Technical Field

The present invention relates to devices based on conductive organic nanoparticles having calcium-containing electrodes, and methods for preparing the devices.

Background of the Invention

Although the inherent properties of conventional organic photovoltaic materials are well-suited to low-cost fabrication using reel-to-reel manufacturing techniques, two key aspects of current OPV fabrication are not well-suited to building large area PV modules using high speed printing. First, using current fabrication approaches to control phase segregation across large areas is problematic. Second, using volatile, flammable organic solvents presents difficulties for high speed industrial printing line development. The present inventors have recently shown that multilayered photovoltaic devices can be fabricated from aqueous nanoparticle dispersions (solar paint) that are more efficient than the corresponding bulk heterojunction blend devices (PCT/AU201 1/0001001). This enhanced performance arises from a control of device morphology that is not achievable by simple blending of bulk materials. As such, these solar paints, which can be readily synthesized using the miniemulsion technique, offer a potentially attractive path to large scale OPV manufacturing since they simultaneously provide nanomorpological control whilst eliminating toxic and hazardous solvents from the fabrication process.

The present inventors have now surprisingly discovered that the use of calcium aluminium cathodes instead of aluminium cathodes in devices fabricated from aqueous nanoparticle dispersions results in an unprecedented doubling of the efficiency of the devices.

Summary of the Invention

In a first aspect the present invention provides a device comprising at least one nanoparticle layer, the nanoparticles comprising at least one conductive organic compound, and wherein the device includes a cathode comprising calcium.

The nanoparticles may comprise at least one conductive organic compound selected from the group consisting of: polyacetylenes, porphyrins, phthalocyanins, fullerenes, polyparaphenylenes, polyphenylenevinylenes, polyfluorenes, polythiophenes, polypyrroles, polypyridines, polycarbazoles, polypyridinevinylenes, polyarylvinylenes, poly (p-phenylmethylvinylenes), including derivatives and co-polymers thereof.

The at least one conductive organic compound may be selected from the group consisting of: poly(9,9-dioctylfIuorene-2,7-diyl-co-bis-N,N-(4-butylphenyl) -bis-N,N- phenyl-1 ,4-phenylenediamine) (PFB), poly(9,9-dioctylfluorene-2,7-diyl~co- benzothiadiazole) (F8BT), poly-3-hexylthiophene, (6,6)-phenyl-C6i-butyric acid methyl ester and poly(2-methoxy-5-(2'-ethyl-hexyloxy)-l ,4-phenylene vinylene).

The nanoparticles may comprise at least two conductive organic compounds.

In one embodiment the nanoparticles comprise poly(9,9-dioctylfluorene-2,7-diyl-co- bis-N,iV-(4-butylphenyl)-bis-N,N-phenyl- 1 ,4-phenylenediamine) and poly(9,9- dioctylfluorene-2,7-diyl-co-benzothiadiazole).

The device may comprise multiple nanoparticle layers, for example two, three, four, five, six, seven, eight, nine, ten, eleven, twelve, thirteen, fourteen, fifteen or more layers.

In one embodiment, the device comprises four or five layers.

The nanoparticles may have a surface energy between about 20 and 60 J/m ³ .

The nanoparticles may have a mean diameter between about 5 nm and 200 nm, and a mean domain size between about 2 nm and 1 10 nm.

The nanoparticles may have a mean diameter between about 45 nm and 60 nm and a mean domain size between about 15 nm and 30 nm.

The surface of the nanoparticles may be free, or substantially free, of surfactant.

The cathode may further comprise aluminium.

The cathode may comprise a layer of aluminium and a l ayer of calcium.

The nanoparticles may be core shell nanoparticles.

The device may be an electronic device, for example an LED, a transistor or a photovoltaic device, such as a sensor, an array, a memory element or a circuit. In one embodiment the device is a photovoltaic device.

In an embodiment of the first aspect the present invention provides a device comprising multiple nanoparticle layers, the nanoparticles comprising PFB and F8BT, and wherein the device includes a cathode comprising calcium.

The device may comprise four nanoparticle layers.

The surface of the nanoparticles may be free, or substantially free, of surfactant. The nanoparticles may have a mean diameter between about 45 nm and 60 nm and a mean domain size between about 15 nm and 30 nm. The cathode may comprise calcium and aluminium.

The nanoparticles may have a surface energy between about 20 J/m ³ and 60 J/m ³ , or between about 30 J/m ³ and 40 J/m ³ , or about 38 J/m ³ .

In a second aspect, the present invention provides a process for preparing a device comprising:

(i) providing an aqueous emulsion comprising an organic solvent, a surfactant and at least one conductive organic compound;

(ii) removal of the organic solvent to provide an aqueous suspension of conductive nanoparticles comprising the at least one conductive organic compound;

(iii) depositing the nanoparticles onto a substrate to form a nanoparticle layer;

(iv) applying to the device a cathode comprising calcium; and

(v) annealing the nanoparticle layer.

The nanoparticles may comprise at least one conductive organic compound selected from the group consisting of: polyacetylenes, porphyrins, phthalocyanins, fullerenes, polyparaphenylenes, polyphenylenevinylenes, polyfluorenes, polythiophenes, polypyrroles, polypyridines, polycarbazoles, polypyridinevinylenes, polyarylvinylenes, poly (p-phenylmethylvinylenes), including derivatives and co-polymers thereof.

The at least one conductive organic compound may be selected from the group consisting of: poly(9,9-dioctylfluorene-2,7-diyl-co-bis-N,N-(4-butylphenyl) -bis-N,N- phenyl- 1 ,4-phenylenediamine) (PFB), poly(9,9-dioctylfiuorene-2,7-diyl-co- benzothiadiazole) (F8BT), poly-3-hexylthiophene, (6,6)-phenyl-C6i-butyric acid methyl ester and poly(2-methoxy-5-(2'-ethyl-hexyIoxy)-l,4-phenylene vinylene).

The nanoparticles may comprise at least two conductive organic compounds.

In one embodiment the nanoparticles comprise poly(9,9-dioctylfluorene-2,7-diyl-co- bis-N,N-(4-butylphenyl)-bis-N,N-phenyI-l ,4-phenylenediamine) and poly(9,9- dioctylfluorene-2,7-diyl-co-benzothiadiazole).

The nanoparticles may have a mean diameter between about 5 nm and 200 nm, and a mean domain size between about 2 nm and 1 10 nm.

The nanoparticles may have a mean diameter between about 45 nm and 60 nm and a mean domain size between about 15 nm and 30 nm.

The process may further comprise dialysis of the aqueous suspension of nanoparticles so as to minimise the amount of surfactant therein.

Dialysis may be performed until the surface tension of a filtrate is less than about 50 mN/ra.

The process may comprise repeating step (iii) so as to provide multiple nanoparticle layers.

Step (iii) may be repeated once, twice, three, four, five, six, seven, eight, nine, ten, eleven, twelve, thirteen, fourteen, fifteen or more times so as to provide multiple nanoparticle layers.

Step (iii) may be repeated two, three or four times.

Step (iii) may be repeated three times so as to provide four nanoparticle layers.

Step (iii) may be repeated four times so as to provide five nanoparticle layers.

Following step (iii), and each repetition thereof, the nanoparticle layer(s) may be dried.

Following step (iii), and each repetition thereof, the nanoparticle layer(s) may be dried at a temperature between about 50 °C and 150 °C.

Following step (iii) and each repetition thereof, with the exception of the final nanoparticle layer, the nanoparticle layer(s) may be dried at a temperature between about 50 °C and 100 °C.

Following step (iii) and each repetition thereof,, with the exception of the final nanoparticle layer, the nanoparticle layer(s) may be dried at a temperature between about 50 °C and 100 °C for a period of time between about 10 minutes and 20 minutes.

Following deposition of the final nanoparticle layer in step (iii) the nanoparticle layer(s) may be dried at a temperature between about 100 °C and 160 °C.

Following deposition of the final nanoparticle layer in step (iii) the nanoparticle layer(s) may be dried at a temperature between about 100 °C and 160 °C for a period of time between about 10 minutes and 20 minutes.

Step (iv) may comprise applying to the device a cathode comprising calcium and aluminium.

Step (v) may be carried out by heating the nanoparticle layer.

Step (v) may be carried out by heating the nanoparticle Iayer(s) at a temperature between about 70 °C and 150 °C.

Step (v) may be carried out by heating the nanoparticle layer(s) at a temperature between about 100 °C and 150 °C.

Step (v) may be carried out by heating the nanoparticle layer(s) at a temperature between about 100 °C and 150 °C for a period of time between 1 minute and 10 minutes. In a third aspect, the present invention provides a device when prepared by the process of the second aspect.

Brief Description of the Figures

A preferred embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings wherein:

Figure 1 : Variation of power conversion efficiency (PCE) with number of deposited layers for PFB:F8BT nanoparticulate OPV devices fabricated with an Al cathode (filled circles) and a Ca Al cathode (open circles). Dotted and dashed lines have been added to guide the eye. An average error has been determined based upon the variance for a minimum of six devices for each layer.

Figure 2: Variation of current density as a function of applied voltage (J-V) for the most efficient PFB:F8BT 1 : 1 nanoparticle (NP) devices fabricated with Ca/Al (4 NP layers, solid line) and Al (5 NP layers, dashed line) cathodes. Also plotted are the J-V curves for PFB:F8BT 1 : 1 bulk heteroj unction (BHJ) devices fabricated with Ca Al (filled circles) and Al (open circles) cathodes.

Figure 3: External quantum efficiency (EQE) as a function of incident wavelength for PFB:F8BT 1 : 1 NP devices fabricated with Ca/Al (4 NP layers, solid line) and Al (5 NP layers, dashed line) cathodes. Also plotted are the EQE spectra for PFB:F8BT 1 : 1 BHJ devices fabricated with Ca/Al (filled circles) and Al (open circles) cathodes.

Figure 4: A comparison of the integrated short circuit currents obtained from the EQE spectra with the short circuit currents obtained under AM 1.5 conditions for a series of PFB:F8BT devices. The open circles correspond to 1 :1 PFB:F8BT BHJ devices with Al cathodes of similar V _oc (1.1 ± 0.1V) but differing J _so s. The closed circles correspond to the NP and BHJ devices presented in Table 1. The integrated J _sc values systematically underestimate the AM 1.5 values (primarily as a result of low wavelength cut-off of the optical fibre used to transmit the light to the sample). The e ^c values listed in Table 1 are obtained by projecting the 'sc values onto the straight line fit to the calibration data.

Figure 5: Energy level diagrams for PFB:F8BT nanoparticles in the presence of calcium. (A) Calcium diffuses to nanoparticle surface. (B) Calcium dopes PFB-rich shell producing gap states. Electron transfer occurs from calcium producing filled gap states. (C) An exciton generated on PFB approaches the doped PFB material (PFB*) and a hole transfers to the filled gap state producing a more energetic electron. (D) Electron transfer from an exciton generated on F8BT to either the higher energy PFB LUMO or the filled lower energy PFB* LUMO is hindered.

Definitions

Throughout this specification and the claims which follow, unless the context requires otherwise, the word "comprise", and variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

The articles "a" and "an" are used herein to refer to one or to more than one (i.e., to at least one) of the grammatical object of the article. By way of example, "an element" means one element or more than one element.

In the context of this specification, the term "about" is understood to refer to a range of numbers that a person of skill in the art would consider equivalent to the recited value in the context of achieving the same function or result.

Detailed Description of the Invention

The present inventors have surprisingly discovered that the use of calcium/aluminium cathodes instead of aluminium cathodes in devices fabricated from aqueous nanoparticle dispersions results in an unprecedented doubling of the efficiency of the devices. Accordingly, the present invention provides a device comprising at least one nanoparticle layer, the nanoparticles comprising at least one conductive organic compound, and wherein the device includes a cathode comprising calcium.

The present invention also provides a process for preparing a device comprising:

(i) providing an aqueous emulsion comprising an organic solvent, a surfactant and at least one conductive organic compound;

(ii) removal of the organic solvent to provide an aqueous suspension of conductive nanoparticles comprising the at least one conductive organic compound;

(iii) depositing the nanoparticles onto a substrate to form a nanoparticle layer;

(iv) applying to the device a cathode comprising calcium; and

(v) annealing the nanoparticle layer.

The device of the first aspect may be prepared by the process of the second aspect.

In the first and second aspects the nanoparticles may comprise any conductive organic compounds, including, but not limited to: poly acetylenes, porphyrins, phthalocyanins, fullerenes, polyparaphenylenes, polyphenylenevinylenes, polyfluorenes, polythiophenes, polypyrroles, polypyridines, polycarbazoles, polypyridinevinylenes, polyarylvinylenes, poly (p-phenylmethylvinylenes), including derivatives and co-polymers thereof.

In one embodiment the at least one conductive organic compound is selected from the group consisting of: poly(9,9-dioctylfluorene-2,7-diyl-co-bis-N,N-(4-butylphenyl) -bis- N,N-phenyl-l ,4-phenylenediamine) (PFB), poly(9,9-dioctylfluorene-2,7-diyl-co- benzothiadiazole) (F8BT), poly-3-hexylthiophene, (6,6)-phenyl-C6i -butyric acid methyl ester and poly(2-methoxy-5-(2'-ethyl-hexyloxy)-l ,4-phenylene vinylene).

In some embodiments, the nanoparticles comprise two, or at least two conductive organic compounds. In one embodiment the nanoparticles comprise PFB and F8BT. Where the device comprises multiple conductive organic compounds, the ratio by weight of each compound may be between about 1 :2 and 2: 1 or about 1 : 1 . Where the device comprises PFB and F8BT, the weight ratio may be between about 1 :2 and 2: 1 , or about 1 : 1.

The devices may comprise multiple nanoparticle layers, for example two, three, four, five, six, seven, eight, nine, ten, eleven, twelve, thirteen, fourteen, fifteen, sixteen, seventeen, eighteen, nineteen or twenty layers. In some embodiments, the device comprises four or five layers.

The nanoparticles may have a surface energy between about 20 and 60 J/m , or between about 30 and 40 J/m ³ , or about 38 J/m ³ .

The surface of the nanoparticles may be free, or substantially free, of surfactant. Reference to "surface" in this context is understood to mean the outermost surface of the nanoparticles. In this context, the term "substantially free" means that the surface composition of the nanoparticles comprises less than 10%, or less than 9%, or less than 8%, or less than 7%, or less than 6%, or less than 5%, or less than 4%, or less than 3%, or less than 2%, or less than 1 %, or less than 0.5%, or less than 0.1%, or less than 0.01%, or less than 0.001%, surfactant by weight.

The cathode of the device comprises calcium, and in some embodiments may further comprise aluminium such that the cathode is a Ca/Al cathode. The cathode may comprise a layer of aluminium and a layer of calcium. The thickness of the calcium layer may be between about 5 nm and about 50 nm, or between about 10 nm and about 40 nm, or between about 10 nm and about 30 nm, or about 20 nm. The thickness of the aluminium layer may be between about 20 nm and about 100 nm, or between about 30 nm and about 90 nm, or between about 40 nm and about 80 nm, or between about 60 nm and about 80 nm, or about 70 nm.

The nanoparticles may have a mean diameter between about 5 nm and about 200 nm, and a mean domain size between about 2 nm and about 1 10 nm, or the nanoparticles may have a mean diameter between about 5 nm and about 150 nm, and a mean domain size between about 2 nm and about 75 nm, or the nanoparticles may have a mean diameter between about 15 nm and about 120 nm, and a mean domain size between about 5 nm and about 60 nm, or the nanoparticles may have a mean diameter between about 25 nm and about 100 nm, and a mean domain size between about 10 nm and about 50 nm, or the nanoparticles may have a mean diameter between about 30 nm and about 90 nm, and a mean domain size between about 15 nm and about 45 nm, or the nanoparticles may have a mean diameter between about 40 nm and about 80 nm, and a mean domain size between about 20 nm and about 45 nm, or the nanoparticles may have a mean diameter between about 45 nm and about 60 nm, and a mean domain size between about 15 nm and about 30 nm. In one embodiment, the nanoparticles have a mean diameter of about 50 nm and a mean domain size between about 20 nm to 25 nm.

In embodiments where the conductive organic compounds are PFB and F8BT, the nanoparticles may have a mean diameter between about 5 nm and about 200 nm, and a mean domain size between about 2 nm and about 1 10 nm, or the nanoparticles may have a mean diameter between about 25 nm and about 100 nm, and a mean domain size between about 10 nm and about 50 nm, or the nanoparticles may have a mean diameter between about 40 nm and about 80 nm, and a mean domain size between about 20 nm and-about 45 nm, or the nanoparticles may have a mean diameter between about 45 nm and about 60 nm, and a mean domain size between about 15 nm and about 30 nm. In one embodiment where the conductive organic compounds are PFB and F8BT the nanoparticles have a mean diameter of about 50 nm and a mean domain size between about 20 nm to 25 nm

When preparing nanoparticles using emulsion techniques, the particle diameter and domain size may be controlled by varying the nature and amount of the surfactant present (an increase in the concentration of the surfactant results in a decrease in particle diameter), and/or by the application of shear force (for example ultrasound or high pressure homogenisation). The use of shear force allows the preparation of nanoparticles in which up to 98% of the nanoparticles have a diameter which differs from the mean diameter of all nanoparticles by not more than 10%. Particle diameter and domain size may also be controlled by annealing films comprising the nanoparticles as described herein.

The device may be an electronic device, for example an LED, a transistor or a photovoltaic device, such as a sensor, an array, a memory element or a circuit. The device may comprise a substrate comprising a PEDOT:PSS layer on ITO, however those skilled in the art will appreciate that alternative substrates may be used such as those defined below in connection with the second aspect.

Organic solvents suitable for use in the process of the second aspect include any organic solvents which are capable of dissolving, or partially dissolving, the at least one conductive organic compound. Examples of suitable organic solvents include, but are not limited to: alcohols, ethers, ketones, glycol ethers, hydrocarbons and halogenated hydrocarbons. In one embodiment, the solvent is a halogenated solvent, such as chloroform, dichloroethane, dichloromethane or chlorobenzene. The ratio of water to organic solvent in the aqueous emulsion may be between about 2:1 and about 6: 1, or between about 3: 1 and about 6: 1, or between about 4:1 and. about 6:1 , or alternatively about 4: 1.

The surfactant used in the process of the second aspect may be any suitable compound comprising at least one hydrophilic group and at least one hydrophobic group. Suitable surfactants include, but are not limited to: alkyl benzenesulfonates, alkyl sulfates, alkyl sulfonates, fatty alcohol sulfates, alkyl phosphates and alkyl ether phosphates. In one embodiment, the surfactant is sodium dodecyl sulfate (SDS).

The process of the second aspect preferably comprises agitation of the aqueous emulsion of step (i) so as to produce a mini- or micro-emulsion. Agitation may be achieved by methods well known to those skilled in the art, including the use of shear force, for example ultrasound or high pressure homogenisation.

The process of the second aspect may further comprise dialysis of the aqueous suspension of nanoparticles so as to minimise the amount of surfactant therein. Dialysis may be performed Until the surface tension of the filtrate is less than about 200 mN/m, or less than about 100 mN/m, or less than about 50 mN/m, or less than about 40 mN/m, or about 38mN/m. Dialysis may be performed by ultracentrifuge.

The process of the second aspect may comprise repeating step (iii) so as to provide multiple nanoparticle layers. Step (iii) may be repeated once, twice, three, four, five, six, seven, eight, nine, ten, eleven, twelve, thirteen, fourteen, fifteen or more times so as to provide multiple nanoparticle layers. In one embodiment step (iii) is repeated three times so as to provide four nanoparticle layers. In another embodiment step (iii) is repeated four times so as to provide five nanoparticle layers. The inventors have found that a four layer device prepared in accordance with the process of the invention provides unprecedented power conversion efficiency (under AM 1.5 illumination) of 0.82% (see Example below). This is more than double the efficiency of the corresponding bulk heteroj unction device and more than double the efficiency of the corresponding nanoparticle device having an aluminium cathode.

Following step (iii), and each repetition thereof, the nanoparticle layer(s) may be dried. Following step (iii), and each repetition thereof, the nanoparticle layer(s) may be dried at a temperature between about 50 °C and 150 °C. Following step (iii) and each repetition thereof, the nanoparticle layer(s) may be dried at a temperature between about 30 °C and 180 °C, or between about 40 °C and 170 °C, or between about 50 °C and 150 °C, or between about 60 °C and 150 °C. The drying may be continued for a period of time between about 30 seconds and 30 minutes, or between about 1 minute and 20 minutes or between about 2 minutes and 20 minutes, or between about 10 minutes and 20 minutes.

Following step (iii) and each repetition thereof, optionally with the exception of the final nanoparticle layer, the nanoparticle layer(s) may be dried at a temperature between about 30 °C and 120 °C, or between about 40 °C and 110 °C, or between about 50 °C and 100 °C, or between about 50 °C and 90 °C, or between about 50 °C and 80 °C, or between about 60 °C and 80 °C, or between about 65 °C and 75 °C. The drying may be continued for a period of time between about 30 seconds and 30 minutes, or between about 30 seconds and 20 minutes, or between about 1 minute and 20 minutes, or between about 2 minutes and 20 minutes, or between about 4 minutes and 20 minutes, or between about 5 minutes and 20 minutes, or, between about 10 minutes and 20 minutes, or between about 12 minutes and 16 minutes, or about 15 minutes. In one embodiment, following step (iii), and/or each repetition thereof, optionally with the exception of the final nanoparticle layer, the nanoparticle layer is dried at a temperature between about 65 °C and 75 °C for a period of time between about 12 and 18 minutes.

Following deposition of the final nanoparticle layer in step (iii) the nanoparticle layer(s) may be dried at a temperature between about 40 °C and 180 °C, or between about 60 °C and 180 °C, or between about 70 °C and 170 °C, or between about 90 °C and 170 °C, or between about 100 °C and 160 °C, or between about 120 °C and 160 °C, or between about 130 °C and 150 °C or between about 135 °C and 145 °C, or about 140 °C. The drying may be continued for a period of time between about 30 seconds and 30 minutes, or between about 30 seconds and 20 minutes, or between about 1 minute and 20 minutes or between about 2 minutes and 20 minutes, or between about 5 minutes and 20 minutes, or between about 10 minutes and 20 minutes, or between about 12 minutes and 16 minutes, or about 15 minutes. In one embodiment, the final nanoparticle layer may be dried at a temperature between about 135 °C and 145 °C for a period of time between about 12 minutes and 18 minutes.

Step (iii) may be performed by methods well known to those skilled in the art including, but not limited to: electroplating, vapour phase deposition, spin coating, screen printing, inkjet printing, slot-dye printing, spray coating, draw bar coating or derived coating/printing techniques thereof, painting, gravure, roller and embossing.

In the process of the second aspect step (iv) may be carried out by methods well known amongst those skilled in the art, for example by evaporation in a vacuum chamber.

In the process of the second aspect step (v) may be carried out by heating the nanoparticle layer(s). Step (v) may be carried out by heating the nanoparticle layer(s) at a temperature between about 70 °C and 180 °C, or between about 80 °C and 170 °C, or between about 80 °C and 160 °C, or between about 90 °C and 160 °C, or between about 100 °C and 160 °C, or between about 110 °C and 160 °C, or between about 120 °C and 160 °C, or between about 130 °C and 150 °C, or between about 135 °C and 145 °C. The heating may be continued for a period of time between about 30 seconds and 30 minutes, or between about 30 seconds and 20 minutes, or between about 1 minute and 15 minutes or between about 2 minutes and 15 minutes, or between about 2 minutes and 10 minutes, or between about 2 minutes and 6 minutes, or about 4 minutes. In one embodiment, step (v) is carried out by heating the nanoparticle layer(s) at a temperature between about 135 °C and 145 °C for a period of time between about 2 minutes and 10 minutes. Step (v) may be carried out following application of electrodes to the device. Step (v) may be carried out under an inert atmosphere.

In the process of the second aspect the substrate may be PEDOT:PSS on ITO, glass on ΙΊΌ, ITO on a flexible substrate, conducting transparent coatings on transparent substrates, carbon, graphene, carbon nanotubes, a thin metal layer, or any other suitable substrate known to those skilled in the art. In one embodiment, the substrate is a PEDOT:PSS layer on ITO. The process may further comprise annealing the PEDOT:PSS layer following coating on the ITO. The PEDOTrPSS layer may be annealed by heating according to the annealing conditions described herein, for example by heating at a temperature between about 100 °C and 160 °C for a period of time between about 1 minute and 1 hour. The device of the first aspect may comprise a substrate as defined above.

Step (v) is typically carried out following application of an electrode or electrodes to the device, however those skilled in the art will recognise that step (v) may be carried out prior to application of an electrode or electrodes to the device.

The thickness of the nanoparticle layer(s) may be between about 100 nm and about 500 nm, or between about 50 nm and about 350 nm, or between about 100 nm and about 350 nm.

In an embodiment of the first aspect the present invention provides a device comprising multiple nanoparticle layers, the nanoparticles comprising PFB and F8BT, and wherein the device includes a cathode comprising calcium. In this embodiment:

• The device may comprise two, three, four, five, six, seven, eight, nine or ten nanoparticle layers, and/or

• The surface of the nanoparticles may be free, or substantially free, of surfactant, and/or

• The nanoparticles may have a mean diameter between about 45 nm and 60 nm and a mean domain size between about 15 nm and 30 nm, and/or

• The cathode may comprise calcium and aluminium, and/or

• The nanoparticles may have a surface energy between about 20 J/m ³ and 60 J/m ³ , or between about 30 J/m ³ and 40 J/m ³ , or about 38 J/m ³ .

In an embodiment of the second aspect the invention provides a process for preparing a device comprising the following steps:

(i) providing an aqueous emulsion comprising an organic solvent, a surfactant and at least one conductive organic compound;

(ii) removal of the organic solvent to provide an aqueous suspension of conductive nanoparticles comprising at least one conductive organic compound;

(iii) depositing the nanoparticles onto a substrate to form a nanoparticle layer;

(iv) repeating step (iii) so as to provide multiple nanoparticle layers;

(v) following performance of step (iii) and each repetition thereof, drying the nanoparticle layer(s);

(vi) applying to the device a cathode comprising calcium; and (vii) annealing the nanoparticle layer(s).

In this embodiment:

• The nanoparticles may comprise PFB and F8BT, and/or

• The nanoparticles may have a mean diameter between about 45 nm and 60 nm and a mean domain size between about 15 nm and 30 nm, and/or

• Step (iii) may be repeated one, two, three, four, five, six, seven, eight, nine or ten times, and/or

• The process may further comprise dialysis of the aqueous suspension of nanoparticles so as to minimise the amount of surfactant therein, and/or

• Dialysis may be performed until the surface tension of the filtrate is less than about 200 mN/m, or less than about 100 mN/m, or less than about 50 mN/m, or less than about 40 mN/m, or about 38mN/m, and/or

• Step (vi) may comprise applying to the device a cathode comprising calcium and aluminium, and/or

• Step (v) may be performed by heating at a temperature between about 40 °C and 150 °C, and/or

• Step (v) may be performed by heating at a temperature between about 40 °C and 150 °C for a period of time between about 10 minutes and 20 minutes, and/or

• Step (vii) may be performed by heating at a temperature between about 100 °C and 160 °C, and/or

• Step (vii) may be performed by heating at a temperature between about 100 °C and 160 °C for a period of time between about 2 minutes and 10 minutes.

In an embodiment of the second aspect the invention provides a process for preparing a device comprising the following steps:

(i) providing an aqueous emulsion comprising an organic solvent, a surfactant and at least one conductive organic compound;

(ii) removal of the organic solvent to provide an aqueous suspension of conductive nanoparticles comprising at least one conductive organic compound;

(iii) depositing the nanoparticles onto a substrate to form a nanoparticle layer;

(iv) repeating step (iii) so as to provide multiple nanoparticle layers;

(v) following performance of step (iii) and each repetition thereof, drying the nanoparticle layer(s);

(vi) applying a final nanoparticle layer; (vii) following performance of step (vi), drying the nanoparticle layer(s);

(viii) applying to the device a cathode comprising calcium; and

(ix) annealing the nanoparticle layer(s).

In this embodiment:

• The nanoparticles may comprise PFB and F8BT, and/or

• The nanoparticles may have a mean diameter between about 45 nm and 60 nm and a mean domain size between about 15 nm and 30 nm, and/or

• Step (iii) may be repeated one, two, three, four, five, six, seven, eight, nine or ten times, and/or

• The process may further comprise dialysis of the aqueous suspension of nanoparticles so as to minimise the amount of surfactant therein, and/or

• Dialysis may be performed until the surface tension of the filtrate is less than about 200 mN/m, or less than about 100 mN/m, or less than about 50 mN/m, or less than about 40 mN/m, or about 38mN/m, and/or

• Step (viii)-may comprise applying to the device a cathode comprising calcium and aluminium, and/or

• Step (v) may be performed by heating at a temperature between about 50 °C and 100 °C, and/or

• Step (v) may be performed by heating at a temperature between about 50 °C and 90 °C for a period of time between about 10 minutes and 20 minutes, and/or

• Step (vii) may be performed by heating at a temperature between about 120 °C and 160 °C, and or

• Step (vii) may be performed by heating at a temperature between about 120 °C and 160 °C for a period of time between about 2 minutes and 10 minutes, and/or

• Step (ix) may be performed by heating at a temperature between about 120 °C and 160 °C, and/or

• Step (ix) may be performed by heating at a temperature between about 120 °C and 160 °C for a period of time between about 2 minutes and 10 minutes.

The device of the first aspect may be prepared by the processes described and exemplified herein.

Examples

The invention will now be described in more detail, by way of illustration only, with respect to the following example. The example is intended to serve to illustrate this invention and should in no way be construed as limiting the generality of the disclosure of the description throughout this specification.

Preparation of a device based on PFB:F8BT nanoparticles having a Ca/AI electrode

Described below is a process for preparing a photovoltaic device in accordance with one embodiment of the invention.

An aqueous dispersion of semi-conducting PFB.F8BT (American Dye Source Inc) nanoparticles (1 : 1 blend) was prepared using the miniemulsion technique as outlined in Burke et al. (Nanotechnology, 22, 265710, (201 1)). Dynamic light scattering (Zetasizer Nano-ZS, Malvern Instruments, UK) was used to measure the distribution of particle sizes in the aqueous dispersion and gave a mean particle size of 51.9 ± 1.3 nm. PEDOT:PSS (Baytron P) films were spin-coated (5000 rpm) on pre-cleaned patterned ITO glass slides and annealed at 140 °C for 30 min to eliminate water in the films. PFB:F8BT nanoparticle layers were deposited by spin coating 35 μΐ, of the dispersion (2000 rpm for 1 minute) in air. Following the deposition of each layer, the film was dried at 70 °C for 15 min. For the final layer, the film drying takes place at 140 °C for 15 min and the films were then transferred into a vacuum chamber for electrode evaporation. Bulk heteroj unction layers were spin coated from PFB:F8BT blend solutions (1 : 1 PFB:F8BT, 10 mg/ml in chloroform) to give a total layer thickness of approximately 110 to 120 nm as measured using a KLA-Tencor Alpha-step 500 surface profilometer. The calcium/aluminium (Ca/AI) and aluminium (Al) electrodes were evaporated on the active layers under vacuum (2* 10 ^'6 Torr). The thickness of the Ca and Al layers were measured to be about 20 nm and 70 nm respectively using a quartz crystal monitor. After evaporation, fabricated devices were annealed at 140°C on a hot plate for 4 min under a nitrogen atmosphere and then tested. The photocurrent density- voltage (J-V) measurements were conducted using a Newport Class A solar simulator with an AM 1.5 spectrum filter. The light intensity was measured to be 100 mWcm ^"2 by a silicon reference solar cell (FHG-ISE) and the J-V data were recorded with a Keithley 2400 source meter. Figure 1 shows that NP devices with Al and Ca/AI cathodes exhibit qualitatively very similar behaviour, with a peak PCE of ~ 0.4% for Al and -0.8% for Ca/AI, and that there is a distinct optimised thickness for the NP devices. The J-V characteristics under illumination and the solar cell performance (for NP and BHJ devices for both cathodes) are summarized in Figure 2 and Table 1 respectively. Table 1: Comparison Of device characteristics for the most efficient PFB:F8BT 1 : 1 NP devices fabricated with Ca/Al (4 NP layers) and Al (5 NP layers) cathodes and the corresponding BHJ devices. The data presented is for the best device of a minimum of ten devices and the APCE error is obtained from the standard deviation of the device

, EQE _C

efficiencies. The »sc values are calculated from the calibrated short circuit currents

EQE

obtained by combining the measured EQE spectra with the AM 1.5 spectrum (he ) (see Figure 4)

The data show that: (a) changing from an Al to a Ca/Al electrode results in an approximate doubling of the measured PCE for both the NP and BHJ devices, and (b) the PCE of the NP devices is double that of the corresponding BHJ devices for both Al and Ca/Al cathodes. The increased PCE of the NP devices relative to the BHJ devices is driven primarily by a significant increase in J _sc and demonstrates that this improvement is inherent to the NP structure and is not dependent upon the choice of cathode material.

In addition, this PCE increase is not due to increased thickness of the NP devices since the efficiency of F8BT:PFB BHJ devices is effectively constant for thicknesses between 50 to 500 ran. The inventors have previously shown that the NPs have a phase- segregated core-shell domain structure with length scales close to the optimal size for exciton dissociation for the PFB-F8BT system. Thus, without wishing to be bound by theory, the inventors hypothesise that the improved PCE of the NP devices arises from enhanced exciton dissociation resulting from an optimized domain structure in the NP active layer; a result not achievable by simple blending of bulk materials. Moreover,.the low R _s values for the NP structures indicates that charge transport pathways are not detrimentally affected by the nanoparticulate structure and interparticle connectivity must be high.

The NP devices with an Al cathode exhibit a significantly lower V _M than the corresponding BHJ devices, which can be attributed to increased recombination. By contrast, the use of a Ca/Al cathode restores V _oc to the value observed for an optimized BHJ device whilst retaining the high J _sc values expected from the NP structure, suggesting that the addition of calcium reduces recombination without detrimentally affecting charge generation and transport in the device. This result is consistent with previous observations that have shown that electron transfer occurs at the Ca/polymer interface resulting in a decrease in the HOMO of the polymer and an increase in V _oc .

The EQE spectra (see Figure 3) for the BHJ and NP devices are consistent with the device characteristics and, moreover, the J _sc values obtained under AM 1.5 illumination agree with those obtained by combining the measured EQE spectra with the AMI .5 spectrum (he ^e ) with an average deviation of less than ± 5 % (see Figure 4). However, the different shapes of the NP EQE plots reveal that for the Ca/Al cathode NP device there is a significantly increased contribution from charges generated b the PFB component (peak -380 nm). It has been established that PFB:F8BT NPs adopt a core shell morphology with a majority PFB component located in the shell, and as such the increased PFB contribution to the EQE spectrum indicates that Ca decreases the recombination of charges generated by the PFB component.

Again without wishing to be bound by theory, whilst there is likely to exist a rich complexity of interfacial structure and interactions in these devices, our results are consistent with a model (Figure 5) whereby Ca penetrates into the multilayered structure creating interfacial states at the surface of the NP. These states are created primarily in the PFB component since this polymer preferentially segregates to the surface of the nanoparticle. Subsequently, electron transfer from Ca (Figure 5b) to the doped polymer (PFB*) results in a filled interband-gap state at the Ca-NP interface; excitons approaching these interband-gap states are likely to dissociate and contribute their holes to the polaron. For an exciton generated on PFB (Figure 5 c), holes moving into the filled interband state create excess energy, which is transferred to the electron to conserve pair energy resulting in enhanced charge separation and the observed increase in EQE contribution from PFB. Furthermore, the observed increase in V _oc for the calcium doped devices is explained by the reduction in PFB HOMO energy level. By contrast, for an exciton generated on F8BT, electron transfer to either the higher energy PFB LUMO, or the filled lower energy PFB* LUMO, is unfavourable (Figure 5d) Thus, regions of the PFB-rich shell that are Ca-doped act as a partial blocking layer for charges generated on the F8BT explaining the reduced EQE contribution from this component. Furthermore, any Ca-doping of the minority F8BT in the outer shell of the NP results in additional electron trap states at the interface.

In summary, the present inventors have shown that the intrinsic morphology of NP PFB:F8BT OPV devices enhances exciton dissociation relative to the corresponding BHJ structure. Moreover, the use of a Ca/Al cathode results in the creation of interfacial gap states, which reduce recombination of charges generated by the PFB in these devices and restores V _oc to the level obtained for an optimized BHJ device, resulting in a PCE approaching 1 %.

Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. It is to be understood that the invention includes all such variations and modifications.