Synthesis and photovoltaic properties of the copolymers containing zinc porphyrin derivatives as pendant groups

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Synthesis and photovoltaic properties of the copolymers containing zinc porphyrin derivatives as pendant groups Xi Luo, Fen Wu, Haibin Xiao, Huan Guo, Yijiang Liu, Songting Tan* Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of Education, College of Chemistry, Xiangtan University, Xiangtan 411105, PR China

A R T I C L E I N F O

Article history: Received 24 May 2016 Received in revised form 11 November 2016 Accepted 22 November 2016 Available online xxx Keywords: Porphyrin Pendant groups Diketopyrrolopyrrole Copolymers Synthesis Polymer solar cells

A B S T R A C T

A series of copolymers containing zinc porphyrin (ZP) derivatives as pendant groups and diketopyrrolopyrrole (DPP) as electron-withdrawing units were designed and synthesized by Stillecoupling polymerization. The introduction of zinc porphyrin derivatives enhances the light-harvesting capability and leads to a complementary absorption at short wavelength band. The absorption of the copolymers can be well-tuned by varying the ratio of zinc porphyrin derivatives and DPP units. The results indicated that copolymer P(ZP-BT-DPP4) with ZP/DPP ratio of 1:4 showed an strong absorption in the range of 400–900 nm. The power conversion efficiency (PCE) of 2.44% with an increased Jsc of 6.14 mA/ cm2 was achieved due to the better light-harvesting ability and suitable active film morphology. ã 2016 Elsevier B.V. All rights reserved.

1. Introduction In the past decade, polymer solar cells (PSCs) based on conjugated polymers as donor and fullerene derivatives as acceptor have attracted considerable interests from both the academic and industrial communities due to their potential application in large-area, flexible, and cost-effective devices [1–4]. Among the conjugated polymers, the donor–acceptor (D–A) copolymers [5–8] containing electron-donating and electron-withdrawing building blocks have been widely researched to various D and A units to adjust the HOMO level, LUMO level and the intrinsic bandgap (Eg) conveniently, which are determinative to the open-circuit voltage (Voc) and short-circuit current (Jsc) of PSCs [9–19]. One of the most effective method to get excellent photovoltaic performance is to tune the absorption spectrum of D-A copolymer to match the solar radiation. Porphyrins are effective light-harvesting compounds with strong absorption in blue (Soret or B band) and red (Q-bands) part of visible spectrum, thus they are important participants in the photosynthesis of plants. In addition, large p-conjugation macrocycle, good thermal stabilities and alterable structures make them a kind of promising photovoltaic materials. Therefore, investigators pay more attention

to porphyrin and their derivatives for fabricating photovoltaic devices. Porphyrins were initially introduced either on the side chains or in the backbone of conjugated polymers to increase light absorption [20–23], but poor PCEs were obtained. To broaden light absorption range and reduce band gap of porphyrins-based conjugated polymers is an effective way to improve the PCEs. Much effort has been made to design and synthesize D–A conjugated porphyrin polymers with broad absorption and low bandgap [24–26]. In 2012, Shi and co-workers reported porphyrincontaining D–p–A conjugated copolymer PCTTQP based on a 2,7-carbazole donor unit and a porphyrin-based acceptor unit with terthiophene p bridge, which exhibited an enhanced PCEs of 2.5%. When incorporating zinc in center of porphyrin, they found that the HOMO/LUMO energy levels are upshifted and the hole mobilities increased due to the enhancing the packing structure in the blends [25]. Recently, Wang et al. synthesized a series of low bandgap conjugated polymers including two acceptor units of quinoxalino [2,3-b’]porphyrin (QP) and diketopyrrolopyrrole (DPP), and oligothiophene donor units. The introduction of para-linked zincporphyrin complex not only improved coplanarity and extended p-conjugation along the polymer backbone, and also broadened

* Corresponding author. E-mail address: [email protected] (S. Tan). http://dx.doi.org/10.1016/j.synthmet.2016.11.026 0379-6779/ã 2016 Elsevier B.V. All rights reserved.

Please cite this article in press as: X. Luo, et al., Synthesis and photovoltaic properties of the copolymers containing zinc porphyrin derivatives as pendant groups, Synthetic Met. (2016), http://dx.doi.org/10.1016/j.synthmet.2016.11.026

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and enhanced the absorptions of short wavelength photons. They showed that the content of QP in the polymers influenced the photovoltaic performance of the conjugation polymers, and the highest PCE of 5.07% was obtained with 10% QP in the polymer backbone [26]. These results deduced from porphyrins acceptor in the polymers main chains and not refer to the porphyrins conjugation side chains. Hsu and co-workers reported the highest PCE of 8.5% for porphyrin-based conjugation polymers through introducing porphyrin-pyrene pendant on the side chain of the copolymer [27]. They focus on the difference between with and without porphyrin-pyrene pendant while less refer to the effect of content of porphyrin-pyrene pendant on the photovoltaic properties. Herein, a series of new copolymers based on 5-(2,5-bibromothiophene)-10,15,20-tris(4-(octyloxy)phenyl) zinc porphyrin (ZP) and diketopyrrolopyrrole (DPP) were designed and synthesized. Zinc porphyrin derivatives were used as a complementary lightharvesting unit to broaden absorption at Soret band. DPP was chosen as the electron-deficient unit for it possesses good photoconductive, hole-transporting properties [28,29]. The effects of the ZP/DPP ratios on the photophysical, electrochemical properties and aggregation of the random terpolymers were carefully studied. The results indicated that the copolymer P(ZP-BT-DPP4) (ZP: DPP = 1:4) showed stronger absorption and more favorable film morphology than the copolymers with other ZP/DPP ratios, so the polymer solar cell based on the copolymer P (ZP-BT-DPP4) showed the highest short-circuit current (Jsc) and the best power conversion efficiency (PCE).

2. Experimental 2.1. Materials The compounds and copolymers were synthesized according to Scheme 1. All the chemicals and thiophene-3-carbaldehyde, 4-hydroxybenzaldehyde, 1H-pyrrole, 2,20 -bithiophene-5,50 -bis(trimethylstannane) (BT) were purchased from Alfa Aesar, Chem Greatwall and SunaTech Inc. China, respectively. 3,6-Bis(5bromothien-2-yl)-2,5-bis(2-octyldodecyl)pyrrolo-[3,4-c]pyrrole1,4-dione and 5-(2,4-dibromo thiophene)-10,15,20 tris(4-(octyloxy)phenyl) zinc porphyrin was the first synthesized according to our previously reported procedures [30,31]. 2.2. Characterization Nuclear magnetic resonance (NMR) spectra of the compounds and copolymers were performed on a Bruker AVANCE 400 spectrometer. Matrix-assisted laser desorption/ionization time of flight (MALDI-TOF) mass Spectrometry were measured with a Bruker Autoflex III. UV–vis–Near Infrared (UV–vis–NIR) absorption spectra of the copolymers were determined on a Perkin–Elmer Cary 100 UV–vis–NIR spectrometer. The average molecular weight of the copolymers were conducted using Waters 1515 gel permeation chromatography (GPC) analysis with THF as eluent and polystyrene as standard. Thermogravimetric analysis (TGA) measurement was measured with a Netzsch TG 209 analyzer. Cyclic voltammetry (CV) were obtained by using a Pt plate as the

Scheme 1. Synthetic routes of monomers and copolymers.

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working electrode, Pt slice as the counter electrode, and Ag/Ag+ (saturated 10 mmol/L AgNO3) electrode as the reference electrode at a scan rate of 100 mV s1. The supporting electrolyte is 0.1 M tetrabutylammonium-n-butylammonium hexafluorophosphate (Bu4NPF6) in anhydrous acetonitrile solution. Atomic force microscopy (AFM) were measured on a Veecomulty mode 8 instrument in a tapping mode. 2.3. Synthesis of 2,5-dibromothiophene-3-carbaldehyde (2) In a 250 mL one-necked flask, after 3-thiophenecarboxaldehyde (2.24 g, 20 mmol) was dissolved in 80 mL DMF, a mixture of Nbromosuccinimide (NBS) (8.95 g, 50 mmol) in DMF (30 mL) was added dropwise and the solution was stirred for 24 h in the dark. Then, the mixture was poured into water and extracted with diethyl ether. The organic phase was dried over anhydrous MgSO4, and the solvent was removed by rotary evaporation. The crude product was purified by column chromatography (silica gel, petroleum ether/dichloromethane mixture = 3:1) to afford a white solid of compound 2 (4.96 g, 92%).1H NMR (400 MHz, CDCl3, d, ppm): 9.80 (s,1H).7.35 (s, 1H). 2.4. Synthesis of 4-(octyloxy)benzaldehyde (4) In a 250 mL one-necked flask, 4-hydroxybenzaldehyde (4.4 g, 36 mmol), n-octylbromide (7.7 g, 40 mmol) and potassium carbonate (5.5 g, 40 mmol) were dissolved in 100 mL DMF, and the mixture was heated to 140  C for 8 h. The mixture was stirred at room temperature for 30 min, then cooling to room temperature and stirred for another 30 min. The reaction was quenched by adding cooled water. The mixture was extracted with chloroform, and the organic layer was washed with saturated brine for three times and dried over anhydrous MgSO4. After the solvent was removed by rotary evaporation, the residue was purified by column chromatography (silica gel, petroleum ether/acetic mixture = 3:1) to afford a colorless liquid of compound 4 (6.4 g, 76%). 1H NMR (400 MHz, CDCl3) d 9.88 (s, 1H), 7.83 (d, J = 7.6 Hz, 2H), 6.99 (d, J = 7.6 Hz, 2H), 4.04 (t, J = 5.7 Hz, 2H), 2.01–1.66 (m, 2H), 1.38 (d, J = 69.9 Hz, 10H), 0.89 (s, 3H). 2.5. Synthesis of 5-(2,5-dibromothiophene)-10,15,20-tris(4-(octyloxy) phenyl) porphyrin (6) In a 250 mL three-necked round-bottomed flask, 2,5-dibromothiophene-3- carbaldehyde (2.3 g, 8.5 mmol) and 4-(octyloxy) benzaldehyde (5.97 g, 25.5 mmol) were dissolved in 100 mL propionicacid. The solution was heated and refluxed at 140  C. Pyrrole (5.6 mL, 80.0 mmol) was added dropwise and stirred for another 30 min. After cooling to room temperature, half of the solvent was evaporated and 100 mL CH3OH was added. The mixture was placed in a refrigeratory overnight and filtrated under suction. The crude production was purified by column chromatography (petroleum ether/dichloromethane mixture = 3:1 as an eluent). After recrystallization from the mixture of CHCl3 and CH3OH (1: 5), a desired purple solid of compound 6 was obtained (1.8 g, 18%). 1H NMR (400 MHz, CDCl3, d, ppm): 8.82 (d, J = 22.3 Hz, 6H), 8.02 (d, J = 21.9 Hz, 7H), 7.68 (s, 1H), 4.16 (s, 6H), 1.90 (s, 6H), 1.48 (dd, J = 195.8, 92.6 Hz, 30H), 0.84 (d, J = 24.2 Hz, 9H), 2.83 (s, 2H). MALDI-TOF MS (C66H74Br2N4O3S) m/z: calcd: 1163.19; found, 1163.42.

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quenched with water (100 mL), and the mixture was extracted with DCM (2  100 mL). The combined extracts were washed with water and dried over anhydrous MgSO4. The solvent was removed under reduced pressure to give the product. Yield (0.24 g, 93%). 1H NMR (400 MHz, CDCl3) d 9.23–9.09 (m, 1H), 9.09–8.90 (m, 3H), 8.08 (dd, J = 41.5, 33.6 Hz, 3H), 7.89 (s, 1H), 7.76 (s, 1H), 4.27 (t, J = 6.2 Hz, 6H), 1.99 (d, J = 7.0 Hz, 6H), 1.64–1.05 (d, J = 7.1 Hz, 26H), 0.95 (d, J = 6.4 Hz, 12H). Anal. calcd for C66H72Br2N4O3SZn: C, 64.63; H, 5.92; N, 4.57; S, 2.61. Found: C, 63.16; H, 6.27; N, 4.58; S, 2.65. MALDI-TOF MS calcd. for (C66H72Br2N4O3SZn) m/z: 1226.56, found: 1226.37. 2.7. Synthesis of P(ZP-BT-DPP1) 5-(2,4-Dibromothiophene)-10,15,20-tris(4-(octyloxy)phenyl) zinc porphyrin (61.30 mg, 0.05 mmol), 2,20 -bithiophene-5,50 -bis(trimethylstannane) (BT) (49.20 mg, 0.10 mmol), and 3,6-bis(5bromothien-2-yl)-2,5-bis(2-octyldodecyl)pyrrolo[3,4-c] pyrrole1,4-dione (DPP) (50.96 mg, 0.05 mmol) were added in a twonecked flask and subsequently dissolved in 6 mL of chlorobenzene. The mixture was purged with nitrogen for 15 min, and then Pd2(dba)3 (5.00 mg, 0.005 mmol) and P(o-tol)3 (12.50 mg, 0.04 mmol) were added. After being purged for another 15 min, the reaction mixture was stirred at 135  C for 72 h. After being cooled to room temperature, the reaction mixture was added dropwise to 200 mL of methanol and then collected by filtration and washed with methanol. Then the solid was subjected to Soxhlet extraction with methanol, hexane, and chloroform. Subsequently, the fraction extracted by chloroform was evaporated under reduced pressure and then precipitated in methanol, filtered, and finally dried under vacuum to obtain a black solid 89.0 mg, yield 78.9%. 2.8. Synthesis of P(ZP-BT-DPP3) 5-(2,4-Dibromothiophene)-10,15,20-tris(4-(octyloxy)phenyl) zinc porphyrin (36.78 mg, 0.03 mmol), 2,20 -bithiophene-5,50 -bis(trimethylstannane) (BT) (58.92 mg, 0.12 mmol), and 3,6-bis(5bromothien-2-yl)-2,5-bis(2-octyldodecyl)pyrrolo[3,4-c] pyrrole 1,4-dione (DPP) (91.71 mg, 0.09 mmol) were added in a twonecked flask and subsequently dissolved in 6 mL of chlorobenzene. The mixture was purged with nitrogen for 15 min, and then Pd2(dba)3 (5.0 mg, 0.005 mmol) and P(o-tol)3 (12.5 mg, 0.04 mmol) were added. The concrete steps were operated using the same procedure as P(ZX-BT-DPP1), and finally dried under vacuum to obtain a purple solid 70 0.0 mg, yield 66.2% 2.9. Synthesis of P(ZP-BT-DPP5) 5-(2,4-Dibromothiophene)-10,15,20-tris(4-(octyloxy)phenyl) zinc porphyrin (24.52 mg, 0.02 mmol), 2,20 -bithiophene-5,50 -bis(trimethylstannane) (BT) (58.92 mg, 0.12 mmol), and 3,6-bis(5bromothien-2-yl)-2,5-bis(2-octyldodecyl)pyrrolo[3,4-c] pyrrole1,4-dione (DPP) (101.90 mg, 0.10 mmol) were added in a twonecked flask and subsequently dissolved in 6 mL of chlorobenzene. The mixture was purged with nitrogen for 15 min, and then Pd2(dba)3 (5 mg, 0.005 mmol) and P(o-tol)3 (12.5 mg, 0.04 mmol) were added. The concrete steps were operated using the same procedure as P(ZP-BT-DPP1), and finally dried under vacuum to obtain a blue solid 108.0 mg, yield 85.0%. 3. Results and discussion

2.6. Synthesis of ZP 3.1. Synthesis and characterization A suspension of compound 6 (0.245 g, 0.21 mmol) and Zn (OAc)22H2O (0.460 g, 2.10 mmol) in a mixture of DCM (60 mL) and methanol (40 mL) were stirred at 23  C for 3 h. The reaction was

The synthetic route of copolymers is outlined in Scheme 1. All copolymers were synthesized by palladium-catalyzed Stille-

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Table 1 Molecular weights and thermal properties of the copolymers. Copolymers

Mn (kDa)

Mw (kDa)

PDI (Mw/Mn)

Td ( C)a

ZP-BT-DPP1 ZP-BT-DPP3 ZP-BT-DPP4 ZP-BT-DPP5

22.8 58.1 86.2 27.8

54.3 234.8 257.4 98.3

2.4 4.0 3.0 3.5

419 414 404 421

3.3. Optical properties

The 5% weight-loss temperature in nitrogen.

coupling polymerization. The detailed procedure is described in the experimental section. 5-(2,4-Dibromothiophene)-10,15,20-tris (4-(octyloxy)phenyl) zinc porphyrin was synthesized according to our previously reported procedures [32,33]. The polymers P(ZPBT-DPP1), P(ZP-BT-DPP3), P(ZP-BT-DPP4) and P(ZP-BT-DPP5) were synthesized with different feed ratio of ZP. All copolymers had good solubility in common organic solvents such as chloroform, tetrahydrofuran, and dichlorobenzene. The numberaverage molecular weight (Mn), weight-average molecular weight (Mw), and polydispersity index (PDI) were confirmed by gelpermeation chromatography (GPC). As shown in Table 1, the copolymers synthesized in this work have Mn in the range of 20–90 kg mol1. 3.2. Thermal stability of copolymers The thermal properties of the copolymers containing zinc porphyrin were determined by thermogravimetric analysis (TGA). As shown in Fig. 1, the copolymers showed good thermal stability

100

Weight(%)

90 80 70

ZP-BT-DPP1 ZP-BT-DPP3 ZP-BT-DPP4 ZP-BT-DPP5

60 50 40 0

100

200

300

400

500

600

Temperature（°C） Fig. 1. TGA plots of the copolymers with a heating rate of 20  C/min under a nitrogen atmosphere.

ε/10 4 （g/ml ）-1 cm-1

1.6 1.4

(a)

ZP-BT-DPP1 ZP-BT-DPP3 ZP-BT-DPP4 ZP-BT-DPP5

1.2 1.0 0.8 0.6 0.4 0.2 0.0 300

400

500

600

700

Wavelength(nm)

800

900

The UV–vis absorption spectra of the copolymers in chloroform solution and thin films are shown in Fig. 2. The correlated optical data are summarized in Table 2, including the absorption peak wavelengths (labs), the absorption edge wavelengths (lonset), and the optical bandgap (Egopt). As shown in Fig. 2a, all the copolymers in the solution exhibit two distinct absorption bands. The Soret band in the region of 400–500 nm can attribute to the introduction of ZP. The other absorption bands at longer wavelengths (600–900 nm) with the maximum absorption peak at 678, 745, 747, and 759 nm for P(ZP-BT-DPP1), P(ZP-BT-DPP3), P(ZP-BTDPP4) and P(ZP-BT-DPP5), respectively, are attributed to intramolecular charge transfer (ICT) between the electron-donating and electron-withdrawing units [34]. It is worth noting that, among the copolymers, P(ZP-BT-DPP1) with 25 mol% ZP feed ratio shows a strong absorption in the short wavelength region of 400– 450 nm and the extinction coefficient at the peak wavelength is 1.52  104 (g/mL)1 cm1, but a relatively weak absorption in the long wavelength region of 500–800 nm. P(ZP-BT-DPP4) with 10 mol% ZP feed ratio shows a reasonable enhanced absorption in the short wavelength region, meanwhile maintaining the extended absorption to near-infrared region at 600–800 nm with the extinction coefficients are 1.01 104(g/mL)1 cm1 at 427 nm and 0.88  104(g/mL)1 cm1 at 747 nm. Obviously, the appropriate feed ratio of ZP is a key matter for achieving optimized absorption. In solid films (Fig. 2b), the absorption spectra of the copolymers show broader absorption with red-shifts compared with the solution spectra because of the stronger intermolecular interaction on the solid state. We can found that, with the increase of ZP, the absorption spectra blue-shift. This is because, on one hand, ZP is a bulky unit, introducing excessive amount of ZP unit would weaken the intermolecular interaction of the copolymer and lead to the decrease of p–p stacking distance. On the other hand, ZP is a less electron-deficient acceptor unit than the DPP unit, introducing excessive amount of ZP unit would also weaken the ICT effect between the electron-donating and electron-withdrawing units, resulting in the weaken and blue-shifted absorption in the long wavelength region [35]. The optical gaps of the copolymers were estimated to be 1.46, 1.43, 1.42 and 1.41 eV for P(ZP-BT-DPP1), P(ZP-BT-DPP3), P(ZP-BT-DPP4), P(ZP-BT-DPP5), respectively, which were determined by the onset of their light absorptions [36]. In conclusion, P(ZP-BT-DPP4) with the ZP feed ratio of 10 mol% is the best ratio for the integrated optical properties. The

Normalized absorption(a.u.)

a

with onset decomposition temperatures (Td) with 5% weight loss surpass 400  C. Obviously, these copolymers exhibited adequate thermal stability for fabricating PSCs devices.

(b)

ZP-BT-DPP1 ZP-BT-DPP3 ZP-BT-DPP4 ZP-BT-DPP5

1.0 0.8 0.6 0.4 0.2 0.0 300

400

500

600

700

800

900

Wavelength(nm)

Fig. 2. UV–vis absorption spectra of copolymers (a) in chloroform solution and (b) in thin films.

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Table 2 Optical and electrochemical properties of the copolymers. Copolymers

labs

labs

426, 678 427, 745 427, 747 427, 759

436, 436, 435, 436,

in solna (nm) ZP-BT-DPP1 ZP-BT-DPP3 ZP-BT-DPP4 ZP-BT-DPP5 a b c d

in film (nm)b 709 751 777 777

lonset

Egopt (eV)c

HOMO/Eox (eV/V)

LUMO/Ered (eV/V)

Egec (eV)d

in film (nm) 850 869 870 874

1.46 1.43 1.42 1.41

5.14/0.42 5.16/0.44 5.18/0.46 5.14/0.42

3.66/1.06 3.72/1.0 3.64/1.08 3.76/1.08

1.48 1.44 1.54 1.50

Measured in chloroform solution. Cast from chloroform solution. Bandgap estimated from the onset wavelength (lonset) of the optical absorption: Egopt = 1240 nmeV/lonset. Egec = e (EoxEred) (eV).

results indicate that it is feasible to modulate the absorption range and bandgaps of the copolymers by controlling the feed ratio of the ZP units. 3.4. Electrochemical properties Cyclic voltammetry (CV) was used as an effective tool in investigating electrochemical properties of copolymers. Cyclic voltammograms of the copolymer films are shown in Fig. 3, The redox potential of ferrocene is 0.08 V vs Ag/Ag+ electrode. Which is assumed to an absolute energy level of 4.8 eV under vacuum. related to the Ag/Ag+ (saturated AgNO3 solution) electrode. The corresponding HOMO and LUMO energy levels of copolymers were calculated according to the equations [37–39]: EHOMO = –e (Eox + 4.72) (eV)

(1)

ELUMO = –e (Ered + 4.72) (eV)

(2)

From the value of Eox and Ered of the polymers, the HOMO, LUMO, and electrochemical bandgaps (Egec) were calculatedand shown in Fig. 4 and Table 2. The HOMO levels are 5.14, 5.16, 5.18 and 5.14 eV for P(ZP-BT-DPP1), P(ZP-BT-DPP3), P(ZP-BTDPP4) and P(ZP-BT-DPP5), respectively. Among them, P(ZP-BTDPP4) has a deeper HOMO levels which was beneficial for a higher Voc in PSCs and good air stability. The LUMO levels of P(ZP-BTDPP1), P(ZP-BT-DPP3), P(ZP-BT-DPP4) and P(ZP-BT-DPP5) were 3.66, 3.72, 3.64 and 3.76 eV, respectively. Their LUMO values were higher than the LUMO energy level of PCBM which suggested that the energy offsets between the LUMO energy levels of the copolymer and PCBM were sufficient to facilitate efficient charge transfer/separation at the interfaces of polymer and PCBM [40]. This result demonstrates that the introduction of a reasonable amount of ZP can effectively enhance the light harvesting ability and decrease the HOMO level. That is, increase Jsc and Voc of the PSCs. 3.5. Film morphology

Current(a.u.)

ZP-BT-DPP1 ZP-BT-DPP3 ZP-BT-DPP4 ZP-BT-DPP5 + Fc / Fc

-1.5

-1.0

-0.5

0.0

0.5

1.0

Potential(V) Fig. 3. Cyclic voltammograms of the polymer films on a platinum electrode in 0.1 mol/L Bu4NPF6acetonitrile solution at a scan rate of 100 mV/s.

To gain a better understanding of the relationship between the PSCs performance and the film microstructures, we have also investigated the morphologies of the four copolymers: PC61BM (1: 2, w/w) blends using atomic force microscopy (AFM), as shown in Fig. 5. The blend films of P(ZP-BT-DPP1), P(ZP-BT-DPP3), P(ZPBT-DPP4) and P(ZP-BT-DPP5) showed the morphology with average surface roughness (Ra) of 1.63, 2.01, 0.95 and 1.25 nm, respectively. The blend film based on P(ZP-BT-DPP4) showed a clearly nanoscaled interpenetrating network, which is beneficial to the exciton dissociation and charge carriers transport. Therefore, the photovoltaic device based on P(ZP-BT-DPP4)/PC61BM will obtain a better Jsc and FF. This large discrepancy of Ra in morphologies among these copolymers may be explained by the intrinsic structure of the copolymers. P(ZP-BT-DPP1) and P(ZPBT-DPP3) with 25 mol% and 12.5 mol% ZP feed ratio tend to form excessive stacked copolymer clusters because of a strong pp interaction in ZP skeletons, leading to the rough morphology. P(ZP-

Fig. 4. Energy level diagrams for P(ZP-BT-DPP1), P(ZP-BT-DPP3), P(ZP-BT-DPP4) and P(ZP-BT-DPP5).

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Fig. 5. AFM height (top) and phase (bottom) images (3  3 mm2) of (a, e) P(ZP-BT-DPP1)/PC61BM (1:2, w/w), (b, f) P(ZP-BT-DPP3)/PC61BM (1:2, w/w), (c, g) P(ZP-BT-DPP4)/ PC61BM (1:2, w/w), and (d, h), P(ZP-BT-DPP5)/PC61BM(1:2, w/w).

BT-DPP4) and P(ZP-BT-DPP5) with 10 mol% and 8.3 mol% ZP feed ratio have a weak pp interaction in ZP skeletons to cause less stacked copolymer clusters, so the smooth and homogeneous blend films were observed, but excessive DPP unit in copolymer P (ZP-BT-DPP5) can also cause a slight increase of roughness, so the copolymer P(ZP-BT-DPP4) showed a minimum value of Ra. The results indicate that by incorporating the appropriate amount of ZP can form suitable nanoscaled phase separation structure of the interpenetrating network. 3.6. Photovoltaic properties The photovoltaic properties of the copolymers were investigated by fabricating BHJ-PSCs devices with a configuration of ITO/PEDOT:PSS (30 nm)/copolymer: PC61BM (100 nm)/LiF (0.5 nm)/Al (100 nm). Photoactive layers were spin-coated from

Current Density (mA cm-2 )

8 ZP-BT-DPP1 ZP-BT-DPP3 ZP-BT-DPP4 ZP-BT-DPP5

6 4 2 0

1,2-dichlorobenzene (DCB) of the copolymers and PC61BM. The current densityvoltage characteristics of the PSCs based on copolymer:PC61BM under illumination of AM 1.5 G, 100 mW/cm2 are shown in Fig. 6, and representative data of the solar cells are summarized in Table 3. Copolymers with ZP-incorporation showed different photovoltaic properties depending on the feed ratio of ZP. P(ZP-BT-DPP4) with 10 mol% ZP feed ratio showed the highest PCE of 2.44% among these copolymers, along with a high Jsc of 6.14 mA/ cm2 and high FF of 0.59. The high FF attribute to the suitable film morphology, and the high Jsc attributed to the enhance absorption of the copolymer in the short wavelength region and simultaneously retains the excellent absorption in the long wavelength region. Fig. 7 shows the incident photon-to-current conversion efficiencies (IPCE) curves of the solar cell devices. All the devices showed broad IPCE response range covering from 300 to 900 nm. P (ZP-BT-DPP4) exhibited higher IPCE than other three copolymers which was consistent with its higher photocurrent. The significantly broadened IPCE responses in the visible region can be attributed to both the intrinsic absorption of the copolymers and the response of PC61BM. The calculated Jsc values obtained by integration of the IPCE data for these devices only showed minor mismatch (2–5%) with the Jsc from J-V curves.

Table 3 Photovoltaic performance of the PSCs under illumination of AM 1.5G, 100 mW/cm2.

-2

PCE(%)

-4 Copolymersa

-6 -8 -0.2

0.0

0.2

0.4

0.6

0.8

1.0

Voltage (V ) Fig. 6. Current density–voltage characteristics of PSCs based on copolymer:PC61BM under illumination of AM 1.5G, 100 mW/cm2.

ZP-BT-DPP1 ZP-BT-DPP3 ZP-BT-DPP4 ZP-BT-DPP5 a b

Voc (V)

Jsc (mA/cm2)

FF

0.66 0.65 0.68 0.61

4.03 5.18 6.14 4.87

0.38 0.47 0.59 0.42

max

averageb

mh

(cm2V1S1) 1.13 1.58 2.44 1.24

1.05 1.46 2.28 1.22

1.0  104 1.3  104 1.9  104 1.2  104

The copolymer/PC61BM = 1:2 (w/w). The average PCE values were obtained from the data of over five devices.

Please cite this article in press as: X. Luo, et al., Synthesis and photovoltaic properties of the copolymers containing zinc porphyrin derivatives as pendant groups, Synthetic Met. (2016), http://dx.doi.org/10.1016/j.synthmet.2016.11.026

G Model SYNMET 15515 No. of Pages 7

X. Luo et al. / Synthetic Metals xxx (2016) xxx–xxx

IPCE(%)

30

ZP-BT-DPP1 ZP-BT-DPP3 ZP-BT-DPP4 ZP-BT-DPP5

20

10

0 300

400

500

600

700

800

900

Wavelength(nm) Fig. 7. IPCE spectra of the PSCs based on copolymer/PC61BM blends.

4. Conclusion In summary, a series of new copolymers with oligothiophene as donor units, diketopyrrolopyrrole as acceptor units and zinc porphyrin as pendant group were designed, synthesized and characterized. Incorporation of zinc porphyrin broadened and enhanced the absorption of short wavelength photons as well as keeps the well-tuned HOMO and LUMO energy levels. The copolymer P(ZP-BT-DPP4) with proper ZP feed ratio (10 mol%) shows the strong absorption in the short wavelength region as well as maintains the absorption in the long wavelength region and an appropriate microphase separation. The copolymers with ZP feed ratio either higher or lower than 10 mol% would cause undesired absorption characteristic and inferior microphase separation. So the photovoltaic devices based on P(ZP-BT-DPP4) as donor and PC61BM as acceptor show the highest PCE of 2.44% with an increased Jsc of 6.14 mA/cm2 and an high FF of 0.59. Acknowledgments This work was supported by the National Natural Science Foundation of China (21474081). References [1] G. Yu, J. Gao, J.C. Hummelen, F. Wudl, A.J. Heeger, Science 270 (1995) 1789– 1791. [2] X. Li, W.C. Choy, L. Huo, F. Xie, W.E. Sha, B. Ding, X. Guo, Y. Li, J. Hou, J. You, Adv. Mater. 24 (2012) 3046–3052. [3] L. Dou, W.H. Chang, J. Gao, C.C. Chen, J. You, Y. Yang, Adv. Mater. 25 (2013) 825– 831. [4] J. You, L. Dou, K. Yoshimura, T. Kato, K. Ohya, T. Moriarty, K. Emery, C.-C. Chen, J. Gao, G. Li, Nat. Commun. 4 (2013) 1446–1455. [5] D.F. Zeigler, K.S. Chen, H.L. Yip, Y. Zhang, A. K-Y Jen, J. Polym. Sci. A Polym. Chem. 50 (2012) 1362–1373. [6] Y. Liu, J. Zhao, Z. Li, C. Mu, W. Ma, H. Hu, K. Jiang, H. Lin, H. Ade, H. Yan, Nat. Commun. 5 (2014) 5293–5300.

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Please cite this article in press as: X. Luo, et al., Synthesis and photovoltaic properties of the copolymers containing zinc porphyrin derivatives as pendant groups, Synthetic Met. (2016), http://dx.doi.org/10.1016/j.synthmet.2016.11.026