WO2013102035A1 - Thienothiadiazole based polymer semiconductors and uses in electronics and optoelectronics - Google Patents

Abstract

Provided herein are new materials for application in the field of organic semiconductors, organic thin-film transistors, and broadband photodetectors and specifically to thienothiadiazole-based polymer semiconductors and their applications in organic electronic devices such as thin-film field-effect transistors, photodetectors, and solar cells. Many embodiments disclosed herein relate to conjugated thienothiadiazole-based copolymers comprising at least one thienothiadiazole unit in the repeat unit. Organic electronic devices comprising the thienothiadiazole-based copolymers, such as OLEDs, transistors, photodetectors and solar cells are also disclosed and described.

Description

THIENOTHIADIAZOLE BASED POLYMER SEMICONDUCTORS AND USES IN ELECTRONICS AND OPTOELECTRONICS

RELATED APPLICATIONS

This application claims priority to US provisional application serial no. 61/582,192 filed December 30, 2011, which is hereby incorporated by reference in its entirety for all purposes.

FEDERAL FUNDING STATEMENT

The inventions were made with United States Government support under Grant No. DMR-0805259 of the National Science Foundation. The Government has certain rights in the inventions.

BACKGROUND

Solution processable conjugated polymer semiconductors are of growing interest for diverse applications in electronics and optoelectronics.^1"6 The fine-tuning of the electronic structure, charge transport, and optoelectronic properties of π-conjugated polymers is greatly facilitated by the donor-acceptor (D-A) approach, whereby electron-donating (D) and electron-accepting (A) units are incorporated into the conjugated polymer chain in a modular fashion. 7-^"11 The resulting intramolecular charge transfer (ICT) in such a D-A copolymer can be a powerful means of tailoring the electronic structure and properties of this class of organic semiconductors, depending on the electron-releasing and electron- withdrawing strengths of the D and A building blocks. A large number of D-A conjugated polymer semiconductors based on benzodiazole acceptors such as benzo[c][l,2,5]thiadiazole

12 15 16 17

(BTD), ^" benzoselenadiazole (BSe), and benzoxadiazole (BX) have been extensively studied. D-A copolymers containing these benzodiazole acceptor units are known to exhibit absorption maximum (λ_^) in the range of 535-689 nm with optical band gaps of 1.5-2.3 eV.¹²-¹⁷

Thienothiadiazole (TTD) is a stronger electron acceptor than benzothiadiazole (BTD) and the others in the homologous series. Compared to the related electron-poor rings such as benzothiadiazole, benzoselenadiazole and benzoxadiazole, which have been extensively investigated in the construction of D-A conjugated copolymers, thienothiadiazole (TTD) has rarely been explored as a building block. This is surprising considering that the first TTD- containing conjugated polymer, poly(4,6-di(2-thienyl)thieno[3,4-c][l ,2,5]thiadiazole),

18

was introduced almost 2 decades ago. Recently, a TTD-containing copolymer, poly(5,7- bis(4-decanyl-2-thienyl)-thieno(3,4-b)diathiazole-thiophene-2,5) (PDDTT), was used to fabricate ultrasensitive polymer photodetectors exhibiting performance that was comparable

22 to high performance inorganic photodetectors in the 400-1450 nm spectral range. In contrast, bulk heterojunction solar cells made form PDDTT/fullerene blends had a poor

22

performance with the highest efficiency of only 0.11 %. The high lying HOMO energy level of - 4.71 eV and low molecular weight of PDDTT appear to be a reason for the poor

21 22

photovoltaic properties. ' More recently, another TTD-based copolymer with fluorene as a donor moiety was reported by Kminek et al.^23-26 Unfortunately, the observed photovoltaic response was also poor under non-standard and not meaningful illumination (Xe lamp with UV filter WG35 for 45 mW/cm ). These previous studies of TTD-containing polymers

(Figure 1) are able to show only limited potential of TTD in the development of new conjugated copolymers for electronics and optoelectronics. The molecular weight of these TTD-containing polymers synthesized by Stille or Suzuki coupling polymerization is generally low to moderate (M_n = 3.1-31.7 kDa).^21"26

Moreover, most or all of the previously-studied thienothiadiazole-based polymers lack good thermal or oxidative stability, or the practical processability characteristics needed in order to make commercially practical electronic devices. Therefore, there exists an unsatisfied need for new and improved polymeric and/or copolymeric materials, and/or solid materials or compositions derived therefrom that can provide the needed properties for electron or hole transport, as well as improved processability, performance, cost, and thermal and oxidative stability for use in organic electronic devices, especially transistors and solar cells.

SUMMARY

Provided herein are new D-A conjugated copolymers based on thieno[3,4- c][l ,2,5]thiadiazole acceptor and various donor moieties. Examples of the new

poly(thienothiadiazole)s (PTTDs), including poly(4,6-bis(3-ethylhexyl-2-thienyl)thieno[3,4- c][l ,2,5]thiadiazole-alt-phenylene) (PTTP), poly(4,6-bis(3-ethylhexyl-2-thienyl)thieno[3,4- c][l ,2,5]thiadiazole-alt-4,8-bis(2-ethylhexyloxy)benzo[l ,2-b:4,5-b']dithiophene) (PTTBDT), poly(4,6-bis(3-ethylhexyl-2-thienyl)thieno[3,4-c][l ,2,5]thiadiazole-alt-vinylene) (PTTV) and poly(4,6-bis(3-ethylhexyl-2-thienyl)thieno[3,4-c][l,2,5]thiadiazole-alt-N-(l- hexyldecyl)dithieno[3,2-b:2',3'-d]pyrrole) (PTTDTP), were synthesized by Stille coupling polymerization. The molecular structures of the new examples of PTTDs are given in Figure 2. Optical absorption spectroscopy shows that the new PTTDs have very narrow band gaps of 0.9-1.2 eV. Electrochemically derived electronic structures show that the new polymers have a LUMO energy level of -3.4 to -3.6 eV and a HOMO energy level of -4.9 to -5.1 eV. The charge transport and photovoltaic properties of the PTTDs were investigated by organic field- effect transistor (OFETs) and bulk heterojunction (BHJ) solar cells, respectively. The thienothiadiazole-based copolymers had a moderate field-effect mobility of holes of 4.6^X 10^" cm /V s under simple thin film processing conditions while a photovoltaic efficiency of 0.38 % was obtained in non-optimized bulk heterojunction devices.

Embodiments provided herein include compositions, devices, and articles, as well as methods of making and methods of using the compositions, devices, and articles.

For example, provided herein is a composition comprising at least one conjugated copolymer wherein the copolymer is represented by:

wherein: a) n is 1 or more; b) yi and y₂ are each 0, 1, 2, 3 or 4; c) Xi, X₂ and X₃ are each independently a heteroatom; d) Ri and R₂ are each independently a hydrogen, a fluorine, or an optionally substituted linear, branched, or cyclic Ci-C₂₄ organic group; e) L is represented by:

wherein each X is independently S, O, N, Se, or Te, and wherein R₃ and R₄ are each independently a hydrogen, a fluorine, or an optionally substituted linear, branched, or cyclic C1-C24 organic group.

In one embodiment, X_ls X₂ and X3 are each independently O, N, S, Se or Te. In another embodiment, X_ls X₂ and X3 are each S. In one embodiment, y₁ and y₂ are each 1.

In one embodiment, L is

In another embodiment, L is

In a further embodiment, L i

In yet another embodiment, L is

In yet a further embodiment, L i

In an additional embodiment, L i

In one embodiment, R_{l s} R₂, R3, and R₄ are each independently an optionally substituted Ci-C₂₄ alkyl, alkoxy, or thioalkyl. In another embodiment, R_{l s} R₂, R3, and R₄ are each independently a branched alkyl group. In a further embodiment, wherein R_{l s} R₂, R3, and R₄ are each independently a hydrogen, a fluoride, a cyano, or an optionally substituted C₄-C₂₄ alkyl, alkoxy, or thioalkyl.

In one embodiment, the copolymer has a weight-average molecular weight (M_w) of 5 kDa or higher. In one embodiment, the copolymer has an onset decomposition temperature (T_d) of 250°C or higher. In one embodiment, the copolymer has an ionization potential of 4.5 eV or higher. In one embodiment, the copolymer has an optical band gap of 1.2 eV or lower. In one embodiment, the copolymer has an electrochemical band gap of 1.7 eV or lower.

Also provided herein are devices comprising the conjugated copolymer described above. In one embodiment, the device is a transistor. In another embodiment, the device is a photodetector. In a further embodiment, the device is a photovoltaic device. In an additional embodiment, the device is a light-emitting device.

In one embodiment, the device is a field-effect transistor. In another embodiment, the device is a field-effect transistor comprising a thin-film of the copolymer. In an further embodiment, the device is a field-effect transistor comprising a thin-film of the copolymer annealed at a temperature of 150 °C or more. In one embodiment, the device is a field-effect transistor having a carrier mobility of

1 x 10 -^"4 cm 2 /Vs or higher, or 1 x 10 -^"2 cm 2 /Vs or higher. In another embodiment, the device is a field-effect transistor having a on/off current ratio of 10² to 10⁶.

Moreover, provided herein is a thin-film field-effect transistor comprising at least one conjugated copolymer, wherein the copolymer comprises at least one donor moiety and at least one acceptor moiety, and wherein the acceptor moiety is an optionally substituted thieno[3 ,4-c] [ 1 ,2,5]thiadiazole.

Furthermore, provided herein are transistors comprising at least one conjugated copolymer, wherein the copolymer is represented by:

wherein: a) n is 1 or more; b) each a is independently 0, 1, 2, 3 or 4, b is 0 or 1; c) each X is independently O, S, Se, Te or NR' wherein R' is hydrogen, a C₁-C30 normal, branched, or cyclic alkyl group; d) each X' is independently S, Se or Te; e) each Y and Y' is N or CR", wherein R" is hydrogen, fluorine, cyano, or a C₁-C30 normal, branched, or cyclic alkyl, perfluoroalkyl, alkoxy, thioalkyl, or thioalkoxy group; and f) L is represented by:

In one embodiment, each a and b is 1. In one embodiment, each X and X' is S.

In one embodiment, each Y and Y' is CR". In another embodiment, each Y is CH, each Y' is CR" with R" being an optionally substituted C₁-C30 alkyl, alkoxy, or thioalkyl. In a further embodiment, each Y is CH, each Y' is CR" with R" being a branched alkyl.

In one embodiment, the transistor is a field-effect transistor comprising a thin-film of the copolymer. In another embodiment, the transistor is a field-effect transistor comprising a thin-film of the copolymer annealed at a temperature of 150 °C or higher. In a further embodiment, the transistor is a field-effect transistor having a carrier mobility of lxlO^"4 cm /Vs or higher. In yet another embodiment, the transistor is a field-effect transistor having a carrier mobility of 1x10 -^"2 cm 2 /Vs or higher. In yet a further embodiment, the transistor is a field-effect transistor having a on/off current ratio of 10 2 -106. In an additional embodiment, the conjugated copolymer comprises at least one donor moiety and at least one acceptor moiety, and wherein the acceptor moiety is an optionally substituted thieno[3,4- c][l,2,5]thiadiazole.

DESCRIPTION OF THE FIGURES

Figure 1 shows molecular structures of thieno[3,4-c][l,2,5]thiadiazole-based conjugated copolymers.

Figure 2 shows molecular structures of newly synthesized thieno[3,4- c][l,2,5]thiadiazole-based conjugated copolymers.

Figure 3 illustrates synthetic route to 4,6-bis(5-bromo-3-ethylhexyl-2- thienyl)thieno[3,4-c] [ 1 ,2,5]thiadiazole.

Figures 4- A and 4-B illustrate methods for synthesizing PTTDs. Figure 5 shows cyclic voltammograms of PTTDs thin films in 0.1 M Bu4NPF6 solution in acetonitrile at a scan rate of 40 mV/s: oxidation scans (A) and reduction scans (B).

Figure 6 shows optical absorption spectra of PTTDs in dilute chloroform solution (A) and as thin films on glass substrates (B).

Figure 7 shows output (A-D) and transfer (E) characteristics of the OFETs based on the PTTDs. Forward and backward scans are overlaid in both output and transfer curves.

Figure 8 shows J-V curves (A) and absorption spectra (B) of PTTD:PC₇iBM (1 :2 wt/wt) solar cells.

Figure 9 shows embodiments of the thienothiadiazole-based conjugated copolymers for electronics and optoelectronics.

Figure 10 shows 1H NMR spectra of 4,6-bis(5-bromo-3-ethylhexyl-2- thienyl)thieno[3,4-c] [ 1.2.5]thiadiazole.

Figure 11 shows liquid chromatograph mass spectrum of 4,6-bis(5-bromo-3- ethylhexyl-2-thienyl)thieno[3,4-c][1.2.5]thiadiazole.

Figure 12-A, 12-B and 12-C show 1H NMR spectra of PTTDs.

Figure 13 shows TGA thermograms of PTTDs in N₂.

Figure 14 shows the second heating DSC scans of PTTDs.

Figure 15 shows XRD spectra of PTTDS as thin films on glass substrates.

Figure 16 shows CV oxidation scan of PTTV (A) and reduction scan of PTTDTP (B) as thin films in 0.1 M Bu₄NPF6 solution in acetonitrile at a scan rate of 40 mV/s.

DETAILED DESCRIPTION

All references described herein are hereby incorporated by reference in their entirety. Priority US provisional application serial no. 61/582,192 filed December 30, 2011, is hereby incorporated by reference in its entirety for all purposes.

Various terms are further described herein below:

"A", "an", and "the" refers to "at least one" or "one or more" unless specified otherwise.

"Optionally substituted" groups refers to, for example, functional groups that may be substituted or unsubstituted by additional functional groups. For example, when a group is unsubstituted by an additional group it can be referred to as the group name, for example alkyl or aryl. When a group is substituted with additional functional groups it may more generically be referred to as substituted alkyl or substituted aryl. "Alkyl" refers to, for example, straight chain and branched monovalent alkyl groups having from 1 to 24 carbon atoms. This term is exemplified by groups such as for example methyl, ethyl, n-propyl, iso-propyl, n-butyl, t-butyl, n-pentyl, ethylhexyl, dodecyl, isopentyl, and the like.

"Aryl" refers to, for example, a monovalent aromatic carbocyclic group of from 6 to 14 carbon atoms having a single ring (e.g., phenyl) or multiple condensed rings (e.g., naphthyl or anthryl) which condensed rings may or may not be aromatic provided that the point of attachment is at an aromatic carbon atom. Preferred aryls include phenyl, naphthyl, and the like.

"Heteroalkyl" refers to, for example, an alkyl group wherein one or more carbon atom is substituted with a heteroatom. The heteroatom can be, for example, O, S, N, Se, Te, Ge, etc.

"Heteroaryl" refers to, for example, an aryl group wherein one or more carbon atom is substituted with a heteroatom. The heteroatom can be, for example, O, S, N, Se, Te, Ge, etc.

"Alkoxy" refers to, for example, the group "alkyl-O-" which includes, by way of example, methoxy, ethoxy, n-propyloxy, iso-propyloxy, n-butyloxy, t-butyloxy, n-pentyloxy, 1-ethylhex-l-yloxy, dodecyloxy, isopentyloxy, and the like.

"Aryloxy" refers, for example, to the group "aryl-O-" which includes, by way of example, phenoxy, naphthoxy, and the like.

"Thioalkyl" refers to, for example, the group "alkyl-S-" which includes, by way of example, thiomethyl, thioethyl, and the like.

"Thioaryl" refers, for example, to the group "aryl-S-" which includes, by way of example, thiophenyl, thionaphthyl, and the like.

"Salt" refers to, for example, derived from a variety of organic and inorganic counter ions well known in the art and include, by way of example only, sodium, potassium, calcium, magnesium, ammonium, tetraalkylammonium, and the like; and when the molecule contains a basic functionality, salts of organic or inorganic acids, such as hydrochloride, hydrobromide, tartrate, mesylate, acetate, maleate, oxalate and the like.

FIRST POLYMER SUBUNIT

Many embodiments described herein relate to a copolymer comprising a first polymer subunit and a second polymer subunit, wherein the first polymer subunit is represented by:

wherein yi and y₂ are each 0, 1 , 2, 3 or 4; X_{l s} X₂ and X₃ are each independently a heteroatom; and Ri and R₂ are each independently a hydrogen, a fluorine, or an optionally substituted linear, branched, or cyclic C₁-C24 organic group.

Xi, X₂ and X₃ can be, for example, each a heteroatom such as O, N, S, Se, Ge or Te. In one embodiment, X_{l s} X₂ and X₃ are each S. In another embodiment, X₁ and X₂ are each S, and X₃ is N, O, Se, Ge or Te.

yi and y₂ can be, for example, both 0, or both 1 , or both 2, or both 3, or both 4, or 0 and 1 respectively, or 1 and 0 respectively, or 0 and 2 respectively, or 2 and 0 respectively, or 0 and 3 respectively, or 3 and 0 respecticely, or 0 and 4 respectively, or 4 and 0 respectively, or 1 and 2 respectively, or 2 and 1 respectively, or 1 and 3 respectively, or 3 and 1

respectively, or 1 and 4 respectively, or 4 and 1 respectively, or 2 and 3 respectively, or 3 and 2 respectively, or 2 and 4 respectively, or 4 and 2 respectively, or 3 and 4 respectively, or 4 and 3 respectively.

Ri and R₂ can be, for example, each a hydrogen. Ri and R₂ can be, for example, each a fluorine. Ri and R₂ can also be, for example, each an optionally substituted linear, branched, or cyclic C1-C24 organic group.

Said optionally substituted C₁-C24 organic group can be, for example, an optionally substituted alkyl, an optionally substituted aryl, an optionally substituted heteroaryl, or an optionally substituted heteroaryl. An optionally substituted aryl can be, for example, a perfluoroalkyl or an aryl-substituted alkyl group. An optionally substituted aryl can be, for example, a perfluoroaryl or an alkyl-substituted aryl group. An optionally substituted heteroalkyl can be, for example, an alkoxy, a perfluoroalkoxy, a thioalkyl, or a

perfluorothioalkyl. An optionally substituted heteroaryl can be, for example, an aryloxy, a perfluoroaryloxy, a thioaryl, or a perfluorothioaryl. The Ci-C₂4 organic group can comprise linear, branched, or cyclic functional groups.

Examples of the optionally substituted Ci-C₂4 organic group also include alkyl sulfoxide, perfluoroalkyl sulfoxide, alkyl sulfone, perfluoroalkyl sulfone, pyridyl, thiophene, furan, pyrrole, diazole, triazole, oxadiazole, carbonyl alkyl/aryl (e.g., "alkyl/aryl-C(0)-"), carboxyl alkyl/aryl (e.g., "alkyl/aryl-C(0)-0-"), ether (e.g., "alkyl/aryl-O-alkylene/arylene-"), ester (e.g., "alkyl/aryl-C(0)-0-alkylene/arylene-"), ketone(e.g. , "alkyl/aryl-C(0)- alkylene/arylene-"), and cyano. One or more hydrogen atoms and/or carbon atoms of said C₁-C24 organic group can be further substituted with known chemical groups.

In some embodiments, Ri and R₂ are each independently a hydrogen, a fluoride, a cyano, or an optionally substituted C4-C24 alkyl, alkoxy, or thioalkyl. In further embodiments, Ri and R₂ are each independently a C4-C24 branched alkyl group.

In other embodiments, the first polymer subunit is represented by:

wherein each "a" is independently 0, 1 , 2, 3 or 4, and "b" is 0 or 1.

Each X and X' can be, for example, independently a heteroatom. Each X can be, for example O, S, Se, Te or NR' wherein R' is hydrogen, a C₁-C30 normal, branched, or cyclic alkyl group. Each X can be different or the same. In one embodiment, each X is S.

X' can be, for example, S, Se or Te. In one embodiment, X' is S.

Each Y and Y' can be, for example, N or CR", wherein R" is hydrogen, fluorine, cyano, or a C₁-C30 normal, branched, or cyclic alkyl, perfluoroalkyl, alkoxy, thioalkyl, or thioalkoxy group. Each Y and Y' can be, for example, CR" . Each Y and Y' can be different or the same. In one embodiment, each Y is CH, each Y' is CR" with R" being an optionally substituted C₁-C30 alkyl, alkoxy, or thioalkyl. In another embodiment, each Y is CH, each Y' is CR" with R" being a branched alkyl. Additionally, each pair of Y and Y' can form a ring.

Each "a" can be different or the same, "a" and "b" can be different or the same. In one embodiment, each "a" and "b" is 1.

can be for example, independently selected from:

In a articular embodiment, the first polymer subunit is represented by:

wherein each Rl is independently a linear or branched alkyl, alkoxy, thioalkyl or polyether group. In a particular embodiment, each Rl is a branched alkyl group such as 2-ethylhexyl.

SECOND POLYMER SUBUNIT (L)

Many embodiments described herein relate to a copolymer comprising a first polymer subunit and a second polymer subunit, wherein the second polymer subunit (L) can be either an electron donating moiety or electron accepting moiety. The second polymer subunit (L) can be represented by, for example, the following structures:

wherein each X is independently S, O, N or Se, and wherein R₃ and R₄ are each independently a hydrogen, a fluorine, or an optionally substituted linear, branched, or cyclic C1-C24 organic group.

R₃ and R₄ can be, for example, each a hydrogen. R₃ and R₄ can be, for example, each a fluorine. R₃ and R₄ can also be, for example, each an optionally substituted linear, branched, or cyclic Ci-C₂₄ organic group.

Said optionally substituted Ci-C₂₄ organic group can be, for example, an optionally substituted alkyl, an optionally substituted aryl, an optionally substituted heteroaryl, or an optionally substituted heteroaryl. An optionally substituted aryl can be, for example, a perfluoroalkyl or an aryl-substituted alkyl group. An optionally substituted aryl can be, for example, a perfluoroaryl or an alkyl-substituted aryl group. An optionally substituted heteroalkyl can be, for example, an alkoxy, a perfluoroalkoxy, a thioalkyl, or a

perfluorothioalkyl. An optionally substituted heteroaryl can be, for example, an aryloxy, a perfluoroaryloxy, a thioaryl, or a perfluorothioaryl. The Ci-C₂₄ organic group can comprise linear, branched, or cyclic functional groups.

Examples of the optionally substituted Ci-C₂₄ organic group also include alkyl sulfoxide, perfluoroalkyl sulfoxide, alkyl sulfone, perfluoroalkyl sulfone, pyridyl, thiophene, furan, pyrrole, diazole, triazole, oxadiazole, carbonyl alkyl/aryl (e.g., "-C(0)-alkyl/aryl"), carboxyl alkyl/aryl (e.g., "-0-C(0)-alkyl/aryl"), ether (e.g., "-alkylene/arylene-O-alkyl/aryl"), ester (e.g., "-alkylene/arylene-0-C(0)-alkyl/aryl"), ketone(e.g. , "-alkylene/arylene-C(0)- alkyl/aryl"), and cyano. One or more hydrogen atoms and/or carbon atoms of said Ci-C₂₄ organic group can be further substituted with known chemical groups.

In some embodiments, R₃ and R₄ are each independently (a hydrogen), a fluoride, a cyano, or an optionally substituted C₄-C₂₄ alkyl, alkoxy, or thioalkyl. In further embodiments, Ri and R₂ are each independently a C₄-C₂₄ branched alkyl group.

In some embodiments the second polymer subunit (L) is selected from

In additional, the second polymer subunit can be selected from the following:

and X ; wherein each X is independently O, S, Se, Te or NR', said R' is hydrogen or a C₁-C30 linear, branched or cyclic alkyl group; wherein each Y is independently N or CR", said R" is hydrogen, fluorine, cyano, or a C1-C30 linear, branched or cyclic alkyl, perfluoroalkyl, alkoxy, thioalkyl, or thioalkoxy group; and wherein R is hydrogen, fluorine, cyano, or a C₁-C30 linear, branched or cyclic alkyl, perfluoroalkyl, alkoxy, thioalkyl, or thioalkoxy group.

In some embodiments, the second polymer subunit is different from the first polymer subunit.

COPOLYMER Many embodiments described herein relate to a copolymer represented by:

above; and L represents the second polymer subunit described above.

Other embodiments described herein relate to a copolymer represented by:

; wherein n is 1 or more;

represents the first polymer subunit described above; and

L represents the second polymer subunit described above.

As known in the art of synthetic polymers, n can represent an average number of repeat units derived from number average molecular weight and the molecular weight of the repeat unit. The value "n" can be 1 or more, 2 or more, 5 or more, 10 or more, or 50 or more, for example.

The weight-average molecular weight (M_w) of the copolymer can be, for example, 5 kDa or higher, or 8 kDa or higher, or 10 kDa or higher, or 12 kDa or higher, or 15 kDa or higher, or 20 kDa or higher, or 25 kDa or higher, or 30 kDa or higher, or 40 kDa or higher, or 50 kDa or higher, or 60 kDa or higher. The number-average molecular weight (M_n) of the copolymer can be, for example, 3 kDa or higher, or 4 kDa or higher, or 5 kDa or higher, or 6 kDa or higher, or 8 kDa or higher, or 10 kDa or higher, or 12 kDa or higher, or 15 kDa or higher, or 20 kDa or higher, or 25 kDa or higher, or 30 kDa or higher

The onset decomposition temperature (T_d) of the copolymer can be, for example, 200 °C or higher, or 220 °C or higher, 240 °C or higher, or 260 °C or higher, or 280 °C or higher, or 300 °C or higher, or 320 °C or higher, or 340 °C or higher, or 360 °C or higher, or 380 °C or higher, or 400 °C or higher.

The ionization potential (IP) of the copolymer can be, for example, 4.5 eV or higher, or 4.6 eV or higher, or 4.7 eV or higher, or 4.8 eV or higher, or 4.9 eV or higher, or 5.0 eV or higher, or 5.1 eV or higher, or 5.2 eV or higher, or 5.3 eV or higher.

The electron affinity (EA) of the copolymer can be 3.0 eV or higher.

The optical band gap of the copolymer can be, for example, 1.8 eV or lower, or 1.7 eV or lower, or 1.6 eV or lower, or 1.5 eV or lower, or 1.4 eV or lower, or 1.3 eV or lower, or 1.2 eV or lower, or 1.1 eV or lower, or 1.0 eV or lower, or 0.9 eV or lower, or 0.8 eV or lower. The electrochemical band gap of the copolymer can be, for example, 2.0 eV or lower, or 1.9 eV or lower, or 1.8 eV or lower, or 1.7 eV or lower, or 1.6 eV or lower, or 1.5 eV or lower, or 1.4 eV or lower, 1.3 eV or lower, or 1.2 eV or lower, or 1.1 eV or lower.

The absorption maximum (λ_^) of the higher energy band due to π-π* transition of the copolymer in solution can be in the range of 350-650 nm. The absorption maximum

(λωκ) of the higher energy band due to π-π* transition of the copolymer in thin film can be in the range of 400-650 nm.

The absorption maximum (λ_^) of the intramolecular charge transfer (ICT) band of the copolymer in solution can bein the range of 700-1500 nm. The absorption maximum

(λωκ) of the ICT band of the copolymer in thin film can bein the range of 700-2000 nm.

Examples of the copolymer include, but are not limited to, the following:

SYNTHESIS OF COPOLYMER

Methods for synthesizing thienothiadiazole groups are known in the art, including the cited references, and also described in the working examples and Figure 3. Methods for synthesizing copolymers comprising thienothiadiazole groups are known in the art, including the cited references, and also described in the working examples and Figures 4-A and 4-B.

DEVICES COMPRISING PTTDs

Many embodiments described herein relate to novel organic electronic devices comprising the copolymers described herein, including transistors including field effect transistors, photodetectors, photovoltaic devices and light-emitting devices. Each of these applications typically comprises the formation of a film of the copolymers of the invention on a substrate. Organic films of the polymer described herein can be prepared by known methods such as spin coating methods, casting methods, dip coating methods, inkjet methods, doctor blade coating methods, screen printing methods, and spray coating methods. By using such methods, organic films having good properties such as mechanical strength, toughness, and durability can be prepared without forming cracks in the films. Therefore, the organic films can be preferably used for organic electronic devices such as organic field-effect transistor (OFETs), photodetectors, solar cells and organic light-emitting diodes (OLEDs).

Films of the copolymer described herein are typically prepared by coating polymer solution, which is prepared by dissolving the copolymer in a solvent such as dichloromethane, tetrahydrofuran, chloroform, toluene, chlorobenzene, dichlorobenzene, or xylene, on a substrate. Specific examples of the coating methods include spray coating methods, spin coating methods, blade coating methods, dip coating methods, cast coating methods, roll coating methods, bar coating methods, die coating methods, ink jet methods, dispense methods, etc. In this regard, methods and solvents are selected in consideration of the properties of the polymer used or intended device application.

Suitable materials for use as the substrate on which a film of the polymer described herein is formed include inorganic substrates such as glass plates, silicon plates, indium tin oxide (ITO) plates, FTO plates, ITO-coated glass plates, and FTO-coated glass plates, and organic substrates such as plastic plates (e.g., PET films, polyimide films, and polystyrene films) and ITO or FTO coated plastic plates, which can be optionally subjected to a surface treatment. It is preferable that the substrate has a smooth surface.

The thickness of the organic film and the organic semiconductor layer of the organic thin film transistor described herein are not particularly limited. However, the thickness is determined such that the resultant film or layer is a uniform thin layer (i.e., the film or layer does not include gaps or holes adversely affecting the carrier transport property thereof). The thickness of the organic semiconductor layer is generally not greater than 1 micron, and preferably from 5 to 200 nm.

TRANSISTORS COMPRISING PTTDs

In some embodiments, the devices described herein comprise a field-effect transistor comprising at least one conjugated copolymer, wherein the copolymer comprises at least one donor moiety and at least one acceptor moiety, and wherein the acceptor moiety is an optionally substituted thieno[3,4-c][l,2,5]thiadiazole.

In one embodiment, the field-effect transistor comprises a thin- film of the copolymer described herein. The thin film can be deposited from a solution of the copolymer. The thin- film can be fabricated by spin coating. The thin-film can be fabricated by vacuum vapor deposition. The thin- film can be annealed at a temperature of, for example, 150 °C or higher, or 170 °C or higher, or 190 °C or higher, or 210 °C or higher, or 230 °C or higher, or 250 °C or higher.

The carrier mobility of the field-effect transistor can be, for example, 1 x 10 -^"4 cm 2 /V s or higher, or 2.5 x 10 -^"4 cm 2 /Vs or higher, or 5 ^x 10 -^"4 cm 2 /Vs or higher, or 7.5 x 10 -^"4 cm 2 /Vs or higher, or 1 x 10 -^"3 cm 2 /Vs or higher, or 2.5 ^x 10 -^"3 cm 2 /Vs or higher, or 5 ^x 10 -^"3 cm 2 /Vs or higher, or 7.5 x 10 -^"3 cm 2 /Vs or higher, or 1 x 10 -^"2 cm 2 /Vs or higher. 2

The on/off current ratio of the field-effect transistor can be, for example, about 10 - 10⁶, or about 10²-10³, or about 10³-10⁴, or about 10⁴-10⁵, or about 10⁵-10⁶.

The organic thin film transistors described herein typically have a configuration such that an organic semiconductor layer including the copolymer described herein is formed therein while also contacting the source electrode, drain electrode and insulating dielectric layer of the transistor.

The organic thin film transistor prepared above is typically thermally annealed.

Annealing is performed while the film is set on a substrate, and is believed (without wishing to be bound by theory) to allow for at least partial self-ordering and/or π-stacking of the copolymer chains to occur in the solid state. The annealing temperature is determined depending on the property of the polymer, but is preferably from room temperature to 300 °C, and more preferably from 50 to 300 °C. In many embodiments, thermal annealing is carried out at 150 °C or more, or preferably at 170 ° C or more, or at 200 ° C or more. When the annealing temperature is too low, the organic solvent remaining in the organic film cannot be well removed therefrom. In contrast, when the annealing temperature is too high, the organic film can be thermally decomposed.

Annealing is preferably performed in a vacuum, or under nitrogen, argon or air atmosphere. In some embodiments annealing is performed in an atmosphere including a vapor of an organic solvent capable of dissolving the polymer so that the molecular motion of the polymer is accelerated, and thereby a good organic thin film can be prepared. The annealing time is properly determined depending on the aggregation speed of the polymer.

An insulating (dielectric) layer is used in the organic thin film transistors comprising the copolymers described herein, situated between the gate electrode and the organic thin film comprising the polymers. Various insulating materials can be used for the insulating layer. Specific examples of the insulating materials include inorganic insulating materials such as silicon oxide, silicon nitride, aluminum oxide, aluminum nitride, titanium oxide, tantalum oxide, tin oxide, vanadium oxide, barium strontium titanate, barium zirconate titanate, lead zirconium titanate, lead lanthanum titanate, strontium titanate, barium titanate, barium magnesium fluoride, bismuth tantalate niobate, hafnium oxide, and trioxide yttrium; organic insulating materials such as polymer materials, e.g., polyimide, polyvinyl alcohol, polyvinyl phenol, polystyrene, polyester, polyethylene, polyphenylene sulfide, unsubstituted or halogen-atom substituted polyparaxylylene, polyacrylonitrile, and cyanoethylpullulan; etc. These materials can be used alone or in combination. Among these materials, materials having a high dielectric constant and a low conductivity are preferably used. Suitable methods for forming such an insulating layer include dry processes such as CVD methods, plasma CVD methods, plasma polymerization methods, and vapor deposition methods; wet processes such as spray coating methods, spin coating methods, dipcoating methods, inkjet coating methods, castcoating methods, blade coating methods, and bar coating methods; etc.

In order to improve the adhesion between the insulating layer and organic

semiconductor layer, to promote charge transport, and to reduce the gate voltage and leak current, an organic thin film (intermediate layer) can be employed between the insulating layer and organic semiconductor layer. The materials for use in the intermediate layer are not particularly limited as long as the materials do not chemically affect the properties of the organic semiconductor layer, and for example, molecular films of organic materials, and thin films of polymers can be used therefore. Specific examples of the materials for use in preparing the molecular films include coupling agents such as octadecyltrichlorosilane, octyltrichlorosilane, octyltrimethoxysilane, hexamethyldisilazane (HMDS), and

octadecylphosphonic acid. Specific examples of the polymers for use in preparing the polymer films include the polymers mentioned above for use in the insulating layer. Such polymer films can serve as the insulating layer as well as the intermediate layer.

The materials of the electrodes (such as gate electrodes, source electrodes and drain electrodes) of the organic thin film transistor described herein are not particularly limited as long as the materials are electrically conductive. Specific examples of the materials include metals such as platinum, gold, silver, nickel, chromium, copper, iron, tin, antimony, lead, tantalum, indium, aluminum, zinc, tungsten, titanium, calcium, and magnesium; alloys of these metals; electrically conductive metal oxides such as indium tin oxide (ITO); inorganic or organic semiconductors, whose electroconductivity is improved by doping or the like, such as silicon single crystal, polysilicon, amorphous silicon, germanium, graphite, carbon nanotube, polyacetylene, polyparaphenylene, polythiophene, polypyrrole, polyaniline, polythienylenevinylene, polyparaphenylenevinylene, and complexes of

polyethylenedioxythiophene (PEDOT) and polystyrene sulfonic acid.

SOLAR CELLS COMPRISING PTTDs

Solar cells described herein can be fabricated by first spin-coating a PEDOT buffer layer on top of ITO-coated glass substrates (10 Ω/sq, Shanghai B. Tree Tech. Consult Co., Ltd, Shanghai, China) at 1500 rpm for 60 s and dried at 150°C for 10 min under vacuum. The thickness of PEDOT was around 40 nm. The active layer of the solar cells comprising the polymers of the invention normally comprise a mixed "heterojunction" active layer that is a phase separated blend of the polymers or copolymers described above and an electron acceptor material. The electron acceptor material can comprise a variety of organic materials (small molecules, oligomers, polymers, or copolymers) that have a LUMO energy level that is at least about 0.2 to 0.6 eV more negative than the LUMO energy level of the copolymers described herein, and a

HOMO energy level that is more negative than the HOMO energy level of the copolymers described herein. In many embodiments, the electron acceptor material can be a fullerene or a modified fullerene (e.g., C6i-phenyl-butyric acid methyl ester, PC₆iBM, or C₇i-phenyl- butyric acid methyl ester, PC₇iBM). In other embodiments, the electron acceptor material can be an electron accepting semiconducting organic small molecule, oligomer, or polymer having appropriate LUMO and HOMO energies (at least about 0.2-0.6 eV more negative than the LUMO energy level and a more negative HOMO energy level than the HOMO energy level of the copolymers described herein). Examples of such electron acceptor materials can include small molecules, oligomers, polymers, or copolymers having highly electron deficient functional groups, such as for example napthalene diimide, perylene diimide, rylene, phalimide, and related derivatives comprising electron accepting groups.

In many embodiments of the solar cells of Applicants' inventions, a composition comprising a solution or dispersion of one or more of Applicants' polymers or copolymers and one or more acceptor materials (for example fullerene derivatives) is spin-coated on top of the PEDOT layer, for example at a speed of 1000 rpm for 30 seconds, to form a layer comprising the one or more copolymers and one or more electron accepting materials. In some embodiments, the solution or dispersion is applied using a hot solvent, and dried under vacuum immediately after the deposition the copolymers.

The coated device precursor can then be annealed, for example on a hot plate at 130 ± 10 °C for 10 min in a glove box, to form the active layer. The active layer can also be spin- coated in air and dried in a vacuum oven without thermal annealing. The solvents used for dissolving the mixture of copolymers of the invention and the electron acceptors can be chloroform, chlorobenzene, 1 ,2-dichlorbenzene, etc. The solvents for copolymer/fullerene blend can be a single solvent such as chloroform, chlorobenzene, 1 ,2-dichlorbenzene or a mixture of two or three different solvents, the second (third) solvent can be 1,8-diiodooctane, 1,8-dibromoctane, 1,8-octanedithiol, etc.

Optionally, the solvents can be heated so as to increase the solubility of the polymer and/or electron acceptor, as an aid to film formation. Thermal annealing is believed to induce at least partial phase separation between the polymers of the invention and the electron acceptors, forming the "heterojunctions" on the nanometer scale that are believed to be the site of light-induced charge separation.

After cooling down, the solar cell precursors comprising the polymer-coated substrates were taken out of the glove box and loaded in a thermal evaporator (BOC Edwards, 306) for the deposition of the cathode. The cathode consisting of 1.0 nm LiF and 80 nm aluminum layers was sequentially deposited through a shadow mask on top of the active layers in a vacuum of 8x 10—7 torr. Each substrate contained 5 solar cells with an active area of 4 mm.

The maximum photovoltaic efficiency of the solar cells described herein can be, for example, 0.1% of more, or 0.3% or more, or 0.5% or more, or 1% or more.

WORKING EXAMPLES

Materials. 2,5-Bromo-3,4-dinitrothiophene and trans- l,2-bis(tri-n-butyl-stannyl)ethylene were purchased from Fisher Scientific Inc. 2,6-Dibromo-4,8-bis(2-ethylhexyloxy)benzo[l,2- b:4,5-b']dithiophene was purchased from Luminescence Technology Corp (Taiwan). All other chemicals were purchased from Sigma-Aldrich. 4,6-Bis(5-bromo-3-ethylhexyl-2- thienyl)thieno[3,4-c][1.2.5]thiadiazole (5)²⁴ and 2,6-di(trimethyltin)-N-(l-

27

hexyldecyl)dithieno[3,2-b:2',3'-d]pyrrole were synthesized according to known procedures. See Figures 10 and 11, respectively, for 1H NMR results and chromatograph mass spectrum results of (5).

Example 1 - Synthesis of Poly(4,6-bis(3-ethylhexyl-2-thienyl)thieno[3,4- c] [l,2,5]thiadiazole-alt-phenylene) (PTTP)

4,6-Bis(5-bromo-3-ethylhexyl-2-thienyl)thieno[3,4-c][1.2.5]thiadiazole (5) (269.5 mg, 0.39 mmol), 1 ,4-bis(tributylstannyl)benzene (256.8 mg, 0.39 mmol), Pd₂(dba)₃ (7.16 mg, 0.0078 mmol) and P(o-tolyl)₃ (9.53 mg, 0.0313 mmol) were added into a 100 mL three-neck round-bottom flask. The flask equipped with a condenser was then degassed and filled with argon three times. Afterwards, 15 mL of chlorobenzene was added and degassed and filled with argon three times. The reaction mixture was refluxed for 24 h under argon. After cooling down to room temperature, the polymerization mixture was poured and stirred into 200 mL methanol and 5 mL hydrochloric acid solution for 5 h. The polymer precipitated out as a dark green solid and was filtered using a filter paper. The polymer was purified by Soxhlet extraction with methanol and acetone (183 mg; yield = 73.6 %). Figure 12-B shows 1H NMR results (CDC1₃, 300 MHz): δ (ppm) 7.7 (6H), 3.0 (4H), 1.83 (2H), 1.3-0.8 (28H).

Example 2 - Synthesis of Poly(4,6-bis(3-ethylhexyl-2-thienyl)thieno[3,4- c] [1 ,2,5] thiadiazole-alt-4,8-bis(2-ethylhexyloxy)benzo [ 1 ,2-b: 4,5-b ' ] dithiophene)

(PTTBDT)

4,6-Bis(5-bromo-3-ethylhexyl-2-thienyl)thieno[3,4-c][l .2.5]thiadiazole (5) (385.42 mg, 0.562 mmol), 2,6-di(tributylstannyl)-4,8-bis(2-ethylhexyloxy) benzo[l,2-b:4,5- b']dithiophene (432.7 mg, 0.562 mmol), Pd₂(dba)₃ (10.29 mg, 0.011 mmol) and P(o-tolyl)₃ (13.7 mg, 0.045 mmol) were added into a 100 mL three-neck round-bottom flask. The flask equipped with a condenser was then degassed and filled with argon three times. Afterwards, 35 mL of chlorobenzene was added and degassed and filled with argon three times. The reaction mixture was refluxed for 72 h under argon. After cooling down to room temperature, the polymerization mixture was poured and stirred into 200 mL methanol and 5 mL

hydrochloric acid solution for 5 h. The polymer precipitated out as a dark green solid and was filtered using a filter paper. The polymer was purified by Soxhlet extraction with methanol and hexane (320 mg; yield = 60 %). Figure 12-A shows 1H NMR results (CDC1₃, 300 MHz): δ (ppm) 7.16 (2H), 7.0 (2H), 3.99 (4H), 2.9 (4H), 1.9-0.9 (60 H).

Example 3 - Synthesis of Poly(4,6-bis(3-ethylhexyl-2-thienyl)thieno[3,4- c] [l,2,5]thiadiazole-alt-vinylene) (PTTV)

4,6-Bis(5-bromo-3-ethylhexyl-2-thienyl)thieno[3,4-c][1.2.5]thiadiazole (5) (612.6 mg, 0.89 mmol), trans- l,2-bis(tri-n-butyl-stannyl)ethylene (539.2 mg, 0.89 mmol), Pd₂(dba)₃ (16.3 mg, 0.018 mmol) and P(o-tolyl)₃ (21.66 mg, 0.071 mmol) were added into a 100 mL three-neck round-bottom flask. The flask equipped with a condenser was then degassed and filled with argon three times. Afterwards, 30 mL of chlorobenzene was added and degassed and filled with argon three times. The reaction mixture was refluxed for 16 h under argon. After cooling down to room temperature, the polymerization mixture was poured and stirred into 200 mL methanol and 5 mL hydrochloric acid solution for 5 h. The polymer precipitated out as a dark solid and was filtered using a filter paper. The polymer was purified by Soxhlet extraction with methanol, hexane and chloroform (450 mg; yield = 86.5 %). 1H NMR (CDC1₃, 300 MHz): δ (ppm) 7.14 (2H), 7.0 (2H), 4.08 (4H), 2.03 (2H), 1.35-0.8 (28H). Example 4 - Synthesis of Poly(4,6-bis(3-ethylhexyl-2-thienyl)thieno[3,4- c] [l,2,5]thiadiazole-alt-N-(l-hexyldecyl)dithieno[3,2-b:2',3'-d]pyrrole) (PTTDTP)

4,6-Bis(5-bromo-3-ethylhexyl-2-thienyl)thieno[3,4-c][1.2.5]thiadiazole (5) (221.8 mg, 0.323 mmol), 2,6-di(trimethyltin)-N-(l-hexyldecyl)dithieno[3,2-b:2',3'-d]pyrrole (235.8 mg, 0.323 mmol), Pd₂(dba)₃ (5.92 mg, 0.00646 mmol) and P(o-tolyl)₃ (7.87 mg, 0.0258 mmol) were added into a 100 mL three-neck round-bottom flask. The flask equipped with a condenser was then degassed and filled with argon three times. Afterwards, 13 mL of chlorobenzene was added and degassed and filled with argon three times. The reaction mixture was refluxed for 48 h under argon. After cooling down to room temperature, the polymerization mixture was poured and stirred into 200 mL methanol and 5 mL

hydrochloride solution for 5 h. The polymer precipitated out as a dark solid and was filtered by a filter paper. The polymer was purified by Soxhlet extraction with methanol and hexane (220 mg; yield = 70.8 %). Figure 12-C shows 1H NMR results (CDC1₃, 300 MHz): δ (ppm) 7.14 (2H), 6.99 (2H), 4.07 (4H), 2.03 (2H), 1.3-0.7 (61H).

Example 5 - Synthesis of 2,6-Di(tributylstannyl)-4,8-bis(2-ethylhexyloxy)benzo[l,2- b:4,5-b']dithiophene

2,6-Dibromo-4,8-bis(2-ethylhexyloxy)benzo[l,2-b:4,5-b']dithiophene (330 mg, 0.546 mmol) was added into a 100 mL three-neck round-bottom flask. Afterwards, 14 mL THF was added under argon. The mixture was cooled to -78 °C in a dry ice bath and n-BuLi (0.75 mL, 1.19 mmol) was added dropwise. After stirring the mixture for 45 min at -78 °C,

trimethyltinchloride (1.27 mL, 1.27 mmol) was added in one portion. Dry ice bath was removed after 5 min and the mixture warm up to room temperature. After stirring 2 h at room temperature, the reaction mixture was poured into water and extracted with diethyl ether 3 times. The organic phase was dried with sodium sulfate anhydrous and the solvent was evaporated by vacuum rotary evaporator. Yellow crystals were obtained and subsequently used in polymerization without further purification (553 mg; yield = 92 %). 1H NMR (CDC1₃, 300 MHz): δ (ppm) 6.99 (2H), 4.08 (2H), 2.05 (1H), 1.33-0.63 (30 H), 0.33 (18H).

Example 6 - Characterization of the PTTDs

To analyze the molecular and physical properties of the four copolymers, 1H NMR, thermogravimetry analysis (TGA), X-ray diffraction (XRD) and gel permeation

chromatography (GPC) analysis were performed. 1H NMR spectra at 300 MHz were recorded on a Bruker-AF300 spectrometer. TGA thermograms were obtained on a TA

Instruments Q50 TGA at a heating rate of 20 °C/min under nitrogen gas flow. XRD data were obtained from Bruker AXS D8 Focus diffractometer with a Cu Ka beam, and the samples were prepared by drop-casting of polymer solutions with chlorobenzene onto glass substrates. GPC analysis of the copolymers was performed on a Waters 1515 gel permeation

chromatograph against polystyrene standards in chlorobenzene at 60 °C as eluent with UV and RI detectors. Absorption spectra of the copolymers were measured on a Perkin-Elmer model Lambda 900 UV/vis/near-IR spectrophotometer. Solution and solid state absorption spectra were obtained from polymer solutions in chloroform and as thin films on glass substrates, respectively.

Cyclic voltammetry (CV) experiments were done on an EG&G Princeton Applied Research potentiostat/galvanostat (model 273 A) in an electrolyte solution of 0.1 M

tetrabutylammonium hexafluorophosphate (Bu₄NPF₆) in acetonitrile. A three-electrode cell was used in this analysis. Platinum wires were used as counter and working electrodes, and Ag/Ag⁺ (Ag in 0.1 M AgN0₃ solution, Bioanalytical System, Inc.) was used as a reference electrode. Ferrocene/ferrocenium was used as an internal standard by running CV at the end, and this data was used to convert the potential to saturated calomel electrode (SCE) scale. The films of the copolymers were coated onto the Pt wires by dipping the wires into 1 wt % chloroform or chlorobenzene polymer solutions.

Example 7 - Fabrication and characterization of field-effect transistors

Organic field-effect transistors (OFETs) were made on top of n-doped silicon with thermally grown oxide (200 nm) substrates. Gold source-drain electrodes (W=800 μιη and L=40 μιη) with a thin chromium adhesive layer were photographically patterned to form bottom-contact transistor devices. The surface of the oxide was treated with

octyltrichlorosilane (OTS8) by spin-coating a chloroform solution of OTS8 (4 mM) onto a cleaned substrate, followed by rinsing with toluene and annealing at 100 °C for 10 min in a fume hood. Polymer thin films spun from a solution in 1 ,2-dichlorobenzene (4-8 mg/mL) were annealed at 200 °C for 10 min under argon environment. Devices were tested by using an HP4145B semiconductor parameter analyzer in nitrogen- filled dry box. Electrical parameters were calculated by using the standard equation for metal-oxide-semiconductor field-effect transistors in the saturation region similar to previous reports.²⁸' ^5(b^ Example 8 - Fabrication and characterization of photovoltaic cells

Solar cells were fabricated by first spin-coating a PEDOT buffer layer on top of ITO- coated glass substrates (10 Ω/α) at 3500 rpm for 40 s and annealing at 150 °C for 10 min under vacuum. The thickness of PEDOT was around 30 nm. The active layer was spin-coated on top of the PEDOT in a glove box at a speed of 620 rpm for 20 s and annealed at 180 °C for 10 min. The device substrates were then loaded into a thermal evaporator for cathode deposition. The cathode, consisting of 1.0 nm LiF and 100 nm Al layers, was sequentially deposited through a shadow mask on top of the active layers after the chamber vacuum

-7 2

reached 8 x 10^" torr. The solar cells have an active area of 9.0 mm . The active layer had thickness of 60-80 nm. Film thickness was measured on an Alpha-Step 500 profilometer. Current- voltage characteristics were measured by using an HP4155A semiconductor parameter analyzer. The light intensity of AM 1.5 sunlight from a filtered Xe lamp was calibrated by a Si photodiode calibrated at the National Renewable Energy Laboratory (NREL). All the characterization steps were carried out under ambient laboratory air and

29

further details can be found in previous report.

Example 9 - PTTDs Synthesis and Characterization

The synthetic route to the dibromide monomer 5 is presented in Figure 3. The final monomer 5 was obtained in three steps from dinitroterthiophene 8. Reduction of compound 8 with hydrochloric acid and tin powder gave the compound 7, and following ring closing reactions of compound 7 with N-thionylaniline and chlorotrimethylsilane in pyridine the thienothiadiazole compound 6 was obtained. Finally, boromination of compound 6 with N- bromosuccinimide (NBS) gave the monomer 5. Monomer 5 was obtained as a blue solid and its molecular structure was verified by 1H NMR and LC mass spectrometry. The new polythienothiadiazoles (PTTDs) were synthesized by Stille coupling polymerization in the presence of Pd₂(dba)₃ and P(o-tolyl)3 using chlorobenzene as the solvent (Figure 4- A). Color changes from blue to green were observed during the polymerization reaction, and as a result, PTTP and PTTBDT were collected as dark green solids. In the case of PTTV and PTTDTP, the color of the polymerization solutions changed from blue to reddish purple whereas dark solids were obtained. PTTP, PTTBDT and PTTDTP are readily soluble in common organic solvents (e.g. chloroform, chlorobenzene) at room temperature. In the case of PTTV, the polymer precipitated after only 5 h Stille coupling polymerization; consequently, the final dark solid was only partially soluble in chlorobenzene even at 120 °C. The low solubility of the vinylene-linked PTTV comes from its highly coplanar and rigid backbone.

The molecular structures of the new PTTDs were verified primarily by 1H NMR spectra, which were in good agreement with the proposed structures of the copolymers. The molecular weight and polydispersity of the copolymers were measured by gel permeation chromatography (GPC) against polystylene standards in chlorobenzene at 60 °C. The number-average molecular weight (M_n) varied from 6.4 kDa in PTTBDT to 24.0 kDa in PTTDTP. In the case of PTTV, 1H NMR and GPC measurement could not be done because of the low solubility in organic solvents. The molecular weight of the PTTDs is thus moderate and can probably be increased further by optimizations of the polymerization conditions, including the monomer concentration of solution, catalyst and temperature. The thermal stability and thermal transition properties of the PTTDs were evaluated by thermo gravimetric analysis (TGA) and differential scanning calorimetry (DSC), respectively. The onset decomposition temperature (T_d) of all four polymers under nitrogen flow was in the range of 320-380 °C, indicating good thermal stability of the copolymers (Figure 13). The DSC scans of the PTTDs did not show any distinct transitions up to 350 °C (Figure 14).

Example 10 - Electrochemical Properties of PTTDs

The redox properties and electronic structures of the new D-A conjugated copolymers were investigated by cyclic voltammetry (CV) of thin films on platinum (Pt) electrodes. The oxidation and reduction cyclic voltammograms of PTTDs are shown in Figure 5. The cyclic voltammograms (CVs) of all four copolymers (PTTP, PTTBDT, PTTV, PTTDTP) showed a much larger oxidation current than reduction current. The oxidation waves were all irreversible whereas the reduction waves were quasi-reversible (Figure 5; see also Figure 16).

The ionization potential (IP)/electron affinity (EA) and their associated

HOMO/LUMO energy levels were estimated from the onset redox potentials extracted from the cyclic voltammograms (IP = eE_ox ^onset + 4.4 eV, EA = eE_red ^onset + 4.4 eV).³⁰ The IP and EA values are summarized in Table 2. The IP value or HOMO level of the copolymers at 4.9-5.1 eV vary only slightly with donor moiety. The EA value or LUMO level of the PTTD copolymers also varied in a very narrow range from 3.4 eV for PTTP to 3.6 eV for PTTV and PTTDTP. Compared to the known TTD-containing copolymer, poly(5,7-bis(4-decanyl-2- thienyl)-thieno(3,4-b)diathiazole-thiophene-2,5), which has HOMO/LUMO energy levels of 4.71 eV/3.59 eV,²¹ the present PTTDs have HOMO/LUMO energy levels that vary slightly with the donor moieties. The electrochemical band gap (E_g ^el = IP - EA) of the PTTDs varied from 1.3 eV in PTTDTP to 1.6 eV in PTTP and PTTBDT; the electrochemical band gap is 0.40-0.60 eV larger than the optical band gap E_g ^opt (Table 2) and this can in part be explained

31 32

by the strongly bound excitons in the materials. ' We note that the N-(l - hexyldecyl)dithieno[3,2-b:2',3'-d]pyrrole (DTP)-linked copolymer PTTDTP, had the smallest band gap among the four new PTTDs as a result of increased HOMO level and decreased LUMO level. In contrast, the weak donor moieties of PTTBDT and PTTP brought decreased HOMO level and increased LUMO level and thus a larger band gap energy.

Example 11 - Optical properties of PTTDs

The normalized optical absorption spectra of the PTTDs in dilute (10^~6 M) chloroform solutions and as spin-coated thin films on glass substrates are shown in Figure 6. All four copolymers are characterized by two absorption bands in both dilute solutions and as thin films. The higher energy absorption band can be assigned to a π-π* transition whereas the lower energy absorption band can be assigned to an intramolecular charge transfer (ICT) interaction between the TTD acceptor and the various donor moieties.⁴ In dilute solution, the absorption maximum (λ_^) of the higher energy band varied from 445 nm in PTTP to 521 nm in PTTDTP. The of the ICT absorption band varied from 760 nm in PTTP to 909 nm in PTTDTP.

The ICT band of the thin film absorption spectra of the PTTDs is significantly red- shifted compared to the solution spectra. Although the higher energy band centered at 481- 515 nm shifted very little from the solution spectra, the ICT absorption maximum varied from 843 nm in PTTBDT to 941 nm in PTTV. Compared to the solution spectra, PTTP and PTTV showed a large redshift of 104 nm and 106 nm, respectively, in the ICT absorption band whereas PTTBDT had a little redshift of 37 nm and PTTDTP was unchanged. The large redshift in the thin film absorption spectra compared to the solution spectra, can be explained by the increased planarization and strong intermolecular interactions of the copolymer chains in the solid state.

The optical band gap of the PTTDs varied from 1.2 eV in PTTBDT to 0.9 eV in PTTV and PTTDTP (Table 2). The relatively weak donor moieties of PTTP and PTTBDT and thus smaller ICT interactions can explain their larger optical band gaps compared to PTTV and PTTDTP. The relatively narrow band gap of PTTV can be explained by the increased coplanarity enabled by the rigid vinylene group in its backbone. The rather small optical band gap of PTTDTP is derived from the strong electron-donating N-(l- hexyldecyl)dithieno[3,2-b:2',3'-d]pyrrole (DTP) donor moiety, which facilitates the strongest ICT interaction among the four polymers. Compared to other organic semiconductors already reported based on different acceptor moieties, such as BTD or the others in the homologous

12-17

series including BSe and BX, " the new thienothiadiazole-based PTTDs have a broad absorption that extends to near infrared region. The four new copolymers are thus good candidates for applications in photodetectors and photovoltaics.

Example 12 - Field-Effect Transistors

The charge transport properties of the new copolymer semiconductors were

investigated by fabricating and testing field-effect transistors with bottom-contact and bottom-gate geometry on hydrophobically modified gate dielectric layer. All of the PTTD transistors exhibited p-channel characteristics with good current modulation and saturation, as shown in Figure 7. The electrical parameters of the PTTD OFETs are collected in Table 3. PTTP and PTTBDT showed saturation hole mobilities of 4.6 10^"3 cm²/Vs and 3.2x l0^~3 cm /V s, respectively. On the other hand, PTTV and PTTDTP had lower carrier mobilities of

-4 2 -4 2

2.5 x 10^" cm /V s and 6.1 x 10^" cm /V s, respectively. The one order of magnitude lower carrier mobility of PTTV and PTTDTP compared to the other PTTDs can be explained by the difference in crystallinity as revealed by XRD (Figure 15). PTTBDT films showed a lamellar diffraction peak at 2Θ = 5.54° with the corresponding d-spacing of 15.94 A, and PTTP showed lamellar diffraction peak at 2Θ = 6.06° with the d-spacing of 14.6 A. These peaks are considered to be from (100) diffraction of lamellar planes of edge-on oriented polymers. We were not able to resolve any peaks from other diffractions. In contrast, PTTDTP and PTTV films did not show any X-ray diffraction peaks, indicating largely amorphous morphology. The more crystalline PTTD thin films had higher carrier mobility. No sign of significant contact resistance was observed in the output curves of the PTTD transistors. The HOMO energy levels of the copolymers (4.9-5.1 eV) are well-matched with the work function of the gold source/drain electrodes (5.1 eV), suggesting that the hole transport is not limited by charge injection barrier in the series of PTTDs. Electron-transport was not observed despite the relatively low- lying LUMO energy levels (3.4-3.6 eV) of PTTDs. The on/off current ratios of PTTP and PTTV OFETs were 10⁴ and those of PTTBDT and PTTDTP devices were 10 . Overall, small hysteresis between forward and backward scans was observed except for the transfer curves of PTTBDT and PTTV. The PTTBDT transistor also exhibited a large positive threshold voltage of 18.3 V on average and a relatively large off-current of 0.01-0.1 μΑ. The OFETs based on the other copolymers had negative threshold voltages (-8.1 V for PTTV, -12.5 V for PTTP, and -16.2 V for PTTDTP) and lower off-current of less than 1 nA. The origin of the large hysteresis, the large off- current, and a positive threshold voltage in PTTBDT transistors are not clear at present. Extrinsic molecules and/or impurities might have caused unintentional doping, resulting in the large hysteresis and off-current as well as the positive threshold voltage.

Figure 9 shows embodiments of the thienothiadiazole-based conjugated copolymers for electronics and optoelectronics.

Example 13 - Photovoltaic Properties

Bulk heterojunction (BHJ) polymer solar cells using a PTTD as donor and the fullerene derivative [6,6]-phenyl-C71 -butyric acid methyl ester (PC₇₁BM) as acceptor were fabricated and characterized. The ratio of PTTD:PC₇iBM was fixed at 1 :2 (w wt) in all the devices. Representative current density— voltage (J—V) curves of the PTTD:PC₇iBM solar cells are shown in Figure 8 A and the solar cell parameters are summarized in Table 3. The low solubility of PTTV in dichlorobenzene precluded fabrication of PTTV solar cells. The average power conversion efficiency (PCE) of PTTP, PTTBDT, and PTTDTP solar cells was rather low at 0.05%, 0.35%>, and 0.09%>, respectively. The corresponding short-circuit current density (J_sc), open circuit voltage ( oe), and fill factor (FF) of these solar cells are 1.04-2.33 mA/cm , 190-410 mV and 0.28-0.36, respectively. The peak efficiency obtained in the best solar cell of PTTBDT:PC₇iBM was 0.38% with a J_sc of 2.55 mA/cm², a V_oc of 0.42 V, and a FF of 0.36. Although the absorption of the PTTDs and thus light harvesting in BHJ devices extends to the near infrared (Figure 8B), all the photovoltaic parameters (J_sc, V_oc, FF) are rather low. The low power conversion efficiency of BHJ solar cells based on the PTTDs and fullerenes may be because of the following reasons. The high-lying HOMO energy levels (- 4.9 to -5.1 eV) of the PTTDs relative to the LUMO energy level of the PCBM (-4.0 eV) resulted in small V_oc of 0.19-0.36 V. The poor quality of the spin coated polymer/fullerene blend films and the low to moderate mobility of holes in the polymers could explain the low fill factor of the solar cells.

Table 1. Molecular Weight and Thermal Stability of PTTDs.

M_w ^a M_w / M_n ^a

Copolymer (kDa) (kDa) CO

PTTBDT 13.0 6.4 2.08 320

PTTP 10.0 8.3 1.23 360

PTTV - - - 380

PTTDTP 46.0 24.0 1.9 350

a Molecular weights were determined by GPC using polystyrene standards. * Onset decomposition temperature measured from TGA under nitrogen.

Table 2. Optical and Electrochemical Properties of PTTDs. d

Copolymer ΕΑ^Ω IP" ' nax £g°^pt

(eV) (eV) (eV) (nm) (nm) (eV)

PTTBDT 3.45 5.06 1.61 485, 806 499, 843 1.16

PTTP 3.36 4.98 1.62 445, 760 481 , 864 1.08

PTTV 3.6 5.1 1.5 484, 835 514, 941 0.9

PTTDTP 3.6 4.9 1.3 521 , 909 515, 911 0.9 ^a Electron affinity was obtained based on EA = eE_redox ^onse + 4.4 eV. Ionization potential was obtained based on IP = eE_ox ^onset + 4.4 eV. The absorption maximum in dilute solution. ^d The thin film absorption maximum. Table 3. Field-effect Charge Transport and Photovoltaic Properties of PTTDs.

Copolymer lon/Ioff v_t / ^a v ^a FF^a

lmax)

(cm²/Vs) (V) (niA/cm ) (V) (%)

PTTBDT 3.2x l0^"3 10³ 18.3 2.33 0.41 0.36 0.35 (0.38)

PTTP 4.6X 10^"3 10⁴ -12.5 1.04 0.19 0.28 0.05 (0.06)

PTTV 2.54x l0^"4 10⁴ -8.1 - - - -

PTTDTP 6.1 X 10^"4 10³ -16.2 1.45 0.22 0.31 0.09 (0.1)

Photovoltaic properties of PTTDs:PC₇iBM (1 :2 wt/wt) solar cells.

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Claims

WHAT IS CLAIMED IS:

1. A composition comprising at least one conjugated copolymer, wherein the copolymer is represented by:

a) n is 1 or more;

b) yi and y₂ are each 0, 1, 2, 3 or 4;

c) Xi, X₂ and X₃ are each independently a heteroatom;

d) Ri and R₂ are each independently a hydrogen, a fluorine, or an optionally substituted linear, branched, or cyclic C1-C24 organic group;

e) L is represented by:

, wherein each X is independently S, O, N, Se or Te, and wherein R₃ and R4 are each independently a hydrogen, a fluorine, or an optionally substituted linear, branched, or cyclic C1-C24 organic group.

2. The composition of claim 1, wherein X_ls X₂ and X₃ are each independently O, N, S, Se or Te.

3. The composition of claim 1, wherein X_ls X₂ and X₃ are each S.

4. The composition of claim 1 , wherein y₁ and y₂ are each 1.

The composition of claim 1 , wherein L is

The composition of claim 1 , wherein L

The composition of claim 1 , wherein L is

The composition of claim 1 , wherein L is

The composition of claim 1 , wherein L is

The composition of claim 1, wherein R_ls R₂, R₃, and R₄ are each independently an optionally substituted Ci-C₂₄ alkyl, alkoxy, or thioalkyl.

The composition of claim 1, wherein R_ls R₂, R₃, and R₄ are each independently a branched alkyl group.

The composition of claim 1, wherein R_ls R₂, R₃, and R₄ are each independently a hydrogen, a fluoride, a cyano, or an optionally substituted C₄-C₂₄ alkyl, alkoxy, or thioalkyl.

14. The composition of claim 1, wherein the copolymer has a weight-average molecular weight (M_w) of 5 kDa or higher

15. The composition of claim 1, wherein the copolymer has an onset decomposition temperature (Td) of 250°C or higher.

16. The composition of claim 1, wherein the copolymer has an ionization potential of 4.5 eV or higher.

17. The composition of claim 1, wherein the copolymer has an optical band gap of 1.2 eV or lower.

18. The composition of claim 1, wherein the copolymer has an electrochemical band gap of 1.7 eV or lower.

19. A device comprising the composition of claim 1.

20. The device of claim 19, wherein the device is a transistor.

21. The device of claim 19, wherein the device is a field-effect transistor.

22. The device of claim 19, wherein the device is a photodetector.

23. The device of claim 19, wherein the device is a photovoltaic device.

24. The device of claim 19, wherein the device is a light-emitting device.

25. A thin film field-effect transistor comprising at least one conjugated copolymer, wherein the copolymer comprises at least one donor moiety and at least one acceptor moiety, and wherein the acceptor moiety is an optionally substituted thieno[3,4- c] [ 1 ,2,5]thiadiazole.

26. A transistor comprising at least one conjugated copolymer, wherein the copolymer is represented by:

wherein:

a) n is 1 or more;

b) each a is independently 0, 1, 2, 3 or 4, b is 0 or 1;

c) each X is independently O, S, Se, Te or NR' wherein R' is hydrogen, a C₁-C30 normal, branched, or cyclic alkyl group;

d) each X' is independently S, Se or Te;

e) each Y and Y' is N or CR", wherein R' ' is hydrogen, fluorine, cyano, or a C₁-C30 normal, branched, or cyclic alkyl, perfluoroalkyl, alkoxy, thioalkyl, or thioalkoxy group; and

f L is represented by:

27. The transistor of claim 26, wherein each X and X' is S.

28. The transistor of claim 26, wherein each Y and Y' is CR".

29. The transistor of claim 26, wherein each Y is CH, each Y' is CR" with R" being an optionally substituted C1-C30 alkyl, alkoxy, or thioalkyl.

30. The transistor of claim 26, wherein each Y is CH, each Y' is CR" with R" being a branched alkyl.

31. The transistor of claim 26, wherein each

is independently selected from:

The transistor of claim 26, wherein each a and b is 1.

The transistor of claim 26, wherein the transistor is a field-effect transistor comprising a thin-film of the copolymer.

34. The transistor of claim 26, wherein the transistor is a field-effect transistor comprising a thin- film of the copolymer annealed at a temperature of 150 °C or higher.

35. The transistor of claim 26, wherein the transistor is a field-effect transistor having a carrier mobility of 1 x 10 -^"4 cm 2 /Vs or higher.

36. The transistor of claim 26, wherein the transistor is a field-effect transistor having a carrier mobility of 1 x 10 -^"2 cm 2 /Vs or higher.

37. The transistor of claim 26, wherein the transistor is a field-effect transistor having a on/off current ratio of 10²-10⁶.

38. The transistor of claim 26, wherein the conjugated copolymer comprises at least one donor moiety and at least one acceptor moiety, and wherein the acceptor moiety is an optionally substituted thieno[3,4-c][l,2,5]thiadiazole.