WO2017191466A1

WO2017191466A1 - Organic ternary blends

Info

Publication number: WO2017191466A1
Application number: PCT/GB2017/051256
Authority: WO
Inventors: Andrew WADSWORTH; Christian Nielsen; Sarah HOLLIDAY; Iain Mcculloch; Mindaugas KIRKUS; Derya Baran; Shahid ASHRAF
Original assignee: Imperial Innovations Limited
Priority date: 2016-05-06
Filing date: 2017-05-05
Publication date: 2017-11-09
Also published as: EP3452474A1; WO2017191468A1

Abstract

The invention provides compositions comprising a blend of two or more organic electron acceptor compounds and an organic electron donor compound, wherein at least one of the electron acceptor compounds is a compound of Formula (I), which may be used in organic optical or electronic devices.

Description

ORGANIC TERNARY BLENDS

FIELD OF THE INVENTION The invention relates to organic blends comprising more than one non-fullerene electron acceptors for use in organic optical or electronic devices.

BACKGROUND Over the past decade significant improvements in the performance of organic solar cells (OSC) have been made via material and device design, leading to measurement of power conversion efficiencies (PCE) values of over 11 % in single and multi-junction bulk heterojunction (BHJ) solar cells. However, insufficient light harvesting, and large internal energy losses in these materials still contribute to limit these values. To address these issues, different strategies have been adopted by the use of multiple active materials with complementary absorption in a single junction (ternary) or multi-junction (tandem) devices, while also ensuring that intrinsic energetic losses from too large orbital energy offsets are minimized. Due to complex design and multistep fabrication process, tandem solar cells development for commercialization purposes remains elusive. On the other hand, a ternary system offers a simple and alternative approach to enhance the light harvesting in a single junction device. A typical ternary BHJ system consists of two electron donor and an electron acceptor usually a fullerene derivative or vice versa. Recently, many groups have reported different types of ternary blend systems, such as two electron donor polymers or one polymer and a small molecule donor and a fullerene acceptor, one polymer donor and two fullerene derivatives as electron acceptors. In most cases, ternary blends have surpassed the corresponding binary blends mainly due to increase in short circuit current (J_sc).

The working mechanism of ternary blends depends on a particular blend structure and can be of following four types, charge transfer (CT), energy transfer, parallel and alloy structure. One or all of the four mechanisms may possibly be present in a particular ternary system. However, the resultant performance greatly depends on the blend morphology, miscibility and surface energy of donor or acceptor components. The third component plays an important role in the blend morphology and can act as a vitrifying or crystallisation template to facilitate a favourable BHJ morphology. Careful design and judicial positioning of energy levels are crucial for efficient charge transport. In a ternary system with poor miscibility and a relatively large difference in HOMO energy levels of donor materials, the low band gap polymer or molecule can act as a sensitizer and transfer holes to the wide band gap donor component. In this type of case, the open circuit voltage (V_oc) is usually pinned to the smallest V_oc of corresponding binary blend. On the other hand, if either donor or acceptor is structurally compatible and miscible, the working mechanism can be parallel-like or alloy type. In case of parallel-like ternary solar cells, excitons generated in an individual donor would migrate to the respective donor/acceptor interface and then dissociate into free electrons and holes. Meanwhile, in an alloy type of ternary solar cell, instead of each component forming its own independent charge carrier network, forms an electronic alloy with similar HOMO and LUMO energies, a same type of variation of valence and conduction band energies of an inorganic semiconductor alloy. In both these models the V_oc usually is tunable and varies with the composition of donor or acceptor. Although, several groups have reported ternary systems consisting two or more donor polymers/molecules and one acceptor (usually a fullerene derivative) (see Ameri, T. et al., J. Organic Ternary Solar Cells: A Review. Advanced Materials 25, 4245-4266, (2013); Zhang, Y. et al., Advanced Materials 27, 1071-1076, (2015); Yang, L. et al., J. Physical Chemistry Letters 4, 1802-1810, (2013); Yang, Y. et al., Nat Photon 9, 190-198 (2015); Zhang, J. et al., Journal of the American Chemical Society 137, 8176-8183, (2015); Lu, L. et al., Nat Photon 9, 491-500 (2015), the entire contents of which are herein incorporated by reference in their entirety)., very few examples are presented of two acceptors and one donor ternary systems, and these are limited to fullerene derivatives as acceptors (see Khlyabich, P. P. et al., J. American Chemical Society 133, 14534-14537 (201 1); Ko, S.-J. et al., Advanced Energy Materials 5 (2015); Kang, H. et al, ACS Applied Materials & Interfaces 5, 4401-4408 (2013), the entire contents of which are herein incorporated by reference in their entirety). It has been demonstrated that V_oc can be tuned across its full composition range in such ternary systems and record efficiencies are achieved with low band gap polymers. However, the materials used in these systems present difficulties to scale up, issues with solubility, device irreproducibility and photochemical instability.

At the moment, P3HT is the only polymer being scaled above 10 Kg scale, making it a strong candidate for OPV commercialisation, and it has already been widely employed in large area, printed solar cells. However, as P3HT is often neglected due to narrow absorption, it is important to develop new acceptors that can be used with P3HT. Recently, a non-fullerene acceptor (FBR), showing improved performance with P3HT in solar cells compared to 60PCBM (S. Holliday et al., J Am Chem Soc, 2015, 137, 898-904, the contents of which is herein incorporated by reference in its entirety. Despite this, there remains a need for organic photovoltaic devices having improved efficiencies.

SUMMARY OF THE INVENTION

Described herein are ternary blends comprising non-fullerene electron acceptor compounds and their use in optical and electronic devices. Accordingly, in a first aspect, the invention provides a composition comprising a blend of two or more organic electron acceptor compounds and an organic electron donor compound, wherein at least one of the electron acceptor compounds is a compound of Formula (I):

T¹-(B¹)_a-(A)-(B²)_b-T²

Formula (I)

wherein:

A is a divalent conjugated fused ring system containing aromatic groups directly conjugated to groups B¹ and B², and having the structure:

wherein:

X_! is C, Ge or Si;

R¹ is, at each occurrence, independently, H, or optionally substituted Ci.₃₀ aliphatic, aryl or heteroaryl;

Cy^1"10 are, at each occurrence, independently, absent or a 5 or 6-membered ring having 0, 1 or 2 ring heteroatoms, or a fused polycyclic (for example, bicyclic or tricyclic) ring optionally having one or more ring heteroatoms, provided that at least one of Cy^1"5 and at least one of Cy^6"10 is not absent, and wherein each of Cy^1"10, when present, is optionally substituted by one or more groups R²; R² is, at each occurrence, independently, halo, Ci.₃₀ aliphatic, aryl, heteroaryl, =0, =S, =R°, -CN, -NC, -NCO, -NCS, -OCN, -SCN, -C(=0)NR°R°°, -C(=0)X°, -C(=0)R°, - C(=0)OR°, -C(=S)R°, -C(=S)OR°, -OC(=0)R°, -OC(=S)R°, -C(=0)SR°, -SC(=0)R°, -NH₂, - NR°R⁰⁰, -NR°C(0)R°, -SH, -SR°, -S0₃H, -S0₂R°, -OH, -N0₂, -CF₃, -CF₂-R°, -SF₅, silyl or hydrocarbyl with 1 to 40 C atoms and which optionally comprises one or more hetero atoms, wherein Ci.₃₀ aliphatic, aryl, heteroaryl, silyl or hydrocarbyl are optionally substituted, and wherein X° is halogen and R° and R⁰⁰ are, independently, H or optionally substituted Ci.₄₀ hydrocarbyl; or two R², with the intervening atoms form an optionally substituted fused ring, having 0, 1 or 2 ring heteroatoms;

each occurrence of B¹ and B² is, independently, -CY¹=CY²-, -C≡C-, or a cyclic hydrocarbyl group with 5 to 30 ring atoms optionally including one or more heteroatoms, preferably aryl or heteroaryl, wherein each occurrence of B¹ and B² is, independently, unsubstituted or substituted by one or more R³, wherein R³ has the meaning of R²;

Y¹ and Y² are, independently, H, F, CI or CN;

a and b are, independently of each other, 0, 1 or 2; and

T¹ and T² are, independently of each other, an electron deficient group conjugated to group B¹ or B², respectively, or wherein when a and/or b are 0, T¹ and T² are, independently of each other, an electron deficient group conjugated to group A, respectively; and

wherein A contains an optionally substituted aromatic ring having 0, 1 , 2 or more ring heteroatoms directly bonded to groups B¹ and B², respectively, or wherein when a and/or b are 0, A contains an optionally substituted aromatic ring having 0, 1 , 2 or more ring heteroatoms directly bonded to groups T¹ and T², respectively. It will be appreciated that each occurrence of * represents the bond to B¹ and B², respectively. Where any one or more of Cy^1"10 is absent, the point of attachment to B¹ and B² is instead on the next adjacent one of Cy^1"10 that is not absent. For example, if Cy¹ is absent, but Cy² is present, the bond between A and B¹ will be between Cy² and B¹. Where a or b, respectively, is not 0, the one of Cy^1"10 which is bonded to B¹ or B², respectively, is an optionally substituted aromatic ring having 0, 1 , 2 or more ring heteroatoms . When a and/or b is 0, the one of Cy^1"10 which is bonded to T¹ or T², respectively, is an optionally substituted aromatic ring having 0, 1 , 2 or more ring heteroatoms directly bonded to groups T¹ and T², respectively. Cy^1"10 are preferably, at each occurrence, independently, absent or a 5 or 6-membered ring having 0, 1 or 2 ring heteroatoms, each optionally substituted by one or more groups R², provided that at least one of Cy^1"5 and at least one of Cy^6"10 is not absent.

Preferably the one of Cy^1"5 and at the one of Cy^6"10 that are directly bonded to groups B¹ and B², or wherein when a and/or b are 0, the one of Cy^1"5 and at the one of Cy^6"10 that are directly bonded to groups T¹ and T², are each independently a 5 or 6-membered aromatic ring having 0, 1 or 2 ring heteroatoms (preferably phenyl or thiophenyl), each optionally substituted by one or more groups R².

Any one or more of Cy^1"10 may have the , wherein X_! is C, Ge or Si. Each

of Cy^1"10 is preferably, independently, abs

ent, or a 5 or 6-membered aromatic ring having 0, 1 or 2 ring heteroatoms (preferably phenyl or thiophenyl), each optionally substituted by one or more groups R². For example, in a compound of Formula (I) as

Cy^4"10 are at each occurrence, independently a 5 or 6-membered aromatic ring having 0, 1 or 2 ring heteroatoms (preferably phenyl or thiophenyl), each optionally substituted by one or more groups R². Preferably A may be selected from:

optionally substituted by one or more groups R , preferably

R².

Preferably X_! is C or Ge. In any of the structures illustrated above, R¹ may, at each occurrence, independently, be Ci-3o aliphatic or aryl optionally substituted with CM₀ aliphatic, preferably C₆-io aliphatic (for example, linear or branched C₈ aliphatic) or aryl (for example, phenyl) substituted with Ci_₆ aliphatic. In any of the structures illustrated above, R² may, at each occurrence, independently, be H, optionally substituted Ci.₃₀ aliphatic, aryl or heteroaryl, preferably Ci.₃₀ aliphatic or aryl substituted with CM₀ aliphatic, preferably C₆-s aliphatic (for example, linear or branched C₈ aliphatic) or aryl (for example, phenyl) substituted with Ci_₆ aliphatic.

Within any of the structures described above for a compound of Formula (I), T¹ and T² may be, independently of each other, -CR⁴=Y, -CR⁴=CR⁴-Y, -L-Y or -Y; Y is an optionally substituted cyclic hydrocarbyl group, preferably optionally substituted aryl or heteroaryl; and L is a divalent alkylenyl chain of 3 to 10 carbon atoms, having alternating double and single bonds, optionally substituted by one or more R⁴; and R⁴ is H or has the meaning of R², preferably wherein R⁴ is H.

Prefera is:

in which * marks the point of attachment to -CR⁴=;

X² is S, O or C(R⁶)₂;

W is S, O or C(R⁶)₂;

R⁵ is H, halo, aliphatic, heteroaliphatic, aryl, heteroaryl, -CN, -NC, -NCO, -NCS, - OCN, -SCN, -C(=0)NR°R°°, -C(=0)X°, -C(=0)R°, -C(=0)OR°, -C(=S)R°, -C(=S)OR°, - OC(=0)R°, -OC(=S)R°, -C(=0)SR°, -SC(=0)R°, -NH₂, -NR°R⁰⁰, -NR°C(0)R°, -SH, -SR°, - S0₃H, -SO2R⁰, -OH, -NO2, -CF₃, -CF2-R⁰, -SF₅, silyl or hydrocarbyl with 1 to 40 C atoms and which optionally comprises one or more hetero atoms, wherein aliphatic,

heteroaliphatic, aryl, heteroaryl, silyl or hydrocarbyl are optionally substituted, and wherein X° is halogen and R° and R⁰⁰ are, independently, H or optionally substituted C1-40 hydrocarbyl (preferably optionally substituted aliphatic, heteroaliphatic, aryl or heteroaryl);

R⁶ is, at each occurrence, independently, H, halo, aliphatic, heteroaliphatic, aryl, heteroaryl, -CN, -NC, -NCO, -NCS, -OCN, -SCN, -C(=0)NR°R°°, -C(=O)X⁰, -C(=O)R⁰, - C(=0)OR°, -C(=S)R°, -C(=S)OR°, -OC(=O)R⁰, -OC(=S)R°, -C(=O)SR⁰, -SC(=O)R⁰, -NH₂, - NR°R⁰⁰, -NR°C(0)R°, -SH, -SR°, -SO₃H, -S0₂R°, -OH, -N0₂, -CF₃, -CF₂-R°, -SF₅, silyl or hydrocarbyl with 1 to 40 C atoms and which optionally comprises one or more hetero atoms, wherein aliphatic, heteroaliphatic, aryl, heteroaryl, silyl or hydrocarbyl are optionally substituted, and wherein X° is halogen and R° and R⁰⁰ are, independently, H or optionally substituted C1-40 hydrocarbyl; .-·^■' may be present or absent and represents a fused mono-, bi- or tri- cyclic hydrocarbyl group, preferably aryl or heteroaryl, optionally substituted by one or more R₇, wherein R⁷ has the meaning of R²;

R⁸ is, at each occurrence, independently, halo, aryl, heteroaryl, -CN, -NC, -NCO, - NCS, -OCN, -SCN, -C(=0)NR°R°°, -C(=0)X°, -C(=0)R°, -C(=0)OR°, -C(=S)R°, - C(=S)OR°, -OC(=0)R°, -OC(=S)R°, -C(=0)SR°, -SC(=0)R°, -NH₂, -NR°R⁰⁰, -NR°C(0)R°, - SH, -SR°, -SO₃H, -SO₂R⁰, -OH, -NO₂, -CF₃, -CF₂-R⁰, -SF₅, silyl or hydrocarbyl with 1 to 40 C atoms and which optionally comprises one or more hetero atoms, wherein aryl, heteroaryl, silyl or hydrocarbyl are optionally substituted, and wherein X° is halogen and R° and R⁰⁰ are, independently, H or optionally substituted C1-40 hydrocarbyl; and

n is 0-4.

Preferably, at least one of T¹ or T² is -CR⁴=Y, and Y is:

Preferably, T¹ or T² are both -CR⁴=Y and Y is as defined above.

Preferably, at least one of T ' and V is -CR⁴=CR⁴-Y or -Y and Y is

m is 0-3 and o is 0-2; and R⁵, R⁶ and X₂ are as defined above. In preferred embodiments, X₂ is O.

In any of the above embodiments of Y, R⁵ is preferably CM₂ aliphatic, preferably CM alkyl, more preferably Ci_₈ alkyl. Within any of the structures described above for a compound of Formula (I), preferably a and b are, independently, 1 or 2, more preferably a and b are both 1.

Within any of the structures described above for a compound of Formula (I), each occurrence of B¹ and B² may preferably be, independently, mono-, bi- or tri-cyclic aryl or heteroaryl group, unsubstituted or substituted by one or more groups R³, wherein the aryl or heteroaryl group may optionally include a non-aromatic carbocyclic or heterocyclic ring fused thereto. Preferably, one or more occurrences of B¹ and B² is:

wherein p is 0, 1 or 2; and

R⁹ is, at each occurrence, independently, halo, aryl, heteroaryl, -CN, -NC, -NCO, -NCS, - OCN, -SCN, -C(=0)NR°R°°, -C(=0)X°, -C(=0)R°, -C(=0)OR°, -C(=S)R°, -C(=S)OR°, - OC(=0)R°, -OC(=S)R°, -C(=0)SR°, -SC(=0)R°, -NH₂, -NR°R⁰⁰, -NR°C(0)R°, -SH, -SR°, - S0₃H, -SO2R⁰, -OH, -NO2, -CF₃, -CF2-R⁰, -SF₅, silyl or hydrocarbyl with 1 to 40 C atoms and which optionally comprises one or more hetero atoms, wherein aryl, heteroaryl, silyl or hydrocarbyl are optionally substituted, and wherein X° is halogen and R° and R⁰⁰ are, independently, H or optionally substituted C1-40 hydrocarbyl.

A compound of Formula elected from:

11

A compound of Formula (I) as defined herein, may preferably be selected from:



Preferably, R¹ is Ci_-8 aliphatic, preferably -CsH_{! 7} or -CH2C(C2H5)HC₄H₉ and R⁵ is methyl or ethyl (preferably ethyl).

A compound of Formula (I) as defined herein, may preferably be selected from:

22

A composition according to any embodiment of the first aspect of the invention comprises a first electron acceptor compound and a second electron acceptor compound. Preferably, the first electron acceptor compound is a compound of formula (I) as defined herein and the second electron acceptor compound is a compound

(i) having an electron affinity and ionization potential between that of the electron donor and the first electron acceptor, for example such that there is an

exothermic charge transfer on exciton dissociation, and it does not provide a source for charge trapping;

(ii) intimately mixing with the first acceptor, rather than phase separating, thus preventing pinning of the open circuit voltage to the lowest ionization potential

component;

(iii) not disrupting or suppressing the molecular organisation of the donor phase; and/or

(iv) facilitating charge transport, improves charge carrier collection and extends device lifetime. The second electron acceptor compound may be a small molecule, for example a compound having a molecular weight of 1000Da or less. Preferably both the first and the second electron acceptor compounds are a compound of formula (I) as defined herein, provided that the first and electron acceptor compounds of formula (I) are not the same.

Preferably, the composition comprises a first electron acceptor compound and a second electron acceptor compound, wherein the second electron acceptor compound has an electron affinity and ionization potential between that of the electron donor and the first electron acceptor compound. Alternatively, the composition comprises a first electron acceptor compound and a second electron acceptor compound, wherein the first electron acceptor compound has an electron affinity and ionization potential between that of the electron donor and the second electron acceptor compound. Preferably, the first electron acceptor compound is compound of formula (I) and the second electron acceptor compound is a compound as depicted in Figure 7. In a composition according to any embodiment of the first aspect of the invention, the electron donor is preferably a polymer or small molecule, e.g. a small molecule light absorber. Preferabl 5-diyl) (P3HT) or a

polymer of structure

, wherein n is 1-20000

(referred to herein as Y4). Other exemplary electron donor compounds include the polymeric compounds disclosed in WO 2013/000532, US 2015/0255725 and US

2015/0108409, the entire contents of which are herein incorporated by reference in their entirety.

Preferably the composition comprises an IDTBR moiety

and a P3HT electron donor, and more preferably the composition comprises IDTBR and IDFBR at a weight ratio of 0.6-0.7:0.3-0.4.

A composition of the invention may be provided, for example, in the form of a bulk material or a film, for example a thin film. As would be understood by a skilled person, a thin film is a film with a thickness of about 100μηι or less, preferably from about 5nm to about 100μηι, more preferably from about 5 to about 500nm.

In a second aspect, the invention provides an optical or electronic device comprising a composition according to any one of the preceding claims. Preferably, the device is a photovoltaic cell (optionally an organic solar cell), an organic transistor, a light emitting diode, a photodetector or a photocatalytic device. The device may further comprise an anode and a cathode. The composition may forms an active layer between the anode and the cathode. Preferably, the device is an organic solar cell comprising a bulk

heterojunction active layer comprising the composition according to the first aspect of the invention. Preferably, the device further comprises a hole transport layer and an electron transport layer.

In a third aspect, the invention provides a process for producing a composition according to the first aspect of the invention, the process comprising:

selecting a first organic electron acceptor compound;

selecting a second organic electron acceptor compound

selecting an organic electron donor; and

blending the compounds to provide the composition.

Preferably, the first and second organic electron acceptors and the organic electron donor are selected such that the second electron acceptor compound has an electron affinity and ionization potential between that of the electron donor and the first electron acceptor compound.

In a fourth aspect, the invention provides a process for producing a device according to the second aspect of the invention, comprising

providing a substrate; and

depositing a composition according to the first aspect of the invention on a surface of the substrate to form an active layer. Preferably, the process further comprises depositing an electrode on the active layer. In a another aspect, the invention provides a compound of Formula (IA)

T¹-(B¹)_a-(A)-(B²)_b-T²

Formula (IA)

wherein

A is a divalent conjugated fused ring system having the structure:

wherein:

Xi is C, Ge or Si (preferably C);

Cy^1"8 are, at each occurrence, independently, absent or a 5 or 6-membered ring having 0, 1 or 2 ring heteroatoms, or a fused polycyclic (for example bicyclic or tricyclic) ring optionally having one or more ring heteroatoms, provided that at least one of Cy^1"4 and at least one of Cy^5"8 is not absent, and wherein each of Cy^1"8, when present, is optionally substituted by one or more groups R²;

R² is, at each occurrence, independently, halo, Ci.₃₀ aliphatic, aryl, heteroaryl, =0, =S, =R°, -CN, -NC, -NCO, -NCS, -OCN, -SCN, -C(=0)NR°R°°, -C(=0)X°, -C(=0)R°, - C(=0)OR°, -C(=S)R°, -C(=S)OR°, -OC(=0)R°, -OC(=S)R°, -C(=0)SR°, -SC(=0)R°, -NH₂, - NR°R⁰⁰, -NR°C(0)R°, -SH, -SR°, -S0₃H, -S0₂R°, -OH, -N0₂, -CF₃, -CF₂-R°, -SF₅, silyl or hydrocarbyl with 1 to 40 C atoms and which optionally comprises one or more hetero atoms, wherein , Ci_₃₀ aliphatic, aryl, heteroaryl, silyl or hydrocarbyl are optionally substituted, and wherein X° is halogen and R° and R⁰⁰ are, independently, H or optionally substituted C1-40 hydrocarbyl; or two R², with the intervening atoms form an optionally substituted fused ring, having 0, 1 or 2 ring heteroatoms;

and wherein within A it is required that at least 3 of the Cy^1"4 or Cy^5"8 groups are 6- membered rings;

each occurrence of B¹ and B² is, independently, -CY¹=CY²-,— C≡C— , or a cyclic hydrocarbyl group with 5 to 30 ring atoms optionally including one or more heteroatoms, preferably aryl or heteroaryl, wherein each occurrence of B¹ and B² is, independently, unsubstituted or substituted by one or more groups R³, wherein R³ has the meaning of R²;

Y¹ and Y² are, independently, H, F, CI or CN;

a and b are, independently of each other, 0, 1 or 2;

wherein A contains an optionally substituted aromatic ring having 0, 1 , 2 or more ring heteroatoms directly bonded to groups B¹ and B², respectively, or wherein when a and/or b are 0, A contains an optionally substituted aromatic ring having 0, 1 , 2 or more ring heteroatoms directly bonded to groups T¹ and T², respectively.

It will be appreciated that each occurrence of * represents the bond to B¹ and B², respectively. Where any one or more of Cy^1"10 is absent, the point of attachment to B¹ and B² is instead on the next adjacent one of Cy^1"10 that is not absent. For example, if Cy¹ is absent, but Cy² is present, the bond between A and B¹ will be between Cy² and B¹. Where a or b, respectively, is not 0, the one of Cy^1"10 which is bonded to B¹ or B², respectively, is an optionally substituted aromatic ring having 0, 1 , 2 or more ring heteroatoms . When a and/or b is 0, the one of Cy^1"10 which is bonded to T¹ or T², respectively, is an optionally substituted aromatic ring having 0, 1 , 2 or more ring heteroatoms directly bonded to groups T¹ and T², respectively.

Cy^1"10 are preferably, at each occurrence, independently, absent or a 5 or 6-membered ring having 0, 1 or 2 ring heteroatoms, each optionally substituted by one or more groups R², provided that at least one of Cy^1"5 and at least one of Cy^6"10 is not absent.

Preferably the one of Cy^1"5 and at the one of Cy^6"10 that are directly bonded to groups B¹ and B², or wherein when a and/or b are 0, the one of Cy^1"5 and at the one of Cy^6"10 that are directly bonded to groups T¹ and T², are each independently a 5 or 6-membered aromatic ring having 0, 1 or 2 ring heteroatoms (preferably phenyl), each optionally substituted by one or more groups R².

V¹

Any one or more of Cy^1"8 may have the structure V— . For example, in a compound of Formula (IA) as defined herein, A may preferably be selected from:

wherein Cy^1"8 are as defined above. Preferably, Cy 1-8 are, at each occurrence, independently a 5 or 6-membered aromatic ring having 0, 1 or 2 ring heteroatoms (preferably phenyl), optionally substituted by one or more groups R².

Preferably A is

, optionally substituted by one or more groups R².

In any of the structures illustrated above, R¹ may, at each occurrence, independently, be Ci-3o aliphatic or aryl optionally substituted with CM₀ aliphatic, preferably C₆-io aliphatic (for example, linear or branched C₈ aliphatic) or aryl (for example, phenyl) substituted with Ci_₆ aliphatic.

In any of the structures illustrated above, R² may, at each occurrence, independently, be H, optionally substituted Ci.₃₀ aliphatic, aryl or heteroaryl, preferably Ci.₃₀ aliphatic or aryl substituted with CM₀ aliphatic, preferably C₆-s aliphatic (for example, linear or branched C₈ aliphatic) or aryl (for example, phenyl) substituted with Ci_₆ aliphatic.

Within any of the structures described above for a compound of Formula (IA), T¹ and T² may be, independently of each other, -CR⁴=Y, -CR⁴=CR⁴-Y, -L-Y or -Y; Y is an optionally substituted cyclic hydrocarbyl group, preferably optionally substituted aryl or heteroaryl; and L is a divalent alkylenyl chain of 3 to 10 carbon atoms, having alternating double and single bonds, optionally substituted by one or more R⁴; and R⁴ is H or has the meaning of R², preferably wherein R⁴ is H.

Prefera is:

in which * marks the point of attachment to -CR⁴=; X² is S, O or C(R⁶)₂;

W is S, O or C(R⁶)₂ (preferably S);

R⁵ is H, aliphatic, heteroaliphatic, aryl, heteroaryl , -CN, -NC, -NCO, -NCS, -OCN, - SCN, -C(=0)NR°R°°, -C(=0)X°, -C(=0)R°, -C(=0)OR°, -C(=S)R°, -C(=S)OR°, -OC(=0)R°, -OC(=S)R°, -C(=0)SR°, -SC(=0)R°, -NH₂, -NR°R⁰⁰, -NR°C(0)R°, -SH, -SR°, -S0₃H, - S0₂R°, -OH, -N0₂, -CF₃, -CF₂-R°, -SF₅, silyl or hydrocarbyl with 1 to 40 C atoms and which optionally comprises one or more hetero atoms, wherein aliphatic, heteroaliphatic, aryl, heteroaryl, silyl or hydrocarbyl are optionally substituted, and wherein X° is halogen and R° and R⁰⁰ are, independently, H or optionally substituted C1-40 hydrocarbyl

(preferably optionally substituted aliphatic, heteroaliphatic, aryl or heteroaryl);

R⁶ is, at each occurrence, independently, H, halo, aliphatic, heteroaliphatic, aryl, heteroaryl, -CN, -NC, -NCO, -NCS, -OCN, -SCN, -C(=0)NR°R°°, -C(=0)X°, -C(=0)R°, - C(=0)OR°, -C(=S)R°, -C(=S)OR°, -OC(=0)R°, -OC(=S)R°, -C(=0)SR°, -SC(=0)R°, -NH₂, - NR°R⁰⁰, -NR°C(0)R°, -SH, -SR°, -S0₃H, -S0₂R°, -OH, -N0₂, -CF₃, -CF₂-R°, -SF₅, silyl or hydrocarbyl with 1 to 40 C atoms and which optionally comprises one or more hetero atoms, wherein aliphatic, heteroaliphatic, aryl, heteroaryl, silyl or hydrocarbyl are optionally substituted, and wherein X° is halogen and R° and R⁰⁰ are, independently, H or optionally substituted C1-40 hydrocarbyl;

--·^' may be present or absent and represents a fused mono-, bi- or tri- cyclic hydrocarbyl group, preferably aryl or heteroaryl, optionally substituted by one or more R₇, wherein R⁷ has the meaning of R²;

R⁸ is, at each occurrence, independently, halo, aryl, heteroaryl, -CN, -NC, -NCO, - NCS, -OCN, -SCN, -C(=0)NR°R°°, -C(=O)X⁰, -C(=O)R⁰, -C(=O)OR⁰, -C(=S)R°, - C(=S)OR°, -OC(=0)R°, -OC(=S)R°, -C(=O)SR⁰, -SC(=O)R⁰, -NH₂, -NR°R⁰⁰, -NR⁰C(O)R°, - SH, -SR°, -S0₃H, -S0₂R°, -OH, -N0₂, -CF₃, -CF₂-R°, -SF₅, silyl or hydrocarbyl with 1 to 40 C atoms and which optionally comprises one or more hetero atoms, wherein aryl, heteroaryl, silyl or hydrocarbyl are optionally substituted, and wherein X° is halogen and R° and R⁰⁰ are, independently, H or optionally substituted C1-40 hydrocarbyl; and

n is 0-4.

Preferably, at least one of T¹ or T² is -CR⁴=Y, and

Preferably, T¹ or T² are both -CR⁴=Y and Y is as defined above. Preferably, at least one of T¹ and T² is -CR⁴=CR⁴-Y or -Y and Y is:

m is 0-3 and o is 0-2 and R⁵, R⁶ and X₂ are as defined above. In preferred embodiments, X₂ is O.

In any of the above embodiments of Y, R⁵ is preferably CM₂ aliphatic, preferably CM₂ alkyl, more preferably Ci_₈ alkyl.

Within any of the structures described above for a compound of Formula (IA), preferably a and b are, independently, 1 or 2, more preferably a and b are both 1 .

Within any of the structures described above for a compound of Formula (IA), each occurrence of B¹ and B² may preferably be, independently, mono-, bi- or tri-cyclic aryl or heteroaryl group, unsubstituted or substituted by one or more groups R³, wherein the aryl or heteroaryl group may optionally include a non-aromatic carbocyclic or heterocyclic ring fused thereto.

Preferably, one or more occurrences of B¹ and B² is:

p is 0, 1 or 2; and

R⁹ is, at each occurrence, independently, halo, aryl, heteroaryl, -CN, -NC, -NCO, -NCS, - OCN, -SCN, -C(=0)NR°R°°, -C(=O)X⁰, -C(=O)R⁰, -C(=O)OR⁰, -C(=S)R°, -C(=S)OR°, - OC(=0)R°, -OC(=S)R°, -C(=0)SR°, -SC(=O)R⁰, -NH₂, -NR°R⁰⁰, -NR⁰C(O)R°, -SH, -SR°, - S0₃H, -SO2R⁰, -OH, -NO2, -CF₃, -CF2-R⁰, -SF₅, silyl or hydrocarbyl with 1 to 40 C atoms and which optionally comprises one or more hetero atoms, wherein aryl, heteroaryl, silyl or hydrocarbyl are optionally substituted, and wherein X° is halogen and R° and R⁰⁰ are, independently, H or optionally substituted C1-40 hydrocarbyl.

A compound of Formula (IA) as defined herein, may preferably be selected from:

preferably wherein A is:

A compound of Formula (IA) as defined herein, may preferably be selected from:

Preferably, R¹ is Ci_-8 alkyl, preferably -C₈H₁₇ or -CH₂C(C2H5)HC₄H9 and R⁵ is methyl ethyl (preferably ethyl).

DESCRIPTION OF THE FIGURES

Figure 1 (a to b) shows exemplary electron acceptor and donor compounds.

Figure 2 shows a) the UV-absorption spectra for IDTBR, OIDFBR and P3HT, b) the energy level diagrams of P3HT, FBR, IDFBR, IDTBR and 60PCBM and c) the change in thin films absorption of binary P3HT: IDTBR and ternary P3HT: IDTBR: IDFBR blends upon annealing in inert atmosphere at 130 °C for 10 mins..

Figure 3 shows a) the JV and b) EQE spectra of IDTBR: IDFBR: P3HT ternary solar cells. Figure 4 shows variations in Voc, FF, Jsc and PCE(%) of ternary blends of

IDTBR: IDFBR: P3HT as a function of change in IDFBR weight ratio.

Figure 5 shows microstructural analysis of P3HT: IDTBR: IDFBR ternary blend a) DSC heating profiles of individual P3HT, IDTBR and IDFBR along with binary P3HT: IDTBR, P3HT:IDFBR and ternary P3HT: IDTBR: IDFBR blends; b) illustration of the binary

P3HT:IDTBR blend without IDFBR presence, where crystallinity of both P3HT and IDTBR is preserved; c) illustration of the ternary P3HT: IDTBR: IDFBR (1 :0.7:0.3) blend, where the acceptor crystallinity is vitrified and P3HT crystallinity is preserved; and d) illustration of the ternary P3HT: IDTBR: IDFBR blend >30% IDFBR, where the acceptor phase is an amorphous solid and P3HT crystallinity is preserved.

Figure 6 shows the oxidative stability of a IDTBR:IDFBR:P3HT device efficiencies (PCE) compared with IDTBR:P3HT and other high performance polymer:fullerene systems. Devices were exposed to ambient conditions over the course of 1200 hours.

Figure 7 (a to d) shows further exemplary electron acceptor compounds that may be used in devices described herein.

DETAILED DESCRIPTION OF THE INVENTION As used herein, the terms "donor" or "donating" and "acceptor" or "accepting" will be understood to mean an electron donor or electron acceptor, respectively. "Electron donor" will be understood to mean a chemical entity that donates electrons to another compound or another group of atoms of a compound. "Electron acceptor" will be understood to mean a chemical entity that accepts electrons transferred to it from another compound or another group of atoms of a compound, (see also U.S. Environmental Protection Agency, 2009, Glossary of technical terms, http://www.epa.gov/oust/cat/TUMGLOSS.HTM, or "Glossary of terms used in physical organic chemistry (lUPAC recommendations 1994)" in Pure and Applied Chemistry, 1994, 66, 1077, pages 1109-11 10). It will be appreciated that any organic electron acceptor compound referenced in any aspect or embodiment of the invention as described herein is preferably a non-fullerene organic electron acceptor compound, i.e. an organic compound comprising no fullerene components. An organic electron donor compound referenced in any aspect or embodiment of the invention as described herein is preferably a non-fullerene organic electron donor compound, i.e. an organic compound comprising no fullerene components.

Preferably, a composition of the invention does not contain any fullerene components. By modifying the molecular structure of non-fullerene electron acceptors, through aromatic ring expansion and restricting steric torsion, it is possible to tune the absorption profile of these non-fullerene acceptors in visible region while specifically and independently controlling the frontier molecular orbital energy levels responsible for V_oc. Ternary compositions comprising two electron acceptor compounds enable improved efficiencies due to, for example, providing a broader and/or stronger absorption of the active layer. In a ternary composition of the invention, or an optical or electronic device comprising a ternary composition of the invention, there are a number of considerations to optimise the parameters of the device. This enables the photovoltaic parameters to be improved over those of a binary device. In relation to the second electron acceptor compound, preferably it may be selected such that it has one or more of the following properties:

(i) it has an electron affinity and ionization potential between that of the electron donor and the first electron acceptor, such that there is an exothermic charge transfer on exciton dissociation, and it does not provide a source for charge trapping;

(ii) it intimately mixes with the first acceptor, rather than phase separates, thus preventing pinning of the open circuit voltage to the lowest ionization potential

component;

(iii) it does not disrupt or suppress the molecular organisation of the donor phase; and

(iv) it facilitates charge transport, improves charge carrier collection and extends device lifetime.

Preferably, the second electron compound may be a compound having energy levels that satisfy the following: a lower electron affinity than the primary acceptor, to facilitate an energy cascade heterojunction leading to larger open circuit voltages than can be obtained with just the first acceptor; an ionisation potential that is not too large to inhibit hole transfer to the electron donor; and a bandgap at a wavelength which can contribute to the cell external quantum efficiency. Preferably, the surface energy of the electron acceptor (either the first electron acceptor or the second electron acceptor) with the lowest ionisation affinity should be in-between that of the electron donor and other electron acceptor.

Measurements of Ionisation potential and electron affinity can be obtained by many standard techniques known to a skilled person in the art including, for example, cyclic voltammetry, ultraviolet photoelectron spectroscopy and inverse photoelectron

spectroscopy.

Exemplary second electron acceptor compounds are set out in Figure 7.

As used herein, the term "n-type" or "n-type semiconductor" will be understood to mean an extrinsic semiconductor in which the conduction electron density is in excess of the mobile hole density, and the term "p-type" or "p-type semiconductor" will be understood to mean an extrinsic semiconductor in which mobile hole density is in excess of the conduction electron density (see also, J. Thewlis, Concise Dictionary of Physics, Pergamon Press, Oxford, 1973).

As used herein, the term "conjugated" will be understood to mean a compound (for example a small molecule or a polymer) that contains mainly C atoms with sp2- hybridisation (or optionally also sp-hybridisation), and wherein these C atoms may also be replaced by hetero atoms. In the simplest case this is for example a compound with alternating C— C single and double (or triple) bonds, but is also inclusive of compounds with aromatic units like for example 1 ,4-phenylene. The term "mainly" in this connection will be understood to mean that a compound with naturally (spontaneously) occurring defects, which may lead to interruption of the conjugation, is still regarded as a conjugated compound. In the original meaning a conjugated system is a molecular entity whose structure may be represented as a system of alternating single and multiple bonds: e.g. CH2=CH-CH=CH₂, CH₂=CH-C≡N. In such systems, conjugation is the interaction of one p-orbital with another across an intervening o-bond in such structures. (In appropriate molecular entities d-orbitals may be involved.) The term is also extended to the analogous interaction involving a p-orbital containing an unshared electron pair, e.g. :CI-CH=CH₂. Accordingly, a conjugated system is a system of connected p-orbitals with delocalized electrons in molecules with alternating single and multiple bonds. A conjugated system has a region of overlapping p-orbitals, bridging the adjacent single bonds. They allow a derealization of electrons across all the adjacent aligned p-orbitals. The conjugated system may be cyclic, acyclic, linear, branched or mixed. A conjugated system according to the present invention is a system which may be partly or completely conjugated.

In the context used herein, asmall molecule may be a compound having a molecular weight of 1000Da or less. Unless stated otherwise, groups or indices like Cy, Ar, R^1"4, n etc. in case of multiple occurrences are selected independently from each other and may be identical or different from each other. Thus several different groups may be represented by a single label like

"R¹".

The term "aliphatic" includes both saturated and unsaturated, nonaromatic, straight chain (i.e., unbranched), branched, acyclic, and cyclic (i.e., carbocyclic) hydrocarbons, which are optionally substituted with one or more functional groups. As will be appreciated by one of ordinary skill in the art, "aliphatic" is intended herein to include, but is not limited to, alkyl, alkenyl, alkynyl, cycloalkyi, cycloalkenyl, and cycloalkynyl moieties containing from 1 to 40 carbon atoms, preferably 1 to 20 carbon atoms. Heteroaliphatic is an aliphatic group where one or more carbon atoms are replaced with a heteroatom, such as O, N, S, P etc.

The term 'alkyl', 'aryl', 'heteroaryl' etc also include multivalent species, for example alkylene, arylene, 'heteroarylene' etc.

The term "carbyl group" as used above and below denotes any monovalent or multivalent organic radical moiety which comprises at least one carbon atom either without any non- carbon atoms (like for example -C≡C-), or optionally combined with at least one non- carbon atom such as N, O, S, P, Si, Se, As, Te or Ge (for example carbonyl etc.). The term "hydrocarbyl group" denotes a carbyl group that does additionally contain one or more H atoms and optionally contains one or more hetero atoms like for example N, O, S, P, Si, Se, As, Te or Ge. A carbyl or hydrocarbyl group comprising a chain of 3 or more C atoms may also be linear, branched and/or cyclic, including spiro and/or fused rings.

Preferred carbyl and hydrocarbyl groups include alkyl, alkenyl, alkynyl, alkoxy, alkylcarbonyl, alkoxycarbonyl, alkylcarbonyloxy and alkoxycarbonyloxy, each of which is optionally substituted and has 1 to 40, preferably 1 to 25, more preferably 1 to 20 or 1 to 18 C atoms, and optionally substituted aryl, arylalkyl, alkylaryl, or aryloxy having 5 to 40, preferably 5 to 25 C atoms, alkylaryloxy, arylcarbonyl, aryloxycarbonyl, arylcarbonyloxy and aryloxycarbonyloxy, each of which is optionally substituted and has 5 to 40, preferably 5 to 25 C atoms. The carbyl or hydrocarbyl group may be a saturated or unsaturated acyclic group, or a saturated or unsaturated cyclic group. Unsaturated acyclic or cyclic groups are preferred, especially aryl, alkenyl and alkynyl groups (especially ethynyl). Where the CrC₄₀ carbyl or hydrocarbyl group is acyclic, the group may be linear or branched.

The Ci-C₄₀ carbyl or hydrocarbyl group includes for example: a C C₄₀ alkyl group, a C₂- C₄₀ alkenyl group, a C₂-C₄₀ alkynyl group, a C₃-C₄₀ allyl group, a C₄-C₄₀ alkyldienyl group, a C₄-C₄₀ polyenyl group, a C₆-Ci₈ aryl group, a C₆-C₄₀ alkylaryl group, a C₆-C₄₀ arylalkyl group, a C₄-C₄₀ cycloalkyl group, a C₄-C₄₀ cycloalkenyl group, and the like. Preferred among the foregoing groups are a C C₂₀ alkyl group, a C₂-C₂o alkenyl group, a C₂ -C₂₀ alkynyl group, a C₃-C₂₀ allyl group, a C₄-C₂₀ alkyldienyl group, a C₅-Ci₂ aryl group, a C₆ - C₂₀ arylalkyl group, a 5 to 20 membered heteroaryl and a C₄-C₂₀ polyenyl group, respectively. Also included are combinations of groups having carbon atoms and groups having hetero atoms, like e.g. an alkynyl group, preferably ethynyl, that is substituted with a silyl group, preferably a trialkylsilyl group.

Further preferred carbyl and hydrocarbyl groups include straight-chain, branched or cyclic alkyl with 1 to 40, preferably 1 to 25 C-atoms, which is unsubstituted, mono- or

polysubstituted by F, CI, Br, I or CN, and wherein one or more non-adjacent CH₂ groups are optionally replaced, in each case independently from one another, by -0-, -S-, -NH-, - NR⁰-, -SiR°R⁰⁰-, -CO-, -COO-, -OCO-, -0-CO-0-, -S-CO-, -CO-S-, -S0₂-, -CO-NR⁰-, -NR°- CO-, -NR°-CO-NR⁰⁰-, -CY¹=CY²- or -C≡C- in such a manner that O and/or S atoms are not linked directly to one another, wherein Y¹ and Y² are independently of each other H, F, CI, Br, I or CN, and R° and R⁰⁰ are independently of each other H or an optionally substituted aliphatic or aromatic hydrocarbon with 1 to 20 C atoms. Preferred carbyl and hydrocarbyl groups are aliphatic and heteroaliphatic groups.

Halogen is F, CI, Br or I. As used herein, an alkyl group is a straight chain or branched, cyclic or acyclic, substituted or unsubstituted group containing from 1 to 40 carbon atoms, from 1 to 25 carbon atoms, from 1 to 20 carbon atoms, from 1 to 18 carbon atoms, from 1 to 12 carbon atoms or from 1 to 8 carbon atoms, inclusive. An alkyl group may optionally be

substituted at any position. Preferred alkyl groups include, without limitation, methyl, ethyl, n-propyl, i-propyl, n-butyl,

1- butyl, s-butyl, t-butyl, 2-methylbutyl, n-pentyl, s-pentyl, cyclopentyl, n-hexyl, cyclohexyl,

2- ethylhexyl, n-heptyl, cycloheptyl, n-octyl, cyclooctyl, dodecanyl, tetradecyl, hexadecyl, trifluoromethyl, perfluoro-n-butyl, 2,2,2-trifluoroethyl, perfluorooctyl, perfluorohexyl etc.

Preferred alkenyl groups include, without limitation, ethenyl, propenyl, butenyl, pentenyl, cyclopentenyl, hexenyl, cyclohexenyl, heptenyl, cycloheptenyl, octenyl, cyclooctenyl etc.

Preferred alkynyl groups include, without limitation, ethynyl, propynyl, butynyl, pentynyl, hexynyl, octynyl etc.

Preferred alkoxy groups include, without limitation, methoxy, ethoxy, 2-methoxyethoxy, n- propoxy, i-propoxy, n-butoxy, i-butoxy, s-butoxy, t-butoxy, 2-methylbutoxy, n-pentoxy, n- hexoxy, n-heptoxy, n-octoxy etc.

Preferred amino groups include, without limitation, dimethylamino, methylamino, methylphenylamino, phenylamino, etc.

Aromatic rings are cyclic aromatic groups that may have 0, 1 or 2 or more, preferably 0, 1 or 2 ring heteroatoms. Aromatic rings may be optionally substituted and/or may be fused to one or more aromatic or non-aromatic rings, which may contain 0, 1 , 2, or more ring heteroatoms, to form a polycyclic ring system.

Aromatic rings include both aryl and heteroaryl groups. Aryl and heteroaryl groups may be mononuclear, i.e. having only one aromatic ring (like for example phenyl or phenylene), or polynuclear, i.e. having two or more aromatic rings which may be fused (like for example napthyl or naphthylene), individually covalently linked (like for example biphenyl), and/or a combination of both fused and individually linked aromatic rings. Preferably the aryl or heteroaryl group is an aromatic group which is substantially conjugated over substantially the whole group. Aryl groups may contain from 5 to 40 ring carbon atoms, from 5 to 25 carbon atoms, from 5 to 20 carbon atoms, or from 5 to 12 carbon atoms. Heteroaryl groups may be from 5 to 40 membered, from 5 to 25 membered, from 5 to 20 membered or from 5 to 12 membered rings, containing 1 or more ring heteroatoms selected from N, O, S and P. An aryl or heteroaryl may be fused to one or more aromatic or non-aromatic rings to form a polycyclic ring system. Aryl and heteroaryl preferably denote a mono-, bi- or tricyclic aromatic or heteroaromatic group with up to 25 ring atoms that may also comprise condensed rings and is optionally substituted. Preferred aryl groups include, without limitation, benzene, biphenylene, triphenylene, [1 ,1 ':3',1 "]terphenyl-2'-ylene, naphthalene, anthracene, binaphthylene, phenanthrene, pyrene, dihydropyrene, chrysene, perylene, tetracene, pentacene, benzpyrene, fluorene, indene, indenofluorene, spirobifluorene, etc. Preferred heteroaryl groups include, without limitation, 5-membered rings like pyrrole, pyrazole, silole, imidazole, 1 ,2,3-triazole, 1 ,2,4-triazole, tetrazole, furan, thiophene, selenophene, oxazole, isoxazole, 1 ,2-thiazole, 1 ,3-thiazole, 1 ,2,3-oxadiazole,

1.2.4- oxadiazole, 1 ,2,5-oxadiazole, 1 ,3,4-oxadiazole, 1 ,2,3-thiadiazole, 1 ,2,4-thiadiazole,

1.2.5- thiadiazole, 1 ,3,4-thiadiazole, 6-membered rings like pyridine, pyridazine, pyrimidine, pyrazine, 1 ,3,5-triazine, 1 ,2,4-triazine, 1 ,2,3-triazine, 1 ,2,4,5-tetrazine, 1 ,2,3,4- tetrazine, 1 ,2,3,5-tetrazine, and fused systems like carbazole, indole, isoindole, indolizine, indazole, benzimidazole, benzotriazole, purine, naphthimidazole, phenanthrimidazole, pyridimidazole, pyrazinimidazole, quinoxalinimidazole, benzoxazole, naphthoxazole, anthroxazole, phenanthroxazole, isoxazole, benzothiazole, benzofuran, isobenzofuran, dibenzofuran, quinoline, isoquinoline, pteridine, benzo-5,6-quinoline, benzo-6,7-quinoline, benzo-7,8-quinoline, benzoisoquinoline, acridine, phenothiazine, phenoxazine, benzopyridazine, benzopyrimidine, quinoxaline, phenazine, naphthyridine, azacarbazole, benzocarboline, phenanthridine, phenanthroline, thieno[2,3b]thiophene,

thieno[3,2b]thiophene, dithienothiophene, dithienopyridine, isobenzothiophene, dibenzothiophene, benzothiadiazothiophene, or combinations thereof. The heteroaryl groups may be substituted with alkyl, alkoxy, thioalkyl, fluoro, fluoroalkyl or further aryl or heteroaryl substituents.

Preferred arylalkyl groups include, without limitation, 2-tolyl, 3-tolyl, 4-tolyl, 2,6- dimethylphenyl, 2,6-diethylphenyl, 2,6-di-i-propylphenyl, 2,6-di-t-butylphenyl, o-t- butylphenyl, m-t-butylphenyl, p-t-butylphenyl, 4-phenoxyphenyl, 4-fluorophenyl, 3- carbomethoxyphenyl, 4-carbomethoxyphenyl etc.

Preferred alkylaryl groups include, without limitation, benzyl, ethylphenyl, 2-phenoxyethyl, propylphenyl, diphenylmethyl, triphenylmethyl or naphthalinylmethyl. Preferred aryloxy groups include, without limitation, phenoxy, naphthoxy, 4- phenylphenoxy, 4-methylphenoxy, biphenyloxy, anthracenyloxy, phenanthrenyloxy etc.

As used herein, the term "fused" refers to a cyclic group, for example an aryl or heteroaryl group, in which two adjacent ring atoms , together with additional atoms, forms a fused ring to give a polycyclic (for example, a bicyclic) ring system.

Any of the above groups (for example, those referred to herein as "optionally substituted", including aryl, heteroaryl, carbyl and hydrocarbyl groups) may optionally comprise one or more substituents, preferably selected from silyl, sulpho, sulphonyl, formyl, amino, imino, nitrilo, mercapto, cyano, nitro, halogen, -NCO, -NCS, -OCN, -SCN, -C(=0)NR°R°°, - C(=0)X°, -C(=0)R°, -NR°R⁰⁰, C_1-12alkyl, Ci.i₂alkenyl, Ci.i₂alkynyl, C₆-i₂ aryl, heteroaryl having 5 to 12 ring atoms, Ci_i₂ alkoxy, hydroxy, CM₂ alkylcarbonyl, CM₂ alkoxy-carbonyl, Ci-12 alkylcarbonlyoxy or CM₂ alkoxycarbonyloxy wherein one or more H atoms are optionally replaced by F or CI and/or combinations thereof; wherein X° is halogen and R° and R⁰⁰ are, independently, H or optionally substituted C1-40 hydrocarbyl. The optional substituents may comprise all chemically possible combinations in the same group and/or a plurality (preferably two) of the aforementioned groups (for example amino and sulphonyl if directly attached to each other represent a sulphamoyl radical).

Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of the words, for example "comprising" and "comprises", mean "including but not limited to", and are not intended to (and do not) exclude other components.

It will be appreciated that variations to the foregoing embodiments of the invention can be made while still falling within the scope of the invention. Each feature disclosed in this specification, unless stated otherwise, may be replaced by alternative features serving the same, equivalent or similar purpose. Thus, unless stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

All of the features disclosed in this specification may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. In particular, the preferred features of the invention are applicable to all aspects of the invention and may be used in any combination. Likewise, features described in nonessential combinations may be used separately (not in combination). It will be appreciated that many of the features described above, particularly of the preferred embodiments, are inventive in their own right and not just as part of an embodiment of the present invention. Independent protection may be sought for these features in addition to or alternative to any invention presently claimed.

EXAMPLES

Example 1 - Syntheses of Electron Acceptors

FBR electron acceptor compounds

Scheme 1. Synthesis of EH-FBR electron acceptor The synthesis of EH-FBR (see Scheme 1) followed the synthesis procedure for O-FBR detailed in Holliday et al. J. Am. Chem. Soc , 2015, 137, 898-904 (the contents of which are herein incorporated by reference it its entirety). The fluorene core was coupled to a bromo benzothiadiazole carboxaldehyde unit on both sides using a Suzuki cross coupling reaction. A Knoevenagel condensation reaction, between 3-ethylrhodanine and the aldehyde group of the benzothiadiazole units, allowed the introduction of rhodanine groups to flank the acceptor. The identity of the product formed was confirmed to be EH- FBR with the use of ¹ H and ¹³C NMR along with mass spectrometry.

The phenyl-phenyl links between the fluorene core and the benzothiadiazole units of EH- FBR lead to twisting in the backbone of the molecule and therefore the acceptor is not planar. Backbone twisting is also caused by the relatively small amount of quinoidal character of the phenyl-phenyl link. This lack of planarity disrupts the crystal packing of the acceptor, reducing its crystallinity. The branched 2-ethylhexyl pendant alkyl chains also are expected to disrupt the crystal packing, relative to the linear n-octyl chains, which would give it a slightly different morphology in blends compared to O-FBR. The optoelectronic properties of the acceptor were expected to be very similar to those of O- FBR.

IDFBR electron acceptor compounds

Scheme 2. Syntheses of O-IDFBR electron acceptors

The syntheses of EH-IDFBR and O-IDFBR were carried out in accordance with Scheme 2. The alkylated indenofluorene units were brominated, followed by the formation of the boronic acid pinacol esters according to standard procedures, so that they too could undergo Suzuki cross coupling reactions with the bromo benzothidiazole carboxaldehyde units. Indenofluorene has similar sterics to fluorene, and is not so electron rich as to cause significant deboronation, hence it was deemed suitable to use the Suzuki reaction with the same conditions as for the synthesis of EH-FBR and O-FBR. The flanking 3- ethylrhodanine units were introduced using a Knoevenagel condensation reaction with the aldehyde functionality of the benzothiadiazole units. The identities of the products formed were confirmed to be O-IDFBR and EH-IDFBR with the use of ¹ H and ¹³C NMR along with mass spectrometry. The indenofluorene core is larger and more conjugated than fluorene; hence the IDFBR acceptors were designed to be slightly more crystalline than their fluorene counterparts. This is due to increased ττ-π stacking of the larger, more conjugated indenofluorene core Again, the phenyl-phenyl link should lead to backbone twisting in order to avoid the steric clash of ortho hydrogens. The reduced quinoidal character is also likely to contribute to this reduced planarity, which would limit the crystallinity of the acceptors to some degree. Again, the acceptor bearing the branched 2-ethylhexyl alkyl side chains was expected to be more amorphous than the acceptor with the linear n-octyl chains for crystal packing reasons. Despite the slightly more crystalline order expected in these acceptors, they still exhibit good solubility in common organic solvents, hence are suitable for solution processing.

The more conjugated indenofluorene core was also expected to raise the HOMO level slightly, whilst having little effect on the LUMO, relative to the FBR acceptors. The HOMO is expected to be delocalized across the entire molecule, as discussed for O-FBR; hence increasing the conjugation of the aromatic core will raise the HOMO. But the LUMO is expected to be localized onto the electron deficient periphery of the molecule

(benzothiadiazole and rhodanine units) as a result of the twisting between the aromatic core and the periphery units. Therefore, varying the conjugation of the core is unlikely to significantly affect the LUMO. The overall effect of this would be a slightly reduced bandgap for these acceptors, and hence a slightly red shifted absorption profile.

Procedure

Compound 1 was prepared according to the methods set out in Zhang, W. et al., J Am Chem Soc 131 , 10814-10815, (2009) and Setayesh, S. et al., Macromolecules 33, 2016- 2020, (2000), the contents of which are herein incorporated by reference in their entirety.

1 (1.414 g, 1.65 mmol) was dissolved in dry THF (60 mL) and left to cool to -78 °C. 2.5 M n-butyl lithium (1.78 mL, 4.44 mmol) was added dropwise and solution allowed to stir for 15 min. 2-lsopropoxy-4,4,5,5- tetramethyl-1 ,3,2-dioxaborolane (1.34 mL, 6.58 mmol) was added slowly, then the mixture was allowed to warm to room temperature overnight, with stirring. The reaction mixture was quenched with water, before being extracted with CHCI3 and dried over MgS04, followed by the removal of solvent under reduced pressure. The crude product was recrystallized from acetone to yield 5b as a white crystalline solid (0.701 g, 0.74 mmol, 44.6%). ¹ H NMR (400 MHz, CDCI3) δ: 7.84 (d, J = 8.0 Hz, 2H), 7.77 (m, 4H), 7.66 (s, 2H), 2.05 (m, 8H), 1.42 (m, 26H), 1.18-1.04 (m, 30H), 0.81 (m, 16H), 0.63 (m, 12H).

2 (0.701 g, 0.74 mmol) and 7-bromo-2,1 ,3-benzothiadiazole-4-carboxaldehyde (0.536 g, 2.20 mmol) were dissolved in dry dimethoxyethane before the solution was degassed. Pd(PPh3)4 (0.034 g, 0.029 mmol) was added before a solution of degassed 2 M K2CO3 (2.94 ml_, 5.88 mmol) was also added. The solution was heated to 85 °C with stirring and left overnight. The reaction mixture was quenched with water and extracted with CHCI3, before being dried over MgS04. The crude product was purified by column chromatography on silica gel with hexanes/acetone (2:1), and solvent removed under reduced pressure to afford 6b as an orange solid (0.265 g, 0.26 mmol, 35.1 %). ¹ H NMR (400 MHz, CDCI3) δ: 10.84 (s, 2H), 8.37 (d, J = 8.1 Hz, 2H), 8.1 1 (d, J = 8.1 Hz, 2H), 8.06 (m, 2H), 8.00 (m, 2H), 7.77 (s, 2H), 2.15 (m, 8H), 1.11 (m, 32H), 0.79 (m, 20H). 3 (0.265 g, 0.26 mmol) and 3-ethylrhodanine (0.129 g, 0.80 mmol) were dissolved in warm tert-butyl alcohol (20 ml_) before 1 drop of piperidine was added. The mixture was then allowed to reflux at 85 °C with stirring overnight. The product was extracted with CHCI3, dried with MgS04 and solvent removed to yield a crude dark red solid. This was purified by flash column chromatography on silica gel from CH2CI2 and solvent removed under reduced pressure and the resulting crude product was recrystallized from acetone to yield IDFBR as a red solid (0.323 g, 0.25 mmol, 95.2%). mp = 180-190 °C. ¹ H NMR (400 MHz, CDCI3) δ: 8.60 (s, 2H), 8.12 (dd, J = 8.1 Hz and 1.6 Hz, 2H), 8.05 (m, 2H), 7.97 (q, J = 8.0 Hz, 4H), 7.85 (d, J = 8.0 Hz, 2H), 7.77 (s, 2H), 4.29 (q, J = 8.0 Hz, 4H), 2.14 (m, 8H), 1.36 (t, J = 8.0 Hz, 6H), 1.17-1.11 (m, 42H), 0.83-0.78 (m, 18H). ¹³C NMR (101 MHz, CDCI3) δ: 193.16, 167.52, 154.65, 153.57, 151.80, 151.15, 142.63, 140.52, 137.19, 135.00,

131.17, 128.58, 127.38, 127.38, 125.41 , 125.33, 124.04, 119.84, 1 14.55, 55.08, 40.56, 39.96, 31.84, 30.05, 29.29, 29.22, 23.93, 22.61 , 14.09, 12.35. MS (MALDI-ToF): m/z calculated for C76H92N6O2S6: 1312.6; m/z found 1314.4 (M + H)⁺. IDTBR electron acceptor compounds

Scheme 3. Syntheses of IDTBR electron acceptors The syntheses of EH-IDTBR and O-IDTBR were carried out as specified in Scheme 3. The indacenodithiophene core is more electron rich than fluorene or indenofluorene, hence deboronation of the aryl boronic acid pinacol ester would occur readily. As such, the boronic acid pinacol ester was instead incorporated onto the benzothiadiazole carboxaldehyde unit. A Suzuki cross coupling reaction was then carried out to add the benzothiadiazole units to the indacenodithiophene core. Once the coupling reaction was completed, the 3-ethylrhodanine units were incorporated using a Knoevenagel condensation reaction with the aldehyde functionality of the benzothiadiazole unit. The identities of the products formed were confirmed to be O-IDTBR and EH-IDTBR with the use of ¹ H and ¹³C NMR along with mass spectrometry.

The indacenodithiophene core contains a thiophene-like moiety at either end of the unit; hence the IDTBR acceptors contain a thiophene-phenyl link rather than the phenyl-phenyl link in the FBR acceptors. This link leads to far less steric repulsion of ortho protons than its phenyl-phenyl counterpart, hence these acceptors are expected to have a much more planar structure. Also, the thiophene-phenyl link has more quinoidal character than a phenyl-phenyl link, which further enhances the planarity of the molecule. A consequence of this is that the crystallinity of these acceptors is expected to be much higher than the FBR and IDFBR acceptors, since there is likely to be increased ττ-π stacking in the more planar IDTBR acceptors. As has been the case for the other acceptors, it is expected that EH-IDTBR to be less crystalline than O-IDTBR, since the branched chains disrupts the crystal packing. Again, despite the expected increase in crystalline order for the IDTBR acceptors, they still exhibit good solubility in common organic solvents, and therefore are suitable to be solution processed.

The enhanced planarity of the IDTBR acceptors would allow much greater conjugation throughout the molecules. One would also expect the LUMO to become slightly more delocalized across the entire structure of the molecule. A result of this is that the bandgap of these acceptors is expected to be much narrower than for the FBR and IDFBR acceptors. This would red shift the absorption profile further, leading to an even larger fraction of the solar spectrum being utilized in devices with P3HT.

GelDTBR electron acceptor compounds

O-GelDTBR was synthesised according to scheme 4 and the experimental procedures below.

GelDTBR

Scheme 4

Dichlorobis(octyl)germane

CI , ,CI C₈H₁₇ZnBr CI CI

Ge ^► _Ge

CI ' CI THF, diethyl ether C₈H₁₇' ^'C₈H₁₇ Perchlorogermane (21.5g, 100 mmol) was diluted in 100 ml_ of anhydrous THF and cooled to 0°C. A 1 M solution of (octyl)magnesium bromide (200 ml_, 200 mmol) in diethyl ether was added dropwise over one hour. The reaction mixture was stirred overnight, whilst warming up to room temperature. 200 ml_ of n-hexane were added to the reaction mixture and large amounts of precipitate form. The precipitate is filtered-off and the solvent removed from the filtrate on the rotary evaporator. The cloudy viscous oil is distilled under reduced pressure. The dichlorobis(octyl)germane (16.7 g, 45.1 mmol) was recovered as a colourless oil in the temperature range of 100 to 140°C at 0.4 mbar. ¹ H NMR (400 MHz, CDCI₃): δ: 1.78-1.68 (m, 4H), 1.51-1.19 (m, 24H), 0.94-0.83 (m, 6H). ¹³C NMR (101 MHz, CDCI₃) δ: 31.83, 31.63, 29.10, 29.03, 28.06, 24.27, 22.66, 14.13. -(boronic acid pinacol ester)-2, 1 ,3-benzothiadiazole-4-carboxaldehyde

1 ,4 dioxane

7-bromo-2, 1 ,3-benzothiadiazole-4-carboxaldehyde (2 g, 8.23 mmol) was added to bis(pinacolato)diboron (4.8 g, 18.92 mmol), PdCI₂(dppf) CH₂CI₂ (336 mg, 0.41 mmol), and KOAc (4.85 g, 49.36 mmol). The flask was sealed and heated to 80°C under high vacuum for an hour. The reaction vessel was filled with nitrogen and 25 ml of degassed anhydrous 1 ,4-dioxane was added and stirred at 80 °C overnight. The reaction was quenched by adding water, and the resulting mixture was extracted into ethyl acetate. The organic layers were washed with brine, dried over MgS0₄, and concentrated under reduced pressure to yield dark brown solid. The solid was triturated with heptane, filtered through celite and the solvent was evaporated to give the crude product as a yellow colored solid (1.18g, Yield -50%). ¹ H NMR (400 MHz, CHCI₃): δ 10.85 (s, 1 H), 8.35 (d, J = 6.8 Hz, 1 H), 8.22 (d, J = 6.8 Hz, 1 H), 1.42 (s, 12H). ¹³C NMR (101 MHz, CDCI₃): δ 189.51 , 137.58, 130.66, 129.30, 84.91 , 24.92.

1) 1 ,4-dibromo-2,5-diiodobenzene

20 g (85 mmol) of 1 ,4-dibromobenzene were dissolved in 250 mL of concentrated sulphuric acid at 80°C. Iodine (47.3 g, 187 mmol) was added to the reaction flask in several portions. After complete addition the reaction temperature was increased to 130°C and the mixture was heated during 2 days. The reaction mixture was cooled to room temperature and carefully poured into ice-water. The black solid was filtered-off and extensively washed with water, before it was dissolved in warm chlorobenzene (300 mL). The organic chlorobenzene layer was washed several times with a concentrated aqueous sodium thiosulfate solution and water. The organic layer was separated and dried over anhydrous magnesium sulphate. The solution was concentrated and then precipitated into well stirred methanol. The formed solid was filtered off and the title compound was recovered as a white solid (23.3 g, 48 mmol, 56% yield). ¹ H NMR (400 MHz, CDCI₃): <5 8.04 (s, 2H). ¹³C NMR (100 MHz, CDCI₃): 6 142.44, 129.34, 101 .48. MS (El): m/z calcd for C₆H₂Br₂l2 (M⁺) 488, 486, 490, 489 found 488, 486, 490, 489.

2) 2,2'-(2,5-dibromo-1 ,4-phenylene)bis(3-bromothiophene)

To an oven dried round bottom flask was added 1 (14.5 g, 29.8 mmol) and

tetrakistriphenylphosphinepalladium(O) (1 .7 g, 1 .5 mmol) before a 0.5 M (3- bromothiophen-2-yl)zinc(ll) bromide solution in THF (125 mL, 62.5 mmol) was added. The reaction mixture was stirred and heated at 65 °C (oil bath temperature) overnight. The mixture was cooled to room temperature and poured into 150 mL of saturated aqueous ammonium chloride solution. The precipitate was filtered off and washed with water, acetone and diethyl ether. Compound 2 was recovered as an off-white solid (1 1 .6 g, 20.8 mmol, 70% yield). ¹ H NMR (400 MHz, CDCI₃): 6 7.74 (s, 2H), 7.44 (d, J = 5.3 Hz, 2H), 7.1 1 (d, J = 5.3 Hz, 2H). ¹³C NMR (100 MHz, CDCI₃): <5 136.63, 136.1 1 , 135.17, 130.51 , 126.97, 123.53, 1 1 1.79, 77.16. HRMS (El): m/z calcd for Ci₄H₆Br₄S2 (M⁺) 557.6603 found 557.6603.

3) (5,5'-(2,5-dibromo-1 ,4-phenylene)bis(4-bromothiophene-5,2- diyl))bis(trimethylsilane)

2,2'-(2,5-dibromo-1 ,4-phenylene)bis(3-bromothiophene) 2 (11 g, 19.7 mmol) was dissolved in anhydrous THF (400 mL) and cooled to -78°C. A 1.8 M solution of lithium diisopropylamide in THF/heptanes/ethylbenzene (33 mL, 59.4 mmol) was added slowly to the reaction. The temperature was kept at all times below -70°C. After complete addition the reaction was stirred during 1 hour at -78°C and then quenched by the addition of chlorotrimethylsilane (8.8 mL, 69.3 mmol). The reaction mixture was warmed to room temperature and stirred for another 30 minutes. The solvent was removed under reduced pressure and the crude product plugged through a silica pad using petroleum ether (60- 80°C) as eluent. The solvent was evaporated and the product recrystallized from ethyl acetate to afford 3 as white needles (11.7 g, 16.7 mmol, 84% yield). ¹ H NMR (400 MHz, CDCIs): δ 7.70 (s, 2H), 7.17 (s, 2H), 0.36 (s, 18H). ¹³C NMR (100 MHz, CDCI₃): δ 142.4, 139.8, 136.5, 136.3, 136.1 , 123.0, 1 12.6, 0.30. MS (El): m/z calcd for CaoH^B^SaSia (M⁺) 701.7, 699.7, 703.7, 702.7 found 701.7, 699.7, 703.7, 702.7. 4 and 5) 2,7-dibromo-4,4,9,9-tetrakis(octyl)-4,9-dihydro-benzo[1 ",2":4,5;4",5":4',5']

ithiophene

In an oven dried three-necked round bottom flask, compound 3 (3 g, 4.27 mmol) was dissolved in 60 mL of anhydrous THF and cooled to -90°C. In a second dry three-necked round bottom flask were introduced 30 mL of anhydrous THF, which were cooled down to -90°C before a 1.7 M solution of te/f-butyllithium (20.7 mL, 35.11 mmol) in pentane was added. The solution containing compound 3 was added dropwise to the f-butyllithium solution, whilst maintaining the temperature below -85°C. After complete addition, the resulting dark brown solution was stirred during one hour at -90°C.

Dichlorobis(octyl)germane (3.48 g, 9.40 mmol) diluted in 10 mL of dry THF was added dropwise to the reaction mixture. The resulting solution was stirred for additional three hours at low temperature, before the temperature was slowly raised to room temperature overnight. The reaction mixture was diluted with n-hexane and quenched by addition of 80 ml of saturated ammonium chloride solution. The organic layer was separated and the aqueous layer was extracted twice with n-hexane. The combined organic layers were washed with brine and dried over sodium sulfate. After solvent evaporation, the orange crude oil was purified by column chromatography on silica using n-hexane as eluent. The trimethylsilyl protecting groups were easily cleaved on the column and it was not possible to isolate the product. Therefore the crude product was dissolved in 100 mL of THF and cooled to 0°C. /V-bromosuccinimide (1.8 g, 10.2 mmol) was added and the reaction was stirred during one hour. The reaction progress was followed by TLC and once the bromination had come to completion, the reaction mixture was diluted with 50 mL of n- hexane and quenched by the addition of 100 mL of water. The organic layer was separated and the aqueous layer extracted two more times with hexane (50 mL). The combined organic layers were dried over sodium sulfate and the solvent evaporated. The crude product was purified by column chromatography on silica gel using n-hexane as eluent. After solvent evaporation, compound 5 was recovered as an orange-yellow oily solid (0.700 g, 0.704 mmol, 10% yield). ¹ H NMR (400 MHz, CDCI₃): δ 7.46 (s, 2H), 7.06 (s, 2H), 1.51-1.41 (m, 8H), 1.33-1.08 (m, 48H), 0.84-0.75 (m, 12H). ¹³C NMR (101 MHz, CDCIs) δ 154.35, 142.73, 141.11 , 140.90, 132.63, 125.71 , 1 11.74, 32.91 , 31.87, 29.26, 29.12, 25.42, 22.68, 14.50, 14.13. HRMS (ESI-ToF): m/z calcd for C₄6H₇₂Br₂Ge₂S₂ (M+) 994, 992, 996, 990, 992 found 994, 992, 996, 990, 993. 6)

5 (0.343g, 0.345 mmol) and 7-(boronic acid pinacol ester)-2, 1 ,3-benzothiadiazole-4- carboxaldehyde (0.250g, 0.862 mmol) were dissolved in anhydrous toluene (30 mL) along with a few drops of Aliquat 366, before being degassed for 2 hours.

Tris(dibenzylideneacetone)dipalladiumO (0.0158 g, 0.0172 mmol) and tri(o-tolyl)phosphine (0.0105 g, 0.0345 mmol) were added to the solution before degassing for a further 30 mins. Degassed 2M sodium carbonate solution (1.38 mL, 2.758 mmol) was then added and the reaction was heated to 110 °C with stirring overnight, under an inert argon atmosphere. The mixture was then poured into H₂0, extracted with CH₂CI₂, and the organic phase was washed with H₂0 and brine, before drying over anhydrous magnesium sulphate. The resulting solid was purified by column chromatography over silica using CH₂CI₂, and precipitated out from CH₂CI₂/ methanol to yield 6 as a dark purple solid (0.186g, 0.160 mmol, 46.4% yield). ¹ H NMR (400 MHz, CDCI₃) δ 10.73 (s, 2H), 8.41 (s, 2H), 8.25 (d, J = 7.8 Hz, 2H), 8.05 (d, J = 8.0 Hz, 2H), 7.82 (s, 2H), 1.60-1.51 (m, 8H), 1.40-1.14 (m, 48H), 0.82 (m, 12H). ¹³C NMR (101 MHz, CDCI₃) δ 188.50, 158.54, 153.96, 144.97, 143.06, 141.84, 140.05, 133.71 , 132.89, 126.80, 124.98, 123.40, 32.98, 31.88, 29.29, 29.15, 25.52, 22.67, 14.59, 14.10. HRMS (MALDI-ToF): m/z calcd for

C₆oH₇₈Ge₂N₄0₂S4 (M+) 1 160, 1 158, 1162, 1161 , 1156 found 1 160, 1 158, 1162, 1 161 , 1 156. -GelDTBR

6 (0.150 g, 0.129 mmol) and 3-ethylrhodanine (0.062 g, 0.387 mmol) were dissolved in t- butanol (10 ml_) and the mixture heated to 85 °C with stirring. 2 drops of piperidine were added and the mixture was heated at 85 °C with stirring overnight. The reaction mixture was then poured into water, extracted with CH₂CI₂ and the organic phase was washed with water and brine and dried over anhydrous magnesium sulphate. The crude product was purified by flash column chromatography over silica with CH₂CI₂ and precipitated from CH₂CI₂/methanol and was then recrystallized from acetone to afford O-GelDTBR as a dark blue solid (0.159 g, 0.110 mmol, 85.1 % yield). ¹ H NMR (400 MHz, CDCI₃) δ 8.52 (s, 2H), 8.34 (s, 2H), 8.00 (d, J = 8.0 Hz, 2H), 7.79(s, 2H), 7.72 (d, J = 8.0 Hz, 2H), 4.25 (q, J = 8.0 Hz, 4H), 1.60-1.51 (m, 8H), 1.40-1.14 (m, 54H), 0.82 (m, 12H). ¹³C NMR (101 MHz, CDCIs) δ 193.07, 167.57, 157.73, 154.59, 151.89, 144.86, 143.01 , 141.81 , 140.36, 132.84, 131.34, 130.12, 127.26, 126.68, 124.65, 124.42, 39.94, 32.98, 31.88, 29.30, 29.15, 25.53, 22.68, 14.59, 14.11 , 12.33.

NIDTBR electron acceptor compounds

EH-NIDTBR was synthesised according to scheme 5 and the experimental procedures below.

Scheme 5 Naphthalene-2,6-diyl bis(diethylcarbamate) (7)

10 g (1 equiv.) of naphthalene 2,6-diol were dissolved in THF and added to a stirred suspension of NaH (3 equiv.) in THF at 0°C. Then, the resulting suspension was stirred at 0°C for one hour before 23.7 ml_ of diethylcarbamoyl chloride (3 equiv.) were added dropwise. The reaction was allowed to warm up to room temperature and stirred overnight. Then, the reaction was carefully quenched by adding a few drops of water. Subsequently, the THF was removed by distillation and the residue was extracted with H₂0 and ethyl acetate. The organic layer was washed with aq. KOH (1 M) and H₂0, dried over MgS0₄ and evaporated. The retrieved product could be used without further purification. Yield: (-99%). ¹ H NMR (400 MHz, CDCI₃, δ): 7.77 (d, 2H), 7.57 (dd, 2H), 7.28 (dd, 2H), 3.45 (m, 8H), 1.25 (m, 12H) ¹³C-NMR (100 MHz, CDCI₃, δ): 154.20, 148.74, 131.37, 128.51 , 122.08, 1 18.19, 42.16, 41.82, 14.16, 13.29

N², N², N⁶, N⁶-tetraethyl-3,7-dihydroxynaphthalene-2,6-dicarboxamide (8)

Under an argon atmosphere, 162 ml_ of LDA solution (5 equiv.) were slowly added via syringe to a solution of 23.2 g (1.0 equiv.) of naphthalene-2,6-diyl bis(diethylcarbamate) in THF at -78 °C. The resulting mixture was allowed to warm to room temperature overnight while it turned deep green. Then, the reaction mixture was carefully quenched with HCI 2M solution, and the formed precipitate was filtered off and washed with Et₂0. After drying, a yield of (49 %) of a pale yellow solid were obtained which could be used without further purification. ¹ H NMR (400 MHz, DMSO-d⁶, δ): 9.72 (s, 2H), 7.46, (s, 2H), 7.12 (s, 2H), 3.45 (m, 4H), 3.13 (m, 4H), 1.16 (t, 6H), 1.00 (t, 6H)¹³C NMR (100 MHz, DMSO-d⁶, δ): 167.6, 149.4, 129.0, 128.1 , 124.4, 109.3, 44.2, 42.3, 13.9, 12.9ESI-TOF-MS m/z:

calc'd for C20H27N2O4 [M+] 359.1971 ; found, 359.1989.

Dimethyl 3,7-dihydroxynaphthalene-2,6-dicarboxylate (9)

N²,N²,N⁶,N⁶-tetraethyl-3,7-dihydroxynaphthalene-2,6-dicarboxamide (1 equiv.) were dissolved in DMF and imidazole (3.5 equiv.) was added. Then, TBSCI (3 equiv.) was added portionwise and the reaction mixture stirred at room temperature for 24 h. The reaction was quenched by pouring into water and the resulting white precipitate was filtered off, washed with copious amounts of water, and dried in vacuum. The crude product was dissolved in anhydrous DCM and (CH₃)₃OBF₄ (2.4 equiv.) was added in portions. After consumption of the amide was complete, as indicated by TLC (ca. 18 h), the reaction mixture was evaporated to dryness and methanol was added followed by a saturated solution of Na₂C0₃ and solid Na₂C0₃. The resulting mixture was filtered and acidified with HCI to a pH of 1. The formed solid was recovered by filtration as a first fraction, which could be used without further purification (34%). The organic layer was dried, evaporated and purified by silica gel filtration (chloroform as eluent) to yield a second fraction 49% yield was obtained in total. ¹H NMR (400 MHz, CDCI₃, δ): 10.23 (s, 2H), 8.36 (s, 2H), 7.32 (s, 2H), 4.04 (s, 6H) ¹³C NMR (100 MHz, CDCI₃, δ): only sparingly soluble in chloroform: 130.6 (CH, arom, naptht), 112.7 (CH, arom, naptht), 52.8 (CH₃) 3,7-Di(thiophen-2-yl)naphthalene-2,6-dicarboxylic acid dimethylester (10)

Dimethyl 3,7-dihydroxynaphthalene-2,6-dicarboxylate (1 equiv.) were dissolved

(suspended) in DCM and a few drops of dry pyridine were added. Then, the reaction mixture was cooled to 0°C and (2.2 equiv.) of triflic anhydride were added dropwise. The reaction mixture was allowed to warm up to room temperature and was stirred overnight. Then, water and 2M HCI were added and the aqueous phase was subsequently extracted two times with DCM. The combined organic layers were extracted with sat. NaHC0₃ solution and brine, dried over MgS0₄ and evaporated to dryness. A white solid was retrieved which could be directly used for the next step. A mixture of the crude product, 2- thienylzinc bromide (2.5 equiv.) and Pd(PPh₃)₄ (0.05 equiv) was heated to reflux for 3 h. The reaction was allowed to cool to room temperature and sat. NH₄CI solution was added, after which a white precipitate formed. The product was recovered by filtration, washed with water and methanol and dried in vacuum to give dimethyl 3,7-di(thiophen-2- yl)naphthalene-2,6-dicarboxylic acid dimethyl ester as a pale yellow solid (82%). ¹ H NMR (400 MHz, CDCIs, δ): 8.26 (s, 2H), 8.01 (s, 2H), 7.40 (dd, 2H), 7.12 (m, 4H), 3.81 (s, 6H). A ¹³C NMR could not be recorded due to poor solubility.

4, 10-Dihydro-naphtho[3",2":3,4;7",6":3',4'] dicyclopenta[2,1-b:2',1 '-b'] dithiophene-4, 10- dione (11) To a solution of 3,7-di(thiophen-2-yl)naphthalene-2,6-dicarboxylic acid dimethylester (1 equiv.) in ethanol, a solution of sodium hydroxide (16 equiv.) was added. The reaction mixture was heated to reflux for 15 h. Then, the ethanol was removed on a rotary evaporator. The remaining aqueous solution was then acidified with concentrated hydrochloric acid. The precipitated product was isolated by filtration, washed with water and methanol and dried in vacuo. A crude yellow solid (98%) was obtained which could be used without further purification. To a suspension of 3,7-di(thiophen-2-yl)naphthalene-2,6- dicarboxylic acid (4 equiv.) in anhydrous DCM, oxalyl chloride (1 equiv.) was added, followed by dropwise addition of anhydrous DMF. The resultant mixture was stirred overnight at room temperature. Then, the solvents were removed in vacuo and after drying, the formed crude acid chloride (yellow solid) was redissolved in anhydrous DCM. This solution was then added dropwise (via cannula) to a suspension of anhydrous AICI₃ (4.6 equiv.) in DCM which was cooled to 0°C. The reaction mixture was stirred overnight while being allowed to warm up to room temperature. Then, it was poured onto ice containing HCI. A red precipitate was formed which was collected by filtration and washed with 2M HCI solution, water and acetone. After drying in vacuo, a red solid was obtained. ¹ H NMR (400 MHz, CDCI₃, δ): 7.83 (s, 2H) 7.49 (s, 2H), 7.29 (d, J = 4.8Hz, 2H), 7.21 (d, J = 4.8Hz, 2H) A ¹³C NMR spectrum could not be recorded due to poor solubility.

4, 10-Dihydro-naphtho[3",2":3,4;7",6":3',4']-dicyclopenta[2,1-b:2',1 '-b']-dithiophene (12)

A mixture of 4, 10-dihydro-naphtho[3",2":3,4;7",6":3',4'] dicyclopenta[2, 1-b:2', 1 '-b'] dithiophene-4, 10-dione (1 equiv.), hydrazine monohydrate (20 equiv.) and KOH (20 equiv.) in diethylene glycol was heated at 180 °C for 24 h, then poured into ice containing hydrochloric acid. The precipitate was collected by filtration and washed with water and acetone, and dried in vacuo to give the title compound as pale yellow solid. ¹ H NMR (400 MHz, CDCI3, δ): 7.91 (s, 2H, Ar-H), 7.85 (s, 2H, Ar-H), 7.38 (d, J = 4.8Hz, 2H, Ar-H), 7.15 (d, J = 4.8Hz, 2H, Ar-H), 3.88 (s, 4H, CH₂). A ¹³C NMR spectrum could not be recorded due to poor solubility.

4,4, 10, 10-tetrakis-(2-ethylhexyl)-4,10-dihydro-naphtho[3",2":3,4;7",6'':3',4']- dicyclopenta[2,1-b:2',1 '-b']-dithiophene (13)

To a suspension of 4, 10-dihydro-naphtho[3",2":3,4;7",6":3',4']-dicyclopenta[2, 1-b:2',1 '-b']- dithiophene (1 equiv.) in anhydrous DMSO was added sodium tert-butoxide (6 equiv.) in parts. The reaction mixture was heated at 80 °C for 1 h, followed by the addition of 1- bromohexadecane (6 equiv.) dropwise. After complete addition, the resultant mixture was heated at 85-90 °C for 5 h, then poured into ice-water. The resulting brown solution was extracted with dichloromethane (three times) and the organic layer was dried over magnesium sulfate and evaporated to dryness. The received brown oil was purified by column chromatography on silica, eluting with hexanes, to give a colourless oil (10%). ¹ H NMR (400 MHz, CDCI₃, δ): 7.73 (s, 2H, Ar-H), 7.64 (s, 2H, Ar-H), 7.30 (d, J = 4.8 Hz, 2H, Ar-H), 7.00 (d, J = 4.8 Hz, 2H, Ar-H), 1.99 (m, 8H, CH2), 1.05-0.45 (m, 60H, CH, CH₂ and CH₃) ¹³C NMR (100 MHz, CDCI₃, δ): 151.7, 141.4, 136.5, 131.5, 127.3, 122.6, 121.9, 1 16.2, 77.3, 77.0, 76.7, 53.2, 44.6, 35.0, 28.5, 27.2, 22.8, 14.1 , 10.6. MALDI-TOF-MS: m/z: calc'd for C52H76S2 [C52H76S2+ = M+] 764.5; found, 764.8.

2,8-Dibromo-4,4,10,10-tetrakis-(2-ethylhexyl)-4, 10-dihydro-naphtho[3",2":3,4;7",6":3',4']- dicyclopenta[2, 1 -b:2', 1 '-b']-dithiophene (14)

A solution of 4,4, 10, 10-tetrakis-(2-ethylhexyl)-4,10-dihydro-naphtho[3",2":3,4;7",6":3',4'] dicyclopenta[2,1-b:2',1 '-b']-dithiophene (1 equiv.) in chloroform was cooled to 0°C under argon in the absence of light. N-bromosuccinimide (4.4 equiv.) dissolved in chloroform was added in portions and the reaction progress was monitored by TLC. After full conversion had been detected, the reaction mixture was extracted with water, dried over magnesium sulphate and evaporated to dryness. The crude was purified by column chromatography (using hexanes as mobile phase). This yielded a colourless oil (81 %). ¹ H NMR (400 MHz, CDCI₃, δ): 7.66 (s, 2H, Ar-H), 7.63 (s, 2H, Ar-H), 7.02 (s, 2H, Ar-H), 1.96 (m, 8H, CH₂), 1.05-0.46 (m, 60H, CH, CH₂ and CH₃). MALDI-TOF-MS: m/z: calc'd for C₅2H₇4Br₂S2 [C₅2H₇4Br₂S2⁺ = MH⁺] 922.4; found, 922.8. (15)

2,8-Dibromo-4,4,10,10-tetrakis-(2-ethylhexylW

dicyclopenta[2,1-b:2',1 '-b']-dithiophene (1 equiv.) and 7-(boronic acid pinacol ester)-2, 1 ,3- benzothiadiazole-4-carboxaldehyde (2.5 equiv.) were dissolved in anhydrous toluene along with a few drops of Aliquat 366, before being degassed for 2 hours.

Tris(dibenzylideneacetone)dipalladiumO (0.05 equiv.) and tri(o-tolyl)phosphine (0.1 equiv.) were added to the solution before degassing for a further 30 mins. Degassed 2M sodium carbonate solution (8 equiv.) was then added and the reaction was heated to 110 °C with stirring overnight, under an inert argon atmosphere. The mixture was then poured into H₂0, extracted with CH₂CI₂, and the organic phase was washed with H₂0 and brine, before drying over anhydrous magnesium sulphate. The resulting solid was purified by column chromatography over silica using CH₂CI₂, and precipitated out from CH₂CI₂/ methanol to yield 15.

NIDTBR

15 (1 equiv.) and 3-ethylrhodanine (3 equiv.) were dissolved in t-butanol and the mixture heated to 85 °C with stirring. 2 drops of piperidine were added and the mixture was heated at 85 °C with stirring overnight. The reaction mixture was then poured into water, extracted with CH₂CI₂ and the organic phase was washed with water and brine and dried over anhydrous magnesium sulphate. The crude product was purified by flash column chromatography over silica with CH₂CI₂ and precipitated from CH₂CI₂/methanol and was then recrystallized from anhydrous toluene to afford NIDTBR.

GeNIDTBR electron acceptor compounds

O-GeNIDTBR was synthesised according to scheme 6 and the experimental procedures below.

Scheme 6 2,2'-(3,7-dibromonaphiha!ene-2,6-diyi)bis(3-bromoihiophene) (16)

To an oven dried round bottom flask was added 3,7-dibromo-2,6- bis(trifluoromethanesulfonyloxy)naphthalene (1 equiv.) and (1 ,3- Bis(diphenylphosphino)propane)palladium(ll) chloride (0.05 equiv.). Anhydrous THF was added to dissolve the solids and 0.5 M (3-bromothiophen-2-yl)zinc(ll) bromide solution in THF (2 equiv.) was added dropwise to the resulting solution at 0 °C. The reaction mixture was stirred at room temperature overnight. The mixture was quenched with saturated aqueous ammonium chloride solution and THF was removed to precipitate the solid. The precipitate was filtered off and washed with water, methanol and acetone. The product was recovered as an off-white solid (56% yield). ¹H NMR (400 MHz, CDCI₃): <5 8.21 (s, 2H), 7.89 (s, 2H), 7.46 (d, 2H, J = 5.2 Hz), 7.14 (d, 2H, J = 5.2 Hz). ¹³C NMR (100 MHz, CDCIs): 6 136.5, 133.29, 132.67, 131.79, 131.17, 130.37, 126.57, 123.3, 11 1.92.

(5,5'-(3 -dibromonaphthalene-2,6-diyl)bis(4-bromothiophene-5,2-diyl))bis(trimethylsilane)

(17)

2,2'-(3,7-dibromonaphthalene-2,6-diyl)bis(3-bromothiophene) (1 equiv) was dissolved in anhydrous THF and cooled to -78°C. A 2 M solution of lithium diisopropylamide in

THF/heptanes/ethylbenzene (3 equiv.) was added slowly to the reaction. After complete addition the reaction was stirred for an hour at -78°C and then quenched by the addition of chlorotrimethylsilane (4 equiv.). The reaction mixture was slowly warmed to room temperature and stirred overnight. The solvent was removed under reduced pressure and the crude product was filtered, washed with water followed by methanol to give the product as an off-white solid (85% yield). ¹ H NMR (400 MHz, CDCI₃): δ 8.19 (s, 2H), 7.86 (s, 2H), 7.22 (s, 2H), 0.4 (s, 18H). ¹³C NMR (100 MHz, CDCI₃): δ 142.06, 141.33, 136.5, 133.6, 132.61 , 131.72, 123.06, 112.9, 0.12.

(19)

In an oven dried two-necked round bottom flask, (5,5'-(3,7-dibromonaphthalene-2,6~ diyl)bis(4-bromothiophene-5,2-diyl))bis(trimethylsilane) (1 equiv.) was dissolved in anhydrous THF. In a second dry two-necked round bottom flask anhydrous THF was introduced, which was cooled down to -90°C before a 1.7 M solution of te/f-butyllithium (8.2 equiv.) in pentane was added. The solution containing TMS protected reactant was added dropwise to the f-butyllithium solution at -90°C. After complete addition, the resulting dark brown solution was stirred for two hour at -90°C. Dichloro-di-n-octylgermane (2.2 equiv.) diluted in dry THF was added dropwise to the reaction mixture. The resulting solution was stirred for additional two hours at -90°C, before the temperature was slowly raised to room temperature overnight. The reaction mixture was diluted with petroleum ether and quenched by addition of saturated ammonium chloride solution. The organic layer was separated and the aqueous layer was extracted twice with petroleum ether. The combined organic layers were washed with brine and dried over magnesium sulfate. After solvent evaporation, the orange crude oil was purified by column chromatography on silica using petroleum ether as eluent. Solvent was removed under reduced pressure to give the intermediate product which was dissolved in THF and cooled to 0°C. /V-bromosuccinimide (2.1 equiv. based on the crude intermediate) was added and the reaction was stirred for an hour. The reaction mixture was then diluted with petroleum ether and quenched by the addition of water. The organic layer was separated and the aqueous layer extracted twice with petroleum ether. The combined organic layers were dried over magnesium sulfate and the solvent evaporated. The crude product was purified by column chromatography on silica gel using petroleum ether as eluent. After solvent evaporation, the product was recovered as yellow oil, which slowly crystallized into a solid on storage under

refrigeration (16% yield). (20)

19 (1 equiv.) and 7-(boronic acid pinacol ester)-2,1 ,3-benzothiadiazole-4-carboxaldehyde (2.5 equiv.) were dissolved in anhydrous toluene along with a few drops of Aliquat 366, before being degassed for 2 hours. Tris(dibenzylideneacetone)dipalladiumO (0.05 equiv.) and tri(o-tolyl)phosphine (0.1 equiv.) were added to the solution before degassing for a further 30 mins. Degassed 2M sodium carbonate solution (8 equiv.) was then added and the reaction was heated to 110 °C with stirring overnight, under an inert argon

atmosphere. The mixture was then poured into H₂0, extracted with CH₂CI₂, and the organic phase was washed with H₂0 and brine, before drying over anhydrous magnesium sulphate. The resulting solid was purified by column chromatography over silica using CH₂CI₂, and precipitated out from CH₂CI₂/ methanol to yield 20.

GeNIDTBR

20 (1 equiv.) and 3-ethylrhodanine (3 equiv.) were dissolved in t-butanol and the mixture heated to 85 °C with stirring. 2 drops of piperidine were added and the mixture was heated at 85 °C with stirring overnight. The reaction mixture was then poured into water, extracted with CH₂CI₂ and the organic phase was washed with water and brine and dried over anhydrous magnesium sulphate. The crude product was purified by flash column chromatography over silica with CH₂CI₂ and precipitated from CH₂CI₂/methanol and was then recrystallized from anhydrous toluene to afford GeNIDTBR.

NIDFBR and GeNIDFBR electron acceptor compounds

NIDFBR may be synthesised according to scheme 7, below.

Scheme 7 GeNIDFBR may be synthesised according to scheme 8, below.

Scheme 8

Example 2 - Optoelectronic Properties of Electron Acceptors

Figure 2a shows the absorption spectra of P3HT, IDFBR and IDTBR. The absorption profiles of both acceptors are quite different, depending on the structure of the middle core. In comparison to the IDFBR, reduced steric twisting from adjacent alpha C-H bonds on the coupled phenyl rings and increased quinoidal character of the phenyl-thienyl bond in IDTBR leads to enhanced planarity and more electron-rich thiophene-based core of IDTBR acts to raise the highest occupied molecular orbital (HOMO), leading to a significantly red-shifted UV-vis absorption spectrum relative to that of IDFBR. In the as- cast thin film, the absorption of IDTBR is red-shifted by 170 nm relative to that of IDFBR and a further 41 nm bathochromic shift is observed for IDTBR upon annealing above 110 °C. By contrast, no apparent red shift was observed for IDFBR. The LUMO energy levels of P3HT, IDFBR and IDTBR were measured to be -3.2, -3.7 and 3.9 eV; with HOMO energy levels of -5.2, -5.7 and -5.5 eV, respectively, determined by cyclic voltammetry (Figure 2b). Table 1. Optoelectronic Properties of IDTBR and IDFBR Electron Acceptors

Measured in ^a) CHCI₃ solution; ^b> thin film spin-coated from 10 mg ml^"1 chlorobenzene solution; ^c) thin film annealed at 130 °C for 10 min; ^d) cyclic voltammetry carried out on the unannealed thin film with 0.1 M TBAPF₆ electrolyte in acetonitrile; ^e) estimated from the electrochemical EA and the optical E_g.

Example 3 - Device Data

The photovoltaic performances of the binary P3HT: IDTBR and ternary blends are measured with a device architecture comprising: indium tin oxide (ITO)/ zinc oxide (ZnO)/active layer (90±5 nm) /molybdenum oxide (Mo0₃)/Ag, where the active layer consists of binary P3HT:electron acceptor compound or ternary P3HT:IDTBR:second electron acceptor compound blends. The active layer blends were spin-coated from chlorobenzene solution under ambient conditions without the use of additives. Thermal annealing (10 min at 130 °C) was carried out to promote ordering of the polymer, as is typical in P3HT solar cells, as well as to induce acceptor crystallisation. The weight ratio of IDFBR in IDTBR was changed from 10-70%. In all cases, the active layers were spin coated from chlorobenzene (CB) under ambient conditions and for all acceptor ratios; the weight ratio of acceptor to P3HT was kept at 1 : 1. Figure 3 shows the JV and EQE Spectra of binary and ternary blends under simulated AM 1.5 G illuminations at l OOmWcm^"2.

Table 2. Photovoltaic performance of optimised IDTBR:IDFBR:P3HT ternary solar cells measured under simulated AM1.5G illumination at 100 mW cm^-2. Average values are obtained from 10-15 devices. The IDTBR and IDFBR compounds are all octyl substituted, i.e. O-IDTBR and O-IDFBR.

The P3HT:IDTBR device exhibits a J_sc and FF of 13.9 mA/cm² and 0.60, respectively and a relatively high V_∞ of 0.73 V for P3HT based solar cells (0.58V for P3HT:60PCBM) (Table S3) which results in a PCE of 6.3%. By adding 30% (with respect to IDTBR) 60PCBM into the P3HT:IDTBR (1 :0.7) blend, all J_sc, V_∞ and FF values substantially decrease with an overall efficiency of 3.6 % mainly due to reduced V_∞ (0.59V) and low FF values around 0.5 compared to binary devices. Low FF values of P3HT:IDTBR:60PCBM devices are an indication of immiscibility and suboptimal morphology in the blend. (ref) FBR is then used to replace 60PCBM, as the third component, which is structurally more similar to IDTBR compared to 60PCBM, is expected to facilitate improved mixing of the acceptor components. FBR can replace 60PCBM and works better with P3HT

(4.1 %).P3HT: IDTBR:FBR (1 :0.7:0.3) ternary solar cells exhibits an increased V_∞of 0.8V compared to binary blend. However, the overall efficiency did not surpass the binary blend, mainly caused by lower J_sc values due to an effective dilution of the ratio of IDTBR in the blend, which absorbs low energy photons in the visible region.

Finally, the third component is selected to be IDFBR. The optimized P3HT:IDFBR reference binary devices are achieved with 1 :1 (w:w) ratio in the same solvent as P3HT:IDTBR system (CB) with a remarkably high V_∞ and FF values up to 0.88V and 0.64, respectively, with an overall efficiency of 4.5%. The small amount (10% by weight) addition of IDFBR as third component into P3HT:IDTBR binary blend increased both J_sc and V_oc in the ternary blend with a slightly lower FF values. The best ternary

P3HT:IDTBR: IDFBR (100:70:30) devices exhibit a J_sc of 14.4 mAcm^"2 and a 100 meV higher V_∞ than the binary P3HT:IDTBR blend of 0.82V and relatively high FF of 0.64 with a remarkable power conversion efficiency of 7.7% which means an average of over 20% PCE improvement compared to binary P3HT:IDTBR devices. Further addition (above 30%) of IDFBR did not further increase the V_oc or FF values (up to 70%) but decreased the Jsc mainly due to diluted IDTBR amount in the ternary blend, and subsequently lower photocurrent generation.

It is also noteworthy to mention that P3HT:IDTBR:IDFBR devices retain the high FF values (65%) with slightly lower V_oc and J_sc (0.78 V and 11.3 mAcm^"2) with an overall efficiency of 5.7% even at high thicknesses (-200 nm). Furthermore, larger area

P3HT:IDTBR:IDFBR devices (~1 cm²) were also successfully demonstrated with efficiencies as high as 6.5%, with the slightly lower efficiencies attributed to the lower FF.

External quantum efficiency (EQE) spectra of both binary and ternary blends are shown in Figure 3b. The P3HT: IDTBR binary device exhibits an EQE of around 55% across the visible region between 400 and 800 nm. In comparison, photo-response enhancement in the 400-500 nm region of ternary blends compared to P3HT: IDTBR blend shows the distinctive photocurrent contribution from third components. However, low absorption strength of 60PCBM and FBR and the blue shift in the onset of EQE from 800 to 750 nm limits the overall photocurrent obtained from these ternary blends compared to binary devices. On the contrary, the stronger absorption strength of IDFBR contributes to the higher EQE than P3HT:IDTBR. The blue shift in EQE of ternary blends is in agreement with the absorption profiles shown in Figure 2c suggesting that the presence of a third component has a significant impact on the blend morphology.

Due to higher Jsc, better FF and Voc, the PCE of ternary blends significantly surpassed the individual reference binary blends. The devices with 60-70% IDTBR weight ratio with respect to the other acceptor component have shown PCE between 7.2-7.7%. This value is the highest reported for a non-fullerene ternary blend with P3HT as the donor polymer. To understand the impact of third component and especially the changes in EQE spectra and Voc, it is important to gain information about the film morphology of ternary system. It is important to know how third component is effecting the host matrix, either it is located at donor:acceptor interface, embedded in one of the host domain, forms own channel or an alloy. Surface energy, miscibility and crystallinity of the host matrix are shown to play an important role in determining the position of the third component and interactions between third component and the host can lead to unexpected changes in morphology, resulting in different performances. To study the role of IDFBR on the morphology in the ternary blend and gain insight into the thermal phase behaviour and molecular orientation, differential scanning calorimetry (DSC) of ternary blends was carried out. Figure 5 shows the DSC profiles of P3HT, IDTBR and IDFBR neat, binary and the best P3HT:IDTBR: IDFBR blend. The heat flow profiles reveal that both the IDFBR and IDTBR binary blends with P3HT exhibits broad endothermic transitions at temperatures above 200 °C, attributed to a P3HT crystalline phase melt. In comparison to pristine P3HT film, the melting transition of P3HT is significantly broadened and supressed in all blends. The P3HT melting transition enthalpy for the IDFBR binary blend is also reduced by a factor of 5, whereas it is only slightly lower than the pristine P3HT in the case of the IDTBR blend. This difference indicates that both small molecules can diffuse into the P3HT phases, being more prominent for IDFBR leading to extensive disorder in the polymer. However, in the P3HT:IDTBR blend film the endotherm is prominent, although its peak has broadened, and there is a reduction in melting temperature and melting enthalpy. In the ternary blend film, the P3HT crystalline phase still persists, with a broad melt endotherm. The P3HT melting point and melting enthalpies in the ternaries are intermediate between those of the respective binaries. The IDTBR crystalline transition in the ternary blends exhibits a melting point depression and lower enthalpy in comparison to the IDTBR binary film, indicating that the IDFBR has been able to also diffuse into the IDTBR phase. No evidence of any IDFBR thermal transitions is present. A cooling scan shows a strongly super-cooled

crystallisation of P3HT, but no small molecule crystallisation. The ternary film, therefore, can be described as having three partially miscible components, comprising of a crystalline P3HT phase, which also hosts a molecular dispersion of both IDFBR and IDFBR molecules, as well as an IDTBR rich crystalline phase that also contains IDFBR. There is also likely to be a disordered phase containing a molecularly dissolved mix of the three components. Example 4 - Stability Data

Both efficiency and stability are essential for the development of OPV materials on a commercial scale. Recently, several low-bandgap polymers demonstrated very high efficiencies but device fabrication and measurement are carried out under inert conditions. In most of these devices, solvent additives were used to obtain high performances. By contrast, it has been shown that IDTBR:P3HT devices could be processed and measured in air, excluding a brief thermal annealing step. To explore the oxidative stability of the ternary solar cells, OPV devices of the best ternary blend IDTBR:IDFBR:P3HT (0.7:0.3:1) was chosen and J- \/ characteristics were measured over the course of 1200 hours of exposure to air alongside reference P3HT:IDTBR, 60PCBM:P3HT devices, as well as devices with the high performance polymers PTB-7, PCE-10 and PCE-1 1. The reduction in normalised PCE over time exposed to air for each blend is shown in Figure 6. It is clear from this data that the ternary blend device performance shows the least degradation of the materials studied, and retains value above 80%. By contrast, the efficiency of the commercial polymers, PCE-10, PCE-1 1 and PTB-7 drops to below 50% of its initial value after 110 hours in air and it falls to zero by the end of the period of study.

Embodiments of the invention have been described by way of example only. It will be appreciated that variations of the described embodiments may be made which are still within the scope of the invention.

Claims

Claims:

1. A composition comprising a blend of two or more organic electron acceptor compounds and an organic electron donor compound, wherein at least one of the electron acceptor compounds is a compound of formula (I)

T¹-(B¹)_a-(A)-(B²)_b-T² Formula (I)

wherein

A is a divalent conjugated fused ring system having the structure:

wherein:

X_! is C, Ge or Si;

Cy^1"10 are, at each occurrence, independently, absent or a 5 or 6-membered ring having 0, 1 or 2 ring heteroatoms, or a fused polycyclic (for example, bicyclic or tricyclic) ring optionally having one or more ring heteroatoms, provided that at least one of Cy^1"5 and at least one of Cy^6"10 is not absent, and wherein each of Cy^1"10, when present, is optionally substituted by one or more groups R²;

R² is, at each occurrence, independently, halo, Ci.₃₀ aliphatic, aryl, heteroaryl, =0,

=S, =R°, -CN, -NC, -NCO, -NCS, -OCN, -SCN, -C(=0)NR°R°°, -C(=0)X°, -C(=0)R°, - C(=0)OR°, -C(=S)R°, -C(=S)OR°, -OC(=0)R°, -OC(=S)R°, -C(=0)SR°, -SC(=0)R°, -NH₂, - NR°R⁰⁰, -NR°C(0)R°, -SH, -SR°, -S0₃H, -S0₂R°, -OH, -N0₂, -CF₃, -CF₂-R°, -SF₅, silyl or hydrocarbyl with 1 to 40 C atoms and which optionally comprises one or more hetero atoms, wherein Ci_₃₀ aliphatic, aryl, heteroaryl, silyl or hydrocarbyl are optionally substituted, and wherein X° is halogen and R° and R⁰⁰ are, independently, H or optionally substituted Ci.₄₀ hydrocarbyl; or two R², with the intervening atoms form an optionally substituted fused ring, having 0, 1 or 2 ring atoms;

Y¹ and Y² are, independently, H, F, CI or CN;

a and b are, independently of each other, 0, 1 or 2; and

The compound , wherein each of Cy^1"10 are, at each occurrence,

independently, absent,

a 5 or 6-membered aromatic ring having 0, 1 or 2 ring heteroatoms (preferably phenyl or thiophenyl), each optionally substituted by one or more groups R².

, optionally wherein Cy^1"10 are, at each occurrence, independently a 5 or 6-membered aromatic ring having 0, 1 or 2 ring heteroatoms, each optionally substituted by one or more groups R², optionally wherein A is

e groups R².

4. The composition of any one of the preceding claims, wherein T¹ and T² are, independently of each other, -CR⁴=Y, -CR⁴=CR⁴-Y, -L-Y or -Y; Y is an optionally substituted cyclic hydrocarbyl group, preferably optionally substituted aryl or heteroaryl; and

L is a divalent alkylenyl chain of 3 to 10 carbon atoms, having alternating double and single bonds, optionally substituted by one or more R⁴; and

R⁴ is H or has the meaning of R², preferably wherein R⁴ is H.

5. one of T¹ or T² is -CR⁴=Y, and Y is:

in which * marks the point of attachment to -CR⁴=;

X² is S, O or C(R⁶)₂;

W is S, O or C(R⁶)₂;

R⁵ is H, halo, aliphatic, heteroaliphatic, aryl, heteroaryl, -CN, -NC, -NCO, -NCS, - OCN, -SCN, -C(=0)NR°R°°, -C(=0)X°, -C(=0)R°, -C(=0)OR°, -C(=S)R°, -C(=S)OR°, - OC(=0)R°, -OC(=S)R°, -C(=0)SR°, -SC(=0)R°, -NH₂, -NR°R⁰⁰, -NR°C(0)R°, -SH, -SR°, - S0₃H, -S0₂R°, -OH, -NO2, -CF₃, -CF2-R⁰, -SF₅, silyl or hydrocarbyl with 1 to 40 C atoms and which optionally comprises one or more hetero atoms, wherein aliphatic,

heteroaliphatic, aryl, heteroaryl, silyl or hydrocarbyl are optionally substituted, and wherein X° is halogen and R° and R⁰⁰ are, independently, H or optionally substituted C1-40 hydrocarbyl, preferably optionally substituted aliphatic, heteroaliphatic, aryl or heteroaryl;

R⁶ is, at each occurrence, independently, H, halo, aliphatic, heteroaliphatic, aryl, heteroaryl, -CN, -NC, -NCO, -NCS, -OCN, -SCN, -C(=0)NR°R°°, -C(=O)X⁰, -C(=O)R⁰, - C(=0)OR°, -C(=S)R°, -C(=S)OR°, -OC(=O)R⁰, -OC(=S)R°, -C(=O)SR⁰, -SC(=O)R⁰, -NH₂, - NR°R⁰⁰, -NR°C(0)R°, -SH, -SR°, -SO₃H, -S0₂R°, -OH, -N0₂, -CF₃, -CF₂-R°, -SF₅, silyl or hydrocarbyl with 1 to 40 C atoms and which optionally comprises one or more hetero atoms, wherein aliphatic, heteroaliphatic, aryl, heteroaryl, silyl or hydrocarbyl are optionally substituted, and wherein X° is halogen and R° and R⁰⁰ are, independently, H or optionally substituted C1-40 hydrocarbyl;

R⁸ is, at each occurrence, independently, halo, aryl, heteroaryl, -CN, -NC, -NCO, - NCS, -OCN, -SCN, -C(=0)NR°R°°, -C(=O)X⁰, -C(=O)R⁰, -C(=O)OR⁰, -C(=S)R°, - C(=S)OR°, -OC(=0)R°, -OC(=S)R°, -C(=0)SR°, -SC(=0)R°, -NH₂, -NR°R⁰⁰, -NR°C(0)R°, - SH, -SR°, -SO₃H, -SO2R⁰, -OH, -NO2, -CF₃, -CF2-R⁰, -SF₅, silyl or hydrocarbyl with 1 to 40 C atoms and which optionally comprises one or more hetero atoms, wherein aryl, heteroaryl, silyl or hydrocarbyl are optionally substituted, and wherein X° is halogen and R° and R⁰⁰ are, independently, H or optionally substituted C1-40 hydrocarbyl; and

n is 0-4.

6.

R₅ is C1-12 aliphatic.

7. The composition of any one of the preceding claims, wherein at least one of T¹ and T² is -CR⁴=CR⁴-Y or -Y and Y is:

m is 0-3 and o is 0-2.

8. The composition of any one of the preceding claims, wherein a and b are both 1.

9. The composition of any one of the preceding claims, wherein each occurrence of of B¹ and B² is, independently, mono-, bi- or tri-cyclic aryl or heteroaryl, unsubstituted or substituted by one or more groups R³, wherein the aryl or heteroaryl group may optionally include a non-aromatic carbocyclic or heterocyclic ring fused thereto.

10. The composition of any one of the preceding claims, wherein one or more occurrences of B¹ and B² is:

p is 0, 1 or 2; and

1 1. The composition of any one of the preceding claims, wherein the compound of formula (I) is

74

12. The composition of any one of the preceding claims, wherein the compound of formula (I) is

5

77



81

82

13. The composition of any one of the preceding claims, wherein the composition comprises a first electron acceptor compound and a second electron acceptor compound, wherein both the first and the second electron acceptor compounds are a compound of formula (I) as defined in any one of claims 1 to 12, wherein the first and electron acceptor compounds of formula (I) are not the same.

14. The composition of any one of the preceding claims, wherein the electron donor is a polymer or small molecule light absorber.

15. The composition of claim 14, wherein the electron donor is poly(3-hexylthiophene- -diyl) (P3HT) or a polymer of structure

, wherein n is 1-20000.

16. The composition of any one of the preceding claims, wherein the composition comprises a first electron acceptor compound and a second electron acceptor compound and wherein the second electron acceptor compound has an electron affinity and ionization potential between that of the electron donor and the first electron acceptor compound.

17. The composition of any one of the preceding claims, wherein the composition is provided in the form of a bulk material or a film.

18. The composition of any one of the preceding claims, wherein the composition

and a P3HT electron donor, preferably wherein the composition comprises IDTBR and IDFBR at a weight ratio of 0.6-0.7:0.3-0.4.

19. An optical or electronic device comprising a composition according to any one of the preceding claims.

20. The device of claim 19, wherein the device is a photovoltaic cell (optionally an organic solar cell), an organic transistor, a light emitting diode, a photodetector or a photocatalytic device.

21. The device of claim 20, wherein the device further comprises an anode and a cathode.

22. The device of claim 21 , wherein the composition forms an active layer between the anode and the cathode.

23. The device of any one of claims 19 to 22, wherein the device is an organic solar cell comprising a bulk heterojunction active layer comprising the composition according to any one of claims 1 to 16. 24. The device of any one of claims 19 to 23, wherein the device further comprises a hole transport layer and an electron transport layer.

25. A process for producing a composition according to any one of claims 1 to 18, the process comprising:

selecting a first organic electron acceptor compound;

selecting a second organic electron acceptor compound

selecting an organic electron donor; and

blending the compounds to provide the composition. 26. The process of claim 25, wherein the first and second organic electron acceptors and the organic electron donor are selected such that the second electron acceptor compound has an electron affinity and ionization potential between that of the electron donor and the first electron acceptor compound. 27. A process for producing a device as claimed in any of claims 19 to 24, comprising providing a substrate; and

depositing a composition according to any one of claims 1 to 18 on a surface of the substrate to form an active layer. 28. The process of claim 27, wherein the process further comprises depositing an electrode on the active layer.

29. A composition, device or process as substantially described herein with reference to or as illustrated in one or more of the examples or accompanying figures.