JP2012004251A - Solar battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a solar battery which is made easier to take out electric current from a mini band by making a barrier smaller at the time of transferring a flow of electrons excited by sunlight in an i-layer to a p-layer or n-layer.SOLUTION: The solar battery includes an n-layer 12, an i-layer 13 formed on the n-layer 12, and a p-layer 14 formed on the i-layer 13, which sunlight goes into. The i-layer 13 is composed of an intrinsic semiconductor layer. The n-layer 12 and p-layer 14 are composed of n-type and p-type semiconductor layers respectively, but both are quantum dot layers. Therefore, electrons excited in the i-layer 13 travel through mini bands of the quantum dot layers including the n-layer 12 and flow into an electrode.

Description

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

  Quantum dots use the properties of electrons as quantum mechanical waves by confining electrons in a region of about 10 nm or less. Recently, attempts have been made to develop solar cells using the properties of the quantum dots. By changing the size of the quantum dot, the wavelength of the light that is absorbed changes according to the size (quantum size effect), the small quantum dot absorbs short wavelength light (blue light), and the large quantum dot Absorbs long wavelength light (red light). When quantum dots are arranged, a new light absorption band is formed by the interaction, and the wavelength range for absorbing light can be widened. Thereby, theoretically, a solar cell with extremely high conversion efficiency can be obtained. In order to fully exhibit the effect of such quantum dots, it is necessary to arrange quantum dots of uniform size regularly and densely in a three-dimensional manner.

  In the so-called first generation, crystalline silicon is used as a solar cell, but in the second generation, amorphous silicon and microcrystalline silicon thin-film silicon cells are used. In this second-generation thin-film silicon cell, various researches have been made to improve its conversion efficiency. Recently, the theoretical conversion greatly exceeds the theoretical limit of conversion efficiency of conventional materials (up to about 28%). Quantum dot solar cells are attracting attention as solar cells that may have an efficiency (over 60%) (Non-Patent Document 1). In Non-Patent Document 1, this quantum dot is created by a method of epitaxially growing a nanocrystal structure of several nm to several tens of nm on a substrate crystal.

  FIG. 5 shows the structure of the quantum dot solar cell described in Non-Patent Document 1, and FIG. 6 shows its energy band diagram. As shown in FIG. 5, a back electrode 101 is formed on the back surface of the semiconductor substrate 100, and an n (n-type semiconductor) layer 102 is formed on the semiconductor substrate 100. An i (intrinsic semiconductor) layer 103 is formed on the n layer 102, and a p (p-type semiconductor) layer 104 is further formed on the i layer 103. A grid-like grid electrode 106 is formed on the p layer 104, and a transparent antireflection film 105 is formed in the opening of the grid electrode 106. Sunlight passes through the antireflection film 105 at the opening of the grid electrode 106 and enters the p layer 104, the i layer 103, and the n layer 102.

  The i layer 103 is formed by alternately stacking quantum dot layers 107 and intermediate layers 108. When the intermediate layer 108 inserted between the quantum dot layers 107 is thick, the energy band structure is as shown in FIG. As shown in FIG. Electrons excited by the absorption of sunlight escape from the wells of the quantum dots by photoexcitation or thermal excitation and are extracted as current. At this time, non-radiative recombination and radiative recombination of electrons occur, resulting in energy loss.

  On the other hand, when the thickness of the intermediate layer is reduced to about several nanometers, as shown in FIG. 6B, a miniband is formed between the quantum dots, and electrons and holes excited by absorption of sunlight are less energy loss. The current generated in this i layer is large.

Science & Technology Trends January 2008 feature article 02 (http://www.nistep.go.jp/achiev/ftx/jpn/stfc/stt082j/0801_03_featurearticles/0801fa02/200801_fa02.html

  As described above, since the mini-band is formed by forming the i layer 103 by the quantum dot layer 107, the energy required to excite electrons and holes by sunlight may be small and the absorbed energy is small. Since the electrons and holes are excited by this, a current is generated efficiently and the conversion efficiency is high. The excited electrons conduct through the miniband and reach the n layer 102.

  However, when the electrons excited as described above move to the n layer 102, it is necessary to overcome the barrier from the miniband to the conduction band. For this reason, the conventional quantum dot solar cell has a limit in the electric current which can be taken out.

  The present invention has been made in view of such problems, and it is possible to reduce the barrier when transferring the flow of electrons or holes excited by sunlight in the i layer to the n layer or the p layer, An object is to provide a solar cell in which current can be easily taken out from a miniband.

  The solar cell according to the present invention includes a back electrode, an n-type semiconductor layer formed on the back electrode, an intrinsic semiconductor layer formed on the n-type semiconductor layer, and an intrinsic semiconductor layer. A p-type semiconductor layer formed on the p-type semiconductor layer, and an electrode that is formed on the p-type semiconductor layer and transmits sunlight to be incident on the intrinsic semiconductor layer. The intrinsic semiconductor layer, the n-type semiconductor layer, and the p-type semiconductor layer Each of the semiconductor layers is a quantum dot layer, and both are layers having a miniband inside the conduction band and the valence band.

  Another solar cell according to the present invention includes a back electrode, an n-type semiconductor layer formed on the back electrode, an intrinsic semiconductor layer formed on the n-type semiconductor layer, and an upper surface of the intrinsic semiconductor layer. A p-type semiconductor layer formed on the p-type semiconductor layer, and an electrode that is formed on the p-type semiconductor layer and transmits sunlight to be incident on the intrinsic semiconductor layer. The intrinsic semiconductor layer, the n-type semiconductor layer, Each of the p-type semiconductor layers is a quantum dot layer made of nanoparticles having a particle diameter of 5 to 1000 nm, and both are layers having a miniband inside the conduction band and the valence band. .

  In this solar cell, for example, the intrinsic semiconductor layer is one in which Si particles and Ag particles are regularly arranged, and the P-type semiconductor layer includes Si particles doped with P-type impurities, Ag particles, Are regularly arranged, and the N-type semiconductor layer is one in which Si particles doped with N-type impurities and Ag particles are regularly arranged.

  According to the present invention, since the n-type semiconductor layer (n layer) and the p-type semiconductor layer (p layer) are also composed of quantum dot layers, the barrier when electrons move from the i layer to the n layer is small, and a large current Can be taken out at high speed.

It is sectional drawing which shows the layer structure of the cell of the solar cell which concerns on embodiment of this invention. It is a figure which shows the band structure of each layer of the said solar cell. It is a schematic diagram which shows the particle structure of a quantum dot layer. It is a schematic diagram which shows the manufacturing method of a quantum dot layer. It is sectional drawing which shows the layer structure of the conventional quantum dot solar cell. It is a figure which shows the band structure of each layer of the said conventional solar cell.

  Hereinafter, embodiments of the present invention will be specifically described with reference to the accompanying drawings. FIG. 1 is a cross-sectional view showing a thin film structure of a solar cell according to an embodiment of the present invention, and FIG. 2 is an energy band gap diagram thereof. As shown in FIG. 1, the solar cell of this embodiment has an n-type semiconductor layer (n layer) 12, an intrinsic semiconductor layer (i layer) 13, and a p-type semiconductor layer (p layer) on the back electrode 11. 14 are laminated. A grid-like grid electrode 16 is formed on the p layer 14, and an antireflection film 15 is formed in the opening of the grid electrode 16. Sunlight passes through the antireflection film 15 at the opening of the grid electrode 16 and enters the p layer 14, the i layer 13, and the n layer 12. The antireflection film 15 is a layer for preventing sunlight from being reflected and efficiently incorporating sunlight into the thin film structure. In addition, electrons excited in the stacked body of the n layer 12, the i layer 13, and the p layer 14 flow from the n layer 12 to the back electrode 11. On the other hand, holes from the p layer 14 flow to the grid electrode 16.

  As shown in FIG. 3, for example, the i layer 13 has a base structure composed of a large number of silicon (Si) particles having a particle diameter of 5 to 1000 nm and Ag (silver) particles having a particle diameter of 5 to 1000 nm. These Si particles and Ag particles are regularly arranged. Similarly, the p layer 14 has a large number of silicon (Si) particles having a particle diameter of 5 to 1000 nm and Ag (silver) particles having a particle diameter of 5 to 1000 nm regularly arranged. In the layer 14, the Si particles are doped with a p-type impurity such as B (boron). Furthermore, similarly, the n layer 12 is regularly arranged with a large number of silicon (Si) particles having a particle diameter of 5 to 1000 nm and Ag (silver) particles having a particle diameter of 5 to 1000 nm. In the layer 12, the Si particles are doped with an n-type impurity such as As (arsenic). Note that these doping elements are not limited to the above elements, and known p-type doping elements and n-type doping elements can be used. Further, the Fermi level of the p layer 14 and the n layer 12 can be controlled by adjusting the amount of each doping element.

  The base-structured Ag particles are regularly arranged so that the intervals are constant. Next, an example of a method for forming the quantum dot layer having the base structure will be described. In the particle structure shown in FIG. 3, the nano-particle Si particles are present at the positions of the lattice points of the face-centered cubic lattice, and four Si particles having three axes orthogonal to each other on one surface of the face-centered cubic lattice. It is in contact with Ag particles at the face center position.

  Ag particles are conductor particles and are nanoparticles. However, rare metal nanoparticles such as indium (In), tantalum (Ta), and tungsten (W), and noble metal nanoparticles such as gold (Au) can also be used. Indium, tantalum, and tungsten are amorphous, but the average particle diameters of commercially available products are 25 μm, 0.8 μm, and 0.6 μm, respectively. Gold and silver are spherical, but the average particle diameter of those commercially available is 0.5 μm and 0.2 μm. Also, commercially available Au particle sources that supply gold (Au) nanoparticles in solution include those with particle sizes of 3.5 to 5.5 nm, 3 to 5 nm, and 2 to 4 nm. In addition, commercially available Ag particle sources that supply silver (Ag) nanoparticles in a solution include those having a particle size of 3 to 7 nm and 5 to 15 nm.

  The base structure of the particle structure shown in FIG. 3 can be formed as follows. As shown in FIG. 4, a solution 6 in which Ag particles and Si particles (intrinsic semiconductor, p-type semiconductor, or n-type semiconductor Si particles) are uniformly dispersed is stored in a container 5. Water can be used as the solvent of the solution 6. The substrate 7 is suspended from a support member 8, and the support member 8 can be moved up and down by an elevating device 9. Then, the substrate 7 is lowered by the lifting device 9 via the support member 8, and the substrate 7 is immersed in the solution 6. Thereby, the layer of the solution 6 adheres to the surface of the substrate 7. Then, when the substrate 7 is pulled up by the lifting device 9, the layer of the solution 6 attached to the surface of the substrate 7 is shrunk by the surface tension in the drying process, and among the particles in the solution, the large-diameter particles are the closest. Solidify in the filling position. As a result, when the close-packed crystal structure is a face-centered cubic lattice, as shown in FIG. In position, these Si particles and Ag particles are arranged. Such a face-centered cubic lattice crystal structure (particle arrangement structure) is formed three-dimensionally. The pulling speed of the substrate 7 is, for example, several hundred nm / s to several tens μm / s. As the material of the substrate 7, glass, iron plate, aluminum foil or the like can be used, and further, a thin film resin sheet or the like can be used.

  Thus, as shown in FIG. 3, large-diameter Si particles are present in contact with the small-diameter Ag particles at the face-centered position at the positions of the lattice points of the face-centered cubic lattice, and these Si particles and Ag particles A nanoparticle arrangement structure consisting of At this time, if the particle diameters of the Si particles and the Ag particles are constant (uniform), the Ag particles can be arranged in a region of about 10 nm or less, and the electrons can be confined to about 10 nm. The arrangement of the nanoparticles is not limited to the position of the face-centered cubic lattice as in the above embodiment, and various arrangement modes are conceivable. For example, the nanoparticles can be arranged at the position of the body-centered cubic lattice. .

  In this way, a semiconductor layer of quantum dots can be formed very easily and with high productivity. Quantum dots are a technique for forming a semiconductor band gap by confining electrons in a region of about 10 nm or less. Thereby, as shown in FIG. 2, a miniband is formed inside the conduction band and the valence band, and electrons and holes excited by light can move through the miniband. .

  Next, the operation of the solar cell configured as described above will be described. Since the i layer 13 is provided between the pn junction between the p layer 14 and the n layer 12, sunlight is not a conduction band portion of a sharp band energy change of the pn junction, but a gentle i layer conduction band portion. In order to excite electrons and holes, the electrons and holes can be excited efficiently, and the absorption efficiency of sunlight is high. The conduction band electrons generated in large quantities in the i layer 13 flow toward the semiconductor substrate 10 and the back electrode 11 via the miniband because the i layer is a quantum dot layer. For this reason, even if the excitation energy of the sunlight with respect to an electron and a hole is low, an electron and a hole are fully excited and an electric current can be taken out.

  On the other hand, conventionally, since the n layer 12 and the p layer 14 are an n-type semiconductor layer and a p-type semiconductor layer, respectively, when excited electrons are transferred from the i layer 13 to the n layer 12, FIG. As shown in b), it is necessary to overcome the conduction band of the n layer 12, and the flow of electrons is obstructed in this portion. Moreover, it is necessary for the holes to get over the valence band of the p layer 14, and the flow of holes is obstructed in this portion.

  On the other hand, in this embodiment, since the p layer 14 and the n layer 12 are also constituted by quantum dot layers, the excited electrons are conducted through the miniband of the n layer 12 to the back electrode 11. Therefore, the conventional barrier is eliminated, and the current can be taken out at high speed with high efficiency. Similarly, the holes conduct through the mini-band of the p layer 14 and reach the grid electrode 16, and are conducted to the grid electrode 16 with high efficiency without the conventional barrier becoming an obstacle.

  Thus, in the present invention, since the quantum dot layers are formed as the n layer and the p layer, electrons efficiently excited by the solar cell can be extracted with high efficiency, and a large current can be extracted at high speed. be able to.

  In addition, this invention is not limited to the said embodiment, For example, p layer 14, i layer 13, and n layer 12 immerse the board | substrate 7 in the solution 6, as shown in FIG. 4 of the said embodiment. Then, the layer of the solution 6 is attached to the surface of the substrate 7, and in the process of drying the layer of the solution 6, the surface tension is shrunk to form a close-packed crystal structure (face-centered cubic lattice), thereby forming a quantum dot For example, the quantum dot structure may be formed by epitaxial growth. The quantum dot structure itself can be formed by various methods. Further, the stacked body of the p layer 14, the i layer 13, and the n layer 12 may be formed using an appropriate semiconductor substrate as a support. The quantum dot layer is a layer having a miniband inside the conduction band and the valence band.

  INDUSTRIAL APPLICABILITY The present invention is extremely useful as a solar battery cell that converts sunlight into electric energy with high efficiency and can extract a large current with high efficiency and at high speed.

6: Solution 7: Substrate 11, 101: Back electrode 12: n layer (quantum dot layer)
13: i layer (quantum dot layer)
14: p layer (quantum dot layer)
15, 105: Antireflection film 16, 106: Grid electrode 102: n layer 103: i layer 104: p layer

Claims (3)

A back electrode, an n-type semiconductor layer formed on the back electrode, an intrinsic semiconductor layer formed on the n-type semiconductor layer, a p-type semiconductor layer formed on the intrinsic semiconductor layer, An electrode that is formed on the p-type semiconductor layer and transmits sunlight to be incident on the intrinsic semiconductor layer. The intrinsic semiconductor layer, the n-type semiconductor layer, and the p-type semiconductor layer are all quantum A solar cell, which is a dot layer, both of which are layers having a miniband inside a conduction band and a valence band. A back electrode, an n-type semiconductor layer formed on the back electrode, an intrinsic semiconductor layer formed on the n-type semiconductor layer, a p-type semiconductor layer formed on the intrinsic semiconductor layer, An electrode that is formed on the p-type semiconductor layer and transmits sunlight to be incident on the intrinsic semiconductor layer. The intrinsic semiconductor layer, the n-type semiconductor layer, and the p-type semiconductor layer are all particles. A solar cell characterized in that it is a quantum dot layer made of nanoparticles having a diameter of 5 to 1000 nm, both of which are layers having a miniband inside the conduction band and the valence band. In the intrinsic semiconductor layer, Si particles and Ag particles are regularly arranged, and in the P-type semiconductor layer, Si particles doped with P-type impurities and Ag particles are regularly arranged. 3. The solar cell according to claim 2, wherein the N-type semiconductor layer includes regularly arranged Si particles doped with N-type impurities and Ag particles.

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