阅读说明：本技术 新型叠层硅量子点异质结太阳能电池及其制备方法 (Novel laminated silicon quantum dot heterojunction solar cell and preparation method thereof ) 是由 单丹 周寿斌 唐明军 杨瑞洪 曹蕴清 钱松 仇实 陈雪圣 于 2019-10-12 设计创作，主要内容包括：本发明公开了一种新型叠层硅量子点异质结太阳能电池及其制备方法,属于光电技术领域,该太阳能电池包括依次叠加的Al电极层、n型硅衬底、n型硅纳米线、p型叠层渐变带隙硅量子点多层膜、石墨烯层和Au电极层。该方法包括：n型硅纳米线的刻蚀,通过等离子体增强气相沉积工艺在n型硅衬底和n型硅纳米线上生长p型叠层渐变带隙硅量子点多层膜；通过气相沉积工艺在p型叠层渐变带隙硅量子点多层膜上制备石墨烯层；在石墨烯层上蒸镀Au电极层；在n型硅衬底背面蒸镀Al电极层。本发明形成异质结结构,硅材料成本可节约30%,利用叠层硅量子点多层薄膜的渐变带隙来拓宽吸收层中的光响应范围,改善了器件的光电性能,提高电池的光电转换效率至7.4%。(The invention discloses a novel laminated silicon quantum dot heterojunction solar cell and a preparation method thereof, belonging to the technical field of photoelectricity. The method comprises the following steps: etching the n-type silicon nanowire, and growing a p-type laminated graded band gap silicon quantum dot multilayer film on the n-type silicon substrate and the n-type silicon nanowire by a plasma enhanced vapor deposition process; preparing a graphene layer on the p-type laminated graded band gap silicon quantum dot multilayer film by a vapor deposition process; evaporating an Au electrode layer on the graphene layer; and evaporating an Al electrode layer on the back surface of the n-type silicon substrate. The invention forms a heterojunction structure, the cost of silicon materials can be saved by 30%, the gradual change band gap of the laminated silicon quantum dot multilayer film is utilized to widen the photoresponse range in the absorption layer, improve the photoelectric performance of devices and improve the photoelectric conversion efficiency of the cell to 7.4%.)

1. The laminated silicon quantum dot heterojunction solar cell is characterized in that: the graphene-based multilayer silicon quantum dot structure comprises an Al electrode layer (1) at the bottom and an Au electrode layer (6) at the top, wherein the Al electrode layer (1) is evaporated on the surface of one side of an n-type silicon substrate (2), a plurality of n-type silicon nanowires (3) which are vertically distributed are etched on the surface of the other side of the n-type silicon substrate (2), the n-type silicon substrate (2) and the n-type silicon nanowires (3) are of an integral structure with the same material, a p-type laminated gradient band gap silicon quantum dot multilayer film (4) is uniformly deposited on the surfaces of the n-type silicon substrate (2) and the n-type silicon nanowires (3), a graphene layer (5) is laid on the surface of the p-type laminated gradient band gap silicon quantum dot multilayer film (4), and the Au electrode layer (6) is evaporated on the surface.

2. The tandem silicon quantum dot heterojunction solar cell of claim 1, wherein: the p-type laminated graded band gap silicon quantum dot multilayer film (4) comprises 6 layers of silicon quantum dot films, the thicknesses of every two layers of silicon quantum dot films are consistent, three thickness specifications are provided, a silicon carbide film is arranged between every two adjacent silicon quantum dot films, the silicon quantum dot film with the largest thickness is arranged on one surface close to the n-type silicon substrate (2) and the n-type silicon nanowire (3), and the silicon quantum dot film with the smallest thickness is close to the graphene layer (5).

3. The tandem silicon quantum dot heterojunction solar cell of claim 2, wherein: the thickness of the silicon quantum dot film is 8 nm, 4 nm and 2 nm in sequence, and the thickness of the silicon carbide film is 2 nm.

4. The tandem silicon quantum dot heterojunction solar cell of claim 1, wherein: the thickness of the Al electrode layer (1) is 20 nm-100 nm; the height of the n-type silicon nanowire (3) is 700 nm; the thickness of the p-type laminated graded band gap silicon quantum dot multilayer film (4) is 56 nm; the thickness of the graphene layer (5) is 30 nm; the thickness of the Au electrode layer (6) is 20 nm.

5. The preparation method of the laminated silicon quantum dot heterojunction solar cell is characterized by comprising the following steps of: the method comprises the following steps:

first step, etching of n-type silicon nanowires (3)

Etching an n-type silicon nanowire (3) with a cylindrical structure on an n-type silicon substrate (2) by a metal ion assisted chemical etching method;

secondly, preparing a p-type laminated gradient band gap silicon quantum dot multilayer film (4)

Growing a p-type laminated graded band gap silicon quantum dot multilayer film (4) on an n-type silicon substrate (2) and an n-type silicon nanowire (3) by a plasma enhanced vapor deposition process;

thirdly, preparing a graphene layer (5)

Preparing a graphene layer (5) on the p-type laminated graded band gap silicon quantum dot multilayer film (4) by a vapor deposition process;

fourthly, evaporating an Au electrode layer (6) on the graphene layer (5);

and fifthly, evaporating an Al electrode layer (1) on the back surface of the n-type silicon substrate (2).

6. The method for preparing a tandem silicon quantum dot heterojunction solar cell according to claim 5, wherein:

in the first step, the specific process of chemically etching the silicon nanowire with the assistance of metal ions is as follows:

(1) in plastic burningThe cup is filled with 5M/L hydrofluoric acid (HF) and 0.02M/L silver nitrate (AgNO)₃) Soaking the cleaned n-type silicon wafer in the mixed solution, and etching at room temperature until the etching depth of the etched n-type silicon wafer reaches 700nm, wherein the chemical reaction equation of the etching is 4Ag⁺(aq)+Si⁰(s)+6F^-(aq)→4Ag(s)+SiF6^2-(aq)；

(2) And (3) soaking the etched silicon wafer in dilute nitric acid until the tree-shaped reaction residues on the surface are removed, and then washing and drying the silicon wafer by using deionized water to obtain the nanowire array structure vertically distributed on the silicon substrate, wherein the height of the nanowire array structure is 700 +/-30 nm.

7. The method for preparing a tandem silicon quantum dot heterojunction solar cell according to claim 5, wherein:

in the second step, the preparation process of the p-type laminated graded band gap silicon quantum dot multilayer film (4) is as follows:

(1) placing the n-type silicon substrate (2) with the etched n-type silicon nanowires (3) in a reaction chamber, introducing hydrogen with the flow of 20sccm, and pretreating for 5 minutes under the condition that the radio-frequency power is 20W;

(2) the reaction chamber was evacuated and methane (CH) was added₄) And Silane (SiH)₄) The flow ratio of the reaction gas R = [ CH4 ] is fixed by using the mixed gas of (1) as the reaction gas]/[SiH4]5sccm, preparing an amorphous silicon carbide thin film, wherein the deposition time is 20s, and the thickness of the deposited thin film is 2 nm, namely the silicon carbide thin film; then, the reaction chamber is evacuated and Silane (SiH) is introduced₄) And borane (B)₂H₆) The mixed gas is used as a reaction gas, and the flow ratio R = [ B ] of the reaction gas is fixed₂H₆]/[SiH₄]5sccm, depositing a boron-doped amorphous silicon thin film for 80 s, wherein the thickness of the deposited thin film is 8 nm, namely a silicon quantum dot thin film, and performing two periods alternately to form a silicon carbide thin film with the thickness of 2 nm, a silicon quantum dot thin film with the thickness of 8 nm, a silicon carbide thin film with the thickness of 2 nm and a silicon quantum dot thin film with the thickness of 8 nm alternately;

changing the deposition time to 40 s, depositing the boron-doped amorphous silicon film with the thickness of 4 nm, and alternately performing two periods of the two processes without changing the other processes to form a silicon carbide film with the thickness of 2 nm, a silicon quantum dot film with the thickness of 4 nm, a silicon carbide film with the thickness of 2 nm and a silicon quantum dot film with the thickness of 4 nm alternately;

finally, changing the deposition time to 20s, depositing a boron-doped amorphous silicon film with the thickness of 2 nm, then depositing an amorphous silicon carbide film with the thickness of 2 nm, and alternately performing two periods of the two processes without changing the other processes to form a silicon carbide film with the thickness of 2 nm, a silicon quantum dot film with the thickness of 2 nm, a silicon carbide film with the thickness of 2 nm and a silicon quantum dot film with the thickness of 2 nm, so that a boron-doped amorphous silicon/amorphous silicon carbide gradient structure multilayer film with the thicknesses of 8 nm, 4 nm and 2 nm from bottom to top is deposited, the substrate temperature is maintained at 250 ℃ in the whole growth process, and the radio frequency power is 30W;

(3) in order to prevent the p-type laminated graded band gap silicon quantum dot multilayer film (4) from cracking in subsequent high-temperature annealing, firstly, the boron-doped amorphous silicon/amorphous silicon carbide graded structure multilayer film sample prepared in the process (2) is subjected to constant-temperature dehydrogenation treatment to enable a large amount of hydrogen contained in the multilayer film to stably escape from the film, the constant-temperature dehydrogenation treatment temperature is 450 ℃, the time is 1 hour,

then, carrying out high-temperature annealing treatment on the sample subjected to dehydrogenation treatment, wherein due to the restrictive crystallization principle, the boron-doped amorphous silicon sublayer is subjected to crystallization nucleation and is longitudinally limited by the silicon carbide layers on two sides to form silicon quantum dots with controllable size, the annealing temperature is 1000 ℃, the annealing time is 1 hour,

in order to ensure that the p-type laminated graded band gap silicon quantum dot multilayer film (4) is not oxidized in the high-temperature treatment process, the dehydrogenation and annealing processes are carried out under the atmosphere of high-purity nitrogen.

8. The method for preparing a tandem silicon quantum dot heterojunction solar cell according to claim 5, wherein:

in the third step, a multilayer film of p-type laminated graded band gap silicon quantum dots is prepared by a chemical vapor deposition CVD method(4) Growing graphene layer on the substrate, introducing a mixed gas of methane and hydrogen in the growth process, wherein the gas flow ratio is CH₄: H₂10sccm, a vacuum degree of 3 Torr, and a growth temperature of 1000 ℃ until a graphene layer with a thickness of 30nm is obtained.

Technical Field

The invention relates to a novel laminated silicon quantum dot heterojunction solar cell and a preparation method thereof, belonging to the technical field of photoelectricity.

Background

With the continuous development of the new generation of solar cells, the nano-silicon structure is considered as a material capable of better adjusting the forbidden bandwidth to realize a wide spectral response, wherein the heterojunction solar cell formed by the nano-silicon structure and the monocrystalline silicon substrate has been a research hotspot of people with wide attention. On the one hand, however, the absorption efficiency for light is low due to the small thickness of the active layer. Thereby leading to lower photoelectric conversion efficiency (5-6%) of the nano silicon-monocrystalline silicon heterojunction solar cell. On the other hand, increasing the thickness of the active layer introduces more surface states and defect states into the device, resulting in a decrease in device performance. Therefore, it is necessary to explore an effective way to realize the broad spectrum absorption and response of thin film batteries by using nanotechnology.

Disclosure of Invention

In order to solve the technical problems, the invention discloses a novel laminated silicon quantum dot heterojunction solar cell and a preparation method thereof, and particularly relates to an n-type silicon/n-type silicon nanowire/p-type laminated graded band gap silicon quantum dot/graphene heterojunction solar cell based on a silicon nanowire light trapping structure and a laminated silicon quantum dot structure with graded band gap and a preparation method thereof, wherein the specific technical scheme is as follows:

the laminated silicon quantum dot heterojunction solar cell comprises an Al electrode layer at the bottom and an Au electrode layer at the top, wherein the Al electrode layer is evaporated on the surface of one side of an n-type silicon substrate, a plurality of n-type silicon nanowires (3) which are vertically distributed are etched on the surface of the other side of the n-type silicon substrate, the n-type silicon substrate and the n-type silicon nanowires are of an integral structure with the same material, a p-type laminated graded band gap silicon quantum dot multilayer film is uniformly deposited on the surfaces of the n-type silicon substrate and the n-type silicon nanowires, a graphene layer is laid on the surface of the p-type laminated graded band gap silicon quantum dot multilayer film, and the Au electrode layer is evaporated on the surface of the graphene layer.

Furthermore, the p-type laminated graded band gap silicon quantum dot multilayer film comprises 6 layers of silicon quantum dot films, the thicknesses of every two layers of silicon quantum dot films are consistent, three thickness specifications are provided, a silicon carbide film is arranged between every two adjacent silicon quantum dot films, the thickness of the silicon quantum dot film close to the n-type silicon substrate and one surface of the n-type silicon nanowire is the largest, and the silicon quantum dot film with the smallest thickness is close to the graphene layer.

Further, the thicknesses of the silicon quantum dot film are 8 nm, 4 nm and 2 nm in sequence, and the thickness of the silicon carbide film is 2 nm.

Further, the thickness of the Al electrode layer is 20 nm-100 nm; the height of the n-type silicon nanowire is 700 nm; the thickness of the p-type laminated graded band gap silicon quantum dot multilayer film is 56 nm; the thickness of the graphene layer is 30 nm; the thickness of the Au electrode layer is 20 nm.

The preparation method of the laminated silicon quantum dot heterojunction solar cell comprises the following steps:

first, etching of n-type silicon nanowires

Etching the n-type silicon nanowire (3) with a cylindrical structure on the n-type silicon substrate by a metal ion assisted chemical etching method;

secondly, preparing the p-type laminated gradient band gap silicon quantum dot multilayer film

Growing a p-type laminated graded band gap silicon quantum dot multilayer film on an n-type silicon substrate and an n-type silicon nanowire by a plasma enhanced vapor deposition process;

step three, preparing a graphene layer

Preparing a graphene layer on the p-type laminated graded band gap silicon quantum dot multilayer film by a vapor deposition process;

fourthly, evaporating an Au electrode layer on the graphene layer;

and fifthly, evaporating an Al electrode layer on the back surface of the n-type silicon substrate.

Further, in the first step, the specific process of chemically etching the silicon nanowire with the assistance of metal ions is as follows:

(1) 5M/L hydrofluoric acid (HF) and 0.02M/L silver nitrate (AgNO) are arranged in a plastic beaker₃) Soaking the cleaned n-type silicon wafer in the mixed solution, and etching at room temperature until the etching depth of the etched n-type silicon wafer reaches 700nm, wherein the chemical reaction equation of the etching is 4Ag⁺(aq)+Si⁰(s)+6F^-(aq)→4Ag(s)+SiF6^2-(aq)；

(2) And (3) soaking the etched silicon wafer in dilute nitric acid until the tree-shaped reaction residues on the surface are removed, and then washing and drying the silicon wafer by using deionized water to obtain the nanowire array structure vertically distributed on the silicon substrate, wherein the height of the nanowire array structure is 700 +/-30 nm.

Further, in the second step, the preparation process of the p-type laminated graded bandgap silicon quantum dot multilayer film is as follows:

(1) introducing hydrogen with the flow of 20sccm into the n-type silicon substrate with the etched n-type silicon nanowires, and pretreating for 5 minutes under the condition that the radio-frequency power is 20W;

(2) the reaction chamber was evacuated and methane (CH) was added₄) And Silane (SiH)₄) The flow ratio of the reaction gas R = [ CH4 ] is fixed by using the mixed gas of (1) as the reaction gas]/[SiH4]5sccm, preparing an amorphous silicon carbide thin film, wherein the deposition time is 20s, and the thickness of the deposited thin film is 2 nm, namely the silicon carbide thin film; then, the reaction chamber is evacuated and Silane (SiH) is introduced₄) And borane (B)₂H₆) The mixed gas is used as a reaction gas, and the flow ratio R = [ B ] of the reaction gas is fixed₂H₆]/[SiH₄]5sccm, depositing a boron-doped amorphous silicon thin film for 80 s, wherein the thickness of the deposited thin film is 8 nm, namely a silicon quantum dot thin film, and performing two periods alternately to form a silicon carbide thin film with the thickness of 2 nm, a silicon quantum dot thin film with the thickness of 8 nm, a silicon carbide thin film with the thickness of 2 nm and a silicon quantum dot thin film with the thickness of 8 nm alternately;

changing the deposition time to 40 s, depositing the boron-doped amorphous silicon film with the thickness of 4 nm, and alternately performing two periods of the two processes without changing the other processes to form a silicon carbide film with the thickness of 2 nm, a silicon quantum dot film with the thickness of 4 nm, a silicon carbide film with the thickness of 2 nm and a silicon quantum dot film with the thickness of 4 nm alternately;

finally, changing the deposition time to 20s, depositing a boron-doped amorphous silicon film with the thickness of 2 nm, then depositing an amorphous silicon carbide film with the thickness of 2 nm, and alternately performing two periods of the two processes without changing the other processes to form a silicon carbide film with the thickness of 2 nm, a silicon quantum dot film with the thickness of 2 nm, a silicon carbide film with the thickness of 2 nm and a silicon quantum dot film with the thickness of 2 nm, so that a boron-doped amorphous silicon/amorphous silicon carbide gradient structure multilayer film with the thicknesses of 8 nm, 4 nm and 2 nm from bottom to top is deposited, the substrate temperature is maintained at 250 ℃ in the whole growth process, and the radio frequency power is 30W;

(3) in order to prevent the p-type laminated gradient band gap silicon quantum dot multilayer film from cracking in subsequent high-temperature annealing, firstly, a boron-doped amorphous silicon/amorphous silicon carbide gradient structure multilayer film sample prepared in the process is subjected to constant-temperature dehydrogenation treatment to enable a large amount of hydrogen contained in the multilayer film to stably escape from the film, the constant-temperature dehydrogenation treatment temperature is 450 ℃, the time is 1 hour,

then, carrying out high-temperature annealing treatment on the sample subjected to dehydrogenation treatment, wherein due to the restrictive crystallization principle, the boron-doped amorphous silicon sublayer is subjected to crystallization nucleation and is longitudinally limited by the silicon carbide layers on two sides to form silicon quantum dots with controllable size, the annealing temperature is 1000 ℃, the annealing time is 1 hour,

in order to ensure that the p-type laminated graded band gap silicon quantum dot multilayer film is not oxidized in the high-temperature treatment process, the dehydrogenation and annealing processes are carried out under the atmosphere of high-purity nitrogen.

Further, in the third step, a graphene layer is prepared on the p-type laminated graded band gap silicon quantum dot multilayer film by a chemical vapor deposition CVD method, and in the growth process, mixed gas of methane and hydrogen is introduced, and the gas flow ratio CH₄:H₂10sccm, a vacuum degree of 3 Torr, and a growth temperature of 1000 ℃ until a graphene layer with a thickness of 30nm is obtained.

The invention has the beneficial effects that:

the n-type silicon, the n-type silicon nanowire, the p-type laminated graded band gap silicon quantum dot and the graphene layer form a heterojunction structure, and compared with the traditional silicon-based thin-film solar cell (the active layer is about 200 nm), the solar cell with the heterojunction structure has the advantage that the production cost (silicon material cost) can be saved by 30%.

The solar cell has simple design and preparation process and low material consumption, and can effectively reduce the production cost of the traditional silicon-based thin-film solar cell.

The method of the invention enhances the light absorption in the absorption layer by utilizing the light trapping effect of the nanowire structure, widens the light response range in the absorption layer by utilizing the gradual change band gap of the laminated silicon quantum dot multilayer film, improves the photoelectric performance of the device and improves the photoelectric conversion efficiency of the battery to 7.4 percent.

Drawings

Figure 1 is an enlarged perspective view of the present invention,

figure 2 is a cross-sectional view of the present invention,

figure 3 is a side cross-sectional view of the p-type stacked graded bandgap silicon quantum dot multilayer film of figure 2,

reference numerals: the silicon quantum dot structure comprises 1-Al electrode layer, 2-n type silicon substrate, 3-n type silicon nanowire, 4-p type laminated gradient band gap silicon quantum dot multilayer film, 5-graphene layer and 6-Au electrode layer.

Detailed Description

The present invention is further illustrated by the following figures and specific examples, which are to be understood as illustrative only and not as limiting the scope of the invention, which is to be given the full breadth of the appended claims and any and all equivalent modifications thereof which may occur to those skilled in the art upon reading the present specification.

As shown in fig. 1, the laminated silicon quantum dot heterojunction solar cell includes an Al electrode layer 1 at the bottom and an Au electrode layer 6 at the top, the Al electrode layer 1 is evaporated on one side surface of an n-type silicon substrate 2, a plurality of n-type silicon nanowires 3 vertically distributed are etched on the other side surface of the n-type silicon substrate 2, the n-type silicon substrate 2 and the n-type silicon nanowires 3 are of an integral structure with the same material, a p-type laminated graded band gap silicon quantum dot multilayer film 4 is uniformly deposited on the surfaces of the n-type silicon substrate 2 and the n-type silicon nanowires 3, a graphene layer 5 is laid on the surface of the p-type laminated graded band gap silicon quantum dot multilayer film 4, and the Au electrode layer 6 is evaporated on the surface of the graphene layer 5.

As can be seen from fig. 1 and 2, the Al electrode layer 1, the n-type silicon substrate 2, the n-type silicon nanowire 3, the p-type laminated graded band gap silicon quantum dot multilayer film 4, the graphene layer 5, and the Au electrode layer 6 are sequentially stacked from bottom to top. The thickness of the Al electrode layer 1 is 20 nm-100 nm; the height of the n-type silicon nanowire 3 is 700 nm; the thickness of the p-type laminated gradient band gap silicon quantum dot multilayer film 4 is 56 nm; the thickness of the graphene layer 5 is 30 nm; the thickness of the Au electrode layer 6 was 20 nm.

As can be seen by combining the attached figure 3, the p-type laminated graded band gap silicon quantum dot multilayer film 4 comprises 6 layers of silicon quantum dot films, the upper two layers are 2 nm silicon quantum dot films, the middle two layers are 4 nm silicon quantum dot films, the lower two layers are 8 nm silicon quantum dot films, and a silicon-rich silicon carbide film with the thickness of 2 nm is arranged between the layers. The light absorption in the absorption layer is enhanced by utilizing the light trapping effect of the nanowire structure, the light response range in the absorption layer is widened by utilizing the gradually-changed band gap of the laminated silicon quantum dot multilayer film, and the photoelectric performance of the device is improved.

The invention relates to a preparation method of a laminated silicon quantum dot heterojunction solar cell, which comprises the following steps:

first, etching of n-type silicon nanowires 3

Etching an n-type silicon nanowire 3 with a cylindrical structure on an n-type silicon substrate 2 by a metal ion assisted chemical etching method, which comprises the following specific steps:

(1) 5M/L hydrofluoric acid (HF) and 0.02M/L silver nitrate (AgNO) are arranged in a plastic beaker₃) Soaking the cleaned n-type silicon wafer in the mixed solution, and etching at room temperature for 5 min to obtain an etched n-type silicon wafer with an etching depth of 700nm and an etching chemical reaction equation of 4Ag⁺(aq)+Si⁰(s)+6F^-(aq)→4Ag(s)+SiF6^2-(aq)；

(2) And (3) soaking the etched silicon wafer in dilute nitric acid for 15 min to remove 'tree-like' reaction residues on the surface, then washing with deionized water, and drying to obtain the vertically distributed nanowire array structure on the silicon substrate, wherein the height is about 700 +/-30 nm.

Secondly, preparing a p-type laminated gradient band gap silicon quantum dot multilayer film 4

A p-type laminated graded band gap silicon quantum dot multilayer film 4 grows on an n-type silicon substrate 2 and an n-type silicon nanowire 3 through a plasma enhanced vapor deposition process, and the preparation process is as follows:

(1) placing the n-type silicon substrate 2 with the etched n-type silicon nanowires 3 in Plasma Enhanced Chemical Vapor Deposition (PECVD) equipment, introducing hydrogen with the flow of 20sccm, and pretreating for 5 minutes under the condition that the radio-frequency power is 20W;

(2) the reaction chamber was evacuated and methane (CH) was added₄) And Silane (SiH)₄) The flow ratio of the reaction gas R = [ CH4 ] is fixed by using the mixed gas of (1) as the reaction gas]/[SiH4]5sccm, preparing an amorphous silicon carbide thin film, wherein the deposition time is 20s, and the thickness of the deposited thin film is 2 nm, namely the silicon carbide thin film; then, the reaction chamber is evacuated and Silane (SiH) is introduced₄) And borane (B)₂H₆) The mixed gas is used as a reaction gas, and the flow ratio R = [ B ] of the reaction gas is fixed₂H₆]/[SiH₄]5sccm, depositing a boron-doped amorphous silicon thin film for 80 s, wherein the thickness of the deposited thin film is 8 nm, namely a silicon quantum dot thin film, and performing two periods alternately to form a silicon carbide thin film with the thickness of 2 nm, a silicon quantum dot thin film with the thickness of 8 nm, a silicon carbide thin film with the thickness of 2 nm and a silicon quantum dot thin film with the thickness of 8 nm alternately;

changing the deposition time to 40 s, depositing the boron-doped amorphous silicon film with the thickness of 4 nm, and alternately performing two periods of the two processes without changing the other processes to form a silicon carbide film with the thickness of 2 nm, a silicon quantum dot film with the thickness of 4 nm, a silicon carbide film with the thickness of 2 nm and a silicon quantum dot film with the thickness of 4 nm alternately;

finally, changing the deposition time to 20s, depositing a boron-doped amorphous silicon film with the thickness of 2 nm, then depositing an amorphous silicon carbide film with the thickness of 2 nm, and alternately performing two periods of the two processes without changing the other processes to form a silicon carbide film with the thickness of 2 nm, a silicon quantum dot film with the thickness of 2 nm, a silicon carbide film with the thickness of 2 nm and a silicon quantum dot film with the thickness of 2 nm, so that a boron-doped amorphous silicon/amorphous silicon carbide gradient structure multilayer film with the thicknesses of 8 nm, 4 nm and 2 nm from bottom to top is deposited, the substrate temperature is maintained at 250 ℃ in the whole growth process, and the radio frequency power is 30W;

(3) in order to prevent the p-type laminated graded band gap silicon quantum dot multilayer film 4 from cracking in subsequent high-temperature annealing, firstly, the boron-doped amorphous silicon/amorphous silicon carbide graded structure multilayer film sample prepared in the process 2 is subjected to constant-temperature dehydrogenation treatment to enable a large amount of hydrogen contained in the multilayer film to stably escape from the film, the constant-temperature dehydrogenation treatment temperature is 450 ℃, the time is 1 hour,

then, carrying out high-temperature annealing treatment on the sample subjected to dehydrogenation treatment, wherein due to the restrictive crystallization principle, the boron-doped amorphous silicon sublayer is subjected to crystallization nucleation and is longitudinally limited by the silicon carbide layers on two sides to form silicon quantum dots with controllable size, the annealing temperature is 1000 ℃, the annealing time is 1 hour,

in order to ensure that the p-type laminated graded band gap silicon quantum dot multilayer film 4 is not oxidized in the high-temperature treatment process, the dehydrogenation and annealing processes are carried out under the atmosphere of high-purity nitrogen.

Thirdly, preparing the graphene layer 5

Preparing a graphene layer on the p-type laminated gradient band gap silicon quantum dot multilayer film 4 by a chemical vapor deposition CVD method, and introducing a mixed gas of methane and hydrogen in the growth process to obtain a gas flow ratio CH₄: H₂10sccm in vacuum degree of 3 Torr, and the growth temperature is controlled at 1000 ℃ for 10 min until a graphene layer with a thickness of 30nm is obtained.

Fourthly, an Au electrode layer 6 is vapor-plated on the graphene layer 5, wherein the thickness of the Au electrode layer 6 is 20 nm; the evaporation process is a conventional evaporation method, pure Au is heated to be changed into steam, and the steam is evaporated on the surface of the graphene layer 5 in a vacuum environment or an inert protective gas environment.

And fifthly, evaporating an Al electrode layer 1 on the back surface of the n-type silicon substrate 2, wherein the thickness of the Al electrode layer 1 is 20-100 nm, the evaporation process is a conventional evaporation method, pure Al is heated to be changed into steam, and the steam is evaporated on the surface of the n-type silicon substrate 2 in a vacuum environment or an inert protective gas environment.

It will be evident to those skilled in the art that the invention is not limited to the details of the foregoing illustrative embodiments, and that the present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein. Any reference sign in a claim should not be construed as limiting the claim concerned.

Furthermore, it should be understood that although the present description refers to embodiments, not every embodiment may contain only a single embodiment, and such description is for clarity only, and those skilled in the art should integrate the description, and the embodiments may be combined as appropriate to form other embodiments understood by those skilled in the art.