WO2019215448A1

WO2019215448A1 - Polythiophene electron donors in organic devices

Info

Publication number: WO2019215448A1
Application number: PCT/GB2019/051278
Authority: WO
Inventors: Andrew WADSWORTH; Zeinab HAMID; Iain Mcculloch; Jun Yan; Elham REZASOLTANI; Anne A.Y. GUILBERT; Jenny NELSON
Original assignee: Imperial College Of Science, Technology And Medicine
Priority date: 2018-05-09
Filing date: 2019-05-09
Publication date: 2019-11-14
Also published as: GB201807553D0

Abstract

The invention relates to compositions comprising a blend of an organic electron acceptor compound and an organic electron donor compound and their use in organic optical or electronic devices.

Description

POLYTHIOPHENE ELECTRON DONORS IN ORGANIC DEVICES

FIELD

The invention relates to organic blends comprising a polythiophene electron donor and non-fullerene electron acceptors for use in organic optical or electronic devices.

BACKGROUND

Since their inception over twenty years ago, organic solar cells (OSCs) have progressed from single component layers of organic semiconducting materials to highly tuned, multiple component blends. This added complexity in the design of devices has been mirrored by the increasing complexity of the organic semiconductors used, where rational design has allowed fine tuning of morphological and optoelectronic properties of the active layer blend, and has resulted in power conversion efficiencies (PCEs) of over 10% to now become commonplace. The most popular strategy that has been employed is the use of a bulk heterojunction, in which an electron donor polymer is paired with a small molecule electron acceptor. Until recently, fullerene-based electron acceptors, such as phenyl-C_6i butyric acid methyl ester (PC_6IBM), had been dominant in the field of OSCs; owing to their exceptional electron accepting and transport properties, in addition to their ability to form domains on the scale of the exciton diffusion length (-10 nm in organic semiconductors) when blended with donor polymers. However, fullerene-containing devices have not been able to exceed a PCE of 7% with the early wide bandgap donor polymers developed for OSCs, such as poly (3-hexylthiophene) (P3HT), poly (2-methoxy-5-(2-ethylhexyloxy)-1 ,4- phenylenevinylene) (MEH-PPV) and polyfluorenes, though PCEs are often much lower than the record value. This can be traced back to a number of drawbacks that fullerene containing blends possess; (i) the weak absorption of visible light exhibited by fullerenes, which hampers the active layer’s ability to efficiently harvest photons in the visible region of the solar spectrum, (ii) the deep lying lowest unoccupied molecular orbital (LUMO) of fullerenes, which limit the voltage an OSC can achieve and (iii) the long-term

morphological instability of fullerenes in polymer blends, resulting in device failure within a matter of days.

The poor optical properties of fullerenes were mitigated, more recently, by the use of donor polymers, which were tuned to absorb in more abundant regions of the solar spectrum. The push-pull hybridization, which occurs in donor-acceptor copolymers, was utilized in order to create narrow bandgap materials that were able to compensate for the lack of visible light absorption by fullerenes. This strategy, along with further tuning of the structural properties of the donor polymers to form optimal blend morphologies, led to great strides in improving the performance of OSCs. Despite this success, the added synthetic complexity of the donor polymers required to produce such device performance render them incompatible with the large-scale industrialization of OSCs.

More recently, non-fullerene acceptors (NFAs) have been developed to replace the traditionally favored PC61BM, and its analogues. The main principle behind their design was to maintain the favorable electron accepting and charge transport properties of fullerenes, whilst also improving the Voc that can be achieved in devices, through the development of materials with higher lying LUMOs. Additionally, improved visible light absorption and reduced aggregation tendencies of the acceptors was preferable to maximize the performance in devices. The emergence of acceptor-donor-acceptor (A-D- A) NFAs has led to great progress in the field of OSCs in recent times. These acceptors consist of an electron rich core flanked by electron deficient peripheral units; making use of push-pull hybridization to create low bandgap materials with high absorption coefficients, without the need for such large planar units in most cases. Another benefit of these acceptors is the ease with which their frontier molecular orbitals (FMOs) can be tuned by varying the strength or electron-rich core, the electron-poor peripheral units, or the inclusion of electron withdrawing (or donating) functional groups. This allows the A-D- A acceptors to achieve exceptional open circuit voltage (Voc) in devices. The donor polymers most commonly used with both perylene diimide and A-D-A acceptors are medium or low bandgap polymers such as PTB7-Th, PffBT4T-20D and PBDB-T.

Although the pairing of NFAs with these complex push-pull copolymers has yielded record breaking OSCs, with several examples of 1 1-13% PCEs being achieved, the large-scale viability of such acceptors is a significant hurdle (A. Wadsworth, et al., ACS Energy Lett. 2017, 2, 1494-1500, the contents of which are herein incorporated by reference in its entirety).

P3HT is a donor polymer that is suitable for the large-scale commercialization of OSCs, due to its relatively straightforward synthesis and availability of monomer. P3HT is a wide bandgap donor homopolymer (1.9 eV) that consists of a repeating alkylated thiophene monomer unit. The planar nature of thiophene-thiophene linkages renders P3HT a relatively crystalline polymer with extended tt-electron delocalization along its backbone, as such it has been shown to have good charge transport properties and is often able to form domains on the appropriate lengthscale when blended with an acceptor. As a simple homopolymer, only one monomer is needed, the synthesis is facile and comparatively cheap to most alternative donor polymers, and P3HT has been synthesized in flow with excellent control over the number average molecular weight (M_w), polydispersity index (PDI) and regioregularity (RR) (J.H. Bannock et al., Adv. Funct. Mater. 2013, 23, 2123- 2129, the contents of which are herein incorporated by reference in its entirety). Whilst the best performing single-junction P3HT devices are not able to replicate the >10% PCEs seen with a number of the push-pull copolymers, they have been reported to achieve respectable efficiencies of 5-7% to date (S. Holliday et al., Nat. Commun. 2016, 7, 11585, D. Baran et al., Nat. Mater. 2017, 16, 363-369, the contents of which are herein incorporated by reference in their entirety). It is likely that P3HT-based OSCs can present a strong case for commercialization, by improving upon the PCE and stability that can be achieved. A number of NFA are disclosed in WO2017191468 and WO2017191466, the entire contents of which are herein incorporated by reference in their entirety.

The microstructural characteristics of the polymensmall molecule blends should be assessed to ensure optimal device performance can be attained. The influence of the properties of the donor polymer (for example, P3HT), on device performance, has been probed in fullerene-containing devices, but very few studies involving fullerene-free organic photoelectric devices (such as OSCs) have been undertaken.

Thus, there remains a need for an optimized donor polymer for use with NFA acceptor compounds in organic photoelectric devices.

SUMMARY OF INVENTION

The invention provides optimized electron donor-acceptor blends based on polythiophene donors (such as P3HT) for use in organic optical or electronic devices (such as organic solar cells). Such devices provide greater power conversion efficiencies than previously reported devices based on P3HT and non-fullerene acceptor compounds.

Accordingly, in a first aspect, the invention provides a composition comprising a blend of an organic electron acceptor compound and an organic electron donor compound, wherein the organic electron donor compound is a polythiophene having a number average molecular weight (M_w) of about 25 to about 60 kDa, and wherein the electron acceptor compound 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:

Xi is C, Ge or Si;

R¹ is, at each occurrence, independently, H, or optionally substituted Ci_-3o 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, C_1-30 aliphatic, aryl, heteroaryl, =0, =S, =R°, -CN, -NC, -NCO, -NCS, -OCN, -SCN, -C(=O)NR⁰R⁰⁰, -C(=O)X⁰, -C(=O)R⁰, - C(=O)OR⁰, -C(=S)R°, -C(=S)OR°, -OC(=O)R⁰, -OC(=S)R°, -C(=O)SR⁰, -SC(=O)R⁰, -NH₂, - NR°R⁰⁰, -NR⁰C(O)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 C_1-30 aliphatic, aryl, heteroaryl, silyl or hydrocarbyl are optionally substituted, and wherein X° is halogen and R° and R⁰⁰ are, independently, H or optionally substituted C_1-40 hydrocarbyl; or two R², with the intervening atoms form an optionally substituted fused ring, having 0, 1 or 2 ring atoms;

each occurrence of B¹ and B² is, independently, -CY¹=CY²-, -CºC-, 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, Cl 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.

The organic electron donor compound may be a polythiophene comprising a repeat unit having formula:

wherein, each R^T is independently selected from the group comprising:

(i) H;

(ii) C,-2o aliphatic, preferably C3-15 aliphatic, C3-12 aliphatic or C3-10 aliphatic;

(iii) aryl optionally substituted with C1-20 aliphatic, preferably aryl optionally substituted with C3-15 aliphatic, C3-12 aliphatic or C6-10 aliphatic;

(iv) -C(O)O-Ci-20 aliphatic, preferably -C(0)0-C3-is aliphatic or -C(0)0-Cio-i₅ aliphatic;

(v) -O-C,-2o aliphatic, preferably -O-C3-15 aliphatic; and

(vi) a polyalkylene glycol chain having 2-20 alkylene glycol units, preferably a polyethylene glycol chain having 2-20 alkylene glycol units.

Each R^T may preferably be independently selected from H, C3-15 aliphatic, aryl optionally substituted with C3-15 aliphatic, and -C(0)0-C₃-is aliphatic. Preferably, within a polythiophene organic electron donor compound, each R^T is:

(i) independently H or C_3-15 aliphatic;

(ii) independently H or aryl optionally substituted with C_3-15 aliphatic; or

(iii) independently H or -C(0)0-C₃-is aliphatic.

Each R^T may preferably be C3-12 aliphatic or C3-10 aliphatic.

The organic electron donor compound may be poly(3-hexylthiophene-2,5-diyl) (P3HT), poly(3-(4-octylphenyl)thiophene) (P30T), or poly[5,5'-bis(2-butyloctyl)-(2,2'-bithiophene)- 4,4'-dicarboxylate-alt-5,5'-2,2'-bithiophene (PDCBT), optionally P3HT. The organic electron donor compound may have a number average molecular weight of about 30 to about 50 kDa, preferably about 30 to about 40 kDa, preferably about 30 to about 38 kDa preferably about 34 kDa.

The regioregularity of the organic electron donor compound is about 90 to about 98 %, preferably about 91 to about 97 %, about 92 to about 96 %, about 93 to about 95 %, preferably about 94 %.

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 Xi is C, Ge or Si. Each

of Cy^{1 10} is preferably, 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. For example, in a compound of Formula (I) as defined herein, A may preferably be selected from:

are as defined above. Preferably,

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 R², preferably

optionally substituted by one or more groups

R²

Preferably Xi is C or Ge.

In any of the structures illustrated above, R¹ may, at each occurrence, independently, be C-i-30 aliphatic or aryl optionally substituted with C-MO aliphatic, preferably C6-10 aliphatic (for example, linear or branched Cs aliphatic) or aryl (for example, phenyl) substituted with C1-6 aliphatic.

In any of the structures illustrated above, R² may, at each occurrence, independently, be H, optionally substituted C1-30 aliphatic, aryl or heteroaryl, preferably C1-30 aliphatic or aryl substituted with C-MO aliphatic, preferably C_6-8 aliphatic (for example, linear or branched Cs 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(=O)NR⁰R⁰⁰, -C(=O)X⁰, -C(=O)R⁰, -C(=O)OR⁰, -C(=S)R°, -C(=S)OR°, - OC(=O)R⁰, -OC(=S)R°, -C(=O)SR⁰, -SC(=O)R⁰, -NH₂, -NR°R⁰⁰, -NR⁰C(O)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 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(=O)NR⁰R⁰⁰, -C(=O)X⁰, -C(=O)R⁰, - C(=O)OR⁰, -C(=S)R°, -C(=S)OR°, -OC(=O)R⁰, -OC(=S)R°, -C(=O)SR⁰, -SC(=O)R⁰, -NH₂, - NR°R⁰⁰, -NR⁰C(O)R°, -SH, -SR°, -SO3H, -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;

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(=O)NR⁰R⁰⁰, -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°, - SO3H, -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 aryl, heteroaryl, silyl or hydrocarbyl are optionally substituted, and wherein X° is halogen and R° and R°° 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 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 C-i-s 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(=O)NR⁰R⁰⁰, -C(=O)X⁰, -C(=O)R⁰, -C(=O)OR⁰, -C(=S)R°, -C(=S)OR°, - OC(=O)R⁰, -OC(=S)R°, -C(=O)SR⁰, -SC(=O)R⁰, -NH₂, -NR°R⁰⁰, -NR⁰C(O)R°, -SH, -SR°, -

SO3H, -S0₂R°, -OH, -N0₂, -CF3, -CF₂-R°, -SF_5I 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 R°° are, independently, H or optionally substituted C1 -40 hydrocarbyl. A compound of Formula (I) as defined herein, may preferably be selected from

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

5



. Preferably, R¹ is C-i-₈ aliphatic, preferably -CsHiz 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:



20

21

22





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 100pm or less, preferably from about 5nm to about 100pm, more preferably from about 5 to about 500nm.

A composition of the invention may preferably comprise IDTBR (optionally O-IDTBR), and/or P3HT, optionally having a M_w of about 34 kDa, and optionally a regioregularity of about 94 %. The composition may preferably comprise about 20 to about 50 wt% IDTBR.

A composition of the invention may comprise a second organic electron acceptor compound of formula (I). Such a composition may be referred to as a ternary blend.

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 form 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 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 fourth aspect, the invention provides the use of a composition described herein as an active layer in an optical or electronic device, optionally an organic solar cell.

In a fifth aspect, the invention provides the use of a polythiophene (optionally P3HT) having a number average molecular weight (M_w) of about 25 to about 60 kDa as an organic electron donor compound in an optical or electronic device (for example, a device as described herein), in combination with one or more organic electron acceptor compounds, wherein the one or more organic electron acceptor compounds are independently as described herein.

In a sixth aspect, the invention provides a composition, device, use or process as substantially described herein with reference to or as illustrated in one or more of the examples or accompanying figures.

Embodiments described herein in relation to the first aspect of the invention apply mutatis mutandis to the second to sixth aspects of the invention.

BRIEF SUMMARY OF FIGURES

Figure 1 shows a) structures of P3HT and the IDTBR acceptors, b) box plots of the PCE achieved in P3HT:0-IDTBR devices, with varying polymer M_w, c) box plots of the PCE achieved in P3HT:EH-IDTBR devices, with varying polymer M_w.

Figure 2 shows a) box plots of the photocurrent (Jsc) of P3HT:0-IDTBR devices, with varying M_w, b) box plots of the open circuit voltage (Voc) of P3HT:0-IDTBR devices, with varying M_w, c) box plots of the fill factor (FF) of P3HT :0-IDTBR devices, with varying M_w, d) box plots of the photocurrent (Jsc) of P3HT:EH-IDTBR devices, with varying M_w, e) box plots of the open circuit voltage (Voc) of P3HT:EH-IDTBR devices, with varying M_w, f) box plots of the fill factor (FF) of P3HT:EH-IDTBR devices, with varying M_w.

Figure 3 shows (a) normalized photoluminescence spectrum (at open circuit condition) and (b) normalized electroluminescence spectrum with 30 mA injection current density for P3HT:0-IDTBR devices, with varying P3HT M_w

Figure 4 shows (a) extended external quantum efficiency and electroluminescence; (b) voltage losses comparisons with respect to varying P3HT M_w in P3HT:0-IDTBR devices. Figure 5 shows phase diagrams of different molecular weight P3HTs and (EH-) and O- IDTBR binaries obtained on the basis of the DSC thermograms. The endset of the melting transition at each composition was used a) P3HT: EH-IDTBR, 20 kDa (red) 34 kDa (blue) and 94 kDa (grey) the eutectic point relative to the EH-IDTBR is located at 0.5 for P3HT 20 kDa and 94 kDa, b) P3HT: O-IDTBR 20 kDa (red), 35 kDa (blue), 64 kDa (green) and 94 kDa (grey).

Figure 6 shows a schematic depiction of the balance between phase separation and percolation pathways in polymenacceptor blends. Purple represents pure NFA, dark red represents pure P3HT, red represents P3HT-rich amorphous phase, blue represents NFA-rich amorphous phase.

DEFININTIONS

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 (IUPAC recommendations 1994)” in Pure and Applied Chemistry, 1994, 66, 1077, pages 1 109-1110).

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.

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.

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-CºN. In such systems, conjugation is the interaction of one p-orbital with another across an intervening s-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 delocalization 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, a small 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

“P1”

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, cycloalkyl, 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 -CºC-), 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 C1-C40 carbyl or hydrocarbyl group is acyclic, the group may be linear or branched.

The C₁-C₄₀ carbyl or hydrocarbyl group includes for example: a C₁-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-C₁₈ 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 C₁-C₂₀ alkyl group, a C₂-C₂₀ alkenyl group, a C₂ -C₂₀ alkynyl group, a C₃-C₂₀ allyl group, a C₄-C₂₀ alkyldienyl group, a C₅-C₁₂ 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, Cl, 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-, -O-CO-O-, -S-CO-, -CO-S-, -S0₂-, -CO-NR⁰-, -NR°- CO-, -NR°-CO-NR⁰⁰-, -CY¹=CY²- or -CºC- 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, Cl, 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, Cl, 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, peril uorooctyl, 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-methyl butoxy, 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.

As used herein, the term polyalkylene glycol refers to a group having 2-20 alkylene (preferably ethylene or propylene, preferably ethylene) glycol repeat units of the following formula -(aikylene-0-)_n-R, wherein R is H or methyl, preferably methyl, and n is an integer representing the number of repeat units. The alkylene of each -(alkylene-O)- unit may be the same or different (preferably the same) and may, for example, be C2-6, C2-4 or C2-3 alkylene. The polyalkylene glycol may be polyethylene glycol or polypropylene glycol. A polyethylene glycol has repeat units of the following formula -(CH₂-CH₂-0-)_n-R. A polypropylene glycol has repeat units of the following formula -(CH₂-CH₂-CH₂-0-)_n-R. A polyalkylene glycol substituent may be linked to another moiety via an oxygen linker (e.g., to form a structure of formula -O-(alkylene-O-)_n-R). This structure is encompassed within the term“polyalkylene glycol” as referenced herein. 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, sulfo, sulfonyl, formyl, amino, imino, nitrilo, mercapto, cyano, nitro, halogen, -NCO, -NCS, -OCN, -SCN, -C(=O)NR⁰R⁰⁰, - C(=O)X⁰, -C(=O)R⁰, -NR°R⁰⁰, Ci-i₂alkyl, Ci-i₂alkenyl, C_M2alkynyl, C_6-i2 aryl, heteroaryl having 5 to 12 ring atoms, Ci-i₂ alkoxy, hydroxy, C_M2 alkylcarbonyl, C_M2 alkoxy-carbonyl, C_M2 alkylcarbonlyoxy or C_M2 alkoxycarbonyloxy wherein one or more H atoms are optionally replaced by F or Cl 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 sulfonyl if directly attached to each other represent a sulfamoyl radical).

As used herein, a polythiophene is a polymer comprising repeat unit of formula (which may be substituted as described herein):

Each repeat unit may be optionally substituted. A polythiophene may consist essentially of a structure of the formula:

with termination of the polymer at the position marked ^*, wherein n is an integer chosen as appropriate based on the number average molecular weight of the polythiophene and each repeat unit may be optionally substituted. A skilled person will appreciate that termination may be with any suitable moiety such that a stable polymer is formed. For example, termination may be with H or a thiophene, such that a stable polymer is formed. For example a polythiophene may consist essentially of:

wherein each repeat unit may be optionally substituted.

The M_w of a polythiophene may be measured using Gel Permeation Chromatography (GPC). For example, GPC with in-situ optical measurements may be performed using an Agilent Technologies 1260 Infinity GPC System with 1260 RID and DAD VL attachments. Measurements may be performed at 80 °C, using analytical grade chlorobenzene as eluent with two PLgel 10 pm MIXED B columns in series. The molar mass as a function of elution time through the columns may be calibrated using Agilent InfinityLab High EasiVial narrow dispersity polystyrene standards of known molecular weights. Samples may be prepared using analytical grade chlorobenzene in

concentrations of ~1-2 mg mL ¹ and filtered with VWR PES membrane 0.45 pm syringe filters before submission. An injection volume of 50 pL and GPC flow rate of 1.00 mL min ¹ may be used. See, for example, Active Standard ASTM D5296 - 1 1“Standard Test Method for Molecular Weight Averages and Molecular Weight Distribution of Polystyrene by High Performance Size-Exclusion Chromatography” for determination of the number average molecular weight.

The regioregularity of a polythiophene may be measured using NMR spectroscopy. The asymmetry of 3-substituted thiophenes results in three possible couplings when monomers are linked between the 2- and the 5-positions. These couplings are:

2,5', or head-tail (HT), coupling.

2,2', or head-head (HH), coupling

5,5', or tail-tail (TT), coupling

These can be combined into four distinct conformations: head-to-tail head-to-tail (HT-HT), tail-to-tail head-to-head (TT-HH), head-to-tail head-to-head (HT-HH) and tail-to-tail head- to-tail (TT-HT), shown below:

The triads are distinguishable by ¹H NMR spectroscopy as each confirmation has a different characteristic ¹H NMR peak. Integration of each peak enables the calculation of the regioregularity. For example, a polythiophene that is completely regioregular (i.e., having a regioregularity (RR) of 100%) refers to the case when you have complete head- to-tail head-to-tail (HT-HT) coupling between each 3-substituted thiophene monomeric unit. An exemplary method for determining the regioregularity of a polythiophene is set out in G. Barbarella et al., Macromolecules, 1994, 27, 3039-3045, the contents of which is herein incorporated by reference in its entirety. Further exemplary methods for determining the regioregularity of a polythiophene are set out in K. Sivula et al., J. Am. Chem. Soc., 2006, 128, 13988-13989 and T. Chen et al., J. Am. Chem. Soc., 1995, 117, 223-244, the contents of which are herein incorporated by reference in their entirety. 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 non- essential 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.

DETAILED DESCRIPTION

In most cases, the primary considerations to select an appropriate donor polymer to pair with an electron acceptor have been optoelectronic in nature. Though such considerations are of great significance, the microstructural characteristics of the polymensmall molecule blends should also be assessed to ensure optimal device performance can be attained. The influence of the molecular weight (M_w) of the polymer, on device performance, has been probed in fullerene-containing devices, but very few studies involving fullerene-free OSCs have been undertaken.

When investigating the effect of molecular weight on the J-V characteristics in

P3HT:PC_6iBM devices, it has been suggested that an optimal M_w within the range of 15- 40 kDa provided the best device performance (A. Ballantyne et al., J. Adv. Funct. Mater. 2008, 18, 2373-2380, the contents of which are herein incorporated by reference in its entirety). It was noted that lower or higher P3HT M_w was detrimental to solar cell performances in P3HT: ROb-iBM devices. Another study obtained a variety of molecular weight batches of P3HT with virtually identical RR, and came to a similar conclusion, that a molecular weight of between 20-30 kDa was optimal for use in bulk heterojunction P3HT:PC_6iBM cells (P. Schilinsky et ai, J. Chem. Mater. 2005, 17, 2175-2180, the contents of which are herein incorporated by reference in its entirety).

Molecular weight impacts the charge mobility of P3HT. Increasing molecular weight has been reported to increase mobilities in the range of 2 to 19 kDa, 3 to 40 kDa as measured in field-effect transistors while decreasing mobilities in the range of 13 to 121 kDa as measured by time-of-flight techniques. This points towards an optimum in the middle range P3HT M_w, although it must be noted that the mobilities of the lowest studied M_w are sensitive to processing. When the processing was optimized, the films for the lower P3HT M_w were observed to form highly ordered nanorod structures, whereas the middle range M_w P3HT formed less ordered isotropic nodule structures. No direct correlation between crystallinity and mobility was found but it was rather suggested that the mobility of the low M_w P3HT was limited by a combination of disorder at the grain boundaries and the inherent effects of chain lengths. Low M_w P3HT also contains a higher density of chain- ends and defects arising during their synthesis, both of which have been shown to act as traps and may also account for the observed reduced mobilities. For higher M_w, chains start participating in different aggregates leading to an increase in interconnection between high mobilities aggregates. For very high M_w, since the polymer chains are much longer, they possess much higher viscosity, and are more likely to entangle with one another, hindering crystallization.

Whilst M_w affects the hole mobility in P3HT, variations in efficiency of R3HT:RObiBM solar cells could not be assigned solely to changes in hole mobility, and it was suggested that active layer morphology was also likely to play a role. As an example of the effect of M_w dependent microstructure on device performance, the greater crystallinity of low M_w P3HT has been shown to lead to a decrease in charge transfer state energy, and therefore a corresponding decrease in the open circuit voltage. Another consequence of the greater degree of crystallinity in the low M_w P3HT was stronger light absorption, especially at longer wavelengths, which resulted in greater short circuit current in devices. Polymer M_w dependent effects on solar cell PCE have been observed in other systems. One study focused on the dependence of PBDTT-FTTE:perylene diimide device performance on polymer M_w and the crystallinity of the perylene diimide acceptor (N. Eastham et ai, J. Chem. Mater. 2017, 29, 4432-4444, the contents of which are herein incorporated by reference in its entirety). It was found that the most crystalline acceptor suppressed the PCE M_w dependence by dominating the morphology formation during processing, while blends incorporating the less crystalline acceptors led to different trends in PCE with varying M_w.

The influence of the RR of P3HT has also been explored in fullerene-containing OSCs; highly regioregular P3HT batches (> 98%) possess a greater degree of crystallinity in neat films, though it is uncertain whether this improves the ordering, and therefore charge carrier mobilities in blends. Despite achieving lower PCEs initially, the use of less regioregular P3HT (86%) has been reported to improve the thermal stability of

P3HT:PC_6iBM devices by hindering the crystallization-induced phase segregation of PCeiBM.

It was, therefore important, to explore whether these relationships were also prevalent in polymenNFA blends.

The following examples of the invention are provided to aid understanding of the invention but should not be taken to limit the scope of the invention.

Described herein is a comparison of five batches of P3HT, with a variety of molecular weights, to elucidate any relationship between M_w and device performance when used in bulk heterojunction OSCs with two NFAs; namely O-IDTBR and EH-IDTBR. The five P3HT batches, detailed in Table 1 , were selected to span a wide variety of M_w values, whilst keeping the RR relatively constant. It must be noted that there are some variations in the polydispersity index (PDI) of the batches. However, we do not believe that this small PDI variation complicated our observations. The two different NFAs were selected because of differences in their crystallinity: previous reports have shown that O-IDTBR is more crystalline in nature than EH-IDTBR, its branched chain analogue

Table 1. Summary of the properties of each neat batch of P3HT

P3HT M_n PDI RR

Batch [kDa] [kDa] [%]^a)

A 17 13 1 .31 96

B 20 15 1 .34 96

C 34 26 1 .35 95

D 64 35 1 .83 96

E 94 59 1 .59 97 a⁾ measured from ¹ H NMR with CDCh as the solvent. An inverted architecture was utilized to fabricate all devices, as per previous reports using these NFAs (S. Holliday et al., Nat. Commun. 2016, 7, 11585, D. Baran et al., Nat. Mater. 2017, 16, 363-369). As can be seen from Figure 1b and Table 2 the performance of P3HT:0-IDTBR devices displays a strong dependence on the polymer M_w, with a maximum PCE of 7.0% being achieved using P3HT-C (34 kDa), and lower PCE being observed in both higher and lower M_w P3HT batches. Similar to the findings in

P3HT:PC_6iBM devices, the lower M_w P3HT led to a reduced Voc; with P3HT-A only able to achieve 0.67 V, over 50 mV lower than the Voc achieved with the higher M_w P3HT batches. Upon increasing the P3HT M_w from 34 kDa to 94 kDa the photocurrent decreases from 14.6 to 12.1 mA cm ² However, the lowest M_w P3HT, batches A and B, were only able to achieve a photocurrent of 11.8 and 12.0 mA cm ² respectively. This is considerably lower than those achieved with the higher M_w P3HT-C, which is contrary to the trend observed in fullerene-containing devices reported previously. P3HT-A exhibited significantly lower FF values than the highest performing P3HT C devices (0.50 and 0.66 respectively). The higher M_w batches achieved slightly lower FF values of 0.62.

Interestingly, when O-IDTBR is replaced with the less crystalline analogue, EH-IDTBR, as the electron acceptor, the strong dependence of device performance on the M_w of P3HT was not observed.

Table 2. Summary of the J-V characteristics of inverted architecture P3HT:IDTBR

OSCs

Active Layer Voc FF Max PCE/Average PCE³* Standard

P3HT A:0-IDTBR 17 0.67 1 1 .8 0.50 4.0/3.6 0.24 P3HT B:0-IDTBR 20 0.74 12.0 0.61 5.4/4.7 0.38 P3HT C:0-IDTBR 34 0.73 14.6 0.66 7.0/6.7 0.23 P3HT D:0-IDTBR 64 0.74 13.1 0.62 6.0/5.7 0.22 P3HT E:0-IDTBR 94 0.74 12.1 0.62 5.5/5.3 0.13

P3HT A:EH-IDTBR 17 0.79 12.0 0.65 6.3/5.9 0.34

P3HT B:EH-IDTBR 20 0.77 12.6 0.690 6.3/6.0 0.24

P3HT C:EH-IDTBR 34 0.77 12.0 0.68 6.1/5.9 0.28

P3HT D:EH-IDTBR 64 0.78 11.3 0.69 6.2/5.9 0.18

P3HT E:EH-IDTBR 94 0.78 11.7 0.68 6.2/5.9 0.20 a) average PCE and standard deviation were calculated over 8 devices

As Figure 1c and Table 2 show, the J-V characteristics of devices using all five P3HT batches are virtually identical, with all devices able to achieve a PCE of approximately 6%. Figure 2 clearly shows that each of the P3HT batches were able to achieve very similar Jsc and Voc in devices. The FF of P3HT:EH-IDTBR devices using P3HT A is marginally lower than those using the higher M_w polymers, however this is a substantially smaller difference than was observed in O-IDTBR devices. Although the P3HT:EH-IDTBR devices were not able to match the 7% attained by the P3HT-C:0-IDTBR blends, the insensitivity that EH-IDTBR exhibits to P3HT M_w may render it a more versatile NFA when used with P3HT. We hypothesize that these observations are linked with the dependence of blend microstructure on polymer M_w.

To better characterize the M_w dependence of the photovoltaic performance of our

P3HT:0-IDTBR devices, we analyzed the contributions to the voltage loss relative to the voltage of ideal devices, using luminescence spectroscopy. In this approach the principle of reciprocity between photon absorption and emission is used to quantify the radiative and non-radiative voltage losses via measurement of the EQE and electroluminescence (EL). Photoluminescence (PL) spectra showed that the excitonic state energy lies at around 1.55 eV in every case (see Figure 3a). This value is consistent with the absorption onset energy as estimated from the EQE when plotted on a linear scale (not shown). Figure 3b shows the normalized EL spectra of the devices based on different P3HT M_w. We see a clear broadening effect of the spectrum as M_w increases from 17 to 34 kDa before narrowing again for the higher M_w batches of P3HT. This is also reflected in the EQE measurements (see Figure 4a), where the EQE edge broadens as M_w increases, peaking in the 34 kDa P3HT device and subsequently narrowing in the 64 and 94 kDa P3HT devices.

The observed broadening in the EL and EQE for the 34 kDa P3HT device leads to a lower radiative limit to Voc, Voc, rad and since the optical gap and the corresponding Shockley- Queisser limit to Voc (Voc _,SQ), does not change, it leads to a greater voltage loss due to broadening of the absorption edge AVoc.abs. These contributions to the voltage loss are shown in Figure 4b. However, the effect of M_w on AVoc_abs is relatively small compared to the effect on the non-radiative voltage loss, AVoc.nrad (defined as Voc,rad-Voc). The resulting voltage losses are summarized in Table 3, where the 34 kDa P3HT device shows the highest AVoc.abs (0.15 V), but the lowest AVoc.nrad (0.39 V). Since the absorption broadening voltage loss in this system is relatively small (less than 0.15 V compared to 0.3-0.5 eV for the non-radiative loss), we can conclude that non-radiative recombination limits the Voc of the cells. Accordingly, the best device also shows the lowest non-radiative voltage loss. The improvement in the non-radiative voltage losses are due to an enhanced LED quantum efficiency of the cell, where the ratio between radiative and non-radiative recombination rate increase. The best P3HT:Q-IDTBR shows a AVoc.nrad similar to that of P3HT:PC_6iBM, but much lower overall voltage loss, i.e. 0.81 V versus 1 .35 V. The significant improvement in the Voc losses is mainly due to the reduced broadening of the EQE edge, compared to that of P3HT:PC_6iBM.

Table 3. Summarized voltage losses for different M_w P3HT:0-IDTBR devices

Mw [kDa] E_gap [eV] Voc.SQ [V] V_oc,rad [V] AVoc.abs [V] Voc [V] AVoc.nrad [V]

17 1.55 ΪT28 Ϊ Td M O 066 052

20 1.55 1 .28 1.16 0.12 0.74 0.42

34 1.55 1 .28 1.13 0.15 0.74 0.39

64 1.55 1 .28 1.17 0.1 1 0.75 0.42

94 1.55 1 .28 1.16 0.12 0.76 0.40

In an effort to understand the PCE dependence on M_w for the O-IDTBR systems, and M_w independence in the P3HT:EH-IDTBR devices, the phase behavior of the binary blends of these materials was investigated using differential scanning calorimetry (DSC) to construct the non-equilibrium phase diagram for each binary. The endset of the melting transition at each composition was used to construct the liquidus. Note that to obtain the phase diagrams, the samples are drop-cast. The phase diagrams deduced from the DSC measurements for four P3HT batches (P3HT-B, P3HT-C, P3HT-D and P3HT-E) blended with O-IDTBR, and the three batches of P3HT with EH-IDTBR (P3HT-B, P3HT-C and P3HT E) are depicted in Figure 5. Eutectic behavior was observed for both blend systems at all studied molecular weights. Figure 5 shows that in the case of P3HT:0-IDTBR, the eutectic composition becomes increasingly rich in O-IDTBR as the molecular weight of P3HT increases (40 to 60 wt% O-IDTBR on increase of M_w from 20 to 94 kDa) while for EH-IDTBR, the eutectic composition is insensitive to molecular weight. The active layer composition (1 :1 P3HT:0-IDTBR) is hyper-eutectic when expressed in terms of the fraction of O-IDTBR for low molecular weights including the best performing device (with 34 kDa) while it is hypo-eutectic for higher M_w. In the case of EH-IDTBR, the active layer composition is hyper-eutectic regardless of M_w. In R3HT:RObiBM blends, the optimal R3HT:RObiBM ratio was found to be slightly hyper-eutectic. It was suggested that the excess acceptor is required to ensure the formation of percolation pathways which improve electron collection at the cathode. The eutectic composition of R3HT:RObiBM also becomes increasingly rich in ROb-iBM with increasing P3HT M_w but for all P3HT M_w, 1 :1 is always hyper-eutectic. Furthermore, DSC endotherms of the heating cycle of P3HT:EH-IDTBR blends show an absence of a clear melting transition around the eutectic composition for P3HT:EH- IDTBR. This indicates either that the mixture is mainly amorphous or that the crystals are too small (nanometer size) to be detected by DSC. This can also be seen in the phase diagram where a significant jump in the melting point depression is observed between 20 and 50 wt% EH-IDTBR for the blend with 20 kDa P3HT, and between 20 and 30 wt% in the highest M_w blend. This suggests that the P3HT:EH-IDTBR blends are significantly more amorphous in these composition windows, which might explain insensitivity of the eutectic composition to polymer M_w.

In contrast, DSC endotherms of the heating cycle of P3HT:0-IDTBR blends show a jump in melting point depression in blends with O-IDTBR occuring within the range of 20 to 40 wt% O-IDTBR for the 20 kDa P3HT but which is absent in the higher M_w blends, which also show a maximum melting point depression at higher O-IDTBR contents. This difference in behavior between the blend systems aligns with the fact that O-IDTBR is a more crystalline acceptor than EH-IDTBR, leading to a stronger competition between polymer and acceptor crystallization in the blends, hence a variation in eutectic composition with polymer M_w.

In summary, the microstructure of P3HT:0-IDTBR blends is complex and presents features on different length scales. In particular, the microstructures observed for high M_w polymers are likely more dominated by kinetics of drying rather than by thermodynamics. Although miscibility changes with molecular weight, the temperature dependence of the miscibility and the spinodal demixing behavior also change significantly. Therefore, drying rates and donor/acceptor mixing ratios become more relevant for the higher molecular weight fractions. We observe micron-size features by AFM and phase-separation at the nanoscale between P3HT-rich and O-IDTBR-rich domains. While the size of the micron- size features decrease slightly with P3HT M_w, the phase separation at the nanoscale level seems to increase leading to more prominent O-IDTBR-rich domains with higher M_w. We suggest that at the lower P3HT molecular weights, the 1 :1 ratio is hyper-eutectic leading to the formation of acceptor percolation pathways that lead to improve charge collection. Around these percolation pathways, the eutectic microstructure develop with nanoscale crystals of both P3HT and O-IDTBR, unlikely to be captured by DSC but measured by GIWAXS, and an amorphous mixture of P3HT and O-IDTBR. P3HT and O-IDTBR are likely to exhibit finite miscibility. Thus, we suggest that the phase separation observed at the nanoscale is due to a spinodal decomposition of the amorphous mixture of P3HT and O-IDTBR. The difference in nanoscale phase separation with P3HT M_w is only a reflection of the change of eutectic composition. Thus, we suggest that P3HT-C combines the advantage of being hyper-eutectic, with associated good charge collection due to an acceptor percolation pathway, and an optimal phase separation at the nanoscale, leading to good charge generation. When the P3HT M_w increases, the devices suffer from reduce charge collection due to a lack of acceptor percolation pathways; while when the P3HT M_w is too low, the devices suffer from reduce charge generation due to a lack of phase separation at the nanoscale. This is summarized in Figure 6.

Interestingly, on the other hand, P3HT:EH-IDTBR blend devices do not exhibit the same sensitivity to P3HT molecular weight, but do not reach as high performance as the best optimized O-IDTBR devices.

To conclude, considerable progress has been made in designing P3HT OSC systems.

The established fullerene-containing devices have progressed from the use of simple soluble fullerenes, such as PC61BM, to the use of less symmetric C70 cages to improve light absorption (PC71BM) and bis-adducts to raise the acceptor’s LUMO (IC60BA and IC70BA). These developments have led to the ability to achieve PCEs as high as 7.4% with IC70BA, though a high boiling additive was necessary to achieve the optimal morphology. Despite being unable to match the record performance in P3HT:fullerene OSCs, the use of ternary blends have also shown potential in device optimization, moving forwards. In several cases, the addition of a second acceptor into the blend has improved the blend morphology, leading to simultaneous improvements in Jsc, Voc and FF. Issues of morphological instability still persist in fullerene devices owing to their strong

aggregation tendency and leading to the formation of micrometer sized aggregates in blends over time. This has been successfully addressed with the use of cross-linking, however the improved stability is accompanied with a loss in PCE and the need for expensive cross-linkable acceptors. The stability issues, coupled with the large Voc losses exhibited in P3HT:fullerene OSCs, leave such blends unlikely to succeed.

The recent success of A-D-A nonfullerene acceptors have provided a promising alternative to fullerene acceptors. They have been designed to have higher lying LUMOs, improved photon absorption and a reduced aggregation tendency, thereby overcoming the issues that have limited fullerene acceptors. Though early A-D-A acceptors were unable to form domains on the correct lengthscale when blended with P3HT, due to their twisted structures, the development of planar acceptors such as O-IDTBR and H1 led to improved phase separation and complementary absorption, by narrowing the bandgap of the acceptors. P3HT:0-IDTBR devices were able to achieve a OCE as high as 6.4%, with impressive stability exhibited relative to fullerene-containing devices. This system was further developed with the addition of a third component, IDFBR, which led to the formation of a favorable three-phase microstructure, and OSCs that were able to achieve a maximum PCE of 7.6%. This progress has led to the record performance in single- junction P3HT devices, and paired with the excellent stability, renders P3HT:NFA blends likely to be among the most realistic for commercialization, at present. However, many of the more recently reported NFAs have been designed to work well with low bandgap polymers. These polymers are not currently scalable on the industrial level and as such, more emphasis should be apportioned to developing new acceptors suited to perform well with P3HT.

This work on P3HT:IDTBR blends highlights the care that must be taken when optimizing P3HT:NFA OSCs. We observed a clear M_w dependence on device performance in P3HT:0-IDTBR blends, and therefore suggest that it is pertinent to consider the ideal polymer M_w to use in such systems. We propose that the optimal device performance (7.0%), when P3HT-C is used with O-IDTBR, is a result of improved acceptor percolation pathways and optimal phase separation at the nanoscale, in comparison to blends with either lower or higher M_w polymer batches. This can be related to the competition between the crystallization of the P3HT and O-IDTBR, which varies with polymer M_w, as shown by the DSC measurements. When the less crystalline EH-IDTBR is used as the electron acceptor, no variation in device performance with M_w was observed, though the maximum PCE was slightly lower (6.3%) than achieved with P3HT-C:0-IDTBR. A result of the reduced crystallinity of EH-IDTBR, in comparison to O-IDTBR, is the more amorphous nature of P3HT:EH-IDTBR blends. Hence, the competition between the crystallization of P3HT and EH-IDTBR is reduced significantly, leading to an apparent insensitivity to device performance.

EXPERIMENTAL DETAILS

Materials

P3HT batches were purchased from Ossila and BASF, or synthesized according to the procedure outlined in J.H. Bannock et al., J. C. Adv. Funct. Mater. 2013, 23, 2123-2129. O-IDTBR and EH-IDTBR were synthesized according to the procedure outlined in S. Holliday et al., Nat. Commun. 2016, 7, 11585. All other chemicals were purchased from Sigma Aldrich and used as received. Device Fabrication and Characterization

Bulk heterojunction solar cells were fabricated in an inverted architecture

(glass/ITO/ZnO/P3HT:IDTBR/Mo03/Ag). Glass substrates, pre-patterned with ITO (15 W sheet resistance per square), were cleaned by sonication in acetone, detergent, deionized water and isopropanol before ozone plasma treatment for 10 min. A layer of ZnO, approximately 30 nm, (from Zn(OAc)2 in 60 pl_ monoethanolamine and 2 mL of 2- methoxyethanol) was deposited by spin-coating onto the ITO substrate at 4,000 r.p.m. for 40 s, followed by annealing at 150 °C for 20 min. Active layer solutions (P3HT:IDTBR, weight ratio 1 :1 ) were prepared from CB with a total concentration of 24 mg mL ¹. The solutions were heated to 70 °C overnight, and the active layer was deposited by spincoating at 2,500 r.p.m. for 1 min. The active layers were then annealed at 125 °C for 12 min, under an inert atmosphere. A M0O3 anode interlayer (10 nm) and Ag anode (100 nm) were then deposited by thermal evaporation through a shadow mask, giving an active area of 0.045 cm² per device. The J-V characteristics were measured under AM1 5G (100 mW cm ²) irradiation using an Oriel Instruments Xenon lamp calibrated to a Si reference cell to correct for spectral mismatch, and a Keithley 2400 source meter.

Electroluminescence (EL) experiments consisted of injecting current from the anode, then collecting the emitted photons as function of wavelength/energy. The injection current, normally 10-30 mA, was provided from a constant flow source by Keithley 2400. The emission spectrum was then collected by a Shamrock 303 spectrograph with an iDUS InGaAs array detector cooled to -90 °C. The obtained EL spectra intensity was calibrated with the spectrum from a calibrated halogen lamp. Photoluminescence (PL)

measurements were carried out by illuminating the device under a 473 nm laser beam, normally keeping it under open circuit condition, in which case we can normally only see excitonic state emission. These measurements used the same detector as EL

measurements, mentioned above, to collect the emission spectrum from the cell. The external quantum efficiency (EQE) was measured using a whole spectrum (300 to 1 100 nm) of monochromatic light generated by the CVI DIGIKROM 240 type grating

spectrometer combined with a tungsten halogen light source. A chopper with a frequency of ~80 Hz was used for reducing the noise of the monochromatic light from the ambient, the light beam coming from the chopper was split into two equal intensity light beam, one for the silicon photodiode and one for the device, and the photocurrent of both of the silicon and the device were detected at the same time, using a Stanford Research Systems SR380 lock-in amplifier, which provides an in-situ calibration. Long pass filters (610, 715, 780, 850, and 1000 nm) were used to filter out the scattered light from the monochromator. Morphological Characterization

Samples for differential scanning calorimetry (DSC) were prepared by drop-casting 150 pl_ of chlorobenzene solutions (25 mg mL ¹). The films were scrapped off and ~ 2-3 mg were transferred into hermetic DSC pans, which were sealed with punctured lids. A Mettler Toledo DSC 1 was used; two heating and two cooling cycles were recorded at a 5 °C.min

¹ rate. The first heating cycles were used to construct the liquidus curves, for which the endotherm endset temperatures were used. 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

1 . A composition comprising a blend of an organic electron acceptor compound and an organic electron donor compound, wherein the organic electron donor compound is a polythiophene having a number average molecular weight (M_w) of about 25 to about 60 kDa, and wherein the electron acceptor compound 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:

Xi is C, Ge or Si;

R¹ is, at each occurrence, independently, H, or optionally substituted C1-30 aliphatic, aryl or heteroaryl;

Y¹ and Y² are, independently, H, F, Cl 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.

2. The composition of claim 1 , wherein the organic electron donor compound is a polythiophene comprising a repeat unit having formula:

wherein, each R^T is independently H, C1-20 aliphatic, aryl optionally substituted with C1-20 aliphatic, -C(O)O-Ci-20 aliphatic, -O-C1-20 aliphatic or a polyalkylene glycol chain having 2- 20 alkylene glycol units.

3. The composition of claim 1 or 2, wherein the organic electron donor compound is poly(3-hexylthiophene-2,5-diyl) (P3HT), poly(3-(4-octylphenyl)thiophene) (P30T), or PDCBT, optionally P3HT.

4. The composition of claim 3, wherein the regioregularity of the organic electron donor compound is about 90 to about 98 %.

5. The composition of any preceding claim, wherein the organic electron donor compound has a M_w of about 30 to about 50 kDa, optionally about 30 to about 40 kDa, optionally about 30 to about 38 kDa optionally about 34 kDa.

6. The compound of any preceding claim, 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.

7. The composition of any preceding claim, wherein A is:

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

each optionally substituted by one or more groups R².

8. The composition of any preceding claim, 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. 9. The composition of claim 8, wherein at least 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(=O)NR⁰R⁰⁰, -C(=O)X⁰, -C(=O)R⁰, -C(=O)OR⁰, -C(=S)R°, -C(=S)OR°, -

OC(=O)R⁰, -OC(=S)R°, -C(=O)SR⁰, -SC(=O)R⁰, -NH₂, -NR°R⁰⁰, -NR⁰C(O)R°, -SH, -SR°, -

S0₃H, -S0₂R°, -OH, -N0₂, -CF_3I -CF₂-R°, -SF_5I 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(=O)NR⁰R⁰⁰, -C(=O)X⁰, -C(=O)R⁰, - -NH₂, -

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 R°° 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(=O)NR⁰R⁰⁰, -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, -CF_3I -CF₂-R°, -SF_5I 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 R°° are, independently, H or optionally substituted C1 -40 hydrocarbyl; and

n is 0-4.

10. The composition of claim 9, wherein Y is:

R₅ is C-i-₁₂ aliphatic.

1 1. The composition of any preceding claim, 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.

12. The composition of any preceding claim, wherein a and b are both 1.

13. The composition of any preceding claim, wherein each occurrence 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.

14. The composition of any preceding claim, wherein 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(=O)NR⁰R⁰⁰, -C(=O)X⁰, -C(=O)R⁰, -C(=O)OR⁰, -C(=S)R°, -C(=S)OR°, - OC(=O)R⁰, -OC(=S)R°, -C(=O)SR⁰, -SC(=O)R⁰, -NH₂, -NR°R⁰⁰, -NR⁰C(O)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 R°° are, independently, H or optionally substituted C1 -40 hydrocarbyl. The composition of any preceding claim, wherein the compound of formula (I) is

16. The composition of any preceding claim, wherein the composition comprises IDTBR (optionally O-IDTBR) and P3HT having a M_w of about 34 kDa, and optionally a regioregularity of about 94 %, optionally wherein the composition comprises about 20 to about 50 wt% IDTBR.

17. The composition of any preceding claim, wherein the composition further comprises a second organic electron acceptor compound of formula (I).

18. An optical or electronic device comprising a composition according to any preceding claim.

19. The device of claim 18, 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.

20. The device of claim 19, wherein the device further comprises an anode and a cathode, optionally wherein the composition forms an active layer between the anode and the cathode.

21. The device of any one of claims 18 to 20, 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 17.

22. A process for producing a device as claimed in any of claims 18 to 21 , comprising providing a substrate; and

depositing a composition according to any one of claims 1 to 17 on a surface of the substrate to form an active layer.

23. The process of claim 22, wherein the process further comprises depositing an electrode on the active layer.

24. Use of a composition as defined in any one of claims 1 to 17 as an active layer in an optical or electronic device, optionally an organic solar cell.

25. Use of a polythiophene (optionally P3HT) having a number average molecular weight (M_w) of about 25 to about 60 kDa as an organic electron donor compound in an optical or electronic device, in combination with one or more organic electron acceptor compounds, wherein the one or more organic electron acceptor compounds are independently as defined in any one of claims 1 and 6 to 15.