# Patent

## Title
Photovoltaic device including flexible substrate or inflexible substrate and method for manufacturing the same

## Abstract

Disclosed is a photovoltaic device. The photovoltaic device includes: a substrate; a first electrode placed on the substrate; a second electrode which is placed opposite to the first electrode and which light is incident on; a first unit cell being placed between the first electrode and the second electrode, and including an intrinsic semiconductor layer including crystalline silicon grains making the surface of the intrinsic semiconductor layer toward the second electrode textured; and a second unit cell placed between the first unit cell and the second electrode.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims priority under 35 U.S.C. §119 to Korean Patent Application No. 10-2010-0027280 filed on Mar. 26, 2010, the entirety of which is hereby incorporated by reference.
FIELD OF THE INVENTION
The present invention relates to a photovoltaic device including a flexible substrate or an inflexible substrate and a method for manufacturing the same.
BACKGROUND OF THE INVENTION
Recently, because of a high oil price and a global warming phenomenon based on a large amount CO2 emission, energy is becoming the most important issue in determining the future life of mankind. Even though many renewable energy technologies including wind force, bio-fuels, a hydrogen/fuel cell and the like have been developed, a photovoltaic device using sunlight is in the spotlight in that solar energy, the origin of all energies, is an almost infinite clean energy source.
The sunlight incident on the surface of the earth has an electric power of 120,000 TW. Thus, theoretically, if a photovoltaic device having a photoelectric conversion efficiency of 10% covers only 0.16% of the land surface of the earth, it is possible to generate electric power of 20 TW that is twice as much as the amount of energy globally consumed during one year.
Actually, the world photovoltaic market has explosively grown by almost 40% of an annual growth rate for the last ten years. Now, a bulk-type silicon photovoltaic device occupies a 90% of the photovoltaic device market share. The bulk-type silicon photovoltaic device includes a single-crystalline silicon photovoltaic device and a multi-crystalline or a poly-crystalline silicon photovoltaic device and the like. However, productivity of a solar-grade silicon wafer which is the main material of the photovoltaic device is not able to fill the explosive demand thereof, so the solar-grade silicon wafer is globally in short supply. Therefore, this shortage of the solar-grade silicon wafer is a huge threatening factor in reducing the manufacturing cost of a photovoltaic device.
Contrary to this, a thin-film silicon photovoltaic device based on a hydrogenated amorphous silicon (a-Si:H) allows to reduce a thickness of a silicon layer equal to or less than 1/100 as large as that of a silicon wafer of the bulk-type silicon photovoltaic device. Also, it makes possible to manufacture a large area photovoltaic device at a lower cost.
Meanwhile, a single-junction thin-film silicon photovoltaic device is limited in its achievable performance. Accordingly, a double junction thin-film silicon photovoltaic device or triple junction thin-film silicon photovoltaic device having a plurality of stacked unit cells has been developed, pursuing high stabilized efficiency.
The double junction or triple junction thin-film silicon photovoltaic device is referred to as a tandem-type photovoltaic device. The open circuit voltage of the tandem-type photovoltaic device corresponds to a sum of each unit cell's open circuit voltage. Short circuit current is determined by a minimum value among the short circuit currents of the unit cells.
Regarding the double junction thin-film silicon photovoltaic device or the triple junction thin-film silicon photovoltaic device, research is being devoted to an electrode surface texture in order to improve efficiency by means of light scattering.
SUMMARY OF THE INVENTION
One aspect of the present invention is a photovoltaic device. The photovoltaic device includes: a substrate; a first electrode placed on the substrate; a second electrode which is placed opposite to the first electrode and which light is incident on; a first unit cell being placed between the first electrode and the second electrode, and including an intrinsic semiconductor layer including crystalline silicon grains making the surface of the intrinsic semiconductor layer toward the second electrode textured; and a second unit cell placed between the first unit cell and the second electrode.
Another aspect of the present invention is a method for manufacturing the photovoltaic device of the present invention. The method includes: forming an n-type semiconductor layer of the first unit cell; forming an intrinsic semiconductor layer of the first unit cell on the n-type semiconductor layer, the intrinsic semiconductor layer including an amorphous silicon based material surrounding crystalline silicon grain; etching the surface of the intrinsic semiconductor layer; and forming a p-type semiconductor layer of the first unit cell on the intrinsic semiconductor layer.



BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1a and 1b show a photovoltaic device according to an embodiment of the present invention.
FIGS. 2a to 2h show a method for manufacturing the photovoltaic device according to the embodiment of the present invention.



DETAILED DESCRIPTION
Hereinafter, a photovoltaic device according to an embodiment of the present invention will be described in detail with reference to the accompanying drawings.
FIG. 1 shows a photovoltaic device according to an embodiment of the present invention. As shown in FIG. 1, the photovoltaic device according to the embodiment of the present invention includes a substrate 100, a first electrode 110, a second electrode 120, a first unit cell 130 and a second unit cell 140.
The substrate 100 is a flexible substrate or an inflexible substrate. The flexible substrate includes either a metal substrate made of stainless steel or aluminum foil, or a plastic substrate made of Polyethylene Naphthalate (PEN) or Poly Ethylene Terephthalate (PET). In the metal substrate, an insulating film is formed on the surface of the metal substrate in order to insulate the metal substrate from an electrode formed on the metal substrate. The inflexible substrate may include a glass substrate.
The first electrode 110 is placed on the substrate 100. In an n-i-p type photovoltaic device, since light is incident through the second unit cell 140, the first electrode 110 adjacent to the substrate 100 may or may not have light transmittance. Therefore, the first electrode 110 may be formed of a metallic material or be formed of a light-transmitting conductive material like transparent conductive oxide (TCO).
The second electrode 120 is placed opposite to the first electrode 110. Light is incident on the second electrode 120. As described above, in the n-i-p type photovoltaic device, since light is incident through the second unit cell 140, light is incident on the second electrode 120 prior to the first electrode 110. Therefore, the second electrode 120 may be formed of a light transmitting conductive material.
The first unit cell 130 is located between the first electrode 110 and the second electrode 120, and includes an intrinsic semiconductor layer 133 including crystalline silicon grains forming a textured surface toward the second electrode 120. The first unit cell 130 includes an n-type semiconductor layer 131, an intrinsic semiconductor layer 133 and a p-type semiconductor layer 135, which are sequentially stacked. Therefore, among the n-type semiconductor layer 131, the intrinsic semiconductor layer 133 and the p-type semiconductor layer 135, the n-type semiconductor layer 131 is the closest to the first electrode 110.
In the embodiment of the present invention, since light is incident on the first unit cell 130 posterior to the second unit cell 140, the first unit cell 130 absorbs more light of a long wavelength than light of a short wavelength. Therefore, in order to easily absorb the light of a long wavelength, the intrinsic semiconductor layer 133 of the first unit cell 130 includes a hydrogenated microcrystalline silicon based material layer. The hydrogenated microcrystalline silicon based material layer is composed of hydrogenated microcrystalline silicon (i-μc-Si:H) or hydrogenated microcrystalline silicon germanium (i-μc-SiGe:H).
The hydrogenated microcrystalline silicon based material layer includes a hydrogenated amorphous silicon based material 133a and crystalline silicon grains 133b surrounded by the hydrogenated amorphous silicon based material 133a. The crystalline silicon grains 133b projects toward the second electrode 120, so that the surface toward the second electrode 120 among the surfaces of the intrinsic semiconductor layer 133 is textured. A method for projecting the crystalline silicon grains 133b toward the second electrode 120 will be described later in detail.
As such, due to the textured surface formed by the crystalline silicon grains 133, the surfaces of an n-type semiconductor layer 141, an intrinsic semiconductor layer 143 and a p-type semiconductor layer 145 of the second unit cell 140 are also textured. Hereby, light incident on the second unit cell 140 may be scattered.
In the n-i-p type photovoltaic device, though the surface of either the substrate 100 or the first electrode 110 is textured, the surface of the second unit cell 140 on which light is incident is not textured enough since the thicknesses of the first unit cell 130 and the second unit cell 140 are large. That is, since the thicknesses of the first unit cell 130 and the second unit cell 140 are large, the surfaces of the first unit cell 130 and the second unit cell 140 can be flattened while the first unit cell 130 and the second unit cell 140 are formed by deposition.
On the contrary, in the photovoltaic device according to the embodiment of the present invention, the crystalline silicon grains of the first unit cell 130 project toward the second unit cell 140. Therefore, the surface of the second unit cell 140 is textured when the second unit cell 140 is formed on the first unit cell 130.
As described above, an average crystal volume fraction of the intrinsic semiconductor layer 133 including the hydrogenated microcrystalline silicon based material layer is equal to or more than 25% and equal to or less than 75%.
When the average crystal volume fraction of the intrinsic semiconductor layer 133 is equal to or more than 25%, an incubation layer including hydrogenated amorphous silicon can be prevented from being formed either between the intrinsic semiconductor layer 133 and the p-type semiconductor layer 135 or between the intrinsic semiconductor layer 133 and the n-type semiconductor layer 131. Accordingly, hole transition or electron transition is easily performed, so that electron-hole recombination is reduced and photoelectric conversion efficiency is improved.
When the average crystal volume fraction of the intrinsic semiconductor layer 133 is equal to or less than 75%, the size of the crystalline silicon grain is prevented from excessively increasing and the volume of grain boundary is hereby prevented from increasing.
The second unit cell 140 is located between the first unit cell 130 and the second electrode 120. The second unit cell 140 includes an n-type semiconductor layer 141, an intrinsic semiconductor layer 143 and a p-type semiconductor layer 145, which are sequentially stacked. Therefore, among the n-type semiconductor layer 141, the intrinsic semiconductor layer 143 and the p-type semiconductor layer 145, the p-type semiconductor layer 145 is the closest to the second electrode 120.
Here, the p-type semiconductor layer 135 of the first unit cell 130 and the p-type semiconductor layer 145 of the second unit cell 140 are doped with group III impurities. The n-type semiconductor layer 131 of the first unit cell 130 and the n-type semiconductor layer 141 of the second unit cell 140 are doped with group V impurities.
Meanwhile, as shown in FIG. 1b, the photovoltaic device according to the embodiment of the present invention further includes an intermediate reflector 150 located between the first unit cell 130 and the second unit cell 140. An average refractive index of the intermediate reflector 150 is equal to or more than 1.7 and is equal to or less than 2.5 at a wavelength of 600 nm. In such a state, refractive index matching occurs. As a result, light of a short wavelength is reflected to the second unit cell 140, so that the photoelectric conversion efficiency of the photovoltaic device is improved.
Next, a method for manufacturing the photovoltaic device according to the embodiment of the present invention will be described in detail with reference to the drawings.
As shown in FIG. 2a, provided is a substrate 100 on which the first electrode 110 is formed by a sputtering method. Before the first electrode 110 is formed, a cleaning process may be performed on the substrate 100. When the first electrode 110 is made of a metallic material, a ZnO layer may be formed between the first electrode 110 and the substrate 100 in order to increase an adhesive strength between the first electrode 110 and the substrate 100. Since the substrate 100 has been described above in detail, the description thereof will be omitted.
As shown in FIG. 2b, the n-type semiconductor layer 131 of the first unit cell 130 is formed on the first electrode 110.
As shown in FIG. 2c, the intrinsic semiconductor layer 133 of the first unit cell 130 is formed on the n-type semiconductor layer 131. Here, the intrinsic semiconductor layer 133 of the first unit cell 130 includes the amorphous silicon based material 133a surrounding the crystalline silicon grains 133b. That is, the intrinsic semiconductor layer 133 of the first unit cell 130 can include a hydrogenated microcrystalline silicon based material layer having a mixed phase. As described above, the average crystal volume fraction of the intrinsic semiconductor layer 133 is equal to or more than 25% and is equal to or less than 75%.
As shown in FIG. 2d, the surface of the intrinsic semiconductor layer 133 is etched. Here, the surface of the intrinsic semiconductor layer 133 is etched by using a dry-etching process such as a hydrogen plasma etching process or an argon plasma etching process. When the photovoltaic device according to the embodiment of the present invention includes a flexible substrate, a chemical etching process transforms or damages the flexible substrate composed of a metallic material or a polymer. However, the dry-etching process has less influence on the flexible substrate than the chemical etching process.
Accordingly, the intrinsic semiconductor layer 133 is selectively etched. In other words, as described above, the intrinsic semiconductor layer 133 includes the crystalline silicon grains and the amorphous silicon based material, which have different etch rates from each other. The amorphous silicon based material is etched more quickly than the crystalline silicon grain under the same condition. Therefore, through the dry-etching process, the crystalline silicon grains surrounded by the amorphous silicon based material are projected. The surface of the intrinsic semiconductor layer 133 is textured due to the crystalline silicon grains projected by the aforementioned method.
Further, hole transport within the intrinsic semiconductor layer 133 is enhanced by removing the amorphous silicon based material. Accordingly, an open circuit voltage of the photovoltaic device and the photoelectric conversion efficiency of the photovoltaic device can be improved.
As shown in FIG. 2e, a passivation layer (PL) having a thickness of about 5 nm is formed on the etched intrinsic semiconductor layer 133. When the surface of the intrinsic semiconductor layer 133 is etched, electron-hole recombination can be increased on the surface of the intrinsic semiconductor layer 133. Therefore, the passivation layer (PL) is formed on the etched surface of the intrinsic semiconductor layer 133. Here, the passivation layer (PL) consists of the hydrogenated microcrystalline silicon based material. Since the intrinsic semiconductor layer 133 also includes the hydrogenated microcrystalline silicon based material, the manufacturing process thereof can be simplified.
As shown in FIG. 2f, the p-type semiconductor layer 135 of the first unit cell 130 is formed on the intrinsic semiconductor layer 133. Here, the p-type semiconductor layer 135 is composed of hydrogenated microcrystalline silicon (p-μc-Si:H).
As shown in FIG. 2g, the n-type semiconductor layer 141, the intrinsic semiconductor layer 143 and the p-type semiconductor layer 145 are sequentially stacked on the first unit cell 130. Since the second unit cell 140 is formed on the first unit cell 130 having the textured surface, the surface of the second unit cell 140 is also textured. The intrinsic semiconductor layer 143 of the second unit cell 140 may be made of hydrogenated amorphous silicon (i-a-Si:H), hydrogenated amorphous silicon carbide (i-a-SiC:H), hydrogenated amorphous silicon oxide (i-a-SiO:H), hydrogenated proto-crystalline silicon (i-pc-Si:H) or hydrogenated proto-crystalline silicon having a multi-layer structure.
As shown in FIG. 2h, the second electrode 120 is formed on the second unit cell 140 by means of a sputtering method or an LPCVD method. Here, the second electrode 120 may be made of indium tin oxide (ITO), SnO2:F or ZnO.
As such, the surface of the second electrode 120 is textured by the crystalline silicon grains 133b. The textured surface of the second electrode 120 has an average pitch equal to or more than 50 nm and equal to or less than 500 nm in order to scatter incident light. As shown in FIG. 2h, the average pitch corresponds to an average of distances “P” between two adjacent convex-up portions of the textured surface. The average pitch of the second electrode on which light is incident is equal to or more than 50 nm and equal to or less than 500 nm, the textured surface of the second electrode 120 is able to sufficiently scatter visible light.
Meanwhile, since the surface of the second electrode 120 is textured by the crystalline silicon grains 133b, it is not necessary to form the intermediate reflector 150 with a large thickness. For example, for the purpose that the surface of the second electrode 120 is textured by depositing the intermediate reflector 150 including ZnO, the surface of the intermediate reflector 150 should be fully textured. In order that the surface of the intermediate reflector 150 is textured as mentioned above, the thickness of the intermediate reflector 150 is required to be at least 1.6 μm. The processing time thereof is hereby increased.
Particularly, when the intermediate reflector 150 is formed in a roll-to-roll type manufacturing equipment, the substrate 100 is continuously moving, so that a time during which the substrate 100 stays in a process chamber is constant. Therefore when the thickness and processing time of the intermediate reflector 150 are increased, the number of the process chambers for forming the intermediate reflector 150 may increase.
Meanwhile, in the photovoltaic device according to the embodiment of the present invention, since the surface of the second electrode 120 is textured enough by the crystalline silicon grains 133b, the thickness of the intermediate reflector 150 can be prevented from being increased. The thickness of the intermediate reflector 150 according to the embodiment of the present invention is equal to or more than 20 nm and equal to or less than 200 nm. When the thickness of the intermediate reflector 150 is less than 20 nm, the refractive index matching is hard to obtain. When the thickness of the intermediate reflector 150 is larger than 200 nm, the amount of light absorbed by the intermediate reflector 150 is increased according to the increase of the thickness of the intermediate reflector 150. As a result, even though the photovoltaic device according to the embodiment of the present invention includes the flexible substrate 100, it is possible to prevent the processing time from being increased.
In the embodiment of the present invention, though the double-junction tandem photovoltaic device has been described, the present invention is applicable to an intermediate cell or a bottom cell of a triple-junction tandem photovoltaic device.
While the embodiment of the present invention has been described with reference to the accompanying drawings, it can be understood by those skilled in the art that the present invention can be embodied in other specific forms without departing from its spirit or essential characteristics. Therefore, the foregoing embodiments and advantages are merely exemplary and are not to be construed as limiting the present invention. The present teaching can be readily applied to other types of apparatuses. The description of the foregoing embodiments is intended to be illustrative, and not to limit the scope of the claims. Many alternatives, modifications, and variations will be apparent to those skilled in the art. In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents but also equivalent structures.

## Claims

1. A method for manufacturing an n-i-p type photovoltaic device including a first unit cell and a second unit cell on which light is incident, the method comprising:
forming an n-type semiconductor layer of the first unit cell on a substrate;
forming an intrinsic semiconductor layer of the first unit cell on the n-type semiconductor layer, the intrinsic semiconductor layer comprising a mixed phase hydrogenated microcrystalline silicon based material comprising an amorphous silicon based material and crystalline silicon grains;
etching a surface of the intrinsic semiconductor layer to partially expose the silicon grains, such that the intrinsic semiconductor layer has a first surface that is textured by the exposed silicon grains and a non-textured opposing second surface; and
forming a p-type semiconductor layer of the first unit cell on the intrinsic semiconductor layer,
wherein an average crystal volume fraction of the intrinsic semiconductor layer of the first unit cell ranges from 25% to 75%.

2. The method of claim 1, wherein the substrate is a flexible substrate.

3. The method of claim 1, wherein the etching the surface of the intrinsic semiconductor layer is performed by a dry-etching process.

4. The method of claim 3, wherein the dry-etching process is a hydrogen plasma etching process or an argon plasma etching process.

5. The method of claim 1, wherein the amorphous silicon based material is etched more quickly than the crystalline silicon grain.

6. The method of claim 1 wherein a passivation layer is formed on the etched surface of the intrinsic semiconductor layer.

7. The method of claim 6, wherein the passivation layer comprises a hydrogenated microcrystalline silicon based material.

8. The method of claim 1, further comprising forming an electrode on the second unit cell,
wherein an average pitch of a textured surface of the electrode is equal to or more than 50 nm and equal to or less than 500 nm.