GB2496200A - Nitride Photovoltaic or Photoconductive Devices - Google Patents

Nitride Photovoltaic or Photoconductive Devices Download PDF

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GB2496200A
GB2496200A GB201119197A GB201119197A GB2496200A GB 2496200 A GB2496200 A GB 2496200A GB 201119197 A GB201119197 A GB 201119197A GB 201119197 A GB201119197 A GB 201119197A GB 2496200 A GB2496200 A GB 2496200A
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nanoparticles
nitride
layer
electrode
nanoparticle
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GB201119197D0 (en
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Katherine Louise Smith
Richard Simon Tuley
Ian Robert Sellers
Stephen Day
Matthias Kauer
Peter Neil Taylor
Michael Alan Schreuder
Marcelo Jose Pastorello Alcocer
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Sharp Corp
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Sharp Corp
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Abstract

A photoconductive or photovoltaic device comprising: a first electrode 2; a second electrode 4; and a layer of core-shell nitride nanoparticles 3 disposed between the first and second electrodes 2,4. Also disclosed is a method of making a photoconductive or photovoltaic device, the method comprising: forming a layer of nitride nanoparticles 3 in contact with a first electrode 2; and forming a second electrode 4 in contact with the layer of nitride nanoparticles 3 but not in contact with the first electrode 2. Further disclosed is a method of forming nitride nanoparticles comprising: reacting nanoparticles precursors in solution in a first solvent in an oxygen-free atmosphere, the first solvent being a high boiling point solvent; and adding an acetonitrile precipitant to the solution to precipitate the nitride nanoparticles out of the solution. The nitride nanoparticles may consist of III-N, or II-III-N or II-N compounds. The shell of the core-shell nitride nanoparticles may consist of zinc sulphide (ZnS).

Description

Nitride Photovoltaic or Photoconductive Devices
Technical field
The present invention is in the field of nanostructures. More specifically, the invention discloses the use of nitride nanoparticles in photovoltaic devices.
Background art
Historically almost all of the world's energy has been supplied through the consumption of fossil fuels, such as coal, oil and natural gas. However, these are finite resources and the world is entering a crucial transition period, from nearly total reliance on fossil fuels to a sustainable energy future based on renewable energy sources. There is a growing market in the use of renewable energy sources and solar energy is a major component of this trend. There are other factors than basic cost which determine the competitiveness of an energy technology, such as modularity, image and government subsidy, but in order be truly competitive with fossil fuels and for any real massive market penetration to be viable, the cost of solar energy ideally needs to reach grid parity (the point at which renewable electricity is equal to or cheaper than grid power).
The rising cost of conventional methods of energy production will contribute towards this but a reduction in cost and increase in efficiency of solar energy is also required.
Solar energy will become cheaper as its market share grows. One of the major differences between solar energy and fossil fuels is that the major cost for solar energy occurs during the installation as capital costs while the cost of fossil fuels are dominated by the fuel costs. To date, silicon has been the main semiconductor used in commercial solar technology. However, it has problems of high equipment cost and high manufacturing cost. Alternative solar cell technologies which can reduce the capital costs of energy production would allow easier transition to the adoption of solar cell technologies. The other vital factor to be considered when evaluating solar technology is the efficiency of the photovoltaic module. The best in class crystalline silicon modules have materials with theoretical limits of 33% efficiency and in production as modules these devices have an efficiency of typically around 15%. The successful development of alternative materials and device structures could result in a four-fold increase in the overall efficiency.
Semiconductor nanoparticles, otherwise known as quantum dots or nanocrystals, are nanometer scale structures that are composed of semiconductor materials.
Due to the small size of the crystals (typically mm -lOOnm) quantum confinement effects occur. Therefore the optical and electronic properties are dependent on the size and shape of the nanoparticles as well as the properties of the bulk materials of the material(s) of which the nanoparticle is composed.
Colloidal nanoparticles may have their surface capped by an organic overcoating (surfactant) which confers additional properties compared to non-coated nanoparticles. The organic layer helps to protect the nanoparticles from oxidation and chemical attack; passivates' the surface traps (i.e., offers atoms to compensate for dangling bonds at the particle surface) with the result of increasing the luminescence; preventing aggregation between nanoparticles; offering tuneable reactivity and surface functionalization. Colloidal inorganic nanoparticles retain the primary advantages of organics; scalable, controlled synthesis and solution processing, which will allow colloidal inorganic nanoparticle devices to be produced at much cheaper costs than conventional silicon solar cells. Colloidal nanoparticles can be synthesized by a large number of routes [Cushing et al., Chem. Rev., 2004, 104, 3893]. Major progress has been achieved in the synthesis of, for example, colloidal cadmium selenide nanoparticles through various wet chemistry routes that allow excellent size control and band-gap tunability. However cadmium selenide is a very toxic material and many efforts have been made over the recent years to develop synthetic methods to yield high quality nanoparticles of alternative semiconductor materials that are non-toxic and can offer similar optical properties. Semiconductors nanoparticles made from a wide range of materials have been studied including ll-V, Il-VI and Ill-V semiconductors. In addition to spherical nanoparticles rod-, arrow-, teardrop-and tetrapod-shaped nanoparticles [Alivisatos et. al., J. Am. Chem. Soc, 2000, 122, 12700; W003054953] and core/shell structures [Bawendi, J. Phys. Chem. B, 1997, 1010, 9463; Li and Reiss, J. Am. Chem. Soc., 2008, 130, 11588] have also been prepared.
Ill-V semiconductors are numerous and one of the most interesting classes of III-V semiconductors is the Ill-nitrides, such as AINJ, GaN, InN and their respective alloys. These are used for the manufacture of blue light-emitting diodes, laser diodes and power electronic devices. Nitrides are also chemically inert, are resistant to radiation, and have large breakdown fields, high thermal conductivities and large high-field electron drift mobilities, making them ideal for high-power applications in caustic environments [Neumayer at. al., Chem., Mater., 1996, 8, 25]. The band gaps of aluminium nitride (6.2eV), gallium nitride (3.5eV) and indium nitride (0.7 eV) [Gillan et. al., J. Mater. Chem., 2006, 38, 3774] mean that nitrides span much of the ultraviolet, visible and infrared regions of the electromagnetic spectrum. The fact that alloys of these materials have direct optical band gaps over this range makes these very significant for optical devices.
0B2467161 patent discloses a nitride nanoparticle, the nanoparticle being a light-emissive nanoparticle and having a photoluminescence quantum yield of at least 1%. It also discloses the increased stability and improved photoluminescent quantum yield of lnN-ZnS core-shell nanoparticles in solution compared with core-only material. In addition, GB2467162 teaches how to make such nanoparticles, but does not teach how to form a solar cell using nitride nanoparticles, nor does it show that nitride nanoparticles may be stable enough to be formed into a device when precipitated out of solution. It does not teach that the increased stability of core-shell nanoparticles observed in solution is expected to also apply to layers of nanoparticles in a device, since in other similar material systems core-only nanoparticles would be used in devices.
As described in Adv. Mater. 2011, 23, 12-29. In the solution phase, a core/shell approach has been found to improve the photoluminescence yield of CdSe CODs by the growth of ZnS shell. Even when the shell is not epitaxial to all facets of the core, such as in the PbS/CdS core/shell system, photoluminescence emission intensity has been seen to improve appreciably. Related systems such as PbSe/PbS and PbSe/PbSeS1 core-shell CODs, as well as PbSe/PbS core-shell nanowires, have seen similar study. To date, only air stability and photoluminescence yield -and not absolute device performance -have been reported for lead chalcogenide core/shell CODs. Shell growth, while reducing surface defects, is expected to come at a cost to carrier transport. Core/shell CODs generally confine one (type II core-shell) or two (type I core-shell) carrier wave function inside the core, militating against delocalization of either both or one charge carrier. A delicate balance between surface passivation and carrier transport necessitates the careful engineering of ultrathin shell growth. All of the best device results reported with these materials are with core-only nanoparticles.
Co-pending UK patent application No. 1012646.4 discloses group Il-Ill-V and Il-V semiconductor compounds, for example ZnGaN, ZnInN, ZnlnGaN, ZnAIN, ZnAIGaN, ZnAIInN or ZnAlGalnN. The type group Il-Ill-V means that the semiconductor compound consists of one or more group II elements from the periodic table, one or more group Ill elements from the periodic table and one or more group V elements from the periodic table. A Il-Ill-V alloy material is defined as containing at least 1% of each group II, Ill and V element atoms. Co-pending UK patent application No. 1012644.9 teaches how to make such nanoparticle compounds, but not how to produce an electrically active device. Devices made from synthesized nanoparticles as described in these applications do not produce photoconductive devices without further preparation steps.
US27012355A1 discloses a solar cell including a semiconductor base layer, a semiconductor nanoparticle complex over the semiconductor base layer, and a semiconductor emitter layer formed over the semiconductor nanoparticle complex. The semiconductor nanoparticle complex includes nanoparticle cores dispersed in an inorganic matrix material. A corresponding method is also disclosed.
US2009/0217973A1 discloses a photovoltaic device having a first electrode layer, a high resistivity transparent film disposed on the first electrode, a second electrode layer, and an inorganic photoactive layer disposed between the first and second electrode layers, wherein the inorganic photoactive layer is disposed in at least partial electrical contact with the high resistivity transparent film, and in at least partial electrical contact with the second electrode. The photoactive layer has a first inorganic material and a second inorganic material different from the first inorganic material, wherein the first and second inorganic materials exhibit a type II band offset energy profile, and wherein the photoactive layer has a first population of nanostructures of a first inorganic material and a second population of nanostructures of a second inorganic material.
US7750235 discloses nanocomposite photovoltaic devices that include semiconductor nanoparticles as at least a portion of a photoactive layer.
Photovoltaic devices and other layered devices that comprise core-shell nanostructures and/or two populations of nanostructures, where the nanostructures are not necessarily part of a nanocomposite, are also features of the invention. Varied architectures for such devices are also provided including flexible and rigid architectures, planar and non-planar architectures and the like, as are systems incorporating such devices, and methods and systems for fabricating such devices. Compositions comprising two populations of nanostructures of different materials are also a feature of the invention.
US6512172 discloses a method of producing a photo device containing a layer of nanometer sized particles and a conducting polymer in solid state, wherein the layer is made by mixing the nanometer sized particles in colloid with precursor polyparaphenylenevinylene or a derivative thereof and wherein the nanometer sized particle is selected from the group consisting of Ti02, ZnO, 7r02 and Sn02.
US2O11/0146766 discloses a solar cell comprising of a nanoparticle film of a single material between two electrodes, but does not mention nitride devices.
US 2011/0240106 and W02011126778 disclose a photovoltaic device with light absorbing metal chalcogenide nanoparticles (such as PbS nanoparticles). They disclose nanoparticles with a double shell structure of an inorganic anion and cation, which shell structure replaces the organic ligands that would normally surround the nanoparticles.
Description of figures
Figure 1 shows a solar panel system Figure 2 shows a photovoltaic device with a nitride nanoparticle layer.
Figure 3 shows a summary of a typical process flow for fabrication of a device.
Figure 4 shows the current-voltage characteristics of a device with a structure such as that in Figure 2 and demonstrates the existence of photovoltaic behaviour.
Figure 5 shows the varying current with excitation wavelength of a device with a structure such as that shown in Figure 2.
Figure 6 shows the current-voltage characteristics of a device with a structure such as that in Figure 2.
Figure 7 shows a device with a nitride nanoparticle layer where the nitride nanoparticles have a core/shell structure.
Figure 8 shows a comparison of core/shell and core-only nitride nanoparticles as the photoactive layer.
Figure 9 shows a first electrode layer patterned on a substrate.
Figure 10 shows a patterned first electrode layer and a patterned second electrode layer.
Figure 11 shows a comparison between annealed and unannealed devices Figure 12 shows a device with the nitride nanoparticles in a polymer matrix.
Figure 13 shows a device with particles present in the active layer in addition to the nitride nanoparticles.
Figure 14 shows a device with more than one population of nitride nanoparticles.
Figure 15 shows a device with more than one population of nitride nanoparticles where the nitride nanoparticles are randomly dispersed.
Figure 16 shows a device with an active layer consisting of one monolayer of nitride nanoparticles.
Figure 17 shows a multijunction device with two different populations of nitride nanoparticles.
Summary of the invention
A first aspect of the present invention provides a photovoltaic or photoconductive device comprising a first electrode, a second electrode, and a layer of nitride nanoparticles between the two electrodes. The layer of nanoparticles may be in contact with the first electrode and/or the second electrode, or alternatively there may be one or more intervening layers.
The nitride nanoparticles consist of compounds having the composition Ill-N, or Il-Ill-N or Il-N, and the nitride narioparticle layer exhibits photovoltaic or photoconductive behaviour.
For the purposes of this application, by a "nanoparticle" is meant a particle that is of nanoscale dimensions in all three dimensions, for example that is of the order of 1 to lOOnm and more preferably of the order of 1 to 3Onm in all three dimensions (in contrast to, for example, a nanowire which is of nanoscale dimensions in only two dimensions). Such nanoparticles can produce quantum confinement effects, and include the nanoparticles sometimes referred to as a "quantum dot". A nanoparticle of the invention may have a crystalline or polycrystalline structure and so form a nanocrystal, or it may have an amorphous structure. The nitride nanoparticles preferably have approximately the same size in all three dimensions, so that the nanoparticles are approximately spherical or tear-drop in shape.
For the purpose of this invention specifying that a compound has a composition containing certain components is intended to mean that the compound must consist of at least 1% of each of the stated components. So for example in a nanoparticle having the composition Il-Ill-N: at least 1% of the atoms will be one or more group II atoms, at least 1% of the atoms will be one or more group Ill atoms and at least 1% of the atoms will be nitrogen atoms. This is in contrast with, for example, the known doping of a Ill-N semiconductor with a group II element (e.g. Mg) to change its electrical conductivity -in this case the only a tiny amount of the group II element is typically needed to dope a Ill-V semiconductor (well below 1%), and the addition of the group II element as a dopant does not lead to the formation of an Il-Ill-N compound.
The layer may consist of Ill-nitride quantum dots, of Ill-nitride colloidal nanoparticles, of Il-nitride or Il-Ill-nitride quantum dots, or of Il-nitride or Il-Ill-nitride colloidal nanoparticles. For a nanoparticle to be a "quantum dot" implies that the exciton is quantum confined in all three dimensions i.e. the quantum dot is smaller than the exciton in the bulk material. All quantum dots are nanoparticles, but not all nanoparticles are quantum dots.
The nitride nanoparticles in the layer may be shelled.
The shell may be composed of a material which is not a nitride compound, such as ZnS The shell may be composed of a second Il-N or II-Ill-N or Ill-N compound, the second Il-N or II-Ill-N or Ill-N compound having a different composition to the first Il-N or II-Ill-N or Ill-N compound. The shell may be surrounded by organic ligands. These ligands may include hexadecanethiol and stearate ligands.
The first electrode may be a transparent electrode. Suitable materials include, but are not limited to, a transparent conductive oxide such as indium tin oxide (ITO).
The device may further comprise a substrate, the first electrode being disposed on the substrate. The substrate may, for example, be made of a glass or of a plastics material.
The first electrode may form an ohmic contact to the layer of nanoparticles.
The second electrode may be a metal electrode. Suitable materials include, but are not limited to, aluminium.
A Schottky barrier may be formed, in use, at the interface between the second electrode and the layer of nanoparticles. A Schottky barrier refers to the electrostatic potential barrier between the electrode and the nanoparticles, resulting in an in-built electric field in the nanoparticle layer. This effect arises from differences in work function between the electrode and the nanoparticle layer, and the effect of surface states.
The layer may comprise a first population of nitride nanoparticles and a second population of nitride nanoparticles different from the first population.
Nanoparticles of the first population may have different light-absorbing properties to nanoparticles of the second population.
The first population of nitride nanoparticles may be comprised in a first sub-layer and the second population of nitride nanoparticles may be comprised in a second sub-layer.
Alternatively, nanoparticles of the first population and nanoparticles of the second population may be randomly dispersed throughout the layer.
The device may comprise a second layer of nitride nanoparticles disposed over the second electrode, and a third electrode disposed over the second layer of nitride nanoparticles.
The layer may comprise a first population of nitride nanoparticles and the second layer may comprise a second population of nitride nanoparticles different from the first population.
Nanoparticles of the first population may have different light-absorbing properties to nanoparticles of the second population.
Nanoparticles of the first population may have a different shape and/or size to nanoparticles of the second population.
A second aspect of the present invention provides a method of making a nitride photoconductive or photovoltaic device, the method comprising the steps of: forming a layer of nitride nanoparticles in contact with a first electrode; and forming a second electrode in contact with the layer of nitride nanoparticles but not in contact with the first electrode.
The nanoparticles may be core-shell nanoparticles.
The shell may be composed of a material which is not a nitride compound, such as ZnS The shell may be composed of a second Il-N or II-Ill-N or Ill-N compound, the second Il-N or II-Ill-N or Ill-N compound having a different composition to the first Il-N or II-Ill-N or Ill-N compound. The shell may be surrounded by organic ligands. These ligands may include hexadecanethiol and stearate ligands.
Forming the layer of nitride nanoparticles may comprise: reacting nanoparticle precursors in solution in a first solvent under an oxygen-free atmosphere thereby to form nitride nanoparticles, removing the nitride nanoparticles from solution, and disposing the nitride nanoparticles on the first electrode thereby to form the layer of nitride nanoparticles.
The first solvent may be a high-boiling point solvent. By a "high boiling point" solvent is meant a solvent that has a boiling point that is sufficiently greater than the desired reaction temperature, so that the reaction does not need to be performed in a pressure vessel. Preferably the boiling point is greater than 50°C higher than the reaction temperature.
Removing the nitride nanoparticles from solution may comprise precipitating the nitride nanoparticles from solution.
The method may comprise adding a precipitant to the nitride nanoparticle solution, the precipitant being more polar than the high-boiling point solvent.
Many different quantitative scales exist to define the polarity of a solvent, but the approximate scale adopted here is the dielectric constant of the solvent, with a more polar solvent possessing a higher dielectric constant.
The precipitant may be acetonitrile.
To improve miscibility of the high-boiling point solvent and the precipitant, a third component may be added.
This third component may be toluene.
Removing the nitride nanoparticles from solution may comprise distilling the solution.
The method may comprise dissolving the removed nitride nanoparticles in a second solvent, and precipitating the nitride nanoparticles from the second solvent. This removes impurities from the nitride nanoparticles. The process of dissolving the removed nitride nanoparticles in the second solvent and precipitating the nitride nanoparticles may be repeated several times to purify the nitride nanoparticles to a desired level.
The nitride nanoparticles may be deposited on the first electrode by spin coating.
The method may comprise annealing the layer of nitride nanoparticles. This is found to increase the conductivity of the layer of nanoparticles.
The method may comprise, after the annealing, forming a second layer of nitride nanoparticles in contact with the first layer of nitride nanoparticles. It can happen that annealing a layer of nanoparticles leads to cracks or pinholes forming in the layer, and forming second layer of nitride nanoparticles (this second layer is a layer of nanoparticles having the same composition as the nanoparticles of the first layer of nitride nanoparticles) will close up cracks or pinholes caused by the annealing. The process of annealing the layer of nitride nanoparticles and forming a further layer of nitride nanoparticles over the annealed layer may be repeated several times.
A third aspect of the invention provides a device formed by a method of the second aspect.
A third aspect of the invention provides a method of forming nitride nanoparticles comprising: reacting nanoparticle precursors in solution in a first solvent under an oxygen-free atmosphere thereby to form nitride nanoparticles, the first solvent being a high-boiling point solvent; and adding a precipitant to the solution thereby to precipitate the nitride nanoparticles from solution, the precipitant being acetonitrile.
The main object of the present invention is to demonstrate photovoltaic behaviour from nitride nanoparticles. This will enable the development of low-cost efficient nitride nanoparticle solar cell technology.
As shown in Figure 2 a first layer (2) is deposited on a substrate (1) to provide a first electrode. A layer of nitride nanoparticles (3) is deposited on the first electrode (2) then a second electrode (4) is deposited on the nitride nanoparticle layer (3). The substrate layer (1) may consist of any suitable material including but not limited to silicon, glass, quartz or plastic. The first electrode layer (2) may consist of any suitable material including but not limited to indium tin oxide (ITO) or fluorine-doped tin oxide (FTO). Layers (1) and (2) may also be commercially supplied preformed for example as ITO on glass, in which case the resultant PV device may be illuminated from below, through the substrate, thereby allowing use of a conventional metal electrode as the second electrode 4 (although in principle a PV device could be embodied using a transparent second electrode 4 to allow illumination from above). The thickness of the first electrode layer (2) layer may be in the range 5nm -500nm. The thickness of the first electrode layer (2) layer may be 300nm.
The nitride nanoparticle layer may consist of any nitride nanoparticles with the composition Ill-N, Il-Ill-N or Il-N. This includes but is not limited to GaN, InN, AIN, InGaN, AIGaN, InAIN, InGaAIN, ZnN, ZnGaN, ZnlnN, ZnlnGaN, ZnAIN, ZnAIGaN, ZnAIInN or ZnAIGaInN. The nitride nanoparticles may have a core-only structure or may have a core-shell structure. The nitride nanoparticles preferably have approximately the same size in all three dimensions, so that the nanoparticles are approximately spherical or tear-drop in shape. The size of the individual nitride nanoparticles may be in the range mm -lOOnm. The size of the individual nitride nanoparticles may be in the range 5nm -SOnm. The thickness of the nanoparticle layer may be between mm and l000nm, and may for example be between lOOnm and 200nm.
The thickness of the second electrode layer (4) may be in the range mm -l000nm. The thickness of the second electrode layer (4) may be in the range 5Onm -200nm.
Figure 4 shows the current-voltage characteristics of a device with a structure such as that in Figure 2 when illuminated with O.93Wcm2 of light at 400nm from a Ti-sapphire laser and demonstrates the existence of photovoltaic behaviour.
This demonstration of photovoltaic behaviour shows for the first time that nitride nanoparticles are suitable for use in photodevices. Figure 5 shows the varying photocurrent with excitation wavelength of a device with a structure such as that in Figure 2 and demonstrates the existence of photoconductive behaviour in a device with a nitride nanoparticle layer (3) consisting of an InN core and a ZnS shell. Figure 6 shows the current-voltage characteristics of a device with a structure such as that in Figure 2 when illuminated with lWcm2 of light from a xenon bulb and demonstrates the existence of photoconductive behaviour in a device with a nitride nanoparticle layer (3) consisting of an InN core and a ZnS shell.
In a second aspect of the present invention the nitride nanoparticles have a core-shell structure as shown in Figure 7. Layers 1, 2 and 4 of Figure 7 are as described in Figure 2. The nitride nanoparticle layer (3) consists of nanoparticle with a core/shell structure where a shell of a second material is grown directly onto the surface of the nitride nanoparticle (which forms the core). The core may be a nitride nanoparticle with the composition Ill-N, Il-Ill-N or Il-N. The size of the core nitride nanoparticles may be in the range mm -lOOnm. The size of the core nitride nanoparticles may be in the range 5nm -SOnm. The shell may be formed from any suitable inorganic material that is different from the core material, this includes but is not limited to ZnS, ZnSe, GaN, InN, AIN, InGaN, AIGaN, InAIN, InGaAIN, ZnN, ZnGaN, ZnlnN, ZnlnGaN, ZnAIN, ZnAIGaN, ZnAIInN or ZnAlGalnN. The overall core-shell nanoparticle size may be in the range mm -lOOnm. The overall core-shell nanoparticle size may be in the range mm -5Onm.
The prior art teaches that the best nanoparticle solar cells are obtained using core-only structures as this provides the highest density layer and best carrier transport characteristics. However, we have discovered that as well as improving the stability of the nanoparticles in solution, a shell layer also improves the characteristics of a photodevice as shown in Figure 8. This result is in contrast to the nanoparticle solar cell prior art in which core-only structures devices give the best results as described in Adv. Mater. 2011, 23, 12-29.
Description of embodiments
A first embodiment of the present invention is shown in Figure 2; a nitride nanoparticle layer between two electrodes exhibits photovoltaic behaviour and forms part of a solar panel. When illuminated by the sun the solar panel generates electric current which is routed to either a battery system or provide dc power or an inverter to provide AC power.
In a second embodiment of the present invention a colloidal core-shell nitride nanoparticle is synthesized for use in a photovoltaic cell. A summary of the process flow is shown in Figure 3. The reaction should be carried out in an oxygen-free atmosphere, for example a nitrogen atmosphere, by using a glovebox. Alternatively a Schlenk line or an argon atmosphere may be used. A high boiling point solvent, eg octadecene, is preferably used to avoid the need to perform the reaction under pressure. Mix 300mg lnl3, 380mg zinc stearate and 500mg sodium amide; and add to a flask. Add 20m1 octadecene (ODE) then 75omicroL hexadecanethiol (HDT) and heat while stirring the solution. The mixture may be heated to between 150°C and 300°C. The mixture may be heated to 225°C. The nanoparticle size will increase with increasing reaction time, therefore the reaction time depends on the size required. The reaction time may be greater than I mm and less than 10 hours. The reaction time may be 75mm.
After the reaction, toluene may be added to aid nanoparticle solubility, for example by adding lOmI of toluene. Solid impurities are removed from the reaction mix by centrifugation. To shell the core-only nanoparticles with ZnS, add 500mg zinc diethyldithiocarbamate ("ZnDEDTC") to a new flask, add the core-only nanoparticle solution and heat while stirring. The solution may be heated to between 100°C and 300°C. The solution may be heated to 175°C. The reaction time may be between 1mm and lOhours. The reaction time may be 60mm. Once cooled, toluene may be added to improve solubility and solid impurities are removed from the reaction mix by centrifugation. In order to deposit nanoparticles films, the nanoparticles must be purified and removed from the high boiling point solvent (octadecene). This can either be achieved by distillation or precipitation to remove the high boiling point solvent, and further precipitation to purify them.
In the preferred embodiment, a multi-step precipitation method is used. Many chemicals are found to precipitate the nanoparticles, including but not limited to chemicals with a higher polarity than toluene, such as isopropanol, ethanol, and acetonitrile. This precipitation step can lead to damage of the nanoparticles, manifested as a blue shift in their photoluminescence and/or a loss of solubility.
Using acetonitrile to precipitate the nanoparticles has been found to lead to the smallest blue shift, by rendering the QDs insoluble while minimizing the presence of any oxygen species. Since acetonitrile and octadecene are immiscible, a three component system must be initially used to aid miscibility. The third component can be toluene. For example, to precipitate the nanoparticles from ODE add l4ml toluene and 18m1 acetonitrile to lOmI of reaction mix and mix then centrifugate (for example at 3000rpm for 10mm) to separate the nanoparticles from the solution. The quantity of toluene may be in the range 1-1 OOml and the quantity of acetonitrile in the range 1-lOOmI. Dispose of the supernatant and redissolve the nanoparticles in toluene. Add further acetonitrile to precipitate the nanoparticles again, and re-isolate them by centrifugation. Dispose of the supernatant and allow the precipitate to dry. The nanoparticles can be precipitated and redissolved in this way four times. The nanoparticles can be precipitated and redissolved in this way between one and twenty times. Finally dissolve the nanoparticles in a small volume of toluene (e.g. 0.2ml). Other nanoparticles solvent and precipitant combinations may be used as well as or instead of toluene and acetonitrile in this purification stage. To improve final film quality, it is advantageous to remove any particulates or aggregates from the solution. This can be done by sonication, centrifugation and filtering, for example, sonicate for 20mm to break up any nanoparticle aggregates, centrifuge for 15mm at 3000rpm to remove insoluble impurities then pass the solution through a 0.2pm filter in a glovebox. It is also advantageous to thoroughly dry the final precipitate to remove any remaining precipitant, for example by gentle heating (e.g. at 40°C) or by placing the precipitate under vacuum. The purified mixture may be diluted with toluene to control the device thickness, for example dilute in the ratio 5OuL toluene to 3OuL of nanoparticle mixture to achieve a nanoparticle concentration in the range 0.001-lmg/uL. To form this mixture into a device, spin a layer of the mixture onto a patterned ITO substrate at 4000rpm for 6osec. The spin speed may be in the range 500-6000rpm, and can be used to control film thickness.
Alternatively the device layer may be deposited by doctor blade or dip coating techniques. The patterned ITO substrate may consist of ITO (2) deposited on glass (1) and formed into electrodes as shown in Figure 9. The electrode width (Elw) may be in the range 0.1-100mm. The electrode width may be 2mm. The nanoparticle layer thickness may be between mm and l000nm. The layer thickness may be between lOOnm and 200nm. Deposit the second electrode.
The second electrode may be deposited by evaporation, sputtering or screen printing. In the preferred embodiment the electrode is thermally evaporated. The second electrode may be formed of aluminium. The second electrode may be between 5nm and l000nm thick. The second electrode may be lOOnm thick. The second electrode fingers may be deposited perpendicular to the ITO electrodes on the patterned substrate as shown in Figure 10. The width of the second electrode (E2w) may be in the range 0.1-100mm. The electrode width may be 2mm.
In a third embodiment of the present invention a colloidal core-shell nitride nanoparticle is synthesized for use in a photovoltaic cell where the shell is built up layer by layer. The reaction should be carried out in a nitrogen atmosphere, such as a glovebox. The synthesis of the core nanoparticles proceeds as in the second embodiment. The layer by layer shelling method restricts the amount of shelling precursor in the reaction mix at any one time, allowing more control over the shell formation. For a layer-by-layer shell process add 135mg ZnDEDTC in I.5m1 ODE and 1.5m1 octylamine then shake well to dissolve. Add a set volume to the reaction mix, which is limited in precursor quantity to only allow a set thickness (for example, nominally around 0.5 monolayers) to grow then heat at 185°C for 20mm. Allow to cool for 10mm then repeat as many times as required.
For example these quantities will provide up to 13 shelling steps. Proceed as described in the second embodiment to precipitate and deposit the nanoparticles and add the second electrode.
In the fourth embodiment, the solar cell is produced as in the second embodiment, but the core nanoparticles can be made with a different nitride material, such as replacing the indium precursor with an aluminium precursor.
For example, the core nanocrystals can be made using 102mg All3 (0.25mmole), 474mg Zinc Stearate (0.75mmole), 390mg Sodium Amide (lOmmole) in 25ml ODE (octadecene) and 462microL HDT (hexadecanethiol).
In the fifth embodiment, after the fabrication of nanocrystals films as in the previous embodiments, the films may be annealed in an oxygen free environment, such as under nitrogen or under vacuum. The temperature of the annealing step may be in the range 50 -500°C. The temperature of the annealing step may be 275°C under nitrogen. The duration of the annealing step may be in the range 1-S0ominutes. The duration of the annealing step may be 3Ominutes. Such an annealing step results in a decrease in film thickness, while the nanoparticle absorption is maintained, and the nanoparticles become insoluble in toluene, suggesting a partial ligand removal. Annealed films result in more conductive and photoconductive devices, as shown in Figure 11, due to better transport due to the reduced nanoparticle to nanoparticle distance.
Annealed films can however crack or form pinholes, leading to device shunting.
Therefore in the preferred embodiment, a film is annealed followed by spincoating a further nitride nanoparticle film of the same composition as the first film (as the first film is no longer soluble in the solvent), leading to any cracks being covered. A further annealing step may be applied. Multiple cycles of annealing and spincoating further layers may be used to build up the layer thickness. The second electrode is deposited as in the second embodiment.
In a sixth embodiment as shown in Figure 12 the nitride nanoparticles (3a) are disposed in and may be coupled to a conductive polymer matrix (3b). Either the nitride nanoparticles (3a) and the matrix (3b) or both may conduct electrons and holes.
In a seventh embodiment as shown in Figure 13 there are other particles (3c) present in layer 3 in addition to the nitride nanoparticles (3a). The additional particles (3c) may be conductive particles such as hO2, ZnO, Zr02 and Sn02 or any other suitable material. Both these populations of particles (3a and 3c) may or may not be disposed in and may or may not be coupled to a polymer matrix (3b). Such a structure may, for example, be used in a dye-sensitized solar cell device.
In an eighth embodiment as shown in Figure 14 there may be more than one population of nitride nanoparticles (3a). A first population of nitride nanoparticles (3ai) may be different from a second population of nitride nanoparticles (3aii) by having different conduction properties for electrons or holes or both (eg, the first population of nitride nanoparticles and the second population of nitride nanoparticles have different compositions to one another and/or have different sizes to one another). Both these populations of particles (3ai and 3aii) may or may not be disposed in and may or may not be coupled to a polymer matrix (3b).
There may be more than two populations of nitride nanoparticles (3a). There may be other particles (3c), as described in the seventh embodiment, in addition to the two populations of nitride nanoparticles.
In a ninth embodiment there may be more than one population of nitride nanoparticles (3a). A first population of nitride nanoparticles (3ai) may be different from a second population on nitride nanoparticles (3aii) by being a different size or shape and hence a different absorption profile. For example the absorption onset and range may be shifted in wavelength by 5Onm or more between the first population and the second population. The different populations of nitride nanoparticles may be dispersed randomly as shown in Figure 15. The different populations of nitride nanoparticles may be formed into separate layers as shown in Figure 14. There may be more than two different populations of nitride nanoparticles with different sizes or shapes resulting in more than two absorption profiles. These populations of particles (3ai and 3aii) may or may not be disposed in and may or may not be coupled to a polymer matrix (3b). There may be other particles (3c), as described in the seventh embodiment, in addition to the two populations of nitride nanoparticles.
To obtain two (or more) different, randomly dispersed populations of nitride nanoparticles as shown in figure 15, one option is to mix solutions of different nanoparticles. The different nanoparticles will be randomly dispersed in solution, and hence will be randomly dispersed when removed from solution, for example by precipitation.
In a tenth embodiment a nitride nanoparticle layer (3a) exhibits carrier multiplication or multiple exciton generation. Since the threshold for multiple exciton generation effects is typically two to four times the nanoparticle band gap, depending on the material, a low band gap material is required for this effect to result in an overall increase in solar cell efficiency. In a preferred embodiment, a material with a band gap less than 1.5eV is used, which is fulfilled by InN or other nitride nanoparticle material discussed in the previous embodiments. Other features may be required to observe efficient multiple exciton generation. An additional feature could include high multiple exciton generation rates, for example due to the increased Coulomb interaction between photogenerated carriers, and the relaxation of the momentum conservation requirements, as a result of the small nanoparticle size. Another additional feature leading to efficient multiple exciton generation could include a low carrier thermalisation rate, for example due to a phonon bottle neck in the nanoparticles. Another additional feature leading to efficient multiple exciton generation could include rapid exciton dissociation and carrier extraction, which is faster than the biexciton lifetime, but slower than the multiple exciton generation timescale, for example due to the small spacing between nanoparticles or a favourable band offset at the contacts.
In order to achieve multiple exciton generation, there may be additional populations of nitride nanoparticles, there may be other particles present (3c).
The nanoparticles and/or particles may or may not be disposed in and may or may not be coupled to a polymer matrix (3b).
In an eleventh embodiment the nitride nanoparticle layer (3a) may have a thickness of one nanoparticle monolayer as shown in Figure 16. The nanoparticles may or may not be disposed in and may or may not be coupled to a polymer matrix (3b).
In a twelfth embodiment the present invention is used as part of a multijunction or tandem solar cell. The separate junctions in the multijunction or tandem device may each consist of a nitride nanoparticle solar cell as described in the present invention, each with a different population of nitride nanoparticles such that each had a different absorption profile. The different absorption profile may be achieved by varying the size of the nitride nanoparticles as shown in Figure 17.
Layers 1, 2, 3ai, 3aii, and 4 are as previously described. Further layers (5) may be included inbetween the different populations of nitride nanoparticles to create the required junction. The layers may consist of electrode layers, recombination layers or a tunnel junction. There may be more than two solar cells comprising the multijunction solar cell to create more than two junctions. The different absorption profiles may be obtained by varying either the core composition or the shell composition or both. The nitride nanoparticle solar cell may only form one of the junctions as part of a multijunction or tandem solar cell.
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