âJacketingâ Effect Liquid Crystalline Polymer with Perylenediimide as

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“Jacketing” Effect Liquid Crystalline Polymer with Perylenediimide as Side Chain: Synthesis, Liquid Crystalline Phase, and Photovoltaic Performances Lei Tao, Huang Chen, Ze-Yang Kuang, Chao Weng, Ping Wang, Nie Zhao, and He-Lou Xie ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00771 • Publication Date (Web): 18 Jul 2018 Downloaded from http://pubs.acs.org on July 19, 2018

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ACS Applied Energy Materials

“Jacketing” Effect Liquid Crystalline Polymer with Perylenediimide as Side Chain: Synthesis, Liquid Crystalline Phase, and Photovoltaic Performances Lei Tao a, Huang Chen a, Ze-Yang Kuang a, Chao Weng a *, Ping Wang a , Nie Zhao b, He-Lou Xie a * a

Key Lab of Environment-friendly Chemistry and Application in Ministry of

Education, and Key Laboratory of Advanced Functional Polymer Materials of Colleges, Universities of Hunan Province and College of Chemistry, Xiangtan University, Xiangtan 411105, Hunan Province, China b

College of Materials Science and Engineering, Xiangtan University, Xiangtan 411105, Hunan Province, China *To whom the correspondence should be addressed. E-mail: [email protected] (HLX) and [email protected] (CW)

Abstract: All polymer solar cells (all-PSCs) is one of the important emerging renewable energy technologies. In this work, we use “Jacketing” effect liquid crystalline polymer (LCP) with perylenediimide as side chain to fabricate all-PSCs. Firstly, poly (2,5-bis {[6-(4-alkoxy-4’-perylenediimide) -6-hexyl] oxycarbonyl} styrene) (abbreviated as PPDCS) is successfully synthesized via chain polymerization. The resultant polymer PPDCS forms stable smectic C (SmC) structure until decomposition. The electrochemical experiment indicates PPDCS shows deep LUMO energy level of –3.81 eV, thus, the non-conjugated PPDCS can be employed as acceptor materials to build all-PSCs. Atomic force microscopy (AFM) experiments show that the PBT7/PPDCS blend film forms a bicontinuous network-domains and the resultant film shows extensive absorption spectrum (300-800 nm) on UV-Vis 1

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spectra. All-PSCs device fabricated by PTB7/PPDCS presents the best power conversion efficiency (PCE) of 1.23% after optimization, where the short-circuit current density (Jsc) is 4.34 mA cm-2, an open-circuit voltage (Voc) is 0.65 V, and a fill factor (FF) is 0.37. This work suggests that the non-conjugated LCP shows potential application for solar cell. KEWORDS: Liquid crystalline polymer , “Jacketing” effect, Perylenediimide, Non-conjugated structure, Polymer solar cells.

Introduction Due to the light weight, mechanical flexibility, and low cost, all polymer solar cells (all-PSCs) as one of the emerging renewable energy technologies have greatly attracted research attentions over the few decades.[1-4] Generally, all-PSCs are fabricated by the donor and acceptor materials. Fullerene and its derivatives with the donor materials favor to form the nano-networks structure, which result in high electron mobility, high electron affinity, charge-transfer isotropy. Thus, this kind of compounds have been the dominant acceptor materials in all-PSCs.[5-11] However, some inherent drawbacks, such as narrow and weak absorption, poor photostability, poor solar energy collection, complicated synthesis, low economic benefit,[12-14] limit their further application. Thus, it is particularly important to further develop the non-fullerene acceptor.[15-18] As for the non-fullerene acceptor materials, perylenediimide (PDI) units are the most typical example,[19-23] which has been well developed as organic semiconductors 2


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materials with photochemical and high thermal stabilities. Since the well-placed LUMO energy and promising electron mobility, some molecules based PDI generally show a powerful absorption at 400 and 650 nm with excellent electron affinity and an excellent molar extinction coefficient (εmax).[24-28] However, the planar nature of PDI molecules favors to a tendency for π–π stacking, which leads to the serious aggregation and a poor solubility. The aggregation results in the exciton to strongly self-capture, which severely limits the diffusion/separation of the exciton. Meantime, the aggregation also reduces relatively short-circuit current density (Jsc) and fill factor (FF), which can decrease the power conversion efficiency (PCE). To overcome the aggregation, different methods have been developed, such as attaching the alkyl chains on the bay locations of PDI, tailoring the substituted groups via direct attachment or small heterocycles at the bay locations or imide locations. These approaches do not inhibit their charge transfer, which help to achieve relatively high PCEs.[29-37] For example, TPE-PDI4 with the strong steric hindrance and highly twisted and weak intermolecular interactions results in high PCE of 5.53%.[38] Generally, the polymers fabricated by PDI for all-PSCs are conjugated, which are synthesized through multistep complicated reactions.[39-41] Different from the complex conjugated system, the non-conjugated polymer can be fabricated through using PDI as side chain and vinyl polymer as main chain. This kind of polymer can be synthesized via chain polymerization, which can result in large enough molecular weight and excellent film-forming property. Herein, we proposed a kind of non-conjugated liquid crystalline polymer (LCP) using PDI side chain and 3



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“Jacketing” effect LCP as main chain. “Jacketing” effect LCP (MJLCP) is a kind of special LCPs different from general main-chain LCPs and side-chain LCPs.[42-44] The typical feature of MJLCP is that the bulky side group directly or via very short spacer attached to the flexible backbone. Due to the spatial requirement, the whole polymer chain is forced to form the extended conformation, which result in the formation of cylinder by the whole molecular chain. Thus, MJLCPs show the chemical structure of the typical side chain LCPs and the similar property of the main chain LCPs.[45-51] Meantime, the non-conjugated polymer using the MJLCPs as the main chain can lead to versatile hierarchical liquid crystalline structures, which can be benefit to improving processability and the performance of the device based on PDI. In addition, this kind of chemical structure is the first-time reported, which can provide a new method to fabricate the non-conjugated acceptor materials. With this mind, poly (2,5-bis {[perylenediimide-6-hexyl] oxycarbonyl} styrene) (PPDCS) was designed and synthesized. The chemical structure and molecular model of PPDCS are shown in Scheme 1. In order to increase the solubility, the imide position of PDI group is modified by swallow-tailed hexylheptyl and the other imide position is linked with styrene via alkyl chain. The experimental results showed PPDCS formed stable smectic C (SmC) structures during the whole temperature area. Further,

the

commercial

poly(3-hexylthiophene)

(P3HT)

and

polythieno[3,4-b]-thiophene-co-benzodithiophene (PTB7) were used as the donor materials to build all-PSCs. The PCE value of PTB7/PPDCS-based device was 1.23%, which showed the potential application of non-conjugate structure LCP for all-PSCs. 4


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Scheme 1. The chemical structure and molecular model of PPDCS and chemical structure of PTB7, P3HT.

Experimental Section Materials. 7-Trideaanone (97%, Energy Chemical), Sodium cyanoborohydride (95%, Energy Chemical), Perylenediimide (98%, Energy Chemical), 6-Amino-1-hexanol (95%, Energy Chemical), PTB7 and P3HT (1-Material company, Canada). 4-dimethylaminopyridine (DMAP) (Shanghai Chemical Reagents Co.,) were directedly used without any treatment. 2,2’-azo-bis-isobutyronitrile (AIBN) was purified by recrystallization in alcohol solution, and tetrahydrofuran (THF) was dried with metal sodium and evaporated by vacuum distillation.

Synthesis. The detail synthetic route is shown in Scheme 2. The information of the intermediate compounds is shown in Supporting Information, and the detail information of the monomer and corresponding polymer (PPDCS) is as following. Scheme 2. Synthetic route of the monomer and PPDCS.

5



Synthesis of 2,5-bis {[perylenediimide-6-hexyl] oxycarbonyl} styrene (PDCS). In a 100 mL flask, N-(1-heptyl)-perylene-N’-(6-hydroxyhexyl)-tetracarboxylbisimide (1 g, 1.5 mmol), DMAP (0.097 mg, 0.8 mmol), and TEA (0.26 g, 2.6 mmol) were dissolved in refined THF (50 mL) cooling to 0 ºC. Then, vinyl p-biphthalic acid (125 mg, 0.65 mmol), thionyl chloride 10 mL and two drops benzoquinone was added to the round bottom flask under the solution clarification. Subsequently, the resultant product was concentrated in vacuum with dried petroleum ether (3 × 10 mL), diluted by THF (10 mL) and drop into the above-mentioned flask. Further stirring overnight and filtering and the resultant filtrate was removed the solvent under vacuum. The achieved crude product was purified by column chromatography using SiO2 as filler and petroleum ether/ethyl acetate =1/5 solution. The final red solid product was 0.37 g with a yield of 37%. 1H NMR (CDCl3) δ (ppm): 8.66 (dd, 8H, perylene), 8.21 (s, 1H, Ar−H), 7.90 (d, 2H, Ar−H), 7.35 (d, 2H, =CH−), 5.73 (d, 1H, =CH2), 5.42 (d, 1H, =CH2), 5.18 (m, 2H, -NCH(C6H13)2), 4.33 (t, 4H, -CH2COO-Ar), 4.20 (t, 2H, -NCH2CH2-), 2.23 (m, 4H, -CH2CHN-), 1.83 (m, 4H, -CH2CH2COO-), 1.55 (m, 8H, 6


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-CH2CHN-), 1.24-1.33 (m, 28H, 14 CH2), 0.83 (t, 12H, 4 CH3). 13C NMR (CDCl3) δ (ppm):166.08 (C=O), 165.54 (C=O), 159.28 (PDI C=O), 147.80, 138.79, 127.89, 123.87 (PDI), 139.56 (aromatic C-CH=CH2), 135.08 (aromatic C-C=O), 133.44 (aromatic and =CH-), 132.83 (aromatic C-C=O), 130.35 (aromatic meta C-CH=CH2), 127.66 (=CH2), 67.93 (-OCH2-), 65.53 (-OCH2-), 45.38 (-CH-N-), 40.65 (-CH2-N-), 32.69 (-CH2-), 32.06 (-CH2-), 29.60 (-CH2-), 29.03 (-CH2-), 27.22 (-CH2-), 27.16 (-CH2-), 22.90 (-CH2-), 22.87 (-CH2-), 22.75 (-CH2-), 13.36 (-CH3). MALDI-TOF (m/z): [M + Na]+ calcd for C96H100N4O12Na, 1523.739; found, 1524.168. Synthesis of PPDCS. The radical polymerization was employed to synthesize PPDCS (see Scheme 2). PDCS (0.3 g, 0.2 mmol), AIBN (2 × 10

-4

g, 1.6 x 10

-3

mmol) and THF (0.8 mL) were put into glass tube. The glass tube was further sealed under vacuum environment and reacted at 65 °C for 12 h. At the end of the reaction the reactant was diluted with THF. Subsequently, the mixture was precipitated in petroleum ether. Yield 56%. Fabrication of all-PSCs. The sandwich structure was used to construct all-PSCs. The pre-cleaned glass-ITO as substrate was used to fabricate thin film of poly-(3,4-ethylenedioxy-thiophene) (PEDOT) and polystyrene sulfonic acid (PSS) via spin-coating. Further, the resultant thin film was thermal annealing at 160 °C for 10 min. The active layer was constructed from the solution of P3HT/PTB7 and PPDCS with/without

1,8-diiodooctane

(DIO)

via

spin-coating

on

the

surface

of

above-mentioned thin film. At last, LiF and Al electrodes were deposited on the surface of active layer under vacuum with 5.0 × 10-4 Pa pressure. The device effective 7



surface was 3.8 mm2, which was determined by the overlap of metal electrode and ITO.

Results and Discussion Synthesis and Characterization of PPDCS. The detailed synthesis process is shown in Scheme 2. The chemical structures of the intermediate compounds, monomer and polymer were identified by NMR and MALDI-TOF. As shown in Fig. 1, the hydrogen displacements of the PDI and benzene ring for the monomer are located at 8.60 - 8.43 ppm, 8.20 ppm and 7.92 ppm. While the peaks at 7.40 ppm, 5.70 ppm, 5.41 ppm are the typical feature of the vinyl substituent. Further calculation revealed that the ratios of all the peak areas well machted with the hydrogen atom numbers, revealing the targeted monomer was successfully obtained. After polymerization, the vinyl peaks disappeared, and other peaks became broader, which indicated that PPDCS was successfully synthesized. Further GPC results showed the MW was 1.2× 104 and the polymer polydispersity index was 1.53.

8


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Figure 1. 1H NMR spectra of Monomer and PPDCS LC properties of the PPDCS. Thermogravimetric analysis (TGA) experimental result showed that the PPDCS decomposition temperatures of at 5% weight-loss was 340 °C, which meant PPDCS showed very good thermal stability. The good thermal stability would help to all-PSCs application. Differential scanning calorimetry (DSC) was employed to investigate the phase transition. Fig. 2a depicts DSC traces of PPDCS from 0 °C to 230 °C after erasing the thermal history. Obviously, only a glass transition at about 196 °C could be observed, which was typical feature of MJLCPs. The PDI side chain did not show any crystallinity, which indicated that the polymer chains significant destroyed the packing of the side chain PDI. Polarized optical microscopy (POM) experiment was carried out to investigate the LC property. The POM samples with ~10 µm thickness were prepared by spin-coated from the THF solution. While the temperature exceeds the glass transition temperature (Tg), typical birefringence is shown in Fig. 2b. Meantime, 9



this birefringence phenomenon did not disappear below the decomposition temperature. During the cooling process, the birefringence always remained, revealing stable LC phase always remained during the whole temperature range. This experiment result was typical LC phenomenon of MJLCPs.

Figure 2. (a) DSC spectra and (b) POM micrographs of PPDCS. The phase structure was further confirmed by wide-angle X-ray diffraction (WAXD). The polymer solution was dropwise added to the groove of tin-foil and drying at normal temperature. The one-dimensional (1D) WAXD profiles at different temperatures of PPDCS are shown in Fig. 3. In the low angle, three obvious diffraction peaks could be observed located at 1.72°, 3.45° and 5.20°, respectively. The corresponding q-ratio was 1:2:3 corresponding to a smectic phase. The diffuse halo angles (>10°) in the high angle were arisen from the liquid-like N-substituents, and another angle (25.5°) was information of the perylene-perylene stacking.[52] During the whole variational temperature process, the intensity of diffraction peaks remained unchanged, indicating PPDCS formed stable smectic structure during the whole variational temperature range. According to the diffractions, the d100 layer period was calculated as 5.14 nm. Assuming the all trans conformation for both the tail and spacer, the molecular length of PPDCS was 6.77 nm, which was longer than 10


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d100. This indicated the side chain of PPDCS partially interdigitated packing or adjusting incline packing. According to our previous work, the rod-like main-chain favored to forming smectic C (SmC).[53] Herein, the data revealed that PPDCS formed SmC structure. Meantime, combined the d100 value and the calculated molecular length, the inclined angle between the side chain and layer was 49o. The packing model of PPDCS is shown in Fig. 3c.

Figure 3. 1D WAXD profiles of PPDCS during the cooling process in the low-angle (a) and high-angle regions (b), and the assuming molecular packing model for PPDCS (c).

Optoelectronic Properties. The absorption spectra of P3HT, PTB7 and PPDCS were firstly investigated in dilute solution and thin films. In dilute chloroform solution, the normalized spectrum of optical absorption of PPDCS shows a narrow absorption 11



range at 490 nm and 532 nm (see Fig. S1). In thin film, the PPDCS shows a wider absorption with a similar profile, and about 5 nm red shift of absorption peak can be observed (see Fig. 4a), indicating the weak interactions and aggregation of molecules in thin films. The absorption spectrum (λonset) of the PPDCS film was 630 nm, then the optical band gaps (Egopt) of PPDCS was 1.97 eV deduced from their absorption edges. The detailly results about absorption data are shown in Table 1. As shown in Fig. 4b, the thin film fabricated by the mixture of PPDCS and P3HT (or PTB7) with 3% DIO shows broader absorption than those of the pure P3HT and PTB7 film. Moreover, the PTB7:PPDCS with 3% DIO blend film exhibited a more wider UV-Vis absorption compared with the P3HT:PPDCS with 3% DIO blend film, (from 350 nm to 850 nm). Further the UV-Vis absorption indicated the PTB7 with PPDCS formed better matched complementary absorption than that of P3HT with PPDCS. Meanwhile, the UV-Vis absorption of PTB7:PPDCS blend film demonstrated that the active layer significantly absorbed more photons, which help to improve the property of the short circuit current density (Jsc) of the photovoltaic device.

Figure 4. UV–Vis absorption spectra of pure polymers film (a) and blend films (b).

12


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Electrochemical Properties. The electrochemical property of PPDCS were detailly investigated by cyclic voltammetry (CV) method using Ag/AgCl as electrode. As show in Fig. 5a, the onset reduction potentials (Ered) and reduction potentials (Eox) are -0.52 V and 1.65 V compared with Ag/Ag+. With respect to the previous formal potential of 4.8 eV of the Fc/Fc+,[54] it was 0.48V vs. Ag/Ag+. Thus, according to the empirical equation, the LUMO energy levels (ELUMO) could be calculated: ELUMO = (Ered + 4.32) eV and EHOMO = - (Eox + 4.32) eV. The ELUMO and the HOMO of PPDCS were -3.81 eV and -5.98 eV, respectively. The energy level comparison between the donor polymers P3HT and PTB7 and the acceptor polymer PPDCS is displayed in Fig. 5b. The LUMO value of P3HT and PTB7 were -2.74 eV and -3.31 eV,[55] and calculated the offset between LUMO of P3HT, PTB7 with PPDCS were 1.07 eV and 0.6 eV, respectively. This value was much larger than 0.3eV of the accepted minimal value,[56] which was high enough to drive the electron transportation and exciton separation in the polymer blends.

Figure 5. (a) Cylic Voltammogram for PPDCS and (b) Energy levels of P3HT, PTB7 and PPDCS.

13



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Table 1. Summary of optical properties of PPDCS. λmax (nm)

PPDCS

Solution

film

532

537

λonset Film (nm) a

ELUMO (eV)b

EHOMO (eV)b

Egopt (eV)c

630

-3.81

-5.98

1.97

a

Obtained from film absorption; Measured by cyclic voltammetry; c Calculated from Egopt =1240/λonset. b

Photovoltaic performances. UV-Vis and CV results revealed that P3HT, PTB7 with PPDCS showed complementary absorption spectra and well-matched energy levels. So, we fabricated all-PSCs devices with PTB7 and P3HT as donor materials and PPDCS as the acceptor materials, respectively. Device performance and corresponding J-V curves are summarized in Fig. 6a and Table 2, respectively. Without any additive, the two devices with 1:1 weight ratio yielded the low PCE of 0.30% and 0.63%, respectively. Obviously, the PCE of PTB7:PPDCS device was slightly higher than that of P3HT:PPDCS, which may be resulted from the complementary absorption spectrum. As shown in Fig. 4, the PTB7:PPDCS blend film shows a broader UV-Vis absorption than that of the P3HT:PPDCS blend film, which means the devices used PTB7 as donor can absorb more energy and reach the relatively high Jsc (2.84 mA cm-2). The small amount of 3% DIO (v/v) was added to the device to improve device performance. For P3HT:PPDCS device, the PCE value almost remained unchanged (0.25% vs 0.3%), but for the PTB7:PPDCS device, the 14


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PCE value significantly increased (from 0.63% to 1.23%). The increased PCE of the PTB7:PPDCS device was benefited from the remarkable increasing Jsc (from 2.84 mA cm-2 to 4.34 mA cm-2), which probably attributed to the bicontinuous network formed by the donor and acceptor domains.[57] As shown in Fig. 6b the incident photon-to-current efficiency (IPCE) curves of the best devices are consistent with the Jsc variation of J-V characteristics. For the PTB7:PPDCS with 3% DIO device, the PSC displayed extensive IPCE spectrum from 300 to 800 nm, indicating that both PPDCS and PTB7 contributed to Jsc and IPCE. Meanwhile, the PTB7:PPDCS with 3% DIO device showed the highest value of 26.42% at 500 nm (the absorption of PPDCS), indicating that PPDCS significantly facilitate the overall photocurrent of the whole device. Specifically, the P3HT:PPDCS with 3% DIO device only showed a narrow IPCE spectra from 300 to 650 nm, and the maximum IPCE value was 12.83% at 436 nm, which corresponded to the low Jsc values of 1.94 mA cm-2.

Figure 6. (a) J-V curves and (b) IPCE curves of all-PSCs, the P3HT, PTB7:PPDCS = 1:1 (weight ratio).

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Table 2. Device performance of PPDCS with P3HT, PTB7. Weight

*

DIO(%)

Active layer

Voc(V)

Jsc(mA cm-2)

PCEmax(PCEave ) FF

Ratio

(v/v)

P3HT:PPDCS

1:1

0

0.63

1.13

0.43

0.30(0.28)

P3HT:PPDCS

1:1

3

0.37

1.94

0.35

0.25(0.22)

PTB7:PPDCS

1:1

0

0.68

2.84

0.32

0.63(0.61)

PTB7:PPDCS

1:1

3

0.76

4.34

0.37

1.23(1.17)

(%)

Blend film Morphologies. The microphase separation of the active layers was significantly crucial to the performance of PSCs. The P3HT, PTB7:PPDCS blend films was prepared through spin-coating from chlorobenzene solution, and the surface behaviors of the resultant thin films were investigated by atomic force microscopy (AFM) with tapping mode. As shown in Fig. 7, P3HT:PPDCS with 3% DIO blend film shows a rough morphology with 19.0 nm of root-mean-square (RMS) roughness, while PTB7:PPDCS with 3% DIO blend films shows relatively smooth and homogeneous morphology with RMS roughness of 4.97 nm. The smoother surfaces which afforded a better contact with the top electrode was beneficial to achieve a high Jsc. The suitable crystalline domains and phase separation helped to achieve high PCE for all-PSCs. As shown in Fig.7b and d, the phase image shows smaller domain sizes of PTB7:PPDCS with 3% DIO blend films (about 200 nm) than those of the P3HT:PPDCS with 3% DIO blend film (about 600 nm). This phenomenon indicated 16


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that the PTB7:PPDCS system exhibited good interpenetrating networks and suitable phase separation scale, which was beneficial to improve PCE. On the other hand, the domain side of the PTB7:PPDCS system was still too large, which prevented the improvement of PCE. o

35nm

10

150nm

10

o

Figure 7. The AFM height images (left) and phase images (right) for the (a), (b) PTB7:PPDCS with 3% DIO blend films and (c), (d) P3HT:PPDCS with 3% DIO blend films with tapping-mode. All the image size: 3 x 3 µm. Conclusion In summary, “Jacketing” effect LCP (PPDCS) with PDI as side chain was successfully synthesized through radical polymerization. Combined experimental results of POM and 1D WAXD showed that PPDCS formed stable SmC structure. Further the PPDCS was used as an acceptor polymer to fabricate all-PSCs. PPDCS exhibited a complementary absorption and an appropriate energy level to donor material. For the donor polymer of P3HT and PTB7, PTB7 with PPDCS showed well-matched complementary absorption in UV-Vis. Further optimization, all-PSCs with PTB7 and PPDCS could reach a better PCE of 1.23%. Evidently, non-conjugated

17



structure of PPDCS showed potential application for all-PSCs. Further selecting appropriate donor material with PPDCS may form a better phase state that will be beneficial to enhance the capability of photovoltaic equipment, which provides a novel method to design new acceptor materials and all-PSCs.

Acknowledgements We acknowledge the National Natural Science Foundation of China (NNSFS 21374092, 21674088 and 51503174), the Nature Science Foundation of Hunan Province (NSFH 2016JJ2127), Hunan graduate scientific research innovation project (CX2017B299) and Hunan 2011 Collaborative Innovation Center of Chemical Engineering & Technology with Environmental Benignity and Effective Resource Utilization for financial support.

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For Table of Contents use only

“Jacketing”

Effect

Liquid

Crystalline

Polymer

with

Perylenediimide as Side Chain: Synthesis, Liquid Crystalline Phase, and Photovoltaic Performances Lei Tao, Huang Chen, Ze-Yang Kuang, Chao Weng, Ping Wang, Nie Zhao, He-Lou Xie

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âJacketingâ Effect Liquid Crystalline Polymer with Perylenediimide as

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