JP2014179368A - Solar cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a solar cell to which a quantum dot is applied, capable of obtaining photoelectric conversion characteristics even if the size is large.SOLUTION: A solar cell has a quantum dot layer 5 having a plurality of quantum dots 5a on the main surface 3 of a semiconductor substrate 1. The quantum dots 5a are a columnar body whose aspect ratio is 2 or more and arranged in parallel in the same direction. As a result, since an intermediate band having wide width is formed in the quantum dot 5a and a wave function having large amplitude can be formed, the amount of electronic tunnel current between the quantum dots 5a can be increased. As a result, the photoelectric conversion efficiency becomes possible to be enhanced.

Description

  The present invention relates to a solar cell using quantum dots.

  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. 5 is a schematic cross-sectional view showing a conventional quantum dot solar cell typified by the solar cell disclosed in Patent Document 1. As shown in FIG. In FIG. 5, 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 is composed of semiconductor particles that are the quantum dots 105a and a matrix 105b that is a high resistance layer formed around the semiconductor particles.

  In the quantum dot type solar cell, when the light 106 is irradiated to the quantum dot 105a, the electrons in the quantum dot 105a reach the quantum level of the energy gap higher than the band gap inherent in the semiconductor due to the confinement effect of the quantum dot 105a. Excited. 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 considered that this can increase the photoelectric conversion efficiency.

  In this case, the semiconductor particles to be the quantum dots 105a are spherical particles having a shape that can realize a so-called “0-dimensional” world in which the confined electrons cannot move in any of the vertical / horizontal / height directions. Is ideal, and the size is considered to be about several nanometers.

  However, in practice, it has been difficult to achieve a quantum dot layer in which quantum dots composed of spherical particles having a maximum diameter of only a few nm are integrated.

JP 2006-114815 A

  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 solar cell of the present invention is a solar cell comprising a quantum dot layer having a plurality of quantum dots on a main surface of a semiconductor substrate, wherein the quantum dots are columnar bodies having an aspect ratio of 2 or more, and the longitudinal direction Are arranged in parallel in the same direction.

  According to the present invention, it is possible to obtain quantum dots with high photoelectric conversion efficiency and a solar cell to which the quantum dots are applied even if the size is large.

It is a perspective view which shows typically one Embodiment of the solar cell of this invention. (A) is a schematic diagram which shows the band structure of the quantum dot which comprises the solar cell of this embodiment, (b) is a schematic diagram which shows the band structure in case a quantum dot is a spherical particle (0 dimension). . It is a perspective view which shows typically the 2nd aspect of the solar cell of this invention. It is sectional drawing which shows typically the 3rd aspect 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 perspective view schematically showing one embodiment of a solar cell of the present invention. The solar cell of this embodiment has a configuration in which a quantum dot layer 5 including a plurality of quantum dots 5a is disposed on the main surface 3 on the light incident side of the semiconductor substrate 1, and the quantum dots 5a have an aspect ratio. Two or more columnar bodies are arranged in parallel in the same direction in the longitudinal direction.

  Although FIG. 1 only shows a state in which one quantum dot 5 a is arranged in the quantum dot layer 5 on the main surface 3 of the semiconductor substrate 1, the quantum dot 5 a is actually the thickness of the quantum dot layer 5. Several tens of layers are stacked in the direction.

  The quantum dot formed in the quantum dot solar cell has a shape that can efficiently exhibit photoelectric conversion at a wavelength different from the photoelectric conversion wavelength region of the single junction solar cell having a pn junction so far due to the electron confinement effect. It is preferable that the particles be in a so-called zero-dimensional state in which electrons cannot move in any of the vertical / horizontal / height directions and are completely confined, and the size should be 6 nm or less. I came.

  However, in practice, it is extremely difficult to form the quantum dot layer 5 by accumulating particulate quantum dots 5a having a diameter of 6 nm or less.

  Therefore, as a result of various investigations on the quantum dots 5a, the present applicant has produced quantum dots 5a having different shapes and examined the relationship between the size and the energy gap. Conventionally, the quantum dots 5a are preferably spherical particles. It has been found that, even if the quantum dots 5a are elongated and arranged in parallel in the same direction, the electron confinement effect is exhibited even if they are not spherical particles.

FIG. 2A is a schematic diagram showing a band structure of quantum dots constituting the solar cell of the present embodiment, and FIG. 2B is a schematic diagram showing a band structure when the quantum dots are spherical particles (0-dimensional). It is. In FIG. 2 (a), the quantum dots 5a are shown in a round shape because they show a cross-sectional view of the solar cell. Actually, the quantum dots 5a are located at the back of the paper surface of FIG. 2 (a). It has a long and narrow shape.

  In the solar cell of the present embodiment, the electrons move in the longitudinal direction inside the quantum dots 5a. However, when the electrons are easy to move in only one direction, the quantum dots 5a are shown in FIG. Unlike the case of the spherical particles as shown in FIG. 9), the intermediate band formed in the quantum dot 5a is wider than that of the spherical particles, and a wave function having a large amplitude is formed. As a result, the overlapping width of wave functions generated between adjacent quantum dots 5a can be increased, so that the amount of electron tunneling current between the quantum dots 5a can be increased, and as a result, the photoelectric conversion efficiency can be increased. Become.

  In this case, the quantum dot 5a is preferably a columnar body having an aspect ratio of 2 or more because the overlapping width of the electron wave function is provided. Here, the aspect ratio is a ratio L / S where L is the length in the longitudinal direction of the quantum dots 5a and S is the maximum diameter (diameter) of the cross section.

  The length of the quantum dots 5a in the longitudinal direction is preferably 6 nm or more and 7 nm or more, particularly 10 nm or more, and the maximum diameter (diameter) of the cross section (end face) is 1 nm or more and 5 nm or less. Is preferred.

  As shown in FIG. 1, the quantum dots 5a are usually surrounded by a material (matrix 5b) having a larger energy gap than the quantum dots 5a.

  Thus, when the quantum dot layer 5 is irradiated with the light 6, in addition to the function of converting visible light having a wavelength of 400 to 1100 nm, the long wavelength (1200 to 1700 nm of which cannot be normally absorbed) (Wavelength) light 6 can be absorbed and used effectively for power generation.

  Note that the arrangement in the same direction means a state in which the orientations of the quantum dots 5a in the longitudinal direction are substantially parallel as shown in FIG.

  Next, in the solar cell of the present embodiment, when the quantum dots 5a are arranged so that the longitudinal direction of the quantum dots 5a is parallel to the main surface 3 of the semiconductor substrate 1, the wave function of the electrons overlapped with a wide width is the light. The direction is directed from the incident side toward the semiconductor substrate 1, whereby electrons generated by the quantum dot layer 5 are easily moved toward the semiconductor substrate 1, and a high photoelectric conversion efficiency is obtained.

  FIG. 3 is a perspective view schematically showing a second aspect of the solar cell of the present invention. In the solar cell shown in FIG. 3, the quantum dots 5 a are arranged so that the longitudinal direction is perpendicular to the main surface 3 of the semiconductor substrate 1.

  When the quantum dots 5a are arranged so that the longitudinal direction of the quantum dots 5a stands perpendicular to the main surface 3 of the semiconductor substrate 1 as in the solar cell of FIG. 3, the direction of the electrons generated in the quantum dots 5a is originally Since it is in the direction of the semiconductor substrate 1, the electron mobility from the quantum dot layer 5 to the semiconductor substrate 1 is high, and it is possible to generate power more efficiently.

FIG. 4 is a cross-sectional view schematically showing a third aspect of the solar cell of the present invention. In FIG. 4, in order to show the difference in the size of the quantum dot 5a in each layer when the two quantum dot layers 5 are laminated, for convenience, it is shown in a sectional view and shows the difference in the diameter of the quantum dot 5a existing in each layer. However, also in this case, the quantum dots 5a in each quantum dot layer 5 are columnar bodies. In addition, the length of the longitudinal direction of a columnar body may differ in each layer.

  In the solar cell of the present embodiment, when a plurality of quantum dot layers 5 are stacked on the main surface 3 of the semiconductor substrate 1, the size of the quantum dots 5a is preferably different for each layer.

  If the quantum dots 5a have different sizes (for example, volume and maximum length), the wavelength of light that can be absorbed varies. When a plurality of quantum dot layers 5 are formed on the main surface 3 of the semiconductor substrate 1, if the size of the quantum dots 5 a is different for each layer, light of different wavelengths can be absorbed in each quantum dot layer 5. Photoelectric conversion can be performed in a wider range of wavelengths, thereby further increasing power generation efficiency.

  In this case, in each quantum dot layer 5, it is desirable that the sizes of the existing quantum dots 5a are substantially the same. For example, the average value of the maximum length in the longitudinal direction is x, and the standard deviation thereof is σ. Sometimes σ / x is desirably 20% or less.

  In addition, the energy gap of the semiconductor particles constituting the quantum dots 5a is preferably at a level higher than 1.1 eV, for example, 1.3 to 2.0 eV.

  The quantum dots 5a constituting the quantum dot layer 5 are mainly composed of semiconductor particles, and the energy gap (Eg) varies depending on the materials used, but those having 0.15 to 2.50 ev are preferable. It is. Specific materials for the quantum dots 5a include germanium (Ge), silicon (Si), gallium (Ga), indium (In), arsenic (As), antimony (Sb), copper (Cu), iron (Fe). It is desirable to use any one selected from sulfur (S), lead (Pb), tellurium (Te) and selenium (Se) or a compound semiconductor thereof.

  The quantum dots 5a are usually surrounded by a material having a larger energy gap than the quantum dots 5a, and the material surrounding the quantum dots 5a may be referred to as a matrix 5b. The material of the matrix 5b is preferably a material having an energy gap of about 2 to 15 times that of the semiconductor particles, and preferably has an energy gap (Eg) of 1.0 to 10.0 ev. . The matrix material is preferably a compound (semiconductor, carbide, oxide, nitride) containing at least one element selected from Si, C, Ti, Cu, Ga, S, In and Se.

  As described above, the solar cell of the present embodiment has the quantum dot layer 5 provided on the main surface 3 of the semiconductor substrate 1, but the semiconductor substrate is also provided on the upper surface side of the quantum dot layer 5. In this case, for example, when the semiconductor substrate 1 disposed on the lower surface side of the quantum dot layer 5 is a p-type (carrier is a hole) semiconductor, the semiconductor substrate disposed on the upper surface side of the quantum dot layer 5 is n-type. Note that the p-type and n-type may be reversed. The semiconductor substrate 1 may be either polycrystalline or single crystal, but is 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 dots 5a in the present embodiment are prepared using a method of depositing a metal component from a metal solution containing the semiconductor material described above.

First, for example, silicon powder is prepared as the metal containing the above-described semiconductor particles as a main component. On the other hand, hydrofluoric acid, acetic acid, nitric acid, and ethanol are prepared as solvents, and after mixing these solvents at a predetermined ratio, silicon powder is added and dissolved to prepare a solution in which silicon is dissolved.

  Next, an ultrasonic wave is applied to the solution in which the silicon is dissolved. The time for applying the ultrasonic waves is preferably about 50 to 100 minutes. By applying ultrasonic energy to a solution in which silicon is dissolved, nano-sized columnar silicon particles are formed in the solution.

  Next, the solution to which ultrasonic waves are applied is filtered to extract silicon particles, and then the silicon particles are washed with pure water. The cleaned silicon particles are dispersed in ethanol when stored.

  In this case, when the time for applying the ultrasonic wave is shorter or longer than the above time, or when water is added to the solution in which silicon is dissolved, the formed silicon particles tend to be spherical, and are columnar. It becomes difficult to form body silicon particles.

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

  Note that the semiconductor particles (quantum dots) can also be deposited in the same direction on the surface of the semiconductor substrate 1 by a method of immersing and pulling up the semiconductor substrate 1 in a slurry containing silicon particles.

  Next, the semiconductor substrate 1 on which the semiconductor particles 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 to sinter the semiconductor particles. . In this case, the oxide film formed on the surface of the semiconductor particles serves as a matrix.

  In the solar cell obtained as described above, the quantum dots 5a constituting the quantum dot layer 5 are columnar bodies having an aspect ratio of 2 or more, and are arranged in parallel in the same direction. Since an intermediate band is formed and a wave function having a large amplitude can be formed, the amount of electron tunneling current between the quantum dots 5a can be increased, and as a result, the photoelectric conversion efficiency can be increased.

DESCRIPTION OF SYMBOLS 1, 7 ..... Semiconductor substrate 3 ..... Main surface 5, 105 ..... Quantum dot layer 5a, 105a ..... Quantum dot 5b, 105b .... Matrix 6, 106 ... ・ ・ ・ ・ ・ Light

Claims (4)

  A solar cell including a quantum dot layer having a plurality of quantum dots on a main surface of a semiconductor substrate, wherein the quantum dots are columnar bodies having an aspect ratio of 2 or more and are arranged in parallel in the same longitudinal direction A solar cell characterized by being made.   The solar cell according to claim 1, wherein the quantum dots are arranged so that a longitudinal direction thereof is parallel to the main surface of the semiconductor substrate.   The solar cell according to claim 1, wherein the quantum dots are arranged so that a longitudinal direction thereof is perpendicular to the main surface of the semiconductor substrate.   4. The sun according to claim 1, wherein a plurality of the quantum dot layers are stacked on the main surface of the semiconductor substrate, and the size of the quantum dots is different for each layer. 5. battery.