Synthesis of Anthracene-Based Donor–Acceptor Copolymers with a

Article pubs.acs.org/Macromolecules

Synthesis of Anthracene-Based Donor−Acceptor Copolymers with a Thermally Removable Group for Polymer Solar Cells Chunchen Liu,† Wenzhan Xu,† Xing Guan,† Hin-Lap Yip,*,† Xiong Gong,*,†,‡ Fei Huang,*,† and Yong Cao† †

Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, P. R. China ‡ College of Polymer Science and Polymer Engineering, The University of Akron, Akron, Ohio 44325, United States S Supporting Information *

ABSTRACT: A highly soluble anthracene cyclic adduct with a thermally cleavable substituent was synthesized, and it was used as a donor unit in a series of donor−acceptor type conjugated copolymers with improved processability. The removable group was eliminated under elevated temperature through retro Diels−Alder reaction, which offered the corresponding copolymers with better planarity and rigidity. Thermogravimetric analysis (TGA), FT-IR, and UV−vis spectroscopy were carried out to study the thermal cleavage process. Uniform films were easily formed from these precursor copolymers due to their good solution processabilty. Polymer solar cells were successfully fabricated through applying thermal annealing treatment on the blend films that were spin-coated from solutions of the precursor copolymers blended with fullerene derivatives. The best polymer solar cell device with a power conversion efficiency (PCE) of 2.15% was achieved based on copolymer PCOAEHDPP.

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INTRODUCTION Over the past decade, polymer solar cells (PSCs) with bulk heterojunction (BHJ) structures have attracted considerable attention due to their potential application as a low cost, sustainable energy source.1−11A significant research effort has been dedicated on developing new organic semiconductors with better photovoltaic properties. Although PSCs with power conversion efficiency (PCE) over 9% have been realized with new materials,12−17 the long-term stability of PSCs needs to further improve before they can be commercialized. Therefore, development of new materials with improved thermal and morphological stability is critically needed. Nowadays, the use of alternating electron-rich (donor) and electron-deficient (acceptor) units along the backbone of conjugated polymers has emerged as a popular method to develop novel copolymers with high PCEs.18−25 Through such a design strategy, one can easily tune the molecular frontier orbital by rational choice of the electron-donor and acceptor units. Among different molecular building units, anthracene has been widely used as the electron-donor moiety to construct donor−acceptor (D−A) based copolymers and these polymers had been applied in various types of organic electronic devices. Because of the molecular planarity and rigidity nature of the anthracene building block, the corresponding copolymers showed good charge transporting properties, while the weak donating property of anthracene resulted in polymers with a deep HOMO level, which is favorable to produce PCSs with high Voc and PCEs.26−32,47 Despite anthracene shows several good properties as a donor building block, a major problem for © XXXX American Chemical Society

using unsubstituted anthracene to construct the copolymer is its poor solution processability. Typically, bulky side chains are introduced on the anthracene units at its 9,10-positions to ensure good solution processability of the resulted copolymers. However, such substitutions will influence the intermolecular packing between the copolymers and therefore adversely affect the charge transport property in the thin films. Therefore, it will be interesting to apply novel molecular design concept to synthesize new anthracene-based copolymers with very good solution processability while can still maintain a good packing in the thin film. Anthracene, as a conjugated diene, can be easily reacted with dienophile such as maleic anhydride through Diels−Alder reaction, and the obtained cyclic adduct is highly soluble because of the perturbed rigidity and coplanarity of anthracene as well as the solubilizing effect of the maleic anhydride. Additionally, such cyclic adduct can be successfully converted to the original anthracene structure by retro Diels−Alder reaction at elevated temperature. Several precursor copolymers containing Diels− Alder adduct of anthracene have already been developed for the application in organic thin-film transistors (OFET), and it is demonstrated that these precursor copolymers can be easily converted into the target semiconducting copolymers with desirable charge carrier mobilities, which is originated from the enhanced intermolecular packing between the polymers Received: September 26, 2014 Revised: November 16, 2014

A

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Chart 1. Schematic Diagram for the Thermal Elimination of the Leaving Group and the Steps for Preparation of the Thin Films of the Target Semiconducting Copolymers

poured into water and extracted with ethyl acetate three times. The combined organic phase was then dried over anhydrous magnesium sulfate. After removing the solvent under reduced pressure, a viscous liquid was obtained and washed with petroleum ether several times. The crude product was used for the next step without further purification. Dimethyl 2,6-Dibromo-9,10-dihydro-9,10-ethanoanthracene11,12-dicarboxylate (2). Monomer 1 (4.66 g, 10 mmol) obtained from aforementioned way was dissolved with 20 mL of methanol and then transferred into a 100 mL three-necked round-bottom flask. To this solution, 1 mL of concentrated H2SO4 was added immediately. The mixture was refluxed overnight. After cooled to room temperature, the reaction mixture was poured into water and extracted with ethyl acetate three times. The combined organic phase was then dried over anhydrous magnesium sulfate. After removing the solvent under reduced pressure, the residue was purified using silica gel column chromatography (petroleum ether: dichloromethane (1:4, v:v) to afford the product as white solid (3.3 g, 6.8 mmol, yield 68%). 1H NMR (CDCl3, 300 MHz): 7.46 (t, 2H), 7.30 (m, 2H), 7.21 (m, 2H), 4.54 (s, 2H), 3.55 (d, 6H), 3.24 (t, 2H). 13C NMR (CDCl3, 75 MHz): 171.62, 171.45, 144.30, 142.00, 140.76, 138.58, 129.52, 129.47, 128.23, 127.00, 126.86, 125.43, 120.23, 120.17, 51.99, 51.94, 46.90, 46.73, 45.92, 45.75. MS (EI, M+, C20H16Br2O4): calcd, 480.15; found, 481.0. Dimethyl 2,6-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)9,10-dihydro-9,10-ethanoanthracene-11,12-dicarboxylate (3). Monomer 2 (4.8 g, 10 mmol), bis(pinacolato)diboron (10.16 g, 40 mmol), potassium acetate (3.93 g, 40 mmol), and Pd(dppf)2Cl2 (0.69 g) were added into a three-necked round-bottom flask which was subsequently degassed; then to this mixture was added 150 mL of anhydrous 1,4-dioxane under the protection of nitrogen. The solution was heated to 80 °C overnight. After cooling to room temperature, the reaction solution was poured into water and extracted with dichloromethane three times, and the combined organic phase was dried over anhydrous magnesium sulfate. Removal of the solvent under reduced pressure gave a crude product, which was purified by flash chromatography using dichloromethane and then recrystallized from methanol to obtain final product (4.2 g, 7.3 mmol, yield 73%). 1 H NMR (CDCl3, 300 MHz):δ (ppm) 7.76 (d, 2H) 7.60 (m, 2H), 7.32 (m, 2H), 4.61 (s, 2H), 3.52 (d, 6H), 3.21 (s, 2H), 1.36 (m, 24H). 13 C NMR (CDCl3, 75 MHz): 172.02, 171.99, 146.05, 143.64, 141.65, 139.14, 133.31, 133.20, 131.26, 129.78, 124.57,123.16, 83.80, 83.56, 51.80, 51.76, 47.22, 47.18, 46.70, 46.65, 24.95, 24.82. MS (EI, M+, C32H40B2O8): calcd, 574.28; found, 575.5. Synthesis of the Copolymers. PCOA8OTBT. In a 25 mL dry flask, monomer 3 (57.4 mg, 0.1 mmol), monomer 4 (71.4 mg,

and also the favorable orientation of the polymers on the substrate.33,34 Here we reported a new molecular design concept by applying thermally cleavable group on the anthracene unit to improve its initial solubility while strong packing of the polymers can be achieved after removing the side chains through postannealing of the polymer films. A series of novel precursor copolymers, alternately consisting of the cyclic adduct of anthracene and different acceptor units, such as benzo[c][1,2,5]thiadiazole (BT) and diketopyrrolopyrrole (DPP), have been designed and synthesized. All obtained precursor copolymers, PCOA8OTBT, PCOA12OTBT, PCOAODDPP, and PCOAEHDPP can be completely converted to the target conjugated copolymers with better molecular planarity after being heated at 250 °C for 20 min (Chart 1). It was found that the absorption, frontier molecular orbital energy levels, as well as the solid-state molecular packing of the resulting copolymers could be significantly influenced by the choice of alternating acceptor unit. In the BHJ solar cells, the molecular packing of the target copolymers and their miscibility with [6,6]-phenyl C60-butyric acid methyl ester (PCBM) are the key factors that determine the ultimate PCEs. The best PCE of 2.15% was achieved based on the copolymer PCOAEHDPP with the ethylhexyl substituted DPP as the acceptor moiety.

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EXPERIMENTAL SECTION

Materials. All reagents and solvents, unless otherwise specified, were obtained from Aldrich and Alfa-Aesar Chemical Co. and were used as received. The solvent 1,4-dioxane was distilled from anhydrous magnesium sulfate before use. Monomers 4,35 5,36 6,37 and 737 were synthesized according to the reported procedures. Monomers 1 and 2 were prepared according to the modified methods.34 All of the other compounds were synthesized with the following procedures described below. Synthesis of Monomers. 2,6-Dibromo-12-(methoxycarbonyl)9,10-dihydro-9,10-ethanoanthracene-11-carboxylic Acid (1). 2,6Dibromoanthracene (3.36 g, 10 mmol) and maleic anhydride (3.43 g, 35 mmol) were added into a 100 mL three-necked round-bottom flask under nitrogen atmosphere. Then 40 mL of toluene was added. The mixture was stirred under reflux overnight. To the mixture, 20 mL of methanol was added in five portions. The solution was refluxed overnight. After cooled to room temperature, the reaction mixture was B

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Scheme 1. Synthesis of the Monomers and the Precursor Copolymersa

a

Synthetic routes of monomers and copolymers: (i) toluene, reflux, overnight, then methanol, reflux, overnight; (ii) concentrated H2SO4, methanol, reflux, overnight; (iii) Pd(dppf)2Cl2, KOAc, 1,4-dioxane, 80 °C, overnight; (iv) Pd(OAc)2, tris(2,4,6-trimethoxyphenyl) phosphine, Aliquat 336, 2 M Na2CO3 (aq.), toluene, 100 °C, 4 h. PCOA12OTBT. Yield: 64.1 mg, 63%. 1H NMR (CDCl3, 300 MHz): 8.48 (d, 2H), 7.73 (d, 2H), 7.48 (t, 2H), 7.39 (m, 4H), 4.85 (s, 2H), 4.16 (s, 4H), 3.59 (t, 6H), 3.54 (m, 2H), 1.97 (d, 4H), 1.26 (m, 40H), 0.89 (t, 6H). GPC: Mn = 26.0 kg mol−1, Mw = 53.0 kg mol−1, Mw/Mn = 2.0. PCOAODDPP. Yield: 95.5 mg, 79%.1H NMR (CDCl3, 300 MHz): 8.91 (s, 2H), 7.67 (d, 2H), 7.42 (m, 4H), 7.32 (d, 2H), 4.85 (d, 2H), 4.06 (s, 4H), 3.69 (d, 6H), 3.51 (s, 2H), 2.24 (t, 2H), 1.25 (m, 70H), 1.02 (t, 6H). GPC: Mn = 31.0 kg mol−1, Mw = 47.0 kg mol−1, Mw/Mn = 1.51. PCOAEHDPP. Yield: 67.8 mg, 73%). 1H NMR (CDCl3, 300 MHz): 8.93 (d, 2H), 7.66 (d, 2H), 7.45 (m, 6H), 4.85 (d, 2H), 4.07 (s, 4H), 3.58 (m, 6H), 3.32 (s, 2H), 1.93 (s, 2H), 1.29 (m, 18H), 0.93 (t, 6H). GPC: Mn = 20.0 kg mol−1, Mw = 31.0 kg mol−1, Mw/Mn = 1.55. Measurement and Characterization. 1H and 13C NMR were characterized with Bruker-300 spectrometer operating at 300 and 75 MHz in deuterated chloroform solution at 298 K. Chemical shifts were recorded as δ values (ppm) with the internal standard of tetramethylsilane (TMS).The number-average (Mn) and weightaverage (Mw) molecular weights were determined with Waters GPC 2410 in THF using a calibration curve with standard polystyrene as a reference. Mass spectra were carried out on a ACQ-TQD#QBB1460 instrument. Thermogravimetric analyses (TGA) were performed on a Netzsch TG 209 under nitrogen at a heating rate of 10 °C min−1.

0.1 mmol), and 2 drops of Aliquat 336 were dissolved in degassed toluene (4 mL) under the protection of nitrogen. The mixture was vigorously stirred at 95 °C for 30 min, then Pd(OAc)2 (4 mg) and tris(2,4,6-trimethoxyphenyl) phosphine (20 mg) were added quickly. After stirred for another 30 min, 1 mL of Na2CO3 (aq.) (2 M) was slowly dropped into the reaction mixture, and then the solution was stirred at 100 °C for 4 h. After cooling to room temperature, the mixture was dropped into methanol. The polymer was precipitated and then collected by filtration. Then the polymer was washed in a Soxhlet extractor with acetone and hexane for 24 h sequentially. After that, the polymer was Soxhlet-extracted with chloroform. The chloroform fraction was collected and concentrated under reduced pressure, and which was then precipitated in methanol. The final product was collected by filtration and dried under vacuum at 40 °C overnight (60.5 mg, yield 68%).1H NMR (CDCl3, 300 MHz): 8.48 (d, 2H), 7.72 (d, 2H), 7.51 (t, 2H), 7.40 (m, 4H), 4.84 (s, 2H), 4.16 (s, 4H), 3.69 (d, 6H), 3.56 (t, 2H), 1.99 (t, 4H), 1.34 (m, 24H), 0.88 (t, 6H). GPC: Mn = 18.0 kg mol−1, Mw = 31.0 kg mol−1, Mw/Mn = 1.67. The other copolymers PCOA12OTBT, PCOAODDPP, and PCOAEHDPP were prepared according to a similar procedure as PCOA8OTBT. C

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Table 1. Molecular Weights and Thermal Properties of the Copolymers: Theoretical (a) and Experimental (b) Weight Loss of the Dimethyl Maleate Group with Td Being the First Decomposition Temperature copolymers

Mn (kg mol−1)

Mw (kg mol−1)

PDI

Td (°C)

weight lossa (%)

weight lossb (%)

PCOA8OTBT PCOA12OTBT PCOAODDPP PCOAEHDPP

18.0 26.0 31.0 20.0

31.0 53.0 47.0 31.0

1.67 2.00 1.51 1.55

223 231 244 232

16.4 14.6 12.2 16.0

15.6 12.7 10.8 15.2

FT-IR spectra were observed on a NEXUS 670 instrument. UV−vis absorption spectra were recorded on a HP 8453 spectrophotometer. Cyclic voltammetry (CV) was performed on a CHI600D electrochemical workstation with a working electrode of ITO-coated glass and a Pt wire counter electrode at a scanning rate of 50 mV s−1 against a reference electrode of saturated calomel electrode (SCE) with a nitrogen saturated anhydrous solution of tetra-n-butylammonium hexafluorophosphate in acetonitrile (0.1 mol L−1). Atomic force microscopy (AFM) measurements were carried out using a Digital Instrumental DI Multimode Nanoscope III in a taping mode. Device Fabrication and Characterization. The indium tin oxide (ITO) coated substrates were first cleaned by ultrasonic treatment in deionized water, acetone, and isopropyl alcohol and then dried in a nitrogen stream. Polymer solar cells with inverted structures were then fabricated with the structure of ITO/ZnO/copolymer: PCBM/MoO3/Al under the conditions as follows: a 35 nm thick layer of sol−gel processed ZnO as the interlayer was prepared on the ITO substrates according to the literature reported procedures,38 the precursor copolymer and PCBM were codissolved in a mixed solvent (1,2-dichlorobenzene: chlorobenzene = 1.1, v:v) with a weight ratio of 1:1, and the concentration of the precursor copolymers/PCBM blending solution used in this study was 10 mg mL−1. All the active layers were then spincoated from the corresponding solutions with a thickness of ca. 90 nm, and then the films were annealed at 250 °C for 20 min for the cleavage of the leaving group. Finally, 10 nm of MoO3 and 90 nm of Al were subsequently evaporated onto the active layer to form the top electrode of the device. The effective device area was 0.16 cm2. The current density−voltage (J−V) characteristics were investigated with a Keithley 236 source meter. The spectral responses were measured with a commercial photomodulation spectroscopic setup (Oriel). The EQEs (external quantum efficiencies) were recorded on a Hyper monolight System (CROWNTECH-1000).

copolymers exhibited excellent solubility in common organic solvents such as tetrahydrofuran, chloroform, chlorobenzene, and 1,2-dichlorobenzene due to inferior planarity and rigidity of the precursor copolymer backbone and solubilizing effect of the thermally removable side chains. Uniform films could be easily prepared from the corresponding solutions of all the precursor copolymers, indicating their great potential for application in PSCs. The Mn of the copolymers are in the range of 18.0−31.0 kg mol−1, with the corresponding polydispersity indexes (PDI) in the range of 1.51−2.0, which were summarized in Table 1. Thermal and FT-IR Properties. The thermal properties of the precursor copolymers were evaluated by thermogravimetric analysis (TGA), which demonstrated that all copolymers undergo two thermal decomposition steps at elevated temperature (Figure 1). The first decomposition correlated to the

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RESULTS AND DISCUSSION Synthesis. The synthetic routes of all the monomers and polymers are illustrated in Scheme 1. Treatment of 2,6dibromoanthracene with an excess of maleic anhydride in toluene at reflux temperature gave the Diels−Alder adduct, which was then reacted with methanol at reflux to give the half ester 2,6-dibromo-12-(methoxycarbonyl)-9,10-dihydro-9,10ethanoanthracene-11-carboxylic acid (1). Monomer 1 was further reacted with methanol catalyzed by concentrated H2SO4 to offer the diester dimethyl 2,6-dibromo-9,10-dihydro-9,10-ethanoanthracene11,12-dicarboxylate (2). Monomer 3 was obtained by a [1,1′bis(diphenylphosphino)ferrocene]dichloropalladium(II) (Pd(dppf)2Cl2) catalyzed Miyaura reaction of 2 with an excess amount of 4,4,4′,4′,5,5,5′,5′-octamethyl-2,2′-bi(1,3,2-dioxaborolane). The copolymers were prepared by Suzuki copolymerization of 3 with 4, 5, 6, and 7, respectively, and the corresponding precursor copolymers were denoted as PCOA8OTBT, PCOA12OTBT, PCOAODDPP, and PCOAEHDPP, respectively. Purifications of the crude copolymers were performed by Soxhlet extraction sequentially with acetone and hexane to remove low molecular weight impurities and then with chloroform to extract the polymer product. Finally, the chloroform fraction was collected, concentrated and reprecipitated from methanol to achieve the desired copolymers. All precursor

Figure 1. Thermogravimetric profiles of the precursor copolymers.

thermal cleavage of dimethyl maleate from anthracene unit through retro Diels−Alder reaction, which occurred at 223 °C for PCOA8OTBT, 231 °C for PCOA12OTBT, 244 °C for PCOAODDPP, and 232 °C for PCOAEHDPP, respectively. This result revealed that all precursor copolymers could be successfully converted at the temperature around 250 °C. Moreover, the first weight losses of 15.6%, 12.7%, 10.8% and 15.2% for PCOA8OTBT, PCOA12OTNT, PCOAODDPP, and PCOAEHDPP, respectively, corresponded well with the theoretical weight losses of the dimethyl maleate group as gaseous products from the corresponding precursor copolymers, indicating the nearly quantitative yield of target polymers (Table 1). An additional weight loss step occurred at over 300 °C, as can be attributed to the decomposition of the polymer backbones, which indicated good thermal stability of final polymers after cleavage of dimethyl maleate group. FT-IR spectroscopy (Figure 2) was also performed to investigate the thermal conversion process of the precursor copolymers. The FT-IR spectra of all the precursor copolymers before heating showed a strong carbonyl stretching peak at ca. D

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Figure 2. FT-IR spectra for the films of the precursor copolymers: as-coated (black curves), with thermal annealing at 250 °C for 20 min (red curves).

1700 cm−1. However, after heating the films of all the precursor copolymers at 250 °C for 20 min, the carbonyl stretching peak disappeared completely. It further suggested that that the dimethyl maleate leaving group could be completely removed from all the precursor copolymers after thermal treatment. Optical Properties. The UV−vis absorption studies for all the precursor copolymers in chloroform solutions (ca. 2 mg mL−1) and as thin films without and with heating were performed to investigate their absorption properties. Copolymers PCOA8OTBT and PCOA12OTBT with benzo[c][1,2,5]thiadiazole (BT) as acceptor unit showed similar absorption profiles in chloroform solution and as thin films, where the short wavelength peaks located at ca. 360 nm can be ascribed to the π−π* transition, and the long wavelength peaks can be attributed to the intramolecular charge transfer (ICT) interaction between donor and acceptor (BT) moieties.39 Copolymers PCOAODDPP and PCOAEHDPP with DPP as electron deficient unit showed the typical absorption profiles of the DPP-based D−A type copolymers both in chloroform solutions and as thin films, where two weak absorption bands in the short wavelength and a strong absorption band in the long wavelength region could be found.40 The broad absorption band in the long wavelength region can be ascribed to the intramolecular charge transfer (ICT) interaction between donor and acceptor (DPP) moieties.39 It should be noted that the absorption spectra of copolymers based on DPP exhibited a much red-shifted absorption band with respect to that based on BT unit due to the stronger electron withdrawing ability of the former unit.41 It is also worth pointing out that the long wavelength absorption band of all the precursor copolymers films before heating exhibited a ca. 10 nm red shift compared with that in solution, indicating that intermolecular π−π stacking is formed in thin films. Impressively, a ca. 100 nm red shift in the long wavelength absorption band can be found in all the thermally annealed thin films with respect to those before

heating (Figure 3). The significant red shift in the absorption spectrum can be attributed to the recovery of the good planarity and rigidity of the anthracene unit, which further enhanced the packing between the polymers and the effective conjugated length was also increased when the dimethyl maleates were eliminated from the precursor copolymers backbone. The optical bandgaps (Egopt) of the precursor copolymers before and after heating were both deduced from the absorption onset of the corresponding thin films. It could be found that the Egopt of all copolymers after annealing were smaller than those before heating, which is due to the enhanced intramolecular charge transfer (ICT) effect between the recovered anthracene and acceptor units. The Egopt of precursor copolymers PCOA8OTBT and PCOA12OTBT were 2.17 and 2.18 eV before heating while after heating they reduced to 1.91 and 1.93 eV, respectively. Similarly, precursor copolymers PCOAODDPP and PCOAEHDPP exhibited Egopt of 1.86 and 1.85 eV before heating and they reduced to 1.60 and 1.56 eV after heating, respectively (Table 2). Electrochemical Properties. The cyclic voltammetry (CV) measurements were carried out under an inert atmosphere by using tetra-n-butylammonium hexafluorophosphate (Bu4NPF6, 0.1 M in acetonitrile) as the supporting electrolyte with a ITO-coated glass working electrode, a platinum wire counter electrode and a reference electrode of saturated calomel electrode (SCE). The potential of the ferrocene/ ferrocenium (Fc/Fc+) redox couple was measured as the standard. It is assumed that the redox potential of Fc/Fc+ has an absolute energy level of −4.8 eV to vacuum.42 Under the same experimental conditions the half-wave potential of Fc/Fc+ was measured to be 0.31 V with respect to the reference electrode of SCE. Therefore, the HOMO energy levels of the copolymers were calculated according to the following equation E HOMO = −e(Eox + 4.49) (eV) E

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Figure 3. Absorption spectra of copolymers in chloroform solution (s-PCOA8OTBT, s-PCOA12OTBT, s-PCOAODDPP, s-PCOAEHDPP), as films before heating (PCOA8OTBT, PCOA12OTBT, PCOAODDPP, PCOAEHDPP) and as films after heating (H-PCOA8OTBT, H-PCOA12OTBT, H-PCOAODDPP, H-PCOAEHDPP).

We attributed this to the enhanced electron donating ability of the anthracene unit after elimination of the leaving group. The calculated LUMO energy levels for copolymer PCOA8OTBT and PCOA12OTBT after heating were −3.67 and −3.69 eV, respectively, which were higher than the −3.92 and −3.73 eV for PCOAODDPP and PCOAEHDPP due to the stronger electron withdrawing ability of the DPP unit.41 It is worth noting that the LUMO energy levels of all the copolymers after heating were more than 0.3 eV higher than that of the PCBM acceptor, providing good driving force for the dissociation of the excitons.43,44 Bulk Heterojunction Photovoltaic Device Performance. BHJ PSCs were fabricated with an inverted device structure of ITO/ZnO/copolymer: PCBM (without heating or with heating at 250 °C for 20 min)/MoO3/Al, which was proven to have better stability,45 and the device performance was measured under a simulated AM 1.5 illumination of 100 mW/cm2. The devices were fabricated by spin-coating the blend solutions of precursor copolymers and PCBM with an optimized weight ratio of 1:1 in the cosolvent of dichlorobenzene and chlorobenzene (ODCB: CB = 1.1, v:v). Here a layer of ZnO (ca. 35 nm) was prepared on the ITO substrate as

Table 2. Photophysical Properties of the Copolymers: (a) Parameters without Annealing; (b) Parameters after Thermal Cleavage at 250 °C for 20 min copolymers PCOA8OTBT PCOA12OTBT PCOAODDPP PCOAEHDPP

λabs (nm) in solution 345, 345, 341, 340,

482 483 612 611

λ(a)abs (nm) as films

λ(b)abs (nm) as films

350, 352, 350, 348,

351, 352, 350, 350,

493 492 623 625

558 550 737 717

Egopt (a) Egopt (b) 2.17 2.18 1.86 1.85

1.91 1.93 1.60 1.56

where the Eox is the onset of the oxidation potential vs SCE. The detailed electrochemical data is summarized in Table 3 and Figure S1. HOMO energy levels of the precursor copolymers before and after heating were calculated to be in the range of −5.32 to −5.61 eV and −5.29 to −5.58 eV, respectively. It was observed that the HOMO energy levels of the precursor copolymers PCOA8OTBT, PCOA12OTBT, PCOAODDPP, PCOAEHDPP before heating were −5.59, −5.61, −5.53, and −5.32 eV, respectively, while after heating, the HOMOs slightly raised up to −5.58, −5.55, −5.52, and −5.29 eV, respectively.

Table 3. Electrochemical Properties of the Copolymers: (a) Parameters without Annealing; (b) Parameters after Thermal Cleavage at 250 °C for 20 min copolymers

Eox(a) (V)

Eox(b) (V)

HOMO(a) (eV)

HOMO(b) (eV)

LUMO(a) (eV)

LUMO(b) (eV)

PCOA8OTBT PCOA12OTBT PCOAODDPP PCOAEHDPP

1.10 1.12 1.04 0.83

1.09 1.06 1.03 0.80

−5.59 −5.61 −5.53 −5.32

−5.58 −5.55 −5.52 −5.29

−3.42 −3.43 −3.67 −3.47

−3.67 −3.69 −3.92 −3.73

F

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and PCOA12OTBT based on BT unit, this can be attributed to the improved overlap of the former copolymer absorption spectrum with the solar spectrum. The morphologies of the pure polymer films and BHJ films were studied by tapping-mode atomic force microscopy (AFM). The BHJ films were prepared in the same conditions as those for photovoltaic properties measurements, while the pure polymer films were spin-coated from solutions of polymers in ODCB. All the films from the precursor copolymers were heated at 250 °C for 20 min. As shown in Figure 5, we can clearly observe that the surface morphology of all the BHJ films except the one based on PCOAODDPP improved after heating. The BHJ of the copolymer PCOAODDPP before heating showed large aggregates (Figure 5c) while after heating the morphology altered significantly and pinholes with sizes over hundreds of nanometers were developed across the film (Figure 5g), which eventually lead to a roughfilm with roommean-square (RMS) roughness of 44.3 nm. This can be the reason why the lowest Jsc and Voc were achieved in the device based on copolymer PCOAODDPP. The poor morphology of the PCOAODDPP-based BHJ films can be attributed to the low miscibility between the copolymer and PCBM and also the intrinsic poor film formation property of PCOAODDPP (Figure S2c,g). In contrast, the analogue copolymer PCOAEHDPP with shorter 2-ethylhexyl side chains showed smaller phase separation in the corresponding BHJ film after the annealing treatment (Figure 5d,h), which can be attributed to the relatively weak tendency for PCOAEHDPP to aggregate in the pure film as shown in Figure S2(d) and Figure S2(h). Moreover, the phase separation in the BHJ films of copolymers PCOA8OTBT and PCOA12OTBT with annealing treatment (Figure 5e,f) is smaller than that of copolymers PCOAODDPP and PCOAEHDPP (Figure 5g,h) due to their weaker tendency for aggregation as observed in Figure S2. Nevertheless, higher PCE of copolymer PCOAEHDPP-based deviceswere achieved with respect to those based on PCOA8OTBT and PCOA12OTBT due to the broader absorption of PCOAEHDPP (Figure 3) and higher hole mobility of the PCOAEHDPP: PCBM blend film (Table 4), which resulted in improved Jsc and FF. EQE measurements were also performed to study the spectral response of the BHJ solar cells and the EQE spectra are shown in Figure 6. The devices based on copolymer PCOAEHDPP showed the most efficient photo responsively in a broad range from 400 to 750 nm, which accounted for its high Jsc. In contrary, EQE lower than 5% was observed for the device based on copolymer PCOAODDPP, leading to extremely low photocurrent. The devices based on copolymers PCOA8OTBT and PCOA12OTBT exhibited comparable EQE spectrum with moderate efficiency in the main range from 450 to 600 nm. Charge Transport Property. In order to investigate the charge transport properties of the copolymers in the BHJ films, the hole mobilities were measured by using space charge limited current (SCLC) method with the hole-only device configuration of ITO/MoO3 (10 nm)/copolymer: PCBM (90 nm)/MoO3

the electron transporting layer due to its excellent stability at the high temperature, making it compatible for the thermalcleavage process for the precursor copolymers. Device performances with active layer after heating were characterized by J−V curves as shown in Figure 4; nevertheless, almost no

Figure 4. J−V characteristics of the devices with configuration of ITO/ZnO/copolymer: PCBM/MoO3/Al under illumination AM 1.5 from a solar simulator (100 mW cm−2).

photovoltaic response was observed in the devices without thermal treatment. As shown in Table 4, considerable Vocs in the range of 0.26−0.76 V were achieved due to the low-lying HOMO energy levels of these four copolymers after heating at 250 °C for 20 min (Table 4). Copolymers PCOA8OTBT and PCOA12OTBT showed comparable performance with the PCE of 0.79% and 0.85%, respectively. This result can be ascribed to their similar copolymer backbone, absorption spectra as well as frontier orbital molecular energy levels. Interestingly, it should be noted that the performances of copolymers PCOAODDPP and PCOAEHDPP showed a big difference although their molecular structures, molecular weights and optical properties were almost the same. The best PCE of the PCOAEHDPP-based devices was 2.15%, which is much better than that based on PCOAODDPP, which only showed a PCE of 0.06%. The FF of copolymers PCOAODDPP and PCOAEHDPP were comparable, however, the Jsc and Voc of 0.58 mA cm−2 and 0.26 V for PCOAODDPP were much lower than 7.78 mA cm−2 and 0.76 V for PCOAEHDPP with shorter side chains. We attribute the drastic difference in performance for the two DPP-based polymers to the different morphologies of their BHJ films, which will be discussed in next section. It should be also noted that copolymer PCOAEHDPP based on acceptor unit (DPP) with the stronger electron withdrawing ability exhibited a higher Jsc with respect to those of copolymers PCOA8OTBT

Table 4. Photovoltaic Parameters of Devices Measured under Illumination of Simulated AM 1.5G Conditions (100 mW cm−2) and Hole Mobilities of Blend Films Measured by SCLC Methods copolymer

Jsc (mA cm−2)

Voc (V)

FF

PCE (%)

PCOA8OTBT PCOA12OTBT PCOAODDPP PCOAEHDPP

4.19 3.51 0.58 7.78

0.61 0.73 0.26 0.76

30.83 33.19 38.55 36.32

0.79 0.85 0.06 2.15

G

μ (cm2 V−1 s−1) 2.4 1.7 2.3 6.2

× × × ×

10−6 10−5 10−6 10−5

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Figure 5. Surface topographic AFM images (5 μm × 5 μm) of the polymer:PCBM blend films: (a) PCOA8OTBT, (b) PCOA12OTBT, (c) PCOAODPP, and (d) PCOAEHDPP without thermal annealing; (e) PCOA8OTBT, (f) PCOA12OTBT, (g) PCOAODPP, and (h) PCOAEHDPP with annealing at 250 °C for 20 min.

devices. This indicated that side chains with different lengths in the acceptor BT unit could also affect the charge transport properties of the target copolymers despite the effect is less severe when compared to the case of DPP-based copolymers. The comparable mobilities of the copolymers based on BT unit also explain why similar Jsc and FF were observed in the devices based on copolymers PCOA8OTBT and PCOA12OTBT.

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CONCLUSION In conclusion, donor−acceptor precursor copolymers based on anthracene with thermally cleavable group have been synthesized. All precursor copolymers can be completely converted into the target semiconducting copolymers with good coplanarity and rigidity through heating at moderate temperatures. Uniform films can be formed due to the good solubility of the precursor copolymers in common solvents. A big difference of absorption spectra and frontier orbital energy levels was observed between the films with and without heating. The photophysical and electrochemical properties can be also easily tuned through changing the electron deficient unit of the copolymers. The copolymers based on DPP units showed a redshifted and broader absorption compared with those based on BT units. PSCs devices were successfully fabricated through applying thermal annealing treatment on the polymer:fullerene blend films. It was observed that the morphology and charge transport properties of the active layers were dramatically influenced by the choice of side chains of the DPP-based copolymers. The highest PCE of 2.15% was achieved based on copolymer PCOAEHDPP with shorter side chains on the DPP unit.

Figure 6. EQEs spectra of copolymer:PCBM based solar cells illuminated by monochromatic light.

(10 nm)/Al,46 As shown in the plots of J1/2 vs V (Figure S3), the hole mobilities of the blend films based on different copolymers could be deduced from the following equation,

J=

9 V2 ε0εr μ0 3 8 L

where J is the current density, μ is the hole mobility, ε0 is the permittivity of free space, εr is the dielectric constant of the blend films, assumed to be 3 in all the cases, L is the thickness of the blend film, and V is the voltage drop across the device. V = Vappl − Vrs − Vbi, where V is the effective voltage, Vappl is the applied voltage, Vrs is the voltage drop, and Vbi is the buildin voltage. The calculated SCLC hole mobilities are summarized in Table 4. It was found that the devices based on copolymer PCOAEHDPP showed the highest mobility of 6.2 × 10−5 cm2 V−1 s−1. which is 1 order of magnitude higher than that based on copolymer PCOAODDPP with larger side chains. This indicated that polymer with DPP unit composed of smaller side chains was more favorable for charge transportation in the blend film. This result also explained why a much lower Jsc was obtained in the PCOAODDPP-based devices. The hole mobility of the blend film based on PCOA12OTBT was 1.7 × 10−5 cm2 V−1 s−1, which is a slightly higher than that of PCOA8OTBT-based

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ASSOCIATED CONTENT

* Supporting Information S

Electrochemical properties of the copolymers, AFM images of the pure copolymer films, and the hole mobilities of the devices. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*(F.H.) E-mail: [email protected]. *(H.-L.P.) E-mail: [email protected]. *(X.G.) E-mail: [email protected]. H

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Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

The work was financially supported by the Ministry of Science and Technology (No. 2014CB643501 and 2014CB643505), the Natural Science Foundation of China (No. 21125419, 51323003 and 51361165301), and the Guangdong Natural Science Foundation (Grant No. S2012030006232).

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