Patent Application
20120080070
This invention generally relates to photovoltaic devices with improved efficiency by enhanced down conversion of photons. More particularly, the invention relates at least in part to a down-converting layer for improving energy conversion in photovoltaic devices.
One of the main focuses in the field of photovoltaic devices is the improvement of energy conversion efficiency (from electromagnetic energy to electric energy or vice versa). The devices often suffer reduced performance due to loss of light. Therefore, research in optical designs of these devices includes light collection and trapping, spectrally matched absorption, and up/down light energy conversion.
Typically, the photovoltaic devices suffer loss of efficiency due to a thermalization mechanism in which carriers generated by high-energy photons are lost as phonons in the crystal. The absorption of incident photons with energies greater than the threshold energy for the absorption leads to the generation of typically only one electron-hole pair per absorbed photon, regardless of the photon energy. The excess energy of an incident photon above the threshold energy is wasted during the thermalization of the generated electron-hole pairs. Certain cell designs, employing a heterojunction window layer, lose high-energy photons due to parasitic absorption in the window layer. It is therefore desirable to convert these high-energy photons (short wavelength) to lower energy photons (long wavelength) that can be effectively absorbed in an absorber photovoltaic layer, and converted to collectable charge carriers.
One well-known method to overcome loss of light and related loss mechanisms involves “down-conversion” of high electromagnetic energy from a shorter wavelength to a longer wavelength. Because the absorption of high-energy photons in undesired regions/layers of optoelectronic devices must be avoided, a down-converting layer may be disposed on a surface of the device, exposed to electromagnetic radiation.
Therefore, it would be desirable to produce improved photovoltaic devices having down-converting properties, in order to meet various performance requirements.
One embodiment of the invention is a photovoltaic device. The device includes a glass plate having a first surface and a second surface. The first surface is exposed to ambient radiation. A transparent conductive layer is disposed adjacent to the second surface of the glass plate. The device further includes a first type semiconductor layer disposed adjacent to the transparent conductive layer and a second type semiconductor layer disposed adjacent to the first type semiconductor layer. A down-converting layer is interposed between the second surface of the glass plate and the transparent conducting layer. The down-converting layer exhibits an effective refractive index that has a value between the respective refractive indices of the glass plate and the transparent conductive layer.
Another embodiment is a photovoltaic module having a plurality of photovoltaic devices as described above.
These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings, wherein:
As discussed in detail below, some of the embodiments of the present invention provide a layer or a coating for optical surfaces to improve energy conversion. These embodiments advantageously reduce loss of light due to parasitic absorption and thermalization mechanisms. The embodiments of the present invention describe a photovoltaic device with improved efficiency having such a layer disposed on a surface of the photovoltaic device.
Approximating language, as used herein throughout the specification and claims, may be applied to modify any quantitative representation that could permissibly vary without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term or terms, such as “about”, is not limited to the precise value specified. In some instances, the approximating language may correspond to the precision of an instrument for measuring the value.
In the following specification and the claims, the singular forms “a”, “an” and “the” include plural referents unless the context clearly dictates otherwise.
As used herein, the terms “may” and “may be” indicate a possibility of an occurrence within a set of circumstances; a possession of a specified property, characteristic or function; and/or qualify another verb by expressing one or more of an ability, capability, or possibility associated with the qualified verb. Accordingly, usage of “may” and “may be” indicates that a modified term is apparently appropriate, capable, or suitable for an indicated capacity, function, or usage, while taking into account that in some circumstances the modified term may sometimes not be appropriate, capable, or suitable. For example, in some circumstances, an event or capacity can be expected, while in other circumstances the event or capacity cannot occur—this distinction is captured by the terms “may” and “may be”.
The term “transparent”, as used herein, means that a layer of a material allow the passage of a substantial portion of incident solar radiation. The substantial portion may be at least about 80% of the incident solar radiation.
As discussed in detail below, some embodiments of the invention are directed to improved photovoltaic (PV) device designs. The device includes a glass plate that has a first surface and a second surface. The first surface of the glass plate is exposed to ambient radiation and a transparent conductive layer is disposed adjacent to the second surface of the glass. A first type semiconductor layer is disposed adjacent to the transparent conductive layer and a second type semiconductor is disposed adjacent to the first type semiconductor layer. The device further includes a down-converting layer interposed between the glass plate and the transparent conductive layer. The down-converting layer exhibits an effective refractive index (also, referred to as refractive index of the layer) that has a value between the respective refractive indices of the glass plate and the transparent conductive layer.
As used herein “down-converting layer” may be a single layer, or may include multiple sub-layers. “Down-conversion” represents a method for the generation of multiple electron-hole pairs, per incident high-energy photon, and can be used to reduce the thermalization losses. It is a material property that can be achieved if the material contains states or bands of intermediate energies. Incident high-energy photons can be transformed by the material into one or multiple lower energy photons. In a particular embodiment, the material is capable of emitting one photon per absorbed photon.
According to an embodiment of the invention, the down-converting layer comprises a phosphor material. Typically, such down-converting material contains a host material activated by a dopant (activator). A host material can be described as a transparent host lattice. A dopant adds desired energy levels at which incoming radiation is absorbed, such as an external photon, and a generated internal photon is preferentially emitted, based on the underlying absorber properties. Therefore, the down-converting material, at the basic level, contains an absorber and an emitter.
A variety of dopants may be used, based on the desired energy level of the emitted photon. In one embodiment, dopant ions that may be used for 1 to 1 down-conversion include lanthanide ions, transition metal ions and rare-earth ions. Examples of suitable dopant ions include Ce3+, Eu2+, Cr3+, Mn2+ and Mn4+. In addition, sensitizers may be doped into the host materials along with the dopants. Sensitizers are useful if the dopant ions cannot be excited, for example, because of forbidden transitions. The exciting energy is absorbed by the sensitizers and subsequently transferred to the dopant ions. For example, transition metal ions may be sensitized by the lanthanide ions.
Although
Suitable examples of the phosphor material may include halides, oxides and phosphates. Suitable examples of fluorides include, but are not limited to, samarium-doped BaAlF5, samarium-doped (Ba, Sr, Ca)MgF4. Other examples include mixed halides such as samarium-doped (Ca, Sr, Ba)XX″ (X=F; X″=Cl, Br, I). Non-limiting examples of other suitable phosphors may include samarium-doped strontium borate (SrB4O7:Sm2+), samarium-doped (Sr, Ca, Ba)BPO5 and europium-doped (Sr, Ca)SiO4.
Other down-converting material may include organic materials. For example, an organic down-converting material may include an organic dye, such as BASF LUMOGEN dye. Furthermore, a hybrid organic-inorganic dye may also be used for down-conversion. In another embodiment the down converting material comprises a quantum dot, such as a shore-shell giant quantum dot system.
The optical properties of the down-converting layer can be determined, in large part, by its material composition, particle size of down-converting material, thickness of the layer etc. By controlling the amount, particle size, and refractive index of the down-converting material, the refractive index and conversion properties of the down-converting layer may be tailored to minimize energy losses.
The down-converting material may absorb radiation of a particular wavelength, or a particular range of wavelengths, while not scattering the radiation. The material may absorb radiation from UV, to visible, to near infrared, to infrared and converts the absorbed radiation to usable radiation. The term “usable radiation” as used herein, refers to photons of a particular wavelength, or a particular range of wavelength that take part in energy conversion with high internal and external quantum efficiency. That is, the probability of collecting an electron-hole pair in that spectral range is high, usually greater than about 60%, and often greater than about 80%. Thus, the down-converting material emits such photons that can be absorbed by a semiconductor layer of the device to produce electron-hole pairs. In a certain embodiment for solar energy conversion, the material absorbs radiation with wavelength below about 525 nm, and produces radiation with wavelength longer than 550 nm. Moreover, the excitation and absorption properties of the down-converting layer, as well as the emission spectrum, are designed to enhance external quantum efficiency (EQE) of the PV device.
In addition to down-conversion properties, the material exhibits a refractive index value that should typically match well with refractive indices of adjacent mediums. This configuration advantageously provides reduced reflection at interfaces because of improved matching of refractive indices. Thus, the down-converting layer described herein benefits the photovoltaic device in two ways: (1) reduces absorption losses and (2) reduces reflection losses, and thus improves over all energy conversion.
In one embodiment, the down-converting layer has a uniform refractive index. The reflectance of the layer having a uniform refractive index may be minimum, for a particular wavelength. In another embodiment, the down-converting layer may have a graded refractive index. The graded refractive index may be defined as a variation in the refractive index as a function of position in a selected direction, typically a direction perpendicular to a substrate supporting the layer. A value of refractive index at a first surface of the layer varies from a value of refractive index at a second surface of the layer. The variation in refractive index may be continuous in the selected direction or may occur in a series of discrete steps. In both the instances, the value of refractive index of the layer at a particular point is between the respective refractive indices of the glass plate and the transparent conductive layer.
The gradient of refractive index can be achieved by compositional variations. The graded composition provides a graded refractive index to the layer. The “graded composition” as defined herein refers to a variation in the composition of the down-converting material in one direction, although the gradation may not always be constant. In this manner, the excitation and emission profile of the layer can be broadened, which will have beneficial effect of providing a coverage of broad wavelength ranges for the overall down-converting layer. In one embodiment, the down-converting layer is a single layer. In another embodiment, the down-converting layer includes more than one sub-layer. Multiple sub-layers of different refractive indices may be deposited, one over another, to attain the desired grading.
In some embodiments, the particles 10 of the down-converting material may be dispersed or embedded in a transparent matrix 12, as illustrated in
In general, the refractive index of a medium is defined as the ratio of the velocity of light in a vacuum to that of the medium. In a real material, the refractive index can be defined as n=n′+ik, where n′ is the refractive index indicating the phase speed, while k is the extinction coefficient, which indicates absorption loss when an electromagnetic wave propagates through the material. Both n and k are dependent on the wavelength.
“Effective refractive index”, as used herein, refers to refractive index of the composite down-converting layer having down-converting particles embedded in a matrix. The effective refractive index, as defined herein, is used to determine the phase lag and attenuation of the coherent wave as electromagnetic radiation propagates through the layer. The parameters such as size, local volume fraction or area fraction, down-converting material fraction, matrix fraction and material refractive index, determine the effective refractive index of the layer. The effective refractive index of the down-converting layer may be given as:
n
eff=(1−α)nm+αnp
where nm and np represent refractive indices of the matrix and the down-converting particles, and α represents volume fraction of the down-converting particles in the matrix.
As indicated above, the refractive index of a material or medium may vary with wavelength. This effect is typically known as dispersion. In the case of a composite down-converting layer, the refractive indices of the down-converting material particles 10 and the matrix 12 may vary differently with wavelength. By tailoring the difference in respective refractive indices (Δn) of the down-converting particles 10 and the matrix 12, absorption of the spectral radiation within the composite layer can be engineered. In some embodiments, the dispersion of refractive indices for the down-converting particles 10 and the matrix 12 are chosen such that the refractive indices are well matched in the long wavelength range (>about 550 nanometers) of the solar spectrum so that scattering is minimized for incoming radiation in that range. However, the dispersion in the lower wavelength region, specifically below about 525 nm, is chosen such that the refractive indices diverge such that photon-trapping in the composite layer can occur to improve absorption.
Thus, the down-converting material may contain particles 10 of various shapes and sizes depending on refractive index of the constituents' materials, difference in refractive indices (Δn) and scattering effects. In other words, the size of particles is in part, a function of Δn. In some instances, nanosize particles of the down-converting material are desirable, especially for Δn larger than about 0.05. As used herein, “nanosize” refers to average size of the down-converting particles in a range from about 1 nanometer to about 500 nanometers, and in some specific embodiments, from about 10 nanometers to about 100 nanometers. In some other instances, bigger particles may be used for Δn less than about 0.05. In these instances, the average particle size ranges from about 0.5 micron to about 10 microns, and in specific embodiments, from about 1 micron to about 5 microns.
In some embodiments, the matrix 12 may include a non-conductive, non-crystalline material such as glass. Non-limiting examples of glasses may include soda-lime glass, alumino-silicate glass, boro-silicate glass, silica, and low-iron glass. In some embodiments, the matrix 12 may include a non-conductive crystalline material. Other suitable materials such as a dielectric material or a hybrid organic-inorganic material may also be used.
In some embodiments, the down-converting material particles 10 may be present in the matrix 12 in any amount (percentage) that is appropriate for the desired function. Suitably, the down-converting particles 10 may be present at a level of between about 0.001% to about 60% by volume, depending on the type of the matrix material and type of down-converting material. In some specific embodiments, the percentage (amount) may be in a range of from about 10% to about 25% by volume.
The down-converting materials may also contain additional layers on them, for the purposes of surface passivation or improved refractive index matching (e.g. a core-shell structure).
In another embodiment, the down-converting particles 10 are coated with a thin layer of metal nanoparticles (not shown). These particles have strong plasmon resonance that helps to improve the emission efficiency (luminescent quantum efficiency) of down converted radiation from the down-converting particles 10. In some instances, the metal nanoparticles are placed in direct contact with the down-converting particles 10, and in some other instances, the metal nanoparticles are separated by a thin dielectric shell that is first coated on the down-converting particles 10. The thickness of the shell layers may be about 1 nanometer to about 10 nanometers. These coated particles are then mixed with a liquid precursor matrix solution, which is deposited and solidified to form the composited down-converting layer.
In certain instances, to achieve refractive index gradient, the down-converting particles 10 form a density gradient from a lower region to an upper region within the matrix 12. This density gradient provides gradation in refractive index of the layer in one direction, although the gradation may not always be constant. In one embodiment, the down-converting layer is a single layer having density gradation as illustrated in
Generally, the down-converting layer has a thickness greater than about 100 nanometers. In some embodiments, the thickness of the layer may be in a range of about 500 nanometers to about 1 micron. In case of multiple sub-layers, the thickness of each of the sub-layer may be in a range of about 500 nanometers to about 800 nanometers, in some instances. In some other embodiments, the down-converting layer has a thickness from about 1 micron to about 3000 microns, and in some specific embodiments from about 1 to about 100 microns.
A down-converting layer characterized by a graded index profile provides good matching of refractive index at the interfaces, resulting in less reflection than may be achieved with uniform refractive index. The refractive index of the layer may increase or decrease with position from a first surface towards a second surface. Furthermore, the variation of the refractive index may also depend on the position of the layer in the device so that the values of refractive indices at the first and the second surfaces substantially match with the respective adjacent layers or mediums. In certain embodiments, the refractive index may increase in a direction extending from the first surface that is near to the glass plate, to the second surface, which is near to the transparent conductive oxide layer.
The down-converting layer can be formed (deposited) by a variety of techniques, such as physical vapor deposition, chemical deposition, sputtering, solution growth, and solution deposition. Other suitable techniques include dip-coating, spray-coating, spin-coating, slot-die coating, roller coating, gravure printing, ink-jet printing, screen printing, capillary printing, tape casting, flexo coating, extrusion coating, and combinations thereof.
According to some embodiments of the present invention, the photovoltaic device is a thin-film heterojunction device, for example a CdTe PV device or a Cu(In,Ga,Al)(Se,S)2 (also referred to as “CIGS”) PV device.
The glass plate 102 may have a substantially planar surface. A “substantially planar surface”, as defined herein, usually refers to a substantially flat surface. The surface can be smooth, although it may include a relatively minor degree (e.g., an RMS roughness that is less than about 1 micron, or more specifically less than about 300 nm) of texture, indentations, and various irregularities. These irregularities, textures, or patterns, may be useful in minimizing light trapping in the down-converting layer and channeling the converted radiation to the device by refraction at the dimpled surface. In one embodiment, in a CdTe PV device, the glass plate 102 is a substrate.
In another embodiment, for example in a CIGS PV device, the glass plate 102 acts as a cover and the device 100 further includes a substrate 114. Substrate selection, in these instances, may include substrates of any suitable material, including, but not limited to, metal, semiconductor, doped semiconductor, amorphous dielectrics, crystalline dielectrics, and combinations thereof.
A transparent conductive layer 108 is disposed adjacent to the down-converting layer 104 such that the down-converting layer is interposed between the glass plate 102 and the transparent conductive layer 108. Suitable materials for transparent conductive layer 108 may include an oxide, sulfide, phosphide, telluride, or combinations thereof. These transparent conductive materials may be doped or undoped. In an exemplary embodiment, the conductive oxide may include titanium dioxide, silicon oxide, zinc oxide, tin oxide, aluminum doped zinc oxide, fluorine-doped tin oxide, cadmium stannate (cadmium tin oxide), or zinc stannate (zinc tin oxide). In another embodiment, the conductive oxide includes indium-containing oxides. Some examples of suitable indium containing oxides are indium tin oxide (ITO), Ga—In—Sn—O, Zn—In—Sn—O, Ga—In—O, Zn—In—O, and combinations thereof. Suitable sulfides may include cadmium sulfide, indium sulfide and the like. Suitable phosphides may include indium phosphide, gallium phosphide, and the like.
A first type semiconductor layer 110 is disposed adjacent to the transparent conductive layer 108 and a second type semiconductor layer 112 is disposed adjacent to the first type semiconductor layer 110. The first type semiconductor layer 110 and the second type semiconductor layer 112 may be doped with a p-type doping or n-type doping such as to form a heterojunction. As used in this context, a heterojunction is a semiconductor junction, which is composed of layers of dissimilar semiconductor material. These materials usually have non-equal band gaps. As one example, a heterojunction can be formed by contact between a layer or region of one conductivity type with a layer or region of opposite conductivity, e.g., a “p-n” junction). In addition to solar cells, other devices, which utilize the heterojunction, include thin film transistors and bipolar transistors.
The second type semiconductor material layer 112 includes an absorber layer. The absorber layer is a part of a photovoltaic device where the conversion of electromagnetic energy of incident light (for instance, sunlight) to electron-hole pairs (that is, to electrical current), occurs. A photo-active material is typically used for forming the absorber layer. In one embodiment, the second type semiconductor material used for the absorber layer includes Cu(In,Ga,Al)(Se,S)2 (also referred to as “CIGS”). In some instances, CIGS may further be substituted with an additional element, for example silver. CIGS layer or film may be manufactured by various known methods. Examples of such methods include vacuum-based processes, which co-evaporate, or co-sputter copper, gallium and indium, reactive sputtering, ion beam deposition, solution based deposition of nanoparticles precursors, and metal-organic chemical vapor deposition.
Cadmium telluride (CdTe) is another photo-active material, which may be used for the absorber layer, in one embodiment. CdTe is an efficient photo-active material that is used in thin-film photovoltaic devices. CdTe is relatively easy to deposit and therefore is considered suitable for large-scale production. A typical method to deposit CdTe is closed-space sublimation.
Quite generally, in the interest of brevity of the discussions herein, photovoltaic devices including CdTe as the photo-active material may be referred to as “CdTe PV devices” and those including CIGS may be referred to as “CIGS PV devices.” Other examples of photovoltaic devices, according to a photo-active material used in the device, include a micromorph tandem silicon thin film cell, a copper-zinc-tin-sulfide (CZTS) thin film cell, a metal sulfide thin film cell and a metal phosphide thin film cell.
Moreover, the above-mentioned photo-active semiconductor materials may be used alone or in combination. Also, these materials may be present in more than one layer, each layer having different type of photo-active material or having combinations of the materials in separate layers. One of the ordinary skills in the art would be able to optimally configure the construction and the amount of the photo-active materials to maximize the efficiency of the photovoltaic cell.
An example of the first type semiconductor 110 includes cadmium sulfide (CdS). Cadmium sulfide absorbs radiation strongly at wavelengths below about 500 nanometers and significantly reduces the quantum efficiency of a PV device in this wavelength region. To avoid such losses, the down-converting layer 104 is disposed on the device 100 in front of the cadmium sulfide layer 110 that may absorb radiation with wavelength lower than about 525 nanometers and convert them to longer wavelengths, in these instances.
One embodiment is a photovoltaic module. The photovoltaic module may have an array of a number of photovoltaic devices described above electrically connected to in series or in parallel. Substantially all photovoltaic devices include down-converting layer disposed on the device as discussed in above embodiments. In some embodiments, edges of the module are painted with a diffuse reflecting paint to reduce reflection and escape of emitted photon from the edge of the module
The following examples are presented to further illustrate certain embodiments of the present invention. These examples should not be read to limit the invention in any way.
13 mm thick glass substrate is first coated with cerium-doped yttrium aluminum garnet oxide for down conversion. A layer of about 2 microns is deposited using RF sputtering technique. Subsequently, a CdTe PV cell is fabricated by depositing an indium tin oxide on the down-converting oxide layer, following by a CdS layer, and CdTe layer, and standard back contacts known in the art. Laser scribing is used to connect the CdTe PV cells in series to form a module. The scribe laser has wavelength longer than about 550 nm.
Method I. Phosphor particles were formed by high temperature reaction process, followed by mechanical ball milling Milling was continued for the time required to achieve desired particle size. These particles of desired amount were dispersed in a liquid glass precursor solution by mixing them ultrasonically.
Method II. Phosphor particle of desired size are prepared as described in method I. Prior to incorporation in the liquid glass precursor solution, the particles are subject to TEOS-based chemistry in chemical baths for deposition of various transparent oxide layer on the particle surface. These shell layers provide a graded index on the particle surface that is more effectively allow light to enter the particles for down conversion.
A composite solution is prepared as described in method I or II containing cerium-doped yttrium aluminum garnet oxide particles. The mean particle size of the oxide particles is about 100 nm, and difference in refractive indices is more than 0.05, i.e. about 1.7. The amount of particles in the precursor solution is about 33 weight percent. The composite solution is applied on a glass substrate to form a down-converting layer. Subsequently, a CdTe PV cell is fabricated as explained in example 1.
Three different solutions are prepared with about 30 weight percent, about 20 weight percent, and about 10 weight percent of oxide particles in the precursor solution as described in example 3. The solution containing the lowest weight percent of particles is deposited on a glass substrate first, followed by a solution with the second high loading, and then a solution with the highest loading to form a down-converting layer. Subsequently, a CdTe PV cell is fabricated as explained in example 1.
While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention
This invention was made with Government support under contract number DE-EE0000568 awarded by Department of Energy. The Government has certain rights in the invention.
