WO2018119780A1

WO2018119780A1 - Weak electron-donating building blocks, copolymers thereof and related devices

Info

Publication number: WO2018119780A1
Application number: PCT/CN2016/112715
Authority: WO
Inventors: Xugang GUO; Yulun Wang; Qiaogan LIAO; Han GUO; Shengbin SHI
Original assignee: Southwest University of Science and Technology
Current assignee: Southwest University of Science and Technology
Priority date: 2016-12-28
Filing date: 2016-12-28
Publication date: 2018-07-05
Anticipated expiration: 2019-06-28

Abstract

A compound having a formula (I) is provided, in which each of A and B is independently and respectively one of formula (II) and (III), given that at least one of A and B comprise an alkynyl, each of R₁ and R₂ is independently and respectively a C_1-20 alkyl, a thienyl or a phenyl, and each of the thienyl and the phenyl is optionally and independently substituted with a C_1-20 alkyl. An organic semiconductor polymer formed by an electron donor unit and an electron acceptor unit, a film formed by the organic semiconductor polymer, and a semiconductor device are also provided.

Description

WEAK ELECTRON-DONATING BUILDING BLOCKS, COPOLYMERS THEREOF AND RALATED DEVICES

FIELD

The present disclosure relates to a field of semiconductor technology, in particular to organic semiconductor compound, organic semiconductor polymer, organic semiconductor film, organic thin-film transistors, organic solar cells, and semiconductor devices.

BACKGROUND

Si-based amorphous semiconductors occupy the majority of the market presently due to the excellent workability thereof. However it is expected for the organic semiconductor to be applied to a wearable device because of such characteristics referred to as lightweight and flexibility.

Impressive progresses have been achieved over the last two decades in polymer-based opto-electrical devices such as organic thin-film transistors (OTFTs) and polymer solar cells (PSCs) . The performance improvement in OTFTs and PSCs are mainly driven by materials innovation in combination with device engineering. During the course of developing high-performance polymer semiconductors, the invention of new building blocks plays a critical role, which should afford the resulting semiconductors with improved solution processability and well tailored opto-electrical properties. High-degree of polymer backbone coplanarity typically is a highly desired characteristics for achieving improved device performance in both OTFTs and PSCs.

However, polymer-based opto-electrical devices are still to be improved.

SUMMARY

The present application is based on inventors’ discoveries and recognitions of the following facts and issues.

During the course of developing high-performance polymer semiconductors, the invention of new building blocks plays a critical role, which should afford the resulting semiconductors with improved solution processability and well tailored opto-electrical properties. After deep research, the present inventors surprisingly found a new building block, alkynyl-functionalized head-to-head linkage containing bithiophene, and the new building block may be a promising donor unit for high-performance polymer semiconductors.

Embodiments of the present disclosure seek to solve at least one of the problems existing in the related art to at least some extent.

Accordingly, a first object of the present disclosure is to provide a compound having a formula of

in which each of A and B is independently and respectively one of

and

given that at least one of A and B comprise a alkynyl, each of R₁ and R₂ is independently and respectively a C_1-20 alkyl, a thienyl or a phenyl, and each of the thienyl and the phenyl is optionally and independently substituted with a C_1-20 alkyl. According to embodiments of present disclosure, the compound may be used as a promising donor unit for high-performance polymer semiconductors. It was found by the present inventors that replacing alkyl chains with less steric demanding alkynyl chains should greatly reduce the steric hindrance due to the elimination of two H atoms on the sp hybridized carbon, and some experimental results demonstrate that alkynyl is a versatile side chain for enabling semiconductors with good solublizing ability, high degree of backbone planarity, and optimized opto-electrical property.

A second object of the present disclosure is to provide a compound having a formula of ,

wherein A and B is defined previously, Me is methyl. The compound may form a polymer with an electron acceptor, and the resulting polymer may exhibit excellent properties for use in semiconductor devices such as opo-electrical devices for example organic thin-film transistors and polymer solar cells.

A third object of the present disclosure is to provide an organic semiconductor polymer (copolymer) formed by an electron donor unit and an electron acceptor unit, wherein the electron donor is a compound described above, and the electron acceptor unit is at least one selected from a group comprising:

The organic semiconductor polymer may exhibit excellent properties for use in semiconductor devices such as opo-electrical devices for example organic thin-film transistors and polymer solar cells.

A fourth object of the present disclosure is to provide a semiconductor film formed by the organic semiconductor polymer described above. The semiconductor film may exhibit excellent properties for use in semiconductor devices such as opo-electrical devices for example organic thin-film transistors and polymer solar cells.

A fifth object of the present disclosure is to provide a semiconductor device comprising the film described above or the polymer described above.

According to embodiments of present disclosure, new building blocks having high degree of backbone planarity, good solublizing ability, and well-tailored physicochemical property are provided for constructing high-performance polymer semiconductors. Due to the detrimental steric hindrance created by the alkyl chains at the 3 and 3’ positions of bithiophene, the head-to-head linkage containing 3, 3’ -dialkyl-2, 2’ -bithiophene (BTR) is highly avoided in materials design. Replacing alkyl chains with less steric demanding alkynyl chains should greatly reduce the steric hindrance due to the elimination of two H atoms on the sp hybridized carbon. According to embodiments of present disclosure, a novel electron donor unit, 3, 3’ -dialkynyl-2, 2’ -bithiophene (BTRy) , was invented and incorporated into polymer backbones. The alkynyl-functionalized head-to-head linkage containing bithiophene enables the resulting polymers with good solubility without sacrificing backbone planarity, hence the BTRy-based polymers show high degree of conjugation with a narrow bandgap of ～1.6 eV. When incorporated into organic thin-film transistors, the polymers show substantial hole mobility up to 0.13 cm² V^-1s^-1 in top-gated devices. The weak electron-withdrawing alkynyl chains lower the energy levels of frontier molecular orbitals, therefore the difluorobenzothiadiazole and difluorobenzoxadiazole copolymers show remarkable ambipolarity with electron mobility up to 0.06 cm² V^-1s^-1 in bottom-gated transistors. When incorporated into polymer solar cells, the BTRy-based polymers show promising PCEs approaching 8％with remarkable V_ocs of 0.91-1.0 V, reflecting the weak electron withdrawing characteristics of the alkynyl chain. The results demonstrate that alkynyl is versatile side chain for enabling semiconductors with good solublizing ability, high degree of backbone planarity, and optimized opto-electrical property. The study offers a new approach for materials innovation in organic electronics.

Additional aspects and advantages of embodiments of present disclosure will be given in part in the following descriptions, become apparent in part from the following descriptions, or be learned from the practice of the embodiments of the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects and advantages of embodiments of the present disclosure will become apparent and more readily appreciated from the following descriptions made with reference to the drawings, in which:

Fig. 1 shows materials design strategies employed in developing polymer semiconductors with high degree of backbone coplanarity: (a) inserting non-alkylated π-spacers； (b) conformation locking through covalent bonds； (c) conformation locking through intramolecular non-covalent interaction； (d) introducing alkynyl side chains. The elimination of two hydrogen atoms on the sp hybridized C should reduce steric hindrance and promote backbone planarity (present disclosure) .

Fig. 2 shows chemical structures, optimized geometries, and energy levels of the frontier molecular orbitals of (a) 3, 3’ -dialkyl-2, 2’ -bithiophene (BTR) ； (b) 3, 3’ -dialkoxy-2, 2’ -bithiophene (BTOR) ； (c) 3-alkyl-3’ -alkoxy-2, 2’ -bithiophene (TRTOR) ； (d) 3, 3’ -dialkynyl-2, 2’ -bithiophene (BTRy) . Calculations were carried out at the DFT//B3LYP/6-31G*level. The alkyl substituents were truncated here to simplify the calculations.

Fig. 3 shows (a) UV-vis absorption spectra of polymers PffBT-BTRy and PffBX-BTRy in solution (1 ×10^-5 M in o-dichlorobenzene) and as thin film (spin coated from 1 mg mL^-1 o-dichlorobenzene solution) ； (b) Cyclic voltammograms of polymer films measured in 0.1 M (n-Bu) ₄N^. PF₆ acetonitrile solution with the F_c/F_c ⁺redox couple as the internal standard at scan rate of 50 mV s^-1； (c) PffBT-BTRy and (d) PffBX-BTRy UV-vis absorption spectra of polymer solution (1 × 10^-5 M in o-dichlorobenzene) at various temperatures as indicated.

Fig. 4 shows chemical structures and optimized geometries for the trimers of the repeating units of (a) PffBT-BTRy and (b) PffBX-BTRy. Calculations were carried out at the DFT//B3LYP/6-31G^*level； dihedral angles between neighboring arenes are indicated by red circles. Alkyl substituents are truncated to simplify the calculation.

Fig. 5 shows electrical characteristics of bottom-gate/top-contact organic thin-film transistors containing PffBT-BTRy as the active layer. (a) Output curve and (b) transfer curve in p-type regime； (c) output curve and (d) transfer curve in n-type regime. Inset of (a) shows the transistor structure, the device dimension is 90 μm × 1.8 mm.

Fig. 6 shows (a) J-V characteristics of the optimized polymer solar cells under simulated AM 1.5 G illumination (100 mW cm^-2) ； (b) external quantum efficiency spectra of the corresponding PSCs.

Fig. 7 shows tapping-mode AFM height images of (a) PffBT-BTRy: PC₇₁BM and (b) PffBX-BTRy: PC₇₁BM blend films； TEM images of (c) PffBT-BTRy: PC₇₁BM and (d) PffBX-BTRy: PC₇₁BM blend films. The films are prepared with the processing additive DPE under the same conditions for the optimal PSC fabrication.

Fig. 8 shows PL spectra obtained from the films of (a) PffBT-BTRy neat film； (b) PffBT-BTRy: PC71BM (1: 1.5, w: w) blend； (c) PffBX-BTRy neat film； (d) PffBX-BTRy: PC71BM (1: 1.5, w: w) blend.

Fig. 9 shows (a) Thermogravimetric analysis (heating ramp: 10 ℃ min^-1) of polymers PffBT-BTRy and PffBX-BTRy； (b) DSC thermograms of polymers PffBT-BTRy and PffBX-BTRy for the second heating and cooling scans (heating ramp: 10 ℃ min^-1) . Nitrogen was used as the purge gas for TGA and DSC measurements.

Fig. 10 shows electrical characteristics of bottom-gate/top-contact organic thin-film transistors containing PffBX-BTRy as the active layer. (a) Output curve and (b) transfer curve in p-type regime； (c) output curve and (d) transfer curve in n-type regime. The device dimension is 90 μm × 1.8mm.

Fig. 11 shows electrical characteristics of top-gate/bottom-contact organic thin-film transistors containing PffBT-BTRy and PffBX-BTRy as the active layer in p-type regime. (a) Output curve and (b) transfer curve of PffBT-BTRy active layer； (c) output curve and (d) transfer curve PffBX-BTRy active layer. The device dimension is 100 μm × 5 mm.

Fig. 12 shows the corresponding J^1/2–V curves for the hole-only (left) and electron-only (right) devices based on the polymer: PC₇₁BM blend films with or without 3％DPE (in dark) .

Fig. 13 shows tapping-mode AFM height (a, b, e, and f) and phase images (c, d, g, and h) of polymer: PC₇₁BM blend films prepared without processing additive diphenyl ether (DPE) (a, b, c and d) and with DPE (e, f , g and h) under the same conditions for the optimal PSC fabrication.

Fig. 14 shows TEM images of polymer: PC₇₁BM blend films prepared without processing additive diphenyl ether (DPE) and with DPE under the same conditions for the optimal PSC fabrication.

DETAILED DESCRIPTION

Reference will be made in detail to embodiments of the present disclosure. The same or similar elements and the elements having same or similar functions are denoted by like reference numerals throughout the descriptions. The embodiments described herein with reference to drawings are explanatory, illustrative, and used to generally understand the present disclosure. The embodiments shall not be construed to limit the present disclosure.

During the course of developing high-performance polymer semiconductors, the invention of new building blocks plays a critical role, which should afford the resulting semiconductors with improved solution processability and well tailored opto-electrical properties. High-degree of polymer backbone coplanarity typically is a highly desired characteristics for achieving improved device performance in both OTFTs and PSCs. The planar backbone can result in highly delocalized intramolecular charge carrier transport due to the substantially overlapped π-orbitails. In addition, the planar backbone can facilitate three-dimensional lamellar packing of polymer chains and assist in achieving long-range of materials ordering and film crystallinity, thus intermolecular charge carrier hopping can be greatly enhanced versus the amorphous polymer semiconductors. As the result, polymer semiconductors with highly planar backbone conformation typically result in greatly improved charge carrier mobility in OTFTs. In PSC field, high degree of backbone planarity enables polymer semiconductors with narrowed bandgaps and results in enhanced harvesting of solar spectrum, which is essential for maximizing the short-circuit currents (J_scs) . The backbone planarity is beneficial to charge carrier transport and extraction, hence charge carrier recombination can be greatly suppressed, resulting in improved J_scs and fill factors (FFs) in PSCs.

In polymer semiconductors, solubilizing alkyl chains are essential for enabling materials solution processability. However, introducing such chains typically generates undesired steric hindrance, which is detrimental to backbone planarity, polymer chain packing, and film crystallinity. As shown in Fig. 1, in order to address the challenges, several materials design strategies have been developed to achieve high degree of polymer backbone planarity without sacrificing materials solubility. By inserting non-alkylated π-spacers (Fig. 1a) , such as bithiophene or thienothiophene, high mobility polymer semiconductors PQT and PBTTT are developed respectively, and the polymers show high-degree of backbone planarity and film crystallinity. The strategy generates a vast number of polymer semiconductors, showing promising performance in both OTFTs and PSCs. However the spacers are typically thiophene or thiazole-related arenes, which limit the materials diversity and the tuning of physicochemical properties over a wide range. In addition, this class of polymer semiconductors typically suffer from unsatisfactory solubility. Conformational lockings through both covalent bond (Fig. 1b) or intramolecular non-covalent interaction (Fig. 1c) are another two highly successful strategies with their distinctive pros and cons. In the strategy using covalent bond-based conformational locking, the locking atoms (X in Fig. 1b) , such as C, Si, and Ge, are usually sp³ hybridized, and hence the attached solublizing chains adopt an out-of-plane orientation, which is detrimental to close π-stacking and film crystallinity. In addition, the locking atom-containing building blocks are axisymmetric, which are not ideal for polymer chain packing versus centrosymmetric ones. Hence the polymers typically show limited charge carrier mobilities in OTFTs and sub-optimal FFs in PSCs. While in the strategy using non-covalent bond-based conformational locking, highly polarizable heteroatoms, sush as oxygen (Fig. 1c) , are usually used to promote such non-covalent interaction, which also allow the attachment of solublizing chains on the 3 and 3’ positions of bithiophene to improve solubility. However, as shown in Fig. 2, the resonance effect of the alkoxy chains leads to elevated energy levels of the highest occupied molecular orbitals (HOMOs) for the resulting polymer semiconductors (Fig. 2b and 2c) , which limits the device performance stability in OTFTs and open-circuit voltge (V_ocs) and power conversion effciencies (PCEs) in PSCs.

As shown in Fig. 2， the head-to-head linkage containing 3, 3’ -dialkyl bithiophene (BTR, Fig. 2a) is highly avoided in polymer semiconductors due to the substantial backbone torsion induced by the steric hindrance. Thanks to the smaller van der waals radius

of oxygen versus that

of the methylene group, the oxygen insertion beteween thiophene backbone and alkyl chain should greatly mitigate the steric hindrance, which in combination with the non-covalent interaction-based conformation locking affords a new materials design strategy (Fig. 1c) . In spite of the great success of this strategy, the alkoxy-functionalized bithiophenes, BTOR and TRTOR (Fig. 2) , suffer from the high-lying frontier molecular orbitals (FMOs) with the HOMOs of -4.67 and-4.94 eV, respectively, which lead to the resulting polymers with degraded device stability with large off-currents in OTFTs and small V_ocs (0.4–0.7 V) in PSCs.

After deep research, the present inventors surprisingly found a new building block, 3, 3’ -dialkynyl-2, 2’ -bithiophene unit (BTRy, Fig. 2d) . In comparison to the methylene group, the sp hybridized carbon on the alkynyl should lead to greatly decreased steric hindrance due to the elimination of two H atoms. The van der waals radius

of the sp hybridized C is comparable to that

of O atom, which can result in greatly reduced steric hindrance in the head-to-head linkage containing 3, 3’ -dialkynyl-2, 2’ -bithiophene. Density functional theory (DFT) computation reveals a highly coplanar BTRy backbone (Fig. 2d) with a dihedral angle of 0.01° between the two thiophene planes. The alkynyl substituents should afford good materials solubility. In addition, ethynylene or acetylene moieties have been widely used in polymeric semiconductors, typically incorporated into backbone, which results in a class of polymers, poly (phenylene ethynylene) s (PPEs) . The sp hybridized C can stablize the frontier molecualr orbitals (FMOs) of PPEs due to its weak electron withdrawing ability.

By incorporating the acetylene unit into polythiophene backbone, the resulting semiconductor shows a 0.3 eV lower-lying HOMO than the parent polymer P3HT. In spite of the distinctive advantages of acetylene moiety, it is mainly incorporated into semiconductor backbone and its use as the side chain has been greatly overlooked. The inventors surprisingly found that the new building block BTRy, contains two solubilizing alkynyl chains on the 3 and 3’ positions of bithiophene, which should enable the resulting polymers with good solubility. The weak electron-withdrawing alkynyl group affords BTRy unit with a low-lying HOMO (-5.16 eV, Fig. 2d) , which should be beneficial to the device stability in OTFTs and achieving large V_ocs in PSCs. When incorporated into polymers, the BTRy-based semiconductors show promising performance in both OTFTs and PSCs. The results demonstrate that the alkynyl-functionalized head-to-head linkage containing bithiophene is a promising donor unit for high-performance polymer semiconductors.

in which each of A and B is independently and respectively one of

and

In addition, the compound according to embodiments of the present disclosure may further have at least one of the following additional technical features:

In an embodiment, the compound is represented by the following formula:

wherein each R is C1～20 alkyl independently.

In some embodiments of present disclosure, the compound may act as an electron donor and form polymer with an electron receptor at the sites shown in the following formulas:

In an embodiment, each R is C10 alkyl independently.

In an embodiment, each R is C10 branched alkyl.

In an embodiment, each R is

In an embodiment, the compound is represented by the following formula:

A second object of the present disclosure is to provide a compound having a formula of

In an embodiment, the compound is represented by the following formula:

A third object of the present disclosure is to provide an organic semiconductor polymer formed by an electron donor unit and an electron acceptor unit, wherein the electron donor is a compound described above, and the electron acceptor unit is at least one selected from a group comprising:

In some embodiments of present disclosure, the electron accpetor may form the organic semiconductor polymer at the sites shown in the following formulas:

In addition, the organic semiconductor polymer according to embodiments of the present disclosure may further have at least one of the following additional technical features:

In an embodiment, the organic semiconductor polymer is represented by the following formula:

wherein n is an integer ranging from 1 to10, and x is S or O.

In some embodiments , n is an integer ranging from 1 to 5, for example, 1, 2, 3, 4, and 5. In some embodiments, n is an integer sufficient to afford an polymer dispersity index (PDI) of 4.7 and 2.8 respectively.

In an embodiment, the organic semiconductor polymer shows a band gap of about 1.6eV.

In an embodiment, the organic semiconductor polymer shows a substantial hole mobility up to 0.13 cm² V^-1s^-1 in a top-gated organic thin-film transistor.

In an embodiment, the organic semiconductor polymer shows an electron mobility up to 0.06 cm² V^-1s^-1 in a bottom-gated organic thin-film transistor.

In an embodiment, the organic semiconductor polymer shows a maximum Power Conversion Efficiency approaching 8％and/or a V_ocs of 0.91-1.0 V in a polymer solar cell.

wherein n is suitable to achieve a number average molecular weight of 43.8 kDa.

wherein n is suitable to achieve a number average molecular weight of 33.0 kDa.

In an embodiment, the semiconductor device is an opto-electrical device comprising at least one of organic thin-film transistor and polymer solar cell.

In an embodiment, the organic thin-film transistor is a top-gated organic thin-film transistor or a bottom-gated organic thin-film transistor.

In an embodiment, the film or the polymer shows: a substantial hole mobility up to 0.13 cm² V^-1s^-1 in a top-gated organic thin-film transistor, an electron mobility up to 0.06 cm² V^-1s^-1 in a bottom-gated organic thin-film transistor, or a maximum Power Conversion Efficiency approaching 8％and/or a V_ocs of 0.91-1.0 V in a polymer solar cell.

According to embodiments of present disclosure, new building blocks having high degree of backbone planarity, good solubilizing ability, and well-tailored physicochemical property are provided for constructing high-performance polymer semiconductors. Due to the detrimental steric hindrance created by the alkyl chains at the 3 and 3’ positions of bithiophene, the head-to-head linkage containing 3, 3’ -dialkyl-2, 2’ -bithiophene (BTR) is highly avoided in materials design. Replacing alkyl chains with less steric demanding alkynyl chains should greatly reduce the steric hindrance due to the elimination of two H atoms on the sp hybridized carbon. According to embodiments of present disclosure, a novel electron donor unit, 3, 3’ -dialkynyl-2, 2’ -bithiophene (BTRy) , was invented and incorporated into polymer backbones. The alkynyl-functionalized head-to-head linkage containing bithiophene enables the resulting polymers with good solubility without sacrificing backbone planarity, hence the BTRy-based polymers show high degree of conjugation with a narrow bandgap of ～1.6 eV. When incorporated into organic thin-film transistors, the polymers show substantial hole mobility up to 0.13 cm² V^-1s^-1 in top-gated devices. The weak electron-withdrawing alkynyl chains lower the energy levels of frontier molecular orbitals, therefore the difluorobenzothiadiazole and difluorobenzoxadiazole copolymers show remarkable ambipolarity with electron mobility up to 0.06 cm² V^-1s^-1 in bottom-gated transistors. When incorporated into polymer solar cells, the BTRy-based polymers show promising PCEs >7％with remarkable V_ocs of 0.91-1.0 V, reflecting the weak electron withdrawing characteristics of the alkynyl chain. The results demonstrate that alkynyl is versatile side chain for enabling semiconductors with good solubilizing ability, high degree of backbone planarity, and optimized opto-electrical property. The study offers a new approach for materials innovation in organic electronics.

The present disclosure will be further described in details as follows. It should be understood that the following embodiments are only used for description, and the scope of the present disclosure should not be limited in any manner. In addition, unless specified or limited otherwise, methods with conditions or steps not specified are all common methods and agents and materials are all commercially available.

All reagents and chemicals were commercially available and were used without further purification unless otherwise stated. Anhydrous tetrahydrofuran and toluene were distilled from Na/benzophenone, and anhydrous diisopropylamine and acetonitrile were distilled from CaH₂. The monomer 4, 7-dibromo-5, 6-difluorobenzo [c] [1, 2, 5] thiadiazole was purchased from Derthon Optoelectronic Materials Science Technology Co., Ltd. (Shenzhen, Guangdong, China) and monomer 4, 7-dibromo-5, 6-difluorobenzo [c] [1, 2, 5] oxadiazole was prepared following the published procedure. The cathode interfacial layer PFN for solar cells was purchased from Solarmer Materials Inc.. PC₇₁BM was bought from American Dye Source, Inc.. The P (VDF-TrFE) (

300) dielectrics for the top-gate transistors was purchased from Solvay S.A.. ¹H and ¹³C NMR spectra of monomers and their precursors were measured on

Bruker Ascend

400 and 500 MHz spectrometers, respectively. ¹H NMR of the polymers were recorded on Bruker Ascend 400 MHz spectrometer at 80 ℃. Chemical shifts were referenced to residual protio-solvent signals. DSC curves were recorded on a differential scanning calorimetry (Mettler, STARe, heating rate ＝ 10 ℃ min^-1, nitrogen purge) . TGA curves were collected on a TA Instrument (Mettler, STARe) . UV-vis absorption spectra of polymer solution and film at room temperature were collected on a Shimadzu UV-3600 UV-VIS-NIR spectrophotometer. Temperature dependent UV-vis absorption spectra of polymer solutions at various temperatures were collected on Perkin Elmer Lambda 950 UV/VIS/NIR Spectrometer. Steady-state photoluminescence (PL) spectra were conducted using a Horiba iHR320 spectrometer with the Andor Newton EMCCD detector. PL spectra were excited using a Coherent 532 CW laser. Cyclic voltammetry measurements of polymer films were carried out under argon atmosphere using a CHI760 Evoltammetric analyzer with 0.1 M tetra-n-butylammoniumhexafluorophosphate in acetonitrile as the supporting electrolyte. A platinum disk working electrode, a platinum wire counter electrode, and a silver wire reference electrode were employed, and Fc/Fc⁺ redox couple was used as the internal reference for all measurements. The scanning rate was 50 mV S^-1. AFM measurements of polymer: PCBM blend films were conducted using a Dimension Icon Scanning Probe Microscope (Asylum Research, MFP-3D-Stand Alone) in tapping mode. TEM specimens were prepared following identical conditions as the actual devices, but were drop-cast onto 40 nm PEDOT: PSS covered substrate. After drying, substrates were transferred to deionized water and the floated films were transferred to TEM grids. TEM images were obtained on Tecnai Spirit (20 kV) TEM.

Example 1. Synthesis of the 3, 3’ -dialkynyl-2, 2’ -bithiophene (BTRy)

Scheme 1 depicts the synthetic route to BTRy (6) . Briefly, 5-ethynylundecane 4 was synthesized following the published procedures starting from the commecial alcohol. 4-butyloctanol is oxidized using pyridinium chlorochromate (PCC) to form 2-butyloctanal 2, then the Corey-Fuchs sequence is carried out to provide the terminal alkyne 4. Sonogashira coupling between 4 and the dibromobithiophene 5 affords the key compound 6, which is subjected to column chromatography on silica and then careful purification using C18 reversed-phase chromatography. High purity 6 is lithiated and then quenched with Me₃SnCl to afford the monomer 7. After recrystallization in ethanol, 7 is obtained with good yield and high purity (See Fig. s S1 and S2 for the NMR spectra) . Difuorobenzothiadiazole⁶ and difluorobenzoxadiazole are chosen as the comonomers due to their easy accessability and strong electron-withdrawing ability, and have been shown to be good acceptor units. Polymerimaztions are performed under a typical Stille coupling protocol using microwave as the heating source and Pd₂ (dba) ₃ and P (o-tolyl) ₃ as the catalyst and the ligand, respectively (Scheme 1) . After polymerizations, the polymer chains are end-capped with mono-functionalized thiophene. The product polymers are collected by precipitation into methanol and then are purified by Soxhlet extraction. Polymers PffBT-BTRy and PffBX-BTRy exhibit good solubility in common organic solvents for device fabrication. Molecular weights are measured by high-temperature (140 ℃) gel permeation chromatography (GPC) versus polystyrene standards. Number average molecular weight (M_n) and polydispersity index (PDI) data of PffBT-BTRy and PffBX-BTRy are summarized in Table 1. Compared to PffBT-BTRy (M_n ＝ 43.8 kDa) , PffBX-BTRy shows slightly smaller molecuar weigh (M_n ＝ 33.0 kDa) .

Scheme 1. Synthetic route to the BTRy monomer and the BTRy-based polymer semiconductors.

Reagents and conditions: (i) PCC, dichloromethane； (ii) PPh₃, CBr₄, dichloromethane； (iii) n-BuLi, THF, H₂O； (iv) CuI, Pd (PPh₃) ₄, DIPA, toluene, 135 ℃； (v) n-BuLi, Me₃SnCl, THF； (vi) Pd₂ (dba) ₃, P (o-tolyl) ₃, toluene, microwave, 140 ℃.

Monomer and Polymer Synthesis

2-butyloctanal 2. Pyridiniumchlorochromate (32.39 g, 150.3 mmol) was added in small portions to a stirred solution of 2-butyloctan-1-ol (10 g, 53.7 mmol) in dichloromethane (300 mL) . The mixture was stirred at room temperature for 2 h. The reaction mixture was carefully filtered through a short gel column eluting with dichloromethane. Then product was purified twice by flash chromatography on silica gel with petroleum ether as an eluent to afford a colorless oil (6.65 g, 67％) . ¹H NMR (400 MHz, CDCl₃, ppm) : δ 9.53 (s, 1H) , 2.25 –2.16 (m, 1H) , 1.64 –1.55 (m, 2H) , 1.46 –1.37 (m, 2H) , 1.27 (m, 12H) , 0.87 (dd, J ＝ 14.0, 6.9 Hz, 6H) .

5- (2, 2-dibromovinyl) undecane 3. Triphenylphosphine (32.83 g, 125.2 mmol) was added slowly to a solution of tetrabromomethane (20.69 g, 62.4 mmol) in dichloromethane (170 mL) at 0 ℃. 2 (5.75 g, 31.2 mmol) was then added dropwise over a period of 30 min. The reaction mixture was stirred at room temperature for 2 h and was then poured into stirring brine (200 mL) followed by extraction with dichloromethane several times. The combined extract was dried with anhydrous magnesium sulfate, and the solvent was removed under reduced pressure to give a residue which was purified by flash chromatography on silica gel with petroleum ether as an eluent to afford a colorless oil (4.45 g, 42％) . ¹H NMR (400 MHz, CDCl₃, ppm) : δ 6.11 (d, J ＝ 9.9 Hz, 1H) , 2.36 (s, 1H) , 1.40 (m, 2H) , 1.31 (m, 14H) , 0.92 –0.86 (m, 6H) .

5-ethynylundecane 4. n-Butyllithium solution in hexane (20.6 mmol, 2.4 mol/L) was added dropwise over a period of 40 min to a solution of 3 (3.18 g, 9.35 mmol) in anhydrous tetrahydrofuran (40 mL) at-78℃, followed by stirring at-78℃ for 1 h. Cold water (40 mL) then was carefully added, and resulting reaction mixture was stirred overnight, allowing the reaction temperature to rise to room temperature. The reaction mixture was extracted with petroleum ether several times, and the combined extract was washed with brine and dried with anhydrous magnesium sulfate. The solvent was removed under reduced pressure and the residue was purified by flash chromatography on silica gel with petroleum ether as an eluent to afford a colorless oil (0.696 g, 41％) . ¹H NMR (400 MHz, CDCl₃, ppm) : δ 2.34 –2.25 (m, 1H) , 2.03 (d, J ＝ 2.4 Hz, 1H) , 1.43 (dd, J ＝ 12.4, 6.6 Hz, 6H) , 1.33 (m, 10H) , 0.94 –0.85 (m, 6H) .

3, 3'-bis (3-butylnon-1-yn-1-yl) -2, 2'-bithiophene 6. To a mixture of 3, 3'-dibromo-2, 2'-bithiophene (0.5 g, 1.54 mmol) , CuI (29.39 mg, 0.154 mmol) and tetrakis (triphenylphosphine) palladium (0) (178.3 mg, 0.154 mmol) , anhydrous toluene (10 mL) , diisopropylamineand (10 mL) and 4 (0.695 mg, 3.86 mmol) were added in turn under argon protection. The whole mixture was stirred for 24 h at 135 ℃. Then the reaction mixture was carefully filtered and washed by dichloromethane. The solvent was removed under reduced pressure to give a residue which was purified by flash chromatography on C18 reversed-phase silica gel column with methanol and deionized water as an eluent to afford a reddish oil (0.54 g, 67％) . ¹H NMR (500 MHz, CDCl₃, ppm) : δ 7.13 (d, J ＝ 5.2 Hz, 2H) , 7.01 (d, J ＝ 5.2 Hz, 2H) , 2.62 –2.55 (m, 2H) , 1.61 –1.50 (m, 14H) , 1.47 –1.39 (m, 2H) , 1.38 –1.24 (m, 16H) , 0.89 (m, 12H) .

3, 3'-bis (3-butylnon-1-yn-1-yl) -5, 5'-bis (trimethylstannyl) -2, 2'-bithiophene 7. To a flask was added 6 (436.2 mg, 0.83 mmol) and 5 mL anhydrous tetrahydrofuran. The resulting clear solution was cooled to-78 ℃ using dry ice/acetone bath. Then n-butyllithium solution in hexane (1.08 mmol, 2.4 mol/L) was added dropwise. After stirring at-78 ℃ for 1 h and room temperature for 1 h, trimethyltin chloride (1.16 mmol, 1 mol/L) was added in one portion, and then the cooling bath was removed. After being stirred at room temperature for 4 h, the reaction mixture was quenched with water carefully and then extracted with diethyl ether (10 mL) . After the removal of solvent, the monomer was obtained as a brownish solid, which was further purified by recrystallization in ethanol at-78 ℃ to provide the product as yellowish powder (0.637 g, 93％) . ¹H NMR (500 MHz, CDCl₃, ppm) : δ 7.06 (s, 2H) , 2.60 (m, 2H) , 1.64 –1.50 (m, 14H) , 1.48 –1.40 (m, 2H) , 1.39 –1.22 (m, 16H) , 0.93 –0.83 (m, 12H) , 0.37 (s, 18H) . ¹³C NMR (100 MHz, CDCl₃, ppm) : δ 142.83, 138.52, 135.59, 121.09, 77.94, 34.89, 34.56, 33.04, 31.86, 29.78, 29.34, 27.55, 22.71, 14.15, -8.32.

General Procedure for Polymerizations via Stille Coupling for Synthesis of Polymers PffBT-BTRy and PffBX-BTRy. To an air-free glass tube was charged with two monomers (0.1 mmol each) , tris (dibenzylideneacetone) dipalladium (0) (Pd₂ (dba) ₃) _, and tris (o-tolyl) phosphine (P (o-tolyl) ₃) (1: 8, Pd₂(dba) ₃: P (o-tolyl) ₃ molar ratio； Pd loading: 0.03 equiv) . The tube and its contents were subjected to 3 pump/purge cycles with argon, followed by the addition of anhydrous toluene (3-4 mL) via syringe. The tube was sealed under argon flow and then stirred at 80 ℃ for 10 minutes, 100 ℃ for 10 minutes, and 140 ℃ for 3 h under microwave irradiation. Then, 0.05 mL of 2- (tributylstanny) thiophene was added and the reaction mixture was stirred under microwave irradiation at 140 ℃ for 0.5 h. Finally, 0.10 mL of 2-bromothiophene was added and the reaction mixture was stirred at 140 ℃ for another 0.5 h. After cooling to room temperature, the reaction mixture was slowly dripped into 100 mL of methanol (containing 2 mL 12 N hydrochloric acid) under vigorous stirring. After stirring for 1h, the solid precipitate was transferred to a Soxhlet thimble. After drying, the crude product was subjected to sequential Soxhlet extraction. After final extraction, the polymer solution was concentrated to approximately 10 mL, and then dripped into 100 mL of methanol under vigorous stirring. The polymer was collected by filtration and dried under reduced pressure to afford a deep colored solid as the product.

PffBT-BTRy. The solvent sequence for Soxhlet extraction was methanol, acetone, hexane, dichloromethane, chloroform and chlorobenzene. The chlorobenzene fraction was concentrated by removing most of solvent and precipitated into methanol. The solid was collected by filtration and dried in vacuum to afford the polymer as a deep colored solid (38.4 mg, 41.8％) . ¹H NMR (400 MHz, 1, 2-C₆D₄Cl₂) δ 8.30 (br, 2H) , 2.80 (br, 2H) , 2.38–0.87 (m, 32H) , 0.87 –0.45 (m, 12H) . M_n ＝ 25.3 kDa, PDI ＝ 1.8.

PffBX-BTRy. The solvent sequence for Soxhlet extraction was methanol, acetone, hexane, dichloromethane and chloroform. The chloroform fraction was concentrated by removing most of solvent and precipitated into methanol. The solid was collected by filtration and dried in vacuum to afford the polymer as a deep colored solid (72 mg, 71.4％) . ¹H NMR (400 MHz, 1, 2-C₆D₄Cl₂) δ 8.15 (br, 2H) , 2.90 (br, 2H) , 2.33–0.89 (m, 32H) , 0.88–0.50 (m, 12H) . Mn ＝ 18.2 kDa, PDI ＝ 2.1.

The solvent sequence for Soxhlet extraction was methanol, acetone, hexane, dichloromethane, and chloroform. The chloroform fraction was concentrated by removing most of solvent and precipitated into methanol. The solid was collected by filtration and dried in vacuum to afford the polymer as a deep colored solid (63.8％) .

The solvent sequence for Soxhlet extraction was methanol, acetone, hexane, dichloromethane, and chloroform. The chloroform fraction was concentrated by removing most of solvent and precipitated into methanol. The solid was collected by filtration and dried in vacuum to afford the polymer as a deep colored solid (56.3％) .

The solvent sequence for Soxhlet extraction was methanol, acetone, hexane, dichloromethane, and chloroform. The chloroform fraction was concentrated by removing most of solvent and precipitated into methanol. The solid was collected by filtration and dried in vacuum to afford the polymer as a deep colored solid (48.3％) .

The solvent sequence for Soxhlet extraction was methanol, acetone, hexane, dichloromethane, and chloroform. The chloroform fraction was concentrated by removing most of solvent and precipitated into methanol. The solid was collected by filtration and dried in vacuum to afford the polymer as a deep colored solid (59.4％) .

The solvent sequence for Soxhlet extraction was methanol, acetone, hexane, dichloromethane, and chloroform. The chloroform fraction was concentrated by removing most of solvent and precipitated into methanol. The solid was collected by filtration and dried in vacuum to afford the polymer as a deep colored solid (48.1％) .

Example 2 Thermal Properties and GIWAXS Measurements of Polymers prepared in Example 1

The thermal properties of polymers synthesized in Example 1 were tested in this example, and the results were shown in Fig. 9, (a) Thermogravimetric analysis (heating ramp: 10 ℃ min^-1) of polymers PffBT-BTRy and PffBX-BTRy； (b) DSC thermograms of polymers PffBT-BTRy and PffBX-BTRy for the second heating and cooling scans (heating ramp: 10 ℃ min^-1) . Nitrogen was used as the purge gas for TGA and DSC measurements.

Grazing incidence wide-angle x-ray scattering (GIWAXS) measurements were performed at Beamline 8-ID-E of the Advanced Photon Source at Argonne National Laboratory. Polymer samples were prepared on Si substrate using identical spin speeds, solvents, concentrations and annealing temperature and times to the relevant OTFT and PSC devices. All spectra were collected in air. The photon energy is 7.35 keV

and data were collected on a Pilatus 1M pixel array detector at a sample-detector distance of 204 mm. Spectra were collected at an incidence angle of 0.2°； the films were exposure for 20 seconds. To account for the gaps in the detector array, two images were taken per sample, one with the detector in the standard position and the other translated 23 mm down to fill the gap, the two images are then merged.

1D line cuts were taken from the 2D scattering spectra in the in-plane and out-of-plane directions using the GIXSGUI software package developed by the beamline scientists. To account for air scatter, the line cuts were background subtracted utilizing an exponential fit. The background-subtracted peaks were fit using the multipeak fit function in igor pro. Scherrer analysis was performed utilizing the method by Smiglies to account for instrumental broadening and detection limits in the 2d detector. The values presented represent a lower limit for correlation length, as the Scherrer analysis does not account for broadening due to defects in the crystallites.

Example 4 Optical and Electrochemical Properties of Polymers

Absorption spectra of the BTRy-based polymers PffBT-BTRy and PffBX-BTRy are shown in Fig. 3 and the relevant data and bandgaps are summarized in Table 1. From solution to film state (Fig. 3a) , both polymers show minimal shifts of absorption maximum (λ_max) as well as absorption onsets (λ_onset) , which indicate the strong aggregation of the BTRy-based polymers in solution. The temperature dependent absorption shows the gradual vanishment of the absorption shoulder and the absorption profile becomes less structured as the temperature is increased (Fig. 3c and 3d) . On the basis of the absorption profiles, the benzoxadiazole-based polymer PffBX-BTRy exhibits a stronger aggregation than the benzothiadiazole-based polymer PffBT-BTRy since PffBX-BTRy shows sharper and more structured absorption at elevated temperatures. The stronger PffBX-BTRy aggregation is likely attributed to the more electron-deficient nature of difluorobenzoxadiazole versus that of difluorobenzothiadiazole, resulting in more intense inter-polymer chain interactions. In addition to the absorption maximum, both polymers show distinct absorption shoulder, an indicative of a high degree of polymer backbone coplanarity and ordering, in good accord with the DFT computation. The DFT results show complete planar backbone formation for the trimmers of the repeating units of the BTRy-based polymers (Fig. 4) .

The optical bandgaps derived from the absorption onsets of PffBT-BTRy and PffBX-BTRy films are 1.66 and 1.62 eV, respectively. The slightly smaller bandgap of PffBX-BTRy is a reflection of the stronger electron-withdrawing ability of difluorobenzoxadiazole (versus difluorobenzothiadiazole) . It is interesting to note that the bandgap (1.66 eV) of PffBT-BTRy is comparable to those (1.60-1.65 eV) of the difluorobenzothiadiazole-oligothiophene copolymers, which show high degree of polymer backbone coplanarity and film crystallinity. Therefore, on the basis of the polymer absorption spectra, the head-to-head linkage containing BTRy-based polymers should maintain a high-degree of polymer backbone planarity, which is attributed to the reduced steric hindrance due to the elimination of two hydrogen atoms on the sp hybridized carbon.

The electrochemical properties of the BTRy-based polymers are investigated using cyclic voltammetry and the ferrocene/ferrocium (Fc/Fc⁺) redox couple is used as the internal standard. Both polymers show distinct reduction and oxidation peaks (Fig. 3b) , and the derived HOMO/LUMOs are-5.54/-3.88 and-5.71/-4.09 eV for PffBT-BTRy and PffBX-BTRy, respectively. Compared to the HOMOs of the high-performance difluorobenzothiadiazole-oligothiophene copolymers, the HOMO of the difluorobenzothiadiazole-BTRy copolymer PffBT-BTRy is further decreased, which reflects the weak electron withdrawing capability of alkynyl chains. The low-lying HOMOs should be beneficial to the V_ocs of the PSCs. In comparison to PffBT-BTR, PffBX-BTR shows suppressed HOMO and LUMO, which are attributed to the higher electron negativity of oxygen in benzoxadiazole versus sulfur in benzothiadiazole. The low-lying LUMOs in combination with the substantial reduction peaks indicate that the BTRy-based polymers can function as n-type semiconductors, which is in good accord with the OTFT performance (vide infra) .

Table 1. Molecular weights, optical absorption, and electrochemical data for polymers PffBT-BTRy and PffBX-BTRy.

^a GPC versus polystyrene standards, trichlorobenzene as the eluent, at 140 ℃. ^b Solution absorption spectra (1×10^-5 M in chloroform) . ^c Thin film absorption spectra of pristine film cast from 5 mg/mL chlorobenzene solution. ^d E_HOMO ＝ - (E_ox ^onset + 4.80) eV, and E_ox ^onset determined electrochemically using Fc/Fc⁺ internal standard. ^e E_LUMO ＝ - (E_red ^onset + 4.80) eV, and E_red ^onset determined electrochemically using Fc/Fc⁺ internal standard. ^f E_LUMO ＝ E_HOMO + E_g ^opt. ^g Optical bandgap estimated from absorption edge of as-cast polymer thin film.

Example 5 Device Fabrication and Characterization.

Bottom-Gate/Top-Contact (BGTC) Thin-Film Transistors: p-doped Silicon with 200 nm thermal oxide layer was sonicated in acetone and isopropanol followed by SC-1 cleaning. After further UV-ozone treatment and plasma cleaning, octadecyltrimethoxysilane (OTS) monolayer was spin-coated from 3 mM solution in trichloroethylene (TCE) according to an earlier report. The substrates were subsequently exposed to ammonium vapor for 15 h, sonicated in toluene and isopropanol, then rinsed with DI-water. The polymer active layers were spin coated from 5 mg mL-1 CB solutions, and then they were thermally annealed at various temperatures for 20 min. Finally, 40 nm Au source-drain electrodes were evaporated on top through shadow mask.

Top-gate/bottom-contact (TGBC) thin-film transistors: Source–drain electrodes (3 nm Cr and 30 nm Au) were patterned on borosilicate glass by photolithography. The substrates were cleaned by sonication in acetone and isopropanol followed by UV-ozone treatment. The polymer active layers were spin coated from 5 mg mL^-1 chlorobenzene solutions, and then they were thermally annealed at various temperatures for 20 min. The P(VDF-TrFE) (

300) dielectric layers were spin-coated from 60 mg mL^-1 2-butanone (MEK) solutions, then they were annealed at 60 ℃ for 30 min. Finally, 50 nm Al was evaporated on top as the gate electrode.

Polymer Solar Cell Fabrication and Characterization.

The indium tin oxide (ITO) -coated glass substrates (10 Ω/square) were cleaned through ultrasonic treatment in detergent, DI water, acetone, and isopropanol and then dried in an oven overnight. PEDOT: PSS (Clevios P VP A1 4083) (～30 nm) was spin coated onto ultraviolet ozone-treated ITO substrates. After annealing at 140 ℃ for 15 min. in air, the substrates were transferred into a N₂ glove-box. The ODCB blend solution stirred at 110 ℃ overnight (4 mg/mL for PffBT-BTRy and 8 mg/mL for PffBX-BTRy) was spin coated on top of the PEDOT: PSS layer. The blend film thickness was controlled at～60-130 nm (KLA-TENCOR Alpha-Step Surface Profiler) . Finally, ～6 nm PFN (0.2 mg/ml ethanol solution) was spin-coated at 2500 rpm for 20 s on the active layer and then 100 nm Al cathode was deposited (area 4.5 mm² defined by metal shadow mask) on the active layer under high vacuum (1 × 10^-4 Pa) using a thermal evaporator. All current-voltage (I-V) characteristics of the devices were measured under simulated AM1.5G irradiation (100 mW/cm²) using a Xe lamp-based Newport 91160 300-W Solar Simulator. A Xe lamp equipped with an AM1.5G filter was used as the white light source. The light intensity was controlled with an NREL-calibrated Si solar cell with a KG-5 filter. The external quantum efficiency (EQE) was measured by a QE-R3011 measurement system (Enli Technology, Inc. ) .

SCLC Mobility Measurements.

Hole and electron mobility were measured using the space charge limited current (SCLC) method. Device structures of ITO/PEDOT: PSS/Polymer: PC₇₁BM/MoO₃/Ag was used for hole-only devices and ITO/ZnO/Polymer: PC₇₁BM/PFN/Al was used for electron-only devices, respectively. The SCLC mobilities were calculated by MOTT-Gurney equation:

Where J is the current density, ε_r is the relative dielectric constant of active layer material usually 2-4 for organic semiconductors, herein we use relative dielectric constants of 3.9 for polymer and 3.0 for PC₇₁BM respectively, ε₀ is the permittivity of empty space, μ is the mobility of hole or electron and d is the thickness of the active layer, V is the internal voltage in the device, and V ＝ V_app-V_bi, where V_app is the voltage applied to the device, and V_bi is the built-in voltage resulting from the relative work function difference between the two electrodes (in the hole-only and the electron-only devices, the V_bi values can be neglected) .

1. Bottom-Gate/Top-Contact OTFTs under Various Annealing Temperature.

Table S1. Device performance of bottom-gate/top-contact organic thin-film transistors containing PffBT-BTRy as the active layer under various annealing temperature.

Table S2. Device performance of bottom-gate/top-contact organic thin-film transistors containing PffBX-BTRy as the active layer under various annealing temperature.

Fig. 10 shows electrical characteristics of bottom-gate/top-contact organic thin-film transistors containing PffBX-BTRy as the active layer. (a) Output curve and (b) transfer curve in p-type regime； (c) output curve and (d) transfer curve in n-type regime. The device dimension is 90 μm × 1.8 mm.

2. Top-Gate/Bottom-Contact OTFTs.

Table S3. Device performance of top-gate/bottom-contact organic thin-film transistors containing PffBT-BTRy and PffBX-BTRy as the active layers.

Fig. 11 shows lectrical characteristics of top-gate/bottom-contact organic thin-film transistors containing PffBT-BTRy and PffBX-BTRy as the active layer in p-type regime. (a) Output curve and (b) transfer curve of PffBT-BTRy active layer； (c) output curve and (d) transfer curve PffBX-BTRy active layer. The device dimension is 100 μm × 5 mm.

3. PSC Performance under Various Fabrication Conditions.

Table S4. Device performance parameters of conventional PSCs with architecture of ITO/PEDOT: PSS/PffBT-BTRy: PC₇₁BM/PFN/Al fabricated under CB and ODCB processing solvent.

Blend	V_oc (V)	J_sc (mA/cm²)	Fill Factor (％)	PCE_Max (PCE_av ) (％)
CB	0.90	14.58	56.6	7.43 (7.27)
ODCB	0.91	14.13	59.8	7.69 (7.42)

Table S5. Device performance parameters of conventional PSCs with architecture of ITO/PEDOT: PSS/PffBT-BTRy: PC₇₁BM/PFN/Al fabricated under various D/Aratios.

D/Aratio (w/w)	V_oc (V)	J_sc (mA/cm²)	Fill Factor (％)	PCE_Max (PCE_av ) (％)
1: 1	0.91	13.50	54.2	6.64 (6.56)
1: 1.5	0.91	14.13	59.8	7.69 (7.42)
1: 2	0.90	13.64	58.4	7.17 (7.10)

Table S6. Device performance parameters of conventional PSCs with architecture of ITO/PEDOT: PSS/PffBT-BTRy: PC₇₁BM/PFN/Al fabricated with different additives and under different thermal treatments.

Table S7. Device performance parameters of conventional PSCs with architecture of ITO/PEDOT: PSS/PffBT-BTRy: PC₇₁BM/PFN/Al fabricated under with different contents of DPE.

DPE (vol％)	V_oc (V)	J_sc (mA/cm²)	Fill Factor (％)	PCE_Max (PCE_av ) (％)
0	0.93	11.97	46.6	5.19 (5.12)
1	0.94	11.65	50.9	5.59 (5.53)
3	0.91	14.13	59.8	7.69 (7.42)
5	0.93	11.63	50.4	5.47 (5.38)
7	0.95	11.77	45.7	5.09 (5.03)

Table S8. Device performance parameters of conventional PSCs with architecture of ITO/PEDOT: PSS/PffBT-BTRy: PC₇₁BM/PFN/Al fabricated with different thickness of blend film.

Table S9. Device performance parameters of conventional PSCs with architecture of ITO/PEDOT: PSS/PffBX-BTRy: PC₇₁BM/PFN/Al fabricated under various D/Aratios..

^a The additive is 3vol％DPE； ^b thermal annealing is under 80 ℃ for 5 min.

Table S10. Device performance parameters of conventional PSCs with architecture of ITO/PEDOT: PSS/PffBX-BTRy: PC₇₁BM/PFN/Al fabricated with different thickness of blend film.

4. SCLC Mobility Measurements.

Table S11. SCLC mobility measurements of the polymer: PC₇₁BM blend films with or without 3％DPE (in dark) . The films were prepared under the same condition for the optimal solar cell performance.

5. Film Morphology.

6. Photoluminescence (PL) quenching measurements

PL quenching efficiency was calculated with an equation:

Where the PL_blend and PL_polymer are the PL intensity of the blend films and neat polymer films, respectively.

Table S12. PL quenching Efficiency of optimal blend films.

Blend film	△PL (％)
PffBT-BTRy: PC₇₁BM	94.52
PffBX-BTRy: PC₇₁BM	85.59

Example 6 Device Performance in Organic Thin-Film Transistors and Polymer Solar Cells.

Organic thin-film transistors (OTFTs) in two different architectures of bottom-gate/top-contact (BGTC) and top-gate/bottom-contact (TGBC) are fabricated to investigate the charge carrier transport properties of the BTRy-based polymers and the relevant device performance parameters are compiled in Table 2 and Table S1-S3. In BGTC architectures, both polymers show ambipolar transport (Fig. 5) . Thermal annealling was carried out to optimize the OTFT performance and annealling at 200 C led to the optimal performance for the PffBT-BTRy BGTC OTFTs with an average saturation hole mobility (μ_h, _OTFT) of 0.014 cm² V^-1s^-1 and electron mobility (μ_e, OTFT) of 0.048 cm² V^-1s^-1 (Table 2) . The highest μ_e, OTFT of 0.058 cm² V^-1s^-1 was obtained for the film annealled at 240 C. It is interesting to note that the the BTRy-based polymer PffBT-BTRy shows remarkable electron mobility since difluorobenzothiadiazole-based polymers typically exhibit hole dominating transport. The ambipolarity is in good accord with the electrochemical property, showing both oxidation and reduction peaks (Fig. 3) . In the linear region, the PffBT-BTRy OTFTs exhibit low off-currents of 10^-11–10^-10 A in both p- and n-channels, which are remarkable for ambipolar OTFTs. The suppressed off-currents are likely attributed to the low-lying HOMO of the BTRy-based polymers.

The PffBX-BTRy OTFTs exhibit further decreased off-currents (10^-12–10^-11 A) and one order of magnitude higher I_on/I_off ratios (Fig. 10) due to its lower HOMO versus that of PffBT-BTRy. In the n-type regime, the PffBT-BTRy OTFTs typically show large threshold voltages (V_T: ～65 V) , which are likely attributed to the large electron inject barrier (～1.2 eV) between the fermi level of Au electrode and the LUMO lvele of PffBT-BTRy. The benzoxadiazole-based polymer PffBX-BTRy exhibits smaller V_T (～45 V) for the n-channel operation and slightly larger V_T (～-45 V) for the p-channel operation, consistent with the FMO evolution, which results in reduced electron injection barrier and enlarged hole injection barrier, respectively.

Top-gate/bottom-contact (TGBC) OTFTs with the fluorinated dielectrics, poly (vinylidenefluoride-trifluoroethylene) P (VDF-TrFE) , are also fabricated, which exhibit hole dominating transport with negligible electron mobility. In TGBC OTFTs, the μ_h, OTFTs are greatly enhanced to 0.13 and 0.097 cm² V^-1s^-1 for PffBT-BTRy and PffBX-BTRy, respectively. The representative transfer and output curves are given in Fig. 11 and the device performance parameters are summerized in Table 2. The carrier mobility of polymer semiconductors generally increases with carrier concentrations, but this is not the case here since the dielectric capacitance of the TG/BC and BG/TC devices are highly comparable (1.5 × 10^-8 F cm^-2 for TG/BC and 1.7 × 10^-8 F cm^-2 for BG/TC) and the mobility values are extracted at the same voltages. The mobility increase could be linked to the improved molecule packing and polymer chain orientation at the top surface of the semiconductor film in the TG/BC OTFTs versus that at the burried bottom surface in the BG/TC OTFTs, as the result of the different liquid-air and liquid-solid interfaces during the spin-coating process. In addition, the directional interface state modulation by the C-F dipole in P (VDF-TrFE) can decrease the hole injection barrier, but increase the electron injection barrier. Hence the hole mobility is improved and the electron mobility is greatly suppressed in the top-gated OTFTs containing the P (VDF-TrFE) dielectrics. Moreover the hole mobility increase could be attributed to the improved semiconductor/dielectrics interfacial properties in the top-gate OTFTs. The electron transport in the P (VDF-TrFE) -containing devices is substantially suppressed, which could be attributed to the presence of electron trapping groups at the semiconductor/P (VDF-TrFE) interface, similar to the earlier observation in ambipolar P (NDI2OD-T2) transistors. Despite the large hole injection barrier (～ 0.4 –0.6 eV) , both polymers show substantial hole mobilities in the optimized OTFTs, which reflect the high degree of backbone planarity and good film crystallinity of the BTRy-based polymer semiconductors (vide infra) .

Table 2. Organic thin-film transistor performance of polymers PffBT-BTRy and PffBX-BTRy.

^a BGTC: bottom-gate/top-contact； TGBC: top-gate/bottom-contact. ^b Data represent device with the best mobilities and the average mobilities from more than 5 devices are shown in parentheses.

Polymer solar cells with a conventional device configuration of ITO/PEDOT: PSS/polymer: PC₇₁BM/PFN/Al are fabricated to investigate the photovoltaic response of the BTRy-based polymer semiconductors, and the current density-voltage (J-V) plots of the optimized solar cells are illustrated in Fig. 6a with the relevant device performance parameters collected in Table 3. Among various processing additives used for device optimization, it was found that the addition of 3％of diphenyl ether (DPE) led to the optimal PCEs (Table S6) , which is attributed to the improved film morphology. Under the optimized fabrication condition, the PffBT-BTRy PSCs show a PCE of 7.69％with a J_sc of 14.13 mA cm^-2 and a FF of 59.8％and the PffBX-BTRy PSCs show a PCE of 3.35％with a J_sc of 6.56 mA cm^-2 and a FF of 49.0％. The J_scs integrated from the external quantum efficiency spectra (Fig. 6b) of the optimized PSCs are 13.51 and 5.86 mA cm^-2 for the PffBT-BTRy and PffBX-BTRy, respectively, showing good internal consistency. It is remarkable to note that the BTRy-based polymers exhibit remarkable V_ocs of 0.91 and 1.04 V for the PffBT-BTRy and PffBX-BTRy PSCs, respectively. The V_oc (0.91 V) of PffBT-BTRy is 0.03 V larger than that (0.88 V) of difluorobenzothiadiazole-bithiophene copolymer. Considering that both share the same polymer backbone, the attachment of alkynyl side chain results in a larger V_oc, which is attributed to the suppressed HOMO of PffBT-BTRy. The further increased V_oc of PffBX-BTRy is in good accord with its HOMO due to the higher electronegativity of O on benzoxathiazole versus that of S in benzothiadiazole. Both the BTRy-based polymers exhibit sub-optimal FFs (<60％) in PSCs. In spite of the comparable bandgap and charge carrier mobility in OTFTs, the PffBX-BTRy shows greatly smaller J_sc (6.56 mA cm^-2) and lower FF (49.0％) compared to those of PffBT-BTRy, which could be related to its very low-lying LUMO and hence substantial monomolecular recombination due to the small energetic driving force for charge separation across the polymer: fullerenen heterojunction. In addition, the polymer molecular weight may also play a role considering their M_n difference is not negligible.

Fig. 6. Shows (a) J-V characteristics of the optimized polymer solar cells under simulated AM 1.5 G illumination (100 mW cm^-2) ； (b) external quantum efficiency spectra of the corresponding PSCs.

Table 3. Device performance parameters for PffBT-BTRy and PffBX-BTRy-based PSCs with the architecture of ITO/PEDOT: PSS/polymer: PC₇₁BM (1: 1.5) /PFN/Al under the optimized fabrication conditions. 3％ (v/v) diphenyl ether (DPE) is used as the processing additive, device area: 0.045 cm².

^a Hole mobility of devices fabricated from blend solutions with 3％DPE calculated from SCLC model. ^b Electron mobility of devices fabricated from blend solutions with 3％DPE calculated from SCLC model. ^c Data represent the best performing device, and average PCEs from > 15 devices are shown in parentheses.

Film Morphology and the Correlation with Device Performance。

The blend film morphology was also characterized using atomic force microscopy (AFM) and transmission electron microscopy (TEM) . The AFM height images of PffBT-BTRy: PC₇₁BM (Fig. 7a) and PffBX-BTRy: PC₇₁BM (Fig. 7b) blend films reveal that the DPE addition leads to increased root mean square roughness (RMS) from 1.45 to 3.75 nm and from 2.87 to 5.94 nm (see Fig. 13 for the films without using DPE) , respectively, which is likely indicative of improved polymer chain packing and film crystallinity. Such large film roughness increases the contact area between the blend film and the interfacial layer/electrode, which can facilitate charge extraction. Hence distinct increased J_sc and FF are observed in both PffBT-BTRy: PC₇₁BM and PffBX-BTRy: PC₇₁BM after DPE addition. Fig. 7c and Fig. 7d show the TEM images of the PffBT-BTRy: PC₇₁BM and PffBX-BTRy: PC₇₁BM blends under the optimized fabrication condition, respectively. In comparison to the blend films without using DPE (Fig. 14) , the phase separation at finer scale occurs and the interpenetrating bicontinuous network develops after DPE addition, which can contribute to more efficient exciton dissociation and charge carrier transportation/collection. It is interesting to note that the PffBT-BTRy: PC₇₁BM blend shows distinct finer fibril structure after adding DPE. In contrast, the addition of DPE in the PffBX-BTRy: PC₇₁BM blends results in the smearing of TEM image and the fibril structure is less obvious. The charge carrier mobilities of the blend films are measured using space charge limited current (SCLC) method, and the derived μ_h, SCLC/μ_e, SCLCs are 9.61 × 10^-5/1.10 × 10^-4 and 6.96 × 10^-5/4.40 × 10^-4 cm² V^-1s^-1 for the PffBT-BTRy and PffBX-BTRy blends (Table 3 and Fig. 12) , respectively. Hence, for the cells fabricated with DPE, the more balanced charge transport property and the finer scale phase separation with the improved interpenetrating network result in the higher PCE for the PffBT-BTRy: PC₇₁BM cells versus the PffBX-BTRy: PC₇₁BM cells.

Photoluminescence (PL) quenching measurements were carried out to investigate exciton dissociation in the bulk hetrojunction films. The steady-state PL spectra of the neat BTRy polymer films and the corresponding blend films are shown in Fig. 8. The PL quenching efficiency (△PL) is calculated using PL intensity of the blend films relative to those of neat polymer films, which to some extent can be employed to characterize the degree of exciton dissociation at the polymer: PC₇₁BM interface in blend films. The PL emission peaks of the neat polymers PffBT-BTRy and PffBX-BTRy are 774 and 791 nm, respectively. As depicted in Table S12, the optimized blend films in PSCs show different △PL. The △PL of the PffBT-BTRy: PC₇₁BM blend film reachs 94.52％, while that of PffBX-BTRy: PC₇₁BM blend film is lower (85.49％) . The higher △PL (94.52％) of PffBT-BTRy: PC₇₁BM blend reflects more efficient exciton dissociation, which is consistent with its energy level and film morphology. In contrast, the lower △PL (85.49％) of PffBX-BTRy: PC₇₁BM blend film indicates lower degree of exciton dissociation, in good accord with the PSC performance.

Conclusions

A novel building block 3, 3’ -dialkynyl-2, 2’ -bithiophene (BTRy) was invented and incorporated into polymer semiconductors. The sp hybridized carbon on the alkynyl chain greatly reduces steric hindrance due to the elimination of two hydrogen atoms, hence the alkynyl-functionalized head-to-head linkage containing BTRy enables the resulting polymers with good solubility without sacrificing backbone planarity. In addition, the weak electron withdrawing alkynyl chain enables the resulting polymer semiconductors with low-lying frontier molecular orbitals, hence the difluorobenzothiadiazole and difluorobenzoxadiazole copolymers show remarkable ambipolarity with electron mobility up to 0.06 cm² V^-1s^-1. In top-gated organic thin-film transistors, the BTRy-based polymers exhibit hole mobility up to 0.13 cm² V^-1s^-1. When incorporated into polymer solar cells, the BTRy-based polymers show promising PCEs approaching 8％with remarkable V_ocs of 0.91-1.0 V, which is attributed to the low-lying polymer HOMOs (-5.5 –-5.7 eV) . The result indicates that the head-to-head linkage containing 3, 3’ -dialkynyl-2, 2’ -bithiophene (BTRy) is a weak donor unit with promising physicochemical properties and the study demonstrates that incorporating alkynyl side chain is an effective strategy for materials innovation in organic electronics.

Reference throughout this specification to “an embodiment, ” “some embodiments, ” “one embodiment” , “another example, ” “an example, ” “a specific example, ” or “some examples, ” means that a particular feature, structure, material, or characteristic described in connection with the embodiment or example is included in at least one embodiment or example of the present disclosure. Thus, the appearances of the phrases such as “in some embodiments, ” “in one embodiment” , “in an embodiment” , “in another example, ” “in an example, ” “in a specific example, ” or “in some examples, ” in various places throughout this specification are not necessarily referring to the same embodiment or example of the present disclosure. Furthermore, the particular features, structures, materials, or characteristics may be combined in any suitable manner in one or more embodiments or examples.

Although explanatory embodiments have been shown and described, it would be appreciated by those skilled in the art that the above embodiments cannot be construed to limit the present disclosure, and changes, alternatives, and modifications can be made in the embodiments without departing from spirit, principles and scope of the present disclosure.

Claims

A compound having a formula of
wherein

each of A and B is independently and respectively one of
and
given that at least one of A and B comprise an alkynyl,

each of R₁ and R₂ is independently and respectively a C_1-20 alkyl, a thienyl or a phenyl, and each of the thienyl and the phenyl is optionally and independently substituted with a C_1-20 alkyl.
The compound of claim 1, wherein the compound is represented by the following formula:

wherein each R is C_1-20 alkyl independently.
The compound of claim 2, wherein each R is C₁₀ alkyl independently.
The compound of claim 3, wherein each R is C₁₀ branched alkyl.
The compound of claim 4, wherein each R is
The compound of claim 5, wherein the compound is represented by the following formula:
A compound having a formula of
wherein A and B is defined in any one of claims 1 to 6, Me is methyl.
A compound of claim 7, wherein the compound is represented by the following formula:
An organic semiconductor polymer formed by an electron donor unit and an electron acceptor unit, wherein the electron donor is a compound claimed in any one of claims 1 to 6, and the electron acceptor unit is at least one selected from a group comprising:

wherein each R is C1～20 alkyl independently.
The organic semiconductor polymer of claim 9, represented by the following formula:

wherein n is an integer ranging from 1 to 10, and x is S or O.
The organic semiconductor polymer of claim 9 or 10, showing a band gap of about 1.6 eV.
The organic semiconductor polymer of any one of claims 9 to 11, showing a substantial hole mobility up to 0.13 cm² V^-1s^-1 in a top-gated organic thin-film transistor.
The organic semiconductor polymer of any one of claims 9 to 11, showing an electron mobility up to 0.06 cm² V^-1s^-1 in a bottom-gated organic thin-film transistor.
The organic semiconductor polymer of any one of claims 9 to 11, showing a Power Conversion Efficiency of at least >7％ and/or a V_ocs of 0.91-1.0 V in a polymer solar cell.
The organic semiconductor polymer of any one of claims 9 to 14, represented by the following formula:
wherein n is suitable to achieve a number average molecular weight of 43.8 kDa.
The organic semiconductor polymer of any one of claims 9 to 15, represented by the following formula:
wherein n is suitable to achieve a number average molecular weight of 33.0 kDa.
A semiconductor film formed by the organic semiconductor polymer of any one of claims 9 to 16.
A semiconductor device comprising the film of claim 17 or the polymer of any one of claims 9 to 16.
The semiconductor device of claim 18, wherein the semiconductor device is an opto-electrical device comprising at least one of organic thin-film transistor and polymer solar cell.
The semiconductor device of claim 19, wherein the organic thin-film transistor is a top-gated organic thin-film transistor or a bottom-gated organic thin-film transistor.
The semiconductor device of any one of claims 18 to 20, wherein the film or the polymer shows:

a substantial hole mobility up to 0.13 cm² V^-1s^-1 in a top-gated organic thin-film transistor,

an electron mobility up to 0.06 cm² V^-1s^-1 in a bottom-gated organic thin-film transistor, or

a maximum Power Conversion Efficiency approaching 8％ and/or a V_ocs of 0.91-1.0 V in a polymer solar cell.