The synergistic effects of fluorination and alkylthiolation on the

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Cite This: ACS Appl. Energy Mater. 2018, 1, 4686−4694

Synergistic Effects of Fluorination and Alkylthiolation on the Photovoltaic Performance of the Poly(benzodithiophenebenzothiadiazole) Copolymers Kangkang Weng,†,‡ Xiaonan Xue,†,‡ Feng Qi,† Yu Zhang,† Lijun Huo,*,† Jianqi Zhang,§ Donghui Wei,⊥ Meixiu Wan,† and Yanming Sun† †

School of Chemistry, Beihang University, Beijing 100191, P. R. China National Center for Nanoscience and Technology, Beijing 100190, P. R. China ⊥ College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou 450001, P. R. China

ACS Appl. Energy Mater. 2018.1:4686-4694. Downloaded from pubs.acs.org by UNIV OF SUNDERLAND on 10/08/18. For personal use only.

§

S Supporting Information *

ABSTRACT: Atom functional modifications (e.g., fluorination and alkylthiolation) in conjugated building blocks, have attracted a lot of attention due to their promising potential in finely tuning optical-electronic properties. To have a better understanding of the influence of polymeric fluorinated and alkylthiolated effects on photovoltaic performance, here, four low band gap benzothiophene-benzothiadiazole (BDT-BT) copolymers, PBBT, PB-FBT, PBS-BT, and PBS-FBT, featuring a step-by-step approach to using fluorination and alkylthiolation were designed and synthesized. Their optical and electronic properties, crystallinity, and the corresponding photovoltaic performance in bulk heterojunctions (BHJ) with fullerene as acceptor were systematically investigated. First, it is found that the fluorination in BT moieties offers several advantages including lowering the highest occupied molecular orbital (HOMO) levels, strengthening π−π stacking, and improving thermal stability. Second, alkylthiolation in the BDT moiety shows weak effects in modulating the polymeric HOMO levels and the crystallinity. Finally, it is interesting to note that the alkylthiolation added to fluoro-substituted copolymer synergistically strengthens the ordered π−π stacking and improves its crystallinity. Subsequently, superior photovoltaic performance was rendered in PBS-FBT based devices due to reasonable phase separation and enhanced fibrous morphology, which was characterized by utilizing a combination of atomic force microscopy (AFM), transmission electron microscope (TEM), grazing-incidence wide-angle X-ray scattering (GIWAXS), and other microscopy measurements. This work provides important insights into designing high-performance photovoltaic polymers by fluorinated and alkylthiolated modifications. KEYWORDS: synergistic effects, fluorination, alkylthiolation, polymer solar cell, molecular design

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INTRODUCTION

by introducing specific atoms such as O, S, Se, F, Si, Ge, and Cl into conjugated polymeric main backbones or side chains.29−41 So far, numerous research works regarding the fluorination of polymers have been carried out.42−44 For example, You et al. first reported D−A copolymers named PBnBT-DTffBT, and it is found that inserting fluorine substituents into polymers is identified as an effective method to lower the highest occupied molecular orbital (HOMO) levels, suppress charge recombination, and subsequently noticeably improve the performance of polymer solar cells.39 Meanwhile, the alkylthiolation of polymers also presents a better performance than alkylsubstituted polymers.45−47 For instance, Huo et al. first introduced 3-alkylthio as the side groups to replace 3-alkyl in

Organic photovoltaics (OPVs) are considered to be a very promising next-generation solar cell technology as they have a large number of distinguished advantages such as light weight, flexibility, and solution processability.1−9 In the past decades, extensive efforts have been done in developing new materials including conjugated polymers, small molecules, fullerene derivatives, and nonfullerenes.10−18 In terms of conjugated polymers, donor−acceptor (D−A) copolymers are identified as potential donor materials to be used in OPVs due to the simply tunable polymeric optical−electronic properties such as energy levels, bandgaps, and light absorption range.19 To date, the single junction solar cells with over 12% power conversation efficiency (PCE) based on D−A conjugated/ nonfullerenes have been reported in some groups.20−28 Among the various molecular design strategy of D−A polymeric donor, atom functional modifications have attracted a lot of attention © 2018 American Chemical Society

Received: May 24, 2018 Accepted: August 6, 2018 Published: August 6, 2018 4686

DOI: 10.1021/acsaem.8b00819 ACS Appl. Energy Mater. 2018, 1, 4686−4694

Article

ACS Applied Energy Materials Scheme 1. Synthetic Routes of PB-BT, PB-FBT, PBS-BT, and PBS-FBT

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RESULT AND DISCUSSION Synthesis. The synthetic routes of the copolymers, PB-BT, PB-FBT, PBS-BT, and PBS-FBT, are shown in Scheme 1. The intermediates of (4,8-bis(4-(2-butyloctyl)phenyl)benzo[1,2b:4,5-b′]dithiophene-2,6-diyl)bis(trimethylstannane) (BDT) and (4,8-bis(4-((2-butyloctyl)thio)phenyl)benzo[1,2-b:4,5b′]dithiophene-2,6-diyl)bis(trimethylstannane) (BDTS) were prepared according to our previous work.48 For the acceptor unit, 4,7-bis(4-(2-ethylhexyl)-2-thienyl)-2,1,3-benzothiadiazole (BT) and 5,6-difluoro-4,7-bis(4-(2-ethylhexyl)-2-thienyl)2,1,3-benzothiadiazole (FBT) were purchased from Aldrich Co. and used without further purification. The four copolymers, PB-BT, PB-FBT, PBS-BT, and PBS-FBT, were synthesized through typical Stille-coupling polymerization in toluene, with Pd(PPh3)4 as the catalyst under refluxing reaction for 12 h, reactively. The obtained four copolymers show good solubility in toluene, chloroform, and odichlorobenzene. The molecular weights of these polymers were measured by gel permeation chromatography (GPC) in a chloroform eluent with a polystyrene standard; as a result, the number-average molecular weights (Mns) of PB-BT, PB-FBT, PBS-BT, and PBS-FBT are 33.1, 32.9, 29.0, 24.2 kDa, and the corresponding polymer dispersivity indexes (PDIs) are 2.1, 2.8, 3.9, and 2.5, respectively. Thermogravimetric analysis (TGA) was used to investigate the thermal stabilities of the four copolymers. The TGA curves of PB-BT, PB-FBT, PBS-BT, and PBS-FBT were obtained at a heating rate of 10 °C min−1 as shown in Figure S1. The 5% weight-loss temperatures (Tds) of four polymers were 448, 451, 393, and 398 °C, respectively. It was found that PB-FBT with fluorine substituents owns the highest thermal stability among these copolymers; however, the copolymers of PBS-BT and PBS-FBT obviously show lower weight-loss temperatures compared to PB-BT due to the alkylthiolation. Considering that the device fabrication temperature is far lower than 390 °C under nitrogen, all these copolymers have adequate thermal stability beyond processing conditions. Optical and Electrochemical Properties. The UV−vis absorptions of the four polymers in dilute chloroform solution (10−5 M) and in solid films are presented in Figure 1. In chloroform solution, all copolymers exhibit two notably absorption peaks. One locates at short wavelength band

the polyalkylthiophene (PTh), significantly boosting the PCE, and it worked very well in different families of conjugated systems such as benzo[1,2-b:4,5-b′]dithiohene (BDT) based polymers.38 In spite of this, little research in this field has investigated the detailed effects of the fluorination and alkylthiolation in the electron-deficient and electron-rich moieties, respectively. On the basis of this consideration, we adopted a D−A copolymer named PB-BT and three analogues, PB-FBT with fluorinated substitutents, PBS-BT with alkylthiolated substitutents, and PBS-FBT with both fluorinated and alkylthiolated substitutents, to systematically investigate the atom functional modification effects on their optical and electronic properties, crystallinity, and the corresponding photovoltaic performance. The molecular structures of the four polymers are shown in Scheme 1. The properties of these four polymers were studied in terms of photophysics, electrochemistry, theoretical simulation, thermal properties, photovoltaics, and morphology. As a result, we found that introducing fluorine atoms into PBBT offers several advantages including deeper energy levels, strengthened π−π stacking, and better thermal stability. But the blend film of fluorine-substituted polymer and PC71BM suffers from severe phase separation. In addition, we also found that instead of using alkyl-substitutents, incorporating alkylthio-substituents to polymers can also slightly lower the HOMO level of conjugated polymers. What is more, alkythiosubstituents can lead a reasonable phase separation of the blend film since the fibrous morphology is emerged in the blend film. As a result, higher hole mobility and better photovoltaic performance can be achieved. Interestingly, when the fluorination and alkylthiolation of polymer were carried out together to get PBS-FBT, the target polymer showed the synergistic effects on several aspects such as lowering the HOMO level, strengthening π−π ordered stacking, enhancing hole mobility, and forming better fibrous morphology in the blend film, etc. Consequently, the blend of PBS-FBT exhibits the best photovoltaic performance among these four copolymers. This work provides a comprehensive understanding of the effects of the fluorination and alkylthiolation modifications on polymers and shows the great potential of the approach to designing polymers for high-performance OPVs. 4687

DOI: 10.1021/acsaem.8b00819 ACS Appl. Energy Mater. 2018, 1, 4686−4694

Article

ACS Applied Energy Materials

Figure 1. UV−vis absorption spectra of PB-BT, PB-FBT, PBS-BT, and PBS-FBT (a) in chloroform solution and (b) in thin films.

Figure 2. (a) Cyclic voltammogram curves of PB-BT, PB-FBT, PBSBT, and PBS-FBT (from bottom to top); (b) schematic energy diagram of active layer component.

(300−450 nm), which is attributed to π−π* transitions,49 and the other one appears at longer wavelength band (500−650 nm) due to intramolecular charge transfer. In addition, vibronic shoulder peaks at long wavelength are observed in two solutions of fluorine-substituted copolymers of PB-FBT and PBS-FBT, indicating strong intermolecular forces between polymer chains. Moreover, the temperature-dependent UV− vis absorption spectra of PBS-FBT was measured to investigate the aggregation of the polymers in chlorobenzene solution. As shown in Figure S2, the UV−vis absorption spectra of PBS-FBT solution shows a marked blue-shift when the temperature rises, indicating that PBS-FBT is well dissolved and disaggregated at elevated temperature in chlorobenzene solution. This strong temperature-dependent aggregation behavior for PBS-FBT will lead to more potential in forming better morphology and device performance. In the solid state, the absorption spectra of both PB-BT and PBS-BT exhibit a large bathochromic shift of ca. 50 nm from solution to film. Compared with the absorption spectra of PB-BT and PBS-BT films, that of PB-FBT shows a small bathochromic shift of 14 nm from 566 nm in solution to 581 nm in film and there is a strengthened vibronic shoulder appearing at 628 nm, suggesting stronger aggregation and more effective π−π stacking in the solid state.50,51 Meanwhile, although the absorption spectrum of PBS-FBT film is similar with that of PB-FBT film, the former show more bathochromic shift of ca. 18 nm than the latter mostly due to the introduction of the electron-rich property of the alkythio-substituent. Moreover, it is interesting to note that the vibronic shoulder peak of the PBS-FBT film is enhanced compared to that of PB-FBT, which is primarily assumed to be due to the fact that the S−F interaction causes the stronger aggregation and π−π stacking of polymer chains.52,53 And the maximum absorption coefficient of PB-BT, PB-FBT, PBS-BT, and PBS-FBT films are 3.64 × 104, 4.8 × 104, 3.96 × 104, and 5.12 × 104 cm−1, respectively, indicating that inserting fluorine substituents and alkylthiolated substitutents could enhance light-harvesting capability of polymers. The energy levels were estimated by an electrochemical cyclic voltammetry (CV) method. The CV curves of four copolymers in 0.1 M tetrabutylammonium hexafluorophosphate (Bu4NPF6) acetonitrile solution are shown in Figure 2a. The HOMO and LUMO levels were calculated according to their onset oxidation and reduction potentials, respectively. As shown in Figure 2b and summarized in Table 1, the HOMO levels of PB-BT, PB-FBT, PBS-BT, and PBSFBT are −5.52, −5.58, −5.55, and −5.62 eV, respectively. Both the fluorination of PB-FBT and the alkylthiolation of PBS-BT lead to lower HOMO levels than PB-BT, and the former has the stronger effect on lowing the HOMO level. Meanwhile, the HOMO level of PBS-FBT shows the lowest

Table 1. Optical Properties and Molecular Energy Levels of PB(S)-(F)BT HOMO/LUMO (eV)

polymer

λonset film (nm)

PB-BT

709

PBFBT PBSBT PBSFBT

690 714 705

DFT

CV

Egcalc (eV)

Egec (eV) Egopt (eV)

−2.73/4.70 −2.81/4.77 −2.76/4.73 −2.85/4.80

−3.53/5.52 −3.50/5.58 −3.51/5.55 −3.55/5.62

1.97

1.99

1.75

1.95

2.08

1.80

1.98

2.04

1.73

1.95

2.07

1.76

values, which implies that the final accumulated effect on lowing the HOMO level is obtained by the fluorination and alkylthiolation together and agrees well with the following density functional theory (DFT) simulated results (Figure S3− S6). Theoretical Simulation. DFT calculations were performed to investigate the alkythio- and fluoro-substituted effects on the geometrical configuration and electronic structure of these four polymers, respectively. To simply the calculation, all side chains were replaced by methyl groups. DFT calculations are based on three repeat units. As shown in Figure 3, the four copolymers present comparable geometrical structures. The dihedral angles between conjugated side chains and the BDT backbone are almost identical values of ca. 57− 58° for all the polymers. On other hand, there is a larger

Figure 3. Optimized molecular geometries (three repeat units) of PBBT, PB-FBT, PBS-BT, and PBS-FBT (side view) 4688

DOI: 10.1021/acsaem.8b00819 ACS Appl. Energy Mater. 2018, 1, 4686−4694

Article

ACS Applied Energy Materials

Figure 4. J−V characteristics and the IPCE curves of PB-BT:PC71BM (a, e), PB-FBT:PC71BM (b, f), PBS-BT:PC71BM (c, g), and PBSFBT:PC71BM (d, h) solar cells with different additive contents.

Table 2. Device Parameters of PB-BT:PC71BM, PB-FBT:PC71BM, PBS-BT:PC71BM, and PBS-FBT:PC71BM Solar Cells with Different Solvent Additives under the Illumination of AM 1.5 G, 100 mW/cm2 active layer PB-BT:PC71BM

PB-FBT:PC71BM

PBS-BT:PC71BM

PBS-FBT:PC71BM

additive [%] 0 0.5% 1% 0 0.5% 1% 0 0.5% 1% 0 0.5% 1%

Voc [V] 0.874 0.876 0.816 0.954 0.895 0.840 0.920 0.885 0.857 0.952 0.942 0.927

± ± ± ± ± ± ± ± ± ± ± ±

0.012 0.003 0.004 0.005 0.012 0.027 0.014 0.005 0.003 0.002 0.003 0.004

Jsc [mA cm−2] 6.6 8.1 7.5 8.2 6.5 4.7 9.9 11.3 11.1 10.8 13.1 11.4

± ± ± ± ± ± ± ± ± ± ± ±

0.6(6.88) 0.1(7.83) 0.1(7.2) 0.20(8.07) 0.08(6.29) 0.04 (4.70) 0.19(9.8) 0.08(11.19) 0.2(11.00) 0.46(10.98) 0.13(12.60) 0.2(11.35)

FF [%] 0.33 0.50 0.47 0.50 0.39 0.38 0.43 0.58 0.58 0.59 0.65 0.63

± ± ± ± ± ± ± ± ± ± ± ±

0.02 0.00 0 0.01 0.02 0.05 0.01 0.02 0.01 0.00 0.00 0.01

PCEa [%]

PCEmax [%]

± ± ± ± ± ± ± ± ± ± ± ±

2.3 3.5 2.9 4.0 2.4 1.8 4.0 6.0 5.6 6.3 8.1 6.8

2.0 3.5 2.9 3.9 2.3 1.5 3.9 5.8 5.5 6.0 8.1 6.7

0.3 0 0.1 0.1 0.1 0.2 0.1 0.1 0.1 0.3 0.1 0.1

The values in parentheses are average PCEs from five devices.

a

Al) was fabricated. The ZrAcac was adopted as the cathode interlayer since the energy level of ZrAcac matched well with Al, which was beneficial to cathode electrode to collect electron efficiently. Besides, the weight ratios of donor vs PC71BM and the additive content of 1,8-diiodooctane (DIO) were explored. The current density−voltage (J−V) curves for the devices with different DIO concentrations and the corresponding incident photon conversion efficiency (IPCE) spectra are presented in Figure 4. The device parameters are summarized in Table 2. All four polymer photovoltaic devices exhibit optimum IPCE at the D−A ratio of 1:1 (Figure S8, Table S1). Without DIO additive, all these three polymers of PB-BT, PB-FBT, and PBS-FBT showed the low PCEs. When adding a small optimized amount of DIO (0.5%, v/v), it is a pronounced enhancement in PCEs for all these polymeric devices. As for PB-FBT based devices, all the key parameters decreased upon addition of DIO. Under the optimized conditions, PB-FBT exhibited a PCE of 3.9%, with a Voc of 0.954 V, a Jsc of 8.2 mA cm−2, and an FF of 50%. Meanwhile, an inferior PCE of 3.5% with a Voc of 0.876 V was obtained for PB-BT. The higher Voc of PB-FBT than that of PB-BT is agrees well with the CV results. As for PBS-BT, the performance of which improved to obtain a PCE of 5.8%, a Voc of 0.885 V, a Jsc of 11.3 mA cm−2, and an FF of 58%. The comparable Voc values of PB-BT and PBS-BT show that the alkylthiolation in PBS-BT has little effect in changing Voc.

dihedral angle of 6° between the thienyl-bridge and electrondeficient BT unit for PB-BT and PBS-BT; however, a smaller dihedral angle of 2° is observed between them for PB-FBT and PBS-FBT, which is mainly attributed to the better planarity created by the fluorination. Meanwhile, a similar alkoxyphenylsubstituted polymer of PBDTP-DTBTff was compared with PBS-FBT.54 Although there is an identical dihedral angle ca. 58° for alkoxyphenyl and alkythiophenyl side chains, the latter produces a greater dihedral angle than the former by ca. 5° between the thienyl-bridge and BT unit, which may be caused by the larger atomic size of sulfur than oxygen atom. Therefore, it is concluded that the fluorination in polymeric main backbone can obviously increase molecular rigidity of the polymer. The primarily conclusion is also coincident with aforementioned polymeric optical properties. As results, among these four copolymers, PBS-FBT has the most planar molecular conformation which could be attributed to the synergistic effect of the fluorination and alkylthiolation in the copolymers and the former is the main factor in improving molecular planarity. What is more, all polymers show continuous positive molecular electrostatic potentials, which contribute to the migration of charge carriers (Figure S7, Supporting Information) Photovoltaic Properties. In order to investigate the photovoltaic performance of these four copolymers, a conventional structure (ITO/PEDOT:PSS/donor:PC71BM/ZrAcac/ 4689

DOI: 10.1021/acsaem.8b00819 ACS Appl. Energy Mater. 2018, 1, 4686−4694

Article

ACS Applied Energy Materials

modifications, respectively. Furthermore, when the hole mobility of PBS-FBT was measured, it showed the highest value of 2.33 × 10−4 cm2 V−1 s−1 among these four blends, which proves the effective synergistic effects in the improvement of hole mobility. Moreover, the balanced electron and hole mobility with a ratio of 1.17 assisted to explain the higher FF achieved in PBS-FBT:PC71BM devices (Table S2). Morphology. Atom force microscopy (AFM) and transmission electron microscopy (TEM) were used to gain insight into the nanoscale morphology of active layers. Without additive DIO, the root-mean-square roughness (Rq) of the blend surfaces of PB-BT, PB-FBT, PBS-BT, and PBS-FBT are 0.994, 0.703, 0.502, and 2.61 nm, respectively. Under optimum conditions, as shown in Figure 6, all AFM topography blend films became more coarse because the addition of DIO promoted blend aggregation. The corresponding Rq values of 1.29, 2.18, 0.636, and 3.83 nm were obtained for PB-BT, PBFBT, PBS-BT, and PBS-FBT, respectively. It is found that stronger aggregation exists in PB-FBT and PBS-FBT blend films than in those of PBS-BT and PB-BT, which suggests that the fluorination of PB-FBT and PBS-FBT can increase the aggregation of polymers and lead to higher values of Rq. Among these four blends, the PBS-FBT blend showed the highest Rq value mostly due to the fluorination and alkylthiolation together further strengthening the aggregation. Moreover, as shown in the TEM images of Figure 6, in comparison with the amorphous morphology of PB-BT, both PB-FBT and PBS-BT show more nanoscale aggregated morphologies. Compared to the serious phase separation of PB-FBT caused by the fluorination, that of PBS-BT containing alkythio-substituent exhibits a more fibrillar type topology. For PBS-FBT, it is noted that the TEM of PBS-FBT seems to have more strengthened fibrillar features, which implies that the introduction of alkylthiolation can effectively alleviate the severe phase separation caused by the fluorination modifications in the polymer, leading to a preferable phase separation. What is more, the mild developed fibrillar structures of the polymer (PBS-BT, PBS-FBT) blend films are beneficial to increase charge transport, which also agree well with the test results of charge carrier transport.51 To further clarify the influence of fluorination and alkylthiolation on crystallinity and orientation of polymer neat films and blend films, the grazing incidence wide-angle Xray scattering (GIWAXS) was investigated (Figure S9) . In the

Notably, PBS-FBT shows the best performance among these four polymers, with a champion PCE of 8.1%, a Voc of 0.942 V, a Jsc of 13.1 mA cm−2, and an FF of 65%. The high Voc of 0.94−0.95 V for both PB-FBT and PBS-FBT further prove that the fluorination in polymers can effective improve Voc. In addition, as shown in Figure 4, these polymer devices have broad IPCE spectra from 300 to 750 nm. In comparison with the lower IPCE (