Article pubs.acs.org/Macromolecules
Regioregular D1‑A‑D2‑A Terpolymer with Controlled Thieno[3,4‑b]thiophene Orientation for High-Efficiency Polymer Solar Cells Processed with Nonhalogenated Solvents Hyojung Heo,† Honggi Kim,† Donghwa Lee,† Seokhoon Jang,† Lyeojin Ban,† Bogyu Lim,‡ Jaechol Lee,‡ and Youngu Lee*,† †
Department of Energy Systems Engineering, Daegu Gyeongbuk Institute of Science and Technology (DGIST), 333, Techno jungang-daero, Hyeonpung-Myeon, Dalseong-Gun, Daegu 42988, Republic of Korea ‡ Future Technology Research Center, Corporate R&D, LG Chem Research Park, 188, Moonji-ro, Yuseong-gu, Daejeon 34122, Republic of Korea S Supporting Information *
ABSTRACT: A regioregular D1-A-D2-A terpolymer PDTSTTBDT incorporating dithieno[3,2-b:2′,3′-d]silole (DTS, D1) and benzo[1,2-b:4,5-b]dithiophene (BDT, D2) units with perfectly controlled thieno[3,4-b]thiophene (TT, A) orientation was synthesized for the first time. The thermal, optical, and electrochemical properties of the regioregular PDTSTTBDT were characterized and compared with the random PDTSTTBDT without structural regioregularity. The regioregular PDTSTTBDT showed ideal optical bandgap (1.45 eV), lower lying HOMO energy level, and higher degree of crystallinity compared to the random PDTSTTBDT. Moreover, it exhibited excellent solubility in nonhalogenated solvents as well as halogenated solvents. The inverted bulk-heterojunction polymer solar cells (PSCs) based on the regioregular PDTSTTBDT and o-xylene process solvent showed a power conversion efficiency as high as 6.14%, which is 500% higher than the random PDTSTTBDT-based PSCs. It was found that the remarkable enhancement of photovoltaic performance in regioregular PDTSTTBDT-based PSCs is mainly due to improved light absorption, effective polymer ordering, and high charge carrier mobility.
■
INTRODUCTION Bulk-heterojunction (BHJ) polymer solar cells (PSCs) consist of a blended film based on a conjugated polymer donor and a fullerene acceptor, which has been found to be the most successful device architectures. 1 The BHJ PSCs have remarkably progressed in the past decade and now attain power conversion efficiencies (PCEs) over 10% in single junction solar cell.2−6 The most polymer donors in BHJ PSCs contain an electron-donating (push) unit (donor, D) and an electron-accepting (pull) unit (acceptor, A) on their conjugated backbones.7−9 This push−pull system has been confirmed to be effective in tuning the photo responsibility, energy bandgap, energy levels, carrier mobility, crystallinity, and film morphology of the polymer donors by employing the various push and pull segments. However, only a limited number of donor and acceptor units have allowed the high performance of the PSCs.10,11 Recently, terpolymers have emerged for an alternative molecular design strategy of conjugated polymers with distinct optoelectronic properties.12,14−21 The terpolymers comprise three components with two different donor units and one acceptor unit or with one donor unit and two different acceptor units on their conjugated polymer backbones.12,14−20 There© XXXX American Chemical Society
fore, the terpolymers allow the integration of the advantages of different donor and acceptor units to attain well-controlled physical properties.12,16,18,19 In addition, the physical properties of the terpolymers can be flexibly tuned by varying the composition of the monomers in the polymerization.12,13 However, the terpolymers tend to cause variation in the molecular sequences of different three moieties, which can lead to randomly arranged donor and acceptor units on their polymer backbones.12,14,15 This random arrangement in the terpolymers can give rise to the numerous molecular chromophores with intricate hybridization of molecular orbitals and reduce intermolecular π−π stacking interactions which may result in unanticipated trap sites and limit charge transport in polymer films.12,17,19,21 Furthermore, the batch-to-batch variation of the terpolymers can be intensified due to their random arrangement, which makes it difficult to control and reproduce their physical properties, such as light absorption, electronic energy levels, charge mobility, and solubility as well as to define their polymer structures.14,15,21 These drawbacks of Received: February 4, 2016 Revised: April 12, 2016
A
DOI: 10.1021/acs.macromol.6b00269 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules Scheme 1. Synthetic Procedures and Polymer Structures of Regioregular and Random Terpolymersa
a
Reaction conditions: (i) compound 5, Pd(PPh3)4, toluene, 110 °C; (ii) NBS, CHCl3, rt.
regioregular PDTSTTBDT showed a power conversion efficiency as high as 6.14%, which is 500% higher than the random PDTSTTBDT-based PSCs.
the random terpolymers can be controlled by adjusting the donor/acceptor arrangement on their polymer backbone.20 The regioregular terpolymers with perfectly arranged donor and acceptor moieties tend to reduce the batch-to-batch variation and maintain uniformly their physical properties owing to their well-defined structures which can provide precise electronic structures with molecular ordering tendency and more favorable BHJ morphology for high-efficiency PSCs.12,16−20,22 Several research groups have reported the promising prospect in the photovoltaic performance of the regioregular terpolymers with an D1-A-D2-A repeating units using diketopyrrolopyrrole (DPP), benzo[2,1,3]thiadiazole (BT), or fluoro2,1,3-benzothiadiazole (FBT).18−20 However, the regioregular terpolymers containing thieno[3,4-b]thiophene (TT) segment which is one of the most famous acceptor unit for highefficiency PSCs have not been synthesized yet because of synthetic difficulty in controlling TT orientation in the polymer backbone. Moreover, most of the regioregular terpolymers were processed using halogenated solvents such as chlorobenzene (CB) and dichlorobenzene (DCB), which are highly toxic to the environment and human health.16,18−20 Therefore, it is required to synthesize new regioregular terpolymers containing TT segment with excellent photovoltaic performance as well as processability in nonhalogenated solvent. Very recently, we successfully synthesized regioregular donor−acceptor (D−A) copolymers with perfectly controlled TT orientation and demonstrated that the PSCs based on the regioregular D−A copolymers exhibited a highly enhanced PCE due to effective molecular ordering between polymer chains and increase in charge carrier mobility.23,24 In this article, we report design and synthesis of new regioregular terpolymer denoted as D1-A-D2-A structure incorporating dithieno[3,2-b:2′,3′-d]silole (DTS, D1) and benzo[1,2-b:4,5-b]dithiophene (BDT, D2) units with perfectly controlled TT (A) orientation for the first time. Furthermore, the solubility of the regioregular terpolymer was controlled by alkyl side chains attached over all monomer segments, resulting in high solubility in nonhalogenated solvents as well as halogenated solvents. The regioregular PDTSTTBDT exhibited lower lying HOMO energy level and higher degree of crystallinity compared to the random PDTSTTBDT without structural regioregularity. The inverted BHJ PSCs based on the
■
RESULTS AND DISCUSSION Synthesis and Thermal Stability. The synthetic procedures of the regioregular and random PDTSTTBDT are shown in Scheme 1. The regioregular and random PDTSTTBDT were prepared by the Stille polycondensation reaction. The regioregular PDTSTTBDT was synthesized with bis-brominated TT-BDT-TT monomer (3) with perfectly controlled TT orientation and bis-trimethylstannylated DTS monomer (4) by using the Stille polycondensation reaction. The random PDTSTTBDT was synthesized with compounds 4 and 5 and dibrominated TT monomer (6) by using the Stille polycondensation reaction. The regioregular and random PDTSTTBDT showed high solubility in nonhalogenated solvents such as toluene and oxylene as well as halogenated solvents such as CB, DCB, and chloroform. The number- and weight-average molecular weights (Mn, Mw) and polydispersity index (PDI) of both polymers were measured by gel permeation chromatography (GPC) against polystyrene standards by using tetrahydrofuran (THF) as an eluent. The regioregular PDTSTTBDT has Mn of 21.7 kg/mol and Mw of 50.5 kg/mol with PDI of 2.26. The random PDTSTTBDT has Mn of 20.8 kg/mol and Mw of 54.5 kg/mol with PDI of 2.62. Both polymers have similar molecular weight, thereby minimizing the potential complication from the influence of different molecular weights. The thermal stabilities of both polymers were investigated using thermogravimetric analysis (TGA). The decomposition temperatures (Td) at 5% loss of the regioregular and random PDTSTTBDT were observed at 372 and 361 °C, respectively (Figure S7). Optical Properties. The UV−vis absorption spectra of the regioregular and random PDTSTTBDT in chloroform solutions and spin-coated films on glass substrates are presented in Figure 1. The optical properties of both polymers are summarized in Table 1. For the UV−vis absorption spectra of both polymers in chloroform solutions, the regioregular PDTSTTBDT solution exhibited maximum absorption (λmax) at 666 nm while the λmax for the random PDTSTTBDT was slightly blue-shifted to 642 B
DOI: 10.1021/acs.macromol.6b00269 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
As a result, the optical bandgap (Eg) of the regioregular and random PDTSTTBDT were 1.45 and 1.28 eV, respectively. Electrochemical Properties. Cyclic voltammetry (CV) measurement was used to investigate electrochemical properties of both polymers such as the energy levels of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) (Figure S9 and Table 1). The onset oxidation potentials of the regioregular and random PDTSTTBDT were 0.80 and 0.69 V, corresponding to HOMO energy levels of −5.19 and −5.08 eV, respectively. This results verified that the random PDTSTTBDT possesses higher HOMO energy level than the regioregular PDTSTTBDT, which can be caused by TT-DTS-TT-DTS rich segments. Thus, it is expected that the lower lying HOMO energy level of the regioregular PDTSTTBDT could increase open circuit voltage (Voc) of the corresponding PSCs. The onset reduction potentials of the regioregular and random PDTSTTBDT were the identical value of −0.77 V, corresponding to LUMO energy level of −3.62 eV. Photovoltaic Performances. To investigate the influence of the structural regioregularity of the regioregular and random PDTSTTBDT on the photovoltaic performance of the solar cell devices, inverted BHJ PSCs were fabricated with a structure of ITO/ZnO/polymer:PC71BM/MoO3/Ag. The active layers were carefully optimized in terms of thickness, polymer− fullerene ratio, and solvent composition of the spin-coating solution. The active layer thickness was about 98 nm. Both polymers had identical optimized processing conditions and displayed the photovoltaic performance when spin-coated from a chlorobenzene solution containing DIO of 5 vol % as additive and using a 1:1.5 polymer−fullerene weight ratio. The current density−voltage (J−V) characteristics of inverted BHJ PSCs based on the regioregular and random PDTSTTBDT under AM 1.5 G illumination (100 mW/cm2) are shown in Figure 2a. Representative characteristics of inverted BHJ PSCs such as open circuit voltage (Voc), short circuit current (Jsc), fill factor (FF), and power conversion efficiency (PCE) are summarized in Table 2. The device based on the random PDTSTTBDT showed a Voc of 0.50 V, a Jsc of 5.61 mA/cm2, a FF of 39.19%, and a PCE of 1.11%. On the other hand, the regioregular PDTSTTBDTbased device exhibited remarkably improved photovoltaic performance with a Voc of 0.64 V, a Jsc of 14.35 mA/cm2, a FF of 61.81%, and a PCE of 5.64%. It is noteworthy that the regioregular PDTSTTBDT-based device showed approximately 500% higher PCE than the random PDTSTTBDT-based device. The regioregular PDTSTTBDT-based device obtained relatively higher Voc which was ascribed to the lower lying HOMO energy level. However, clearer improvement of the regioregular PDTSTTBDT-based device was mostly due to improved Jsc and FF values. Interestingly, the regioregular PDTSTTBDT-based device showed about 250% higher Jsc value despite its wider energy bandgap and narrower absorption band than the random PDTSTTBDT. Moreover, the regioregular PDTSTTBDT also showed about 50% higher FF
Figure 1. UV−vis absorption spectra of regioregular and random PDTSTTBDT (a) in chloroform and (b) in film.
nm. This result clearly indicates that the intramolecular charge transfer (ICT) transition among regioregularly oriented segments is much stronger than that among randomly orientated segments. The regioregular PDTSTTBDT showed sharp onset of the electronic transitions and a narrow band, which can be attributed to the frontier orbitals extended over all different segments on its backbone.18,21 In contrast, the random PDTSTTBDT exhibited an onset at lower energies, a much broader spectrum. Similar differences were also observed in the spectra of thin films. It can be conjectured that longer segments enriched in DTS or BDT electron rich moieties led to regional differences in the HOMO and LUMO energy levels along the conjugated backbones of the random terpolymer and thus a broadened absorption.12 The broadening at lower wavelength can be assigned to -TTBDT-TT-BDT- rich sections, and the broadening toward higher wavelengths can be ascribed to -TT-DTS-TT-DTSenriched segments. It is well-known that the absorption broadening of the random terpolymers can be originated from the various sections with different chemical composition on its backbone.12,14 Furthermore, it is noteworthy that the absorption coefficient of the regioregular PDTSTTBDT ranging from 550 to 750 nm is much higher than that of the random PDTSTTBDT. 15 This result implies that the regioregular PDTSTTBDT might possess relatively excellent light harvesting ability compared to the random PDTSTTBDT.
Table 1. Optical and Electrochemical Properties of Regioregular and Random Terpolymers compound
λmax,sol (nm)
ε ((g/mL)−1 cm−1)
λmax,film (nm)
Egopt (eV)
HOMOa (eV)
LUMOa (eV)
666 642
5.36 × 10 3.29 × 104
691 649
1.45 1.28
−5.19 −5.08
−3.62 −3.62
Regioregular PDTSTTBDT Random PDTSTTBDT a
4
As measured by cyclic voltammetry. C
DOI: 10.1021/acs.macromol.6b00269 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
the best performance with a Voc of 0.62 V, a Jsc of 14.77 mA/ cm2, a FF of 66.88%, and a PCE of 6.14% with increased Jsc and FF compared to the device with DIO of 5 vol %. To further confirm the accuracy of the measurements, the external quantum efficiency (EQE) of the inverted BHJ PSCs with the regioregular and random PDTSTTBDT was measured and was shown in Figure 2b. As for the regioregular PDTSTTBDT-based device, the maximum EQE was over 53% at 460 nm and was ∼47% in the range from 450 to 850 nm. However, the random PDTSTTBDT-based device exhibited a relatively lower EQE, about 30% in the range from 400 to 900 nm, with a maximum of 36% at 385 nm. The calculated Jsc values from integration of the EQE spectra were well matched (within 5% error) with the measured Jsc values of the optimized devices. Crystalline Properties and Hole Mobility. To investigate ordered structures of the regioregular and random PDTSTTBDT in spin-coated films, grazing incidence wide-angle X-ray scattering (GIWAXS) analysis was performed on the neat polymer and blended films. Figure 3 and Figure S22 show the GIWAXS images of the neat polymer and polymer:PC71BM (1:1.5, wt:wt) blended films cast from CB, o-xylene solvent with DIO of 5 vol %, and coadditives of AA of 2 vol % and DIO of 3 vol %. The GIWAXS image of the neat regioregular PDTSTTBDT film on Si substrate clearly showed an intense X-ray reflection at qz ≈ 0.31 Å−1, corresponding to multistacked crystal layers containing edge-on, with a layer spacing, d(100) of about 20.20 Å. Additional X-ray reflection was strongly measured at qz ≈ 1.55 Å−1, corresponding to vertically stacked crystal planes of face-on chains on the substrate, with an intermolecular π-overlap spacing, d(010) of approximately 4.04 Å (Figure 3a). The GIWAXS image of the neat random PDTSTTBDT film showed similar crystal reflections along the qz-axis, indicating d(100) and d(010) values of 19.95 and 3.98 Å, respectively (Figure 3b). Although the regioregular and random PDTSTTBDT films had edge-on and face-on orientations, the regioregular PDTSTTBDT film showed more favorable face-on orientation for the photovoltaic application, showing stronger X-ray reflection on the qz-axis. The GIWAXS image of the regioregular polymer:PC71BM blended film cast from CB solvent with DIO of 5 vol % showed weak X-ray reflection peak at 1.61 Å−1 toward the qz-axis, corresponding to the π−π stacking between polymer chains on face-on orientation (Figure 3c). However, there was no reflection peak, corresponding to π−π stacking interaction in the random PDTSTTBDT:PC71BM blended film (Figure 3d), showing the only reflection peak corresponding to edge-on orientation. The GIWAXS images of the regioregular PDTSTTBDT:PC71BM blended film cast from o-xylene solvent with
Figure 2. (a) J−V characteristics and (b) external quantum efficiencies for ITO/ZnO/polymers:PC71BM/MoO3/Ag configuration devices fabricated using CB solvent and o-xylene solvents under the illumination of AM 1.5, 100 mW/cm2.
value. In addition, the regioregular PDTSTTBDT was applied to the BHJ PSCs using o-xylene as a processing solvent, which is one of the nonhalogenated solvents used eco-friendly for large-scale fabrication of the PSCs. The PSCs fabricated using o-xylene with DIO of 5 vol % showed a Voc of 0.62 V, a Jsc of 14.65 mA/cm2, a FF of 62.86%, and a PCE of 5.75%, which was slightly increased by enhanced Jsc and FF compared to the device fabricated using CB with DIO of 5 vol %. For further increase in the photovoltaic performance of the regioregular PDTSTTBDT, we added another promising additive, panisaldehyde (AA), which is eco-compatible. Recently, Sprau et al. reported the good combination of o-xylene/AA for highly efficient PSCs. In their reported work, AA induced the improvement of the polymer ordering and reduced the fullerene agglomeration, leading to enhanced Jsc and FF.28 However, the regioregular PDTSTTBDT-based PSCs with AA of 2 vol % showed a lower PCE of 4.22% (Figure S17 and Table S8). By adding the AA of 2 vol % and DIO of 3 vol % to the device, the regioregular PDTSTTBDT-based device achieved
Table 2. Photovoltaic Performances of Regioregular and Random PDTSTTBDT compound Random PDTSTTBDT Regioregular PDTSTTBDT
a
solvent
additive
thickness (nm)
CB
DIO, 5 vol %
98
CB
DIO, 5 vol %
97
o-xylene
DIO, 5 vol %
99
o-xylene
AA, 2 vol % DIO, 3 vol %
98
Voc,max (Voc,ave)a (V) 0.50 (0.50 ± 0.11) 0.64 (0.64 ± 0.01) 0.62 (0.62 ± 0.01) 0.62 (0.62 ± 0.01)
Jsc,max (Jsc,ave) (mA/cm2)a
FFmax (FFave)a (%)
5.61 (5.51 ± 0.11) 14.35 (14.31 ± 0.15) 14.65 (14.43 ± 0.31) 14.77 (14.46 ± 0.25)
39.19 (38.09 ± 0.67) 61.81 (61.55 ± 0.70) 62.86 (62.83 ± 0.04) 66.88 (67.54 ± 0.65)
PCEmax (PCEave)a (%) 1.11 (1.06 ± 0.03) 5.64 (5.61 ± 0.05) 5.75 (5.66 ± 0.13) 6.14 (6.07 ± 0.06)
calcd Jscb (mA/cm2) 5.35 13.61 13.78 13.96
The average values in parentheses are obtained from over 16 devices. bThe calculated Jsc values are obtained from EQE spectra. D
DOI: 10.1021/acs.macromol.6b00269 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
Figure 3. GIWAXS images of (a) neat regioregular PDTSTTBDT film, (b) neat random PDTSTTBDT film, (c) regioregular PDTSTTBDT:PC71BM blended film with DIO of 5 vol %, (d) random PDTSTTBDT:PC71BM blended film with DIO of 5 vol %, (e) regioregular PDTSTTBDT:PC71BM films cast from o-xylene solution with DIO of 5 vol %, and (f) with AA of 2 vol % and DIO of 3 vol %.
the additives exhibited similar crystal reflections with the regioregular PDTSTTBDT:PC71BM blended film cast from CB solvent (Figure 3e,f). It is well-known that face-on stacking of the donor polymers in the active layer induces vertical πoverlap between the polymer backbones for effective charge transport to the electrodes, in comparison with edge-on stacked films.29,32 Accordingly, the regioregular PDTSTTBDT possessed more favorable molecular ordering for the photovoltaic application compared with the random PDTSTTBDT. To examine how the structural regioregularity of both polymers affects the charge transport, the space-charge-limitedcurrent (SCLC) measurements were used to evaluate the hole mobility of both polymers (Figure 4). In blended films, hole
mobility (μh) values of the regioregular and random PDTSTTBDT were 7.04 × 10−5 and 1.70 × 10−6 cm2/(V s), respectively. This result clearly indicated that the μh value of the regioregular PDTSTTBDT was 41 times higher than that of the random PDTSTTBDT. The highly enhanced μh value of the regioregular PDTSTTBDT originated from the effective crystallinity and vertically orientated π-overlap, as determined by GIWAXS analysis, in comparison with the random PDTSTTBDT with discernible π-overlap oriented parallel to the electrode surfaces.29−32 The resulting μh values were consistent with the photovoltaic performances of the regioregular PDTSTTBDT system showing higher Jsc and FF values than those of the random PDTSTTBDT-based devices. Morphology Study. The nanoscale morphologies of polymer:PC71BM blend films were studied using transmission electron microscopy (TEM). As shown in Figure 5, large domains with diameters 50−200 nm were exhibited in the random and regioregular PDTSTTBDT blended films cast from CB solvent without DIO, whereas both polymer blended films with DIO of 5 vol % showed the internetwork structure of nanofibrillar crystallites. Interestingly, the regioregular PDTSTTBDT blended films cast from o-xylene with DIO of 5 vol % showed more definite nanofibrillar structures originating from a bicontinuous interpenetrating network and much smoother surface roughness of 2.24 nm. In particular, the regioregular PDTSTTBDT blended film with AA of 2 vol % and DIO of 3 vol % tended to form highly pronounced and uniform nanofibrillar structures with much smoother surface roughness
Figure 4. Hole carrier mobility (μh) of regioregular and random PDTSTTBDT. E
DOI: 10.1021/acs.macromol.6b00269 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
mobility. This study implies that the regioregular terpolymers can be new electron donors for high performance polymer solar cells.
■
EXPERIMENTAL SECTION
Materials and Synthesis. All reagents were obtained from SigmaAldrich, ACROS, and TCI, used without further purification. 6Bromothieno[3,4-b]thiophene-2-carboxylic acid-2-ethylhexyl ester (1),24 4,4′-bis(2-ethylhexyl)-5,5′-bis(trimethyltin)dithieno[3,2-b:2′,3′d]silole (4),25 2,6-bis(trimethyltin)-4,8-bis(5-(2-ethylhexyl)thiophen2-yl)benzo[1,2-b:4,5-b′]dithiophene (5),26 and 4,6-dibromothieno[3,4-b]thiophene-2-carboxylic acid-2-ethylhexyl ester (6)27 were synthesized according to literature procedures. All reactions were carried out under nitrogen. 1H NMR and 13C NMR spectra were measured by NMR spectrometer (AVANCE 3, Bruker) using tetramethylsilane (TMS) as internal reference. CDCl3 was used as a solvent for NMR analysis. Bis(2-ethylhexyl)-6,6′-(4,8-bis(5-(2-ethylhexyl)thiophen-2yl)benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl)bis(thieno[3,4-b]thiophene-2-carboxylate) (2). Compound 1 (1.46 g, 3.90 mmol, 2.5 equiv) and compound 5 (1.41 g, 1.56 mmol, 1 equiv) with tetrakis(triphenylphosphine)palladium(0) (Pd(PPh3)4) (0.09 g, 0.078 mmol, 5 mol %) were dissolved in anhydrous toluene (20 mL). The mixture was refluxed and stirred at 110 °C for 24 h under N2. The mixture was cooled to room temperature, and then the organic solvent was removed under vacuum. The residue was purified by column chromatography on silica gel using methylene chloride (CH2Cl2) and n-hexane (1:3) as an eluent. Pure compound 2 (0.51 g, 28%) was obtained as a red solid. 1H NMR (400 MHz, CDCl3): δ 8.02 (s, 2H), 7.79 (s, 2H), 7.37 (d, 2H), 7.23 (s, 2H), 6.95 (d, 2H), 4.31−4.20 (m, 4H), 2.91 (s, 2H), 2.89 (s, 2H), 1.79−1.67 (m, 4H), 1.52−1.28 (m, 32H), 1.01−0.85 (m, 24H). 13C NMR (100 MHz, CDCl3): δ 163.10, 146.56, 142.72, 141.13, 140.88, 139.43, 137.37, 136.73, 136.67, 129.01, 128.14, 125.86, 123.96, 123.94, 120.73, 111.48, 68.33, 41.64, 39.07, 34.57, 32.79, 30.75, 29.17, 29.15, 25.99, 24.23, 23.23, 23.17, 14.37, 14.25, 11.32, 11.12. HRMS (m/z): calcd for C64H78O4S8, m/z = 1166.37; found, 1166.50 [M]+. Bis(2-ethylhexyl)-6,6′-(4,8-bis(5-(2-ethylhexyl)thiophen-2yl)benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl)bis(4-bromothieno[3,4-b]thiophene-2-carboxylate) (3). A solution of N-bromosuccinimide (NBS) (0.157 g, 0.55 mmol, 2.2 equiv) in chloroform (CHCl3) (20 mL) was added dropwise to the compound 2 (0.47 g, 0.40 mmol, 1 equiv) in CHCl3 (30 mL) over 30 min at room temperature. The reaction mixture was poured into DI water and extracted with CH2Cl2 three times. The organic phase was dried with anhydrous sodium sulfate (Na2SO4), and the organic solvent was removed under vacuum. The residue was purified by column chromatography on silica gel using CH2Cl2 and n-hexane (1:3) as an eluent. Compound 3 (0.38 g, 71%) was obtained as a deep red solid. 1H NMR (400 MHz, CDCl3): δ 7.99 (s, 2H), 7.71 (s, 2H), 7.34 (d, 2H), 6.94 (d, 2H), 4.31−4.21 (m, 4H), 2.91 (s, 2H), 2.89 (s, 2H), 1.78−1.67 (m, 4H), 1.51−1.28 (m, 32H), 1.00−0.87 (m, 24H). 13C NMR (100 MHz, CDCl3): δ 162.65, 146.70, 142.63, 141.78, 141.10, 139.44, 137.31, 136.36, 135.90, 130.35, 128.16, 125.89, 124.68, 124.05 120.99, 98.18, 68.45, 41.58, 39.02, 34.52, 32.75, 30.67, 29.14, 29.12, 25.95, 24.15, 23.22, 23.12, 14.36, 14.22, 11.27, 11.08 HRMS (m/z): calcd for C64H76O4Br2S8, m/z = 1324.19; found, 1324.60 [M]+. Regioregular PDTSTTBDT. Compound 3 (0.201 g, 0.151 mmol, 1 equiv) and compound 4 (0.113 g, 0.151 mmol, 1 equiv) were dissolved in anhydrous toluene (2 mL) and anhydrous dimethylformamide (DMF) (0.2 mL) with tris(dibenzylideneacetone)dipalladium(0) (Pd2(dba)3) (0.0027 g, 2 mol %) and triphenylphosphine (PPh3) (0.0031 g, 8 mol %). The reaction mixture was refluxed at 110 °C for 30 h and cooled to room temperature. Then, methanol (50 mL) was added to the reaction mixture, and a black solid was filtered over a thimble filter. Sequentially, the polymer solid was purified by Soxhlet extractions with methanol, n-hexane, and CHCl3. The polymer solid was reprecipitated in methanol and CHCl3 solution and filtered out. The regioregular PDTSTTBDT (201 mg, 82.3%) was
Figure 5. TEM images of (a) random polymer:PC71BM blended films cast from CB without DIO, (b) with DIO of 5 vol %, (c) regioregular polymer:PC71BM blended film cast from CB without DIO, (d) with DIO of 5 vol % and (e) cast from o-xylene with DIO of 5 vol %, and (f) with AA of 2 vol % and DIO of 3 vol %.
of 1.78 nm (Figure S24). The well-ordered nanofibrillar morphology of the regioregular PDTSTTBDT resulted in much higher charge carrier transport.30,31 These results clearly confirmed that the higher μh and better film morphology of the regioregular PDTSTTBDT dramatically enhanced Jsc, and FF, leading to the highly improved PCE.
■
CONCLUSION In summary, we successfully synthesized new regioregular terpolymer, PDTSTTBDT, denoted as D1-A-D2-A structure incorporating DTS (D1) and BDT (D2) units with perfectly controlled TT (A) orientation for the first time. The regioregular PDTSTTBDT showed more excellent optical properties and higher degree of crystallinity than the random PDTSTTBDT. Moreover, it exhibited excellent solubility in nonhalogenated solvents as well as halogenated solvents. The regioregular PDTSTTBDT exhibited lower lying HOMO energy level and higher degree of crystallinity compared to the random PDTSTTBDT without structural regioregularity. The inverted BHJ PSCs based on the regioregular PDTSTTBDT and o-xylene processing solvent showed a power conversion efficiency as high as 6.14%, which is 500% higher than the random PDTSTTBDT-based PSCs. The remarkable enhancement of photovoltaic performance in regioregular PDTSTTBDT-based PSCs might be due to effective polymer ordering, improved light absorption, and high charge carrier F
DOI: 10.1021/acs.macromol.6b00269 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules obtained as a black solid. 1H NMR (400 MHz, CDCl3): δ 8.18−6.73 (br, 10H), 4.70−3.89 (br, 4H), 3.36−2.65 (br, 4H), 2.10−0.56 (br, 94H). Elemental analysis calcd (%) for (C88H112O4S10)n: C, 66.79; H, 7.13; S, 20.26. Found: C, 66.48; H, 6.91; S, 20.33. Random PDTSTTBDT. Compound 4 (0.130 g, 0.17 mmol, 1 equiv), compound 5 (0.158 g, 0.17 mmol, 1 equiv), and compound 6 (0.158 g, 0.34 mmol, 2 equiv) were dissolved in anhydrous toluene (2 mL) and anhydrous DMF (0.2 mL) with Pd2(dba)3 (0.0032 g, 2 mol %) and PPh3 (0.0036 g, 8 mol %). The reaction mixture was refluxed at 110 °C for 72 h and cooled to room temperature. Then, the purification of the random polymer was conducted in the same way preparing the regioregular polymer. The random PDTSTTBDT (236 mg, 83.9%) was obtained as a black solid. 1H NMR (400 MHz, CDCl3): δ 8.30−6.66 (br, 10H), 4.75−3.95 (br, 4H), 3.31−2.67 (br, 4H), 2.06−0.44 (br, 94H). Elemental analysis calcd (%) for (C88H112O4S10)n: C, 66.79; H, 7.13; S, 20.26. Found: C, 65.73; H, 6.94; S, 20.03. Device Fabrication and Characterization. BHJ PSC devices are prepared as follow. An indium tin oxide (ITO)-coated glass was cleaned by ultrasonic treatment for 10 min in acetone, DI water, and isopropyl alcohol and dried by using nitrogen gas. The cleaned ITOcoated glass was treated in an UV-ozone chamber for 20 min and immediately spin-coated with a ZnO solution. The ZnO layer was formed by the sol−gel method. The ZnO solution was prepared by dissolving zinc acetate dihydrate (Zn(CH3COO)2·2H2O, 1.0 g) and ethanolamine (0.28 g) in 2-methoxyethanol (10 mL) under vigorous stirring for 24 h in the air. The ZnO-coated ITO glass was annealed on a hot plate for 1 h at 200 °C in air. The thickness of the ZnO layer was approximately 40 nm. The polymer and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM), which were blended in different weight ratio and concentration, were dissolved in CB and o-xylene with different amount (vol %) of 1,8-diiodooctane (DIO) and p-anisaldehyde (AA). Then, the polymer/PC71BM solution was spin-coated on top of the ZnO layer and dried for 90 min at room temperature. Finally, an anode electrode composed of a MoO3 layer (10 nm) and an Ag layer (100 nm) was deposited by thermal evaporation with the shadow mask in a high vacuum thermal evaporator (
