Donor–Acceptor Random versus Alternating Copolymers for Efficient

Article pubs.acs.org/Macromolecules

Donor−Acceptor Random versus Alternating Copolymers for Efficient Polymer Solar Cells: Importance of Optimal Composition in Random Copolymers Tae Eui Kang,† Joonhyeong Choi,† Han-Hee Cho,† Sung Cheol Yoon,‡ and Bumjoon J. Kim*,† †

Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, Korea ‡ Advanced Materials Division, Korea Research Institute of Chemical Technology (KRICT), Daejeon 305-600, Korea S Supporting Information *

ABSTRACT: The backbone composition of conjugated copolymers is of great importance in determining the conjugated structure and intermolecular assembly and in manipulating their optical, electrochemical, and electronic properties. However, limited attention has been directed at controlling the backbone composition of donor− acceptor (D−A) type low bandgap polymers. Herein, we developed a series of D−A random copolymers (P(BDTT-r-DPP)) composed of different compositions of electron-rich (D) thienyl-substituted benzo[1,2b:4,5-b′]dithiophene (BDTT) and electron-deficient (A) pyrrolo[3,4c]pyrrole-1,4-dione (DPP). The optical and electrical properties of D−A random copolymers could be controlled by tuning the ratios of BDTT to DPP (4:1, 2:1, 1:1, 1:2, and 1:4) in the polymer backbone; an increase in BDTT resulted in increased absorption in the range of 400−600 nm and a lower-lying highest occupied molecular orbital energy level, while a higher proportion of DPP induced stronger absorption in the range of 700−900 nm. The P(BDTT-r-DPP) copolymer with a D:A ratio of 2:1 produced the highest power conversion efficiency (PCE) of 5.63% in the polymer solar cells (PSCs), which outperformed the D−A alternating copolymer, P(BDTT-alt-DPP) (1:1)-based PSCs (PCE = 5.03%), because of the improved light absorption and open-circuit voltage. Thus, we highlight the importance of developing random copolymers with controlled D:A compositions for optimizing their optoelectronic properties and performances of PSCs. Also, we compared the polymer packing structure and the electrical properties between the P(BDTT-r-DPP) and P(BDTT-alt-DPP) copolymers and developed a quantitative understanding of the effect of the D:A monomer sequence on the structural, electrical, and photovoltaic properties of the D−A copolymers.

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electron-accepting materials.20−25 However, limited attention has been paid to tune the composition ratio of D−A copolymers because most of D−A copolymers were typically synthesized from coupling reaction between D and A monomers, and thus a control in the D:A composition has been difficult.26−28 And, most of the reported examples were limited to copolymers based on thiophene monomers.29−31 In addition, the sequential distribution of D and A units in the polymer backbone is another important parameter on the semiconducting properties of the polymers, but reports on the direct comparison between D−A random copolymers and D− A alternating copolymers are scarce.27,32 Such studies can investigate the regularity effect of the monomer sequence on the photovoltaic performance of the polymers and also are critical for understanding the compositional effect of D−A

INTRODUCTION Bulk heterojunction (BHJ)-type polymer solar cells (PSCs) have been studied extensively, and power conversion efficiencies (PCEs) of 9−10% have been reported.1−9 These achievements were mainly attributed to the advance in electron-rich unit (D)−electron-deficient unit (A) alternating low band gap polymers.10−14 However, while developing new D and A units has been more difficult, a limited number of new backbone units for D−A alternating copolymers have been successful in achieving high PCE values.15−18 In addition, their PCE values have been close to a plateau, and some of the efficient D−A alternating copolymers have already achieved the maximum internal quantum efficiency of close to 100%.19 While all of D−A alternating copolymers have a fixed D:A ratio of 1:1, the D:A composition could be a very important parameter for further enhancement of photovoltaic performance. For example, the D:A composition will affect important properties of conjugated polymers, including light absorption, energy levels, charge mobility, and solubility/miscibility with © 2016 American Chemical Society

Received: December 25, 2015 Revised: March 3, 2016 Published: March 10, 2016 2096

DOI: 10.1021/acs.macromol.5b02772 Macromolecules 2016, 49, 2096−2105

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methanol, hexane, acetone, dichloromethane, and chloroform. The polymer in the chloroform fraction was concentrated under reduced pressure and precipitated into cold methanol. The polymer was dried under vacuum for 24 h. Stille Polymerization for P(BDTT-r-DPP) (4:1) (PR1). A mixture of M1 (0.221 mmol, 0.2 g, 1 equiv), M2 (0.084 mmol, 0.07 g, 0.4 equiv), and M3 (0.133 mmol, 0.097 g, 0.6 equiv) in chlorobenzene was used to synthesize PR1 according to the procedure described above. Yield: 0.281 g (51%). Anal. Calcd: C = 71.04; H = 7.43; N = 0.95; S = 19.51%. Found: C = 69.54; H = 7.42; N = 0.97; S = 19.07%. Stille Polymerization for P(BDTT-r-DPP) (2:1) (PR2). A mixture of M1 (0.221 mmol, 0.2 g, 1 equiv), M2 (0.148 mmol, 0.117 mg, 0.67 equiv), and M3 (0.073 mmol, 0.053 g, 0.33 equiv) in chlorobenzene was used to synthesize PR2 according to the procedure described above. Yield: 0.275 g (50%). Anal. Calcd: C = 70.09; H = 7.65; N = 1.56; S = 17.90%. Found: C = 70.48; H = 7.71; N = 1.59; S = 17.39%. Suzuki Polymerization for P(BDTT-r-DPP) (1:1) (PR3). A mixture of M2 (0.11 mmol, 0.081 g, 0.5 equiv), M3 (0.11 mmol, 0.088 g, 0.5 equiv), M4 (0.11 mmol, 0.098 g, 0.5 equiv), and M5 (0.11 mmol, 0.092 g, 0.5 equiv) in toluene was used to synthesize PR3 according to the procedure described above. Yield: 0.230 g (41%). Anal. Calcd: C = 71.65; H = 8.86; N = 4.40; S = 10.07%. Found: C = 68.83; H = 8.51; N = 4.67; S = 9.46%. Suzuki Polymerization for P(BDTT-r-DPP) (1:2) (PR4). A mixture of M2 (0.148 mmol, 0.109 g, 0.67 equiv), M3 (0.073 mmol, 0.058 g, 0.33 equiv), and M4 (0.221 mmol, 0.196 mg, 1 equiv) in toluene was used to synthesize PR4 according to the procedure described above. Yield: 0.265 g (48%). Anal. Calcd: C = 71.46%; H = 8.18%; N = 3.03%; S = 13.87%. Found: C = 70.58; H = 8.29; N = 3.21; S = 12.70%. Suzuki Polymerization for P(BDTT-r-DPP) (1:4) (PR5). A mixture of M2 (0.084 mmol, 0.062 g, 0.4 equiv), M3 (0.133 mmol, 0.105 g, 0.6 equiv), and M4 (0.221 mmol, 0.196 mg, 1 equiv) in toluene was used to synthesize PR5 according to the procedure described above. Yield: 0.251 g (46%) Anal. Calcd: C = 71.63; H = 8.34; N = 3.59; S = 12.34%. Found: C = 71.31; H = 8.43; N = 3.59; S = 11.77%. Stille Polymerization for P(BDTT-alt-DPP) (1:1) (PA). A mixture of M1 (0.221 mmol, 0.2 g, 1 equiv) and M3 (0.221 mmol, 0.175 g, 1 equiv) in chlorobenzene was used to synthesize PA according to the procedure described above. Yield: 0.362 g (66%) Anal. Calcd: C = 71.24; H = 7.97; N = 2.31; S = 15.85%. Found: C = 71.14; H = 8.06; N = 2.45; S = 15.80%. Instruments. 1H NMR spectra were measured on a Liquid 400 NB NMR instrument, and o-dichlorobenzene-d4 and CDCl3 were used as solvent. The molecular weights of the synthesized polymers were measured using a size exclusion chromatography (SEC) system equipped with a Waters 1515 Isocratic HPLC pump, a temperature control module, and a Waters 2414 refractive index detector. Polystyrene was used as a standard by using o-dichlorobenzene as an eluent at 80 °C (flow rate: 1 mL/min). Thermogravimetric analysis (TGA) curves were obtained with a NETZSCH TG 209 F3 under a nitrogen atmosphere at a heating rate of 10 °C/min from 10 to 600 °C. Elemental analysis was conducted with a Thermo Scientific Flash 2000 Series. The optical properties of the polymers were measured using a Shimadzu UV spectrophotometer, UV-1800. Electrochemical cyclic voltammetry (CV) (CHI 600C electrochemical analyzer) was conducted with a Pt disk working electrode, a Pt counter electrode, and an Ag wire quasi-reference electrode in anhydrous acetonitrile containing 0.1 M tetrabutylammonium hexafluorophosphate (Bu4NPF6) at a potential scan rate of 50 mV/s. For electrochemical measurements, the polymer films were coated from chloroform solution with a concentration of approximately 10 mg/mL. Grazing incidence X-ray scattering (GIXS) measurements were performed at beamline 3C in the Pohang Accelerator Laboratory (South Korea) at a wavelength of 1.1010 Å. The thin films of polymers:PC71BM were prepared by spin-coating on Si wafers from blending solutions. Fabrication and Characterization of BHJ PSCs. PSC devices with the structure ITO/PEDOT:PSS/polymers:PC71BM/LiF/Al were fabricated under the following conditions: ITO-coated glass substrates were subjected to ultrasonication sequentially in acetone, deionized water, and isopropyl alcohol. The substrates were then dried for

random copolymers on the device performance, in terms of both charge generation and transport as well as structural and morphological properties. Herein, we developed a series of D−A random copolymers (P(BDTT-r-DPP)) based on electron-rich thienyl-substituted benzo[1,2-b:4,5-b′]dithiophene (BDTT) and electron-deficient pyrrolo[3,4-c]pyrrole-1,4-dione (DPP) with different BDTT:DPP compositions of 4:1 (PR1), 2:1 (PR2), 1:1 (PR3), 1:2 (PR4), and 1:4 (PR5). We also synthesized the D− A alternating copolymer, P(BDTT-alt-DPP) (PA), as a reference polymer and compared it with the D−A random copolymers. The use of BDTT and DPP segments produced complementary light absorption over a broad range of 400−900 nm, providing an ideal system for studying the compositional and sequential effects of the D−A random copolymers on PSC performance.33−38 Varying the ratios of BDTT to DPP influenced the optical and electrical behavior of the D−A random copolymers and showed much broader absorption than that of the D−A alternating copolymer. The best PCE value of 5.63% was achieved for P(BDTT-r-DPP) with a D:A ratio of 2:1, which was higher than those of P(BDTT-alt-DPP) and P(BDTT-r-DPP) with a D:A ratio of 1:1. We investigated the effect of (1) the D:A composition and (2) the D:A monomer sequence on the structural, optical, electrical, and photovoltaic properties of the D−A copolymers. Thus, controlling the D−A composition in the backbone of random copolymers provides an important means for optimization of the polymer properties and the photovoltaic performances.

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EXPERIMENTAL SECTION

Materials. 4,8-Bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5b′]dithiophene-2,6-diyl)bis(trimethylstannane) (M1), 2,6-dibromo4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene (M2), 3,6-bis(5-bromothiophen-2-yl)-2,5-bis(2-butyloctyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (M3), 2,5-bis(2-butyloctyl)-3,6-bis(5-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (M4), and 2,2′-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl)bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane) (M5) were purchased from SunaTech, Inc., and used as received without purification. [6,6]Phenyl-C71-butyric acid methyl ester (PC71BM) was purchased from Nano-C. Unless otherwise stated, all of the chemicals were purchased from Aldrich and used as received. Synthesis. General Polymerization Procedure by the Stille Coupling. Compounds M1, M2, and M3 were weighed and added to a microwave vial equipped with a magnetic stirrer. After Pd2(dba)3 (3 mol %) and P(o-tolyl)3 (12 mol %) were added, the cap was sealed. This mixture was degassed three times, and then dry chlorobenzene was added. The vial was placed in a microwave reactor and stirred at 180 °C for 6 h. After cooling to room temperature, the polymer was precipitated into methanol and filtered through a Soxhlet thimble. The precipitate was purified via Soxhlet extraction sequentially with methanol, hexane, acetone, dichloromethane, and chloroform. The polymer in the chloroform fraction was concentrated under reduced pressure and precipitated into cold methanol. The polymer was dried under vacuum for 24 h. General Polymerization Procedure by the Suzuki Coupling. Compounds M2, M3, and M4 were weighed and added to a microwave vial equipped with a magnetic stirrer. After Aliquat 336 and toluene were added, the mixture was degassed with nitrogen for 15 min, after which 2 M K2CO3, Pd2(dba)3 (3 mol %), and P(o-tolyl)3 (12 mol %) were added. After degassing for another 15 min, the cap was sealed, and then, the vial was stirred at 180 °C for 6 h in a microwave reactor. After cooling to room temperature, the polymer was precipitated into methanol and filtered through a Soxhlet thimble. The precipitate was purified via Soxhlet extraction sequentially with 2097

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Figure 1. (a) Structures of D−A random copolymers (PR1−PR5) and alternating copolymer (PA). (b) Energy level diagrams for PR1−PR5 and PA. several hours in an oven at 80 °C. The ITO substrates were treated with UV-ozone prior to the deposition of PEDOT:PSS. A filtered dispersion of PEDOT:PSS in water (PH 500) was applied by spincoating at 3000 rpm for 40 s and baking for 20 min at 150 °C in air. The blend ratio of polymers:PC 71 BM was 1:1.5, and the concentrations of the polymers in chlorobenzene were 12−14 mg/ mL. Each active blending solution was spin-coated on top of the PEDOT:PSS layer at 1500−2500 rpm for 40−60 s. Finally, the LiF (0.7 nm) and Al (100 nm) electrodes were deposited on the active layer using a thermal evaporator under high vacuum (less than 10−6 Torr). The active area of the fabricated devices was 0.09 cm2 as measured by optical microscopy. The photovoltaic performances of the devices were measured under simulated AM 1.5G solar irradiation (100 mW cm−2, Peccell: PEC-L01) in ambient conditions. This solar simulator system satisfies the Class AAB, ASTM Standards. The intensity of the solar simulator was calibrated carefully by using a standard silicon 3 reference cell with a KG-5 visible color filter. The J− V behavior was collected using a Keithley 2400 SMU. The external quantum efficiency (EQE) was obtained using a spectral measurement system (K3100 IQX, McScience Inc.). This system applied monochromatic light from a xenon arc lamp at 300 W filtered by a monochromator (Newport) and an optical chopper (MC 2000 Thorlabs) at ambient conditions. The EQE data were taken in dark conditions. The theoretical short-circuit current density (Jsc) value was acquired by integrating the product of the EQE with the AM 1.5G solar spectrum.

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DPP (4:1, 2:1, 1:1, 1:2, and 1:4) were used during the copolymerization to produce five different D−A random copolymers: P(BDTT-r-DPP) (4:1) (PR1), P(BDTT-r-DPP) (2:1) (PR2), P(BDTT-r-DPP) (1:1) (PR3), P(BDTT-r-DPP) (1:2) (PR4), and P(BDTT-r-DPP) (1:4) (PR5) (Figure 1). Figure 2 presents the synthetic routes for producing a series of P(BDTT-r-DPP) random copolymers and the D−A alternating copolymer, P(BDTT-alt-DPP). PR1 and PR2 with BDTT as the major component were synthesized by reacting trimethyltin-functionalized BDTT (M1) with different ratios of brominated BDTT (M2) and brominated DPP (M3). For example, a mixture of M1 (1 equiv), M2 (0.33 equiv), and M3 (0.67 equiv) was used to synthesize PR2. PR4 and PR5, with DPP as the major component, were produced by coupling borated DPP (M4) with controlled ratios of brominated DPP (M3) and brominated BDTT (M2). For example, for PR4, a mixture of M2 (0.67 equiv), M3 (0.33 equiv), and M4 (1 equiv) was used for the Suzuki coupling polymerization. And, a mixture of two different BDTT monomers, M2 (0.5 equiv) and M5 (0.5 equiv), and two other DPP monomers, M3 (0.5 equiv) and M4 (0.5 equiv), was used to produce PR3 polymers having D:A= 1:1 with a random sequence. As a control polymer, D−A alternating copolymer, P(BDTT-alt-DPP) (D:A= 1:1) (PA), was also synthesized via Stille coupling by using Pd2(dba)3 (3 mol %) and P(o-tolyl)3 (12 mol %) (Figures 1 and 2). After Soxhlet extraction with methanol, hexane, acetone, dichloromethane, and chloroform, all polymers were obtained by precipitating the chloroform fraction. The weight-average molecular weight (Mw), number-average molecular weight (Mn), and polydispersity index (PDI) of the polymers were measured by SEC at 80 °C with o-dichlorobenzene as an eluent. The characteristics of the polymers are summarized in Table 1. To determine the actual ratio between BDTT and DPP in the polymers, they were characterized by 1H NMR spectroscopy (Figure S1) and elemental analysis (Table 1). In elemental analysis, the actual ratios between BDTT and DPP in the copolymers were determined by comparing the contents between nitrogen and carbon atoms because only the DPP unit

RESULTS AND DISCUSSION

Synthesis and Characterization of Polymers. To investigate the effects of the D:A composition of D−A random copolymers on their properties, we developed a series of novel P(BDTT-r-DPP) random copolymers with BDTT as the electron-donating unit and DPP as the electron-deficient unit. DPP has strong π−π interactions and electron-withdrawing effects due to the presence of the electron-deficient lactam rings, while BDTT exhibits a large planar conjugated structure and small steric hindrance between the adjacent repeating units.22,39,40 Therefore, the use of BDTT and DPP segments can produce highly planar structures with excellent charge transport properties. Different monomer ratios of BDTT to 2098

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Figure 2. Synthetic routes and structures of polymers (PR1-PR5 and PA).

(5% weight loss) were 427, 412, 392, 387, and 383 °C, respectively, indicating that the thermal stability of the polymers was enhanced with increasing amounts of BDTT.41,42 Interestingly, the Td (405 °C) of PA was higher than that of PR3 (392 °C), which was probably due to the difference in the inter- and intramolecular interactions caused by the change of the monomer sequence in the polymer backbone. However, all of the polymers exhibited excellent thermal stability and showed no thermal transition from 20 to 300 °C in differential scanning calorimetry measurements. Optical and Electrochemical Properties. The optical properties of the P(BDTT-r-DPP) and P(BDTT-alt-DPP) polymers in thin films and dilute chloroform solutions are shown in Figure 3 and Figure S3, respectively. The characteristics are summarized in Table 2. The comparison of the absorption spectra in the film and the solution indicated that all of the polymers exhibited red-shifted absorption peaks in thin films due to the enhanced intermolecular π−π interactions.43,44 The optical band gaps (EOPT g ) of the PR1−PR5 films were estimated to be 1.54, 1.46, 1.29, 1.27, and 1.24 eV, respectively, based on the onset of the light absorption. Thus, the absorption onset for PR1 to PR5 was gradually red-shifted due to the increased amount of the electron-deficient DPP unit and the

Table 1. Composition, Molecular Weights, and Thermal Properties of Polymers BDTT:DPP polymers

feed ratio

actual ratioa

Mwb (kg/mol)

Mnb (kg/mol)

PDIb (Mw/Mn)

Tdc (°C)

PR1 PR2 PR3 PR4 PR5 PA

4:1 2:1 1:1 1:2 1:4 1:1

3.75:1 1.90:1 1:1.01 1:2.03 1:4.14 1:1.06

289 243 90 193 194 222

84 90 26 48 46 74

3.43 2.71 3.46 4.02 4.21 3.00

427 412 392 387 383 405

a

Molar ratio of BDTT:DPP in the copolymers measured by elemental analysis. bMn, Mw, and PDI were determined by SEC using polystyrene standards in DCB at 80 °C. cObtained from the TGA (5% weight loss temperature).

includes nitrogen atoms. The actual ratios and the feed ratios of the copolymers are in good agreement with each other and within experimental error. Thermal Properties. Thermal stability of the polymers was compared by thermogravimetric analysis (TGA), as shown in Figure S2. The decomposition temperatures (Td) of PR1-PR5 2099

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Figure 3. UV−vis absorption spectra of (a) PR1−PR5 and (b) PR3 and PA in thin films.

respectively. As the DPP content increased, the HOMO level increased gradually, resulting from the more electron-deficient characteristics of DPP. And the LUMO levels of PR1−PR5 were estimated to be −3.49, −3.49, −3.54, −3.55, and −3.55 eV, respectively. As the DPP content increased from 20 to 50%, the LUMO levels were decreased from −3.49 (PR1) to −3.54 (PR3). However, when the DPP content was increased further, the LUMO levels of PR3−PR5 were almost unaffected. The main reason for the similar LUMO levels of PR3−PR5 might be the saturation effect by incorporation of large number of DPP units. The HOMO and LUMO levels of D−A type copolymers could be saturated, at relatively short repeating units (4−6).47,48 Photovoltaic Properties and Theoretical Calculations. To explore the effect of the D:A composition of the random copolymers on their photovoltaic properties, PSCs with the structure of ITO/PEDOT:PSS/BHJ active layer/LiF/Al were fabricated, and their performances were measured under AM 1.5G simulated solar illumination (Figure 4). Table 3

Table 2. Optical and Electrochemical Properties of Polymers ox

red

ECV g

polymers

E (V)

E (V)

HOMO (eV)

LUMO (eV)

(eV)

EOPT g (eV)

PR1 PR2 PR3 PR4 PR5 PA

0.60 0.58 0.52 0.52 0.52 0.53

−1.31 −1.31 −1.26 −1.25 −1.25 −1.28

−5.40 −5.38 −5.32 −5.32 −5.32 −5.33

−3.49 −3.49 −3.54 −3.55 −3.55 −3.52

1.91 1.89 1.78 1.77 1.77 1.81

1.54 1.46 1.29 1.27 1.24 1.42

appearance of absorption band around 800−900 nm, which was attributed to the π−π* transitions of DPP. Interestingly, broader absorptions of the P(BDTT-r-DPP) random copolymers were observed from 400 to 1000 nm compared to the alternating copolymer, PA. And, the EOPT of PR3 (1.29 eV) was g significantly decreased compared to that of PA (1.42 eV), which might be attributed to the existence of the consecutive DPP segments (i.e., DPP−DPP units) in the random copolymer.32 In comparison, the alternating PA polymer had higher absorption coefficient in the range 650−800 nm due to the stronger intermolecular interactions induced by the regular sequence of monomers. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of the polymers were determined by CV measurements. Their HOMO and LUMO energy levels are summarized in Table 2, and their CV curves are shown in Figure S4. The HOMO and LUMO energy levels and the energy gap (ECV g ) of the polymers were calculated from the onset oxidation potentials red (Eox onset) and the onset reduction potentials (Eonset) of the polymers, using a ferrocene/ferrocenium (Fc/Fc+) redox couple (4.80 eV below the vacuum level) as an external standard.45,46 The HOMO energy levels of PR1−PR5 were determined to be −5.40, −5.38, −5.32, −5.32, and −5.32 eV,

Table 3. Photovoltaic Properties of the PSCs polymers

VOC (V)

JSC (mA cm−2)

FF

PR1 PR2 PR3 PR4 PR5 PA

0.75 0.74 0.71 0.70 0.68 0.73

15.06 14.84 6.41 5.10 2.98 12.20

0.44 0.51 0.53 0.46 0.40 0.57

PCEmax (PCEave)a (%) 5.04 5.63 2.42 1.64 0.79 5.03

(4.94) (5.52) (2.22) (1.58) (0.72) (4.92)

calc JSCb (mA cm−2) 14.64 14.40 6.26 4.91 2.86 11.43

a

The average PCE values were obtained from more than 10 separate devices. bCalculated JSC values from EQE spectra.

summarizes the open-circuit voltage (VOC), short-circuit current density (JSC), fill factor (FF), and PCE values of the

Figure 4. (a) Current−voltage (J−V) characteristics of BHJ-type PSCs based on PR1−PR5 and PA under illumination of AM 1.5G, 100 mW/cm2. (b) EQE values of the PSCs at the optimized conditions. 2100

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To gain a deeper insight into the different spectral responses of the PSCs with different D:A compositions, the molecular geometries and the electron density distributions were simulated using density functional theory (DFT) at the B3LYP/6-31G(d,p) level with the Spartan 08 software package. The two model compounds of random sequence (BDTTDPP)-(DPP-BDTT)-(DPP-BDTT) and alternative sequence (BDTT-DPP)3 were used for calculations in order to understand the effect of the monomer sequence on the electron density distribution over the conjugated backbone. Also, the long alkyl chains in the BDTT and DPP monomers were replaced with methyl groups to simplify computation. The simulated molecular geometries and frontier molecular orbitals are shown in Figure 5. In the case of the alternative sequence

PSC devices at optimized conditions. To optimize the device performance, six different polymer donors of PR1−PR5 and PA blended with PC71BM were fabricated with different polymer concentrations, polymer:PC71BM blend ratios, and spin-coating conditions. The devices had the thicknesses of 90−100 nm for their active layers. Different volume percentages of diphenyl ether (DPE) and 1,8-diiodooctane (DIO) were added to the blends of polymers: PC71BM to optimize their photovoltaic performances.49−53 The VOC of the PSCs decreased slightly in the order of PR1 (0.75 V) > PR2 (0.74 V) > PA (0.73 V) > PR3 (0.71 V) > PR4 (0.70 V) > PR5 (0.68 V). The variation in the VOC of the PSCs was consistent with the trend in the HOMO energy levels of the polymer donors because the VOC is typically proportional to the difference between the HOMO level of the polymer and the LUMO level of the PC71BM.54−56 However, the PCE values of the PSCs from P(BDTT-r-DPP) polymers showed a nonlinear, compositional dependence in the order of PR2 (5.63%) > PR1 (5.04%) > PR3 (2.42%) > PR4 (1.64%) > PR5 (0.79%), and their variations were significantly large, ranging from 0.79 to 5.63%. This dramatic difference in the PCE number was mainly attributed to the changes in the JSC values. In spite of the relatively large bandgaps of PR1 and PR2 when compared to other random copolymers, PR1 and PR2 showed dramatically higher JSC values. Among all of the devices, the PR2:PC71BM device exhibited the highest PCE value of 5.63% (VOC = 0.74 V; JSC = 14.84 mA cm−2; and FF = 0.51), outperforming the PA:PC71BM device (PCE = 5.03%; VOC = 0.73 V; JSC = 12.20 mA cm−2; and FF = 0.57) and the PR3:PC71BM device (PCE = 2.42%; VOC = 0.71 V; JSC = 6.41 mA cm−2; and FF = 0.53). These results suggested that PA, the D−A alternating copolymer, was not the optimal polymer for PSC applications and controlling the ratio of BDTT and DPP in conjugated polymers can enhance the optical and electrical properties as well as the performance of PSCs. External quantum efficiencies (EQEs) were measured for different PSC devices under optimized device conditions (Figure 4b). The measured JSC values for the PSC devices were well matched (within 3% error) with the integrated JSC values obtained from the EQE spectra. Even though the changes in the JSC values for the PR1−PR5 based PSCs were well reflected in the EQE curves, it was seen that their spectral responses were very different. In spite of the increased the DPP content for the PR3, PR4, and PR5 polymers, the EQE values of the PR3-, PR4-, and PR5-based PSCs were greatly reduced particularly in the wavelength range of 700−900 nm, the light absorption of which was mainly contributed by the DPP units. This EQE trend became obvious when we compared the spectral responses of PR3- and PA-based PSCs. Although the PR3 and PA differed only in the monomer sequence and had the same D:A ratio of 1:1, the JSC value of PR3 was considerably lower than that of PA. In particular, we observed that the contribution at high wavelengths of 700−900 nm showed a dramatic contrast. For example, whereas the EQE values of the PA and PR3 devices at 490 nm were not much different (i.e., 49 and 36%, respectively), at 750 nm, the EQE value of the PR3 device was only 8%, which was in stark contrast to the EQE value (41%) of the PA device at the same wavelength of 750 nm. This low photocurrent generation of the PR3 device at the wavelengths range of 700−900 nm suggested that the photogenerated excitons absorbed by the DPP part of PR3 were particularly inefficient in producing their dissociation and charge transfer in the PSCs.

Figure 5. Simulated HOMO and LUMO orbitals for the alternative sequence (BDTT-DPP)3 (top); HOMO and LUMO orbitals for the random sequence (BDTT-DPP)-(DPP-BDTT)-(DPP-BDTT) (bottom).

(BDTT-DPP)3, the HOMO wave function was well-distributed along the conjugated backbone, while the LUMO wave function is slightly centralized on the electron-deficient unit (DPP). In contrast, the LUMO wave functions for the random sequence (BDTT-DPP)-(DPP-BDTT)-(DPP-BDTT) were significantly more localized on the DPP−DPP linkage. This strong localization of orbitals at the DPP−DPP units in random copolymers may trap excitons or charges, thus interrupting the preseparation of an exciton into a loosely Coulombically bound exciton and suppressing electron transfer from the polymer donor to PC71BM, while recombination at the interface is made easier.58−60 Indeed, Janssen et al. recently reported that the homocoupled DPP−DPP units in the conjugated backbone act as defects to trap excitons and charges, resulting in a large decrease in the photocurrent and a significant deterioration of the photovoltaic performances.57 Also, the LUMO level of the (BDTT-DPP)-(DPP-BDTT)-(DPP-BDTT) was significantly lower (by 0.12 eV) than that of the (BDTT-DPP)3, indicating that the presence of the DPP−DPP units decreased their LUMO levels of the polymers.57 The decrease of the LUMO level could induce insufficient driving force for charge separation and prevent the dissociation at the interface between polymer donors and PC71BM.61−63 Therefore, the presence of consecutive DPP−DPP units in the PR3−PR5 polymers clearly 2101

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Figure 6. (a) GIXS patterns of PR1−PR5 blends with PC71BM under optimized device conditions. (b) In-plane line cuts (qxy) of the GIXS images.

explained a remarkable difference of the EQE spectral responses between BDTT-rich PR1−PR2 PSCs and DPPrich PR3−PR5 PSCs. The conversion of the excitons generated in the DPP−DPP units to free charges was particularly suppressed, resulting in a significant reduction in the photoresponse beyond 700 nm, where the light absorption of the DPP unit contributes. This trend became more evident for PR4 and PR5 because the contribution to the JSC from light absorption of DPP units was reduced even further despite larger content of DPP units in PR4 and PR5. Interestingly, this feature of reduced charge generation in the DPP−DPP units explained why the JSC value of the randomly sequenced PR3 was significantly lower than that of the alternatively bonded PA, in spite of the same D:A composition. Whereas the PA alternating polymer should not have any consecutive DPP− DPP units, such DPP−DPP units may exist in the PR3 random copolymer. Consequently, the change of photovoltaic properties in terms of different D:A compositions and monomer sequences is well explained by the DFT calculations and the above experimental results. Structural Properties of Polymers and BHJ Blend Morphology. The effect of the BDTT:DPP composition on the polymer packing structure in thin films was investigated by grazing incidence X-ray scattering (GIXS) measurements. Figure 6a,b presents the GIXS patterns of the PR1−PR5 blend films prepared under optimized device conditions and their in-plane line profiles (qxy). All PR1−PR5 blends showed distinct reflection peaks from the (100) crystal plane, corresponding to the lamellar spacing (d100). As more DPP was introduced into the backbone of the random copolymers, the d100 spacing of PR1, PR2, PR3, PR4, and PR5 gradually decreased from 20.1, 19.0, 19.0, 17.9 to 17.5 Å, respectively. This feature is likely due to the increased content of DPP units, which have structurally shorter side chains. However, we did not observe significant difference in the spacing of the π−π stacking of the PR1−PR5 polymers and the polymer orientation in the thin film. For further characterization, the blend morphologies of P(BDTT-r-DPP) and P(BDTT-altDPP) were investigated by atomic force microscopy (AFM) (Figure S5 and Figure 7c). The AFM images of the blends of PR1−PR5 with PC71BM showed similar morphologies of interpenetrating network with small length scales of 20−30 nm and root-mean-square (RMS) roughness values of 0.79−1.27 nm. Therefore, the composition of the random copolymers did not have a significant influence on the macroscopic aspect (i.e., domain spacing) of the BHJ blend morphology. We also compared the GIXS patterns and the AFM images of the PR3/ PC71BM and PA/PC71BM blends to examine the influence of

Figure 7. (a) GIXS patterns of PR3/PC71BM and PA/PC71BM blends prepared under optimized device conditions. (b) In-plane line cuts of the GIXS images. (c) AFM images of PR3/PC71BM (root-meansquare (RMS) roughness = 1.27 nm) and PA/PC71BM (RMS roughness = 1.63 nm). The scale bar is 1 μm.

monomer sequence on the polymer packing structure and the blend morphology (Figure 7). The d100 spacing (18.5 Å) of PA in the blend thin film was decreased slightly compared to that (19.0 Å) of PR3 in the blend film, while both polymers had dominant (100) reflections in the in-plane direction. This discrepancy in d100 spacing might be attributed to the difference in the D−A monomer sequence because the regular D−A sequence in the PA polymer was expected to promote tighter interchain packing. This feature became more prominent when we compared the correlation lengths (LC) of the polymer crystallites in the PR3/PC71BM and PA/PC71BM films. The LC values were calculated from the full widths at half-maximum (FWHM) of the scattering peaks in the GIXS reflections using the Scherrer equation.64,65 The decrease in the FWHM of the scattering peaks of PA compared to that of PR3 was evident, indicating that the PA blend film had much larger crystallite size (LC = 11.2 nm) than PR3 blend film (LC = 3.1 nm) (Figure 7b). Thus, the alternative and regular monomer sequence was highly beneficial for enhancing the local ordering of the polymers. In addition, the blend of PA with PC71BM showed higher RMS roughness of 1.63 nm than that of PR3 (1.27 nm), which may be attributed to the formation of larger crystalline domains in the PA polymers (Figure 7c). Thus, regulating the D:A monomer sequence is very important in promoting the intermolecular assembly and tuning the BHJ blend morphology, both of which are crucial factors in influencing the photovoltaic performances.

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CONCLUSIONS A new series of P(BDTT-r-DPP) random copolymers with different D:A compositions were designed and synthesized 2102

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(3) Li, W. W.; Furlan, A.; Hendriks, K. H.; Wienk, M. M.; Janssen, R. A. J. Efficient Tandem and Triple-Junction Polymer Solar Cells. J. Am. Chem. Soc. 2013, 135, 5529−5532. (4) Cabanetos, C.; El Labban, A.; Bartelt, J. A.; Douglas, J. D.; Mateker, W. R.; Frechet, J. M. J.; McGehee, M. D.; Beaujuge, P. M. Linear Side Chains in Benzo[1,2-b:4,5-b ′]dithiophene-Thieno[3,4c]pyrrole-4,6-dione Polymers Direct Self-Assembly and Solar Cell Performance. J. Am. Chem. Soc. 2013, 135, 4656−4659. (5) Cui, R. L.; Zou, Y. P.; Xiao, L.; Hsu, C. S.; Keshtov, M. L.; Godovsky, D. Y.; Li, Y. F. Efficient solar cells based on a new polymer from fluorinated benzothiadiazole and alkylthienyl substituted thieno[2,3-f]benzofuran. Dyes Pigm. 2015, 116, 139−145. (6) He, Z. C.; Zhong, C. M.; Su, S. J.; Xu, M.; Wu, H. B.; Cao, Y. Enhanced power-conversion efficiency in polymer solar cells using an inverted device structure. Nat. Photonics 2012, 6, 591−595. (7) Zhao, W. C.; Ye, L.; Zhang, S. Q.; Sun, M. L.; Hou, J. H. A universal halogen-free solvent system for highly efficient polymer solar cells. J. Mater. Chem. A 2015, 3, 12723−12729. (8) Chen, J. D.; Cui, C. H.; Li, Y. Q.; Zhou, L.; Ou, Q. D.; Li, C.; Li, Y. F.; Tang, J. X. Single-Junction Polymer Solar Cells Exceeding 10% Power Conversion Efficiency. Adv. Mater. 2015, 27, 1035−1041. (9) Hu, H. W.; Jiang, K.; Yang, G. F.; Liu, J.; Li, Z. K.; Lin, H. R.; Liu, Y. H.; Zhao, J. B.; Zhang, J.; Huang, F.; Qu, Y. Q.; Ma, W.; Yan, H. Terthiophene-Based D-A Polymer with an Asymmetric Arrangement of Alkyl Chains That Enables Efficient Polymer Solar Cells. J. Am. Chem. Soc. 2015, 137, 14149−14157. (10) Bian, L. Y.; Zhu, E. W.; Tang, J.; Tang, W. H.; Zhang, F. J. Recent progress in the design of narrow bandgap conjugated polymers for high-efficiency organic solar cells. Prog. Polym. Sci. 2012, 37, 1292− 1331. (11) Chao, Y. H.; Jheng, J. F.; Wu, J. S.; Wu, K. Y.; Peng, H. H.; Tsai, M. C.; Wang, C. L.; Hsiao, Y. N.; Wang, C. L.; Lin, C. Y.; Hsu, C. S. Porphyrin-Incorporated 2D D-A Polymers with Over 8.5% Polymer Solar Cell Efficiency. Adv. Mater. 2014, 26, 5205−5210. (12) Jung, J. W.; Liu, F.; Russell, T. P.; Jo, W. H. Semi-crystalline random conjugated copolymers with panchromatic absorption for highly efficient polymer solar cells. Energy Environ. Sci. 2013, 6, 3301− 3307. (13) Liu, D. L.; Zhao, W. C.; Zhang, S. Q.; Ye, L.; Zheng, Z.; Cui, Y.; Chen, Y.; Hou, J. H. Highly Efficient Photovoltaic Polymers Based on Benzodithiophene and Quinoxaline with Deeper HOMO Levels. Macromolecules 2015, 48, 5172−5178. (14) Cheng, S. W.; Tsai, C. E.; Liang, W. W.; Chen, Y. L.; Cao, F. Y.; Hsu, C. S.; Cheng, Y. J. Angular-Shaped 4,9-Dialkylnaphthodithiophene-Based Donor-Acceptor Copolymers for Efficient Polymer Solar Cells and High-Mobility Field-Effect Transistors. Macromolecules 2015, 48, 2030−2038. (15) Cao, J. M.; Liao, Q. G.; Du, X. Y.; Chen, J. H.; Xiao, Z.; Zuo, Q. Q.; Ding, L. M. A pentacyclic aromatic lactam building block for efficient polymer solar cells (vol 6, pg 3224, 2013). Energy Environ. Sci. 2013, 6, 3224−3228. (16) Zhong, H. L.; Li, Z.; Deledalle, F.; Fregoso, E. C.; Shahid, M.; Fei, Z. P.; Nielsen, C. B.; Yaacobi-Gross, N.; Rossbauer, S.; Anthopoulos, T. D.; Durrant, J. R.; Heeney, M. Fused Dithienogermolodithiophene Low Band Gap Polymers for High-Performance Organic Solar Cells without Processing Additives. J. Am. Chem. Soc. 2013, 135, 2040−2043. (17) Ohshita, J.; Nakashima, M.; Tanaka, D.; Morihara, Y.; Fueno, H.; Tanaka, K. Preparation of a D-A polymer with disilanobithiophene as a new donor component and application to high-voltage bulk heterojunction polymer solar cells. Polym. Chem. 2014, 5, 346−349. (18) Li, S. G.; Yuan, Z. C.; Yuan, J. Y.; Deng, P.; Zhang, Q.; Sun, B. Q. An expanded isoindigo unit as a new building block for a conjugated polymer leading to high-performance solar cells. J. Mater. Chem. A 2014, 2, 5427−5433. (19) Park, S. H.; Roy, A.; Beaupre, S.; Cho, S.; Coates, N.; Moon, J. S.; Moses, D.; Leclerc, M.; Lee, K.; Heeger, A. J. Bulk heterojunction solar cells with internal quantum efficiency approaching 100%. Nat. Photonics 2009, 3, 297−303.

from electron-rich BDTT and electron-deficient DPP by coupling polymerization. We investigated (1) the effect of the D:A composition and (2) the effect of the D:A monomer sequence in the polymer backbone on the polymer properties. Varying the D:A composition of the polymers significantly affected their optical and electrical properties, and the highest PCE value was obtained for the D−A random copolymers, not the alternating copolymers. The highest PCE value of 5.63% was achieved for P(BDTT-r-DPP) with D:A ratio of 2:1 due to improved light absorption and VOC value, which was higher than those of P(BDTT-r-DPP) and P(BDTT-alt-DPP) with D:A ratio of 1:1. In addition, the results of DFT calculations demonstrated the impact of the monomer sequence on the electrical properties of the D−A random copolymers. The consecutive DPP−DPP units induced the localization of the electron density distribution, thereby acting as exciton or charge trap sites and generating adverse effects on the PSC performance with respect to charge transfer and energy level offset.57 This result also explained very different photovoltaic performances between the P(BDTT-r-DPP) and P(BDTT-altDPP) at the same D:A ratio of 1:1. Consequently, the control of the D:A composition and the monomer sequence in random copolymers is crucial for optimizing their optical, electrical, and photovoltaic properties.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b02772. Detailed characterization data of polymers including 1H NMR spectra, TGA plots, UV−vis absorption spectra, CV curves, and AFM images (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (B.J.K.). Author Contributions

T.E.K. and J.C. contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Global Frontier R&D Program on Center for Multiscale Energy System (2012M3A6A7055540), funded by the Korean Government. This research was also supported by the New & Renewable Energy Program of KETEP Grant (20133030000130, 20133030011330), funded by the Ministry of Trade, industry & Energy, Republic of Korea. We acknowledge the support from the Research Project of the CRH (Climate Change Research Hub) of KAIST.

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REFERENCES

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DOI: 10.1021/acs.macromol.5b02772 Macromolecules 2016, 49, 2096−2105

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DOI: 10.1021/acs.macromol.5b02772 Macromolecules 2016, 49, 2096−2105