WO2014088155A1 - Solar cell and method for manufacturing same - Google Patents

Abstract

The present invention provides a solar cell comprising: first and second semiconductor layers which have different conductive forms; first and second electrodes which are respectively formed so as to come into contact with the first and second semiconductor layers; and a conversion layer which is formed on the second semiconductor layer, and which absorbs light of a wavelength having first energy and converts the light into light of a wavelength having second energy that is lower than the first energy. Thus, the present invention can improve efficiency by absorbing visible light and visible light converted from ultraviolet light.

Description

Solar cell and manufacturing method thereof

TECHNICAL FIELD This invention relates to a solar cell. Specifically, It is related with the silicon solar cell which can improve efficiency, and its manufacturing method.

A solar cell is a kind of semiconductor device that converts solar energy directly into electrical energy and has a junction structure between a p-type semiconductor and an n-type semiconductor. The basic structure is the same as that of a diode.

The performance of such solar cells is highly dependent on the efficiency of converting light energy into electrical energy. Therefore, a lot of research is being conducted to increase the efficiency of solar cells, and one of them is a method of maximizing absorption of light by texturing a portion where light is incident. This texturing increases the light absorption through light scattering.

However, it is difficult to further increase the amount of incident light even by texturing. Therefore, there is a need for a method capable of increasing the amount of incident light and thereby improving efficiency.

The present invention provides a solar cell and a method of manufacturing the same that can improve the efficiency.

The present invention provides a solar cell and a method of manufacturing the same that can improve efficiency by increasing the amount of visible light through energy down conversion.

The present invention provides a solar cell capable of increasing the amount of incident light by absorbing light having an ultraviolet wavelength through energy down conversion using quantum dots and emitting light having a visible wavelength, and absorbing it with visible light. It provides a manufacturing method.

A solar cell according to an aspect of the present invention includes first and second semiconductor layers of different conductivity types; First and second electrodes respectively formed to contact the first and second semiconductor layers; And a conversion layer formed on the second semiconductor layer and absorbing light of a wavelength having a first energy and converting the light into a light having a lower second energy.

The first semiconductor layer may be formed by doping a semiconductor substrate with a first impurity of a first conductivity type, and at least one surface may be textured.

The second semiconductor layer may be formed by doping a second impurity of a second conductivity type to a predetermined depth of the first semiconductor layer.

The first and second semiconductor layers may include crystalline silicon.

The conversion layer converts light in an ultraviolet region to emit light in the visible region, and the first and second semiconductor layers absorb visible light and visible rays converted from ultraviolet rays by the conversion layer.

The conversion layer includes a core and a quantum dot including a shell surrounding the quantum dot, wherein the core is formed of any one selected from CdSe, InP, InAs, CuInS ₂ , PbS, and PbTe, and any one of ZnS, ZnSe, and CdSe A shell is formed, wherein the core is formed of CdSe, and the shell may be formed of ZnS.

The wavelength of the conversion layer is adjusted according to the size of the core and shell.

The core may be formed in a size of 0.1 nm to 10 nm, the shell may be formed in a size of 8 nm to 20 nm. In addition, as the core size increases, the wavelength to be converted increases, and as the core size decreases, the wavelength to be converted may be reduced.

An anti-reflection film may be formed on the second semiconductor layer, the conversion layer may be formed on the anti-reflection film, and may further include a protective layer formed on the conversion layer.

An anti-reflection film may be formed on the second semiconductor layer so that the anti-reflection film is textured, and the conversion layer may be formed in a recess of the anti-reflection film.

The conversion layer may be formed on the second semiconductor layer, and an anti-reflection film may be formed to cover the conversion layer.

According to another aspect of the present invention, a method of manufacturing a solar cell includes forming a second semiconductor layer on one surface of a first semiconductor layer; Forming a first electrode on the other surface of the first semiconductor layer, and forming a second electrode in a predetermined region on the second semiconductor layer; And forming a conversion layer on the second semiconductor layer to absorb light of a wavelength having a first energy and convert the light into a light having a lower second energy.

The first and second semiconductor layers may form the second semiconductor layer by doping the first impurity to a semiconductor substrate and then doping the second semiconductor to a predetermined depth in the first semiconductor layer. can do.

The semiconductor substrate may include a crystalline substrate and an amorphous substrate.

The method may further include texturing at least one surface of the first semiconductor layer.

The method may further include forming an anti-reflection film between the second semiconductor layer and the conversion layer, and may further include forming an anti-reflection film on the conversion layer.

The conversion layer includes a core and a quantum dot including a shell surrounding the core.

Embodiments of the present invention form a quantum dot of the core / shell structure on the semiconductor layer, by absorbing the high energy in the ultraviolet region by using the quantum dot to energy down conversion (energy down conversion) to emit the visible light Thereby generating visible light. Therefore, the solar cell can increase the incident light by absorbing visible light converted from ultraviolet rays in addition to visible light, compared to the conventional method of absorbing only visible light. In addition, the efficiency can be improved by increasing the incident light.

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

2 is a partially enlarged photograph of a solar cell according to an embodiment of the present invention.

3 and 4 are cross-sectional views of solar cells according to other embodiments of the present invention.

5 is a diagram showing energy according to the wavelength of sunlight.

6 is an energy bandgap of a quantum dot of a core / shell structure according to the present invention;

7 and 8 are graphs measuring the optical luminescence intensity and absorbance according to the structure of the quantum dot.

9 and 10 are photographs and size distribution diagrams of morphology (morphology) according to the structure of the quantum dot.

11, 12 and 13 is a graph measuring reflectivity after forming quantum dots of various structures at various concentrations.

14, 15 and 16 are graphs of the external quantum efficiency measured according to the structure of the quantum dot.

17 and 18 are graphs measuring photo-voltaic performance of a solar cell according to the structure of a quantum dot.

Hereinafter, with reference to the accompanying drawings will be described an embodiment of the present invention; However, the present invention is not limited to the embodiments disclosed below, but may be implemented in various forms, and only the embodiments are intended to complete the disclosure of the present invention and to those skilled in the art. It is provided for complete information.

1 is a cross-sectional view of a solar cell according to an embodiment of the present invention, Figure 2 is a partially enlarged photograph of a solar cell according to an embodiment of the present invention.

Referring to FIG. 1, a solar cell according to an embodiment of the present invention includes a first semiconductor layer 110, a first electrode 120 formed on a rear surface of the first semiconductor layer 110, and a first semiconductor layer ( The second semiconductor layer 130 formed on the 110, the antireflection film 140 formed on the second semiconductor layer 130, the conversion layer 150 formed on the antireflection film 140, and the antireflection film 140 And the second electrode 160 connected to the second semiconductor layer 130 through a predetermined region of the conversion layer 150.

The first semiconductor layer 110 includes a semiconductor layer doped with a first impurity of a first conductivity type. Here, the first semiconductor layer 110 may use a semiconductor substrate, and may be formed by doping the semiconductor substrate with a first impurity of a first conductivity type. As the semiconductor substrate, a silicon substrate such as a single crystal silicon substrate or a polycrystalline silicon substrate can be used. That is, a silicon wafer can be used as a semiconductor substrate. Of course, semiconductor substrates other than a silicon substrate can also be used. In addition, the first impurity may be a p-type impurity or an n-type impurity, the p-type impurity includes a group III element such as boron (B), aluminum (Al), gallium (Ga), and the n-type impurity is phosphorus ( Group V elements such as P), arsenic (As), and antimony (Sb) may be included. For example, the first semiconductor layer 110 may be doped with a group III element such as boron or a group V element such as phosphorous on the single crystal silicon substrate. Meanwhile, in order to maximize absorption of light, the first semiconductor layer 110 may be textured, thereby improving the efficiency of the solar cell. That is, since the first semiconductor layer 110 is textured, the anti-reflection film 140 formed on the first semiconductor layer 110 is also textured. Therefore, the incident light is not reflected and is not lost, and light absorption is increased through light scattering. At this time, the first semiconductor layer 110 is textured to form a pyramid or inverted pyramid structure on the surface, or to form a porous or uneven structure. In addition, the first semiconductor layer 110 may be formed by texturing a rear surface as well as an upper surface.

The first electrode 120 is formed on the rear surface of the first semiconductor layer 110. For example, the first electrode 120 is formed on the back surface of the first semiconductor layer 110 that is textured so that the surface is planarized. The first electrode 120 may be formed of a material having excellent conductivity, for example, titanium (Ti), chromium (Cr), gold (Au), aluminum (Al), nickel (Ni), silver (Ag). It can form using metals, such as these, or these alloys. In addition, the first electrode 120 may be formed as a single layer, or at least two or more layers may be laminated. In the present embodiment, the first electrode 120 may be formed using aluminum.

The second semiconductor layer 130 is formed on the first semiconductor layer 110. The second semiconductor layer 130 includes a semiconductor layer doped with a second impurity of the second conductivity type. Here, the first impurity may be a p-type impurity or an n-type impurity. However, each of the first and second semiconductor layers 110 and 130 should have opposite conductive characteristics to form a junction structure that may cause photoelectric effect when incident light is absorbed. Therefore, if the first semiconductor layer 110 is a p-type semiconductor layer doped with p-type impurities, the second semiconductor layer 130 should be an n-type semiconductor layer doped with n-type impurities. However, since the first semiconductor layer 110 of the embodiment of the present invention uses a silicon substrate, the second semiconductor layer 130 may be formed by doping n-type impurities to a silicon substrate at a predetermined depth.

The anti-reflection film 140 is formed to lower reflectance of light incident on the first and second semiconductor layers 110 and 130. The anti-reflection film 140 has a refractive index of 1.0 to 4.0 silicon nitride (SiNx), titanium oxide (TiO ₂ ), aluminum oxide (Al ₂ O ₃ ), niobium oxide (Nb ₂ O ₅ ), magnesium oxide (MgO) , Silicon oxide (SiO ₂ ) or the like. That is, the anti-reflection film 140 may be formed of oxide, nitride, or the like having a low refractive index. In addition, the anti-reflection film 140 may be formed in a single layer structure or may be formed in a multilayer structure. When the anti-reflection film 140 is formed in a multilayer structure, at least two layers having different refractive indices may be stacked. In this case, at least two layers may be alternately formed repeatedly. Meanwhile, since the anti-reflection film 140 is formed on the textured first semiconductor layer 110, the anti-reflection film 140 has a textured structure. As the anti-reflection film 140 is textured, incident light is not reflected and is not lost, and light absorption may be increased through light scattering.

The conversion layer 150 is formed on the anti-reflection film 140. That is, the conversion layer 150 is formed along the surface of the textured anti-reflection film 140. The conversion layer 150 includes a plurality of quantum dots 151 adsorbed on the anti-reflection film 140. The quantum dot 151 has a core / shell structure, and a shell is formed to surround the core. The quantum dot 151 has a core formed of any one selected from CdSe, InP, CuInS ₂ , PbS, and CdTe, and may be a shell made of any one selected from ZnS, ZnSe, and CdSe. Preferably, the quantum dot 151 may be formed in a core / shell structure using CdSe / ZnS. Meanwhile, CdSe / ZnS quantum dots can be synthesized by a hot injection method, which will be described below. First, a predetermined amount of cadmium (Cd) raw material powder and zinc (Zn) raw material powder are introduced into a predetermined reactor maintaining room temperature. CdO may be used as the cadmium raw material, and zinc acetate may be used as the zinc raw material. In this way, the solvent is introduced into the reactor in which the cadmium raw material and the zinc raw material are added. Here, the solvent may be used, for example, oleic acid (Oleic Acid) and octadecene (1-Octadecene). Then, when the temperature of the reactor is increased, the raw powder begins to melt in the solvent, for example, at a temperature of about 120 ° C. When the temperature of the reactor rises to about 150 ° C., cadmium (Cd) ions and zinc (Zn) ions are generated and dissolved in acetate. The acetate is then evaporated when the reactor is kept in vacuo, e.g. 30 minutes, to evaporate all the acetate. In addition, after raising the temperature of the reactor to about 310 ° C., for example, CdSe / ZnS quantum dots are formed by rapidly injecting TPO (Trioctylphosohine) in which selenide (Se) and sulfur (S) are dissolved and maintaining the same for a predetermined time. In this case, the size of the quantum dot may be adjusted according to the retention time after TPO injection and the molar ratio of the mixed materials. The synthesized quantum dot 151 of the core / shell structure may be formed by, for example, mixing with a dispersing agent such as chloroform at a predetermined ratio, being formed on the antireflection film 140 by a spin coating process, and then drying. Quantum dots 151 of the core / shell structure have different amounts of adsorption on the antireflection film 140 according to the molar ratio mixed with the dispersant, and the characteristics of the solar cell are also different. For example, FIG. 2 (a) is a photograph of adsorption of quantum dots of CdSe / ZnS structure at a concentration of 0.3wt%, and FIG. 2 (b) is a photograph of adsorption of quantum dots of CdSe / ZnS structure at a concentration of 1.0wt%. to be. As shown, it can be seen that the quantum dots of the core / shell structure are well adsorbed on the <111> textured surface, and as the concentration of the core / shell quantum dots is increased, the amount of adsorbed on the textured surface increases. The quantum dot 151 of the core / shell structure absorbs light in the ultraviolet region to generate light in the visible region. That is, the quantum dot 151 of the core / shell structure generates light in the visible region by energy down conversion. The quantum dot 151 absorbs the high energy in the ultraviolet region to lower the energy and recombines the visible light. Generates the light of the line. As the quantum dot 151 having the core / shell structure is formed, the visible light converted from ultraviolet rays in addition to the visible light is absorbed to increase incident light, thereby improving efficiency. On the other hand, the quantum dot 151 is formed in a size of nm, the core is formed in a size of 0.1 nm to 10 nm, for example, the shell is formed in a size larger than the core to surround the core, for example 8 nm to It may be formed in a size of 20 nm. In addition, the color of the light generated by the conversion may be adjusted according to the size of the core. For example, green light may be generated when using a 2.9 nm CdSe core and a ZnS shell of 10.082 nm, and red light may be generated when using a 4.7 nm CdSe core and a 8.989 nm ZnS shell. That is, as the size of the core becomes smaller, the energy band gap becomes larger to convert ultraviolet rays into short wavelength visible light, and as the size of the core becomes larger, the energy band gap becomes smaller to convert ultraviolet rays into long wavelength visible light. For example, if the ultraviolet ray is changed into visible light of the first wavelength by using the core of the first size, the ultraviolet ray may be changed into visible light of the second wavelength having a longer wavelength than the first wavelength by using the core larger than the first size. . Therefore, the core can be formed in a size that is larger than the size capable of converting ultraviolet light into visible light, for example, 0.1 nm or more. And, the shell can be formed to a certain size or more that can wrap the core. On the other hand, the shape of the quantum dot 151 may have a variety of shapes such as spheres, rods, wires, pyramids, cubes, and the like, although not particularly limited, it is preferable to have a spherical shape.

The second electrode 160 is formed to contact the second semiconductor layer 130. That is, the second electrode 160 is formed in contact with the exposed second semiconductor layer 130 by removing a predetermined region of the anti-reflection film 140 and the conversion layer 150. In this case, the second electrode 160 is partially formed in order to prevent incident light from being reduced by the second electrode 160. Like the first electrode 120, the second electrode 160 may be formed of a material having excellent conductivity. For example, titanium (Ti), chromium (Cr), gold (Au), aluminum (Al), and nickel ( It can be formed using a metal material such as Ni), silver (Ag), or an alloy thereof. In addition, the second electrode 160 may be formed as a single layer, or at least two or more layers may be stacked. Here, the second electrode 170 may be formed of the same material as the first electrode 120 or may be formed of a different material. In the present embodiment, the second electrode 160 may be formed of silver (Ag).

3 and 4 are cross-sectional views of solar cells according to other embodiments of the present invention. As illustrated in FIG. 3, the conversion layer 150 may be formed on the second semiconductor layer 130, and an anti-reflection film 140 may be formed thereon. The anti-reflection film 140 may function as a protective film for protecting the conversion layer 150. Of course, a protective film can also be formed separately. That is, the conversion layer 150 may be formed on the anti-reflection film 140, and a protective film may be formed on the conversion layer 150. In this case, the protective film may be formed of a transparent insulating material capable of transmitting light. In addition, FIG. 4 may be formed only in a concave portion of the anti-reflection film 140 in which the conversion layer 150 is textured. That is, the conversion layer 150 may be filled in the concave portion of the anti-reflection film 140 to planarize the surface, and may be formed thicker than that to form the flat surface on the anti-reflection film 140.

As described above, embodiments of the present invention form a conversion layer having a core / shell structure, and use energy down conversion to absorb and absorb high energy in the ultraviolet region by using the conversion layer to reduce and recombine energy. Produces visible light. Therefore, the semiconductor layer can increase incident light by absorbing visible light converted from ultraviolet light in addition to visible light, thereby improving efficiency. In this way, the efficiency of the solar cell is improved by energy down conversion by the core / shell structure conversion layer in more detail.

5 is a diagram showing energy according to the wavelength of sunlight, showing the spectrum of sunlight and the wavelength and energy (blue display) converted by the silicon solar cell. 6 is a diagram illustrating an energy band gap of a quantum dot of a core / shell structure according to the present invention.

As shown in FIG. 5, a conventional solar cell absorbs light having a wavelength of 1100 nm or less of visible light from sunlight emitting infrared light, visible light, and ultraviolet light to generate a photo current. Conventional solar cells absorb visible light and convert it into a photocurrent, but there is a limit in the photocurrent generated because the ultraviolet light does not absorb and converts into a photocurrent. However, the solar cell according to the present invention absorbs ultraviolet rays having a wavelength of 400 nm or less by applying quantum dots of a core / shell structure to lower the energy of ultraviolet rays and then recombine to generate visible light. This will be described in more detail as follows. In the quantum dot of the core / shell structure according to the present invention, the nano-sized shell surrounds the nano-sized core, and as shown in FIG. 6, the energy band gap Eg of the shell is larger than the energy band gap Eg of the core. . Thus, the electrons are quantized outside the conduction energy band and the balance energy band of the core, and electron quantum levels of E1, E2, and E3 and hole quantum levels of H1, H2, and H3 are formed. In addition, the shell's high energy bandgap (Eg) serves as an energy barrier for the quantized electrons and holes in the core. When the quantum dot of the core / shell structure absorbs ultraviolet rays, electrons in the H3 quantum level of the balance band of the core transition to the E3 quantum level of the high conduction band, as shown in FIG. 6. And electron and hole pairs. Afterwards, the electron and hole pairs move to lower quantum levels of electrons and holes, which are then transferred to the E2 and H2 quantum levels, and then, when electrons and holes in the E2 and H2 quantum levels are recombined, they emit visible light. In addition, absorbing ultraviolet light with lower energy causes the electrons in the H2 quantum level of the balance band of the core to transition to the E2 quantum level of the high conduction band, and the electron and hole pair Will make Afterwards, the electron and hole pairs move to lower quantum levels of electrons and holes, respectively, and are transferred to the E1 and H1 quantum levels, and then, when electrons and holes in the E1 and H1 quantum levels are recombined, they emit visible light. At this time, the wavelength of the visible light emitted depends on the energy band gap Eg of the core, and the energy band gap Eg of the core depends on the size of the core and the energy band gap Eg of the shell. Light emitted from the quantum dots of the core / shell structure is absorbed by the surface of the <111> textured layer to increase the photocurrent inside the silicon solar cell. That is, the quantum dot of the core / shell structure absorbs ultraviolet rays and then converts energy to emit visible light, and the visible light emitted from the quantum dot of the core / shell structure is absorbed by the surface of the semiconductor layer, thereby shorting current of the silicon solar cell. The circuit current can be increased to improve the power conversion efficiency of the solar cell.

To prove this, quantum dots of various concentrations of core / shell structures are adsorbed onto the surface of the layer textured by <111> by spin coating, followed by surface reflectance and light emission intensity. Photo luminescence, external quantum efficiency, photovoltaic performance, and the like were measured.

The color and intensity of the light emitted according to the structure of the quantum dots were measured, and as a comparison example, CdSe quantum dots and quantum dots of CdSe / ZnS structures having different sizes were compared as embodiments of the present invention. CdSe quantum dots consist of only 4.7 nm cores, and CdSe / ZnS quantum dots consist of 2.89 nm CdSe cores, 10.092 nm ZnS shells, and 4.91 nm CdSe cores and 8.969 nm ZnS shells. That is, Example 1 of the present invention consists of a 2.89 nm CdSe core and 10.092 nm ZnS shell, and Example 2 of the present invention consists of a 4.91 nm CdSe core and 8.969 nm ZnS shell.

When the quantum dots were dissolved in a solvent and irradiated with light of different wavelengths, the wavelengths of the light emitted by the respective quantum dots were measured. That is, visible light and ultraviolet rays of 254 nm and 365 nm wavelength were irradiated. CdSe quantum dots emit light of 578nm wavelength, CdSe / ZnS quantum dots of Example 1 emits light of 562nm, CdSe / ZnS quantum dots of Example 2 emits light of 603nm wavelength Able to know. The color of the light emitted by the CdSe quantum dots emits red light when irradiated with visible light, and emits red light that is slightly different depending on the wavelength when irradiated with ultraviolet light. In addition, the CdSe / ZnS quantum dots of Example 1 emit green light when both visible and ultraviolet rays are irradiated. In the case of the CdSe / ZnS quantum dots of Example 2, the red light is emitted when the visible light is irradiated, and the red light is slightly different depending on the wavelength when the ultraviolet light is irradiated. From this, it can be seen that the quantum dot absorbs light and emits visible light through energy down conversion.

The surface of the silicon nitride antireflection film textured with <111> was spin-coated at various concentrations and irradiated with ultraviolet rays at a wavelength of 254 nm, and then illuminance of the emitted light was measured. In the case of CdSe quantum dots, it is difficult to find the concentration dependence of the quantum dots as the emission illuminance is below the detection limit. However, in the case of the CdSe / ZnS quantum dots of Example 1, the light emission illuminance increases as the concentration of the quantum dots increases. However, in the case of the CdSe / ZnS quantum dots of Example 2, it is difficult to find the concentration dependence of the quantum dots as the emission illuminance is below the detection limit.

As described above, it can be seen that the quantum dot of the core / shell structure absorbs light in the ultraviolet region and then emits light in the visible region by energy down conversion. In addition, it can be seen that the emission of visible light of different colors according to the size of the core, the light emission intensity increases as the concentration increases.

7 and 8 are graphs measuring the intensity and absorbance of photo luminescence (hereinafter referred to as PL) according to the structure of quantum dots. 7 and 8 (a) shows the PL intensity and absorbance according to the concentration of the CdSe quantum dots, Figure 7 and 8 (b) shows the concentration of the CdSe / ZnS quantum dots emitting green light PL intensity and absorbance are shown, and FIG. 7 and FIG. 8C show PL intensity and absorbance according to the concentration of CdSe / ZnS quantum dots emitting red light.

As a result of the PL measurement, as shown in FIG. 7, a CdSe quantum dot detected a light emission peak at 575 nm, and a CdSe / ZnS quantum dot emitting green light detected a light emission peak at 542 nm and 583 nm. CdSe / ZnS quantum dots emitting light detected a light emission peak at 607 nm. When the PL intensity is 0.5 wt%, the CdSe / ZnS quantum dot emitting green light is about 20,000 (arb, unit), the CdSe / ZnS quantum dot emitting red light is about 3000, and the CdSe quantum dot is about 600 is shown, respectively. Thus, CdSe / ZnS quantum dots emitting green light exhibit the highest PL intensity.

Absorbance shown in Figure 8 was measured by using a quantum dot UV-vis equipment. CdSe quantum dots have an absorption peak at 559 nm, 483 nm, and 266 nm. This is because there is a discontinuous quantum energy level above and below the conduction band and the balance band of CdSe, and the absorption peaks are absorbed because the light is absorbed by these discontinuous quantum energy levels. exist. In addition, it can be seen that the degree of absorption increases as the concentration increases. However, in the case of the quantum dot of the core / shell structure, such absorbance peaks are not confirmed, and it can be confirmed only that the wavelength region where light is absorbed increases as the concentration increases. This tendency is because CdSe / ZnS quantum dots that emit green light have a larger energy bandgap due to the smaller size of the core (CdSe) because only light with higher energy than the energy bandgap of the core is absorbed. ), I.e., CdSe / ZnS quantum dots that begin to absorb at light with a wavelength below about 500 nm and emit red light begin to absorb at higher wavelengths (lower energy), that is, at wavelengths below 600 nm. You can see that.

Comparing the absorbance and PL intensity of each quantum dot, the CdSe quantum dot absorbs light having a wavelength of about 250 nm to 560 nm, and emits light having a wavelength of 575 nm through energy down conversion. CdSe / ZnS quantum dots that emit green light absorb light in the 250 nm to 500 nm wavelength region and emit light with 542 nm and 583 nm wavelengths through energy down conversion. CdSe / ZnS quantum dots that emit red light absorb light with a wavelength of 250 nm to 600 nm and emit red light with a wavelength of 607 nm through energy down conversion.

9 and 10 are TEM analysis of morphology according to the structure of a quantum dot and a distribution chart for each size. Here, (a) of the angle is a photograph and distribution of the CdSe quantum dot, (b) is a photograph and distribution of the quantum dot of the CdSe / ZnS structure that emits green light, (c) is a CdSe / ZnS structure that emits red light Photos and distribution maps of quantum dots.

As shown in FIG. 9, the CdSe quantum dots have a size of about 4.7 nm and have an irregular shape. However, the CdSe / ZnS quantum dots emitting green light have a size of 10.098 nm, and the CdSe / ZnS quantum dots emitting red light have a size of 8.969 nm and have an irregular shape. In addition, as shown in FIG. 10, the size-by-size distribution according to the structure of the quantum dot shows that the size of the CdSe / ZnS quantum dots emitting green light is uniform and thus the dispersion is better than that of the CdSe quantum dots.

11, 12 and 13 are graphs of reflectivity measured after forming quantum dots at various concentrations using a spin coating method on a silicon nitride anti-reflection film textured with <111>. Here, FIGS. 11 to 13 (a) show reflectances according to the wavelengths of the ultraviolet and visible light regions, and (b) show reflectances according to the wavelengths of the ultraviolet light regions.

As shown in FIG. 11, when the CdSe quantum dot is coated, the concentration of the CdSe quantum dot increases as compared with the reference where the surface reflectance does not form the quantum dot in the ultraviolet region (200 nm to 450 nm). Increases. In particular, about 1 wt% CdSe quantum dots decrease from about 25% to about 17.5%. As shown in FIG. 12, when the CdSe / ZnS quantum dots emitting green light are coated, the surface reflectivity decreases from about 25% to 15% as the concentration of the quantum dots increases in the ultraviolet region. In addition, as shown in FIG. 13, when the CdSe / ZnS quantum dots emitting red light are coated, when the quantum dot concentration increases in the ultraviolet region, the surface reflectivity decreases from about 25% to 15%. These results indicate that the quantum dots absorb light in the ultraviolet region. However, in the regions other than ultraviolet light (450 nm to 1100 nm), the surface reflectivity slightly increases as the concentration of quantum dots increases in both CdSe quantum dots, CdSe / ZnS quantum dots emitting green light, and CdSe / ZnS structures emitting red light. However, the efficiency is further increased because it is less than the light that is energy down-converted into ultraviolet light and converted into visible light.

14, 15, and 16 are results of measuring external quantum efficiency (EQE) according to the structure of a quantum dot by using the IPCE equipment. 14 to 16 (a) show the external quantum efficiency according to the wavelength of the ultraviolet and visible light region, (b) shows the external quantum efficiency according to the wavelength of the ultraviolet region.

As shown in FIG. 14, in the case of the CdSe quantum dot, the EQE data hardly changes. Although the quantum dots absorbed the light and the reflectance was reduced, it was confirmed that there was no change in the EQE data because the amount of light emitted was not enough. However, as shown in FIGS. 15 and 16, the CdSe / ZnS quantum dots emitting green light and red light may increase efficiency between 300 nm and 500 nm, unlike CdSe quantum dots. The silicon solar cell did not have good conversion efficiency at a wavelength of 300 nm to 500 nm, but it was confirmed that the efficiency of the silicon solar cell was increased by converting the light having a wavelength of 500 nm or more. On the other hand, in the 500 nm to 1100 nm region, the conversion efficiency decreases as the concentration increases due to the decrease in reflectivity. However, the efficiency is further increased because it is less than the light that is energy down-converted into ultraviolet light and converted into visible light.

 17 and 18 are graphs illustrating photo-voltaic performance of a solar cell in which quantum dots are formed on a surface of a silicon nitride antireflection film textured with <111>. FIG. 17 illustrates the short-circuit current density Jsc of the solar cell according to the concentration of the quantum dots, and FIG. 18 illustrates the efficiency of the solar cell according to the concentration of the quantum dots. 17 and 18, (a), (b) and (c) show the short-circuit current densities and efficiencies of CdSe quantum dots, CdSe / ZnS quantum dots emitting green light, and CdSe / ZnS quantum dots emitting red light, respectively. It is shown.

As shown in FIGS. 17A and 18A, the short-circuit current density (Jsc) and power conversion efficiency (PCE) of the solar cell in which the CdSe quantum dots are formed do not increase compared to the reference in which the quantum dots are not formed. Did.

However, in the case of a solar cell having a CdSe / ZnS structure that emits green light, as shown in FIG. 17 (b), the short-circuit current density (Jsc) has a concentration of about 0.2 wt% of the quantum dot compared to the reference. After a sharp increase, after about 0.5wt%, it decreases slightly as the quantum dot concentration increases. However, at a concentration of about 0.2 wt%, the short-circuit current density is increased by about 2.2 mA / cm 2 compared to the reference, and thus it can be seen that the improvement is about 6.34%. In addition, as shown in FIG. 18B, when the concentration of CdSe / ZnS quantum dots emitting green light is increased to 0.2wt%, PCE increases by 0.91% compared to the reference, which is about 6.5%. have. However, CdSe / ZnS quantum dots emitting green light decrease in PCE when the concentration increases to 0.3 wt% or more. The increase in PCE up to 0.3 wt% is due to the absorption of ultraviolet rays and the absorption of visible light emitted through energy down conversion, and when the concentration of quantum dots is more than 0.3 wt%, the reflectance in visible light in addition to energy down conversion. Decreases the PCE.

Meanwhile, in the case of a solar cell in which CdSe / ZnS quantum dots emit red light, as shown in FIG. 18 (b), the short-circuit current density (Jsc) increases rapidly with respect to the reference to the concentration of the quantum dots by about 0.4 wt%. After about 0.5wt%, it decreases slightly as the concentration of quantum dots increases. However, at a concentration of about 0.4 wt%, the short-circuit current density is increased by 1.9 mA / cm 2 compared to the reference, and accordingly, it can be seen that the improvement is about 5.58%. In addition, as shown in FIG. 18C, the PCE increases as the concentration of the quantum dots increases to 0.4 wt%, increasing by 0.54% relative to the reference, and improving by about 3.8%. However, as the quantum dot concentration increases at 0.5 wt% or more, it is slightly decreased. Thus, the PCE increase (0.91%) of solar cells with CdSe / ZnS quantum dots emitting green light is greater than the PCE (0.54%) of solar cells with CdSe / ZnS quantum dots emitting red light.

On the other hand, although the technical spirit of the present invention has been described in detail according to the above embodiment, it should be noted that the above embodiment is for the purpose of explanation and not for the limitation. In addition, those skilled in the art will understand that various embodiments are possible within the scope of the technical idea of the present invention.

Claims (23)

First and second semiconductor layers of different conductivity types; First and second electrodes respectively formed to contact the first and second semiconductor layers; And And a conversion layer formed on the second semiconductor layer and configured to absorb light having a wavelength having a first energy and convert the light having a second wavelength having a lower energy. The solar cell of claim 1, wherein the first semiconductor layer is formed by doping a semiconductor substrate with a first impurity of a first conductivity type. The solar cell of claim 2, wherein at least one surface of the first semiconductor layer is textured. The solar cell of claim 3, wherein the second semiconductor layer is formed by doping a second impurity of a second conductivity type to a predetermined depth of the first semiconductor layer. The solar cell of claim 4, wherein the first and second semiconductor layers comprise crystalline silicon. The solar cell of claim 5, wherein the conversion layer converts light in an ultraviolet region by energy down conversion to emit light in a visible region. The solar cell of claim 6, wherein the first and second semiconductor layers absorb visible light converted from ultraviolet rays by energy down conversion of the visible light and the conversion layer. The solar cell of claim 6, wherein the conversion layer comprises a quantum dot including a core and a shell surrounding the conversion layer. The solar cell of claim 8, wherein the quantum dot has a core formed with any one selected from CdSe, InP, CuInS ₂ , PbS, and CdTe, and a shell is formed with any one selected from ZnS, ZnSe, and CdSe. The solar cell of claim 9, wherein the core is formed of CdSe, and the shell is formed of ZnS. The solar cell of claim 9, wherein the conversion layer has a wavelength that is converted according to the size of the core and the shell. The solar cell of claim 9, wherein the core is formed in a size of 0.1 nm to 6 nm, and the shell is formed in a size of 8 nm to 20 nm. The solar cell of claim 6, wherein an antireflection film is formed on the second semiconductor layer, and the conversion layer is formed on the antireflection film. The solar cell of claim 13, further comprising a protective layer formed on the conversion layer. The solar cell of claim 6, wherein an anti-reflection film is formed on the second semiconductor layer so that the anti-reflection film is textured, and the conversion layer is formed in a recess of the anti-reflection film. The solar cell of claim 6, wherein the conversion layer is formed on the second semiconductor layer, and an antireflection film is formed to cover the conversion layer. Forming a second semiconductor layer on one surface of the first semiconductor layer; Forming a first electrode on the other surface of the first semiconductor layer, and forming a second electrode on a predetermined region of the second semiconductor layer; And Forming a conversion layer on the second semiconductor layer to absorb light of a wavelength having a first energy and convert the light into a light having a lower second energy. The method of claim 17, wherein the first and second semiconductor layers, And forming the second semiconductor layer by doping the first impurity to a semiconductor substrate to form the first semiconductor layer and then doping the first semiconductor layer with a second impurity to a predetermined depth. The method of claim 18, wherein the semiconductor substrate comprises a crystalline silicon substrate. 19. The method of claim 18, further comprising texturing at least one surface of the first semiconductor layer. 20. The method of claim 19, further comprising forming an anti-reflection film between the second semiconductor layer and the conversion layer. 20. The method of claim 19, further comprising forming an anti-reflection film on the conversion layer. The method of claim 18, wherein the conversion layer comprises a quantum dot including a core and a shell surrounding the core.

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