JP2014165198A - Solar battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a solar battery capable of enhancing a photoelectric conversion efficiency, in a configuration having a quantum dot layer on a semiconductor substrate.SOLUTION: On a principal surface 3 of a semiconductor substrate 1, provided is a quantum dot film 5 that has a quantum dot 5a and a barrier layer 5b surrounding the quantum dot 5a and that is configured by spherical quantum dot complexes 5CS. The quantum dot film 5 is configured by coupling the quantum dot complexes 5CS via neck parts N. Thereby, a band structure continuous three-dimensionally is formed in the quantum dot film 5, and a current is likely to be supplied to the semiconductor substrate 1 side from the quantum dot film 5. As a result, the photoelectric conversion efficiency of the solar battery can be improved.

Description

  The present invention relates to a solar cell in which a quantum dot film in which fine semiconductor particles are closely arranged is applied as a quantum dot layer.

  Solar cells have the advantage that they do not emit carbon dioxide and do not require fuel for 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, since the theoretical limit of photoelectric conversion efficiency of single-junction solar cells (hereinafter referred to as “theoretical limit efficiency”) is only about 30%, a new method for further improving the theoretical limit efficiency is 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 layer and a barrier layer (barrier layer) containing the quantum dot. A solar cell characterized in that the energy band structure of a quantum dot layer comprising a film forms type II (a superlattice structure system in which the lower end of the conduction band of one semiconductor and the valence band of the other semiconductor overlap) is disclosed. ing.

  Quantum dots formed in a quantum dot solar cell are semiconductor nanocrystals having a size of about 10 nm. Electrons and holes generated in the semiconductor quantum dots by irradiating the semiconductor quantum dots with light (hereinafter referred to as the quantum dots) , Sometimes collectively referred to as “carrier”)). For example, by confining electrons in a semiconductor quantum dot, it becomes possible to use the properties of electrons as a quantum mechanical wave, and also absorb the solar spectrum in a band that could not be absorbed by conventional solar cells. It becomes possible. Furthermore, according to the quantum dot solar cell, it is possible to reduce the energy lost as heat, and thus it is considered that the theoretical limit efficiency can be improved to 60% or more.

  FIG. 4 is a schematic longitudinal sectional view showing a conventional quantum dot solar cell represented by the solar cell disclosed in Patent Document 1. In FIG. 4, the number of layers of the quantum dot film 105 is simplified on the surface of the semiconductor substrate 101 and only two layers are shown, but the quantum dot film 105 has a structure in which at least several tens of layers are stacked. In this case, the semiconductor particles which are the quantum dots 105a have a structure formed in the barrier layer 105b which is a high resistance layer formed around the semiconductor particles.

  In this case, when the semiconductor quantum dot 105a is applied as an element for photoelectric conversion of a solar cell, it is indispensable to form a band (intermediate band) for electronically coupling carriers between the quantum dots 105a. For this reason, in order to increase the state density of the wave function between the adjacent quantum dots 105a, it is recommended that the quantum dots 105a be densely arranged.

However, in the quantum dot film 105 having the structure shown in FIG. 4, the thickness of the matrix 105b formed around the quantum dot 105a (corresponding to the interval between the closest quantum dots 105a) is the direction around the quantum dot 105a. Therefore, there is a case where the carrier moving speed (escape speed from the quantum dots 105a) is slow, and it is difficult to take out the current as a current, and the photoelectric conversion efficiency cannot be increased.

JP 2006-114815 A

  This invention is made | formed in view of the said subject, and it aims at providing the solar cell which can improve a photoelectric conversion efficiency in the structure which has a quantum dot film | membrane on a semiconductor substrate.

  A quantum dot film having a quantum dot and a barrier layer surrounding the quantum dot on a main surface of the semiconductor substrate and including a spherical quantum dot composite is provided. The quantum dot composite is connected through a neck portion.

  According to the present invention, a solar cell with high photoelectric conversion efficiency can be obtained.

(A) is a longitudinal cross-sectional schematic diagram which shows one Embodiment of the solar cell of this invention, (b) is the sectional view on the AA line of (a). It is a schematic diagram which shows the method of calculating | requiring the ratio of the number of the triple point grain boundary with respect to the number sum of the interfacial grain boundary and the triple point grain boundary in the cross section of a quantum dot film, (a) is the number ratio of a triple point grain boundary. When the number is large, (b) shows the case where the number ratio of triple point grain boundaries is small. The vertical cross-sectional schematic diagram which shows one Embodiment of the other solar cell of this invention, and shows the structure where the quantum dot film | membrane containing the quantum dot composite from which an average diameter differs on the main surface of a semiconductor substrate was laminated | stacked in multiple layers. It is. It is a longitudinal cross-sectional schematic diagram which shows the conventional quantum dot type solar cell.

  FIG. 1A is a schematic longitudinal sectional view showing an embodiment of the solar cell of the present invention, and FIG. 1B is a sectional view taken along line AA in FIG. The solar cell of the present embodiment has a configuration in which a quantum dot film 5 is stacked on the main surface 3 of the semiconductor substrate 1. In FIG. 1A, the quantum dot film 5 is simplified and only one layer is shown, but in reality, the quantum dot film 5 extends to several tens of layers.

  The quantum dot film 5 includes a quantum dot 5a and a barrier layer 5b surrounding the quantum dot 5a, and is configured by a quantum dot complex 5CS having a spherical outer shape. In addition, the quantum dot film 5 is formed by connecting the quantum dot composites 5CS through the neck portion N.

The quantum dot composite 5CS has a so-called core-shell structure. In this case, the quantum dot 5a is the core portion C, and the shell portion S surrounding the core portion is the barrier layer 5b. Therefore, in the quantum dot film 5 of the present embodiment, the spherical quantum dot composite 5CS is in contact with each other via a grain boundary 5B that is thinner than the thickness of the barrier layer 5b constituting the quantum dot composite 5CS. In the quantum dot film 5, the quantum dots 5a in the quantum dot composite 5CS that are in contact with each other are coupled to each other via a neck portion N between two adjacent barrier layers 5b. Here, the neck portion N is formed, for example, by diffusion of the material of the barrier layer 5b, and constitutes a structure that is sintered to such an extent that grain boundaries are recognized, such as a fired ceramic. In this case, when the cross section of the quantum dot film 5 is observed using an electron microscope or the like, a contour representing the outer edge of the quantum dot composite 5CS is partially recognized.

  For example, if two quantum dot composites 5CS are adjacent to each other, the barrier layer 5b of the quantum dot composite 5CS exists between the two quantum dots 5a with a thickness of only two layers, Two adjacent quantum dots 5a are arranged at an interval of about twice the thickness of the barrier layer 5b. For this reason, the thickness of the barrier layer 5b formed around the quantum dots 5a (corresponding to the interval between the closest quantum dots 5a) has substantially the same interval in all directions around the quantum dots 5a. ing.

  As a result, the moving speed of carriers between the quantum dots 5a in the quantum dot film 5 (escape speed from the quantum dots 5a) is the same in any direction, and it is easy to form a three-dimensional continuous band structure. Current is easily supplied from the dot film 5 to the semiconductor substrate 1 side, and as a result, the photoelectric conversion efficiency of the solar cell can be improved.

  In this case, the quantum dots 5a are also preferably spherical, and the barrier layer 5b surrounding the quantum dots 5a preferably covers the entire surface of the quantum dots 5a. The quantum dot composite 5CS preferably has a ratio (L / S) of 1 to 1.2 or less, where L is the maximum diameter measured in the cross section and S is the shortest diameter. It is preferable that the dots 5a have the same L / S ratio.

  Further, in the quantum dot film 5 of this embodiment, as shown in FIGS. 1A and 1B, a triple point grain boundary 5BC is formed between the quantum dot composites 5CS, and the triple point grain boundary 5BC It is desirable to have pores 5D. In the quantum dot film 5, when the pores 5D are present in the triple point grain boundary 5BC, the light 6 is scattered by the pores 5D, so that the amount of the light 6 that does not contribute to power generation through the quantum dot film 5 is reduced. As a result, the absorption efficiency into each quantum dot 5a can be increased, and the photoelectric conversion efficiency can be further improved. The pores 5D of the triple point grain boundary 5BC can be formed, for example, by changing the area of the triple point grain boundary 5BC by changing the sinterability of the quantum dot composite 5CS.

  In addition, the quantum dot film 5 in this embodiment will be described. When the quantum dot composite 5CS is close to a cubic close-packed structure or a hexagonal close-packed structure, as described later, 2 The number ratio of the triple point grain boundary 5BC to the number sum of the interplane grain boundary 5BW and the triple point grain boundary 5BC is 20% or more.

  Here, when three or more quantum dot composites 5CS are gathered, the state where the triple point grain boundary 5BC is formed instead of the inter-surface grain boundary 5BW is that the cross section of the quantum dot film 5 is planar from the light incident side. When viewed, the three quantum dot composites 5CS are in contact with each other at an interval narrower than the thickness of the barrier layer 5b constituting the quantum dot composite 5CS, and the grain boundary between the two quantum dot composites 5BC The width of 5BW is equal to or less than ¼ of the diameter of the quantum dot composite 5CS. Specifically, the width of the interfacial grain boundary 5BW is 5 nm or less. Even when three quantum dot composites 5CS are adjacent to each other, when one of the two interfacial grain boundaries 5BW has a part wider than 1/4 of the diameter of the quantum dot composite 5CS The triple point grain boundary 5BC is not formed.

On the other hand, in a state where the neck portion N is not recognized between the quantum dot composites 5CS, or when the quantum dots 5a are arranged at a predetermined interval in the material of the barrier layer 5b. In this state, since the neck portion N is not formed between the quantum dot composites 5CS, the carrier moving speed (the escape speed from the quantum dots 5a) between the quantum dots 5a in the quantum dot film 5 depends on the direction. Since it is greatly different, it is difficult to form a three-dimensional continuous band structure, so that it is difficult for current to flow from the quantum dot film 5 to the semiconductor substrate 1, and as a result, the photoelectric conversion efficiency of the solar cell is lowered. Resulting in.

  In a state where the neck portion N is not recognized between the quantum dot composites 5CS, or in a state where the quantum dots 5a are arranged at a predetermined interval in the material of the barrier layer 5b, The number ratio of the triple point grain boundary 5BC to the number sum of the interfacial grain boundary 5BW and the triple point grain boundary 5BC in the cross section is lower than 20%, and the structure in such a state is closely packed with the quantum dots 5a. Since it is no longer in a state, the quantum dots 5a are often arranged at intervals larger than twice the thickness of the barrier layer 5b. Further, since the thickness of the barrier layer 5b formed around the quantum dots 5a (corresponding to the interval between the closest quantum dots 5a) varies greatly depending on the direction of the quantum dots 5a, as described above, It becomes difficult to form a three-dimensionally continuous band structure in the quantum dot film 5, and the photoelectric conversion efficiency of the solar cell is lowered.

  In this embodiment, the ratio of the number of triple point boundaries 5BC with respect to the sum of the numbers of interfacial boundary 5BW and triple point boundary 5BC in the cross section is determined as follows.

  FIG. 2 is a schematic diagram showing a method for obtaining the ratio of the number of triple point grain boundaries to the sum of the numbers of interfacial grain boundaries and triple point grain boundaries in the cross section of the quantum dot film, and (a) is a triple point grain boundary. (B) shows the case where the number ratio of triple point grain boundaries is small.

  The inter-surface grain boundary 5BW and the triple point grain boundary 5BC between the plurality of quantum dot composites 5CS constituting the quantum dot film 5 of this embodiment are, for example, locations as shown in FIG. . In FIG. 2 (a), a place with a number on a white background is a place showing the triple point grain boundary 5BC, and a place with a number on a black background is the interfacial grain boundary 5BW.

  In the example of FIG. 2 (a), there are 6 triple-point grain boundaries 5BC and 13 inter-surface grain boundaries 5BW, so the number of inter-surface grain boundaries 5BW in the cross section (13 places) and 3 The ratio of the number of triple-point grain boundaries 5BC (6 locations) to the sum (19 locations) of the number of important grain boundaries 5BC (6 locations) is 31.6%.

  On the other hand, as shown in FIG. 2B, when the quantum dot composite 5CS does not exist in a dense state in the quantum dot film 5, the quantum dot composite 5CS in the quantum dot film 5 The number of triple-point grain boundaries 5BC formed is greatly reduced. In FIG. 2B, the number of triple-point grain boundaries 5BC with respect to the sum (14 places) of the number (13 places) of interfacial grain boundaries 5BW and the number of triple-boundary grain boundaries 5BC (1 place) in the cross section (1 place). %) Is 7.1%.

  FIG. 3 shows an embodiment of another solar cell of the present invention, and is a schematic cross-sectional view showing a structure in which quantum dot films including quantum dots having different average diameters are stacked in multiple layers on the main surface of a semiconductor substrate. FIG.

  In the case where the quantum dot film 5 has a multilayer structure, the average diameter of the quantum dots 5a in the quantum dot film 5A arranged on the light incident side is set to the quantum in the quantum dot film 5B arranged on the semiconductor substrate 1 side. It is desirable to make it smaller than the dot 5a.

The quantum dot solar cell can control the wavelength of the light 6 to be absorbed by changing the diameter of the quantum dot 5a. By laminating a plurality of quantum dot films 5 having different absorption wavelengths, for example, light 6 having a wide wavelength from the ultraviolet light region to the near infrared light region can be converted into electric power, so that the total photoelectric conversion efficiency is improved. Is possible. In this case, the average diameter of the quantum dots 5a is preferably gradually increased from the light incident side to the semiconductor substrate 1 side. Here, the average diameter of the quantum dots 5a gradually increases, which means that the average diameter of the quantum dots 5a included in the quantum dot film 5 disposed on the semiconductor substrate 1 side is always adjacent to the light incident side. Not only is it larger than the average diameter of the quantum dots 5a included in the arranged quantum dot layer 5, but the average diameter of the quantum dots 5a is large when viewed from the light incident side to the semiconductor substrate 1 side. It also includes the case that it seems to be. In this case, the size (average diameter) of the quantum dot composite 5CS may be changed with the same tendency by changing the thickness of the barrier layer 5b together with the size of the quantum dots 5a.

  As the quantum dot 5a which comprises the quantum dot film | membrane 5 mentioned above, it consists of what has a semiconductor particle as a main body, and the thing whose energy gap (Eg) has 0.15-1.20ev is suitable. 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.

  In this case, the shape of the quantum dot 5a is preferably spherical because it is easy to form a three-dimensionally continuous band structure with the adjacent quantum dots 5a as described above. Moreover, as for the size of the quantum dot 5a, it is desirable that the maximum diameter is 3 nm to 20 nm.

  In addition, the material of the barrier layer 5b surrounding the quantum dot 5a preferably has an energy gap (Eg) of 2.0 to 10.0 eV, and is an alloy or intermetallic compound of the semiconductor material or the semiconductor. The oxide of the material is preferred. In this case, the thickness of the barrier layer 5b is preferably 1 to 10 nm.

  In addition, the variation in the average diameter of the quantum dots 5a and the quantum dot composite 5CS is desirably 10% or less in terms of a ratio (σ / x) where σ is the standard deviation and x is the average diameter.

  As described above, in the solar cell of this embodiment, the quantum dot film 5 is provided on the main surface 3 of the semiconductor substrate 1, but as shown in FIG. The semiconductor substrate 7 may also be provided on the upper surface side of 5. In this case, for example, when the semiconductor substrate 1 disposed on the lower surface side of the quantum dot film 5 is a p-type (carrier is a hole) semiconductor, the semiconductor substrate 7 disposed on the upper surface side of the quantum dot film 5. Becomes n-type. Note that the p-type and n-type may be reversed. The semiconductor substrates 1 and 7 disposed on both surfaces of the quantum dot film 5 may be either polycrystalline or single crystal, but are preferably polycrystalline in terms of high mass productivity and low cost.

  Next, the quantum dot film 5 in the present embodiment and a method for manufacturing a solar cell to which the quantum dot film 5 is applied 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,
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 it is possible to synthesize particles that are close to a spherical shape, and it is possible to obtain particles with small variations in particle size.

  Next, the semiconductor particles are taken out from the ferritin having the synthesized semiconductor particles. 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 semiconductor particles (quantum dots 5a) 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, the zeta potential of the semiconductor particles is adjusted so that the semiconductor particles are aligned and deposited on the semiconductor substrate 1. The zeta potential is preferably -90 to -110 mV. In addition, a solvent is selected in consideration of the viscosity and evaporability of the slurry containing semiconductor particles. As the solvent, it is preferable to use phthalic acid ester or glycerin.

  Next, the semiconductor substrate 1 is once immersed in a slurry containing semiconductor particles, and then the semiconductor particles are deposited on the surface of the semiconductor substrate 1 by a method of pulling up. In this case, the surface of the semiconductor substrate 1 is preferably processed to be hydrophilic.

  By the above manufacturing method, the semiconductor particles to be the quantum dots 5a are in contact with each other so as to form the neck portion N on the surface of the semiconductor substrate 1, and one or more layers are attached in the thickness direction. The precursor of the quantum dot film | membrane 5 arrange | positioned uniformly on the surface of this can be obtained.

  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. . At this time, an oxide film to be the barrier layer 5b is formed on the surface of the semiconductor particles to form the quantum dot composite 5CS.

  In the solar cell obtained from the above, the quantum dot composite 5CS constituting the quantum dot film 5 has a spherical shape, and the quantum dot film 5 is formed by connecting the quantum dot composite 5CS via the neck portion N. Since a continuous band structure is easily formed in the quantum dot film 5 and the amount of light absorbed by the quantum dots 5a can be increased, the photoelectric conversion efficiency can be improved.

DESCRIPTION OF SYMBOLS 1 ... Semiconductor substrate 3 ... Main surface 5, 105 ... Quantum dot film 5a ... Quantum dot 5b ... Barrier layer 5B ... Grain boundary 5BW ... Interfacial grain boundary 5BC ... Triple point grain Boundary 6, 106 ... Optical C ... Core N ... Neck S ... Shell

Claims (3)

  A quantum dot film having a quantum dot and a barrier layer surrounding the quantum dot on a main surface of the semiconductor substrate and including a spherical quantum dot composite is provided. A solar cell, wherein quantum dot composites are connected via a neck portion.   2. The solar cell according to claim 1, wherein the quantum dot film has a triple-point grain boundary formed between the quantum dot composites and has pores at the triple point grain boundary. The plurality of quantum dot films are stacked, and the quantum dots have an average diameter that decreases from the semiconductor substrate side to a light incident side above the semiconductor substrate. The solar cell described.

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