Patent Application
20140332075
1. Field of the Invention
The present invention relates generally to the field of optoelectronic transducers that convert optical energy into electrical energy, such as for example photovoltaic cells.
2. Description of the Related Art
For over a century fossil fuel such as coal, oil, and natural gas has 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 the 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 main cause of global warming. Thus there is an urgent 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 (PV) cells convert optical energy in to electrical energy and thus can be used to convert solar energy into electrical power. Photovoltaic solar cells can be made very thin and modular. PV cells can range in size from a few millimeters to 10's of centimeters. The individual electrical output from one PV cell may range from a few milliwatts to a few Watts. Several PV cells may be connected electrically and packaged to produce sufficient amount of electricity. PV cells can be used in wide range of applications such as providing power to satellites and other spacecraft, providing electricity to residential and commercial properties and charging automobile batteries. However, the use of solar energy as an economically competitive source of renewable energy is hindered by low efficiency in converting light energy into electricity.
What is needed therefore are photovoltaic devices and methods that provide increased efficiency in converting optical energy into electrical energy.
Certain embodiments of the invention include interferometrically tuned photovoltaic cells wherein reflection from interfaces of layered PV devices coherently sum to produce an increased electric field in an active region of the photovoltaic cell where optical energy is converted into electrical energy. Such interferometrically tuned or interferometric photovoltaic devices (iPVs) increase the absorption of optical energy in the active region of the interferometric photovoltaic cell and thereby increase the efficiency of the device. In various embodiments, one or more optical resonant cavities and/or optical resonant layers are included in the photovoltaic device to increase the electric field concentration and the absorption in the active region. The optical resonant cavities and/or layers may comprise transparent non-conducting materials, transparent conducting material, air gaps, and combinations thereof. Other embodiments are also possible.
In one embodiment, a photovoltaic device comprises an active layer configured to produce an electrical signal as a result of light absorbed by the active layer. A reflector layer is disposed to reflect light transmitted through the active layer; and an optical resonance cavity is disposed between the active layer and the reflector layer. The presence of the optical resonance cavity can increase the amount of light absorbed by the active layer. In some embodiments, the optical resonance cavity may comprise a dielectric. In some embodiments, the optical resonance cavity may comprise an air gap. In certain embodiments, the optical resonance cavity may comprise a plurality of layers.
In another embodiment, a photovoltaic device comprises at least one active layer configured to produce an electrical signal as a result of light absorbed by the active layer. The photovoltaic device also comprises at least one optical resonance layer, wherein the at least one active layer has an absorption efficiency for wavelengths in the solar spectrum, and the absorption efficiency integrated over the wavelengths in the solar spectrum increases by at least about 20% with the presence of the at least one optical resonance layer.
In one embodiment, a photovoltaic device comprises an active layer configured to produce an electrical signal as a result of light absorbed by the active layer. The photovoltaic device also comprises at least one optical resonance layer, wherein the photovoltaic device has an overall conversion efficiency for wavelengths in the solar spectrum, and the overall conversion efficiency integrated over the wavelengths in the solar spectrum increases by at least about 15% by the presence of the at least one optical resonance layer.
In another embodiment, a photovoltaic device comprises an active layer configured to produce an electrical signal as a result of light absorbed by the active layer. The photovoltaic device further comprises an optical resonance layer, the optical resonance layer having a thickness such that the photovoltaic device has an overall conversion efficiency integrated over the solar spectrum that is greater than 0.7.
In one embodiment, a photovoltaic device comprises an active layer configured to produce an electrical signal as a result of light absorbed by the active layer. The photovoltaic device further comprises at least one optical resonant layer that increases the average electric field intensity in the active layer, wherein the active layer has an average electric field intensity therein for wavelengths in the solar spectrum when the photovoltaic device is exposed to sunlight. The presence of the at least one optical resonant layer produces an increase in the average electric field intensity integrated over the solar spectrum that is greater for the active layer than the increase in average electric field intensity integrated over the solar spectrum for any other layers in the photovoltaic device.
In one embodiment, a photovoltaic device comprises an active layer configured to produce an electrical signal as a result of light absorbed by the active layer. The active layer has an average electric field intensity and absorbed optical power therein for wavelengths in the solar spectrum when the photovoltaic device is exposed to sunlight. The photovoltaic device further comprises at least one optical resonant layer that increases the average electric field intensity and absorbed optical power in the active layer, wherein the presence of the at least one optical resonant layer produces an increase in the absorbed optical power integrated over the solar spectrum that is greater for the active layer than the increase in absorbed optical power integrated over the solar spectrum for any other layers in the photovoltaic device.
In one embodiment, a photovoltaic device comprises a substrate; an optical stack disposed on the substrate; and a reflector layer disposed on the optical stack. The optical stack further comprises at least one active layer and one or more layers; wherein the at least one active layer comprises an absorption efficiency greater than 0.7 for light at approximately 400 nm.
In one embodiment, a method of increasing light absorption inside an active layer in a photovoltaic device using interference principles comprises providing at least one active layer for absorbing light and converting it into electrical energy; and positioning at least one optically resonant layer with respect to the active layer, wherein interference principles of electromagnetic radiation increases absorption of solar energy in the at least one active layer by at least 5%, the absorption being integrated for wavelengths in the solar spectrum.
In certain embodiment, a photovoltaic device comprises at least one active layer for absorbing electromagnetic radiation and converting it into electrical energy. The photovoltaic device further comprises at least one optically resonant layer disposed with respect to the active layer, wherein the optical resonance layer increases absorption of solar energy in the at least one active layer by at least 5% as a result of optical interference, the absorption being integrated across the solar spectrum.
In one embodiment, a photovoltaic device comprises an active layer configured to produce an electrical signal as a result of light absorbed by the active layer. A reflector layer is disposed to reflect light transmitted through the active layer, the reflector layer being partially optically transmissive such that the photovoltaic device is partially transmissive for some wavelengths. The photovoltaic device further comprises at least one optical resonance layer disposed between the active layer and the reflector layer, the presence of the at least one optical resonance layer increasing the amount of light absorbed by the active layer.
In one embodiment, a photovoltaic device comprises an active layer configured to produce an electrical signal as a result of light absorbed by the active layer. The photovoltaic device further comprises at least one optical resonance layer, the presence of the at least one optical resonance layer increasing the amount of light absorbed by the active layer, wherein the thickness of the at least one optical resonance layer is adjustable with application of a control signal for controlling the thickness.
In one embodiment, a method of optimizing absorption efficiency of a photovoltaic cell comprises providing a photovoltaic cell comprising a stack of layers, wherein at least one layer comprises at least one active layer, wherein providing a photovoltaic cell comprises using interference principles to optimize absorption efficiency of the at least one active layer in the photovoltaic cell at a plurality of wavelengths.
In one embodiment, a photovoltaic comprises a substrate; an optical stack disposed on the transparent substrate; and a reflector disposed on the substrate. The optical stack further comprises one or more thin film layers and an active layer optimized for absorbing a selected wavelength of light based upon a thickness of the one or more thin film layers, wherein the absorption of the active layer is optimized via an analysis of coherent summation of reflections from a plurality of interfaces.
In one embodiment, a photovoltaic device comprises first and second active layers configured to produce an electrical signal as a result of light absorbed by the active layers. The photovoltaic device further comprises a first optical resonance layer between the first and second active layers, the presence of the optical resonance layer increasing the amount of light absorbed by at least one of the first and second active layers.
In one embodiment, a photovoltaic device comprises a means for absorbing light. The light absorbing means is configured to produce an electrical signal as a result of light absorbed by the light absorbing means. The means for reflecting light is disposed to reflect light transmitted through the at least one light absorbing means. The means for producing optical resonance is disposed between the light absorbing means and the light reflecting means. The optical resonance producing means is configured to increase the amount of light absorbed by the at least one light absorbing means, wherein the optical resonance producing means comprises means for electrically insulating.
In another embodiment, a method of manufacturing a photovoltaic device comprises providing an active layer, the active layer configured to produce an electrical signal as a result of light absorbed by the active layer. The method further comprises disposing a reflector layer to reflect light transmitted through the active layer; and disposing an optical resonance cavity between the active layer and the reflector layer. In one embodiment, the optical resonance cavity comprises a dielectric. In another embodiment, the optical resonance cavity comprises an air gap.
In one embodiment, a photovoltaic device comprises means for absorbing light. The light absorbing means is configured to produce an electrical signal as a result of light absorbed by the light absorbing means. The photovoltaic device further comprises means for reflecting light disposed to reflect light transmitted through the light absorbing means and means for producing optical resonance between the light absorbing means and the light reflecting means. The optical resonance producing means is configured to increase the amount of light absorbed by the at least one light absorbing means, wherein the optical resonance producing means comprising a plurality of means for propagating light therethrough.
In another embodiment, a method of manufacturing a photovoltaic device comprises providing an active layer, the active layer configured to produce an electrical signal as a result of light absorbed by the active layer. The method further comprises disposing a reflector layer to reflect light transmitted through the at least one active layer; and forming an optical resonance cavity between the active layer and the reflector layer, wherein the optical resonance cavity comprises a plurality of layers.
In an alternate embodiment, a means for converting light energy into electrical energy comprises means for absorbing light, the light absorbing means being configured to produce an electrical signal as a result of light absorbed by the light absorbing means. The means for converting light energy into electrical energy further comprises means for reflecting light disposed to reflect light transmitted through the at least one light absorbing means; and means for producing optical resonance disposed between the light absorbing means and the light reflecting means, wherein the light absorbing means has an absorption efficiency for wavelengths in the solar spectrum, and the absorption efficiency integrated over the wavelengths in the solar spectrum increases by at least about 20% with the presence of the optical resonance producing means.
In one embodiment, a method of manufacturing a photovoltaic device comprises providing at least one active layer, the active layer being configured to produce an electrical signal as a result of light absorbed by the active layer. The method further comprises disposing a reflector layer to reflect light transmitted through the at least one active layer and disposing at least one optical resonance layer between the active layer and the reflector layer, wherein the at least one active layer has an absorption efficiency for wavelengths in the solar spectrum, and the absorption efficiency integrated over the wavelengths in the solar spectrum increases by at least about 20% with the presence of the at least one optical resonant layer.
In one embodiment, a means for converting light energy into electrical energy comprises means for absorbing light, the light absorbing means configured to produce an electrical signal as a result of light absorbed by the light absorbing means. The means for converting light energy into electrical energy further comprises means for reflecting light disposed to reflect light transmitted through the at least one light absorbing means; and means for producing optical resonance disposed between the light absorbing means and the light reflecting means. The means for converting light energy into electrical energy has an overall conversion efficiency for wavelengths in the solar spectrum, and the overall conversion efficiency integrated over the wavelengths in the solar spectrum increases by at least about 15% with the presence of the optical resonance producing means.
In one embodiment, a method of manufacturing a photovoltaic device comprises providing an active layer, the active layer configured to produce an electrical signal as a result of light absorbed by the active layer. The method further comprises disposing a reflector layer to reflect light transmitted through the at least one active layer; and disposing at least one optical resonance layer between the at least one active layer and the reflector layer. The photovoltaic device has an overall conversion efficiency for wavelengths in the solar spectrum, and the overall conversion efficiency integrated over the wavelengths in the solar spectrum increases by at least about 15% with the presence of the at least one optical resonant layer.
In one embodiment, a means for converting light energy into electrical energy comprises means for absorbing light, the light absorbing means configured to produce an electrical signal as a result of light absorbed by the light absorbing means. The means for converting light energy into electrical energy further comprises means for producing optical resonance, wherein the optical resonance producing means increases the average electric field intensity in the light absorbing means. The light absorbing means has an average electric field intensity for wavelengths in the solar spectrum therein when the means for converting light energy into electrical energy is exposed to sunlight. The presence of the optical resonance producing means produces an increase in the average electric field intensity integrated over the solar spectrum that is greater for the light absorbing means than the increase in average electric field intensity integrated over the solar spectrum for any other layers in the means for converting light energy into electrical energy.
In one embodiment a method of manufacturing a photovoltaic device comprises providing an active layer, the active layer configured to produce an electrical signal as a result of light absorbed by the active layer. The method further comprises providing at least one optical resonance layer, wherein the optical resonance cavity increases the average electric field intensity in the active layer. The active layer has an average electric field intensity for wavelengths in the solar spectrum therein when the photovoltaic device is exposed to sunlight, and the presence of the at least one optical resonance layer produces an increase in the average electric field intensity integrated over the solar spectrum that is greater for the active layer than the increase in average electric filed intensity integrated over the solar spectrum for any other layers in the photovoltaic device.
In another embodiment, a means for converting light energy into electrical energy comprises means for absorbing light configured to produce an electrical signal as a result of light absorbed by the light absorbing means, the light absorbing means having an average electric field intensity and absorbed optical power therein for wavelengths in the solar spectrum when the means for converting light energy into electrical energy is exposed to sunlight. The means for converting light energy into electrical energy further comprises means for producing optical resonance which increases the average electric field intensity and absorbed optical power in the light absorbing means, wherein the presence of the optical resonance producing means produces an increase in the absorbed optical power integrated over the solar spectrum that is greater for the light absorbing means than the increase in absorbed optical power integrated over the solar spectrum for any other layers in the means for converting light energy into electrical energy.
In one embodiment, a method of manufacturing a photovoltaic device comprises providing an active layer, the active layer configured to produce an electrical signal as a result of light absorbed by the active layer, the active layer having an average electric field intensity and absorbed optical power for wavelengths in the solar spectrum therein when the photovoltaic device is exposed to sunlight. The method further comprises providing at least one optical resonance layer, wherein the optical resonance cavity increases the average electric field intensity and absorbed optical power in the active layer, wherein the presence of the at least one optical resonance layer produces an increase in the absorbed optical power integrated over the solar spectrum that is greater for the active layer than the increase in absorbed optical power integrated over the solar spectrum for any other layers in the photovoltaic device.
In one embodiment, a photovoltaic device comprises a means for supporting. The photovoltaic device further comprises a means for interacting with light disposed on the supporting means, the light interacting means comprising at least one means for absorbing light and one or more means for propagating light. The photovoltaic device also comprises a means for reflecting light disposed on the light interacting means, wherein the at least one light absorbing means comprises an absorption efficiency greater than 0.7 for light at approximately 400 nm.
In one embodiment, a method of manufacturing a photovoltaic device comprises providing a substrate. The method also comprises disposing an optical stack on the substrate, the optical stack comprising at least one active layer and one or more layers; and disposing a reflector layer on the optical stack, wherein the at least one active layer comprises an absorption efficiency greater than 0.7 for light at approximately 400 nm.
In certain embodiment, a photovoltaic device comprises means for absorbing light, the light absorbing means configured to absorb light and convert the absorbed light into electrical energy. The photovoltaic device further comprises means for producing optical resonance, wherein interference principles of electromagnetic radiation increases absorption of solar energy in the light absorbing means by at least 5%, the absorption being integrated for wavelengths in the solar spectrum.
In certain embodiment, a photovoltaic device comprises means for absorbing light configured to produce an electrical signal as a result of light absorbed by the means for absorbing light. The photovoltaic device further comprises a means for reflecting light disposed to reflect light transmitted through the at least one light absorbing means; and means for producing optical resonance between the light absorbing means and the light reflecting means, the presence of the optical resonance producing means increasing the amount of light absorbed by the light absorbing means, wherein the reflecting means is partially optically transmissive such that the means for converting light energy into electrical energy is partially transmissive for some wavelengths.
In one embodiment, a method of manufacturing a photovoltaic device comprises forming an active layer configured to produce an electrical signal as a result of light absorbed by the active layer; forming a reflector layer disposed to reflect light transmitted through the at least one active layer; and forming at least one optical resonance layer between the active layer and the reflector layer, the presence of the at least one optical resonance layer increasing the amount of light absorbed by the active layer, wherein the reflector layer is partially optically transmissive such that the photovoltaic device is partially transmissive for some wavelengths.
In certain embodiment, a photovoltaic device comprises means for absorbing light configured to produce an electrical signal as a result of light absorbed by the light absorbing means. The photovoltaic device further comprises means for reflecting light disposed to reflect light transmitted through the at least one light absorbing means; and means for producing optical resonance disposed between the light absorbing means and the light reflecting means, the presence of the optical resonance producing means increasing the amount of light absorbed by the light absorbing means, wherein the thickness of the optical resonance producing means is adjustable with application of a control signal for controlling the thickness.
In one embodiment, a method of manufacturing a photovoltaic device comprises forming at least one active layer configured to produce an electrical signal as a result of light absorbed by the active layer. The method further comprises forming a reflector layer disposed to reflect light transmitted through the at least one active layer and forming at least one optical resonance layer between the at least one active layer and the reflector layer, the presence of the at least one optical resonance layer increasing the amount of light absorbed by the active layer, wherein the thickness of the at least one optical resonance layer is adjustable with application of a control signal for controlling the thickness.
In one embodiment, a photovoltaic device comprises first and second means for absorbing light configured to produce an electrical signal as a result of light absorbed by the first and second light absorbing means. The photovoltaic device further comprises first means for producing optical resonance. The presence of the first optical resonance producing means increasing the amount of light absorbed by the first and second light absorbing means.
In one embodiment, a method of manufacturing a photovoltaic device comprises forming first and second active layers configured to produce an electrical signal as a result of light absorbed by the first and second active layers and forming a first optical resonance layer, the presence of the first optical resonance layer increasing the amount of light absorbed by the first and second active layer.
Example embodiments disclosed herein are illustrated in the accompanying schematic drawings, which are for illustrative purposes only.
The following detailed description is directed to certain specific embodiments of the invention. However, the invention can be embodied in a multitude of different ways. In this description, reference is made to the drawings wherein like parts are designated with like numerals throughout. As will be apparent from the following description, the embodiments may be implemented in any device that comprises a photovoltaic material. MEMS devices may be coupled to photovoltaic devices as described herein below.
An optically transparent dielectric film or layer such as shown in
A ray of light 103 that is incident on surface 101 of the optical resonant cavity is partially reflected (e.g., due to Fresnel reflection) as indicated by the light path 104 and partially transmitted through surface 101 along light path 105. The transmitted light may be partially reflected (e.g., again due to Fresnel reflection) along light path 107 and partially transmitted out of the resonant cavity along light path 106. The amount of light transmitted and reflected may depend on the refractive indices of the material comprising the optical resonant cavity and of the surrounding medium.
For purposes of the discussions provided herein, the total intensity of light reflected from the optical resonant cavity is a coherent superposition of the two reflected light rays 104 and 107. With such coherent superposition, both the amplitude and the phase of the two reflected beams contribute to the aggregate intensity. This coherent superposition is referred to as interference. Generally, the two reflected rays 104 and 107 may have a phase difference with respect to each other. In some embodiments, the phase difference between the two waves may be 180 degrees and cancel each other out. If the phase and the amplitude of the two light rays 104 and 107 are configured so as to reduce the intensity, then the two light beams are referred to as interfering destructively. If on the other hand, the phase and the amplitude of the two light beams 104 and 107 are configured so as to increase the intensity, then the two light rays are referred to as interfering constructively. The phase difference depends on the optical path difference of the two paths, which depends both on the thickness of the optical resonator cavity and the index of refraction and thus the material between the two surface 101 and 102. The phase difference is also different for different wavelengths in the incident beam 103. Accordingly, in some embodiments the optical resonant cavity may reflect a specific set of wavelengths of the incident light 103 while transmitting other wavelengths in the incident light 103. Thus, some wavelengths may interfere constructively and some wavelengths may interfere destructively. In general, the colors and the total intensity reflected and transmitted by the optical resonant cavity therefore depend on the thickness and the material comprising the optical resonant cavity. The reflected and transmitted wavelengths also depend on angle, with different wavelengths being reflected and transmitted at different angles.
In
In some embodiments, the dielectric (e.g. glass, plastic, etc.) between the top and bottom reflector layers 201, 202 may be replaced by an air gap. The optical interference cavity may reflect one or more specific colors of the incident light. The color or colors reflected by the optical interference cavity may depend on the thickness of the air gap. The color or colors reflected by the optical interference cavity may be changed by changing the thickness of the air gap.
In certain embodiments, the gap between the top and the bottom reflectors 201, 202 may be varied for example by a microelectromechanical systems (MEMS). MEMS include micro mechanical elements, actuators, and electronics. Micromechanical elements may be created using deposition, etching, and/or other micromachining processes that etch away or remove parts of substrates and/or deposited material layers or that add layers to form electrical and electromechanical devices. Such MEMS devices include interferometric modulators (“IMODs”) having an optical resonant cavity that can be adjusted electrically. As used herein, the term interferometric modulator or interferometric light modulator refers to a device that selectively absorbs and/or reflects light using the principles of optical interference regardless of whether or not the device can be adjusted or whether movement within the device is possible (e.g. static IMOD). In certain embodiments, an interferometric modulator may comprise a pair of conductive plates, one of which is partially reflective and partially transmissive and the other of which is partly or totally reflective. The conductive plates are capable of relative motion upon application of an appropriate electrical signal. In a particular embodiment, one plate may comprise a stationary layer deposited on a substrate and the other plate may comprise a metallic membrane separated from the stationary layer by an air gap. As described herein in more detail, the position of one plate in relation to another can change the optical interference of light incident on the interferometric modulator. In this manner, the color of light output by the interferometric modulator can be varied.
Using this optical interference cavity it is possible to provide at least two states. In one embodiment, for example, a first state comprises an optical interference cavity of a certain dimension whereby light of a selected color (based upon the size of the cavity) interferes constructively and is reflected out of the cavity. A second state comprises a visible black state produced either due to constructive and/or destructive interference of light, such that visible wavelengths are substantially absorbed.
In the embodiment shown as
To illustrate how an IMOD can produce dark output,
In
The reflected rays 403a, 404a, 405a, 406a and 407a may be transmitted out of each of the layers of the IMOD and may be finally transmitted out of the device as indicated in
As described with reference to
The description above is an approximation of the optical process. More details may be included in a higher order analysis. For example, as described above, only a single pass and the reflections generated were discussed above. Of course, light reflected from any of the layers may be again reflected backward toward another interface. Light may thus propagate multiple times within any of the layers including the optical resonant cavity 304. The effect of these additional reflections is not illustrated in
The IMOD stack 300 can be static. In a static IMOD stack, the thickness and the material of the various layers is fixed by the manufacture process. Some embodiments of a static IMOD stack include an air gap. In other embodiments, for example, instead of an air gap, the optical resonant cavity may comprise a dielectric or an ITO. The light output by the static IMOD stack 300, however, depends on the view angle, the wavelength of light incident thereon, and the interference condition at the viewing surface of the IMOD stack for that particular wavelengths incident thereon. By contrast, in a dynamic IMOD stack, the thickness of the optical resonant cavity 304 can be varied in real time using, for example, a MEMS engine, thereby altering the interference condition at the viewing surface of the IMOD stack. Similar to the static IMOD stack, the light output by the dynamic IMOD stack depends on the view angle, the wavelength of light, and the interference condition at the viewing surface of the IMOD stack.
In the “open” state, one or more frequencies of the incident light interfere constructively above the surface of the substrate 301 as described with reference to
Referring to
The spectral response of the various layers of the IMOD in the “closed” state for normally incident light viewed normal to the IMOD is shown in
Generally, the IMOD stack has a view angle dependency that may be taken into consideration during the design stage. More generally, the spectral response of the IMOD can depend on the angle of incidence and the view angle.
PV cells can have many different sizes and shapes, e.g., from smaller than a postage stamp to several inches across. Several PV cells can often be connected together to form PV cell modules that may be up to several feet long and a few feet wide. The modules can include electrical connections, mounting hardware, power-conditioning equipment, and batteries that store solar energy for use when the sun is not shining. Modules, in turn, can be combined and connected to form PV arrays of different sizes and power output. The size of an array can depend on several factors, such as the amount of sunlight available in a particular location and the needs of the consumer.
A photocell has an overall energy conversion efficiency (ii, “eta”) that may be determined by measuring the electrical power output from a photocell and the optical power incident on the solar cell and computing the ratio. According to one convention, the efficiency of the solar cell can be given by the ratio of the amount of peak electrical power in Watts produced by a photocell having 1 m2 of surface area that is exposed to the standard solar radiation (known as the “air mass 1.5”). The standard solar radiation is the amount of solar radiation at the equator at noon on a clear March or September equinox day. The standard solar radiation has a power density of 1000 watts per square meter.
A typical PV cell comprises an active region disposed between two electrodes and may include a reflector. The reflector may have a reflectivity of greater than 50%, 60%, 70%, 80%, 90% or more in some embodiments. The reflector may have lower reflectivity in other embodiments. For example, the reflectivity may be 10%, 20%, 30%, 40% or more. In some embodiments, the PV cell additionally comprises a substrate as well. The substrate can be used to support the active layer and electrodes. The active layer and electrodes, for example, may comprise thin films that are deposited on the substrate and supported by the substrate during fabrication of the photovoltaic device and/or thereafter. The active layer of a PV cell may comprise a semiconductor material such as silicon. In some embodiments, the active region may comprise a p-n junction formed by contacting an n-type semiconductor material 903 and a p-type semiconductor material 904 as shown in
The layers 903 and 904 are sandwiched between two electrodes that provide an electrical current path. The back electrode 905 can be formed of aluminum or molybdenum or some other conducting material. The back electrode can be rough and unpolished. The front electrode 901 is designed to cover a large portion of the front surface of the p-n junction so as to lower contact resistance and increase collection efficiency. In embodiments wherein the front electrode is formed of an opaque material, the front electrode may be configured to have holes or gaps to allow illumination to impinge on the surface of the p-n junction. In such embodiments, the front electrode can be a grid or configured in the shape of a prong or fingers. In some other embodiments, the electrodes can be formed from a transparent conductor, for example, transparent conducting oxide (TCO) such as tin oxide (SnO2) or indium tin oxide (ITO). The TCO can provide good electrical contact and conductivity and simultaneously be optically transmissive to the incoming light. In some embodiments, the PV cell can also comprise a layer of anti-reflective (AR) coating 902 disposed over the front electrode 901. The layer of AR-coating 902 can reduce the amount of light reflected from the surface of the n-type layer 903 shown in
When the surface of the p-n junction is illuminated, photons transfer energy to electrons in the active region. If the energy transferred by the photons is greater than the band-gap of the semiconducting material, the electrons may have sufficient energy to enter the conduction band. An internal electric field is created with the formation of the p-n junction. The internal electric field operates on the energized electrons to cause these electrons to move thereby producing a current flow in the external circuit 907. The resulting current flow can be used to power various electrical devices such as a light bulb 906 as shown in
The efficiency at which optical power is converted into electrical power corresponds to the overall efficiency as described above. The overall efficiency depends at least in part on the efficiency at which light is absorbed by the active layer. This efficiency, referred to herein by the absorption efficiency, ηabs, is proportional to the index of refraction, n, the extinction coefficient, k, and the square of the electric field amplitude, |E(x)|2, in the active layer shown by the relationship set forth below,
ηabs∝n×k×|E(x)|2
The value, n, is the real part of the complex index of refraction. The absorption or extinction coefficient k is generally the imaginary part of the complex index of refraction. The absorption efficiency, ηabs, can thus be calculated based on the material properties of the layer and the electric field intensity in the layer (e.g., active layer). The electric field intensity for a particular layer may be referred to herein as the average electric field intensity wherein the electric field is averaged across the thickness of the particular layer.
As described above, light absorbed in the active layer generates free carriers, e.g., electron hole pairs, that may be used to provide electricity. The overall efficiency or overall conversion efficiency depends in part on the efficiency at which these electrons and holes generated in the active material are collected by the electrodes. This efficiency is referred to herein as collection efficiency, ηcollection. Thus, the overall conversion efficiency depends on both the absorption efficiency, ηabs, and the collection efficiency, ηcollection.
The absorption efficiency ηabs and the collection efficiency ηcollection of the PV cell are dependent on a variety of factors. The thickness and material used for the electrode layers 901 and 905, for example, can affect both the absorption efficiency ηabs and the collection efficiency ηcollection simultaneously. Additionally, the thickness and the material used in the PV material 903 and 904 can affect the absorption and collection efficiencies.
The overall efficiency can be measured by placing probes or conductive lead to the electrode layers 901 and 905. The overall efficiency can also be calculated using a model of the photovoltaic device.
As used herein, these efficiencies are for standard solar radiation—air mass 1.5. Also, the electric field, absorption efficiencies, etc. may be integrated for wavelengths over the solar spectrum. The solar spectrum is well known and comprises the wavelengths of light emitted by the sun. These wavelengths include visible, UV, and infrared wavelengths. In some embodiments, the electric field, absorption efficiency, overall efficiency etc. are integrated over a portion of the solar spectrum, for example, over the visible range of wavelengths, infrared range of wavelengths or the ultraviolet wavelength range. In certain embodiments, the electric field, absorption efficiency, overall efficiency etc. are computed over smaller wavelength ranges e.g. ranges having a bandwidth of 10 nm, 100 nm, 200 nm, 300 nm, 400 nm, 500 nm or 600 nm, etc.
In some embodiments, the p-n junction shown in
The active region can be formed of a variety of light absorbing materials such as crystalline silicon (c-Silicon), amorphous silicon (α-silicon), cadmium telluride (CdTe), copper indium diselenide (CIS), copper indium gallium diselenide (CIGS), light absorbing dyes and polymers, polymers having light absorbing nanoparticles disposed therein, III-V semiconductors such as GaAs etc. Other materials may also be used. The light absorbing material where photons are absorbed and transfer energy for example to electrons is referred to herein as the active layer of the PV cell. The material for the active layer can be chosen depending on the desired performance and the application of the PV cell.
In some embodiments, the PV cell can be formed by using thin film technology. For example, in one embodiment, the PV cell may be formed by depositing a first layer of TCO on a substrate. A layer of active material (or light absorbing material) is deposited on the first TCO layer. A second TCO layer can be deposited on the layer of active material. In some embodiments, a layer of AR coating can be deposited over the second TCO layer. The layers may be deposited using deposition techniques such as physical vapor deposition techniques, chemical vapor deposition techniques, electro-chemical vapor deposition techniques etc. Thin film PV cells may comprise polycrystalline materials such as thin-film polycrystalline silicon, CIS, CdTe or CIGS. Some advantages of thin film PV cells are small device footprint and scalability of the manufacturing process, among others.
Comparing
In a conventional PV cell such as the one illustrated in
Additionally, in some conventional designs, the thickness of the electrode layer 1005 and the reflector layer 1006 is varied to minimize the total amount of light reflected from the PV cell. The assumption is that if light is not reflected from the PV cell, this light is absorbed and the overall efficiency of the photovoltaic device is increased. To this end, the surface of the reflector 1006 may be roughened to be more diffuse to reduce specular reflection from the reflector. These methods can potentially produce a PV cell that looks black. However, the above described methods directed to reducing reflection from the PV device and producing a PV cell that looks black alone may be insufficient to increase the absorption in the absorbing or active layer 1003 and thus may be insufficient to increase the efficiency of the photovoltaic device.
The success of such conventional approaches to increasing efficiency of the PV cell have been limited. As described above, however, interference principles can be used to “tune” the one or more layers in the PV device and optimize the PV cell such that more light can be absorbed by the absorbing layer 1003. For example, the principles of interference used in the design of IMODs can be applied to the fabrication of PV cell. Optical resonant cavities that produce electric field resonances in the active layer, can be included in the PV cell thereby increasing electric field strength and absorption in the active layer. As will be shown, for example, increasing absorption in the active layer (or light absorbing layer 1003) can be accomplished by replacing the second transparent electrode layer 1005 with an optical resonant cavity comprising an air gap or a transparent non-conducting dielectric such as SiO2. By replacing the transparent electrode layer 1005 with an optical resonant cavity, the reflection of the reflector is not necessarily enhanced, however, the optical resonant cavity comprises a low absorption layer that can interferometrically increase absorption in the active layer.
To demonstrate how the efficiency of a solar cell can be increased, a conventional solar cell design shown in
In
The optical resonant cavity (or layer) can be dynamic in some embodiments. As shown in
In general, an optical stack may comprise multiple layers wherein each interface between layers will reflect some portion of the incident light. In general, the interfaces also allow some portion of incident light to pass through (except maybe the last layer).
At the interfaces of the various layers, a portion of the incident radiation is reflected as well. For example, electric field Ej+1,l represents the portion of the electric field Ej+1,r that is reflected from the boundary of layers 1204 and 1205 and thus propagates toward the left of the drawing. Similarly the electric fields E′j,l; Ej,l; E′j−1,l and E1,l represent the waves propagating in the iPV device towards layer 1201. The reflected wave Er is given by a superposition of the waves reflected from the various layers of the iPV device. The electric fields going into and coming out of a given interface can be calculated using matrix methods and values for the reflection coefficient and the transmission coefficient for various interfaces and the phase due to traversing the layers. Once the electric fields in a given layer, e.g. the active layer, are known, the absorption therein may be determined. The time averaged magnitude of the Poynting vector or the time averaged energy flux (time-averaged power per unit of normal area) going into the absorber layer 1203 and coming out of e.g. the absorber layer, can be calculated. The total power absorbed by the absorber layer 1203 can thus be calculated by subtracting the amount of power going out of the absorber layer 1203 from the total power going into the absorber layer 1203. In various embodiments, the ratio of the time averaged magnitude of the Poynting vector going into the absorber layer 1203 to the time averaged magnitude of the Poynting vector coming out of the absorber layer 1203 can be increased to increase the efficiency of the iPV device.
The power absorbed in any layer of the iPV cell, e.g., the absorber layer, can depend on the entire iPV stack as described above. The amount of energy absorbed in any layer of the iPV cell is directly proportional to the refractive index n of the layer, the extinction coefficient k of the layer, the square of the electric field amplitude |E(x)|2 in the layer and the thickness of the layer, t. One approach to increasing or optimizing the energy absorption in the iPV device is to decrease the amount of energy absorbed in the layers surrounding the absorber layer and increase the amount of energy absorbed in the absorber layer. The amount of energy absorbed in the layers surrounding the absorber layer can be decreased, for example, by choosing materials with low n×k value, reducing the thickness of the surrounding layers or by decreasing the electric field strength in the surrounding layers or any combination of these approaches. For example, in one optimization method, the electric field in the absorber layer of the iPV cell can be increased using one or more of the following. A) The material and the thickness of the various layers of the iPV stack can be adjusted so the reflected and transmitted electric fields reaching the active layer interfere constructively. B) The electric field strength in the layers of the iPV device other than the active layer can be reduced, for example, as a result of at least in part from destructive interference. C) A material can be selected for the optical resonant cavity having a desirable or optimum refractive index n that provides, for example, appropriate phase shift and reflections, and a low index of refraction, n, and/or low extinction coefficient constant k, so that the optical resonant cavity has a low absorption for wavelengths corresponding to the band-gap of the active layer such that less light converted into electrical energy by the active layer is absorbed by optical resonant cavity. In some embodiments, the composition and the thickness of the optical resonant cavity may be such that the electric field in the absorber is increased, for example, for wavelengths having an energy equivalent to the band-gap of the active layer. D) More generally, materials wherein the product of refractive index n and extinction coefficient k is low, for example, for wavelengths having an energy equivalent to the band-gap of the active layer, may be used in those layers other than the active layer. By reducing the electric field strength in the layers of the iPV device other than the active layer and/or reducing the absorption using materials with low refractive index and/or extinction coefficient k value in those layers, a decrease in the energy absorption in all the layers except the active or absorber layer of the iPV device can be achieved. E) Materials with low n and/or k value and thus low absorption may also be used, in particular, in those layers other than the active layer where electric field strength is high.
To optimize the iPV device for increased absorption in the active or absorber layer, the thickness of the optical resonant cavity can be selected to, through interference effects, increase the intensity of light in the active region. In some embodiments, the thickness of the gap in the optical resonant cavity is selected or optimized during the design stage of the iPV cell by using modeling software and numerical routines. The thickness of the gap in the optical resonant cavity can also be varied dynamically in real time by further incorporating a MEMS engine or platform in the IMOD incorporated PV cell structure of
The method then moves to state 1306, wherein the parameters that are not constrained are selected or optimized to increase efficiency (e.g. absorption efficiency) of the active layer. In one embodiment, optimizing efficiency comprises identifying a maximum in efficiency based upon at least one design characteristic. In some embodiments, the efficiency can be optimized for a particular wavelength or a range of wavelengths (e.g. solar spectrum, visible spectrum, infrared spectrum, ultraviolet spectrum). The range may be at least 100 nm wide, 200 nm wide, 300 nm wide, 400 nm wide, 500 nm wide, 600 nm wide, etc. The process for increasing or optimizing absorption in a particular layer at a particular wavelength or wavelength range can involve a calculation based upon all or most of the layers in the optical stack. For certain embodiments, the precise thickness of each layered material may be calculated to increase or optimize the absorption in the active layer for a particular wavelength or a particular range of wavelengths.
In some embodiments, the layers comprise thin film layers. Accordingly, the layers are treated as thin films in the design process. “Thin films” can have a thickness less than or on the order of coherence length of the incident light, e.g. less than 5000 nm. For thin films, the phase of the light is considered in what is referred to as coherent superposition for determining the intensity levels resulting from multiple reflections. As described above, the absorption efficiency of the active layer can be optimized via an analysis of coherent summation of reflections from the plurality of interfaces of the iPV device. In some embodiments, such coherent summations are used to calculate the energy input and output from a given layer to determine the absorption in the layer, e.g., the active layer, and likewise the absorption efficiency thereof. Poynting vectors may be used in this process. Other steps in the method may also include the deletion of or replacement of layers within a conventional photovoltaic device.
In some embodiments, the overall efficiency is increased or optimized by increasing or optimizing the absorption efficiency, ηabs. As described above, however, the overall absorption efficiency, ηoverall, is dependent on both the efficiency at which light is absorbed in the active layer to form electron hole pairs, ηabs, and the efficiency of which the electron hole pairs are collected by the electrodes, ηcollection.
Interferometric principles can be used to increase or optimize the overall conversion efficiency ηoverall by increasing or optimizing one or both of the above defined parameters ηabs and ηcollection. For example, in some embodiments, the absorption efficiency ηabs can be optimized or maximized without taking into account the collection efficiency ηcollection. However, parameters varied to increase or optimize the absorption efficiency, ηabs, may also affect the collection efficiency, ηcollection. For example, the thickness of the electrodes or the thickness of the active layer may be altered to increase absorption in the active layer, however, this thickness adjustment may also impact the collection efficiency. Accordingly, in some embodiments an optimization can be performed such that both the collection efficiency, ηcollection, and the absorption efficiency, ηabs, are considered and/or optimized to achieve an increased or optimized overall efficiency ηoverall. In certain other embodiments, the absorption efficiency, ηabs, and the collection efficiency, ηcollection, can be optimized recursively to maximize the overall efficiency, ηoverall. Other factors may also be included in the optimization process. In some embodiments, for example, optimizing the overall efficiency of the iPV device can be based upon heat dissipation or absorption in one or more inactive layers.
The method then proceeds to state 1308, wherein the photovoltaic device is fabricated in accordance with the fabrication constraints and optimized elements. Once the designer has completed state 1308, the method terminates at an end state 1310. It will be understood that other steps may be included to improve or optimize a photovoltaic device.
The PV stack of
In general, layers may be included in the PV device that provide increased absorption in the active layer by appropriate selection of parameters, e.g., materials and dimensions, associated with these layers. One or more parameters of one of these layers may be adjusted while holding the parameters of other layers fixed, or, in certain embodiments one or more parameters of one or more layers may be adjusted to provide for increased absorption in the active layer. In some embodiments, one or more parameters of all the layers may be adjusted to obtain increased absorption in the active layer. In various embodiments, these parameters may be adjusted at the design stage, for example, by calculating the effects of different parameters on the absorption. Optimization procedures may be used. A range of other techniques may also be used to obtain values for the parameters that yield improved performance.
In some embodiments, the optical resonant layer 2506 and the optical resonant cavity 2503 may comprise electrode layers. In various embodiments, however, either or both the optical resonant layer 2506 and the optical resonant cavity 2503 may comprise a material with a low extinction (or absorption) coefficient k and/or low index of refraction, n that yield a low n×k value. One or both of the optical resonant layer 2506 and the optical resonant cavity 2503 may comprise, for example, a low n×k value. As described above, for example, the optical resonant cavity 2503 may comprise air or a dielectric such as SiO2 or an electrically conducting material such as a TCO, like ITO or ZnO. Other materials with low or approximately zero k may also be used so as to provide low n×k value. Still other materials are possible. Similarly, the optical resonant layer 2506 may comprise air, a dielectric material with a low extinction (or absorption) coefficient k; or an electrically conducting material such as a TCO, like ITO or ZnO; or any other material with low n×k value. Also, other materials may also be used.
In certain embodiments hybrid or composite structures are used for the optical resonant cavity and/or optical resonant layer. For example, the optical resonant cavity and/or optical resonant layer may comprise an air/dielectric, conductor/dielectric, air/conductor combination or mixture.
In the embodiment shown, the active layer of the PV cell comprises an n-type CDS layer 2505 and a p-type CIGS layer 2504. In other embodiments, the active layer may comprise other materials. The optical stack can be deposited on a substrate 2501 by using thin film fabrication techniques. The substrate 2502 may comprise glass or other suitable material. In some embodiments, a reflector 2502 may be deposited between the substrate and the remainder of the optical stack comprising the active layer surrounded by the optical resonant layer and optical resonant cavity. The reflector may be formed of Al, Mo or other reflecting material such as a metal or dielectric. In some embodiments, the reflector may comprise single or composite material.
The reflector 2502 of
As described above, the optical resonant cavity 2503 and the optical resonant layer 2506 may comprise TCO such as ITO or SnO2. In other embodiments, the optical resonant cavity and the optical resonant layer may comprise transparent dielectric material or an air gap or combination thereof. The materials used for the optical resonant cavity 2503 and the optical resonant layer 2506 need not be the same.
As described above, one or more parameters of one or more of the layers in these devices shown in
Parameters of different layers may be selected based on their spectral properties. For example, gold has a high extinction coefficient, k, in the wavelength region around red and has a relatively low extinction coefficient, k, in the wavelength region around blue. However, the refractive index n of gold is low in the wavelength region around red and high in the wavelength region around blue. As a result, the product n×k is low for gold in the wavelength region around red and high in the wavelength region around blue. Therefore, a reflector comprising gold will predominantly reflect wavelengths around red and absorb wavelengths around blue. Thus a reflector can be used to tune the absorption by choosing a material for the reflector that has a low n×k value in the wavelength range that corresponds to the useful optical absorption range of the active layer (where light is absorbed and converted into electrical power) and a high n×k value in wavelengths that are not in the useful optical absorption range of the active layer (for example, where optical energy is converted into heat, which may degrade the operation of the device). For example, if it is advantageous to not let blue light into the iPV device, then it may be desirable to form the reflector 1104 of gold. In some embodiments, the reflector material may be chosen so as to absorb infrared wavelengths.
Likewise, as described above, the selection of a particular gap distance will dictate whether a particular color is reflected by the reflector layer (for example, 1104 of
In a multi junction photovoltaic device, there are numerous approaches to optimize energy absorption in each of the junctions of the photovoltaic device. For example, one approach can be to dispose an optical resonant cavity between the combined stack of multi junction active layers (for example, 2706A-2706C) and the reflector 2708. Another approach can be to dispose an optical resonant layer between each active layer that forms the multi junction photovoltaic device and dispose an optical resonant cavity between the last active layer of the photovoltaic device and the reflector. These two approaches are described in detail below.
In some embodiments, the multi junction photodiode include less optical resonant layers than shown in
As described above, the composition and/or the thickness of each layer in the different embodiments of the photovoltaic device may be optimized in the design and fabrication stage using the methods described above to increase absorption in the active layers and decrease reflection. The iPV embodiments, for example, can be optimized using the IMOD design principles as described above. In some embodiments, a MEMS engine or platform can be provided to vary the thickness of the optical resonant cavities or layers in these embodiments dynamically while the iPV cell is in operation. The iPV embodiments described above can thus be improved as a result of interferometric effects. An increase in the absorption of energy in the PV absorber/active region may result in an increase in the overall efficiency of the iPV device.
The designs, however, are not truly optimal in every respect. For example, in those embodiments comprising a TCO layer in the optical resonant cavity, electrical losses may be negligible. However, the TCO may introduce some optical loss. The embodiments comprising air or SiO2 in the optical resonant cavity may exhibit a small decrease in the optical absorption due to the presence of vias. In some embodiments, the presence of vias for electrical connection may result in optical aperture loss.
In some embodiments of the iPV device, increased or optimized absorption efficiency in the active layer may not be necessarily dependent upon the orientation of the incident light with respect to the iPV device. For example, the absorption efficiency when the incident light is substantially normal to the iPV device can be approximately the same as the absorption efficiency when the incident light is at high grazing incidence (for example, approximately 89 degrees from the normal to the iPV device). The orientation of the photovoltaic cell thus need not be completely aligned for optimal absorption efficiency. Nevertheless, the angle of incidence does affect the intensity of light reaching the active layer and thus affects the energy available to be absorbed by the active layer; the less light reaching the photovoltaic cell, the less energy is present to be absorbed by the active layer. Thus, it should be emphasized that for a given area of the photovoltaic device, without active tracking (e.g., moving the photovoltaic to align with the path of the sun), the total absorbed energy diminishes, as the angle of incident θi increases, by a factor of cos(θi).
In some embodiments, however, where the absorption efficiency changes as a function of the angle of incidence, the iPV stack can be designed for particular angles of incidence using the IMOD principles and interferometric effects. For example, the thickness of the optical cavity can be adjusted to cause increased absorption of desired wavelengths of light incident on the device at non-normal angles. In some embodiments, the optical cavity may be variable (as opposed to fixed) so as to provide for different incident angles, for example, of the sun at different times of the day.
The principles described herein are applicable to both completely reflective (e.g., opaque) as well as transmissive PV devices.
To optimize the semi-transparent PV cell of
The multi junction PV shown in
A partially transmissive reflector layer may be used in other designs disclosed herein. For example, a partially optically transmissive reflector layer may be used in PV devices having a single active layer. Still other configurations are possible. As
Although in various embodiments described herein, the absorption in the active layer has been optimized, as described above, in certain embodiments, the overall efficiency can be increased or optimized by additionally considering the effects of other factors such as collection efficiency. For example, one or more parameters may be adjusted to increase the aggregate effect of both the absorption efficiency and the collection efficiency. In such embodiments, for example, the overall efficiency may be monitored in the optimization process. Other figures of merit, however, may also be used and may be incorporated in the optimization, design or manufacturing process.
As described above, the devices or systems in which the device is integrated may be modeled and calculations performed to assess the performance of the device or system. In some embodiments, the actual performance may be measured. For example, the overall efficiency may be measured by making electrical connection with the electrodes contacting the active layer. Electrical probes 3110 and 3112, for example, are shown in
A wide range of variations of the methods and structures described herein are possible.
Accordingly, in various embodiments described herein, the performance of photovoltaic devices may be improved using interferometric techniques. In some embodiments, an optical resonator cavity disposed between an active layer and a reflector may increase absorption in the active layer or layers. However, as described above, optical resonator layers located elsewhere may also provide an increase in absorption in one or more active layers and correspondingly increase efficiency. Thus, as described above, one or more parameters of one or more layers may be adjusted to increase, for example, the efficiency of the device in converting optical power into electrical power. These one or more layers may be the layers employed in conventional photovoltaic devices and not layers added to such structures to obtain improved performance. Accordingly, the optical resonant layers are not to be limited to layers added to a structure to obtain improvement. Additionally, the optical resonant layers are not limited to the layers described above, but may include any other layers that are tuned to provide increased absorption in the active layer using interferometric principles. The optical resonant layers or cavities can also have other functions such as operating as an electrode. The design or optimization may be implemented to increase absorption and efficiency in one or more active layers.
Additionally, although various techniques have been described above as providing for optimization, the methods and structures described herein are not limited to true optimal solutions. The techniques can be used to increase, for example, but not necessarily maximize, absorption in the active layer or overall optical efficiency of the device. Similarly, techniques can be used to decrease and not necessarily minimize absorption in layers other than the active layer. Similarly, the resultant structures are not necessarily the optimal result, but may nevertheless exhibit improved performance or characteristics.
The methods and structures disclosed herein, however, offer a wide range of benefits including performance advantages for some photovoltaic devices. For example, by using an optical resonant cavity or other optical resonant layers in the PV cell, the absorption efficiency of the photovoltaic device may be improved. In some embodiments, for example, the absorption efficiency of the active layer or layers increases by at least about 20% with the presence of at least one optical resonant cavity or layer. Here the absorption value is integrated over the wavelengths in the solar spectrum. In some other photovoltaic devices, the absorption efficiency integrated over the wavelengths in the solar spectrum can increase by at least 25%, 30%, 40%, 50%, 60%, 70%, 80%, 90% or more due to the presence of the optical resonant cavity or layer. In other embodiments, the increase may be 5% or more, 10% or more or 20% or more. For some embodiments, these values may apply when integrating over smaller wavelength ranges as well.
Accordingly interference principles can be applied to increase or optimize the efficiency of the active layer for one or more wavelengths. For example, at least one of the active layers may be configured to absorb light at wavelength of approximately 400 nm with an absorption efficiency greater than 0.7. At least one of the active layers may be configured to absorb light at wavelengths between 400 nm and 450 nm or between 350 nm and 400 nm with an absorption efficiency greater than 0.7. In some embodiments, the active layer or layers may be configured to absorb light between 350 nm and 600 nm with an absorption efficiency greater than 0.7. In other embodiments, the absorption efficiency can be increased or optimized for a single wavelength between 250 nm and 1500 nm, or alternately for a bandwidth of at least 50 nm, 100 nm or 500 nm in the wavelength range between 250 nm and 500 nm. For some embodiments, these values may apply when integrating over smaller wavelength ranges as well.
The overall efficiency of the photovoltaic device may increase as well. For example, in some photovoltaic devices the overall conversion efficiency integrated over the wavelengths in the solar spectrum can increase by at least 15%, 20%, 25% or 30%, 40%, 50%, 60%, 70%, 80%, 90% or more with suitable optical resonant layer or layers. In certain embodiments, the increase may be 5% or more or 10% or more. In some embodiments, the overall conversion efficiency of the photovoltaic device is greater than 0.7, 0.8, 0.9, or 0.95. In other embodiments, the overall conversion efficiency may be less. For example, the overall conversion efficiency may be at least 0.3, 0.4, 0.5, 0.6 or higher. In one embodiment, the overall conversion efficiency may be 0.1 or 0.2 or higher. For some embodiments, these values may apply when integrating over smaller wavelength ranges as well.
An increase in absorption of solar energy in the active layer or active layers of at least 5%, 10%, 20%, 25%, 30% or more may be obtained as a result of optical interference. These absorption values may be determined by integrating over the solar spectrum. For some embodiments, these values may apply when integrating over smaller wavelength ranges as well.
In some embodiments, the presence of at least one optical resonant cavity or layer can increase the average field intensity in the active layer or layers by at least 20%, 25% or 30% when the photovoltaic device is exposed to electromagnetic radiation such as solar spectrum. In other embodiments, the increase in average field intensity is at least 40%, 50%, 60% 70%, 80%, 90% or more. In certain embodiments, the increase is 5% or more, 10% or more or 15% or more. As described below, the average electric field intensity corresponds to the electric field is averaged across the thickness of the particular layer of interest, e.g., the active layer. For some embodiments, these values may apply when integrating over smaller wavelength ranges as well.
In certain embodiments, the presence of at least one optical resonant cavity or layer can produce an increase in the average electric field intensity integrated over the solar spectrum that is greater for the active layer or active layers than the increase in average electric field intensity integrated over the solar spectrum for any other layers in the photovoltaic device. In some embodiments, average electric field intensity in the active layer or layers of the photovoltaic device can increase by at least 1.1 times the average electric field intensity in the active layer or layers of a PV cell without an optical resonant layer. In some other embodiments, the average electric field intensity in the active layer or layers of the photovoltaic device can be at least 1.2 times or 1.3 times the average electric field in the active layer or layers of a PV cell without an optical resonant layer. In other embodiments the increase is at least 1.4 times, 1.5 time, 1.6 times, or 1.7 times the average electric field in the active layer of a PV cell without one or more resonant layer. For some embodiments, these values may apply when integrating over smaller wavelength ranges as well.
In some embodiments, the increase in the average electric field intensity may be greater in another layer of the photovoltaic device other than the active layer or layers. In such embodiments, the absorption in this other layer of the photovoltaic device may, however, be lesser than the absorption in the active layer or layers. In certain embodiments, the average electric field in the active layer or layers is higher than in any other layer, although in other embodiments, a layer other than the active layer has the highest average electric field intensity. Such conditions may be achieved for wavelengths over the solar spectrum or over smaller wavelength ranges.
In various embodiments disclosed, the optical power absorbed by the active layer or layers is increased. In certain embodiments, the increase in the optical power absorbed by the active layer or layers is greater than the optical power absorbed by all the other inactive layers of the photovoltaic device combined. The increase in optical power absorbed by the active layer or layers may be more than 1.1 times, 1.2 times, or 1.3 times the increase in absorbed optical power for any other layer in the PV device. In other embodiments, the increase is more than 1.4 times, 1.5 times, 1.6 times or 1.7 times the increase in absorbed optical power for any other layer in the PV cell.
As described above, these values may be determined by integrating over the solar spectrum. Additionally, these values may be determined for standard solar radiation known as the “air mass 1.5”.
As noted above, in certain embodiments these values apply over a wavelength range smaller than the solar spectrum. The values may apply, for example, to the visible wavelength spectrum, the ultraviolet wavelength spectrum or the infrared wavelength spectrum. The values may apply to a wavelength range of 100 nm, 200 nm, 300 nm, 400 nm, 500 nm, 600 nm, 700 nm, 800 nm, 900 nm, 1000 nm or more. The values may apply for larger or smaller wavelength ranges as well. Thus, in certain embodiments these values apply when the parameter e.g. absorption efficiency, overall efficiency, electric field, optical power etc. are integrated over smaller wavelength range other than the entire solar spectrum.
Additionally, these values may be for one or more active layers. For example, the PV cell may be designed to increase absorption in one or more active layer (such as a p type layer, intrinsic semiconducting layer or n type layer) together or separately. Accordingly these values may apply to any of these layers individually or any combination of these layers.
Similarly one or more optical resonant layers may contribute to the level of performance recited herein. Likewise, the performance values listed above may depend on the presence of one or more design parameters of one optical resonant layer or of a group of two or more optical resonant layers.
A wide variety of alternative configurations are possible. For example, components (e.g., layers) may be added, removed, or rearranged. Similarly, processing and method steps may be added, removed, or reordered. Also, although the terms film and layer have been used herein, such terms as used herein include film stacks and multilayers. Such film stacks and multilayers may be adhered to other structures using adhesive or may be formed on other structures using deposition or in other manners. Likewise, the term active layer may be used to include p and n doped regions and/or intrinsic portions of an active region. Similarly, other types of materials may be used. For example, although the active layer may comprise semiconductor, other materials such as organic materials may also be used in some embodiments.
Numerous applications are possible for devices of the present disclosure. The photovoltaic devices may, for example, be used on architectural structures such as homes, or buildings, or in stand alone structures such as in a solar farm. The solar devices may be included on vehicles such as automobiles, planes, marine vessels, spacecraft, etc. The solar cells may be used on electronics devices including but not limited to cell phones, computers, portable commercial devices. The solar cells may be used for military, medical, consumer industrial and scientific applications. Applications beyond those specifically described herein are also possible.
It will also be appreciated by those skilled in the art that various modifications and changes may be made without departing from the scope of the invention. Such modifications and changes are intended to fall within the scope of the invention, as defined by the appended claims.
This application is a divisional application of U.S. application Ser. No. 12/730,112, filed on Mar. 23, 2010, titled “INTERFEROMETRIC PHOTOVOLTAIC CELL,” which is a divisional application of U.S. application Ser. No. 11/949,699, filed on Dec. 3, 2007, titled “INTERFEROMETRIC PHOTOVOLTAIC CELL,” which claims priority under 35 U.S.C. §119(e) to U.S. Provisional Application Ser. No. 60/995,259, filed on Sep. 24, 2007, titled “INTERFEROMETRIC LIGHT TRAPPING FOR PHOTOVOLTAIC CELL” (Atty. Docket No. QCO.207PR2). Each of the above-identified application is hereby expressly incorporated by reference herein in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 60995259 | Sep 2007 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 12730112 | Mar 2010 | US |
| Child | 14339204 | US | |
| Parent | 11949699 | Dec 2007 | US |
| Child | 12730112 | US |
