KR101593398B1 - Booster of solar cell - Google Patents

Abstract

A solar cell booster is provided. A solar cell booster according to an embodiment of the present invention includes an absorption layer made of a semiconductor material, a first graphene layer stacked on the absorption layer, an insulating layer stacked on the first graphene layer, A solar cell comprising a stacked second graphene layer; And a DC power supply having its both ends connected to the first and second graphene layers, wherein only a voltage is applied between the first and second graphene layers by the insulating layer.

Description

Solar cell booster {BOOSTER OF SOLAR CELL}

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a solar cell booster, and more particularly, to a solar cell booster using metal having a large work function difference from an absorber to minimize light interruption and maximizing efficiency by utilizing graphene as a transparent electrode .

Since the discovery of graphene, a two-dimensional material of carbon atoms, has attracted much attention due to its many new and excellent properties. In particular, the Nobel Prize for Physics in 2010 has been awarded to two Nobel Prize winners who have separated the unit graphene for the first time and attract a great deal of interest from researchers worldwide as well as the general public.

Graphene is a very thin crystal composed of a single atomic layer, and many studies have been done in various fields about the application technology using the characteristics of already known graphene. Graphene is the thinnest of all known materials, yet it is the strongest and most flexible material, as well as the best able to conduct electricity and heat.

Utilizing such excellent properties of graphene, it is expected to be used as a structural material or to replace a Si electronic device. GRAPHIN is being used as a new material in next generation display fields such as flexible display and touch panel, energy industry such as solar cell, smart window, RFID and various electronic industries.

In recent years, hydroelectric power generation, wind power generation, and solar power generation have been attracting attention due to increased interest in environmentally friendly energy. Among them, researches on solar power generation using solar energy are very active. As a typical example of this solar power generation, there is a solar cell using a semiconductor. Such a solar cell is a device that converts light energy into electrical energy using a photovoltaic effect generated by electrons when the light is irradiated.

In order to increase the efficiency of such a semiconductor solar cell, an absorber having an energy band gap of an appropriate size capable of efficiently absorbing solar energy and a built-in voltage capable of efficiently separating electrons and holes generated by absorption of light voltage, and fast movement of electrons and holes are important factors.

However, the elements required for solar cell construction can interfere with light reaching the absorber. That is, in order to increase the efficiency of the solar cell, it is necessary to minimize the obstacle that light reaches the absorption layer.

Japanese Laid-Open Patent No. 2011-108883

In a solar cell, light is incident on an absorber, light arrives in an appropriate region for separating holes and electrons generated by light absorption, and all possible light is absorbed to obtain high efficiency. That is, the absorption of light increases exponentially with the travel distance of light, so that the wider the area, the greater the probability that the absorbed light will become current. Therefore, a solar cell is constructed using an efficient electrode structure to form a high builtin voltage, almost all the light is absorbed in a region where a builtin voltage is applied, and photoelectron and photohole are absorbed. It is necessary to configure the structure of the device so as to efficiently separate and move the device. Here, the builtin voltage means a voltage spontaneously formed in the inside.

In order to form a high built-in voltage, an absorber (p-n junction solar cell), a absorber and a Schottky junction solar cell having a very large work function may be used. In the case of these solar cells, since the builtin voltage is formed at the interface, a high efficiency solar cell can be expected when light is absorbed not at the surface of the absorber but at the interface between the absorber and the absorber or the interface between the absorber and the conductor. Therefore, in the case of a p-n junction solar cell, a material on which light is incident has a material whose energy band gap is larger than light energy. In the Schottky junction solar cell, when a conductor with a high work function is placed in the path of light, the light is blocked. Therefore, as the area ratio of the metal covering the solar cell increases, the efficiency of the solar cell decreases sharply do.

In order to overcome this, it is possible to reduce the light blocking efficiency by using a transparent electrode or by using a very narrow metal electrode strip. Materials such as ITO (Indium Tin Oxide) can be used as the transparent electrode, but they require an alternative material due to instability of the indium market price. In particular, materials such as indium tin oxide (ITO), which are fragile, are not suitable for the development of flexible flexible solar cells. In addition, when a narrow electrode band is used, photoelectrons and photo holes generated far from the electrode are not a big problem when the photoelectrons and the photohole are generated around the electrode, but the photoelectrons and the photohole generated far away from the electrode do not contribute to the current and are recombined again to be released as heat energy and lost.

Therefore, in order to obtain a high built-in voltage, a metal having a large difference in work function from the absorber is used, and in particular, a very narrow metal electrode is required. However, when a high efficiency solar cell is developed by such a method, The blocking effect of light can not be avoided and it is difficult to expect high efficiency.

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide a light emitting device that uses a metal having a large work function difference from that of an absorber, and minimizes light interruption and generates a builtin voltage spontaneously generated by a photo- And to provide a solar cell booster which can easily separate photoelectrons and optical holes and can move fast to the electrodes.

Another object of the present invention is to provide a solar cell booster which can maximize efficiency by adjusting the thickness of the insulating layer and the graphene layer stacked on the absorption layer so as to minimize light reflection.

Another problem to be solved by the present invention is to provide a solar cell booster which can increase the built-in voltage by utilizing graphene itself instead of metal to obtain a high Schottky junction barrier.

The problems to be solved by the present invention are not limited to the above-mentioned problems, and other matters not mentioned can be clearly understood by those skilled in the art from the following description.

According to an aspect of the present invention, there is provided a solar cell booster comprising an absorption layer made of a semiconductor material, a first graphene layer stacked on the absorption layer, an insulating layer stacked on the first graphene layer, And a second graphene layer stacked on the insulating layer; And a direct current power source having both ends connected to the first and second graphene layers, wherein only a voltage is applied between the first and second graphene layers by the insulating layer.

Other specific details of the invention are included in the detailed description and drawings.

According to the present invention, both photoelectrons generated by absorption of light and a photohole generated by a built-in voltage generated spontaneously by minimizing light interruption are easily separated, and photoelectrons and optical holes can be easily separated, A high efficiency solar cell can be realized.

In addition, the insulating layer and the graphene layer stacked on the absorption layer can maximize the efficiency of the solar cell by adjusting the thickness so as to minimize reflection of light.

Instead of metal, graphene itself can be used to increase the builtin voltage to obtain a high Schottky junction barrier.

1 is a cross-sectional view of a solar cell according to an embodiment of the present invention.
2 is a top view of the solar cell of FIG. 1 viewed from AA.
FIG. 3 is a graph showing a potential distribution due to Schottky junction formation in the solar cell of FIG. 1; FIG.
4A is a graph showing Igs vs Vgs curves according to presence or absence of light.
4B is a graph showing Ids vs Vgs curves depending on the presence or absence of light.
4c is a graph showing Ids vs Vds curves for various Vgs.
5 is a cross-sectional view of a solar cell according to another embodiment of the present invention.
6 is a flowchart of a method of manufacturing a solar cell according to an embodiment of the present invention.
7 is a first drawing showing each step in the method of manufacturing the solar cell of FIG.
8 is a second drawing showing each step in the method of manufacturing the solar cell of Fig.
9 is a first sectional view of a solar cell according to another embodiment of the present invention.
10 is a second sectional view of a solar cell according to another embodiment of the present invention.
11 is a cross-sectional view of a solar cell booster according to an embodiment of the present invention.
12A is a diagram showing the structure of a solar cell booster in the case where the absorption layer is p-type.
FIG. 12B is a diagram showing the energy band structure in FIG. 12A. FIG.
13A is a diagram showing the structure of a solar cell booster in the case where the absorption layer is n-type.
Fig. 13B is a diagram showing the energy band structure in Fig. 13A.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS The advantages and features of the present invention and the manner of achieving them will become apparent with reference to the embodiments described in detail below with reference to the accompanying drawings. The present invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Is provided to fully convey the scope of the invention to those skilled in the art, and the invention is only defined by the scope of the claims. Like reference numerals refer to like elements throughout the specification.

Unless defined otherwise, all terms (including technical and scientific terms) used herein may be used in a sense commonly understood by one of ordinary skill in the art to which this invention belongs. Also, commonly used predefined terms are not ideally or excessively interpreted unless explicitly defined otherwise.

Hereinafter, the present invention will be described in more detail with reference to the accompanying drawings.

1 is a cross-sectional view of a solar cell according to an embodiment of the present invention. 2 is a top view of the solar cell of FIG. 1 as seen from A-A.

1 and 2, a solar cell according to an exemplary embodiment of the present invention includes an absorption layer 10 for absorbing light energy, a plurality of through holes 22 formed on the absorption layer 10, A metal 30 is deposited on the through hole 22 of the insulating layer 20 including the insulating layer 20 and the graphene layer 40 formed on the insulating layer 20. The solar cell according to an embodiment of the present invention uses a metal 30 having a large work function difference from the absorption layer 10 and utilizes a patterned metal 30 having a small area instead of a metal band . The surface of the absorbing layer 10 is covered with an insulating layer 20 and a patterned through hole 22 is formed in the insulating layer 20 to deposit the metal 30. The metal 30 is absorbed by the absorbing layer 10 ) And a Schottky junction. The graphene layer 40, which is chemically stable, functions as a transparent electrode.

The absorption layer 10 may be made of a semiconductor material having an energy band gap smaller than that of light energy for absorption of light. Thus, the absorption layer 10 may be at least one of an n-type semiconductor or a p-type semiconductor. This absorbent layer 10 should have little light reflection at its surface. When most of the light is absorbed in the region where the spontaneous built-in voltage is applied, the efficiency of light to current conversion becomes very high. In addition, various losses such as ohmic losses are minimized at high currents. Therefore, the energy band gap and the absorption coefficient of light of the absorption layer 10, the mobility of electrons and holes, the position of the energy level, and the refractive index for light should be considered.

Of the semiconductor materials that can be used as the absorber layer 10, GaAs semiconductor crystals are known to be most efficient, but will be apparent to those skilled in the art. For example, a Si semiconductor crystal can also be used as the absorber layer 10. In addition to these monocrystalline semiconductors, polycrystalline and amorphous semiconductor materials may also be considered as the absorber layer 10. The efficiency of a solar cell made of an organic semiconductor absorber is somewhat lower than that of a solar cell made of an inorganic semiconductor absorber, but the organic semiconductor absorber is flexible and has an energy band gap of various sizes. However, conductivity of the organic semiconductor, that is, mobility of the charge carrier is low, and thus it is difficult to achieve high efficiency. Therefore, when the amorphous and organic semiconductor are used as the absorption layer 10, the absorption layer 10 must be very thin in order to effectively extract the photoelectrons and the photo holes, and a structure in which solar cells are laminated in series is preferable. The series lamination structure of the solar cells will be described later.

The insulating layer 20 may be formed of an insulator material having an energy band gap larger than light energy. As the insulating layer 20, any material having an insulator property such as an inorganic material, an oxide, or an organic dielectric material may be used. The refractive index of light is smaller than that of the absorbing layer 10, which is suitable for reducing reflection of light. The insulating layer 20 includes a through hole 22 through which a metal 30 is deposited, and the through hole may be formed by etching using a mask and PR (photoresist). The insulating layer 20 is formed so that the depth of the through hole 22 is in the range of 1 to 10 μm and the through hole 22 is formed in the depth direction of the through hole 22. In order to realize a high efficiency solar cell, Sectional length of 1 to 10 mm and a distance between the through holes 22 of 1 to 10 cm. The structure of the patterned metal 30 is shown in Fig. At this time, when the thickness of the insulating layer 20, that is, the depth of the through hole is formed to be between 1 and 10 μm, the single-layer absorbent layer 10 may be formed to have a thickness of 10 to 100 μm. The through holes 22 may be formed in various shapes such as a circle, an ellipse, and a square. Although the through-holes 22 are formed in various shapes, the shortest cross-sectional length, that is, the minimum cross-sectional length may be between 1 and 10 mm. Of course, those skilled in the art will appreciate that the size of the through-holes formed in the insulating layer 20 and the spacing between the through-holes may be formed in different sizes.

The metal 30 may be deposited on the etched through-hole 22 using a mask or the like. The metal 30 is deposited on the through-hole 22 to form a Schottky junction with the absorption layer 10. In order to form a Schottky junction, it is preferable that the metal 30 has a large work function difference from the absorption layer 10. For example, when the absorption layer 10 is an n-type semiconductor, Au, Ag, Cu or the like having a high work function can be used. When the absorption layer 10 is a p-type semiconductor, Al, In, Ti, MgAg, an alkali metal or the like having a low work function can be used. In the Schottky junction solar cell, as the area ratio of the metal 30 is increased, the efficiency of the solar cell drastically decreases, so that the ratio of the metal pattern deposited on the through hole 22 to the surface of the absorption layer 10 is adjusted to be within 1% . In this case, the degree to which the metal 30 interferes with the progress of the light can be ignored, and the light absorption rate can be maximized. High conductivity is required to efficiently implement optoelectronic and optical holes generated by light. Since the depth of the through hole 22 can be formed to be between 1 and 10 μm, the electrical resistance by the metal 30 is very small. However, an additional electrode is required to transfer the current delivered from the electrode made of these metals 30 to the outside, and this can be replaced with graphene.

The graphene layer 40 may be either a multilayer or a single layer, but a graphene layer 40 of a single layer is suitable. As the number of graphene layers 40 increases, the transmittance of light decreases, but the electrical conductivity increases. That is, it can be changed according to the current density generated in the solar cell, but since a current density generated in an ordinary solar cell is about tens mA / cm 2, it is possible to utilize a single layer. Due to the nature of the graphene composed of a single atomic layer, graphene can be easily bent or unfolded even by very small forces, and its warpage can be from several nm. Further, when the graphene and other materials are laminated in a multi-layered structure of more than two layers, the shape structure of the graphene can be freely controlled. And graphene can be easily deposited on the surface of an object by the van der Waals force acting between the surface of the object and graphene. Thus, graphene, which has high electrical conduction and light transmittance and is chemically stable, is very suitable as a transparent electrode. Although a normal transparent electrode may be used, graphene is superior to a general transparent electrode because of low light reflectance and high transmittance in a high light wavelength region. Graphene is also easy to apply to flexible solar cells.

FIG. 3 is a graph showing a potential distribution due to Schottky junction formation in the solar cell of FIG. 1; FIG. 4B is a graph showing Ids vs Vgs curves depending on the presence or absence of light, and FIG. 4C is a graph showing Ids vs Vds curves for various Vgs. FIG. 4A is a graph showing Igs vs Vgs curves according to presence or absence of light, FIG.

3, a point A is a Schottky contact, and a point B is an Ohmic contact. In the absence of the graphene layer 40, the Schottky junction effect is formed only on the electrode level, but when the graphene layer 40 is present, the potential distribution is formed on the entire surface, Efficiency can be maximized. The role of graphene is largely divided into a photoelectron generated by the absorption of light and an electrode collecting the photohole, a role of assisting the transmission of light, a metal 30 formed on the surface of the absorption layer 10 and a builtin and a diffusion barriers for protecting the insulating layer 20 and the metal 30 made of non-conductive materials. FIGS. 4A to 4C show various graphs of voltages according to currents, and are converted into ON currents when the light is illuminated.

5 is a cross-sectional view of a solar cell according to another embodiment of the present invention.

As described above, when the absorption layer 10 having a very low mobility of the organic substance and the charge carrier is used, a structure in which solar cells are connected in series in a laminated structure is suitable. 5, the absorption layer 10, the insulating layer 20 and the graphene layer 40 are successively laminated on the absorption layer 10, the insulating layer 20 and the graphene layer 40 in order, The efficiency of the solar cell can be increased. Here, the thickness of the absorption layer 10 is reduced so that light is absorbed only in a region where both the photoelectrons generated by absorption of light and the photohole can be converted into current, and the light not absorbed in this layer is absorbed in the following layers The absorption rate of light and the current generation efficiency, that is, the energy conversion rate can be increased. At this time, it is preferable to form the laminated structure by controlling the thickness of the absorbing layer 10 to within micrometers (~ m).

6 is a flowchart of a method of manufacturing a solar cell according to an embodiment of the present invention. FIG. 7 is a first drawing showing each step in the manufacturing method of the solar cell of FIG. 6, and FIG. 8 is a second drawing showing each step of the manufacturing method of the solar cell of FIG.

Referring to FIG. 6, a method of manufacturing a solar cell according to an embodiment of the present invention includes forming an absorbing layer 10 made of a semiconductor material that absorbs light energy (S10) A plurality of through holes 22 are formed by etching the insulating layer 20 to form a plurality of through holes 22 and a plurality of through holes 22 are formed in the plurality of through holes 22, (S10), and a graphene layer 40 is formed on the insulating layer 20 (S10).

7, when the absorbing layer 10 is formed (a), the thickness of the absorbing layer 10 may be 100 μm or less. When the insulating layer 20 is formed (b), the thickness of the insulating layer 20 may be 10 m or less. In the case of forming the through hole 22, the insulating layer 20 can be etched using the photoresist and the mask. In the case of depositing the metal 30, can do. In this case, the thicknesses of the absorbing layer 10 and the insulating layer 20 may be different.

8, the structure of the absorption layer 10, the insulating layer 20, the metal 30 deposited on the through hole 22, and the graphene layer 40 may be formed in a series-connected laminated structure (f). That is, each step of FIG. 6 may be repeated at least once in order to form a laminated structure. In this case, since the absorbent layer 10 is at least two layers or more, it is preferable that the thickness of the absorbent layer 10 is 10 m or less. Further, the thickness of the insulating layer 20 may be formed to be 10 占 퐉 or less. More preferably, when the thickness of the insulating layer 20 is about 10 mu m, the thickness of the absorbing layer 10 can be formed to be 1 mu m or less.

9 is a first sectional view of a solar cell according to another embodiment of the present invention. 10 is a second sectional view of a solar cell according to another embodiment of the present invention.

In the above method, a high Schottky junction barrier is obtained as a patterned metal (30) electrode in the insulating layer 20, a high light transmittance is obtained, and electrons and holes are efficiently separated and converted into current In the solar cell shown in Figs. 9 and 10, instead of using the patterned electrode, the metal 30 having a high work function difference from the light absorber is directly deposited on the absorber layer 10, A graphene layer 40 is deposited on the surface of the deposited metal 30 and a graphene layer 40 is also deposited on the bottom surface of the absorbent layer 10. The thickness of the graphene layer 40 is determined by the skin depth The thickness of the metal layer 30 is also thinner than the skin depth of the light to maintain a high transmittance of light. ) Layer is reduced, but the graphene layer 40 is deposited Without affecting the transmission of light and improve the electrical conductivity.

It is preferable to deposit the metal 30 thicker than the percolation limit in order to cause a high builtin voltage to appear over the entire surface of the absorbent layer 10. In the case of depositing the thin metal 30, Electrical conductivity is reinforced. That is, if the thickness of the metal 30 layer is below the percolation limit, or if the growth method does not completely wet the metal 30 surface, the electrical conductivity is significantly lowered, but this is reinforced by graphene.

The metal layer 30 may have a continuous surface as shown in Fig. 9, or may have a discontinuous surface as shown in Fig. That is, it may have a thickness greater than the percolation limit or have a smaller value. 9, the thickness of the metal 30 layer is thinner than the skin depth of light, but uses a work function of the metal 30 to form a high builtin voltage and a high conductivity using graphene And the light transmittance can be maintained. In FIG. 10, the thickness of the metal layer 30 may be very thin to form a discontinuous surface, or metal nano particles may be deposited to form a discontinuous surface. Thus, the builtin voltage is formed by the metal 30 and the absorption layer 10, and the electrical conductivity is improved by the graphene layer 40. [ In particular, a discontinuous metal (30) layer causes a plasmon effect, thereby amplifying the absorption of light. That is, the discontinuous metal thin film can reduce the reflection of light and promote the generation of bluzzons that enhance the absorption of light, thereby realizing a solar cell with high efficiency. Therefore, it can be maximized by using nano particles. Here, since there is no need to deposit the insulating layer 20, the process becomes simpler, and the plasmon effect and the reflectance due to graphene are minimized.

11 is a cross-sectional view of a solar cell booster according to an embodiment of the present invention.

11, a solar cell booster according to an embodiment of the present invention includes an absorbing layer 10 made of a semiconductor material, a first graphene layer 40 laminated on the absorbing layer 10, A solar cell including the insulating layer 20 stacked on the graphene layer 40 and the second graphene layer 40 stacked on the insulating layer 20 and the first and second graphene layers 40, And a DC power source 50 connected at both ends thereof. At this time, only the voltage is applied between the first and second graphene layers 40 by the insulating layer 20. That is, no current flows between the first and second graphene layers 40.

Through this solar cell booster, graphene itself is used instead of metal to obtain a high Schottky junction barrier. The graphene work function has a normal size, but its work function is chemically and electrically adjustable. In the solar cell booster according to the embodiment of the present invention, the work function is electrically controlled to increase the built-in voltage of the solar cell. In order to control the work function of graphene, graphene directly contacts the absorbing layer 10, and an insulating layer 20 made of at least one of dielectric or electrolyte is raised on the graphene, and then graphenes are deposited thereon, Applying a voltage between the two graphenes. At this time, since only a voltage is applied between the first and second graphene layers 40 and no current flows, the thickness of the insulating layer 20 must be sufficiently thick so as to prevent a leakage current from flowing, . Since sufficient Fermi level movement must be induced, it is preferable to use a dielectric material having a very high dielectric constant or to use a high concentration of electrochemically stable electrolyte.

Fig. 12A is a diagram showing the structure of a solar cell booster when the absorption layer is p-type, and Fig. 12B is a diagram showing the energy band structure in Fig. 12A. 13A is a diagram showing the structure of a solar cell booster when the absorption layer is n-type, and FIG. 13B is a diagram showing the energy band structure in FIG. 13A.

The absorption layer 10 is at least one of an n-type semiconductor and a p-type semiconductor. The polarity of the DC power source 50 connected to the graphene layer 40 is different according to the absorption layer 10.

12A and 12B, in the case where the absorption layer is the p-type semiconductor 12, the DC power source 52 is connected to the first graphene layer with the anode and the cathode to the second graphene layer. As a result, the Fermi level of graphene is controlled.
That is, when the voltage of two graphenes is applied, it acts as a battery. As a result, the graphene connected to the anode is positively charged, and the graphene connected to the cathode is negatively charged, thereby causing the Fermi level of the graphene to move. This is due to the nature of graphene with a two-dimensional structure.
The energy band structure for this is shown in Fig. 12B. A Schottky junction is formed between the graphene and the p-type semiconductor 12, and the builtin voltage is determined by the voltage of the DC power source 52 and the dielectric constant and thickness of the insulating layer 20. [
The graphene connected to the cathode is negatively charged, and as a result, the Fermi level of the graphene moves upward. That is, the Fermi level of graphene approaches the conduction band of the semiconductor, while the Fermi level of the semiconductor is close to the valence band of the semiconductor, and the Schottky junction between the graphene and the semiconductor, .
As a result, band bending occurs in the energy band of the semiconductor and a built-in potential is formed.
In particular, the Fermi level of graphene changes the charge amount according to the applied voltage and the dielectric constant of the dielectric between the two grapins, so that the Fermi level of the graphene can be freely changed.

13A and 13B, when the absorbing layer is the n-type semiconductor 14, the DC power source 54 is connected to the cathode of the first graphene layer and to the anode of the second graphene layer. As a result, the Fermi level of graphene is controlled. The energy band structure for this is shown in Fig. 13B. Schottky junctions are formed between the graphene and the n-type semiconductor 14, and the builtin voltage is determined by the voltage of the DC power supply 54 and the dielectric constant and thickness of the insulating layer 20.
Here, in the case of an n-type semiconductor, it may be used in reverse to the case of the above-described p-type semiconductor.

Therefore, through the solar cell booster according to the embodiment of the present invention, the builtin voltage can be actively controlled using graphene without using a metal.

While the present invention has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, You will understand. It is therefore to be understood that the above-described embodiments are illustrative in all aspects and not restrictive.

10: Absorption layer
20: Insulation layer
30: Metal
40: graphene layer

Claims (5)

1. A solar cell comprising: an absorbing layer made of a semiconductor material; a first graphene layer stacked on the absorbing layer; an insulating layer stacked on the first graphene layer; and a second graphene layer stacked on the insulating layer; And
And a DC power supply having both ends connected to the first and second graphene layers,
Wherein only a voltage is applied between the first and second graphene layers by the insulating layer. The method according to claim 1,
Wherein the absorption layer is at least one of an n-type semiconductor and a p-type semiconductor. 3. The method of claim 2,
Wherein the direct current power source has a positive electrode connected to the first graphene layer and a negative electrode connected to the second graphene layer when the absorbing layer is a p-type semiconductor. 3. The method of claim 2,
Wherein the direct current power source has a negative electrode connected to the first graphene layer and a positive electrode connected to the second graphene layer when the absorbing layer is an n-type semiconductor. The method according to claim 1,
Wherein the insulating layer is a dielectric.

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