WO2018076247A1

WO2018076247A1 - A weak electron-donating building block, copolymers thereof and their preparation methods as well as their applications

Info

Publication number: WO2018076247A1
Application number: PCT/CN2016/103620
Authority: WO
Inventors: Xugang GUO; Jun Huang; Yumin TANG; Han GUO
Original assignee: South University Of Science And Technology Of China
Priority date: 2016-10-27
Filing date: 2016-10-27
Publication date: 2018-05-03

Abstract

A weak electron-donating building block, copolymers thereof and their preparation methods as well as their applications. The weak electron-donating building block provided by the present invention is of Formula I. In the present invention, incorporating strong electron-withdrawing substituents into the electron-rich 3, 3' -dialkyoxy-2, 2' -bithiophene leads to new monomers with weaker electron donating abilities and hence lower-lying HOMOs.

Description

A weak electron-donating building block, copolymers thereof and their preparation methods as well as their applications

FIELD

The present invention belongs to the field of semiconductor materials, in particular to a weak electron-donating building block, copolymers thereof and their preparation methods, as well as their applications.

BACKGROUND

Organic semiconductors have received much attention since their potential for fabricating diverse opto-electrical devices using solution-based processing techniques, which enable the fabrication of cost-effective, large area, and mechanically flexible electronic devices. In order to achieve enhanced device performance, great efforts have been made to tune the highest occupied molecular orbitals (HOMOs) , lowest unoccupied molecular orbitals (LUMOs) as well as the bandgaps of the semiconducting materials.

In addition to energetic consideration, the materials solubility and the film morphology and microstructure play critical roles for performance enhancement. In order to improve solution processability, solubilizing alkyl chains are usually attached to semiconductor backbone. Due to the accompanying steric hindrance, the alkylation position must be strategically located to minimize backbone torsion, which is detrimental to materials packing and film crystallinity, and hence limits π-orbital overlapping and intramolecular/intermolecular charge delocalization. For example, head-to-head linkage in poly (3-alkylthiophene) leads to twisted polymer backbone conformation and reduced film crystallinity, thus degraded performance in PSCs and OTFTs is observed versus the regioregular poly (3-alkylthiophene) , in which the head-to-head linkage is highly eliminated. In addition to controlling regioregularity, inserting non-alkylated π-spacers, such as thienothiophene or bithiophene, is another effective strategy to develop organic semiconductors without head-to-head linkage. However, the resulting polymers typically suffer from limited solubility.

For addressing the issue of reduced solubility and maintaining backbone planarity, a novel materials strategy by incorporating head-to-head linkage into polymer backbone is designed. It has been reported that intramolecular non-covalent S…O interaction can lock conformation and promote backbone planarity in the head-to-head linkage containing bithiophene. By inserting oxygen atom into the 3, 3’ -dialkyl-2, 2’ -bithiophene, a novel electron donor unit, 3, 3’ -dialkyoxy-2, 2’ -bithiphene (BTOR, Formula 1) , was designed and synthesized successfully.

Two solubilizing alkoxy chains on the 3, 3’ -positions of bithiophene afford polymers with excellent solubility and the conformation locking via intramolecular non-covalent S…O interaction leads to polymers with high degree of backbone planarity, narrow band gap, and substantial crystallinity. When copolymerized with phthalimide and naphthalene diimide, the BTOR-based copolymers show large hole mobility and remarkable ambipolar transport in OTFTs, respectively. In spite of the good absorption in visible region and substantial charge carrier mobilities, the phthalimide-BTOR copolymers show moderate power conversion efficiencies (PCEs) of ～ 4％in PSCs. The highly electron rich BTOR results in greatly elevated HOMOs (-5.1 eV) of the resulting copolymers, and hence the BTOR-based semiconductors typically show small open-circuit voltages (V_ocs) of ～ 0.5 V. The electron rich BTOR also results in unsatisfactory device performance stability in OTFTs.

For lowering semiconductor HOMOs, more electron-deficient thiazole was used to replace thiophene and a new 3, 3’ -dialkoxy-2, 2’ -bithiazole (BTzOR, Formula 2) was innovated and incorporated into polymer semiconductors. The resulting semiconductors maintain good mobilities (0.06 –0.25 cm²V^-1s^-1) but with slightly lower-lying HOMO level (-5.2 eV) and improved OTFT current modulation ratios and device stabilities. The results demonstrate that the electron-deficient thiazole can offset the electron-donating tendencies of the alkoxy substitutes, resulting in a weak donor unit.

For further lowering the HOMOs of polymer semiconductors, a novel monoalkoxy functionalized head-to-head linkage containing bithiophene, 3-alkyl-3’ -alkoxy-2, 2’ -bithiophene (TRTOR, Formula 3) is developed. In comparison to the BTOR-based polymers, the TRTOR analogous semiconductors exhibit ～ 0.2 -0.3 eV lower HOMOs without sacrificing backbone planarity and film crystallinity. When incorporated into PSCs, the TRTOR-phthalimide copolymers demonstrate improved PCEs of 6.3％with enlarged V_ocs of 0.7 –0.8 V, which are 0.2 –0.3 V greater than those of BTOR-based polymers.

As can be seen from the above, from BTOR to BTzOR and to TRTOR, the HOMOs of the alkoxy functionalized head-to-head linkage containing bithiophene are gradually lowered and the corresponding polymers show enlarged V_ocs and improved PCEs in PSCs via optimizing backbone arenes and side chain substitutes. For further lowering polymer HOMOs and achieving larger V_ocs in PSCs, the electron-donating capability of the head-to-head linkage containing dialkoxy bithiophene should be further optimized.

SUMMARY

To this end, incorporating strong electron-withdrawing substituents into the electron-rich BTOR should lead to new monomers with weaker electron donating capabilities and hence lower-lying HOMOs.

In order to achieve this purpose, the present invention employs the following technical solutions:

A weak electron-donating building block of Formula I,

Wherein

R is a straight or branched alkyl, preferably having 5–15 carbon atoms, and more preferably having 7–12 carbon atoms.

X is an electron-withdrawing group, preferably being fluorine, chloride, cyano, or nitro-group.

Y is bromine, iodine, trimethyltin, or tributyltin group.

Z is sulfur, oxygen, or selenium atom.

Fluorine, chloride, cyano and nitro-group are electron-withdrawing group and used to tune the energy level of semiconducting materials. For example, cyano is a strong electron-withdrawing group, which is highly effective to lower both HOMOs and LUMOs of organic semiconductors. Functionalizing poly (phenylenevinylene) s(PPVs) with cyano group can change the polarity of charge carrier and the cyano-modified PPVs can function as electron acceptor in PSCs due to the greatly lowered frontier molecular orbitals (FMOs) . Gradual addition of cyano group onto 2, 1, 3-benzothiadiazole acceptor was found to systematically lower polymer FOMs. You et al. recently reports an effective synthetic methodology to introduce cyano substituents in triazole-based polymers to tune semiconductor FMOs.

In comparison to the common strategy to functionalize acceptor unit with cyano, functionalization of donor units with cyano is greatly overlooked. Due to its strong electron withdrawing capability, we expect that the attachment of cyano onto the electron rich dialkoxy bithiophene should afford a weak donor with optimized electrical property and the incorporation of the monomer into polymer backbone should generate a series of organic semiconductors with improved device performance.

Compared to BTOR, BTzOR and TRTOR (Figure 1a-1c) , the newly designed BTCNOR (I) shows the lowest-lying HOMO of -6.06 eV in the series, attributed to the introduction of strong electron-withdrawing cyano group onto the 4, 4’ -positions of bithiophene. The cyano additions also afford the BTCNOR with the lowest-lying LUMO and comparable HOMO-LUMO gap. Therefore, BTCNOR-based polymer semiconductors are likely to realize the lowest-lying HOMOs, which should result in the highest V_ocs in PSCs. Moreover, the calculated distance between the sulfur and oxygen atoms in BTORCN is

substantially smaller than the sum of the sulfur and oxygen van der Waals radii

indicative of a close intramolecular non-covalent S…O interaction. The interaction should promote the BTCNOR to achieve a planar backbone conformation, which should result in reduced bandgap and benefit charge carrier delocalization in the corresponding polymers (Figure 1d) . The DFT computation results explicitly reveal that the new BTCNOR is a promising building block to construct polymer semiconductors with low-lying HOMO, high degree of backbone planarity, and good solubility.

In another aspect, the present invention provides a copolymer of the weak electron-donating building block described herein having Formula II,

Wherein

X is an electron-withdrawing group, preferably being fluorine, chlorine, cyano, or nitro-group.

Z is sulfur, oxygen, or selenium atom.

П is an aromatic unit and can be the same as or different from the building block.

n is 5-100.

Preferably, on the basis of the technical solution provided by the present invention, П is selected from the following groups:

wherein R’ is identical or different with each other, and is a straight or branched alkyl, preferably having 5–15 carbon atoms, and more preferably having 7–12 carbon atoms. R is a straight or branched alkyl, X is an electron-withdrawing group, such as cyano, fluorine, or chlorine, Z is sulfur, oxygen, or selenium atom；

As a proof of materials design, the BTCNOR is copolymerized with benzodithiophene (BDT) . It will be seen that the resulting polymer semiconductors exhibit very low-lying HOMOs as well as planar backbones. When incorporated into polymer: PCBM bulk heterojunction (BHJ) PSCs, remarkable V_ocs are obtained, which are greatly higher than those of polymers incorporating other dialkoxy bithiophenes without strong electron-withdrawing substituents, such as BTOR, BTzOR and TOTOR.

Moreover, BTCNOR-based polymer semiconductors show good performance in OTFTs. The results demonstrate that functionalizing highly electron rich dialkoxy bithiophenes with strong electron-withdrawing substituents can afford polymers with large V_ocs without sacrificing backbone planarity and materials solubility. Through a wide spectrum of materials and device characterization techniques, the materials structure-property-device performance correlations of the new BTCNOR polymeric semiconductors are established, which offers insights into materials innovation for high performance organic electronics.

In another aspect, the present invention provides a preparation method of the weak electron-donating building block described herein, wherein R is straight alkyl or branched alkyl, comprising:

(1) adding 3, 3’ -bis (alkyloxy) - [2, 2’ -bithiophene] -4, 4’ -dicarbonitrile, an organic solvent into a reaction vessel, and purging the solution with an inert gas；

(2) adding n-BuLi；

(3) adding Br₂, or I₂, or tributylchlorotin, or trimethylchlorotin, warming the mixture to 0–60 ℃；

(4) extracting the reaction mixture and washing；

(5) concentrating the organic layer and purifying to give the weak electron-donating building block； wherein the alkyloxy is straight or branched.

preferably, on the basis of the technical solution provided by the present invention, in step (1) , the ratio of the organic solvent to the 3, 3’ -bis (alkyloxy) - [2, 2’ -bithiophene] -4, 4’ -dicarbonitrile is 2–10 mL/mmol, for example, 3 mL/mmol, 5 mL/mmol, 8 mL/mmol, 9.5 mL/mmol and so on, preferably being 3–7 mL/mmol.

preferably, the organic solvent is selected from THF, diethyl ether, hexane or a mixture of at least two of them.

preferably, time of the purging is more than 10 minutes, preferably more than 20 minutes, more preferably 30 minutes.

preferably, the inert gas is selected from any one of Ar, N₂, He, Ne, or a mixture of at least two of them.

preferably, in step (2) , the mole ratio of the n-BuLi to the 3, 3’ -bis (alkyloxy) - [2, 2’ -bithiophene] -4, 4’ -dicarbonitrile is 2–4: 1, for example, 2.5, 3.3, 3.9 and so on, preferably 2–3: 1, more preferably 2.5: 1.

preferably, adding n-BuLi is conducted under low temperature.

preferably, in step (3) , the mole ratio of Br₂, or I₂, or tributylchlorotin, or trimethylchlorotin to the 3, 3’ -bis (alkyloxy) - [2, 2’ -bithiophene] -4, 4’ -dicarbonitrile is 2-15: 1, for example, 2.5: 1, 4: 1, 7: 1, 9: 1, 11: 1, 13: 1 and so on, preferably 2–8: 1, more preferably 6: 1.

preferably, the Br₂, or I₂, or tributylchlorotin, or trimethylchlorotin is added dropwise.

preferably, the mixture is warmed at 20-50 ℃.

preferably, in step (4) , the reaction mixture is extracted with organic solvent, preferably with DCM.

preferably, the washing is conducted with water and brine.

preferably, in step (5) , the concentrating is conducted under reduced pressure.

preferably, the purifying is conducted by column chromatography or recrystallization.

In another aspect, the present invention provides a preparation method of the copolymer described herein comprising:

(1) adding the weak electron-donating building block described herein, a precursor of an aromatic unit, tris (dibenzylideneacetone) dipalladium (Pd₂ (dba) ₃) _, and tris (o-tolyl) phosphine (P (o-tol) ₃) into a reaction vessel, and subjecting the reaction vessel and the mixture to an inert gas；

(2) adding an organic solvent； sealing the reaction vessel under an inert gas flow and then stirring while heating；

(3) adding 2- (tributylstanny) thiophene and stirring the reaction mixture while heating； finally, adding 2-bromothiophene and stirring the reaction mixture while heating；

(4) after cooling to room temperature, dripping the reaction mixture into methanol containing hydrochloric acid；

(5) drying the solid precipitate obtained in step (4) to give the crude product, and then extracting the crude product；

(6) after final extraction, concentrating the polymer solution, and then being dripped into methanol again, collecting the polymer and drying to give the target copolymer.

Preferably, in step (1) , the precursor of aromatic unit is an aromatic compound. While synthesizing a copolymer by copolymerizing with the weak electron-donating building block, the precursor can introduce the aromatic unit into the copolymer. The aromatic unit is selected from the following group:

wherein R’ is identical or different with each other, and is a straight or branched alkyl, preferably having 5–15 carbon atoms, and more preferably having 7–12 carbon atoms.

preferably, the mole ratio of the weak electron-donating building block to the aromatic unit in the precursor is 1: 0.5–2, for example, 1: 0.9, 1: 1.2, 1: 1.8 and so on, preferably 1: 0.8–1.5, more preferably 1: 1.

preferably, the mole ratio of the tris (dibenzylideneacetone) dipalladium (Pd₂ (dba) ₃) to tris (o-tolyl) phosphine (P (o-tolyl) ₃) is 1: 4-15, for example, 1: 6, 1: 9, 1: 13 and so on, preferably 1: 6-10, more preferably 1: 8； the Pd loading is 0.005–0.1 equiv, preferably 0.01–0.06.

preferably, the reaction vessel and the mixture are subjected to 1–5 pump/purge cycles with Ar.

preferably, in step (2) , the inert gas is selected from any one of Ar, N₂, He, Ne, or a mixture of at least two of them.

preferably, the organic solvent is selected from any one of anhydrous toluene, benzene, chlorobenzene, DMF, or a mixture of at least two of them.

preferably, the ratio of the organic solvent to the weak electron-donating building block is 10–75 mL/mmol, preferably 5–50 mL/mmol.

preferably, the heating is conducted at 50–170 ℃ for 1–72 h, preferably at 80–150 ℃ for 3–50 h.

preferably, the heating is conducted under microwave irradiation.

preferably, the heating is conducted by 80 ℃ for 10 minutes, 100 ℃ for 10 minutes, and 140 ℃ for 3 h under microwave irradiation.

preferably, in step (3) , the heating is conducted at 80–170 ℃ for more than 0.2 h, preferably at 100–160 ℃ for more than 0.4 h.

preferably, the heating is conducted under microwave irradiation.

preferably, the heating is conducted under microwave irradiation at 140 ℃ for 0.5 h； finally, adding 2-bromothiophene and stirring the reaction mixture at 140 ℃for another 0.5 h.

preferably, the mole ratio of the 2- (tributylstanny) thiophene to the weak electron-donating building block is 0.1–0.5: 1, for example, 0.2: 1, 0.3: 1, 0.45: 1 and so on, preferably 0.2: 0.4–1.

preferably, the mole ratio of the 2-bromothiophene to the weak electron-donating building block is 0.2–1.5: 1, for example, 0.2: 1, 0.3: 1, 0.45: 1, 0.6: 1, 0.9: 1, 1.2: 1, 1.4: 1 and so on, preferably 0.4: 0.8-1.

preferably, in step (4) , the methanol contains 0.5–10mL hydrochloric acid， preferably 0.5–10 mL of 5–20 mol/L hydrochloric acid.

preferably, the dripping is conducted under vigorous stirring, preferably is conducted for at least 0.5 h, preferably at least 1 h.

preferably, in step (6) , the concentrating is conducted under vacuum.

preferably, the dripping is conducted under vigorous stirring.

preferably, the collecting is conducted by filtration.

preferably, the drying is conducted under reduced pressure.

In still another aspect, the present invention provides the use of the copolymer according to the present invention in PSCs or OTFTs.

The raw materials used in above preparation method can be prepared by known method in the art or by the following method described below or buying on the market.

In this invention, we report the design and synthesis of a novel head-to-head linkage containing bithiophene, which is functionalized with strong electron withdrawing group and features non-covalent S…O interaction. Functionalization of the dialkoxy bithiophene (BTOR) with strong electron-withdrawing substituents enables the new building block as a weak electron donor.

As a proof of materials design, the new building block is copolymerized with benzodithiophene (BDT) , thieno [3, 4-c] pyrrole-4, 6-dione (TPD) , bithiophene. Espcially, the resulting polymer based on BDT exhibit very low-lying HOMOs (～-5.5 –-5.6 eV) as well as planar backbones. When incorporated into polymer: PCBM bulk heterojunction (BHJ) PSCs, remarkble V_ocs up to ～1.0 V are obtained, which are greatly higher than those of polymers incorporating other dialkoxy bithiophenes without strong electron-withdrawing substituents, such as BTOR, BTzOR and TOTOR.

The results demonstrate that functionalizing highly electron rich dialkoxy bithiophenes with strong electron-withdrawing substituents can afford polymers with large V_ocs without sacrificing backbone planarity and materials solubility. Through a wide specrum of materials and device characterization techniques, the materials structure-property-device performance correlations of polymeric semiconductors are established, which offers insights into materials innovation for high performance organic electronics.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows Chemical structures, optimized geometries, and energy levels of the frontier molecular orbitals of (a) 3, 3’ -dialkoxy-2, 2’ -bithiophene (BTOR) ； (b) 4, 4’ -dialkoxy-5, 5’ -bithiazole (BTzOR) ； (c) 3-alkyl-3’ -alkoxy-2, 2’ -bithiophene (TRTOR) , and (d) 3, 3’ -dialkoxy-4, 4’ -dicyano-2, 2’ -bithiophene (BTCNOR) . Calculations are carried out at the DFT//B3LYP/6-31G**level. Alkyl substituents are truncated here to simplify the calculations.

Figure 2 wherein (a) shows optical absorption spectras of polymers P1-P3 in o-dichlorobenzene solutions (1×10^-5 M) at room temperature； (b) shows Temperature dependant optical absorption spectras of polymer solutions (1×10^-5 M) in o-dichlorobenzene at various temperature of 50, 70, 90, and 100 ℃； (c) shows Optical absorption spectras of polymer films casted from o-dichlorobenzene solutions (5 mg mL^-1) .

Figure 3 wherein (a) shows cyclic voltammograms of P1-P3 films in 0.1 M (n-Bu) ₄N. PF₆ acetonitrile solution with the Fc/Fc⁺ redox couple as the internal standard； (b) shows DSC thermograms of P1-P3 for the second heating and cooling scans at a temperature ramp rate of 10 ℃/min under N₂.

Figure 4 shows TGA curves of P1-P3 at a scan of 20 ℃ min^-1 under the nitrogen atmosphere.

Figure 5 shows (a) structure of 3, 3’ -dimethoxy-4, 4’ -dicyano-2, 2’ -bithiophene viewed from； stacking of the model compound alogn with (a) a axis； (b) b axis； (c) c axis. The S…O interaction in the model compound is indicated by a dashed line. The dihedral angle of the bithiophene is ～1.0e and the S…O distance is

Figure 6. wherein (a) shows the BTCNOR-BDT trimer showing the calculated torsion angles； (b) shows front view and (c) shows side view of the computed trimer； (d) shows calculated HOMO orbital and (e) shows LUMO orbital of the trimer. The calculation was carried out using the density functional theory (DFT) at the B3LYP/6-31G*level.

Figure 7 shows photovoltaic performances of the optimized polymer: PC₇₁BM solar cells with and without using 2％ (v/v) chloronaphthalene (CN) : (a) J-V curves； (b) corresponding EQE plots.

Figure 8 shows J-V characteristics of (a) the hole-only devices and (b) the electron-only devices of polymer: PC₇₁BM blend films without CN additive.

Figure 9 shows J-V characteristics of (a) the hole-only devices and (b) the electron-only devices of polymer: PC₇₁BM blend films using CN as the processing additive.

Figure 10 shows linear J-V characteristics of TGBC FETs with semiconductor films annealed at 190 ℃, wherein Channel length is 20 μm and channel width is 5 mm.

Figure 11 shows tapping mode atomic force microscopy images (5×5 μm) of (a) P1: PC₇₁BM blend film without CN, RMS roughness ＝ 2.30 nm； (b) P2: PC₇₁BM blend film without CN, RMS roughness ＝ 2.13 nm； (c) P3: PC₇₁BM blend film without CN, RMS roughness ＝ 2.58 nm； (d) P1: PC₇₁BM blend film with CN, RMS roughness ＝ 1.97 nm； (e) P2: PC₇₁BM blend film with CN, RMS roughness ＝ 1.92 nm； (f) P3: PC₇₁BM blend film with CN, RMS roughness ＝ 1.58 nm.

Figure 12 shows transmission electron microscopy (TEM) images of (a) P1: PC₇₁BM blend film without CN； (b) P2: PC₇₁BM blend film without CN； (c) P3: PC₇₁BM blend film without CN； (d) P1: PC₇₁BM blend film with CN； (e) P2: PC₇₁BM blend film with CN； (f) P3: PC₇₁BM blend film with CN. The scale bar is 200 nm.

Figure 13 wherein (a) shows optical absorption spectra of polymers P4-P7 in o-dichlorobenzene solutions (1×10^-5 M) at room temperature； (b) shows optical absorption spectra of polymer films casted from o-dichlorobenzene solutions (5 mg mL^-1) .

Figure 14 shows optical absorption spectra of polymers P8-P9 in chloroform solutions (1×10^-5 M) at room temperature and optical absorption spectra of polymer P10 in chloroform solution (1×10^-5 M) or in solid state at room temperature.

Figure 15 shows ¹H NMR and ¹³C NMR of 5, 5’ -dibromo-3, 3’ -bis (dodecyloxy) - [2, 2'-bithiophene] -4, 4’ -dicarbonitrile and 3, 3’ -bis (dodecyloxy) -5, 5’ -bis (trimethylstannyl) - [2, 2’ -bithiophene] -4, 4’ -dicarbonitril e.

DETAILED DESCRIPTION

To facilitate understanding of the present invention, the embodiment of the present invention is exemplified as follows. Skilled in the art should be appreciated that the embodiments are merely used to help understand the present invention and should not be regarded as specific limits on the invention.

Materials and Instruments

All reagents and chemicals were commercially available and were used without further purification unless otherwise stated.

THF and toluene were distilled from Na/benzophenone. ¹H NMR and ¹³C NMR) spectra were recorded on a Bruker AV 400 spectrometer with tetramethylsilane (TMS) as the internal reference. Molecular weights of the polymers were obtained on a PL GPC 220 (Polymer Laboratories) at 140 ℃ using a calibration curve of polystyrene standards, with 1, 2, 4-trichlorobenezene as the eluent. Elemental analysis was measured on Vario EL Cube. Mass spectra were recorded on Agilent Technologies 5977A MSD or Q-Exactive. Thermogravimetric (TGA) measurements were carried out with a METTLER TOLEDO (TGA 1 STAR^e System) apparatus at a heating rate of 20 ℃ /min under N₂. Differential scanning calorimetry (DSC) analysis was performed on a METTLER TOLEDO (DSC 1 STAR^e System) apparatus at a heating or cooling rate of 10 ℃ /min under N₂ and the second cycle was collected. UV-vis absorption spectra were recorded on a SHIMADZU UV-3600 spectrophotometer. Cyclic voltammetry (CV) was performed on a CHI660A electrochemical workstation with platinum electrodes at a scan rate of 50 mV s^-1 against an Ag/Ag⁺reference electrode with N₂-saturated solution of 0.1 M tetrabutylammonium hexafluorophosphate (Bu₄NPF₆) in acetonitrile (CH₃CN) . Potentials were referenced to the ferrocenium/ferrocene redox couple as an internal standard.

The deposition of a copolymer on the electrode was completed by the evaporation of a chlorobenzene solution. AFM measurements of polymer: PC₇₁BM blend films were performed by 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 device fabrication, 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.

Conventional polymer solar cells with a structure of glass/ITO/MoO₃/polymer: PC₇₁BM/Ca/Al is used for device fabrication and optimization. Pre-patterned ITO-coated glass with a sheet resistance of < 10 Ω sq^-1 is used as the substrate, which is cleaned by sequential sonication in H₂O containing detergent, deionized H₂O, acetone, and isopropanol followed by UV/ozone (BZS250GF-TC, HWOTECH, Shenzhen) treatment for 15 min. MoO₃ (10 nm) is deposited onto ITO glass substrates via thermal evaporation as the hole extraction layer. The active layer with an optimal thickness of ～90 nm is spin-coated onto MoO₃ from polymer: PC₇₁BM solutions (10 mg mL^-1 for polymer) in o-DCB with various polymer: PC₇₁BM weight ratios. Then Ca (15 nm) and Al (100 nm) are sequentially deposited atop the active layer via thermal evaporation (ca. 2 × 10^-6 mbar) . The size of PSCs is 0.045 cm². Device characterization is carried out in the N₂-filled glovebox using a Xeno-lamp-based solar simulator (Newport, Oriel AM 1.5G, 100 mW cm^-2) with a computerized Keithley 2400 source meter. The light intensity is carefully calibrated using an NREL-certified monocrystalline Si diode coupled to a KG3 filter to bring spectral mismatch to unity.

Top-gate/bottom-contact transistors are fabricated to characterize the field-effect mobility. Source and drain electrodes (3 nm Cr and 30 nm Au) are patterned on borosilicate glass by photolithography. The substrates are cleaned by sonication in acetone and isopropanol, sequentially, and then treated with UV-ozone. Semiconductor films are spin-coated from 5 mg mL^-1 o-DCB solutions, then are thermal annealed at elevated temperatures (190 ℃) for 15 minutes. Dielectric layers are spin-coated from diluted CYTOP solutions (CTL-809M: CT-SOLV180 ＝ 2: 1 (v: v) , Asahi Glass Co.， Ltd. ) , then are annealed at 100 ℃ for 20 minutes. Finally, 50 nm Al is thermally evaporated on top as the gate electrode. The devices are characterized using a Keithley 4200 SCS. All device fabrication and measurement are carried out in a N₂-filled glove box.

Hole-only devices with a structure of ITO/PEDOT: PSS/polymer: PC₇₁BM/MoO₃/Ag and electron-only devices with a structure of ITO/ZnO/Polymer: PC₇₁BM/Ca/Al are used to measure hole and electron mobilities in SCLC regime, respectively. The mobility is calculated by MOTT-Gurney equation:

where J is the current density, ε_r is the relative dielectric constant of active layer material typically 2-4 for organic semiconductors. Herein relative dielectric constants of 3.9 and 3.0 are used for polymer and for PC₇₁BM, respectively. ε₀ is the permittivity of free space, μ is the charge mobility, and L is the active layer thickness. The thickness of film is determined using a surface profilometer (KLA TENCOR D-120) . V is the internal voltage in the device, and V ＝ V_app-V_bi, where V_app is the voltage applied to the devices, 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) .

The synthesis of the new monomer, 3, 3’ -dialkoxy-4, 4’ -dicyano-2, 2’ -bithiophene (BTCNOR) is straightforward, and Scheme 1 depicts the synthetic route and corresponding polymer semiconductors. Compounds 3 with different side chains are prepared according to the synthetic pathway reported in the literature. Model compound for single crystal analysis is also synthesized, the synthetic route is depicted in Scheme 2.

Reagents and conditions in Scheme 1 are: (i) Mesyl chloride, trimethylamine, dichloromethane； (ii) 4-cyano-3-tetrahydrothiophenone, CsCO₃, DMF； (iii) DDQ, THF； (iv) n-BuLi, triisopropylsilyl chloride, THF； (v) n-BuLi, ferric acetylacetonate； (vi) tetrabutylammonium fluoride, THF； (vii) (a) n-BuLi, THF； (b) Br₂, THF； (viii) Pd₂(dba) ₃, P (o-tolyl) ₃, toluene, microwave, 80 ℃ for 10 min, 100 ℃ for 10 min, and 140 ℃ for 3 h.

Reagents and conditions in Scheme 2 are: (i) 4-cyano-3-tetrahydrothiophenone, CsCO₃, DMF； (ii) DDQ, THF； (iii) n-BuLi, triisopropylsilyl chloride, THF； (iv) n-BuLi, ferric acetylacetonate； (v) tetrabutylammonium fluoride, THF.

More detailed monomer and polymer synthetic and characterization information is reported as follows. Compounds 3 with different side chains are prepared according to the synthetic pathway reported in the literature. Since the proton (5-position) adjacent to the cyano group has a higher acidity versus the proton (2-position) next to the alkoxy chain, triisopropylsilyl (TIPS) is introduced to 3 at 5-position as the protecting group for future reaction, which is highly similar to the methodology for the synthesis of dialkoxy bithiazole (BTzOR) .

Then the 2-position of compound 4 can be successfully lithiated with n-BuLi and the subsequent Fe-mediated coupling affords compound 5 in good yield. The TIPS group on compound 5 can be readily deprotected by treating with tetrabutylammonium fluoride (TBAF) to give the key compound 6 with nearly quantitative yield. It should be noted that the target monomer 7 is obtained in poor yield by directly brominating compound 6 with Br₂. To our delight, the n-BuLi/Br₂ sequential treatment on compound 6 gives the target monomer 7 in high yield. The identity and purity of all compounds are supported by ¹H NMR, ¹³C NMR, and mass spectrum (or elemental analysis) .

Monomer and Polymer Synthesis-wherein R is a linear alkane or branched alkane

2-Ethylhexyl methanesulfonate (1a) synthesized by following scheme 3

To a solution of 2-ethylhexan-1-ol (31.3 mL, 200 mmol) and triethylamine (33.3 mL, 1.2 equiv) in THF (150 mL) was added dropwise methanesulfonyl chloride (18.6 mL, 1.2 equiv) at 0 ℃ for 30 minutes under argon atmosphere. The resulting light brown suspension was warmed to r.t. and stirred overnight. Then, a saturated solution of ammonium chloride (100 mL) was added. The aqueous layer was extracted with dichloromethane (100 mL × 3) . The organic layer was combined and dried over anhydrous magnesium sulfate followed by filtration. After rotary evaporation, a light brown oil was obtained as the crude product, which was subjected to flash chromatograph. Finally, the product was obtained as a colorless solid (38.2 g, 92％yield) .

¹H NMR (400 MHz, CDCl₃) δ (ppm) : 4.15 (t, 2H) , 3.02 (d, 2H) , 1.61 (m, 1H) , 1.46-1.25 (m, 8H) , 0.93 (m, 3H) . ¹³C NMR (100 MHz, CDCl₃) , δ (ppm) : 72.16, 39.17, 37.17, 29.84, 28.77, 23.26, 22.88, 14.02, 10.84.

n-Decyl methanesulfonate (1b) synthesized by following scheme 4.

Compound 1b was prepared following the same procedures employed for compound 1a with a yield of 91％.

¹H NMR (400 MHz, CDCl₃) , δ (ppm) : 4.21 (t, 2H) , 2.99 (s, 2H) , 1.74 (m, 1H) , 1.39-1.24 (m, 16H) , 0.87 (m, 3H) . ¹³C NMR (100 MHz, CDCl₃) , δ (ppm) : 70.35, 37.48, 31.98, 29.59, 29.54, 29.39, 29.25, 29.15, 25.54, 22.79, 14.23.

n-Dodecyl methanesulfonate (1c) synthesized by following scheme 5.

Compound 1c was prepared following the same procedures employed for compound 1a with a yield of ～100％.

¹H NMR (400 MHz, CDCl₃) , δ (ppm) : 4.24 (t, 2H) , 3.02 (d, 2H) , 1.76 (m, 2H) , 1.43-1.28 (m, 18H) , 0.88 (t, 3H) . ¹³C NMR (100 MHz, CDCl₃) , δ (ppm) : 70.36, 37.46, 32.02, 29.73, 29.63, 29.54, 29.45, 29.24, 29.15, 25.54, 22.8, 14.24. (Note: one peak in ¹³C NMR spectrum overlap) .

4- (2-Ethylhexyloxy) -2, 5-dihydrothiophene-3-carbonitrile (2a) synthesized by following scheme 6.

To a solution of 4-cyano-3-tetrahydrothiophene (1 g, 7.86 mmol) in DMF (10 mL) , CsCO₃ (2.8 g, 1.1 equiv) and 2-ethylhexyl methanesulfonate (1a, 2 g, 1.2 equiv) were added. The solution was irradiated in microwave oven (T ＝ 80 ℃) for 10 min. After the completion of the reaction, the mixture was poured into water and then extracted with dichloromethane (100 mL × 3) . The combined organic layer was dried over anhydrous MgSO₄, filtrated and then concentrated to give the crude product. The crude product was subjected to column chromatograph using petroleum ether: ethyl acetate (10: 1) as the eluent to afford a pale yellow oil as the product (1.2 g, 64％yield) .

¹H NMR (400 MHz, CDCl₃) , δ (ppm) : 4.20 (t, 2H) , 3.76 (s, 4H) , 1.62 (m, 1H) , 1.33-1.28 (m, 8H) , 0.87 (m, 6H) . ¹³C NMR (100 MHz, CDCl₃) , δ (ppm) : 168.31, 115.81, 80.55, 74.53, 39.55, 36.01, 34.05, 29.95, 28.86, 23.42, 23.1, 13.85, 10.84. EIMS: C₁₃H₂₁NOS calcd: 239.13, found: 239.1.

4- (n-Decyloxy) -2, 5-dihydrothiophene-3-carbonitrile (2b) synthesized by following scheme 7.

Compound 2b was prepared following the same procedures employed for compound 2a with a yield of 92％.

¹H NMR (400 MHz, CDCl₃) δ (ppm) : 4.31 (t, 2H) , 3.76 (s, 4H) , 1.72 (m, 2H) , 1.39-1.29 (m, 16H) , 0.83 (t, 3H) . ¹³C NMR (100 MHz, CDCl₃) δ (ppm) : 169.11, 115.94, 80.72, 72.49, 36.25, 34.29, 32.0, 29.63, 29.6, 29.51, 29.42, 29.33, 25.71, 22.8, 14.24. EIMS: C₁₅H₂₅NOSS calcd: 267.17, found: 267.1.

4- (n-Dodecyloxy) -2, 5-dihydrothiophene-3-carbonitrile (2c) synthesized by following scheme 8.

Compound 2c was prepared following the same procedures employed for compound 2a with a yield of 95％.

¹H NMR (400 MHz, CDCl₃) δ (ppm) : 4.31 (t, 2H) , 3.77 (s, 4H) , 1.74-1.67 (m, 2H) , 1.43-1.27 (m, 18H) , 0.88 (t, 3H) . ¹³C NMR (400 MHz, CDCl₃) δ (ppm) : 169.11, 115.92, 80.64, 72.44, 36.2, 34.23, 32.01, 29.73, 29.72, 29.64, 29.57, 29.47, 29.44, 29.3, 25.67, 25.52, 22.79, 14.23. EIMS: C₁₇H₂₉NOS calcd: 295.2, found: 295.1.

4- (2-Ethylhexyloxy) thiophene-3-carbonitrile (3a) synthesized by following scheme 9.

To a solution of 4- ( (2-ethylhexyl) oxy) -2, 5-dihydrothiophene-3-carbonitrile (2a, 2.5 g, 10.44 mmol) in dichloromethane (50 mL) was added 2, 3-dicyano-5, 6-dichlorobenzoquinone (DDQ) (2.85 g, 1.2 equiv) in tetrahydrofuran (20 mL) at 50 ℃ under argon atmosphere. After addition, the mixture was stirred at the temperature overnight. The mixture was cooled to RT and extracted with dichloromethane (200 mL × 3) . The combined organic layer was dried over anhydrous magnesium sulfate followed by filtration. After rotary evaporation, the residue was subjected to column chromatograph using petroleum ether: ethyl acetate (10: 1) as the eluent to afford a colorless oil as the product (1.1 g, 85％yield) .

¹H NMR (400 MHz, CDCl₃) δ (ppm) : 7.77 (d, 1H) , 6.25 (d, 1H) , 3.89 (d, 2H) , 1.75 (m, 1H) , 1.49-1.27 (m, 8H) , 0.94 (m, 6H) . ¹³C NMR (100 MHz, CDCl₃) δ (ppm) : 157.96, 134.18, 113.54, 104.28, 97.92, 73.82, 39.3, 30.49, 29.18, 23.89, 23.11, 14.21, 11.24. EIMS: C₁₃H₁₉NOS calcd: 237.12, found: 237.1.

4- (n-Decyloxy) thiophene-3-carbonitrile (3b) synthesized by following scheme 10.

Compound 3b was prepared following the same procedures employed for compound 3a with a yield of 83％.

¹H NMR (400 MHz, CDCl₃) δ (ppm) : 7.77 (d, 1H) , 6.25 (d, 1H) , 4.0 (t, 2H) , 1.81 (m, 2H) , 1.46-1.32 (m, 15H) , 0.87 (t, 3H) . ¹³C NMR (100 MHz, CDCl₃) δ (ppm) : 157.81, 134.19, 113.48, 104.28, 98.07, 71.44, 32.03, 29.68, 29.66, 29.45, 29.03, 26.02, 22.82, 14.25. (Note: one peak in ¹³C NMR spectrum overlap) . EIMS: C₁₅H₂₅NOS calcd: 265.15, found: 265.1.

4- (n-dodecyloxy) thiophene-3-carbonitrile (3c) synthesized by following scheme 11.

Compound 3c was prepared following the same procedures employed for compound 3a with a yield of 74％.

¹H NMR (400 MHz, CDCl₃) δ (ppm) : 7.76 (s, 2H) , 6.25 (s, 2H) , 4.13 (t, 2H) , 1.85-1.78 (m, 2H) , 1.31-1.24 (m, 16H) , 0.86 (t, 3H) . ¹³C NMR (100 MHz, CDCl₃) δ (ppm) : 157.78, 134.21, 113.6, 104.22, 98.04, 71.4, 32.06, 29.79, 29.77, 29.73, 29.66, 29.49, 29.45, 29.02, 26.02, 22.83, 14.27. EIMS: C₁₇H₂₇NOS calcd: 293.2, found: 293.1.

4- (2-Ethylhexyloxy) -2- (triisopropylsilyl) thiophene-3-carbonitrile (4a) synthesized by following scheme 12.

To a solution of 4- (2-ethylhexyloxy) thiophene-3-carbonitrile (3a, 865 mg, 3.64 mmol) in tetrahydrofuran (30 mL) was added n-BuLi (2.4 M, 1.67 mL, 1.1 equiv) slowly at -78 ℃ under argon atmosphere. After addition, the mixture was stirred at the temperature for 1 h. Then triisopropylsilyl chloride (0.86 mL, 1.1 equiv) was then added in one portion. The mixture was warmed to RT and stirred for overnight. The mixture was quenched with water and extracted with dichloromethane (100 mL × 3) . The combined organic layer was dried over anhydrous magnesium sulfate followed by filtration. After rotary evaporation, the crude product was subjected to column chromatograph using petroleum ether as the eluent to afford a colorless oil as the product (1.43 g, 93％yield) .

¹H NMR (400 MHz, CDCl₃) δ (ppm) : 6.51 (s, 1H) , 3.88 (d, 2H) , 1.68 (m, 1H) , 1.55 (m, 3H) , 1.53-1.2 (m, 8H) , 1.13 (d, 18H) , 0.92 (m, 6H) . ¹³CNMR (100 MHz, CDCl₃) δ (ppm) : 160.62, 145.8, 115.42, 110, 102.69, 73.96, 39.39, 30.52, 29.22, 23.91, 23.12, 18.63, 14.23, 11.81, 11.27. EIMS: C₂₂H₃₉NOSSi calcd: 393.25, found: 393.3.

4- (n-Decyloxy) -2- (triisopropylsilyl) thiophene-3-carbonitrile (4b) synthesized by following scheme 13.

Compound 4b was prepared following the same procedures employed for compound 4a with a yield of 81％.

¹H NMR (400 MHz, CDCl₃) δ (ppm) : 6.51 (s, 1H) , 3.99 (t, 2H) , 1.82 (m, 2H) , 1.58 (m, 3H) , 1.55-1.27 (m, 14H) , 1.12 (d, 18H) , 0.87 (t, 3H) . ¹³C NMR (100 MHz, CDCl₃) δ (ppm) : 160.3, 145.7, 115.3, 109.8, 102.6, 71.4, 31.89, 29.56, 29.53, 29.36, 29.32, 29.02, 25.94, 22.68, 18.49, 14.11, 11.89. EIMS: C₁₅H₂₃NOS calcd: 265.15, found: 265.1.

4- (n-Dodecyloxy) -2- (triisopropylsilyl) thiophene-3-carbonitrile (4c) synthesized by following scheme 14.

Compound 4c was prepared following the same procedures employed for compound 4a with a yield of 96％.

¹H NMR (400 MHz, CDCl₃) δ (ppm) : 6.45 (s, 1H) , 3.91 (t, 2H) , 1.78 (m, 2H) , 1.49 (m, 2H) , 1.71 (s, 3H) , 1.31-1.20 (m, 18H) , 1.06 (d, 18H) , 0.86 (t, 3H) . ¹³C NMR (100 MHz, CDCl₃) δ (ppm) : 160.32, 145.72, 115.93, 109.81, 102.37, 71.39, 31.94, 30.37, 29.69, 29.65, 29.63, 29.55, 29.38, 29.03, 25.96, 22.72, 18.50, 14.15, 11.68. EIMS: C₂₆H₄₇NOSSi calcd: 449.31, found: 449.3.

3, 3’ -Bis (2-ethylhexyloxy) -5, 5’ -bis (triisopropylsilyl) - [2, 2’ -bithiophene] -4, 4’ -dicarbonitrile (5a) synthesized by following scheme 15.

To a solution of 4- (2-ethylhexyloxy) -2- (triisopropylsilyl) thiophene-3-carbonitrile (4a, 400 mg, 1.06 mmol) in tetrahydrofuran (30 mL) , n-BuLi (2.4 M, 0.5 mL, 1.1 equiv) was slowly added at -78 ℃ under argon atmosphere. After addition, the mixture was stirred at this temperature for 45 min and warmed to RT for 1h. The mixture was cooled to 0 ℃ and a solution of Fe (acac) ₃ (400 mg, 1.1 equiv) in THF (10 mL) was added in one portion. The mixture was stirred at 80 ℃ for 2 h. Then the reaction was quenched with water and extracted with dichloromethane (75 mL × 3) . The organic layer was dried over anhydrous magnesium sulfate followed by filtration. After the removal of solvent, the residue was subjected to column chromatograph using petroleum ether as the eluent to afford a white solid as the product (288 mg, 69％yield) .

¹H NMR (400 MHz, CDCl₃) δ (ppm) : 4.16 (d, 2H) , 1.84 (m, 1H) , 1.63 (m, 3H) , 1.55-1.25 (m, 10H) , 1.15 (d, 18H) , 0.90 (m, 6H) . ¹³C NMR (100 MHz, CDCl₃) δ (ppm) : 156.26, 145.07, 122.72, 116.19, 111.63, 78.23, 40.97, 30.63, 29.61, 24, 23.49, 19, 14.52, 12.2, 11.55. HRMS (ESI) : m/z calcd for C₄₄H₇₆N₂O₂S₂Si₂, 785.3940； found, 785.4937.

3， 3’ -Bis (decyloxy) -5, 5’ -bis (triisopropylsilyl) - [2, 2’ -bithiophene] -4, 4’ -dicarbonitrile (5b) synthesized by following scheme 16.

Compound 5b was prepared following the same procedures employed for compound 5a with a yield of 70％.

¹H NMR (400 MHz, CDCl₃) δ (ppm) : 4.27 (t, 2H) , 1.89 (m, 2H) , 1.62 (m, 3H) , 1.60-1.27 (m, 14H) , 1.15 (d, 18H) , 0.88 (t, 3H) . ¹³C NMR (100 MHz, CDCl₃) δ (ppm) : 155.67, 144.45, 123.16, 115.79, 111.37, 75.33, 32.04, 30.49, 29.76, 29.74, 29.49, 26.22, 22.83, 18.66, 14.27, 11.82. (Note: one peak was ¹³C NMR spectrum overlap) . HRMS (ESI) : m/z calcd for C₄₈H₈₄N₂O₂S₂Si₂, 841.5020； found, 841.5560.

3, 3’ -Bis (dodecyloxy) -5, 5’ -bis (triisopropylsilyl) - [2, 2’ -bithiophene] -4, 4’ -dicarbonitrile (5c) synthesized by following scheme 17.

Compound 5c was prepared following the same procedures employed for compound 5a with a yield of82％.

¹H NMR (400 MHz, CDCl₃) δ (ppm) : 4.19 (t, 2H) , 1.82 (m, 2H) , 1.70 (m, 2H) , 1.66 (s, 3H) 1.71 (s, 3H) , 1.54-1.19 (m, 18H) , 0.98 (d, 18H) , 0.81 (t, 3H) . ¹³C NMR (100 MHz, CDCl₃) δ (ppm) : 155.64, 144.32, 123.03, 116.66, 111.25, 75.21, 31.94, 30.37, 29.71, 29.68, 29.67, 29.64, 29.62, 29.38, 26.09, 22.71, 18.53, 14.15, 11.70. HRMS (ESI) m/z: [M+H] ⁺ calcd for C₅₂H₉₂N₂O₂S₂Si₂, 897.6139； found, 897.6186.

3, 3’ -Bis ( (2-ethylhexyl) oxy) - [2, 2’ -bithiophene] -4, 4’ -dicarbonitrile (6a) synthesized by following scheme 18.

To a solution of 3, 3’ -bis (2-ethylhexyloxy) -5, 5’ -bis (triisopropylsilyl) - [2, 2’ -bithiophene] -4, 4’ -dicarbon itrile (5a, 500 mg, 0.64 mmol) in tetrahyrofuran (25 mL) was slowly added tetrabutylammonium fluoride (1 M, 1.9 mL, 3 equiv) at -78 ℃ under argon atmosphere. After addition, the mixture was warmed to room temperature. The mixture was quenched with water and extracted with dichloromethane (70 mL × 3) . The organic layer was dried over anhydrous magnesium sulfate followed by filtration. After the removal of solvent, a white solid was obtained as the product (302 mg, 99％) , which can be used without further optimization.

¹H NMR (400 MHz, CDCl₃) δ (ppm) : 7.77 (s, 1H) , 4.19 (d, 2H) , 1.84 (m, 1H) , 1.51-1.25 (m, 9H) , 0.92 (m, 6H) . ¹³C NMR (100 MHz, CDCl₃) δ (ppm) : 152.7, 133.51, 117.29, 114.12, 104.54, 77.58, 40.36, 30.18, 29.07, 23.64, 23.14, 14.2, 11.17. HRMS (ESI) : m/z calcd for C₂₆H₃₆N₂O₂S₂, 472.2218； found, 472.2282.

3, 3’ -Bis (decyloxy) - [2, 2’ -bithiophene] -4, 4’ -dicarbonitrile (6b) synthesized by following scheme 19.

Compound 6b was prepared following the same procedure employed for compound 6a with a yield of 86％.

¹H NMR (400 MHz, CDCl₃) δ (ppm) : 7.76 (s, 1H) , 4.3 (d, 2H) , 1.88 (m, 2H) , 1.49-1.27 (m, 14H) , 0.87 (t, 3H) . ¹³C NMR (100 MHz, CDCl₃) δ (ppm) : 152.39, 133.35, 118.04, 114.02, 104.63, 75.09, 32.03, 30.16, 29.67, 29.66, 29.46, 29.45, 25.88, 22.83, 14.27.

HRMS (ESI) m/z: [M+H] ⁺ calcd for C₃₀H₄₄N₂O₂S₂, 529.2844； found, 529.2965.

3, 3’ -Bis (dodecyloxy) - [2, 2’ -bithiophene] -4, 4’ -dicarbonitrile (6c) synthesized by following scheme 20.

Compound 6c was prepared following the same procedures employed for compound 6a with a yield of 100％.

¹H NMR (400 MHz, CDCl₃) δ (ppm) : 7.76 (s, 2H) , 4.30 (t, 2H) , 1.88 (m, 2H) , 1.50-1.25 (m, 18H) , 0.88 (t, 3H) . ¹³C NMR (100 MHz, CDCl₃) δ (ppm) : 152.26, 133.23, 117.92, 113.90, 104.51, 74.97, 31.94, 30.04, 29.68, 29.66, 29.60, 29.54, 29.38, 29.35, 25.76, 22.72, 14.15. HRMS (ESI) : m/z calcd for C₃₄H₅₂N₂O₂S₂, 584.3470； found, 584.3429.

5, 5’ -dibromo-3, 3’ -bis (2-ethylhexyloxy) - [2, 2’ -bithiophene] -4, 4’ -dicarbonitrile (7a) synthesized by following scheme 21.

To a solution of 3, 3’ -bis (2-ethylhexyloxy) - [2, 2’ -bithiophene] -4, 4’ -dicarbonitrile (6a, 225 mg, 0.48 mmol) in tetrahyrofuran (20 mL) was added n-BuLi (2.4 M, 0.41 mL, 2.1equiv) slowly at -78 ℃ under argon atmosphere. After addition, the mixture was stirred at this temperature for 1h. Then the solution was warmed to r.t. and stayed for 1h. The mixture was cooled to -78 ℃ again and Br₂ (0.16 mL, 6.7 equiv) was added in one portion. The mixture was warmed to r.t. and stirred overnight. The reaction was quenched with water and extracted with dichloromethane (50 mL × 3) . The combined organic layer was dried over anhydrous magnesium sulfate followed by filtration. After the removal of solvent, the residue was subjected to flash chromatograph using dichloromethane: petroleum ether (1: 1) as eluent. A pale yellow solid was obtained as the product (302 mg, 80％) .

¹H NMR (400 MHz, CDCl₃) δ (ppm) : 4.3 (d, 2H) , 1.85 (m, 1H) , 1.55-1.25 (m, 8H) , 0.9 (m, 6H) . ¹³C NMR (100 MHz, CDCl₃) δ (ppm) : 151.34, 122.68, 117.96, 112.94, 107.93, 77.99, 40.33, 30.15, 151.34, 122.68, 117.96, 112.94, 107.93, 77.99, 40.33, 30.15, 29.02, 23.6, 23.16, 14.23, 11.16. E_LEM. A_NAL. calcd. for C₂₆H₃₄Br₂N₂O₂S₂: C, 49.53； H, 5.44； N, 4.44； S, 10.17. Found: C, 49.65； H, 5.56； N, 4.48； S, 10.13.

5, 5’ -dibromo-3, 3’ -bis (decyloxy) - [2, 2’ -bithiophene] -4, 4’ -dicarbonitrile (7b) synthesized by following scheme 22.

Compound 7b was prepared following the same procedures employed for compound 7a with a yield of 83％.

¹H NMR (400 MHz, CDCl₃) δ (ppm) : 4.3 (d, 2H) , 1.89 (m, 2H) , 1.56-1.25 (m, 14H) , 0.88 (t, 3H) . ¹³C NMR (100 MHz, CDCl₃) δ (ppm) : 150.99, 122.63, 118.16, 112.83, 107.94, 75.36, 32.03, 30.05, 29.66, 29.46, 29.4, 25.8, 22.83, 14.28. (Note: one peak was ¹³C NMR spectrum overlap) . E_LEM. A_NAL. calcd. for C₃₀H₄₂Br₂N₂O₂S₂: C, 52.48； H, 6.17； N, 4.08； S, 9.34. Found: C, 52.16； H, 6.15； N, 4.06； S, 9.18.

5, 5’ -dibromo-3, 3’ -bis (dodecyloxy) - [2, 2’ -bithiophene] -4, 4’ -dicarbonitrile (7c) synthesized by following scheme 23.

Compound 7c was prepared following the same procedures employed for compound 7a with a yield of 94％.

¹H NMR (400 MHz, CDCl₃) δ (ppm) : 4.30 (t, 2H) , 1.87 (m, 2H) , 1.55-1.26 (m, 18H) , 0.88 (t, 3H) . ¹³C NMR (100 MHz, CDCl₃) δ (ppm) : 150.85, 122.50, 118.03, 112.70, 107.80, 75.23, 31.95, 29.93, 29.68, 29.66, 29.59, 29.54, 29.38, 29.28, 25.68, 22.72, 14.16. E_LEM. A_NAL. calcd. for C₃₄H₅₀Br₂N₂O₂S₂: C, 54.98； H, 6.79； N, 3.77； S, 8.63. Found: C, 55.49； H, 6.89； N, 3.84； S, 8.21.

General polymerization procedure of polymers P1-P9:

A glass tube was charged with dibromo monomer (1.0 equiv) , distannylated monomer (1.0 equiv) , (Pd₂ (dba) ₃) (1.5％) , and (P (o-tolyl) ₃) (12％) . The tube and its contents were subjected to 3 pump/purge cycles with argon, followed by the addition of anhydrous toluene (5 mL) via syringe. The tube was sealed under argon flow and then stirred at 80 ℃ for 10 minutes, 100 ℃ for 10 minutes, and 150 ℃ for 3 h under microwave irradiation. Then, 0.1 mL of 2- (tributylstanny) thiophene was added and the reaction mixture was stirred under microwave irradiation at 140 ℃ for 0.5 h. Finally, 0.2 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 solution was dripped into 200 mL methanol (containing 5 mL 12 N hydrochloric acid) under vigorous stirring. After stirring for 1 h, the precipitation was poured into a Soxhlet thimble, and then extracted with methanol, acetone, hexane, dichloromethane, and chloroform, sequentially. The chloroform fraction was concentrated and dripped into methanol under vigorously stirring. The title polymer was obtained after vacuum filtration and drying.

Poly {4, 8-bis (5- ( (2-ethylhexyl) thio) thiophen-2-yl) benzo [1, 2-b: 4, 5-b’ ] dithiophene-alt-3, 3’ -bis (2-ethylhexyloxy) - [2, 2’ -bithiophene] -4, 4’ -dicarbonitrile) thiophene} (P1) synthesized by following scheme 24.

The polymer was obtained as blue solid with a yield of 81％. M_n: 34.0 kg/mol； M_w/M_n: 2.5. E_LEM. A_NAL. calcd. for (C₆₀H₇₆N₂O₄S₈) _n: C, 64.71； H, 6.88； N, 2.52； S, 23.03. Found: C, 64.9； H, 6.84； N, 2.49； S, 22.59.

Poly {4, 8-bis (5- ( (2-ethylhexyl) thio) thiophen-2-yl) benzo [1, 2-b: 4, 5-b’ ] dithiophene-alt-3, 3’ -bis (decyloxy) - [2, 2’ -bithiophene] -4, 4’ -dicarbonitrile) thiophene} (P2) synthesized by following scheme 25.

The polymer was obtained as blue solid with a yield of 56％. M_n: 34.0 kg/mol； M_w/M_n: 2.5. E_LEM. A_NAL. calcd. for (C₆₄H₈₄N₂O₂S₈) _n: C, 65.71； H, 7.24； N, 2.39； S, 21.39. Found: C, 65.86； H, 7.48； N, 2.25； S, 22.18.

Poly {4, 8-bis (5- ( (2-ethylhexyl) thio) thiophen-2-yl) benzo [1, 2-b: 4, 5-b’ ] dithiophene-alt-3, 3’ -bis (dodecyloxy) - [2, 2’ -bithiophene] -4, 4’ -dicarbonitrile) thiophene} (P3) synthesized by following scheme 26.

The polymer was obtained as blue solid with a yield of 89％. M_n: 34.0 kg/mol； M_w/M_n: 2.5. E_LEM. A_NAL. calcd. for (C₆₈H₉₂N₂O₂S₈) _n: C, 66.62； H, 7.56； N, 2.29； S, 20.92. Found: C, 66.91； H, 7.94； N, 2.2； S, 21.24.

Poly {4, 8-bis (2-ethylhexyl) thiophen-2-yl) benzo [1, 2-b: 4, 5-b’ ] dithiophene-alt-3, 3’ -bis (dodecyloxy) - [2, 2’ -bithiophene] -4, 4’ -dicarbonitrile) thiophene} (P4) synthesized by following scheme 27.

The polymer was obtained as blue solid with a yield of 95％. The corresponding UV-Vis absorption spectra of chlorobenzene solution (10^-5 M) and in film solid state are shown in Figure 13a and Figure 13b.

Poly {4, 8-bis (5- (2-ethylhexyl) thiophen-2-yl) benzo [1, 2-b: 4, 5-b’ ] dithiophene-alt-3, 3’ -bis (dodecyloxy) - [2, 2’ -bithiophene] -4, 4’ -dicarbonitrile) thiophene} (P4) synthesized by following scheme 28.

The polymer was obtained as blue solid with a yield of 92％. The corresponding UV-Vis absorption spectra of chlorobenzene solution (10^-5 M) and in film solid state are shown in Figure 13a and Figure 13b.

Poly {3, 3’ -bis (dodecyloxy) -2, 2’ -bithiophene-alt-3, 3’ -bis ( (2-ethylhexyl) oxy) - [2, 2’ -bithiophene] -4-carbonitrile} (P6) synthesized by following scheme 29.

The polymer was obtained as blue solid with a yield of 62％. The corresponding UV-Vis absorption spectra of chlorobenzene solution (10^-5 M) and in film solid state are shown in Figure 13a and Figure 13b.

Poly {4, 4’ -didodecyl-2, 2’ -bithiophene-alt-3, 3’ -bis ( (2-ethylhexyl) oxy) - [2, 2’ -bithiophene] -4-carbonitrile} (P6) synthesized by following scheme 30.

The polymer was obtained as black solid with a yield of 70％. The corresponding UV-Vis absorption spectra of chlorobenzene solution (10^-5 M) and in film solid state are shown in Figure 13a and Figure 13b. 5- (4-cyano-3- (dodecyloxy) -5- (trimethylstannyl) thiophen-2-yl) -4- (dodecyloxy) -2- (trimethylstannyl) thiophene-3-carbonitrile (7) synthesized by following scheme 31.

To a solution of 5- (4-cyano-3- (dodecyloxy) thiophen-2-yl) -4- (dodecyloxy) thiophene-3-carbonitrile (176 mg, 0.3 mmol) in tetrahyrofuran (20 mL) was slowly added n-BuLi (2.4 M, 0.31 mL, 2.5 equiv) at -78 ℃ under argon atmosphere. After addition, the mixture was stirred at this temperature for 1h. Then the solution was warmed to RT and stayed for 1h. The mixture was cooled to -78 ℃ again and a solution of trimethylstannyl chloride (1 M, 0.81 mL, 2.7 equiv) was added in one portion. The mixture was warmed to RT and stirred overnight. The reaction was quenched with water and extracted with dichloromethane (50 mL) . The organic layer was dried over anhydrous magnesium sulfate, filtration and concentration gave the crude white product. The product was recrystallized with methanol. White solid, 230 mg, 84％. The 1HNMR and 13CNMR spectra are shown in Figure 15 (right column) . ¹H NMR (CDCl₃, 300 MHz) , δ (ppm) : 4.26 (t, 2H) , 1.85 (m, 2H) , 1.53-1.26 (m, 18H) , 0.88 (t, 3H) , 0.47 (s, 9H) . ¹³C NMR (CDCl₃, 75 MHz) , δ (ppm) : 154.79, 149.60, 123.14, 115.98, 111.58, 74.80, 31.94, 30.19, 29.71, 29.67, 29.64, 29.51, 29.39, 25.98, 22.72, 14.15, -8.21. LC-MASS: m/z ＝ 911.28.

Poly {3, 3’ -bis (dodecyloxy) - [2, 2’ -bithiophene] -4, 4’ -dicarbonitrile-alt-5- (2-ethylhexyl) -4H-thieno [3, 4-c] pyrrole-4, 6 (5H) -dione} (P8) synthesized by following scheme 32.

The polymer was obtained as blue solid with a yield of 80％. The corresponding UV-Vis absorption spectrum of chloroform solution (10^-5 M) is shown in Figure 14.

Poly {3, 3’ -bis (dodecyloxy) - [2, 2’ -bithiophene] -4, 4’ -dicarbonitrile-alt-5- (2-hexyldecyl) -4H-thieno [3, 4-c] pyrrole-4, 6 (5H) -dione} (P9) synthesized by following scheme 33.

The polymer was obtained as blue solid with a yield of 65％. The corresponding UV-Vis absorption spectrum of chloroform solution (10^-5 M) is shown in Figure 14a.

Poly {3, 3’ -bis (dodecyloxy) - [2, 2’ -bithiophene] -4, 4’ -dicarbonitrile-alt-4, 4, 9, 9-tetrakis (4-hexylphenyl) -4, 9-dihydro-sindaceno [1, 2-b: 5, 6-b’ ] dithiophene} (P10) synthesized by following scheme 34.

The polymer was obtained as black solid with a yield of 70％. The corresponding UV-Vis absorption spectra of chloroform solution (10^-5 M) and solid state are shown in Figure 14b.

2- (alkythio) thienyl substituted benzodithiophene (BDT) has shown great success for enabling high performance semiconductors and the stannylated BDT is chosen here as the comonomer. Polymers P1-P3 are synthesized under conventional Stille coupling condition using microwave as the heating source. After polymerization, polymer chains are end-capped with mono-functionalized 2- (tributylstannyl) thiophene and 2-bromothiophene, sequantially. P1-P3 are collected by precipitation in methanol, and then subjected to Soxhlet extractions using methanol, acetone, hexane, dichloromethane, and chloroform. All polymers show good solubilities in common organic solvents, such as chloroform, chlorobenzene, and dichlorobenzene. Molecular weights of BTCNOR-based polymers were measured by high temperature gel permeation chromatograph (GPC) at 140 ℃ using 1, 2, 4-trichlorobenzene as the eluent versus polystyrene standards. The M_ns of P1-P3 are 34.0, 57.4 and 44.1 kg mol^-1, with polydisperity index (PDI； M_w/M_n) of 2.5, 2.5 and 3.4, respectively. The corresponding data are summarized in Table 1.

Table 1. Molecular weights, optical absorption properties, and electrochemical properties of the BTCNOR-based polymers P1-P3.

^a E_HOMO ＝ - (E_ox ^onset + 4.80) eV, and E_ox ^onset determined electrochemically using Fc/Fc⁺ internal standard； ^b E_LUMO ＝ E_HOMO + E_g, opt； ^c Optical bandgap derived from absorption onset of as-cast thin film: E_g, opt ＝ 1240/λ_onset (eV) .

Opto-electrical and Thermal Properties of Polymers

UV-vis absorption spectra of BTCNOR-based polymers P1-P3 in o-dichlorobenzene solutions and in thin film state are shown in Figure 2a and Figure 2c. The detailed absorption parameters, including absorption maxima (λ_max) , absorption edge (λ_onset) , and optical band gap (E_g, _opt) are summarized in Table 1. The temperature dependent absorption (Figure 2b) of polymer solution indicates strong aggregation of the BTCNOR-based polymers, which is attributed to the high degree of polymer backbone planarity and strong interchain interactions. The λ_maxs of P1-P3 in solution are located at 620, 622 and 625 nm, respectively. Compared to those of P2 and P3, the slightly blue-shifted λ_max of P1 manifests its lower degree of aggregation, which is attributed to the better solubilizing capability of branched 2-ethylhexyl on the BTCNOR. From solution to film, the λ_maxs of P1 and P2 show minimal change, a further indicative of the strong aggregation of polymers in solution. In film state, all polymers show structured absorption profile, which indicates a certain degree of ordering of the polymer films. It should be noted that the branched 2-ethylhexyl chain in P1 leads to slightly larger bandgap versus those of P2 and P3 due to the lower degree of ordering of P1, which results in degraded device performance in OTFTs and PSCs (vide infra) . The optical bandgaps derived from absorption onsets are 1.82, 1.77 and 1.78 eV for P1, P2 and P3, respectively.

The cyclic voltammograms of the BTCNOR-based polymers show distinct oxidation peaks, indicating their p-type characteristics (Figure 3a) . The HOMO energy levels of the BTCNOR-based polymer films are determined from the onsets of oxidation peaks (versus the half wave potentials of the Fc/Fc⁺ redox couple as the internal standard) . The HOMOs are calculated using the following equation: E_HOMO ＝– (E_ox ^onset + 4.80) eV, where E_ox ^onset is the onset oxidation potential versus Fc/Fc⁺. The caculated HOMOs of P1, P2 and P3 are –5.60, –5.54 and –5.56 eV, respectively. In comparison to the high-lying HOMOs (～ -5.1 –-5.3 eV) of other alkoxy-functionalized bithiophene or bithiazole, functionalizing dialkoxy bithiophene (BTOR) with strong electron-withdrawing cyano group can substantially lower the HOMOs of the resulting polymer semiconductors, which should lead to large V_ocs as the BTCNOR-based polymers are used as the donor layers in BHJ PSCs. Among P1-P3, the longer chain and the branched chain on BTCNOR lower polymer HOMOs, likely attributed to less compact polymer chain packing, which results in low degree of interchain π-orbital overlapping. The low-lying HOMOs are also beneficial to the stability of OTFT performance, which has been a challenge for typical alkoxy thiophene-based polymer semiconductors. The LUMOs of P1, P2, and P3 are –3.78, –3.77 and –3.78 eV, respectively, which are derived from their HOMOs and optical bandgaps using the equation of E_LUMO ＝E_HOMO + E_g, _opt. On the basis of the polymer FMOs, the HOMOs should lead to large V_ocs and the LUMOs ensure efficient exciton dissociation when the BTCNOR-based polymers are blended with fullerene derivatives in BHJ PSCs.

Thermal properties of P1-P3 are investigated by thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) . TGA reveals that the onset decomposition temperatures (T_ds) of P1, P2, and P3 under N₂ with a 5％weight-loss are 312, 320, and 331 ℃ (Figure 4) , respectively. The thermal stability of the BTCNOR-based polymers is sufficient for device PSC fabrication. DSC measurement shows that P1 and P2 have featureless thermogram (Figure 3b) , indicating amorphous character or low crystallinity. However, P3 exhibits distinct thermal transitions at 156/111 ℃ during the second heating/cooling cycles. Therefore, the replacement of branched 2-ethylhexyl chain and extending the side chain length from n-decyl to n-dodecyl result in improved film crystallinity for polymer P3, which results in improved device performance in PSCs (vide infra) .

Molecular Modeling and Conformation Study

The model compound 6 with a short methoxy substituent, 3, 3’ -dimethoxy-4, 4’ -dicyano-2, 2’ -bithiophene, is synthesized to study the intramolecular non-covalent S…O interaction and the backbone conformation of the head-to-head linkage containing BTCNOR. The detailed synthesis and characterization can be found in Supporting Information. The single crystal of the model compound is obtained by diffusing methanol into its chloroform solution and the crystallographic structure of the single crystal has been analyzed by X-ray diffraction. As shown in Figure 5, the two thiophenes in the model compound adopt anti-conformation with a high degree of planarity having a dihedral angle of 1.0°, which is good consistent with the density functional theory (DFT) computation result. The distance between the sulfur atom and oxygen atom is

which is significantly shorter than the sum of their van der Waals radii

The data of the single crystal show direct evidence of the intramolecular noncovalent S…O interaction, resulting in self-planarization and rigidification of backbone in the head-to-head linkage containing BTCNOR. High degree of polymer backbone planarity would be beneficial to charge carrier delocalization. Moreover, the model compound shows a typical lamellar arrangement motif with a π-π stacking distance of

In addition to the analysis of single crystal structure of the model compound, theoretical calculation (Figure 6) of BTCNOR-BDT trimer using DFT at the B3LYP/6-31*level was performed to gain insight of the molecular geometry of the polymer semiconductor. The trimmer shows a torsion angle of 3.8° between BDT and thiophene and a torsion angle of 4.6° between two thiophenes (Figure 6a) . Such small angles indicate good coplanarity of BTCNOR-based polymers (Figure 6b and 6c) . As depicted in Figure 6d and 6e, the electron density in the HOMO and LUMO wave function is delocalized along the polymer backbone, which should facilitate intramolecular charge transport.

Photovoltaic Performance of Polymer Solar Cells

Table 2. Summary of the performance for PSCs based on polymer: PC₇₁BM ＝ 1: 1 or 1: 3.

^a The ratio of polymer to PC₇₁BM. ^b Data in the parentheses are the averaged values based on over 12 devices.

To evaluate the photovoltaic performance of the BTCNOR-based polymer semiconductors, single junction BHJ PSCs are fabricated using P1-P3 as the electron donating materials and (6, 6) -phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) as the electron accepting material with a conventional device structure of glass/ITO/MoO₃/polymer: PC₇₁BM/Ca/Al. The active layers are spin-coated from warm o-dichlorobenzene solutions. In order to achieve optimized photovoltaic performance, the polymer: PC₇₁BM blend ratios are varied from 1: 1 to 1: 2 and to 1: 3 (Table 2) . It is found that the polymer: PC₇₁BM ratio of 1: 2 leads to the optimal performance (Table 3) . The current density-voltage (J-V) curves of the PSCs having the optimal polymer: PC₇₁BM ratio under the illumination of air mass (AM) 1.5G, 100 mW cm^-2 are shown in Figure 7a and the corresponding data are compiled in table 3. The average PCEs of 4.76％, 4.79％, and 6.22％are obtained for PSCs based on P1, P2, and P3, respectively. Processing additives are widely used during PSC fabrication for performance enhancement due to the improved film morphology. Among various additives tested, it was found that 1-chloronaphthalene (CN) is the most effective one for the BTCNOR-based polymer series.

In comparison to the cells without using processing additive, the PSCs based on P1 and P2 casted from blend solutions containing 2％ (v/v) CN show small PCE increment from 4.76％to 4.98％and from 4.79％to 4.93％, respectively, while the addition of CN leads to distinct PCE increment from 6.22％to 7.06％for P3-based PSCs. The highest PCE of 7.13％is obtained from P3-based PSCs with a J_sc of 12.4 mA cm^-2, a FF of 64％, and a V_oc of 0.92 V, which is ascribed to the higher charge carrier mobilities and optimized nanoscale blend film morphology than those of P1 and P2-based PSCs.

Table 3. Device performance parameters of PSCs under optimized conditions and the charge transport mobilites of hole and electron only devices based on polymer: PC₇₁BM ＝ 1: 2 blend films with or without CN.

^a PSCs were prepared from blend solutions without (N) or with (Y) 2％ (v/v) 1-chloronaphthalene (CN) as the processing additive. ^bData in the parentheses are the averaged values based on over 12 devices. ^c hole mobility of devices fabricated from blend solutions with or without 2％CN calculated from SCLC model； ^d electron mobility of devices fabricated from blend solutions with or without 2％CN calculated from SCLC model.

Figure 7b shows the external quantum efficiency (EQE) of the optimized P1-P3-based PSCs. Broad EQE spectra can be observed in the range of 300 to 750 nm for all devices. The optimized P1-based PSCs show a maximum EQE of ～50％and the P2-based PSCs exhibit higher EQE than that of devices based on P1, which is consistent with increased J_scs of the P2-based PSCs. The P3-based PSCs without using CN additive show EQE greater than 50％in the range of 400 to 650 nm, the addition of CN additive results in increased EQE with the highest EQE approaching to 70％at 400 nm. For P2-based PSCs the current (11.16 mA cm^-2) integrated from EQE is larger than that (10.31 mA cm^-2) from the J-V curve, for P1 and P3-based PSCs the mismatch between the currents (8.66 and 11.82 mA cm^-2) integrated from EQEs and the currents (8.76 and 12.21 mA cm^-2) from the J-V curves are 1％and 3％, respectively, indicating good internal consistency.

Among all PSCs performance parameters, it is remarkable to note that the BTCNOR-based polymers show very large V_oc (> 0.9 V) and the largest V_oc of 1.0 V is obtained from the P1-based PSCs. The large V_ocs are contributed to their very low-lying HOMOs (-5.54 –-5.60 eV) of P1-P3. The results demonstrate that incorporating strong electron-withdrawing cyano group into highly electron-rich dialkoxy bithiophene can lead to polymer semiconductors with substantial V_ocs. Functionalizing organic semiconductors with alkoxy chain has shown great success for enabling materials with narrow bandgap, high degree of backbone planarity, and enhanced solubility, however the strategy is mainly limited by the high-lying HOMOs of the resulting semiconductors. This work clearly demonstrates that attachment of strong electron-withdrawing groups onto highly electron-rich dialkoxy-functionalized bithiophenes affords an effective solution to lower polymer HOMOs without sacrificing the advantages enabled by the dialkoxy bithiophene.

Charge Transport Properties

Table 4. Top-gate/bottom-contact organic thin-film transistor performance parameters of polymers P1-P3.

In table 4, data represent the best mobilities with average mobilities in parentheses. The mobilities and threshold voltages are averaged from more than 5 devices. Device structure: glass/Cr-Au/polymer/CYTOP/Al.

The charge transport properties of the neat BTCNOR-based polymer semiconductors are investigated by fabricating organic thin-film transistors (OTFTs) and the hole mobilities (μ_hs) of top-gate/bottom-contact (BGTC) OTFTs are collected in Table 4 and the transfer curves are shown in Figure 10. The OTFTs show very low off-currents of 10^-12 –10^-11 A, which are two to three orders of magnitude lower than the those of OTFTs using the BTOR or BTzOR-based polymer semiconductors as the active layers. The greatly suppressed off-currents of P1-P3 OTFTs are attributed to their low-lying HOMOs. The calculated μ_hs in saturated regime are 1.6×10^-3, 3.0×10^-3, and 4.4×10^-3 cm² V^-1s^-1 for the P1, P2, and P3 neat films annealed at 190 ℃. The mobilities are likely limited by the moderate film crystallinity (vide infra) . On the other hand, BDT is not a typical unit for high mobility polymers due to the limited charge carrier delocalization, and the mobilities of the BTCNOR-BDT copolymers are among the highest values for BDT-based polymer semiconductors. Moreover, the low-lying HOMOs of the BTCNOR-based polymers results in inefficient hole injection from source electrodes (Au) due to the large charge injection barrier, which results in the moderate mobilities for the BTCNOR-based polymers. From P1 to P2 and to P3, the OTFTs show gradually increased μ_h, which is in accord with the materials crystallinity.

In addition to the OTFT mobilities, the charge carrier mobilities of polymer: PC₇₁BM blend films are also investigated using space charge limited current (SCLC) model, which is widely applied to determine the hole and electron transporting ability of active layers between electrodes in PSCs field. The corresponding current-voltage plots are presented in Figure 8 and Figure 9, and the values of charge mobility are summarized in Table 3 and Table 5. It was found that both the μ_h and the electron mobility (μ_e) of the P3: PC₇₁BM blend films are one to two orders of magnitude higher than those of P1: PC₇₁BM and P2: PC₇₁BM blend films. As 2％CN adding to the polymer: PC₇₁BM solutions, all the hole or electron only device show improved mobilites (P1: μ_h ＝ 4.4×10^-5 cm² V^-1s^-1, μ_e ＝ 1.9×10^-6 cm² V^-1s^-1； P2: μ_h ＝ 3.0×10^-6 cm² V^-1s^-1, μ_e ＝ 1.2×10^-5 cm² V^-1s^-1； P3: μ_h ＝ 2.4×10^-4 cm² V^-1s^-1, μ_e ＝ 2.0×10^-4 cm² V^-1s^-1) than those of devices without CN additive. Moreover, the μ_h/μ_eratios are 0.23, 0.25, and 1.20 for P1, P2, and P3-based blend films fabricated using 2％CN as the processing additive, respectively. The more balanced and the higher P3: PC₇₁BM blend film mobilities compared those of P1 and P2-based blend films result in more effective charge transport and collection in PSCs, which affords the highest J_sc (12.21 mA cm^-2) and FF (64％) for P3-based PSCs in the polymer series.

Table 5. The mobilities calculated from the hole-only or electron-only devices by fitting the J-V curves in the SCLC regime and the charge balance.

Film Morphologies and Microstructures

Film morphology of active layer plays an important role in determining the photovoltaic performance of PSCs. Atomic force microscopy (AFM) is used to investigate the surface morphologies of the polymer: PC₇₁BM blend films fabricated without and with the processing additive CN. The measurements are carried out under N₂ on the exposed organic layers between the Al electrodes. As shown in Figure 11a-b and 1d-e, the P1 and P2-based active layers show no significant morphology variation after CN addition, and the root mean square (RMS) roughnesses are slightly decreased from 2.30 nm to 1.97 nm, and from 2.13 nm to 1.92 nm for the P1 and P2 blend films, respectively. The reduced roughness likely indicates a phase separation at finer scale, which results in slightly improved PCEs for the cells fabricated using the processing additive. For the P3-based blend film, the CN addition shows remarkable effect on the blend film morphology (Figure 11c and 11f) . Without the processing additive, the P3: PC₇₁BM blend film shows many dark regions with a RMS roughness of 2.58 nm. The dark regions are likely due to the aggregates of one component, which can cause inefficient exciton dissociation since their sizes exceed the typical exciton diffusion length (20 nm) . The P3: PC₇₁BM film using CN as the processing additive shows that the dark region is absent and the RMS roughness is greatly reduced to 1.58 nm, which reflects phase separation at finer scale. At the same time, fibrillary structures with interpenetrating network are developed for the blend film processed with CN, which result in more efficient exciton dissociation and provide continuous pathway for charge carriers to reach their corresponding electrodes. Hence the J_sc is enhanced from 10.94 to 12.21 mA cm^-2 and the PCE is increased from 6.28％to 7.13％for the P3-based PSCs fabricated with the CN additive. Therefore, among the series, the P3-based PSC is the most sensitive one to the processing additive, which is likely due to its optimal combination of aggregation and solubility.

Transmission electron microscopy (TEM) is also used to investigate film morphology and phase separation. Figure 12 shows the TEM images of polymer: PC₇₁BM blend films with or without the additive. The most pronounced feature is that the blend films without using CN display large dark regions, corresponding PC₇₁BM-rich domains (Figure 12a-c) . After adding the CN, the degree of PC₇₁BM aggregation is slightly mitigated for the P1 and P2-based blend films (Figure 12d and Figure 12e) . For the P3-based blend film, the CN addition results in highly uniform film morphology (Figure 12f) . Phase separation at nanoscale becomes dominating and a bicontinious interpenetrating network with defined structure is clearly developed, which result in the highest J_sc (12.21 mA cm^-2) for the P3 cells fabricated using CN as additive. In comparison to the P1 and P2 blend films using CN, the CN addition results in complete elimination of aggregates with size larger than 20 nm in P3 blend. The optimal P3: PC₇₁BM blend film morphology is likely driven by its highest crystallinity as revealed by the DSC and GIXD studies. The TEM results of the P1-P3 blend films are highly compatible with their photovoltaic performance.

In summary, we have designed and synthesized a novel head-to-head linkage containing dialkoxy bithiophene unit, 3, 3’ -dialkoxy-4, 4’ -dicyano-2, 2’ -bithiophene (BTCNOR) . Incorporating the strong electron-withdrawing cyano group into the highly electron-rich dialkoxy bithiophene can greatly lower the FOMs of the building block, which enables BTCNOR as an optimized donor unit with improved electrical properties versus its analogues, such as BTOR, BTzOR, and TRTOR. Single crystal analysis reveals direct evidence of intramolecular non-covalent S…O interaction, which leads to the self-planarization and rigidification of BTCNOR backbone.

The cyano group on the head-to-head linkage bithiophene shows positive effect on the opto-electrical properties of BTCNOR-based polymer semiconductors. As applied in BHJ PSCs, the devices exhibit remarkable V_ocs approaching 1.0 V, which is ～0.4 –0.5 V larger than the devices using polymers without cyano substituents. In spite of the same polymer backbone, their photovoltaic performance is sensitive to side chain on the BTCNOR. The P3 with higher degree of film crystallinity shows an impressive performance with a PCE up to 7.13％in PSCs. Considering its medium bandgap, large V_oc, and encouraging PCE, P3 could be a promising candidate for the front cells in tandem/multijunction solar cells. Our results demonstrate that BTCNOR is a promising unit to construct high performance organic semiconductors and balancing the electron-donating ability of alkoxy chain using strong electron-withdrawing group offers a new strategy for materials innovation with optimized opto-electrical properties.

Embodiments of the invention have been described above and, obviously, modifications and alterations will occur to others upon the reading and understanding of this specification. The invention and any claims are intended to include all modifications and alterations insofar as they come within the scope of the claims or the equivalent thereof.

Claims

A weak electron-donating building block of Formula I,

Wherein

R is a straight or branched alkyl,

X is an electron-withdrawing group, such as cyano, fluorine, chlorine, or nitro-group,

Y is bromine, iodine, trimethyltin, or tributyltin,

Z is sulfur, oxygen, or selenium atom.
The weak electron-donating building block according to claim 1, characterized in that, R has 5–15 carbon atoms, and preferably 7–12 carbon atoms, Preferably, X is fluorine, chloride, cyano, or nitro-group.
A copolymer of the weak electron-donating building block according to claim 1 or 2 having Formula II,

Wherein

R is a straight or branched alkyl,

X is an electron-withdrawing group, such as cyano, fluorine, chlorine, or nitro-group,

Z is sulfur, oxygen, or selenium atom,

Πis an aromatic unit can be the same as or different from the building block,

n is 5-100.
The copolymer according to claim 3, characterized in that, R has 5–15 carbon atoms, and preferably 7–12 carbon atoms,

Preferably, X is fluorine, chlorine, cyano, or nitro-group.
The copolymer according to claim 3 or 4, characterized in that, Π is selected from the following groups:

wherein R’ is identical or different with each other, and is a straight or branched alkyl, preferably having 5–15 carbon atoms, and more preferably having 7–12 carbon atoms, R is a straight or branched alkyl, X is an electron-withdrawing group, such as cyano, fluorine, or chlorine, Z is sulfur, oxygen, or selenium atom.
Preparation method of the weak electron-donating building block according to claim 1 or 2, wherein R is straight or branched alkyl, comprising:

(1) adding 3, 3’ -bis (alkyloxy) - [2, 2’ -bithiophene] -4, 4’ -dicarbonitrile, an organic solvent into a reaction vessel, and purging the solution with an inert gas；

(2) adding n-BuLi；

(3) adding Br₂, or I₂, or tributylchlorotin, or methylchlorotin, warming the mixture to 0-60 ℃；

(4) extracting the reaction mixture and washing；

(5) concentrating the organic layer and purifying to give the weak electron-donating building block； wherein the alkyloxy is straight or branched.
The preparation method according to claim 6, characterized in that, in step (1), the ratio of the organic solvent to the 3,3’ -bis (alkyloxy) - [2, 2’ -bithiophene] -4, 4’ -dicarbonitrile is 2–10 mL/mmol, preferably being 3–7 mL/mmol；

preferably, the organic solvent is selected from THF, diethyl ether, hexane or a mixture of at least two of them；

preferably, time of the purging is more than 10 minutes, preferably more than 20 minutes, more preferably 30 minutes；

preferably, the mixture is cooled to 0 –- 90 ℃； preferably to -78 ℃；

preferably, the inert gas is selected from any one of Ar, N₂, He, Ne, or a mixture of at least two of them；

preferably, in step (2) , the mole ratio of the n-BuLi to the 3,3’ -bis (alkyloxy) - [2, 2’ -bithiophene] -4, 4’ -dicarbonitrile is 2–4: 1, preferably 2–3: 1, more preferably 2.5: 1；

preferably, adding n-BuLi is conducted under low temperature；

preferably, in step (3) , the mole ratio of Br₂, or I₂, or tributylchlorotin, or trimethylchlorotin to the 3, 3’ -bis (alkyloxy) - [2, 2’ -bithiophene] -4, 4’ -dicarbonitrile is 2-15: 1, preferably 2-8: 1, more preferably 6: 1；

preferably, the Br₂, or I₂, or tributylchlorotin, or trimethylchlorotin is added dropwise；

preferably, the mixture is warmed at 20–50 ℃；

preferably, in step (4) , the reaction mixture is extracted with organic solvent, preferably with DCM；

preferably, the washing is conducted with water and brine；

preferably, in step (5) , the concentrating is conducted under reduced pressure；

preferably, the purifying is conducted by column chromatography or recrystallization.
Preparation method of the copolymer according to any one of claims 3–5 comprising:

(1) adding the weak electron-donating building block according to claim 1 or 2, a precursor of aromatic unit, tris (dibenzylideneacetone) dipalladium (Pd₂ (dba) ₃) , and tris (o-tolyl) phosphine (P (o-tol) ₃) into a reaction vessel, and subjecting the reaction vessel and the mixture to an inert gas；

(2) adding an organic solvent； sealing the reaction vessel under an inert gas flow and then stirring while heating；

(3) adding 2- (tributylstanny) thiophene and stirring the reaction mixture while heating, finally, adding 2-bromothiophene and stirring the reaction mixture while heating；

(4) after cooling to room temperature, dripping the reaction mixture into methanol containing hydrochloric acid；

(5) drying the solid precipitate obtained in step (4) to give the crude product, and then extracting the crude product；

(6) after final extraction, concentrating the polymer solution, and then being dripped into methanol again, collecting the polymer and drying to give the target copolymer.
The preparation method according to claim 8, characterized in that, in step (1), the precursor of aromatic unit is selected from precursors of the following group:

wherein R’ is identical or different with each other, and is a straight or branched alkyl, preferably having 5–15 carbon atoms, and more preferably having 7–12 carbon atoms, R is a straight or branched alkyl, X is an electron-withdrawing group, such as cyano, fluorine, or chlorine, Z is sulfur, oxygen, or selenium atom；

preferably, the mole ratio of the weak electron-donating building block of claim 1 or 2 to the aromatic unit in the precursor is 1: 0.5–2, preferably 1: 0.8–1.5, more preferably 1: 1；

preferably, the mole ratio of the tris (dibenzylideneacetone) dipalladium (Pd₂ (dba) ₃) to tris (o-tolyl) phosphine (P (o-tolyl) ₃) is 1: 4–15, preferably 1: 6–10, more preferably 1: 8； the Pd loading is 0.005–0.1 equiv, preferably 0.01–0.06；

preferably, the reaction vessel and the mixture are subjected to 1–5 pump/purge cycles with Ar；

preferably, the inert gas is selected from any one of Ar, N₂, He, Ne, or a mixture of at least two of them；

preferably, in step (2) , the inert gas is selected from any one of Ar, N₂, He, Ne, or a mixture of at least two of them；

preferably, the organic solvent is selected from any one of anhydrous toluene, benzene, chlorobenzene, DMF, or a mixture of at least two of them；

preferably, the ratio of the organic solvent to the weak electron-donating building block is 10–75 mL/mmol, preferably 5–50 mL/mmol；

preferably, the heating is conducted at 50–170 ℃ for 1–72 h, preferably at 80–150 ℃ for 3–50 h；

preferably, the heating is conducted under microwave irradiation；

preferably, the heating is conducted by 80 ℃ for 10 minutes, 100 ℃ for 10 minutes, and 140 ℃ for 3 h under microwave irradiation；

preferably, in step (3) , the heating is conducted at 80–170 ℃ for more than 0.2 h, preferably at 100–160 ℃ for more than 0.4 h；

preferably, the heating is conducted under microwave irradiation；

preferably, the heating is conducted under microwave irradiation at 140 ℃ for 0.5 h； finally, adding 2-bromothiophene and stirring the reaction mixture at 140 ℃ for another 0.5 h；

preferably, the mole ratio of the 2- (tributylstanny) thiophene to the weak electron-donating building block is 0.1–0.5: 1, preferably 0.2: 0.4–1；

preferably, the mole ratio of the 2-bromothiophene to the weak electron-donating building block is 0.2–1.5: 1, preferably 0.4: 0.8–1；

preferably, in step (4) , the methanol contains 0.5–10mL hydrochloric acid， preferably 0.5–10 mL of 5–20 mol/L hydrochloric acid；

preferably, the dripping is conducted under vigorous stirring, preferably is conducted for at least 0.5 h, preferably at least 1 h；

preferably, in step (6) , the dripping is conducted under vigorous stirring；

preferably, the collecting is conducted by filtration；

preferably, the drying is conducted under reduced pressure.
Use of the copolymer according to any one of claims 3–5 in organic thin-film transistors (OTFTs) , polymer solar cells (PSCs) and other organic electronics.