TW201503392A - Structure of heterojunction thin film epitaxy silicon solar cell and preparation method thereof - Google Patents

Abstract

The invention relates to a structure of heterojunction thin film epitaxy silicon solar cell and its preparation method using purification metallurgical grade silicon chip having lower purity silicon as substrate of preparing solar cell, decrease usage of high-purity silicon material. After firstly disposing epitaxial silicon thin film on the purification metallurgical grade silicon chip, level such as amorphous-silicon thin film and transparent conductive film then are disposed to construct HIT (Hetero-junction with Intrinsic Thin- layer) solar cell structure. The manufacturing cost of heterojunction thin film epitaxy silicon solar cell can be greatly reduced under usage of decreasing high-purity silicon material.

Description

Structure of heterojunction film epitaxial germanium solar cell and preparation method thereof

The invention relates to a structure of a solar cell and a preparation method thereof, in particular to an upgraded metallurgical grade silicon (UMG-Si) with a lower purity of germanium in the preparation of a heterojunction thin film epitaxial solar cell. As a substrate, it is a novel heterojunction film epitaxial solar cell structure and a preparation method thereof.

The main source of energy used by humans today is fossil energy, which is mainly used in the form of fuel oil and gasoline. As the population increases year by year and the progress of civilization, the demand for energy continues to increase, and the oil source is limited, so that oil prices continue to rise. Coupled with the long-term use of fossil energy by humans, the global greenhouse effect is becoming more and more serious, so the search for environmentally friendly renewable energy has become an important option. Although nuclear fission energy can produce high-density power sources, there are problems such as radioactive pollution caused by the leakage of radioactive materials and the storage of nuclear waste. The damage to biological and environmental environment is serious, and it is still difficult to have an effective solution.

Solar energy is considered one of the most renewable energy options that are most likely to replace fossil energy compared to renewable energy such as wind, fire and geothermal. Therefore, there are quite a few research teams around the world who are actively researching how to develop low-cost and high conversion efficiency solar cells. The world's currently mass-produced solar cells are solar cells made of silicon bulk materials, such as single crystal germanium (c-Si) and poly-Si wafers (wafer-based). The solar cell of the substrate. However, the production processes of high-purity single crystal germanium and polycrystalline germanium materials are required to be carried out in a high temperature environment, resulting in a high cost of the twinned solar cells.

Sanyo, Japan, has revealed a heterojunction solar cell that uses amorphous germanium to fabricate a-Si(p)/a-Si(i)/c-Si(n) junction structures, also known as HIT ( Hetero-junction with Intrinsic Thin-layer). The intrinsic amorphous germanium (a-Si(i)) is used to passivate the surface of the germanium wafer, and a layer of n-type or p-type amorphous germanium is deposited. The structure is divided into single-sided or double-sided, and the double-sided side is a backside-deposited intrinsic amorphous germanium layer and an n-type or p-type amorphous germanium layer. Since the structural process is carried out in a low temperature environment of about 200 ° C or less, in addition to maintaining the quality of the tantalum material, it is also more energy efficient than the conventional twin crystal solar cell, but the structure must utilize a relatively high quality and high cost single. Crystalline substrate (FZ-c-Si) materials can achieve high efficiency.

Therefore, the use of a single crystal germanium substrate material which provides a structure and method to reduce high quality and high cost, so that the production cost of the solar cell is lowered, is a problem to be solved by the present invention.

The main object of the present invention is to provide a structure of a heterojunction thin film epitaxial germanium solar cell, which uses a purified metallurgical germanium wafer having a lower purity as a substrate of a solar cell, thereby reducing the use of a high purity germanium material source. Significantly reduced production costs.

A secondary object of the present invention is to provide a structure of a heterojunction thin film epitaxial germanium solar cell, which is provided with an epitaxial germanium film on the purified metallurgical germanium wafer, so that the combination of the purified metallurgical germanium wafer and the epitaxial germanium film remains The solar cell can maintain good energy conversion efficiency.

A further object of the present invention is to provide a method for preparing a heterojunction thin film epitaxial germanium solar cell, which is characterized by purifying a metallurgical germanium wafer as a core, and gradually depositing each film layer to construct an HIT thin film epitaxial germanium with high energy conversion efficiency. Solar battery.

A further object of the present invention is to provide a method for preparing a heterojunction thin film epitaxial germanium solar cell, which can be generally applied to single-sided and double-sided heterojunction thin film epitaxial solar cells, and has a wide application range.

Therefore, in order to achieve the above object, the present invention discloses a structure of a heterojunction thin film epitaxial germanium solar cell and a method for fabricating the same, which comprises: a back electrode; and a purified metallurgical grade germanium wafer; Above the back electrode; an epitaxial film deposited on the purified metallurgical grade germanium wafer; a first amorphous germanium film disposed on the epitaxial germanium film; a first transparent conductive film And disposed on the first amorphous germanium film; and a front electrode disposed on the first transparent conductive film. According to the structure and the corresponding preparation method, the purified metallurgical tantalum wafer can be used as the core of the heterojunction thin film epitaxial solar cell, and the function of reducing cost can be exerted.

The first figure is a schematic structural view of a single-sided heterojunction thin film epitaxial solar cell in the present invention;
The second figure is a schematic structural view of a double-sided heterojunction film epitaxial solar cell in the present invention;
Third: it is a flow chart of the steps of the present invention;
Figure 4 is a schematic view showing the structure of a single-sided heterojunction thin film epitaxial germanium solar cell having an intrinsic amorphous germanium film; and a fifth figure: it is an essential type in the present invention. Schematic diagram of a double-sided heterojunction thin film epitaxial solar cell of a germanium film.

For a better understanding and understanding of the features and advantages of the present invention, the preferred embodiments and the detailed description are described as follows:

First, please refer to the first figure, which is a schematic cross-sectional view of the present invention in the form of a single-sided hetero-junction thin film epitaxial solar cell, which comprises: a purified metallurgical grade The germanium wafer 1, an epitaxial germanium film 2, a first amorphous germanium film 31, a first transparent conductive film 41, a back surface electrode 51, and a front surface electrode 52. The epitaxial germanium film 2 is disposed on the purified metallurgical grade germanium wafer 1, the first amorphous germanium film 31 is disposed on the epitaxial germanium film 2, and the first transparent conductive film 41 is disposed on the first amorphous film. The ruthenium film 31 and the front surface electrode 52 are disposed under the purification metallurgical grade ruthenium wafer 1 and above the first transparent conductive film 41, respectively.

If the design of a double-sided hetero-junction thin-film epitaxial solar cell is adopted, the schematic cross-sectional view of the present invention is referred to the second figure, which is to purify the metallurgical grade germanium wafer 1. The second amorphous germanium film 32 and the second transparent conductive film 42 are sequentially disposed, and then the back surface electrode 51 is provided.

The germanium substrate used in the invention has a thickness of about two hundred micrometers, and is a purified metallurgical grade germanium wafer 1 having a relatively low price and a purity of 99.9% to 99.999%, instead of a solar grade germanium substrate of up to 99.99998%, so Reduce the manufacturing cost of thin film epitaxial germanium solar cells.

In the preparation method, please refer to the step flow shown in the third figure, the core steps of which include:
Step S1: polishing a purified metallurgical grade germanium wafer;
Step S2: depositing an epitaxial germanium film on the purified metallurgical grade germanium wafer;
Step S3: depositing a first amorphous germanium film on the epitaxial germanium film;
Step S4: depositing a first transparent conductive film on the first amorphous germanium film; and step S5: printing a front electrode on the first transparent conductive film.

In the step S1, the invention uses the purified metallurgical grade germanium wafer 1 as the main body of the twinned solar cell, and the n-type or p-type can be selected. Since it has mechanical damage to the surface due to wire cutting at the time of preparation, it needs to be polished before it can be used. In the present invention, the purified metallurgical grade germanium wafer 1 is immersed in at least one acid liquid for polishing the surface mechanical damage selected from the group consisting of hydrofluoric acid (HF), nitric acid (HNO ₃ ), and sulfuric acid (H ₂ ). At least one of the group consisting of SO ₄ ), hydrochloric acid (HCl), and acetic acid (CH ₃ COOH). After the metallurgical grade ruthenium wafer 1 is immersed in several times of acid immersion and rinsed with deionized water, the surface mechanical damage layer can be effectively removed to achieve the polishing effect.

Next, the polished metallurgical grade germanium wafer 1 is polished on one side of the epitaxial germanium wafer 2, and the type of the epitaxial germanium film 2 can be intrinsic or corresponding to the purification of the metallurgical grade germanium wafer 1. The impurity type, for example, when the metallurgical grade germanium wafer 1 is n-type, the n-type epitaxial germanium film 2 is deposited at this time.

After depositing on the surface of the purified metallurgical grade germanium wafer 1, the epitaxial germanium film 2 provides the function of a solar grade germanium substrate with a purity of 99.99998%, which not only avoids the use of pure metallurgical grades of lower quality. The problem of poor conversion efficiency faced by the wafer 1 as a substrate also reduces manufacturing costs.

The present invention can further perform a texturing process on the surface of the epitaxial germanium film 2, and at this time, a rough structure is formed on the surface of the epitaxial germanium film 2, so that light can enter the inside of the germanium block. Multiple reflections increase the chance of being absorbed.

The present invention is followed by direct setup of the amorphous germanium film layer without the conventional phosphorus thermal diffusion process. For example, a single-sided heterojunction thin film epitaxial solar cell is used, and the setting of the back electrode 51 is completed before the amorphous germanium film layer is disposed. Here, the aluminum metal is deposited under the purified metallurgical grade germanium wafer 1 by screen printing aluminum paste or evaporation.

If the back electrode 51 is disposed using a screen printing method, further baking and sintering treatment are required to convert the aluminum paste into a solid metal layer; if the evaporation method is used, only the annealing is required. The back electrode 51 is completed without additional baking or sintering.

After the single-sided heterojunction film epitaxial germanium solar cell is prepared, the first amorphous germanium film 31 is deposited on one side of the epitaxial germanium film 2 after the back electrode 51 is completed. The type of the first amorphous germanium film 31 is different depending on the doping type of the epitaxial germanium film 2. If the epitaxial germanium film 2 is p-type, the deposited first amorphous germanium film 31 is n. Type and vice versa. On the other hand, if the epitaxial germanium film 2 is intrinsic, the doping type of the first amorphous germanium film 31 is of the opposite type depending on the doping type of the metallurgical grade germanium wafer 1.

If the target is a double-sided heterojunction thin film epitaxial solar cell, the back electrode 51 is not formed first, but an amorphous germanium film is deposited on both sides. In addition to depositing the first amorphous germanium film 31 over the epitaxial germanium film 2, a second amorphous germanium film 32 is also deposited under the purified metallurgical grade germanium wafer 1. The doping type of the first amorphous germanium film 31 and the second amorphous germanium film 32 are the same.

Please refer to the fourth and fifth figures, respectively, which are schematic diagrams of the structure of a single-sided and double-sided heterojunction thin film epitaxial solar cell, wherein the amorphous amorphous germanium film can be deposited before depositing the amorphous germanium film. That is, the first intrinsic amorphous germanium film 61 exists between the epitaxial film 2 and the first amorphous germanium film 31, and there is a difference between the purified metallurgical grade germanium wafer 1 and the second amorphous germanium film 32. Two intrinsic amorphous germanium films 62.

Above the amorphous germanium film is a deposition arrangement of a transparent conductive film (TCO), as shown in the first figure and the second figure, or a structure as in the fourth figure and the fifth figure, which has the first The transparent conductive film 41 is disposed on the first amorphous germanium film 31, and the second transparent conductive film 42 is formed on the second amorphous germanium film under the double-sided deposition process of the double-sided heterojunction thin film epitaxial solar cell. Below 32. The most commonly used materials for these transparent conductive films are Indium Tin Oxide (ITO), but other transparent conductive materials can also be used for deposition.

After the preparation of each of the above film layers is completed, the electrode can be disposed; wherein the back electrode 51 of the single-sided heterojunction film epitaxial solar cell has been set first, and the double-sided cell can be used at this time. The same method was used for the preparation. In the portion of the front electrode 52, the present invention employs a screen printing silver paste electrode process which protects other film structures that are disposed from being damaged by high temperatures. The silver paste can be selected from a low temperature process with a baking temperature lower than 200 ° C or a normal temperature silver paste process. The baking process is carried out using a Vertical Dryer or an IR Belt Dryer.

The following are examples of actual operation for preparing single-sided/double-sided heterojunction thin film epitaxial solar cells:

[Double-sided heterojunction film epitaxial solar cell]
1. The purified metallurgical grade ruthenium wafer is placed in a mixed solution of HF, HNO ₃ and CH ₃ COOH at a ratio of 3:5:3, and the immersion time is about 10 to 30 seconds until the surface is polished. Take it out again. The purified metallurgical grade ruthenium wafer was further cleaned with deionized water (DI water) for about 10 seconds. The solution was placed in a 2% by weight potassium hydroxide (KOH) solution, and the soaking time was about 10 seconds to achieve the acid-base neutralization effect, and then taken out. The purified metallurgical grade ruthenium wafer was placed in a solution prepared by HF and H ₂ O in a ratio of 10:1, and the immersion time was about 10 seconds, and then taken out. The purified metallurgical grade ruthenium wafer was placed in a solution prepared by the ratio of H ₂ SO ₄ and hydrogen peroxide (H ₂ O ₂ ) in a ratio of 4:1, and the immersion time was about 10 minutes, and then taken out. The purified metallurgical grade twin is placed in a solution prepared by HF and H ₂ O in a ratio of 10:1, and the immersion time is about 10 seconds, and then taken out. The purified metallurgical grade ruthenium wafer was then rinsed with deionized water for about 10 seconds. The purified metallurgical grade ruthenium wafer was placed in a solution prepared at a temperature of 80 ° C, HCl, H ₂ O ₂ and H ₂ O at a ratio of 1:1:6, and the immersion time was about 10 minutes, and then taken out. The metallurgical grade ruthenium wafer is then purified in deionized water for a cleaning time of about 10 seconds. The purified metallurgical grade ruthenium wafer is placed in a solution prepared by HF and H ₂ O in a ratio of 10:1, and the immersion time is about 10 seconds, and then taken out, thereby completing the step of removing the mechanical cutting damage process.
2. The purified metallurgical grade ruthenium wafer is placed in an atmospheric pressure chemical vapor deposition (APCVD) machine. When the deposition temperature is 1100~1150 °C, the main gas stream is hydrogen and the Si ₂ H _{2 is introduced.} Cl ₂ (flow rate is about 10~30 sccm) deposits an intrinsic epitaxial germanium film, and the thickness of the epitaxial germanium film is about 15~30 μm. In the case of n-type or p-type, additional PH ₃ or B ₂ H ₆ gas is required. After the process is completed, after cooling, the metallurgical grade germanium wafer is taken out.
3. The purified metallurgical grade ruthenium wafer is placed in a plasma-enhanced chemical vapor deposition (PECVD) machine, hydrogen is introduced, and the RF generator is turned on to generate plasma, which is treated by hydrogen plasma. ~30 minutes later. The purified metallurgical grade ruthenium wafer is placed in a Reactive Ion Etching (RIE) machine, and a gas such as O ₂ and CF _{4 is introduced} , the RF generator is turned on to generate a plasma, and reactive ion etching is performed for about 5 to 30 minutes. After the process is completed, the purified metallurgical grade germanium wafer is taken out.
4. The purified metallurgical grade ruthenium wafer is placed in a plasma enhanced chemical vapor deposition machine. When the deposition temperature is 180-220 ° C, the intrinsic amorphous ruthenium layer is deposited by gas such as SiH ₄ and H ₂ ; For the n-type amorphous germanium layer or the p-type amorphous germanium layer, an additional gas such as PH ₃ or B ₂ H _{6 is required} . The detailed process parameters are shown in Table 1 below. After the end of the process, the temperature is lowered to room temperature, and the metallurgical grade silicon wafer is removed.


5. Place the purified metallurgical grade ruthenium wafer into a sputter machine and place the ITO target. The test piece stage is adjusted to the rotation mode. When the deposition temperature is 150 ~ 220 °C, Ar (flow rate 30 sccm) gas is introduced, the RF generator is turned on, and the power is slowly increased to 100~200 W. The deposition thickness is about 700~900 Å. After the end of the process, the temperature was lowered to room temperature, and the metallurgical grade germanium wafer was removed.
6. The purified metallurgical grade bismuth wafer is placed in a screen printing machine, and screen printing is performed using low temperature silver paste. The screen mesh number is 280 and the emulsion thickness is 18 μm. After the process is completed, the purified metallurgical grade germanium wafer is taken out.
7. Place the purified metallurgical grade silicon wafer into the hot air oven (Vertical Dryer), set the temperature and constant temperature time to 130~180 °C and the process time is about 5~60 minutes. After the process is finished, drop to room temperature. The purification of the metallurgical grade germanium wafer is completed, that is, the solar cell component process is completed.

[Single-sided heterojunction film epitaxial solar cell]
(Items 1 to 3 are the same as those of the above-mentioned double-sided heterojunction film epitaxial solar cell)
4. The purified metallurgical grade bismuth wafer is placed in the screen printing machine, and the aluminum paste is used for screen printing (you can also choose to use low temperature silver paste). The stencil mesh number can be 200~250, and the emulsion thickness is 20 μm. After the process is completed, the purified metallurgical grade germanium wafer is taken out.
5. The purified metallurgical grade silicon wafer is placed in a hot air oven, the set temperature and constant temperature are 180~200 °C and the process time is about 6~12 minutes. After the process is finished, the temperature is lowered, and then the metallurgical grade silicon wafer is removed. And then placed in the Rapid Thermal Annealing (RTA) machine, the peak temperature can be 750 ~ 880 ° C, the gas can be air or only oxygen. After the end of the process, the temperature was lowered to room temperature, and the metallurgical grade germanium wafer was removed.
(Continued to follow the 4th to 7th items of the double-sided heterojunction film epitaxial solar cell, that is, complete the solar cell component process)

In combination with the structures and steps disclosed above, the present invention is further utilized for purifying metallurgical grade germanium wafers, which perform epitaxial process of thin film germanium on pure metallurgical grade germanium wafers, reducing the use of high purity germanium material sources, and The efficiency can reach the level of traditional twin solar cell efficiency. Therefore, the present invention undoubtedly provides a structure of a heterojunction thin film epitaxial solar cell with practical value and a preparation method thereof under the advantages of cost and efficiency.

The above is only the preferred embodiment of the present invention, and is not intended to limit the scope of the present invention, and the variations, modifications, and modifications of the shapes, structures, features, and spirits described in the claims of the present invention. All should be included in the scope of the patent application of the present invention.

1‧‧·Refining metallurgical grade germanium wafers
2‧‧‧Emission film
31‧‧‧First amorphous germanium film
32‧‧‧Second amorphous film
41‧‧‧First transparent conductive film
42‧‧‧Second transparent conductive film
51‧‧‧Back electrode
52‧‧‧Front electrode
61‧‧‧First intrinsic amorphous germanium film
62‧‧‧Second intrinsic amorphous germanium film
S1~S5‧‧‧Steps

S1~S5‧‧‧Steps

Claims (10)

A structure of a heterojunction thin film epitaxial germanium solar cell, comprising:
a back electrode;
An upgraded metallurgical grade silicon (UMG-Si) is disposed on the back electrode;
An epitaxial film deposited on the purified metallurgical grade germanium wafer;
a first amorphous germanium film is disposed on the epitaxial germanium film;
A first transparent conductive film is disposed on the first amorphous germanium film; and a front electrode is disposed on the first transparent conductive film. The structure of claim 1, wherein the back electrode and the purified metallurgical grade germanium wafer further comprise:
A second amorphous germanium film is disposed under the purified metallurgical grade germanium wafer; and a second transparent conductive film is disposed under the second amorphous germanium film and disposed over the back electrode. The structure of claim 1, further comprising a first intrinsic amorphous germanium film disposed between the epitaxial germanium film and the first amorphous germanium film. The structure of claim 2, further comprising a second intrinsic amorphous germanium film disposed between the purified metallurgical grade germanium wafer and the second amorphous germanium film. A method for preparing a heterojunction thin film epitaxial germanium solar cell, comprising the steps of:
Polishing and purifying a metallurgical grade germanium wafer;
Depositing an epitaxial germanium film on the purified metallurgical grade germanium wafer;
Depositing a first amorphous germanium film on the epitaxial germanium film and simultaneously depositing a second amorphous germanium film under the purified metallurgical grade germanium wafer;
Depositing a first transparent conductive film on the first amorphous germanium film and simultaneously depositing a second transparent conductive film under the second amorphous germanium film; and separately printing a front electrode on the first transparent A conductive film and a back electrode are under the second transparent conductive film. The preparation method according to claim 5, wherein in the step of polishing the metallurgical grade ruthenium wafer, the purified metallurgical grade ruthenium wafer is immersed in at least one acid solution. The preparation method according to claim 6, wherein the acid solution is at least one selected from the group consisting of hydrofluoric acid, nitric acid, hydrochloric acid, sulfuric acid, and acetic acid. The preparation method of claim 5, wherein after the step of depositing the epitaxial germanium film on the purified metallurgical grade germanium wafer, the method further comprises the step of: structuring the surface of the epitaxial germanium film. A method for preparing a heterojunction thin film epitaxial germanium solar cell, comprising the steps of:
Polishing and purifying a metallurgical grade germanium wafer;
Depositing an epitaxial germanium film on the purified metallurgical grade germanium wafer;
Providing a back electrode under the purified metallurgical grade germanium wafer;
Depositing a first amorphous germanium film on the epitaxial germanium film;
Depositing a first transparent conductive film on the first amorphous germanium film; and screen printing a front electrode on the first transparent conductive film. The preparation method according to claim 9, wherein in the step of disposing the back electrode under the purified metallurgical grade germanium wafer, a screen printing method or an evaporation method is used.