Patent Application
20130087190
This disclosure relates to photovoltaic devices.
For over a century fossil fuels such as coal, oil, and natural gas have provided the main source of energy in the United States. The need for alternative sources of energy is increasing. Fossil fuels are a non-renewable source of energy that is depleting rapidly. The large scale industrialization of developing nations such as India and China has placed a considerable burden on available fossil fuel. In addition, geopolitical issues can quickly affect the supply of such fuel. Global warming is also of greater concern in recent years. A number of factors are thought to contribute to global warming; however, widespread use of fossil fuels is presumed to be a major contributor to global warming. Thus, there is a need to find a renewable and economically viable source of energy that is also environmentally safe. Solar energy is an environmentally safe renewable source of energy that can be converted into other forms of energy such as heat and electricity.
Photovoltaic cells convert optical energy to electrical energy and thus can be used to convert solar energy into electrical power. Photovoltaic cells can be made very thin and modular, and can range in size from about a few millimeters to tens of centimeters, or larger. The individual electrical output from one photovoltaic cell may range from a few milliwatts to a few watts. Several photovoltaic cells may be connected electrically and packaged in arrays to produce a sufficient amount of electricity. Additionally, photovoltaic cells can be used in a wide range of applications, such as providing power to satellites and other spacecraft, providing electricity to residential and commercial properties, charging automobile batteries, and powering mobile devices, such as smart phones or personal computers.
While photovoltaic devices have the potential to reduce reliance upon hydrocarbon fuels, the widespread use of photovoltaic devices has been hindered by a variety of factors, including energy inefficiency. Accordingly, there is a need for photovoltaic devices having improved efficiency.
The systems, methods and devices of the disclosure each have several innovative aspects, no single one of which is solely responsible for the desirable attributes disclosed herein.
One innovative aspect of the subject matter disclosed in this disclosure can be implemented in a photovoltaic device including an anode contact structure, a cathode contact structure, and an inorganic solar cell disposed between the anode and cathode contact structures. The inorganic solar cell includes a p-type photovoltaic layer adjacent the anode contact structure, an n-type photovoltaic layer disposed between the cathode contact structure and the p-type photovoltaic layer, and a hole blocking layer also disposed between the cathode contact structure and the p-type photovoltaic layer. The hole blocking layer provides an energy barrier to holes that is more than an energy barrier provided to electrons.
In some implementations, a boundary between the cathode contact structure and the inorganic solar cell defines a first interface surface, and the hole blocking layer prevents holes in the p-type photovoltaic layer from reaching the first interface surface and permits electrons to cross when the inorganic solar cell is generating current.
In some implementations, the hole blocking layer has a band gap that is about 0.2 eV to about 1.0 eV greater than a band gap of the p-type photovoltaic layer.
Another innovative aspect of the subject matter disclosed in this disclosure can be implemented in a method of forming a thin film solar cell device, the method including providing an anode contact structure, providing an inorganic solar cell adjacent the anode contact structure, and providing a cathode contact structure adjacent the inorganic solar cell on a side of the inorganic solar cell opposite the anode contact structure. Providing the inorganic solar cell includes providing a p-type photovoltaic layer adjacent the anode contact structure, providing an n-type photovoltaic layer between the cathode contact structure and the p-type photovoltaic layer, and providing a hole blocking layer between the cathode contact structure and the p-type photovoltaic layer. The hole blocking layer provides an energy barrier to holes that is more than an energy barrier provided to electrons.
In some implementations, forming the hole blocking layer includes forming the hole blocking layer between the p-type and n-type photovoltaic layers.
Another innovative aspect of the subject matter disclosed in this disclosure can be implemented in a photovoltaic device including an anode contact structure, a cathode contact structure, and an inorganic solar cell disposed between the anode and cathode contact structures. The inorganic solar cell includes a p-type photovoltaic layer adjacent the anode contact structure, an n-type photovoltaic layer disposed between the cathode contact structure and the p-type photovoltaic layer, and a means for blocking holes disposed between the cathode contact structure and the p-type photovoltaic layer. The hole blocking means provides an energy barrier to holes that is more than an energy barrier provided to electrons.
Another innovative aspect of the subject matter disclosed in this disclosure can be implemented in a photovoltaic device including an anode contact structure, a cathode contact structure, and an inorganic solar cell disposed between the anode and cathode contact structures. The inorganic solar cell includes an n-type photovoltaic layer adjacent the cathode contact structure, a p-type photovoltaic layer disposed between the anode contact structure and the n-type photovoltaic layer, and an electron blocking layer also disposed between the anode contact structure and the n-type photovoltaic layer. The electron blocking layer provides an energy barrier to electrons that is more than an energy barrier provided to holes.
In some implementations, a boundary between the anode contact structure and the inorganic solar cell defines a first interface surface, and the electron blocking layer prevents electrons in the n-type photovoltaic layer from reaching the first interface surface and permits holes to cross when the inorganic solar cell is generating current.
In some implementations, the electron blocking layer has a band gap that is about 0.2 eV to about 1.0 eV greater than a band gap of the n-type photovoltaic layer.
Another innovative aspect of the subject matter disclosed in this disclosure can be implemented in a method of forming a thin film solar cell device, the method including providing a cathode contact structure, providing an inorganic solar cell adjacent the cathode contact structure, and providing an anode contact structure adjacent the inorganic solar cell on a side of the inorganic solar cell opposite the cathode contact structure. Providing the inorganic solar cell includes providing an n-type photovoltaic layer adjacent the cathode contact structure, providing a p-type photovoltaic layer between the anode contact structure and the n-type photovoltaic layer, and providing an electron blocking layer between the anode contact structure and the n-type photovoltaic layer. The electron blocking layer provides an energy barrier to electrons that is more than an energy barrier provided to holes.
In some implementations, forming the electron blocking layer includes forming the electron blocking layer between the p-type and n-type photovoltaic layers.
Another innovative aspect of the subject matter disclosed in this disclosure can be implemented in a photovoltaic device including an anode contact structure, a cathode contact structure, and an inorganic solar cell disposed between the anode and cathode contact structures. The inorganic solar cell includes an n-type photovoltaic layer adjacent the cathode contact structure, a p-type photovoltaic layer disposed between the anode contact structure and the n-type photovoltaic layer, and a means for blocking electrons disposed between the anode contact structure and the n-type photovoltaic layer. The electron blocking means provides an energy barrier to electrons that is more than an energy barrier provided to holes.
Details of one or more implementations of the subject matter described in this specification are set forth in the accompanying drawings and the description below. Other features, aspects, and advantages will become apparent from the description, the drawings, and the claims. Note that the relative dimensions of the following figures may not be drawn to scale.
Photovoltaic devices including an anode contact structure, a cathode contact structure, and an inorganic solar cell disposed between the anode and cathode contact structures are disclosed. The inorganic solar cell includes a p-type photovoltaic layer, an n-type photovoltaic layer, and one or more minority carrier blocking layers for improving the efficiency of the solar cell by preventing minority carriers within the solar cell from reaching areas where recombination is possible, such as interface recombination surfaces associated with the anode and cathode contact structures. For example, the inorganic solar cell can include a hole blocking layer configured to block holes in the p-type photovoltaic layer from reaching an interface surface between the cathode contact structure and the inorganic solar cell. Similarly, the inorganic solar cell can include an electron blocking layer configured to block electrons in the n-type photovoltaic layer from reaching an interface surface between the anode contact structure and the inorganic solar cell. By including at least one minority carrier blocking layer to prevent minority carriers from reaching interface surfaces, minority carrier recombination can be reduced and solar cell efficiency can be improved.
Implementations of the subject matter described in this disclosure can increase power efficiency of a photovoltaic device, thereby increasing the magnitude of a photocurrent generated from a given amount of light. Additionally, some implementations can be used to reduce recombination losses in thin-film photovoltaic devices such as a-Si, CdTe, and/or CIGS solar cells.
The cathode and/or anode contact structures 4 and 6 can be any suitable conductor. For example, the cathode and/or anode contact structures 4 and 6 can include a transparent conductor, including, for example, a transparent conducting oxide (TCO) of zinc oxide (ZnO) or indium tin oxide (ITO). A TCO or other transparent conductor in the photovoltaic device 10 can provide electrical connectivity to the inorganic solar cell 2, while permitting light to pass through the cathode and/or anode contact structures 4 and 6 and reach the inorganic solar cell 2. However, the cathode and/or anode contact structures 4 and 6 need not be transparent. For example, in configurations in which light is configured to pass through only one side of the photovoltaic device 10, the contact structure receiving the light can be transparent, while the other contact structure can be formed from an opaque material, such as a reflector configured to reflect light back toward the inorganic solar cell 2. In some implementations, the anode and/or cathode contact structures 4 and 6 can be formed of an opaque material and can include one or more openings that provide a path for light to reach the inorganic solar cell 2.
The inorganic solar cell 2 can include one or more light absorbing inorganic photovoltaic materials, including, for example, silicon (Si), germanium (Ge), crystalline silicon (c-Si), a-Si, CdTe, copper indium diselenide (CIS), copper indium gallium diselenide (CIGS), and/or III-V semiconductors. The inorganic solar cell 2 can operate as a photodiode 14, which can convert light energy into electrical energy or current. When the inorganic solar cell 2 is illuminated with light, photons from the light transfer energy to the inorganic solar cell 2, which can result in the creation of electron-hole pairs. For example, photons having energy greater than the band-gap of the material(s) in the inorganic solar cell 2 can generate electron-hole pairs within the inorganic solar cell 2 by band-to-band excitation. In addition, high-energy photons can generate electron-hole pairs by impact ionization or via recombination-generation centers within the lattice of the inorganic solar cell 2.
When photons create electron-hole pairs within or near a depletion region of the inorganic solar cell 2, the electric field of the depletion region can sweep the electrons and holes to the anode and cathode contact structures of the photovoltaic device 10, thereby generating a photocurrent. The electron-hole pairs can also move via diffusion through photovoltaic device 10. The generated photocurrent can be used to provide power to any suitable load 12. For example, the load 12 can be power supplied to an electrical grid, or electrical requirements of a mobile device.
The efficiency of the photovoltaic device 10 can be limited by minority carrier recombination occurring at the interfaces between the solar cell 2 and cathode and/or anode contact structures 4 and 6. For example, a cathode interface surface 11 at the boundary between the n-type photovoltaic layer 3a and the cathode contact structure 4 can include interface energy states resulting from dangling bonds, lattice defects, and/or other surface related defects that can operate as recombination centers for minority holes in the n-type photovoltaic layer 3a. Similarly, an anode interface surface 13 at the boundary between the p-type photovoltaic layer 3b and the anode contact structure 6 can include interface energy states that operate as recombination centers for minority electrons in the p-type photovoltaic layer 3b.
The current resulting from the flow of electrons to the interface between the anode contact structure 6 and the p-type layer 3b and the current resulting from the flow of holes to the interface between the cathode contact structure 4 and the n-type layer 3a are in a direction opposite the flow of the photocurrent IPHOTO delivered to the load. Thus, minority carrier recombination at these interfaces reduces the magnitude of the photocurrent generated by the solar cell. Although surface passivation techniques can be employed to reduce the number of interface energy states, there remains a need for reducing minority carrier recombination.
Inorganic solar cells with at least one minority carrier blocking layer are provided. The minority carrier blocking layer can be used to reduce minority carrier recombination, thereby improving efficiency of the inorganic solar cell. For example, a hole blocking layer can be used to block holes from reaching an interface between the solar cell and the cathode contact and/or an electron blocking layer can be used to block electrons from reaching an interface between the solar cell and the anode contact. Since lattice defects and/or surface energy states associated with the interfaces to the anode and cathode contacts can cause minority carrier recombination, blocking the flow of minority carriers to these interfaces can improve efficiency of the inorganic solar cell.
In some implementations, the minority carrier blocking layer can be an electron blocking layer configured to prevent electrons from reaching an interface between the solar cell and the anode contact, as shown, for example, in the configurations illustrated in
The electron blocking layer 26 can operate as a barrier for electrons in the p-type photovoltaic layer 22 from reaching the interface between the inorganic solar cell 24 and the anode contact structure 28. By providing a barrier for electrons from reaching this interface, recombination of electrons at the interface can be decreased, thereby reducing carrier recombination and increasing the probability that the electrons will reach the cathode contact structure 29 and contribute to the generated photocurrent. To avoid hindering the operation of the photovoltaic device 20, the electron blocking layer 26 can be configured to have an energy band level that permits holes to pass across the electron blocking layer 26. Thus, the electron blocking layer 26 can reduce minority carrier recombination without hindering the flow of majority carriers.
The hole blocking layer 25 can operate as a barrier for holes in the p-type photovoltaic layer 22 from reaching the interface between the inorganic solar cell 31 and the cathode contact structure 29. By providing a barrier for holes in this manner, recombination of holes at the interface can be decreased, thereby reducing carrier recombination and increasing the probability that the holes will reach the anode contact structure and contribute to the generated photocurrent. To avoid hindering the operation of the photovoltaic device 30, the hole blocking layer 25 can be configured to have an energy band level that permits electrons to pass across the hole blocking layer 25.
Minority carrier blocking layers described herein, including but not limited to the electron blocking layer 26 of
The inorganic solar cell 34 of
The inorganic solar cell 37 of
The hole blocking layer has generated an energy barrier 55 for holes in the p-type photovoltaic layer from reaching the n-type photovoltaic layer. In certain implementations, a hole blocking layer can be configured to provide an energy barrier 55 to holes that is have about 0.2 eV to about 1.0 eV greater than the energy barrier provided to electrons, for example about 0.2 eV to about 0.5 eV. Likewise, in certain implementations, an electron blocking layer can be configured to provide an energy barrier to electrons that is about 0.2 eV to about 1.0 eV greater than the energy barrier provided to holes.
As illustrated in
The minority carrier blocking layer can be relatively thin. Providing a relatively thin minority carrier blocking layer can allow a greater number of majority carriers to cross the minority carrier blocking layer when the inorganic solar cell is generating a photocurrent relative to a scheme in which the minority carrier blocking layer has a relatively large thickness. In some implementations, the minority carrier blocking layer has a thickness ranging between about 1 nm and about 10 nm.
The transparent substrate 61 can be a glass substrate or any other suitable transparent substrate, such as an optical plastic. The transparent substrate 61 can be employed to structurally support the TCO layer 68, the solar cell 64, and the cathode contact structure 69, each of which can be formed on the transparent substrate 61 using thin film technology. Accordingly, the TCO layer 68, the solar cell 64, and the cathode contact structure 69 can be formed from a plurality of thin film layers deposited on a surface of the transparent substrate 61. The transparent substrate 61 can have any suitable thickness, such as a thickness ranging between about 0.025 mm to about 10 mm.
The solar cell 64 can receive light through the transparent substrate 61 and the TCO structure 68. The solar cell 64 shown in
The solar cell 64 also includes the hole blocking layer 65, which has been disposed along the path between the n-type a-Si layer 63c and the cathode contact structure 69 so as to prevent holes from reaching an interface between the solar cell 64 and the cathode contact structure 69. In certain implementations, the hole blocking layer 65 can be an n-type a-Si layer having a band gap that is about 0.05 eV to about 0.3 eV greater than a band gap of the n-type a-Si layer 63c. The hole blocking layer 65 can have any suitable thickness, such as a thickness in the range of about 1 nm to about 10 nm.
The TCO structure 68 can operate as an anode contact structure for the solar cell 64. The solar cell 64 also includes the cathode contact structure 69, which can include any material suitable for making electrical contact with the solar cell cathode. In certain implementations, the cathode contact structure 69 can include a reflective metal, such as aluminum (Al) and/or silver (Ag). Including a reflective metal in the cathode contact structure 69 can help reflect light that passes through the inorganic solar cell 64 back toward the solar cell, thereby increasing the energy efficiency of the photovoltaic device 60.
The photovoltaic device 70 of
The electron blocking layer 66 has been disposed along the path between the p-type a-Si layer 63a and the TCO structure 68 so as to prevent electrons from reaching an interface between the solar cell 76 and the TCO structure 68. In certain implementations, the electron blocking layer 66 can be a p-type a-Si layer having a band gap that is about 0.05 eV to about 0.5 eV greater than a band gap of the p-type a-Si layer 63a. In some implementations, the electron blocking layer 66 can have, for example, a thickness in the range of about 1 nm to about 10 nm.
Certain features of the photovoltaic device 75 of
Features of photovoltaic device 77 of
The transparent substrate 81 can be a glass substrate or any other suitable transparent substrate, such as an optical plastic. The transparent substrate 81 can be employed to structurally support the TCO layer 88, the solar cell 84, and the anode contact structure 89, each of which can be formed on the transparent substrate 81 using thin film technology. Additional details of the transparent substrate 81 can be similar to those described earlier.
The solar cell 84 can receive light through the transparent substrate 81 and the TCO structure 88. The solar cell 84 shown in
The TCO structure 88 can operate as a cathode contact structure for the solar cell 84. The solar cell 84 also includes the anode contact structure 89, which can include any material suitable for making electrical contact with the solar cell anode. In certain implementations, the anode contact structure 89 can include a reflective metal, such as nickel-aluminum (Ni—Al) and/or gold (Au).
Certain features of photovoltaic device 90 of
The electron blocking layer 86 has been disposed along the path between the p-type CdTe layer 83a and the anode contact structure 89 so as to prevent electrons from reaching an interface between the solar cell 96 and the anode contact structure 89. In certain implementations, the electron blocking layer 86 includes at least one of ZnSe, an alloy of cadmium telluride and zinc telluride in a ratio of x and 1-x (CdxZn1-xTe), an alloy of cadmium selenide and zinc selenide in a ratio of x and 1-x (CdxZn1-xSe), an alloy of cadmium sulfide and cadmium telluride in a ratio of x and 1-x (CdxTe1-xSx), an alloy of cadmium selenide and cadmium telluride in a ratio of x and 1-x (CdTe1-xSex), an alloy of cadmium selenide and cadmium sulfide in a ratio of 1-x and x (CdSe1-xSx), an alloy of mercury sulfide and mercury telluride in a ratio of x and 1-x (HgTe1-xSx), and an alloy of mercury selenide and mercury sulfide in a ratio of 1-x and x (HgSe1-xSx). The electron blocking layer 86 can have any suitable thickness, such as a thickness in the range of about 1 nm to about 10 nm.
Additional details of the photovoltaic device 95 of
Although
The substrate 101 can be a glass substrate or any other suitable substrate, including an opaque substrate. The substrate 101 can be used to structurally support the anode contact structure 108, the solar cell 104, and the TCO structure 109, each of which can be formed on the substrate 101 using thin film technology. The anode contact structure 108 can include any material suitable for making electrical contact with the solar cell anode, including, for example, molybdenum (Mo). The TCO structure 109 can operate as the cathode contact structure of the solar cell 104.
The solar cell 104 can receive light through the TCO structure 109. In some implementations, the solar cell 104 has a thickness in the range of about 1 μm to about 10 μm. The solar cell 104 shown in
The photovoltaic device 110 of
The electron blocking layer 106 has been disposed along the path between the p-type CIGS layer 103a and the anode contact structure 108 so as to prevent electrons from reaching an interface between the solar cell 116 and the anode contact structure 108. In certain implementations, the electron blocking layer 106 includes at least one of ZnSe, CdxZn1-xTe, CdxZn1-xSe, CdTe1-xSx, CdTe1-xSex, CdSe1-xSx, HgTe1-xSx, and HgSe1-xSx. The electron blocking layer 106 can have any suitable thickness, such as a thickness in the range of about 1 nm to about 10 nm.
Additional details of the photovoltaic device 115 of
Although
The process 250 continues at a block 253, in which a cathode contact structure is provided. In certain implementations, the cathode contact structure is a TCO structure. In other implementations the cathode contact structure includes Ni, Al, Au, and/or Mo.
In an ensuing block 254, a p-type photovoltaic layer is provided adjacent the anode contact structure. In certain implementations, the p-type photovoltaic layer includes at least one of a-Si, copper indium gallium selenide (CuInxGa1-xSe2), and CdTe. In the block 255, an n-type photovoltaic layer is provided between the cathode contact structure and the p-type photovoltaic layer. The n-type photovoltaic layer can be, for example, CdS or n-type amorphous silicon.
The process 250 continues at a block 256, in which a hole blocking layer is provided between the p-type photovoltaic layer and the cathode contact structure. The p-type photovoltaic layer, the n-type photovoltaic layer, and the hole blocking layer can collectively form an inorganic solar cell. The hole blocking layer can provide an energy barrier to holes in the p-type photovoltaic layer from reaching an interface between the inorganic solar cell and the cathode contact structure provided in block 253. In some implementations, after block 256 other subsequent steps may also be performed.
The process 260 continues at a block 266, in which an electron blocking layer is provided between the n-type photovoltaic layer and the anode contact structure. The p-type photovoltaic layer, the n-type photovoltaic layer, and the electron blocking layer can collectively form an inorganic solar cell. The electron blocking layer can provide an energy barrier to electrons in the n-type photovoltaic layer from reaching an interface between the inorganic solar cell and the anode contact structure provided in block 263. By providing a barrier for electrons in this manner, recombination of holes at the interface can be decreased. In certain implementations, the electron blocking layer can include a material having a relatively high optical transparency, a relatively low defect density, and a relatively high hole carrier concentration.
The method is illustrated as ending at 266, however, other subsequent steps may also be performed. Additionally, in certain implementations, the methods of
Various modifications to the implementations described in this disclosure may be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other implementations without departing from the spirit or scope of this disclosure. Thus, the disclosure is not intended to be limited to the implementations shown herein, but is to be accorded the widest scope consistent with the claims, the principles and the novel features disclosed herein. The word “exemplary” is used exclusively herein to mean “serving as an example, instance, or illustration.” Any implementation described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other implementations.
Certain features that are described in this specification in the context of separate implementations also can be implemented in combination in a single implementation. Conversely, various features that are described in the context of a single implementation also can be implemented in multiple implementations separately or in any suitable subcombination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a subcombination or variation of a subcombination.
Similarly, while operations are depicted in the drawings in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. In certain circumstances, multitasking and parallel processing may be advantageous. Moreover, the separation of various system components in the implementations described above should not be understood as requiring such separation in all implementations, and it should be understood that any described program components and systems can generally be integrated together in a single software product or packaged into multiple software products. Additionally, other implementations are within the scope of the following claims. In some cases, the actions recited in the claims can be performed in a different order and still achieve desirable results.
