WO2010132951A1

WO2010132951A1 - Oligothiophenes and uses thereof in photovoltaic devices

Info

Publication number: WO2010132951A1
Application number: PCT/AU2010/000612
Authority: WO
Inventors: Richard Evans; Akhil Gupta
Original assignee: Commonwealth Scientific And Industrial Research Organisation
Priority date: 2009-05-22
Filing date: 2010-05-21
Publication date: 2010-11-25

Abstract

Compounds of formula (I) wherein EWG is an electron withdrawing group or combination of groups, R¹ and R² are each independently selected from the group consisting of alkyl, aromatic or heteroaromatic groups, or R¹ and R² may together comprise a linked alkyl, aromatic or heteroaromatic group. R³ and R⁴ are each independently selected from the group consisting of alkyl, alkoxy or H, or R³ and R⁴ may together comprise a divalent alkyl group, a divalent alkoxy or alkyldioxy group, or R³ and R⁴ may together comprise a heterocyclic, heteroaromatic or aromatic group linked or fused to the thiophene group and n is an integer between 2 and 10. The compounds are capable of charge transportation and have application in organic photovoltaic devices such as dye sensitised solar cells.

Description

OLIGOTHIOPHENES AND USES THEREOF IN PHOTOVOLTAIC DEVICES

Field

The present application relates to new chemical compounds useful in organic photovoltaic applications, and to photovoltaic devices containing such compounds.

Background

Photovoltaic devices include heterojunction and bilayer organic photovoltaic cells, sometimes referred to as organic photovoltaics (OPVs), hybrid solar cells and dye sensitised solar cells, which are also known as Cratzel cells.

Photovoltaic devices contain a combination of electron acceptor materials and electron donor materials (or hole accepting materials) in the active layer. Absorption of a photon results in the generation of a weakly-bound electron- hole pair (or exciton) in the active layer. Dissociation of the bound electron-hole pair is facilitated by the interface between the electron donor and electron acceptor materials. The separated holes and electrons travel towards respective electrodes and consequently generate a voltage potential at the electrodes.

Poly 3-hexylthiophene is an example of a polymeric organic material used as an electron donor material in photovoltaic devices, together with fullerene as an example of an electron acceptor material. The two materials may be present as layers, forming a bilayer photovoltaic cell, or may be present as a blend, forming a bulk heterojunction photovoltaic cell. In bulk heterojunction photovoltaic cells the donor material (or p-type conductor) and acceptor material (n-type conductor) are presented in a tight blend in the active (specifically, photoactive) layer of a device, and the concentration of each component often gradually increases when approaching to the corresponding electrode. This provides an increase in the total surface area of the junctions between the materials and facilitates the exciton's dissociation. In organic solar cells the electron donor and acceptor materials are both organic materials. In hybrid solar cells, one type of which is a dye sensitised solar cell, one material is typically an inorganic material and the other is an organic material. In dye sensitised solar cells, dye materials, also known as "sensitisers" or charge transporting chromophores, are used as a charge generating material, typically with an inorganic semiconductor. One example of this is the use of electron donor dyes with an n-type semi conductor such as titania, as the electron acceptor material.

There has been an emerging trend to develop new chemical compounds capable of charge transportation (as either the electron donor or electron acceptor material) for use in organic photovoltaic applications.

In charge transportation materials recently developed for such applications, the trend has been towards the use of compounds containing a donor electron group (such as an N,N-diarylamino group) at one end, a combination of an oligothiophene and an acceptor electron group at the other end, and a highly aromatic linker based on a pi system, such as phenyl, linking the two ends.

There is a need for further chemical compounds that can be used in such applications., which may provide improved charge derealization in the compound. There is also a need for devices containing these new compounds.

Summary

According to the present invention there is provided a compound of formula I:

formula I

wherein:

EWG is an electron withdrawing group or combination of groups, R¹ and R² are each independently selected from the group consisting of alkyl, aromatic or heteroaromatic groups, or R¹ and R² may together comprise a linked alkyl, aromatic or heteroaromatic group, R³ and R⁴ are each independently selected from the group consisting of alkyl, alkoxy or H, or R³ and R⁴ may together comprise a divalent alkyl group, a divalent alkoxy or alkyldioxy group, or R³ and R⁴ may together comprise a heterocyclic, heteroaromatic or aromatic group linked or fused to the thiophene group,

R⁵ is H, or R⁵ is H or CN when EWG is =C(CN)₂, and n is an integer between 2 and 10.

According to some embodiments, the compounds are selected from compounds other than:

Thus, according to one embodiment, the following provisos apply to the compounds:

- when EWG is =C(CN>₂ and R³ and R⁴ are H, either:

(i) n is 2 or an integer between 4 and 10, (preferably an integer between 4 and 10), or

(H) R¹ and R² are each other than unsubstituted phenyl (for example, R¹ and R² are independently selected from the group consisting of optionally substituted C ₁ -C_h alky I, optionally substituted C3-C8 cycloalkyl, substituted aromatic, and optionally substituted heteroaromatic groups or Ri and R₂ together with the nitrogen atom to which they are attached comprise an optionally substituted saturated or unsaturated ring which may optionally contain further heteroatoms selected from the group consisting of O₁ N and S₁ and may optionally be further fused to one or more other rings), or (iii) R⁵ is CN;

- when EWG is a thiobarbituric acid of formula:

and R³ and R⁴ are H, either:

(i) n is an integer between 3 and 10, or

(ii) R¹ and R² are other than unsubstituted phenyl (for example, R¹ and R² are independently selected from the group consisting of optionally substituted CrC_2O alkyl, optionally substituted C₃-Cs cycloalkyl, substituted aromatic, and optionally substituted heteroaromatic groups or Ri and R_∑ together with the nitrogen atom to which they are attached comprise an optionally substituted saturated or unsaturated ring which may optionally contain further heteroatoms selected from the group consisting of O, N and S₁ and may optionally be further fused to one or more other rings),

- when EWG is an isoxazolone of formula:

and R³ and R⁴ are H₁ either:

(i) n is an integer between 3 and 10, or

(ii) R¹ and R² are other than unsubstituted phenyl (for example, R¹ and R² are independently selected from the group consisting of optionally substituted CrC₂O alkyl, optionally substituted C₃-Ce cycloatkyl, substituted aromatic, and optionally substituted heteroaromatic groups or R₁ and R₂ together with the nitrogen atom to which they are attached comprise an optionally substituted saturated or unsaturated ring which may optionally contain further heteroatoms selected from the group consisting of O, N and S, and may optionally be further fused to one or more other rings).

According to the present invention there is also provided a photovoltaic device comprising: - a first electrode,

- a second electrode, and

- an active material in electrical contact with the first an second electrodes, the active material comprising a compound of formula I and a second material which is either an electron donor material or an electron acceptor material, wherein the device generates an electrical potential upon the absorption of photons.

Preferred details of the compound and the device are set out in the detailed description below.

Brief Description of the Figures

Figure 1 is a schematic illustration of a photovoltaic device, in the form of a bilayer photovoltaic cell, according to one embodiment of the invention.

Figure 2 is a schematic illustration of a photovoltaic device, in the form of a bulk heterojunction photovoltaic cell, according to a second embodiment of the invention.

Figure 3 is a schematic illustration of a photovoltaic device, in the form of a dye sensitised solar cell, according to a third embodiment of the invention.

Figure 4 is an I-V curve or graph of voltage vs current density for a photovoltaic device according to one embodiment of the invention incorporating AG 4-36 as the compound of formula I.

Figure 5 is a an I-V curve or graph of voltage vs current density for a photovoltaic device according to another embodiment of the invention incorporating AG 4-44 as the compound of formula I.

Detailed Description

The present invention relates to novel compounds, and their use in photovoltaic devices.

The compounds of the present application are based on a donor-acceptor design which has greater absorption of visible light than current o/igothiophene- based materials due to the highly efficient electron donor-acceptor configuration of the substltuents on a thiophene (or oligothiophene) core.

The structure includes a direct link between the amino nitrogen atom and the thiophene (or oligothiophene) unit, which is then directly linked to a strongly electron withdrawing group. The absence of a highly aromatic benzene or fluorene group between the thiophene and amine, and the inclusion of the thiophene linking group provides a better energy balance and better charge delσcalisation which serves to produce resonance delocalisatioπ to give further absorption. Photovoltaic devices containing such compounds will benefit from these properties.

In formula I n is an integer between 2 and 10. The compounds of the invention may be referred to as oligothiophene compounds. According to some embodiments, n is between 2 and 6.

In formula I, R³ and R⁴ are each independently selected from the group consisting of alkyl, alkoxy or H, or R³ and R⁴ may together comprise a divalent alkyl group, a divalent alkoxy or alkyldioxy group, or R³ and R^A may together comprise a heterocyclic, heteroaromatic or aromatic group linked or fused to the thiophene group. Expressed in another way, R3 and R₄ are independently selected from the group consisting of hydrogen, optionally substituted C₁-C₁₈ alkyl, optionally substituted C3-C₁₈ cycloalkyl and optionally substituted Ci-C₁S alkoxy groups, or R₃ and R₄ may together with the carbon atoms to which they are attached comprise an optionally substituted saturated or unsaturated ring which may optionally contain one or more heteroatoms selected from the group consisting of O, N and S, and may optionally be further fused to one or more other rings.

According to some embodiments, R³ and R⁴ are each independently selected from the group consisting of alkyl, alkoxy and H. Alkyl encompasses straight chained, branched or cyclic alkyl groups of C1 to C18, and encompasses groups of the formula -C_xH_2x+I, where x is an integer between 1 and 18, such as between 1 and 10, or between 1 and 8. Examples include methyl, ethyl, propyl, hexyl, /so-butyl, tert-butyl, and so forth. Unless the context requires otherwise, alkyl also encompasses alkyl groups containing one less hydrogen atom, such that the group is attached via two positions. Such groups are also referred to as "alkylene" groups. Alkoxy refers to the group -OC_xH_2x+I , where x is an integer between 1 and 18. Examples include methoxy, ethoxy, and so forth. The oxygen atom may be located along the hydrocarbon chain, and need not be the atom linking the group to the remainder of the compound.

According to some embodiments R³ Js H. According to some embodiments R⁴ is H. According to some embodiments, one of R³ and R⁴ is H, and the other of R³ and R⁴ is alkyl. According to one embodiment, R³ and R⁴ are both H.

According to some embodiments R³ and R⁴ may together comprise an aliphatic divalent linking group such as -alkyl-, -alkoxy-, dioxy, and dioxyalkyl, or R³ and R⁴ may together comprise a heterocyclic, heteroaromatic or aromatic groups linked or fused to the thiophene group. Examples of embodiments containing divalent linking groups for R³ and R⁴ (i.e. containing -R³-R⁴-) are diazo thiophenes, alkylenedioxythiophene (such as ethylenedioxythiophene) and isobenzothiophene. Divalent alkyl groups have been described previously, and encompass groups of the formula -C_nH_2n- where n is a positive integer, amongst others. Divalent alkoxy groups encompass groups containing one or more alkyl or alkylene segments, and one or more oxygen atoms. Examples include -CH₂OCH₂-. Dioxy groups are groups comprising two oxygen atoms.

Dioxyalkyl groups encompass groups comprising two oxygen atoms and an alkyl group, such as ethylenedioxy (-OEtO-).

R³ and R⁴ may together comprise a heterocyclic, heteroaromatic or aromatic group linked or fused to the thiophene group. An example of an aromatic group is benzene, and when such an aromatic group is fused to the thiophene, R³ and R⁴, together with the thiophene ring form a substituted or unsubstituted isobenzothiophene. More generally, "aromatic group" is used in accordance with its usual meaning in the art and refers to an aromatic ring containing system. The aromatic group may contain one ring, or up to 3 fused or linked rings, in which the linking groups are alkyl (specifically, alkylene) groups, and thus the term encompasses phenyl, napthyl, fluoreπyl, biphenyl and so forth. Such groups may be substituted or uπsubstituted. The term "heteroaromatic group" refers to aromatic group containing one or more heteroatoms. The groups may be substituted or unsubstituted. The heteroaromatic group may comprise one or more rings, with one or more of the rings containing a heteroatom. The heteroatom or heteroatoms in the heteroaromatic group may be selected from one or more of O₁ N and S. The term "heterocyclic group" refers to any ring or ring systems, including linked or fused ring systems, containing at least one heteroatom, selected from O, N and S. The ring or rings may contain single and/or double bonds, but the electron configuration is such that the ring or ring system is not aromatic. The heterocyclic, heteroaromatic or aromatic groups may be fused to the thiophene unit, or may be linked through direct bonds or other linking atoms.

R¹ and R² are each independently selected from the group consisting of alkyl, aromatic or heteroaromatic groups, or may together comprise a linked alkyl, aromatic or heteroaromatic group. To avoid any doubt, this encompasses the situation where Ri and R₂ are independently selected from the group consisting of optionally substituted C1-C₂₀ alkyl, optionally substituted C₃-C₈ cycloalkyl, optionally substituted aromatic, and optionally substituted heteroaromatic groups or Ri and R₂ together with the nitrogen atom to which they are attached comprise an optionally substituted saturated or unsaturated ring which may optionally contain further heteroatoms selected from the group consisting of O, N and S, and may optionally be further fused to one or more other rings.

R¹ and R² are each alkyl, aromatic or heteroaromatic groups, or may together with the nitrogen atom to which they are attached comprise a linked alkyl, aromatic or heteroaromatic group. The term "aromatic group" refers to any group containing an aromatic ring system. The aromatic groups are attached to the nitrogen atom through an aromatic ring carbon atom. Such groups may contain fused ring systems, linked ring systems (such as biphenyl and fluoreπe groups), and may be substituted or unsubstituted. Any substituents that do not adversely impact on the electronic properties of the ring system are permissible, and suitable examples include one or more substituents selected from alkyl, alkoxy, hydroxyl, carbonyl, carboxylic acid, halo, aryl, thioalkyl, cyano, haloalkyl such as perfluorinated alkyl, dialkylamino, diarylamine, N- carbazol, heteroaryl, biphenyl, silyl, trimethylsilyl, silylether, methacryloxy, acryloxy, hydroxyalkyneneoxy and 2-bromo-2methylpropanoate. Halo refers to a halogen. Haloalkyl refers to an alkyl substituted with one or more halogen. Thioalkyl is the thio (S-containing) equivalent of alkoxy. Carbonyl encomposses, carboxylic acids, esters, aldehydes and ketones.

The term "heteroaryl" or similarly "heteroaromatic group" refers to any group containing a heteroaromatic ring system. The heteroatoms in the heteroaromatic group may be selected from one or more of O₁ N and S. Where one or both of R¹ and R² are heteroaromatic groups, the groups are attached to the nitrogen atom through an atom in the heteroaromatic ring. Such groups may be substituted or unsubstituted, and may contain fused ring systems, including a fused heteroaromatic and carbon-based aromatic rings, and linked ring systems. Suitable substituents are the same as those listed above for the aromatic group.

Linked aromatic or heteroaromatic groups refers to a single group which comprises at least two rings, each of the two rings being directly attached to the nitrogen atom of the compound of formula I, and a linking group linking the two aromatic/heteroaromatic rings. The linking group may be, for one example, an alkyl group, or more specifically an alkylene group, of the formula -C_xHa_x- , wherein x is an integer between 1 and 18. The linking group may be, for another example, a direct bond between the aromatic/heteroaromatic rings.

In embodiments where R¹ and R² comprise alkyl, the term alkyl has the same definition as provided above in the context of R³ and R⁴. Where R¹ and R² together comprise a linked alkyl group, the linked alkyl group may for instance be of the formula -C_nH2n-, where n is an integer between 4 and 10. The alkyl group may be substituted by any suitable substituent such as one or more substituents selected from alkyl, alkoxy, carbonyl carboxylic acid, halo, aryl, thioalkyl, cyano, and haloalkyl such as perfluorinated alkyl.. A suitable subset of substituents is alkyl, alkoxy, carbonyl, carboxylic acid, halo, thioalkyl, cyano and haloalkyl such as perfluorinated alkyl. According to some embodiments, R¹ and R² are each independently selected from the group consisting of phenyl, substituted phenyl, fluorenyl, and substituted fluorenyl. According to other embodiments, R¹ and R² are independently selected from the group consisting of - phenyl, - phenyl substituted by one or more alkyl, alkyoxy, carboπyl, carboxylic acid, aryl or heteroaryt, - an aryl other than phenyl, - an aryl other than phenyl substituted by one or more alkyl, alkyoxy, carbonyl, carboxylic acid, aryl or heteroaryl, - heteroaryl, or - heteroaryl substituted by one or more alkyl, alkyoxy, carbonyl, carboxylic acid, aryl or heteroaryl.

The term "aryl" refers to aromatic groups based on one ring, or up to 3 fused or linked rings, in which the linking groups are alkyl (specifically, alkylene) groups, and thus encompasses phenyl, napthyl, fluorenyl and so forth. The term "heteroaryl" refers to the heteroaromatic equivalent of "aryl" and thus refers to one heteroaromatic ring, or up to 3 fused aromatic or heteroaromatic rings in which the fused combination includes at least one heteroatom. Examples of "heteroaryl" include pyridyl, thieπyl, furyl, iπdolinyl and so forth. Unless otherwise specified, the terms "aryl" and "heteroaryl" refer to unsubstituted groups. An example of a substituted heteroaryl (specifically an alkoxy substituted heteroaryl) is ethylenedioxythiophene.

According to some embodiments, R¹ and R² are each independently selected from the group consisting of - phenyl, - phenyl substituted by one or more alkyl, alkyoxy, carbonyl, carboxylic acid, or aryl, - fluorenyl and - fluorenyl substituted by one or more alkyl.

EWG in formula I is an electron withdrawing group or combination of groups. The term "electron withdrawing group" is well understood in the field of chemistry. Electron withdrawing groups draw electrons towards the group, and away from the remainder of the molecule.

Examples of suitable electron withdrawing groups are =C(CN)₂ and aromatizable groups which becomes aromatized on quarternization of the amine nitrogen in the compound of formula I. Cyclic groups containing carbonyls are further examples of electron withdrawing groups.

According to some embodiments, the EWG is selected from the group consisting of:

(a) a group of the formula:

(b) barbituric acid groups of formula:

(c) thiobarbituric acid groups of formula:

(d) isoxazolone groups of formula:

(e) pyridone groups of formula:

(T) rhodamine groups of formula:

(g) groups of formula:

(h) groups of formula:

(i) groups of the formula:

0) groups of the formula:

(k) groups of the formula:

(I) groups of the formula:

(m) groups of the formula:

and

(n) groups of the formula:

wherein

R" is selected from the group consisting of optionally substituted C₁-C₈ alkyl, optionally substituted Ci-C₈ perfluorinated alkyl, optionally substituted C₃-C₈ cycloalkyl, optionally substituted aromatic, and optionally substituted heteroaromatic groups (in summary, alkyl, perfluoroalkyl, or aryl),

R^y is selected from the group consisting of optionally substituted C₁-C₃₀ alkyl wherein one or more carbon atoms of the alkyl are optionally replaced with one or more of O, S, NR⁶, carbonyl or thiocarbonyl; optionally substituted C₃-C₈ cycloalkyl; optionally substituted C2-C₁₂ alkenyl wherein one or more carbon atoms of the alkenyl are optionally replaced with one or more of O, S, NR⁶, carbonyl or thiocarbonyl; optionally substituted C₂-C₈ alkynyl wherein one or more carbon atoms of the alkynyl are optionally replaced with one or more of O, S, NR^δ, carbonyl or thiocarbonyl; optionally substituted C3-C₁₂ alkoxy; optionally substituted aromatic; and optionally substituted heteroaromatic groups; wherein R^β is hydrogen or R^x ,

R^z is selected from the group consisting of alkyl and alkoxyalkyl, R^w is selected from the group consisting of H and alky!,

R^u is selected from the group consisting of H, alkyl, halo, perfluoroalkyl. carbonyl, carboxylic acid, alkoxy and hydroxyl, or an adjacent pair of R^g substituents may together with the carbon atoms to which they are attached comprise an optionally substituted saturated or unsaturated aromatic ring, and m is 4, and

R' is selected from the group consisting of H, alkyl and alkoxy, or the adjacent pair of R¹ substituents may represent -0-CH₂-CH₂-O-.

As outlined above, R^x is selected from the group consisting of optionally substituted Ci-Ce alkyl, optionally substituted CrCe perfluorinated alkyl, optionally substituted Cs-C₈ cycloalkyl, optionally substituted aromatic, and optionally substituted heteroaromatic groups. The terms "alkyl group",

"cycloalkyl group", "aromatic group" and "heteroaromatic group" are as defined above. In some embodiments, R^x is an optionally substituted C-i-Cβ alkyl, an optionally substituted Ci-C₆ perfluorinated alkyl or an optionally substituted C₃- Ce cycloalkyl. In some embodiments, R^x is selected from the group consisting of methyl, ethyl and CF₃. As defined above, the term "alky! group" encompasses straight chain or branched alkyl groups and in one embodiment R* is an optionally substituted branched C₂-Cs alkyl. In some embodiments, R^x is a thiophenyl group.

As also noted above, R^y is selected from the group consisting of optionally substituted C₁-C₃0 alkyl wherein one or more carbon atoms of the alkyl are optionally replaced with one or more of O, S, NR⁶, carbonyl or thiocarbonyl; optionally substituted C₃-C₈ cycloalkyl; optionally substituted C₂-Ci₂ alkenyl wherein one or more carbon atoms of the alkenyl are optionally replaced with one or more of O, S, NR⁶, carbonyl or thiocarbonyl; optionally substituted C₂-C₈ alkynyl wherein one or more carbon atoms of the alkynyl are optionally replaced with one or more of O, S, NR⁶, carbonyl or thiocarbonyl; optionally substituted C₃-Ci₂ alkoxy; optionally substituted aromatic; and optionally substituted heteroaromatic groups; wherein R⁶ is hydrogen or R\ Suitable substituents are the same as those listed above for the aromatic group.

The terms "alkyl group", "cycloalkyl group", "alkenyl group", "alkyπyl group", "alkoxy group", "aromatic group" and "heteroaromatic group" are defined above.

In some embodiments, R^y is selected from the group consisting of optionally substituted Ci-Ce alkyl wherein the alkyl chain may be optionally interrupted with one or more of O₁ S₁ NR⁶, carbonyl or thiocarbonyl; optionally substituted C₃-C6 cydoalkyl; optionally substituted C2-C6 alkenyl wherein one or more carbon atoms of the alkenyl are optionally replaced with one or more of O, S, NR⁶, carbonyl or thiocarbonyl; optionally substituted C₂-C₆ alkynyl wherein one or more carbon atoms of the alkynyl are optionally replaced with one or more of O, S₁ NR⁶, carbonyl or thiocarbonyl; and optionally substituted C₃-C₆ alkoxy; wherein R₆ is hydrogen or R*. In some embodiments, R^y may be an optionally substituted branched C₂-Ci₀ alkyl wherein the alkyl chain may be optionally interrupted with one or more of O, S, NR⁶, carbonyl or thiocarbonyl; optionally substituted branched C₂-C1₀ alkenyl wherein one or more carbon atoms of the alkenyl are optionally replaced with one or more of O, S, NR⁶, carbonyl or thiocarbonyl; optionally substituted branched C₃-Ci₀ alkynyl wherein one or more carbon atoms of the alkynyl are optionally replaced with one or more of O, S, NR⁶, carbonyl or thiocarbonyl; or optionally substituted branched C3-C₁₂ alkoxy; wherein NR⁶ is hydrogen or R^x.

In some embodiments, R^y is an optionally substituted 5- or 6- heteroaromatic group containing one or more heteroatoms selected from the group consisting of O, N and S. Suitable substituents are the same as those listed above for the aromatic group.

In some embodiments, R^y is an optionally substituted C₁-C₃0 alkyl wherein the optional substituents are selected from the group consisting of hydroxyl, carboxylic acid, methacryloxy, acryloxy, hydroxyalkyleneoxy, 2-bromo-2- methylpropanoate, trimethylsily! and silyl ether.

In some embodiments, R^y is an aromatic group which is substituted with a carboxylic acid group.

In some embodiments, EWG is a pyridone group (e) as described above, and the group R^y is as described in the preceding 4 paragraphs.

In some embodiments, R^y is selected from the group consisting of alkyl, alkoxyalkyl, hydroxyalkyl, aryl and carbonyl (such as carboxylic acid or ester).

In some embodiments, in the group (m):

the CO_∑H substitυent is in the para position. Each R^u may be the same or different. In some embodiments, each R^u is H.

Specific examples of these groups are:

(b) barbituric acid groups in which R^x is alkyl, such as ethyl;

(c) thiobarbituric acid groups in which R^x is alkyl, such as ethyl;

(d) isoxazolone groups in which R^x is aryl, such as phenyl;

(e) pyridone groups in which R^x is alkyl such as methyl; and pyridone groups in which R^y is selected from alkyl such as methyl, ethyl or phenyl, or hydroxyalkyl such as hydroxyethyl;

(f) rhodamine in which R* is alkyl, such as methyl;

(g) and (h) groups of the formulae

(m) groups of the formula

in which the substituent -CO₂H is in the para position.

R⁵ is H when the EWG is other than =C(CN)₂. When the EWG is =C(CN)₂, R⁵ is selected from H and CN. According to some embodiments, R⁵ is H (irrespective of the identity of the EWG).

Unless stated otherwise, references to optional substituents refers to one or more substituents selected from the group consisting of alkyl, alkoxy, hydroxyl, carbonyl, carboxylic acid, halo, aryl, thioalkyi, cyano, haloalkyl such as perfluorinated alkyl, dialkylamino, diarylamine, N-carbazol, heteroaryl, biphenyl, silyl, trimethylsilyl, silylether, methacryloxy, acryloxy, hydroxyalkyneneoxy and 2-bromo-2methylpropanoate.

Compounds containing the electron withdrawing groups selected from the group consisting of =C(CN)₂, (e), (h) and (m) are the subject of some embodiments of the invention.

As indicated by the wavy line in formula I₁ the compounds of the present application are not limited to any particular stereochemistry. The compounds may comprise mixtures of isomers in any ratio, racemic mixtures, a single isomer of the compound, or otherwise. The absence of a wavy line at a position corresponding to that shown in Figure I in other parts of this specification should not be taken to imply specific stereochemistry about the double bond. The actual stereochemistry can only be determined by assessment of the compound as synthesised by the specified synthetic procedure.

Previously studied active materials for use in organic photovoltaic devices have included poly 3-hexyl thiophene (PZHT), which obtains its colour and function by an extended π (pi) system. However, the number of thiophenes in the molecule to be used as an active material in organic photovoltaic devices can be reduced by induction of a dipole in the molecule. These molecules contain broad structural constituents of donor-aromatic liπker-oligothiophene-acceptor. The aromatic groups have previously been highly aromatic groups such as benzene or fluorene, and the oligothiophene has been made of only 2-4 units.

In such systems, we have observed that there are strong inductive dipole generation in the materials, however resonance derealization to give further absorption is not possible. We have found that the removal of the strongly aromatic linker overcomes this problem. We have also explored and identified further features that provide improvements or useful alternatives to the known materials.

(1) Alternative acceptor groups. According to some embodiments, it has been found that if the EWG is an aromatizable group which becomes aromatized on quarternization of the amine ' nitrogen in the compound of formula I, such compounds provide considerable advantages. This phenomenon is illustrated below with respect to the groups thioparbituric acid, hydroxyphridone, phenyl isoxazolone and rhodamine. Rhodamine exhibits the weakest effect of the four illustrated.

polyene form betaine/zwiterlonic form

It is noted that the illustration above is provided with respect to compounds containing a phenyl linking group, and that the compounds illustrated are not themselves compounds of the present invention. However, they do illustrate the same effect that applies to compounds of the present invention. In fact, the replacement of the strongly aromatic phenyl group with the oligothiophene unit has been shown to provide advantages in terms of improved charge delocalisation, as discussed below.

These aromatizable EWGs allow the approach of a phenomenon known as the "cyanine" limit. Note that aromatizatlon of the acceptor group (the EWG) is accompanied by the loss of aromaticity in the aromatic linker between the tertiary amine and acceptor. This is a state whereby the neutral polyene form and the canonical zwitterionic form contribute equally to the structure. This results in (1) the highest degree of conjucation, (2) high dipole moments and forst order hyperpolarizability, and (3) vanishing second order hyperpolarizability, and thus (4) no change in dipole moment on excitation.

This can be seen in x-ray structures where double bond alternation is reduced and sometimes even the partial zwitterionic forms are observed in the polar environment of a crystal. The structure below illustrates a molecule at the cyanine limit with the disappearance of double bond alternation.

Other groups disclosed in the art offer minimal canonical forms and such molecules cannot approach the cyanine limit.

(2) Donor group connecϋon to thiophenes.

Current research directions have been towards having the donor group communicating to the thiophenes via a strongly aromatic benzene or fluorene group. The benzene group's high aromaticity prevents the approach of the cyanine limit. The lower aromaticity of thiophene is superior for charge delocalisation. There is better energy balance between the thiophene linking group, which looses aromaticity, and the aromatizable EWG, which is aromatized. Thus the diaryl amine in the compounds of formula I are directly coupled to the thiophenes rather than via a benzene group. It is noted that there is a large λmax as measured by UV/Vis spectroscopy for the single thiophene dye when subjected to direct coupling and an aromatizable acceptor group.

The above table shows that the benzene ring prevents the approach of the cyanine limit as reflected in a larger Δ dipole and β.

Compounds of formula I in which there is no strongly aromatic linking group between the EWG and the diaryl amine display a red shifted λmax. Compared to similar compounds but containing a strongly aromatic linking group, the red shift is of about at least 20nm, typically at least 30nm or 40nm (measured as a thin film, or in the same solvent). According to some embodiments, the compounds of formula I have a λmax of 550nm or above. According to some embodiments, the compounds of formula I have a λ_max of 560nm or above.

Synthesis of compounds.

Examples demonstrating the synthesis of compounds of a full range of embodiments of the invention are set out in the Example section.

Generally the synthesis involves:

- obtaining an amine of the formula R¹R²NH (noting that many such amines are available for purchase, or can be synthesised by very straight-forward techniques known in the art) - reacting the amine R¹R²NH with:

X = Br₁ CI

to form a tributylstannyl thiophene derivative of the amine:

(Compound A)

(noting that variants with appropriate groups R³ and R⁴ can be purchased or syπthesised by techniques known in the art)

- reacting the tributylstannyl thiophene derivative of the amine with an iodo- thiophene-carbaldehyde of formula:

(Compound B)

(which may be purchased, or may be synthesised from the simple thiophene or oligothiophene by deprotonation with BuLi followed by trapping with CO₂ to give the aldehyde, then iodination) to produce a formyl precursor containing the selected groups R¹, R², R³, R⁴ and n:

and then - reacting the formyl precursor, Compound C, with the EWG to produce the target compound of formula I.

As a further example, the following procedure could be used to prepare the starting materials for the preparation of the compounds of the invention containing longer oligothiophene units:

Compound A

Typically a repetitive use of ϊ) formylation (Vilsmeier-Haack reaction as a non- limiting example), ii) iodination (via N-iodosuccinimide as a non-limiting example) and iii) coupling to a thiophene with R³ and R⁴ substitueπts to the iodo (oligo)thiophene via Suzuki coupling (via a boronic ester or acid with the iodothiophene or via Stifle coupling) will give access to a variety of oligσthiophene lengths with a substituted amine at one end and an aldehyde at the other. The publication by Tao and Wong et al. in Advanced Materials 2008 doi: 10.1002/adma.200703032 illustrates the technique in making oligothiophene of various lengths using a compound containing triarylamine boronic esters that would apply to compounds of this invention.

The following reaction scheme shows the construction of oligothiophenes of a desired length. First a formylation is performed and then cycle of iodination and Suzuki coupling. It is understood that simple variation such as the use of a dithiophene boronic acid/ester or tristhiophene boronic acid/ester would allow oligothiophene length increases of two and three thiopehene units respectively in each cycle.

N-iodosuccinimidx

1 - iodinalion

Compound B

Once in hand, the aldehyde functional oligothiophene may terminated with a boronic ester or iodide (to yield Compound B), to allow subsequent coupling to either disubstituted amine directly or a disubstituted aminothiophene as shown below (illustrating a StUIe coupling).

F*2

It is understood that a variety of synthetic paths may be used to make compounds of the type C and that compounds of type C may have a 1 to 10 thiophene units.

The formy! precursor (Compound C) can then be reacted with the electron withdrawing group to form the target compound of formula I.

A broad range of compounds with R³ and R⁴ being H can be purchased from Sigma Aldrich, Apollo Chemicals, and others, which can be conveniently converted into the starting materials such as Compound B using simpler reactions, examples of which are presented below.

Buy from Acros Orgartics

Maybridge

Apollo Chemicals

imide

Buy from Sigma Aldrich

Many EWG compounds are available for purchase from chemical suppliers. Pyridone groups offer great utility, as these can be prepared by straightforward techniques known in the art from a corresponding amine and may enable various functional groups R^y to be introduced (R^y is represented by -CH₂-R in the figure below). This is demonstrated by the following:

Similarly diethylthiobarbituic acid, rhodanine, malononitrile, indanoπe, isoxazolone are all .commercially available and can be used directly to condense with the terminal aldehyde group on compounds like compound C. This is because such EWG have active methylene groups with acidic hydrogen that will readily react with aldehydes as shown below. This is frequently as simple as refluxing compound C in an alcohol with the EWG. A catalyst (amine base like piperidine) or dehydrating agent (like acetic anhydride) may be required.

When EWG is R⁵ is =C(CN)₂, and R⁵ is CN, the compound is prepared through the following route:

Photovoltaic Devices ₍ The compound of formula I outlined above is suitably used in a photovoltaic device. The photovoltaic device generally comprises:

- a first electrode,

- a second electrode, and

- an active material in electrical contact with the first an second electrodes, the active material comprising

(i) the compound of formula I, and

(ii) a second material which is either an electron donor material or an electron acceptor material. The device generates an electrical potential upon the absorption of photons. In other words, the active material is arranged such that the device generates an electrical potential upon the absorption of the photons.

The compound of formula I may be seen as being "ambi-potar", and may act either as an electron donor material or an electron acceptor material, depending on the relative HOMO and LUMO levels of the compound and those of the second material. In some embodiments, the compound of formula I is an electron donor and the second material is an electron acceptor. In other embodiments, the compound of formula I is an electron acceptor, and the second material is an electron donor.

Where the second material is an electron acceptor material, the material may be selected from any electron acceptor materials known in the art. The materials are generally organic electron acceptors, such as the fullerenes of various sizes (C60, C70, C80 and their soluble analogues PC61BM, PC71BM, PC84BM etc)

Where the second material is an electron donor material, the material may be selected from any electron donor materials known in the art. The materials are generally organic electron acceptors, such as conductive polymers including polythiophenes (including P3HT) and the like.

The photovoltaic device may be in the form of an organic solar cell, such as a bulk heterojunction organic solar cell, a bi layer organic solar cell, or a dye sensitised solar cell.

In the case of bilayer organic solar cells, the compound of formula I and the second material form layers.

In the case of a bulk heterojunction photovoltaic cell, the electron donor material (p-type conductor) and electron acceptor material (n-type conductor) are presented in a tight blend in an active material layer of the device. According to one embodiment, the concentration of each component gradually increases when approaching to the corresponding electrode. The first electrode may be an anode. Any suitable anode materials can be used. The anode material is suitably a transparent anode material. According to some embodiments the anode is a metal oxide anode, including doped metal oxides, such as indium tin oxide, doped tin oxide, doped zinc oxide (such as aluminium-doped zinc oxide), metals such as gold, alloys and conductive polymers and the like. The anode may be supported on a suitable support. Supports include transparent supports, such as glass or polymer plates.

The second electrode may be a cathode. Any suitable cathode material can be used. According to some embodiments the cathode is a metal or metal alloy. Suitable metals and alloys are well known in the art and include aluminium, lithium, and alloys of one or both.

The device may further comprise any additional features known in the art. Some photovoltaic devices contain interfacial layers between one or both of the anodes and the active material, and such features may be incorporated in to the photovoltaic devices of the present application. The devices may be constructed by any techniques known in the art.

In the context of dye sensitised solar cells, the compound of formula I is a sensitiser, and the second material is an inorganic semiconductor material. Suitable n-type inorganic semiconductor materials are well known in the art, and include titanium dioxide (TiOz). Suitable p-type inorganic semiconductor materials are well known in the art and include nickel oxide. The second material is suitably a particulate material. The particulate second material provides a high surface area for the attachment of molecules of the compound of formula I, which allows for high exposure to the incident light, and to high contact between the molecules of formula I and the electrolyte. Particles of a nanometer size are particularly suited, and encompass particles of between 0.1 nm to 100nm in size, such as between 1 and 50nm sized particles.

When the device is a dye sensitised solar cell according to some embodiments, the photovoltaic device comprises a charge transport material, which may be solid or liquid, such as an electrolyte, in contact with the compound of formula I and the second electrode. Suitable electrolytes are well known in the art and include room temperature ionic liquids, organic electrolytes and aqueous electrolytes. The electrolytes may be doped with a charge carrying species. Suitable electrolytes include iodide electrolytes.

Examples

The present invention will now be described in further detail with reference to the following examples, relating to some embodiments of the invention. It will be understood that the invention is not limited to the embodiments provided by way of example.

Example 1 5"-dicyaπovinylidene-5-(N,N-di-p-tolylamino)-2,2':5'2"- terthiophene (AG 4-36)

Part 1. Synthesis of S'-formyl-S^N.N-di-p-tolylamino)^¹^"- terthiophene.

To a degassed solution of N,N-dip-tolyl-5-(tributylstannyr)thiophen-2-amine (1Og₁ 17.6 mmol) and 5'iodo-2,2'bithiophene-5-carbaldehyde (5.64g, 17.6 mmol) in dimethylformamide (100ml) was added bis(triphenylphosphine)palladium(ll) chloride (445mg, 0.634mmol). The mixture was heated to 80⁰C for 10 min, then cooled to 40⁰C and stirred under nitrogen for overnight. The orange coloured solution was worked up with dichloromethane and water and the organic layer was washed with water followed by brine and finally dried over anhydrous sodium sulphate and recovered to get 6.Og (72.3%) of the crude titled material which was subjected to column chromatography (Hexane: Ethyl acetate : : 80:20) to obtain the purified product. 1H NMR (200MHz₁ C6D6) δ9.54 (s, 1H), 6.49 (d, J=3.95Hz, 1H)₁ 6.80 (d, J=3.88Hz. 1 H). 6.74-6.68 <m, 2H), 6.87 (d, J=3.97Hz, 2H), 7.01 (m, 4H), 7.22 (m, 4H), 2.16 (S₁ 6H) Part 2. Synthesis 5"-dicyanovinylidene-5-(N,N-di-p-tolylamino)-2,2'.5'2"- terthiophene (AG 4-36)

To a solution of 5"-forτnyl-5-(N,N-di-p-tolylamino)-2,2':5'2"-terthiopheπe. (2g, 4.25 mmol) and malenonitπϊe (5meq, 21.25mmol, 1.4g) in chloroform (30 ml) was added pyridine (1.5 ml) at RT. The resulting purple solution was stirred at RT for overnight and TLC was checked giving the indication of the desired product. The reaction mixture was worked up with water and dichloromethane and the final organic layer was dried over anhydrous sodium sulphate and the solvent was removed in vacuo. The crude material was subjected to column chromatography to give 1.2g (54.4%) of the desired purified dark black powder. ¹H NMR (400MHz, CD₃COCD₃) 58.37 (s, IH), 7.9 (dd, 1H), 7.55 (d, J=3.96Hz, 1H), 7.52 (d, J=4.15Hz, 1H), 6.48 (d. J=4.00Hz, 1H), 7.16-7.19 (m, 6H), 7.07- 7.10 (m, 4H), 2.31 (s, 6H); ^t3C NMR (200 MHz, CDCI₃) 5154.0, 149.7, 144.9, 140.2, 133.7, 132.9, 132.0, 129.9, 128.2, 126.7, 124.0, 123.8, 123.5, 123.2, 117.3, 114.4, 113.6, 20.8; IR (KBr, cm-1) 2920 (bd), 2200, 1730, 1566, 1457, 1433, 1250, 840; found m/z = 519.1; UV-Vis (CH₂CI₂ film) λ max 572 nm (Onset 764 nm); M. Pt: 170-174^* C

Example 2 Diethylthiobarbituric acid adduct of 5"-formyl~5-(N,N-di-p- tolylamino)-2,2':5'2^B-terthiophene (AG 4-44)

A solution of 5"-formyl-5-(N,N-di-p-tolylamino)-2,2':5'2'^t-terthiophene (980 mg, 2.08 mmol) (as prepared in example 1 part 1 ) and diethylthiobarbituric acid (3 eq, 6.24 mmol, 1.25 g) in methanol (100 ml) was heated to refluxed for 4 hours and then cooled to ambient temperature. The resulting black solid was filtered off, washed with methanol and dried U/V to get the required adduct (1.2 g, 88.5%) of high purity.

¹H NMR (400MHz₁ C6D6) δ8.50 (s, 1H), 6.34 (d, J=3.85Hz, 1H)₁ 6.55 (d, J=3.81Hz, 1H), 6.70 (d, J=4.07Hz, 1 H), 6.77 (d, J=3.86 Hz, 1H), 6.82 (d, J=3.84Hz, 1H)₁ 6.87 (m, 6H), 7.10 (m, 4H), 4.45-4.56 (m, 4H), 2.05 (s, 6H), 1.26-1.33 (m, 6H) ; found m/z = 653.1 ; IR (KBr₁ cm-1) 2920 (bd), 1732, 1456, 1130-1330 (bd), 840; UV-Vis (CH₂CI₂ film) λ max 602 nm (Onset 800 nm); M. R.: 210-215° C

Example 3. 5"- cyanophenylcarboxyvinylidene-5-(N,N-di-p-tolylamino)- 2,2¹:5¹2"-terthiophene (AG5-100)

Part 1. Synthesis of 4-(Cyanomethyl)benzoic acid

C₈H₇CIO₂ C₉H₇NO₂ 170.59 161.16

To a solution of 4-(chloromethyl)benzoic acid (3g, 17.65 mmol) in 20 ml of 5:7 mixture of water: ethanol was added potassium cyanide (2.6ME, 45.89 mmol, 2.98g) at RT. The resulting suspension was stirred at RT for 30 minutes followed by at 75-80° C for 5 Hr¹S. The reaction mixture was cooled to RT and dumped in water. The resulting solution was acidified with 5N hydrochloric acid and extracted with ethyl acetate thrice. The resulting organic layer was washed twice with water followed by brine solution and dried over anhydrous sodium sulphate. The solvent was removed in vacuo to get crude off-white solid (2.5g, 87.93%) which was used as such for the next step.

¹H NMR (400MHz₁ CD₃COCD₃) 58.05 (m, 2H)₁ 7.54 (m, 2H), 4.09 (s, 1H) Part 2 Synthesis 5"- cyanophenylcarboxyvinylidene-5-(N,N-di-p-tolylamino)- 2,2^>:5¹2"-terthiophene (AG5-100)

To a solution of 5"-(N, N-di-p-tolylamino)-2\ 5'-bithiophene-2', 5-bithiophene-2- carboxaldehyde (250mg, 0.53 mmol) and 4-(cyanomethyl)benzoic acid (1.4meq, 0.743 mmol, 119.6 mg) in methanol (20 ml) was added pyrrolidine (0.1 ml) at RT. The resulting red solution was refluxed for overnight and cooled to RT followed by dumping in water (70 ml) and the resulting solution was acidified with 2N hydrochloric acid. The solid appeared was filtered off and washed with water followed by its drying U/V. The dried solid was subjected to column chromatography (chloroform: methanol:: 80:20) to give 100 mg (30.7%) of the desired purified deep red solid.

¹H NMR (400MHz, CD₃SOCD₃) δ8.41 (s, 1H), 8.0 (m, 2H), 7.80 (m, 2H), 7.73 (m, 1H), 7.47(m, 1H), 7.44(m, 1H), 7.13-7.17 (m, 6H), 7.01-7.03 (m, 4H), 6.47(m, 1H), 2.26 (s, 6H).

Example 4. Adduct of ethylhexyl cyanopyridion and 5"-(N₁ N-di-p-tolylamino)- 2", 5'-bithiophene-2\ 5~bithiophene-2-carboxaldehyde (AG5-70)

To a solution of 5"-(N, N-di-p-tolylamino)-2", 5'-bithiophene-2', 5-bithiophene-2- carboxaldehyde (125mg, 0.265 mmol) in methanol (20 ml) was added 1-(2- ethylhexylM-methyl-2, 6-dioxo-1 ,2,5,6-tetrahydropyridine-3-carbonitrile (1.5meq, 0.397mmol, ~105mg) at RT. The resulting solution was refluxed for overnight and the solid appeared in the reaction was filtered off, washed with methanol followed by its drying U/V to get 150 mg (78.8%) of the desired dark black flakes.

¹H NMR (400MHz, CD₂CI₂) 57.91 (s, 1H), 7.73 (m, 1 H)₁ 7.45 (m, 1H), 7.36 (m, 1H), 7.08-7.15 (m, 8H), 7.06 (m, 1H), 7.01 (m, 1H)₁ 6.45 (m, 1H), 3.93 (m, 2H)₁ 2.62 (s, 3H), 2.34 (s, 6H), 1.87 (m, 1H), 1.30-1.35 (m, 8H), 0.90-0.95 (m, 6H)

4. Comparison studies

The above table shows the beneficial effect having the electron donating group (the amine) directly connected to the thiophene molecule rather than via a highly aromatic group such as fluorene. AG 4-36, as compared to PhNOFOT(3)-DCN, shows greater absorbance of the visible spectrum with a maxima at 572 nm (cf 540 nm) and significantly lower band gap 1.62 eV (cf 1.85 eV). The highly aromatic fluorene group reduces the electron donation of the amine in the oligothiophene as it limits formation of resonance structures where the fluorene loses aromaticity.

5 The compound of Example 3 (AG5-100) contains an acceptor group with a carboxylic acid group. This will allow use in dye sensitised solar cell such that the dye must be bound to TiO_∑. The acid group is need for such binding. Conventionally cyanocarboy vinylidene is used however the use of the acceptor group in the compound of Example 3 provides broader absorption and henceo greater light harvesting.

The addition of a stronger electron withdrawing group that is capable of aromatization (AG4-44) shows even more absorption (max 608 nm) and lower band gap of 1.55 eV. Greatest redshifting of absorbance was observed with the5 use of the cyanopyridione acceptor group (Ex4 AG5-70) with absorbance maxima now at 635 nm vs 540 nm of PhNOFOT(3)-DCN. The low LUMOs will improve electron transfer to an electron acceptor material (electron transport material) such as fullerene. The low LUMO can even allow the compounds of the invention to act as electron donor or transport materials in their own right.0

5. Variations

Further compounds within general formula I can be prepared through the selection of appropriate starting materials. In the preparation of the precursor5 as per step 1 above, the starting amine (R¹R²NH) for forming the tributylstannyl thiophene derivative (the equivalent of 568 in the reaction scheme) can be synthesised or purchased with the appropriate groups R¹ and R². The iodo- carbaldehyde can also be prepared with the appropriate groups R³ and R⁴, and with the appropriate number of thiophene units (of n-1 in number). From those o starting materials, the appropriate formyl precursor containing the selected groups R¹, R², R³, R⁴ and n is prepared. Step 2 is prepared as shown above from the reaction of the new formyl precursor with the appropriate EWG to produce the desired target compound. 5 6. Bilayer Solar Cell

A bilayer organic solar cell (1) of one embodiment of the invention is illustrated in Figure 1. The bilayer organic solar cell comprises a transparent layer of indium tin oxide as the anode (2) supported on a transparent thin film support (3), and a cathode (4) in the form of a metal cathode, opposite. Between the anode and cathode are layers of the compound of formula I (5) as the electron donor (or p-conductor), and an electron acceptor material (6) (or n-conductor) such as fullerene. The device may contain multiple layers, and the term "bilayer" should be interpreted as encompassing 2 or more layered devices. The device may be in the form of a single cell, or multiple cells connected in parallel and/or series. The device typically further comprises positive and negative terminals (not illustrated) for connection to an energy storage device or other electrical component(s) or circuit(s).

7. Bulk Heterojunction Solar Cell

A bulk heterojunction organic solar cell (7) of one embodiment of the invention is illustrated in Figure 2. In this figure, elements that are common to the bilayer solar cell (1 ) of Figure 1 are referred to using the same numerals. The bulk heterojunction organic solar cell (7) comprises a transparent layer of indium tin oxide as the anode (2) supported on a transparent thin film support (3), and a cathode (4) in the form of a metal cathode, opposite. Between the anode and cathode is an active layer comprising a blend of electron acceptor material (6) (or n-conductor) such as fullerene, and the compound of formula I (5) as the electron donor (or p-conductor). The concentration of each component (5) and (6) gradually increases when approaching to the corresponding electrode. The device may be in the form of a single cell, or multiple cells connected in parallel and/or series. The device typically further comprises positive and negative terminals (not illustrated) for connection to an energy storage device or other electrical component(s) or circuits).

8. Dye Sensitised Solar Cell

A dye sensitised solar cell (8) of one embodiment of the invention is illustrated in Figure 3. In this figure, elements that are common to the bilayer solar cell (1) of Figure 1 are referred to using the same numerals. The dye sensitised solar cell comprises a transparent layer of indium tin oxide as the anode (2) supported on a transparent thin film support (3). A layer of particulate titanium dioxide (9) of an average particle size of 20nm is located on the surface of the anode (2), which is an n-type inorganic semiconductor material and acts as an electron acceptor material. The titanium dioxide layer (9) is coated on its surface with the compound of formula I, acting as the sensitizer, or electron donor material. This is represented schematically by an area marked with the numeral (5) in Figure 1 , but in reality would be a thin coating on the particles. This is applied by any suitable technique, such as by dissolving in a solvent, and contacting with the titanium dioxide layer, to load the sensitiser onto the surface. A cathode (4) in the form of a metal cathode is placed above the layer of sensitiser (5), and an electrolyte (10) filled in the space between the sensitiser (5) and the cathode (4), contacting the two materials. The electrolyte is of any suitable type, and in the illustrated embodiment is typically the iodine/triodide red/ox couple. Other electrolytes maybe ionic liquid or solid or polymeric electrolytes.. The edges of the device are sealed to encase the electrolyte (10) between the anode (2) and cathode (4). The device may be in the form of a single cell, or multiple cells connected in parallel and/or series. The device typically further comprises positive and negative terminals (not illustrated) for connection to an energy storage device or other electrical component(s) or circuit(s).

9. Test work on photovoltaic devices containing AG 4-46 and AG 4-

44.

Apparatus

Indium tin oxide (ITO) coated glass with a sheet resistance of 15 ohms/square was purchased from Kintek. Polyethylenedioxythiophene/polystyrenesulfonate ("PEDOT/PSS¹") (Baytron P Al 4083) was purchased from HC Starck. PCBM and C60 were purchased from Nano-C. Calcium pellets and 2,9-dimethyl-4,7- diphenyl-1 ,10-phenanthroline (BCP) were purchased from Aldrich. Aluminium pellets (99.999%) were purchased from KJ Lesker.

UV-ozone cleaning of ITO substrates was performed using a Novascan PDS- UVT, UV/ozone cleaner with the platform set to maximum height, the intensity of the lamp is greater than 36 mW/cm² at a distance of 100 cm. At ambient conditions the ozone output of the UV cleaner is greater than 50 ppm.

Aqueous solutions of PEDOT/PSS were deposited in air using a Laurell WS- 400B-6NPP Lite single wafer spin processor. Organic blends were deposited inside a glovebox using an SCS G3P Spincoater. Film thicknesses were determined using a Dektak 6M Profilometer. Vacuum depositions were carried out using an Edwards 501 evaporator inside a glovebox. Samples were placed on a shadow mask in a tray with a source to substrate distance of approximately 25 cm. The area defined by the shadow mask gave device areas of 0.1 cm². Deposition rates and film thicknesses were measured using a calibrated quartz thickness monitor inside the vacuum chamber. C60 was evaporated from a boron nitride crucible wrapped in a tungsten filament. BCP was evaporated from a baffled tantalum boat. Ca and Al (3 pellets) were evaporated from separate, open tungsten boats.

Methods

ITO coated glass was cleaned by standing in a stirred solution of 5% (v/v)

Deconex 12PA detergent at 90 ⁰C for 20 mins. The ITO was successively sonicated for 10 minutes each in distilled water, acetone and /sσ-propanol. The substrates were then exposed to a UV-ozone clean (at room temperature) for 10 minutes. The PEDOT/PSS solution was diluted by 50% in methanol, filtered (0.2 μm RC filter) and deposited by spin coating at 5000 rpm for 60 sec to give a 38 nm layer. The PEDOT/PSS layer was then annealed on a hotplate in the glovebox at 140 ^βC for 10 minutes. Where used, solutions of the organic blends were deposited onto the PEDOT/PSS layer by spin coating inside a glovebox (H₂O and O₂ levels both < 1 ppm). Spinning conditions and film thicknesses were optimised for each blend. The devices were transferred (without exposure to air) to a vacuum evaporator in an adjacent glovebox. Where used, single layers of the organic materials were deposited sequentially by thermal evaporation at pressures below 2x10^""6 mbar. Where used, a layer of Ca was deposited by thermal evaporation at pressures below 2x10^~* mbar. For all devices a layer of Al was deposited by thermal evaporation at pressures below 2*10^"6 mbar. Where noted, the devices were then annealed on a hotplate in the glovebox.

A small amount of silver paint (Silver Print II, GC electronics, Part no.: 22-023) was deposited onto the connection points of the electrodes. Completed devices were encapsulated with glass and a UV-cured epoxy (Lens Bond type J-91) by exposing to 254nm UV-light inside a glovebox (H_ZO and O₂ levels both < 1 ppm) for 10 minutes. Electrical connections were made using alligator clips. The cells were tested with an Oriel solar simulator fitted with a 1000W Xe lamp filtered to give an output of 100mW/cm² at AM 1.5. The lamp was calibrated using a standard, filtered Si cell from Peccell limited (The output of the lamp was adjusted to give a JSC of 0.605 mA). The estimated mismatch factor of the lamp is 0.95. Values were not corrected for this mismatch.

The Incident Photon Collection Efficiency (IPCE) data was collected using an Oriel 150W Xe lamp coupled to a monochromator and an optical fibre. The output of the optical fibre was focussed to give a beam that was contained within the area of the device. The IPCE was calibrated with a standard, unfiltered Si cell.

For both the solar simulator and the IPCE measurements devices were operated using a Keithley 2400 Sourcemeter controlled by Labview Software. The measurements on the solar simulator gave the cell efficiency under AM 1.5 illumination. The measurements on the IPCE setup gave them cell efficiency at individual wavelengths

Results

Device Example 1 The AG 4-36 material of Example 2 was used in a blend device with the fullerene derivative [6,6l-phenyl-C61 -butyric acid methyl ester (PCBM) as the second material.

Device structure: ITO / PEDOTiPSS (38 nm) / AG4-36 : PCBM (1 :1) (90 nm) / Ca (20 nm) /AI (200 nm).

A 3 cm³ solution of AG4-36 (30 mg) and PCBM (30 mg) in chlorobenzene was prepared by stirring in the glovebox for 1 hr. The solution was filtered (0.2 μm RC filter) and spin coated in the glovebox at 1000 rpm. The films were annealed in the glovebox at 100 ⁰C for 15 minutes prior to deposition of the Ca (20 nm) and Al (200 nm) layers.

The I-V curve for the device is shown in Figure 4. The device parameters were Voc = 702 mV, I₈₀ = 5.66 mA/cm², FF = 31%, PCE = 1.22%.

Device Example 2

The AG 4-44 material was used in a blend device with PCBM. Device structure: ITO / PEDOTPSS (38 nm) / AG4-44 : PCBM (1 :1) (-90 nm) / Ca (20 nm) / AI (160 nm).

A 1 cm³ solution of AG4-44 (10 mg) and PCBM (10 mg) in chlorobenzene was prepared by stirring in the glovebox for 10 mins. The solution was not filtered and spin coated in the glovebox at 1250 rpm. The films were annealed in the glovebox at 100 ⁰C for 10 mins prior to deposition of the Ca (20 nm) and Al (160 nm) layers.

The I-V curve for the device is shown in Figure 5. The device parameters were Voc = 731 mV, l_sc = 1.42 mA/cm², FF = 31%, PCE = 0.33%

It will be understood to persons skilled in the art of the invention that many modifications may be made without departing from the spirit and scope of the invention.

Claims

1. A compound of formula I:

formula

wherein:

EWG is an electron withdrawing group or combination of groups,

R¹ and R² are each independently selected from the group consisting of alkyl, aromatic or heteroaromatic groups, or R¹ and R² may together comprise a linked alkyl, aromatic or heteroaromatic group,

R³ and R⁴ are each independently selected from the group consisting of alkyl, alkoxy or H, or R³ and R⁴ may together comprise a divalent alkyl group, a divalent alkoxy or alkyldioxy group, or R³ and R⁴ may together comprise a heterocyclic, heteroaromatic or aromatic group linked or fused to the thiophene group,

R⁵ is H, or R⁵ is H or CN when EWG is =C(CN)₂, and n is an integer between 2 and^' 10;

with the proviso that:

- when EWG is =C(CN)₂ and R³ and R⁴ are H, either:

(i) n is 2 or an integer between 4 and 10, or (H) R¹ and R² are each other than unsubstituted phenyl, or (Hi) R⁵ is CN;

- when EWG is a thiobarbituric acid of formula:

and R³ and R⁴ are H₁ either:

(i) n is an integer between 3 and 10, or

(ii) R¹ and R² are other than unsubstituted phenyl,

- when EWG is an isoxazolone of formula:

and R³ and R⁴ are H₁ either: (i) n is an integer between 3 and 10, or

(ii) R¹ and R² are other than unsubstituted phenyl.

2. The compound of claim 1 , wherein R³ is H.

3. The compound of claim 1 or claim 2, wherein R⁴ is H.

4. The compound of claim 1 , wherein one of R³ and R⁴ is H, and the other of R³ and R⁴ is alkyl.

5. The compound of any one of claims 1 to 4, wherein R⁵ is H.

6. The compound of any one of claims 1 to 5, wherein R¹ and R² are each independently selected from the group consisting of - phenyl, - phenyl substituted by one or more alkyl, alkoxy, carbonyl, carboxylic acid, aryl or heteroaryl, - an aryl other than phenyl, - an aryl other than phenyl substituted by one or more alkyl, alkyoxy, carbonyl, carboxylic acid, aryl or heteroaryl, - heteroaryl, or - heteroaryl substituted by one or more alkyl, alkyoxy, carbonyl, carboxylic acid, aryl or heteroaryl.

7. The compound of claim 6, wherein R¹ and R² are each independently selected from the group consisting of - phenyl, - phenyl substituted by one or more alkyl, cartonyl, carboxylic acid, alkoxy or aryl, - fluorenyl and - fluorenyl substituted by one or more alkyl, carbonyl, carboxylic acid, alkoxy or aryl.

8. The compound of any one of claims 1 to 7, wherein n is an integer between 2 and 6.

9. The compound of any one of claims 1 to 8, wherein EWG is either =C(CN)₂ or an aromatizable group which becomes aromatized on quarternization of the amine nitrogen in the compound of formula I.

10. The compound of any one of claims 1 to 8, wherein EWG is selected from the group consisting of:

(a) a group of the formula:

(b) barbituric acid groups of formula:

(C) thiobarbituric acid groups of formula:

(d) isoxazolone groups of formula:

(e) pyridone groups of formula:

(f) rhodamine groups of formula:

(g) groups of formula:

(h) groups of formula:

(i) groups of the formula:

(j) groups of the formula:

(k) groups of the formula:

([) groups of the formula:

(m) groups of the formula:

and

(π) groups of the formula:

wherein

R^x is selected from the group consisting of optionally substituted Ci-Ce alkyl, optionally substituted Ci-C₈ perfluorinated alkyl, optionally substituted C₃-CΘ cycloalkyl, optionally substituted aromatic, and optionally substituted heteroaromatic groups,

R^y is selected from the group consisting of optionally substituted CrC₃₀ alkyl wherein one or more carbon atoms of the alkyl are optionally replaced with one or more of O, S, NR⁶, carbonyl or thiocarbonyl; optionally substituted C₃-C₈ cycloalkyl; optionally substituted C₂-Ci₂ alkenyl wherein one or more carbon atoms of the alkenyl are optionally replaced with one or more of O, S, NR⁶, carbonyl or thiocarbonyl; optionally substituted C₂-C_βalkynyl wherein one or more carbon atoms of the alkynyl are optionally replaced with one or more of O, S, NR⁶, carbonyl or thiocarbonyl; optionally substituted C3-C1₂ alkoxy; optionally substituted aromatic; and optionally substituted heteroaromatic groups; wherein R⁶ is hydrogen or R^x ,

R^z is selected from the group consisting of alkyl and alkoxyalkyl,

R^w is selected from the group consisting of H and alkyl,

R^u is selected from the group consisting of H, alkyl, halo, perfluoroalkyl, carbonyl, carboxylic acid, alkoxy and hydroxyl, or an adjacent pair of R^u substftuents may together with the carbon atoms to which they are attached comprise an optionally substituted saturated or unsaturated aromatic ring, and m is 4, and

R* is selected from the group consisting of H, alkyl and alkoxy, or the adjacent pair of R¹ substituents may represent -0-CH₂-CH₂-O-.

1 1. The compound of claim 10, wherein EWG is selected from (b) barbituric acid groups of formula:

in which R^x is alkyl.

12. The compound of claim 10, wherein EWG is selected from (c) thiobarbituric acid groups of formula:

in which R^x is alkyl.

13. The compound of claim 10, wherein EWG is selected from (d) isoxazolone groups of formula:

in which R^x is aryl.

14. The compound of claim 10, wherein EWG is selected from (e) pyridone groups of formula:

in which R" is alkyl.

15. . The compound of claim 10 or claim 11 , wherein EWG is selected from (e) pyridone groups of formula:

in which R^y is an optionally substituted C₁-C₃₀ alkyl wherein the optional substituents are selected from the group consisting of hydroxyl, carboxylic acid, methacryloxy, acryloxy, hydroxyalkyleneoxy, 2-bromo-2-methylpropanoate, trimethylsilyl and silyl ether.

16. The compound of claim 10, wherein EWG is selected from (f) rhodamine groups of formula:

in which R^x is alkyl.

17. The compound of claim 10, wherein EWG is selected from (g) groups of formula:

and (h) groups of formula:

in which R^w is H.

18. The compound of claim 10, wherein EWG is selected from (m) groups of formula:

in which the -CO₂H is in the para position.

19. The compound of any one of the preceding claims, in which the compound has a λmax of 550nm or above.

20. The compound of any one of the preceding claims, in which the compound has a λ_max of 560nm or above.

21. A photovoltaic device comprising:

- a first electrode,

- a second electrode, and

- an active material in electrical contact with the first and second electrodes, the active material comprising:

(i) a compound of formula I:

formula I

wherein:

EWG is an electron withdrawing group or combination of groups,

R⁵ is H, or R⁵ is H or CN when EWG is =C(CN)_2l and n is an integer between 2 and 10; and

(ii) a second material which is either an electron donor material or an electron acceptor material, wherein the device generates an electrical potential upon the absorption of photons.

22. The photovoltaic device of claim 21 , wherein the compound of formula I is as defined in any one of claims 2 to 20.

23. The photovoltaic device of claim 21 , wherein EWG in the compound of formula I is selected from:

(a) a group of the formula:

(e) pyridone groups of formula.

0 (d) isoxazolone groups of formula:

24. The photovoltaic device of any one of claims 21 to 23, wherein the5 first electrode is an anode.

25. The photovoltaic device of claim 24, wherein the anode is a metal oxide anode.

o 26. The photovoltaic device of any one of claims 21 to 25, wherein the second electrode is a cathode.

27. The photovoltaic device of claim 26, wherein the cathode is a metal or metal alloy cathode.

28. The photovoltaic device of any one of claims 21 to 27, wherein the second material is an electron acceptor material.

29. The photovoltaic device of any one of claims 21 to 28, wherein the second material is a fullerene.

30. The photovoltaic device of any one of claims 21 to 27, wherein the second material is an electron donor material.

31. The photovoltaic device of any one of claims 21 to 30, wherein the active material comprises a mixture of the compound of formula I and the second material.

32. The photovoltaic device of any one of claims 21 to 31 , wherein the active material comprises layers of the compound of formula I and the second material.