), nickel (Ni), platinum (Pt), chromium (Cr), copper (Cu), tungsten (W) and At least one solar cell selected from the group consisting of oxides thereof. 20. The method of claim 20,
And a plurality of first electrodes positioned on the plurality of first buffer electrodes and a plurality of second electrodes positioned on the plurality of second buffer electrodes. 20. The method of claim 20,
The substrate is a crystalline semiconductor, and the plurality of emitter portions and the plurality of electric field portions are formed of an amorphous semiconductor. 20. The method of claim 20,
And the face of the substrate is opposite the face of the substrate to which light is incident.

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