Patent Application
20120180855
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 power efficiency. Moreover, there is a need for photovoltaic devices that can operate efficiently over a wide range of lighting conditions.
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 described in this disclosure can be implemented in a solar cell device including a transparent insulator, a thin film first solar subcell disposed on a first surface of the transparent insulator, and a thin film second solar subcell disposed on a second surface of the transparent insulator, the second surface on an opposite side of the transparent insulator than the first surface. The first solar subcell is configured to receive ambient light, and the second solar subcell is configured to receive a portion of light that propagates through the first solar subcell. The second solar subcell includes a first electrode including a conductive reflective layer configured to reflect light that propagates through a photovoltaic structure of the second subcell back toward the first solar subcell.
In some implementations, the first solar subcell is characterized by a first absorption spectrum and the second solar subcell is characterized by a second absorption spectrum different from the first absorption spectrum. According to some implementations, the transparent insulator prevents chemical reactions between the first and second solar sub cells.
Another innovative aspect of the subject matter described in this disclosure can be implemented in a solar power system including a stack of thin film solar subcells. The stack of thin film solar subcells includes an optically transparent insulator having a first side and an opposite second side, a thin film first solar subcell disposed on a first side of the insulator, and a thin film second solar subcell disposed on a second side of the insulator. The first solar subcell includes a first conductive layer defining a first electrical terminal, a first photovoltaic structure, and a second conductive layer defining a second electrical terminal. The first and second electrical terminals contact opposite sides of the first photovoltaic structure and are configured to provide electrical power generated by the first photovoltaic structure to an external circuit when the first solar subcell is illuminated with light. The second solar subcell includes a third conductive layer defining a third electrical terminal, a second photovoltaic structure, and a fourth conductive layer defining a fourth electrical terminal. The third and fourth electrical terminals contact opposite sides of the second photovoltaic structure and are configured to provide electrical power generated by the second photovoltaic structure when the second solar subcell is illuminated with light. The insulator is optically transparent to a portion of light absorbed by the second solar subcell.
Another innovative aspect of the subject matter described in this disclosure can be implemented in method of forming a thin film solar cell device. The method includes forming a first conductive layer on a first surface of a transparent substrate, forming a first photovoltaic structure over the first conductive layer, forming a second conductive layer over the first photovoltaic structure, forming a third conductive layer on a second surface of the transparent substrate, forming a second photovoltaic structure over the third conductive layer, and forming a fourth conductive layer over the second photovoltaic structure. The second surface is on an opposite side of the transparent substrate than the first surface.
Another innovative aspect of the subject matter described in this disclosure can be implemented in solar cell device including a transparent insulator, a first means for receiving ambient light, and a second means for receiving ambient light. The transparent insulator includes a first and second surface, the second surface on an opposite side of the transparent insulator than the first surface. The first light receiving means includes a thin film solar subcell disposed on the first surface of the transparent insulator. The second light receiving means includes a thin film second solar subcell disposed on the second surface of the transparent insulator and is configured to receive a portion of light that propagates through the first light receiving means. The second light receiving means includes a first reflective electrode configured to reflect light that propagates through the photovoltaic structure of the second light receiving means back toward the first light receiving means.
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 having a first photovoltaic subcell, a second photovoltaic subcell, and a transparent substrate are disclosed. The first photovoltaic subcell is disposed on a first surface of the transparent substrate and can receive light. The second photovoltaic cell is disposed on a second surface of the transparent substrate opposite to the first surface, and can receive a portion of light that passes through the first photovoltaic subcell. The first and second photovoltaic subcells each include separate electrodes for providing power or current to one or more loads, for example, to an electrical device, to an electrical power system which then provides power to other electrical devices, and/or to an electrical power storage system. By providing separate electrodes, the first and second subcells can be configured to electrically operate in parallel, thereby avoiding limiting the current of the photovoltaic device to the smaller of the photocurrents generated by the first or second subcells. In certain implementations, the second subcell can include a reflector which is positioned and configured to reflect light unabsorbed by the second photovoltaic subcell back toward the first photovoltaic subcell to increase the amount of power generated from a given amount of incident light (e.g., power efficiency) on the photovoltaic device.
Particular implementations of the subject matter described in this disclosure can be implemented to increase power efficiency of a photovoltaic device. Additionally, some implementations can be used to improve robustness of a photovoltaic device to variations in solar spectrum, such as variations that can occur at high altitude, on cloudy or overcast days, during winter or spring, and/or at dusk or dawn. Furthermore, according to some implementations, providing first and second photovoltaic subcells on opposite sides of a transparent substrate facilitates the manufacture of subcells having vastly different chemistries, thereby increasing flexibility in design of the photovoltaic device. Enhancing flexibility in the design of the photovoltaic device permits a broader selection of manufacturing materials for the first and second photovoltaic subcells, including materials having absorption spectrums that are more complimentary relative to existing tandem junction solar cells.
The first and second electrodes 4, 6 can be any suitable conductor. For example, the first and/or second electrodes 4, 6 can be 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 cell 10 can provide electrical connectivity to the p-n junction 2, while permitting light to pass through the first and/or second electrodes 4, 6 and reach the p-n junction 2. However, the first electrode 4 and/or the second electrode 6 need not be transparent. For example, the first electrode 4 can formed of an opaque material and can include one or more openings that provide a path for light to reach the p-n junction 2. Additionally, the second electrode 6 can be configured as a reflector to reflect light that passes through the first electrode 4 and the p-n junction 2 back toward the p-n junction 2.
The p-n junction 2 can be formed from a wide variety of materials, including, for example, silicon (Si), germanium (Ge), cadmium telluride (CdTe), and/or copper indium gallium (di)selenide (CIGS). The p-n junction 2 can operate as a photodiode 14, which can convert light energy into electrical energy or current. When the p-n junction 2 is illuminated with light, photons from the light can transfer energy to the p-n junction 2, which can result in the creation of electron-hole pairs. For example, photons having energy greater than the band-gap of the p-n junction 2 can generate electron-hole pairs within the p-n junction 2 by band-to-band excitation and/or high-energy photons can generate electron-hole pairs by impact ionization or via recombination-generation centers within the lattice of the p-n junction 2. When photons create electron-hole pairs within or near the depletion region of the p-n junction 2, the electric field of the depletion region can sweep the electrons and holes to the first and second electrodes of the photovoltaic cell 10, thereby generating a photocurrent. The photocurrent can be used to provide power to any suitable load, such as the illustrated load 12.
In certain implementations, the photovoltaic cell 10 can include the antireflective structure 8 disposed on a surface of the first electrode 4 opposite the p-n junction 2 (e.g., a incident light surface). The antireflective structure 8 can reduce the amount of light reflected off of the photovoltaic cell 10, thereby increasing the amount of light reaching the p-n junction 2 and the overall power efficiency of the cell.
The first photovoltaic subcell 20 includes a first photovoltaic structure 32, and includes first and second electrodes 36, 37 as electrical terminals of the first photovoltaic subcell 20. In this implementation, the first electrode 36 is disposed adjacent to the first surface of the transparent substrate 18, the first photovoltaic structure 32 is disposed adjacent to the first electrode 36, and the second electrode 37 is disposed adjacent to the first photovoltaic structure 32 opposite to the first electrode 36.
Similarly, the second photovoltaic subcell 22 includes a second photovoltaic structure 34, and third and fourth electrodes 38, 39 as electrical terminals of the second photovoltaic subcell 22. In this implementation, third electrode 38 is disposed adjacent to the second surface of the transparent substrate 18, the second photovoltaic structure 34 is disposed adjacent to the third electrode 38, and the fourth electrode 39 is disposed adjacent to the second photovoltaic structure 34 opposite to the third electrode 38.
The electrodes 36-39 of the first and second subcells 20, 22 can be similar to the first and second electrodes 4, 6 described above with reference to
The transparent substrate 18 can be a glass substrate or any other suitable transparent substrate, such as an optical plastic. The transparent substrate 18 can be employed to structurally support the first and second photovoltaic subcells 20, 22. Additionally, as will be described in detail below with reference to
The transparent substrate 18 can aid in chemically isolating the first and second subcells 20, 22 during manufacture, thereby permitting the subcells to include materials which can chemically interact when in contact and/or which are manufactured using different chemistries. For example, the transparent substrate 18 can include a relatively chemically inert material, such as glass or plastic, and can have a thickness sufficient to chemically isolate opposing sides of the transparent substrate, such as a thickness ranging between about 0.1 mm to about 10 mm. Accordingly, including the transparent substrate 18 can allow a wider selection of materials that can be used in forming the photovoltaic device 30 relative to certain other photovoltaic devices, for example conventional tandem junction cells, which can have material limitations due to certain chemical interactions between subcells and/or conflicting process requirements during manufacture. Accordingly, the first and second photovoltaic subcells 32, 34 can each be chosen from a wide selection of light absorbing photovoltaic materials, including, for example, crystalline silicon (c-silicon), amorphous silicon (a-silicon), cadmium telluride (CdTe), copper indium diselenide (CIS), copper indium gallium diselenide (CIGS), III-V semiconductors, and/or organics such as light absorbing small molecular weight dyes and polymers. The material for the photovoltaic structure can be chosen depending on the desired performance and the application of the photovoltaic device 30. For example, the first and second subcells 20, 22 can be formed from materials having absorption spectrums that are complimentary, as will be described in further detail below.
The first and second photovoltaic subcells 20, 22 of
With continuing reference to
In contrast, the first and second subcells 20, 22 of the illustrated photovoltaic device 30 can generate independent photocurrents, which can be combined and delivered to a load, thereby avoiding limiting the current of the photovoltaic device 30 to the smallest subcell photocurrent. For example, in certain implementations, when the first photovoltaic subcell 20 has a fill-factor FF1, an open-circuit voltage VOC1 and a photocurrent I1, and the second photovoltaic subcell 22 has a fill-factor FF2, an open-circuit voltage VOC2 and a photocurrent I2, the overall power P provided by the photovoltaic device 30 can be given by equation 1 below.
P=I
1
*V
OC1
*FF
1
+I
2
*V
OC2
*FF
2 (1)
The photovoltaic device 30 of
In
In one implementation, a photovoltaic device includes a first subcell and a second subcell, the first subcell having an absorption spectrum with a quantum efficiency greater than about 50% at a wavelength ranging between about 350 nm and about 600 nm, and the second subcell having an absorption spectrum with a quantum efficiency greater than about 50% at a wavelength ranging between about 600 nm and about 800 nm.
As described above with reference to
The first conductive layer can be any suitable conductor, including, for example, a transparent conductive oxide (TCO) structure such as tin oxide (SnO2), zinc oxide (ZnO) and/or indium tin oxide (ITO). Selecting the first conductive layer to be a transparent conductor, such as a TCO structure, can permit more light to pass through the first conductive layer relative to a scheme in which the layer is optically opaque and includes one or more openings for passing light. In one implementation, the first conductive layer has a thickness ranging between about 50 nm to about 5000 nm.
Formation of the first conductive layer may be carried out using deposition techniques, including, for example physical vapor deposition (PVD, e.g., sputtering), chemical vapor deposition (CVD), electrochemical vapor deposition (EVD), or pyrolysis. Forming the first conductive layer can include patterning the conductive layer to form desired electrical connectivity of the photovoltaic device. As used herein, and as will be understood by a person having ordinary skill in the art, the term “patterned” is used to refer to masking as well as etching processes.
The process 100 illustrated in
In a block 108, a second conductive layer is formed over the first photovoltaic structure. As will be described below, the second conductive layer can be configured to be transparent to ambient light. The second conductive layer can be, but need not be, similar to the first conductive layer formed in block 104. In one implementation, the second conductive layer has a thickness ranging between about 50 nm to about 5000 nm.
The first conductive layer, the first photovoltaic structure, and the second conductive layer collectively form a first photovoltaic subcell disposed on the first surface of the transparent substrate. The first and second conductive layers can operate as electrodes of the first photovoltaic subcell.
With continuing reference to
In a block 112, a second photovoltaic structure is formed over the third conductive layer. The second photovoltaic structure can be any of a wide variety of photovoltaic structures, including, for example, an amorphous silicon (a-Si) structure, a cadmium telluride/cadmium selenium (CdTe/CdS) structure, an organic structure, a copper indium gallium selenide (CIGS) structure, or any of the photovoltaic structures described earlier. The second photovoltaic structure can be formed using thin film processing techniques. Additionally, the characteristics of the second photovoltaic structure, such as the material composition, can be selected so that the second photovoltaic structure has an absorption spectrum that is complimentary to the absorption spectrum of the first photovoltaic structure, thereby enhancing the overall optical absorption of the photovoltaic device. In one implementation, the second photovoltaic structure has a thickness ranging between about 50 nm and about 10 μm.
The process 100 illustrated in
The first and second subcells 20, 22 can be formed using thin film techniques, such as deposition processes employing physical vapor deposition (PVD), chemical vapor deposition (CVD), electro-chemical vapor deposition (EVD) and/or plasma enhanced chemical vapor deposition (PE-CVD). The thin film photovoltaic subcells 20, 22 can include amorphous, monocrystalline, or polycrystalline materials, including, for example, silicon, copper indium selenide (CIS), cadmium telluride (CdTe) or copper indium gallium selenide (CIGS). Providing the first and second subcells 20, 22 on opposite surfaces of a transparent substrate permits formation of subcells that are electrically and chemically independent, thus permitting a broader selection of manufacturing materials and avoiding the need to form a tunnel junction between subcells. Additional details of the first and second subcells 20, 22 can be as described earlier.
The first photovoltaic subcell 20 includes a first transparent conductive oxide (TCO) structure 66 positioned adjacent to the first surface 59a of the glass substrate 61, a first photovoltaic structure 62 disposed adjacent to the first TCO structure 66, and a second TCO structure 67 positioned adjacent to the first photovoltaic structure 62 and on the opposite side of the first photovoltaic structure 62 than the first TCO structure 66. The first and second TCO structures 66, 67 can be configured as electrodes of the first photovoltaic subcell 20.
The second photovoltaic subcell 22 includes a third TCO structure 68 positioned adjacent to the second surface 59b of the glass substrate 61, a second photovoltaic structure 64 disposed adjacent to the third TCO structure 68, and a conductive reflector 69 positioned adjacent to the second photovoltaic structure 64 and on the opposite side of the second photovoltaic structure 64 as than the third TCO structure 68. The third TCO structure 68 and the conductive reflector 69 can be configured as electrodes of the second photovoltaic subcell 22.
The first photovoltaic structure 62 shown in
In the implementation illustrated in
Still referring to
To increase the overall amount of light absorbed by the photovoltaic device 60, the second photovoltaic subcell 22 can include the conductive reflector 69 for reflecting light back toward the first and second photovoltaic subcells 20, 22. The conductive reflector 69 can increase the overall efficiency of the photovoltaic device 60. For example, a portion of light 54c can pass through the first and second photovoltaic subcells 20, 22, and can thereafter be reflected by the conductive reflector 69 and absorbed by the second photovoltaic structure 64. Similarly, a portion of light 54d can pass through the first and second photovoltaic subcells 62, 64, and can thereafter be reflected by the conductive reflector 69 and absorbed by the first photovoltaic structure 62. Thus, the conductive reflector 69 can increase the efficiency of the photovoltaic device 60 by increasing the amount of light absorbed by the first and second photovoltaic subcells 20, 22.
The first photovoltaic subcell 20 includes a first transparent conductive oxide (TCO) structure 66 adjacent the first surface 59a of the glass substrate 62, a first photovoltaic structure 62 disposed adjacent the first TCO structure 66, and a second TCO structure 67 for receiving light and disposed adjacent the first photovoltaic structure 62 opposite the first TCO structure 66. The first and second TCO structures 66, 67 can operate as electrodes of the first photovoltaic subcell 20. The illustrated first photovoltaic structure 62 is a p-i-n junction including a p-type layer 63a, an intrinsic layer 63b, and an n-type layer 63c, as was described above with respect to
The second photovoltaic subcell 22 includes a third TCO structure 68 adjacent the second surface 59b of the glass substrate 61, a second photovoltaic structure 74 adjacent the third TCO structure 68, and a conductive reflector 69 adjacent the second photovoltaic structure 74 opposite the third TCO structure 68.
The second photovoltaic structure 74 shown in
The second photovoltaic subcell 22 includes a third TCO structure 68 adjacent the second surface 59b of the glass substrate 61, a second photovoltaic structure 76 adjacent the third TCO structure 68, and a conductive reflector 69 adjacent the second photovoltaic structure 76 opposite the third TCO structure 68.
The second photovoltaic structure 76 is an organic photovoltaic structure, such as a structure including polymers and/or small molecular weight dyes. In one implementation, the second photovoltaic structure has a thickness ranging between about 50 nm to about 1000 nm. As illustrated in
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 the 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.
