CN1416179A - Silicon solar cell of nesa with transparent conductive folm front electrode - Google Patents

Abstract

The transparent conductive film front electrode crystalline silicon solar cell belongs to a photovoltaic cell with a new structure. The main point is to use a transparent conductive film as the front electrode to replace the grid-shaped front electrode and the anti-reflection layer. The front electrode of the transparent conductive film of the present invention has a stacked layer structure, that is, a high-resistance transparent conductive film with a thickness of less than 100nm is fabricated on the silicon substrate that has formed an n-p junction, and then a thickness greater than 150nm is produced according to the requirements of conductance and anti-reflection. High conductivity transparent conductive film. It can increase the conversion efficiency by 25-35%. The production cost is lower than that of the prior art.

Description

Transparent Conductive Film Front Electrode Crystalline Silicon Solar Cell

1. Technical field

The invention belongs to a photovoltaic cell with a new structure.

2. Background technology

The basic structure of existing crystalline silicon (single crystal silicon, polycrystalline silicon) solar cells is shown in FIG. 1 . It consists of a crystalline silicon (B) with a planar semiconductor n-p junction (C), plus a back electrode (A) and a front electrode (E). The front electrode is very critical. It can neither block the incident light to ensure that most of the incident light enters the n-p junction, but also fully collect the photogenerated carriers. These are two contradictory requirements. The current technology is to use a metal grid electrode to solve this contradiction.

In order to reduce the reflection of sunlight on the silicon surface, an optical anti-reflection layer (F) is usually used. They are insulating films such as silicon monoxide (SiO) and silicon nitride (SiN) with a refractive index between 2.0 and 2.2.

It can be seen from Figure 1 that the grid-shaped front electrode leaves a large amount of surface to receive incident light. The photo-generated current relies on the lateral movement of photo-generated carriers in the surface layer (D) of the solar cell and is collected by the grid-shaped front electrode. For n-p type crystalline silicon solar cells, the reason why photogenerated carriers can move laterally for a longer distance is because they have a longer diffusion length in the surface layer of n-Si. Of course, it is also required that the surface layer has sufficient thickness to reduce the resistance of the surface layer as much as possible. In this way, in the case of using a grid-shaped front electrode, another pair of contradictions must be dealt with: on the one hand, it is hoped that the surface layer is thin, that is, the junction is shallow, which is beneficial to improving the short-wave response and short-circuit current of the solar cell, and improving the performance of the battery. Both are beneficial; on the other hand, it is desirable to increase the thickness of the surface layer to reduce the lateral resistance of the surface layer, thereby electrically increasing the short-circuit current and fill factor. The existing technology of crystalline silicon solar cells is to control the surface layer to about 1 micron. Therefore, for the grid-shaped front electrodes used in crystalline silicon solar cells, the distance between each electrode can be relatively wide, that is, the density of the grid is relatively thin, and the shape can be inaccurate.

Among crystalline silicon solar cells, there is also a solar cell with a structure called MIS. The basic structure of the MIS solar cell is shown in Figure 2. It uses the Schottky junction (J) formed by metals and semiconductors to drive photo-generated carriers to generate electromotive force. Between the metal and the semiconductor, there must be a very thin (about tens of angstroms), high-quality oxide insulating layer (I), such as SiO ₂ , Al ₂ O ₃ and so on.

Since the Schottky junction is very shallow and the surface layer is very thin, and because the Schottky junction formed by the metal electrode and the semiconductor needs to cover the entire semiconductor surface, the distance between the grid electrodes (E) must be quite small. That is, the density of the grid is relatively high, and the shape must be very precise.

In summary, the prior art has the following problems:

The grid-shaped front electrode used should cover about 6-10% of the surface area, which reduces the effective area of the solar cell and reduces the conversion efficiency.

(1) As a result of using grid-like front electrodes, photogenerated carriers must move laterally on the surface. This introduces additional series resistance, reducing short-circuit current and fill factor. The calculation shows that: when the surface resistance is 245Ω.cm ² , the effective area equivalent to the solar cell is reduced to 40%; when the surface resistance is 4.6Ω.cm ² , the effective area is reduced to 60%; the surface resistance is 1Ω.cm ² , the effective area is only about 80%. On the other hand, to improve the spectral response, shallow junctions are made. Subsequently, dense grids have to be made. The calculation shows that: when the thickness of the surface layer is 0.15μ, 30 dense grids/cm are needed, which can only be realized by photolithography technology. As a result, the cost of the solar cell is greatly increased.

(2) In order to make the resistance of the surface layer low, the surface layer must be relatively thick, generally about 1μ. Such a thick highly doped layer is called a "dead layer". It can absorb a lot of incident light (for example: 0.5μ silicon can absorb 9% of sunlight; 2μ thick sunlight is almost completely absorbed), but can not be converted into electricity. It also has a large number of structural defects, which will trap photo-generated carriers, making it unable to contribute to the photo-generated current. Therefore, the thick surface layer is also a non-negligible limitation to improve the conversion efficiency of solar cells.

Combining the above factors, a rough calculation shows that even if the existing crystalline silicon solar cell adopts an appropriate gate shallow density and a matching surface thickness, the conversion efficiency of the solar cell will still be lost by about 30%.

3. Contents of the invention

The purpose of the present invention is to overcome the disadvantages of the prior art and obtain a crystalline silicon solar cell with high conversion efficiency and low manufacturing cost.

The technical means that the present invention adopts is, to the solar cell that structure is back electrode/formed on the crystalline silicon of n-p junction/front electrode, the front electrode that adopts is the front electrode of transparent conductive film, to replace grid shape front electrode. Appropriate selection of the type and preparation method of the transparent conductive film can make its refractive index 1.8-2.0, and it will have an anti-reflection effect when it is prepared to an appropriate thickness. Therefore, the front electrode of the transparent conductive film used in the present invention will play the double role of front electrode and anti-reflection.

In order to make the present invention achieve a better effect, consider that if the transparent conductive film adopts a high-conductivity transparent conductive film, it is a wide bandgap degenerate semiconductor with a high doping concentration, and there will be a gap between it and highly doped silicon. Lots of interface states. Therefore, the present invention introduces a barrier layer between the high-conductivity transparent conductive film and n-type silicon, that is, a sufficiently thick high-conductivity transparent conductive film conductive layer and a sufficiently thin high-resistance transparent conductive film barrier layer The front electrode of the transparent conductive film is formed to replace the grid-shaped front electrode and the anti-reflection layer.

In order to realize the present invention, according to the existing preparation technology, the conductive layer of the high-conductivity transparent conductive film in the transparent conductive film front electrode can adopt ITO (In ₂ O ₃ : SnO ₂ ), SnO ₂ , ZnO or Cd ₂ SnO ₄ etc. crystal thin film, the doping concentration is 10 ¹⁹ -10 ²¹ /cm ³ . Their energy gap width is between 3.3-4.2eV, their bulk conductivity is 10 ³ -10 ⁴ (Ω.cm) ^-1 , and their refractive index is 1.8-2.00. The film can be made 300--900nm thick. Using it as a front electrode has three functions and brings significant effects:

(1) Let the incident light pass through. In the prior art, the light absorption coefficient of the transparent conductive film is very small; the transmittance of the transparent conductive film with a thickness of 500-600 nm is about 95%.

(2) As an anti-reflection layer, it increases the incidence of light. Calculation shows that when the transparent medium with a refractive index of about 1.95 is covered on the silicon surface, when the optical thickness of the transparent medium layer satisfies the relationship nL=K(λ ₀ /4), it has an anti-reflection effect on light with a wavelength of λ ₀ . In the formula, n is the refractive index of the transparent medium, L is its thickness, λ ₀ is the selected light wavelength, and K is a positive integer. When n is 2, λ ₀ is 600nm, then the transparent conductive film has the best anti-reflection effect when the thickness is L=75, 150, 225, 300, 375, 525, 600...nm. This is the basic principle for selecting the thickness of the transparent conductive film in the present invention.

(3) As a conductive electrode, it collects photogenerated current. For a transparent conductive film with a thickness of 600nm, its film layer resistance is about 6Ω.cm ² , which is less than one-tenth of the surface resistance of shallow-junction silicon and less than one-third of that of deep-junction silicon. Therefore, the collection of photogenerated current can be greatly improved.

The high-resistance transparent conductive film barrier layer in the transparent conductive film front electrode can be non-doped SnO ₂ , ZnO or Zn ₂ polycrystalline films such as SnO ₄ or non-doped SnO ₂ , ZnO and other amorphous films (in the process of making The impurity concentration of 10 ¹⁶ /cm ³ is allowed in the process. With the change of preparation process, composition and microstructure, its resistivity is 1~1000Ω.cm. It has a passivation effect on the surface of highly doped silicon and will Greatly reducing the interface state on the high conductance transparent conductive film/highly doped silicon interface will also modify the energy band structure of this interface. Therefore, it is called a barrier layer. The thickness of the barrier layer is determined by its own The resistivity is determined. The basic consideration is that it should not only play the role of passivation and energy band modification, but also not bring additional series resistance. For example, when the resistivity is 1-10Ω.cm, the thickness is preferably 100nm. When the volume resistivity is 100-1000Ω.nm, the thickness is reduced to 10nm or less.

The present invention makes the performance of crystalline silicon solar cell have following improvement:

(a) increased the amount of incident light by about 5%;

(b) By improving the lateral conductance, it is equivalent to increasing the effective area of the solar cell by about 15-20%;

(c) The crystalline silicon solar cell can be made into a shallow junction, the short-wave spectrum response can be improved, and the output power can be increased by 5-10%.

The combination of the above three effects can increase the conversion efficiency by 35-40%. It is believed that with the improvement of the performance of the transparent conductive film, its effect will be more significant. In addition, implementation of the invention reduces the cost of crystalline silicon solar cells. Since the cost of making the transparent conductive film is equivalent to the cost of making the anti-reflection film, the preparation cost of the front electrode is much lower than that of making a dense grid by photolithography technology, and is also lower than that of making a common grid electrode by screen printing technology. Therefore, the cost will be reduced by more than 10%.

4. Description of drawings

Fig. 1 is the structural diagram of existing crystalline silicon solar cell;

Figure 2 is a structural diagram of the MIS solar cell;

Fig. 3 is a structural diagram of a crystalline silicon solar cell of the present invention.

5. Specific implementation

Embodiments of the present invention will be described in detail below in conjunction with the accompanying drawings.

According to the solar cell structure shown in Figure 3: back electrode (A)/crystalline silicon (B)/high resistance transparent conductive film barrier layer (G)/high conductance transparent conductive film conductive layer (H) with n-p junction (C) )/metal electrode (K), the order of making the front electrode of the crystalline silicon solar cell transparent conductive film is: on the crystalline silicon substrate that has formed n-p junction, first deposit the high-resistance transparent conductive film barrier layer of 10-100nm, then deposit 300 --900nm thick high-conductivity transparent conductive film conductive layer, and finally make metal electrodes. According to the size of the silicon chip, one or several parallel metal electrodes can be made, and the width between the strip metal electrodes can be equal to or greater than 10mm.

The following implementation processes are all carried out on the silicon substrate (B) on which the n-p junction (C) has been formed.

Example 1:

The high-resistance transparent conductive film barrier layer is made of amorphous SnO ₂ and prepared by PECVD, which can obtain high resistivity. Generally, the resistivity is ≥10 ⁴ Ω.cm. The conductive layer of the high-conductivity transparent conductive film adopts doped SnO ₂ . The specific implementation is as follows: first, a barrier layer (G) with a thickness of 5-10 nm is prepared by PECVD technology. Then prepare the conductive layer (H) of the high-conductivity transparent conductive film by the sputtering method, the low-pressure CVD method or the normal-pressure CVD method. The thickness is controlled to be 150-750 nm or thicker according to the anti-reflection conditions. Finally, the metal front electrode (K) is fabricated by screen printing, sputtering or vacuum evaporation. Generally, the density of front electrode strips is 0.5-2 strips/cm.

When the resistivity of the amorphous SnO ₂ barrier layer is 10 ⁴ Ω.cm and the thickness is 5nm; the conductivity of the doped SnO ₂ transparent conductive film conductive layer is 10 ⁴ (Ω.cm) ^-1 and the thickness is 600nm , The conversion efficiency is about 30% higher than that using grid-shaped front electrodes.

Example 2:

The barrier layer of the high-resistance transparent conductive film is made of amorphous SnO ₂ , and the conductive layer of the high-conductivity transparent conductive film is made of ITO thin film, or doped ZnO thin film or Cd ₂ SnO ₄ thin film. A barrier layer (G) of 5-10 nm is firstly prepared by PECVD technology. Then, the conductive layer (H) of the high-conductivity transparent conductive film is prepared by the sputtering method or the APCVD method, and the thickness is 150-750nm or thicker. Finally, the metal front electrode (K) was prepared as described in Example 1.

When the resistivity of the amorphous SnO ₂ barrier layer is 10 ⁴ Ω.cm and the thickness is 5nm; the conductivity of the conductive layer of the ITO transparent conductive film is 1.5×10 ⁴ (Ω.cm) ^-1 and the thickness is 600nm, when the conversion The efficiency is about 35% higher than using grid-shaped front electrodes.

Example 3:

The high-resistance transparent conductive film barrier layer SnO ₂ adopts polycrystalline thin film, and the high-conductivity transparent conductive film conductive layer adopts doped SnO ₂ thin film or ITO film, or doped ZnO thin film, Cd ₂ SnO ₄ thin film. Firstly, an undoped high-resistance transparent conductive film barrier layer (G) is prepared by low-pressure CVD or normal-pressure CVD. Since its resistivity is between 5-10Ω.cm, its thickness is controlled at about 100nm. Then, according to the technique described in Example 1 or Example 2, a conductive layer (H) of a high-conductivity transparent conductive film is produced, with a thickness of 150-750 nm, or thicker. That is, the conductive layer of _SnO2 polycrystalline thin film transparent conductive film can be prepared by sputtering method, low-pressure CVD method or normal pressure CVD method; or the ITO film can be prepared by sputtering method or normal pressure CVD method; or the doped ZnO thin film can be prepared by other methods , Cd ₂ SnO ₄ film. Finally, the metal front electrode (K) was fabricated according to the method of Example 1.

When the resistivity of the undoped SnO ₂ barrier layer is 1-10Ω.cm and the thickness is 50-100nm; the conductivity of the ITO layer is 1.5×10 ⁴ (Ω.cm) ^-1 and the thickness is 600nm, the conversion efficiency ratio About 25% higher using grid electrodes.

Example 4:

The barrier layer of the high-resistance transparent conductive film is Zn ₂ SnO ₄ thin film, and the material used for the conductive layer of the high-conductivity transparent conductive film can be the same as that in Embodiment 3. First prepare the barrier layer (G) by sputtering or other methods. Since its resistivity is between 10-1000Ω.nm, the thickness of the barrier layer is controlled between 20-50nm. Then, according to the technique of Example 1 or Example 2, a conductive layer (H) of 150-750 nm or thicker transparent conductive film is produced. Finally, the metal front electrode (K) was fabricated according to the technique of Example 1.

When the resistivity of the Zn ₂ SnO ₄ barrier layer is 1000Ω.cm and the thickness is 10nm; the conductivity of the Cd ₂ SnO ₄ layer is 1.5×10 ⁴ (Ω.cm) ^-1 and the thickness is 600nm, the conversion efficiency is higher than that of the gate Shaped electrodes are about 35% higher.

Example 5:

The barrier layer of the high-resistance transparent conductive film adopts an undoped ZnO amorphous film, and the conductive layer of the high-conductivity transparent conductive film can be a doped ZnO film, or ITO, doped SnO ₂ or Cd ₂ SnO ₄ in the above-mentioned embodiments wait. First prepare the barrier layer (G) by PECVD or other methods. Since the resistivity is between 1-100Ω.nm, the thickness of the ZnO barrier layer is controlled between 5-50nm, and then a transparent conductive film conductive layer (H) with a thickness of 150-750nm or thicker is fabricated. Finally, make the metal front electrode (K) according to Example 1.

When the resistivity of the ZnO barrier layer is 1000Ω.cm and the thickness is 10nm; the conductivity of the doped ZnO layer is 0.8×10 ⁴ (Ω.cm) ^-1 and the thickness is 600nm, the conversion efficiency is higher than that of grid electrodes 25% or so.

Embodiment 6:

The high-resistance transparent conductive film barrier layer can be an oxide film with higher resistivity, such as: TiO ₂ or Al ₂ O ₃ , SiO ₂ film. The conductive layer of the high-conductivity transparent conductive film can be made of ITO or other materials used in the above-mentioned embodiments. The barrier layer (G) of the transparent conductive film is prepared first, and the thickness is controlled to be 2-10 nm. Then, according to the method adopted in the above-mentioned embodiments, a conductive layer (H) of 150-750 nm or thicker transparent conductive film is produced. Finally, make the metal front electrode (K) according to Example 1.

When the resistivity of the TiO ₂ barrier layer is higher than 1000Ω.cm and the thickness is 2nm; the conductivity of the ITO layer is 0.8×10 ⁴ (Ω.cm) ^-1 and the thickness is 600nm, the conversion efficiency is 35% higher than that of grid electrodes. %about.

Claims (3)

1. crystal silicon solar energy battery, its structure are back electrode/be shaped on the crystalline silicon/preceding electrode of n-p knot, and electrode is an electrode before the nesa coating before it is characterized in that.

2. solar cell as claimed in claim 1 is characterized in that the preceding electrode of nesa coating is led the nesa coating conductive layer by high electricity and high resistance nesa coating barrier layer constitutes.

3. solar cell as claimed in claim 2 is characterized in that it is 10 that high electricity is led nesa coating conductive layer employing doping content ¹⁹-10 ²¹/ cm ³ITO or ZnO or SnO ₂Or Cd ₂SaO ₄Polycrystal film; Plain SnO is adopted on high resistance nesa coating barrier layer ₂Or ZnO or Zn ₂SnO ₄Polycrystal film or SnO ₂Attitude film or TiO ₂Or Al ₂O ₃Or SiO ₂Film.

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