WO2007060463A2 - Photovoltaic devices with improved efficiency - Google Patents

Abstract

The invention provides a photovoltaic device comprising a first electrode, a second electrode, and, disposed between the first and second electrodes an electron acceptor layer and an electron donor layer. Either the electron acceptor layer or electron donor layer comprises a layer which comprises a photoactive conjugated polymer. The layer which comprises a photoactive conjugated polymer has a thickness less than or equal to three times the exciton diffusion length of the polymer, and/or the polymer has a weight average molecular weight of less than 500,000 g/mol. The invention further relates to processes for producing the photovoltaic devices, hi one embodiment, the photoactive conjugated polymer is annealed by heat treatment.

Description

PHOTOVOLTAIC DEVICES WITH IMPROVED EFFICIENCY

FIELD OF THE INVENTION

The present invention relates to photovoltaic heterojunction devices comprising a layer of an electron acceptor material and a layer of an electron donor material between two electrodes, in which at least one of the layers comprises a photoactive conjugated polymer. In particular, the invention concerns hybrid devices in which the electron donor material is an organic conjugated polymer and the electron acceptor material is an inorganic semiconductor, for instance a metal oxide such as TiO₂.

BACKGROUND TO THE INVENTION

Hybrid conjugated polymer/inorganic semiconductor photovoltaic cells, particularly conjugated polymer/TiO₂ (CPATiO₂) cells, have been intensively investigated. Such cells can be regarded as a solid-state alternative to the liquid electrolyte based Gratzel cell. While for the latter a power conversion efficiency (PCE) of greater than 10% has been reported, the efficiency of CP/TiO₂ cells has thus far been confined to below 0.5%. The best reported efficiency for a bi-layer poly[2-(2- ethylhexyloxy)-5-methoxy-l,4-phenylenevinylene] / TiO₂ (MEH-PP WTiO₂) cell not modified using additional interfacial layers is 0.18%. The use of a conjugated polymer in CP/TiO₂ photovoltaic cells to perform both photogeneration and charge (hole) transportation has been intensively investigated. However attempts to design a conjugated polymer which can perform this dual role and simultaneously satisfy the processing requirements for efficient penetration of the nanoporous titania electrode have not been successful. It is at present unclear how much the currently attained levels of device efficiency reflect the true photovoltaic potential of heterojunction devices incorporating a photoactive conjugated polymer, and how much they are limited by the processing (device fabrication) methods and conditions and/or the device architecture. There is therefore a continuing need to address these issues in order to establish whether, and if so, how photovoltaic cells with improved performance characteristics could be prepared. SUMMARY OF THE INVENTION

The present inventors have found that by reducing the thickness of the polymer film in a bi-layer device to less than or equal to three times the exciton diffusion length of the polymer, the efficiency of the device after postproduction treatment can be increased to 0.50%.

Accordingly, in one aspect the invention provides a photovoltaic device comprising a first electrode; a second electrode; and, disposed between the first and second electrodes, an electron acceptor layer and an electron donor layer, wherein either the electron acceptor layer or the electron donor layer comprises a layer which comprises a photoactive conjugated polymer and which has a thickness less than or equal to three times the exciton diffusion length of the polymer.

It has also been found that reducing the polymer molecular weight of the polymer film in a bi-layer device gives rise to surprisingly improved device performance. Accordingly, in another aspect the invention provides a photovoltaic device comprising a first electrode; a second electrode; and, disposed between the first and second electrodes, an electron acceptor layer and an electron donor layer, wherein either the electron acceptor layer or the electron donor layer comprises a layer which comprises a photoactive conjugated polymer having a weight average molecular weight of less than 500,000 g/mol.

It was additionally found that heat treatment of the polymer layer in devices of the present invention caused a further significant increase in device performance.

Thus a process for producing a photovoltaic device of the invention is provided, which comprises the step of annealing the photoactive conjugated polymer by heat treatment. hi another aspect, the invention provides a process for producing a photovoltaic device comprising the steps of: (i) forming an electron acceptor layer on a substrate comprising a first electrode; (ii) forming an electron donor layer on said electron acceptor layer; and (iii) forming a second electrode on said electron donor layer, wherein either the electron acceptor layer or the electron donor layer comprises a layer which comprises a photoactive conjugated polymer and which has a thickness less than or equal to three times the exciton diffusion length of the polymer, L_ED- In another aspect, the invention provides a process for producing a photovoltaic device comprising the steps of: (i) forming an electron acceptor layer on a substrate comprising a first electrode; (ii) forming an electron donor layer on said electron acceptor layer; and (iii) forming a second electrode on said electron donor layer, wherein either the electron acceptor layer or the electron donor layer comprises a layer which comprises a photoactive conjugated polymer having a weight average molecular weight of less than 500,000 g/mol.

Typically, either the electron acceptor layer or the electron donor layer consists essentially of, or more typically consists of (i.e. is), said layer which comprises a photoactive conjugated polymer.

Typically, said layer which comprises a photoactive conjugated polymer consists of the photoactive conjugated polymer. However, alternative arrangements are also envisaged, in which the photoactive conjugated polymer is part of a blend with one or more other materials in this layer.

BRIEF DESCRIPTION OF THE FIGURES

Figure 1 shows the structure of the bi-layer MEH-PPWTiO₂ solar cells: a) shows a schematic representation of a cell in which (i) is Au, (ii) is MEH-PPV, (iii) is TiO₂, (iv) is ITO and (v) is glass; b) shows a micrograph of the TiO₂ layer prepared by doctor-blading; c) shows the chemical structure of MEH-PPV.

Figure 2 is a graph of current density in units of m A. cm^"2 (y axis) versus bias in units of Volts (x axis) for a bi-liayer MEH-PP WTiO₂ solar cell before (dashed line) and after (solid line) post-production heat treatment at 200°C. The cells were illuminated with a simulated AMI.5 light modified by the use of a 420 nm long-pass filter, (80 mW.cm^"2).

DETAILED DESCRIPTION OF THE INVENTION

Photovoltaic devices according to the present invention comprise an electron acceptor layer and an electron donor layer, both of which are sandwiched between a first electrode and a second electrode. The first and second electrodes are an anode and a cathode, one of which is transparent to allow the ingress of light. Such devices are often termed bilayer heteroj unction devices. For example, a known bilayer heteroj unction device has the structure Cathode/Electron acceptor/Electron donor/ Anode. In the devices of the invention, either the electron acceptor layer or the electron donor layer comprises a layer which comprises a photoactive conjugated polymer. Typically, either the electron acceptor layer or the electron donor layer consists essentially of said layer which comprises a photoactive conjugated polymer. More typically, either the electron acceptor layer or the electron donor layer consists of said layer which comprises a photoactive conjugated polymer (i.e. more typically either the electron acceptor layer or the electron donor layer is said layer which comprises a photoactive conjugated polymer). Typically, said layer which comprises a photoactive conjugated polymer consists of the photoactive conjugated polymer. However, alternative arrangements are also envisaged, in which the photoactive conjugated polymer is part of a blend with one or more other materials in this layer. The one or more other materials may be electron acceptor or electron donor materials: if the electron acceptor layer comprises the layer which comprises a photoactive conjugated polymer, then said one or more other materials may be selected from suitable electron transporting materials. If the electron donor layer comprises the layer which comprises a photoactive conjugated polymer, then said one or more other materials may be selected from suitable hole transporting materials. As used herein, the term "photoactive" refers to a material that absorbs photons to make a contribution to the photocurrent of the device. If the electron donor layer comprises the layer which comprises a photoactive conjugated polymer, then the electron acceptor layer may be selected from any suitable organic or inorganic electron-transporting material, e.g. an n- type semiconductor. Suitable electron transporting materials would be apparent to a person of skill in the art, and include, for example, TiO₂ (titania), ZnO, CdSe, CdTe, perylene, C₆₀, C₇₀ and other fullerenes and carbon nanotubes, including functionalised fullerenes like (6,6)-phenyl C₆₁-butyric acid methyl ester (PCBM) and (6,6)-phenyl C₇₁- butyric acid methyl ester ([7O]PCBM). Similarly, if the electron acceptor layer comprises the layer which comprises a photoactive conjugated polymer (i.e. in a case where the conjugated polymer is used as an electron transporting material), then the electron donor layer may be selected from any suitable organic or inorganic p-type

(hole transporting) semiconducting material. Suitable hole transporting materials would be apparent to a person of skill in the art, and include CdTe, polyphenylene, poly(l,4- phenylenevinylene) (PPV), polythiophene and polypyrrole.

However, the present invention typically concerns "hybrid" organic/inorganic devices in which the electron donor material is the photoactive conjugated organic polymer and the electron acceptor material is an inorganic electron-transporting semiconductor. Thus, typically, the layer of photoactive conjugated polymer is the electron donor layer. The structures of hybrid polymer/metal oxide photovoltaic cells and general descriptions of the device types and method of working are well known. The choice of the first and second electrodes of the photovoltaic devices of the present invention is dependent on the structure type. Typically when a metal oxide is used as the electron acceptor the metal oxide is deposited onto indium-tin oxide (ITO), the first electrode, and the second electrode is a high work function metal such as gold. This is a typical arrangement in the present invention. However, ITO may be used as the transparent (first) electrode in combination with a low work function metal as the second electrode. Suitable high work function materials for use as an electrode in the present invention may be selected from the group comprising ITO, tin oxide, aluminum oxide or indium doped zinc oxide, magnesium-indium oxide, cadmium tin-oxide, gold, silver, nickel, palladium and platinum. ITO is a typically used as the transparent electrode in the claimed photovoltaic devices. However, conducting polymers such as PANI (polyaniline) or PEDOT can also be used. The electrode material is deposited by sputtering, vapour deposition or spin coating, as appropriate. Suitable low work function materials for use as an electrode in the present invention may be selected from the group including Li, Na, K, Rb, Be, Mg, Ca, Sr, Ba, Yb, Sm and Al. In addition, suitable low work function electrode materials may be an alloy of such metals or an alloy of such metals in combination with other metals, for example the alloys MgAg and LiAl. The electrode may thus comprise multiple layers, for example Ca/Al, Ba/ Al, or LiF/ Al. The device may further comprise a layer of dielectric material between the cathode and the emitting layer, such as is disclosed in WO 97/42666. For example, an alkali or alkaline earth metal fluoride may be used as a dielectric layer between the cathode and the organic semiconductor.

Many suitable polymers which may be used as the photoactive conjugated polymer in the devices of the present invention are well known in the art. Particularly suitable polymers include substituted and unsubstituted polyarylene, poly(arylene- vinylene), poly(arylene-acetylene) homo- and copolymers. Such polymers are described by J. L. Segura, Acta. Polym., 1998, 49, 319. Particularly suitable substituents for these polymers include alkyl and alkoxy groups, which can impart polymer solubility in polar aprotic solvents such as toluene, chlorobenzene, tetrahydrofuran and chloroform. C_1-10 alkyl and C_1-I0 alkoxy are examples of particularly typical polymer substituents.

Other suitable polymers for use at the photoactive conjugated polymer may include polymers comprising monomer units of the formula (I):

(I) wherein:

X is selected from C_6-14 arylene, C_6-14 arylene-vinylene and C_6-14 arylene-acetylene units; each A represents hydrogen or a group of formula -(L)]-EWG or

-(LZ)₁-EDG, wherein EWG is an electron- withdrawing group as defined herein and EDG is an electron-donating group as defined herein; each B represents hydrogen or a group of formula -(L)i-EWG or

-(LZ)₁ -EDG; a is 1, 2 or 3; b is 1, 2 or 3;

1 is zero or an integer of from 1 to 10; r is zero or an integer of from 1 to 10;

L and IZ are spacer groups independently selected from C_6-14 arylene,

(C_6-14 arylene)-vinylene, (C_6-14 arylene)-acetylene, 5- to 10-membered heteroarylene, (5- to 10-membered heteroarylene)-vinylene, and (5- to

10-membered heteroarylene)-acetylene groups, wherein the arylene and heteroarylene moieties are unsubstituted or substituted by one or more groups selected from Ci_-I0 alkyl, C_1-I0 alkoxy, EDG groups defined herein and EWG groups defined herein; when 1 is greater than zero, EWG is attached to an arylene, heteroarylene, vinylene or acetylene moiety of L; - when I' is greater than zero, EDG is attached to an arylene or heteroarylene moiety of L'.

Specific polymers commonly used in photovoltaic cells include poly(2- methoxy-5 -(2 ' -ethylhexoxy)- 1 ,4-phenylenevinylene (MEH-PP V), poly(3- hexylthiophene) (P3HT) and poly[2-methoxy-5-(3 ',7'-dimethyloctyloxy)-/?-phenylene- vinylene (OCiCio-PPV). The layer of photoactive conjugated polymer may consist entirely of a polymer comprising monomer units of a suitable polymer (e.g. MEH-PPV, P3HT or OC₁C_1O-PPV). Equally, the polymer layer can consist entirely of a copolymer, terpolymer, tetrapolymer or a polymer of any number of different monomer units from suitable polymers. Alternatively, however, the polymer (or copolymer, terpolymer, tetrapolymer, etc.) of the layer of photoactive conjugated polymer may be blended with other polymers or small molecules to aid light absorption, charge separation and/or charge transport. For example, to aid charge transport, hole transporting materials such as TPD (iV,N'-diphenyl-N,N'-bis(3-methylρhenyl)[l,r-biphenyl]-4,4'-diamine), NPD (4,4'-bis[iV-naphthyl)-N-phenyl-ammo]biphenyl) and MTDATA may be added. Typically, the layer of photoactive conjugated polymer comprises MEH-PPV.

Other suitable polymers for use as the photoactive conjugated polymer include charge separation polymers, comprising monomer units of the formula (II):

(H) wherein: X is selected from C_6-14 arylene, C_6-14 arylene-vinylene and C_6-I4 arylene-acetylene units; each A represents a group of formula -(L)i-EWG wherein EWG is an electron- withdrawing group; a is 1, 2 or 3;

1 is zero or an integer of from 1 to 10;

L is a spacer group selected from C_6-14 arylene, (C_6-14 arylene)-vinylene, (C_6-14 arylene)-acetylene, 5- to 10-membered heteroarylene, (5- to 10- membered heteroarylene)-vinylene, and (5- to 10-membered heteroarylene)-acetylene groups, wherein the arylene and heteroarylene moieties are unsubstituted or substituted by one or more groups selected from C_1-10 alkyl, Ci_-10 alkoxy and EWG groups defined herein; each B represents a group of formula -(L')r-EDG wherein EDG is an electron-donating group; b is 1, 2 or 3;

I' is zero or an integer of from 1 to 10;

I/ is a spacer group selected from C_6-I4 arylene, (C_6-14 arylene)-vinylene, (C_6-14 arylene)-acetylene, 5- to 10-membered heteroarylene, (5- to 10- membered heteroarylene)-vinylene, and (5- to 10-membered heteroarylene)-acetylene groups, wherein the arylene and heteroarylene moieties are unsubstituted or substituted by one or more groups selected from Ci_-10 alkyl, Ci_-I0 alkoxy and EDG groups defined herein; when 1 is greater than zero, EWG is attached to an arylene, heteroarylene, vinylene or acetylene moiety of L; - when Y is greater than zero, EDG is attached to an arylene or heteroarylene moiety of L'; and 1 and I' are not both zero.

The polymers of formula (II) assist in charge separation by employing groups attached to the polymer backbone that will stabilize both the holes and the electrons that are formed when the exciton is separated. This is achieved by having electron- withdrawing and electron-donating groups across the substituents of the backbone. Such an arrangement will give rise to a dipole and it should be noted that the factors that control dipole strength are well known to those skilled in the art of producing second-order non-linear optic materials (see, for example, H Meier, Angew. Chem. Int. Ed., 2005, 44, 2482).

As used herein the term C_1-10 alkyl is a linear or branched alkyl group or moiety containing from 1 to 10 carbon atoms such as a C_1-4 or C_1-6 or C_1-8 alkyl group or moiety. Examples of C_1-4 alkyl groups and moieties include methyl, ethyl, n-propyl, /-propyl, «-butyl, z-butyl and t-butyl. For the avoidance of doubt, where two alkyl moieties are present in a group, the alkyl moieties may be the same or different.

As used herein, a C_2-6 alkenyl group or moiety is a linear or branched alkenyl group or moiety containing from 2 to 6 carbon atoms respectively such as a C_2-4 alkenyl group or moiety. For the avoidance of doubt, where two or more alkenyl moieties are present in a group, the alkenyl moieties may be the same or different.

As used herein, a halogen is typically chlorine, fluorine,. bromine or iodine. It is typically chlorine, fluorine or bromine, more typically fluorine. As used herein the term amino represents a group of formula -NH₂. The term

Ci_-10 alkylamino represents a group of formula -NHR' wherein R' is a C_1-10 alkyl group, typically a Ci_-8 alkyl group, as defined previously. The term di(Ci_-10)alkylamino represents a group of formula -NR 'R" wherein R' and R" are the same or different and represent C_MO alkyl groups, typically Ci_-8 alkyl groups, as defined previously. As used herein the term amido represents a group of formula -C(O)NR 'R" wherein R' and R' are the same or different and are selected from hydrogen and CM_O alkyl groups, more typicallyfrom hydrogen and C_1-8 alkyl groups as defined previously.

As used herein the term aryl refers to C_6-I4 aryl groups which may be mono- or polycyclic, such as phenyl, naphthyl and fluorenyl. An aryl group may be unsubstituted or substituted at any position. Unless otherwise stated, it carries 0, 1, 2 or 3 substituents in addition to any group EWG or EDG that is present. Typical substituents on an aryl group include Ci-I₀ alkyl groups, because such groups improve the solubility in polar aprotic solvents, such as toluene, xylene, chlorobenzene, tetrahydrofuran and chloroform. If the aryl group is part of the group B of formula (II), then the substituents are typically electron-donating groups, such as the groups EDG as exemplified herein. However, if the aryl group is part of the group A of formula (II), then typically the substituents are not strongly electron-donating groups. For example, if the aryl group is part of the group A of formula (II), then the substituents may be groups EWG as exemplified herein or alkyl.

As used herein, a heteroaryl group is typically a 5- to 14-membered aromatic ring, such as a 5- to 10-membered ring, more typically a 5- or 6-membered ring, containing at least one heteroatom, for example 1, 2 or 3 heteroatoms, selected from O, S and N. Examples include pyridyl, pyrazinyl, pyrimidinyl, pyridazinyl, furanyl, thienyl, pyrrolyl, oxadiazolyl, thiadiazolyl, thiazolyl, imidazolyl, oxazolyl, benzofuranyl, indolyl, indazolyl, carbazolyl, purinyl, cinnolinyl, quinoxalinyl, naphthyridinyl, benzimidazolyl, benzoxazolyl, quinolinyl, quinazolinyl and isoquinolinyl.

Typical heteroaryl groups include thiophenyl, pyrrolyl, pyridyl, furanyl, oxadiazolyl and carbazolyl.

When the heteroaryl group is a monocyclic heteroaryl group, typical groups include thiophenyl, pyrrolyl, pyridyl, furanyl and oxadiazolyl. As used herein, references to a heteroaryl group include fused ring systems in which a heteroaryl group is fused to an aryl group. When the heteroaryl group is such a fused heteroaryl group, typical examples are fused ring systems wherein a 5- to 6- membered heteroaryl group is fused to one or two phenyl groups. Examples of such fused ring systems are benzofuranyl, benzopyranyl, cinnolinyl, carbazolyl, benzotriazolyl, phenanthridinyl, indolyl, indazolyl, benzimidazolyl, benzoxazolyl, quinolinyl, quinazolinyl and isoquinolinyl moieties.

A heteroaryl group may be unsubstituted or substituted at any position. Unless otherwise stated, it carries 0, 1, 2 or 3 substituents. Typical substituents on a heteroaryl group include those listed above in relation to aryl groups. As used herein, arylene and heteroarylene groups respectively represent aryl and heteroaryl groups which are capable of bonding to at least two other groups, i.e. which are at least divalent. The aryl and heteroaryl groups are as defined above. As used herein, an alkoxy group is typically a said alkyl group attached to an oxygen atom. Similarly, alkenyloxy groups and aryloxy groups are typically a said alkenyl group or aryl group respectively attached to an oxygen atom. An alkylthio group is typically a said alkyl group attached to a thio group. Similarly, alkenylthio groups and arylthio groups are typically a said alkenyl group or aryl group respectively attached to a thio group. A haloalkyl or haloalkoxy group is typically a said alkyl or alkoxy group substituted by one or more said halogen atoms. Typically, each carbon atom of said group is substituted by one or more halogen atoms, with the maximum number of halogen atoms being the number required to bring the total valency of the carbon atom to four. Haloalkyl and haloalkoxy groups include perhaloalkyl and perhaloalkoxy groups such as -CX₃, -CX₂CX₃ and -OCX₃ wherein X is a said halogen atom, for example chlorine or fluorine, as well as longer alkyl and/or alkoxy chains such as C_2-6 chains substituted by one or more halogen atoms.

Haloaryl groups are, by analogy, typically a said aryl group substituted by one or more said halogen atoms. Typically, it is substituted by 1, 2 or 3 said halogen atoms. As used herein, a sulfoxide group is typically a group of the formula -SOR wherein R is a said alkyl or aryl group. A sulfone group is typically a group of the formula -SO₂R wherein R is a said alkyl or aryl group.

Turning now to the different portions of the polymers of formula (I) and charge separation polymers of formula (II) that may be used in the layer of photoactive conjugated polymer, and discussing each in turn:

Polymer Backbone (X):

Units of the polymer backbone, designated X in formulae (I) and (II), are selected from C_6-I4 arylene, C_6-I4 arylene-vinylene and C_6-14 arylene-acetylene units.

Typical C_6-14 arylene groups include phenylene and fluorenylene, with phenylene being more typical. In addition to the groups A and B that are present as substituents, these arylene groups are further unsubstituted or are further substituted by one, two or three groups selected from C_1-10 alkyl and C_1-I0 alkoxy. Typical further substituents are C_1-8 alkyl groups which are themselves unsubstituted.

Typically, in polymers comprising monomer units of formula (II), the polymer backbone consists only of arylene, vinylene and acetylene units. In particular, in polymers comprising monomer units of formula (II) there are typically no heteroatoms such as nitrogen, oxygen, sulphur or silicon present as atoms in the backbone itself. In one embodiment the polymer backbone consists of groups selected from the arylene, arylene-vinylene and/or arylene-acetylene units defined above, substituted by the groups A and B. In other words, there are no other monomer units present in the polymer backbone. In another embodiment, the polymer backbone also includes other monomers. In other words, the polymer is a copolymer of arylene, arylene-vinylene and/or arylene-acetylene units defined above which are substituted by the groups A and B, along with another monomer or monomers. Examples of the other monomer or monomers include arylene, arylene-vinylene, arylene-acetylene, heteroarylene, heteroarylene-vinylene and heteroarylene-acetylene units. The arylene and heteroarylene moieties in said other monomer or monomers may be unsubstituted or substituted by any of the functional groups described above. The substituents may, for example, be chosen in such a way as to make the spectrum of the copolymer match more fully the solar spectrum.

The polymer backbone units bear groups A and B. Integers a and b define, respectively, the number of A and B units. Typically a is 1 or 2, more typically 1. Typically b is 1 or 2, more typically 1.

Electron Donating Groups (EDGs):

The electron donating groups are in conjugation with the polymer backbone and, particularly in the charge-separation polymers comprising monomer units of formula (II), are capable of stabilising a hole once an exciton has been generated separated. Typical electron donating groups include C_1-Io alkyl, C_1-10 alkoxy, amino, C_1-1O alkylamino and di(Ci-io alkyl)amino. In particular, C_1-1O alkylamino and di(C_1-10 alkyl)amino are typical. Typical alkoxy groups include C_1-8 alkoxy groups which are unsubstituted or substituted by one, two or three groups selected from C_1-4 alkyl groups and Ci_-4 alkoxy groups. More typical alkoxy groups include C_1-8 alkoxy groups such as Ci_-6 alkoxy groups, which are unsubstituted or substituted by one or two Ci_-4 alkyl groups. A more typical alkoxy group is 2-ethylhexyloxy.

Electron Withdrawing Groups (EWGs):

The electron-withdrawing groups are in conjugation with the polymer backbone and, particularly in the charge-separation polymers comprising monomer units of formula (II), are capable of stabilising an electron once an exciton has been generated and separated. Suitable electron withdrawing groups include nitro, cyano, acid amide, ketone, phosphinoyl, phosphonate, ester, sulfone, sulfoxide, halo(C_1-6 alkyl), and halo(C_6-14 aryl) groups. In particular, nitro, cyano, ketone, sulfone, sulfoxide, ImIo(C_1-6 alkyl) and halo(C_6-]₄ aryl) are typical. Typical acid amide groups include tertiary acid amide groups. Typical ketone groups include diarylketones. Typical ester groups include groups of the formula -CO₂R where R is a Ci_-10 alkyl group such as a methyl or ethyl group, or a C_6-J4 aryl group. Typical sulfone groups include groups of the formula -SO₂R where R is a Ci-I₀ alkyl group such as a methyl or ethyl group, or a C_6-I4 aryl group. More typical sulfone groups are -SO₂Me groups. Aryl sulfones are especially typical. Typical sulfoxide groups include groups of the formula -SOR where R is a C₁. io alkyl group such as a methyl or ethyl group, or a C₆-_H aryl group. More typical sulfoxide groups are -SOMe groups. Arylsulfoxides are especially typical. Typical haloalkyl groups include Ci_-6 alkyl groups substituted by one or more halogen atoms, for example trifluoromethyl. Haloalkyl groups may be perhalogenated, e.g. perfluorinated. Typical haloaryl groups include C_6-W aryl groups which may be mono- or polycyclic, such as phenyl, naphthyl and fluorenyl. Haloaryl groups may be perhalogenated, e.g. perfluorinated. Especially typical electron withdrawing groups are cyano, nitro and sulfone groups.

Spacer Groups (L and L'):

The spacer groups L and I/ are selected from C_6-I₄ arylene, (C_6-I₄ arylene)-vinylene, (C_6-I₄ arylene)-acetylene, 5- to 10-membered heteroarylene, (5- to 10-membered heteroarylene)-vinylene and (5- to 10-membered heteroarylene)-acetylene groups, wherein the arylene and heteroarylene moieties are unsubstituted or substituted by one or more groups selected from C_1-1O alkyl and C_1-1O alkoxy. In the case of L, the arylene and heteroarylene moieties can be substituted by further EWG groups defined above. In the case of L', the arylene and heteroarylene moieties can be substituted by further EDG groups defined above.

Typical L and I/ groups include C_6-I4 arylene and (C_6-I4 arylene)vinylene groups, wherein the C_6-I4 arylene groups and the C_6-14 arylene moieties of the (C_6-14 arylene)-vinylene groups are unsubstituted or substituted by one or more groups, typically one or two groups, selected from C₁-Io alkyl and Ci_-I0 alkoxy. Typical C_6-I4 arylene groups and moieties include phenylene, naphthylene and fluorenylene, in particular phenylene and fluorenylene.

Typical 5- to 10-membered heteroarylene groups and moieties within the definition of L include heteroarylene with a relatively high electron affinity, such as pyridine. Typical 5- to 10-membered heteroarylene groups and, moieties within the definition of L' include heteroarylene with a relatively low electron affinity, such as thiophene.

Typical substituents on the arylene and heteroarylene groups include C_1-10 alkyl groups, for example Ci_-4 alkyl groups such as methyl, ethyl, propyl and butyl groups. Additionally, if the arylene group or heteroarylene group is part of the group B in formula (II), then the substituents are typically electron-donating groups, such as the groups EDG as exemplified herein. However, if the arylene group or heteroarylene group is part of the group A in formula (II), then typically the substituents are not strongly electron-donating groups. For example, if the arylene group or heteroarylene group is part of the group A in formula (II), then the substituents may be groups EWG as exemplified herein or alkyl, typically C_1-4 alkyl groups. For example, a particularly typical L group is a fluorenyl group which is disubstituted by «-propyl groups (see, for example, Scheme 1 below).

The 1 and Y subscripts define the number of spacer groups present between the backbone and the EWG and EDG groups respectively. Typically 1 is zero or an integer of from 1 to 5, more typically zero, 1, 2, 3 or 4, even more typically zero, 1, 2 or 3. Most typically, 1 is 1 or 2. Typically 1' is zero or an integer of from 1 to 5, more typically zero, 1, 2 or 3, even more typically zero, 1 or 2. Most typically 1' is zero. Where 1 and/or 1' is an integer of 2 or more, then 2 or more spacer groups are present between the polymer backbone and the group EWG or EDG. In this embodiment, the spacer groups are the same or different. For example, when 1 is 2, a fluorenylene group and a phenylene group could be present between the polymer backbone and EWG. In another embodiment, two fluorenylene groups could be present between the polymer backbone and EWG. The number of spacer groups between EDG and EWG will govern the strength of the dipole. In one embodiment the polymer of the layer of photoactive conjugated polymer is a charge-separation polymer comprising monomer units of one or more of formulae (TLA), (ITB) and (TLC):

wherein A, B, L, I/, 1, Y, EWG and EDG are as defined above with respect to formula (II), each x is zero or one, and each y is zero or one provided that at least one A group and at least one B group are present. Typical values of A, B, L, I/, 1, 1', EWG and EDG are as defined earlier. Typically either x is 1 and y is zero, or x is zero and y is 1.

For the avoidance of doubt, it should be noted that the conjugated polymer may include head-to-head, head-to-tail and tail-to-tail couplings of the monomer units.

Typically, the conjugated polymer comprises monomer units of one or more of formulae (TLIA), (LTLB) and (IIIC):

(MlA) (INB) (IIIC) wherein A, B, L, I/, 1, T₅ EWG and EDG are as defined above with respect to formula (II). Again, typical values of A, B, L, L', 1, Y, EWG and EDG are as defined above.

Processes: The charge separation polymers which may be used in the layer of photoactive conjugated polymer of the devices of present invention may be prepared by known preparation processes, or by analogy with known preparation processes. The strategies for forming poly[(hetero)arylenevinylene], poly[(hetero)aryleneacetylene] and poly[(hetero)arylene] homo- and copolymers are well known and are reviewed in detail by J. L. Segura, Acta. Polym., 1998, 49, 319. Simple conjugated polymers are inherently insoluble and hence unprocessible. The main strategy used to overcome this is to attach side chains to the polymer backbone. For example, alkyl or alkoxy side chains of the appropriate length such can impart solubility in polar aprotic solvents such as toluene, chlorobenzene, tetrahydrofuran and chloroform.

The main route to poly[(hetero)arylenevinylene]s is via the Gilch route or variants thereof, for example, the Xanthate route (S. -C. Lo et al.₅ J. Mater. Chem. 2000, Vol. 10, 275). Poly[(hetero)arylenevinylene]s can either be prepared so they are soluble in their conjugated form, by the attachment of solubilising groups, as are typically used in the present invention, or via a soluble precursor polymer that can be processed and converted in the solid state to the conjugated polymer. The advantage of the latter route is that the no solubilising side-chain may be needed.

Poly[(hetero)arylenevinylene]s can also be formed by Wittig chemistry and palladium catalysed Heck reactions. These latter strategies allow for the simple formation of homo- and copolymers.

Poly[(hetero)aryleneacetylene]s can be formed via Sonogashira type chemistry. For example a homopolymer can be formed from a monomer that contains a

(hetero)arylene unit with a halogen moiety and an acetylene moiety. Alternatively a monomer that has two acetylene units can be polymerized with one containing two halide moieties. With the latter method if the (hetero)arylene unit is the same in both cases a homopolymer is formed, but if they are different a copolymer is formed. Poly[hetero(arylene)]s are generally made from palladium catalysed Suzuki or

Stille couplings with the synthesis of homo- and copolymers following the same strategies as used for the poly[(hetero)aryleneacetylene]s.

The polymers used in the present invention may be prepared using each of the general methods described above.

Polymer Layer Thickness By using a reduced thickness of the layer which comprises a photoactive conjugated polymer in the devices of present invention, the efficiency of the device can be increased. In such devices, the layer which comprises a photoactive conjugated polymer has a thickness less than or equal to three times the exciton diffusion length {L_ED) of the polymer.

Light absorption by the photoactive conjugated polymer layer results in the generation of excitons in the polymer, which can travel through the polymer. An exciton has a finite lifetime in a given polymer, due to radiative and non-radiative decay processes. By "exciton diffusion length" herein is meant the linear distance travelled by the exciton over its lifetime, from an initial site at which the exciton is generated to another site at which the exciton disappears.

The exciton diffusion length of a polymer may be measured using time-resolved photoluminescence, as described, for example, by J. J. Kalinowski et al. J. Appl. Phys. 98 (2005) pO63532, and by K. Kawata et al. 'Description of exciton transport in a TiO2/MEH-PPV heteroj unction photovoltaic material' Solar Energy Materials and Solar Cells 87 715-724 (2004). The exciton diffusion length of MEH-PPV has been estimated to be 20+3 nm (T. J. Savenije et al. Chem. Phys. Lett. 1998, 287, 148).

By "thickness" herein, in relation to the layer which comprises a photoactive conjugated polymer, is meant the average thickness of the polymer layer without taking into account any polymer that penetrates into any pores or columns of the adjacent layers above and below the polymer. For example, where the electron acceptor layer below the polymer layer is a columnar TiO₂ layer deposited by the Glancing Angle Deposition (GLAD) technique, or some other highly porous TiO₂, then the polymer penetrating into the pores or columns of the TiO2 is disregarded for the purpose of measuring the thickness of the polymer layer. Thus, where the electron acceptor layer immediately below the polymer layer is columnar TiO₂, the thickness of the polymer layer is taken to be the average distance from an upper surface of the TiO₂ layer defined by joining together the tops of the TiO₂ columns, to the bottom surface of the layer immediately above the polymer. The thickness of the layer which comprises a photoactive conjugated polymer in the devices of the present invention can be measured using atomic force microscopy in the contact mode, ellipsometry or surface profilometry, for example. For example, the thickness may be estimated by measuring the thickness of a film of the polymer on ITO/glass using atomic force microscopy in the contact mode, and then assuming that the layer of the same polymer in the device deposited under the same conditions is of the same thickness. It has been determined that the thickness of the layer which comprises a photoactive conjugated polymer in devices of the present invention is typically less than or equal to three times the exciton diffusion length of the polymer, L_ED- It is also usual that the minimum thickness of the layer which comprises a photoactive conjugated polymer in devices of the present invention is 0.15 times the exciton diffusion length of the polymer, L_ED- Further typical minimum thicknesses for the layer are 0.25 L_ED, 0.40 LED, 0.50 LED, 0.60 L_ED, 0.75 L_ED, 0.85 L_ED, 1.00 LED, 1.10 LED, 1-20 LED and 1.25 LED- Further typical maximum thicknesses for the layer are 2.75 L_ED, 2.50 L_ED, 2.40 L_ED, 2.25 L_ED, 2.00 L_ED and 1.75 LE_D- Typical ranges for the thickness of the layer which comprises a photoactive conjugated polymer include all possible ranges formed by combining a typical minimum thickness from the above list with a typical maximum thickness from the above list. For example, one typical thickness range for the layer is from 1.10 L_ED to 2.40 L_ED- More typically, the thickness of the layer which comprises a photoactive conjugated polymer in devices of the present invention is 1.50 L_ED, or, for example, from 1.40 LED to 1.60 LED. Where the layer which comprises a photoactive conjugated polymer comprises, or consists of, a charge-separation polymer of formula (II), the thickness of the layer is typically less than 2.50 L_ED, more typically from 1.10 LED to 2.40 LED, even more typically from 1.40 L_ED to 1.60 L_ED, and most typically 1.50 L_ED- Where the layer which comprises a photoactive conjugated polymer does not comprise a charge- separation polymer of formula (II), the thickness of the layer is typically less than or equal to 3.00 L_ED, more typically from 0.15 LE_D to 3.00 L_ED, and even more typically from 0.50 LED to 2.50 L_ED-

In absolute terms, the thickness of the layer which comprises a photoactive conjugated polymer in devices of the present invention is typically less than or equal to 60 nm. The minimum thickness of the layer which comprises a photoactive conjugated polymer in devices of the present invention is typically 3 nm. Further typical minimum thicknesses for the layer are 5 nm, 8 nm, 10 nm, 12 nm, 15 nm, 17 nm, 20 nm, 22 nm, 24 nm and 25 nni. Further typical maximum thicknesses for the layer are 55 run, 50 nm, 48 nm, 45 nm, 40 nm and 35 nm. Typical ranges for the thickness of the layer which comprises a photoactive conjugated polymer include all possible ranges formed by combining a typical minimum thickness from the above list with a typical maximum thickness from the above list. For example, one typical thickness range for the layer is from 22 nm to 48 nm. Most typically, the thickness of the layer which comprises a photoactive conjugated polymer in devices of the present invention is 30 nm, or, for instance, from 27 nm to 33 nm.

Where the layer which comprises a photoactive conjugated polymer comprises, or consists of, a charge-separation polymer of formula (II), the thickness of the layer is typically less than 50 nm, more typically from 22 nm to 48 nm, even more typically from 27 nm to 33 nm, and most typically 30 nm. Where the layer which comprises a photoactive conjugated polymer does not comprise a charge-separation polymer of formula (II), the thickness of the layer is typically less than 60 nm, more typically from 3 nm to 60 nm, and even more typically from 10 nm to 50 nm.

Polymer Molecular Weight

The use of a reduced polymer molecular weight for the layer which comprises a photoactive conjugated polymer in devices of the invention gives rise to surprisingly improved device performance. In such devices, the polymer of the layer has a weight average molecular weight of less than 500,000 g/mol. Typically, the minimum weight average molecular weight of the polymer is 10,000 g/mol. Further typical minimum weight average molecular weight values for the polymer of the layer which comprises a photoactive conjugated polymer are 20,000, 50,000, 100,000, 150,000, and 180,000 g/mol. Typical maximum weight average molecular weight values for the polymer of the layer which comprises a photoactive conjugated polymer are 480,000, 450,000, 400,000, 350,000, 300,000, 250,000 and 220,000 g/mol. Typical ranges for the weight average molecular weight values for the polymer include all possible ranges formed by combining a typical minimum weight average molecular weight from the above list with a typical maximum weight average molecular weight from the above list. For example, typical ranges for the weight average molecular weight value of the polymer are from 20,000 g/mol to 350,000 g/mol; from 100,000 g/mol to 300,000 g/mol; from 150,000 g/mol to 300,000 g/mol; and from 180,000 g/mol to 300,000 g/mol. Most typically, the weight average molecular weight value of the polymer is 200,000 g/mol, or, for instance, from 180,000 g/mol to 220,000 g/mol.

A common technique for measuring weight average molecular weight is gel permeation chromatography. Typically, the measurements are made against a polystyrene standard.

Polymer Heat Treatment

It was found that heat treatment of the polymer in devices of the present invention caused a further significant increase in device performance. Thus, typically, the polymer of the layer which comprises a photoactive conjugated polymer in the devices of the present invention has been annealed by heat treatment. More typically, the heat treatment is conducted at a temperature greater than the polymer glass transition temperature T_g. By "glass transition temperature" or 'T/' herein is meant the bulk glass transition temperature of the polymer.

Accordingly, the invention provides a process for producing a photovoltaic device of the invention, which process comprises annealing the photoactive conjugated polymer by heat treatment of the polymer. Typically, the temperature of the heat treatment is above the glass transition temperature of the polymer. Typically, the temperature of the heat treatment is from 70⁰C to 280⁰C. More typically, the temperature of the heat treatment is from 175 ⁰C to 225 ⁰C. A temperature of 200 ⁰C, or for example, a temperature of from 190⁰C to 210⁰C, is particularly typical. Typically, the process further comprises: (i) forming the electron acceptor layer on a substrate comprising the first electrode; (ii) forming the electron donor layer on the electron acceptor layer; and (iii) forming the second electrode on the electron donor layer.

Typically, the layer which comprises a photoactive conjugated polymer is the electron donor layer. In such a case, the electron acceptor layer typically comprises an inorganic electron-transporting semiconductor such as TiO₂ (titania), ZnO, CdSe, CdTe, C₆₀, C₇₀ and other fullerenes and carbon nanotubes, including functionalised fullerenes like (6,6)-phenyl C₆i -butyric acid methyl ester (PCBM) and (6,6)-phenyl C₇₁-butyric acid methyl ester ([7O]PCBM). However, the electron acceptor layer may comprise an organic electron-transporting material, for instance perylene. More typically, the electron acceptor layer comprises titania (TiO₂), which is cheap and non-toxic.

Typically, the electron acceptor layer is of porous titania. As used herein "porous titania" means either nanoporous titania or columnar titania, as opposed to dense or "solid" TiO₂. Nanoporous titania contains pores with diameters in the nanometer scale, and can be formed synthetically, for example by sintering together titania nanocrystals. Layers of such nanoporous titania can be deposited using doctor blade coating techniques, which are well known to the skilled person. Alternatively, layers of nanoporous titania can be produced using Glancing Angle Deposition (GLAD), which is a technique for fabricating materials with controlled structure. The use of GLAD to fabricate porous structures is outlined in US Patent 5,866,204. GLAD may also be used to produce porous titania in columnar form. An advantage of using a porous electron acceptor layer (such as nanoporous titania or columnar titania produced by GLAD) in conjunction with an electron donor layer of an organic conjugated polymer is that devices can be fabricated in which the polymer infiltrates into the pores of the titania, thus increasing the interfacial area between the two semiconductors for exciton splitting.

Typically, the electron acceptor layer is 20 nm to 2 μm thick, for instance 40 nm to 1 μm nm thick. More typically, the electron acceptor layer has a thickness of 50 nm to 500 nm.

The photovoltaic devices of the present invention may further comprise organic layers between the anode and cathode to improve charge extraction and device efficiency. In particular a layer of conductive or hole-transporting material may be situated over the anode. This layer serves to increase charge conduction through the device. The preferred anode coating in polymer devices is a conductive organic polymer such as polystyrene sulfonic acid doped polyethylene dioxythiophene (PEDOT:PSS) as disclosed in WO98/05187. Other hole transporting materials such as doped polyaniline, TPD, NPD and MTDATA may also be used.

A layer of electron transporting material may be next to the cathode as this can improve device efficiency. Suitable materials for electron transporting layers include BCP, TPBI and PBD. The substrate of the photovoltaic device should provide mechanical stability to the device and act as a barrier to seal the device from the environment. Where it is desired that light enters the device through the substrate, the substrate should be transparent or semi-transparent. Glass is widely used as a substrate due to its excellent barrier properties and transparency. Other suitable substrates include ceramics, and plastics such as acrylic resins, polycarbonate resins, polyester resins, polyethylene terephthalate resins and cyclic olefin resins. Plastic substrates may require a barrier coating to ensure that they remain impermeable. The substrate may comprise a composite material such as the glass and plastic composite. To provide environmental protection the device may be encapsulated.

Encapsulation may take the form of a glass sheet which is glass bonded to the substrate with a low temperature frit material. To avoid the necessity of using a glass sheet to encapsulate the device a layer of passivating material may be deposited over the device. Suitable barrier layers may comprise a layered structure of alternating polymer and ceramic films and may be deposited by PECVD or sputtering, for example.

Photovoltaic devices of the invention maybe produced by any suitable process known to those skilled in the art, or by the processes described herein including in the examples below. A process for producing a photovoltaic device of the invention comprises: (i) forming the electron acceptor layer on a substrate comprising the first electrode; (ii) forming the electron donor layer on said electron acceptor layer; and (iii) forming the second electrode on said electron donor layer, wherein either the electron acceptor layer or the electron donor layer comprises the layer which comprises a photoactive conjugated polymer. Typically the substrate comprising a first electrode is indium tin oxide (ITO) on glass. When the polymer of the layer of photoactive conjugated polymer is soluble this layer may be advantageously deposited by solution processing techniques. Solution processing techniques include selective methods of deposition such as screen printing and ink-jet printing and non-selective methods such as spin coating and doctor blade coating. Typically the polymer is deposited by spin coating. If a precursor polymer is used, then after solution processing it is thermally converted under vacuum or in an inert atmosphere to the conjugated polymer. The other layers, such as the electron donor or acceptor layer may be deposited by evaporation, Glancing Angle Deposition (GLAD), sputtering, spin coating, doctor blading or solution processing techniques, as appropriate, providing that any subsequent solution processing step does not substantially remove the already deposited layers. Doctor blading is a well-known process for producing a layer of material in which a slurry of the material is spread onto the substrate using the edge of a moving blade. The electrode material may be deposited by sputtering, vapour deposition or spin coating. The invention will be described further in the Examples which follow.

EXAMPLES

Example 1

The results described in this Example concern MEH-PPVVTiO₂ photovoltaic cells. The structure of the cells is shown in Figure 1, in which a) shows a schematic representation of a cell; b) shows a micrograph of the TiO₂ layer prepared by doctor- blading; and c) shows the chemical structure of MEH-PPV. The cells consist of a conducting ITO/glass substrate onto which a layer of TiO₂ was formed by doctor- blading a colloidal anatase paste and subsequent sintering of the latter at 450 °C for one hour. This procedure was repeated twice in order to obtain a film with a reduced number of pin-holes. The resulting TiO₂ layer is approximately 1.2 μm thick and, as shown in b) of Figure 1, possesses a well-defined network of randomly oriented and interconnected pores. From gas sorption measurements it was estimated that the average pore size is 9-10 nm. A film of MEH-PPV (average weight average molecular weight ~ 500,000 g.mol^"1) was formed on top of the nanoporous titania layer by spin coating and the solar cells were completed by thermal evaporation of a 45 nm thick gold electrode. The cells were prepared according to the following procedure. A nanoporous layer of TiO₂ was formed on pre-cleaned patterned ITO/glass (20 Ω/sq) by doctor blading a colloidal anatase paste (Ti-Nanoxide HT, Solaronix SA) and sintering for one hour at 450°C. This procedure was repeated twice. MEH-PPV with a molecular weight 513,000 g.mol^"1 was synthesised via the Gilch route. Polymer films, approximately 30 and 60 nm thick, were deposited on top of the TiO₂ layer by spin coating from two chlorobenzene solutions with a different concentration (4.6 and 5.5 mg.ml^"1, respectively) at a rate of 2000 rpm. The back contact of the cells was prepared by thermal evaporation through a shadow mask of a 45 run thick Au layer to define photovoltaic pixels with an active area of 6.3 mm².

The photovoltaic devices prepared for Example 1 can be classified as bi-layer DA cells, although they deviate from the idealised sandwich/planar configuration in two respects. First, the TiO₂ film is not ideally flat but has a finite surface roughness (RMS roughness ~ 10 nm on a 1x1 μm² scale). Second, the possibility of some infiltration of the polymer chains into the porous network of the titania layer close to TiO₂/polynier interface cannot be dismissed. The level of infiltration was not investigated, but our data indicate that the hydrodynamic radii for MEH-PPV with a molecular weight of 500,000 g/mol are of the order of 18-20 nm. The infiltration of the polymer chains into a random network of pores with an average diameter of only 10 nm is therefore bound to be kinetically hindered and rather limited on the time-scale of a spin-coating operation.

(a) Effects of the polymer layer thickness

The thickness of the polymer films was measured on ITO/glass using atomic force microscopy in the contact mode. It was assumed that the films spun on TiO₂ in the tested cells were of the same thickness.

/- V tests of the devices were carried out in vacuum (2.10^"3 mbar) using a Keithley 2400 source meter. The light illumination experiments were performed using a

KH Steuernegel 1200 Solar Simulator and a 420 nm long-pass filter. The light intensity in these measurements is 80 mW.cm^'2 at the sample position.

Table 1 shows the averaged photovoltaic performance parameters of bi-layer devices with a 30 and 60 nm thick MEH-PPV film. The cells built with a 30 nm MEH- PPV film are more efficient (PCE_av = 0.20%) than their 60 nm MEH-PPV counterparts

(PCE_av = 0.13%). For comparison with known devices, the typical PCE of known

ITO/TiO₂/MEH-PPV/Au photovoltaic devices is close to 0.1% (A. J. Breeze et al. Phys.

Rev. B. 2001, 64, 125205; M. Y. Song et al. Synth. Met. 2003, 137, 1387). In the cited works the polymer film thickness was kept to a minimum of 63 nm. Thus, due to the employment of MEH-PPV films with half the previously used thickness, the bi-layer cells exemplified herein are twice as efficient as known devices. The best performing device in the 30 nm MEH-PPV set has an efficiency of 0.23%, with an open circuit voltage of 0.53 V, a short-circuit current density of 1.04 mA.cm^"2, and a fill factor of 0.33.

Table 1 Averaged photovoltaic performance parameters of

ITO/TiO₂/MEH-PPV/Au solar cells with two different MEH-PPV layer thicknesses obtained under simulated AM 1.5 illumination modified using a

420nm long-pass filter (80 mW.cm^"2). The bottom row of the table contains the larger of the two standard deviation sets.

Voc, V J SCj FF, % PCE, % mA/cm²

30 nm MEH-PPV 0.52 0.94 32 0.20

60 nm MEH-PPV 0.63 0.70 24 0.13

Standard deviation 0.04 0.12 2 0.03

As is evident from Table 1, the increased efficiency of the devices with a 30 nm MEH-PPV layer is due to the greater short-circuit current and fill factor of these devices in comparison with those of the 60 nm polymer layer devices.

(b) Effects of post-production heat treatment

In the case of a bi-layer MEH-PP WTiO₂ solar cell with a 30 nm polymer film it was found that heat-treating the polymer layer caused a further significant increase in performance. For MEH-PPV even a short-term heat treatment in the temperature range of from 70°C to 28O⁰C leads to reordering of the polymer structure. Figure 2 shows the changes in photovoltaic performance of a bi-layer MEH-PP WTiO₂ solar cell with a 30 nm polymer film as a result of post-production heat treatment at 200⁰C. The as-made device has an efficiency of 0.16%, with V_oc = 0.56V, J_sc = 0.77 mA.cm^"2 and FF = 0.30. After the post production treatment both J_sc and FF substantially increased, to 1.45 mA.cm^'2 and 0.46, respectively, while the open circuit voltage of the device is little changed (0.59 V). The efficiency after heat treatment is therefore 0.50%. This is nearly three times higher than both the original efficiency of the cell and the highest previously reported PCE of 0.18% (A. J. Breeze et al. Phys. Rev. B. 2001, 64, 125205) for a cell with the basic structure ITO/TiO₂/MEH-PPV/Au.

Without wishing to be bound by theory, the influence of heat treatment on photovoltaic performance (Figure 2) is attributed to two main effects. The first effect is an improvement in the hole-transport (and possibly exciton transport) properties of the polymer as a result of better chain packing. The enhancement of hole transport across the donor phase should improve the efficiency of charge collection. The second effect of heat treatment that needs to be considered is that as the MEH-PPV chains attain a greater motional freedom at the elevated temperature they can reorganise at the interfaces of the polymer film with the TiO₂ electron acceptor layer and the Au top electrode. Such a reorganisation can improve the electrical contact at the two interfaces and result in a greater rate of exciton dissociation at the MEH-PPWTiO₂ interface and/or better hole collection at the cell cathode.

In summary, by reducing the thickness of the polymer film in MEH-PP WTi O₂ bi-layer cells, their efficiency after postproduction treatment has been increased from the highest reported PCE of 0.18% to 0.50%. Without wishing to be bound by theory, this surprising improvement in device performance with MEH-PPV film thickness reduction and heat treatment indicates that the efficiency of the bi-layer cells is limited to a greater extent than had been realised by charge transport in the bulk of the polymer film.

Example 2: Effects of polymer molecular weight

The efficiencies of bi-layer devices manufactured using MEH-PPV of four different weight average molecular weights (namely 200,000; 500,000; 1,000,000 and 1,800,000 g/mol) were compared. Table 2 PV performance of the standard composite solar cells (CSCs) as a

function of the weight average molecular weight (M _w) of MEH-PPV.

M_{w >} g/mo\ PCE, % V_oo V Jsc, mA/cm² FF, % Time after illumination, min

0.14 0.61 0.60 40 22

200,000 0.18 0.55 0.87 37 102 0.19 0.55 0.95 36 ^' 210

500,000 0.064 0.61 0.26 40 112 0.065 0.50 0.37 35 85

1,000,000 0.064 0.61 0.36 29 70 0.079 0.62 0.42 31 68

0.067 0.66 0.30 34 30

1,800,000 0.073 0.60 0.34 36 70 0.085 0.59 0.37 40 63

The MEH-PPV films were formed by spin-coating from chlorobenzene solution. The concentration of each solution was chosen so that a polymer film of nominally 60 nm could be formed upon spin-coating on ITO/glass. Table 2 presents performances

for each of the four groups of CSCs. Devices made with polymers of M_w equal or greater than 500,000 g/mol show similar efficiencies of 0.060-0.085% within the time

of the measurements. The PCE of the device with polymer ofM_w 200,000 g/mol however is approximately twice as great (0.14%) even after only 22 minutes of

illumination. In comparison the PCE of the cell made with polymer of M_w 1,800,000 g/mol is 0.067% after 30 minutes of illumination.

The results of this experiment show a surprisingly improved performance for low molecular weight MEH-PPV devices. Repeat testing of the 200,000 g/mol material freshly prepared and after 1 year storage gave similar results, illustrating the longevity of the material. Example 3: Preparation of a charge separation polymer of formula (ID

Measurements and Materials

NMR spectra were recorded on a Bruker 400 M Hz spectrometer; J values are reported in Hz. IR spectra were recorded on a Spectrum 1000 IR spectrometer and analysed as either a thin film or a KBr disc. UV-visible spectra were recorded on a Perkin-Elmer UV lambda 15 spectrometer as either a thin film or as a solution in spectroscopic grade dichloromethane. Mass spectra were recorded either on a Hewlett Packard 1050 Atmospheric Pressure Chemical Ionisation mass spectrometer (APCI) or VG platform spectrometer. Electronic ionisation was recorded on a Bio-Q spectrometer. Microanalysis was carried out by Mrs. A. Douglas, Inorganic Chemistry Research Laboratory, University of Oxford. Melting points were determined on a Gallenkamp melting point apparatus and are uncorrected. Gel permeation chromatography was carried out with a Polymer Laboratories PL gel 20 μm Mixed A columns (600 mm length and 7mm diameter) calibrated with polystyrene standards (580-11.2 x 10⁶) in tetrahydrofuran with toluene as a flow marker. The UV detector was set at 245 run and solvent was pumped at a flow rate of 1 ml/min.

Preparation of Polv(2-r2-(7-nitro-9.9-dipropyl-9H-fiuoreneΗ-5-(2'-ethyl-hexyloxy)- 1 ,4-phenylenevmylene| (8b)

(a) 2-[4-(2^f-ethyl-hexyIoxy)-2,5-dimethyl-phenyl]-4,4,5,5-tetramethyl- [l,3,2]dioxaborolane (4):

7ert-butyllithium (1.7 M in pentane, 100 mL, 0.17 mol) was added to a solution of l-bromo-4-(2'-ethylhexyloxy)-2,5-dimethylbenzene 3 (29.6 g, 94 mmol) in anhydrous tetrahydrofuran (250 mL) at -78 ⁰C under argon. The reaction mixture was stirred for 1 hour at -78 ⁰C before the addition of 2-isopropoxy-4,4,5,5-tetramethyl- 1,3,2-dioxaborolane (25.0 mL, 0.12 mol). The reaction mixture was stirred at -78 ⁰C for 15 minutes then at room temperature for 20 hours. The reaction was quenched with water (200 mL) and then the aqueous layer was extracted with ether (3 x 200 mL). The combined organic extracts were washed with water (2 x 200 mL), brine (250 mL), and dried over anhydrous magnesium sulphate. The solution was filtered and solvent removed. The residue was purified by column chromatography over silica using light petroleum:dichloromethane (3:1) as eluent to give 4 (20.4 g, 34%) as a yellow oil.

(b) 2-[4-(2'-EthyI-hexyloxy)-2,5-dimethyI-phenyl]-7-nitro-9,9-dipropyI-9iϊ- fluorene (5b):

A mixture of 2-[4-(2'-ethyl-hexyloxy)-2,5-dimemyl-phenyl]-4,4,5,5- tetramethyl-[l,3,2]dioxaborolane 4 (7.0 g, 30 mmol), 2-bromo-7-nitro-9,9-di-n-propyl- 9H- fluorene 2 (6.7 g, 20 mmol), aqueous sodium carbonate (2 M, 160 mL), and toluene (240 mL) was deoxygenated with nitrogen for 10 minutes. Compound 2 can be obtained as described in H. Lambert, Part II Thesis, 2003, University of Oxford, UK. Then tøt7-αAis(triphenylphosphine) palladium (0) (0.5 g, 0.4 mmol) was added whilst maintaining a flow of argon over the reaction mixture. The reaction mixture was heated at reflux in the dark for 24 hours. After cooling, aqueous hydrochloric acid (3 M, 100 mL) was added carefully. The aqueous layer was extracted with ether (3 x 100 mL). The combined organic extracts were washed with water (3 x 100 mL), brine (100 mL), dried over anhydrous magnesium sulphate, filtered and the solvent was then removed. The residue was purified by column chromatography over silica gel using light petroleum:dichloromethane (3:1) as the eluent followed by recrystallisation from a dichloromethane/methanol mixture to give 5b (6.5 g, 76%), mp 115.0-117.0 °C; (Found: C, 79.71; H, 8.60; N, 2.66. C₃₅H₄₅NO₃ requires C, 79.66; H, 8.59; N, 2.65%); δ_H(CDCl₃, 400 MHz) 0.64-0.78 (10 H, m), 0.92-1.05 (6 H, m), 1.35-1.67 (8 H, m), 1.78-1.86 (1 H, m), 1.98-2.13 (4 H, m), 2.29 (3 H, s), 2.31 (3 H, s), 3.94 (2 H, d, J5.4), 6.80 (1 H, s), 7.12 (1 H, s), 7.37 (2 H, m), 7.82 (2 H, d, J 8) and 8.28 (2 H, dd, J 2, J 8).

(c) 2-[2,5-iϊw-acetoxymethyl-4-(2'-ethyl-hexyIoxy)-phenyl]-7-nitro-9,9- dipropyl-9H-fluorene (6b):

A mixture of 5b (16.1 g, 31.4 mmol) and N-bromosuccinimide (11.2 g, 62.9 mmol) in carbon tetrachloride (70 mL) was deoxygenated with argon for 10 minutes. 2,2'-Azo-tø(wo-butyronitrile) (2.1 g, 12.6 mmol) was added and reaction mixture was heated at reflux for 4 hours. The reaction mixture was allowed to cool to room temperature, diluted with dichloromethane (30 mL) and passed through a silica plug using dichloromethane as eluent. The solvent was removed and the residue was taken up in glacial acetic acid (70 mL). Sodium acetate (27.8 g, 0.32 mol) was added and the reaction mixture was heated at reflux for 5 hours. After cooling, water (50 mL) was added and the aqueous layer was extracted with ether (3 x 75 mL). The combined organic extracts were washed with aqueous sodium hydroxide (5% w/v, 50 mL, water (3 x 150 mL) and a saturated solution of sodium bicarbonate (3 x 50 mL). The solution was dried over anhydrous magnesium sulphate, filtered and solvent was removed. The residue was purified by flash chromatography over silica using a gradient elution with light petroleum :dichloromethane (1 : 1-0:1) followed by recrystallisation from a dichloromethaiie/methanol mixture to give 6b (8.6 g, 42%), mp 103.5-104.5 ⁰C; (Found: C, 72.88; H, 7.68; N, 2.18. C₃₉H₄₉NO₇ requires C, 72.76; H, 7.67; N, 2.18%); δ_H (CDCl₃, 400 MHz) 0.68-0.75 (10 H, m), 0.94-1.01 (6 H, m), 1.34-1.58 (8 H, m), 1.75-1.84 (1 H, m), 1.98-2.08 (4 H, m), 2.27 (3 H, s), 2.28 (3 H, s), 3.98 (2 H, d, J5.3), 5.04 (2 H, s), 5.22 (2 H₅ s), 7.07 (1 H, s), 7.38 (3 H, m), 7.83 (2 H, m), 8.28 (2 H, dd, J 2, J 8).

(d) 2-[2,5-5w-chloromethyl-4-(2'-ethyl-hexyloxy)-phenyl]-7-nitro-9,9-dipropyl- 9iϊ-fluorene (7b):

A mixture of 6b (7.4 g, 11.5 mmol), hydrochloric acid (35%, 160 mL), and 1,4- dioxane (160 mL) was heated at reflux for 18 hours under nitrogen. On cooling, the aqueous layer was separated and extracted with ether (3 x 50 mL). Combined organic extracts washed with aqueous sodium hydroxide solution (5% w/v, 50 mL), water (3 x 100 mL), saturated aqueous sodium bicarbonate (50 mL), and brine (50 mL). The organic layer was dried over anhydrous magnesium sulfate, filtered, and then the solvent was removed. The residue was purified by column chromatography over silica using light petroleum:dichloromethane (1 : 1) as the eluent followed by recrystallisation from a dichloromethane/methanol mixture to give 7b (5.6 g, 83%), mp 84.0-85.0 ⁰C; δ_H (CDCl₃, 400 MHz) 0.67-0.79 (10 H, m), 0.95-1.06 (6 H, m), 1.36-1.77 (8 H, m), 1.83-1.89 (1 H, m), 2.06-2.14 (4 H, m), 4.04 (2 H, d, J5.3), 4.53 (2 H, s), 4.72 (2 H, s), 7.12 (1 H, s), 7.44 (2 H, dd, J8, J2); 7.57 (1 H, s), 7.87 (2 H, t, J7.9), 8.30 (2 H, dd, J 8, J2); Exact mass 595.2602. C₃₅H₄₃Cl₂NO₃ requires 595.2620. (e) PoIy{2-[2-(7-nitro-9,9-dipropyl-9H^r-fluorene)]-5-(2'-ethyl-hexyloxy)-l,4- phenylenevinylene} (8b):

Potassium tert-butoxide (0.66 g, 5.86 mmol) in dry tetrahydrofuran (58.6 niL) was added to a stirred solution of 7b (0.7 g, 1.17 mmol) in dry tetrahydrofuran (23.4 niL) at room temperature under nitrogen. The reaction mixture was stirred in dark for 3.5 hours. The solution was poured into ice-cold methanol (100 mL), centrifuged (4500 rpm, 5 minutes), and the supernatant was removed. The residue was taken up in tetrahydrofuran (50 mL), filtered through a cotton wool plug and then poured onto ice- cold methanol (40 mL). The polymer was collected after centrifugation and the process was repeated once more. The residue was dried under vacuum for 16 hours to give 8b (284 mg, 46%); v_max (film, KBr discVcm^"1 969 (C=C-H trans), 1523 (NO₂), 1338 (NO₂); ?w_x (film)/nm 204, 250 sh, 356 and 422 sh; M_w = 3.2 x 10⁵, M_n = 0.5 x 10⁵ and PD = 7.0.

Claims

CLAIMS:

1. A photovoltaic device comprising: a first electrode; a second electrode; and, disposed between the first and second electrodes, an electron acceptor layer and an electron donor layer, wherein either the electron acceptor layer or the electron donor layer comprises a layer which comprises a photoactive conjugated polymer and which has a thickness less than or equal to three times the exciton diffusion length of the polymer, L_ED-

2. A photovoltaic device according to claim 1 wherein the layer which comprises a photoactive conjugated polymer has a thickness less than or equal to 60 run.

3. A photovoltaic device according to claim 1 or claim 2 wherein the layer which comprises a photoactive conjugated polymer has a thickness of from 1.10 L_ED to 2.40

LED-

4. A photovoltaic device according to any one of claims 1 to 3 wherein the layer which comprises a photoactive conjugated polymer has a thickness of from 22 run to 48 nm.

5. A photovoltaic device comprising: a first electrode; a second electrode; and, disposed between the first and second electrodes, an electron acceptor layer and an electron donor layer, wherein either the electron acceptor layer or the electron donor layer comprises a layer which comprises a photoactive conjugated polymer having a weight average molecular weight of less than 500,000 g/mol.

6. A photovoltaic device according to any one of the preceding claims wherein the photoactive conjugated polymer has a weight average molecular weight equal to or less than 300,000 g/mol.

7. A photovoltaic device according to any one of the preceding claims wherein the photoactive conjugated polymer has a weight average molecular weight of from 150,000 g/mol to 300,000 g/mol.

8. A photovoltaic device according to any one of the preceding claims wherein the photoactive conjugated polymer has been annealed by heat treatment.

9. A photovoltaic device according to claim 8 wherein the temperature of the heat treatment is above the glass transition temperature of the polymer.

10. A photovoltaic device according to claim 9 wherein the temperature of the heat treatment is from 70 ⁰C to 280 ⁰C.

11. A photovoltaic device according to any one of the preceding claims wherein the layer which comprises a photoactive conjugated polymer is the electron donor layer.

12. A photovoltaic device according to any one of the preceding claims wherein the photoactive conjugated polymer is a polyarylene, a poly(arylene-vinylene) or a poly(arylene-acetylene).

13. A photovoltaic device according to any one of the preceding claims wherein the photoactive conjugated polymer is poly[2-(2-ethylhexyloxy)-5-methoxy-l,4- phenylenevinylene] .

14. A photovoltaic device according to any one of claims 1 to 11 wherein the photoactive conjugated polymer is a polymer comprising monomer units of the formula (II):

(H) wherein:

X is selected from C_6-I₄ arylene, C_6-14 arylene-vinylene and C_6-I₄ arylene-acetylene units; each A represents a group of formula -(L)i-EWG wherein EWG is an electron- withdrawing group; a is 1, 2 or 3;

1 is zero or an integer of from 1 to 10;

L is a spacer group selected from C_6-14 arylene, (C_6-14 arylene)-vinylene, (C_6-14 arylene)-acetylene, 5- to 10-membered heteroarylene, (5- to 10- membered heteroarylene)-vinylene, and (5- to 10-membered heteroarylene)-acetylene groups, wherein the arylene and heteroarylene moieties are unsubstituted or substituted by one or more groups selected from C_1-10 alkyl, C_1-10 alkoxy and EWG groups defined above; each B represents a group of formula -(L')r-EDG wherein EDG is an electron-donating group; b is 1, 2 or 3;

I' is zero or an integer of from 1 to 10;

I/ is a spacer group selected from C_6-14 arylene, (C_6-14 arylene)-vinylene, (C_6-H arylene)-acetylene, 5- to 10-membered heteroarylene, (5- to 10- membered heteroarylene)-vinylene, and (5- to 10-membered heteroarylene)-acetylene groups, wherein the arylene and heteroarylene moieties are unsubstituted or substituted by one or more groups selected from C_1-10 alkyl, C_1-10 alkoxy and EDG groups defined above; when 1 is greater than zero, EWG is attached to an arylene, heteroarylene, vinylene or acetylene moiety of L; when I' is greater than zero, EDG is attached to an arylene or heteroarylene moiety of L'; and 1 and Y are not both zero.

15. A photovoltaic device according to any one of the preceding claims wherein the electron acceptor layer comprises a metal oxide.

16. A photovoltaic device according to any one of the preceding claims wherein the electron acceptor layer comprises titania.

17. A photovoltaic device according to any one of the preceding claims wherein the first electrode is indium tin oxide and the second electrode is gold.

18. A process for producing a photovoltaic device as defined in any one of the preceding claims, the process comprising annealing the photoactive conjugated polymer by heat treatment.

19. A process according to claim 18 further comprising forming the electron acceptor layer on a substrate comprising the first electrode; forming the electron donor layer on the electron acceptor layer; and forming the second electrode on the electron donor layer.

20. A process according to claim 18 or claim 19 wherein the temperature of the heat treatment is above the glass transition temperature of the polymer.

21. A process according to claim 19 wherein the temperature of the heat treatment is from 70 ⁰C to 280 ⁰C.

22. A process for producing a photovoltaic device comprising: (i) forming an electron acceptor layer on a substrate comprising a first electrode; (ii) forming an electron donor layer on said electron acceptor layer; and (iii) forming a second electrode on said electron donor layer, wherein either the electron acceptor layer or the electron donor layer comprises a layer which comprises a photoactive conjugated polymer and which has a thickness less than or equal to three times the exciton diffusion length of the polymer, L_ED-

23. A process for producing a photovoltaic device comprising: (i) forming an electron acceptor layer on a substrate comprising a first electrode; (ii) forming an electron donor layer on said electron acceptor layer; and (iii) forming a second electrode on said electron donor layer, wherein either the electron acceptor layer or the electron donor layer comprises a layer which comprises a photoactive conjugated polymer having a weight average molecular weight of less than 500,000 g/mol.