CN107644805A - Hole passivation tunnelling film, preparation method and its application in solar cell - Google Patents

Abstract

The invention discloses a hole passivation tunneling film whose main component is silicon nitride (SiNx) and whose film thickness is 0.5-3nm. It has a good chemical passivation effect. The electron affinity of silicon nitride is 1.71eV, the forbidden band width is 5.31eV, and the conduction band of silicon is 2.34eV, and the valence band is 1.85eV. The tunneling barrier is nearly 1eV lower than that of electrons, which is very conducive to the selective tunneling of holes; in addition, silicon nitride has a good impurity blocking ability, which is conducive to reducing the hole selection layer material or other materials. The internal diffusion of impurities in the functional layer keeps the cleanliness of the interface and bulk silicon; while the silicon nitride film with a thickness below 3nm has the best tunneling efficiency, the present invention adopts plasma-assisted or heat-assisted atomic layer deposition method deposition The silicon nitride film can obtain an ultra-thin silicon nitride film of this thickness, and the operation is simple and convenient.

Description

Hole-passivating tunneling film, preparation method and application in solar cells

technical field

The invention relates to a thin film material, in particular to a hole passivation tunneling layer, as well as its preparation method and application in solar cells.

Background technique

In recent years, carrier-selective passivation contact heterocrystalline silicon cells have become a research hotspot in the field of photovoltaics. In traditional crystalline silicon cells, a PN junction is generally formed on the surface of the crystalline silicon substrate by high-temperature thermal diffusion doping, so that the built-in electric field of the PN junction is used to realize the separation of carriers. The carrier selective passivation contact heterogeneous crystalline silicon cell is to achieve selective carrier transport by inserting a selective transport layer between the crystalline silicon substrate and the electrode, that is, the electron transport layer allows electrons to pass through and repels Holes, the hole transport layer allows holes to pass and repels electrons, thereby achieving the separation of carriers. At the same time, the selective transport layer also provides good surface passivation for the crystalline silicon substrate.

Compared with traditional PN crystalline silicon cells, carrier-selective passivation contact heterogeneous crystalline silicon cells have three unique advantages: 1. Avoid the use of heavily doped layers, which can effectively eliminate the recombination loss caused by heavily doped layers; Second, the use of different carrier collection materials can adjust the energy band structure of the heterojunction to a greater extent and reduce parasitic optical absorption; third, realize full-area carrier collection and effectively reduce the carrier transport distance. Therefore, this type of solar cell can obtain higher open-circuit voltage, short-circuit current, and fill factor, and improve the overall performance of the cell. The selective passivation transport layer is composed of a passivation tunneling material and a carrier selective contact material. The requirements for passivation tunneling materials include: good surface passivation performance, precise thickness-controlled growth, high carrier selectivity, etc. According to previous reports, hydrogenated intrinsic amorphous silicon and silicon oxide are generally used as passivation tunneling materials. Although these two are very suitable as electron passivation tunneling layers, they are not ideal for hole passivation tunneling. Floor. Among them, hydrogenated amorphous silicon is used as the passivation tunneling layer for holes. The main problems include:

a. The band gap of amorphous silicon is 1.7eV, its electron affinity is 3.9eV, and the conduction band and valence band of crystalline silicon are 0.15eV and 0.45eV respectively; the choice of amorphous silicon for holes b. Amorphous silicon materials are not resistant to high temperatures and cannot be compatible with high-temperature treatment processes, which greatly restricts the subsequent battery manufacturing process; c. Amorphous silicon is usually prepared by PECVD (Plasma Enhanced Chemical Vapor Deposition), The surface quality of crystalline silicon is very high, it is difficult to obtain high-quality amorphous silicon, and the process stability is difficult to guarantee;

The use of silicon oxide (SiOx) as the hole passivation tunneling layer also has problems, because the electron affinity of silicon oxide is 1eV, the band gap is 8.9eV, and the conduction band step with silicon is 3.05eV. , the valence band order is 4.75eV; for holes, the potential barrier is too large, and the tunneling efficiency is limited. If you want to optimize the hole tunneling efficiency, you need to ensure that the thickness of the silicon oxide layer is below 0.8nm , but it is still impossible to obtain a uniform high-quality silicon oxide film below 0.8nm with the existing preparation technology.

Contents of the invention

In order to solve the above technical problems, the present invention provides a thin film that facilitates hole selective tunneling.

The technical solution of the invention is to provide a hole passivation tunneling film, the main component is silicon nitride, and the film thickness is 0.5-3nm.

Further, the refractive index of the hole passivation tunneling film is 1.8-2.5, so as to ensure sufficient nitriding reaction of the film so as to ensure the hole passivation tunneling function of the film.

The present invention also provides a method for preparing the above-mentioned hole passivation tunneling thin film. The silicon nitride thin film is deposited by plasma-assisted (Plasma) or thermal-assisted (Thermal) atomic layer deposition (ALD) method.

Further, the atomic layer deposition method includes the following steps:

S1. Passing silicon-containing precursor molecules to generate silicon-containing intermediate products on the surface of the silicon wafer;

S2. feed inert gas for cleaning;

S3. Passing nitrogen-containing precursor molecules, nitriding the silicon-containing intermediate product to form a silicon nitride film.

It can be repeated in the order of steps S1-S3.

The reaction temperature of the above atomic layer deposition method is 50-500°C.

Further, in step S1, the silicon-containing precursor molecules are chlorosilane-based gases, and the chlorosilane-based gases are one or more of SiCl ₄ , SiH ₂ Cl ₂ , and Si ₂ Cl ₆ .

Further, the nitrogen-containing precursor molecule in step S3 is one or both of NH ₃ and N ₂ H ₄ .

Further, the generated silicon nitride film is treated with hydrogen plasma, and the method of generating hydrogen plasma is high-frequency (13.56MHz) or microwave (2.45GHz) plasma-enhanced chemical vapor (PECVD).

Further, the prepared silicon nitride film has a thickness of 0.5-3 nm and a refractive index of 1.8-2.5. The refractive index is controlled by controlling the flow rate of precursor molecules, and the flow rate of nitrogen-containing precursor molecules is controlled at 20-100 sccm.

The invention discloses the application of the hole passivation tunneling thin film in crystalline silicon solar cells.

Advantages and beneficial effects of the present invention: the composition of the hole passivation tunneling film is silicon nitride (SiNx), which is stable in nature and resistant to high temperature itself, which is beneficial to integrated high temperature treatment; silicon nitride has good chemical passivation on the surface of crystalline silicon The electron affinity of silicon nitride is 1.71eV, the forbidden band width is 5.31eV, and the conduction band step of silicon is 2.34eV, the valence band step is 1.85eV, and the hole tunneling barrier ratio The tunneling potential barrier of electrons is nearly 1eV lower, which is very conducive to the selective tunneling of holes; in addition, silicon nitride has a good impurity blocking ability, which is conducive to reducing the impurity in the hole selection layer material or other functional layers. internal diffusion, to maintain the cleanliness of the interface and bulk silicon; and the silicon nitride film with a thickness below 3nm has the best tunneling efficiency, and the preparation method of the present invention can obtain an ultra-thin silicon nitride film with this thickness, and the operation is simple and convenient .

Description of drawings

FIG. 1 is a schematic diagram of the current-voltage (I-V) relationship of silicon wafers deposited with different thicknesses of silicon nitride films.

detailed description

The present invention will be further described below in combination with specific embodiments.

In order to illustrate the advantages of the invention, the following examples describe the preparation of passivated contact solar cells using the method proposed by the invention and known methods. An n-type single crystal silicon wafer of 0.5-10Ω·cm is selected as the substrate.

Example 1

The passivation contact solar cell using the silicon nitride thin film as the hole tunneling layer is prepared by using the preparation method of the invention. Thermal ALD method: First, put the n-type single crystal silicon wafer cleaned by RCA standard process into the ALD chamber, evacuate the chamber to above 10 ^-6 Torr, and heat the chamber to 430°C at the same time. Using SiCl ₄ and NH ₃ as precursors, first pass SiCl ₄ steam for 1 second, then pass nitrogen gas with a flow rate of 100 sccm for 30 seconds, then pass ammonia gas with a flow rate of 60 sccm for 5 seconds, and finally pass through a flow rate of 100 sccm Nitrogen for 30 seconds, this is a cycle. After 12 cycles of treatment, a 3nm silicon nitride film can be obtained on the surface of the silicon wafer, and the refractive index of the silicon nitride film prepared by the precursor at this flow rate is 2.0.

Then continue to use the PECVD method on the upper surface to deposit boron-doped p ⁺ amorphous silicon with a thickness of 10nm as a hole-selective contact layer. The body pressure was evacuated to 10 ^-6 Torr, and the cavity was heated to 200°C; 20 sccm silane (purity: 99.999%) and 10 sccm diborane (2%)-hydrogen gas mixture (purity 99.999%) were introduced, and the cavity pressure was adjusted to 300mTorr; turn on the RF power supply (13.56MHz) with a power of 10W to excite the process gas to generate plasma; after 60 seconds, turn off the RF power supply to obtain a 10nm boron-doped p+-type amorphous silicon film. Next, a 80nm thick TCO (transparent conductive oxide) film and a 1μm thick grid silver electrode were deposited on the upper surface by thermal evaporation, and a 1μm thick metallic silver was deposited on the lower surface. The specific process is: put the sample into the fixture Finally, install it at the designated position of the thermal evaporation coating apparatus, place ITO (indium tin oxide) particles in the corresponding crucible, and pump the cavity pressure to 10 ^-6 Torr through the high vacuum system. Turn on the DC power supply of the ITO crucible, slowly tune the power to the evaporation rate of 0.1nm/s, monitor the deposition thickness to 80nm through the film thickness meter, and turn off the power to complete the deposition; then use the same operation method to complete the 1 micron grid silver electrode and the back surface Silver layer deposition.

Another implementation of depositing a 3nm silicon nitride film in this embodiment is to use the Plasma ALD method, which requires a relatively low temperature. The specific process is: place the n-type single crystal silicon wafer cleaned by the RCA standard process on the ALD In the cavity, the cavity is evacuated to above 10 ⁻⁶ Torr, and the cavity is heated to 50° C. at the same time. Using SiCl ₄ and NH ₃ as precursors, first pass SiCl ₄ steam for 1 second, then pass through nitrogen with a flow rate of 100 sccm for 30 seconds, and then pass through 60 sccm of ammonia for 5 seconds. After the plasma is generated in the isolated discharge chamber, it enters the process chamber to react with the SiCl ₄ adsorbed on the substrate, and finally flows nitrogen gas with a flow rate of 100 sccm for 30 seconds. This is a cycle; after 12 cycles of treatment, the silicon wafer can be A 3nm silicon nitride film is obtained on the surface as a hole passivation tunneling layer.

Embodiment 2～6

The number of cycles when depositing the silicon nitride film in Example 1 is controlled at 10, 8, 4, and 2 respectively, so that the thickness of the silicon nitride film obtained is controlled at 2.5nm, 2nm, 1.5nm, and 1nm respectively. , 0.5nm, the refractive index is 2.0, and all the other are with embodiment 1.

Comparative example 1

The number of cycles of the deposited silicon nitride film in Example 1 is controlled at 14, that is, the thickness of the silicon nitride film is changed to 3.5 nm, and the rest are the same as in Example 1.

Comparative example 2

Passivation contact solar cells using hydrogenated intrinsic amorphous silicon as hole passivation tunneling layer, the main preparation steps are:

(1) Deposit 5nm hydrogenated intrinsic amorphous silicon on the surface of the cleaned n-type single crystal silicon wafer by PECVD method as a hole passivation tunneling layer. The specific process is: clean the n-type single crystal with RCA standard process After the silicon wafer is purged or dried with nitrogen gas, it is put into the process chamber of PECVD equipment, and the pressure of the chamber is pumped to 10 ^-6 Torr through the high vacuum system, and the chamber is heated to 200°C; Silane (purity: 99.999%), 20sccm hydrogen (purity 99.999%), control the chamber pressure to 300mTorr; turn on the radio frequency power supply (13.56MHz), power 15W, stimulate the process gas to generate plasma to deposit hydrogenated amorphous silicon film; 30 seconds Finally, turn off the radio frequency power supply to obtain a 5nm hydrogenated amorphous silicon film;

(2) Then, continue to deposit boron-doped p+ amorphous silicon with a thickness of 10nm by ^PECVD method on the upper surface as a hole-selective contact layer. The chamber is heated to 200°C. Introduce 20 sccm silane (purity: 99.999%), 10 sccm diborane (2%)-hydrogen gas mixture (purity 99.999%), adjust the chamber pressure to 300mTorr; turn on the RF power supply (13.56MHz), power 10W, to excite the process gas Generate plasma; after 60 seconds, turn off the radio frequency power supply to obtain a 10nm boron-doped p+-type amorphous silicon film;

(3) Next, a 80nm thick TCO film and a 1μm thick grid silver electrode were deposited on the upper surface by thermal evaporation, and a 1μm thick metallic silver was deposited on the lower surface. The specific process is: put the sample into the fixture and install it on Place ITO particles in the corresponding crucible at the designated position of the thermal evaporation coating instrument, and pump the cavity pressure to 10 ^-6 Torr through the high vacuum system; turn on the DC power supply of the ITO crucible, slowly tune the power to the evaporation rate of 0.1nm/s, and pass the film thickness The instrument monitors that the deposition thickness reaches 80nm, and the power is turned off to complete the deposition; finally, the same operation method is used to complete the deposition of the 1 micron grid silver electrode and the back silver layer.

Comparative example 3

Passivated contact solar cells using silicon oxide as hole passivation tunneling layer, the main preparation steps are:

(1) The cleaned n-type single crystal silicon wafer is first immersed in a nitric acid solution with a concentration of 68% for 10 minutes, so that an ultra-thin silicon oxide of about 1.5 nm is formed on the surface as a hole passivation tunneling layer;

(2) Then, continue to use PECVD method to deposit boron-doped p ⁺ amorphous silicon with a thickness of 10nm as a hole-selective contact layer; the specific process is: put the sample into the process chamber of the PECVD equipment, The system pumps the cavity pressure to 10 ^-6 Torr, and the cavity is heated to 200°C. Introduce 20 sccm silane (purity: 99.999%), 10 sccm diborane (2%)-hydrogen gas mixture (purity 99.999%), adjust the chamber pressure to 300mTorr; turn on the RF power supply (13.56MHz), power 10W, to excite the process gas Plasma is generated; after 60 seconds, the radio frequency power is turned off, and a 10nm boron-doped p+-type amorphous silicon film is obtained.

(3) Next, a 80nm thick TCO film and a 1μm thick grid silver electrode were deposited on the upper surface by thermal evaporation, and a 1μm thick metallic silver was deposited on the lower surface. The specific process is: put the sample into the fixture and install it on The thermal evaporation coater specifies the position, places ITO particles in the corresponding crucible, and pumps the cavity pressure to 10 ^-6 Torr through the high vacuum system. Turn on the DC power supply of the ITO crucible, slowly adjust the power to the evaporation rate of 0.1nm/s, monitor the deposition thickness to 80nm through the film thickness meter, and turn off the power to complete the deposition; then use the same operation method to complete the 1 micron grid silver electrode and the back surface Silver layer deposition.

The performance test of silicon wafers prepared in Examples 1-6 and Comparative Example 1 is shown in Figure 1. The IV curve is a reflection of the current tunneling ability. According to theoretical calculations, when the current value is higher than 0.01A/cm ² , it can guarantee The finished battery can obtain a higher fill factor. Figure 1 shows the IV curve of a silicon wafer using SiNx as the tunneling layer. The figure shows that when SiNx is less than 3nm, it has a relatively good tunneling effect, and it is useful for batteries. It is beneficial to obtain a higher fill factor.

The performance test of the solar cells prepared in Examples 1-6 and Comparative Examples 1-3 is shown in Table 1

Table 1

From the results in the table, the use of ultra-thin silicon nitride as the hole passivation tunneling layer can increase the conversion efficiency of solar cells by 0.4-1.0%, and the short-circuit current, open-circuit voltage and fill factor are all improved, which is benefited from While silicon nitride has good surface passivation for silicon wafers, it also has higher tunneling efficiency for holes.

The materials, reagents and experimental equipment involved in the embodiments of the present invention are all commercially available products in the field of solar cell preparation unless otherwise specified.

The above are only preferred embodiments of the present invention, and it should be pointed out that for those of ordinary skill in the art, improvements and modifications can also be made without departing from the core technology of the present invention. Retouching should also belong to the patent protection scope of the present invention. Any changes within the meaning and scope equivalent to the claims of the present invention should be considered to be included in the scope of the claims.

Claims (11)

1. A hole passivation tunneling film, the main component is silicon nitride, and the film thickness is 0.5-3nm. 2 . The hole passivation tunneling film according to claim 1 , wherein the refractive index of the hole passivation tunneling film is 1.8-2.5. 3. The method for preparing a hole-passivating tunneling film according to any one of claims 1 or 2, characterized in that the silicon nitride film is deposited by plasma-assisted or heat-assisted atomic layer deposition. 4. The preparation method of hole passivation tunneling thin film according to claim 3, is characterized in that, comprises the steps: S1. Passing silicon-containing precursor molecules to generate silicon-containing intermediate products on the surface of the silicon wafer; S2. feed inert gas for cleaning; S3. Passing nitrogen-containing precursor molecules, nitriding the silicon-containing intermediate product to form a silicon nitride film. 5 . The method for preparing a hole passivation tunneling film according to claim 4 , wherein the steps S1 - S3 are repeated in sequence to obtain a required thickness of the silicon nitride film. 6 . 6 . The method for preparing a hole-passivating tunneling film according to claim 4 , wherein the reaction temperature is 50-500° C. 7. The method for preparing a hole passivation tunneling film according to claim 4, characterized in that, in the step S1, the silicon-containing precursor molecule is a chlorosilane gas, and the chlorosilane gas is SiCl ₄ , One or more of SiH ₂ Cl ₂ and Si ₂ Cl ₆ . 8 . The method for preparing a hole-passivating tunneling film according to claim 4 , wherein the nitrogen-containing precursor molecule in the step S3 is one or both of NH ₃ and N ₂ H ₄ . 9 . The method for preparing a hole-passivating tunneling film according to claim 4 , wherein the silicon nitride film formed in step S3 is treated with hydrogen plasma. 10 . The method for preparing a hole-passivating tunneling film according to claim 8 , wherein the method for generating hydrogen plasma is a high-frequency or microwave plasma-enhanced chemical vapor method. 11 . 11. Application of the hole-passivating tunneling film according to any one of claims 1 or 2 in crystalline silicon solar cells.