CN108511537B - Solar cell - Google Patents

Abstract

The present invention relates to a solar cell, comprising: a substrate; a back electrode layer disposed above the substrate, the back electrode layer comprising a first layer and a second layer, the first layer is adjacent to one side of the substrate, the The second layer has a volume density less than that of the first layer; a light absorbing layer disposed over the second layer; and a front electrode layer disposed over the second layer; and wherein the first There is no sharp demarcation between the layer and the second layer. Compared with solar cells with a smooth back electrode surface in the prior art, this kind of solar cell with a rough surface layer with a moderate volume density is conducive to spontaneously forming a light-limiting effect, reducing the light reflectance of the solar cell, and improving the light efficiency. Absorption rate.

Description

a solar cell

technical field

The invention relates to the field of solar photovoltaics, in particular to a solar cell.

Background technique

Rapid economic development has brought about problems such as global energy crisis and environmental pollution, and the development of renewable and clean energy is imminent. In recent years, as a new energy source, solar energy has gradually replaced fossil energy due to its advantages of cheapness, abundant content, easy availability and no pollution. The use of solar energy as energy is mainly reflected in the use of it for power generation.

Thin film solar cell, also known as "solar chip" or "photovoltaic cell", is a device that converts light energy into electrical energy. Thin-film solar cells are mainly composed of copper indium gallium selenide (abbreviated as: CIGS) materials and other materials to form a P-N junction film on the substrate to generate electricity. It has strong light absorption capacity, high conversion efficiency, low manufacturing cost, and flexibility. The advantages of stable power generation and environmental friendliness. The conversion efficiency of thin-film solar cells refers to the percentage of the converted effective electric energy to the incident sunlight energy. At present, the highest conversion efficiency of solar cells in the laboratory has exceeded 22%, but it is still difficult to achieve such a high conversion efficiency in actual industrial production. . Improving the light energy conversion efficiency of solar cells can effectively save production costs and further solve the problem of energy crisis.

Contents of the invention

Aiming at the technical problems existing in the prior art, the present invention proposes a solar cell, comprising: a substrate; a back electrode layer disposed above the substrate, the back electrode layer comprising a first layer and a second layer, the The first layer is adjacent to the substrate side, the volume density of the second layer is smaller than the volume density of the first layer; the light absorbing layer is arranged above the second layer; and the front electrode layer is arranged on the first layer Above the second floor.

In the above solar cell, the first back electrode layer and the second back electrode layer include metal molybdenum.

In the above solar cell, the thickness of the second back electrode layer is 5-30 nm.

According to the above solar cell, the thickness of the first back electrode layer is 300-1000nm.

As for the above solar cell, the molybdenum content per unit volume of the second back electrode layer is about 6 g/cm ³ .

As for the above solar cell, the molybdenum content per unit volume of the first back electrode layer is about 10 g/cm ³ .

In the above solar cell, the second back electrode layer has a rough surface layer, and the roughness Ra is Ra<30nm.

In the above solar cell, the first back electrode layer may further include a first deposition layer and a second deposition layer, the volume density of the first deposition layer is smaller than the volume density of the second deposition layer, wherein the The first deposited layer is proximate to the substrate.

A method for preparing a thin film solar cell, comprising: preparing a first back electrode layer on a substrate; preparing a second back electrode layer on the first back electrode layer, wherein the volume density of the second back electrode layer is smaller than that of the first back electrode layer A volume density of the back electrode layer; a light absorbing layer is prepared on the second back electrode layer; a front electrode layer is prepared on the light absorbing layer.

Compared with solar cells with a smooth back electrode surface in the prior art, a solar cell with a back electrode surface with a moderate volume density rough surface layer is conducive to the spontaneous formation of a light confinement effect, reducing the light reflectance of the solar cell, and improving light absorption Rate.

Description of drawings

Below, preferred embodiment of the present invention will be described in further detail in conjunction with accompanying drawing, wherein:

1A and 1B are schematic diagrams of a thin film solar cell according to an embodiment of the present invention; and

Fig. 2 is a flow chart of the preparation of a thin film solar cell according to an embodiment of the present invention.

Detailed ways

In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments It is a part of embodiments of the present invention, but not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by persons of ordinary skill in the art without making creative efforts belong to the protection scope of the present invention.

In the following detailed description, reference is made to the accompanying drawings which are included in the specification and which illustrate specific embodiments of the application and which are included in this application. In the drawings, like reference numerals describe substantially similar components in different views. Various specific embodiments of the present application are described in sufficient detail below, so that those of ordinary skill in the art can implement the technical solutions of the present application. It should be understood that other embodiments may also be utilized or structural, logical or electrical changes may be made to the embodiments of the present application.

The basis of solar cell work is the photovoltaic effect of the semiconductor PN junction, that is, when sunlight shines on the PN junction in the solar cell, the internal charge distribution state changes, and electromotive force and current are generated on both sides of the PN junction.

1A and 1B are schematic diagrams of a thin film solar cell according to an embodiment of the present invention. According to an embodiment of the present invention, common soda-lime glass is used as the substrate 101, and the sodium element therein enters the CIGS grains (light-absorbing layer 103) in a diffused form to promote the growth of the CIGS grains (light-absorbing layer 103), Optimizing the electrical properties of the light absorbing layer 103 can especially improve its P-type characteristics. As an optional embodiment, the base 101 may also use other rigid materials such as glass, ceramics, etc., or flexible materials such as metals, plastics, etc. It should be noted that some glasses may need to be treated with special processes when used, such as borosilicate glass and polyimide glass; if metal is selected as the substrate 101, the upper surface of the metal needs to be in contact with other components of the solar cell. Plating an insulating barrier layer on the side, such as silicon oxide, silicon nitride, etc.; if you choose plastic as the substrate, you need to pay attention to the temperature limit that the selected plastic can withstand.

The back electrode 102 is plated on the surface of the substrate 101. According to an embodiment of the present invention, metal molybdenum (Mo) is used as the back electrode 102, which has the advantages of good stability, high reflectivity and low resistance. As an optional embodiment, metal tungsten (W) or transparent conductive layer (TCO) may also be used as the back electrode. According to an embodiment of the present invention, the back electrode 102 has a two-layer structure, wherein the first layer 1021 is adjacent to the substrate 101 , and the second layer 1022 includes a rough surface layer. According to an embodiment of the present invention, there is no obvious boundary between the first layer and the second layer. According to an embodiment of the present invention, the volume density of the rough surface layer 1022 is lower than that of the first layer 1021, the volume density is about 4-8 g/cm ³ , and the surface roughness is not more than 30 nm. A rough surface layer with an appropriate roughness can improve the adhesion of the back electrode to the above attachment layer. In the prior art, the volume density of the back electrode is about 9-10g/cm ³ , and the surface roughness is about 4nm, which is a smooth surface. When the rest of the film layers are continuously plated on it, a smooth surface will be formed. In this case, the surface roughness of the light absorbing layer 103 is too low, the bulk density is high, and the reflectivity is high, which is not conducive to sufficient absorption of light energy. Compared with solar cells with a smooth back electrode surface in the prior art, a solar cell with a moderately rough back electrode surface has a lower volume density surface, which is conducive to spontaneously forming a light-limiting effect and reducing the light reflectance of the solar cell. Improve light absorption. According to an embodiment of the present invention, it is found through experiments that the surface volume density of the back electrode is about 4-8 g/cm ³ , and the surface roughness is preferably below 30 nm. Collecting and analyzing the results of the embodiments of the present invention, it can be concluded that when the surface volume density is about 5-7 g/cm ³ and the roughness is about 10-20 nm, the conversion efficiency of the solar cell is significantly improved. According to an embodiment of the present invention, the volume density can also be defined as the volume of a certain material in a unit volume, the volume density of the first back electrode layer is greater than the volume density of the second back electrode layer can be in a unit volume, The content of Mo in the first back electrode layer is greater than that in the second back electrode layer, and it can also be that in a unit volume, the volume occupied by the first electrode layer is greater than the volume occupied by the second back electrode layer.

According to an embodiment of the present invention, the first layer 1021 may further include a first deposition layer and a second deposition layer. Wherein the first deposition layer is adjacent to one side of the substrate, and the second deposition layer is arranged between the first deposition layer and the second layer. According to an embodiment of the present invention, the volume density of the first deposition layer is smaller than that of the second deposition layer, that is to say, the first deposition layer has greater roughness than the second deposition layer, which can increase the adhesion of the back electrode; The second deposition layer is denser than the first deposition layer, has a higher bulk density, smoother surface, lower roughness, and better electrical conductivity. Combined with the rough surface layer of the second layer 1022 in the present invention, the conversion efficiency of the solar cell is obviously improved.

The surface of the back electrode rough surface layer 1022 is the light absorbing layer 103 . If the back electrode has a smooth surface layer, then the light absorbing layer on it also has a smooth surface; if the back electrode has a rough surface layer, then the light absorbing layer on it also has a rough surface layer. According to an embodiment of the present invention, the light absorbing layer may be a CIGS thin film with uniform thickness. CIGS film is a chalcopyrite crystal composed of copper (Cu), indium (In), gallium (Ga), and selenium (Se) in proportion, and has P-type characteristics in solar cells. According to the difference in the ratio of gallium to indium, its bandgap width is continuously adjustable in the range of 1.02eV to 1.65eV, making it applicable to different lighting conditions. If the band gap increases, sulfur can also be used instead of selenium to lower the valence band. According to an embodiment of the present invention, the light absorbing layer is plated on the surface of the back electrode whose roughness is less than 30 nm, and correspondingly has a rough surface layer. When a solar cell with a light-absorbing layer with a rough surface layer is placed in sunlight, the reflection of light energy on the rough surface is reduced and the absorptivity is increased.

The buffer layer 104 on the upper surface of the light-absorbing layer 103 is located between the light-absorbing layer 103 and the front electrode layer, which can reduce the discontinuity of the bandgap between the two, solve the problem of lattice mismatch, and thus solve the problem of mismatching the forbidden band width. According to one embodiment of the present invention, cadmium sulfide (CdS) is used as the buffer layer, which has the characteristics of an N-type semiconductor material. Cadmium sulfide, with high light transmittance, can reduce the light loss of thin-film solar cells, thereby effectively increasing the photoelectric conversion efficiency of solar cells, and is an ideal buffer material for solar cells.

The front electrode layer includes high-resistance zinc oxide and low-resistance zinc oxide, which respectively constitute the high-resistance layer 105 and the transparent electrode layer 106 . According to an embodiment of the present invention, the high resistance layer 105 is an intrinsic zinc oxide layer (i-ZnO). The transparent electrode layer 106 can be used as the upper electrode of the solar cell. In actual production, the transparent electrode layer 106 can be made of n-doped transparent conductive materials (n-ZnO), such as AZO, GZO, IZO, ITO and the like. According to an embodiment of the present invention, the transparent electrode layer 106 is made of aluminum-doped zinc oxide (AZO), which has high visible light transmittance and low surface resistance, which can reduce series resistance loss and improve the conversion efficiency of solar cells. Finally, the front electrode layer together with the light absorbing layer 103 forms the PN junction part of the solar cell to realize the function of solar power generation.

A grid 107 is plated on the front electrode layer for collecting current. Based on the above 101-107, a solar cell that can realize photovoltaic power generation is finally formed.

Fig. 2 is a flow chart of the preparation of a thin film solar cell according to an embodiment of the present invention. As shown in the figure, the method 200 for preparing a solar cell includes the following steps: firstly, a substrate 201 is obtained, and a suitable material is selected as the substrate of the solar cell. According to an embodiment of the present invention, the selected substrate is soda lime glass. According to an optional embodiment of the present invention, in practical application and production, the substrate may also be selected from other rigid materials such as glass, ceramics, etc., or flexible materials such as metals, plastics, etc.

After the substrate 201 is obtained, the back electrode 202 is plated on the selected substrate. According to an embodiment of the present invention, metal molybdenum is selected as the back electrode, and a molybdenum back electrode layer of about 500 nm is plated on the substrate by magnetron sputtering. According to one embodiment of the present invention, first use the first sputtering gas pressure to deposit the first deposition layer of 10-100nm on the substrate as an adhesion layer; A second deposited layer, as a conductive layer, wherein the first sputtering pressure is higher than the second sputtering pressure. This kind of multiple sputtering by changing the sputtering pressure can improve the adhesion and conductivity of the back electrode. Depending on the environment in which the solar cell is installed and the required output power, only one of the sputtering methods can be used to plate the back electrode layer. The first sputtering gas pressure and the second sputtering gas pressure mentioned here do not refer to specific gas pressures, but are relative concepts. In the actual manufacturing process, the sputtering pressure needs to be adjusted according to the actual usage.

Surface modification treatment 203 on the back electrode. According to an embodiment of the present invention, the surface modification treatment of the back electrode is realized by using an ion etching method. That is, after the plating of the back electrode 202 is completed, it enters a vacuum chamber with an Ar ⁺ source, accelerates the Ar ⁺ with a voltage of about 1KV, and etches the surface of the back electrode for about 2-10 minutes to form a rough film surface. According to the relevant experiments of the present invention, under this condition, the roughness of the rough surface layer formed by ion etching for 2-10 minutes is about 10-20 nm. In addition to the ion etching method, according to an embodiment of the present invention, the surface of the back electrode can also be formed with the same roughness by means of mechanical sandblasting and etching.

The method of modifying the surface of the back electrode 203 is not limited to etching. According to an embodiment of the present invention, the magnetron sputtering method can also be used to continuously plate the molybdenum layer on the surface of the plated back electrode by using the third sputtering pressure. Wherein the third sputtering pressure is higher than the second sputtering pressure. Those skilled in the art can easily understand that the film formed by the magnetron sputtering method under high pressure is loose and porous, and the surface is rough. rough surface layer.

A light absorbing layer 204 is plated on the surface of the modified back electrode. According to an embodiment of the present invention, a light-absorbing layer with a thickness of about 2000-3000 nm is plated by a co-evaporation method, and the light-absorbing layers in the present invention are all CIGS layers. According to another embodiment of the present invention, the CIGS layer can also be plated by a selenization method after sputtering. Those skilled in the art should understand that there are many methods for plating a CIGS layer, in addition to the two methods listed above, there are also three-step co-evaporation method, electrochemical deposition method, spray pyrolysis method, screen printing method and so on. The co-evaporation method and the selenization method after sputtering used in the present invention are the methods with the most extensive research, relatively mature technology and high battery efficiency. According to an embodiment of the present invention, the CIGC layer can also be plated by combining the evaporation method with the selenization method after sputtering. Since the light absorbing layer is plated on the basis of the back electrode, and the surface of the back electrode is a rough surface layer, the CIGS layer obtained after plating also has a rough surface.

A buffer layer 205 is plated. According to one embodiment of the present invention, the buffer layer is cadmium sulfide. According to an embodiment of the present invention, a buffer layer with a thickness of about 40 nm is plated on the surface of the light absorbing layer by using a chemical water bath method. As understood by those skilled in the art, other methods can also be used for buffer layer plating, such as vacuum evaporation, sputtering atomic layer chemical vapor deposition, electrodeposition, and the like.

The front electrode layer-high resistance layer 206 is plated. According to an embodiment of the present invention, an intrinsic zinc oxide film with a thickness of about 50 nm is plated on the buffer layer by magnetron sputtering. According to an embodiment of the present invention, the high resistance layer can also be plated by radio frequency sputtering. The plating of the high resistance layer is not limited to these two methods.

The front electrode layer-transparent electrode layer 207 is plated. According to an embodiment of the present invention, the transparent electrode layer is aluminum-doped zinc oxide, and a transparent electrode layer with a thickness of about 300 nm is plated on the high resistance layer by magnetron sputtering. In addition, the radio frequency sputtering method, reactive sputtering method, etc. can also be used to plate the transparent electrode layer. If large-scale plating is required, the DC radio frequency method can also be used.

Finally, the grid 208 is plated. Evaporate the grid on the surface of the battery to finally form a battery with the above layers.

Using the solar cell prepared by the above method, the cell performance is shown in the following table:

Table 1:

As shown in Table 1, the surface roughness in the table is the surface roughness of the back electrode, reflecting the surface layers of the back electrode with different volume densities. In the prior art, the volume density of the back electrode is about 10 g/m ³ , and the roughness is about 4 nm. Compared with the prior art, when the surface roughness of the back electrode increased to 15nm, its open circuit voltage V _OC was constant, the short-circuit current density J _SC was significantly increased, and the conversion efficiency η of the solar cell was significantly increased; when the surface of the back electrode was rough When the density increases to 30nm, although the short-circuit current density J _SC increases, the open-circuit voltage V _OC decreases, the fill factor FF decreases, and the conversion efficiency η of solar cells decreases instead. However, it can be seen from the table that when the surface roughness of the back electrode is 15nm and 30nm, compared with the prior art, the short-circuit current density increases, indicating that increasing the surface roughness is beneficial to increasing the light absorption rate.

It is known through multiple sets of experiments that after the surface modification treatment of the back electrode, when the roughness thereof is in the range of 10-20 nm, it is beneficial to improve the conversion efficiency of the solar cell. An overly rough surface of the back electrode will affect the grain growth of CIGS, and then affect the open-circuit voltage V _OC of the solar cell, thereby affecting the conversion efficiency of the solar cell and reducing the conversion efficiency.

The above-described embodiments are only for illustrating the present invention, rather than limiting the present invention. Those of ordinary skill in the relevant technical field can also make various changes and modifications without departing from the scope of the present invention. Therefore, all Equivalent technical solutions should also belong to the scope of the disclosure of the present invention.

Claims (1)

1. A solar cell, comprising:

a substrate;

the back electrode layer is arranged above the substrate and comprises a first back electrode layer and a second back electrode layer, the first back electrode layer is close to the substrate, and the volume density of the second back electrode layer is smaller than that of the first back electrode layer;

a light absorbing layer disposed over the second back electrode layer;

and

a front electrode layer disposed over the light absorbing layer;

the first back electrode layer and the second back electrode layer comprise metal molybdenum;

the thickness of the second back electrode layer is 5-30nm;

the molybdenum content in the unit volume of the second back electrode layer is 6g/cm ³ ；

The molybdenum content in the unit volume of the first back electrode layer is 10g/cm ³ ；

The second back electrode layer is provided with a rough surface layer, and the roughness Ra is 10-20nm;

wherein the first back electrode layer is adjacent to the substrate, the second back electrode layer comprises a rough surface layer, no obvious boundary exists between the first layer and the second layer, and the volume density of the rough surface layer is less than that of the first back electrode layer; the volume density of the first back electrode layer is larger than that of the second back electrode layer, namely in a unit volume, the content of Mo in the first back electrode layer is larger than that of the second back electrode layer, or in the unit volume, the volume occupied by the first back electrode layer is larger than that of the second back electrode layer;

the first back electrode layer comprises a first deposition layer and a second deposition layer, wherein the first deposition layer is close to one side of the substrate, the second deposition layer is arranged between the first deposition layer and the second deposition layer, and the volume density of the first deposition layer is smaller than that of the second deposition layer;

the surface of the rough surface layer of the back electrode is provided with a light absorption layer; the light absorption layer is a CIGS thin film with uniform thickness, and is plated on the surface of the back electrode with the roughness of 10-20nm;

a buffer layer on the upper surface of the light absorbing layer between the light absorbing layer and the front electrode layer, and cadmium sulfide (CdS) as the buffer layer;

the front electrode layer comprises high-resistance zinc oxide and low-resistance zinc oxide which respectively form a high-resistance layer and a transparent electrode layer, and the high-resistance layer is an intrinsic zinc oxide layer (i-ZnO); the transparent electrode layer is an upper electrode of the solar cell, and aluminum-doped zinc oxide (AZO) is used as the transparent electrode layer;

the grid is plated on the front electrode layer and used for collecting current; finally forming a solar cell capable of realizing photovoltaic power generation; the thickness of the first back electrode layer is 300-1000nm.

Images