Side chain effect on photovoltaic properties of D–A copolymers based on benzodithiophene and thiophene-substituted bithiazole

Organic Electronics 14 (2013) 3152–3162

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Side chain effect on photovoltaic properties of D–A copolymers based on benzodithiophene and thiophene-substituted bithiazole Ping Shen a,b,⇑, Haijun Bin a,b, Xuewen Chen a, Yongfang Li b,⇑ a College of Chemistry and Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of Education, Xiangtan University, Xiangtan 411105, China b Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China

a r t i c l e

i n f o

Article history: Received 15 May 2013 Received in revised form 5 September 2013 Accepted 16 September 2013 Available online 27 September 2013 Keywords: Benzodithiophene Thiophene-substituted bithiazole DA copolymers Side chain Polymer solar cells

a b s t r a c t Two donor–acceptor (DA) copolymers, PEHBDT-BTz and PODBDT-BTz, containing the same backbone of benzodithiophene (BDT) and bithiazole (BTz) units but different side chains were designed and synthesized. Effects of the side chains of BDT and BTz units on solubility, absorption spectra, energy levels, film morphology, and photovoltaic properties of the polymers were investigated. Results showed that the more branched side chains could increase the molecular weight and the introduction of alkylthienyl groups into BTz unit benefits to broaden the absorption and lower the bandgaps as well as deepen HOMO levels, which are propitious to improve the short-circuit current density (Jsc) and open-circuit voltage (Voc) of photovoltaic cells. Polymer solar cells (PSCs) were prepared with the polymers as electron donors and PCBM as an acceptor. The device fabrication conditions, including the additive, the different acceptor and blend ratio of the polymer donor and acceptor, have been optimized. PCE of PSCs based on the copolymers varied from 2.92% for PODBDT-BTz to 3.71% for PEHBDT-BTz, depending on the type and topology of the side chains on the BDT moiety. The results indicate that an appropriate choice of side chains on the backbone is an effective way to improve photovoltaic performance of the related PSCs. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction The ongoing development of polymer solar cells (PSCs) as low cost, lightweight, flexible alternatives to siliconbased inorganic devices is advancing the field of organic photovoltaics and helping address the ever-increasing global energy demand [1–4]. The most successful PSC devices to date are based on the bulk heterojunction (BHJ) concept

⇑ Corresponding authors. Address: College of Chemistry and Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of Education, Xiangtan University, Xiangtan 411105, China (P. Shen). E-mail addresses: [email protected] (P. Shen), liyf@iccas. ac.cn (Y. Li). 1566-1199/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.orgel.2013.09.030

[5], which typically have an interpenetrating network of photoactive electron-donating conjugated polymers and electron-accepting fullerene derivatives (such as [6,6]phenyl-C61-butyric acid methyl ester (PC61BM) and [6,6]phenyl-C71-butyric acid methyl ester (PC71BM)). The key issue in the research of PSCs is to increase the power conversion efficiencies (PCEs) of the devices. The factors that determine PCEs of the PSCs are the short-circuit current density (Jsc), open-circuit voltage (Voc), and fill factor (FF). Jsc and Voc can be roughly predicted from the absorption spectra and the difference between the highest occupied molecular orbital (HOMO) level of a donor (e.g. conjugated polymers) and the lowest unoccupied molecular orbital (LUMO) level of an acceptor (typically, PC61BM), and the HOMO level is predominantly governed by the

P. Shen et al. / Organic Electronics 14 (2013) 3152–3162

structure of the polymer backbones and finely tuned by the peripheral substituents [6,7]. Even if the electronic properties satisfy favorable conditions, the performance of BHJ PSCs still depends on the nanoscale molecular alignments of a polymer donor and a fullerene derivative acceptor. Low bandgap conjugated polymers with electron donor–acceptor (D–A) architecture are particularly attractive, because of the facile tunability of their absorption bands, HOMO/LUMO energy levels and charge carrier mobilities by intramolecular charge transfer (ICT) from donor to acceptor moieties [8–20]. The PCEs of the PSCs based on a number of DA copolymers have reached up to 7–9% [10–18]. Moreover, recently, Yang and Li et al. reported a PCE of 10.6% based on a DA copolymer using tandem structure, which is the highest certified efficiency for PSCs to date [11]. Among the various donor moieties employed in the DA copolymer photovoltaic materials, benzodithiophene (BDT) is currently one of the most preferred donor unit because of its good electron-donating properties, rigid coplanar structure with stronger pp intermolecular interactions, and the possibility of side chain manipulation for suitable solubility and processability [12–16,21–23]. The PSCs based on the BDT-containing polymers showed high PCEs over 7% [12,14,22]. On the other hand, for the acceptor moieties, bithiazole (BTz) unit could lead to lower HOMO energy level of its copolymers due to containing two electron-withdrawing nitrogen of imine (C@N), which is desirable for increasing Voc in PSCs [24–35]. Therefore, BTz-based DA type copolymers recently emerged as a promising class of materials for BHJ solar cells because of its rigid, coplanar, and highly extended p-conjugated structures [24–32]. In previous work, our group have reported some DA copolymers with BTz as the acceptor unit and BDT [29–32], dithienosilole (DTS) [33] or thieno[3,2-b]thiophene (TT) [34] as donor units. These polymers showed relatively high charge mobility, and a PCE up to 4.46% was achieved in combination with PC71BM acceptor [31]. Most research work focused on main chain engineering of the BTz-based polymers, while there are few reports on side chain engineering of the bithiazole copolymers [32]. Equally important is that some studies have been directed towards understanding of the effects of side chains on the photovoltaic performance of PSCs based on DA copolymers [36–43]. Recent work suggests that for the same conjugated polymer backbone the type (alkyl, alkoxy, aryl), shape, topology (linear, branched) and distribution (uniform, alternating, random) of side chains can have a large impact on the morphology (crystallinity, p-stacking orientations), charge transport and photovoltaic properties [36–43] of the polymers. Thus the side chain effect should also be fully explored in the design of new copolymers for highly efficient solar cells. It is worth mentioning that Beaujuge group set a new record of PCE up to 8.5% just by the side chains engineering of a BDT and thieno[3,4c]pyrrole-4,6-dione copolymer [16]. On the basis of the above considerations, herein we report the synthesis and characterization of two new DA copolymers based on benzodithiophene (BDT) and bithiazole (BTz) units (PEHBDT-BTz and PODBDT-BTz, Fig. 1)

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with different alkoxy side chain attached onto the BDT unit and conjugated thienyl side chain onto BTz unit. The most obvious feature of these polymers is that an electrondonating alkyl-substituted thiophene unit is for the first time to be introduced into BTz as the conjugated side chains instead of alkyl side chains. It should help to improve absorptions of the resulting polymers. Furthermore, we chose 2-ethylhexyloxy (EH) and 2-octyldodecyloxy (OD) as side chains of BDT with the aim to systematically investigate the effects of side chain shape and size (EH vs. OD) on the solubility, light absorption, HOMO/LUMO energy levels, and charge transport properties of the resulting polymers as well as on morphology and photovoltaic properties of the PSCs. The other characteristic of two DA polymers is a bithiophene bridge between the donor BDT and acceptor BTz units, which has been proved as a better method [31] to improve the optoelectronic properties compared to the thiophene-bridged countparts [30,32]. PCE of BHJ solar cells with the new copolymers as donors and PCBM as an acceptor varied from 2.92% (PODBDT-BTz) to 3.71% (PEHBDT-BTz), depending on the type and topology of the side chains on the BDT moieties. Our study clearly indicates that an appropriate choice of side chains on the conjugated backbone is an effective way to improve photovoltaic performance of the related PSCs. 2. Experimental section 2.1. Measurements and characterization 1

H NMR spectra were measured on a Bruker DMX-400 spectrometer with chloroform as the solvent and trimethylsilane as the internal reference. UV–visible (UV–vis) absorption spectra were measured on a Hitachi U-3010 UV–vis spectrophotometer. Mass spectra (MS) were recorded on a Shimadzu spectrometer and MALDI-TOF-MS spectra were determined on a Bruker BIFLEX III mass spectrometer. Thermogravimetric analysis (TGA) was conducted on a PerkinElmer TGA-7 thermogravimetric analyzer at a heating rate of 20 °C/min and under a nitrogen flow rate of 100 mL/min. Molecular weights of the polymers were measured by gel permeation chromatography (GPC) method on waters 515–2410 with polystyrenes as standard and tetrahydrofuran (THF) as an eluent. The electrochemical cyclic voltammetry (CV) was performed on a Zahner IM6e Electrochemical Workstation, with a Pt disk coated with the polymer film, Pt wire and Ag/AgCl electrode as the working electrode, counter electrode and reference electrode respectively, in a 0.1 mol L1 tetrabutylammonium hexafluorophosphate (Bu4NPF6) acetonitrile solution. The morphology of blend film was observed by a Nanoscope V (Vecco) atomic force microscope (AFM) in a tapping mode. 2.2. PSC device fabrication and characterization The PSC devices were fabricated with a structure of ITO/ PEDOT:PSS/polymer:PCBM/Ca/Al. The patterned ITO glass was precleaned in an ultrasonic bath of acetone and iso-

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P. Shen et al. / Organic Electronics 14 (2013) 3152–3162 C10H21

C4H9 C2H5

C8H17

C6H13

O S

N S

S

S

C6H13

O C2H5

S

S S

n

PEHBDT-BTz

S C6H13

O C6H13

C4H9

N

S S

S

S

C6H13

S S

S

N

C6H13

O

C6H13

S

S

S S

N

n

S C6H13

C8H17

C10H21

PODBDT-BTz

Fig. 1. Molecular structures of the polymers PEHBDT-BTz and PODBDT-BTz.

propanol and treated in an ultraviolet-ozone chamber (Jelight Co.) for 30 min. A thin layer of PEDOT:PSS (poly(3,4ethylenedioxythiophene): poly(styrenesulfonate)) was spin-cast on precleaned ITO-coated glass from a PEDOT:PSS aqueous solution (Baytron P VP AI 4083 from H.C. Starck) at 2000 rpm and dried subsequently at 150 °C for 30 min in air, then the device was transferred to a glovebox, where the active layer of the blend of the polymer and fullerene derivatives was spin-coated onto the PEDOT:PSS layer. Finally, a Ca/Al metal top electrode was deposited in vacuum onto the active layer at a pressure of ca. 5  105 Pa. The active area of the device was ca. 4 mm2. The current density–voltage (JV) characteristics were measured on a computercontrolled Keithley 236 SourceMeasure Unit. A xenon lamp (150 W) coupled with AM 1.5 solar spectrum filter was used as the light source, and the optical power at the sample was 100 mW/cm2. The external quantum efficiency (EQE) was measured by a Stanford Research Systems model SR830 DSP lock-in amplifier coupled with WDG3 monochromator and a 150 W xenon lamp. The hole mobility was calculated by fitting the dark JV curves for the hole-only devices to SCLC model at low voltages, in which the current density is given by J = 9e0erlV2/8L3exp[0.891c(V/L)0.5], where e0er represents the permittivity of the material, l is the mobility, c is the field activation factor, and L the thickness of the active layer. The applied bias voltage is corrected for the built-in potential so that V = Vapplied  Vbi. 2.3. Materials 2-Hexylthiophene, 3-hexylthiophene, thiophene, 2-bromoacetylbromide, and dithiooxamide were purchased from J&K Chemical. Toluene and THF were distilled from sodium benzophenone under nitrogen before use. All other reagents and solvents used in this work were commercially purchased and used without further purification. All chromatographic separations were carried out on silica gel (200–300 mesh). 2.4. Synthesis of monomers and polymers The synthetic routes of the monomers and polymers are shown in Scheme 1. 4-Hexyl-2-(tributylstannyl)thiophene, 2-(tributylstannyl)thiophene [44], monomers M2 [45] and M3 [45] were synthesized according to the procedure in the literatures. The detailed synthetic processes of other compounds are as follows.

2.4.1. 2-Bromo-1-(5-hexylthiophen-2-yl)ethanone (1) To a solution of anhydrous aluminum chloride (8.01 g, 60.0 mmol) in dichloromethane (50 mL) stirring at 0 °C was added 2-bromoacetylbromide (12.03 g, 60.0 mmol) dropwise. After the addition, 2-hexylthiophene (8.42 g, 50.0 mmol) was added dropwise at 0 °C. The reaction mixture was allowed to warm to room temperature and was stirred for an additional 12 h. After this time, the reaction mixture was cooled to 0 °C and water was added slowly to quench the excess aluminum chloride. The organic layer was isolated and dried over anhydrous MgSO4. After the solvent was removed under vacuum, the residue was purified by column chromatography using hexane and dichloromethane (V/V = 1/2) as eluent to give compound 1 as a yellow oil (8.96 g, yield 62.1%). 1H NMR(CDCl3, 400 MHz): d 7.64 (d, J = 2.0 Hz, 1H), 6.85 (d, J = 2.0 Hz, 1H), 4.31 (s, 2H), 2.85 (t, J = 7.6 Hz, 2H), 1.72–1.68 (m, 2H), 1.56–1.28 (m, 6H), 0.89 (t, J = 6.8 Hz, 3H). MS (EI): m/z [M+] 289. 2.4.2. 4,40 -Bis(5-hexylthiophen-2-yl)-2,20 -bithiazole (2) In a 100 mL round-bottom flask, compound 1 (5.5 g, 19.0 mmol) and dithiooxamide (1.14 g, 9.0 mmol) were dissolved into dry DMF (80 mL) under argon atmosphere. After stirring at 90 °C for 12 h, the mixture was allowed to cool and a brown solid grew. The brown solid was collected and recrystallized from ethanol to give compound 2 as brown crystals (4.12 g, yield 91.6%). 1H NMR(CDCl3, 400 MHz): d 7.34 (d, 4H), 6.75 (d, J = 1.8 Hz, 2H), 2.83 (t, J = 7.6 Hz, 4H), 1.73–1.69 (m, 4H), 1.40–1.31 (m, 12H), 0.89 (t, J = 6.8 Hz, 6H). MS (EI): m/z [M+] 500. 2.4.3. 5,50 -Dibromo-4,40 -bis(5-hexylthiophen-2-yl)-2,20 bithiazole (3) A 250 mL round-bottom flask was charged with the compound 2 (3.0 g, 6.0 mmol) in DMF (50 mL), glacial acetic acid (50 mL) and THF (50 mL) under nitrogen in the dark, and then NBS (2.16 g, 12.0 mmol) was added. After stirring at room temperature for 5 h, a yellow solid was precipitated. The precipitate was filtered, washed with methanol, and then compound 3 was collected as a yellow solid (3.35 g, yield 84.9%). 1H NMR(CDCl3, 400 MHz): d 7.72 (d, J = 1.8 Hz, 2H), 6.81 (d, J = 1.8 Hz, 2H), 2.85 (t, J = 7.6 Hz, 4H), 1.74–1.70 (m, 4H), 1.41–1.32 (m, 12H), 0.90 (t, J = 6.8 Hz, 6H). MS (EI): m/z [M+] 658. 2.4.4. 5,50 -Bis(4-hexylthiophen-2-yl)-4,40 -bis(5hexylthiophen-2-yl)-2,20 -bithiazole (4) 4-Hexyl-2-(tributylstannyl)thiophene (6.86 g, 15 mmol) and compound 3 (3.29 g, 5.0 mmol) were dissolved in dry

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C6H13 O O

Br C CH2Br

C6H13

S

C6H13

AlCl3 CH2Cl2

S

C6H13

S Dithiooxamide

S N S

DMF

Br

1

S

S

3 C6H13

84.9%

C6H13

S

S

C6H13

NBS S

N

N

S

Br

DMF

S

4

C6H13

S

C6H13 N S

Pd(PPh3)4 Toluene

S

Br

N

C6H13 S

SnBu3

N

S

S

DMF

91.6%

C6H13

C6H13

Br

2

62.1%

N

NBS

S

S

C6H13

N

SnBu3

S

Pd(PPh3)4 Toluene

S C6H13

90.9%

C6H13

Br

S

5

C6H13

83.9%

S

C6H13 C6H13

S N

S S

S C6H13

NBS

S

N

N

S

Br

C6H13

92.4%

C10H21

C4H9 C2H5

S

N

M1

C6H13

81.2%

S

S

S

S

DMF

S

6

C6H13

S

S

S

Br

S C6H13

C8H17

O

O S

S

Sn

Sn

Sn

Sn

S

S O

C2H5

O C8H17

C4H9

M2

C10H21

M3

C6H13

+ M3

C6H13

S

M2 Br

N

S S

S

S

S S

N

C6H13

Br

Pd(PPh3)4 Toluene

S

M1

PEHBDT-BTz

PODBDT-BTz

C6H13

Scheme 1. Synthetic route of the monomers and the polymers.

toluene (50 mL) and deoxygenated with argon atmosphere for 30 min. Pd(PPh3)4 (347 mg, 0.3 mmol) was added and the mixture was stirred at 110 °C for 24 h. Then, the mixture was poured into water and extracted with dichloromethane, then dried over anhydrous MgSO4. After concentration, the residue was purified by column chromatography using hexane and dichloromethane (V/V = 2/1) as eluent to afford compound 4 as a yellow solid (3.49 g, yield 83.9%). 1H NMR(CDCl3, 400 MHz): d 7.16 (d, J = 1.8 Hz, 2H), 7.12 (d, 2H), 7.02 (s, 2H), 6.66 (d, J = 1.8 Hz, 2H), 2.79 (t, J = 7.6 Hz, 4H), 2.61 (t, J = 7.6 Hz, 4H), 1.69–1.61 (m, 8H), 1.37–1.30 (m, 24H), 0.87 (t, J = 5.2 Hz, 12H). 13C NMR(CDCl3, 100 MHz): d 160.92, 151.53, 146.85, 135.01, 124.93, 124.66, 112.75, 31.75, 31.71, 30.36, 28.88, 22.72, 14.21. MALDI-TOF: m/z, 833.1.

2.4.5. 5,50 -Bis(5-bromo-4-hexylthiophen-2-yl)-4,40 -bis(5hexylthiophen-2-yl)-2,20 -bithiazole (5) Compound 4 (833 mg, 1.0 mmol) was dissolved in a mixture of DMF (30 mL) and chloroform (10 mL). NBS (378 mg, 2.2 mmol) was then added to the solution and stirred for 4 h in the dark. Then, the mixture was poured into water, extracted with dichloromethane, and dried over anhydrous MgSO4. After concentration, the residue was purified by recrystallization from hexane twice to give compound 5 as a yellow solid (901 mg, yield 90.9%). 1H NMR(CDCl3, 400 MHz): d 7.17 (d, J = 1.8 Hz, 2H), 6.98 (s, 2H), 6.69 (d, J = 1.8 Hz, 2H), 2.81 (t, J = 7.4 Hz, 4H), 2.57 (t, J = 7.6 Hz, 4H), 1.70–1.58 (m, 8H), 1.39–1.30 (m, 24H), 0.89 (t, J = 5.0 Hz, 12H). 13C NMR(CDCl3, 100 MHz): d 157.47, 147.10, 146.08, 141.79, 132.23, 130.14, 129.53,

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126.45, 124.51, 123.48, 30.63, 30.85, 29.17, 28.62, 28.50, 27.83, 27.80, 21.61, 13.10. MALDI-TOF: m/z, 991.3. 2.4.6. 5,50 -Bis(3-hexyl-2,20 -bithiophen-5-yl)-4,40 -bis(5hexylthiophen-2-yl)-2,20 -bithiazole (6) 2-(Tributylstannyl)thiophene (1.12 g, 3.0 mmol) and compound 5 (1.98 g, 2.0 mmol) were dissolved in dry toluene (20 mL) and deoxygenated with argon atmosphere for 30 min. Pd(PPh3)4 (137 mg, 0.12 mmol) was added and the mixture was stirred at 110 °C for 24 h. Then, the mixture was poured into water and extracted with dichloromethane, then dried over anhydrous MgSO4. After concentration, the residue was purified by column chromatography using hexane and dichloromethane (V/V = 4/1) as eluent to afford compound 6 as a yellow brown solid (1.62 g, yield 81.2%). 1H NMR(CDCl3, 400 MHz): d 7.33 (d, J = 1.8 Hz, 2H), 7.15–7.13 (m, 6H), 7.09–7.06 (m, 2H), 6.70 (d, 2H), 2.81 (t, J = 7.6 Hz, 4H), 2.75 (t, J = 7.8 Hz, 4H), 1.71–1.63 (m, 8H), 1.40–1.28 (m, 24H), 0.88 (t, J = 5.8 Hz, 12H). 13C NMR(CDCl3, 100 MHz): d 158.61, 148.27, 147.01, 140.44, 135.87, 134.03, 133.64, 132.63,129.87, 127.91, 127.77, 126.75, 126.22, 124.88, 32.13, 32.04, 30.98, 30.63, 29.68, 29.60, 29.26, 23.08, 23.04, 14.29. MALDI-TOF: m/z, 997.1. 0

0

0

0

2.4.7. 5,5 -Bis(5 -bromo-3-hexyl-2,2 -bithiophen-5-yl)-4,4 bis(5-hexylthiophen-2-yl)-2,20 -bithiazole (M1) Compound 6 (1.62 g, 1.62 mmol) was dissolved in a mixture of DMF (5 mL) and chloroform (25 mL). NBS (0.61 g, 3.4 mmol) was then added to the solution and stirred for 4 h in the dark. Then, the mixture was poured into water, extracted with dichloromethane, and dried over anhydrous MgSO4. After concentration, the residue was precipitated in methanol and filtered, washed with methanol, dried to produce M1 as a red solid (1.72 g, yield 92.4%). 1 H NMR(CDCl3, 400 MHz): d 7.24 (s, 2H), 7.12 (d, 2H), 7.03– 7.00 (m, 2H), 6.88 (d, 2H), 6.70 (d, 2H), 2.81 (t, J = 7.4 Hz, 4H), 2.71 (t, J = 7.6 Hz, 4H), 1.70–1.59 (m, 8H), 1.38–1.23 (m, 24H), 0.88 (t, J = 3.8 Hz, 12H). 13C NMR(CDCl3, 100 MHz): d 158.16, 147.97, 146,64, 140.48, 136.92, 133.40, 132.10, 130.30, 130.02, 127.44, 126.49, 126.15, 124.46, 112.44, 31.68, 31.62, 30.55, 30.21, 29.74, 29.23, 29.16, 28.84, 22.65, 22.63, 14.14. MALDI-TOF: m/z, 1155.2. 2.4.8. General synthetic procedures of the polymers The synthetic routes of the two D–A copolymers are shown in Scheme 1. All of the polymerization procedures were carried out through the palladium (0)-catalyzed Stille coupling reactions. In a 50 mL two-necked flask, M2 (0.2 mmol, or M3) and M1 (0.2 mmol) were added into 10 mL of dry toluene. The mixture was deoxygenated with argon for 30 min, and then Pd(PPh3)4 (14 mg) was added. After another flushing with argon for 20 min, the reaction mixture was reacted for 24 h at 110 °C. After cooled to room temperature, the mixture was poured into methanol (100 mL). A precipitate was collected by filtration and Soxhlet extracted with methanol, hexane and then chloroform. The chloroform extracts were then concentrated and precipitated into methanol, filtered and then wash with methanol. After this, the precipitates were collected and dried under vacuum overnight to afford the polymers.

2.4.8.1. Polymer PEHBDT-BTz. Following the general polymerization procedure, M2 (154.5 mg, 0.2 mmol) and M1 (231.1 mg, 0.2 mmol) were used in this polymerization to obtain a dark purple power (200 mg, yield 69.4%). 1H NMR(CDCl3, 400 MHz): d 7.49–7.46 (br, 2H), 7.08 (br, 6H), 6.87–6.73 (m, 4H), 4.19 (br, 4H), 2.82 (br, 12H), 1.82–1.34 (br, 46H), 1.07–0.90 (br, 24H). GPC: Mw = 8.4 kDa, PDI = 1.20. 2.4.8.2. Polymer PODBDT-BTz. Following the general polymerization procedure, M3 (221.8 mg, 0.2 mmol) and M1 (231.1 mg, 0.2 mmol) were used in this polymerization to obtain a red–purple power (130 mg, yield 36.7%). 1H NMR(CDCl3, 400 MHz): d 7.50 (br, 2H), 7.16–7.11 (br, 6H), 6.73–6.72 (br, 4H), 4.20 (br, 4H), 2.84–2.82 (br, 10H), 1.91 (br, 2H), 1.71–1.68 (br, 16 H), 1.34–1.27 (br, 78H), 0.92–0.86 (br, 24H). GPC: Mw = 30.2 kDa, PDI = 1.42. 3. Results and discussion 3.1. Synthesis and chemical characterization PEHBDT-BTz and PODBDT-BTz were designed to have a same BDT-bithiophene–BTz-bithiophene polymer backbone but different side chains. PEHBDT-BTz has branched ethylhexyloxyl side chains, PODBDT-BTz has more branched octyldodecyloxyl side chains to further improve solubility (Fig. 1). Moreover, all the polymers possess conjugated hexylthiophene side chains attached on the bithiazole (BTz) unit. The choice of these different class side chains is convenient for fine-tuning of the optoelectronic properties of the resulting polymers. As outlined in Scheme 1, two monomers M2 and M3 based on BDT unit were prepared according to the procedure in the literature [45]. Bithiazole-based monomer M1 with hexylthiophene side chain was synthesized by the stepwise synthetic protocol. First, compound 1 was obtained via the acetylation reaction of 2-hexylthiophene and 2-bromoacetylbromide. The bithiazole scaffold (2) formed along with the conjugated thienyl side chain from the reaction between dithiooxamide and compound 1 [46]. Then, the treatment of BTz-based compound 2 with NBS resulted in selective dibromination at position of the bithiazole moiety, yielding compound 3 with a good yield. Finally, monomer M1 with extended conjugated chain was synthesized by the continuous Stille coupling and bromination reactions starting from compound 3. Compounds 1–6 were satisfactorily characterized by 1H NMR and MS or MALDI-TOF spectroscopies. Two new D–A copolymers were synthesized successfully via Stille-type copolymerization of a distannane monomer (M2 or M3) with the dibromobithiazole-based monomer M1. All the copolymers have good solubility in common organic solvents such as chloroform, tetrahydrofuran (THF), chlorobenzene, and o-dichlorobenzene (ODB). The molecular weights and polydispersity indices (PDIs) of the polymers were determined by gel permeation chromatography (GPC) analysis with a polystyrene standard calibration and THF as eluent. The two copolymers had moderate weight-average molecular weights (Mw) of 8.4 and 30.2 kDa with narrow PDIs of 1.2 and 1.42 for

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P. Shen et al. / Organic Electronics 14 (2013) 3152–3162 Table 1 Molecular weights, thermal and optical properties of the two polymers.

d e f

PDI

Td (°C)b

kmax (nm)c

kmax (nm)d

kedge (nm)e

Eopt (eV)f g

PEHBDT-BTz PODBDT-BTz

8.4 30.2

1.20 1.42

357 355

474 468

532 552

645 650

1.92 1.91

Determined by GPC in THF based on polystyrene standards. The 5% weight-loss temperatures in nitrogen. Measured in chloroform dilute solution. Measured on quartz plate by polymers cast from chloroform solution. Absorption edge of the thin films. Estimated from the onset wavelength of the absorption spectra: Eopt g = 1240/kedge.

PEHBDT-BTz and PODBDT-BTz, respectively (Table 1). Clearly, the molecular weight difference between the two polymers is substantial and this should influence the optoelectronic properties of two polymers. It will be discussed in the next sections. As expected, PODBDT-BTz has the much higher molecular weight than PEHBDT-BTz due to the more branched octyldodecyloxyl side chains. Although the molecular weights are relatively low, they are still comparable with those of the similar BDT–BTz-based D– A copolymers with alkyl side chains on BTz unit [31,32]. The thermal stability of conjugated polymers is very important for their application in optoelectronic device. The thermal properties of the polymers were investigated with thermogravimetric analysis (TGA) measurement. The TGA analysis reveals that the onset temperatures with 5% weight loss (Td) of PEHBDT-BTz and PODBDT-BTz are 357 and 355 °C, respectively (Fig. 2), which are not only higher than that of the similar bithiophene-bridged copolymer (343 °C) [31] but also higher than those thiophene-bridged copolymers (317–328 °C) [30,32]. These results show that the introduction of thiophene unit into BTz as the conjugated side chains favors to improve the thermal stability of the resulting polymers. Obviously, the thermal stability of the two polymers is good enough for the applications in photovoltaic devices. 3.2. Optical properties Optical properties of these polymers have been investigated by UV–vis absorption in dilute chloroform solution

PODBDT-BTz 0.8

0.6

0.4

0.2

0.0 300

400

500

600

700

Wavelength (nm)

1.0

(b)

0.8

0.6

0.4 PEHBDT-BTz 0.2

PODBDT-BTz

0.0 300

400

500

600

700

800

Wavelength (nm)

PEHBDT-BTz PODBDT-BTz

80

Weight (%)

PEHBDT-BTz

Fig. 3. Normalized UV–vis absorption spectra of the two polymers in chloroform (a) and in the film state (b).

100 90

(a)

1.0

Normalized absorbance (a.u.)

c

Mw (kDa)a

Normalized absorbance (a.u.)

a b

Polymer

70 60 50 40 30 100

200

300

400

500

600

o

Temperature ( C) Fig. 2. TGA plots of the two polymers with a heating rate of 20 °C/min under an inert atmosphere.

and as thin solid films (Fig. 3). The detailed absorption data including the absorption maximum wavelength (kmax) in both solution and films, the absorption edge (onset wavelength of the absorption peak, kedge) of the polymer films, and optical bandgaps (Eopt g ) are summarized in Table 1. As shown in Fig. 3a, in solutions the kmax of PEHBDTBTz and PODBDT-BTz are 474 and 468, respectively, (Table 1). In comparison with PEHBDT-BTz, PODBDT-BTz exhibits a slightly blue-shifted absorption maximum and narrower spectrum due to the more branched octyldodecyl side chains on BDT unit, which damages the coplanarity of the main chains. Compared to the absorption spectra of the polymer solutions, the absorption maxima of PEHBDT-BTz and PODBDT-BTz films red-shifted by 58 and 84 nm,

P. Shen et al. / Organic Electronics 14 (2013) 3152–3162

respectively, which indicates that there was strong p–p stacking and interchain interaction in the polymer films (Fig. 3b). Though PEHBDT-BTz possessed a blue-shifted absorption maximum (532 nm) relative to PODBDT-BTz, it showed an obvious broader absorption band than that of PODBDT-BTz, meaning this polymer may give a higher Jsc. It has previously been demonstrated in the cases of poly(3-hexylthiophene) [47,48] and other D–A polymers [49] that molecular weight has a significant impact on optoelectronic properties. Here, it is important to note that the effect of substantial difference in the molecular weight of the two polymers on the absorption properties should not to be ignored. The nature of the different side chain on BDT unit should be responsible for the overall shape and wavelength of the absorption peaks, whereas the molecular weight seems to have an influence on the vibronic structures of the peaks. As above mentioned in solution the two polymers behave similarly, in the solid state however the low molecular weight PEHBDT-BTz differs slightly from the high molecular weight PODBDT-BTz. The broader and less red-shifted absorption spectrum of PEHBDT-BTz compared to PODBDT-BTz (Fig. 3b), leading to the assumption that the lower molecular weight polymer aggregates stronger in solid state than the higher molecular weight polymer. Addition, the more obvious red-shift (45 nm) of these polymers relative to their analogues [31] indicate that these BDT–BTz-based polymers with conjugated thiophene side chains on BTz unit can enhance the p–p stacking and ordered aggregation of the polymer main chains in some degree. The absorption edges (kedge) of the polymer films were 645 nm for PEHBDT-BTz and 650 nm for, PODBDT-BTz, and the corresponding optical bandgaps (Eopt g ) estimated from the kedge of the polymer films are 1.92 and 1.91 eV (Eopt g = 1240/kedge), respectively. The absence of obvious difference between the bandgaps of the two polymers indicates that the bandgap is not determined by the alkyl side chain but mainly by the polymer backbone. Furthermore, these bandgaps are not only smaller than that of bithiophene-bridged analogy (1.96 eV) [31] but also than those thiophene-bridged analogues (1.93–1.99 eV) [32] bearing alkyl side chains on BTz unit. These results imply that the introduction of thienyl unit into BTz unit as the conjugated side chains benefits to broaden the absorption bands and lower the bandgaps of the resulting polymers to some extend. 3.3. Electrochemical properties Electrochemical cyclic voltammetry (CV) has been widely employed to investigate the redox behavior of the polymer and to estimate its HOMO and LUMO energy levels. Fig. 4 shows the cyclic voltammograms of the BDT– BTz-based polymer films on a Pt electrode in a 0.1 mol/L Bu4NPF6-acetonitrile solution. The results of the electrochemical properties are summarized in Table 2. The energies of HOMO and LUMO (EHOMO/LUMO) was estimated from the onset of oxidation/reduction potentials (Eox/red) in the voltammograms, measured vs. Ag/AgCl, and calibrated against the ferrocene/ferrocenium couple (Fc/ Fc+ measured as 0.39 V vs. Ag/AgCl). The Fc/Fc+ energy level used in EHOMO/LUMO calculations was assumed to be

0.8

Fc/Fc+

0.6

Current (mA)

3158

PEHBDT-BTz

0.4 0.2

PODBDT-BTz 0.0 -0.2 -0.4 -2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

Potential (V vs Ag/AgCl) Fig. 4. Cyclic voltammograms of the polymer films on platinum electrode in acetonitrile solution containing 0.1 mol/L Bu4NPF6 at a scan rate of 100 mV/s. The cyclic voltammogram of ferrocene was also put in the figure for the potential calibration.

4.80 eV [50]. Thus, EHOMO/LUMO = e (Eox/red + 4.41) (eV), where Eox/red is the oxidation/reduction onset potential of the sample vs. Ag/AgCl. As shown in Fig. 4, the Eox of PEHBDT-BTz and PODBDT-BTz were determined to be 0.78 and 0.85 V (vs. Ag/AgCl), respectively, and the corresponding HOMO energy levels (EHOMO) calculated according to the above equation are 5.19 and 5.26 eV, respectively. The HOMO energy levels obtained electrochemically were deeper than that of literature observation for an analogy of BDT–BTz-based copolymer with alkyl side chains on BTz (5.12 eV) [31], indicating that the conjugated thiophene side chain attached on BTz could deepen the HOMO energy levels. The reason is that the introduction of thienyl side chains on BTz unit increased the steric hindrance and reduced coplanarity of the resulting copolymers, which led to reduce HOMO/LUMO orbital overlap, consequently deepening HOMO levels [51–53]. Additionally, the HOMO value estimated for PODBDT-BTz is slightly lower than their analogue PEHBDT-BTz (albeit, not quite as dramatically only 70 mV). The relatively lower HOMO energy levels of the copolymers are beneficial to the better stability of the polymers against oxidation and higher open-circuit voltage (Voc) of the PSCs based on the polymers as donors [6]. On the other hand, the Ered values were observed as 1.30 and 1.48 V for PEHBDT-BTz and PODBDT-BTz. Accordingly, the calculated LUMO energy levels (ELUMO) of the polymers are 3.11 and 2.93 eV, respectively. Obviously, the different side chains on BDT have great impact on the ELUMO. Moreover, in comparison with the LUMO energy level (2.94 eV) of the analogy of PEHBDT-BTz, the thiophene conjugated side chains on BTz makes the LUMO level downwards shifted [30]. The LUMO energy levels of the polymers are all located within a suitable range (Table 2) and are much higher than that of PC71BM (ca. 3.91 eV) [54], thus, efficient excitons dissociation could be expected to occur in their corresponding PSCs. The electrochemically determined bandgaps of the polymers (Eec g ) were 2.08 eV for PEHBDT-BTz, 2.33 eV for PODBDT-BTz, which were greater than Eopt (Table 1) but g within the range of error (0.2–0.5 eV) [55–57].

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P. Shen et al. / Organic Electronics 14 (2013) 3152–3162 Table 2 Electrochemical properties of the two copolymers in solid films.

a b c

Polymer

Eox (V)a

Ered (V)a

EHOMO (eV)b

ELUMO (eV)b

c Eec g (eV)

PEHBDT-BTz PODBDT-BTz

0.78 0.85

1.30 1.48

5.19 5.26

3.11 2.93

2.08 2.33

Measured by CV. Calculated according to the equation: EHOMO/LUMO = e (Eox/red + 4.41) (eV). Bandgap obtained from ELUMO–EHOMO.

3.4. Photovoltaic properties 2

(a)

Current density (mA/cm 2)

0 -2 -4 -6 D1

-8

D4 D5

-10

D6 -12 -0.2

0.0

0.2

0.4

0.6

0.8

Voltage (V) 2

(b)

0

Current density (mA/cm 2)

To investigate the effects of different side chains on the photovoltaic properties of the BDT–BTz-based copolymers, bulk heterojunction PSC devices with a configuration of ITO/PEDOT:PSS/polymer:PCBM/Ca/Al were fabricated. The active layers were spin-coated from an o-dichlorobenzene (DCB) solution of polymer:PCBM (PC61BM or PC71BM) blend. The thickness of the active layer was controlled by changing the spin speed during the spin-coating process and measured on an Ambios Tech. XP-2 profilometer. The corresponding open-circuit voltage (Voc), short-circuit current (Jsc), fill factor (FF), and power conversion efficiency (PCE) of the devices are summarized in Table 3. Fig. 5 shows the current density–voltage curves (J–V) of the PSCs based on the blends of polymer:PCBM under the illumination of AM1.5G, 100 mW cm2 and Fig. 6 shows the external quantum efficiency (EQE) curves of the PSCs. First, the blend ratio of the polymer donor and PCBM acceptor was optimized (D1–3 and D7–8). As the mixing ratio of PEHBDT-BTz and PC61BM increased from 1:1 to 1:1.5 to 1:2, the PCE of the device decreased from 2.35% to 1.69% to 1.59% for D1, D2 and D3 (Table 3), respectively. The main reason could be attributed to the decrease of the Voc and Jsc along with FF of the cells. The similar result can be found in PODBDT-BTz/PC61BM based cells (D7–8). The values of Jsc and FF of 1:1 cells (D1 and D7) were greater than those of cells with other mixing ratio (D2–3 and D8). The optimal blend ratios for the two polymers were determined to be around 1:1. Next, we investigated the effect of the high boiling point additive 1,8-diiodooctane (DIO) on the photovoltaic properties of the copolymers. Compared D1 with D4, one can find that a small amount of DIO (3%, by volume) can remarkably improve the Voc (0.72 V vs. 0.78 V), Jsc (7.41 mA/cm2 vs. 8.64 mA/cm2) and FF (0.440 vs. 0.487) values of the device and as a result

-2

-4 D7

-6

D9 D10 -8 -0.2

D11 0.0

0.2

0.4

0.6

0.8

Voltage (V) Fig. 5. Typical J–V curves of photovoltaic devices: (a) based on PEHBDTBTz with different fabricated conditions; and (b) based on PODBDT-BTz EH with different fabricated conditions.

Table 3 Photovoltaic properties of the PSCs based on the two copolymers. Device

Active layer

Voc (V)

Jsc (mA/cm2)

FF (%)

PCE (%)

Thickness (nm)

D1 D2 D3 D4 D5 D6 D7 D8 D9 D10 D11

PEHBDT-BTz:PC61BM = 1:1 PEHBDT-BTz:PC61BM = 1:1.5 PEHBDT-BTz:PC61BM = 1:2 PEHBDT-BTz:PC61BM = 1:1, 3% DIO PEHBDT-BTz:PC71BM = 1:1 PEHBDT-BTz:PC71BM = 1:1, 3% DIO PODBDT-BTz:PC61BM = 1:1 PODBDT-BTz:PC61BM = 1:1.5 PODBDT-BTz:PC61BM = 1:1, 3% DIO PODBDT-BTz:PC71BM = 1:1 PODBDT-BTz:PC71BM = 1:1, 3% DIO

0.72 0.70 0.69 0.78 0.74 0.78 0.78 0.78 0.79 0.81 0.82

7.41 6.16 5.95 8.64 8.70 10.30 6.22 5.89 6.29 6.79 7.18

44.0 39.2 38.6 48.7 47.2 46.1 46.2 45.1 46.8 48.8 49.7

2.35 1.69 1.59 3.28 3.04 3.71 2.24 2.07 2.32 2.68 2.92

104 99 96 113 111 102 110 112 120 102 105

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P. Shen et al. / Organic Electronics 14 (2013) 3152–3162

80

D4 D6

70

D9 60

D11

EQE (%)

50 40 30 20 10 0 300

400

500

600

700

Wavelength (nm) Fig. 6. EQE curves of photovoltaic devices based on the two polymers.

to achieve a higher PCE (2.35% vs. 3.28%). As well known, PC61BM and PC71BM are two typical acceptors used in BHJ PSCs. So, we finally optimized the acceptors of these photovoltaic devices. When PC71BM was selected to replace PC61BM as an electron acceptor, the PCEs of devices was further improved with the same fabrication conditions. For example, D5 based on PEHBDT-BTz/PC71BM show a better PCE (3.04%) than that of D1 (2.35%), which ascribed the improvement of the Jsc value (8.70 mA/cm2 vs. 7.41 mA/cm2, Fig. 5a). The effects of the side DIO and

different acceptors on the Jsc can well reflect from the corresponding external quantum efficiency (EQE) curves of photovoltaic devices (Fig. 6). For PEHBDT-BTz-based devices (D7 and D9), when a small amount of DIO was added as the additive the corresponding EQE plot increases with the maximum EQE value improved from 69.9% (D7) to 77.8% (D9) around at 495 nm, which are in agreement with the trend of Jsc based on the two devices (Table 3). For the two polymers, photovoltaic devices with PC71BM as the acceptor (D6 and D11) exhibit significantly higher EQE values than those of devices with PC61BM as the acceptor (D4 and D9) in wavelength range from 350 to 750 nm. This should be attributed to the broader spectral respond of PC71BM relative to PC61BM. In these optimized conditions, the maximal PCEs of 3.71% (Voc = 0.78 V, Jsc = 10.30 mA/cm2, FF = 0.461) and 2.92% (Voc = 0.82 V, Jsc = 7.18 mA/cm2, FF = 0.492) were achieved for the PEHBDT-BTz/PC71BM (D6) and PODBDT-BTz/PC71BM (D11) based devices with 3% DIO as the additive. Clearly, there is a trade-off between Voc and Jsc for devices based on the two polymers, resulting in an obviously higher PCE of PEHBDT-BTz compared to PODBDT-BTz. As shown in Table 3 and Fig. 5, the higher Jsc of PEHBDT-BTz relative to PODBDT-BTz can be ascribed to the relative better absorption ability of the former (see Fig. 3). On the other hand, the Voc of the PSCs based on PODBDT-BTz (above 0.78 V) are higher than that of PEHBDT-BTz (below 0.78 V) due to low HOMO levels of the former (see Table 2). The HOMO level of PODBDT-BTz

Fig. 7. AFM height images of polymer:PC61BM blend films (1:1, w/w) on ITO/PEDOT:PSS: (a) PEHBDT-BTz, (b) PODBDT-BTz; AFM phase images of polymer:PC61BM blend films (1:1, w/w) on ITO/PEDOT:PSS, (c) PEHBDT-BTz, and (d) PODBDT-BTz. The scanned area is 5  5 lm2.

P. Shen et al. / Organic Electronics 14 (2013) 3152–3162

is lower (5.26 eV) than PEHBDT-BTz (5.19), so the Voc of PODBDT-BTz-based PSC is expected to be higher than that of PEHBDT-BTz. Addition, the obvious improvement of Voc for PODBDT-BTz also may be attributed to the longer and bulkier side chains on BDT unit relative to PEHBDT-BTz, which weakens the intermolecular interactions between donor and acceptor materials. This phenomenon had been observed in other polymers with bulky side chains [58–60]. To further explore the substantial diversity in photovoltaic properties of the two polymers with different side chains, the hole mobilities of the polymers in the photosensitive layers (for polymer/PC61BM, 1:1) were measured by the space charge limited current (SCLC) method with the hole-only device of ITO/PEDOT:PSS/polymer:PC61BM/ Au. The hole mobility was calculated by fitting the dark JV curves for the hole-only devices to SCLC model at low voltages, in which the current density is given by J = 9e0erlV2/8L3exp[0.891c(V/L)0.5]. According to the above equation, the hole mobilities obtained are 3.86  103 and 5.91  104 cm2 V1 s1 for PEHBDT-BTz and PODBDTBTz, respectively. Obviously, the shape and size of side chains have a great influence on the hole mobilities and the polymers with small bulky alkoxy side chains on BDT possess the higher hole mobility than that more branched side chain, which should be beneficial for the application as photovoltaic donor materials in PSCs. Consequently, the relatively higher Jsc of the PSCs based on PEHBDTBTz should be as a benefit of its higher hole mobilities and broader absorption band. Moreover, we also investigate the effects of the morphology of the active layers on the photovoltaic performance of the polymers. The morphological structures of the blend films of polymer/PC61BM (1:1, w/w) were analyzed by a tapping mode atom force microscopy (AFM) measurements. Fig. 7 shows the AFM height (a and b) and phase images (c and d) of the active layers based on PEHBDT-BTz and PODBDT-BTz, respectively. The average surface roughness (Ra) of the AFM phase images were 0.923 nm and 1.16 nm, for the blends of PEHBDTBTz:PC61BM, and PODBDT-BTz:PC61BM, respectively. It is clear that the film of 1:1 blends of polymer and PC61BM have a smooth surface and uniform donor–acceptor interpenetrating networks with appropriate domain sizes, which indicate a good nanoscale morphology formation of the active layer in the solar cell device. Consequently, the two polymers of PEHBDT-BTz, PODBDT-BTz showed no obvious difference in PCEs under same fabrication conditions (D1 and D7). 4. Conclusions

Optical and electrochemical properties showed that the introduction of alkylthienyl groups into BTz units as the conjugated side chains benefits to broaden the absorption bands and lower the bandgaps of the resulting polymers as well as deepen the HOMO energy levels, which are propitious to improve the Jsc and Voc of PSCs. The BHJ PSCs based on two polymers were prepared and the device fabrication conditions, including the additive of 1,8-diiodooctane (DIO), the different PCBM acceptor and blend ratio of the polymer donor and acceptor, have been optimized in detail. The best device based on PEHBDT-BTz and PC71BM (1:1, w/w, 3% DIO) displayed a PCE of 3.71% with a Jsc of 10.30 mA/cm2, a Voc of 0.78 V and a FF of 0.461 under the illumination of AM1.5G, 100 mW/cm2. Our study clearly indicates the optoelectronic properties of the copolymers can be fine controlled by the side-chain engineering. Acknowledgments This work was financially supported by the grants from the National Natural Science Foundation of China (Grant No. 21004050) and China Postdoctoral Science Foundation (Grant No. 2012M510554). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]

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Two new D–A copolymers PEHBDT-BTz and PODBDTBTz based on BDT and BTz units with different alkoxy side chain attached onto the BDT unit and alkylthienyl conjugated side chain onto BTz unit were designed, synthesized and applied to PSCs. We found that the type and shape of the side chains have great influences on the solubility, light absorption, HOMO/LUMO energy levels, and charge transport properties of the resulting polymers as well as on morphology and photovoltaic properties of the PSCs.

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