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New Benzo[1,2-d:4,5-d']bis([1,2,3]Thiadiazole) (iso-BBT) Based Polymers for Application in Transistors and Solar Cells Luca Bianchi, Xianhe Zhang, Zhihua Chen, Peng Chen, Xin Zhou, Yumin Tang, Bin Liu, Xugang Guo, and Antonio Facchetti Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b05176 • Publication Date (Web): 29 Jan 2019 Downloaded from http://pubs.acs.org on January 31, 2019

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Chemistry of Materials

Luca Bianchi†, §, Xianhe Zhang†, §, Zhihua Chen‡, Peng Chen†, Xin Zhou†, Yumin Tang†, Bin Liu†, Xugang Guo*, †, Antonio Facchetti*, ‡, # †

Department of Materials Science and Engineering and The Shenzhen Key Laboratory for Printed Organic Electronics, Southern University of Science and Technology No. 1088, Xueyuan Road, Shenzhen, Guangdong 518055, China ‡ Flexterra Inc., 8045 Lamon Avenue, Skokie, Illinois 60077, United States # Department of Chemistry and the Materials Research Center, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208, USA ABSTRACT: In this paper we report the rationale and implementation of a new building block for organic electronics based on 4,8-di(2-thienyl)-benzo[1,2-d:4,5-d']bis([1,2,3]thiadiazole) (isoBBT-T2) and realization of two alkyl-functionalized iso-BBTtetrathienyl (T4) alternating copolymers (P1 with alkyl = 2DT and P2 with alkyl = 2DH). Compared to the previously investigated small molecules/polymers based on the conventional 4,8-di(2-thienyl)-benzo[l,2-c:4,5-c']-bis[ l,2,5]thiadiazole (BBT-T2), the use of the iso-BBT heterocycle widens the polymer bandgap to a region (~ 1.4 eV) compatible for use in single junction solar cells. The influence of iso-BBT vs. BBT structural variation on the molecular structural, electronic structure, and optical properties was accessed by DFT computations, single-crystal determination, optical absorption and electrochemical measurements. In-plane charge transport for P1 and P2 was investigated in an organic thin-film transistor (OTFT) structure demonstrating hole mobilities approaching 1 cm2/Vs and further enhanced by off-center spin-coating method to 1.32 cm2/Vs. Using PC61BM as acceptor, a remarkable PCE of 10.28% was achieved for P1 along with a current density >20 mA/cm2. The substantial PCEs of these devices, despite the relatively narrow donor energy gap, is due to retention of a high open circuit voltages (Voc > 0.8 V) as the result of the small energy loss (Eloss < 0.6 eV). Atomic force microscopy, transmission electron microscopy and X-ray diffraction characterization further support the solar cell trends and rationalize structure-property correlations. These results demonstrate that iso-BBT-T2-based polymers are promising candidates for both organic electronics and photonic applications.

π-Conjugated polymers comprising electron accepting (A) and electron donating (D) units are widely investigated as semiconductors in organic electronic devices such as organic thinfilm transistors (OTFTs) and polymer solar cells (PSCs).1-5 The development of new of these polymers relies on the creative design and synthesis of novel A/D building blocks combined with achieving desired structural and physical properties of the polymers such as backbone planarity, considerable πconjugation and solid-state chain interactions, as well as sufficient solubility for solution-processing.6-12 Furthermore, enabling physical properties specific to a particular device function is required.13 For instance, for TFT applications, molecular orbital energies must be compatible with efficient charge injection from the contacts and stable charge transport. For PSCs, broad optical absorption and molecular orbital energy level match between the two semiconductors in the PSC blend is essential to maximize output power. Another key factor that can influence the PSC performance is the energy loss (E loss) due to VOC reduction.14 The Eloss is linked with the photon efficiency to generate charges and is calculated by the following equation, Eloss = Eg-qVOC, where q is the elemental charge. For highly efficient bulk-heterojunction (BHJ) PSCs this value is usually over 0.7 eV.15 For instance, Yang et al.

Figure 1. Chemical structure of representative polymers. EH = 2-ethyhexyl; OD = 2-octyldecyl; DT = 2-decyltetradecyl obtained a remarkable PCE of 10.30 using a copolymer between bithienyl-benzodithiophene and dithiazole- thieno[3,4b]thiophene (DTBDT-DTzTT) (Figure 1) with an Eloss of 0.77 eV16 using a non-fullerene acceptor. Hou at al. Investigated the polymer donor (PBDB-T-SF) with a new fluorinated indaceno[2,1-b:6,5-b‘]dithiophene and achieved a PCE of 13.1% with an Eloss of 0.66 eV.17 Although values close to 0.6 eV are possible18 and they can fall to 0.5 eV or less for perovskite so-

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lar cells,19-21 very few BHJ solar cells have enabled Eloss lower than 0.6 eV.22-28 For instance, Liu et al. demonstrated allpolymer cells with B-N bridged bipyridine -thiophene (PBNBPP-T) or -selenophene (P-BNBPP-Se) as the electron acceptors and PTB7-Th as the donor reaching in their best result a Eloss of 0.47 eV, unfortunately, with a PCE of only 3.77%.29 Recently Janssen et al. synthesized a dyketopyrrolopyrrole-based polymer bridged with thiazole units and thiophene (PDPP2TzT) using PC71BM they obtained an Eloss of 0.48 eV and a PCE of 1.1%.30 Thus, demonstrating polymers with a low Eloss and high PCEs remain very challenging.

Figure 2. a) Structure of the (iso)BT/BBT units and polymers. b) DFT-computed MO topology and energy of the indicated building blocks. The numbers above the blue arrows are the computed HOMO-LUMO gap. All numbers are in eV. Here we report the implementation of the electron-poor heterocycle, benzo[1,2-d:4,5-d']bis([1,2,3]thiadiazole (iso-BBT, Figure 2a), enabling the new electron-accepting building block 4,8-di(2-thienyl)-benzo[1,2-d:4,5d']bis([1,2,3]thiadiazole (iso-BBT-T2) for organic electronics. The inspiration for using iso-BBT originates from its regioisomer, benzobis[1,2-c;4,5-c’]bis[1,2,5]thiadiazole (BBT), which is a strong nonclassical π-electron acceptor owing to its hypervalent sulfur atom.31, 32 Furthermore, we recently reported new opto-electronic polymers based on the electrondeficient heterocycle benzo[d][1,2,3]thiadiazole (iso-BT), which is the isomer of the widely investigated benzo[d][2,1,3]thiadiazole (BT) (Figure 2a). The iso-BT polymer properties were compared with those of BT showing larger optical bandgaps (1.18-1.80 eV for iso-BT and 1.04-1.12 eV for BT), deeper HOMO energies (-5.7~-5.6 eV for iso-BT and -5.5~-5.6 eV for BT), and comparable TFT/PSC performance [hole mobilities (μh) > 0.7 cm2 V-1 s-1; PCE ~ 9 %].33 Classical BBT derivatives have been investigated for fundamental studies on conducting polymers34 and incorporated in organic light emitting diode (OLED)35, 36 and photothermal therapy37, 38 molecules. In addition, due to its extremely high electron affinity (LUMO level from DFT calculation is -3.56 eV), planar tricyclic fused ring structure, and strong non-covalent interactions, resulted in the development of BBT-based oligomers39, 40 and polymers for OTFT applications.41-48 However, BBToligothiophene-based conjugated polymers exhibit high-lying HOMO energy levels (-4.3~-4.8 eV), which may limit their long-term stability, and very low band-gaps (0.55 eV for oligothiophene = 4T), preventing their use in PSCs. Our data demonstrate that iso-BBT-T2 is an interesting building block enabling both high OTFT carrier mobility and efficient PSC polymers.

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In this section we first report molecular orbital computation to rationalize iso-BBT selection. Next, synthetic access of the new iso-BBT-T2 building blocks and the corresponding polymers as well as details of structural and chemical characterization are summarized. Finally, we detail morphological and opto-electronic properties of the semiconductor films by several techniques and in transistor and solar cell devices. Molecular Design and Synthetic Strategies. Before undertaking synthetic efforts, we performed density functional theory (DFT) computations carried out using the Spartan 08 software package at the B3LYP/6-31G* level. Thus we compared the electronic structure and the energetic of BBT and iso-BBT units and the corresponding BBT-Tn and iso-BBT-Tn (oligothiophenes, Tn = T2 and T4) model compounds (Figure 2b and Table S1). The results indicate that both heterocycles enable equally planar (iso)BBT-Tn units. Furthermore, iso-BBT impacts the frontier molecular orbital energies of all compounds by decreasing those of the HOMO and increasing those of the LUMO. Thus, the strong electron accepting capacity of iso-BBT should enhance the OSC VOC and positively impact OTFT stability to oxidation. Equally important, by comparing BBT-T4 with iso-BBT-T4 models, the latter units being more structurally close to the polymers synthesized here (vide infra), the bandgap should increase by ~ 1eV, thus approaching values compatible with typical single-junction cell operation.49 Based on these results it is worthwhile to synthesize iso-BBT molecules and polymers and to investigate and understand their properties. The iso-BBT heterocycle has been rarely reported in the literature and never investigated for opto-electronics before our recent patent disclosure.50 In previous investigations the unsubstituted iso-BBT was synthesized by reacting pphenylenediaminethiosulphonic acid with nitrous acid, affording the product in 45% yields after filtration and recrystallized from acetic acid.51 The synthesis of the dihydroxy-derivative (65% yields) was reported by Maier et al by using 3,6bis(diazo)cyclohexan-1,2,4,5-tetraone and the Lawesson reagent.52 In our work, we first attempted to synthesize iso-BBTBr2, the key unit to achieve iso-BBT-T2-based polymers, starting from the commercially available 2,5-diamino-1,4benzenedithiol dihydrochloride (1), which can be produced in kilos,53 by first via bromination and then ring closure using HBr (Scheme 1a).54 This reaction resulted in a mixture of the desired iso-BBT-Br2, the monobromo-compound iso-BBTBr, and the non-brominated iso-BBT unit. Due to the limited solubility of these molecules, direct isolation of iso-BBT-Br2 from this mixture was not practical. Thus, the crude mixture was used directly for the following Stille coupling reaction with the tin reagent (4-(2-decyltetradecyl)thiophen-2yl)trimethylstannane (2-DT), and the mono-coupling compound (iso-BBT-T1-DT) was found be to the major product and was obtained in a low yield (35%). Next, a C-H activated direct arylation with 2-bromo-4-(alkyl)thiophene (3-DT) was able to achieve the iso-BBT-T2-DT compound (36% yield). Bromination of the latter with NBS afforded iso-BBT-T2-DTBr2, which was obtained in an overall yield of ~ 4.8%. Thus, inspired by the success of C-H direct arylation, a second approach was carried out for the synthesis of our monomers which reduces the number of steps/reagents and enhances


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Scheme 1. Initial (a) and optimized (b) synthetic routes to iso-BBT-based monomers with different alkyl chains. c) Stille coupling polymerization between iso-BBT-T2-R-Br2 (R = DT and DH) and bis(trimethylstannyl)bithiophene. Table 1. Chemical and opto-electronic properties for the indicated polymers.

a)

Polymera)

Mn [KDa]

PDI

λmax sol b)/film c)[nm]

λonsetc) [nm]

Egf) [eV]

EHOMOd) [eV]

ELUMOe) [eV]

P1

44.3

2.1

815/812

882

1.40

-5.43

-4.03

P2

28.6

2.0

815/816

906

1.37

b) Polymer

c)

-5.38 d) E

-4.01 red-1+4.80)

Chlorobenzene (CB) fractions; in CB solution; Polymer film cast from CB solution; eV deterLUMO = - (E1/2 mined electrochemically using Fc/Fc+ internal standard; e) EHOMO= ELOMO - Egopt; f) Optical bandgap estimated from absorption onset of as/cast polymer film: Eg = 1240/λonset.

yields (Scheme 1b). Here, 1 was converted to iso-BBT in good yields (74%) after reaction with sodium nitrite in presence of concentrated HCl. Next, the C-H activated direct arylation reactions of iso-BBT with a 2-bromo-4-(alkyl)thiophene (3-R) were able to achieve the di-coupled compouds iso-BBTT2-DT and iso-BBT-T2-DH in ~40% yields. A few conditions were evaluated to optimize the yields for the double C-H activation but they all afforded comparable results mainly due to the poor iso-BBT solubility in the reaction media. The best condition employed Pd(OAc)2, P-tBu2MeHBF4, pivalic acid, and K2CO3in toluene at 120 °C. Both compounds were easily brominated with NBS in THF to obtain the key monomers isoBBT-T2-DT-Br2 and iso-BBT-T2-DH-Br2 in good yields (87% and 88%, respectively). Thus, this route afforded the products in an overall yields of 21 and 23 %. Crystal Structure. To corroborate DFT structural results, we synthesized the model compound iso-BBT-T2 and accessed single crystal data. This unit was obtained in a similar way of iso-BBT-T2-DT and iso-BBT-T2-DH, employing a C-H activation direct arylation of iso-BBT with 2-bromo-5triisopropylsilylthiophene (5), followed by the removal of silyl group with tetra-n-butylammonium fluoride (see Supporting Information for detail). Single crystals of iso-BBT-T2 were obtained from Kappa Apex 2 diffractometer and the data was compared with BBT-T2 single crystal already describe by Yamashita et al.34 Figure 3 shows the relative intramolecular

bond lengths (black), angles (blue), torsion (green) and intermolecular packing (red). Due to the different atomic connectivity iso-BBT-T2 exhibits greater structural distortion of the thiadiazole rings (Figure 3a) when compared with BBT-T2 (Figure 3b). Furthermore, when analyzing the bond lengths, considerable elongation of the thiadiazole S-N bonds (~1.69 Å) and contraction of the N-N bounds (1.23 Å) in iso-BBT-T2 vs. the two S-N bonds in BBT-T2 (~1.59 Å) is observed. Combined with reduced bond length alternation of the central benzene ring (1.41 Å and 1.42 Å, average length), this data indicates enhanced classical aromatic character for iso-BBT vs BBT-T2. Interestingly, and similarly to our previous structural observations for the iso-BT vs. BT families,33 despite the greater steric demand of S vs. N, the two molecules are essentially planar and exhibit maximum torsional angles of 9.4° (iso-BBT-T2) and 12.2° (BBT-T2). The C-C bonds between the benzofused central unit and the thiophenes are slightly shorter in BBT-T2 (1.405 Å) vs. iso-BBT-T2 (1.429 Å), in agreement with the greater quinoid character of the benzene unit in BBT. The intermolecular packing distances in the two building blocks are comparable [3.46 Å for iso-BBT and 3.42 Å for BBT], as the result of the highly planar backbone capable of strong intermolecular interactions. Collectively, these results indicate that the corresponding polymers should exhibit comparable backbone π-conjugation and differences in electronic structure will originate from the heterocycle nature and not by backbone torsional distorsions or substantial difference



in interchain polymer interactions. Thus, structural and DFT data indicate that alkyl-substituted iso-BBT-T2 monomers can be promising building blocks for optoelectronic polymers.

a.

1.693 Å

b.

1.233 Å

3.113 Å 1.700 Å

1.589 Å 1.352 Å

1.348 Å

1.464 Å

1.429 Å

9,4° 2,870 Å

hibits a λmax of ~1150 nm with a λonset ~1750 nm, corresponding to a remarkably low optical bandgap of ~0.7 eV. These values corroborates DFT calculations that iso-BBT is effective in expanding the polymer band-gap, as we

2.817 Å

1.481 Å 1.416 Å

1.596 Å

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12.2° 1.459 Å 1.389 Å

92,46°

2,807 Å

101.82°

1.405 Å

1.471 Å 1.405 Å

3.46 Å

3.42 Å

Figure 3. Single crystal structure of a) iso-BBT-T2 and b) BBT-T2. Polymer Synthesis and Characterization. Two iso-BBTtetrathiophene (T4) based polymers P1 and P2 functionalized with different side chains (DT and DH, respectively) were prepared. Note, first was attempted to synthesize P1 via direct C-H arylation polymerization (DARP) under the same conditions used for iso-BBT-T2 monomers, however, only oligomers were obtained. This is not surprising since DARP protocols are not general in scope.55, 56 Thus, P1 and P2 were prepared via conventional Pd-catalyzed Stille polycondensation reaction (Scheme 1c) between the dibrominated monomers iso-BBT-T2-DT-Br2 and iso-BBT-T2-DH-Br2, respectively, and 5,5’-bis(trimethylstannyl)bithiophene (4) using Pd2dba3 and tol3P as catalyst in dry chlorobenzene (CB). Both polymers were purified by soxhlet extraction, with the CB fraction affording the best samples. High temperature gel permeation chromatography (GPC) was used for molecular weight determination, those results with electrochemical and optical properties are summarized in (Table 1). The Mn of the P1 and P2 CB fractions are 44.3 KDa and 28.6 KDa, respectively, with a polydispersity index of ~2. Thermogravimetric analysis (TGA) was carried out using a heating ramp of 10 °C min-1 and indicates that both polymers do not decompose before 350 °C (Figure S6). Remarkably, differential scanning calorimetry (DSC) measurements, carried out using four thermal heating/cooling cycles in a temperature range between 50-270 °C, show a comparable thermal behavior with two main transition peaks at 66 and 202 °C for P1 and 67 and 202 °C for P2 (Figure S7). These data demonstrate the intermolecular forces in these polymers are comparable. The UV-Vis spectra of both polymers in solution and as thin films (Figure 4a) are similar and have a wide absorption in the visible/near-IR regions. Both polymers exhibit identical maximum absorption (λmax) in solution at 815 nm while that of the films differ slightly [812 nm (P1) and 816 nm (P2)]. From the absorption onset wavelength (λonset) of the film spectra [882 and 906 nm, respectively] the optical bangap (Eg) of P1 and P2 are 1.40 and 1.37 eV, respectively. A broad absorption is highly desirable for solar cell applications although an excessively narrow Eg will deteriorate the open circuit voltage.57 Note a BBT-T4 polymer (see structure in Figure 1) with a similar backbone as P1/P2, thus comprising BBT instead of iso-BBT, was pioneered by Wudl et al..44 Solution and thinfilm UV-Vis/NIR data indicate that this BBT-T4 polymer ex-

Figure 4. a) UV-VIS absorption spectra in in CB solution and as thin films and b) Cyclic voltammetry of P1 and P2. observed when comparing the BT- and iso-BT-based polymer families.33 Cyclic voltammetry (CV) plots (Figure 2b) of both polymers using ferrocene/ferrocinum (Fc/Fc+) as internal standard exhibit similar redox features characterized by at least three quasi-reversible reductions [E1/2red-1 = -0.77 V (P1), -0.79 V (P2); E1/2red-2 = -0.98 V (P1), -1.03 V (P2); E1/2red-3 = 1.35 V (P1), -1.37 V (P2)] and irreversible oxidation peaks [+1.48 V (P1) and +1.66 V (P2)]. The onset for reduction (Eonsetred) and oxidation (Eonsetox) are -0.62 V and +1.09 V (P1), respectively, and -0.63 V and +1.18 V (P2), respectively, resulting in an electrochemical gap of 1.71 eV (P1) and 1.81 eV (P2). To correlate with previous studies, the LUMO energy levels of P1 and P2 were calculated using the E1/2red-1 values and the equation ELUMO = -(E1/2red-1 +4.80) eV while the HOMO energies from EHOMO = ELUMO - Egopt. The resulting EHOMO and ELUMO value are -5.43 eV and -4.03 eV for P1 and 5.38 eV and -4.01 eV for P2. These values are considerably shifted toward lower/higher energies when compared to the BBT-T4 reported by Wudl, in agreement with the computed/experimental electronic structure data of iso-BBT vs. BBT based units. Charge Transport in Thin-Film Transistors. Top gate/bottom contact (TG/BC) OTFTs of structure glass/Au/Semiconductor/CYTOP/Al were fabricated to investigate the charge transport properties of the pristine polymer films. The active layer was deposited via both conventional and off-center spin-coating methods and OTFT performance was optimized through thermal annealing of the active layer at various temperatures (130-220 °C). Off-center spin-coating was used to investigate further optimization as demonstrated in previous studies.58, 59 Optimised OTFT performance param-


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eters and representative output and transfer curves are reported in Table 2 and Figure 5, respectively. Both polymers showed distinct ambipolar transport although p-channel characteristics are considerably more pronounced (electron mobilities  ~103

cm2 V-1 s-1). Note, the output/transfer curves reveal non-ideal threshold voltage and contact resistance, which could be mitigated by optimizing hole injection60 and the gate dielectric chemical structure61. At the optimal thermal

Table 2. OTFTs performance parameters of P1 and P2 fabricated under optimal condition by convention and off-center spincoating method. Polymer P1

P2 a)

Method

Tanneal (°C)

μP [cm2 V-1 s-1] a)

VT b) (V)

Ion/Ioff

On-center

160

0.60 (0.40)

-36

104

Off-center

160

0.68 (0.57)

-28

107

On-center

160

0.82 (0.76)

-28

105

Off-center

160

1.32 (0.96)

-23

105

Maximum mobilities from at least 5 devices (average value shown in parentheses); b) Average threshold values.

Figure 5. Transfer and output characteristic curves of representative OTFTs based on P1 (a and c) and P2 (b and d). The XRD image (e) of the pristine film fabricated by on-center and off-center-spin coating and AFM hight images of P1 (f) and P2 (g) film fabricated by off-center spin-coating. The film annealing temperature is 160 ºC. annealing temperature (160 °C), the highest hole mobilities of P1 and P2 are 0.60 cm2 V-1 s-1 and 0.82 cm2 V-1 s-1 (on-center), respectively, and 0.68 cm2 V-1 s-1 and 1.32 cm2 V-1 s-1 (offcenter), respectively. Furthermore, ambient stability was tested for a representative P1 device (Figure S2) after exposure to ambient environment for 30 days, which showed a slight increase of the mobility and a decrease of the threshold voltage. The performance variations, which actually slightly improved transport characteristics by increasing the mobility and shifting the Vt closer to zero, were attributed to weak oxygen doping which pre-feel traps, an effect that has also been seen for other semiconductor polymers62-64 . X-Ray diffraction (XRD) and atomic force microscopy (AFM) were carried to rationalize the OTFT performance of P1 and P2. The XRD data along the z-axis (Figure 5e) demonstrates that both iso-BBT-based polymers are polycrystalline and exhibit distinct (100) reflections. For the pristine films the first

reflection is located at 2θ ~3.65° (P1) and ~3.30° (P2) corresponding to a lamellar d-spacing (dlam) of ~24.21 Å and ~26.77 Å, respectively, in agreement with the alkyl substituent length. Furthermore, P2 also exhibits a second week reflection at 2θ = 25.41° (on-center) and 25.62° (off-center) corresponding to out-of-plane stacking having a close π-π distance (dπ) of 3.51 Å and 3.48 Å, respectively. Furthermore, the film fabricated by off-center spin-coating produces a film with a stronger reflection having a smaller dπ, which demonstrates that offcenter spin-coating method leads to enhanced crystallinity. Compared with previously investigated iso-BT-T4-based polymers,33 the larger heteroaromatic iso-BBT unit enables polymers with shorter π-π staking distances, therefore corroborating the higher TFT mobilities measured here versus those of iso-BT-based transistors (0.03-0.72 cm2 V-1 s-1). Furthermore, it is instructive to point that the π-π staking distances of our iso-BBT-based polymers are comparable/slightly smaller than



those of other BBT-based polymers (see Figure 1 for structures) such as poly(benzobisthiadiazole bithiophenethienothiophene) (PBBTTT, dπ = 3.53 Å),40 which exhibits a μh of ~1.0 cm2 V-1 s-1 and a μe of 0.7 cm2 V-1 s-1, and the corresponding poly(benzobisthiadiazole tetrathiophene) (PBBTQT, dπ=3.50 Å)44, having a μh = 2.50 cm2 V-1 s-1. The μh enhancement of the latter polymer likely originate from the very shallow HOMO level (-4.6 eV), which enables better hole injection from the contacts but results in poor ambient stability. However, due to the higher LUMO level, iso-BBT-polymers exhibit supressed electron mobility ( ~103 cm2 V-1 s-1) compared with BBT-based polymers (7×103 ~ 1 cm2 V-1 s-1).48 Finally, AFM images (Figure 5f, g and Figure S3) provide a deeper understanding of the relation between film morphology and transistor performance. Interestingly, the root-meansquare (RMS) roughness of the as-cast films are 1.04 nm (P1) and 1.84 nm (P2), respectively. After annealing, the roughness values increased to 1.21 nm and 2.69 nm, respectively, which is typical when the film crystallinity is enhanced.8 Interestingly. P1 and P2 film roughness further increases to 1.31 nm and 3.55 nm, respectively, for the off-center spin-coated samples, suggesting further enhanced texturing and in agreement with the charge transport data. Solar Cell Fabrication and Characterization. Bulkheterojunction solar cell performance of the new polymers were investigated using a conventional architecture of structure ITO/PEDOT:PSS/Active layer/PDINO/Al. Two fullerene derivatives, [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) are used as the acceptor although the latter typically provides far better PCEs due to the broader optical absorption in the 300–500 nm region. Performance optimization included blend film thermal annealing and the use of DIO or DPE as a solution additive (Tables S3 and S5). Details are reported in the Supporting Information. Table 3 and Figure 5a collects the performance and representative J-V plots of the best blends, respectively. The optimal PSC performance are achieved without using solvent additives or thermal annealing of the blend films, which has been observed for certain blends.65-70 However, interestingly, PSCs using PC61BM showed statistically higher average PCEs (7.35-9.73 %) than those based on PC71BM (3.44-6.25 %) (Table S3). The best cells based on P1:PC61BM reaches a respectable PCE of 10.28% with an open current voltage (VOC) of 0.81 V, a short circuit current (JSC) of 20.83 mA cm−2 and a fill factor (FF) of 61.03% while that based on P2:PC61BM achieves PCE of 7.89%, a VOC of 0.81 V, a JSC of 15.53 mA cm−2, and a FF of 62.83% (Table 3). Post-thermal annealing the blend films at 120 °C for 10 min before device completion reduces the average PCEs [7.49 % for P1:PC61BM and 6.30 % for P2:PC61BM, Table S4] as does the use of solvent additives [8.21-8.58 % for P1:PC61BM and 5.60-6.35% for P2:PC61BM, Table S4]. Finally, for the best blend (P1:PC61BM) we have also investigated an inverted OSC architecture of structure ITO/ZnO/Active layer/MoO3/Ag and obtained a maximum PCE of 10.02% along with VOC, Jsc, and FF of 0.78 V, 22.36 mA cm−2, and 57.35%, respectively (Table S6 and Figure S4). We also tested the stability of the solar cells based on our polymers under nitrogen for 240 hours, and the results are shown in Figure S8. The data reveal quite stable Jsc and Voc, however, the FF decreases and, thus, the PCE. This result is

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likely due to the poor stability of the small molecular PDINO electron transporting layer.71 The data above suggest that these iso-BBT-based polymers can be PSC donors. Particularly, the data achieved with P1:PC61BM blend is quite interesting since a Jsc > 20 mA/cm2 is the highest value reported to date for a PSC using PC61BM. Even though fullerene derivatives exhibited many shortcomings as acceptors,72 our result indicates that these acceptors could also realize promising performance after further optimizing. Furthermore, although iso-BBT-T4 based polymers have a broad optical absorption extending to >900 nm, the corresponding devices retain high VOC values (> 0.8 V), thus exhibit minimal ELoss of 0.56-0.59 eV. To our knowledge, of the other narrow optical bandgap donor polymers having onset absorption comparable to ours (~1.4 eV), only the naphtho[1,2-c:5,6-c’]bis([1,2,5]thiadiazole)-based polymers achieved comparable performance but only when using PC71BM as the acceptor [PCE ~ 10%, VOC ~ 0.7V, JSC ~ 19 mA cm-2, FF ~ 72%].73 Recently, two studies have reported fullerene-based PSCs with a porphyrin–diketopyrrolopyrrole polymer (PPDPP)74 and poly(4,8-bis(5'-((2''ethylhexyl)thio)4'-fluorothiophen-2'-yl)benzo[1,2-b:4,5-b']dithiophene-2,6diyl)-alt-2'-ethylhexyl-3-fluorothieno[3,4-b]thiophene-2 carboxylate (PBDTT-SF-TT)68 exhibiting low energy losses (< 0.6 eV) with maximum PCEs of 6.44% (ELoss = 0.51 eV) and 9.07% (ELoss = 0.59 eV), respectively. Furthermore, when compared with the iso-BT polymers we recently reported33, which exhibit PCEs from 2.8% to 9.0% in blends with PC71BM depending on the heterocycle degree of fluorination, these new iso-BBT polymers can broader the absorption and enhance PSC performance without employing fluorination. These results imply that iso-BBT-based polymers are promising as mid band gap donor materials. Bulk Heterojunction Characterization. To attempt to rationalize PSC performance results the charge transport and morphology of the best blends were investigated. The bulk transport properties were accessed by space charge limited current (SCLC) measurements for diodes of structure ITO/ZnO/Active layer/PDINO/Al for electron-only devices and ITO/PEDOT:PSS /Active layer/MoO3/Ag for the holeonly devices (Table 3 and Figure S5). All the blend films showed a balance charge transport characteristics (1.1< μh/μe 0.6-1.3 cm2 V−1 s−1 depending on the polymer structure and film processing methodology. Using PC61BM as the acceptor, PSCs with P1 and P2 cast films exhibit PCEs as high as 7.52 % and 10.28 % respectively, a value achieved with a remarkable ELoss < 0.6 eV. Device data was rationalized by a battery of film transport and morphology characterization techniques. Our data demonstrate that iso-BBT-based polymers are promising candidates for realizing organic opto-electronic devices.

General methods. All reagents and chemicals were commercially available and were used without further purification unless otherwise stated. THF and toluene were distilled from



Na/benzophenone. The reagents 2,5-diaminobenzene-1,4dithiol dihydrochloride, were purchased from Ark Pharm, Inc. (Arlington Heights, IL, USA), 4-(2-decyltetradecyl)-2trimethylstannyl thiophene77 and 5,5’-bis(trimethylstannyl)2,2’-bithiophene78 was synthesized according to the literature and PCBM were purchased from Reike Metals, Inc. and American Dye Source, Inc., respectively. 1H NMR and 13C NMR spectra were measured on Bruker Ascend 400 MHz spectrometers. GC-MS was collected on Agilent-7890. High Resolution Mass Spectra (HRMS) were obtained on Thermo Scientific Q Exactive. Chemical shifts were referenced to residual protio-solvent signals. C, H, N, elemental analyses (EAs) of polymers were performed at Shenzhen University (Shenzhen, Guangdong, China). MS Polymer molecular weights were measured on Polymer Laboratories GPC-PL220 high temperature GPC/SEC system at 150 °C vs polystyrene standards using trichlorobenzene as the eluent. DSC curves were recorded on a differential scanning calorimetry (Mettler, STARe, heating rate = 10 °C 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. Cyclic voltammetry measurements of polymers were carried out under argon atmosphere using a CHI760 Evoltammetric analyzer with 0.1 M tetra-n-butylammoniumhexafluorophosphate in acetonitrile as supporting electrolyte. A platinum disk working electrode, a platinum wire counter electrode, and a silver wire reference electrode were employed, and Fc/Fc+ was used as internal standard reference for all measurements. Density Functional Theory (DFT) computations were carried out using the Spartan ’08 software package (Wavefunction, Inc.) at the B3LYP/6-31G* level. Synthesis. Not optimized synthetic route, synthesis of single crystal monomer, 2-bromo-4-(2-decyltetradecyl) thiophene and 2-bromo-4-(2-dodecylhexadecyl)thiophene are reported in supporting info. Benzo[1,2-d:4,5-d‘]bis([1,2,3]thiadiazole) (iso-BBT). To a mixture of 2,5-dimercaptobenzene-1,4-diaminium chloride (1) (5 g, 20.39 mmol) in water (100 mL) was added 12 M HCl solution (77 mL) at 0°C. A solution of sodium nitrile (4.22 g, 61.17 mmol) in water (50 mL) was then added slowly at 0 °C. The reaction was allowed to warm to room temperature and stirred overnight. The suspension was filtred, and the precipitate was collected and washed with water (100 mL) and finally dried under vacuum, leading the crude product (iso-BBT) (2.93 g, 74%), which was used directly in the next step without purification. 1H NMR (400 MHz, CDCl3): 9.37 (s, 2H). 4,8-bis(4-(alkyl)thiophen-2yl)benzo[1,2-d:4,5d‘]bis([1,2,3]thiadiazole) (iso-BBT-T2-DX). Under argon to a mixture of benzo[1,2-d:4,5-d‘]bis([1,2,3]thiadiazole) (isoBBT) (346 mg, 1.78 mmol), 2-bromo-4-(alkyl)thiophene (3) ((3-DT) 2.14 g or (3-DH) 2.38 g, 4.27 mmol), palladium (II) acetate (39.96 mg, 0.178 mmol), ditbutyl(methyl)phosphonium tetrafluoride (88.3 mg, 0.356 mmol), pivalic acid (182 mg, 1.78 mmol), and potassium carbonate (738 mg, 5.34 mmol), anhydrous toluene (17 mL) was added and the solution was stirred at 120 °C for 18 h. Upon cooling to room temperature, the reaction mixture was concentrated in vacuo leaving a solid, which was subjected to column chromatography on silica gel with a mixture of hexanes:CHCl3 (3:1, v/v) as eluent to afford the pure products. For 4,8-bis(4-

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(2-decyltetradecyl)thiophen-2yl)benzo[1,2-d:4,5d‘]bis([1,2,3]thiadiazole) (iso-BBT-T2-DT): red solid (606 mg, 33%) 1H NMR (400 MHz, CDCl3): 8.03 (d, J = 1.1Hz, 2H), 7.29 (s, 2H), 2.70 (d, J = 6.7 Hz, 4H), 1.73 (m, 2H), 1.401.20 (m, 80H), 0.86 (m, 12H). 13C NMR (100 MHz, CDCl3): 154.11, 143.78, 139.40, 137.34, 133.28, 126.16, 120.94, 39.14, 35.14, 33.49, 32.07, 30.19, 29.85, 29.81, 29.51, 26.82, 22.84, 14.27. HRMS (ESI, m/z): calcd for C62H103N4S4 [M+H]: 1031.70601, Found 1031.70654. For 4,8-bis(4-(2dodecylhexadecyl)thiophen-2yl)benzo[1,2-d:4,5d‘]bis([1,2,3]thiadiazole) (iso-BBT-T2-DH): red solid (733 mg, 36%) 1H NMR (400 MHz, CDCl3): 8.02 (d, J = 1.1 Hz, 2H), 7.28 (s, 2H), 2.70 (d, J = 6.7 Hz, 4H), 1.73 (m, 2H), 1.401.20 (m, 96H), 0.87 (m, 12H), 13C NMR (100 MHz, CDCl3): 154.33, 143.99, 139.62, 137.55, 133.48, 126.36, 121.15, 39.34, 35.34, 33.70, 32.27, 31.94, 30.39, 30.05, 30.02, 29.71, 27.02, 23.04, 14.47. HRMS (ESI, m/z): calcd for C70H119N4S4 [M+H]: 1143.83121, Found 1143.83008. 4,8-bis(5-bromo-4-(2-alkyl)thiophen-2-yl)benzo[1,2-d:4,5d‘]bis([1,2,3] thiadiazole) (iso-BBT-T2-XX-Br2). The compound (iso-BBT-T2-DH) or (iso-BBT-T2-DT) (411 mg or 455 mg, respectively, 0.398 mmol) was dissolved in a mixture of THF (8 mL) and acetic acid (0.8 mL) at room temperature. After dissolution, NBS (152.4 mg, 0.856 mmol) was added in one portion. The resulting mixture was stirred overnight in the dark at 50 °C. The solution was concentrated in vacuo, and the residue was sonicated with MeOH:H2O (1:1, 25 mL). The resulting solid was collected by filtration, and then subjected to column chromatography on silica gel with a mixture of hexanes:CHCl3 (6:1, v/v) as eluent to afford the pure product. For 4,8-bis(5-bromo-4-(2-decyltetradecyl)thiophen-2yl)benzo[1,2-d:4,5-d‘]bis([1,2,3] thiadiazole) (iso-BBT-T2DT-Br2): red solid (413 mg, 87%) 1H NMR (400 MHz, CDCl3): 7.73 (s, 2H), 2.64(d, J = 7.1 Hz, 4H), 1.78 (m, 2H), 1.40-1.20 (m, 80H), 0.86 (m, 12H). 13C NMR (100 MHz, CDCl3): 154.13, 143.17, 138.74, 137.05, 132.50, 120.34, 117.14, 38.96, 34.63, 33.76, 32.27, 30.38, 30.04, 30.01, 29.71, 26.95, 23.04, 14.48. HRMS (ESI, m/z): calcd for C62H101N4Br81BrS4 [M+H]: 1189.52499, Found 1189.52637. For 4,8-bis(5-bromo-4-(2-dodecylhexadecyl)thiophen-2yl)benzo[1,2-d:4,5-d‘]bis([1,2,3] thiadiazole) (iso-BBT-T2DH-Br2): red solid (456 mg, 88%) 1H NMR (400 MHz, CDCl3): 7.71 (s, 2H), 2.64 (d, J = 6.9Hz, 4H), 1.78 (m, 2H), 1.40-1.20 (m, 96H), 0.87 (m, 12H). 13C NMR (100 MHz, CDCl3): 153.75, 142.82, 138.31, 136.69, 132.16, 119.95, 116.86, 38.63, 34.29, 33.43, 31.95, 30.06, 29.72, 29.62, 29.39, 26.62, 22.72, 14.15. HRMS (ESI, m/z): calcd for C70H117N4Br81BrS4 [M+H]: 1301.65019, Found 1301.64919 General Polymerization Procedure. A dry Schlenk flask was charged with the two monomers (0.08 mmol each), tris(debenzylideneacetone)dipalladium (0) (Pd 2(dba)3) (0.004 mmol, 3.66 mg), and tris(o-tolyl)phosphine (P(o-tolyl)3) (0.032 mmol, 9.74 mg). The system was placed under argon and then anhydrous chlorobenzene (10 mL) was added. The resulting mixture was stirred and heated to 90 °C for 18 h. Then, 0.05 mL 2-(tributylstannyl)thiophene was added and the reaction mixture was continue to react at 90 °C for an additional 6h. After cooling to room temperature, the reaction mixture was poured into 100 mL of methanol and the resulting solid collected with filtration. Polymer purification was achieved by Soxhlet extraction. For P1. The solvent sequence for Soxhlet was methanol, ethyl acetate, hexanes, chloroform,


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chlorobenzene, dichlorobenzene. The chloroform, chlorobenzene, dichlorobenzene fractions were precipitated in methanol (200 mL). The precipitate was collected by filtration, washed with methanol, and dried under vacuum to afford a greenish solid as final product (9.5 mg from CF, 53.3 mg from CB, 24.7 mg from DCB in total 87.5 mg, 92%). High temperature GPC: Mn = 10.9 KDa, PDI = 1.2 (from CF fraction); Mn = 44.3 KDa, PDI = 2.1 (from CB fraction); Mn = 76.7 KDa, PDI = 1.6 (from DCB fraction). Elemental Analysis: (Calcd.: C: 70.42; H: 8.78; N: 4.69) Found: C: 70.24; H: 8.51; N: 4.67). 1 H NMR (400 MHz, ClCD2CD2Cl, 80 °C): 7.48 (s, 1H). 6.70 (m, br, 2H), 2.34 (m, 2H). 1.32 (m, 1H), 0.90-0.40 (m, 40H), 0.28 (t, 6H). For P2. The solvent sequence for Soxhlet was methanol, ethyl acetate, hexanes. Finally, the polymer product was extracted with chloroform and then chlorobenzene. Both the chloroform and chlorobenzene were precipitated in methanol (200 mL). The precipitate was collected by filtration, washed with methanol, and dried under vacuum to afford a dark greenish solid as final product (23.2 mg from CF fraction and 63.4 mg from CB fraction in total 86.6 mg, 86%). High temperature GPC: Mn = 10.2 KDa, PDI = 2.0 (from CF fraction); Mn = 28.6 KDa, PDI = 2.0 (from CB fraction). Elemental Analysis: (Calcd: C: 71.72; H: 9.26; N: 4.29) Found: C: 71.78; H: 9.05; N: 4.30). 1H NMR (400 MHz, ClCD2CD2Cl, 80 °C): 7.48 (s, 1H). 6.69 (m, br, 2H), 2.34 (m, 2H). 1.33 (m, 1H), 0.90-0.40(m, 43H), 0.28 (t, 6H).

Supporting Information The Supporting Information material is available free of charge via the Internet at http://pubs.acs.org. Synthetic procedures and characterization data; DFT calculation; OTFTs fabrication and characterization; PSCs performance and fabrication process; SCLC mobility measurements; Thermal Gravimetric Analysis (TGA); Differential Scanning Calorimetry (DSC); Single crystal determination

* Xugang Guo: [email protected] * Antonio Facchetti: [email protected] §

These authors contributed equally. All authors have given approval to the final version of the manuscript.

The authors declare no competing financial interest.

We are grateful to the National Science Foundation of China (NSFC, 21774055) and the Shenzhen Peacock Plan project (KQTD20140630110339343).

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