Synergistic Effect of Fluorine Substitution and Thio-Alkylation on

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Synergistic Effect of Fluorine Substitution and Thio-Alkylation on Photovoltaic Performances of Alternating Conjugated Polymers Based on Alkylthio Substituted Benzothiadiazole-Quaterthiophene Ruipeng Peng, Huan Guo, Jingbo Xiao, Guo Wang, Songting Tan, Bin Zhao, Xia Guo, and Yongfang Li ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00238 • Publication Date (Web): 26 Apr 2018 Downloaded from http://pubs.acs.org on April 30, 2018

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ACS Applied Energy Materials

Synergistic Effect of Fluorine Substitution and Thio-Alkylation on Photovoltaic Performances of Alternating Conjugated Polymers Based on Alkylthio Substituted Benzothiadiazole-Quaterthiophene Ruipeng Peng,† Huan Guo,‡ Jingbo Xiao,† Guo Wang,† Songting Tan,*† Bin Zhao,*† Xia Guo*‡ and Yongfang Li‡ †

Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of

Education, College of Chemistry, Xiangtan University, Xiangtan 411105, China ‡

Laboratory of Advanced Optoelectronic Materials, College of Chemistry, Chemical Engineering

and Materials Science, Soochow University, Suzhou 215123, China.

ABSTRACT: Three low bandgap polymers (PSDTBT-DFDT, PSDTfBT-DFDT, and

PSDTffBT-DFDT)

based

on

fluorine

and

alkylthio

substituted

benzothiadiazole-quaterthiophene alternating units are synthesized and applied in polymer solar cells. The effects of fluorination and thio-alkylation on photophysical, electrochemical, photovoltaic properties of the polymers are comparatively investigated. The results indicate that PSDTfBT-DFDT and PSDTffBT-DFDT show better light-harvesting abilities, lower HOMO energy levels and better morphology than that of PSDTBT-DFDT. Bulk heterojunction (BHJ) organic photovoltaic devices based on the three polymers as donor materials were fabricated. The power conversion efficiencies (PCE) of 6.94% for PSDTfBT-DFDT and 7.50% for PSDTffBT-DFDT were attained, respectively, both of which are higher than that of PSDTBT-DFDT (4.48%) due to the increasement of the numbers of fluorine atom. This work demonstrates that both fluorine substitution and thio-alkylation are efficient to improve photovoltaic performances of polymers. KEYWORDS: polymer solar cells, conjugated polymers, fluorine substitution, thio-alkylation, benzothiadiazole

INTRODUCTION Polymer solar cells (PSCs) have made tremendous advances toward commercialization.1-3 It is widely accepted that photovoltaic performances of PSCs highly depend on the conjugated polymers in the BHJ active layers. Therefore, many

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efforts have been focused on molecular design of conjugated polymers and optimization of the devices, and rapid progresses have been achieved.4-7 Until now, the PCE values of single junction PSCs have achieved 10%-13%.8-10 As far as molecular design is concerned, construction of donor-acceptor (D-A) type molecular skeleton is considered to be the most powerful strategy to design a low band gap conjugated polymer due to their advantages in modulation of the absorption spectra, HOMO/LUMO energy levels, intermolecular interactions and molecular orientations. Among the reported D-A copolymers, difluorobenzothiadizole-based D-A copolymers, as one of the most successful molecular system, have obtained intensive investigation in preparation of high–efficiency PSCs.11-17 Moreover, The similar structures of BT have also attracted much attention and made great progress.18,19 The introduction of fluorine atom on conjugated polymer skeleton has been widely investigated owning to its unique advantages20-22. In 2015, Jo et al. reported three fluorinated D-A polymers based on 3,3’-difluoro-2,2’-bithiophene and benzothiadizole (BT), whose the substitution ratios of the F atom and PCE values can be adjusted by fluorination on BT unit. The results demonstrated that, for mono-fluorinated BT-based copolymer (3F), photovoltaic performance is much better than that of the polymers (2F and 4F) with non- and di-fluorination on BT; the inverted PSCs based on 3F:PC71BM blends gave a PCE value of 9.14%.23 Additionally, Zhang et al. reported four D-A type polymers based on BDT-T and TT, in which fluorine atoms were added onto the D-units or/and A-units of the backbone. The PCE of 8.6% was attained for PBT-3F, which higher than the PCE of the device based on PBT-0F (4.5%).24 These results indicated that the introduction of F atoms onto both D or A units of D-A type polymer has a synergistic effect on molecular energy level. Recently, Chen et al. reported three conjugated polyelectrolytes (CPEs) consisting diketopyrrolopyrrple (DPP) and benzene or fluorinated benzene. Among the three CPEs, The device applying CPEs with the highest number of fluorine substitutions achieved the best PCE of 11.51% based on PBDB-ITIC active layer. The results imply that, fluorinated substitution in conjugated backbone is a rational strategy to design highly efficient electrolytes interlayer materials.25 Among various strategies for designing conjugated polymers, side chains engineering have considerable investigated to fine-tune of the solubility of polymers, molecular stacking, and PSC device performances. For example, sulfur atom has some π-acceptor capability due to the formation of pπ(C)-dπ(S) orbital overlap, this makes the alkylthio side chains play an important role in tuning the photovoltaic


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performances of the conjugated polymers.26 In 2012, Lee et al. reported the photovoltaic performance of benzo[1,2-b:4,5-b’]dithiophene (BDT)-based polymer with dialkylthio substituted thiophene as the side chain, exhibiting a high Voc of 0.99 V and a PCE of 4.0%.27 In addition, the introduction of alkylthio modified π-spacers could construct a noncovalent interaction between neighboring segments, which is helpful for enhanced π-π stacking and charge mobility. In 2016, Yang et al. reported a novel D-A type copolymer (PBTI3T-S) using 3-(decylthio)thiophene as the π-spacers. Compared with its analogue PBTI3T, the hole mobility of the PBTI3T-S/PC71BM blend is 1.29×10-2 cm-2v-1s-1, much higher than that of the PBTI3T/PC71BM blend (1.15×10-3 cm-2v-1s-1). The PCE values are all above 7% for the copolymer PBTI3T-S as active layer thickness varies from 115 nm to 271 nm.28 Recently, Bo et al. reported two

alkylthio

substituted

benzothiadiazole-quaterthiophene

based conjugated

polymers. PSCs based on fluorinated polymer P1:PC71BM gave a PCE of 7.76%.29 Meanwhile, some previous works in our group had also demonstrated that the introduction of the side chains was effective to improve the photovoltaic performances of conjugated polymers.30,31 In order to investigate the synergistic effect of fluorination and thio-alkylation on photovoltaic performances of conjugated polymers, we synthesized three D-A polymers (PSDTBT-DFDT, PSDfTBT-DFDT, and PSDffTBT-DFDT) based on 3,3’-difluoro-2,2’-bithiophene (D unit) and 3-alkylthio substituted benzothiadiazole (BT, A unit) with different fluorine atom content (Figure 1). By introduce fluorine atom, the polymers obtained stronger light-harvesting ability, deeper molecule energy level and stronger intermolecular interaction. Bulk heterojunction solar cells based on the three D-A polymers as donor and PC71BM as acceptor were fabricated. The device based on PSDffTBT-DFDT showed a high PCE of 7.50% (a Voc of 0.80 V, a Jsc of 13.72 mA cm-2, a FF of 68.73%).

Figure 1. Molecular structures of the synthetic polymers.



RESULTS AND DISCUSSION Scheme S1 outlines the synthetic routes of three polymers PSDTBT-DFDT, PSDTfBT-DFDT and PSDTffBT-DFDT. The intermediate compound 2 and 4 were synthesized according to previously reported literatures.32,33 The detailed synthesis and characterization of the intermediates and polymers，and preparation method of PSC devices are described in the experimental section of supporting information. Three

polymers

may

be

dissolved

in

chloroform,

chlorobenzene

and

1,2-dichlorobenzene at elevated temperature. The average molecular weights of three polymers were characterized by a high temperature GPC and their results are collected in Table 1. The polymer PSDTBT-DFDT showed a high Mn of 60.7 kDa, but relative low Mn values of 17.0 and 14.9 kDa for polymers PSDTfBT-DFDT and PSDTffBT-DFDT, respectively. This difference is due to the introduction of F atom resulting in the decrease of the solubility of the polymers. Thermal Properties. The thermal stability of three polymers was characterized by thermogravimetric analysis (TGA) under the protection of nitrogen with a heating rate of 20°C/min. As depicted in Figure 2 and Table 1. The decomposition temperatures (Td, 5 % weight loss) of three polymers were 343°C, 339°C, 334°C for PSDTBT-DFDT, PSDTfBT-DFDT and PSDTffBT-DFDT, respectively. Apparently, the thermal stability of three polymers is appropriate for the application in PSCs. PSDTBT-DFDT PSDTfBT-DFDT PSDTffBT-DFDT

100 80

Weight (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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60 40 20 0

100

200 300 400 500 Temperature (ºC)

600

Figure 2. TGA plots of three polymers


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Table1. The Average Molecular Weights and Thermal Properties of Polymers Polymer

Yields (%)

Mn (kDa)

PDI

Td (°C)

PSDTBT-DFDT

68

60.7

2.15

343

PSDTfBT-DFDT

68

17.0

1.68

339

PSDTffBT-DFDT

72

14.9

1.87

334

Optical Properties. To study the optical properties of three polymers, the UV-Vis spectra in diluted CHCl3 solutions (Figure 3a) and on solid films (Figure 3b) were measured, and the relevant parameters are collected in Table 2. Similar absorption peaks were observed in solution and films. The absorption spectra of PSDTBT-DFDT, PSDTfBT-DFDT, and PSDTffBT-DFDT in diluted CHCl3 solution exhibited typical bimodal absorption from 300 nm to 800 nm. The short-wavelength absorption bands (300-500 nm), with the absorption peak (λb) at 454 nm for PSDTBT-DFDT, 451 nm for PSDTfBT-DFDT, and 445 nm for PSDTffBT-DFDT, should be attributed to π-π* transitions of the polymer backbone. The absorption bands in longer wavelength region (500-800 nm), with the absorption peak at 638 nm for PSDTBT-DFDT, 649 nm for PSDTfBT-DFDT, and 638 nm for PSDTffBT-DFDT, are the intramolecular charge transfer (ICT) between the donor and acceptor units. Meanwhile, both PSDTfBT-DFDT and PSDTffBT-DFDT possess obviously higher extinction coefficient (ε) of 4.5×104 (g mL-1)-1cm-1 and 5.0×104 (g mL-1)-1cm-1 than that of PSDTBT-DFDT (4.2×104 (g mL-1)-1cm-1), implying the two fluorinated polymers have better light-harvesting ability. Upon solidification, the peaks of three polymers in thin film state are slightly broader and the shoulder peak obviously appeared nearby 700 nm. According to deduce from the absorption edges of three polymer films, the optical gaps are 779 nm for PSDTBT-DFDT, 782 nm for PSDTfBT-DFDT, and 772 nm for PSDTffBT-DFDT, respectively, according to the equation: Eg,opt = 1240/λonset. It shows both the bandgaps of PSDTBT-DFDT and PSDTfBT-DFDT are almost equal (1.59 eV) while PSDTffBT-DFDT has a slightly wider bandgap of 1.61 eV. To further to investigate the temperature-dependent absorption of three polymers in σ-DCB solution, UV-Vis spectra at temperature range from 25 to 95°C were characterized. As show in Figure 3, similar variances with ICT peaks both blue-shifted

by

about

55

nm

were

observed


for

PSDTBT-DFDT

and


PSDTfBT-DFDT. At 95°C, the two polymers are almost non-aggregated, showing an absorption peak at 580 nm. However, some aggregates appear at low temperature in the solutions of PSDTBT-DFDT and PSDTfBT-DFDT, this variation attributes to donor-acceptor interactions in two polymers. In addition, the ICT peak at 590 nm is blue-shifted about 45 nm for polymer PSDTffBT-DTBT. The weaker shoulder peak at 590 nm still exists even at 95°C σ-DCB. This illustrates that the aggregation of PSDTffBT-DFDT is stronger than that of PSDTBT-DFDT and PSDTfBT-DFDT, which may be attributed to the effect of fluorination. 0.6

ε/105(g/mL)-1cm-1

0.5

(b)

PSDTBT-DFDT PSDTfBT-DFDT PSDTffBT-DFDT

Normalized Absorption

(a)

0.4 0.3 0.2 0.1 0.0 300

400

500

600

700

800

PSDTBT-DFDT PSDTfBT-DFDT PSDTffBT-DFDT

1.0 0.8 0.6 0.4 0.2 0.0 300

400

1.0 0.8

25ºC

0.6 0.4

(d)

25ºC 35ºC 45ºC 55ºC 65ºC 75ºC 85ºC 95ºC

PSDTBT-DFDT

600

700

PSDTfBT-DFDT

1.0


(c)

500

800

Wavelength(nm)

Wavelength(nm)


0.8

25℃

0.6 0.4

95ºC

0.2


95ºC

0.2

0.0

0.0

400

500

600

700

800

400

500

Wavelength(nm)

(e) Normalized Absorption

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1.0

600

700

800

Wavelength(nm)

PSDTffBT-DFDT

0.8

25ºC

0.6

0.4


95ºC

0.2

0.0 400

500

600

700

800

Wavelength(nm)

Figure 3. UV-Visabsorption spectra (a) in CHCl3 solution, (b) thin films on quartz, and the evolutions of absorption spectra of (c) PSDTBT-DFDT, (d) PSDTfBT-DFDT, and (e) PSDTffBT-DFDT in orthodichlorobenzene from 25 to 95°C.

Electrochemical Properties. In order to study the electrochemical properties of


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three polymers, the cyclic voltammetry (CV) was investigated. The HOMO energy levels were calculated on the onsets of the oxidation potentials with a calibrated Ag/AgCl (vs. ferrocene/ferrocenium) as the reference electrode. The oxidation onsets of PSDTBT-DFDT, PSDTfBT-DFDT and PSDTffBT-DFDT are observed at 0.80 V, 0.93 V and 1.02 V and the relevant HOMO energy levels are -5.22 eV, -5.35 eV and -5.44 eV, respectively (Figure 4a). The LUMO energy levels of PSDTBT-DFDT, PSDTfBT-DFDT and PSDTffBT-DFDT are -3.63, -3.76 and -3.83 eV calculated from the optical band gaps and HOMO energy levels of the polymers. It is clear that, both HOMO and LUMO energy level of PSDTffBT-DFDT and PSDTfBT-DFDT are lower than that of PSDTBT-DFDT, owning to the fluorine substitution along the polymer backbone, and the influence is more distinct with the increase of fluorine atom number.34 Interestingly, one can observe that the HOMO energy level of three polymers is decreased by ~0.1 eV with every increase one fluorine atoms onto the benzothiadiazole unit (from 0 to 1 and 2 fluorine atoms). Since Voc of BHJ PSCs is related to the difference between the HOMO of polymer donor and the LUMO of fullerene acceptor, the PSC devices based on the polymers PSDTfBT-DFDT and PSDTffBT-DFDT are expected to afford a higher Voc and better air stability than that of the polymer PSDTBT-DFDT. To make a clearer comparison, the frontier orbital energy level diagram of these polymers was given in Figure 4b. Both the LUMO offsets and HOMO offsets between the polymers and PC71BM are large enough driving force to ensure efficient exciton dissociation and electron transfer. 2.5

(a)

2.0

Current(×10-4A)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.5

PSDTBT-DFDT PSDTfBT-DFDT PSDTffBT-DFDT Ferrocene

1.0 0.5 0.0

-0.5 0.0

0.2

0.4 0.6 0.8 Potential（ V）

1.0

1.2

Figure 4. (a) Cyclic voltammogram plots (b) Energy level diagrams.



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Table 2. Photophysical and Electrochemical Properties of Three Polymers λsolution

λfilm

λonset

Eg,opt

HOMO

LUMO

(nm)

(nm)

(nm)

(eV)

(eV)

(eV)

PSDTBT-DFDT

454,638

454,640, 687

779

1.59

-5.22

-3.63

PSDTfBT-DFDT

451,649

449,642, 689

782

1.59

-5.35

-3.76

PSDTffBT-DFDT

445,638

458,634, 681

772

1.61

-5.44

-3.83

Polymer

Density Functional Theory (DFT) Calculations. To obtain insight into the influence of F atoms in polymers SDTxfBT (x = 0, 1, 2) segments into polymer backbone conformations, DFT calculations were investigated via Gaussian 09 simulations with the B3LYP/6-31G model, alkylthio side chains were replaced by methylthio groups to simply computational process. The calculations afford an effective method to assess the polymer planarity, even though they didn’t reflect the influence of alkythio chains and intermolecular interactions. The planarity of the polymer skeleton can be depicted by dihedral angles of σ–bonds between neighboring structural unit. The calculated values of dihedral angles for three polymers are marked by α1-α3, β1-β3, γ1-γ3, respectively, and the relevant data are collected in Table 3. According to the Figure 5 and Table 3, the following conclusions can be obtained. First, values of γ1, γ2 and γ3 are below 8°. In comparison to the alkyl substituted analogous polymers,23 thio-alkylation polymers exhibited better planar conformation, which would cause extended p-conjugation and enhance intermolecular interactions.29 As the number of substituent fluorine atoms in benzothiadiazole increases, dihedral angles of α1, α2 and α3 decreased gradually, which would improve for intermolecular accumulation and charge transfer. However, it can be observed that with respect to PSDTfBT-DFDT, β2 is three times larger than α2 due to unsymmetrical fluorine substitution in SDfBT segments. Moreover, the directions of mono-F atom between two neighboring structural units cannot order accurately. Hence, this unsymmetrical substitution and twisted polymer skeleton might disrupt the molecular conformation and influence intermolecular accumulation.34 On the other hand, as revealed in wave


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functions of the frontier molecular orbital, the electron density of HOMO in three polymers were distributed over the whole conjugated skeleton, while the electron density of LUMO were mostly located on the SDTxfBT (x = 0, 1, 2) unit. From these features, the photo-induced electrons in PSDTxfBT-DFDT can be more efficiently transported between the polymer and the PCBM.35

Figure 5. DFT optimized geometries (Left) and molecular orbital distributions (Right) of (a) PSDTBT-DFDT, (b) PSDTfBT-DFDT and (c) PSDTffBT-DFDT.

Table 3. Dihedral Angles of the Polymer Backbones Calculated by DFT Polymer

α

β

γ

PSDTBT-DFDT

5.3°

-3.5°

-5.6°

PSDTfBT-DFDT

1.0°

-3.6°

-5.6°

PSDTffBT-DFDT

0.8°

-1.7°

-7.9°

X-ray Diffraction (XRD) Analysis. To investigate the influence of the fluorine substitution and thio-alkylationon the crystallinity and the structural ordering of polymers, the small angle X-ray diffraction (SAXRD) analysis was used to investigate the solid state of three polymers. As depicted in Figure S12, all polymers show three peaks in the XRD curves. The peaks around small-angle region, reflex the distance of polymer backbones separated by the flexible side chains, are located at 2θ of 4.28°, 4.18°, and 4.18° for PSDTBT-DFDT to PSDTffBT-DFDT, corresponding to a distance of 20.59, 21.08, and 21.08Å respectively. It is found that three polymers have similar lamellar stacking distance. PSDTBT-DFDT and PSDTfBT-DFDT exhibit a



weak lamellar stacking around 8°，while the films of the PSDTffBT-DFDT show stronger sharp peak. It implies that PSDffBT-DFDT might be form more orderly stacking with a higher crystallinity, which will facilitate higher charge mobility. Photovoltaic Properties. PSC devices with a conventional structure of ITO/PEDOT:PSS/polymer:PC71BM/PFN-Br/Al were prepared to characterize the photovoltaic performance of three polymers under simulated AM 1.5G illumination (100 mW cm-2). The performance of devices was optimized by adjusting the weight ratio of polymer to PC71BM, additive, the thickness of active layer, and interfacial layers. Chlorobenzene/σ-dichlorobenzene (1:1 v:v) mixed solvent were chosen as the processing solvent, and the optimized weight ratios of polymer:PC71BM were to be 1:1 for all the polymers at a solid total concentration of 10 mg/mL. Figure 6a shows the best J-V curves of the devices, and the relevant photovoltaic parameters including the open-circuit voltage (Voc), the short-circuit current density (Jsc), the fill factor (FF), and power conversion efficiency (PCE), are collected in Table 4. The PCE values of PSDTBT-DFDT, PSDTfBT-DFDT, and PSDTffBT-DFDT-based solar cells are 2.48%, 6.06% and 6.81% without any additive. When 2.5% chloronaphthalene (CN) was added, the PCEs were improved to 4.48%, 6.94% and 7.50% for PSDTBT-DFDT, PSDTfBT-DFDT, and PSDTffBT-DFDT, respectively. Clearly, the devices based on polymers PSDTfBT-DFDT and PSDTffBT-DFDT deliver higher Jsc of 13.56 and 13.72 mA/cm2 than that (10.42 mA/cm2) of PSDTBT-DFDT, because the polymers with more fluorine atom endowed the better light-harvesting abilities due to higher extinction coefficient (ε) compared to the less fluorinated one. As shown in Table 4, polymers PSDTfBT-DFDT and PSDTffBT-DFDT with 3F and 4F as a donor unit showed higher Voc than the 2F counterpart (PSDTBT-DFDT), the Voc of devices based on PSDTBT-DFDT, PSDTfBT-DFDT, and PSDTffBT-DFDT are 0.69, 0.74, and 0.80 V, respectively. This demonstrated more fluorinated benzothiadiazole units are efficient for improvement of Voc due to the lower HOMO energy level. Furthermore, the external quantum efficiencies (EQEs) of the devices were given the similar curves. As shown in Figure 6(b). All the devices exhibit broad photoresponse range from 300-800 nm, which is attributed to the intrinsic absorption of both polymer and PC71BM. Among them, the device based on PSDTBT-DFDT shows a significant lower and narrower of EQE in the wavelength range 300-760 nm


Page 10 of 18

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compared to the other two fluorinated benzothiadiazole polymers, which is consistent with lowest Jsc (10.12 mA cm-2) of devices based on PSDTBT-DFDT. Simultaneously, the EQE values are slightly lower at 400-500 nm but higher at 540-710 nm for PSDTffBT-DFDT than that of the PSDTfBT-DFDT device. This combined result leads to an almost identical Jsc (13.31 vs 13.29 mA cm-2) for above two polymers. These values matched with the J-V measured Jsc of 10.42, 13.72, and 13.56 mA cm-2 for the PSDTBT-DFD, PSDTffBT-DFDT, and PSDTfBT-DFDT-based devices, respectively. Table 4. Photovoltaic Properties of PSCs Based on Three Polymer:PC71BM Blends

Voc

Polymer/PC71BM (1:1, w/w) a

PSDTBT-DFDT

b

PSDTBT-DFDT

PSDTfBT-DFDTa b

PSDTfBT-DFDT

PSDTffBT-DFDTa b

PSDTffBT-DFDT

Jscc

Jsc 2

2

FF

PCEmax

(V)

(mA/cm )

(mA/cm )

(%)

(%)

0.71

5.65

5.41

61.45

2.48

(0.70±0.01)

(5.22±0.43)

(60.33±1.12)

(1.97±0.51)

0.69

10.42

62.10

4.48

(0.68±0.01)

(10.00±0.42)

(61.05±1.05)

(3.95±0.53)

0.72

12.35

68.33

6.06

(0.71±0.01)

(12.01±0.34)

(66.23±2.10)

(5.69±0.37)

0.74

13.56

69.17

6.94

(0.73±0.01)

(13.24±0.32)

(67.12±2.05)

(6.55±0.39)

0.78

13.05

67.35

6.81

(0.77±0.01)

(12.52±0.53)

(65.24±2.11)

(6.38±0.43)

0.80

13.72

68.73

7.50

(0.79±0.01)

(13.21±0.51)

(66.68±2.05)

(7.08±0.42)

10.12

12.23

13.29

12.56

13.31

a

Without any additive. bWith 2.5% CN. cIntegrated from EQE spectra. The average values and the

standard deviation shown in brackets based on 10 devices. 4

70

(b)

60

0

50

PSDTBT-DFDT

-4

EQE(%)

(a)

2 Current Density (mA/cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


PSDTfBT-DFDT PSDTffBT-DFDT

-8

40 30 PSDTBT-DFDT

20

PSDTfBT-DFDT

-12 -16 -0.2

PSDTffBT-DFDT

10 0.0

0.2

0.4

0.6

0.8

0 300

400

500

600

700

800

Wavelength(nm)

Voltage (V)

Figure 6. (a) The J-V curves based on polymer and (b) the corresponding EQE spectra under optimal conditions.

SCLC Mobilities. To further understand the differences in Jsc and FF values and



Page 12 of 18

determine the impact of the fluorine substitutions on the transport properties of charge carriers, the µh and µe of the polymer:PC71BM blend were characterized via the space charge

limited

current

method

in

conventional

device

structures

of

ITO/PEDOT:PSS/polymer:PC71BM/MoO3/Al for hole mobility and ITO/ZnO/ polymer:PC71BM/PFN/Al for electron mobility. The hole and electron mobility data of three polymers are collected in Table 5. The averaged µh of PSDTBT-DFDT, PSDTfBT-DFDT and PSDTffBT-DFDT blend systems were determined to be 5.61×10-4, 6.59×10-4, 8.21×10-4 cm-2v-1s-1, respectively, and the relevant µe are 2.21×10-4, 4.48×10-4, 5.75×10-4 cm-2v-1s-1, respectively. Furthermore, the ratio of hole to electron mobility (µh/µe) for PSDTfBT-DFDT and PSDTffBT-DFDT are 1.47 and 1.43 respectively, which are relatively lower than that of PSDTBT-DFDT (2.54). The higher charge mobility and well-balanced µh/µe values in the PSDTfBT-DFDT and PSDTffBT-DFDT blends are expected to be beneficial to reduce electron-hole recombination and obtain higher Jsc, FF and PCE. Film Morphologies. Atomic force microscopy (AFM) was used to investigate the effect of the surface morphology of the active layer on photovoltaic performances of these polymers. The blend films were prepared with the configuration of the polymer/PCBM for the best performance device (PC71BM as acceptor and 2.5% CN as additive). The height images and phase images of the polymers are shown in Figure 7. It can be observed that the blend film of PSDTBT-DFDT/PC71BM shows relatively rough surface morphology and root-mean-square (RMS) values of 2.16 nm, and the PSDTffBT-DFDT/PC71BM blend film exhibits smoother surfaces with a RMS value of 1.82 nm owning to the suitable phase separation with fullerene acceptor and a nanoscale interpenetrating network structure. Continuous and homogeneous nanoscale phase separation morphology implies that the blend film based on PSDTffBT-DFDT provides an appropriate combination of polymer miscibility with PC71BM, which promotes exciton dissociation and charge transport and provides a higher Jsc compared to the other two polymers. Furthermore, transmission electron microscope (TEM) measurement was performed to further evaluate the morphology of the polymer:PC71BM blends. The blend films were prepared to the best PSC devices condition. As shown in Figure 7, the TEM image of PSDTBT-DFDT and PC71BM blend exhibits relatively larger size phase separations, a smooth and homogeneous morphology can be observed for PSDTfBT-DFDT and PSDTffBT-DFDT-based blend films. This result is consistent with the trend of AFM measurements.


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Figure 7. AFM height images (top), phase images (middle) and TEM images (bottom) of polymers/PC71BM blend films fabricated

under optimized

conditions. (a), (d), (g):

PSDTBT-DFDT; (b), (e), (h): PSDTfBT-DFDT; (c), (f), (i): PSDTffBT-DFDT.

Table 5. The Mobility of the Polymer/PC71BM(1:1, w/w) Measured by Space Charge Limited Current (SCLC) Method Polymer

CN(%)

µh

µe

(cm-2V-1s-1)

(cm-2 V-1s-1)

µh/µe

PSDTBT-DFDT

0

1.18×10-4

0.8×10-4

1.48

PSDTBT-DFDT

2.5

5.61×10-4

2.21×10-4

2.54

PSDTfBT-DFDT

0

6.12×10-4

3.37×10-4

1.82

PSDTfBT-DFDT

2.5

6.59×10-4

4.48×10-4

1.47

PSDTffBT-DFDT

0

7.65×10-4

5.07×10-4

1.51

PSDTffBT-DFDT

2.5

8.21×10-4

5.75×10-4

1.43



Page 14 of 18

CONCLUSION In

summary,

three

conjugated

polymers,

named

PSDTBT-DFDT,

PSDTfBT-DFDT and PSDTffBT-DFDT, were designed, synthesized and applied as the donor materials in PSCs. These polymers possess reasonable solubility and broad absorption spectra. With the number of fluorine substitution on BT varying from 0 (PSDTBT-DFDT) to 1 (PSDTfBT-DFDT) and to 2 (PSDTffBT-DFDT), the polymers exhibited better light-harvesting ability and lower HOMO/LUMO energy levels. Therefore, the Voc of the related PSCs based on these polymers increased by approximately 0.05 V as increasing each of one F atom. At the same time, the Jsc and was increased 30% by varying the numbers of total fluorine atom from 2 to 4. Ultimately, the optimal PSCs based on PSDTBT-DFDT, PSDTfBT-DFDT and PSDTffBT-DFDT obtained the best PCE of 4.48%, 6.94% and 7.50%, respectively. The increased PCE along with the increase of number of the fluorine atom mainly attributed to better light-harvesting ability, lower HOMO energy level and balanced charge carrier mobility as well as optimal morphology of the polymer/PC71BM blend films. This study has demonstrated the fluorination and thio-alkylation in polymer skeleton or side group have significant effect on the photovoltaic performances of the polymers. Supporting Information Additional experimental details and figures, including 1H NMR, 13C NMR, MS-TOF spectra and XRD spectra are shown in Supporting Information, it is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.xxxxx. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (S. T). *E-mail: [email protected] (B.Z.). *E-mail:[email protected] (X. G)


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Author Contributions R. Peng and H. Guo contributed equally to this work.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China(Grants Nos. 21474081，51573153，51503135，51773142), and supported by Hunan 2011 Collaborative Innovation Center of Chemical Engineering & Technology with Environmental Benignity and Effective Resource Utilization.

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Synergistic Effect of Fluorine Substitution and Thio-Alkylation on

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