[2,2′-Bithiophene]-4,4′-dicarboxamide: a novel building block for semiconducting polymers

A novel electron deficient building block [2,2′-bithiophene]-4,4′-dicarboxamide (BTDCA) was designed to lower the highest occupied molecular orbital (HOMO) energy level of polythiophenes in order to achieve a higher open circuit voltage (Voc) and thus a higher power conversion efficiency in polymer solar cells (PSCs). BTDCA dibromo monomers were conveniently synthesized in four steps, and were used to prepare three thiophene-based D-A polymers, P(BTDCA66-BT) (66BT), P(BTDCA44-BT) (44BT) and P(BTDCA44-TT) (44TT). All the polymers exhibited unipolar hole transport properties, exhibiting mobilities in the range of ∼10−4 to 10−2 cm2 V−1 s−1 with the highest hole mobility of up to 1.43 × 10−2 cm2 V−1 s−1 achieved for 44BT in bottom-gate bottom-contact organic thin film transistors (OTFTs). In PSCs, these polymers achieved high Voc's of 0.81–0.87 V when PCBM or ITIC was used as acceptor. When 44TT was used as donor and ITIC was used as acceptor, a power conversion efficiency (PCE) of up to 4.5% was obtained, a significant improvement when compared with the poly(3-hexylthiophene) (P3HT):ITIC devices, which showed the highest PCE of merely 0.92%.

with air plasma for 2 min.Substrates were then immersed in ethanol, chloroform, a 10 mM solution of octadecanethiol in ethanol for 1 hour, and ethanol in a covered petri dish successively.
After that, substrates were immersed in 100 mL DI Water in a petri dish and four drops of 1: 10: 10 (HNO3: HCl: H2O) were added.The substrates were kept for one min, taken out, and rinsed with deionized water, followed by drying with nitrogen gas and subsequently on a hot plate at 120 °C for 10 min.In the next step, the substrates were put in a solution of dodecyltrichlorosilane (DDTS) in toluene (3 v% DDTS in toluene) at room temperature for 20 min.The substrates were then rinsed with toluene and dried under a nitrogen flow.Then a polymer solution in chlorobenzene or dichlorobenzene (ca. 5 mg/mL) was spin-coated onto the substrate at 1000 rpm for 60 s to obtain a polymer film, which was subjected to thermal annealing at different temperatures for 20 min at each temperature in an argon filled glove box.All the OTFT devices have a channel length (L) of 30 μm and a channel width (W) of 1000 μm and were characterized in the same glove box using an Agilent B2912A Semiconductor Analyzer.The hole mobilities in the saturation regime were calculated according to the following equation: where   is the drain-source current, μ is charge carrier mobility,   is the gate dielectric layer capacitance per unit area (~ 11.6 nF cm −2 ),   is the gate voltage,   is the threshold voltage, L is the channel length (30 μm), and W is the channel width (1000 μm).

Fabrication and Characterization Polymer Solar Cells
All the polymer solar cells were fabricated in the following configuration ITO/PEDOT: PSS/Active layer/LiF/Al.The ITO substrates immersed in deionized water and acetone were sonicated in an ultrasonic bath for 20 min each at 40 °C.Then the substrates were taken out and cleaned by Qtips with acetone.Substrates were then sonicated for 20 min at 40 °C in IPA, followed by the cleaning using Q-tips again.The substrates were dried in vacuum and then treated in an air plasma cleaner for 10 min.A ~40 nm thin layer of PEDOT: PSS was deposited by spin-coating a PEDOT: PSS solution (Al 4083) at 4000 rpm and dried subsequently at 150 °C for 20 min in air.
Then the substrates were transferred to a nitrogen filled glove box, where the polymer donor and the acceptor blend layer was spin-coated using a solution of polymer donor : acceptor blend onto Electronic Supplementary Information X. Zhou, et al.
the PEDOT: PSS layer via different spin speed.The substrates were then placed on the hotplate, annealed at different temperatures for 10 min in nitrogen.Finally, a thin layer of LiF (1 nm) and a layer of Al (100 nm) electrode were deposited in vacuum on the substrate at a pressure of ca.5.0 × 10 -6 Pa.The active area of the devices is 0.0574 cm 2 .The current density-voltage (J-V) characteristics of the polymer solar cells were measured on an Agilent B2912A Semiconductor Analyser with a ScienceTech SLB300-A Solar Simulator.A 450 W xenon lamp and an air mass (AM) 1.5 filter were used as the light source.

Synthesis of 5-bromo-N,N-dibutylthiophene-3-carboxamide
To a solution of 5-bromothiophene-3-carboxylic acid (1.64 g, 7.9 mmol) in 12 mL of Electronic Supplementary Information X. Zhou, et al. anhydrous chloroform in a 50 ml heat gun-dried two-neck round bottom flask.The solution was cooled to 0 °C in an ice bath.Oxalyl chloride (2.00 g, 15.8 mmol) was added dropwise followed with one drop of DMF as a catalyst.The solution was allowed to warm to room temperature and stirred for 4 h.The unreacted excess oxalyl chloride was removed under reduced pressure.This intermediate product was used immediately for the next step without further purification.
To a 50 ml heat gun-dried two-neck round bottom flask, dibutylamine (2.05 g, 15.8 mmol) in anhydrous chloroform (10 mL) was added.The solution was cooled to 0 °C in an ice bath.The crude 5-bromothiophene-3-carbonyl chloride synthesized above was dissolved in anhydrous chloroform (5 mL) was added dropwise.Then the solution was allowed to warm to room temperature and stirred overnight.The reaction was quenched by addition of water.The organic phase was washed with brine twice and dried over anhydrous Na 2 SO 4 .Upon removal of solvent in vacuo, the crude product was further purified by column chromatography on silica gel with hexane: ethyl acetate (5:1) to give the target compound.Yield: 2.33 g (95%).To a 25ml two-neck round bottom flask, N 4 ,N 4 ,N 4' ,N 4' -tetrahexyl-[2,2'-bithiophene]-4,4'dicarboxamide (0.74 g, 1.5 mmol) and NBS (0.58 g, 3.15 mmol) were added.The mixture was vacuumed and filled with argon three times and then a mixture of chloroform (7.5 mL) and trifluoroacetic acid (1.5 mL) were added.The mixture was stirred in the dark for 5 h (TLC showed no starting material left).The mixture was extracted with chloroform several times and the organic phase were combined and washed with sodium sulfite aqueous solution.The organic phase was dried with anhydrous Na 2 SO 4 and the solvent was removed under reduced pressure.
The solution of (1) 5-bromothiophene-3-carboxylic acid (1.86 g, 9 mmol) in 12 mL of anhydrous chloroform was put in a 50 mL heat gun-dried two-neck round bottom flask.The solution was cooled to 0 °C in an ice bath.Oxalyl chloride (2.29 g, 18 mmol) was added dropwise followed with one drop of dimethylformamide (DMF) as a catalyst.The solution was allowed to warm to room temperature for 4h.Unreacted oxalyl chloride was removed under reduced pressure.The intermediate product was used immediately for the next step, due to its very poor stability in air.
To a 50 mL heat gun-dried two-neck round bottom flask, dihexylamine (3.34 g, 15 mmol) in 10 mL of anhydrous chloroform was added.The solution was cooled to 0 °C in an ice bath.Crude 5bromothiophene-3-carbonyl chloride was dissolved in 5 mL of anhydrous chloroform and added dropwise carefully.Then the solution was allowed to warm to room temperature and stirred overnight.The reaction was quenched by addition of water.The organic phase was further washed with brine twice and dried over anhydrous Na2SO4.Upon removal of solvent in vacuo, the crude product was further purified by silica gel column chromatography with hexane: ethyl acetate (5:1) to give Compound 2. Yield: 3.23 g, (96%).showing no starting material).

Additional data
Each set of data were obtained from 3-5 OTFT devices.
b The average mobility ± standard deviation (maximum mobility) calculated from the saturation regions of the devices.
c The Vth calculated from the device with maximum mobility.

Figure S1 .
Figure S1.The molecular weight distribution of 66BT obtained by HT-GPC at 140 °C with 1,2,4trichlorobenzene as eluent and polystyrene standards.

Figure S2 .
Figure S2.The molecular weight distribution of 44BT obtained by MALDI-TOF-MS from a matrix of DCTB (2500:1 matrix-to-polymer ratio) casted from chloroform.

Figure S3 .
Figure S3.The molecular weight distribution of 44TT obtained by MALDI-TOF-MS from a matrix of DCTB (2500:1 matrix-to-polymer ratio) casted from chloroform.

Figure S13 .
Figure S13.The TGA curves of A) 66BT B) 44BT and C) 44TT with increasing rate of 10 °C min -1 obtained in nitrogen.

Table S2
Solar cell data for 44BT, 66BT and 44TT.The total concentration of D/A blend is 16 mg ml -1 .For other systems, the total concentration is 20 mg ml -1 . a