Temperature-Modulated Optimization of High-Performance Polymer

Article Cite This: Macromolecules 2019, 52, 4447−4457

pubs.acs.org/Macromolecules

Temperature-Modulated Optimization of High-Performance Polymer Solar Cells Based on Benzodithiophene− Difluorodialkylthienyl−Benzothiadiazole Copolymers: Aggregation Effect Lanqi Huang,† Guangjun Zhang,‡ Kai Zhang,†,§ Qiang Peng,*,‡ and Man Shing Wong*,†

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†

Institute of Molecular Functional Materials, Department of Chemistry and Institute of Advanced Materials, Hong Kong Baptist University, Kowloon Tong, Hong Kong SAR, China ‡ College of Chemistry, Sichuan University, Wangjiang Road 29, Chengdu 610064, Sichuan, China § College of Preclinical Medicine, Southwest Medical University, Luzhou 646000, Sichuan, China S Supporting Information *

ABSTRACT: A novel series of low band gap donor−acceptor copolymers derived from 4,5bis(2-ethylhexyloxy)-benzo[2,1-b:3,4-b′]dithiophene (BDT) and 5,6-difluoro-4,7-bis(4-alkylthien-2-yl)benzo[c][1,2,5]thiadiazole bearing various alkyl side chains, such as PffBB-n (n = 10, 12, 14, and 16), were developed for high-performance bulk-heterojunction (BHJ) polymer solar cells (PSCs). PffBB-n exhibited not only strong and wide absorption but also controllable aggregation behavior in solution and thin films, in which aggregation behavior was greatly influenced by the length of alkyl side chains attached, temperature applied, and solvent used. Aggregation-induced spectral broadening further extended the absorption cutoff to ∼780 nm in thin films, leading to a narrow optical band gap of ∼1.6 eV. Because of the strong aggregation strength, PffBB-n equipped with long alkyl side chains shows enhanced ordered molecular packing and good crystallinity as revealed by X-ray diffraction studies. In addition, temperature-dependent aggregation of the PffBB-n:PC71BM blend was investigated and optimized in the PSC fabrication. PSCs fabricated with the PffBB-n:PC71BM blend, conducted at 80 °C optimized coating temperature, showed relatively high power conversion efficiency (PCE) ranging from 8.22 to 9.93%. The well-ordered BHJ film morphology of the PffBB-14:PC71BM blend led to superior balanced charge carrier mobility, good exciton dissociation, and the least recombination loss and hence PffBB-14-based PSC reached the highest photovoltaic performance with a PCE of 9.93%, a Voc of 0.92 V, a Jsc of 16.77 mA cm−2 and, a FF of 64.36%. Our results demonstrated that the synergetic effect of alkyl side chain modification and processing temperature modulation to control aggregation provides practical and powerful tools to optimize the absorption broadening and optoelectronic properties of an active layer in a BHJ PSC, thus enhancing its ultimate performance.

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INTRODUCTION

to a high-lying highest occupied molecular orbital (HOMO) energy level. Such high energy alignment would not enable to achieve a large open-circuit voltage and is eventually detrimental to the PCE output of PT-based PSCs such as in P3HT/PCBM systems with PCE ranging from 3 to 4%.21,22 Geng and co-workers synthesized two PT-based donor materials by incorporating carboxylate side chains and vinylene linkers between thiophene units. The HOMO and the lowest unoccupied molecular orbital (LUMO) energy levels of the resulting polymers, PBT and PTT, were substantially stabilized when compared to those of the parent P3HT. Such energy level lowering was attributed to the integration of carboxylate and vinylene groups giving rise to a push−pull structural motif, resulting in an enhanced PCE of the PV devices based on them.21

Polymeric materials show promising potential for the applications of future solar cell technology because of their advantages of solution processability, lightweight, cost efficiency, and mechanical flexibility.1 Polymer solar cells (PSCs) with power conversion efficiency (PCE) exceeding 10% have been realized during the last decade, which was benefited from tremendous efforts on the development of advanced donor and/or acceptor materials, device engineering and optimized fabrication processes.2−12 Nevertheless, the poor sunlight harvesting efficiency and low charge carrier mobility of organic semiconducting materials still hinder the further efficiency improvement of PSCs. Polythiophene (PT) materials have been proved to be promising donor materials as most PT materials exhibit high regioregularity for easy formation of well-organized active layer morphology, thus providing an auspicious way to develop new and efficient photovoltaic (PV) materials.13−20 However, thiophene-based units are strongly electron-rich in nature, which often give rise © 2019 American Chemical Society

Received: April 4, 2019 Revised: June 2, 2019 Published: June 13, 2019 4447

DOI: 10.1021/acs.macromol.9b00682 Macromolecules 2019, 52, 4447−4457

Article

Macromolecules Scheme 1. Synthesis of PffBB-n Polymersa

a

Reagents and conditions: (a) (i) n-BuLi, CuBr, LiBr, (ii) oxalyl dichloride, diethyl ether, tetrahydrofuran (THF), 43%; (b) FeCl3, DCM, 92%; (c) Zn, acetic anhydride, triethylamine, DCM, 87%; (d) Cs2CO3, 2-ethylhexyl bromide, MeCN, 87%; (e) (i) n-BuLi, (ii) Sn(C4H9)3Cl, THF, 90− 95%; (f) Mg, alkylbromide, Ni(dppf)Cl2, Et2O, 85−95%; (g) tri-butyl(4-alkylthiophene-2-yl)stannane, Pd(PPh3)4, toluene, reflux, 70−80%; (h) Nbromosuccinimide (NBS), CHCl3/AcOH, ∼90%; (i) 5, Pd(PPh3)4, chlorobenzene, 150 °C, ∼30%.

Over the years, the use of the structural effect to enhance PSC performance has been extensively investigated, particularly the donor−acceptor (D−A) strategy, which has been proved to be an effective approach to develop donor polymers with wide absorption, low-lying energy levels, and enhanced electron properties.23−32 Besides, conjugation expansion on the polymer backbone is an alternative tool to improve the electronic properties of polymers. By replacing a branched 2ethylhexyl side chain with a linear n-octyl group, Zhang and his co-workers synthesized a PTB7-Th derivative namely, PBTT. PV cells based on PBTT:IEICO showed a superior device performance than that of the PTB7-Th-based device with a PCE of 9.5%, a Voc of 0.86 V, a Jsc of 17.9 mA cm−2, and a FF of 61.3%. Such dramatic enhancement in efficiency was attributed to a higher charge mobility and improved morphology of the PBTT:IEICO blend film induced by the less steric hindered linear alkyl side chain.1 Yang and coworkers used selenophene to replace thiophene π-bridge in the indacenodithiophene (IDT)-benzo[1,2-c:4,5-c′]dithiophene4,8-dione (BDD)-based copolymer (PThBDDIDT) to develop a donor−π−acceptor type copolymer, PSeBDDIDT. The

PCE of solar cells based on polymer:PC71BM as an active layer was dramatically increased from 7.04 to 8.65%, respectively. Such improvement could result from stronger intermolecular interactions, expanded absorption range, and better molecular planarity of selenophene contained PSeBDDIDT.33 Random terpolymers comprised of thieno[2′,3′:5′,6′]pyrido[3,4-g]thieno[3,2-c]isoquinoline-5,11(4H,10H)-dione, thiophene (T), and dithiophene (2T) were synthesized by Yang and co-workers in which the crystallinity of terpolymers could be controlled by varying the compositions of T and 2T. The PSC based on a terpolymer with the optimized T/2T ratio of 7:3 showed a high Jsc of 18.3 mA cm−2 and a FF of 71.2%, thus resulting in an enhanced PCE up to 10.8%.34 Fluorination on polymer backbones has also been widely adopted to tune and improve the functional properties of donor polymers such as stabilization of their energy levels and hence positively impacting on PSC performance.35 On the other hand, Li and co-workers employed the difluoro-substituted thiopheneconjugated side chain to construct bi(alkyl-difluorothienyl)benzodithiophene-fluorobenzotriazole copolymer J91, which was shown to exhibit superior cell performance than that of the 4448


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effectively optimize the physical and optoelectronic properties of a material and device for solar cell applications.

nonfluorinated control polymer. The PSC based on J91/mITIC as an active layer with thermal annealing gave a high PCE of 11.63% with a high Voc of 0.98 V and a Jsc of 18.03 mA cm−2.36 The improved cell performance was attributed to the suppression of carrier recombination imposed by the difluorosubstituted side chain. In addition to influencing molecular and functional properties of donor polymers, the structural modification could lead to severe aggregation, which could hamper solution processability of a polymer. On the other hand, aggregation could be desirable and provide a useful tool to control the bulkheterojunction (BHJ) film morphology and then the photovoltaic performance of a solar cell. Yan and co-workers employed the branched alkyl chains attached on the backbone of the difluorobenzothiadiazole-quaterthiophene (PffBT4T) copolymer to acquire desirable temperature-dependent aggregation behavior. By spin-casting the polymer solution at an elevated temperature, they could control the morphology and crystallinity of the polymer films. Taking advantage of this aggregation control approach, high-performance PSCs based on PffBT4T-2OD:PC71BM were fabricated affording a PCE of up to 10.8%.9 Guo and co-workers used a number of fused thiophene spacers to tune the aggregation behavior of difluorobenzoxadiazole (ffBX)-based donor polymers among which PffBX-TT with thieno[3,2-b]thiophene as a spacer showed intermediate temperature-dependent aggregation strength allowing the control of blend film morphology during PSC fabrication. PSCs using PffBX-TT:PC71BM as an active layer gave a maximum PCE of 9.10%.37 Previously, we developed a class of benzodithiophene-based donor polymers with fluoro-dialkylthienylbenzothiadiazole being an acceptor unit that showed interesting temperature-dependent aggregation behavior when equipped with linear alkyl side chains. PSCs based on these polymers:PC71BM blend layers showed good photovoltaic performance with a maximum PCE as high as 9.70%.38 In this contribution, we reported a new series of donor polymer materials based on benzodithiophene (BDT) and difluorinated dialkylthienylbenzothiadiazole, PffBB-n (n = 10, 12, 14, or 16), which exhibited strong aggregation behavior in both solution and thin films. In addition to the size of alkyl side chains attached on the backbone of the polymer, aggregation can be modulated by the solvent used and temperature applied on the film with which the absorption cut-off extended down to ∼780 nm in thin films, resulting in a narrow optical band gap of ∼1.6 eV. Such wide absorption is desirable and complementary to that of the commonly used PC71BM acceptor material in the visible light region, which can increase light harvesting in BHJ PSCs. BHJ PSCs with PffBBn:PC71BM active blend layer spin-coated at different temperatures showed considerably various photovoltaic performance because of the morphological order change of the blend layer at different processing temperatures. Among PffBBn:PC71BM-based PSCs fabricated, PffBB-14 with strongest aggregation strength/behavior exhibited the most promising photovoltaic performance, with a maximum PCE of 9.93%, a large Voc of 0.92 V, a Jsc of 16.77 mA cm−2, and a FF of 64.36%. Such a superior device efficiency was attributed to the greatly enhanced exciton dissociation, higher and balanced charge carrier mobilities, and less recombination losses in the active layer of the PffBB-14:PC71BM-based cells. Our results demonstrated that the modulation of aggregation behavior through structural modification and temperature tuning could

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RESULTS AND DISCUSSION Synthesis and Physical Properties of Polymers. The PffBB-n (n = 10, 12, 14, or 16) polymers were synthesized according to the optimized literature procedures and the synthetic scheme is shown in Scheme 1.39 The Corey−House synthesis of 3-bromothiophene and oxalyl dichloride gave 1,2di(thiophen-3-yl)ethane-1,2-dione (1) as a brown needle-like crystal in a good yield (70%). Oxidative cyclization of 1 in the presence of iron(III) chloride carried out in diluted dichloromethane (DCM) solution yielded a dark purple benzodithiophene-4,5-diketone (2) crystal. Acetylation of 2 with Zn powder and acetic anhydride performed under basic conditions produced yellowish benzodithiophene-4,5-diyl diacetate (3) powder in a good yield of 85%. 3 was then alkylated with 2ethylhexyl bromide in the presence of Cs2CO3 yielding 4,5bis(2-ethylhexyloxy)benzodithiophene (4) as a sticky colorless liquid. Proton abstraction of 4 by n-BuLi followed by the reaction with tri-butyltin chloride gave the polycondensation precursor, 4,5-bis(2-ethylhexyloxy)-2,7-bis(tri-n-butylstannyl)benzodithiophene (5). To preserve high reactivity, the crude product was subjected to flash chromatography and kept under −20 °C for the condensation reaction. Alkylation of the thiophene ring was carried out by Kumada coupling, yielding 3-alkylthiophene as a colorless oil or a white solid (6−9). Lithiation of alkylated thiophene followed by quenching with tri-butyltin chloride afforded tri-butyl(4alkylthiophene-2-yl)stannane (10−13). Subsequent coupling with 4,7-dibromo-5,6-difluorobenzo[c][1,2,5]thiadiazole afforded dithienyl-substituted difluorobenzo[c][1,2,5]thiadiazole, 14−17. NBS bromination of 14−17 produced the acceptor units, 18−21, respectively, which were carefully purified through chromatography and recrystallization to ensure high reactivity for the condensation reaction. The Stille polycondensation reaction of donor unit 5 and acceptor units 18−21 was carried out under the catalyst of Pd(PPh3)4 in chlorobenzene solutions at 150 °C for 3 days that could afford the corresponding polymers as a dark green solid. Gel permeation chromatography (GPC) was used to determine the molecular weight using polystyrene as the standard and 1,2,3-trichlorobenzene as the eluent (Table 1). The decomposition temperature (Td) of PffBB-n was determined by thermal gravimetric analysis (TGA). The results suggested that all polymers exhibited good thermal stability with Td higher than 350 °C (Table 1), indicating that these polymers are feasible for high-temperature device Table 1. Physical Properties of PffBB-n Polymer Series polymer PffBB-10 PffBB-12 PffBB-14 PffBB-16

a

Mn (kDa) 15.3 19.0 11.3 11.9

a

Mw (kDa) 33.9 50.3 20.5 26.3

b

PDI

2.21 2.65 1.81 2.21

c

Td (°C) 356 376 376 363

a

Molecular weight of polymers determined by gel permeation chromatography, using polystyrene as the reference standard, 1,2,3trichlorobenzene as the eluent at 150 °C (flow rate: 1 mL min−1; column set: 3 × PLgel Olexix 300 × 7.5 mm). bThe polydispersity index was calculated from: PDI = Mw/Mn. cEstimated from thermal gravimetric analysis with a ramp rate of 40 °C min−1. 4449


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Figure 1. UV−vis absorption spectra of (a) PffBB-10, (b) PffBB-12, (c) PffBB-14, and (d) PffBB-16 in chlorobenzene at different temperatures.

planarity effect and interchain stacking of PffBB-n caused the bathochromic shift of the absorption cut-off to ∼780 nm, corresponding to a narrow optical band gap of ∼1.61 eV. Density functional theory (DFT) calculations was carried out to probe the molecular and electronic properties of the PffBB-n polymer by means of B3LYP/6-311+(d,p) as a basis set in a vacuum using the repeating conjugated units of the polymer backbone in which all of the alkyl substitutions were replaced by ethyl groups to reduce the calculation time. Figure 2 displays the optimized molecular conformation and the

fabrication. The physical properties of PffBB-n polymers are summarized in Table 1. Detailed synthetic procedures and TGA traces could be found in the Supporting Information (SI). Optical and Electrochemical Properties of Polymers. The UV−vis absorption spectra of PffBB-n (n = 10, 12, 14, or 16) measured in chlorobenzene at different temperatures and in thin films are shown in Figure 1. It is clearly demonstrated that PffBB-n exhibits strong temperature-dependent aggregation behavior. In thin films, PffBB-n showed three major absorption bands with the high-energy absorption peaked at ∼430 nm and two overlapped and strong absorptions peaked at ∼620 and 680 nm. In solution, the first two absorption peaks blue-shift to 380−420 and 520−560 nm, respectively, and the third absorption is highly temperature-dependent which becomes a shoulder peak at high temperature. Therefore, the absorption band located in the high energy region was attributed to π−π* transitions, whereas the broad absorption at around 560 nm corresponded to the intramolecular charge transfer band and the long-wavelength absorption emerged at ∼680 nm was due to the polymer interchain stacking aggregation. This aggregation-induced absorption is temperature-sensitive in which the absorption decreases with the increase of temperature, resulting from polymer chain segregation at high temperature. The incorporation of an alkyl side chain can often render a better solubility of a polymer; however, the long alkyl side chain can exert stronger alkyl−alkyl chain interactions that can give rise to a strong aggregation of polymer chains. As shown in Figure 1, the aggregation ability/strength of PffBB-n polymers also enhanced with an increase in the size of the alkyl side chains (from n = 10 to 16). Furthermore, this class of polymers exhibited solvent-dependent aggregation behavior in which chloroform and toluene often induced stronger aggregation; on the other hand, chlorobenzene and dichlorobenzene induced moderate aggregation (Figure S3). In the solid state, the

Figure 2. (a) DFT-optimized molecular structure, (b) the HOMO and (c) LUMO of the PffBB polymer backbone; and (d) energy level alignment of PffBB-n polymers estimated from cyclic voltammetry (CV) measurements.

frontier molecular orbitals of the ethyl-substituted repeating units of PffBB-n. The dihedral angle between the BDT electron-donating unit and the electron-deficient unit was found to be ∼26.9° in the optimized structure, which is caused by the ethyl substitution on the thienyl ring. The HOMO delocalizes extensively over the π-conjugation framework, whereas the LUMO mainly populated on the benzothiadiazole 4450


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Macromolecules Table 2. Optical and Electrochemical Properties of PffBB-n Polymers polymer PffBB-10 PffBB-12 PffBB-14 PffBB-16

a

λonset (nm) 745 775 770 765

b

Egopt (eV) 1.66 1.60 1.61 1.62

c

Eonsetox (V)

c

Eonsetred (V) −1.31 −1.39 −1.28 −1.26

0.97 1.03 1.03 1.09

d

HOMO (eV) −5.66 −5.73 −5.72 −5.78

d

LUMO (eV) −4.01 −4.11 −4.11 −4.16

e

Egec (eV) 2.28 2.42 2.30 2.35

a

Obtained from UV−vis spectra of thin-film state PffBB-n polymers. bCalculated from the onset absorption of thin-film state PffBB-n, Egopt = 1240/λonset. cDetermined by CV using ferrocene as an external standard, Ag/Ag+ as a reference electrode, platinum disc as a counter electrode, and a platinum−carbon electrode as a working electrode. dHOMO = −(4.7 eV + Eonsetox), LUMO = HOMO + Egopt. eCalculated from CV measurement, Egec = Eonsetox − Eonsetred.

showed a descending trend of 3.84, 3.82, 3.77, and 3.66 Å for PffBB-10, PffBB-12, PffBB-14, and PffBB-16, respectively, which indicated that PffBB-n with longer alkyl side chains exhibited enhanced ordered molecular packing. Notably, the longer the alkyl side chain, the closer the polymer aligned through π−π stacking is, which is attributed to the stronger interchain interactions of long alkyl side chains. Such strong πstacking ability and crystalline nature of PffBB-n may provide a highly ordered network for effective electron transport and exciton diffusion. Photovoltaic Properties. Solar cells using PffBB-n as a donor material and PC71BM as the acceptor material were fabricated with an inverted device structure of ITO/ZnO/ PffBB-n:PC71BM(1:2 w/w)/MoO3/Al. The active blend layer was prepared by means of spin-coating at different temperatures to investigate the impact of coating temperature on photovoltaic device performance, which was characterized under AM 1.5 G simulated solar illumination at an irradiation intensity of 100 mW cm−2. To achieve the high coating temperatures, the high boiling chlorobenzene (132 °C) was used as a solvent even though chloroform is a stronger aggregation-induced solvent. Detailed device fabrication procedures can be found in the electrospray ionisation. The current density−voltage curves of BHJ PSCs fabricated at 60, 80, and 100 °C using a 3% CN as the solvent additive and their respective photovoltaic properties are shown in Figure 4 and summarized in Table 3, respectively. It is important to note that the photovoltaic device performance is greatly affected by the active blend coating temperature. The decyl-substituted PffBB-10-based device showed a good PCE of 8.22% with a high Voc of 0.93 V, a Jsc of 14.62 mA cm−2, and a FF of 60.44% when the active layer was coated at 80 °C. On the other hand, PffBB-10-based devices fabricated at 60 and 100 °C spin-coating temperatures exhibited inferior photovoltaic performance with a lower Jsc and a much lower FF of ∼56% except for a similar Voc. Furthermore, BHJ PSCs based on PffBB-12, PffBB-14, and PffBB-16 also exhibited the same trend in which their highest PCE was achieved by spin-coating the active blend layer at 80 °C. Such a superior device performance is the result of the highest Jsc and FF achieved (Table 3) when using 80 °C as a spin-coating temperature compared to those fabricated at 60 or 100 °C. The decrease in both the Jsc and FF could be due to inferior polymer chain packing in the active layer leading to a poor charge transporting network. Besides, the device performance generally varied with the length of alkyl side chains attached onto the polymer backbone. The device efficiency initially increases upon the incorporation of longer alkyl side chains from decyl to tetradecyl reaching a maximum at tetradecyl-substituted PffBB-14 and then declines with further alkyl side chain extension. Importantly, such chain

unit. Such frontier molecular orbital electron distribution would greatly enhance efficient intramolecular charge transfer. The electrochemical properties of PffBB-n were determined by cyclic voltammetry. Their results are summarized in Table 2, and the respective energy levels are delineated in Figure 2d. The HOMO energy levels of PffBB-10, PffBB-12, PffBB-14, and PffBB-16 were estimated from the onset oxidation potential to be −5.66, −5.73, −5.72, and −5.78 eV, respectively. The corresponding LUMO energy levels were calculated from the HOMO energy level and the onset absorption of polymers in thin films, which were −4.01, −4.11, −4.11, and −4.16 eV, respectively. As a result, the HOMO energy levels of PffBB-n polymers are much lower than that of ITO and their corresponding LUMO energy levels of PffBB-n lied slightly above that of PC71BM. Such energy level alignment would facilitate charge transfer from the donor polymer to the PC71BM acceptor and provide a relatively large difference between the HOMO energy level of donor polymers and the LUMO level of PC71BM, which is important to achieve a large Voc of a solar device. As a result, BHJ PSCs fabricated from these polymers are promising to achieve high device performance. X-Ray Diffraction (XRD). X-ray diffraction measurements were carried out to determine the crystallinity of PffBB-n polymers and the XRD spectra are shown in Figure 3. Despite

Figure 3. XRD patterns of PffBB-n polymers.

various sizes of alkyl side chains attached, polymers with the same backbone show very similar XRD patterns. The sharp peaks at around 5° correspond to the in-plane spacing (d1spacing) between the polymer backbones, whereas the broad peaks lying above 20° (d2-spacing) is characterized as a π−π stacking distance.40,41 As seen in Figure 3, PffBB-n exhibits good crystallinity with d1-spacing of 21.00, 22.63, 24.16, and 25.44 Å for PffBB-10, PffBB-12, PffBB-14, and PffBB-16, respectively, which arises from the strong aggregation ability of these polymers. Meanwhile, with an increase of the length of the alkyl side chains, the d2-spacing of PffBB-n polymers 4451


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Figure 4. J−V characteristics of solar cells with the active layer coated at different temperatures under AM 1.5 G simulated solar illumination (100 mW cm−2).

length-dependent device performance trend is spin-coating temperature independent. The champion PffBB-14-based device affords the highest PCE of 9.93% with a Voc of 0.92 V, a Jsc of 16.77 mA cm−2, and a FF of 64.36%. The size of the alkyl side chain is known to affect the processability and interchain interactions of a polymer, which would provide a tool to optimize the π−π stacking interactions of a polymer in the solid state. A size-optimized alkyl side chain would provide a good balance between the π−π stacking interactions of the polymer backbone and the alkyl−alkyl side chain interactions resulting in a highly ordered molecular packing for efficient charge transport. Figure 5a shows the external quantum efficiency (EQE) spectra of PffBB-n/PC71BM-based PSCs. Strong and broad photo-to-current responses were observed in the range from 330 to 780 nm in all of these PSCs, in which the absorption above 650 nm was attributed to the polymer aggregationinduced absorption broadening. Remarkably, the PffBB-14based device exhibited a much higher photoconversion

Table 3. Photovoltaic Characteristics of Devices Based on PffBB-n at Different Coating Temperatures a

PffBB-10

PffBB-12

PffBB-14

PffBB-16

T (°C)

Voc (V)

Jsc (mA cm−2)

FF (%)

60 80 100 60 80 100 60 80 100 60 80 100

0.93 0.93 0.93 0.94 0.94 0.94 0.92 0.92 0.92 0.95 0.95 0.95

14.21 14.62 14.22 14.62 14.98 14.85 15.23 16.77 15.92 14.42 14.88 14.56

55.69 60.44 56.63 60.46 62.13 61.18 61.98 64.36 62.94 58.67 61.15 60.82

b

PCE (%)

7.26 8.04 7.15 8.14 8.71 8.20 8.52 9.91 9.08 8.01 8.48 8.26

± ± ± ± ± ± ± ± ± ± ± ±

0.11 0.09 0.21 0.10 0.02 0.18 0.10 0.02 0.17 0.02 0.09 0.09

a

Coating temperature. bThe average and statistic data were obtained from five devices.

Figure 5. (a) EQE spectra and (b) integrated current density of devices with the active layer coated at 80 °C under AM 1.5 G simulated solar illumination (100 mW cm−2). 4452


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Figure 6. J1/2 vs V plots of devices based on PffBB-n/PC71BM: (a) hole-only devices and (b) electron-only devices and (c) photogenerated current density against the effective voltage of PSCs based on the PffBB-n:PC71BM blend.

Table 4. Charge Carrier Mobilities of PffBB-n-Based Devices active layer

hole mobility (10−4 cm2 V−1 s−1)

electron mobility (10−4 cm2 V−1 s−1)

μh/μe

charge dissociation probability

PffBB-10 PffBB-12 PffBB-14 PffBB-16

4.80 5.66 9.23 5.35

0.96 2.29 4.92 1.64

5.00 2.47 1.88 3.26

0.86 0.94 0.98 0.92

Figure 7. (a) Plots of Voc and (b) Jsc vs light intensity for PffBB-n:PC71BM-based devices.

dark (JD). Veff = V0 − V, where V0 is the voltage at which JL = JD and V is the applied voltage. When the reverse voltage is greater than 2 V, recombination will be suppressed because of a high internal electric field and thus the current density becomes saturated (Jsat). Consequently, P(E,T) = Jph/Jsat could be used to describe the charge dissociation probability. The charge dissociation probability of PffBB-10, PffBB-12, PffBB14, and PffBB-16-based cells was estimated to be 0.86, 0.94, 0.98, and 0.92, respectively. The highest exciton dissociation of the PffBB-14-based device is anticipated to give rise to the best photovoltaic performance among these series of polymers. Figure 7 demonstrates the dependence of Voc and Jsc against light intensity, which could be used to describe the recombination process in BHJ films. The Voc is directly proportional to the incident light intensity in the semilogarithmic plot with a slope of kT/q, which describes bimolecular recombination in the BHJ film, where k represents the Boltzmann constant, T is the Kelvin temperature, and q is the elementary charge. Trap-assisted recombination exists if the slope is larger than kT/q.42 Among devices fabricated, the PffBB-14-based cell exhibited the smallest slope of 1.10 kT/q, suggesting the least trap-assisted recombination. Bimolecular recombination was also investigated by measuring the dependence of Jsc against light intensity, in which the exponential factor α in the power law of Jsc ∝ Plightα is used

response ranging from 350 to 500 nm than those of other devices, thus affording the highest Jsc among all of these devices. As PffBB-n polymers showed similar absorption properties, the high current density of PffBB-14-based PSC would result from well-ordered polymer:PC71BM network/ packing. Jsc derived from the EQE spectra were estimated to be 14.12, 14.92, 16.29, and 14.21 mA cm−2 for PffBB-10, PffBB12, PffBB-14, and PffBB-16-based devices, respectively, which were in good agreement with those obtained from J−V characterization (Figure 6). Charge Transfer and Recombination. The charge carrier mobilities of PffBB-n-based devices were estimated on hole-only and electron-only devices using the space-chargelimited current (SCLC) model, and the results are summarized in Table 4. The estimated hole/electron mobilities (μh/μe) are 4.80/0.96 × 10−4, 5.66/2.29 × 10−4, 9.23/4.92 × 10−4, and 5.35/1.64 × 10−4 for PffBB-10:PC71BM, PffBB-12:PC71BM, PffBB-14:PC71BM, and PffBB-16:PC71BM blend, respectively. Indeed, the highest and well-balanced charge carrier mobility of the PffBB-14:PC71BM blend affords a higher Jsc and FF than those of other devices. Charge dissociation probability was characterized by photogenerated current density (Jph) vs effective applied voltage (Veff) measurements. Based on the definition, Jph equals to the difference between the current density of devices under 100 mW cm−2 (JL) and that in the 4453


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Figure 8. (a−d) AFM height images and (e−h) phase images of PffBB-10:PC71BM, PffBB-10:PC71BM, PffBB-12:PC71BM, PffBB-14:PC71BM, and PffBB-16:PC71BM blend spin-coated with 3% CN as a solvent additive.

to indicate bimolecular recombination in the BHJ film. The closer the estimated α to 1.0, the weaker the bimolecular recombination occurs in the device.43 Obviously, with α = 0.98, the PffBB-14-based device would exhibit the least occurrence of bimolecular recombination in the active layer. In contrast, the PffBB-10-based device showed an α value of 0.86, indicating that the bimolecular recombination loss was severe in its BHJ film. These results consistently indicate that the PffBB-14:PC71BM blend with superior balanced charge carrier transport exhibits the least recombination loss among PffBBn:PC71BM-based PSCs, thus affording the highest Jsc and FF. Morphology of the Active Layer. AFM images of PffBBn:PC71BM blended thin films were obtained to investigate their microphase separation behavior and the corresponding AFM height and phase images recorded in the tapping mode are shown in Figure 8. Good miscibility could be observed in all of the PffBB-n:PC71BM blended thin films, which gives rise to desirable phase separation. The resulting root-mean-square (rms) roughness of active layer films was estimated to be 2.6, 1.8, 1.0, and 2.2 nm for PffBB-10:PC 71 BM, PffBB10:PC71BM, PffBB-12:PC71BM, PffBB-14:PC71BM, and PffBB-16:PC71BM blend, respectively. Smaller domain sizes in the active blend layer could afford a short exciton diffusion length, which is desirable to suppress the recombination loss.44 The active layer morphologies recorded are in good accordance with the J sc performance of the PffBBn:PC71BM-based devices. Consistent with the observed homogeneous donor:acceptor network with the smallest rms roughness, the PffBB-14-based device exhibited the highest Jsc of 16.77 mA cm−2 and FF of 64.36%, suggesting efficient charge dissociation and transport. In short, AFM results confirm that the PffBB-14:PC71BM blend layer exhibited a desirable morphology for efficient photovoltaic properties. Our results also indicated that the extremely strong aggregation strength of the donor polymer would hinder for the formation of a desirable homogeneous donor/acceptor interpenetration network for efficient photovoltaic performance.

tion behavior, which can be modulated by the size of the alkyl side chain attached on the polymer backbone, processing temperature and solvent used. Because of the strong aggregation behavior, the absorption cut-off of the polymer thin films could extend to ∼780 nm, giving rise to a narrow optical band gap of ∼1.6 eV. Taking advantage of the temperature-dependent aggregation effect, BHJ PSCs with PffBB-n:PC71BM as an active blend layer spin-coated at different temperatures were fabricated and investigated which showed considerably various photovoltaic performance. Among PSCs fabricated, the PffBB-14:PC71BM-based device with an active blend layer spin-coated at 80 °C, showed the best photovoltaic performance with the highest PCE of 9.93%, a large Voc of 0.92 V, a highest Jsc of 16.77 mA cm−2, and a FF of 64.36%. Such an excellent device performance was attributed to the strong and aggregation-broadened absorption, superior exciton dissociation, desirable film morphology, better balanced charge carrier mobilities, and minimal recombination loss. Our results demonstrated that the side chain length modification and processing temperature modulation were proved to be useful for tuning polymer interchain interactions, absorption properties, active layer morphology, and charge carrier properties, thus optimizing the ultimate device performance of BHJ PSCs. Instrumentation. A Bruker-400 NMR spectrometer was employed to record 1H NMR and 13C NMR spectra with reference to the residual CHCl3 signal at 7.26 ppm in 1H NMR and 77.36 ppm in 13C NMR spectra. The decomposition temperatures of compounds were measured by a thermal gravimetric analyzer (PE-TGA6) with a ramping rate of 40 °C min−1 under N2 flow. UV−vis absorption spectra were recorded by a Varian Cary 100-UV−vis spectrophotometer. Solution samples were prepared in CHCl3 and solid-state thin films were coated on a quartz plate using a doctor blade technique from a concentrated polymer/CHCl3 solution. Cyclic voltammetry (CV) measurements were carried out on a CH Instrument 630 C electrochemical workstation. A platinum wire, a sample-coated platinum−carbon electrode, and Ag/Ag+ were used as a counter electrode, a working electrode, and a reference electrode in a supporting electrolyte of 0.02 M hexafluorophosphate (Bu4NPF6) in dry acetonitrile, respectively. Ferrocene was used as an external standard and its half-wave potential was 0.10 V vs Ag/Ag+ under these conditions. The HOMO and LUMO energy levels were calculated from the equation HOMO = −(4.7 + Eonsetox) eV; and LUMO = HOMO + Eg, where Eonsetox was the onset

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CONCLUSIONS We have developed a new series of donor−acceptor copolymer derived from 4,5-bis((2-ethylhexyl)oxy)benzo[2,1-b:3,4-b′]dithiophene and dialkyl-substituted difluorinated 4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole units, such as PffBB-n (n = 10, 12, 14, and 16) as a low band gap donor material for the application of highly efficient BHJ PSCs. PffBB-n exhibited intriguing strong and controllable aggrega4454


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oxidation potential of a sample. The molecular weight of polymers was estimated by the gel permeation chromatography (GPC) method performed at 150 °C, using polystyrene as the reference standard, with the column set 3 × PLgel Olexix 300 × 7.5 mm and 1,2,3-trichlorobenzene as the eluent with a flow rate of 1 mL min−1. X-ray diffraction (XRD) patterns of polymer samples were recorded by a D8 Advance diffractometer (Bruker, Billerica, MA) using Cu Kα radiation. Solar Cell Fabrication. The ITO glass substrates with a sheet resistance of 15 Ω sq−1 were successively washed with a detergent, deionized water, acetone, and isopropanol under ultrasonication, and then finally treated with UV light for 30 min under an ozone atmosphere. The electron extraction layer ZnO (∼30 nm) was first spin-coated on the ITO substrate (ZnO precursor solution: 2 M diethylzinc in toluene, diluted with tetrahydrofuran; spinning rate: 6000 rpm) and baked in dried air at 150 °C for 30 min. The precursor solution of the active layer was prepared with a D/A ratio of 1:2. (polymer concentration: 10 mg mL−1 in chlorobenzene with 3% of 1chloronaphthalene). The active layer precursor solution and the ZnO-coated ITO substrate were preheated on a hot plate at 60, 80, or 100 °C. The active layer was spin-coated on the ITO substrates with a spin-rate of 2000 rmp in an Ar-filled glovebox and then annealed at 60, 80, or 100 °C for 5 min. The active layer-coated ITO was then transferred to the vacuum chamber for thermal evaporation of processing solution. Subsequently, a molybdenum trioxide (MoO3, 10 nm) interlayer and aluminum anode (Al, 100 nm) were deposited onto the surface of the active layer in the evaporation chamber under high vacuum (≤10−6 mbar). The active area of a device was fixed as 4.00 mm2. Photocurrent−Voltage Measurement. A combination of a solar simulator (XES-70S1, SAN-EI) with a computercontrolled Keithley 2400 Source AM 1.5 G (100 mW cm−2) was applied to characterize the I−V performance of devices. The system was calibrated with a standard Si solar cell (AK200, Konica Minolta, INC.). External Quantum Efficiency. The EQE spectra were recorded by Newport QE test Model 77890 (Newport Co. Ltd.) using monochromatic light illumination from a xenon lamp. The integrated current density values were calculated from EQE and the AM 1.5 G irradiation spectrum. Space-Charge-Limited Current. Charge carrier mobilities were measured on hole-only and electron-only devices by adapting the SCLC equation (J = 9ε0εrμV2/8L3, where J is the current density, L is the film thickness of the active blend layer, μ is the hole or electron mobility, εr is the relative dielectric constant of the transport medium, ε0 is the permittivity of free space (8.85 × 10−12 F m−1), and V is the internal voltage in the device and V = Vappl − Vbi − Va, where Vappl is the applied voltage to the device, Vbi is the built-in voltage, and Va is the voltage drop). The device structure of a hole-only device: ITO/PEDOT/donor:PC71BM (1:2 w/w; CB, CN = 3%, (2000 rpm))/MoO 3/Al and electron-only device: Al/ donor:PC71BM (1:2 w/w; CB, CN = 3%, (2000 rpm))/Al. Atomic Force Microscopy. The tapping mode was applied to obtain the AFM images of active blend layers (Bruker Inova atomic microscope).

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.9b00682. Detailed synthetic procedures; TGA traces; CV traces; temperature- and solvent-dependent aggregation behavior (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Q.P.). *E-mail: [email protected] (M.S.W.). ORCID

Qiang Peng: 0000-0002-0536-2313 Man Shing Wong: 0000-0001-8141-9791 Author Contributions

L.Q.H. and G.J.Z. contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Institute of Molecular Functional Materials, a grant from the University Grants Committee, Areas of Excellence Scheme (AoE/P-03/08), the NSFC (51573107, 91633301 and 21432005) and the Foundation of State Key Laboratory of Polymer Materials Engineering (sklpme2017-2-04).

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REFERENCES

(1) Li, G.; Li, W.; Guo, X.; Guo, B.; Su, W.; Xu, Z.; Zhang, M. A new narrow bandgap polymer as donor material for high performance nonfullerene polymer solar cells. Org. Electron. 2019, 64, 241−246. (2) Zhao, W.; Li, S.; Yao, H.; Zhang, S.; Zhang, Y.; Yang, B.; Hou, J. Molecular Optimization Enables over 13% Efficiency in Organic Solar Cells. J. Am. Chem. Soc. 2017, 139, 7148−7151. (3) Dai, S.; Zhao, F.; Zhang, Q.; Lau, T. K.; Li, T.; Liu, K.; Ling, Q.; Wang, C.; Lu, X.; You, W.; Zhan, X. Fused Nonacyclic Electron Acceptors for Efficient Polymer Solar Cells. J. Am. Chem. Soc. 2017, 139, 1336−1343. (4) Shi, X.; Zuo, L.; Jo, S. B.; Gao, K.; Lin, F.; Liu, F.; Jen, A. K. Y. Design of a Highly Crystalline Low-Band Gap Fused-Ring Electron Acceptor for High-Efficiency Solar Cells with Low Energy Loss. Chem. Mater. 2017, 29, 8369−8376. (5) Jin, Y.; Chen, Z.; Dong, S.; Zheng, N.; Ying, L.; Jiang, X. F.; Liu, F.; Huang, F.; Cao, Y. A Novel Naphtho[1,2-c:5,6-c′]Bis([1,2,5]Thiadiazole)-Based Narrow-Bandgap pi-Conjugated Polymer with Power Conversion Efficiency Over 10. Adv. Mater. 2016, 28, 9811− 9818. (6) Kawashima, K.; Fukuhara, T.; Suda, Y.; Suzuki, Y.; Koganezawa, T.; Yoshida, H.; Ohkita, H.; Osaka, I.; Takimiya, K. Implication of Fluorine Atom on Electronic Properties, Ordering Structures, and Photovoltaic Performance in Naphthobisthiadiazole-Based Semiconducting Polymers. J. Am. Chem. Soc. 2016, 138, 10265−10275. (7) Zhao, W.; Qian, D.; Zhang, S.; Li, S.; Inganas, O.; Gao, F.; Hou, J. Fullerene-Free Polymer Solar Cells with over 11% Efficiency and Excellent Thermal Stability. Adv. Mater. 2016, 28, 4734−4739. (8) Zhang, S.; Ye, L.; Zhao, W.; Yang, B.; Wang, Q.; Hou, J. Realizing over 10% efficiency in polymer solar cell by device optimization. Sci. China: Chem. 2015, 58, 248−256. (9) Liu, Y.; Zhao, J.; Li, Z.; Mu, C.; Ma, W.; Hu, H.; Jiang, K.; Lin, H.; Ade, H.; Yan, H. Aggregation and morphology control enables multiple cases of high-efficiency polymer solar cells. Nat. Commun. 2014, 5, No. 5293.

4455


Article

Macromolecules

Based Conjugated Polymers. J. Phys. Chem. C 2009, 113, 21202− 21207. (28) Li, G.; Gong, X.; Zhang, J.; Liu, Y.; Feng, S.; Li, C.; Bo, Z. 4Alkyl-3,5-difluorophenyl-Substituted Benzodithiophene-Based Wide Band Gap Polymers for High-Efficiency Polymer Solar Cells. ACS Appl. Mater. Interfaces 2016, 8, 3686−3692. (29) Ye, L.; Zhang, S.; Huo, L.; Zhang, M.; Hou, J. Molecular design toward highly efficient photovoltaic polymers based on twodimensional conjugated benzodithiophene. Acc. Chem. Res. 2014, 47, 1595−1603. (30) Yao, H.; Zhang, H.; Ye, L.; Zhao, W.; Zhang, S.; Hou, J. Dialkylthio Substitution: An Effective Method to Modulate the Molecular Energy Levels of 2D-BDT Photovoltaic Polymers. ACS Appl. Mater. Interfaces 2016, 8, 3575−3583. (31) Sun, H.; Tang, Y.; Koh, C. W.; Ling, S.; Wang, R.; Yang, K.; Yu, J.; Shi, Y.; Wang, Y.; Woo, H. Y.; Guo, X. High-Performance AllPolymer Solar Cells Enabled by an n-Type Polymer Based on a Fluorinated Imide-Functionalized Arene. Adv. Mater. 2019, 31, No. e1807220. (32) Yu, J.; Chen, P.; Koh, C. W.; Wang, H.; Yang, K.; Zhou, X.; Liu, B.; Liao, Q.; Chen, J.; Sun, H.; Woo, H. Y.; Zhang, S.; Guo, X. Phthalimide-Based High Mobility Polymer Semiconductors for Efficient Nonfullerene Solar Cells with Power Conversion Efficiencies over 13. Adv. Sci. 2019, 6, No. 1801743. (33) Gao, X.; Li, Y.; Yu, L.; Hou, F.; Zhu, T.; Bao, X.; Li, F.; Sun, M.; Yang, R. The regulation of π-bridge of indacenodithiophenebased donor-π-acceptor conjugated polymers toward efficient polymer solar cells. Dyes Pigm. 2019, 162, 43−51. (34) Cho, H. J.; Kim, Y. J.; Chen, S.; Lee, J.; Shin, T. J.; Park, C. E.; Yang, C. Over 10% efficiency in single-junction polymer solar cells developed from easily accessible random terpolymers. Nano Energy 2017, 39, 229−237. (35) Stuart, A. C.; Tumbleston, J. R.; Zhou, H.; Li, W.; Liu, S.; Ade, H.; You, W. Fluorine substituents reduce charge recombination and drive structure and morphology development in polymer solar cells. J. Am. Chem. Soc. 2013, 135, 1806−1815. (36) Xue, L.; Yang, Y.; Xu, J.; Zhang, C.; Bin, H.; Zhang, Z. G.; Qiu, B.; Li, X.; Sun, C.; Gao, L.; Yao, J.; Chen, X.; Yang, Y.; Xiao, M.; Li, Y. Side Chain Engineering on Medium Bandgap Copolymers to Suppress Triplet Formation for High-Efficiency Polymer Solar Cells. Adv. Mater. 2017, 29, No. 1703344. (37) Chen, P.; Shi, S.; Wang, H.; Qiu, F.; Wang, Y.; Tang, Y.; Feng, J. R.; Guo, H.; Cheng, X.; Guo, X. Aggregation Strength Tuning in Difluorobenzoxadiazole-Based Polymeric Semiconductors for HighPerformance Thick-Film Polymer Solar Cells. ACS Appl. Mater. Interfaces 2018, 10, 21481−21491. (38) Huang, L.; Zhang, G.; Zhang, K.; Peng, Q.; Wong, M. S. Benzodithiophene-Dithienylbenzothiadiazole Copolymers for Efficient Polymer Solar Cells: Side-Chain Effect on Photovoltaic Performance. ACS Appl. Mater. Interfaces 2018, 10, 34355−34362. (39) Arroyave, F. A.; Richard, C. A.; Reynolds, J. R. Efficient synthesis of benzo[1,2-b:6,5-b′]dithiophene-4,5-dione (BDTD) and its chemical transformations into precursors for pi-conjugated materials. Org. Lett. 2012, 14, 6138−6141. (40) Wang, N.; Chen, Z.; Wei, W.; Jiang, Z. Fluorinated benzothiadiazole-based conjugated polymers for high-performance polymer solar cells without any processing additives or posttreatments. J. Am. Chem. Soc. 2013, 135, 17060−17068. (41) Kundu, S. K.; Bhaumik, A. A triazine-based porous organic polymer: a novel heterogeneous basic organocatalyst for facile one-pot synthesis of 2-amino-4H-chromenes. RSC Adv. 2015, 5, 32730− 32739. (42) Wan, J.; Xu, X.; Zhang, G.; Li, Y.; Feng, K.; Peng, Q. Highly efficient halogen-free solvent processed small-molecule organic solar cells enabled by material design and device engineering. Energy Environ. Sci. 2017, 10, 1739−1745. (43) Kyaw, A. K.; Wang, D. H.; Gupta, V.; Leong, W. L.; Ke, L.; Bazan, G. C.; Heeger, A. J. Intensity dependence of current-voltage

(10) You, J.; Dou, L.; Yoshimura, K.; Kato, T.; Ohya, K.; Moriarty, T.; Emery, K.; Chen, C. C.; Gao, J.; Li, G.; Yang, Y. A polymer tandem solar cell with 10.6% power conversion efficiency. Nat. Commun. 2013, 4, No. 1446. (11) He, M.; Li, M.; Dong, X.; Tian, H.; Tong, H.; Liu, J.; Xie, Z.; Geng, Y.; Wang, F. A difluorobenzothiadiazole-based conjugated polymer with alkylthiophene as the side chains for efficient, additivefree and thick-film polymer solar cells. J. Mater. Chem. A 2017, 5, 20473−20481. (12) Li, M.; Gao, K.; Wan, X.; Zhang, Q.; Kan, B.; Xia, R.; Liu, F.; Yang, X.; Feng, H.; Ni, W.; Wang, Y.; Peng, J.; Zhang, H.; Liang, Z.; Yip, H.-L.; Peng, X.; Cao, Y.; Chen, Y. Solution-processed organic tandem solar cells with power conversion efficiencies > 12%. Nat. Photonics 2017, 11, 85−90. (13) Hou, J.; Tan, Z.; Yan, Y.; He, Y.; Yang, C.; Li, Y. Synthesis and photovoltaic properties of two-dimensional conjugated polythiophenes with bi(thienylenevinylene) side chains. J. Am. Chem. Soc. 2006, 128, 4911−4916. (14) Ma, W.; Yang, C.; Gong, X.; Lee, K.; Heeger, A. J. Thermally Stable, Efficient Polymer Solar Cells with Nanoscale Control of the Interpenetrating Network Morphology. Adv. Funct. Mater. 2005, 15, 1617−1622. (15) Zhang, Z. G.; Zhang, S.; Min, J.; Cui, C.; Geng, H.; Shuai, Z.; Li, Y. Side Chain Engineering of Polythiophene Derivatives with a Thienylene−Vinylene Conjugated Side Chain for Application in Polymer Solar Cells. Macromolecules 2012, 45, 2312−2320. (16) Huo, L.; Zhou, Y.; Li, Y. Alkylthio-substituted polythiophene: absorption and photovoltaic properties. Macromol. Rapid Commun. 2009, 30, 925−931. (17) Hou, J.; Chen, T. L.; Zhang, S.; Huo, L.; Sista, S.; Yang, Y. An Easy and Effective Method To Modulate Molecular Energy Level of Poly(3-alkylthiophene) for High-VocPolymer Solar Cells. Macromolecules 2009, 42, 9217−9219. (18) Zhang, M.; Guo, X.; Ma, W.; Ade, H.; Hou, J. A polythiophene derivative with superior properties for practical application in polymer solar cells. Adv. Mater. 2014, 26, 5880−5885. (19) Jo, J. W.; Jung, J. W.; Wang, H.-W.; Kim, P.; Russell, T. P.; Jo, W. H. Fluorination of Polythiophene Derivatives for High Performance Organic Photovoltaics. Chem. Mater. 2014, 26, 4214−4220. (20) Po, R.; Bianchi, G.; Carbonera, C.; Pellegrino, A. “All That Glisters Is Not Gold”: An Analysis of the Synthetic Complexity of Efficient Polymer Donors for Polymer Solar Cells. Macromolecules 2015, 48, 453−461. (21) Wang, Q.; Dong, X.; He, M.; Li, M.; Tian, H.; Liu, J.; Geng, Y. Polythiophenes with carboxylate side chains and vinylene linkers in main chain for polymer solar cells. Polymer 2018, 140, 89−95. (22) Scharber, M. C.; Mühlbacher, D.; Koppe, M.; Denk, P.; Waldauf, C.; Heeger, A. J.; Brabec, C. J. Design Rules for Donors in Bulk-Heterojunction Solar CellsTowards 10% Energy-Conversion Efficiency. Adv. Mater. 2006, 18, 789−794. (23) Peng, S.-H.; Tu, W.-Y.; Gollavelli, G.; Hsu, C.-S. Synthesis of diketopyrrolopyrrole based conjugated polymers containing thieno[3,2-b]thiophene flanking groups for high performance thin film transistors. Polym. Chem. 2017, 8, 3431−3437. (24) Kim, J.-H.; Wood, S.; Park, J. B.; Wade, J.; Song, M.; Yoon, S. C.; Jung, I. H.; Kim, J.-S.; Hwang, D.-H. Optimization and Analysis of Conjugated Polymer Side Chains for High-Performance Organic Photovoltaic Cells. Adv. Funct. Mater. 2016, 26, 1517−1525. (25) Kim, J.-H.; Park, J. B.; Jung, I. H.; Grimsdale, A. C.; Yoon, S. C.; Yang, H.; Hwang, D.-H. Well-controlled thieno[3,4-c]pyrrole-4,6(5 H)-dione based conjugated polymers for high performance organic photovoltaic cells with the power conversion efficiency exceeding 9%. Energy Environ. Sci. 2015, 8, 2352−2356. (26) Bon, J. L.; Feng, D.; Marder, S. R.; Blakey, S. B. A C-H functionalization protocol for the direct synthesis of benzobisthiazole derivatives. J. Org. Chem. 2014, 79, 7766−7771. (27) Hou, J.; Chen, H.-Y.; Zhang, S.; Yang, Y. Synthesis and Photovoltaic Properties of Two Benzo[1,2-b:3,4-b′]dithiophene4456


Article

Macromolecules characteristics and recombination in high-efficiency solution-processed small-molecule solar cells. ACS Nano 2013, 7, 4569−4577. (44) Wang, E.; Hou, L.; Wang, Z.; Ma, Z.; Hellström, S.; Zhuang, W.; Zhang, F.; Inganäs, O.; Andersson, M. R. Side-Chain Architectures of 2,7-Carbazole and Quinoxaline-Based Polymers for Efficient Polymer Solar Cells. Macromolecules 2011, 44, 2067−2073.

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