WO2008046147A1

WO2008046147A1 - Up and down conversion using quantum dot arrays

Info

Publication number: WO2008046147A1
Application number: PCT/AU2007/001579
Authority: WO
Inventors: Martin Green
Original assignee: Newsouth Innovations Pty Limited
Priority date: 2006-10-18
Filing date: 2007-10-17
Publication date: 2008-04-24

Abstract

A luminance converter comprises layers of organised, crystalline semiconductor material quantum dots which are formed within the respective layers with size and location determined by parameters of said layer. Quantum dot energy gaps are determined by selecting matrix and quantum dot parameters. The layers of quantum dots are positioned adjacent to each other, and adjacent layers have different quantum dots having different energy gaps. The energy gap of quantum dots in one of the layers of quantum dots will have a differential energy equal to the target energy of output photons of the luminance converter and the energy gap of quantum dots in another of the layers will have differential energy less than or equal to a minimum target energy of input photons of the luminance converter. The quantum dots in adjacent layers are sufficiently closely spaced to allow migration of electrons between energy states either through photon excitation to a higher energy state or relaxation to a lower energy state.

Description

Up and down conversion using quantum dot arrays

Cross-Reference to Related Applications

The present application claims priority from Australian Provisional Patent Application Nos 2006906343 and 2006906076 filed on 18 October 2006 and 1 November 2006 respectively, the contents of which are incorporated herein by reference. The present applicant and inventor are also the applicant and inventor respectively for PCT Patent Application No PCT/AU2005/000614 filed on 29 April 2005, the content of which is incorporated herein by reference.

Introduction

The present invention relates generally to the field of photovoltaics and in particular the invention provides luminance conversion device for matching the energy of incident light falling on a photovoltaic conversion system and the absorption characteristics of a photovoltaic device of the system and a method of forming such a converter.

Background

The main problem in solar energy conversion is the fact that the incident solar spectrum consists of photons in a significant amount in a very broad spectral range up to roughly 4 eV. Conventional photovoltaic devices are based on materials like semiconductors or dye molecules, which have a lower threshold energy for the absorptance. The two major loss mechanisms leading to reduced energy conversion efficiencies in such conventional devices are

1) Transmission losses: Incident photons with energies smaller than the threshold energy for the absorptance are transmitted and cannot be used by the photovoltaic cell.

2) Thermalisation losses: The absorption of incident photons with energies larger than the threshold energy for the absorptance leads to the generation of 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 thermalisation of the generated electron-hole pairs. Previous approaches for improved solar cell systems have tended to try to adapt the solar cell design to the broad spectral range of the incident solar spectrum. However, the conditions on the material quality, which must be fulfilled in order to achieve improved energy conversion efficiencies with these approaches, while achieving low manufacturing cost, are very unlikely to be fulfilled in the near future.

Another approach which has been suggested recently involves using an up or down converter to combine the energy of low energy photons or to create lower energy photons when high energy photons are captured. Such converters can be stacked in front (or behind) conventional photovoltaic devices to condition the light received by the photovoltaic device.

It has been proposed that one way of creating an up-conversion process could be based on quantum-dot "molecules" as illustrated in Fig. 1, with each molecule consisting of two quantum dots I₅ II each with different properties, D. Vanmaekelberg et al. ("Path to Ultra-High Efficiency Photovoltaics", Ispra, October, 2003). The up- conversion process begins with an electron 11a of quantum dot I being excited 18 from its ground state 14 to a higher allowed energy state 15 after absorbing a photon 17a. The energy differential between two allowed energy states will herein after be referred to as an "Energy Gap". Subsequently an electron l ib in the ground state of quantum dot II relaxes to the lower energy ground state of quantum dot I. A second photon 17b is then absorbed by the electron 11a of quantum dot I causing it to be excited to a higher energy state 13 this time associated with quantum dot II. Finally the electron 11a in the higher energy state 13 of quantum dot II relaxes 18c to the ground state 12 (which was vacated by electron 1 Ib) releasing a photon 17c. The photon 17c, which is released when the electron 11a relaxes from a high energy state has a shorter wavelength than the two absorbed photons 17a & 17b which is how the up-conversion effect is achieved.

Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application. Throughout this specification the word "comprise", or variations such as

"comprises" or "comprising", will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps, Summary of the Invention

According to a first aspect, the present invention consists in a luminance converter comprising a plurality of layers of organised quantum dots each layer comprising a plurality of crystalline semiconductor material quantum dots formed within the respective layers with size and location determined by parameters of said layer and wherein the energy gap for the material in the quantum dots of each layer is determined by selecting the material parameters including the size of the quantum dots, the composition of the matrix and the semiconductor material of the quantum dots, the layers of organised quantum dots being positioned adjacent to or in close proximity with each other, and adjacent, or near to adjacent, layers being layers of different quantum dots having different energy gaps, the energy gap of quantum dots in one of the layers of organised quantum dots having a differential energy equal to the target energy of output photons of the luminance converter and the energy gap of quantum dots in another of the layers of organised quantum dots having a differential energy less than or equal to a minimum target energy of input photons of the luminance converter and the quantum dots of layers that are adjacent to one another or in close proximity are sufficiently closely spaced to permit migration of electrons from energy states in quantum dots of one layer to energy states in quantum dots of the layer adjacent or in close proximity either through photon excitation to a higher energy state or through relaxation to a lower energy state of the quantum dot of the layer adjacent or in close proximity.

According to a second aspect of the invention a method is provided for forming a luminance converter, comprising a plurality of layers of organised quantum dots, each layer comprising a plurality of crystalline semiconductor material quantum dots formed within the respective layers with size and location determined by parameters of said layer and quantum dots of a respective layer being different to those of an adjacent, or near to adjacent, layer, the quantum dots of each layer having a selected energy gap, the method comprising; forming a plurality of layers of dielectric material each layer comprising a compound of a semiconducting material, wherein alternating layers are layers of stoichiometric dielectric material and layers of semiconductor rich dielectric material respectively, and heating the layers of dielectric material to cause quantum dots to form in each of the semiconductor rich layers of dielectric material whereby the quantum dots so formed are distributed within each layer of semiconductor rich dielectric material, and wherein the energy gap of the quantum dots in each layer of semiconductor rich dielectric material are determined by selecting the material parameters including the thickness of the layer of semiconductor rich dielectric material, the thickness of the layers of stoichiometric dielectric material on either side of the respective layer of semiconductor rich dielectric material and the semiconductor material of the semiconductor rich dielectric material layer to achieve the desired energy gap parameters of the material, adjacent layers each having quantum dots with different energy gaps, and an energy gap differential energy of the quantum dots of one of the layers of organised quantum dots being essentially equal to the target energy of output photons of the luminance converter and an energy gap differential energy of the quantum dots of another of the layers of organised quantum dots, adjacent or in close proximity to said one of the layers of organised quantum dots, being less than or equal to a minimum target energy of input photons of the luminance converter and the quantum dots of layers that are adjacent to each other or in close proximity are sufficiently closely spaced to permit migration of electrons from energy states in quantum dots of one layer to energy states in quantum dots of the layer adjacent or in close proximity either through photon excitation to a higher energy state or through relaxation to a lower energy state of the quantum dot of the layer adjacent or in close proximity. According to a third aspect, the present invention consists in a luminance converter associated with a photovoltaic junction, the luminance converter comprising a plurality of layers of organised quantum dots each layer having a selected energy gap and comprising a plurality of crystalline semiconductor material quantum dots formed within the respective layers with size and location determined by parameters of said layer and wherein an energy gap of the material in each layer is determined by selecting the material parameters including the size of the quantum dots, the composition of the matrix and the semiconductor material of the quantum dots, and adjacent, or near to adjacent, layers being layers of different quantum dots having different energy gaps, an energy gap of the quantum dots of one of the layers of organised quantum dots having a differential energy equal to the target energy of output photons of the luminance converter and an energy gap of the quantum dots of another of the layers of organised quantum dots, adjacent or in close proximity to said one of the layers of organised quantum dots, having a differential energy less than or equal to a minimum target energy of input photons of the luminance converter and the quantum dots of layers that are adjacent or in close proximity to one another are sufficiently closely spaced to permit migration of electrons from energy states in quantum dots of one layer to energy states in quantum dots of the layer adjacent or in close proximity either through photon excitation to a higher energy state or through relaxation to a lower energy state of the quantum dot of the layer adjacent or in close proximity.

The photovoltaic junction may be a conventional semiconductor junction which is stacked with the luminance converter. Alternatively the photovoltaic junction may be an artificial amorphous semiconductor junction comprising an n-type region of artificial amorphous material adjacent a p-type region of artificial amorphous material, the n-type and p-type artificial amorphous materials being integrally formed as a matrix of dielectric material in which is substantially regularly disbursed a plurality of crystalline semiconductor material quantum dots and wherein the n-type and p-type regions are respectively doped with n-type and p-type dopant atoms, in which case the luminance converter may be either independent of and stacked with the junction device or may be integrated into the junction device.

According to a fourth aspect of the invention a method is provided for forming a luminance converter associated with a photo voltaic cell, the luminance converter comprising a plurality of layers of organised quantum dots, adjacent, or near to adjacent, layers being layers of different quantum dots each layer comprising a plurality of crystalline semiconductor material quantum dots formed within the respective layers with size and location determined by parameters of said layer and each layer having a selected energy gap, the method comprising; forming a plurality of layers of dielectric material each layer comprising a compound of a semiconducting material, wherein alternating layers are layers of stoichiometric dielectric material and layers of semiconductor rich dielectric material respectively, and heating the layers of dielectric material to cause quantum dots to form in each of the semiconductor rich layers of dielectric material whereby the quantum dots so formed are spaced within each layer of semiconductor rich dielectric material, and wherein an energy gap of the quantum dots in each layer of semiconductor rich dielectric material is determined by selecting the material parameters including the thickness of the layer of semiconductor rich dielectric material, the thickness of the layers of stoichiometric dielectric material on either side of the respective layer of semiconductor rich dielectric material and the semiconductor material of the semiconductor rich dielectric material layer to achieve the desired energy gap parameters of the material, adjacent layers each having quantum dots with different energy gaps, and an energy gap differential energy of the quantum dots of one of the layers of organised quantum dots being essentially equal to the target energy of output photons of the luminance converter, and an energy gap differential energy of the quantum dots of another of the layers of organised quantum dots, adjacent or in close proximity to said one of the layers of organised quantum dots, being less than or equal to a minimum target energy of input photons of the luminance converter and the quantum dots of layers that are adjacent to each other or in close proximity are sufficiently closely spaced to permit migration of electrons from energy states in quantum dots of one layer to energy states in quantum dots of the layer adjacent or in close proximity either through photon excitation to a higher energy state or through relaxation to a lower energy state of the quantum dot of the layer adjacent or in close proximity.

The method may comprise stacking the luminance converter with a conventional semiconductor junction. Alternatively the photovoltaic junction may be an artificial amorphous semiconductor junction created by forming a plurality of layers of dielectric material comprising a compound of a semiconducting material, wherein alternating layers are layers of stoichiometric dielectric material and layers of semiconductor rich dielectric material respectively, doping regions of the plurality of layers of dielectric material with p-type and n-type dopants either simultaneously with their formation or subsequently and

Heating the layers of dielectric material to cause quantum dots to form in the semiconductor rich layers, wherein the bandgap and mobility are determined by selecting the material parameters including the size of the quantum dots, the composition of the matrix and the semiconductor material of the quantum dots to achieve the desired parameters., in which case the luminance converter may be either independent of and stacked with the junction device or may be integrated into the junction device.

A region in the vicinity of the junction between the n-type and p-type regions of the artificial amorphous material may be undoped or have a balance of n-type and p- type dopants whereby the region behaves as intrinsic material.

The quantum dots of the photovoltaic cell are distributed in layers throughout the artificial amorphous material and each of the n-type and p-type regions will typically include 20-30 layers of quantum dots and preferably about 25 layers formed by providing that number of each of the alternating stoichiometric and semiconductor rich layers. The n-type and p-type regions are typically each in the range of 75 - 125 nm thick and preferably about lOOnm thick. This is achieved by creating each layer of dielectric material with a thickness in the range of 1.5 to 2.5 nm and preferably about 2 nm and providing 25 of each of the stoichiometric and semiconductor rich layers (i.e. 50 layers in all) in each of the doped regions to give a cell having a thickness of 150 to 250 and preferably 200 nm thick.

While the luminance converter is intended primarily for use with photovoltaic cells other applications are also possible. When the luminance converter is used as an up converter (converting two or more low energy photons to a higher energy photon), a first layer of organised quantum dots will have quantum dots having a lower energy gap than the minimum desired output energy (eg the bandgap energy of an associated photovoltaic device) and a second adjacent (or near to adjacent) layer of organised quantum dots will have quantum dots having an energy gap equal to or higher than the minimum desired output energy.

The dielectric material of the luminance converter is preferably silicon oxide, silicon nitride silicon carbide, a mixture of these materials or a structure including layers of one or more of these materials or mixtures, possibly with layers of other materials included. The semiconductor material of the quantum dots is preferably silicon or a silicon alloy such as silicon alloyed with germanium, but again other semiconductor materials may be used in particular applications, such as germanium, gallium arsenide or Indium phosphide.

Quantum dot luminance converters may be stacked in tandem with other quantum dot luminance converters having different conversion characteristics to cover a wider band of the spectrum of solar energy (or possibly the energy spectrum of another source, in non solar cell applications). Similarly artificial amorphous material photovoltaic cells may be stacked in tandem with other artificial amorphous material photovoltaic cells and/or cells of more conventional material such as poly crystalline silicon cells. When a plurality of cells are stacked in tandem the band gaps of the artificial amorphous material cells are preferably varied from cell to cell (and with respect to any base line silicon cell) whereby each cell is optimised for a different wavelength of incident light on the tandem structure. Conventional material may also be used adjacent to an artificial amorphous semiconductor material layer to assist in connecting to the artificial amorphous semiconductor material.

Brief Description of the Drawings

Embodiments of up and down luminance converters will now be described with reference to the accompanying drawings in which: Fig. 1: diagrammatically illustrates an excitation and relaxation cycle leading to the conversion of two low energy photons to one high energy photon; Fig. 2 diagrammatically illustrates a superlattice structure formed by deposition of alternating stoichiometric and silicon-rich layers;

Fig. 3 shows the layers of Fig. 2 after high temperature treatment showing crystalline silicon quantum dots; Fig. 4 Possible structure for a quantum dot up-converter placed at the rear of a solar cell;

Fig. 5 diagrammatically illustrates an alternative excitation and relaxation cycle leading to the conversion of two low energy photons to one high energy photon;

Fig. 6 diagrammatically illustrates a down-conversion excitation and relaxation cycle leading to the conversion of a high energy photon to two low energy photons;

Fig. 7 diagrammatically illustrates a Quantum dot array formed on a textured surface (not to scale);

Fig. 8 diagrammatically illustrates a generic cell design based on a superlattice of quantum dot material, configured with a quantum dot up-converter and a quantum dot down-converter; and

Fig. 9 diagrammatically illustrates a device comprising a tandem cell structure fabricated according to the present invention including a base line cell and an artificial amorphous semiconductor material cell using crystalline silicon on glass (CSG) technology.

Detailed Description of Up and Down Converters

A luminance converter is a device for converting two low energy photons to one high energy photon. Such a device can help improve the energy conversion efficiency of photovoltaic converters by allowing photons of insufficient energy to create electron-hole pairs in a conventional way to be converted to useful energies. A similar scheme can be used to convert one high energy photon, with energy in excess of that required to create an electron-hole pair, to two low energy photons each of which might create an electron-hole pair, with similar advantages.

The underlying conversion process is shown schematically in Fig. 1. Low energy photons 17a are absorbed by a first quantum dot I with a small energy difference between the ground state 14 and excited state 15. This dot I is in close proximity to a second quantum dot II with its ground state 12 and excited state 13 related in a particular way to that of the first dot I. Electrons lib occupying the ground state 12 of the second dot II flow 18d readily to the ground state 14 of the first dot I, when the latter are vacant. However, the excited state 13 of the second dot II lies at much higher energies. The basic up-conversion process occurs by the excitation 18a of an electron 11a from the ground state 14 to the excited state 15 of the first dot I due to absorption of a photon 17a, followed by the occupation of its ground state 14 by an electron 1 Ib which flows 18d from the ground state 12 of the second quantum dot II. This inhibits relaxation of the excited electron 11a back to this state 14. The next stage of the process involves the absorption of a second photon 17b by the electron 11a in the excited state 15 of the first dot I by excitation 18b to the excited state 13 of the second dot II. From here, the electron 11a can relax 18c to the ground state 12 of this dot II by emission of a single high energy photon 17c and the cycle is ready to be repeated. The embodiments of up and down luminance converters described below use an extension of technology described in co-pending PCT Patent Application No PCT/AU2005/000614 which describes a method of making artificial amorphous semiconductor material. A brief description of that method is given below by way of introduction. A characteristic of artificial amorphous semiconductor material is that a variable band gap can be obtained within the same materials system and this materials system is consistent with the exceptional stability and durability of silicon wafer-based product as well as that based on crystalline silicon films on glass. By varying the structure of the material slightly a material with closely spaced quantum dots having varying energy gaps can be formed, and this characteristic lends itself to the fabrication of luminance conversion devices as will be seen below. The following examples use silicon as the base semiconductor material however the invention is applicable to other semiconductor materials such as germanium, gallium arsenide or Indium phosphide.

General Preparation Approach

Referring to Fig. 2, to prepare the artificial amorphous semiconductor material (in this case using silicon as the quantum dot material), alternating layers 21 of stoichiometric silicon oxide, nitride or carbide are interspersed with layers 22 of silicon-rich material of the same semiconductor type. These layers are formed on a substrate 24 which may be glass, ceramic or other suitable material depending on the particular application. On heating, crystallisation of the excess silicon occurs in the silicon-rich layers 22. As illustrated in Fig. 3, in order to minimise their free energy, the crystallised regions are approximately spherical of a radius determined by the width of the silicon-rich layer, and approximately uniformly dispersed within this layer. If the interspersed layers of stoichiometric material 21 are sufficiently thin, free energy minimisation encourages a symmetric arrangement of quantum dots 23 on neighbouring planes (either in a close-packed arrangement as shown or in related symmetrical configurations) of the dielectric material whereby if the quantum dots are of similar dimensions they will be uniformly distributed and regularly spaced in three dimensions through the dielectric material. Suitable deposition approaches for the layers 21, 22 include physical deposition such as sputtering or evaporation, including these in a reactive ambient, chemical vapour deposition including plasma enhanced processes, or any other suitable processes for depositing the materials involved. Suitable heating processes include heating in a suitable furnace, including belt or stepper furnaces, or heating by rapid thermal processes including lamp or laser illumination amongst others. For approaches resulting in hydrogen incorporation into the layers during deposition, several stages of heating may be required to allow the hydrogen to evolve prior to exposure to the higher crystallisation temperatures

The present approach to implementing a luminance conversion process relies on extending the scheme described above, for forming quantum dots of controlled size and partially controlled location by precipitation from layers of non-stoichiometric composition with only slight modification.

The basic technique is shown in Fig. 2. Alternating layers 21 of stoichiometric material and layers 22 of non-stoichiometric material are deposited on a substrate 24 as described above. On heating, the non-stoichiometric material separates into two near stoichiometric phases. For example, if the layers 22 non-stoichiometric material is silicon dioxide with an excess of silicon and the layers are sufficiently thin, heating will cause the excess silicon to precipitate out of the oxide in the form of silicon quantum dots 23 of a diameter equal to thickness of the layer 22 as seen in Fig. 3. The material described above relies on the precipitation of silicon from layers of constant thickness, this approach can be extended to the deposition of materials other than silicon from layers that can be of different thicknesses to achieve the properties required for efficient up-conversion. The resulting quantum dots 23 will vary in energy gap depending upon the semiconductor material of the quantum dots, the dielectric material and the thickness of the various layers of stoichiometric dielectric material and semiconductor- rich dielectric material.

Part of an up-converter designed for use with a silicon cell is shown in Fig. 4 using silicon, germanium or tin and their alloys to form quantum dots with the desired properties in a silicon oxide, silicon nitride or silicon carbide matrix. The up-converter comprises a plurality of alternating layers of high threshold energy quantum dots 71 and low threshold energy quantum dots 72 in a silicon oxide, silicon nitride or silicon carbide matrix 73. The structure of Fig. 4 is created by forming alternating layers 21 of stoichiometric material and layers 62, 63 of non-stoichiometric or semiconductor rich material and then hearting the structure in the manner described to cause the crystalline quantum dots 71, 72 to precipitate out. The quantum dots 71, 72 are created with different characteristics by altering the thickness of their respective layers 62, 63 and/or by changing the composition of the crystalline dot 71, 72 (by altering the composition of the original non-stoichiometric layer 62, 63). A luminance converter may typically comprise 2 - 50 layers of quantum dots or more. A similar configuration could be used for down-conversion by altering the characteristics of the quantum dot layers. Specifically, the luminance converter structure of Fig. 4 is formed by forming in sequence, a layer 21 of stoichiometric dielectric material such as SiO₂, SiN or SiC (which will become the matrix material 73 of the quantum dots), followed by a layer 63 of silicon rich dielectric material with the required additives to form a layer of the low threshold energy quantum dots 72, followed by another layer 21 of stoichiometric dielectric material, followed by another layer 63 of silicon rich dielectric material with the required additives to form a layer of the low threshold energy quantum dots 72, followed by another layer 21 of stoichiometric dielectric material, followed by a Si rich layer 62 of dielectric material, with additives required to form a layer of the high threshold energy quantum dots 71. This sequence is then repeated until the required number of potential quantum dot layers have been formed each surrounded on either side by a stoichiometric layer. This structure is then heated causing the quantum dots 71, 72 to crystallise and precipitate out of the dielectric material 73. The purpose of providing several layers 63 of silicon rich dielectric material with the required additives to form a layer of the low threshold energy quantum dots 72, between each layer 62 of dielectric material, with additives required to form a layer of the high threshold energy quantum dots 71, is to balance the volume and absorption characteristics of the different quantum dot materials. The actual number of layers of each quantum dot type formed before switching to the other type will depend on the physical characteristics of the quantum dots and possibly also the spectral profile of the light source in the targeted application.

For efficient up-conversion in the proposed mode, the relative energy levels associated with the two types of quantum dots need to be controlled. The energy threshold can be controlled by dot diameter and composition with small diameter favouring large thresholds. Silicon-rich alloys will have the largest threshold for large dots, but the low electron effective mass in germanium in combination with the smaller energy from its valence band edge to vacuum (hole affinity) will make it an appropriate choice for high bandgap quantum dots in the conversion process.

The low threshold dots require low bandgap and high hole affinity. Germanium and tin-rich alloys are favoured for providing low band gap energies, while high silicon content is favoured for high hole affinity. Large dot size is also favoured for small threshold energy, although confinement needs to be sufficiently strong to push higher level conduction band states to high energies so that they do not interfere with the desired conversion process.

Another important feature of the quantum dots in a luminance converter is that for up conversion the ground state energy of the low threshold dots 72 should be lower than the ground state energy of the higher threshold dots 71 so that electrons may easily migrate back to a low threshold quantum dot after it has relaxed to the ground state of a high threshold quantum dot. Conversely, for down conversion the ground state energy of the high threshold dots 71 should be lower than the ground state energy of the lower threshold dots 72 so that electrons may easily migrate back to a high threshold quantum dot after it has relaxed to the ground state of a low threshold quantum dot.

Alternative Path for the Up-conversion Process

An alternative up conversion mechanism is illustrated in Fig. 5, which shows the second stage of the excitation taking place through a second excited state of the first dot, with relaxation taking place to a lower excited state in the second (high threshold) dot.

The up-conversion process starts with the excitation 18a of an electron 11a from the ground state 14 to the excited state 15 of the first dot I due to absorption of a photon 17a, followed by the occupation of its ground state 14 by an electron 1 Ib which flows 18d from the ground state 12 of the second quantum dot II. This inhibits relaxation of the excited electron l la back to this state 14. The next stage of the process involves the absorption of a second photon 17b by the electron 1 Ia in the excited state 15 of the first dot I by excitation 18b to the higher excited state 86 of the first dot I. The electron lla then relaxes 18e to the excited state 13 of the second dot II, which is a slightly lower energy state than the higher excited state 86 of the first dot I. From here, the electron l la can relax 18c to the ground state 12 of this dot II by emission of a single high energy photon 17c and the cycle is ready to be repeated.

The same general type of approach can be used for the down-conversion of light. In this case, the energy level alignments have to be altered to allow for the required electron motion after an excitation event. As an example, Fig. 6 shows the down-conversion equivalent of the up-conversion process of Fig. 5.

The down-conversion process starts with the excitation 88a of an electron l la from the ground state 82 to the excited state 83 of the second, or high threshold energy, dot IV due to absorption of a photon 87, followed by the occupation of its ground state 82 by an electron l ib which flows 88d from the ground state 84 of the first, or low threshold energy, quantum dot III. This inhibits relaxation of the excited electron 1 Ia back to this state 82. The next stage of the process involves the relaxation 88e of the electron 1 Ia to the higher excited state 86 of the first dot III, which is a slightly lower energy state than the excited state 83 of the second dot IV. The electron l la then relaxes 88b to the lower excited state 85 of the first dot III, by emission of a single low energy photon 87b. From here, the electron l la can relax 88c to the ground state 84 of this dot III by emission of a further single low energy photon 87c and the cycle is ready to be repeated. Alternatively, the electron in the higher excited state 83 of the second dot could relax directly to the lower excited state 85 of the first dot while emitting a photon of similar energy to photon 87b.

Other possible energy transition paths are possible for these transitions. For example, for the up-conversion process, the electron in the first excited state 15 of the first quantum dot in Fig. 1 and Fig. 5 could be excited into a conduction band or defect state of the intervening dielectric material by the absorption of photon 17b and from there progress to the excited state 13 of the second quantum dot. Alternatively, it could make a transition from the first excited state 15 to a defect state in the intervening dielectric material and from there be excited by a photon such as photon 17b to the excited state 13 of the second quantum dot.

Solar Cell Application

Since the required dimensions of each layer deposited during the formation of the artificial quantum dot material is small (Fig. 2), much smaller than the wavelength of light in such materials, it follows that the textures normally used in solar cells are on a much larger scale than the spacing of the quantum dots. Referring to Fig. 7 a quantum dot structure 21, 22, 23 similar to that seen in Fig. 3 is shown formed on a textured surface of a glass substrate 124. The local ordered arrangement of the dots 23 will determine the superlattice properties regardless of the roughness of the surface at optical wavelengths. It has been determined experimentally that the strength of the optical emission processes in related quantum well structures increases as the quantum wells become thinner. This is not unexpected since the quantum-mechanical rules that make these processes weak in bulk materials are relaxed in quantum confined geometries.

For silicon-on-glass technology, crystalline layers of 1.6 micron thickness are reported to give good results. The increase in optical strength by confinement depends on experimental details but is of the order of a factor of 10. Hence, required total thickness of the quantum dot artificial semiconductor material will be sub-micron, comparable to the optical wavelength in the material. This allows these layers to double as antirefiection layers, due to a lower "effective-medium" refractive index, and to be used in designs which allow a high intensity standing wave to be established in this material, further boosting the absorption properties of the layer.

Fig. 8 shows a band structure for a generic solar cell configuration where an artificial amorphous semiconductor cell 112 is illustrated surrounded by an up- converter 142 and a down converter 141. The artificial amorphous semiconductor cell 112 is quantum dot superlattice fabricated as described herein. In Fig. 8 the dot width is represented by the width of the lows 131 in the upper square wave, and the matrix width (dot spacing) is represented by the high edges 132 in the upper square wave. The effective bandgap is represented as the gap between the minibands in the valance band 116 and the respective conduction band 117 and increases with increasing quantum confinement (decreasing dot width). Light 67 enters the device of Fig. 8 through the substrate 24 which may be textured as with conventional crystalline silicon on glass (CSG) cells. The down- converter 141 is located between the substrate 24 and the artificial amorphous cell 112 and operates as described with regard to Fig. 6. Therefore the down-converter 141 provides an opportunity to capture high energy light that may have been absorbed inefficiently by the cell 112, before it enters the cell. Light emitted from the down- converter 141 will emit omni-directionally and so 50% will be emitted in directions away from the cell 141 and some of this light will be lost, however some will be reflected back into the cell by total internal reflection from the interfaces encountered in these directions. That part of the spectrum having longer wavelengths corresponding to photon energies less than the band gap of the cell 112 will pass through the down-converter 141 and solar cell 112 and will be potentially absorbed by the up-converter 142. This up-converter 142 is located between the artificial amorphous cell 112 and a reflective surface 143 and operates as described with regard to Fig. 5. Therefore the up-converter 142 provides an opportunity to capture low energy light that would not have otherwise been absorbed by the cell 112, and would have eventually reflected out of the device. Light emitted from the up-converter 142 will also emit omni-directionally and so 50% will be emitted in directions away from the cell 141, however this light will be reflected back towards the cell by the reflector 143 and will therefore have the same chance of absorption by the cell as the light emitted towards the cell. With reference to Fig. 9, a conventional crystalline silicon on glass (CSG) cell

29, 31, 32 is shown sandwiched between two quantum dot luminance converters 66, 65. After deposition of an optional barrier layer 25 (eg a thin silicon nitride layer) onto a textured glass superstrate 24, a quantum dot down-converter 66 is formed using the techniques described above, by a process such as Plasma Enhanced Chemical Vapour Deposition (PECVD) or another suitable deposition process. The heating step to form quantum dots may occur immediately or after subsequent processing. Operation of this down-converter will be as described with regard to Fig. 6. Note that the quantum dot down-converter material may also act as a suitable barrier layer removing the need for the barrier layer 25. The base line silicon cell, which is illuminated 67 through the glass superstrate

24, is then formed before the up-converter. In this case, an n⁺-type silicon layer 29 is deposited over the superlattice structure of the up-converter 142. A p-type silicon layer 31 is then deposited over the n⁺-type silicon layer 29 and a p⁺-type silicon layer 32 is deposited over p-type silicon layer 31. Each of the silicon layers 29, 31 and 32 are deposited as amorphous silicon layers by a process such as Plasma Enhanced Chemical Vapour Deposition (PECVD) or another suitable deposition process. The silicon layers can then be crystallised by solid phase crystallisation, possibly during a thermal anneal step. The step of crystallising the amorphous silicon may also be used to crystallise the quantum dots in the quantum dot down-converter 66 if the temperature of the preceding process steps has not already caused this to happen. Alternatively the step of crystallising the quantum dots in the quantum dot down-converter 66 may be completed as part of a further Rapid Thermal Anneal step towards the end of the processing sequence.

After formation of the doped solar cell layers (and possibly before crystallisation of those layers), a quantum dot up-converter 65 is formed using the techniques described above, again by a process such as Plasma Enhanced Chemical Vapour Deposition (PECVD) or an other suitable deposition process, compatible with the other formation steps of the device. Once again, the heating step to form quantum dots may occur immediately or after subsequent processing. Operation of this up-converter will be as described with regard to Fig. 5. Before contacts are formed on the device, it is scribed to create isolation grooves 39 to separate the individual cells, and a dielectric layer 33 is added (such as a layer of organic resin). The dielectric layer 33 and subsequently formed rear metal layer 36 may also act as a reflector removing the need to use a separate reflecting layer between the dielectric and the up-converter 65. Craters 34 and dimples 35 are then created to expose the front layer 29 and the back layer 32 respectively of the cell and the metal layer 36 is formed over the dielectric and extending into the craters and dimples to contact the front layer 29 and rear layer 32. Finally the metal is scribed to form isolation grooves 41, 42 between n-type and p-type contacts while links 43 are left in place to provide series connections between adjacent cells.

Fig. 9 and the associated description show one method of partitioning the material, initially deposited over the entire substrate or superstrate area, into individual cells and then interconnecting these. Variants of other well-established methods are also suitable. For example, either an up-converter or a down-converter could be used by itself with the cell structure of Fig. 9 or either or both could be used with cells fabricated on standard silicon wafers.

It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.

Claims

CLAIMS:

1. A luminance converter comprising a plurality of layers of organised quantum dots each layer comprising a plurality of crystalline semiconductor material quantum dots formed within the respective layers with size and location determined by parameters of said layer and wherein the energy gap for the material in the quantum dots of each layer is determined by selecting the material parameters including the size of the quantum dots, the composition of the matrix and the semiconductor material of the quantum dots, the layers of organised quantum dots being positioned adjacent to or in close proximity with each other, and adjacent, or near to adjacent, layers being layers of different quantum dots having different energy gaps, the energy gap of quantum dots in one of the layers of organised quantum dots having a differential energy equal to the target energy of output photons of the luminance converter and the energy gap of quantum dots in another of the layers of organised quantum dots having a differential energy less than or equal to a minimum target energy of input photons of the luminance converter and the quantum dots of layers that are adjacent to one another or in close proximity are sufficiently closely spaced to permit migration of electrons from energy states in quantum dots of one layer to energy states in quantum dots of the layer adjacent or in close proximity either through photon excitation to¹ a higher energy state or through relaxation to a lower energy state of the quantum dot of the layer adjacent or in close proximity.

2. The luminance converter of claim 1 wherein the luminance converter is associated with a photovoltaic junction device with which it is stacked.

3 The luminance converter of claim 2 wherein the photovoltaic junction is a junction of two semiconductor layers of p-type and n-type doped semiconductor material.

4. The luminance converter of claim 2 wherein the photovoltaic junction is an artificial amorphous semiconductor junction comprising an n-type region of artificial amorphous material adjacent a p-type region of artificial amorphous material, the n- type and p-type artificial amorphous materials being integrally formed as a matrix of dielectric material in which is substantially regularly disbursed a plurality of crystalline semiconductor material quantum dots and wherein the n-type and p-type regions are respectively doped with n-type and p-type dopant atoms.

5. The luminance converter of claim 1 wherein the luminance converter is associated with a photovoltaic junction device comprising an n-type region of artificial amorphous material adjacent a p-type region of artificial amorphous material, the n- type and p-type artificial amorphous materials being integrally formed as a matrix of dielectric material in which is substantially regularly disbursed a plurality of crystalline semiconductor material quantum dots and wherein the n-type and p-type regions are respectively doped with n-type and p-type dopant atoms and wherein the luminance converter is integrated into the junction device.

6. The luminance converter as claimed in claim 4 or 5 wherein a region in the vicinity of the junction between the n-type and p-type regions of the artificial amorphous material is undoped or has a balance of n-type and p-type dopants whereby the region behaves as intrinsic material.

7. The luminance converter as claimed in claim 4, 5 or 6 wherein the quantum dots of the photovoltaic cell are distributed in layers throughout the artificial amorphous material and each of the n-type and p-type regions include 20-30 layers of quantum dots formed by providing that number of each of the alternating stoichiometric and semiconductor rich layers.

8. The luminance converter as claimed in claim 7 wherein each of the n-type and p-type regions include 25 layers formed by providing that number of each of the alternating stoichiometric and semiconductor rich layers.

9. The luminance converter as claimed in claim 7 or 8 wherein the n-type and p- type regions are each in the range of 75 - 125 run thick

10. The luminance converter as claimed in claim 9 wherein the n-type and p-type regions are each lOOnm thick.

11. The luminance converter as claimed in any one of claims 2 to 10 wherein the luminance converter is an up converter for converting two or more low energy photons to a higher energy photon, and a first layer of organised quantum dots has quantum dots having a lower energy gap than the bandgap energy of the associated photovoltaic device and a second adjacent or near to adjacent layer of organised quantum dots has quantum dots having an energy gap equal to or higher than the bandgap energy of the associated photovoltaic device.

12. The luminance converter as claimed in any one of claims 2 to 11 wherein an artificial amorphous material photovoltaic cell is stacked in tandem with other artificial amorphous material photovoltaic cells and the band gaps of the artificial amorphous material cells are varied from cell to cell whereby each cell is optimised for a different wavelength of incident light on the tandem structure.

13. The luminance converter as claimed in any one of claims 2 to 11 wherein an a polycrystalline photovoltaic cell is stacked in tandem with one or more other photovoltaic cells comprising artificial amorphous material photovoltaic cells and the band gaps of the artificial amorphous material cells are varied from cell to cell and with respect to the polycrystalline cell whereby each cell is optimised for a different wavelength of incident light on the tandem structure.

14. The luminance converter as claimed in any one of claims 1 to 13 wherein the dielectric material of the luminance converter is selected from silicon oxide, silicon nitride silicon carbide, a mixture of these materials or a structure including layers of one or more of these materials or mixtures.

15. The luminance converter as claimed in any one of claims 1 to 14 wherein the semiconductor material of the quantum dots is selected from silicon or a silicon alloy germanium, gallium arsenide or Indium phosphide.

16. The luminance converter as claimed in claim 15 wherein the semiconductor material of the quantum dots is silicon alloyed with germanium.

17. The luminance converter as claimed in any one of claims 1 to 16 wherein a quantum dot luminance converter is stacked in tandem with one or more other quantum dot luminance converters having different energy conversion characteristics.

18. A method of forming a luminance converter, comprising a plurality of layers of organised quantum dots, each layer comprising a plurality of crystalline semiconductor material quantum dots formed within the respective layers with size and location determined by parameters of said layer and quantum dots of a respective layer being different to those of an adjacent, or near to adjacent, layer, the quantum dots of each layer having a selected energy gap, the method comprising; forming a plurality of layers of dielectric material each layer comprising a compound of a semiconducting material, wherein alternating layers are layers of stoichiometric dielectric material and layers of semiconductor rich dielectric material respectively, and heating the layers of dielectric material to cause quantum dots to form in each of the semiconductor rich layers of dielectric material whereby the quantum dots so formed are distributed within each layer of semiconductor rich dielectric material, and wherein the energy gap of the quantum dots in each layer of semiconductor rich dielectric material are determined by selecting the material parameters including the thickness of the layer of semiconductor rich dielectric material, the thickness of the layers of stoichiometric dielectric material on either side of the respective layer of semiconductor rich dielectric material and the semiconductor material of the semiconductor rich dielectric material layer to achieve the desired energy gap parameters of the material, adjacent layers each having quantum dots with different energy gaps, and an energy gap differential energy of the quantum dots of one of the layers of organised quantum dots being essentially equal to the target energy of output photons of the luminance converter and an energy gap differential energy of the quantum dots of another of the layers of organised quantum dots, adjacent or in close proximity to said one of the layers of organised quantum dots, being less than or equal to a minimum target energy of input photons of the luminance converter and the quantum dots of layers that are adjacent to each other or in close proximity are sufficiently closely spaced to permit migration of electrons from energy states in quantum dots of one layer to energy states in quantum dots of the layer adjacent or in close proximity either through photon excitation to a higher energy state or through relaxation to a lower energy state of the quantum dot of the layer adjacent or in close proximity.

19. The method of claim 18 wherein the luminance converter is associated with a photovoltaic junction device and the method further comprises stacking the luminance converter with the photovoltaic junction device, such that in use, the luminance converter will absorb incident light and reemit the light for subsequent absorbtion by the photovoltaic junction device.

20 The method of claim 19 wherein the photovoltaic junction is formed by forming two semiconductor layers of different dopant polarity with a junction therebetween.

21. The method of claim 19 wherein the photovoltaic junction is created by forming a plurality of layers of dielectric material comprising a compound of a semiconducting material, wherein alternating layers are layers of stoichiometric dielectric material and layers of semiconductor rich dielectric material respectively, doping regions of the plurality of layers of dielectric material with p-type and n- type dopants either simultaneously with their formation or subsequently and heating the layers of dielectric material to cause quantum dots to form in the semiconductor rich layers, wherein the bandgap and mobility are determined by selecting the material parameters including the size of the quantum dots, the composition of the matrix and the semiconductor material of the quantum dots to achieve the desired parameters.

22. The method of claim 18 wherein the photovoltaic device is created by forming a plurality of layers of dielectric material comprising a compound of a semiconducting material, wherein alternating layers are layers of stoichiometric dielectric material and layers of semiconductor rich dielectric material respectively, doping regions of the plurality of layers of dielectric material with p-type and n- type dopants either simultaneously with their formation or subsequently and heating the layers of dielectric material to cause quantum dots to form in the semiconductor rich layers, wherein the bandgap and mobility are determined by selecting the material parameters including the size of the quantum dots, the composition of the matrix and the semiconductor material of the quantum dots to achieve the desired parameters., the luminance converter being integrated into the junction device as a continuation of the matrix of the junction device.

23. The method as claimed in claim 21 or 22 comprising forming a region between the n-type and p-type regions of the artificial amorphous material in the vicinity of the junction an undoped region or a region having a balance of n-type and p-type dopants whereby the region behaves as intrinsic material.

24. The method as claimed in claim 21, 22 or 23 comprising forming 20-30 layers of each of the alternating stoichiometric and semiconductor rich layers for each of the n-type and p-type regions.

25. The method as claimed in claim 24 comprising forming 25 layers of each of the alternating stoichiometric and semiconductor rich layers for each of the n-type and p- type regions.

26. The method as claimed in claim 24 or 25 comprising forming each of the alternating layers of stoichiometric and semiconductor rich layers for the n-type and p- type regions to a thickness in the range of 1.5 to 2.5 nm.

27. The method as claimed in claims 26 comprising forming each of the alternating layers of stoichiometric and semiconductor rich layers for the n-type and p-type regions to a thickness of 2.0 nm.

28. The method as claimed in any one of claims 19 to 27 wherein the luminance converter is an up converter for converting two or more low energy photons to a higher energy photon, the method comprising forming a first layer of organised quantum dots which has quantum dots with a lower energy gap than the bandgap energy of the associated photovoltaic device and forming a second adjacent or near to adjacent layer of organised quantum dots which has quantum dots with an energy gap equal to or higher than the bandgap energy of the associated photovoltaic device.

29. The method as claimed in any one of claims 19 to 28 wherein an artificial amorphous material photovoltaic cell is stacked in tandem with other artificial amorphous material photovoltaic cells and the band gaps of the artificial amorphous material cells are varied from cell to cell whereby each cell is optimised for a different wavelength of incident light on the tandem structure.

30. The method as claimed in any one of claims 19 to 28 wherein an a poly crystalline photovoltaic cell is stacked in tandem with one or more other photovoltaic cells comprising artificial amorphous material photovoltaic cells and the band gaps of the artificial amorphous material cells are varied from cell to cell and with respect to the polycrystalline cell whereby each cell is optimised for a different wavelength of incident light on the tandem structure.

31. The method as claimed in any one of claims 18 to 30 comprising forming dielectric material layers of the luminance converter in materials selected from silicon oxide, silicon nitride silicon carbide, a mixture of these materials or a structure including layers of one or more of these materials or mixtures.

32. The method as claimed in any one of claims 18 to 31 comprising forming the quantum dots from semiconductor material selected from silicon or a silicon alloy germanium, gallium arsenide or Indium phosphide.

33. The method as claimed in claim 32 comprising forming the quantum dots from silicon alloyed with germanium.

34. The method as claimed in any one of claims 18 to 33 comprising stacking the quantum dot luminance converter in tandem with one or more other quantum dot luminance converters having different energy conversion characteristics.