WO2010111415A2

WO2010111415A2 - Quantum dot solar concentrator

Info

Publication number: WO2010111415A2
Application number: PCT/US2010/028530
Authority: WO
Inventors: Sayantani Ghosh; Michelle Khine
Original assignee: University of California Berkeley; University of California San Diego UCSD
Current assignee: University of California Berkeley; University of California San Diego UCSD
Priority date: 2009-03-25
Filing date: 2010-03-24
Publication date: 2010-09-30
Anticipated expiration: 2011-09-25
Also published as: WO2010111415A3

Abstract

Quantum dot solar concentrators, enhanced solar cells incorporating the solar concentrators, and methods for making and using the solar concentrators and enhanced solar cells are provided. The solar concentrators include a base layer, an overlayer disposed over the base layer, and a plurality of quantum dots disposed between the base layer and the overlayer. At least a portion of the overlayer may be melted to the base layer to provide a seal around at least a portion of the quantum dots. In some embodiments, the base layer, the overlayer, or both comprise polystyrene. In some embodiments, the quantum dots comprise lead sulfide.

Description

QUANTUM DOT SOLAR CONCENTRATOR

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit under 35 U. S. C. § 119 (e) of U.S. Provisional Serial No. 61/163,343, filed March 25, 2009, the contents of which are incorporated by reference in its entirety.

BACKGROUND

The U.S. electric generation system has roughly 1,000 gigawatts (GW) of generating capacity currently in place. However, the total solar generating capacity is only 0.9 GW, or less than 0.1% of the total capacity. Yet, the solar energy available to be harnessed is enormous. In fact, if every home in the U.S. has a 3 kilowatt (KW) rooftop system, more than 420 MW could be produced — more than 35% of our entire residential demand. Thus, the opportunity to add photovoltaic (PV) solar installations is large and growing.

Silicon accounts for more than 96% of the world- wide photovoltaic and solar cell devices currently in use. The Si-based devices are robust on the scale of a few years and have an average 23% efficiency factor for sunlight to electrical power conversion. Given that almost all the electronic and computing devices utilize Si-based technology in one form or the other, as well as silicon's abundant availability in the elemental form, silicon is a well-suited material for application in renewable energy related uses. Nevertheless, there are some significant drawbacks to Si-based technology. First, there exists an inherent limitation in the efficiency of performance. Although there have been many different solar cell designs (tandem cells, multi-junction cells, intermediate bandgap cells, etc.) and different active materials used in place of or in conjunction with Si (including organic dyes, polymers and quantum dots), improvements in efficiency have been limited. The best efficiency achieved so far in operational devices has not exceeded ~ 40%. , and (2) lack of flexibility in device design.

The reduced efficiency of silicon-based solar cells is due, in part, to the mismatch between the external quantum efficiency (EQE) of silicon and the solar spectrum. As shown in FIG. IA, about 48% of the energy of the solar spectrum is in the infrared range (700-2500 nm), about 44% is in the visible range (400-700 nm) and 7% in the ultraviolet range (< 400 nm). FIG. IB shows the EQE of Si over the UV (ultraviolent)-VIS (visible) spectral region. The reduced efficiency of Si at shorter wavelengths immediately noticeable. As illustrated in the figures, the quantum efficiency of Si is low where the solar spectrum flux is high.

A second disadvantage of silicon-based solar cells is the lack of mechanical flexibility. Silicon panels are large, brittle and inflexible, which make it nearly impossible to combine such panels with any mobile conformable or wearable devices. Although solar cells using plastic sheets with quantum dots and polymer blends in the active regions would be more mobile, lightweight, and ideal for many applications, the reported efficiency of such solar cells is only about 5%.

Solar concentrators including quantum dot-polymer blends have been coupled to solar cells in order to increase the efficiency of the solar cells. These conventional solar concentrators have been based exclusively on cadmium selenide (CdSe) quantum dots, which emit in the visible region, dispersed in a variety of polymer/resin/epoxy blends. Unfortunately, these conventional quantum dot solar concentrators suffer from numerous drawbacks. In particular, the absorption and emission intensity of the CdSe quantum dots is drastically reduced when mixed with the polymer/resin/epoxy blends. In addition, moisture and oxidative degradation rapidly reduce the quantum yield (QY) of the quantum dots. This has limited the lifetime and usefulness of such solar concentrators.

SUMMARY OF THE INVENTION

Provided herein are quantum dot solar concentrators, enhanced solar cells incorporating the solar concentrators, and methods for making and using the solar concentrators and enhanced solar cells. The disclosed solar concentrators are capable of converting high energy photons to lower energy photons which are more readily absorbed by silicon-based solar cells. Thus, the solar concentrators are capable of being coupled to existing silicon-based solar cells, eliminating any need for costly restructuring of such solar cells. The disclosed solar concentrators have very high optical efficiencies and concentration factors as compared with conventional quantum dot solar concentrators. Moreover, the materials used to make the solar concentrators renders the concentrators flexible, durable, lightweight, and inexpensive. Because the solar concentrators increase the power output of even thin, small, and lightweight solar cells, and because the solar concentrators themselves are thin, flexible, and lightweight, enhanced solar cells incorporating the disclosed solar concentrators may be used in a wide variety of applications where portability is paramount. Finally, a disclosed thermal treatment is capable of protecting the quantum dots in the solar concentrators from moisture and oxidative degradation, greatly extending the lifetime of the solar concentrators.

In one aspect, solar concentrators are provided. The solar concentrators include a base layer, an overlayer disposed over the base layer, and a plurality of quantum dots disposed between the base layer and the overlayer. At least a portion of the overlayer may be melted to the base layer to provide a seal around at least a portion of the quantum dots. A variety of compositions for the base layer, the overlayer, and the quantum dots may be used. Exemplary compositions are described below. In some embodiments, the base layer, the overlayer, or both comprise polystyrene. In some embodiments, the quantum dots comprise lead sulfide (PbS). Similarly, the shape and dimensions of the base layer and the overlayer and the surface coverage of the quantum dots may vary. The solar concentrators may include other components, such as a reflective coating disposed on at least a portion of the base layer, the overlayer, or both. Non-limiting examples of reflective coatings are provided.

As noted above, the disclosed solar concentrators exhibit high concentration factors (CF) and optical efficiencies. In some embodiments, the CF value is greater than 1. In other embodiments, the CF value is greater than 2, 6, or 8. However, other CF values are possible. In some embodiments, the optical efficiency is at least about 5%. In other embodiments, the optical efficiency is at least about 8%, about 10%, or about 13%. However, other optical efficiencies are possible.

In another aspect, enhanced solar cells are provided. The enhanced solar cells include any of the disclosed solar concentrators and a solar cell coupled to the solar concentrator. In some embodiments, the solar cell is a silicon-based solar cells, although other types of solar cells may be used. Also provided are articles, including, but not limited to fabrics, incorporating the enhanced solar cells.

In yet another aspect, method for making the solar concentrators and the enhanced solar cells are provided. In some embodiments, the methods involve depositing a plurality of quantum dots on a base layer and placing an overlayer over the plurality of quantum dots. Any of the base layers, overlayers, or quantum dots described herein may be used. The methods may further include melting at least a portion of the overlayer to the base layer to form a seal around at least a portion of the quantum dots. The methods may also include coupling a solar cell to any of the disclosed solar concentrators.

Methods for using the disclosed solar concentrators and enhanced solar cells are also provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. IA is the solar spectrum at 5000 K.

FIG. IB plots the external quantum efficiency of Si over the UV (ultraviolet) - VIS (visible) spectrum.

FIG. 2 shows the spectral properties of PbS quantum dots.

FIG. 3 shows the quantum yield (QY) for PbS quantum dots as a function of incident photon energy normalized to the band gap energy.

FIG. 4 illustrates an exemplary method of determining a suitable concentration of quantum dots.

FIG. 5 illustrates an exemplary method of determining the mean free path of emitted photons in a solar concentrator.

FIG. 6 illustrates an exemplary multi-layer QD concentrator.

FIG. 7 illustrates the formation of an exemplary solar concentrator and an enhanced solar cell.

FIG. 8 A shows a scanning emission image of PbS quantum dots in a solar concentrator before thermal treatment.

FIG. 8B shows a scanning emission image of the solar concentrator of FIG. 8 A. DETAILED DESCRIPTION

Solar concentrators

In one aspect, solar concentrators are provided. By "solar concentrator" it is meant a device that increases the electrical power from a solar cell relative to the electrical power of the solar cell without the solar concentrator. The solar concentrators include a base layer, an overlayer disposed over the base layer, and a plurality of quantum dots disposed between the base layer and the overlayer. As used herein and as understood by those of skill in the art, "quantum dots" are semi-conducting crystals of nanometre (a billionth of a metre) dimensions. They have quantum optical properties that are absent in the bulk material due to the confinement of electron-hole pairs (called excitons) on the particle, in a region of a few nanometres.

An exemplary solar concentrator is shown in FIG. 7C. The composition of the base layer and the overlayer may vary. In some embodiments, the base layer and the overlayer are formed of materials that are capable of melting together upon exposure to a source of energy, such as heat. By "melting" or "melted" it is meant that at least a portion of the base layer has merged with at least a portion of the overlayer to form a single, continuous structure. The melting of the base layer and the overlayer provides a seal, protecting any quantum dots enclosed within the seal from exposure to the atmosphere. This is advantageous because the quantum yield of quantum dots can drop rapidly when exposed to moisture and oxygen. As shown in Example 2, below, the formation of a seal around quantum dots greatly increases emission intensity.

In some embodiments, the base layer, the overlayer, or both include a thermoplastic material. Certain thermoplastic materials are capable of melting together upon exposure to heat, as described above. However, as used herein, "thermoplastic materials" encompasses those plastic materials that also shrink upon heating. Suitable thermoplastic materials include, but are not limited to high molecular weight polymers such as acrylonitrile butadiene styrene (ABS), acrylic, celluloid, cellulose acetate, ethylene-vinyl acetate (EVA), ethylene vinyl alcohol (EVAL), fluoroplastics (PTFEs, including FEP, PFA, CTFE, ECTFE, ETFE), ionomers kydex, a trademarked acrylic/PVC alloy, liquid crystal polymer (LCP), polyacetal (POM or Acetal), polyacrylates (Acrylic), polyacrylonitrile (PAN or Acrylonitrile), polyamide (PA or Nylon), polyamide-imide (PAI), polyaryletherketone (PAEK or Ketone), polybutadiene (PBD), polybutylene (PB), polybutylene terephthalate (PBT), polyethylene terephthalate (PET), Polycyclohexylene Dimethylene Terephthalate (PCT), polycarbonate (PC), polyhydroxyalkanoates (PHAs), polyketone (PK), polyester polyethylene (PE), polyetheretherketone (PEEK), polyetherimide (PEI), polyethersulfone (PES), polysulfone polyethylenechlorinates (PEC), polyimide (PI), polylactic acid (PLA), polymethylpentene (PMP), polyphenylene oxide (PPO), polyphenylene sulfide (PPS), polyphthalamide (PPA), polypropylene (PP), polystyrene (PS), polysulfone (PSU), polyvinyl chloride (PVC), polyvinylidene chloride (PVDC) and spectralon. In some embodiments, the base layer, the overlayer, or both include polystyrene. Polystyrene, as well as a number of other thermoplastic materials, is flexible, durable, lightweight, and inexpensive, each of which is a desirable characteristic for a solar concentrator.

The shape and dimensions of the base layer and the overlayer may vary. The shape of the base layer and the overlayer are not critical, and depend upon the desired application for the solar concentrator. Possible applications are described below. By way of example only, the base layer and the overlayer may be in the form of planar strips or sheets. The area of the base layer and the overlayer may also vary depending upon the application. The area of each layer may be small, about 10 cm , or even smaller, or large, about 1 m , or even larger. The thickness of the base layer and the overlayer may also vary. In some embodiments, the base layer, the overlayer, or both are sufficiently thick so that the amount of light emitted by the quantum dots through the top or bottom surface of the solar concentrator is minimized. In some embodiments, the thickness of the base layer and the overlayer ranges from about 30 μm to about 300 μm. This includes thicknesses of about 60 μm to about 250 μm, about 90 μm to about 200 μm, and about 120 μm to about 150 μm.

For those embodiments in which the base layer, the overlayer, or both include a thermoplastic material, the base layer and the overlayer may be pre-stressed. By "pre- stressed" it is meant that the thermoplastic material has been subjected to heat prior to assembly of the solar concentrator, thereby causing the thermoplastic material to shrink. The thermoplastic material may be "biaxially" pre-stressed, meaning that the shrinking is isotropic. Alternatively, the thermoplastic material may be "uniaxially" pre-stressed, meaning that the shrinking is anisotropic. By way of example only, a rectangular piece of thermoplastic material may be constrained at two edges during the heating process, resulting in shrinking along only one axis of the material. Both the base layer and the overlayer may include a variety of other additives.

As noted above, the disclosed solar concentrators include a plurality of quantum dots. Quantum dots have known optical properties. They are highly absorbent of incident radiation and have very bright emission (fluorescence) under optical excitation. The emission peak of quantum dots may be red-shifted from their absorption spectrum. A variety of quantum dots may be used with the disclosed solar concentrators. In some embodiments, the quantum dots include infrared (IR) emitting quantum dots. By "infrared emitting," it is meant that the quantum dots emit light in the infrared region of the electromagnetic spectrum, i.e., from about 700 nm to about 2500 nm. In some embodiments, the quantum dots include those quantum dots having an emission spectrum that exhibits a maximum between about 750 nm and about 1100 nm. This includes quantum dots exhibiting an emission maximum at about 800 nm, about 850 nm, about 900 nm, and about 1000 nm. However, suitable quantum dots may include quantum dots that emit light in other regions of the electromagnetic spectrum. In some embodiments, the quantum dots comprise lead sulfide (PbS), gallium arsenide (GaAs), cadmium telluride

(CdTe), cadmium selenide (CdSe), or combinations thereof. In still other embodiments, the quantum dots do not comprise cadmium selenide.

In some embodiments, the quantum dots comprise lead sulfide. The absorption and emission spectra of 2.2 nm diameter lead sulfide quantum dots dispersed in toluene at room temperature are shown in FIG. 2. The absorption rises rapidly as wavelength decreases while the emission is peaked around 850 nm with a tail that extends to about 1050 nm. These quantum dots efficiently absorb photons of wavelength less than 900 nm and re-emit them in the spectral band where silicon solar efficiency is higher as shown in FIG. IB. PbS quantum dots exhibit a number of desirable optical properties. However, it is to be understood that other infrared emitting quantum dots besides lead sulfide may exhibit similar advantageous optical properties. First, lead sulfide quantum dots exhibit high absorption efficiency (η_abs) of about 25%, due to the broad absorption peak shown in FIG. 2. By "absorption efficiency" it is meant the fraction of incident photons absorbed. Second, PbS quantum dots exhibit high retention efficiency (η_ret) of greater than about 56%, due to the minimized overlap between the absorption and emission spectra as shown in FIG. 2. By "retention efficiency" it is meant the fraction of emitted photons that reach a solar cell coupled to the solar concentrator. Third, lead sulfide quantum dots exhibit high quantum yield (QY) or quantum efficiency (η_Qy), the fraction of absorbed photons emitted by the quantum dots. In particular, lead sulfide quantum dots have a η_Qy of about 200%, due to an effect called multi exciton generation (MEG). Because of the small band gap of lead sulfide, the absorption of a photon of 400 nm can result in emission of two 825 nm photons, resulting in η_QY of about 200%. FIG. 3 shows a plot of QY for lead sulfide quantum dots as a function of incident photon energy normalized to the band gap energy. The shaded region on the left designates the energy range from about 300 nm to about 850 nm, where the x-axis ranges from about 1 to 2.8. This implies that over the range of the solar spectrum between ultraviolet to the tail end of visible, for every photon the lead sulfide quantum dot absorbs, it can generate at least one or two photons in the infrared. Quantum dots used in conventional solar cells, including cadmium selenide, exhibit η_QY on the order of about 10% to about 60%.

Finally, lead sulfide quantum dots may aid in heat dissipation. By way of example only, photons higher than the band gap in semiconductors, such as silicon, cause heating as the excess energy is dissipated in the form of lattice phonons as the photon moves down to the edge of the band. Heating of silicon solar cells further reduces its electrical efficiency and performance quality. The lower energy photons delivered by infrared emitting quantum dots may reduce this problem by converting visible photons to infrared photons. These infrared photons can contribute to increasing conversion efficiency of the solar cell instead of causing heating.

The surface coverage of the quantum dots disposed on the base layer may vary. In some embodiments, the surface coverage is that which maximizes the absorption of incident photons but minimizes the reabsorption of photons emitted from the quantum dots. Although higher surface coverages increase the fraction of incident photons absorbed (i.e., η_abs) higher surface coverages also increase the probability that a photon emitted by a quantum dot will be absorbed by another quantum dot rather than reaching a solar cell coupled to the solar concentrator. In other words, higher surface coverages may decrease η_ret. Suitable surface coverages for a variety of quantum dots may be readily determined by spectroscopic experiments. One such experiment is illustrated in FIG. 4. In this exemplary experiment, a solution of quantum dots is spin coated onto microscope slides at varying concentrations. The absorption and emission from the coated slides may be measured using an integrating sphere. The detector can be coupled to either a spectrometer to monitor the energy of emission, or to a photodiode to measure integrated emission intensity. The incident photon source may be a laser, tunable across the UV (ultraviolet)-VIS (visible)- NIR (near infrared) spectrum. This enables the measurement of both η_at,_s and η_ret as a function of discrete spectral regions. A solar simulator may also be used as the source of incident radiation to mimic realistic conditions under which the solar concentrator will function.

Another spectroscopic experiment involves determining the mean free path of emitted photons at a particular surface coverage. This experiment is illustrated in FIG. 5. The greater the mean free path of emitted photons, the more likely that the photons will reach a solar cell coupled to the solar concentrator. FIG. 5 illustrates a typical spectroscopic set-up customized with a high numerical aperture objective in the path. This reduces the optical resolution to about 500 nm on the sample (e.g., the solar concentrator), the best possible in a far- field set-up. A steering dichroic mirror in the path allows the collection beam to be deflected independent of the excitation beam. Using this set-up, it is possible to excite and collect emission from spatially separate spots, which will reveal how far a photon generated in one area can travel - an estimate of the path of travel. The sample may be mounted on a motorized positioning stages allowing an exploration of the entire active area of the concentrator to look at the uniformity over the macroscopic region. The layout of FIG. 5 also includes pulsed, tunable lasers as excitation sources and a time-correlated single photon counting (TCSPC) unit, enabling time -resolved measurements.

The disclosed solar concentrators have greater concentration factors and optical efficiency compared with conventional quantum dot solar concentrators. The concentration factor (CF) of a solar concentrator may be defined according to Equation 1 , below.

p CF = ^QDSC Equation 1 reference

In Equation 1, P_QDSC refers to the power output of the solar cell when coupled to the quantum dot solar concentrator (QDSC). P_reference refers to the power output of a control sample that does not include quantum dots. The disclosed solar concentrators exhibit high concentration factors. In some embodiments, the solar concentrators have a CF value of greater than 1. This includes embodiments in which the solar concentrator has a CF value of greater than 2, 3, 4, 5, 6, 7, or 8. In some embodiments, the CF value may be even as high as 9, 10, 11, or 12.

The electrical efficiency (η_ei) of the solar concentrator is proportional to its optical efficiency. The optical efficiency (η_opt) of a solar concentrator may be defined according to Equation 2, below.

rlo_Pt = ^rla_bs ^rlre_t ^rlQγ Equation 2

The factors in Equation 2, η_ab_s, η_ret, and ηoγ have been defined above. The disclosed solar concentrators exhibit high optical efficiencies. In some embodiments, the optical efficiency is at least 5%. This includes embodiments in which the optical efficiency is at least 8%, at least 10%, at least 12%, at least 15%, or even at least 20%.

The disclosed solar concentrators may include other components. In some embodiments, the solar concentrators further include a reflective coating. As used herein, the term "reflective coating" intends a thin film of highly reflective material spread over the surface to produce a mirror-like surface. The reflective coating may serve to minimizes losses of photons from the edges or certain surfaces of the disclosed solar concentrators, by reflecting the photons back towards a solar cell coupled to the solar concentrator. A variety of reflective coatings may be used, including metallic coatings. Suitable metals may include, but are not limited to silver (Ag,) gold (Au), and aluminum (Al). The thickness of the reflective coatings may vary. In some embodiments, the thickness may range from about 30 nm to about 100 nm. This includes ranges of about 40 nm to about 90 nm, about 50 nm to about 80 nm, and about 60 nm to about 70 nm.

Enhanced Solar Cells

In another aspect, enhanced solar cells are provided. In one embodiment, the enhanced solar cell includes any of the disclosed solar concentrators and a solar cell coupled to the solar concentrator. The solar cell may be coupled to the solar concentrator in a variety of configurations and by a variety of coupling mechanisms. In some embodiments, the solar cell may be coupled to an edge of the solar concentrator. An exemplary enhanced solar cell, in which a solar cell is in the process of being coupled to an edge of a solar concentrator, is shown in FIG. 7D. A variety of types of solar cells may be used. In some embodiments, the solar cell is a silicon-based solar cell. By "silicon-based" it is meant a solar cell including silicon as an active material, although silicon may not be the sole active material in the solar cell. Suitable silicon-based solar cells can include, but are not limited to, solar cells using amorphous silicon (a-Si) and solar cells using crystalline silicon (c-Si). Amorphous silicon solar cells respond to longer wavelengths of visible light, while crystalline silicon solar cells respond to shorter wavelengths.

The enhanced solar cells may be incorporated into a variety of articles, including fabrics. By way of example only, the disclosed articles may be attached to clothing, camping gear, and tents. Of course, the articles may be attached to any kind of article in need of a source of electricity from a solar cell. As noted above, solar concentrators formed of certain thermoplastic materials, including polystyrene, are flexible, thin, inexpensive, durable, and lightweight. Accordingly, the enhanced solar cells can be easily attached to such articles without interfering with original purpose, design, or characteristics of the articles. Moreover, the thinness and flexibility of the solar concentrators means that the solar concentrators may be rolled up or folded and are highly portable.

The enhanced solar cells may be characterized by a variety of straightforward measurements. In particular, measurements may be made to determine the incident radiation power to electrical power conversion ratio. By way of example only, a first control measurement may be performed on a solar cell of the same dimensions as that to be attached to the edge of any of the disclosed solar concentrators. The unattached solar cell may be placed parallel to the incident light from a solar simulator and its output power measured as a function of varying incident intensity. A second control measurement involves measuring the input intensity dependent solar cell output for the same unattached solar cell placed face-up to the solar simulator. The results of step 1 and 2 are not directly comparable, but necessary for the next steps. A third control measurement involves measuring the electrical output of the solar cell, but attached to the edge of a structure identical to the solar concentrator except that the structure does not include any quantum dots. This measurement can be used to determine all losses (if any) from base layer and the overlayer. Finally, the measurements are repeated for the solar cell attached to the edge of an actual solar concentrator. The difference between the outputs of the first control measurement and the third control measurement should scale by the geometric gain factor (ratio of the area of the solar concentrator to the area of the solar cell). Also disclosed are collections of enhanced solar cells. An exemplary collection is shown in FIG. 6. The collection includes a stack of three enhanced solar cells. The solar concentrator in each enhanced solar cell includes quantum dots of a different size, and thus, emits a different wavelength of light. By way of example only, the top solar concentrator may include quantum dots of a first size, the middle solar concentrator may include quantum dots of a larger size, and the bottom solar concentrator may include quantum dots of a yet larger size. Thus, the wavelength of light absorbed and emitted increases from the top solar concentrator to the bottom solar concentrator. The solar cells coupled to each solar concentrator may include different materials selected based on the wavelength of light emitted from the solar concentrator. Together, the multi-layer stacked collection works across a broader spectrum than does a single enhanced solar cell.

Methods

In another aspect, methods for making the solar concentrators and enhanced solar cells described above are provided. In one embodiment, the method comprises depositing a plurality of quantum dots on a base layer and placing an overlayer over the plurality of quantum dots. Any of the base layers, overlayers, and quantum dots described above may be used. In some embodiments, the method further comprises melting at least a portion of the overlayer to the base layer to form a seal. The melting step may be accomplished by a variety of ways. By way of example only, a conventional oven may be used. The sandwich structure including the base layer, the quantum dots, and the overlayer may be placed in such an oven at a temperature sufficient to melt the overlayer to the base layer. If the base layer and the overlayer are polystyrene, a temperature of about 16O⁰C may be sufficient to facilitate melting. Alternatively, a laser may be used to melt discrete portions of the overlayer to the base layer. By way of example only, if quantum dots are deposited in discrete regions on a base layer, a laser may be used to melt along the perimeter of these discrete regions forming a seal only around the regions of quantum dots. The disclosed methods may further include coupling any of the disclosed solar cells to the solar concentrator. Coupling methods have been described above.

In yet another aspect, methods for using the disclosed solar concentrators and enhanced solar cells are provided. In one embodiment, a method of converting higher energy photons to lower energy photons is provided. The method may involve exposing any of the disclosed solar concentrators to light. In some embodiments, the light comprises sunlight. In another embodiment, a method of increasing the power output of a solar cell is provided. The method may involve coupling any of the disclosed solar concentrators to a solar cell to provide an enhanced solar cell, and exposing the enhanced solar cell to light. In such embodiments, the power output of the solar cell when coupled to the solar concentrator is greater than the power output of the solar cell when uncoupled to the solar concentrator. In some embodiments, the solar cell is a silicon-based solar cell. In some embodiments, the light comprises sunlight. In yet another embodiment, a method of converting photons to electricity is provided. The method may involve exposing any of the disclosed enhanced solar cells to light. In some embodiments, the light comprises sunlight.

Additional description is provided with use of the following non- limiting examples.

EXAMPLES

Example 1: Preparation of a Solar Concentrator and an Enhanced Solar Cell

A solar concentrator and an enhanced solar cell are formed as illustrated in FIG. 7. First, a solution of lead sulfide quantum dots is spin coated onto a polystyrene base layer as shown in FIG. 7A. The quantum dots are deposited intentionally off-center so that quantum dots will be near one edge of the base layer. The surface coverage of quantum dots is will be that which maximizes the optical efficiency η_opt⁼ ηabsηretHQY, as discussed above.

Second, the layer of quantum dots are covered with an overlayer of polystyrene as shown in FIG. 7B. This configuration allows for total internal reflection of light emitted by the layer of quantum dots at the interface between the polystyrene (n ~ 1.5) and air. Total internal reflection of light at the interface between polystyrene and air has been confirmed by a shining a laser at the center of a polystyrene sheet and observing emitted light at the edges of the sheet (images not shown).

Next, the quantum dots are sealed between the base layer and the overlayer by a thermal treatment. The seal is formed by placing the sandwich structure of FIG. 7C into a conventional oven at about 16O⁰C, which results in melting of the overlayer to the base layer. Alternatively, a laser is used to melt along the perimeter of the area defined by the quantum dots, as shown by the dotted lines in FIG. 3C. Finally, a solar cell (PV cell) is attached to the edge of the solar concentrator as shown in FIG. 7D.

Example 2: Increased emission intensity of a solar concentrator subjected to a thermal treatment

FIG. 8A shows a scanning fluorescence image of a very small area, 100^χ100μm, of lead sulfide quantum dots that had been drop casted on a polystyrene base layer and then covered with a polystyrene overlayer using a scanning confocal set-up similar to the one shown in FIG. 5. The emission was collected by a fiber and dispersed by a spectrometer onto a TE cooled CCD. The intensity of the quantum dot emission was fairly uniform over this area, but showed some inhomogeneity in the quantum dot distribution on the surface towards the top part of the solar concentrator where the intensity was almost a factor of two higher.

Next, the solar concentrator was thermally treated by placing the solar concentrator in an oven to melt the overlayer to the base layer. The image is shown in FIG. 8B. Notably, the emission intensity of the quantum dots has doubled almost over the entire area.

As will be understood by one skilled in the art, for any and all purposes, particularly in terms of providing a written description, all ranges disclosed herein also encompass any and all possible subranges and combinations of subranges thereof. Any listed range can be easily recognized as sufficiently describing and enabling the same range being broken down into at least equal halves, thirds, quarters, fifths, tenths, etc. As a non-limiting example, each range discussed herein can be readily broken down into a lower third, middle third and upper third, etc. As will also be understood by one skilled in the art all language such as "up to," "at least," "greater than," "less than," and the like include the number recited and refer to ranges which can be subsequently broken down into subranges as discussed above.

All publications, patent applications, issued patents, and other documents referred to in this specification are herein incorporated by reference as if each individual publication, patent application, issued patent, or other document were specifically and individually indicated to be incorporated by reference in its entirety. Definitions that are contained in text incorporated by reference are excluded to the extent that they contradict definitions in this disclosure. For the purposes of this disclosure and unless otherwise specified, "a" or "an" means "one or more."

As used herein, the term "comprising" is intended to mean that the compositions and methods include the recited elements, but not excluding others. "Consisting essentially of when used to define compositions and methods, shall mean excluding other elements of any essential significance to the combination for that intended purpose. "Consisting of shall mean excluding more than trace elements of other ingredients and substantial method steps for making or using the concentrators or articles of this invention.

Claims

WHAT IS CLAIMED IS:

1. A solar concentrator comprising:

a base layer, an overlayer disposed over the base layer, and a plurality of quantum dots disposed between the base layer and the overlayer, wherein at least a portion of the overlayer has been melted to the base layer to form a seal around at least a portion of the quantum dots.

2. The solar concentrator of claim 1, wherein the base layer, the overlayer, or both comprise a thermoplastic material.

3. The solar concentrator of claim 1, wherein the base layer, the overlayer, or both comprise polystyrene .

4. The solar concentrator of claim 1, wherein the base layer and the overlayer comprise polystyrene.

5. The solar concentrator of claim 1 , wherein the base layer and the overlayer have thicknesses independently in the range of from about 30 μm to about 300 μm.

6. The solar concentrator of claim 2, wherein the base layer, the overlayer, or both have been pre-stressed.

7. The solar concentrator of claim 1, wherein the quantum dots comprise infrared emitting quantum dots.

8. The solar concentrator of claim 7, wherein the emission the quantum dots have an emission spectra in the range of a maximum between about 750 nm and about 1100 nm.

9. The solar concentrator of claim 1, wherein the quantum dots comprise lead sulfide, gallium arsenide, cadmium telluride, cadmium selenide, or combinations thereof.

10. The solar concentrator of claim 1, wherein the quantum dots comprise lead sulfide.

11. The solar concentrator of claim 1 , wherein the quantum dots do not comprise cadmium selenide.

12. The solar concentrator of claim 1 , wherein the solar concentrator has a CF greater than about 1.

13. The solar concentrator of claim 1, wherein the solar concentrator has a CF greater than about 2.

14. The solar concentrator of claim 1, wherein the solar concentrator has a CF greater than about 6.

15. The solar concentrator of claim 1 , wherein the solar concentrator has a CF of about 8.

16. The solar concentrator of claim 1, further comprising a reflective coating disposed on at least a portion of the base layer, the overlayer, or both.

17. The solar concentrator of claim 16, wherein the reflective coating comprises a metal.

18. The solar concentrator of claim 16, wherein the thickness of the reflective coating ranges from about 30 nm to about 300 nm.

19. A solar concentrator comprising: a base layer, an overlayer disposed over the base layer, and a plurality of quantum dots disposed between the base layer and the overlayer, wherein the base layer, the overlayer, or both comprise polystyrene.

20. The solar concentrator of claim 19, wherein at least a portion of the overlayer has been melted to the base layer to form a seal around at least a portion of the quantum dots.

21. The solar concentrator of claim 19, wherein the quantum dots comprise infrared emitting quantum dots.

22. The solar concentrator of claim 19, wherein the quantum dots comprise lead sulfide.

23. A solar concentrator comprising: a base layer, an overlayer disposed over the base layer, and a plurality of quantum dots disposed between the base layer and the overlayer, wherein the quantum dots comprise infrared emitting quantum dots.

24. The solar concentrator of claim 23, wherein at least a portion of the overlayer has been melted to the base layer to form a seal around at least a portion of the quantum dots.

25. The solar concentrator of claim 23, wherein the quantum dots comprise lead sulfide.

26. The solar concentrator of claim 23, wherein the base layer, the overlayer, or both comprise a thermoplastic material.

27. The solar concentrator of claim 23, wherein the base layer, the overlayer, or both comprise polystyrene.

28. A solar concentrator comprising: a base layer comprising polystyrene, an overlayer disposed over the base layer, the overlayer comprising polystyrene, and a plurality of quantum dots disposed between the base layer and the overlayer, wherein the quantum dots comprise infrared emitting quantum dots.

29. The solar concentrator of claim 28, wherein at least a portion of the overlayer has been melted to the base layer to form a seal around at least a portion of the quantum dots.

30. The solar concentrator of claim 28, wherein the quantum dots comprise lead sulfide.

31. An enhanced solar cell comprising: the solar concentrator of any one of claims 1, 19, 23, or 28, and a solar cell coupled to the solar concentrator.

32. The enhanced solar cell of claim 31 , wherein the solar cell is coupled to an edge of the solar concentrator.

33. The enhanced solar cell of claim 31 , wherein the solar cell is a silicon-based solar cell.

34. An article comprising the enhanced solar cell of claim 31.

35. A method of making a solar concentrator, the method comprising: depositing a plurality of quantum dots on a base layer, placing an overlayer over the plurality of quantum dots, and melting at least a portion of the overlayer to the base layer to form a seal around at least a portion of the quantum dots, wherein the solar concentrator is formed.

36. The method of claim 35, wherein the base layer, the overlayer, or both comprise a thermoplastic material.

37. The method of claim 35, wherein the base layer, the overlayer, or both comprise polystyrene.

38. The method of claim 35, wherein the quantum dots comprise infrared emitting quantum dots.

39. The method of claim 35, wherein the quantum dots comprise lead sulfide.

40. The method of claim 35, further comprising coupling a solar cell to the solar concentrator to provide an enhanced solar cell.

41. A method of making a solar concentrator, the method comprising: depositing a plurality of quantum dots on a base layer, and placing an overlayer over the plurality of quantum dots, wherein the base layer, the overlayer, or both comprise polystyrene, and further wherein the solar concentrator is formed.

42. The method of claim 41 , further comprising melting at least a portion of the overlayer to the base layer to form a seal around at least a portion of the quantum dots.

43. The method of claim 41 , wherein the quantum dots comprise infrared emitting quantum dots.

44. The method of claim 41 , wherein the quantum dots comprise lead sulfide.

45. The method of claim 41 , further comprising coupling a solar cell to the solar concentrator to provide an enhanced solar cell.

46. A method of making a solar concentrator, the method comprising: depositing a plurality of quantum dots on a base layer, and placing an overlayer over the plurality of quantum dots, the quantum dots comprising infrared emitting quantum dots, wherein the solar concentrator is formed.

47. The method of claim 46, further comprising melting at least a portion of the overlayer to the base layer to form a seal around at least a portion of the quantum dots.

48. The method of claim 46, wherein the quantum dots comprise lead sulfide.

49. The method of claim 46, wherein the base layer, the overlayer, or both comprise a thermoplastic material.

50. The method of claim 46, wherein the base layer, the overlayer, or both comprise polystyrene.

51. The method of claim 46, further comprising coupling a solar cell to the solar concentrator to provide an enhanced solar cell.