HK1178318B - High efficiency nanostructured photovoltaic device manufacturing - Google Patents

Description

Fabrication of high efficiency nanostructured photovoltaic devices

Technical Field

The present invention relates generally to the fabrication of photovoltaic devices and light emitting diode devices using micro-and nano-scale structural layers, depositing quantum dots onto the micro-nano structures, and using non-radiative energy transfer for energy conversion.

Background

Photovoltaics is a technical field of direct conversion of sunlight into electricity. Solar cells are a fundamental building block of Photovoltaic (PV) technology. Solar cells are made of semiconductor materials such as silicon. One of the properties that make semiconductors most useful is that their conductivity can be easily changed by introducing impurities into their crystal lattice. On one side of the cell, as impurities with five valence electrons, i.e. phosphorus atoms, and on the other side boron atoms with three valence electrons, they produce a greater affinity for attracting electrons than silicon.

The layers of the photovoltaic cell are made of semiconductor materials, which should be photo-responsive. These materials include group I-III-VI, group IV, and group III-V as well as II-VI semiconductor materials such as CdTe, CdSe, CdS, CdO, ZnS, and the like.

Chanyawadee, s. et al produced a hybrid nanocrystalline quantum dot patterned p-i-n structure that demonstrated a six-fold improvement in photocurrent conversion efficiency compared to a bare p-i-n semiconductor device using non-radiative energy transfer from highly absorbing colloidal nanocrystalline quantum dots to patterned semiconductor slices. Heterostructures are grown by molecular beam epitaxy on (100) GaAs substrates in a p-i-n configuration consisting of 20 periods of 7.5nm thick GaAs quantum wells with 12nm thick AlGaAs barrier layers (PhysicalReviewLetters, 102, 077402, 2009).

In another article by Chanyawadee, s. et al (applied physics letters, 94, 233502, 2009) we demonstrate the photocurrent enhancement of hybrid PV devices consisting of highly absorbing colloidal Nanocrystals (NC) and patterned bulk p-i-n heterostructures at both low 25K and room temperature. The patterning is designed to bring the colloidal NC into close proximity with the intrinsic regions of the p-i-n heterostructure so that the excitation energy of the deposited NC is efficiently transferred to the patterned bulk p-i-n heterostructure by means of non-radiative energy transfer. Such hybrid NC/bulk p-i-n devices provide photocurrents that are about two orders of magnitude higher than hybrid NC/quantum well p-i-nPV devices from their previous work above, and deliver the potential for high efficiency PV cells and optoelectronic devices.

Kiravittaya, s. et al propose the use of Quantum Dots (QDs) of InGaAs size 40-50nm diameter and 4-7nm height on InAs to be used in PV applications because of their broader spectral response, better temperature stability and possibility of carrier storage characteristics (PVConference 2000, 28th ieee conf., P818-821, 2000).

Patent application (WO 2008/137995) discloses an improved photovoltaic device and method. Photovoltaic devices include a semiconductor layer and a photoresponsive layer that form a junction, such as a p-n junction. The photo-responsive layer may comprise a plurality of carbon nanostructures, e.g. carbon nanotubes, located therein. In many cases, the carbon nanostructures may provide a conductive pathway within the photoresponsive layer. In other photovoltaic devices, semiconductor nanostructures are included that can take a variety of forms in addition to the carbon nanostructures. Methods of making photovoltaic devices are also disclosed.

Another patent application US2008/0216894a1 proposes the use of nanostructures and quantum dots outside the active layer in photovoltaic cells or solar cells to improve efficiency and other solar cell performance. In particular, organic photovoltaic cells may benefit. There may be a quantum dot layer between the light source and the active layer or on the side of the active layer opposite the light source. Quantum dots may also be used in the electrode layer.

The prior art proposes to deposit several QD layers in the active layer of a solar cell having several band gaps and fermi levels. In particular, the size and composition of QDs can determine their band gap and fermi level (US 2009/0255580a 1).

Patent application (US 2008/0130120a 1) suggests that IR and/or UV absorbing nanostructure layers in photovoltaic devices will increase the efficiency of solar cells. The nanostructured material is integrated with one or more of: crystalline silicon (single crystal or polycrystalline) and thin film (amorphous silicon, microcrystalline silicon, CdTe, CIGS and III-V materials) solar cells, whose absorption is mainly in the visible region. Nanoparticle materials consist of quantum dots, rods or polypods (polypod) of various sizes.

Detailed Description

Electrode systems comprising an anode and a cathode, and photovoltaic devices comprise an active layer in which light energy is absorbed and converted into electrical energy, and if desired a mechanical support system such as a substrate and other optional layers such as hole injection layers, hole transport layers, additional substrates, reflective layers, encapsulants, barrier layers, adhesives, and the like. The photovoltaic device may contain an organic active layer composition or may be a hybrid (hybrid).

The quantum dot layer comprises one or more nanoparticles. The quantum dots in this layer may be the same material, or may be a mixture of different materials including two or more materials. For example, a quantum dot layer may comprise three or more different quantum dot materials. The different points work together to produce the desired result. The quantum dots in this layer may be the same size or may be a mixture of sizes. The different particles may be combined to provide a mixture. The particle size and particle size distribution provide the desired fluorescent properties of light absorption and light emission, working in conjunction with the light absorption of the active layer. The particle size may be based on a variety of quantum dots. The optical absorption and emission can be shifted to the blue with reduced particle size. Quantum dots can exhibit broad absorption of high energy or blue and UV light energy, and narrower emission to red at the absorption wavelength.

Incident radiation on the quantum dot layer is red-shifted to form red-shifted radiation, and the active layer absorbs the red-shifted radiation. The red-shift of quantum dots is known. Nanostructures are generally known in the art, and quantum dots are also generally known in the art and can be distinguished from quantum wells and quantum wires. The nanostructures comprise nanoparticles. The nanostructures can exhibit fluorescent properties and comprise fluorophores.

The quantum dots may be inorganic materials, metallic materials, and may be semiconductor materials including, but not limited to, elements from, for example, group II, group III, group IV, group V, or group VI, including II-VI and III-V materials. Examples include CdS, CdSe, CdTe, InP, InAs, ZnS, ZnSe, HgTe, GaN, GaP, GaAs, GaSb, InSb, Si, Ge, AlAs, AlSb, PbSe, PbS, and PbTe. Further examples are InGaAs and InGaN, AlInGaP. In particular, quantum dots that absorb in the UV and blue range emit in the visible or near infrared region, and CdS and CdSe can be used in particular.

The layer comprising quantum dots is capable of absorbing radiation in a first wavelength range and may exhibit a peak or maximum absorption, in some limited cases, as well as a shoulder, an overlapping peak, and a cutoff wavelength. The wavelength range for absorption can be determined by methods known in the art. The first wavelength range may include absorption bands consistent with efficient solar collection and conversion to electrical power. The quantum dot layer may have an absorption peak at about 250nm to about 2800 nm. The range of absorption wavelengths and absorption peaks in any given device may span any range within the above limits.

The quantum dot layer can generally be adapted to absorb light that is not absorbed by the active layer. For example, the active layer may absorb light in the red or near infrared region, and the quantum dot layer is capable of absorbing at shorter and higher energies or wavelengths. The quantum dot layer is then able to re-emit radiation in the absorption spectrum (abrupposition spectrum) of the active layer. The maximum emission wavelength of the quantum dots may be selected so as to overlap the maximum absorption wavelength of the active layer.

Quantum dots can be used in colloidal form using wet chemistry means including a carrier solvent. Homogeneous nucleation in a fluid solvent can be performed. Alternatively, the quantum dots may be formed by: thin films are fabricated (e.g., by Molecular Beam Epitaxy (MBE) or Chemical Vapor Deposition (CVD)) and heated to convert the film into dot form, or alternatively quantum dots are formed by nanolithography. Many prior art techniques face difficulties with exciton recombination, charge transport, and limited device efficiency. The present invention is directed to nanostructured layers and quantum dots on epitaxial wafers with greater efficiency.

In the present invention, quantum dots are used on a very thin nanostructure layer near the active material in a photovoltaic cell in order to harvest more light to convert photons into charge carriers. Quantum dots have many physical properties that are desirable in photovoltaic devices, such as tunable bandgap and fermi levels. The energy band gap of a quantum dot can be very different from that of a bulk material, and thus the quantum dot has a small size. In general, the energy band gap of a quantum dot is inversely related to the size of the quantum dot, so that the quantum dot can be adjusted to have a desired energy band gap.

It is important to note that the size of the quantum dot also determines its fermi level. Similar to the energy band gap, the position of the fermi level of a quantum dot is inversely related to the quantum dot size; smaller sized quantum dots generally have a higher fermi level than larger quantum dots of the same composition.

The photovoltaic device includes a QD, a first conductor layer, a second conductor layer, an active layer, and a second nanostructure layer deposited on the first nanostructure layer. The first and second conductive layers may be any material suitable for conducting electrical charges, such as electrons, holes, or any other charge carriers. In operation, photons are absorbed in the active layer and dissociate at least one exciton, thereby generating pairs of charge carriers. These charge carriers are transported to the first and second conductor layers. The first conductor layer and the first nanostructure layer allow photons to pass therethrough and be absorbed in the active layer. Additionally, the second conductor layer may be optically reflective to increase the probability of photon interaction with the active layer.

A method for growing high quality flat and thick compound semiconductors onto foreign substrates uses a nanostructured compliant layer. These methods use structures having a substantially constant diameter along a substantial portion of their length, such as nanorods, or other structures that vary in diameter along their dimensions, such as pyramids, cones, or spheroids. Molecular Beam Epitaxy (MBE), Chemical Vapor Deposition (CVD), Metal Organic Chemical Vapor Deposition (MOCVD), Metal Organic Vapor Phase Epitaxy (MOVPE) or Hydride Vapor Phase Epitaxy (HVPE) methods can be used to grow nanorods of semiconductor material on any heterogeneous substrate. Such nanorods may typically have a diameter of about 10 to 120 nm. Further growth of the continuous compound semiconductor thick film or wafer may be achieved by epitaxial lateral overgrowth. The profile of the nanorods with narrow air gaps allows merging with very thin overgrowth layers. Typically, only a 0.2 μm thickness is required for the layer to be continuously overgrown. For example, growing thick GaN using GaN nanorods as a compliant layer has several advantages. The stress and dislocation are mainly located in the interface between the GaN nanorod and the substrate. Thus, the growth results in the top of the GaN nanorods to be almost stress-and dislocation-free. Thus, high quality thick GaN can be grown on the nanorod compliant layer with little tilt in the merging front on top of the nanorods or on top of the air gaps.

A protective region has been introduced on the wafer edge to reduce the total stress of the surface during the fabrication of the epitaxially grown wafer, and in other words, epitaxial growth will only occur on the nanostructured regions of the wafer that produce stress-free epitaxial wafers.

GaN nanorods with built-in flexibility will generate minimal internal stress due to their aspect ratio and nanoscale dimensions. To easily and reproducibly separate thick GaN from the substrate, an AlN nucleation layer under tensile stress with critical dimensions may be used. Rapid cooling or mechanical twisting will push the local stress above a critical value and thus separate the thick film. An alternative method of separating GaN from the substrate is to use anodic electrochemical etching. To perform this method, a thin p-GaN layer is grown on top of the nanorods before epitaxial lateral overgrowth of thick GaN. Suitable electrolytes and bias voltages cause the p-GaN to be selectively etched away, leaving the n-GaN unaffected.

The above method is used to provide a PV wafer. Which is created by growing an epitaxial-inducing growth surface onto a nanostructure substrate and then growing semiconductor material, such as but not limited to Si, GaAs, InP onto the nanostructure using epitaxial lateral overgrowth with a thickness of 20-50 microns. The grown semiconductor material is separated from the substrate. A nano-imprint lithography method is used to provide nanostructures on the semiconductor material.

The quantum dot composition is selected from the group consisting of PbS, PbSe, PbTe, CdS, CdSe, CdTe, HgTe, HgS, HgSe, ZnS, ZnSe, InAs, InP, GaAs, GaP, AlP, AlAs, Si and Ge. More generally, the quantum dots may include metallic quantum dots, semiconductor quantum dots, or any combination thereof.

As will be appreciated by those skilled in the art, various changes, substitutions and alterations can be made or otherwise implemented without departing from the principles of the invention, e.g., other materials not listed herein can be used for the various layers and quantum dots.

Summary of The Invention

A photovoltaic device comprising an epitaxial wafer comprising a plurality of layers, wherein the wafer is an epitaxially grown material including, but not limited to InP, InAs, ZnS, ZnSe, GaN, GaP, GaAs, GaSb, InSb, Si, SiC, Ge, AlAs, AlSb, PbSe, PbS, PbTe, InGaAs, InGaN, and AlInGaP, wherein the wafer is epitaxially grown on a surface of a nanostructure with space in an outer edge of the surface protected so as to limit epitaxial growth on the outer edge, wherein the wafer is epitaxially grown to a thickness of 20-100 microns on the surface of the nanostructure, wherein the epitaxially grown wafer is to be separated from the surface of the nanostructure, a first nanostructure layer having quantum dots of different composition and of different sizes, wherein the nanostructure layer is produced using a nanoimprint lithography process, wherein lithographically a plurality of the quantum dots are deposited onto the first nanostructure layer, which increases the absorption of radiation from the incident solar spectrum, a first conductive layer, wherein the quantum dots may be inorganic materials, metallic materials, and semiconductor materials, including elements from group II, group III, group IV, group V, or group VI, including II-VI and III-V materials, wherein the family of materials includes, but is not limited to CdS, CdSe, CdTe, InP, InAs, ZnS, ZnSe, HgTe, GaN, GaP, GaAs, GaSb, InSb, Si, Ge, AlAs, AlSb, PbSe, PbS, PbTe, InGaAs, InGaN, and AlInGaP, the active layer comprises at least one np junction, which may be multi-junction, located between the first and second conductive layers, comprising a material exhibiting radiation absorption, a second conductive layer and a second nanostructure layer, wherein the second nanostructure layer is located at the bottom of the photovoltaic cell, which increases internal reflection inside the substrate, and the nanostructure surface is structured by a nanoimprint lithography method;

a light emitting device comprising an epitaxial wafer, the light emitting device comprising a plurality of layers, wherein the wafer is an epitaxially grown material including, but not limited to InP, InAs, ZnS, ZnSe, GaN, GaP, GaAs, GaSb, InSb, Si, SiC, Ge, AlAs, AlSb, PbSe, PbS, PbTe, InGaAs, InGaN, and AlInGaP, wherein the wafer is epitaxially grown on a surface of a nanostructure, wherein a space in an outer edge of the surface is protected so as to limit epitaxial growth on the outer edge, wherein the wafer is epitaxially grown on the surface of the nanostructure to a thickness of 20-100 microns, wherein the epitaxially grown wafer is to be separated from the surface of the nanostructure, a first nanostructure layer having quantum dots of different composition and of different size, wherein the nanostructure layer is produced using a nanoimprint lithography process, wherein a plurality of said quantum dots are deposited onto said first nanostructure layer for the purpose of non-radiative energy transfer in color-converting emission, a first conductive layer, wherein said quantum dots may be inorganic, metallic and semiconductor materials, including elements from group II, III, IV, V or VI, including II-VI and III-V materials, wherein said group of materials includes but is not limited to CdS, CdSe, CdTe, InP, InAs, ZnS, ZnSe, HgTe, GaN, GaP, GaAs, GaSb, InSb, Si, Ge, AlAs, AlSb, PbSe, PbS, PbTe, InGaAs, InGaN, and AlInGaP, an active layer comprising at least one np junction, which may be multi-junction, located between said first and second conductive layers, comprising a material exhibiting radiative excitation, a second conductive layer and a second nanostructure layer, wherein said second nanostructure layer is located at the bottom of the device, which increases the reflection from the back side of the substrate and the nanostructured surface is structured by means of nanoimprint lithography.

Drawings

FIG. 1: a structured silicon substrate (1) and a protective region (10).

FIG. 2: an epitaxial wafer (2) is grown on top of a structured substrate (1) having a protective region (10).

FIG. 3: an epitaxial wafer (2).

FIG. 4: the nanostructure layers (3, 4) are produced on the top and bottom of the epitaxial wafer (2) using NIL.

FIG. 5: the finished device comprises a plurality of layers: a protective glass layer (5), a first conductive layer (6), a first nanostructure layer (3) using NIL and QD (7), an epitaxial wafer comprising an n-p active layer (2), a second nanostructure layer (4), a second conductive layer (8) and optical radiation (9).

FIG. 6: a surface (silicon substrate) is shown which is only partially structured (62) and the outer region is not structured (61). Epitaxial growth will occur only on the nanostructured regions (62) of the wafer that produce a stress-free epitaxial wafer.

Claims (10)

1. A photovoltaic device, comprising:

-an epitaxial wafer comprising a plurality of layers;

-a first nanostructure layer having quantum dots, wherein a plurality of said quantum dots are deposited onto said first nanostructure layer, which increases the absorption of radiation from the incident solar spectrum; wherein the quantum dots are applied onto a very thin nanostructure layer near an active material in a photovoltaic cell in order to harvest more light to convert photons into charge carriers;

-a first electrically conductive layer;

-an active layer, wherein the active layer comprising at least one np-junction is located between the first and second conductive layers, comprising a material exhibiting radiation absorption;

-a second electrically conductive layer;

-a second nanostructure layer, wherein the second nanostructure layer is located at the bottom of the photovoltaic cell, which increases the internal reflection inside the substrate.

2. The device of claim 1, wherein the quantum dots have different compositions.

3. The device of claim 1, wherein the quantum dots have different sizes.

4. The device of claim 1, wherein the nanostructure layer is produced using a nanoimprint lithography method.

5. The device of claim 1, wherein the quantum dots are a material selected from the group consisting of: CdS, CdSe, CdTe, InP, InAs, ZnS, ZnSe, HgTe, GaN, GaP, GaAs, GaSb, InSb, Si, Ge, AlAs, AlSb, PbSe, PbS, PbTe, InGaAs, InGaN, and AlInGaP.

6. The device of claim 1, wherein the np junction can be multi-junction.

7. The device of claim 1, wherein the wafer is an epitaxially grown material including, but not limited to, InP, InAs, ZnS, ZnSe, GaN, GaP, GaAs, GaSb, InSb, Si, SiC, Ge, AlAs, AlSb, PbSe, PbS, PbTe, InGaAs, InGaN, and AlInGaP.

8. The device of claim 1, wherein the wafer is epitaxially grown on the nanostructured surface to a thickness of 20-100 microns.

9. The device of claim 8, wherein the wafer is epitaxially grown on a nanostructured surface, wherein space in an outer edge of the surface is protected so as to limit epitaxial growth on the outer edge.

10. The device of claim 9, wherein the nanostructured surface is structured by a nanoimprint lithography method.