Patent Application
20130125969
This disclosure relates to photovoltaic devices.
For over a century, hydrocarbon fuels including 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. For example, 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 electronic 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 described in this disclosure can be implemented in a photovoltaic device including a substrate including a first surface configured to receive light and a second surface opposite the first surface. The substrate includes a first plurality of substrate features formed on the second surface of the substrate, the first plurality of features having a depth dimension in the range of about 10 μm to about 1000 μm and a width dimension in the range of about 10 μm to about 1000 μm. The photovoltaic device further includes a first electrode formed on the first plurality of substrate features, a thin film solar cell disposed on the first electrode layer, and a second electrode formed on the thin film solar cell. At least a portion of the shape of the first electrode, the thin film solar cell, and the second electrode conform to the shape of the first plurality of substrate features.
In some implementations, the second surface of the substrate includes a second plurality of substrate features having a width dimension in the range of about 1 nm to about 100 nm and a depth dimension of about 1 to about 100 nm.
In some implementations, the shape of the second electrode at least partially conforming to the shape of the first plurality of substrate features such that at least a portion of incident light that reflects from the second electrode propagates through the thin film solar cell at least twice.
Another innovative aspect of the subject matter described in this disclosure can be implemented in a photovoltaic device including a substrate including a first surface configured to receive light and a second surface opposite the first surface. The substrate includes first means for reflecting light formed on the second surface, the first reflecting means having a depth dimension in the range of 10 μm to 1000 μm and a width dimension in the range of 10 μm to 1000 μm. The photovoltaic device further includes a first electrode formed on the first reflecting means, a thin film solar cell disposed on the first electrode layer, and a second electrode formed on the thin film solar cell. At least a portion of the first electrode, the thin film solar cell, and the second electrode conform to the shape of the first plurality of substrate features.
Another innovative aspect of the subject matter described in this disclosure can be implemented in a method of forming a photovoltaic device. The method includes forming a plurality of substrate features on a surface of a glass substrate, the substrate features having a depth dimension in the range of about 10 μm to about 1000 μm and a width dimension in the range of about 10 μm to about 1000 μm. The method further includes forming a thin film solar cell over the surface of the glass substrate including over the plurality of substrate features.
In some implementations, forming the plurality of substrate features includes etching the plurality of substrate features into the substrate using sand blasting.
In some implementations, forming the plurality of substrate features includes partially forming through-glass vias.
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.
In some implementations described herein, photovoltaic devices including a substrate, such as a glass substrate, and a thin film solar cell are disclosed. The glass substrate can have a first surface for receiving light and a second surface including substrate features over which the thin film solar cell is formed. The geometry of the substrate features can be designed to capture reflected light by configuring substrate features to include sloped sides for directing reflected light towards other portions of the thin film solar cell. The geometry of the substrate features can also increase the optical path length of light through the solar cell. The substrate features can have a depth dimension in the range of about 10 μm to about 1000 μm and a width dimension in the range of about 10 μm to about 1000 μm, and in some implementations, the substrate features can be formed using a through-glass via process. The first surface of the glass substrate can also be configured to operate as a boundary for total internal reflection (TIR) of a portion of light reflected off of the thin film solar cell, thus improving the efficiency of the photovoltaic device by reducing the amount of light escaping the photovoltaic device through reflections. In some implementations, the surface of the substrate features can be roughened to include features having a width dimension in the range of about 1 nm to about 100 nm. The roughened surface can help diffuse light incident on the solar cell, which can improve efficiency of the photovoltaic device by reducing the amount of light that escapes the photovoltaic device without being absorbed by the solar cell.
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 increase the optical path length of light through a solar cell so as to increase the probability that the light is absorbed by the solar cell without needing to increase electrical thickness. For example, certain implementations increase the angle of incidence of light reaching the solar cell, thereby increasing the distance light travels through the solar cell while avoiding electrical losses associated with increasing the physical thickness of the solar cell, such as recombination losses associated with charge carriers of opposite polarity recombining. Furthermore, some implementations can be used to reduce the amount of light that is reflected off of a photovoltaic device.
The transparent substrate 1 can be a glass substrate or any other suitable transparent substrate, such as an optical plastic. The transparent substrate 1 can be employed to structurally support the first and second electrodes 4 and 6 and the solar cell 2. The transparent substrate 1 includes a first surface configured to receive light, and a second surface over which the first electrode 4, the solar cell 2, and the second electrode 6 are formed.
The first and second electrodes 4 and 6 can be any suitable conductor. In some implementations, the first electrode 4 can include a transparent conductor, such as a transparent conducting oxide (TCO). Using a TCO or other transparent conductor for the first electrode 4 can provide electrical connectivity to the solar cell 2, while permitting light to pass through the first electrode 4 and reach the solar cell 2. However, the first electrode 4 need not be transparent, such as in configurations in which the first electrode 4 is formed of an opaque material and includes one or more openings that provide a path for light to reach the solar cell 2. In certain implementations, the second electrode 6 is formed from an opaque material, such as a reflector configured to reflect light back toward the solar cell 2. However, other configurations are possible.
Although not illustrated in
The solar cell 2 can be formed from a wide variety of materials, including, for example, silicon (Si), germanium (Ge), cadmium telluride (CdTe), copper indium gallium (di)selenide (CIGS), III-V semiconductors, and/or organics such as light absorbing small molecular weight dyes and polymers. The solar cell 2 can operate as a photodiode 14, which can convert light energy into electrical energy or current. When the photovoltaic device 10 is illuminated with light, photons from the light can transfer energy to the 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 solar cell 2 can generate electron-hole pairs within the 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 solar cell 2.
When photons create electron-hole pairs within or near a depletion region of the solar cell 2, the electric field of the depletion region can sweep the electrons and holes to the first and second electrodes 4 and 6 of the photovoltaic device 10, thereby generating a photocurrent. The electron-hole pairs can also move via diffusion through the photovoltaic device 10. The generated photocurrent can be used to provide power to a load 12, which can be, for example, a load associated with a solar panel.
The optical path length of light through the solar cell 2 can impact the efficiency of the photovoltaic device 10. For example, if the optical path length through the solar cell 2 is relatively long, the probability that light is absorbed by the solar cell 2 can increase. However, increasing the optical path length of light through the solar cell 2 using certain techniques, such as increasing the thickness of the n-type and/or p-type photovoltaic layers 3a and 3b (or an intrinsic layer in p-i-n junction configurations) can increase recombination losses associated with the solar cell 2 and degrade performance. Light reflections can also impact the efficiency of the photovoltaic device 10. For example, the efficiency of the photovoltaic device 10 can be limited by light reflected off of the photovoltaic device 10, such as light reflected off the first electrode 4 and/or light reflected off the solar cell 2. Although antireflective coatings or other structures can be used to reduce reflections, there remains a need for improving the efficiency of the photovoltaic device 10.
In some implementations, the substrate features 23 can each have a depth selected to be in the range of about 10 μm to about 1000 μm and a width in the range of about 10 μm to about 1000 μm. In the configuration illustrated in
The substrate features 23 can be formed on the second surface 22 of the glass substrate 20 using a variety of processes. For example, the substrate features 23 can be formed using sand blasting, laser blasting, and/or chemical etching. Although the substrate features 23 in the illustrated configuration do not extend through the glass substrate 20, in some implementations the substrate features 23 can be formed using a through-glass via process in which blasting or etching associated with the substrate features 23 is stopped before the substrate features 23 extend through the glass substrate 20. Accordingly, in some implementations the substrate features 23 can be partially-formed through-glass vias. For example, through-glass via processing can be performed to form a cavity in the second surface 22 and/or a relatively thick glass substrate can be used to prevent through-glass via formation, such as a glass substrate having a thickness in the range of about 200 μm to about 6 mm. Using a through-glass via process to form the substrate features 23 can reduce the cost of manufacturing the photovoltaic device.
In
In some implementations, the thin film solar cell 26 can include amorphous, monocrystalline, or polycrystalline materials, including, for example, silicon, copper indium selenide (CIS), CdTe, CIGS, dye-sensitized or other organic photovoltaics. In some implementations, the solar cell 26 is a thin film p-i-n junction solar cell.
In
Roughening one or more surfaces disposed between the first electrode 24 and the solar cell 26 can increase the diffusion of light within solar cell 26 and improve efficiency of the device. For example, roughening a surface of the first electrode 24 can diffuse light reaching the solar cell 26, thereby increasing the path length of light through the solar cell 26 and/or reducing the amount of light reflected off the photovoltaic device. In some implementations, a roughened surface disposed between the solar cell 26 and the first electrode 24 includes features having a width dimension in the range of about 1 nm to about 100 nm, and a depth dimension in the range of about 1 nm to about 100 nm.
Any suitable process can be used to roughen a surface between the solar cell 26 and the glass substrate 20. For example, in some implementations a surface of the first electrode 24 facing the solar cell 26 can be roughened using wet etch techniques or can be deposited with a rough surface by, for example, means of chemical vapor deposition (CVD) techniques. However, surface roughening can be achieved in other ways. For example, the first electrode 24 can be formed using a CVD process. Although
The photovoltaic device 30 can be configured to receive light through the first surface 21 of the glass substrate 20. For example, a first portion of light 32 normal to the first surface 21 of the glass substrate 20 can pass through the glass substrate 20 and reach the solar cell 26. As shown in
Forming the solar cell 26 over the substrate features 23a-23d can also improve the efficiency of the photovoltaic device 30 by increasing the amount of unabsorbed light that is reflected back toward the solar cell 26. For example, when the first portion of light 32 reaches a region of the solar cell 26 associated with the first substrate feature 23a, an unabsorbed portion of light 33 can be reflected off of the second electrode 28 toward a region of the solar cell 26 associated with the second substrate feature 23b. Thus, by shaping the second electrode 28 to at least partially conform to the shape of the substrate features 23a-23d, the unabsorbed portion of light 33 that reflects from the second electrode 28 propagates through the solar cell 26 at least twice.
The geometry of the substrate features 23a-23d helps improve the efficiency of the photovoltaic device 30 by reducing the amount of light that escapes the photovoltaic device 30 without being absorbed by the solar cell 26. For example, to reduce the probability that the first portion of light 32 reflects off the second electrode 28 and escapes the photovoltaic device 30 through the first surface 21 of the glass substrate 20, the side of the first substrate feature 23a has been sloped to direct the first portion of light 32 toward the second substrate feature 23b when it is unabsorbed. In one implementation, the substrate features 23a-23d have an angle of slope relative to the second surface 22 that is in the range of about 20 degrees to about 70 degrees. The slope of the substrate features 23a-23d can be selected based on a variety of considerations, including, for example, on the angular distribution of light reaching the photovoltaic device 30 under typical operating conditions. For example, in some implementations, the slope of the substrate features 23a-23d can be selected to increase the probability that light reaching the first surface 21 at the mean angle of incident is absorbed.
Forming the solar cell 26 over the substrate features 23a-23d can also result in a portion of reflected light undergoing total internal reflection in the glass substrate 20. For example, as shown in
Although the glass substrates 50, 51 and 52 of
Still referring to
Although
Although only eight examples of placement and organization of substrate features have been illustrated in
Although only four examples of substrate feature cross-sections have been illustrated in
The multi-layer glass substrate 80 includes a first glass layer 81, a second glass layer 82, a first surface 21, and a second surface 22. A surface of the first glass layer 81 defines the first surface 21 of the multi-layer glass substrate 80, and a surface of the second glass layer 82 defines the second surface 22 of the multi-layer glass substrate 80. The through-glass vias 83 extend from the second surface 22 of the multi-layer glass substrate 80 through the first glass layer 81. The through-glass vias 83 can be formed using a sand blasting process, a chemical etching process, a laser blasting process, and/or another suitable through-glass via process.
The second glass layer 82 can provide support to the first glass layer 81, and can aid in forming part of a support structure over which a solar cell structure can be formed. For example, a first electrode can be deposited over the second surface 22 of the multi-layer glass substrate 80 and can contact portions of the first and second glass layers 81, 82. Then a thin film solar cell can be deposited over the first electrode, followed by a second electrode. Details of some implementations of manufacturing a photovoltaic device on a glass substrate have been discussed above.
The first glass layer 81 can be attached to the second glass layer 82 in a variety of ways. For example, in some implementations the first glass layer 81 is attached to the second glass layer 82 using optically matching glues or fluids.
Although
In block 102, a plurality of substrate features are formed on a surface of a glass substrate. In some implementations, the substrate features can each have a depth selected to be in the range of about 10 μm to about 1000 μm and a width in the range of about 10 μm to about 1000 μm. The substrate features can have a variety of shapes when viewed from above the glass substrate, such as shapes that are circular, elliptical, hexagonal, rectangular and/or square. In some configurations, the substrate features are arranged in an ordered pattern, such as in an array, while in other configurations the substrate features are unordered.
In some implementations, the substrate features are formed using a through-glass via process, such as a sand blasting process, a chemical etching process, and/or a laser blasting process. In certain configurations, the substrate features do not extend through the glass substrate, and can be formed using a through-glass via process in which blasting or etching associated with the substrate features is stopped before through-glass via structures are formed. In other implementations, through-glass vias are formed in a first glass layer, and a second glass layer is provided adjacent the first glass layer to form a multi-layer glass substrate.
The process 100 continues at a block 104, in which a first electrode is formed over the plurality of substrate features. In some implementations, the first electrode can include a transparent conductor, including, for example, a TCO, such as SnO2, zinc oxide ZnO, and/or indium tin oxide ITO.
In some implementations, the glass substrate is roughened before forming the first electrode or the surface of the first electrode is roughened. For example, the surface of the glass substrate can be roughened using dry etching, wet etching, and/or sand blasting to include features having a height dimension in the range of about 1 nm to about 100 nm. In some implementations, the first electrode is roughened, such as by forming the first electrode using a CVD deposition and/or by etching or blasting the first electrode.
In an ensuing block 106, a thin film solar cell is formed over the first electrode and the plurality of substrate features. In some implementations, the solar cell includes at least one of a-Si, copper indium gallium selenide (CuInxGa1-xSe2), CdS, and CdTe.
The process 100 continues at a block 108, in which a second electrode is formed over the thin film solar cell and the plurality of substrate features. In some implementations the second electrode can include a reflective layer, such as aluminum (Al) or silver (Ag).
The method is illustrated as ending at block 108, however, other subsequent processes may also be performed.
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.
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.
