CN116314439A - NiO in solar cell x Hole transport layer and preparation method and application thereof - Google Patents

Abstract

The invention discloses a NiO in a solar cell _x A method of preparing a hole transport layer comprising: placing a crystalline silicon substrate in a vacuum reaction chamber of a magnetron sputtering device, and heating the crystalline silicon substrate to a reaction temperature; introducing a reaction gas into the vacuum reaction chamber, wherein the oxygen flow rate in the reaction gas changes at a linear rate so as to grow NiO with gradually changed band gap at one side of the crystalline silicon substrate _x A hole transport layer; wherein NiO _x The band gap of the hole transport layer gradually rises from the transparent conducting layer in the solar cell to the direction of the crystalline silicon substrate, and the transparent conducting layer is positioned at NiO _x A hole transport layer on one side far away from the crystalline silicon substrate to enable NiO _x The energy band of the hole transport layer near one side of the transparent conductive layer is matched with the energy band of the transparent conductive layer, and NiO _x The hole transport layer is close toThe energy band of one side of the crystalline silicon substrate is matched with the energy band of the crystalline silicon substrate. NiO prepared by the invention _x The hole transport layer is applied to the solar cell, and is beneficial to improving the short-circuit current density and the filling factor of the solar cell.

Description

NiOx hole transport layer in solar cell and its preparation method and application

technical field

At least one embodiment of the present invention relates to a solar cell, in particular to a NiO _x hole transport layer applied in a solar cell and its preparation method and application.

Background technique

Crystalline silicon heterojunction solar cells have become an important pillar of the photovoltaic industry due to their high photoelectric conversion efficiency and good stability. Doped amorphous silicon material, as a commonly used window layer material, has made outstanding contributions to the birth of high-efficiency crystalline silicon heterojunction cells, but the small band gap of the material itself hinders the further improvement of photoelectric conversion efficiency. Therefore, it is necessary to find a wide bandgap material as a window layer to reduce parasitic absorption, thereby improving device efficiency. Nickel oxide, as a wide bandgap p-type semiconductor material, has great development prospects. The photoelectric properties of NiO _x materials are closely related to the Ni vacancies in it. By adjusting the O content, the number of Ni vacancies in the film can be effectively changed, thereby affecting the transmittance of the film and the position of the Fermi level. The energy band structure of NiO _x film greatly affects the performance of solar cell devices. With the increase of oxygen content in the film, the valence band position of NiO _x film gradually decreases, and is close to the valence band position of crystalline silicon to form a smaller The valence band band order is conducive to the transport of holes. However, when the oxygen content increases, the Ni vacancies in the film increase, the carrier concentration increases, and the light scattering phenomenon becomes more serious after passing through the film, so the band gap of the film will decrease with the increase of the oxygen content, which affects the utilization of light. It is also a major factor hindering the improvement of device performance.

Contents of the invention

In view of this, the present invention provides a NiO _x hole transport layer in a solar cell, by forming a NiO _x thin film with an adjustable energy band position gradient, forming a gap between the NiO _x hole transport layer and the crystalline silicon substrate. It has a good selective transmission structure, and forms a good ohmic contact with the transparent conductive oxide electrode, while taking into account its optical properties.

The invention provides a method for preparing a NiO _x hole transport layer in a solar cell, comprising: placing a crystalline silicon substrate in a vacuum reaction chamber of a magnetron sputtering device, and heating the crystalline silicon substrate to a reaction temperature ; Feed reaction gas into the vacuum reaction chamber, and the oxygen flow rate in the reaction gas changes at a linear rate, so as to grow a NiO _x hole transport layer with a gradual change in band gap on one side of the crystalline silicon substrate; wherein, NiO _x hole transport The band gap of the layer increases gradually from the transparent conductive layer in the solar cell to the direction of the crystalline silicon substrate, and the transparent conductive layer is located on the side of the NiO _x hole transport layer away from the crystalline silicon substrate, so that the NiO _x hole transport layer The energy band on the side close to the transparent conductive layer matches the energy band of the transparent conductive layer, and the energy band on the side of the NiO _x hole transport layer close to the crystalline silicon substrate matches the energy band of the crystalline silicon substrate.

The present invention also provides a NiO _x hole transport layer in a solar cell prepared by the above preparation method.

The present invention also provides an application of the above-mentioned NiO _x hole transport layer in a solar cell. The solar cell comprises from bottom to top: a metal back electrode, an electron transport layer, a first passivation layer, a crystalline silicon A substrate, a second passivation layer, a NiO _x hole transport layer, a transparent conductive layer, and a grid line electrode; wherein, the band gap of the NiO _x hole transport layer gradually increases from the transparent conductive layer to the crystal silicon substrate.

According to an embodiment of the present invention, the NiO _x hole transport layer is prepared by magnetron sputtering, and the valence band position of the prepared NiO _x hole transport layer is changed from transparent to conductive by adjusting the flow ratio of oxygen and argon in the reaction gas. layer to the crystalline silicon substrate, so that the energy band of the NiO _x hole transport layer close to the crystalline silicon substrate matches the energy band of the crystalline silicon substrate, and the NiO _x hole transport layer is close to the transparent and conductive The energy band on one side of the layer matches the energy band of the transparent conductive layer, which is beneficial to the transport of holes; because the NiO _x hole transport layer matches the energy bands of the crystalline silicon substrate and the transparent conductive layer, the NiO _x hole transport layer It has a small contact resistance with the transparent conductive layer and the crystalline silicon substrate, thereby reducing the series resistance of the crystalline silicon heterojunction solar cell, increasing the filling factor, and is conducive to obtaining a highly efficient crystalline silicon heterojunction solar cell.

According to the preparation method of the NiO _x hole transport layer provided by the above-mentioned embodiments of the present invention, by adjusting the argon-oxygen flow ratio in the reaction gas, changing the oxygen-nickel ratio in the NiO _x hole transport layer, the prepared NiO _x holes The band gap of the transport layer film gradually increases from the transparent conductive layer to the crystalline silicon substrate, which optimizes the light transmission performance of the NiO _x hole transport layer film, taking into account the energy band structure and optical properties, and reduces the gap between the substrate and the crystalline silicon substrate. The optical loss caused by the bottom energy band matching makes the prepared NiO _x hole transport layer take into account the characteristics of wide band gap and low contact resistivity.

Description of drawings

Fig. 1 is the flowchart of the preparation method of the NiO _x hole transport layer in the solar cell according to the embodiment of the present invention;

2 is a schematic cross-sectional view of a solar cell according to an embodiment of the present invention;

Fig. 3 is the J-V curve comparison diagram of the solar cell in the related art and the solar cell of the embodiment of the present invention; And

FIG. 4 is a comparison chart of the test results of the c-Si/NiO _x /ITO contact resistance in the related art and the c-Si/NiO _x /ITO contact resistance of the embodiment of the present invention.

[Description of Reference Signs]

1- metal back electrode;

2 - Electron transport layer;

31 - a first passivation layer;

32 - second passivation layer;

4-crystalline silicon substrate;

5-NiO _x hole transport layer;

6- Transparent conductive layer;

7-Grid electrodes.

Detailed ways

In order to make the object, technical solution and advantages of the present invention clearer, the present invention will be further described in detail below in conjunction with specific embodiments and with reference to the accompanying drawings. However, this invention may be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this invention will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the drawings, the size and relative sizes of layers and regions may be exaggerated for clarity, and like reference numerals designate like elements throughout.

The terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting of the invention. The terms "comprising", "comprising", etc. used herein indicate the presence of stated features, steps, operations and/or components, but do not exclude the presence or addition of one or more other features, steps, operations or components.

FIG. 1 is a flowchart of a method for preparing a NiO _x hole transport layer in a solar cell according to an embodiment of the present invention. FIG. 2 is a schematic cross-sectional view of a solar cell according to an embodiment of the present invention.

According to an exemplary embodiment of the present invention, the present invention provides a method for preparing a NiO _x hole transport layer in a solar cell, as shown in FIG. 1 and FIG. 2 , comprising: steps S01-S02.

In step S01, the crystalline silicon substrate 4 is placed in a vacuum reaction chamber of a magnetron sputtering device, and the crystalline silicon substrate 4 is heated to a reaction temperature.

According to an embodiment of the present invention, the above-mentioned preparation method further includes: before placing the crystalline silicon substrate 4 in a vacuum reaction chamber, forming a first passivation layer 31 and a second passivation layer 31 on both sides of the crystalline silicon substrate 4, respectively. layer 32, to reduce the interface defects of the crystalline silicon substrate 4 and the _NiO hole transport layer 5, and the interface defects of the crystalline silicon substrate 4 and the electron transport layer 2, thereby reducing the recombination of carriers at the interface defects .

According to an embodiment of the present invention, the crystalline silicon substrate 4 may have a textured structure or a planar structure.

According to an embodiment of the present invention, if the crystalline silicon substrate 4 has a textured structure, the above-mentioned preparation method further includes: before forming the first passivation layer 31 and the second passivation layer 32 respectively on both sides of the crystalline silicon substrate 4 , utilizing alkaline solution and texture agent to corrode the crystalline silicon substrate 4, the alkaline solution can be KOH solution for example, to form a textured structure on the surface of the crystalline silicon substrate 4, and then form a light-trapping structure on the surface of the crystalline silicon substrate 4 , improve light utilization.

According to an embodiment of the present invention, the crystalline silicon substrate 4 is pretreated to obtain a clean crystalline silicon substrate with passivation layers grown on both sides. Specifically, after the surface of the crystalline silicon substrate 4 is cleaned with RCA standard cleaning solution, the surface of the crystalline silicon substrate 4 is etched with an alkaline solution to form a textured structure, and the surface of the crystalline silicon substrate 4 is blown with nitrogen gas. Dry. Put the dried crystalline silicon substrate 4 into 20wt% H ₂ O ₂ solution and soak for 30min to form a first passivation layer 31 and a second passivation layer 32 on both sides of the crystalline silicon substrate 4, wherein the first The material of the passivation layer 31 and the second passivation layer 32 is SiO _x . After the passivation layer is formed on both sides of the crystalline silicon substrate 4 , it is blown dry with nitrogen to obtain a clean crystalline silicon substrate with passivation layers grown on both sides.

According to an embodiment of the present invention, the equipment used to deposit the NiO _x hole transport layer 5 on the side away from the crystalline silicon substrate 4 of the second passivation layer 32 by magnetron sputtering is a radio frequency magnetron sputtering equipment. The target material is a high-purity NiO _x ceramic target, for example, a NiO _x ceramic target with a purity greater than 99.9%. The background vacuum of the vacuum reaction chamber is 10 ^-3 -10 ^-4 Pa.

According to an embodiment of the present invention, the crystalline silicon substrate 4 is heated to a reaction temperature of 100-200° C., and the reaction temperature may be 100° C., 120° C., 150° C., 170° C., or 200° C. for example.

In step S02 , a reaction gas is introduced into the vacuum reaction chamber, and the flow rate of oxygen in the reaction gas changes at a linear rate, so as to grow a NiO _x hole transport layer 5 with a tapered bandgap on one side of the crystalline silicon substrate 4 .

According to an embodiment of the present invention, the _O flow in the reaction gas changes at a linear rate according to formula (1):

F(t) = F0+Ct (1)

Among them, F(t) represents the O ₂ flow rate at time t, F ₀ represents the initial flow rate of O ₂ , C represents the linear change rate of O ₂ flow rate, and t represents the sputtering time from the moment when the reaction gas is initially introduced to time t.

According to an embodiment of the present invention, the thickness of the NiO _x hole transport layer (0<x<1.5) is 5-40 nm. It should be noted that the flow rate of O ₂ increases gradually with F ₀ sccm; the sputtering time t is determined by the required film thickness and sputtering rate.

According to an embodiment of the present invention, in the process of preparing the NiO _x hole transport layer, the pressure of the reaction gas introduced is 0.2-1.0 Pa, and the reaction gas is a mixed gas of Ar and O ₂ ; The initial flow ratio of oxygen and argon may be 4.4%, for example.

According to an embodiment of the present invention, the NiO _x hole transport layer 5 is deposited on the side of the second passivation layer 32 away from the crystalline silicon substrate 4 by magnetron sputtering. The crystalline silicon substrate 4 formed with the first passivation layer 31 and the second passivation layer 32 is placed in the vacuum reaction chamber of the magnetron sputtering equipment, and the background vacuum of the reaction chamber is evacuated to 3×10 ⁻ After ³ Pa, the crystalline silicon substrate 4 is heated to 150° C., and the reaction gas including Ar and O is introduced, wherein the flow of Ar is constant, for example, it can be 18 sccm, and the flow of O ₂ increases linearly, for example, the flow _of O ₂ is according to the formula The growth rate indicated by F(t)=2+0.1t increases from 2 sccm to 4 sccm, and finally a NiO _x hole transport layer 5 with a thickness of about 20 nm is deposited on the second passivation layer 32 . Among them, F(t) represents the O ₂ flow rate at time t, and t represents the sputtering time from the moment when the reaction gas is initially introduced to time t.

According to an embodiment of the present invention, by adjusting the argon-oxygen flow ratio in the reaction gas, the oxygen-nickel ratio in the _NiOx hole-transport layer is changed (that is, the value of x, 0<x<1.5), so that the _NiOx hole-transport layer The energy level range of the valence band is about -5.1eV～-5.4eV, and the valence band position of the NiO _x hole transport layer 5 gradually rises from the light incident surface (transparent conductive layer 6) to the direction of the crystalline silicon substrate 4, so as to Make the energy band matching of NiO _x hole transport layer 5 and crystalline silicon substrate 4 and transparent conductive layer 6, wherein the valence band of crystalline silicon substrate 4 is about-5.17eV, and the valence band of ITO transparent conductive layer 6 is about- 6.8eV; the optical bandgap is 3.6eV-4.0eV, in order to improve the light transmittance of the NiO _x hole transport layer 5 . It should be noted that the greater the light transmittance of the NiO _x hole transport layer, the smaller the parasitic absorption, the reduction of light absorption, and more light can be absorbed by the crystalline silicon substrate 4, increasing the short-circuit current density and improving the filling capacity. factor, thereby improving the photoelectric conversion efficiency of solar cells.

According to an embodiment of the present invention, the sputtering power of the magnetron sputtering is 140-160W, such as 140W, 145W, 150W, 155W, 160W.

The present invention also provides a NiO _x hole transport layer in a solar cell prepared by the above preparation method.

According to an exemplary embodiment of the present invention, the present invention provides an application of the above-mentioned NiO _x hole transport layer in a solar cell, as shown in FIG. 2 , the solar cell includes from bottom to top : metal back electrode 1, electron transport layer 2, first passivation layer 31, crystalline silicon substrate 4, second passivation layer 32, NiO _x hole transport layer 5, transparent conductive layer 6, grid line electrode 7; wherein , the band gap of the NiO _x hole transport layer 5 gradually increases from the transparent conductive layer 6 to the direction of the crystalline silicon substrate 4 .

According to an embodiment of the present invention, after the deposition of the _NiO hole transport layer 5 ends, the crystalline silicon substrate 4 with the _NiO hole transport layer 5 deposited is placed in a plasma-enhanced chemical vapor deposition (PECVD) device , deposit n-type heavily doped electron transport layer 2 on the side of the first passivation layer 31 away from the crystalline silicon substrate 4 . Utilize magnetron sputtering method to deposit transparent conductive layer (TCO) 6 on NiO _x hole transport layer 5 as light-incident surface, TCO can be ITO (Indium Tin Oxide Semiconductor Transparent Conductive Film), for example, thickness can be 80～ 100nm.

According to an embodiment of the present invention, the grid line electrode 7 is deposited on the light incident surface by electron beam evaporation. The material of the grid line electrode 7 can be, for example, Al, and the thickness can be, for example, 1 μm. The electron transport layer 2 is far away from the first passivation layer. A metal back electrode 1 is deposited on one side of 31, the material of the metal back electrode 1 may be, for example, Al, and the thickness may be, for example, 5000 nm.

According to an embodiment of the present invention, the above-mentioned solar cell further includes an anti-reflection layer (not shown in the figure), located between the transparent conductive layer 6 and the grid electrode 7, the material of the anti-reflection layer can be, for example, SiN _x , which is suitable for Reduce light reflection.

It should be noted that the n-type crystalline silicon substrate 4 and the p-type NiO _x hole transport layer 5 form a crystalline silicon heterojunction.

According to the embodiments of the present invention, the NiO _x hole transport layer provided by the present invention can be applied in crystalline silicon heterojunction solar cells (HJT), and can also be applied in Top-Con (tunneling oxide layer passivation contact) cells, DASH (Doping Free, Asymmetric Heterocontact) cells, PERC (Passivated Emitter and Rear Contact), PERL (Passivated Emitter Back Localized) or PERT (Emitter Junction Passivated Full Back Field Diffused) cells .

FIG. 3 is a comparison diagram of J-V curves of a solar cell in the related art and a solar cell in an embodiment of the present invention.

Referring to Fig. 3, the open circuit voltage of the solar cell formed by the NiO _x hole transport layer prepared by the O flow rate from 2sccm to 4sccm is higher than that of the _NiO prepared by the _O flow rate of 2sccm and 4sccm. The open-circuit voltage of the solar cell formed by the hole transport layer; the embodiment of the present invention _adopts O ₂ The flow rate is gradually changed from 2sccm to 4sccm, and the short-circuit current density of the solar cell formed by the _NiOx hole transport layer is higher than that of using O ₂ The flow rate is The short-circuit current densities of the solar cells formed by the NiO _x hole transport layer prepared at 2sccm and 4sccm. Among them, Voc represents the open circuit voltage, and Jsc represents the short circuit current density.

FIG. 4 is a comparison chart of the test results of the c-Si/NiO _x /ITO contact resistance in the related art and the c-Si/NiO _x /ITO contact resistance of the embodiment of the present invention.

Referring to FIG. 4 , the c-Si/NiO _x /ITO contact resistance in the related art and the c-Si/NiO _x /ITO contact resistance in the embodiment of the present invention were respectively detected by TLM test method (Transfer Length Method). The test results show that the c-Si/NiO _{x /ITO contact resistance of the solar cell formed by the NiO x hole} transport layer prepared by the O flow rate from 2sccm to 4sccm _in the embodiment of the present invention is less than the _O flow rate of 2sccm and The c-Si/NiO _x /ITO contact resistance of the solar cell formed by the NiO _x hole transport layer prepared at 4sccm. The contact resistance between the NiO _x hole transport layer 5 and the crystalline silicon substrate 4 and the transparent conductive layer 6 is small, which is conducive to improving the filling factor. Among them, the abscissa represents the O ₂ flow rate, and the ordinate represents the resistance value of the c-Si/NiO _x /ITO contact resistance.

It should be noted that light incident into the solar cell generates electron-hole pairs in the crystalline silicon substrate 4, and the built-in electric field formed by the electron-hole pairs in the crystalline silicon substrate 4 and the NiO _x hole transport layer 5 The holes are separated under the action of the grid line electrode 7, and the electrons are collected by the metal back electrode 1, thereby generating photocurrent.

According to an embodiment of the present invention, the NiO _x hole transport layer is prepared by magnetron sputtering, and the valence band position of the prepared NiO _x hole transport layer is changed from the incident light to surface (transparent conductive layer) to the direction of the crystalline silicon substrate gradually rises, so that the energy band of the side of the NiO _x hole transport layer close to the crystalline silicon substrate matches the energy band of the crystalline silicon substrate, and the NiO _x hole The energy band on the side of the transport layer close to the transparent conductive layer matches the energy band of the transparent conductive layer, which is conducive to the transmission of holes; because the NiO _x hole transport layer matches the energy bands of the crystalline silicon substrate and the transparent conductive layer, NiO There is a small contact resistance between the _x hole transport layer and the transparent conductive layer and the crystalline silicon substrate, thereby reducing the series resistance of the crystalline silicon heterojunction solar cell and improving the fill factor, which is conducive to obtaining efficient crystalline silicon heterojunction solar cells. Mass junction solar cells.

It should be noted that the smaller the oxygen-nickel ratio in the _NiO hole transport layer, the wider the band gap of the _NiO hole transport layer material, the greater the light transmittance, the smaller the parasitic absorption, and the reduced absorption of light, which can Make more light absorbed by the crystalline silicon substrate, increase the short-circuit current density of the solar cell, and then improve the photoelectric conversion efficiency of the solar cell.

According to the preparation method of the NiO _x hole transport layer provided by the above-mentioned embodiments of the present invention, by adjusting the argon-oxygen flow ratio in the reaction gas, changing the oxygen-nickel ratio in the NiO _x hole transport layer, the prepared NiO _x holes The band gap of the transport layer film gradually increases from the light incident surface to the crystalline silicon substrate, which optimizes the light transmission performance of the NiO _x hole transport layer film, takes into account the energy band structure and optical performance, and reduces the The optical loss caused by the bottom energy band matching makes the prepared NiO _x hole transport layer take into account the characteristics of wide band gap and low contact resistivity, so it is an ideal choice for the hole transport layer of crystalline silicon heterojunction solar cells.

According to an embodiment of the present invention, the method of magnetron sputtering is adopted, and the sputtered Ni atoms are directly reacted with _O in _{an O} atmosphere, which has the characteristics of high activity, sufficient reaction, and dense film prepared, and has The preparation process is simple, the repeatability is good, the cost is low, and it is suitable for large-scale production.

Words such as "first", "second", "third" and the like used in the description and claims to modify the corresponding elements do not in themselves mean that the elements have any ordinal numbers, nor The use of these ordinal numbers to represent the sequence of an element with respect to another element, or the order of manufacturing methods, is only used to clearly distinguish one element with a certain designation from another element with the same designation.

The specific embodiments described above have further described the purpose, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above descriptions are only specific embodiments of the present invention, and are not intended to limit the present invention. Within the spirit and principles of the present invention, any modifications, equivalent replacements, improvements, etc., shall be included in the protection scope of the present invention.

Claims (10)

1. NiO in solar cell _x A method for producing a hole transport layer, comprising:

placing a crystalline silicon substrate (4) in a vacuum reaction chamber of a magnetron sputtering device, and heating the crystalline silicon substrate (4) to a reaction temperature;

introducing a reaction gas into the vacuum reaction chamber, wherein the oxygen flow rate in the reaction gas changes at a linear rate so as to grow NiO with gradually changed band gap at one side of the crystalline silicon substrate (4) _x A hole transport layer (5);

wherein the NiO _x The band gap of the hole transport layer (5) gradually rises from the transparent conductive layer (6) in the solar cell to the direction of the crystalline silicon substrate (4), and the transparent conductive layer (6) is positioned on the NiO _x A hole transport layer (5) is arranged on the side far away from the crystal silicon substrate (4) so that the NiO _x The energy band of the hole transport layer (5) near the transparent conductive layer (6) is matched with the energy band of the transparent conductive layer (6), and the NiO _x The energy band of the hole transmission layer (5) near one side of the crystal silicon substrate (4) is matched with the energy band of the crystal silicon substrate (4).

2. The method of manufacturing according to claim 1, further comprising:

before the crystal silicon substrate (4) is placed in the vacuum reaction chamber, a first passivation layer (31) and a second passivation layer (32) are respectively formed on two sides of the crystal silicon substrate (4).

3. The method of manufacturing according to claim 2, further comprising:

and before forming a first passivation layer (31) and a second passivation layer (32) on two sides of the crystalline silicon substrate (4) respectively, etching the crystalline silicon substrate (4) by utilizing alkali solution to form a suede structure on the surface of the crystalline silicon substrate (4).

4. The method of claim 1, wherein the vacuum reaction chamber has a background vacuum of 10 ^-3 ～10 ^-4 Pa。

5. The preparation method according to claim 1, characterized in that the crystalline silicon substrate (4) is heated to a reaction temperature of 100-200 ℃.

6. The method according to claim 1, wherein the pressure of the reaction gas is 0.2 to 1.0Pa;

the reaction gas is Ar and O ₂ Is a mixed gas of (a) and (b).

7. The method of claim 1, wherein NiO is grown _x The sputtering power of the hole transport layer (5) is 140-160W.

8. The method of claim 1, wherein the NiO is grown _x The target material used for the hole transport layer (5) is NiO with the purity of more than 99.9 percent _x A ceramic target.

9. NiO in solar cell prepared by the preparation method of any one of claims 1 to 8 _x And a hole transport layer.

10. A NiO in a solar cell according to claim 9 _x The use of a hole transport layer in a solar cell, characterized in that,

the solar cell comprises, in order from bottom to top: a metal back electrode (1), an electron transport layer (2), a first passivation layer (31), a crystalline silicon substrate (4), a second passivation layer (32), niO _x A hole transport layer (5), a transparent conductive layer (6), and a gate electrode (7);

wherein the NiO _x The band gap of the hole transport layer (5) gradually increases from the transparent conductive layer (6) to the direction of the crystalline silicon substrate (4).

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