JP3893466B2 - Photovoltaic element, solar cell, and method for producing photovoltaic element - Google Patents

Description

  The present invention relates to a photovoltaic device, a solar cell, and a method for manufacturing a photovoltaic device.

  By proposing a pin junction solar cell using a quantum well, the conversion efficiency of the solar cell has been remarkably improved, and has come to replace the conventional tandem solar cell. In the quantum well type solar cell, the absorption edge and the spectral characteristics can be improved by controlling the width and depth of the well, so that the short circuit current and the open circuit voltage can be controlled independently.

  However, in such a quantum well type solar cell, in order for an electron-hole pair generated by light absorption in the quantum well to contribute to power generation, before the electrons and holes are recombined, From the quantum well to the barrier layer, and further, it is required to repeatedly capture and desorb to the adjacent quantum well to reach the electrode. Therefore, even in such conventional quantum well solar cells, recombination of electrons and holes has priority in the transport process of electrons and holes, and the absorbed light may not contribute to power generation effectively. .

An object of the present invention is to provide a novel photovoltaic element that can generate electromotive force with high efficiency by using a wide range of optical spectrum.

In order to achieve the above object, the present invention provides:
a p-type semiconductor portion;
An n-type semiconductor portion provided to face the p-type semiconductor portion;
An island-shaped intrinsic semiconductor layer provided between the p-type semiconductor portion and the n-type semiconductor portion,
In the step of forming the intrinsic semiconductor layer, the p-type semiconductor portion is heated to 700 ° C.
The intrinsic semiconductor layer has a plurality of island-shaped portions arranged in a plane and is stacked in the thickness direction.
The intrinsic semiconductor layer has a plurality of intervening layers that divide the plurality of island-shaped portions for each layer in the thickness direction,
The plurality of island-shaped portions and the intervening layer in the intrinsic semiconductor layer are composed of a lattice-mismatched semiconductor material,
The plurality of island-shaped portions are made of Ge, and the intervening layer is made of Si .

In the photovoltaic device of the present invention, since the intrinsic semiconductor layer that contributes to power generation has an island-like structure, the intrinsic semiconductor layer has a relatively large internal electric field due to the structure. Therefore, electron-hole pairs generated in the intrinsic semiconductor layer by light absorption are separated by the internal electric field, propagate in the intrinsic semiconductor layer without recombination, and contribute to photocurrent. Therefore, highly efficient power generation using a wide optical spectrum can be performed.

The intrinsic semiconductor layer is composed of a plurality of island portions. In this case, the intrinsic semiconductor layer having the island-like structure can be easily formed by laminating the plurality of island-like portions on the plane and in the thickness direction. Further, if the intrinsic semiconductor layer is composed of such a plurality of island-shaped portions, the size of the internal electric field can be controlled by appropriately controlling the size of each island-shaped portion and the number of stacked layers. Further, the upper limit of the light absorption wavelength region can be increased, and high-efficiency power generation can be performed using a wider range of light spectrum.

In this case, when a plurality of intervening layers are provided in the intrinsic semiconductor layer so as to divide the plurality of islands stacked in the thickness direction for each layer, the method for manufacturing a photovoltaic element described above As a result, the island-shaped portion of each layer is formed with the intervening layer as a base layer (base portion), and therefore the size of the island-shaped portion can be arbitrarily set for each layer. Therefore, the stacking state of the plurality of island-shaped portions, that is, the degree of freedom in designing the intrinsic semiconductor layer can be increased, and the internal electric field and the light absorption wavelength region generated in the intrinsic semiconductor layer can be freely set. . Therefore, the design freedom of the power generation efficiency of the photovoltaic element can be increased.

As described above, according to the present invention, it is possible to provide a novel photovoltaic element that can generate an electromotive force with high efficiency by utilizing a wide optical spectrum.

Hereinafter, other features and advantages of the present invention are described in detail.
FIG. 1 is a configuration diagram showing an example of the photovoltaic element of the present invention, and FIG. 2 is a diagram showing a band structure of the photovoltaic element shown in FIG.

A photovoltaic element 10 shown in FIG. 1 includes a p-type Si semiconductor substrate 11 as a p-type semiconductor portion, an n-type Si semiconductor layer 12 as an n-type semiconductor portion formed thereabove, and a gap therebetween. And an intrinsic semiconductor layer 13 provided. The intrinsic semiconductor layer 13 exhibits an island-like structure by arranging a plurality of island-like portions 13A made of Ge on a plane and being stacked in the thickness direction. In addition, a plurality of intervening layers 13B are formed in the intrinsic semiconductor layer 13, and the island-shaped portions 13A stacked in the thickness direction are divided for each layer.

  In order to make the photovoltaic element 10 actually function, an electrode layer is formed on the back surface of the p-type Si semiconductor substrate 11 and the top surface of the n-type Si semiconductor layer 12, but this is omitted in this example.

As described above, since the intrinsic semiconductor layer 13 is composed of a plurality of island-like portions 13A, the intrinsic semiconductor layer 13 has a relatively large internal electric field due to its structure. Therefore, electron-hole pairs generated by light irradiation in the intrinsic semiconductor layer 13 of the photovoltaic element 10 are separated by the internal electric field and propagate in the intrinsic semiconductor layer 13 without recombination, and thus a p-type Si semiconductor. It flows into the electrode layer (not shown) through the substrate 11 and the n-type Si semiconductor layer 13 and contributes to the photocurrent. Therefore, highly efficient power generation using a wide optical spectrum can be performed. Furthermore, the upper limit of the light absorption wavelength region can be increased, and high-efficiency power generation can be performed using a wider optical spectrum.

In the example shown in FIG. 1, since the intrinsic semiconductor layer 13 having the island-shaped portion 13A made of Ge is provided between the p-type Si semiconductor substrate 11 and the n-type Si semiconductor layer 12, the upper limit of the light absorption wavelength region is set. It can be increased from about 1200 nm to about 1400 nm. That is, highly efficient power generation can be performed using a wide optical spectrum up to about 1400 nm.

In addition, in the thickness direction of the intrinsic semiconductor layer 13, the plurality of island-like portions 13 </ b> A has a structure divided by the intervening layer 13 </ b> B for each layer, whereby the plurality of island-like portions 13 </ b> A are separated from the intrinsic semiconductor layer 13. In the thickness direction, the intervening layer 13 can be formed for each layer as a base layer (base), so that the size of the island-like portion located in each layer is changed to the state of the island-like portion located in the lower layer, etc. It can be set arbitrarily without depending on it. Therefore, the stacked state of the plurality of island-shaped portions 13A can be arbitrarily controlled, and the degree of freedom in designing the intrinsic semiconductor layer 13 can be increased. As a result, the magnitude of the internal electric field and the light absorption wavelength region generated in the intrinsic semiconductor layer 13 can be freely changed, and the power generation efficiency can be freely controlled.

  Due to the manufacturing method of the photovoltaic element 10 shown below, when the island-shaped portion 13A is made of Ge, the intervening layer 13B is preferably made of Si.

The height of the island portion 13A is not particularly limited, but is preferably 5 nm to 50 nm. Further, the width of the island-shaped portion 13A is not particularly limited, but is preferably 5 nm to 200 nm. As a result, a sufficiently large internal electric field can be generated in the intrinsic semiconductor layer 13, and the separation of electron-hole pairs generated by light absorption can be increased, so that the power generation efficiency can be increased.

Furthermore, it is preferable that the number of stacked island-shaped portions 13A is 50 to 300 layers. As a result, the quantum efficiency is increased, so that electron-hole pairs can be generated in the intrinsic semiconductor layer 13 even in the light spectrum on the longer wavelength side of the light absorption wavelength region, and the upper limit of the light absorption wavelength region is increased from the conventional about 1200 nm. Is sufficiently large, for example, up to about 1400 nm, and exhibits sufficiently high power generation efficiency.

In FIG. 1, the island portion located above is formed immediately above the island portion located below, but this state is caused by the following method for manufacturing a photovoltaic element. Since the state portion 13A and the whole of the intrinsic semiconductor layer 13 are energetically stable, the stability and long-term reliability of the obtained photovoltaic element 10 are increased.

Next, a method for manufacturing the photovoltaic element shown in FIG. 1 will be described.
First, a p-type Si semiconductor substrate 11 is prepared, the surface is cleaned with sulfuric acid and a hydrogen peroxide solution, rinsed with deionized water or the like, and immersed in a hydrofluoric acid solution to hydrogenate the terminal. Next, the substrate 11 is placed in the MBE apparatus, the inside is evacuated to about 1 × 10 ⁻¹⁰ Torr, and heated to 700 ° C.

Next, a Si buffer layer is formed on the substrate 11 as necessary, and then irradiated with a molecular beam of GeH ₄ to form the island-shaped portion 13A of the first layer. Note that the island-shaped portion 13A is a semiconductor material in which the material (Si) constituting the substrate 11 and the material (Ge) constituting the island-shaped portion 13A are lattice-mismatched semiconductor materials, and GeH _{4 is formed} on the substrate 11. When a Ge layer is formed by irradiating the molecular beam, an increase in surface energy of the Ge layer and a decrease in strain energy are balanced. However, in order to realize the balance, the molecular beam is irradiated so that a Ge layer having a thickness of several atomic layers is formed.

Next, an Si ₂ H ₆ molecular beam is irradiated on the first layer island 13A to form an intervening layer 13B made of Si. Next, GeH ₄ molecular beam is irradiated again on the intervening layer 13B to form a second layer island 13A. By repeating such an operation, an intrinsic semiconductor layer 13 in which island-like portions 13A and intervening layers 13B are alternately stacked is obtained. By forming the intrinsic semiconductor layer 13 using such an MBE method, the island-like portion 13A located above is formed immediately above the island-like portion 13A located below from the energy balance. become.

Next, an Si semiconductor layer is formed by irradiating Si ₂ H ₆ molecular beam on the island-like portion 13A located at the uppermost position, and then an n-type impurity is diffused and implanted to form an n-type Si semiconductor layer 12. Thereafter, an electrode layer (not shown) is formed on the back surface of the p-type Si semiconductor substrate 11 and the top surface of the n-type semiconductor layer 12 to obtain the photovoltaic element 10.

In the example of the manufacturing method described above, the intrinsic semiconductor layer 13 and the n-type semiconductor layer 12 are formed using the MBE method, but other methods may be used. However, since the island-shaped portion constituting the intrinsic semiconductor layer 13 is extremely fine and difficult to control, it is preferable to use the MBE method which is easy to control at the atomic level.

Further, when forming the intrinsic semiconductor layer 13 or the like, it is formed so as to satisfy the above-mentioned preferable requirements.

  A p-type Si (100) semiconductor substrate was prepared, and a photovoltaic device as shown in FIG. 1 was obtained according to the manufacturing method described above. However, the height of the island-shaped portion 13A made of Ge was about 10 nm and the width was about 100 nm. The number of island portions 13A stacked was 50, 100, and 150. The thickness of the intervening layer 13B made of Si was 39 nm, and the thickness of the n-type Si semiconductor layer 12 was 600 nm.

  FIG. 3 is a graph showing the quantum efficiency in the infrared region of each photovoltaic device. As is clear from FIG. 3, the quantum efficiency in the infrared region increases with an increase in the number of stacked island-shaped portions 13A, and further, in the light absorption frequency region as compared with the conventional Si-based photovoltaic device. It can be seen that the upper limit increases from about 1200 nm to about 1400 nm. Therefore, it can be seen that the photovoltaic element obtained in this example exhibits sufficiently high power generation efficiency as compared with the conventional photovoltaic element.

  As described above, the present invention has been described in detail based on the embodiments of the present invention with specific examples. However, the present invention is not limited to the above contents, and all modifications and changes are made without departing from the scope of the present invention. It can be changed.

  For example, in the example shown in FIG. 1, a p-type semiconductor portion is formed from the p-type Si semiconductor substrate 11 and an n-type semiconductor portion is formed from the n-type Si semiconductor layer 12. It is also possible to configure the n-type semiconductor portion and configure the p-type semiconductor portion from the p-type Si semiconductor layer. Furthermore, instead of configuring the p-type semiconductor portion from the p-type Si semiconductor substrate 11, it may be configured from a p-type Si semiconductor layer formed on a predetermined substrate.

  In the example shown in FIG. 1, Si and Ge are selected as the lattice-mismatched semiconductor material, but other materials such as InAs and GaAs can also be selected.

  The present invention can be suitably used for highly efficient pin junction solar cells and the like.

It is a block diagram which shows an example of the photovoltaic element of this invention. It is a figure which shows the band structure of the photovoltaic element shown in FIG. It is a graph which shows the quantum efficiency in the infrared region of the photovoltaic device of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Photovoltaic element 11 P-type Si semiconductor substrate 12 N-type Si semiconductor layer 13 Intrinsic semiconductor layer 13A Island-like part 13B Intervening layer

Claims (19)

a p-type semiconductor portion;
An n-type semiconductor portion provided to face the p-type semiconductor portion;
An island-shaped intrinsic semiconductor layer provided between the p-type semiconductor portion and the n-type semiconductor portion,
In the step of forming the intrinsic semiconductor layer, the p-type semiconductor portion is heated to 700 ° C.
The intrinsic semiconductor layer has a plurality of island-shaped portions arranged in a plane and is stacked in the thickness direction.
The intrinsic semiconductor layer has a plurality of intervening layers that divide the plurality of island-shaped portions for each layer in the thickness direction,
The plurality of island-shaped portions and the intervening layer in the intrinsic semiconductor layer are composed of a lattice-mismatched semiconductor material,
The photovoltaic element, wherein the plurality of island-shaped portions are made of Ge, and the intervening layer is made of Si . 2. The light according to claim 1 , wherein the plurality of island-shaped portions are stacked such that the upper island-shaped portion is positioned immediately above the lower island-shaped portion in the thickness direction. Electromotive force element. Wherein the height of the island-shaped portions is 5 nm to 50 nm, the photovoltaic device according to claim 1 or 2. Wherein the width of the island portion is 5 nm to 200 nm, the photovoltaic device according to any one of claims 1 to 3. Wherein the plurality of islands, in the thickness direction, characterized in that it is laminated in 50 layers to 300 layers, the photovoltaic device according to any one of claims 1 to 4. One of the p-type semiconductor portion and the n-type semiconductor portion is a p-type semiconductor substrate or an n-type semiconductor substrate, and the other of the p-type semiconductor portion and the n-type semiconductor portion is a p-type semiconductor layer or an n-type semiconductor. characterized by comprising the layer, the photovoltaic device according to any one of claims 1 to 5. The plurality of island-shaped portions in the intrinsic semiconductor layer and the p-type semiconductor portion or the n-type semiconductor portion located below the intrinsic semiconductor layer are made of a lattice-mismatched semiconductor material. The photovoltaic device according to any one of claims 1 to 6 , wherein: The photovoltaic element according to claim 7 , wherein the p-type semiconductor part or the n-type semiconductor part located below the intrinsic semiconductor layer is made of Si . Wherein the upper limit of the light absorption wavelength region of approximately 1400 nm, the photovoltaic device according to any one of claims 1 to 8. A solar cell comprising the photovoltaic element according to any one of claims 1 to 9 . forming a p-type semiconductor portion;
Forming an n-type semiconductor portion so as to face the p-type semiconductor portion;
Forming an island-like intrinsic semiconductor layer between the p-type semiconductor portion and the n-type semiconductor portion,
In the step of forming the intrinsic semiconductor layer, the p-type semiconductor portion is heated to 700 ° C.
The intrinsic semiconductor layer is formed by laminating a plurality of island-shaped portions in a plane and laminating in the thickness direction,
In the intrinsic semiconductor layer, forming a plurality of intervening layers that divide the plurality of island-shaped portions for each layer in the thickness direction,
The plurality of island-shaped portions and the intervening layer in the intrinsic semiconductor layer are made of a lattice-mismatched semiconductor material,
The method for manufacturing a photovoltaic device, wherein the plurality of island-shaped portions are made of Ge, and the intervening layer is made of Si . The photovoltaic device according to claim 11 , wherein the plurality of island-shaped portions are stacked such that the upper island-shaped portion is positioned immediately above the lower island-shaped portion in the thickness direction. A method for manufacturing a power element. The method of manufacturing a photovoltaic element according to claim 11 , wherein the island-shaped portion has a height of 5 nm to 50 nm. The photovoltaic element according to claim 11 , wherein a width of the island-shaped portion is 5 nm to 200 nm. The method for manufacturing a photovoltaic device according to claim 11 , wherein the plurality of island-shaped portions are stacked in 50 to 300 layers in a thickness direction. One of the p-type semiconductor portion and the n-type semiconductor portion is formed of a p-type semiconductor substrate or an n-type semiconductor substrate, and the other of the p-type semiconductor portion and the n-type semiconductor portion is a p-type semiconductor layer or an n-type semiconductor substrate. The method for manufacturing a photovoltaic element according to claim 11 , comprising a semiconductor layer. The plurality of island-shaped portions in the intrinsic semiconductor layer and the p-type semiconductor portion or the n-type semiconductor portion positioned below the intrinsic semiconductor layer are made of a lattice-mismatched semiconductor material. The method for producing a photovoltaic element according to claim 11 , wherein the photovoltaic element is produced. 18. The method of manufacturing a photovoltaic element according to claim 17 , wherein the p-type semiconductor part or the n-type semiconductor part located below the intrinsic semiconductor layer is made of Si . The method of manufacturing a photovoltaic element according to claim 11 , wherein the intrinsic semiconductor layer is formed by an MBE method.

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