KR101395028B1 - Tandem Solar Cell and Fabricating Method Thereof - Google Patents

Abstract

An embodiment of the present invention relates to a tandem solar cell and a method of manufacturing the same, wherein a tandem solar cell according to an embodiment of the present invention includes a substrate; At least one of a first gas and a second gas is formed of a conductive material that is chemically reacted with a catalyst and includes a first gas including a conductive material and a material different from the conductive material, A doping layer in which a doping concentration of a conductive material to be grown on the substrate is different according to a change in a flow rate of the doping layer; A buffer layer formed on the doping layer and reducing a stress applied to the growth layer due to a difference in lattice constant between the doping layer and the doping layer in a subsequent process subsequent to the doping layer; And a PN junction layer formed on the buffer layer.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a tandem solar cell,

The present invention relates to a tandem type solar cell and a manufacturing method thereof, and more particularly, to a tandem type solar cell and a method of manufacturing the same using a silicon substrate, Type PN junction on a substrate, and a method of manufacturing the tandem type solar cell.

A solar cell is a device that turns solar light energy into electrical energy. Such a solar cell can be referred to as a "physical cell" having a different structure from the conventional chemical cell. Solar cells generate electricity using two kinds of semiconductors called P-type and N-type. If we look more closely at the principle, when we shine light on a solar cell, electrons and holes are generated inside. The generated electric charges move to the positive and negative electrodes, and a potential difference (or photovoltaic power) is generated between the positive and negative electrodes by this phenomenon. When the load is connected to the solar cell, current flows. This is called photoelectric effect.

Solar cells are largely divided into those using silicon semiconductors and those using compound semiconductors. Silicon semiconductors are again classified as crystalline and amorphous. Of course, if you include what you are currently developing, it is more diverse. Regarding the technology development of solar cells, improvement of conversion efficiency and price adjustment are planned. In addition, solar cells exceeding 20% conversion efficiency and thin film solar cells capable of lowering prices are being developed.

As described above, the silicon solar cell, which is the most widely used semiconductor material for solar cells, is being actively developed and is being mass-produced mainly in polycrystalline silicon. Accordingly, various attempts have been made to increase the photoconversion efficiency of solar cells. In addition, research on solar cell technology through light of ultraviolet ray and infrared band as well as light of visible light band using materials such as gallium arsenide (GaAs) having a wide light energy absorption band is underway.

However, even in the case of a solar cell using a silicon single crystal having the best performance, the maximum conversion efficiency is only about 25%, which is pointed out as a limitation of a solar cell that can be made of silicon.

Silicon-based solar cell technologies and products such as single crystal, polycrystalline, amorphous and thin film solar cells are being actively produced, but compound solar cells still have a market share of less than 10% of solar cell technology There is a problem that the compound semiconductor materials are expensive compared with the silicon of the same area and the technology of the device manufacturing process is not well established yet.

In the embodiment of the present invention, in order to manufacture a solar cell using a high-efficiency tandem system that simultaneously absorbs light of two bands, i.e., an ultraviolet region and a visible light region, a PN junction of silicon and nitride system is formed using MOCVD in a chamber A tandem type solar cell having a high conversion efficiency and performance and a low manufacturing cost, and a method of manufacturing the same.

A tandem solar cell according to an embodiment of the present invention includes a substrate; A first gas comprising a conductive material and a second gas comprising a substance of a property different from that of the conductive material, the conductive material being chemically reacted with a catalyst to be grown on the substrate, A doping layer in which a doping concentration of the conductive material grown on the substrate varies according to a change in the flow rate of at least one of the two gases; A buffer layer formed on the doping layer to reduce stress applied to the growth layer due to a difference in physical properties between the doping layer and the doping layer during a subsequent process subsequent to the doping layer; And a PN junction layer formed on the buffer layer.

Wherein the substrate is an N-type silicon substrate and the doping layer is a P-type silicon single crystal or polycrystalline layer, wherein the N-type silicon substrate and the P-type silicon single crystal or polycrystalline layer form a silicon PN junction.

Wherein the buffer layer is formed of a conductive material and the conductive material is selected based on at least one of a lattice constant and a thermal expansion coefficient between the silicon PN junction and the PN junction layer.

The buffer layer has two or more layers.

The PN junction layer includes an N-type layer and a P-type layer which are sequentially stacked on the buffer layer, and the N-type layer and the P-type layer are formed of a nitride-based material.

The doping layer is characterized in that the surface is non-planar.

Wherein the conductive material is any one of aluminum (Al), gallium (Ga), boron (B), and arsenic (As), and the other material is silicon (Si).

A method of manufacturing a tandem solar cell according to an embodiment of the present invention includes: preparing a substrate; Chemically reacting a first gas comprising a conductive material and a second gas comprising a material of a nature different from the conductive material with a catalyst to form the conductive material as a doping layer on the substrate; Forming a buffer layer on the doped layer in order to reduce a stress applied to the grown layer due to a difference in physical characteristics between the doped layer and the doped layer; And forming a PN junction layer on the buffer layer.

The step of growing the conductive material as a doping layer may vary the flow rate of at least one of the first gas and the second gas to vary the doping concentration of the conductive material to grow the conductive material.

The forming of the buffer layer is characterized in that the buffer layer is formed to have two or more layers.

The forming of the PN junction layer includes: forming an N-type layer on the buffer layer; And forming a P-type layer on the N-type layer.

And the N-type layer and the P-type layer are formed of a nitride-based material.

The step of growing the conductive material as a doping layer may include the steps of using any one of aluminum (Al), gallium (Ga), boron (B), and arsenic (As) as the conductive material, ) Is used.

According to the embodiment of the present invention, a solar cell can be manufactured in a tandem manner, and it can have high efficiency and high performance characteristics. For example, by using MOCVD, it is possible to provide an inexpensive device through a short process time and a simplified manufacturing process do.

As a result, the solar cell using the tandem-type technology can be applied not only to the high-efficiency solar cell for civil use but also because it can be used as the power source for the satellite etc. in the space. It will be possible.

Furthermore, by providing a tandem type solar cell that absorbs both wavelengths of infrared / visible light or visible / ultraviolet light simultaneously, it can be provided with appropriate productivity, which not only can help problems related to future energy, It is possible to reduce the burden on the user.

1 is an outline view of a tandem type solar cell according to an embodiment of the present invention,
2 is a sectional view of a tandem type solar cell according to an embodiment of the present invention,
3 is a graph showing characteristics of a tandem type solar cell,
4A to 4D are views illustrating a manufacturing process of a tandem type solar cell according to an embodiment of the present invention.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a schematic diagram of a tandem type solar cell according to an embodiment of the present invention, and FIG. 2 is a diagram illustrating a cross-sectional structure of a tandem type solar cell according to an embodiment of the present invention.

As shown in FIGS. 1 and 2, a solar cell according to an embodiment of the present invention is a tandem solar cell having a heterojunction structure, for example, a silicon substrate, and a gallium nitride GaN) is formed on a silicon substrate to form a tandem type solar cell (AC). Here, tandem indicates a method of providing a high open-circuit voltage and a high-efficiency high-efficiency by combining a junction made of two different materials into a two-dimensional structure.

The tandem type solar cell according to the embodiment of the present invention includes a part or all of the substrate 200, the doping layer 210, the buffer layer 220 and the PN junction layer 230 as shown in FIG. 2, Further, it may further include upper and lower electrodes (not shown).

Here, the substrate 200 is, for example, an N-type silicon substrate, and a doped layer 210 such as a P-type single crystal or polycrystalline silicon is grown on an N-type silicon substrate. Thus, a tandem type solar cell will complete a silicon PN junction consisting of a silicon substrate and a P-type single crystal or polycrystalline silicon. Such a process is preferably performed before the crystal of the PN junction layer 230, such as the GaN layer, is grown on the N-type silicon substrate in the MOCVD chamber. This may be similar to depositing aluminum (Al), gallium (Ga), boron (B), arsenic (As) or the like on the surface and then doping it at high temperature growth.

Here, MOCVD is a chemical vapor deposition (CVD) in which a raw material gas flows out on a substrate at a high temperature to cause a decomposition reaction on the surface of the substrate to form a thin film, and includes an organic metal complex such as trimethyl gallium in the raw material gas. It is operated at a lower temperature than CVD using a halide gas, and thin film control at atomic order is possible. It is also one of the technologies to make thin films of electronic materials such as GaAs.

The tandem type solar cell according to the embodiment of the present invention is a tandem type solar cell according to the present invention in which a gas containing aluminum (Al) or the like (TMAI) and silicon (Si) such as silane (SiH ₄ ) and monoclosilane (SiH ₃ Cl) And the doping layer 210 of P-type silicon doped at a high concentration is grown through chemical reaction of other catalysts for reacting with the two gases on the N-type silicon substrate to finally complete the PN junction made of silicon.

Substantially, such a doping layer 210 analyzes the doping profile through SIMS (Second Ion Mass Spectroscopy) to determine how the doping concentration of the grown P-type layer varies with the change in the gas flow rate injected into the chamber, Can be obtained by performing the process of finding the optimal growth condition while changing the temperature condition and the pressure condition to obtain the best quality PN junction in the condition.

In order to realize a tandem-type solar cell, for example, a PN junction formed of silicon is first formed. In order to form a P-type region on an N-type substrate, an aluminum (Al) Type silicon doped with aluminum (Al), gallium (Ga), boron (B), arsenic (As) or the like by injecting a gas containing silicon and boron (B) Layer 210. The thin P-type layer thus grown exhibits the characteristics of a thin film having conductivity close to that of the metal. This is because the pre-formed PN junction through the conventional ion implantation is exposed to the high-temperature process for MOCVD 1000 ° C or more for GaN growth, so that carriers are diffused between the junctions, thereby avoiding problems such as damage to the junction itself.

On the other hand, a buffer layer 220 is formed on the doping layer 210 of the substrate 200. The buffer layer 220 may be formed to prevent damage to the PN junction layer 230, for example, the doping layer 210 during GaN growth. In other words, if the PN junction layer 230 such as GaN is grown without the buffer layer 220 on the doping layer 210, the doping layer 210 may be easily damaged.

 With respect to such a buffer layer 220, a suitable buffer layer 220 between the silicon and the GaN should be selected in accordance with the lattice constant and the thermal expansion coefficient, and the optimized thickness must be determined through experimentation. A buffer layer 220 having conductivity for reducing the resistance of the buffer layer 220 may be considered. In other words, the selection of a suitable buffer layer 220 means that the buffer layer 220 of AlN, LT-GaN, AlGaN or the like which is mainly used for the current GaN growth is used and the physical properties such as the lattice constant and the thermal expansion coefficient between silicon and GaN And to mitigate the discrepancy to grow a crack-free GaN layer. Also, in regard to the laminated structure, since it may be difficult to eliminate all the stress applied to the GaN by the single buffer layer 220, it is possible to use a lamination method of several layers.

A PN junction layer 230 is formed on the buffer layer 220. More specifically, the PN junction layer 230 according to the embodiment of the present invention is formed by sequentially stacking an N-type layer 231 and a P-type layer 233 on a buffer layer 220, and a nitride system such as GaN .

In the embodiment of the present invention, for the stable growth of the PN junction layer 230 made of GaN, optimization of process conditions such as growth of a P-type single crystal layer of appropriate thickness and high quality, process temperature and flow rate of injected gases, It is desirable to perform a uniformity test of the joints in the same substrate. In other words, high-resolution X-ray diffraction (XRD) analysis is used to determine the growth of a single crystal to grow a P-type single crystal layer of appropriate thickness and quality, and the silicon surface resistance is measured using a 4-point probe do. In addition, the roughness of the surface is measured with an atomic force microscope (AFM) for stable growth of the GaN layer in the next step. Also, in order to test the uniformity of the junctions in the same substrate, the completeness of the cross section of the grown PN junction was evaluated through an image of a scanning electron microscope (SAM) or a projection electron microscope (TEM) .

Furthermore, the tandem type solar cell according to the embodiment of the present invention may further include upper and lower electrodes (not shown). The upper and lower electrodes may be connected, for example, with the charge of the solar cell, or a load for using electricity in an external device. Accordingly, the upper and lower electrodes may be formed of a conductive material. Although the upper electrode is not shown in detail in the drawings, it may be formed in a shape such as 'E' or 'ㅕ' in order to open as many regions as possible to allow sunlight to be incident, and the lower electrode reflects incident light, It is preferable that it is formed on the whole surface in order to maximize it.

For optimum efficiency of the tandem solar cell, it is necessary to select a metal for resistive contact between the lower electrode formed on the N-type silicon substrate and the upper electrode to be formed on the P-type GaN, optimized heat treatment and surface pretreatment can be performed . Further, a coating method for preventing reflection of light on the GaN surface may be further performed. In addition, there is a possibility that the absorption of light can occur well.

Further, for the optimum efficiency, it is possible to evaluate electrical characteristics, energy conversion efficiency, and the like of the device manufactured through the formation of the vertical type electrode. For this purpose, it is possible to analyze the electrical characteristics through the voltage-current characteristics and to extract the conversion efficiency through the short-circuit current, the open-circuit voltage, and the fill factor. The conversion efficiency of the solar cell is calculated through such analysis and extraction.

FIG. 3 is a graph showing the characteristics of a tandem solar cell. FIG. 3 (a) is a graph showing a red curve of a device manufactured with an n-GaN / pn- FIG. 3 (b) is a graph showing voltage-current characteristics when light is applied in various conditions of a device fabricated in a pn-GaN / pn-Si tandem structure. (c) of FIG. 3 is a graph of pn-1 (blue), 1 sun AM1.5G (green), 20 sun AM1.5G (red), 1 sun AM1.5G + 2 is a graph showing an external quantum efficiency spectrum according to a wavelength change of an applied light of a GaN / pn-Si tandem type solar cell.

When the light of 1 sun AM 1.5G is applied, the open-circuit voltage is 555 mV, the short-circuit current density is 22.9 mA / cm ² , the charging rate is 73% , And a conversion efficiency of about 9.3%. When a solar cell is fabricated in a tandem structure, when a voltage of 1 sun AM 1.5G is applied from the voltage-current characteristics shown in FIG. 3 (b) open-circuit voltage 1.44 V, the short circuit current density of 0.16 mA / cm ^2, and the charging rate 40%, 20 sun and the open-circuit voltage 2.26 V, the short circuit current density of 6.14 mA / cm ² when the light is AM1.5G, 1 sun AM1. When the ultraviolet laser beam of 325 nm wavelength was applied in addition to the light of 5G, the open voltage was 2.4 V, the short circuit current density was 7.3 mA / cm ² , and the charging rate was 59%.

On the basis of this, it can be seen that the open-circuit voltage characteristic of the tandem type, such as MBE (Molecular Beam Epitaxy) or MOCVD, is significantly improved compared with the case of the tandem type. However, We can see that many improvements still need to be made. In addition, tandem-type solar cell technology, which is currently being produced and announced, has high efficiency, but it is difficult to lower the manufacturing cost because the time and cost consumed in the manufacturing process are not suitable for production of low cost devices.

From this point of view, the process technology using MOCVD like the embodiment of the present invention has a great advantage in terms of price competitiveness due to productivity because the process speed is fast. In other words, a tandem solar cell using the MBE method has a merit that crystal growth can be performed at a low temperature, but it is known that the mass productivity is considerably lacking. However, the method using MOCVD ensures mass productivity. For this, in the embodiment of the present invention, a P-type doped layer is formed on an N-type silicon substrate in an MOCVD chamber, thereby manufacturing a MOCVD tandem type solar cell having excellent mass productivity.

4A to 4D are views illustrating a manufacturing process of a tandem type solar cell according to an embodiment of the present invention.

In order to manufacture a tandem solar cell according to an embodiment of the present invention, a step of preparing a substrate 200 is first performed as shown in FIG. 4A. At this time, the preparation of the substrate 200 may include a step of injecting the substrate 200 into the chamber to perform the MOCVD process. Here, the substrate 200 may be a N-type silicon substrate, for example, a wafer or a glass substrate.

A gas containing a conductive material such as aluminum (Al), gallium (Ga), boron (B), and arsenic (As) and silicon is injected into the chamber in which the substrate 200 is prepared. At this time, the gas injected and the catalyst for chemically reacting with the injected gas may be simultaneously injected. Further, the doping concentration of the doping layer 210 may be adjusted by changing the flow rate of the gas.

As a result, a gas containing a conductive material and a gas containing silicon are chemically reacted with the catalyst in the chamber, so that a thin P-type silicon doped layer, such as aluminum (Al) (210) is grown. Here, the grown thin P-type layer, that is, the doped layer 210, forms a thin film having a conductivity close to that of a metal.

A buffer layer 220 is formed on the doping layer 210 as shown in FIG. 4C. At this time, the buffer layer 220 preferably stacks two or more layers. The buffer layer 220 is formed between the lower doping layer 210 and the PN junction layer 230 to be formed later to serve as a buffer. That is, the PN junction layer 230 serves to reduce the stress applied to the doping layer 210. In order to increase the buffering efficiency, it is preferable to form a thin multilayer structure rather than a thick monolayer. Also, the buffer layer 220 preferably has conductivity in order to prevent the resistance from increasing and obstructing the current flow.

A PN junction layer 230 is formed on the buffer layer 220 as shown in FIG. 4D. The PN junction layer 230 includes an N type layer 231 grown on the buffer layer 220 and a P type layer 233 grown on the N type layer 231. At this time, it is preferable that the N-type layer 231 and the P-type layer 233 are made of a nitride-based material and have a bonding structure different from that of the underlying silicon PN junction. It can be understood that this is only for absorbing the wavelengths of the two bands at the same time.

Although not shown in the drawing, the lower electrode and the upper electrode may be formed on the P-type layer 233 forming the PN junction layer 230 and the lower portion of the substrate 200, respectively. In this case, the upper and lower electrodes may be selected in consideration of resistance as a metal, heat treatment conditions, and surface pretreatment. For example, a coating layer or the like may be additionally formed on the GaN surface of the P- There will be.

In this process, for example, the lower electrode may be formed on the lower surface of the substrate 200, but the upper electrode may have various shapes to absorb as much sunlight as possible. For example, the upper electrode may be formed by using a separate device, a printing method, a photolithography process, or the like in the chamber.

While the invention has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention.

200: substrate 210: doped layer
220: buffer layer 230: PN junction layer
231: N-type layer 233: P-type layer

Claims (13)

Board;
A first gas comprising a conductive material and a second gas comprising a substance of a property different from that of the conductive material, the conductive material being chemically reacted with a catalyst and grown on the substrate, A doping layer in which a doping concentration of the conductive material grown on the substrate varies according to a change in the flow rate of at least one of the two gases;
A buffer layer formed on the doping layer to reduce stress applied to the growth layer due to a difference in physical properties between the doping layer and the doping layer during a subsequent process subsequent to the doping layer; And
The PN junction layer formed on the buffer layer
≪ / RTI &
Wherein the substrate is an N-type silicon substrate and the doping layer is a P-type silicon single crystal or polycrystalline layer, wherein the N-type silicon substrate and the P-type silicon single crystal or polycrystalline layer form a silicon PN junction. battery. delete The method according to claim 1,
Wherein the buffer layer is formed of a conductive material and the conductive material forming the buffer layer is selected based on at least one of a lattice constant and a thermal expansion coefficient between the silicon PN junction and the PN junction layer. battery. The method according to claim 1,
Wherein the buffer layer has two or more layers. The method according to claim 1,
Wherein the PN junction layer includes an N-type layer and a P-type layer which are sequentially stacked on the buffer layer, and the N-type layer and the P-type layer are made of a nitride-based material. The method according to claim 1,
Wherein the doped layer has a non-planar surface. The method according to claim 1,
Wherein the conductive material is any one of aluminum (Al), gallium (Ga), boron (B), and arsenic (As), and the other material is silicon (Si). Preparing a substrate;
Growing a doping layer on the substrate by chemically reacting a first gas comprising a conductive material and a second gas comprising a material of a property different from the conductive material with a catalyst;
Forming a buffer layer on the doped layer in order to reduce a stress applied to the grown layer due to a difference in physical characteristics between the doped layer and the doped layer; And
Forming a PN junction layer on the buffer layer
≪ / RTI &
Wherein the substrate is an N-type silicon substrate and the doping layer is a P-type silicon single crystal or polycrystalline layer, wherein the N-type silicon substrate and the P-type silicon single crystal or polycrystalline layer form a silicon PN junction. Gt; 9. The method of claim 8,
The step of growing the doped layer comprises:
Wherein the doping concentration of the conductive material is varied by varying a flow rate of at least one of the first gas and the second gas to grow the doped layer. 9. The method of claim 8,
Wherein the buffer layer is formed such that the buffer layer has two or more layers. 9. The method of claim 8,
Wherein forming the PN junction layer comprises:
Forming an N-type layer on the buffer layer; And
Forming a P-type layer on the N-type layer;
Wherein the tandem-type solar cell is a solar cell. 12. The method of claim 11,
Wherein the N-type layer and the P-type layer are formed of a nitride-based material. 9. The method of claim 8,
The step of growing the doped layer comprises:
Characterized in that any one of aluminum (Al), gallium (Ga), boron (B) and arsenic (As) is used as the conductive material and silicon (Si) Gt;

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