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Diketopyrrolopyrrole-based conjugated polymers containing alkyl and aryl side-chains for bulk heterojunction solar cells
Synthetic Metals 203 (2015) 221–227
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Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Diketopyrrolopyrrole-based conjugated polymers containing alkyl and aryl side-chains for bulk heterojunction solar cells Su Jung Park a , Ji Eun Jung a , Min Kwan Kang a , Yang Ho Na a , Hyun Hoon Song a , Jae Wook Kang b , Nam Seob Baek c, * , Tae-Dong Kim a, * a b c
Department of Advanced Materials, Hannam University, Daejeon 305-811, Republic of Korea Department of Flexible and Printable Electronics, Polymer Materials Fusion Research Center, Chonbuk National University, Jeonju 561-756, Republic of Korea Chem Optics Inc., Daejeon 305-510, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Article history: Received 13 November 2014 Received in revised form 23 February 2015 Accepted 24 February 2015 Available online 6 March 2015
A series of diketopyrrolopyrrole (DPP)-based conjugated polymers were synthesized with varied side groups such as alkyl and aryl side-chains on a DPP backbone. Their thermal, optical, and photovoltaic properties were investigated. After adding 2% of 1,8-diiodooctane (DIO) as a processing additive in active layers for the bulk heterojunction device, we could achieve the highest PCE of 2.02% with Voc = 0.68 V, Jsc = 5.31 mA cm2, and FF = 56.0% for PDPPT-s1:[70]PCBM (1:2). The device for PDPPT-as designed with asymmetrical side groups on the DPP units also showed good solar cell performance comparable with PDPPT-s1. These results suggest that the DIO additives can control side-chain interference on DPP units. ã 2015 Elsevier B.V. All rights reserved.
Keywords: DPP conjugated polymers Side-chain engineering Bulk heterojunction solar cells
1. Introduction Polymer-based organic photovoltaics (OPVs) have generated strong interest in the field of renewable energy because of their potential applications in low-cost, flexible, and light-weight devices [1–5]. In particular, OPVs based on a mixture of electron-rich conjugated polymers and electron-deficient fullerenes, commonly referred to bulk heterojunction (BHJ), have recently demonstrated power conversion efficiency (PCE) exceeding 10% [6,7]. Large number of novel conjugated polymers have been reported to enhance the PCE through utilizing a donor– acceptor (D–A) molecular design resulted in low band gap polymers with tunable energy levels. Among the various D–A conjugated polymers, diketopyrrolopyrrole (DPP)-based polymers are the one of most promising class for OPVs as well as organic field-effect transistors (OFETs) [8–11]. For an example, Jung et al. has coupled an electron donor unit, dithieno[3,2-b:20 ,30 -d]thiophene, with a DPP unit and obtained a conjugated donor polymer having low energy band gap of 1.22 eV with a broad absorption band [12]. They have achieved a PCE of 6.05% with hole mobility of 0.60 cm2 V1 s1 in OFETs contributed from efficient p–p stacking. Dou et al. has demonstrated highly efficient inverted tandem solar
* Corresponding authors. E-mail addresses: [email protected] (N.S. Baek), [email protected] (T.-D. Kim). http://dx.doi.org/10.1016/j.synthmet.2015.02.035 0379-6779/ ã 2015 Elsevier B.V. All rights reserved.
cells based on benzo[1,2-b;4,5-b]dithiophene-containing DPP polymers and achieved a certified PCE of 8.62% with a short circuit current (Jsc) of 8.26 mA cm2, an open circuit voltage (Voc) of 1.56 V, and a fill factor (FF) of 66.8% [13]. Many of the highest performing polymers including DPP-based polymers have branched alkyl groups as side-chains on the conjugated backbones [14,15]. While conjugated backbones determine the optoelectronic properties of the resulting polymers, side-chain substituents can not only provide solution processability and also enhance the charge mobilities of the OFETs and PCEs in OPVs [16–18]. Therefore selecting the side-chains is as important as selecting the conjugated backbones when designing conjugated polymers. Very recently Mei and Bao have reviewed side-chain engineering in solution processable conjugated polymers and advocated expanding the role of side-chains in the polymers during the development of high performance conjugated polymers for OPVs and OFETs [19]. In this paper, we describe the synthesis and properties of new DPP-based conjugated polymers alternating with thieno[3,2-b] thiophene units. The polymers contain varied side groups such as a branched alkyl side-chain and di-tert-butylbenzyl side-chain. Incorporation of di-tert-butylbenzyl groups as bulky pendent groups can provide good solubility due to decreased packing and crystallinity. In addition, an asymmetrically designed DPP derivative has synthesized and polymerized. These structural modifications have led to new DPP-based conjugated polymers with
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improved solubility and solution processability and other desirable properties. We also discuss in terms of the fundamental information on symmetric and asymmetric substituents/property relationships. 2. Experimental details 2.1. Materials 3,6-Di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (DPP) [20], 9-(bromomethyl)nonadecane [21], and 2,5-bis(trimethylstannyl)thieno[3,2-b]thiophene [22] were synthesized according to the literatures. All other chemicals were purchased from commercial sources and solvents were purified by distillation prior to use. All reaction containers were flame dried under vacuum before use. 2.2. 2,5-Bis(2-octyldodecyl)-3,6-di(thiophen-2-yl)pyrrolo[3,4-c] pyrrole-1,4(2H,5H)-dione (DPP-s1) A solution of 9-(bromomethyl)nonadecane (3.0 g, 8.32 mmol) in DMF (5 mL) was added dropwise to a mixture of DPP (1.0 g, 3.33 mmol), K2CO3 (460 mg, 3.33 mmol) and 18-crown-6 (30 mg, 0.11 mmol) in DMF (25 mL) at 120 C and then the mixture was maintained at 120 C overnight. After cooling to room temperature and filtering, the product was dissolved in CH2Cl2, washed with water, brine, and then dried over MgSO4. The solvent was evaporated under reduced pressure and the crude product was purified by flash chromatography (silica gel, hexane/CH2Cl2 = 1:2) to yield a dark pink solid, DPP-s1 (2.3 g, 80%). 1H NMR (CDCl3, ppm): d 8.91 (d, J = 4.8 Hz, 2H), 7.61 (d, J = 5.2 Hz, 2H), 7.27 (dd, J1 = 3.8 Hz, J2 = 4.9 Hz, 2H), 3.98 (d, J = 7.6 Hz, 4H), 1.95–1.85 (m, 2H), 1.37–1.11 (m, 64H), 0.89–0.78 (m, 12H). 13C NMR (CDCl3, ppm) d 162.0, 140.8, 135.5, 130.5, 130.1, 128.4, 108.3, 46.5, 38.1, 32.0, 31.9, 31.6, 31.5, 31.3, 30.1, 29.8, 29.6, 29.4, 29.2, 26.3, 22.8, 14.3. 2.3. 2,5-Bis(3,5-di-tert-butylbenzyl)-3,6-di(thiophen-2-yl)pyrrolo [3,4-c]pyrrole-1,4(2H,5H)-dione (DPP-s2) It was synthesized with a similar procedure as described for the compound DPP-s1 to afford DPP-s2 (yield: 85%) by using 1(bromomethyl)-3,5-di-tert-butylbenzene instead of 9-(bromomethyl)nonadecane. 1H NMR (CDCl3, ppm): d 8.58 (d, J = 4.8 Hz, 2H), 7.53 (d, J = 5.2 Hz, 2H), 7.28 (s, 4H), 7.19 (dd, J1 = 3.8 Hz, J2 = 4.9 Hz, 2H), 7.05 (s, 2H), 5.29 (s, 4H), 1.23 (s, 36H). 13C NMR (CDCl3, ppm) d 161.5, 143.9, 140.0, 138.7, 135.2, 130.4, 130.0, 128.3, 121.6, 120.1, 108.1, 45.1, 34.2, 31.1.
J = 5.2 Hz, 1H), 7.28 (s, 2H), 7.21 (dd, J1 = 3.8 Hz, J2 = 4.9 Hz, 2H), 7.06 (s, 1H), 5.25 (s, 2H), 4.03 (d, J = 7.6 Hz, 2H), 1.95–1.85 (m, 1H), 1.37– 1.11 (m, 50H), 0.89–0.78 (m, 6H). 13C NMR (CDCl3, ppm) d 162.0, 161.4, 143.9, 140.6, 140.0, 138.7, 135.5, 135.3, 130.5, 130.0, 128.4, 121.6, 120.1, 108.3, 108.1, 46.5, 45.1, 38.1, 34.2, 32.0, 31.9, 31.7, 31.6, 31.5, 31.2, 30.1, 29.8, 29.6, 29.4, 29.2, 26.3, 22.8, 14.3. 2.5. 3,6-Bis(5-bromothiophen-2-yl)-2,5-bis(2-octyldodecyl) pyrrolo [3,4-c]pyrrole-1,4(2H,5H)-dione (DPPBr-s1) To the solution of DPP-s1 (1.0 g, 1.16 mmol) in chloroform (100 mL) was added N-bromosuccinimide (0.52 g, 2.90 mmol) in one portion. After stirring at room temperature for overnight, the reaction mixture was poured into cold water and extracted with methylene chloride. The organic phase was washed with brine, dried over MgSO4, and the solvent was evaporated under reduced pressure. The dark-red residue was purified by flash chromatography (silica gel, hexane/CH2Cl2 = 1:1) to yield DPPBr-s1 (1.0 g, 85%). 1 H NMR (CDCl3, ppm): d 8.69 (d, J = 4.8 Hz, 2H), 7.22 (d, J = 4.8 Hz, 2H), 3.99 (d, J = 7.6 Hz, 4H), 1.95–1.85 (m, 2H), 1.37–1.11 (m, 64H), 0.89–0.78 (m, 12H). 13C NMR (CDCl3, ppm) d 161.1, 139.5, 135.1, 131.0, 130.3, 119.5, 108.2, 46.5, 38.1, 32.0, 31.9, 31.7, 31.6, 31.5, 31.3, 30.1, 29.8, 29.6, 29.4, 29.2, 26.3, 22.8, 14.3. 2.6. 3,6-Bis(5-bromothiophen-2-yl)-2,5-bis(3,5-di-tert-butylbenzyl) pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (DPPBr-s2) It was synthesized with a similar procedure as described for the compound DPPBr-s1 to afford DPPBr-s2 (yield: 83%) by using DPPs2 instead of DPP-s1. 1H NMR (CDCl3, ppm): d 8.35 (d, J = 4.8 Hz, 2H), 7.27 (s, 4H), 7.15 (d, J = 4.8 Hz, 2H), 7.04 (s, 2H), 5.25 (s, 4H), 1.23 (s, 36H). 13C NMR (CDCl3, ppm) d 161.1, 143.6, 139.6, 138.3, 135.0, 134.7, 130.2, 128.3, 121.6, 120.0, 108.1, 45.1, 34.2, 31.1. 2.7. 3,6-Bis(5-bromothiophen-2-yl)-2-(3,5-di-tert-butylbenzyl)-5(2-octyldodecyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (DPPBr-as) It was synthesized with a similar procedure as described for the compound DPPBr-s1 to afford DPPBr-as (yield: 87%) by using DPPas instead of DPP-s1. 1H NMR (CDCl3, ppm): d 8.79 (d, J = 2.1 Hz, 1H), d 8.45 (d, J = 2.1 Hz, 1H), 7.26 (s, 2H), 7.19 (d, J = 4.8 Hz, 2H), 7.05 (s, 1H), 5.25 (s, 2H), 4.03 (d, J = 7.6 Hz, 2H), 1.95-1.85 (m, 1H), 1.37–1.11 (m, 50H), 0.89-0.78 (m, 6H). 13C NMR (CDCl3, ppm) d 162.0, 161.4, 143.9, 140.6, 140.0, 138.7, 135.5, 130.5, 130.0, 128.4, 121.6, 120.1, 108.3, 108.1, 46.5, 45.1, 38.1, 34.2, 32.0, 31.9, 31.7, 31.6, 31.5, 31.2, 30.1, 29.8, 29.6, 29.4, 29.2, 26.3, 22.8, 14.3. 2.8. General synthetic procedure of DPP-based polymers
2.4. 2-(3,5-Di-tert-butylbenzyl)-5-(2-octyldodecyl)-3,6-di(thiophen2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (DPP-as) A solution of 9-(bromomethyl)nonadecane (1.2 g, 3.33 mmol) in DMF (5 mL) was added dropwise to a mixture of DPP (1.0 g, 3.33 mmol), K2CO3 (460 mg, 3.33 mmol) and 18-crown-6 (30 mg, 0.11 mmol) in dimethylformamide (DMF) (25 mL) at room temperature and then the mixture was heated up to 80 C for 4 h. To the reaction mixture was added 1-(bromomethyl)-3,5-di-tert-butylbenzene (1.4 g, 4.9 mmol) and K2CO3 (460 mg, 3.33 mmol) in dry DMF (5 mL) under nitrogen atmosphere. The solution was then heated up to 120 C and kept for overnight. After cooling to room temperature and filtering, the product was dissolved in CH2Cl2, washed with water, brine, and then dried over MgSO4. The solvent was evaporated under reduced pressure and the crude product was purified by flash chromatography (silica gel, hexane/CH2Cl2 = 1:2) to yield DPP-as (0.9 g, 35%). 1H NMR (CDCl3, ppm): d 8.91 (d, J = 4.8 Hz, 1H), d 8.59 (d, J = 4.8 Hz, 1H), 7.64 (d, J = 5.2 Hz, 1H), 7.56 (d,
The mixture of DPPBr-s1, -s2 or -as (1.2 mmol), 2,5-bis (trimethylstannyl)thieno[3,2-b]thiophene (0.56 g, 1.2 mmol), tris (dibenzylideneacetylacetonato)dipalladium (0) (22 mg, 0.024 mmol), and tri(o-tolyl)phosphine (29 mg, 0.095 mmol), were dissolved in 1,2-dichlorobenzene (10 mL). The reaction mixture was stirred at 140 C for 72 h in N2 atmosphere. To perform endcapping the polymers, phenyltrimethyltin (0.051 g, 0.21 mmol) was added first and stirred for 8 h. After that, bromobenzene (0.033 g, 0.21 mmol) was added and stirred again for 8 h. The reaction mixture was cooled to room temperature and the mixture was slowly added into a vigorously stirring mixture of methanol (100 mL) containing concentrated HCl (5 mL). The precipitated polymer was collected, dissolved in chloroform and passed through a short silica gel column. The filtrate was concentrated, re-precipitated in methanol and the polymer was purified by subsequent Soxhlet extraction using acetone, hexane, and chloroform. Chloroform fraction was concentrated, filtered through
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0.45 mm teflon filter, precipitated in methanol to afford DPP-based polymers. 2.8.1. PDPPT-s1 (Yield: 71%). 1H NMR (CDCl3, ppm): d 8.88 (br, 2H), 7.30 (br, 2H), 7.01 (br, 2H), 4.00 (b, 4H),1.98–1.88 (br, 2H),1.40–1.10 (br, 64H), 0.89– 0.78 (br, 12H). Mn = 26,000 g mol1, PDI = 13. Tg = N.A., Td = 414 C. 2.8.2. PDPPT-s2 (Yield: 56%). 1H NMR (CDCl3, ppm): d 8.43 (br, 2H), 7.27 (s, 4H), 7.10 (br, 2H), 7.05 (br, 4H), 5.23 (s, 4H), 1.28–1.20 (s, 36H). Mn = 11,000 g mol1, PDI = 4.6. Tg = 187 C, Td = 323 C. 2.8.3. PDPPT-as (Yield: 69%) 1H NMR (CDCl3, ppm): d 8.89 (br, 1H), 8.44 (br, 1H), 7.29 (br, 2H), 7.26 (s, 2H), 7.05 (br, 3H), 5.18 (s, 2H), 4.01 (b, 2H), 1.98–1.89 (br, 1H), 1.40–1.10 (br, 50H), 0.90–0.78 (br, 6H). Mn = 21,000 g mol1, PDI = 2.1. Tg = 195 C, Td = 356 C. 2.9. Characterization 1
H and 13C NMR spectra (300 MHz) were taken on a Varian 300 spectrometer and UV/vis spectra were obtained on a PerkinElmer spectrophotometer. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were performed on a TA instruments Q50 at a ramping rate of 10 C/min under a nitrogen atmosphere. The molecular weight and polydispersity index (PDI) were analyzed by a Waters 1515 gel permeation chromatograph (GPC) with a refractive index detector at room temperature
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(chlorobenzene as an eluent) using polystyrenes as standards. Wide angle X-ray diffraction was measured by the Confocal MaxFluxTM beam conditioning optics (Osmic Inc.,) attached to a rotating anode generator (Rigaku, RU 300) operated at 50 kV and 80 mA. The wavelength was 1.542 (Cu Ka) and sample-to-detector distance was 22 cm. Imaging plate for the data collection and TyphoonTM FLA7000 laser scanner (General Electric) were used. 2.10. Solar cell device fabrication The indium-tin-oxide (ITO) substrates were cleaned by sonicating with detergents, DI water, acetone, and isopropyl alcohol for 20 min each. Then the ITO substrates were treated with O2 plasma for 10 min. A hole transporting layer of PEDOT:PSS (Baytron P VP Al 4083) was spin-coated at 4000 rpm for 30 s and annealed at 120 C for 1 h under N2. Active layers were deposited on the PEDOT:PSS layer by spin-coating at 2000 rpm for 60 s from 1,2-diclorobenzene solution containing 2.0 wt% of DPP polymers: [70]PCBM (1:2 weight/weight). Annealing of the samples at 110 C for 10 min was done prior to thermal evaporation of LiF (0.8 nm) and Al (100 nm). The dark and illuminated J–V characteristics of the solar cells were tested using a Keithley 2400 source measurement unit and an Oriel white light source under AM 1.5 conditions illuminated at 100 mW cm2. 3. Results and discussion The structure and synthesis of DPP-based polymers including intermediates and monomers are shown in Schemes 1 and 2. The
Scheme 1. Synthesis of the DPP derivatives, DPP-s1,DPP-s2, and DPP-as.
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Scheme 2. Synthesis and structure of the DPP conjugated polymers, PDPPT-s1, PDPPT-s2, and PDPPT-as.
intermediate compounds, DPP-s1 and DPP-s2, were synthesized in an one-step reaction of 3,6-di(thiophen-2-yl)pyrrolo[3,4-c] pyrrole-1,4(2H,5H)-dione (DPP) with two equivalent molar ratios of 9-(bromomethyl)nonadecane or 1-(bromomethyl)-3,5-di-tertbutylbenzene under K2CO3 as a base and a catalytic amount of 18crown-6 in DMF at 120 C. Both compounds can be purified by column chromatography to afford them in >80% yields. In order to synthesize DPP-as, we first need to monoalkylate the DPP with one equivalent molar ratio of 9-(bromomethyl)nonadecane using the same reaction conditions for DPP-s1. The crude product were further reacted with 1-(bromomethyl)-3,5-di-tert-butylbenzene and purified to afford DPP-as in total yields of 35% from the reactant of DPP. These DPP derivatives were then reacted with Nbromosuccimide in chloroform to generate dibromonated monomers, DPPBr-s1, DPPBr-s2, and DPPBr-as. The DPP-based conjugated polymers, PDPPT-s1, PDPPT-s2, and PDPPT-as, were prepared via Stille coupling polymerization between 2,5-bis (trimethylstannyl)thieno[3,2-b]thiophene and dibromonated DPP monomers. The resulting intermediates and polymers were
characterized by 1H and 13C NMR, GPC, DSC, TGA, and UV/vis absorption spectroscopy. The number-average molecular weight (Mn) and PDI of PDPPTs1, PDPPT-s2, and PDPPT-as were shown in Table 1. Among three polymers, PDPPT-s1 possessing branched alkyl chains on the DPP backbone showed highest Mn and PDI. However, PDPPT-s2 showed lowest Mn because the rigid bulky side groups of 3,5-di-tertbutylbenzene units could disturb Stille coupling between two monomers. All polymers have good solubility in halogenated organic solvents such as chloroform, chlorobenzene, 1,2-dichlorobenzene, and etc. The thermal properties of PDPPT-s1, PDPPT-s2, and PDPPT-as were evaluated by DSC and TGA. PDPPT-s1 did not show any thermal transition up to 300 C investigated by DSC. However, PDPPT-s2 and PDPPT-as showed glass transition temperatures of 187 C and 195 C, respectively. Additionally they showed good
Table 1 Physical properties of the DPP-based conjugated polymers. Sample PDPPT-s1 PDPPT-s2 PDPPT-as
Mna (g mol1)
PDIb (Mw/Mn)
labsc
labsd
(nm)
(nm)
Ege (eV)
Tgf ( C)
Tdg ( C)
26,000 11,000 21,000
13 4.6 2.1
782 781 784
782 776 789
1.29 1.30 1.31
– 187 195
414 323 356
a,b
Determined by GPC with polystyrene as a standard and chlorobenzene as eluent; number-average molecular weight (Mn), weight-average molecular weight (Mw), and polydispersity index (PDI = Mw/Mn). c The maximum absorption wavelength measured in chlorobenzene solution. d The maximum absorption wavelength measured in a thin solid film on a glass substrate. e Optical energy band gap determined by the absorption onset of the films. f The glass transition temperature of the polymer measured by DSC. g The initial decomposition temperture of the polymer measured by TGA.
Fig. 1. TGA curves for PDPPT-s1 (solid), PDPPT-s2 (dashed), and PDPPT-as (dashed dot).
S.J. Park et al. / Synthetic Metals 203 (2015) 221–227
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Intensity (a.u.)
PDPPT-s1 PDPPT-s2 PDPPT-as
5
10
15
20
25
2 Fig. 3. X-ray intensity profiles of the PDPPT-s1(solid), PDPPT-s2(dashed), and PDPPT-as(dashed dot) in powder.
Fig. 2. Normalized UV/vis absorption spectra for (a) PDPPT-s1, (b) PDPPT-s2, and (c) PDPPT-as in 1,2-dichlorobenzene solution (solid) and thin solid films (dashed).
thermal stability indicating that the initial decomposition temperatures (5% weight loss, Td) were found to be at 414 C, 323 C, and 356 C for PDPPT-s1, PDPPT-s2, and PDPPT-as, respectively, measured by TGA (Fig. 1). UV/vis absorption spectra of the three polymers in the solution and as thin solid films are shown in Fig. 2. In dilute 1,2dichlorobenzene solution, all polymers exhibited broad absorption bands between 500 and 1000 nm, which arose from intramolecular charge transfer between the thieno[3,2-b]thiophene and DPP units. The absorption maxima of PDPPT-s1, PDPPT-s2, and PDPPTas in the solution were located at 782 nm, 781 nm, and 784 nm, respectively. Relative to the solution absorption, the absorption spectrum of the polymer solid film for PDPPT-s1 was almost unchanged. However the film for PDPPT-s2 having 3,5-di-tert-
butylbenzene units as side groups exhibited a slightly hypsochromic absorption shift (5 nm) compared to the solution in which might be due to the twisting the polymer backbone from the steric hindrance. It is noted that PDPPT-as afforded red-shifted and broaden absorption band of the film relative to that of the solution. This result suggests that the asymmetrical side chains for PDPPTas provide better intermolecular interaction and aggregation in solid state. The optical band gap (Eg), calculated from the absorption edge of the solid state films, were 1.29–1.31 eV. Because these polymers exhibited broad absorption range (500–1000 nm) and low optical band gaps, it is highly applicable to the solar cell devices for improving the degree of light harvesting and enhancing the photocurrent of the devices. Fig. 3 shows the X-ray scattering intensity profiles of the polymer in powder form. We can clearly see two peaks for the (1 0 0) and (2 0 0) diffraction related to a lamellar type packing between the polymer molecules. The d-spacing are 2.1 nm for PDPPT-s1, 1.6 nm for PDPPT-s2, and 1.8 nm for PDPPT-as, respectively. The d-spacing well depends on the length of the side chains for the three polymers. The intensity and sharpness of the peak for (1 0 0) diffraction increases in the order of PDPPT-as, PDPPT-s2, and PDPPT-s1. These results suggest that the packing order might be affected by the symmetry than the rigidity in terms of the chemical structure of three polymers. Photovoltaic properties were measured with the conventional device structures of ITO/PEDOT:PSS/Active layer/LiF/Al. The active layers of the solar cells were made by spin-casting a 1:2 (w/w) mixture of the DPP-based polymers with [70]PCBM and thermally annealed for 10 min at 110 C. The best photovoltaic performance obtained for each polymer is summarized in Table 2. The corresponding J–V curves under simulated AM 1.5G solar
Table 2 Photovoltaic properties of the DPP-based conjugated polymers Jsc (mA cm2)
Voc (V)
FF (%)
PCE (%)
(1:2) (1:2) (1:2) (1:2)
2.25 2.29 2.32 5.31
0.70 0.69 0.69 0.68
52.6 31.7 32.6 56.0
0.83 0.50 0.52 2.02
(1:2)
3.64
0.68
41.9
1.04
(1:2)
5.32
0.68
54.6
1.98
Sample PDPPT-s1:[70]PCBM PDPPT-s2:[70]PCBM PDPPT-as:[70]PCBM PDPPT-s1:[70]PCBM with 2% DIO PDPPT-s2:[70]PCBM with 2% DIO PDPPT-as:[70]PCBM with 2% DIO
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irradiation (100 mW cm2) are depicted in Fig. 4(a). Among the devices without 1,8-diiodooctane (DIO) additives, the best performance was obtained by PDPPT-s1:[70]PCBM with Voc = 0.70 V, Jsc = 2.25 mA cm2, FF of 52.6% and an overall PCE of 0.83%. The devices for PDPPT-s2:[70]PCBM and PDPPT-as:[70]PCBM showed similar PCEs of 0.50%. The device performance can be further improved by adding 2% of DIO by volume as a processing additive. The device of PDPPT-s1:[70]PCBM reached PCE of 2.02% with Voc = 0.68 V, Jsc = 5.32 mA cm2, and FF of 56.0%. However, lower device performance of PDPPT-s2:[70]PCBM with PCE of 1.04% (Voc = 0.68 V, Jsc = 3.64 mA cm2, FF = 41.9%) showed under same processing conditions. Interestingly, the device of PDPPT-as:[70] PCBM with DIO additives showed a PCE of 1.98% with Voc = 0.67 V, Jsc = 5.32 mA cm2, and FF of 54.6%, which is similar device performance with PDPPT-s1:[70]PCBM. Generally, DIO additives are known to provide preventing the formation of large and isolated domains of PCBM and enhancing wettability of blend solution on PEDOT:PSS layers, leading to the formation of uniform active layer. However our preliminary results suggest that the DIO additives not only provide their aforementioned advantages, but also control side-chain interference on DPP units which is more preferable to alkyl chain groups than aryl groups. The higher Jscs for PDPPT-s1:[70]PCBM and PDPPT-as:[70]PCBM are also reflected in the improved external quantum efficiency (EQE) at the absorption
Fig 4. (a) Current density–voltage (J–V) characteristics and (b) EQE spectra of OPV devices fabricated with PDPPT-s1 (square), PDPPT-s2 (triangle), and PDPPT-as (circle) blended with [70]PCBM; Open symbols represent the devices without a DIO additive and solid symbols represent the devices with 2% of DIO additives.
wavelength ranges from 400 to 900 nm (Fig. 4(b)), which allowed the active layer to absorb more available photons and enhance the photocurrent of the device. The morphology and phase separation of the active layers were investigated with tapping-mode atomic force microscopy (AFM). Fig. 5 shows their topographic height images (5 5 mm). The rootmean-square (RMS) roughness of the films was 3.35 nm for PDPPTs1:[70]PCBM, 4.56 nm for PDPPT-s2:[70]PCBM, and 3.89 nm for PDPPT-as:[70]PCBM. A relatively smooth surface was found in the film of PDPPT-s1:[70]PCBM and PDPPT-as:[70]PCBM. However, the PDPPT-s2:[70]PCBM film showed large aggregates (100– 200 nm) and phase separation in the overall surface, which might reduce the exciton generation and charge separation at the
Fig. 5. AFM topographic height images (5 mm 5 mm) of a film for (a) PDPPT-s1: [70]PCBM, (b) PDPPT-s2:[70]PCBM, and (c) PDPPT-as:[70]PCBM with 2% of DIO additives.
S.J. Park et al. / Synthetic Metals 203 (2015) 221–227
conjugated donor molecules/[70]PCBM interface leading to a low photocurrent. These results correspond to decrease of the Jsc values of the solar cell devices. 4. Conclusion AseriesofDPP-basedconjugatedpolymersweresynthesizedwith varied side groups such as branched alkyl side-chain and di-tertbutylbenzyl side-chain on the DPP backbone. Their thermal, optical, andphotovoltaicpropertieswereinvestigated.Afteradding2%ofDIO as a processing additive in active layers for the bulk heterojunction device, we could achieve the highest PCE of 2.02% with Voc = 0.68 V, Jsc = 5.31 mA cm2, and FF = 56.0% for PDPPT-s1:[70]PCBM. The device for PDPPT-as designed with asymmetrical side groups on the DPP units also showed good solar cell performance comparable with PDPPT-s1. These results suggest that the DIO additives can control side-chain interference on DPP units. Acknowledgements This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (2012R1A1A2007079) and Korea Research Council Industrial Science & Technology (SK-0903-01). References [1] G. Yu, J. Gao, J.C. Hummelen, F. Wudl, A.J. Heeger, Science 270 (1995) 1789.
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Contents lists available at ScienceDirect
Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Diketopyrrolopyrrole-based conjugated polymers containing alkyl and aryl side-chains for bulk heterojunction solar cells Su Jung Park a , Ji Eun Jung a , Min Kwan Kang a , Yang Ho Na a , Hyun Hoon Song a , Jae Wook Kang b , Nam Seob Baek c, * , Tae-Dong Kim a, * a b c
Department of Advanced Materials, Hannam University, Daejeon 305-811, Republic of Korea Department of Flexible and Printable Electronics, Polymer Materials Fusion Research Center, Chonbuk National University, Jeonju 561-756, Republic of Korea Chem Optics Inc., Daejeon 305-510, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Article history: Received 13 November 2014 Received in revised form 23 February 2015 Accepted 24 February 2015 Available online 6 March 2015
A series of diketopyrrolopyrrole (DPP)-based conjugated polymers were synthesized with varied side groups such as alkyl and aryl side-chains on a DPP backbone. Their thermal, optical, and photovoltaic properties were investigated. After adding 2% of 1,8-diiodooctane (DIO) as a processing additive in active layers for the bulk heterojunction device, we could achieve the highest PCE of 2.02% with Voc = 0.68 V, Jsc = 5.31 mA cm2, and FF = 56.0% for PDPPT-s1:[70]PCBM (1:2). The device for PDPPT-as designed with asymmetrical side groups on the DPP units also showed good solar cell performance comparable with PDPPT-s1. These results suggest that the DIO additives can control side-chain interference on DPP units. ã 2015 Elsevier B.V. All rights reserved.
Keywords: DPP conjugated polymers Side-chain engineering Bulk heterojunction solar cells
1. Introduction Polymer-based organic photovoltaics (OPVs) have generated strong interest in the field of renewable energy because of their potential applications in low-cost, flexible, and light-weight devices [1–5]. In particular, OPVs based on a mixture of electron-rich conjugated polymers and electron-deficient fullerenes, commonly referred to bulk heterojunction (BHJ), have recently demonstrated power conversion efficiency (PCE) exceeding 10% [6,7]. Large number of novel conjugated polymers have been reported to enhance the PCE through utilizing a donor– acceptor (D–A) molecular design resulted in low band gap polymers with tunable energy levels. Among the various D–A conjugated polymers, diketopyrrolopyrrole (DPP)-based polymers are the one of most promising class for OPVs as well as organic field-effect transistors (OFETs) [8–11]. For an example, Jung et al. has coupled an electron donor unit, dithieno[3,2-b:20 ,30 -d]thiophene, with a DPP unit and obtained a conjugated donor polymer having low energy band gap of 1.22 eV with a broad absorption band [12]. They have achieved a PCE of 6.05% with hole mobility of 0.60 cm2 V1 s1 in OFETs contributed from efficient p–p stacking. Dou et al. has demonstrated highly efficient inverted tandem solar
* Corresponding authors. E-mail addresses: [email protected] (N.S. Baek), [email protected] (T.-D. Kim). http://dx.doi.org/10.1016/j.synthmet.2015.02.035 0379-6779/ ã 2015 Elsevier B.V. All rights reserved.
cells based on benzo[1,2-b;4,5-b]dithiophene-containing DPP polymers and achieved a certified PCE of 8.62% with a short circuit current (Jsc) of 8.26 mA cm2, an open circuit voltage (Voc) of 1.56 V, and a fill factor (FF) of 66.8% [13]. Many of the highest performing polymers including DPP-based polymers have branched alkyl groups as side-chains on the conjugated backbones [14,15]. While conjugated backbones determine the optoelectronic properties of the resulting polymers, side-chain substituents can not only provide solution processability and also enhance the charge mobilities of the OFETs and PCEs in OPVs [16–18]. Therefore selecting the side-chains is as important as selecting the conjugated backbones when designing conjugated polymers. Very recently Mei and Bao have reviewed side-chain engineering in solution processable conjugated polymers and advocated expanding the role of side-chains in the polymers during the development of high performance conjugated polymers for OPVs and OFETs [19]. In this paper, we describe the synthesis and properties of new DPP-based conjugated polymers alternating with thieno[3,2-b] thiophene units. The polymers contain varied side groups such as a branched alkyl side-chain and di-tert-butylbenzyl side-chain. Incorporation of di-tert-butylbenzyl groups as bulky pendent groups can provide good solubility due to decreased packing and crystallinity. In addition, an asymmetrically designed DPP derivative has synthesized and polymerized. These structural modifications have led to new DPP-based conjugated polymers with
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improved solubility and solution processability and other desirable properties. We also discuss in terms of the fundamental information on symmetric and asymmetric substituents/property relationships. 2. Experimental details 2.1. Materials 3,6-Di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (DPP) [20], 9-(bromomethyl)nonadecane [21], and 2,5-bis(trimethylstannyl)thieno[3,2-b]thiophene [22] were synthesized according to the literatures. All other chemicals were purchased from commercial sources and solvents were purified by distillation prior to use. All reaction containers were flame dried under vacuum before use. 2.2. 2,5-Bis(2-octyldodecyl)-3,6-di(thiophen-2-yl)pyrrolo[3,4-c] pyrrole-1,4(2H,5H)-dione (DPP-s1) A solution of 9-(bromomethyl)nonadecane (3.0 g, 8.32 mmol) in DMF (5 mL) was added dropwise to a mixture of DPP (1.0 g, 3.33 mmol), K2CO3 (460 mg, 3.33 mmol) and 18-crown-6 (30 mg, 0.11 mmol) in DMF (25 mL) at 120 C and then the mixture was maintained at 120 C overnight. After cooling to room temperature and filtering, the product was dissolved in CH2Cl2, washed with water, brine, and then dried over MgSO4. The solvent was evaporated under reduced pressure and the crude product was purified by flash chromatography (silica gel, hexane/CH2Cl2 = 1:2) to yield a dark pink solid, DPP-s1 (2.3 g, 80%). 1H NMR (CDCl3, ppm): d 8.91 (d, J = 4.8 Hz, 2H), 7.61 (d, J = 5.2 Hz, 2H), 7.27 (dd, J1 = 3.8 Hz, J2 = 4.9 Hz, 2H), 3.98 (d, J = 7.6 Hz, 4H), 1.95–1.85 (m, 2H), 1.37–1.11 (m, 64H), 0.89–0.78 (m, 12H). 13C NMR (CDCl3, ppm) d 162.0, 140.8, 135.5, 130.5, 130.1, 128.4, 108.3, 46.5, 38.1, 32.0, 31.9, 31.6, 31.5, 31.3, 30.1, 29.8, 29.6, 29.4, 29.2, 26.3, 22.8, 14.3. 2.3. 2,5-Bis(3,5-di-tert-butylbenzyl)-3,6-di(thiophen-2-yl)pyrrolo [3,4-c]pyrrole-1,4(2H,5H)-dione (DPP-s2) It was synthesized with a similar procedure as described for the compound DPP-s1 to afford DPP-s2 (yield: 85%) by using 1(bromomethyl)-3,5-di-tert-butylbenzene instead of 9-(bromomethyl)nonadecane. 1H NMR (CDCl3, ppm): d 8.58 (d, J = 4.8 Hz, 2H), 7.53 (d, J = 5.2 Hz, 2H), 7.28 (s, 4H), 7.19 (dd, J1 = 3.8 Hz, J2 = 4.9 Hz, 2H), 7.05 (s, 2H), 5.29 (s, 4H), 1.23 (s, 36H). 13C NMR (CDCl3, ppm) d 161.5, 143.9, 140.0, 138.7, 135.2, 130.4, 130.0, 128.3, 121.6, 120.1, 108.1, 45.1, 34.2, 31.1.
J = 5.2 Hz, 1H), 7.28 (s, 2H), 7.21 (dd, J1 = 3.8 Hz, J2 = 4.9 Hz, 2H), 7.06 (s, 1H), 5.25 (s, 2H), 4.03 (d, J = 7.6 Hz, 2H), 1.95–1.85 (m, 1H), 1.37– 1.11 (m, 50H), 0.89–0.78 (m, 6H). 13C NMR (CDCl3, ppm) d 162.0, 161.4, 143.9, 140.6, 140.0, 138.7, 135.5, 135.3, 130.5, 130.0, 128.4, 121.6, 120.1, 108.3, 108.1, 46.5, 45.1, 38.1, 34.2, 32.0, 31.9, 31.7, 31.6, 31.5, 31.2, 30.1, 29.8, 29.6, 29.4, 29.2, 26.3, 22.8, 14.3. 2.5. 3,6-Bis(5-bromothiophen-2-yl)-2,5-bis(2-octyldodecyl) pyrrolo [3,4-c]pyrrole-1,4(2H,5H)-dione (DPPBr-s1) To the solution of DPP-s1 (1.0 g, 1.16 mmol) in chloroform (100 mL) was added N-bromosuccinimide (0.52 g, 2.90 mmol) in one portion. After stirring at room temperature for overnight, the reaction mixture was poured into cold water and extracted with methylene chloride. The organic phase was washed with brine, dried over MgSO4, and the solvent was evaporated under reduced pressure. The dark-red residue was purified by flash chromatography (silica gel, hexane/CH2Cl2 = 1:1) to yield DPPBr-s1 (1.0 g, 85%). 1 H NMR (CDCl3, ppm): d 8.69 (d, J = 4.8 Hz, 2H), 7.22 (d, J = 4.8 Hz, 2H), 3.99 (d, J = 7.6 Hz, 4H), 1.95–1.85 (m, 2H), 1.37–1.11 (m, 64H), 0.89–0.78 (m, 12H). 13C NMR (CDCl3, ppm) d 161.1, 139.5, 135.1, 131.0, 130.3, 119.5, 108.2, 46.5, 38.1, 32.0, 31.9, 31.7, 31.6, 31.5, 31.3, 30.1, 29.8, 29.6, 29.4, 29.2, 26.3, 22.8, 14.3. 2.6. 3,6-Bis(5-bromothiophen-2-yl)-2,5-bis(3,5-di-tert-butylbenzyl) pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (DPPBr-s2) It was synthesized with a similar procedure as described for the compound DPPBr-s1 to afford DPPBr-s2 (yield: 83%) by using DPPs2 instead of DPP-s1. 1H NMR (CDCl3, ppm): d 8.35 (d, J = 4.8 Hz, 2H), 7.27 (s, 4H), 7.15 (d, J = 4.8 Hz, 2H), 7.04 (s, 2H), 5.25 (s, 4H), 1.23 (s, 36H). 13C NMR (CDCl3, ppm) d 161.1, 143.6, 139.6, 138.3, 135.0, 134.7, 130.2, 128.3, 121.6, 120.0, 108.1, 45.1, 34.2, 31.1. 2.7. 3,6-Bis(5-bromothiophen-2-yl)-2-(3,5-di-tert-butylbenzyl)-5(2-octyldodecyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (DPPBr-as) It was synthesized with a similar procedure as described for the compound DPPBr-s1 to afford DPPBr-as (yield: 87%) by using DPPas instead of DPP-s1. 1H NMR (CDCl3, ppm): d 8.79 (d, J = 2.1 Hz, 1H), d 8.45 (d, J = 2.1 Hz, 1H), 7.26 (s, 2H), 7.19 (d, J = 4.8 Hz, 2H), 7.05 (s, 1H), 5.25 (s, 2H), 4.03 (d, J = 7.6 Hz, 2H), 1.95-1.85 (m, 1H), 1.37–1.11 (m, 50H), 0.89-0.78 (m, 6H). 13C NMR (CDCl3, ppm) d 162.0, 161.4, 143.9, 140.6, 140.0, 138.7, 135.5, 130.5, 130.0, 128.4, 121.6, 120.1, 108.3, 108.1, 46.5, 45.1, 38.1, 34.2, 32.0, 31.9, 31.7, 31.6, 31.5, 31.2, 30.1, 29.8, 29.6, 29.4, 29.2, 26.3, 22.8, 14.3. 2.8. General synthetic procedure of DPP-based polymers
2.4. 2-(3,5-Di-tert-butylbenzyl)-5-(2-octyldodecyl)-3,6-di(thiophen2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (DPP-as) A solution of 9-(bromomethyl)nonadecane (1.2 g, 3.33 mmol) in DMF (5 mL) was added dropwise to a mixture of DPP (1.0 g, 3.33 mmol), K2CO3 (460 mg, 3.33 mmol) and 18-crown-6 (30 mg, 0.11 mmol) in dimethylformamide (DMF) (25 mL) at room temperature and then the mixture was heated up to 80 C for 4 h. To the reaction mixture was added 1-(bromomethyl)-3,5-di-tert-butylbenzene (1.4 g, 4.9 mmol) and K2CO3 (460 mg, 3.33 mmol) in dry DMF (5 mL) under nitrogen atmosphere. The solution was then heated up to 120 C and kept for overnight. After cooling to room temperature and filtering, the product was dissolved in CH2Cl2, washed with water, brine, and then dried over MgSO4. The solvent was evaporated under reduced pressure and the crude product was purified by flash chromatography (silica gel, hexane/CH2Cl2 = 1:2) to yield DPP-as (0.9 g, 35%). 1H NMR (CDCl3, ppm): d 8.91 (d, J = 4.8 Hz, 1H), d 8.59 (d, J = 4.8 Hz, 1H), 7.64 (d, J = 5.2 Hz, 1H), 7.56 (d,
The mixture of DPPBr-s1, -s2 or -as (1.2 mmol), 2,5-bis (trimethylstannyl)thieno[3,2-b]thiophene (0.56 g, 1.2 mmol), tris (dibenzylideneacetylacetonato)dipalladium (0) (22 mg, 0.024 mmol), and tri(o-tolyl)phosphine (29 mg, 0.095 mmol), were dissolved in 1,2-dichlorobenzene (10 mL). The reaction mixture was stirred at 140 C for 72 h in N2 atmosphere. To perform endcapping the polymers, phenyltrimethyltin (0.051 g, 0.21 mmol) was added first and stirred for 8 h. After that, bromobenzene (0.033 g, 0.21 mmol) was added and stirred again for 8 h. The reaction mixture was cooled to room temperature and the mixture was slowly added into a vigorously stirring mixture of methanol (100 mL) containing concentrated HCl (5 mL). The precipitated polymer was collected, dissolved in chloroform and passed through a short silica gel column. The filtrate was concentrated, re-precipitated in methanol and the polymer was purified by subsequent Soxhlet extraction using acetone, hexane, and chloroform. Chloroform fraction was concentrated, filtered through
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0.45 mm teflon filter, precipitated in methanol to afford DPP-based polymers. 2.8.1. PDPPT-s1 (Yield: 71%). 1H NMR (CDCl3, ppm): d 8.88 (br, 2H), 7.30 (br, 2H), 7.01 (br, 2H), 4.00 (b, 4H),1.98–1.88 (br, 2H),1.40–1.10 (br, 64H), 0.89– 0.78 (br, 12H). Mn = 26,000 g mol1, PDI = 13. Tg = N.A., Td = 414 C. 2.8.2. PDPPT-s2 (Yield: 56%). 1H NMR (CDCl3, ppm): d 8.43 (br, 2H), 7.27 (s, 4H), 7.10 (br, 2H), 7.05 (br, 4H), 5.23 (s, 4H), 1.28–1.20 (s, 36H). Mn = 11,000 g mol1, PDI = 4.6. Tg = 187 C, Td = 323 C. 2.8.3. PDPPT-as (Yield: 69%) 1H NMR (CDCl3, ppm): d 8.89 (br, 1H), 8.44 (br, 1H), 7.29 (br, 2H), 7.26 (s, 2H), 7.05 (br, 3H), 5.18 (s, 2H), 4.01 (b, 2H), 1.98–1.89 (br, 1H), 1.40–1.10 (br, 50H), 0.90–0.78 (br, 6H). Mn = 21,000 g mol1, PDI = 2.1. Tg = 195 C, Td = 356 C. 2.9. Characterization 1
H and 13C NMR spectra (300 MHz) were taken on a Varian 300 spectrometer and UV/vis spectra were obtained on a PerkinElmer spectrophotometer. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were performed on a TA instruments Q50 at a ramping rate of 10 C/min under a nitrogen atmosphere. The molecular weight and polydispersity index (PDI) were analyzed by a Waters 1515 gel permeation chromatograph (GPC) with a refractive index detector at room temperature
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(chlorobenzene as an eluent) using polystyrenes as standards. Wide angle X-ray diffraction was measured by the Confocal MaxFluxTM beam conditioning optics (Osmic Inc.,) attached to a rotating anode generator (Rigaku, RU 300) operated at 50 kV and 80 mA. The wavelength was 1.542 (Cu Ka) and sample-to-detector distance was 22 cm. Imaging plate for the data collection and TyphoonTM FLA7000 laser scanner (General Electric) were used. 2.10. Solar cell device fabrication The indium-tin-oxide (ITO) substrates were cleaned by sonicating with detergents, DI water, acetone, and isopropyl alcohol for 20 min each. Then the ITO substrates were treated with O2 plasma for 10 min. A hole transporting layer of PEDOT:PSS (Baytron P VP Al 4083) was spin-coated at 4000 rpm for 30 s and annealed at 120 C for 1 h under N2. Active layers were deposited on the PEDOT:PSS layer by spin-coating at 2000 rpm for 60 s from 1,2-diclorobenzene solution containing 2.0 wt% of DPP polymers: [70]PCBM (1:2 weight/weight). Annealing of the samples at 110 C for 10 min was done prior to thermal evaporation of LiF (0.8 nm) and Al (100 nm). The dark and illuminated J–V characteristics of the solar cells were tested using a Keithley 2400 source measurement unit and an Oriel white light source under AM 1.5 conditions illuminated at 100 mW cm2. 3. Results and discussion The structure and synthesis of DPP-based polymers including intermediates and monomers are shown in Schemes 1 and 2. The
Scheme 1. Synthesis of the DPP derivatives, DPP-s1,DPP-s2, and DPP-as.
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Scheme 2. Synthesis and structure of the DPP conjugated polymers, PDPPT-s1, PDPPT-s2, and PDPPT-as.
intermediate compounds, DPP-s1 and DPP-s2, were synthesized in an one-step reaction of 3,6-di(thiophen-2-yl)pyrrolo[3,4-c] pyrrole-1,4(2H,5H)-dione (DPP) with two equivalent molar ratios of 9-(bromomethyl)nonadecane or 1-(bromomethyl)-3,5-di-tertbutylbenzene under K2CO3 as a base and a catalytic amount of 18crown-6 in DMF at 120 C. Both compounds can be purified by column chromatography to afford them in >80% yields. In order to synthesize DPP-as, we first need to monoalkylate the DPP with one equivalent molar ratio of 9-(bromomethyl)nonadecane using the same reaction conditions for DPP-s1. The crude product were further reacted with 1-(bromomethyl)-3,5-di-tert-butylbenzene and purified to afford DPP-as in total yields of 35% from the reactant of DPP. These DPP derivatives were then reacted with Nbromosuccimide in chloroform to generate dibromonated monomers, DPPBr-s1, DPPBr-s2, and DPPBr-as. The DPP-based conjugated polymers, PDPPT-s1, PDPPT-s2, and PDPPT-as, were prepared via Stille coupling polymerization between 2,5-bis (trimethylstannyl)thieno[3,2-b]thiophene and dibromonated DPP monomers. The resulting intermediates and polymers were
characterized by 1H and 13C NMR, GPC, DSC, TGA, and UV/vis absorption spectroscopy. The number-average molecular weight (Mn) and PDI of PDPPTs1, PDPPT-s2, and PDPPT-as were shown in Table 1. Among three polymers, PDPPT-s1 possessing branched alkyl chains on the DPP backbone showed highest Mn and PDI. However, PDPPT-s2 showed lowest Mn because the rigid bulky side groups of 3,5-di-tertbutylbenzene units could disturb Stille coupling between two monomers. All polymers have good solubility in halogenated organic solvents such as chloroform, chlorobenzene, 1,2-dichlorobenzene, and etc. The thermal properties of PDPPT-s1, PDPPT-s2, and PDPPT-as were evaluated by DSC and TGA. PDPPT-s1 did not show any thermal transition up to 300 C investigated by DSC. However, PDPPT-s2 and PDPPT-as showed glass transition temperatures of 187 C and 195 C, respectively. Additionally they showed good
Table 1 Physical properties of the DPP-based conjugated polymers. Sample PDPPT-s1 PDPPT-s2 PDPPT-as
Mna (g mol1)
PDIb (Mw/Mn)
labsc
labsd
(nm)
(nm)
Ege (eV)
Tgf ( C)
Tdg ( C)
26,000 11,000 21,000
13 4.6 2.1
782 781 784
782 776 789
1.29 1.30 1.31
– 187 195
414 323 356
a,b
Determined by GPC with polystyrene as a standard and chlorobenzene as eluent; number-average molecular weight (Mn), weight-average molecular weight (Mw), and polydispersity index (PDI = Mw/Mn). c The maximum absorption wavelength measured in chlorobenzene solution. d The maximum absorption wavelength measured in a thin solid film on a glass substrate. e Optical energy band gap determined by the absorption onset of the films. f The glass transition temperature of the polymer measured by DSC. g The initial decomposition temperture of the polymer measured by TGA.
Fig. 1. TGA curves for PDPPT-s1 (solid), PDPPT-s2 (dashed), and PDPPT-as (dashed dot).
S.J. Park et al. / Synthetic Metals 203 (2015) 221–227
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Intensity (a.u.)
PDPPT-s1 PDPPT-s2 PDPPT-as
5
10
15
20
25
2 Fig. 3. X-ray intensity profiles of the PDPPT-s1(solid), PDPPT-s2(dashed), and PDPPT-as(dashed dot) in powder.
Fig. 2. Normalized UV/vis absorption spectra for (a) PDPPT-s1, (b) PDPPT-s2, and (c) PDPPT-as in 1,2-dichlorobenzene solution (solid) and thin solid films (dashed).
thermal stability indicating that the initial decomposition temperatures (5% weight loss, Td) were found to be at 414 C, 323 C, and 356 C for PDPPT-s1, PDPPT-s2, and PDPPT-as, respectively, measured by TGA (Fig. 1). UV/vis absorption spectra of the three polymers in the solution and as thin solid films are shown in Fig. 2. In dilute 1,2dichlorobenzene solution, all polymers exhibited broad absorption bands between 500 and 1000 nm, which arose from intramolecular charge transfer between the thieno[3,2-b]thiophene and DPP units. The absorption maxima of PDPPT-s1, PDPPT-s2, and PDPPTas in the solution were located at 782 nm, 781 nm, and 784 nm, respectively. Relative to the solution absorption, the absorption spectrum of the polymer solid film for PDPPT-s1 was almost unchanged. However the film for PDPPT-s2 having 3,5-di-tert-
butylbenzene units as side groups exhibited a slightly hypsochromic absorption shift (5 nm) compared to the solution in which might be due to the twisting the polymer backbone from the steric hindrance. It is noted that PDPPT-as afforded red-shifted and broaden absorption band of the film relative to that of the solution. This result suggests that the asymmetrical side chains for PDPPTas provide better intermolecular interaction and aggregation in solid state. The optical band gap (Eg), calculated from the absorption edge of the solid state films, were 1.29–1.31 eV. Because these polymers exhibited broad absorption range (500–1000 nm) and low optical band gaps, it is highly applicable to the solar cell devices for improving the degree of light harvesting and enhancing the photocurrent of the devices. Fig. 3 shows the X-ray scattering intensity profiles of the polymer in powder form. We can clearly see two peaks for the (1 0 0) and (2 0 0) diffraction related to a lamellar type packing between the polymer molecules. The d-spacing are 2.1 nm for PDPPT-s1, 1.6 nm for PDPPT-s2, and 1.8 nm for PDPPT-as, respectively. The d-spacing well depends on the length of the side chains for the three polymers. The intensity and sharpness of the peak for (1 0 0) diffraction increases in the order of PDPPT-as, PDPPT-s2, and PDPPT-s1. These results suggest that the packing order might be affected by the symmetry than the rigidity in terms of the chemical structure of three polymers. Photovoltaic properties were measured with the conventional device structures of ITO/PEDOT:PSS/Active layer/LiF/Al. The active layers of the solar cells were made by spin-casting a 1:2 (w/w) mixture of the DPP-based polymers with [70]PCBM and thermally annealed for 10 min at 110 C. The best photovoltaic performance obtained for each polymer is summarized in Table 2. The corresponding J–V curves under simulated AM 1.5G solar
Table 2 Photovoltaic properties of the DPP-based conjugated polymers Jsc (mA cm2)
Voc (V)
FF (%)
PCE (%)
(1:2) (1:2) (1:2) (1:2)
2.25 2.29 2.32 5.31
0.70 0.69 0.69 0.68
52.6 31.7 32.6 56.0
0.83 0.50 0.52 2.02
(1:2)
3.64
0.68
41.9
1.04
(1:2)
5.32
0.68
54.6
1.98
Sample PDPPT-s1:[70]PCBM PDPPT-s2:[70]PCBM PDPPT-as:[70]PCBM PDPPT-s1:[70]PCBM with 2% DIO PDPPT-s2:[70]PCBM with 2% DIO PDPPT-as:[70]PCBM with 2% DIO
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irradiation (100 mW cm2) are depicted in Fig. 4(a). Among the devices without 1,8-diiodooctane (DIO) additives, the best performance was obtained by PDPPT-s1:[70]PCBM with Voc = 0.70 V, Jsc = 2.25 mA cm2, FF of 52.6% and an overall PCE of 0.83%. The devices for PDPPT-s2:[70]PCBM and PDPPT-as:[70]PCBM showed similar PCEs of 0.50%. The device performance can be further improved by adding 2% of DIO by volume as a processing additive. The device of PDPPT-s1:[70]PCBM reached PCE of 2.02% with Voc = 0.68 V, Jsc = 5.32 mA cm2, and FF of 56.0%. However, lower device performance of PDPPT-s2:[70]PCBM with PCE of 1.04% (Voc = 0.68 V, Jsc = 3.64 mA cm2, FF = 41.9%) showed under same processing conditions. Interestingly, the device of PDPPT-as:[70] PCBM with DIO additives showed a PCE of 1.98% with Voc = 0.67 V, Jsc = 5.32 mA cm2, and FF of 54.6%, which is similar device performance with PDPPT-s1:[70]PCBM. Generally, DIO additives are known to provide preventing the formation of large and isolated domains of PCBM and enhancing wettability of blend solution on PEDOT:PSS layers, leading to the formation of uniform active layer. However our preliminary results suggest that the DIO additives not only provide their aforementioned advantages, but also control side-chain interference on DPP units which is more preferable to alkyl chain groups than aryl groups. The higher Jscs for PDPPT-s1:[70]PCBM and PDPPT-as:[70]PCBM are also reflected in the improved external quantum efficiency (EQE) at the absorption
Fig 4. (a) Current density–voltage (J–V) characteristics and (b) EQE spectra of OPV devices fabricated with PDPPT-s1 (square), PDPPT-s2 (triangle), and PDPPT-as (circle) blended with [70]PCBM; Open symbols represent the devices without a DIO additive and solid symbols represent the devices with 2% of DIO additives.
wavelength ranges from 400 to 900 nm (Fig. 4(b)), which allowed the active layer to absorb more available photons and enhance the photocurrent of the device. The morphology and phase separation of the active layers were investigated with tapping-mode atomic force microscopy (AFM). Fig. 5 shows their topographic height images (5 5 mm). The rootmean-square (RMS) roughness of the films was 3.35 nm for PDPPTs1:[70]PCBM, 4.56 nm for PDPPT-s2:[70]PCBM, and 3.89 nm for PDPPT-as:[70]PCBM. A relatively smooth surface was found in the film of PDPPT-s1:[70]PCBM and PDPPT-as:[70]PCBM. However, the PDPPT-s2:[70]PCBM film showed large aggregates (100– 200 nm) and phase separation in the overall surface, which might reduce the exciton generation and charge separation at the
Fig. 5. AFM topographic height images (5 mm 5 mm) of a film for (a) PDPPT-s1: [70]PCBM, (b) PDPPT-s2:[70]PCBM, and (c) PDPPT-as:[70]PCBM with 2% of DIO additives.
S.J. Park et al. / Synthetic Metals 203 (2015) 221–227
conjugated donor molecules/[70]PCBM interface leading to a low photocurrent. These results correspond to decrease of the Jsc values of the solar cell devices. 4. Conclusion AseriesofDPP-basedconjugatedpolymersweresynthesizedwith varied side groups such as branched alkyl side-chain and di-tertbutylbenzyl side-chain on the DPP backbone. Their thermal, optical, andphotovoltaicpropertieswereinvestigated.Afteradding2%ofDIO as a processing additive in active layers for the bulk heterojunction device, we could achieve the highest PCE of 2.02% with Voc = 0.68 V, Jsc = 5.31 mA cm2, and FF = 56.0% for PDPPT-s1:[70]PCBM. The device for PDPPT-as designed with asymmetrical side groups on the DPP units also showed good solar cell performance comparable with PDPPT-s1. These results suggest that the DIO additives can control side-chain interference on DPP units. Acknowledgements This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (2012R1A1A2007079) and Korea Research Council Industrial Science & Technology (SK-0903-01). References [1] G. Yu, J. Gao, J.C. Hummelen, F. Wudl, A.J. Heeger, Science 270 (1995) 1789.
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