JP2014165288A - Quantum dot and solar cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a quantum dot ensuring photoelectric conversion characteristics even if the size is large, and to provide a solar cell to which the quantum dot is applied.SOLUTION: A quantum dot is composed of a polycrystalline particle 1 mainly composed of a semiconductor material, and having a plurality of domains 1a, 1b, 1c. The quantum dot exhibits an electron confinement effect even if it is a polycrystalline particle. A solar cell has a quantum dot layer including the quantum dots, mainly composed of the polycrystalline particles 1 having a plurality of domains 1a, 1b, 1c, on the principal surface of a semiconductor substrate 11. Consequently, the transportation efficiency of carrier can be enhanced, and a solar cell having a high photoelectric conversion efficiency can be obtained.

Description

  The present invention relates to a quantum dot and a solar cell using the quantum dot.

  Solar cells have the advantage that they do not emit carbon dioxide and do not require fuel during power generation. For this reason, research on various types of solar cells has been actively promoted. Currently, single-junction solar cells having a pair of pn junctions using single-crystal silicon or polycrystalline silicon are the mainstream among solar cells in practical use.

  However, in a conventional semiconductor structure, light with a short wavelength having high energy does not excite electrons only in the pn junction region, but also excites electrons in each p-type or n-type semiconductor region. Since the carriers generated in each of these semiconductor regions are dissipated as thermal energy due to the interaction with minority carriers due to impurity levels, thermal energy, etc. existing in the p-type or n-type semiconductor region, the theoretical limit efficiency Was less than 30%. For this reason, new methods for further improving the theoretical limit efficiency are being studied.

  One of the new methods studied so far is a solar cell using semiconductor quantum dots (hereinafter referred to as “quantum dot solar cell”).

  As a technique related to a quantum dot solar cell, for example, Patent Document 1 includes a quantum dot having a three-dimensional quantum confinement action on a main surface of a silicon substrate, and a quantum dot including a quantum dot and a barrier layer containing the quantum dot A solar cell having a layer is disclosed.

  FIG. 3 is a schematic cross-sectional view showing a conventional quantum dot solar cell represented by the solar cell disclosed in Patent Document 1. As shown in FIG. In FIG. 3, the number of quantum dot layers 105 is simplified and only one is shown, but the quantum dot layer 105 has a structure in which at least several tens of layers are stacked. Here, the quantum dot layer 105 includes semiconductor particles that are the quantum dots 105a and a matrix 105b that is a high resistance layer formed around the semiconductor particles.

  The quantum dot 105a formed in the quantum dot solar cell is a semiconductor nanocrystal having a size of about 10 nm. When the quantum dot 105a is irradiated with light 106, electrons in the quantum dot 105a are excited to a quantum level having an energy gap higher than the band gap inherent in the semiconductor due to the confinement effect of the quantum dot 105a. As a result, the solar spectrum in a short wavelength region that could not be absorbed by the conventional solar cell is efficiently absorbed in the quantum dot layer 105 formed at the boundary between the p-type semiconductor and the n-type semiconductor. It is possible to increase the photoelectric conversion efficiency.

  The size of a quantum dot having a quantum level with an energy gap higher than the band gap inherent in the single junction semiconductor constituting the solar cell is assumed to be 4 nm or less in the case of a spherical body.

JP 2006-114815 A

  However, in practice, there is a problem that it is extremely difficult to obtain quantum dots that exhibit photoelectric conversion characteristics with a particulate material having a diameter of 4 nm or less.

  This invention is made | formed in view of the said subject, and it aims at providing the quantum dot from which a photoelectric conversion characteristic is acquired even if it is large, and a solar cell to which it is applied.

  The quantum dot of the present invention is characterized by being composed of polycrystalline particles having a semiconductor material as a main component and having a plurality of domains.

  The solar cell of the present invention has a quantum dot layer containing the above quantum dots on the main surface of a semiconductor substrate.

  According to the present invention, it is possible to obtain a quantum dot with high photoelectric conversion efficiency and a solar cell to which the quantum dot is applied.

It is a cross-sectional schematic diagram which shows one Embodiment of the quantum dot of this invention. It is a cross-sectional schematic diagram which shows one Embodiment of the solar cell of this invention. It is a cross-sectional schematic diagram which shows the conventional quantum dot type solar cell.

  FIG. 1 is a schematic cross-sectional view showing an embodiment of the quantum dot of the present invention. Quantum dots formed on quantum dot solar cells have a size that can efficiently exhibit photoelectric conversion at a wavelength different from the photoelectric conversion wavelength region of a single junction solar cell having a pn junction, due to an electron confinement effect. In the case of a spherical body, it has been said that the diameter should be 4 nm or less.

  However, in practice, there is a problem that it is extremely difficult to obtain quantum dots that exhibit photoelectric conversion characteristics with a particulate material having a diameter of 4 nm or less.

  Therefore, the present applicant tried to produce semiconductor particles with higher crystallinity for quantum dots, but conventionally, quantum dots with high crystallinity were considered good, It has been found that even the polycrystalline particles 1 in which the main body of the quantum dots has a plurality of domains 1a, 1b, 1c exhibit an electron confinement effect.

  This is because the quantum dots are polycrystalline particles 1 as a whole, but the domains 1a, 1b, and 1c constituting the quantum dots are single crystals, and the domains 1a, 1b, and 1c are grain boundaries 3 respectively. This is because each of them exhibits a characteristic as a quantum dot.

  Here, the polycrystalline particles 1 having a plurality of domains 1a, 1b, and 1c are delimited by grain boundaries 3 formed by dislocations, defects, and the like when the quantum dots are observed with a transmission electron microscope, for example. In other words, a state where two or more single crystals exist is observed. In this case, each of the domains 1a, 1b, and 1c is a single crystal.

  In other words, the quantum dot of this embodiment has a fine single crystal present through the grain boundary 3 in a particle that appears to be one appearance, and this fine single crystal individually has the properties of a quantum dot. It will be shown. In this case, the size (maximum diameter) of the single crystal is preferably 1 to 3 nm.

  Such quantum dots mainly composed of the polycrystalline particles 1 may be combined with quantum dots made of a single crystal having the same size as the polycrystalline particles 1 to form a quantum dot layer.

  The polycrystalline particles 1 in this embodiment preferably have an L / S ratio of 1 to 1.2 when the maximum diameter is L and the minimum diameter is S. The closer the L / S ratio is to 1, the more electrons The three-dimensional confinement effect is excellent. Here, the L / S ratio indicates sphericity.

  For example, when the semiconductor material is silicon, such polycrystalline particles 1 have a higher energy gap (1.1 eV) than the size of polycrystalline silicon (usually 100 nm or more).

  In this case, the size (diameter) of the polycrystalline particles 1 that can make the energy gap higher than 1.1 eV, particularly 1.3 to 2.0 eV, is desirably 5 nm or more and 10 nm or less.

  In addition, when the quantum dot is a polycrystalline particle 1 having a plurality of domains 1a, 1b, and 1c, the change in the energy gap with respect to the variation in size is small as compared with a quantum dot that is a single crystal as a whole. From this, the particle size may have some variation. In this case, when the average particle size is x and the standard deviation is σ, the particle size variation represented by σ / x is up to 30% or more. Is acceptable. In addition, 50% is desirable as the upper limit of the variation in particle size represented by σ / x.

  Thus, since the quantum dot of this embodiment has a quantum effect even if the particle size and its variation are large, the outer shape is not limited to a spherical shape, and is any one of a cylindrical shape and a polygonal shape. There may be a mixture of spherical, cylindrical, and polygonal shapes, which is suitable for mass production.

  FIG. 2 is a schematic cross-sectional view showing one embodiment of the solar cell of the present invention. The solar cell of this embodiment can be applied to a solar cell as shown in FIG. 2 having a quantum dot layer containing the above quantum dots on the main surface of a semiconductor substrate.

  The solar cell of the present embodiment has a quantum dot layer 15 on the main surface 13 of the semiconductor substrate 11. The quantum dot layer 15 includes a quantum dot 15a and a matrix 15b that includes the quantum dot 15a and serves as a base. In this case, the quantum dots 15a included in the matrix 15b are mainly composed of the polycrystalline particles 1 having a plurality of domains 1a, 1b, and 1c as a main component. Although FIG. 1 only shows a simple structure in which quantum dots 15a are arranged in a horizontal row, quantum dots 15a are multilayered in the quantum dot layer 15 constituting the solar cell of the present embodiment. The quantum dot layer 15 is also multilayered.

  Thereby, when the quantum dot layer 15 is irradiated with the light 16, in addition to the function of converting visible light having a wavelength of 400 to 1100 nm, the long wavelength (1200 to 1700 nm that cannot be normally absorbed), for example. Of light 16) can be absorbed and used effectively for power generation.

Quantum dots 15a (polycrystalline particles 1 having a semiconductor material as a main component and having a plurality of domains 1a, 1b, 1c) constituting such a quantum dot layer 15 have an energy gap (Eg) depending on the component structure. Different ones with 0.15 to 2.50 ev are preferred. Specific materials for the quantum dots 15a include germanium (Ge), silicon (Si), gallium (Ga), indium (In), arsenic (As), and antimony (Sb).
It is desirable to use any one selected from copper (Cu), iron (Fe), sulfur (S), lead (Pb), tellurium (Te) and selenium (Se), or a compound semiconductor thereof.

  The material of the matrix 15b is preferably a material having an energy gap of about 2 to 15 times that of semiconductor particles, and preferably has an energy gap (Eg) of 1.0 to 10.0 ev. . As a material of the matrix 15b, a compound (semiconductor, carbide, oxide, nitride) containing at least one element selected from Si, C, Ti, Cu, Ga, S, In, and Se is preferable.

  When the matrix 15b is arranged around the quantum dot 15a, the confinement effect of the electrons generated in the domains 1a, 1b, and 1c in the quantum dot 15a in the quantum dot 15a can be enhanced. In this case, it is desirable that the matrix 15b surrounds the entire surface of the quantum dot 15a in that the effect of confining electrons in the quantum dot 15a can be further enhanced. That is, it is preferable to have a core-shell structure in which the quantum dot 15a is a core part and the matrix 15b is a shell part.

  In the solar cell of the present embodiment, the quantum dot layer 15 is provided on the main surface 13 of the semiconductor substrate 11 as described above, but the upper surface of the quantum dot layer 15 is shown in FIG. A semiconductor substrate 17 is also provided on the side. In this case, for example, when the semiconductor substrate 11 disposed on the lower surface side of the quantum dot layer 15 is a p-type (carrier is a hole) semiconductor, the semiconductor substrate 17 disposed on the upper surface side of the quantum dot layer 15. Becomes n-type. Note that the p-type and n-type may be reversed. The semiconductor substrates 11 and 17 may be either polycrystalline or single crystal, but are preferably polycrystalline in terms of high productivity and low cost.

  Next, a method for manufacturing the quantum dots and solar cells of the present embodiment will be described.

  The quantum dot 5a in the present embodiment uses a method in which a metal component is precipitated by biomineralization from a solution of a metal compound containing the semiconductor material described above.

  First, a metal compound containing the above-described semiconductor particles as a main component, a solvent, and ferritin are prepared and mixed while heating to synthesize semiconductor particles.

  As an example of a compound containing Si, for example, one kind selected from sodium silicate, hexafluorosilicate, organosilane, and the like is used as the metal compound.

  On the other hand, an apoferritin (horse spleen-derived) solution is prepared as ferritin, and the above-mentioned compound containing Si is added thereto. Here, the pH is preferably about 7 to 10.

  Next, a compound containing Si is dispersed in an apoferritin (horse spleen-derived) solution, and Si is adhered to the inner wall of ferritin as a metal. Since ferritin is a protein, it is possible to control the bio-size, and because the size is limited by the volume of the internal space of ferritin, it is possible to synthesize particles that are close to a spherical shape, and the variation in particle size Can also get a small one.

  In this case, the semiconductor material crystallizes inside ferritin. For example, by increasing the concentration of a compound containing Si and increasing the growth rate of the crystal, a crystal nucleus is partially formed inside ferritin. In the crystal growth process, a plurality of domains 1a, 1b, 1c are formed.

Next, polycrystalline particles 1 containing the synthesized semiconductor material as a main component (hereinafter referred to as polycrystalline particles 1)
The polycrystalline particles 1 are taken out from the ferritin having the following. In this case, for example, an alkaline aqueous solution is added to the ferritin solution so that the pH of the solution is 10 or more and ferritin is dissolved.

  Next, the obtained polycrystalline particles 1 (quantum dots) are dispersed in a solvent to prepare a slurry, and this slurry is applied to the surface of the semiconductor substrate 11 and dried. In this case, a solvent is selected in consideration of viscosity and evaporability so that the polycrystalline particles 1 are deposited in alignment on the surface of the semiconductor substrate 11. Specifically, phthalate ester, glycerin and the like are suitable as the solvent.

  The polycrystalline particles 1 (quantum dots) can also be deposited on the surface of the semiconductor substrate 11 by a method of immersing and pulling up the semiconductor substrate 11 in the slurry containing the polycrystalline particles 1.

  Next, the semiconductor substrate 11 on which the polycrystalline particles 1 are deposited is heated to a temperature of 300 to 1000 ° C. in an inert gas such as argon or nitrogen or in a reducing gas containing hydrogen. Is sintered. In this case, the oxide film formed on the surface of the polycrystalline particle 1 becomes the matrix 15b.

  In the solar cell obtained as described above, the quantum dots 15a constituting the quantum dot layer 15 are composed of polycrystalline particles having a semiconductor material as a main component and having a plurality of domains, thereby forming a continuous band structure. And the amount of light 16 absorbed by the quantum dots 15a can be increased, so that the photoelectric conversion efficiency can be improved.

1 ·················· Polycrystalline grains 1a, 1b, 1c ··· Domain 3 ········ Grain boundary Substrate 13... Main surface 15, 105... Quantum dot layers 15a, 105a... Quantum dots 15b, 105b.

Claims (4)

  A quantum dot comprising a semiconductor material as a main component and polycrystalline particles having a plurality of domains.   The quantum dot according to claim 1, wherein a maximum diameter of the polycrystalline particles is 5 to 10 nm.   3. The quantum dot according to claim 1, wherein the polycrystalline particles have an L / S ratio of 1 to 1.2 when the maximum diameter is L and the minimum diameter is S. 4. A solar cell comprising a quantum dot layer containing the quantum dots according to any one of claims 1 to 3 on a main surface of a semiconductor substrate.