Article pubs.acs.org/Macromolecules
Facile Enhancement of Open-Circuit Voltage in P3HT Analogues via Incorporation of Hexyl Thiophene-3-carboxylate Sangtaik Noh, Nemal S. Gobalasingham, and Barry C. Thompson* Department of Chemistry and Loker Hydrocarbon Research Institute, University of Southern California, Los Angeles, California 90089-1661, United States S Supporting Information *
ABSTRACT: Two families of copolymers featuring varying percentages of hexyl thiophene-3-carboxylate, herein referred to as 3-hexylesterthiophene (3HET), were synthesized via Stille polymerization to generate a series of random poly(3hexylthiophene-co-3-hexylesterthiophene) (P3HT-co-3HET) copolymers as well as a series of semi-random poly(3hexylthiophenethiophene3-hexylesterthiophene− diketopyrrolopyrrole) (P3HTT-DPP-HET) copolymers. Through the introduction of the strongly withdrawing 3HET component, HOMO energy levels can be tuned, and the opencircuit voltage of the resulting polymer solar cells can be significantly enhanced compared to their parent structures, regioregular P3HT and semi-random P3HTT-DPP. We demonstrate that 50% HET content in random copolymers (P3HT50-co-3HET50) outperforms reference P3HT solar cells. In addition to solar cell performance, the influence of 3HET content on copolymers is evaluated by UV−vis absorption, electrochemical HOMO determination, GIXRD crystallinity measurements, DSC, and SCLC hole mobilities. Favorable properties of the parent structures are preserved, including semicrystallinity, high hole mobilities, and absorption coefficients as well as reproducibly good yields and molecular weights. Copolymers featuring 3HET content showed comparable short-circuit currents with parent copolymers up to 50% 3HET loading and further demonstrate the ability to tune electronic properties with the incorporation of strategic components in semi-random and random copolymers.
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steps, which will increase the cost of production.14 D/A copolymers also often require additives for optimal performance,15 which can hasten degradation and are detrimental to solar cell stability.16 Furthermore, they sometimes require high fullerene ratios (even as high as 1:4) for working devices, which inevitably limit polymer light absorption in active layers with confined thicknesses.4 Despite the increasing complexity of structural design in D/A polymers, the thiophene unit has steadfastly remained the signature motif of conjugated polymers.4,17,18 Indeed, the most studied conjugated polymer, regioregular poly(3-hexylthiophene) (rr-P3HT), overcomes many of these drawbacks typically associated with D/A copolymers. It is one of the easiest and cheapest conjugated polymers to synthesize (even more so now with the emergence of direct arylation polymerization (DArP)19−24) and is compatible with roll-toroll processing.25 Furthermore, it is semicrystalline and exhibits excellent morphology with PC61BM at favorable mixing ratios (P3HT:PC61BM; 1:0.8),26 without the need for additives or complex device architectures. This combined with its high peak absorption coefficient, high hole mobility, and impressive charge transfer rate with PC61BM27 makes P3HT highly
INTRODUCTION The past 15 years have witnessed the emergence of numerous conjugated polymers in pursuit of high performance materials for organic photovoltaics,1 field-effect transistors,2 and electrochromic devices.3 Specifically, the promise of organic photovoltaics as a flexible, lightweight, low-cost, and easily processed technology has motivated the steady improvement of these materials.4,5 Recently, polymer-based single-junction bulkheterojunction (BHJ) solar cells have regularly exhibited power conversion efficiencies (PCEs) of 9−10%.6−9 The PCE of an organic solar cell is defined by η = (JscVocFF)/Pin where Jsc is the short-circuit current density,10 Voc is the opencircuit voltage,11 FF is the fill factor,12 and Pin is the solar input power. Efforts to improve the efficiency typically include reducing the electronic band gap of the polymer for increased Jsc or lowering the polymer HOMO for increased Voc (or some combination of both), since the Voc correlates to the HOMODONOR−LUMOACCEPTOR offset.13 As a result, much research has focused on perfectly alternating donor−acceptor (D/A) copolymers, which allow for very finely tuned electronpoor moieties and electron-rich moieties to achieve precise control over the HOMO and LUMO and thus Voc. While D/A copolymers are a viable strategy, the pursuit of such finely controlled structures has led to a number of drawbacks. Achieving each monomer often requires numerous synthetic © XXXX American Chemical Society
Received: June 2, 2016 Revised: August 22, 2016
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DOI: 10.1021/acs.macromol.6b01178 Macromolecules XXXX, XXX, XXX−XXX
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Scheme 1. Synthesis of Monomer 2-Bromo-5-trimethyltin-3-hexylestertyhiophene (3) (a), Stille Polymerization for Poly(3hexylthiophene-co-3-hexylesterthiophene) Random Copolymers (b), and Stille Polymerization for Poly(3-hexylthiophene− thiophenediketopyrrolopyrrole3-hexylesterthiophene) Semi-random Copolymers (c)
of side chains has been acknowledged as a critically important design strategy for controlling the electronic properties of polymers.32,35,42−44 In addition to their ability to influence crystallinity, lamellar stacking distance, glass transition and melting temperatures, and miscibility with fullerenes, side chains can also modify the HOMO of the polymer.42 For example, we reported that the incorporation of 2-ethylhexyl side chains into rr-P3HT could lower the HOMO of the resulting polymer but maintain its Eg.32 Similarly, incorporation of 3-cyanothiophene into rr-P3HT successfully lowered the HOMO without sacrificing important properties of rrP3HT,45,46 though the percentage of incorporation was limited to 20% due to reduced solubility. We also showed polymer properties could be finely tuned by incorporating 3-hexyloxythiophene units in semi-random polymers47 and demonstrated the ability to fine-tune the surface energy of rr-P3HT through side-chain engineering without significantly modifying optoelectronic properties.48 Recently, it has been shown that an unsymmetrical arrangement of alkyl chains in polymers that feature 75% thiophene content can enable single-junction polymer solar cells with efficiencies greater than 10% without processing additives.9 Shrewd design strategies, straightforward syntheses, scalability, and stability are critical to the viability of polymer solar cells as a successful commercial technology. A goal of the present work is to expand the toolkit of side chains for P3HT analogues, as side chains are a modular and facile route to elicit specific HOMOs in polymers. The alkyl ester functional group has the advantage of being an electronwithdrawing group, lowering the HOMO in the same vein as cyano- and fluoroalkyl side chains, but with better solubility in common organic solvents. Here we report the synthesis of 3substituted thiophene-based random and semi-random copolymers containing hexyl thiophene-3-carboxylate, or 3-hexylesterthiophene (3HET), which is electron withdrawing but can be functionalized with a long alkyl chain for sustained solubility even at high monomer incorporation. Polymers containing alkyl ester thiophene units have been reported previously, which have higher Voc than P3HT in solar cells, but the reported polymers were perfectly alternating due to synthetic challenges of regiospecific monomers.49−51 The most notable being the work from Pomerantz et al., who prepared low molecular
attractive; however, these traits do not overshadow its most significant shortcoming. Because of unfavorable positioning of the frontier energy levels, as well as its wide band gap (Eg), P3HT’s maximum Jsc and Voc in fullerene blends are restricted and cap the achievable efficiency of P3HT solar cells.28 To combat these drawbacks, our group has championed a new polymer architecture based on rr-P3HT, semi-random rrP3HT analogues.29−34 These copolymers feature small amounts (typically 5−15%) of an acceptor monomer that is dispersed in the polymer chain via Stille polycondensation. Because of the rational utilization of functional groups, these polymers exhibit restricted linkage patterns, where discrete acceptor units are distributed along a rr-P3HT backbone. As a result, the absorption is broadened, but importantly, the most favorable features of rr-P3HT are preserved (high hole mobility, semicrystallinity, and miscibility with fullerenes).30 This polymer approach, which calls for the strategic implementation of simple monomers instead of a more synthetically focused monomer approach, is most compatible with the realization of OPVs as a easily deployable and costeffective complement to traditional inorganic solar cells.4 Although a considerable array of narrow band gap polymers have been explored (in order to maximize harvesting of the solar spectrum for higher Jsc values), there is still a need for efficient wide band gap polymers with a low-lying HOMO for generating higher Voc values. This strategy can also enable higher efficiencies but also generates materials that are compatible with transparent devices, bottom cells in tandem OPV architecture, and as a component in ternary blends. Until fairly recently, when it came to the design of conjugated polymers, it was widely believed that the backbone primarily determines the electronic properties. Attention to the side chains, which were thought to be necessary primarily for improved solubility and processability, was limited.35,36 Regardless, it was acknowledged that side chains could play an important role in morphology, the most obvious example being the distinct differences between rr-P3HT and regiorandom P3HT, the latter being amorphous and inefficient in solar cells.37,38 The prevelance of alkoxy chains and their electronic influence also hinted at the greater role side chains could play in electronic properties.36,39−41 Steadily, the pronounced influence B
DOI: 10.1021/acs.macromol.6b01178 Macromolecules XXXX, XXX, XXX−XXX
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10 min, N,N′-dicyclohexylcarbodiimide (DCC) (2.737 g, 13.27 mmol, 1.1 equiv) was added. The mixture was allowed to stir at room temperature for 2 days. Precipitated urea was filtered off, and the filtrate was subjected to flash column chromatography eluted with 1:1 hexanes:DCM. After vacuum distillation, the product was obtained as a colorless liquid (2.21 g, 10.4 mmol, 86.4%). 1H NMR (400 MHz, CDCl3): δ 8.10 (dd, 1H), 7.53 (dd, 1H), 7.30 (dd, 1H), 4.27 (t, 2H), 1.74 (m, 2H), 1.42 (m, 2H), 1.34 (m, 4H), 0.90 (t, 3H). 13C NMR (150 MHz, CDCl3): δ 163.0 (CO), 134.1, 132.5, 128.0, 126.0, 64.94 (C−O), 31.6, 28.8, 25.8, 22.7, 14.1. 2-Bromo-3-hexylesterthiophene (2). Similar to previous literature.60 Two 3-neck flasks were flame-dried under vacuum and backfilled with dry N2. Lithium diisopropylamine (LDA) was prepared freshly in flask 1 by the following steps. Diisopropylamine (DIA) (1.71 mL, 12.203 mmol 1.2 equiv) was added and dissolved in 12 mL of THF, and the solution was cooled down to −78 °C before adding nbutyllithium (7 mL, 11.19 mmol, 1.1 equiv) dropwise. After 5 min, the mixture was heated to 0 °C for 20 min and cooled down backed to −78 °C. In flask 2, 3-hexylesterthiophene (2.159 g, 10.17 mmol) was dissolved in 6 mL of THF and cooled down to −78 °C. LDA was transferred from flask 1 to flask 2 via cannula transfer and reacted for about 2 h at −78 °C. Carbon tetrabromide (CBr4) (3.541 g, 10.68 mmol, 1.05 equiv) dissolved in 5 mL of THF was added rapidly to the reaction mixture. After about an hour, the reaction mixture was heated up to room temperature and stirred overnight. The solvent was evaporated under reduced pressure, and the residue was purified by column chromatography with 1:1 hexanes/DCM and vacuum distilled to afford the product as a colorless liquid (1.367 g, 4.69 mmol, 46.2%). 1 H NMR (400 MHz, CDCl3): δ 7.37 (d, 1H), 7.22 (d, 1H), 4.28 (t, 2H), 1.75 (m, 2H), 1.44 (m, 2H), 1.34 (m, 4H), 0.90 (t, 3H). 13C NMR (150 MHz, CDCl3): δ 162.2 (CO), 131.5, 129.6, 125.9, 119.7, 65.3 (C−O), 31.6, 28.7, 25.9, 22.7, 14.2. 2-Bromo-5-trimethyltin-3-hexylesterthiophene (3). To a flamedried 3-neck flask, 2-bromo-3-hexylesterthiophene (1.367 g, 4.696 mmol) was dissolved in 2.8 mL of THF and cooled down to −78 °C. A 0.61 M solution of 2,2,6,6-tetramethylpiperidinylmagnesium chloride−lithium chloride complex (TMPMgCl·LiCl) in THF (9.24 mL, 5.635 mmol, 1.2 equiv) was added dropwise under N2. The mixture was kept at −78 °C for 3 h before a 1.0 M solution of trimethyltin chloride in hexane (5.635 mL, 5.635 mmol, 1.20 equiv) was added slowly. The mixture was allowed to warm up to room temperature and stirred overnight. After extraction with diethyl ether and water, the organic layer was dried over MgSO4. The solvent was evaporated under reduced pressure, the mixture was subjected to column chromatography with 1:1 hexanes/DCM, and the product was obtained with a trace amount of starting material. The product mixture was placed under high vacuum and heated at 60−70 °C overnight to remove starting material impurity. Purified product was achieved as a yellowish oil (0.783.1 g, 1.725 mmol, 36.7%). 1H NMR (400 MHz, CDCl3): δ 7.41 (s, 1H), 4.28 (t, 2H), 1.76 (m, 2H), 1.45 (m, 2H), 1.34 (m, 4H), 0.90 (t, 3H), 0.39 (s, 9H). 13C NMR (150 MHz, CDCl3): δ 162.6 (CO), 139.7, 137.3, 132.6, 124.1, 65.2 (C−O), 31.6, 28.8, 25.9, 22.7, 14.2, −7.99 (Sn−C). ESI-MS: C17H31BrO2SSn (M = 453.9624) m/z 454.9693 [M + H]+. General Procedures for Stille Polymerization. All monomers were dissolved in dry DMF to afford a 0.04 M solution. The solution was then degassed by purging with N2 for 15 min before 0.04 equiv of Pd(PPh3)4 (relative to the total moles of all comonomers) was added in one portion. The solution was degassed for an additional 15 min and then allowed to stir at 95 °C for 48 h. Then the reaction mixture was cooled briefly and precipitated into methanol. Purification was achieved through Soxhlet extractions with a sequence of solvents (methanol, hexane, and chloroform). The last fraction was concentrated under reduced pressure, precipitated in methanol, vacuum filtered, and then dried overnight under high vacuum. P3HT75-co-3HET25. Soxhlet extracted with methanol, hexanes, and finally chloroform. Yield 56%. 1H NMR (400 MHz, CDCl3): δ 7.80 (m, 0.01H), 7.49 (m, 0.13H), 7.36 (m, 0.13H), 6.98 (m, 0.27H), 4.30 (t, 0.25H), 2.80 (t, 0.78H), 1.71 (m, 1.11H), 1.44, 1.34 (m, 3.34H), 0.91 (m, 1.53H).
weight (6 kDa) poly(3-alkyl ester thiophenes) via Ullman and Kumada methods.52−55 Recently, Noonan et al. reported high molecular weight ester-functionalized polythiophene via a controlled Suzuki catalyst-transfer polycondensation (CTP) using nickel precatalysts.56 We achieve highly regioregular poly(3-hexylesterthiophene) (P3HET) homopolymers with good yields, molecular weight, and solubility via the preparation of the P3HET Stille monomer, 2-bromo-5-trimethyltin-3hexylesterthiophene, which was used as a precursor for Stille polymerizations in random and semi-random copolymers as well (Scheme 1).
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EXPERIMENTAL SECTION
Materials and Methods. The monomers 2-bromo-5-trimethyltin3-hexylthiophene, 2,5-bis(trimethyltin)thiophene, and 2,5-diethylhexyl-3,6-bis(5-bromothiophene-2-yl)pyrrolo[3,4-c]-pyrrole-1,4-dione were synthesized following published procedures.29,30 All reagents from commercial sources were used as received unless otherwise noted. Solvents were purchased from VWR and used without further purification except for THF, which was dried over sodium/ benzophenone before being distilled. All reactions were performed under dry N2 in glassware that was predried in oven, unless otherwise noted. Flash chromatography was performed on a Teledyne CombiFlash Rf instrument with RediSep Rf normal phase disposable columns. 1H NMR spectra were recorded in CDCl3 on a Varian Mercury 400 NMR spectrometer. Accurate mass spectra were obtained on a Waters Synapt G2-Si ESI MS. Number-average molecular weight (Mn) and polydispersity (PDI) were determined by size exclusion chromatography (SEC) using a Viscotek GPC Max VE 2001 separation module and a Viscotek Model 2501 UV detector, with 60 °C HPLC grade 1,2-dichlorobenzene (oDCB) as eluent at a flow rate of 0.6 mL/min on one LT6000L Mixed High Org column (Viscotek). The instrument was calibrated vs polystyrene standards (1050−3 800 000 g/mol), and data were analyzed using OmniSec 4.6.0 software. Polymer samples for SEC measurements were prepared by dissolving a polymer in HPLC grade o-DCB at a concentration of 0.5 mg/mL at 40 °C and then allowed to cool to room temperature prior to filtering through a 0.2 μm PTFE filter. Cyclic voltammetry (CV) was performed on Princeton Applied Research VersaStat3 potentiostat under the control of VersaStudio Software. A standard three-electrode cell based on a Pt wire working electrode, a silver wire pseudo-reference electrode (calibrated vs Fc/ Fc+ which is taken as 5.1 eV vs vacuum),57,58 and a Pt wire counter electrode was purged with nitrogen and maintained under a nitrogen atmosphere during all measurements. Polymer films were made by drop-casting an o-DCB solution of polymer (10 mg/mL) and tetrabutylammonium hexafluorophosphate (TBAPF6) (30 mg/mL) directly onto the Pt wire and dried under nitrogen prior to measurement. Acetonitrile was distilled over CaH2 prior to use, and TBAPF6 (0.1 M) was used as the supporting electrolyte. UV−vis absorption spectra were obtained on a PerkinElmer Lambda 950 spectrophotometer. For thin film measurements, polymers were spin-coated onto precleaned glass slides from 7 mg/ mL o-DCB solutions. Thicknesses of the samples and grazing incidence X-ray diffraction (GIXRD) measurements were obtained using a Rigaku diffractometer Ultima IV using a Cu Kα radiation source (λ = 1.54 Å) in the reflectivity and grazing incidence X-ray diffraction mode, respectively. DSC profiles were recorded on a PerkinElmer DSC 8000 under N2 with a scan rate of 10 °C/min. Sample size was about 5 mg, and polymers were used as obtained after Soxhlet extraction. The second cycle is provided in the Supporting Information. Synthetic Procedures. 3-Hexylesterthiophene (1). Modified from the literature.59 To a solution of thiophene-3-carboxylic acid (1.545 g, 12.06 mmol) in 36 mL of dichloromethane (DCM) were added 4-(dimethylamino)pyridine (DMAP) (515.7 mg, 4.22 mmol, 0.35 equiv) and 1-hexanol (2.464 g, 24.12 mmol, 2 equiv). After about C
DOI: 10.1021/acs.macromol.6b01178 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules P3HT50-co-3HET50. Soxhlet extracted with methanol, hexanes, and finally chloroform. Yield 59%. 1H NMR (400 MHz, CDCl3): δ 7.82 (m, 0.10H), 7.53 (m, 0.17H), 7.37 (m, 0.17H), 7.04 (m, 0.06H), 4.30 (t, 0.50H), 2.80 (t, 0.43H), 1.72, 1.57 (m, 1.46H), 1.40, 1.33 (m, 3.03H), 0.89 (m, 1.38H). P3HT25-co-3HET75. Soxhlet extracted with methanol, hexanes, and finally chloroform. Yield 50%. 1H NMR (400 MHz, CDCl3): δ 7.86 (m, 0.23H), 7.56 (m, 0.09H), 7.41 (m, 0.09H), 7.00 (m, 0.01H), 4.30 (t, 0.75H), 2.80 (t, 0.18H), 1.74, 1.57 (m, 1.21H), 1.33 (m, 2.09H), 0.89 (m, 1.36H). P3HET. Soxhlet extracted with methanol, hexanes, and finally chloroform. Yield 69%. 1H NMR (400 MHz, CDCl3): δ 7.86 (s, 0.42H), 4.30 (t, 1H), 1.75 (m, 1.02H), 1.39 (m,0.97H), 1.32 (m, 1.96H), 0.89 (m, 1.41H). P3HTT-DPP-HET10%. Soxhlet extracted with methanol, hexanes, and finally chloroform. Yield 98%. 1H NMR (400 MHz, CDCl3): δ 8.92 (m, 0.13H), 7.48 (m, 0.07H), 7.35 (m, 0.07H), 7.12 (m, 0.18H), 6.99 (m, 0.29H), 4.29 (t, 0.12H), 4.05 (m, 0.21H), 2.79 (t, 0.90H), 1.93 (m, 0.14H), 1.70 (m, 1.17H), 1.34 (m, 4.72H), 0.90 (m, 2.50H). P3HTT-DPP-HET20%. Soxhlet extracted with methanol, hexanes, and finally chloroform. Yield 95%. 1H NMR (400 MHz, CDCl3): δ 8.93 (m, 0.12H), 7.80 (m, 0.03H), 7.52 (m, 0.13H), 7.37 (m, 0.12H), 7.15 (m, 0.22H), 7.00 (m, 0.20H), 4.30 (t, 0.25H), 4.06 (m, 0.23H), 2.80 (t, 0.73H), 1.94 (m, 0.14H), 1.71 (m, 1.38H), 1.35 (m, 4.69H), 0.91 (m, 2.41H). P3HTT-DPP-HET40%. Soxhlet extracted with methanol, hexanes, and finally chloroform. Yield 91%. 1H NMR (400 MHz, CDCl3): δ 8.93 (m, 0.11H), 7.82 (m, 0.08H), 7.52 (m, 0.16H), 7.40 (m, 0.16H), 7.16 (m, 0.12H), 4.30 (t, 0.50H), 4.06 (m, 0.22H), 2.80 (t, 0.49H), 1.94 (m, 0.22H), 1.73 (m, 1.23H), 1.34 (m, 4.44H), 0.91 (m, 2.23H). P3HETT-DPP. Soxhlet extracted with methanol, hexanes, and finally chloroform. Yield 93%. 1H NMR (400 MHz, CDCl3): δ 8.92 (m, 0.13H), 7.86 (m, 0.29H), 7.68 (m, 0.07H), 7.52 (m, 0.12H), 7.36 (m, 0.11H), 4.30 (t, 1H), 4.06 (m, 0.24H), 1.92 (m, 0.20H), 1.74 (m, 1.07H), 1.32 (m, 4.55H), 0.89 (m, 2.42H). Device Fabrication and Characterization. All steps of device fabrication and characterization were performed in air. ITO-coated glass substrates (10 Ω/sq, Thin Film Devices Inc.) were sequentially cleaned by sonication in detergent, deionized water, tetrachloroethylene, acetone, and isopropyl alcohol and dried under a nitrogen stream. PEDOT:PSS (Clevios PH 500, filtered with a 0.45 μm PVDF syringe filter, Pall Life Sciences) was spin-coated on the precleaned ITO-coated glass substrates and then annealed at 120 °C for 60 min under vacuum to generate a 40 nm thick film. Separate solutions of the polymers and PC61BM were prepared in o-DCB. The solutions were stirred for 6 h before they were mixed at the desired ratios and stirred overnight to form a homogeneous solution. Subsequently, the polymer:PC61BM active layer was spin-coated (filtered with a 0.45 μm PVDF syringe filter, Pall Life Sciences) on top of the PEDOT:PSS layer. The optimized ratio for random copolymers:PC61BM were 1:0.8 except 1:0.9 for P3HT50-co-3HET50. For semi-random copolymers with DPP:PC61BM were all 1:1. The concentrations of the blends were 10 mg/mL in polymer (except 11 mg/mL for P3HTT-DPP, P3HTTDPP-3HET10%, and P3HETT-DPP and 12 mg/mL for P3HTT-DPP3HET20%). For optimized conditions, devices of all the polymers were kept in a nitrogen box for 20 min after spin-coating and then placed in the vacuum chamber for aluminum deposition. The substrates were pumped down to high vacuum (
