A Medium Bandgap D–A Copolymer Based on 4-Alkyl-3,5

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

A Medium Bandgap D−A Copolymer Based on 4‑Alkyl-3,5difluorophenyl Substituted Quinoxaline Unit for High Performance Solar Cells Tao Wang,†,‡ Tsz-Ki Lau,§ Xinhui Lu,§ Jun Yuan,† Liuliu Feng,† Lihui Jiang,† Wei Deng,‡ Hongjian Peng,† Yongfang Li,∥ and Yingping Zou*,† †

College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China State Key Laboratory of Chemo/Biosensing and Chemometrics, Hunan University, Changsha 410082, China § Department of Physics, Chinese University of Hong Kong, Shatin, New Territories, Hong Kong ∥ Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China ‡

S Supporting Information *

ABSTRACT: Development of high-performance donor−acceptor (D−A) copolymers has been indicated as a promising strategy to improve the power conversion efficiencies (PCEs) of organic solar cells (OSCs). In this work, a new medium bandgap conjugated D−A copolymer, HFAQx-T, based on 4,8-bis(5-(2-ethylhexyl)thiophen-2yl)benzo[1,2-b:4,5-b′]dithiophene (BDT-T) as donor unit, 4-alkyl3,5-difluorophenyl substituted quinoxaline (HFAQx) as the acceptor unit, and thiophene as the spacer, was designed and synthesized. HFAQx-T is a well-compatible donor polymer; OSCs based on HFAQx-T exhibit excellent performance in both fullerene and fullerene-free based devices. The optimized conventional single junction bulk heterojunction (BHJ) OSCs of HFAQx-T:PC71BM showed a PCE of 9.2%, with an open circut voltage (Voc) of 0.9 V, a short circuit current (Jsc) of 14.0 mA cm−2, and a fill factor (FF) of 0.74. Also, when blended with 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)indanone)-5,5,11,11-tetrakis(4-hexylphenyl)dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]-dithiophene (ITIC), the HFAQx-T-based device exhibited a PCE of 9.6%. HFAQx-T is among a few D−A copolymers that can deliver >9% efficiency in both fullerene and fullerene-free solar cells. This work demonstrates that the 4-alkyl-3,5-difluorophenyl substituted quinoxaline (Qx) is a promising electron-accepting building block in constructing ideal D−A copolymers for OSCs.

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INTRODUCTION

stability, which is a bottleneck to the development of fullerene solar cells.12,13 Thus, to address these problems and to further promote the development of OSCs, considerable efforts have been devoted to the development of fullerene-free acceptors; recently, fullerenefree acceptors have made big progress, especially small-molecular acceptors (such as ITIC,14 IEIC,15 IDIC,16 etc.) are successfully designed and synthesized toward fullerene free solar cells because of strong and broad light absoption, easily tunable energy levels, and better morphology stability. The most representative fullerene-free acceptors are low bandgap n-OS, for instance, ITIC. This fused ring-based “push−pull” structured acceptor ITIC exhibits good miscibility with polymer donors, strong absorption in the wavelength range of 600−800 nm, and suitable energy levels.14 To date, some highly efficient fullenrene-

Solution-processed organic solar cells (OSCs) with bulk heterojunction (BHJ) architecture, based on the blend of a conjugated polymer (p-type organic semiconductor, p-OS) donor and a n-type organic semiconductor (n-OS) acceptor, have been intensively investigated in recent years due to their advantages of light weight, mechanical flexibility, and roll-to-roll solution processability.1−4 In the past few years, rapid and significant progress has been made, and the power conversion efficiencies (PCEs) over 10% have been realized.5−8 The application of fullerenes and their derivatives (e.g., [6,6]phenyl-C61/C71-butyric acid methyl ester (PC61BM/PC71BM)) as electron acceptors has greatly contributed to the advancement of the OSC field owing to their high electron mobility even in blended films, isotropic electron transporting properties, and formation of appropriate phase separation with donor materials.9−11 However, fullerene materials have some inherent limitations, such as weak absorption in visible and near-infrared (NIR) region, high production costs, and poor morphology © XXXX American Chemical Society

Received: February 12, 2018 Revised: March 26, 2018

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DOI: 10.1021/acs.macromol.8b00326 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules free devices based on ITIC and derivatives were fabricated; the performances of 10−13% with single-junction structure have been realized by using medium bandgap conjugated polymer materials.17−22 Further improvement of PCE is still required for commercialization. In order to obtain high efficiency OSCs, it is necessary not only to design high-performance acceptors but also to develop matched polymer donors. In principle, the donor material should possess the following features: (i) the absorption spectra of donor and non-fullerene acceptor should be complementary that results in high incident photon-to-electron conversion efficiency (IPCE) values in the 400−600 nm region and the short-circuit current density (Jsc); (ii) the energy levels of the donor should be appropriate for keeping efficient charge separation with low energy loss and high open-circuit voltage (Voc); (iii) the selection of donor should have morphology compatibility with the acceptor to form nanoscale phase separation, bicontinuous interpenetrating networks, and high fill factor (FF).23−25A conjugated polymer can be divided into two constituting parts: (a) π-conjugated backbones and (b) peripheral flexible solubilizing side chains.26 The development of novel π-conjugated backbones has always been the central issue in this field due to π-conjugated backbones which determine the optoelectronic properties of the polymers. It is also wellestablished that side-chain engineering has played crucial role in key properties of conjugated polymers, such as molecular weight, absorption, emission, inter- and intramolecular interactions, charge transport, and active layer morphology.27,28 Selecting appropriate side chain is as important as selecting conjugated backbones. Currently, the development of conjugated polymers with donor−acceptor (D−A) alternating molecular structure is a successful and universal strategy for preparing high efficiency donor materials, which can tune molecular energy levels, optical absorption properties, and carrier mobility by selecting different donor or acceptor units.29,30 Therefore, the design of new donor and acceptor building motifs has always been one of the most important researches to construct D−A copolymers for high efficiency OSCs. Until now, a large number of D−A copolymers have been developed and shown good photovoltaic properties with PCEs as high as 10% in single-junction OSCs.31−33 This indicates that the development of novel conjugated polymers will certainly play a pivotal role in driving this research. Among the huge variety of D−A polymers for optoelectronic applications, a series of polymers based on benzo[1,2-b:4,5-b′]dithiophene (BDT) unit and quinoxaline (Qx) unit were synthesized. Indeed, similarly to BDT, Qx is a popular and promising building block.34−36 It has already been widely utilized in industry to inhibit metal corrosion, in pharmacology against bacteria, malaria, and cancer, etc.,37,38 and also BHJ solar cells.39,40 The two nitrogen atoms at the 1,4-positions make the quinoxaline unit as an electron-deficient building block. Futhermore, the Qx motif can provide the versatility of introducing different substituents easily on the 2- and 3-positions, which could be used to tune the solubility, bandgap, and energy levels of the resulting polymers (see Scheme 1). Some Qx-based conjugated polymers have been described previously with PCE increasing from 0.3%41 to 11.47%.42 Recently, intensive researches have been devoted to exploring fluorination in conjugated polymers for solar cells since it has a relatively small van der Waals radius of 147 pm, which can minimize the undesired steric interactions. Meanwhile, the introduction of fluorine onto backbone of conjugated polymer has been proven to be an effective method to reduce HOMO and

Scheme 1. Chemical Structure of Quinoxaline (Qx) (R = Alkyl or Aryl Group)

LUMO levels due to the most electronegative element of the fluorine atom with a Pauling electronegativity of 4.0. In addition, noncovalent interactions exist for fluorine, such as F···H, F···S, and F···π, resulting in a better charge mobility of the polymers.43−45 Our group first introduced six fluorine atoms on the Qx unit and obtain an ideal acceptor unit HFQx.46 Therefore, developing new methods to utilizing the fluorination more effectively is of great importance to design conjugated polymer for photovoltaic applications. Motivated by the above-mentioned considerations, in this work, we reported a new D−A copolymer by using 4,8-bis(5-(2ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene (BDT-T) as electron donor unit, 5,8-dibromo-2,3-bis(4-(2ethylhexyl)-3,5-difluorophenyl)-6,7-difluoroquinoxaline as the acceptor unit, and thiophene as the spacer, namely, poly{5-(5(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-6-methylbenzo[1,2b:4,5-b′]dithiophen-2-yl)thiophen-2-yl)-2,3-bis(4-(2-ethylhexyl)-3,5-difluorophenyl)-6,7-difluoro-8-(5-methylthiophen-2-yl)quinoxaline} (HFAQx-T), which was synthesized and used as donor material for OSCs. The 4-alkyl-3,5-difluorophenyl group was first introduced as the lateral side chain to Qx unit (HFAQx). Interestingly, HFAQx-T is a typical medium bandgap donor material due to the weak electron donating ability of the 4-alkyl3,5-difluorophenyl47 substituted Qx unit. The photovoltaic performance of the p-type polymer was investigated by fabricating fullerene-free OSCs with the HFAQx-T as donor and narrow bandgap n-type ITIC as acceptor. After optimizing the phase-separation morphology of the donor and acceptor in the active layer by thermal anneaing, a PCE of 9.6% and a high Jsc of 16.42 mA cm−2 were obtained for the non-fullerene OSCs based on HFAQx-T:ITIC (0.8:1.0, w:w). Single-junction OSCs based on HFAQx-T as the donor and PC71BM as the acceptor have also been fabricated. A PCE of 3.49% without any solvent additives and postannealing treatments. When 1,8-diiodooctane (DIO) was used as the solvent additive, HFAQx-T exhibited a surprisingly increased PCE of 9.2% with a large Voc (0.90 V) and a high FF (0.74). To the best of our knowledge, the PCE of 9.2% is the highest efficiency reported in the literature to date for BDT-Qx fullerene OSCs. More importantly, HFAQx-T is among a few D−A copolymers that can deliver >9% efficiency in both fullerene and fullerene-free solar cells.32 The results indicated that this novel donor polymer HFAQx-T is a well-compatible donor material and probably work as another platform with various small molecular acceptor materials to achieve high performance OSCs.

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RESULTS AND DISCUSSION Synthesis of the Polymer. As outlined in Scheme 2, the polymer HFAQx-T was synthesized via a typical Stille polymerization using toluene as solvent and tetrakis(triphenylphosphine)palladium (0) [Pd(PPh3)4] as catalyst. The detailed synthetic procedures are described in the Supporting Information. The polymer HFAQx-T was carefully purified by B

DOI: 10.1021/acs.macromol.8b00326 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 2. Synthetic Route of the D−A Copolymer HFAQx-T

state. The optical bandgap (Egopt) value of HFAQx-T film was calculated to be 1.73 eV, estimated from the absorption edge (λonset) of its film. Moreover, at the short wavelength range (300−500 nm), HFAQx-T showed distinct peaks. The maximum absorption coefficient of pure HFAQx-T film (αfilm = 5.2 × 104 cm−1) was measured as shown in Figure S5b and listed in Table 1. We also measured the absorption spectra of the blend films of the HFAQx-T:PC71BM and HFAQx-T:ITIC, as illustrated in Figure S5c,d. The blend film of HFAQx-T:ITIC provides a better complementary absorption range from 500 to 750 nm with thermal annealing, in comparison with that of the blend film of HFAQx-T:PC71BM. This strong and broad absorption of the blend film of HFAQx-T:ITIC will benefit the Jsc enhancement of OSCs based on HFAQx-T:ITIC. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) levels of HFAQxT can be determined by cyclic voltammetry (CV). The CV curve is shown in Figure 1c, and the electrochemical properties are summarized in Table 1. The onset potentials for oxidation (Eox) and reduction (Ered) are 1.09 and −0.97 V, respectively. The HOMO level and the LUMO level of HFAQx-T were caculated according to the equations HOMO = −e(Eox + 4.41) (eV) and LUMO = −e(Ered + 4.41) (eV), respectively,48 corresponding to a HOMO level of −5.50 eV and a LUMO level of −3.44 eV, respectively. Obviously, HFAQx-T prensents a similar EHOMO value and larger ELUMO value than the previous analogue of HFQx-T;46 that is, grafting two alkyl side chains at the phenyl have little influence on the HOMO energy level, but they can cause a higher LUMO level for HFAQx-T. The electrochemical bandgap (Egec) of the HFAQx-T calculated from the difference

continuous Soxhlet extractions with methanol, hexane, acetone, and chloroform; then chloroform fraction was concentrated under vacuum evaporation, precipitated into methanol, and collected by filtration. The polymerization yield of 69% was obtained. The NMR spectra of intermediates and polymer are shown in Figures S1−S3.The number-average molecular weight (Mn) is 23.8 kDa with a corresponding polydispersity index (PDI: Mw/Mn) of 1.85 (Figure S4), evaluated by hightemperature gel permeation chromatography (HT-GPC) at 150 °C using 1,2,4-tricholorobenzene as the eluent and monodispersed polystyrene as the standard. The thermal property of the polymer was determined by thermogravimetric analysis (TGA). As shown in Figure S5a, the TGA results reveal that the onset temperature with 5% weight loss (Td) for HFAQxT is 388 °C; this value indicates that the thermal stability of the HFAQx-T is suitable for OSCs. Figure 1a presents the molecular structures of materials employed in this work. HFAQx-T was used as a donor material. PC71BM and ITIC were chosen as acceptor materials to match this medium bandgap polymer HFAQx-T. Optical and Electrochemical Properties. Photophysical properties of HFAQx-T in dilute chloroform solution and thin film were investigated by ultraviolet−visible (UV−vis) absorption spectroscopy (see Figure 1b), and corresponding absorption properties are summarized in Table 1. The spectral profiles of the dilute solution and thin film are similar, with mainly two absorption peaks. The maximum absorption peak for HFAQx-T in chloroform and film states is located at 618 and 635 nm, respectively. Compared with the solution, the 17 nm red-shift in the film state indicates that there is some aggregation of HFAQxT backbone and π−π intermolecular interaction in the solid film C

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Figure 1. (a) Molecular structures of HFAQx-T as an electron donor and PC71BM and ITIC as electron acceptors. (b) Absorption spectra for HFAQx-T in solution and as a film, PC71BM and ITIC as films. (c) Cyclic voltammogram of HFAQx-T in CH3CN/0.1 M Bu4NPF6. (d) Device architecture of the BHJ OSCs. (e) Energy level diagram of the materials used in the OSCs.

Table 1. Optical and Electrochemical Data of HFAQx-T film

solution

a

polymer

Mn (kDa)

PDI

Td (°C)

λmax (nm)

λonset (nm)

α (cm−1)

λmax (nm)

λonset (nm)

Egopt a (eV)

HOMOb (eV)

LUMOb (eV)

Egec

HFAQx-T

23.8

1.85

388

618

721

5.2 × 104

635

718

1.73

−5.5

−3.44

2.04

Calculated from the absorption onset of the polymer,

Egopt

= 1240/λonset. bMeasured by cyclic voltammetry.

As is well-known, it is essential to match energy levels of donor and acceptor materials for high performance OSCs. The molecular energy level alignments in the two types of the blends are shown in Figure 1e. For the HFAQx-T:PC71BM system, the gaps of HOMO and LUMO are larger than 0.3 eV, which is

between HOMO and LUMO values was 2.04 eV. The deep HOMO energy level can be anticipated that HFAQx-T based device would achieve a high Voc, since Voc of BHJ OSCs is closely related to the gap between the LUMO of acceptor material and the HOMO of the donor polymer in the active layer. D

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Figure 2. (a) J−V curves of PSCs baesd on HFAQx-T:acceptor under the illumination of AM 1.5G, 100 mW cm−2. (b) EQE curves of the corresponding devices. (c) Light intensity dependence of Jsc of the corresponding devices based on HFAQx-T:acceptor. (d) Jph versus Veff of the devices.

Table 2. Photovoltaic Performance Parameters of the Optimized OSCs Based on HFAQx-T:Acceptor under Illumination of AM 1.5G (100 mW cm−2) devices

Voc (V)

Jsc (mA cm−2)

FF (%)

PCEmaxa (%)

Rsb (Ω cm2)

Rshc (kΩ cm2)

thickness (nm)

HFAQx-T:PC71BM HFAQx-T:ITIC

0.90 0.94

14.0 15.4

74.0 66.0

9.2 (9.05) 9.6 (9.43)

9.39 9.71

2.94 1.01

96 95

a

The average PCE values obtained from 15 devices are shown in parentheses. bCalculated from the slope at V = Voc in J−V curves under illumination. cCalculated from the inverse slope at V = 0 in J−V curves under illumination.

cac)49,50 or perylene diimide functionalized with amino N-oxide (PDINO)51/Al (100 nm) were fabricated under the illumination of AM 1.5G (100 mW cm−2), where PEDOT:PSS is used as the hole transport layer (HTL); ZrAcac and PDINO are adopted as the cathode buffer layer for lowering the work function of Al. The active layer was spin-coated from chloroform solution of the donor and acceptor. The typical current density versus voltage (J−V) characteristic curves and corresponding external quantum efficiency (EQE) spectra for the solar cells are shown in Figure 2a,b. The photovoltaic data are listed in Table 2 and Table S1. For HFAQx-T:PC71BM solar cells, the best devices gave a PCE of 9.2%, with a Voc of 0.90 V, a Jsc of 14.0 mA cm−2, and a FF of 0.74. These devices have a D/A ratio of 1:1.2 (w/w), an active layer thickness of 95 nm, and 0.25% DIO (v/v) as the additive (see Table S1). The PCE of 9.2% is the highest efficiency reported in the literature to date for Qx-based polymer/fullerene OSCs, which might be due to the good light-harvesting capability of HFAQx-T and the ideal phase separation in the active layer. DIO was found very crucial in improving the performance of HFAQx-T:PC71BM devices. The devices without DIO gave a poor PCE of 3.5%, with reduced Jsc and FF of 8.9 mA cm−2 and 0.43, respectively. After adding 0.25% (v/v) DIO, the EQE

enough for exciton separation of the donor HFAQx-T and PC71BM. However, for the blend of HFAQx-T:ITIC, the HOMO difference is 0.02 eV. To explore the exciton separation and charge transfer behavior, the exciton dissociation and charge transfer in the HFAQx-T:acceptor blends were measured by photoluminescence (PL) quenching experiments. A blend with smaller polymer domain size has higher PL quenching efficiency compared to big one; hence, PL quenching efficiency can directly reflect polymer domain size in a blend. Figure S6 displays the PL spectra of the HFAQx-T (excited at 630 nm) and ITIC (excited at 700 nm) films as well as the blend films of HFAQx-T:PC71BM (1:1.2, w/w) and HFAQx-T:ITIC (0.8:1.0, w/w) (excited at 630 and 700 nm). Both PC71BM and ITIC can effectively quench the PL of HFAQx-T, which suggests that HFAQx-T excitons can be well diffused to the D:A interface and dissociated into carriers. The PL of ITIC can also be quenched by HFAQx-T though the HOMO offset is 0.02 eV, indicating that the hole transfer from ITIC’s excitons to HFAQx-T could be happen. Photovoltaic Properties. BHJ OSCs with a conventional device structure of indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene)poly(styrenesulfonate) (PEDOT:PSS)/ HFAQx-T:PC71BM or ITIC/zirconium acetylacetonate (ZrAE

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Figure 3. AFM topography images (5 × 5 μm2) of HFAQx-T:PC71BM (1:1.2, w/w) blend films (a) as-cast, (b) with 0.25 vol % DIO additive treatment; HFAQx-T:ITIC (0.8:1.0, w/w) blend films (c) as-cast, and (d) after thermal annealing treatment. The bright-field TEM images for the blend films. HFAQx-T:PC71BM (1:1.2, w/w) film (e) as-cast, (f) with 0.25 vol % DIO additive treatment, HFAQx-T:ITIC (0.8:1.0, w/w) film (g) as-cast, and (h) with thermal annealing. Note: scale bar 200 nm.

reduce recombination and built up space charge density, which contribute to high FF and better photovoltaic performance of the devices based on HFAQx-T:PC71BM. For HFAQx-T:ITIC blend films, the μh and μe values are 2.87 × 10−5 cm2 V−1 s−1 and 1.60 × 10−4 cm2 V−1 s−1, respectively, with the μh/μe ratio of 0.12 for the film without thermal annealing, while after thermal annealing, μh and μe values were increased to 8.63 × 10−5 cm2 V−1 s−1 and 2.10 × 10−4 cm2 V−1 s−1, respectively, with the better ratio of 0.41, which could be responsible for the improvement of its photovoltaic performance of the devices based on HFAQxT:ITIC with thermal annealing. To further investigate the effects of the 4-alkyl-3,5difluorophenyl side chain on the device performance, here, we measured the photocurrent versus light intensity (Plight) curves to study the charge recombination behavior of the HFAQxT:acceptor in the devices (see Figure 2c). In general, the relationship between Jsc and Plight could be expressed as Jsc ∝ Plighta, where the power-law component (a) value approaches unity when bimolecular recombination of charge carriers is negligible.53 The values of α for the HFAQx-T:PC71BM (1:1.2, w/w) and HFAQx-T:ITIC (0.8:1.0, w/w) as-cast devices are 0.94 and 0.98, respectively, indicates the existence of the bimolecular recombination. However, good α values of 0.97 and 0.99 were obtained for optimized fullerene- and fullerene-freebased devices; the results showed that bimolecular recombination was more suppressed after optimization and should make a great contribution to the enhanced FF and Jsc. Moreover, we investigated the process of exciton generation, exciton dissociation and charge collection efficiency of the BHJ OSCs by measuring the photocurrent density (Jph) versus the effective voltage (Veff) curves. Veff can be described as Veff = V0 − Vbias, where V0 is the voltage when Jph = 0 and Vbias is the applied external voltage bias. Jph can be described as Jph = JL − JD, where JL and JD are the current densities under illumination and in the dark, respectively. Figure 2d shows a log−log plot of Jph versus Veff for HFAQx-T:acceptor blend films. As shown in Figure 2d, Jph reaches saturation (Jsat) at Veff ≥ 2 V, indicating that almost all of the photogenerated free carriers in the devices can be collected by electrodes. The charge collection probability at short-circuit condition can be calculated from Jph/Jsat. The Jph/Jsat value for the

maximum increased from 49.9% to 74.2% at 500 nm, accounting for the enhancement of Jsc. Highly efficient photovoltaic performance is achieved when ITIC is used as acceptor material. For these fullerene-free based OSCs, the as-cast device of HFAQx-T:ITIC (0.8:1.0, w/w) exhibited a PCE of 8.8%, while it was distinctly enhanced to 9.6% efficiency after annealing at 130 °C for 10 min, with a Voc of 0.94 V, a Jsc of 15.4 mA cm−2, and a FF of 0.66. Compared to the as-cast devices, the increased PCE of the devices with thermal annealing treatment is largely a result of the increased Jsc and FF. Furthermore, compared with PC71BM devices, ITIC based devices gave a higher Voc and Jsc, which is due to the enlarged energy gap between donor’s HOMO and acceptor’s LUMO and the increased absorption in the NIR region, respectively. According to the EQE curves, the calculated Jsc values of the devices agree well with Jsc from J−V measurements within 5% mismatch, indicating that our photovoltaic results are reliable. The series resistance (Rs) and shunt resistance (Rsh) of OSCs are measured to elucidate the effects of additive and thermal annealing from their respective J−V curves under illumination. As shown in Table 2, the optimized fullerene- and fullerene-free based devices present a relatively lower Rs and higher Rsh in comparison to their corresponding as-cast devices, suggesting better overall diode characteristics after the optimization process. To better understand the effect of DIO treatment or thermal annealing on the photovoltaic performance of the HFAQxT:acceptor blend films, the hole and electron mobilities (μh and μe) were evaluated by the space limited current (SCLC) method52 with the hole-only device structure, ITO/PEDOT:PSS/HFAQx-T:acceptor/Au and electron-only device structure ITO/ZnO/HFAQx-T:acceptor/ZrAcac or PDINO/ Al. Figure S7 shows the J1/2−V plots of the HFAQx-T:acceptorbased devices with or without DIO treatment and thermal annealing; the related mobility data are listed in Table S2. For the as-cast film of HFAQx-T:PC71BM, the μh and μe are caculated to be 6.46 × 10−6 cm2 V−1 s−1 and 1.10 × 10−4 cm2 V−1 s−1, respectively, with μh/μe ratio of 0.06. While after DIO treatment, μh and μe values were increased to 7.62 × 10−5 cm2 V−1 s−1 and 1.63 × 10−4 cm2 V−1 s−1, respectively, with the better ratio of 0.47. The higher and more balanced charge carrier mobilities will F

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Figure 4. GISAXS patterns of (a) as-cast HFAQx-T:PC71BM blend film and (b) HFAQx-T:PC71BM blend film processed with 0.25% DIO. (c) As-cast HFAQx-T:ITIC blend film. (d) HFAQx-T:ITIC blend film processed with thermal annealing. (e) The corresponding in-plane intensity profiles.

films of HFAQx-T:ITIC as the π−π peak can still be observed along the qz axis (Figure S8f,g). In contrast, the blended films of HFAQx-T:PCBM exhibit relatively isotropic molecular packing, evident by the ringlike scattering features. 2D GISAXS patterns and the corresponding horizontal intensity profiles are shown in Figure 4a−e and Figure S10 to estimate the sizes of each phase. Here, the scattering contribution from pure HFAQx-T or ITIC phases is fitted by the fractal-like network model,6 whereas the pure PC71BM phases are modeled as poly dispersed hard spheres.55 The Debye−Anderson−Brumberger (DAB) model is used to account for the scattering due to amorphous intermixing region. The scattering of the as-cast HFAQx-T:PC71BM blend film presents a clear shoulder at q ∼ 0.004 Å−1, indicating the formation of excessively large PC71BM domains (Rg = 67.3 nm), which is detrimental to the device performance. With the DIO treatment, the PC71BM domain size shrinks to a reasonable value of Rg = 19.0 nm, in agreement with the observed improvement in device performance. The GISAXS profiles of blended HFAQxT:ITIC with or without thermal annealing are very similar: the estimated domain sizes of the intermixing region, pure ITIC, and pure HFAQx-T phases are 44.1, 4.52, and 43.7 nm for the as-cast film and 32.7, 5.55, and 37.7 nm for the annealed film, consistent with the similar device performance with or without thermal annealing.

optimized devices of HFAQx-T:PC71BM and HFAQx-T:ITIC is 92.4% and 90.0%, respectively, implying that both optimized fullerene- and fullerene-free OSCs based on HFAQx-T have highly efficient exciton dissociation and charge collection. Morphology Investigation. The morphology of the active layer has a significant influence on photovoltaic performance of OSCs. Here, we studied the morphologies of the blend films by atomic force microscopy (AFM) (Figure 3a−d) and transmission electron microscopy (TEM) (Figure 3e−h). For the HFAQx-T:PC71BM blend film, the AFM image exhibits a relatively serious self-aggregation with a large root-mean-square (RMS) value of 5.12 nm which would limit the probability for exciton dissociation and suffer serious trap-assisted recombination consistent with the charge separation and recombination studies (Figure 3a). The corresponding TEM image shows high phase separation and severe aggregation (Figure 3e), which may be attributed to the poor miscibility of HFAQx-T with PC71BM. However, a smooth surface morphology was observed (RMS = 1.57 nm) after optimizing by 0.25 vol % DIO additive treatment (Figure 3b), which would be favorable to get more efficient exciton dissociation and charge transport for OSCs, and thus higher FF and PCE can be achieved, while HFAQx-T:ITIC film with thermal annealing became rougher, with RMS roughness increasing from 0.97 to 1.04 nm (Figure 3c,d), indicating that slightly enhanced aggregation which would be benefit the charge transport. As shown in Figure 3g,h, TEM images from the blend films exhibited a similar tendency with AFM studies to develop more uniform and well-distributed interpenetrating nanoscale networks after thermal annealing. We also carried out grazing-incidence wide- and small-angle Xray scattering (GIWAXS/GISAXS) on pure HFAQx-T film and blend films with different acceptors for a better understanding on the bulk morphology.54 Two-dimensional (2D) GIWAXS patterns and the corresponding intensity profiles in the out-ofplane and in-plane directions are presented in Figures S8 and S9. HFAQx-T in the pure film (Figure S8a) is preferentially face-on oriented with the lamella peak concentrated along the qx axis (q = 0.30 Å−1; d = 20.9 Å) and the π−π peak concentrated along the qz axis (q = 1.74 Å−1; d = 3.61 Å). The favorable face-on ordering of the HFAQx-T crystalline domains is preserved in the blended

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CONCLUSION In summary, a new medium bandgap Qx copolymer, HFAQx-T, was synthesized and used as the donor polymer in OSCs. HFAQx-T showed good solubility in common organic solvents, a broad absorption from 300 to 700 nm, and a low-lying HOMO energy level of −5.50 eV. The optimized conventional singlejunction device of HFAQx-T:PC71BM achieved a PCE of 9.2%, with a Voc of 0.9 V, a Jsc of 14.0 mA cm−2, and a FF of 0.74, which is the highest efficiency reported in the literatures to date for BDT-Qx based fullerene OSCs. When blended with ITIC, the HFAQx-T based device showed a PCE of 9.6%, attributing to the more complementary absorption and better energy level match of the HFAQx-T and ITIC. HFAQx-T is among a few D−A copolymers that can deliver >9% efficiency in both fullerene and non-fullerene solar cells, which is inspiring for the design of new G

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donors compatible with fullerene and fullerene-free acceptors. The results indicated that incorporation of two weak electrondonating alkyl side chain onto the 4-positions of the phenyl at the Qx unit by side-chain engineering would be a feasible approach to improve photovoltaic properties. Through further efforts on modifying the chemical structure of HFAQx-T, device optimization and selection of matched acceptor, higher PCE can be anticipated.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b00326. Experimental details including synthesis, related figures, and tables (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (Y.Z.). ORCID

Yongfang Li: 0000-0002-2565-2748 Yingping Zou: 0000-0003-1901-7243 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been financially supported by the National Natural Science Foundation of China (51673205,21506258), National Key Research & Development Projects of China (2017YFA0206600), Hunan Provincial Natural Science Foundation for Distinguished Young Scholars (2017JJ1029), and Natural Science Foundation of Hunan Province (No. 2016JJ3134). X. Lu and T.-K. Lau gratefully thank the beam time and technical supports provided by 19U2 beamlines at the Shanghai Synchrotron Radiation Facility.

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