5-Alkyloxy-6-fluorobenzo[c][1,2,5]thiadiazole- and ... - ACS Publications

Jun 30, 2014 - Adam Creamer , Abby Casey , Adam V. Marsh , Munazza Shahid , Mei Gao , and Martin Heeney. Macromolecules 2017 50 (7), 2736-2746...
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5‑Alkyloxy-6-fluorobenzo[c][1,2,5]thiadiazole- and SilafluoreneBased D−A Alternating Conjugated Polymers: Synthesis and Application in Polymer Photovoltaic Cells Guangwu Li,† Chong Kang,† Xue Gong,† Jicheng Zhang,† Cuihong Li,*,† Youchun Chen,‡ Huanli Dong,§ Wenping Hu,§ Fenghong Li,‡ and Zhishan Bo*,† †

Beijing Key Laboratory of Energy Conversion and Storage Materials, College of Chemistry, Beijing Normal University, Beijing 100875, China ‡ State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Changchun 130023, China § Key Laboratory of Organic Solids, Institute of Chemistry CAS, Beijing 100190, China S Supporting Information *

ABSTRACT: Three donor−acceptor (D−A) alternating conjugated polymers with silafluorene as the donor unit, 5-alkyloxy-6fluorobenzothiadiazole as the acceptor unit, and thiophene as the spacer has been synthesized and used as donor materials for polymer solar cells (PSCs). The introduction of a fluorine atom on the benzothiadiazole unit can lower the HOMO and LUMO energy level of the resulted polymers to afford higher open circuit voltage (Voc); whereas the introduction of a flexible alkyloxy chain on benzothiadiazole unit can increase the solubility of the resulted polymers without interfering the close packing of polymer chains in the solid state. High molecular weight polymers P-1a, P-1b, and P-1c, which are fully soluble in 1,2-dichorobenzene (DCB) at elevated temperature, have been prepared by Suzuki polycondensation. Among these polymers, P-1c exhibited the highest hole mobility up to 1.36 × 10−2 cm2 V−1 s−1. PSCs based P-1b:PC71BM demonstrated the highest Voc up to 0.98 V. P-1a:PC71BM based PSCs gave the highest power conversion efficiency (PCE) of 6.41%, which is the highest value among solar cells with benzothiadiazole- and silafluorene-containing polymers as the donor material.

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(DFBT)24−27 have been used for the synthesis of donor− acceptor (D−A) alternating conjugated polymers for PSCs. The introduction of two flexible alkoxy chains on the BT unit can increase the solubility of polymers without interfering the ordered packing of polymer chains.18,28 The introduction of two fluorine atoms in the 5,6-positions of BT unit can lower the HOMO and LUMO energy levels of the resulted conjugated polymers to achieve higher open circuit voltage (Voc) as well as increase the intramolecular polymer chain interaction to afford higher hole mobility and PSC performance.24−27,29 However, compared with nonfluorinated conjugated polymers, the corresponding fluorinated conjugated polymers usually exhibited much poorer solubility.30 To achieve high performance polymer donor materials, careful side chain design is usually required to balance the solubility and the chain packing of polymers. Herein, we focus on the design and synthesis of novel silafluorene (SiF) and BT based soluble conjugated polymers for high efficiency PSCs. As shown in Chart 2, the first SiF and BT based conjugated polymer PBSDTBT with a number-average molecular weight (Mn) of 15 kDa was

INTRODUCTION In recent years, the research on polymer solar cells (PSCs) has experienced a rapid development since a large number of new donor−acceptor (D−A) alternating conjugated polymers have been synthesized and used as donor component of the active layer.1−8 Donor−acceptor design is a very appealing design style; one can easily tune the absorption and energy level of polymers by changing the donor or/and acceptor unit.9,10 Benzo[c][1,2,5]thiadiazole (BT) is a commonly used strong acceptor unit, as shown in Chart 1, including the parent BT five kinds of derivatives, namely, BT,11−16 5-alkoxybenzo[c][1,2,5]thiadiazole (ABT),17 5,6-di(alkoxy)benzo[c][1,2,5]-thiadiazole (DABT),18,19 5-fluorobenzo[c][1,2,5]-thiadiazole (FBT),17,20−23 and 5,6-difluorobenzo[c][1,2,5]thiadiazole Chart 1. Chemical Structures of Benzothiadiazole Derivatives Used for the Synthesis of Conjugated Polymer Donor Materials

Received: February 25, 2014 Revised: June 23, 2014

© XXXX American Chemical Society

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Chart 2. Chemical Structures of SiF- and BT-Based Conjugated Polymers Used for PSCs

Scheme 1. Synthesis of Monomers and Copolymera

a Key: (i) aniline, SOCl2, toluene, 100 °C; (ii) NIS, AcOH, H2SO4, room temperature; (iii) Pd2(dba)3, P(o-tol)3, 4,4,5,5-tetramethyl-2-(thiophen-2yl)-1,3,2-dioxaborolane, NaHCO3, THF/H2O, reflux; (iv) KOBut, alkanol, THF, 80 °C; (v) NBS, CHCl3, room temperature; (vi) Pd(PPh3)4, NaHCO3, NBu4Br, toluene/H2O, reflux.

supramolecular interaction, the replacement of one fluorine atom on DFBT unit with alkoxy substituent can markedly enhance the solubility of polymers without interfering the close packing of polymer chains in the solid state.18,28,37 To study the influence of side chains on the performance of PSCs, we have synthesized three polymers with different alkoxy substituent on the BT unit. These polymers are of high molecular weight and fully soluble in organic solvents such as chlorobenzene (CB) and dichlorobenzene (DCB). Pure polymer films exhibited field effect transistor (FET) hole mobilities ranging from 1.51 × 10−3 to 1.36 × 10−2 cm2 V−1 s−1, which match the electron mobility of PC71BM. Bulk heterojunction PSCs based on these polymers and PC71BM exhibited high Voc up to 0.98 V. A high PCE up to 6.41% has been obtained. To the best of our knowledge, a PCE of 6.41% is the best result for PSCs based on silafluorene and benzothiadiazole containing polymers, indicating these polymers are promising donor materials for high efficiency PSCs.

prepared by Leclerc et al. and a power conversion efficiency (PCE) of 1.6% was achieved.31 Later, Cao et al. reported the PCE of high molecular weight PSiF-DBT based PSCs could reach 5.4%.32 We previously reported the synthesis of SiF and 5,6-bis(octyloxy)benzo[c][1,2,5]thiadiazole based conjugated polymer P2, and P2 based PSCs gave a PCE of 6.05%.33 You et al. have recently shown that the introduction of fluorine atom onto the polymer backbone can lower the HOMO level of polymers to acquire high Voc and suppress the charge recombination to enhance short circuit current (Jsc) and fill factor (FF).20,23 At the beginning of this research, we first designed and synthesized DFBT and SiF based polymer PSiFDTDFBT, whose structure is shown in Chart 2. However, PSiF-DTDFBT is unfortunately insoluble in any organic solvent even at elevated temperature. As we known, conjugated polymers are of rod-liked structures, in order to endow them with good solubility, flexible side chains are required to act as the inherent solvent.34−36 To achieve soluble polymers, we for the first time designed and synthesized 5-alkoxy-6-fluorobenzo[c][1,2,5]thiadiazole (AFBT) and SiF based conjugated polymer PSiF-DTAFBT. Because of the intramolecular S−O B

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RESULTS AND DISCUSSION Material Synthesis and Characterization. The syntheses of monomers M-1a, M-1b, and M-1c and polymers P-1a, P-1b, and P-1c are outlined in Scheme 1. The syntheses of monomers M-1a, M-1b, and M-1c were rather straightforward. Starting from commercially available 4,5-difluorobenzene-1,2diamine (1), 5,6-difluoro-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole (4) was synthesized according to the literature procedures.38 The treatment of compound 4 with alkyl alcohols (octan-1-ol, 2-ethylhexan-1-ol, and 2-hexyldecan-1-ol) and potassium tert-butoxide in THF at reflux temperature afforded mono fluoro and mono alkoxy substituted compounds 5a, 5b, and 5c, respectively, in yields of 56−72%. Bromination of compounds 5a, 5b, and 5c with NBS in chloroform containing a few drops of acetic acid as the catalyst afforded monomers M1a, M-1b, and M-1c, respective, in yields of 91−99%. Suzuki polycondensations of M-1a, M-1b, and M-1c with 9,9-dioctyl9H-silafluorene-2,7-bis(boronic acid pinacol ester) were carried out with Pd(PPh3)4 as the catalyst precursor and tetrabutylammonium bromide as the phase transfer catalyst in a solvent mixture of aqueous NaHCO3 and toluene to afford polymers P1a, P-1b, and P-1c, respectively, in yields of 82−90%. Since the introduction a flexible alkoxy chain on the benzothiadiazole ring, all the as-synthesized polymers are fully soluble in CB, DCB, and 1,2,4-trichlorobenzene (TCB) at elevated temperature. Molecular weights and molecular weight distributions, which were measured by gel permeation chromatography (GPC) at 150 °C with TCB as an eluent and narrowly distributed polystyrenes as the calibration standards, are summarized in Table 1. P-1a with an octyloxy chain on the

Figure 1. Powdery XRD curves of polymers.

are located at 2θ of 22.45, 22.11, and 20.51° for P-1a, P-1b, and P-1c, respectively, corresponding to π−π stacking distances of 3.96, 4.01, and 4.31 Å. Optical Properties. UV−vis absorption spectra of polymers P-1a, P-1b, and P-1c in DCB solutions at 100 °C and as thin films are shown in Figure 2. To eliminate the influence of polymer chain aggregation in solution at room temperature, the UV−vis absorption spectra of polymer solutions were therefore measured at 100 °C. Both in solutions and as thin films, all the

Table 1. Physical Properties of the Novel Polymers polymer

Mn (kg/mol)

Mw (kg/mol)

PDI

Tg (°C)

P-1a P-1b P-1c

34.8 97.8 114.9

80.6 360.2 247.1

2.31 3.68 2.15

357 338 338

benzothiadiazole unit showed a Mn of 34.8 kg/mol and a polydispersity index (PDI) of 2.31. Of the three polymers, P-1b and P-1c with branched alkoxy chain on benzothiadiazole unit were better soluble in hot toluene, which was the organic solvent used for Suzuki polycondensation, allowing us to achieve high molecular weight polymers. P-1b showed a Mn of 97.8 kg/mol and a PDI of 3.68, and P-1c showed a Mn of 114.9 kg/mol and a PDI of 2.15. Thermo gravimetric analysis (TGA) and differential scanning calorimetry (DSC) were used to investigate the thermal properties of these polymers. Under nitrogen atmosphere at a heating rate of 10 °C/Min, all the three polymers exhibited good thermal stability with the 5% decomposition temperature up to 357 °C for P-1a, 338 °C for P-1b, and 338 °C for P-1c. DSC measurements under nitrogen atmosphere at a heating rate of 20 °C/Min in the range of 20 to 300 °C also showed that there was no obvious glass transition for all the three polymers. Powdery XRD measurement (Figure 1) was used to investigate the packing of polymer chains in the solid state. All these polymers showed one diffraction peak at small angle region and one broad diffraction peak at wide angle region. The first diffraction peak at small angle region is located at 2θ = 3.95° for P-1a, 2θ = 4.44° for P-1b, and 2θ = 3.70° for P-1c, revealing the distance of polymer chains separated by the flexible chains is 22.35 Å for P-1a, 19.89 Å for P-1b, and 23.86 Å for P-1c. The broad diffraction peaks at the wide angle region

Figure 2. UV−vis absorption spectra of P-1a (a), P-1b (b), and P-1c (c) in DCB solutions at 100 °C and as thin films. C

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Polymer thin films were spin-coated from DCB solutions onto the OTS-modified Si/SiO2 substrates, and Au electrodes with a thickness of 25 nm were vacuum deposited atop polymer thin films. The transfer and output characteristic curves of FET devices are shown in Figure 4. Hole mobilities of P-1a, P-1b, and P-1c were calculated to be 1.51 × 10−3, 8.0 × 10−3, and 1.36 × 10−2 cm2 V−1 s−1, respectively, which are close to the electron mobility of PC71BM (2 × 10−2 cm2 V−1 s−1).28 The hole mobilities, VT and the on/off ratio are summarized in Table 3. The hole mobility of polymer films is usually related to two main factors, namely, the π−π stacking distance and molecular weight of polymers. Usually shorter π−π stacking distance and higher molecular weight are prone to endow polymer films with higher hole mobility.39 The π−π stacking distance of P1-a is shorter than that of P1-c, but the molecular weight of P1-a is much smaller than that of P1-c. Therefore, it is reasonable that the mobility of P1-c is larger that of P-1a. Photovoltaic Properties. Photovoltaic properties were investigated in a device configuration of ITO/PEDOT:PSS/ active layer/interfacial layer/Al. The thickness of active layer was about 100 nm. Two different interfacial layers, LiF and FNC60, were tested. As shown in Chart 3, FN-C60 is an alcoholsoluble cationic fullerene derivative, which can be spin-coated atop the active layer without damaging the underlayer. We have screened the weight ratio of polymer to PC71BM, the concentration of blend solutions, the spin-coating speed, and the volume of 1,8-diiodooctane (DIO) additive to achieve the better device performance. The preliminary results are summarized in Table 4. For all the three polymers, a polymer to PC71BM weight ratio of 1:3 gave the best photovoltaic performances. For P-1a, devices fabricated from DCB solutions using LiF as the interfacial layer for cathode gave a PCE of 4.56% with a Voc of 0.91 V, a Jsc of 9.46 mA/cm2, and an FF of 0.53. When DIO (2.0%, by volume) was used as the processing additive, the PCE was slightly enhanced to 5.80%. The increasing of PCE was mainly attributed to the increasing of Jsc and FF. When FN-C60 was used as the interfacial layer instead of LiF, the PCE was further increased to 6.41%, accompanied by the simultaneously increasing of the Voc, Jsc, and FF. It is believed that the use of FN-C60 as the interfacial layer for cathode can enhance the interfacial contact, electron transportation, and collection. For P-1b, PSC devices with active layers spin-coated from DCB solutions containing DIO (2.0%, by volume) also gave better photovoltaic performance than that fabricated from DCB solutions. The best device performance with a PCE of 4.85%, a Voc of 0.98 V, a Jsc of 8.5 mA/cm2, and an FF of 0.59 was achieved when FN-C60 was used instead of LiF as the interfacial layer for cathode. For P-1c, devices fabricated from DCB solutions containing DIO (2.0%, by volume) also gave the best device performance with a PCE of 5.06%, a Voc of 0.96 V, a Jsc of 8.54 mA/cm2, and an FF of 0.62. The trend of Jsc for the devices is P-1a > P-1c > P-1b, which is inconsistent with the trend of hole mobility of the FET devices based on these polymers. The inconsistency is probably due to that the FET mobility is measured for the pure polymer films in FET devices. Namely, the FET mobility reflects the transport property along the thin polymer film. The short circuit current (Jsc) of solar cells is related to the transport property of polymer:PC71BM blend instead of pure polymer films. The Jsc is mainly related to the nanostructures of polymer:PC71BM blend films. The above preliminary device results indicated that the substituent on the benzothiadiazole unit had significant influence on the performance of PSCs.

three polymers exhibited two absorption peaks, and their locations are summarized in Table 2. Compared with their Table 2. Optical and Electrochemical Properties of Polymers polymer

λmax [nm] solutiona

P-1a

387, 533

P-1b

387, 529

P-1c

387, 525

λmax [nm] film 393, 561 394, 564 392, 563

λedge [nm] film

Eg,opt (eV)b

HOMO (eV)

LUMO (opt, eV)c

624

1.99

−5.54

−3.55

621

1.99

−5.56

−3.57

618

2.00

−5.59

−3.59

a Measured at 100 °C. bCalculated from the absorption onset the polymer film, Eg,opt = 1240/λedge. cCalculated by the equation ELUMO = EHOMO + Eg,opt

solution absorption spectra, the corresponding film ones were red-shifted due to the aggregation of polymer chains in the solid state. The film absorption onsets (λedge) of P-1a, P-1b, and P-1c were 624, 621, and 618 nm, respective. The increasing of the volume of substituent group on the benzothiadiazole moiety could increase the π−π stacking distance of polymer chains in films and resulted in a blue-shift of the absorption peak. The optical band gaps (Eg,opt) of P-1a, P-1b, and P-1c films were therefore calculated to be 1.99, 1.99, and 2.00 eV, respectively, according to the equation: Eg,opt = 1240/ absorption onset. Electrochemical Properties. Electrochemical properties of P-1a, P-1b, and P-1c were investigated by cyclic voltammetry (CV) using a standard three electrodes electrochemical cell. As shown in Figure 3, these three polymers exhibited quasi

Figure 3. Cyclic voltammograms of the polymers.

reversible redox processes. The onset oxidation potentials of P1a, P-1b, and P-1c are 0.83, 0.85, and 0.88 V, respectively. HOMO levels of P-1a, P-1b, and P-1c were determined, using the equation EHOMO = -e(Eox + 4.71), to be −5.54, −5.56, and −5.59 eV, respectively. LUMO levels of P-1a, P-1b, and P-1c were therefore calculated according to the equation ELUMO = EHOMO + Eg,opt to be −3.55, −3.57, and −3.59 eV, respectively. The data are also summarized in Table 2. The above results indicated that the introduction of big volume alkoxy chain on the mono fluoro benzothiadiazole unit can only slightly lower the HOMO energy level of polymers. Their low-lying HOMO level is favorable for achieving PSCs with higher Voc. The changing of alkoxyl substituent group on the benzothiadiazole unit has a subtle influence on their electrochemical properties. Transport Properties. Hole transport properties of P-1a, P1b, and P-1c were investigated by fabrication of bottom-contact field effect transistors (FETs) on Si/SiO2 substrates with the low resistance Si as gate and SiO2 (300 nm) as gate insulator. D

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Figure 4. Transfer characteristics and output characteristics of P-1a (a, b), P-1b (c, d), and P-1c (e, f).

As shown in Figure 6, external quantum efficiencies (EQE) were measured to examine the accuracy of Jsc of devices fabricated under the optimized conditions for each polymer. All polymers exhibited broad photo to current responses in the range of 350 to 700 nm. In the range of 400 to 600 nm, the EQE value of P-1a is higher than 60% with the maximum close to 70%. For P-1b and P-1c, these two polymers exhibited similar EQE curves, in the range of 400 to 600 nm EQE values are around 50%. For all the three polymers, the Jsc calculated from the integration of EQE curves matched well with that obtained from J-V measurements. This good agreement could confirm the accuracy of PCE obtained from J−V measurements. Film Morphology. DIO is a higher boiling point solvent than the processing solvent DCB, exhibits better solubility for PC71BM, and can be used as additive to affect the morphology of polymer:PC71BM blend films.18,40 In previous studies, it has been shown that polymer component can form connected fibrous network when using DIO as the processing additive.40 Atomic force microscopy (AFM) and transmission electron microscopy (TEM) investigations on blend films fabricated without and with DIO as the processing additive were performed to study if DIO acts a similar effect in our system. TM-AFM was utilized to study the blend film morphology of polymer and PC71BM, AFM images of P-1a:PC71BM, P1b:PC71BM, and P-1c:PC71BM (1:3 by weight) blend films (2.0 μm × 2.0 μm) fabricated from DCB solutions without and with DIO as additives are shown in Figure 7. All blend films showed nanoscale phase separation. As shown in Figure 7, the

Table 3. FET Properties of the Pure Polymer Films μ (cm2/(V s))

polymers

on/off

P-1a P-1b P-1c

2.3 × 10 9.4 × 105 4.3 × 105 5

−3

1.51 × 10 8.00 × 10−3 1.36 × 10−2

VT (V) −25 −10 −42

Chart 3. Chemical Structure of FN-C60

Devices based on the monooctyloxy- and monofluoro substituted benzothiadiazole containing conjugated polymer P-1a gave the best device performance, even where the molecular weight of P-1a was the lowest one among the three polymers. Further increasing the photovoltaic performance was anticipated after intensive device optimization. J−V curves for the solar cells are found in Figure 5. E

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Table 4. Photovoltaic Parameters of Devices Based on LiF/Al Cathode and FN-C60/Al Cathode without and with DIO as the Processing Additive

a

device structure

solvent

Voc (V)

Jsc (mA/cm2)

FF

PCE (%)

ITO/PEDOT:PSS/P-1a:PC71BM/LiF/Al ITO/PEDOT:PSS/P-1a:PC71BM/LiF/Al ITO/PEDOT:PSS/P-1a:PC71BM/FN-C60/Al ITO/PEDOT:PSS/P-1b:P71BM/LiF/Al ITO/PEDOT:PSS/P-1b:P71BM/LiF/Al ITO/PEDOT:PSS/P-1b:PC71BM/FN-C60/Al ITO/PEDOT:PSS/P-1c:PC71BM/LiF/Al ITO/PEDOT:PSS/P-1c:PC71BM/LiF/Al ITO/PEDOT:PSS/P-1c:PC71BM/FN-C60/Al

DCB DCBa DCBa DCB DCBa DCBa DCB DCBa DCBa

0.91 0.88 0.92 0.80 0.94 0.98 0.93 0.92 0.96

9.46 12.00 12.07 7.50 8.13 8.50 7.83 8.60 8.54

0.53 0.55 0.58 0.53 0.49 0.59 0.52 0.57 0.62

4.56 5.80 6.41 3.17 3.72 4.86 3.77 4.54 5.06

With 2% DIO (by volume).

Figure 5. J−V curves for the solar cells.

Figure 7. AFM images (2 μm × 2 μm) of P-1a:PC71BM (1:3), P1b:PC71BM (1:3), and P-1c:PC71BM (1:3) blend films.

1c blend films, respectively, after the addition of DIO (2.0%, by volume) as the additive. The AFM results are consistent with the device results. Since AFM can only probe the surface roughness of the polymer:PC71BM blend films, TEM experiments were therefore performed to investigate the nanostructures of the blend films without and with DIO. TEM images of polymer:PC71BM blends spin-coated from DCB solutions without and with 2.0% DIO are shown in Figures 8 and S1 (Supporting Information). As shown in Figure 8a, TEM image of P-1a:PCBM blends spin-coated from DCB solutions exhibited apparent phase separation in nanoscale with

Figure 6. EQE of PSCs fabricated under optimized conditions.

morphology of all blend films became smoother when DIO (2%, by volume) was used as the additive. The root-meansquare (rms) roughness values were decreased from 1.36, 0.93, and 3.91 nm to 0.43, 0.61, and 0.53 nm for P-1a, P-1b, and PF

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1,3,2-dioxaborolan-2-yl)-9,9-dioctyl-9H-silafluoren-7-yl)-1,3,2dioxaborolane and the newly developed 4,7-bis(5-bromothiophen-2-yl)-5-ethoxy-6-fluorobenzo[c][1,2,5]thiadiazole monomer. The introduction of a fluorine atom on the benzothiadiazole unit can usually lower the HOMO and LUMO level of the resulted polymers; whereas the introduction of an alkoxy chain on the benzothiadiazole unit can enhance the solubility of polymers without interfering the dense packing of polymers in the solid state. The as-synthesized three polymers, which are of high molecular weight and fully soluble in CB, DCB, and TCB at elevated temperature, can be used to fabricate PSCs by solution processing. FET hole mobilities of these polymers are in the range of 1.51 × 10−3 to 1.36 × 10−2 cm2 V−1 s−1, which are close to the electron mobility of PC71BM. Because of their deep HOMO energy level, blending with PC71BM one can obtain BHJ PSCs with the highest Voc up to 0.98 V. High PCE of 6.41% indicated that 5-alkyloxy-6-fluorobenzo[c][1,2,5]thiadiazole could be a promising acceptor unit for the synthesis of donor materials for PSC applications.

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ASSOCIATED CONTENT

S Supporting Information *

Syntheses and characterizations of monomers and polymers, experimental details for the fabrication and characterization of OFETs and PSCs, TEM images, solar cell results, and 1H and 13 C NMR spectra. This material is available free of charge via Internet at http://pubs.acs.org.

Figure 8. TEM images of P-1a:PC71BM blends (a), P-1a:PC71BM blends with 2.0% DIO (b), P-1b:PC71BM blends (c), P-1b:PC71BM blends with 2.0% DIO (d); P-1c:PC71BM blends (e), and P1c:PC71BM blends with 2.0% DIO (f). The scale bar is 200 nm.

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AUTHOR INFORMATION

Corresponding Authors

*(Z.B.) E-mail: [email protected]. *(C.L.) E-mail: [email protected].

PC71BM rich isolated dark domains. After the use of 2.0% DIO as the additive, the PC71BM rich domains blurred and entangled nanofibers formed as shown in Figure 8b. As shown in Figure 8c, TEM image of P-1b:PCBM blend films spin-coated from DCB solutions also exhibited similar larger scale phase with many isolated PC71BM rich dark domains. The TEM image of P-1b:PC 71 BM spin-coated from DCB containing 2% DIO is shown in Figure 8d. After the use of DIO as the additive, the PC71BM rich domains disappeared and nanofibrous structures are formed. TEM image of P1c:PC71BM blend films spin-coated from DCB solution is shown in Figure 8e. It can be clearly seen that the blend films are of homogeneous structure without apparent large scale phase separation between polymer and PC71BM; whereas after the use of 2.0% DIO as the processing additive, as shown in Figure 8f, the blend films comprised a large number of entangled nanofibers, which are probably formed by the aggregation of the polymers. From TEM investigations, we can conclude that the enhance of device performance is mainly due to the better phase separation between PC71BM and polymer components and the formation of connected fibrous polymer networks after the addition of DIO as the processing additive.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the financial support from the NSFC of China (Grant Nos. 21161160443 and 91233205).

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REFERENCES

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CONCLUSION We have developed a novel building block, 5-alkoxy-6fluorobenzo[c][1,2,5]thiadiazole, which can be used for the construction of conjugated polymers with finely tuned properties. Three novel D−A alternating conjugated polymers P-1a, P-1b, and P-1c were synthesized by Suzuki polycondensation of 4,4,5,5-tetramethyl-2-(2-(4,4,5,5-tetramethylG

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dx.doi.org/10.1021/ma500417r | Macromolecules XXXX, XXX, XXX−XXX