Fused Perylene Diimide-Based Polymeric Acceptors for Efficient All

Sep 29, 2017 - Two polymeric electron acceptors (PFPDI-2T and PFPDI-2FT) based on the fused perylene diimide (PDI) and bithiophene or difluorobithioph...
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Fused Perylene Diimide-Based Polymeric Acceptors for Efficient AllPolymer Solar Cells Ming Liu,†,§ Jing Yang,‡ Caili Lang,† Yong Zhang,*,† Erjun Zhou,*,‡ Zhitian Liu,§ Fengyun Guo,† and Liancheng Zhao† †

School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China CAS Key Laboratory of Nanosystem and Hierarchical Fabrication, CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology, Beijing 100190, China § School of Materials Science and Engineering, Wuhan Institute of Technology, Wuhan 430073, China ‡

S Supporting Information *

ABSTRACT: Two polymeric electron acceptors (PFPDI-2T and PFPDI-2FT) based on the fused perylene diimide (PDI) and bithiophene or difluorobithiophene units were synthesized via the Stille polymerization. Both polymers exhibit the strong absorption between 350 and 650 nm, which have the good absorption compensation with the low band gap conjugated polymer in polymer solar cells (PSCs). PFPDI-2T and PFPDI2FT have the LUMO energy levels of around −4.12 to −4.15 eV, which are comparable with other PDI-based polymers and fullerene derivatives. All-polymer solar cells (all-PSCs) based on PFPDI-2T or PFPDI-2FT as the polymeric electron acceptor were fabricated with PTB7-Th as the polymeric electron donor. Power conversion efficiency of as high as 6.39% based on PFPDI-2T/PTB7-Th was achieved under the standard illumination of simulated sunlight (AM 1.5, 100 mW cm−2), which is significant higher than that of the all-PSC based on the nonfused PDI counterpart. The results demonstrate that the direct fusion of PDI unit is an effective design strategy to enhance the photovoltaic performances of all-PSCs.

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INTRODUCTION Recently, all-polymer solar cells (all-PSCs) with growing remarkable power conversion efficiencies (PCEs) have received much more attention.1−8 In all-PSCs, both the p-type donor and n-type acceptor materials in the bulk heterojunction layer are the conjugated polymers, which have the advantages of enhanced absorption, easy tuned absorption and energy levels, superior thermal, morphological, and mechanical stability, etc.9−12 The first all-polymer solar cell was reported by Friend et al. in 1995, in which they selected poly(cyanated phenylenevinylene) (CN-PPV) as the n-type polymeric acceptor.13 Since then, various n-type conjugated polymer acceptors based on cynated phenylenevinylene, benzothiadiazole (BT), thienopyrroledione (TPD), diketopyrrolopyrrole (DPP), nanphthalene diimide (NDI), perylene diimide (PDI) and B−N bridged bipyridine, etc., have been developed,14−29 and the considerable endeavors by either designing new n-type polymer acceptors or running the device optimizations have advanced the performances of all-PSCs to be over 8%; however, it still lags behind the current state-of-the-art performance of polymer solar cells with fullerene (i.e., PC71BM) or small molecular nonfullerene (i.e., ITIC) acceptors.30,31 One of the limiting factors for all-PSCs is considered to be the availability © XXXX American Chemical Society

of n-type conjugated polymer acceptors with the good properties of such as the matched absorption and energy levels, good morphological stability, and high charge carrier mobilities. PDI and their derivatives have received a great deal of interest in developing novel polymer acceptors as one of the most promising electron accepting materials because of their outstanding chemical and physical properties, such as cheap cost, high molar absorption efficiency, good electron mobilities, and easily functionalization.32,33 The first n-type PDI-based polymeric acceptor for all-PSC was synthesized by Zhan et al. in 2007, and the photovoltaic properties with a polythiophene derivative as the p-type donor were investigated.34 Since then, extensive research efforts have been devoted to the new design and modification of the PDI-based polymers as the n-type polymeric acceptors. With the advancement of p-type polymer donor and morphological optimization, the device performances with PDI-based polymeric acceptor have gradually been improved up to 3−8%.32 For example, Zhao and Yan et al. Received: July 19, 2017 Revised: September 22, 2017

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Figure 1. Chemical structures of reported PDI-diTh and the synthesized PFPDI-2T and PFPDI-2FT in this work.

Scheme 1. Synthetic Routes of Monomer FPDI-Br, Polymer PFPDI-2T, and PFPDI-2FT

synthesized a PDI and vinylene-based polymer acceptor and applied it in all-PSC with PTB7-Th as the n-type polymer donor, achieving the power conversion efficiency (PCE) of 7.57%.6 Further exploration of the new PDI-based polymeric acceptors with the above-mentioned benefits will be able to provide the promising opportunities to increase the performances of all-PSCs. It is well-known that the extended aromatic unit with the large conjugated coplanar area plays an important role in determining the molecular orientations and the device performances in PSCs.35−37 Among the diimide compounds as the electron acceptor for all-PSCs, naphthalene diimide has been modified, for example, by fusing two thiophene to form the naphtho[2,3-b:6,7-b′]dithiophene-4,5,9,10-diimide, which copolymers with thiophene showed the improved charge transport properties and the enhanced photoresponses in the near-infrared region.38,39 However, when the similar strategy was applied to perylene diimide, the all-PSCs with the dithienocoronene diimide polymers, which are synthesized by fusing the thiophenes in the bay regions of PDI, only gave the PCE of up to 0.84%.40 Beside the low photovoltaic performance, one of the dilemmas in the modifications of PDI polymer is the limited synthetic method to be used. To further explore and improve the photovoltaic performances of all-PSCs based on PDI polymer electron acceptor with taking the benefits of PDI polymers, it is highly necessary and urgent to develop the novel PDI polymers possessing the matched absorption spectra and energy levels as well as the promising device performances.

Herein, we design two new polymeric acceptors (PFPDI-2T and PFPDI-2FT, Figure 1) based on the fused PDI, which is the fusion of two PDI units through the bay region, for the application in all-PSCs. The extended fused unit is believed to have large π-electron delocalization, relatively high electron mobilities, and good electron accepting ability. In addition, the relative longer and nonplanar structure due to the steric congestion in the bay region within the fused PDI will also affect the conformation and molecular packing of polymer chains toward the favorable trend with the enhanced photovoltaic performance in all-PSC. In this study, the all-PSCs fabricated with PFPDI-2T or PFPDI-2FT as the electron acceptor and PTB7-Th as the polymer donor exhibited the PCE of up to 6.39%, which is among the best photovoltaic performances of all-PSCs based on PDI polymeric acceptors (i.e., PPDI-T and PPDI-diTh, Figure 1) and is significantly higher than the PCE of 1.42% for polymer PDI-diTh with PTB7-Th donor (Figure 1).41 We also noted that during the manuscript preparation Zhao and Yan et al. also reported a fused PDI-based polymer for the application of all-PSC.42

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DISCUSSION Synthesis. The synthetic routes toward the fused dipyrene diimide monomer FPDI-Br and polymer acceptors PFPDI-2T and PFPDI-2FT are shown in Scheme 1. The FPDI-Br was started from 1 via three steps. First, compound 1 was reacted with 1,2-bis(tributylstannyl)vinylene to give the vinylene-linked pyrene diimide dimer 2, which was then cyclized under UV

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Figure 2. Absorption spectra of polymer acceptors PFPDI-2T (a) and PFPDI-2FT (b) in chlorobenzene solution and thin film states and the absorption spectra of the PTB7-Th thin film.

Figure 3. Cyclic voltammetry of PFPDI-2T and PFPDI-2FT films (a) and the energy levels diagram of the donor PTB7-Th and acceptors PFPDI-2T and PFPDI-2FT (b).

Optical and Electrochemical Properties. Figure 2 shows the absorption spectra of PFPDI-2T and PFPDI-2FT in chlorobenzene solution and thin film states. As seen from Figure 2, both polymers have similar absorption features in either solution or film state. The absorption peak at around 380 nm was originated from the n−π* transitions of the polymer backbone, which is mainly determined by FPDI unit. The absorption band with a peak at around 550 nm was attributed to π−π* transitions of the fused perylene diimide backbone. The molar extinction coefficients of both polymers are higher than 104 M−1 cm−1 (Figure S1). Upon comparing Figures 2a and 2b, it can see that the absorption bands at 500−700 nm in PFPDI-2T thin film becomes stronger than that in solution due to the stronger intermolecular/intramolecular interactions in film state, indicating the polymer chains of PFPDI-2T are less aggregated or stacking in solution compared to that in film. In PFPDI-2FT, we can find the absorption spectra in solution and film are almost same, which indicates that the PFPDI-2FT chains already have a strong aggregation or stacking even in solution as a result of the “molecular lock” effect from difluorosubstituted bithiophene unit in PFPDI-2FT.46 The absorption edges of PFPDI-2T and PFPDI-2FT are 728 and 694 nm, respectively, corresponding to the optical band gaps of 1.70 and 1.79 eV. This value is smaller than those PDI polymers with annulated bay region. As shown in Figure 2, the absorption

light to produce the fused dipyrene diimde 3. The bromination of 3 with bromine generates the monomer FPDI-Br. The 2octyldecyl branched alkyl chains in FPDI-Br were selected to ensure the solubility of the monomers and polymers. The polymer acceptors PFPDI-2T and PFPDI-2FT were synthesized by Stille polymerization of FPDI-Br with ([2,2′bithiophene]-5,5′-diyl)bis(trimethylstannane) (4) and (3,3′difluoro-[2,2′-bithiophene]-5,5′-diyl)bis(trimethylstannane) (5), respectively. Both polymers have good solubilities in chlorinated solvents, such as chloroform, chlorobenzene, and dichlorobezene. The number-average molecular weights (Mns) of PFPDI-2T and PFPDI-2FT, which were measured using gel permeation chromatography (GPC) with trichlorobenzene at 150 oCas eluent, were 70.8 and 85.6 kDa with a polydispersity index of 2.05 and 1.87, respectively, corresponding to the repeat units of ∼34 and ∼40. The fluorine-substituted bithiophene was selected in PFPDI-2FT because the strong electron-withdrawing property of fluorine atom will be able to effectively lower the HOMO energy levels of the resulted polymer,43 in this case which will increase the difference between the HOMO energy levels of the p-type polymer donor and PFPDI-2FT. In addition, the fluorine will also be able to form the so-called “molecular lock” effect with the sulfur atom of adjacent thiophene so as to generate the large planar structure with the strong interchain interactions.44−46 C

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Figure 4. J−V curves (a) and EQE spectra (b) of PTB7-Th:PFPDI-2T and PTB7-Th:PFPDI-2FT devices.

Table 1. Photovoltaic Performances of All-PSCs Based on PFPDI-2T, PFPDI-2FT, and PDI-diTh as the Electron Acceptor

a

acceptor/donor

Voc (V)

Jsc (mA/cm2)

FF

PCEmax (%)

PCEavga (%)

reference

PFPDI-2T/PTB7-Th PFPDI-2FT/PTB7-Th PDI-diTh/PTB7-Th r-PDI-diTh/P3HT i-PDI-diTh/P3HT

0.73 0.67 0.88 0.52 0.49

13.47 13.31 4.90 7.65 6.25

0.65 0.60 0.32 0.55 0.51

6.39 5.35 1.42 2.17 1.55

6.31 ± 0.08 5.26 ± 0.09 1.36 ± 0.10

this work this work ref 41 ref 47 ref 47

Average data from 10 cells.

2T (or PFPDI-2FT) with different donor−acceptor weight ratios. The active layers were prepared by the spin-coating of PTB7-Th:PFPDI-2T (or PFPDI-2FT) blend in chlorobenzene solution containing a small amount of 1-chloronaphthalene (CN) as solvent additive at a total polymer concentration of 20 mg/mL. The optimized amount of CN is 3% for PTB7Th:PFPDI-2T and 1% for PTB7-Th:PFPDI-2FT. The detailed device performances at different amounts of CN are shown in Table S2. Figure 4a shows the J−V curves of the all-PSCs under the standard AM 1.5G, 100 mW/cm2 illumination condition. It can be found that the all-PSC based on PTB7-Th:PFPDI-2T demonstrated a high PCE of 6.39% with an open-circuit voltage (Voc) of 0.73 V, a current density (Jsc) of 13.47 mA/cm2, and a fill factor (FF) of 0.65, while the PCE of PTB7-Th:PFPDI-2FT device was 5.35% with a Voc of 0.67 V, a Jsc of 13.31 mA/cm2, and a FF of 0.60 (Table 1). It is found that the PCE difference between PFPDI-2T and PFPDI-2FT devices are mainly due to the lower-lying Voc in PFPDI-2FT device, which is believed to attribute to its lower LUMO energy level as a result of fluorine substitution, because the Voc is directly related to the difference of donor’s HOMO energy level and acceptor’s LUMO energy level. The energy loss for PFPDI-2T and PFPDI-2FT devices was calculated to be 0.87 and 0.93 eV, respectively. As shown in Table 1, the PDI-diTh, which is the nonfused PDI copolymer with bithiophene (Figure 1), only showed the PCEs of 1.42% and 0.49−0.52% with PTB7-Th and P3HT as the donor, respectively.41,47 In addition, the performances of PFPDI-2T/ PTB7-Th and PFPDI-2FT/PTB7-Th devices were also significantly higher than that of PDI-T/PTB7-Th device, where the PDI-T is the copolymer of PDI and thiophene units.41 The results demonstrated that the strategy to increase the coplanarity by fusing PDI unit is very efficient to improve

spectra of PTB7-Th has good complementation with that of PFPDI-2T and PFPDI-2FT between 300 and 800 nm, showing the good potential in achieving high current density of all-PSC. The reduction potentials of PFPDI-2T and PFPDI-2FT were measured by the cyclic voltammetry with ferrocene as the internal standard since Fc+/Fc = 0. As shown in Figure 3a, the lowest unoccupied molecular orbital (LUMO) energy levels of PFPDI-2T and PFPDI-2FT are determined to be −4.12 and −4.15 eV, respectively. These values are slighter lower than PDI−thiophene or PDI−bithiophene polymers and are comparable with PCBM. The HOMO energy levels are found to be at −5.82 and −5.94 eV for PFPDI-2T and PFPDI-2FT, respectively, as calculated from the LUMO energy levels and optical band gap. Figure 2b shows the diagram of the energy levels of PTB7-Th, PFPDI-2T, and PFPDI-2FT. It can find that the LUMO differences between the donor PTB7-Th and the polymeric acceptor are around 0.33−0.36 eV, and the LUMO differences are calculated to be around 0.43−0.45 eV. It is well-known that the energy difference of not less than 0.3 eV is normally required for efficient exciton dissociation. Thus, the low-lying LUMO and HOMO energy levels of PFPDI-2T and PFPDI-2FT are matched well with the PTB7-Th and other donor polymers (Figure 3b). Photovoltaic Properties. To evaluate the photovoltaic properties of PFPDI-2T and PFPDI-2FT as the electron acceptor, all-PSCs with the device structure of ITO/ PEDOT:PSS/PTB7-Th:PFPDI-2T or PFPDI-2FT/Ca/Al were fabricated. The PTB7-Th was selected as the donor because it has suitable energy difference with that of PFPDI-2T and PFPDI-2FT (Figure 3b), and PTB7-Th has the promising device performance in all-PSCs. The optimized weight ratio of PTB7-Th:acceptor in the active layer of each device is 1:1. Table S1 shows the device performances for PTB7-Th:PFPDID

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Figure 5. J−V curves in the space-charge-limited-region for the hole-only (a, ITO/PEDOT:PSS/PTB7-Th:polymer acceptor/Au) and electron-only (b, ITO/TiOx/PTB7-Th:polymer acceptor/Al) devices.

Figure 6. AFM height images of PTB7-Th:PFPDI-2T (a) and PTB7-Th:PFDPI-2FT (b) blend films spin-coated from chlorobenzene.

the photovoltaic performance of all-PSCs that based on PDI polymer as the electron acceptor. Figure 4b shows the incident-photon-to-converted-current efficiency (IPCE) spectra of the PTB7-Th/PFPDI-2T and PTB7-Th/PFPDI-2FT devices, which have the broad photoresponses from 300 to 800 nm. The external quantum efficiency (EQE) peak values for PTB7-Th/PFPDI-2T and PTB7-Th/PFPDI-2FT devices are ∼67% and ∼62% at ∼640 nm, respectively. Both devices have the EQE values of more than 60% between ∼650 and 800 nm. From Figure 4b, it can obviously see that both the absorption of PTB7-Th and PFPDI2T or PFPDI-2FT have significant contributions to the photocurrent. Specifically, the photon responses between 300 and 630 nm are mainly contributed by the polymeric electron acceptor PFPDI-2T or PFPDI-2FT, while PTB7-Th provides the dominant role on the photon responses from 630 to 800 nm. The slight lower and higher photoresponse at shorter and longer wavelength regions, respectively, for PTB7-Th/PFPDI-

2T device have a trade-off on the overall photocurrent, and vice versa for PTB7-Th/PFPDI-2FT device, which can explain the similar measured Jsc in both devices. To understand charge transport properties of the PFPDI-2T and PFPDI-2FT based all-PSCs, the charge carrier mobilities of both devices were measured by the space-charge-limitedcurrent (SCLC) method using the Mott−Gurney equation with the structures of ITO/PEDOT:PSS/PTB7-Th:polymer acceptor/Au and ITO/TiOx/PTB7-Th:polymer acceptor/Al for hole-only and electron-only devices, respectively. The typical J−V curves of the hole-only and electron-only devices are shown in Figure 5, in which the Ohmic (slope = 1) and SCLC (slope = 2) regions can be clearly found. The calculated hole and electron mobilities are 2.67 × 10−4 cm2 V−1 s−1 and 3.84 × 10−5 cm2 V−1 s−1 for PTB7-Th/PFPDI-2T device and are 2.32 × 10−4 cm2 V−1 s−1 and 3.32 × 10−5 cm2 V−1 s−1 for PTB7-Th/PFPDI-2FT device, respectively. The relatively low hole and electron mobilities in PTB7-Th/PFPDI-2FT device E

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All-Polymer Solar Cell Fabrication. The detailed fabrication is listed in the Supporting Information. A patterned ITO was precleaned by detergent, deionized water, acetone, and 2-propanol for 15 min under ultrasonic conditions. The cleaned ITO was then treated by UV oxygen to form the hydrophilic surface. The PEDOT:PSS solution was spin-coated on the ITO at 3500 rpm for 30 s and dried over 150 °C for 15 min under air. The PTB7-Th:polymer acceptor solution (optimized weight ratio of 1:1) in chlorobenzene with 0%, 0.5%, 1%, 3%, or 5% CN was then spin-coated above PEDOT:PSS layer at a total concentration of 20 mg/mL within a glovebox. The Ca (20 nm) and Al (80 nm) cathodes were evaporated onto the active layer under a pressure of below 10−6 bar with an active area of 4 mm2. The current density−voltage curves were collected by a Keithley 2420 under Oriel Newport 150 W solar simulator (AM 1.5G). The EQEs were measured by an Oriel Newport System. All above measurements were done at room temperature. The hole-only device for the hole mobility was fabricated with a device structure of ITO/PEDOT:PSS/PTB7-Th:polymer acceptor/ Au. The electron-only device for the electron mobility was fabricated with a device structure of ITO/TiOx/PTB7-Th:polymer acceptor/Al. Both the hole and electron mobilities by space charge limited current (SCLC) were calculated with the following Mott−Gurney equation in the SCLC region: J = (9/8)ε0εrμ(V2/L3), in which ε0 is the permittivity of the vacuum, εr is the dielectric constant of the polymer and assumed to be 3, and L is the thickness of active layer. Synthesis of 3. In a flask reactor, 2 (500 mg, 0.26 mmol) and I2 (100 mg) in toluene (1200 mL) were illuminated using a 250 W UV light for 48 h with slowly bubbling of air. After removing the solvent, the crude product was purified by silica column chromatography (hexane:dichloromethane, from 1:1 to 1:1.5) to give a pure title compound (260 mg, 52%). 1H NMR (400 MHz, CDCl3): δ (ppm) = 9.71 (s, 4H), 9.15−9.09 (m, 8H), 4.16 (s, 8H), 1.99 (s, 4H), 1.33− 1.13 (m, 128H), 0.84−0.75 (m, 24H). 13C NMR (75 MHz, CDCl3): δ (ppm) = 163.5, 132.8, 130.0, 125.5 124.4 123.6, 123.2, 122.3, 45.0, 36.8, 31.9, 31.8, 31.7, 30.1, 29.7, 29.6, 29.3, 26.5, 22.6, 14.1. HRMS (MALDI-TOF), m/z: calcd for C130H176N4O8, 1922.8104; found, 1922.9139. Elemental Analysis: found, C, 80.10%, H, 9.32%, N, 2.87%. Synthesis of FPDI-Br. In a 50 mL flask, 3 (400 mg, 0.21 mmol) was dissolved into dichloromethane (20 mL). After stirring for minutes, Br2 (8 mL) was added quickly to the solution. Then, the mixture solution was heated to reflux overnight. After cooling to room temperature, the excess Br2 was removed by flash using air flow, and then the crude product was purified by silica column chromatography (hexane:dichloromethane 1:1) to give the pure FPDI-Br (260 mg, 60%). 1H NMR (400 MHz, CDCl3): δ (ppm) = 10.71 (dd, J = 8.4, 1.6 Hz, 2H), 10.04 (d, J = 25.2 Hz, 4H), 9.45 (d, J = 2.8 Hz, 2H), 9.17− 9.13 (m, 2H), 4.21 (d, J = 5.2 Hz, 8H), 2.05 (s, 4H), 1.40−1.15 (m, 128H), 0.80−0.77 (m, 24H). 13C NMR (75 MHz, CDCl3): δ (ppm) = 163.2, 134.9, 133.5, 131.9, 129.7, 128.5, 125.9, 125.5, 125.1, 124.7, 124.4, 124.1, 122.5, 45.1, 45.0, 36.8, 36.7, 32.0, 31.7, 30.1, 29.7, 29.6, 29.3, 26.5, 22.6, 14.1. HRMS (MALDI-TOF), m/z: calcd for C130H174Br2N4O8, 2080.6026, found, 2082.7521. Elemental Analysis: found, C, 78.62%, H, 9.60%, N, 2.75%. Synthesis of PFPDI-2T. In a 25 mL flask, FPDI-Br (160 mg, 0.077 mmol), 4 (38 mg, 0.077 mmol), Pd2(dba)3 (3.2 mg, 0.0035 mmol), and P(o-tol)3 (6.4 mg, 0.021 mmol) were added. The flask was then refilled with nitrogen, and toluene (6 mL) was added using a syringe under nitrogen. The reaction mixture was evacuated and refilled with nitrogen for three times and then was heated to 110 °C for 72 h. For end-capping, 2-(tributylstannyl)thiophene (0.2 mL) was added into the mixture and stirred for 12 h; 2-bromothiophene (0.5 mL) was then added and stirred for 12 h. After cooling to the room temperature, the mixture was poured into methanol. The precipitation was collected and dissolved into cholorbenzene. The chlorobenzene solution was concentrated and poured into acetone. The precipitation was collected and dissolved into cholorbenzene again, and the solution was concentrated and poured into hexane. The collected solid was then dried under vacuum to give PFPDI-2T (125 mg, 78%). 1H NMR (400 MHz, CDCl3): δ (ppm) = 9.23 (14H, br), 4.33 (8H, br), 2.08 (4H, br), 1.28−0.84 (152H, br). Elemental Analysis: found, C, 77.95%, H,

compared to that of PTB7-Th/PFPDI-2T device could be the factor for its slight low Jsc and FF. To further explore the difference in the photovoltaic performance between PFPDI-2T and PDPI-FT based devices, the atomic force microscopy (AFM) images of both PTB7-Th/ PFPDI-2T and PTB7-Th/PFPDI-2FT blend films were measured. As shown in Figure 6, no significant phase separation for both blend films was observed, and both films showed the uniform and smooth surfaces, indicating that the PFPDI-2T and PFPDI-2FT has good miscibility with PTB7-Th. The room-mean-square (RMS) roughness of PTB7-Th/PFPDI-2T and PTB7-Th/PFPDI-2FT blend films are 1.52 and 1.13 nm, respectively. It is known that too large or small phase separation in the blend film are not favorable for the charge separation/ transport and will increase the possibility of charge recombination, as a result leading to lower Jsc and FF values. In this case, the lower RMS in PTB7-Th/PFPDI-2FT blend film may be a partial factor for its lower FF in comparison with that of PTB7-Th/PFPDI-2T device.

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CONCLUSION

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EXPERIMENTAL SECTION

We have synthesized two polymeric electron acceptors PFPDI2T and PFPDI-2FT through the fused PDI unit and bithiophene or difluorobithiophene. PFPDI-2T and PFPDI2FT showed the strong absorption in the region of 350−650 nm with the optical band gaps of 1.70 and 1.79 eV, respectively. The LUMO energy levels of PFPDI-2T and PFPDI-2FT were found to be −4.12 and −4.15 eV, respectively, which are comparable with other PDI-based polymers. The all-PSCs with PTBT-Th as the electron donor at the device structure of ITO/ PEDOT:PSS/PTB7-Th:PFPDI-2T/Ca/Al showed a PCE of as high as 6.39% with Voc of 0.73 V, Jsc of 13.47 mA/cm2, and FF of 0.65, which is much higher than its nonfused PDI copolymer with bithiophene. Both all-PSCs exhibited the broad photoresponses between 300 and 800 nm with the EQE value of more than 60% at most wavelength region. The SCLC mobilities and the film morphologies of both devices were also investigated. This results indicated that the direct fusing PDI unit to form the large coplanar aromatic system is an effective design strategy to improve the photovoltaic performances of all-PSCs that based on PDI polymeric electron acceptors, and this method can also be applied in other diimide derivatives in aiming to achieve the promising photovoltaic performance.

Materials. Compounds 1, 2, 4, and 5 were synthesized according to the literature method.37,48−50 PTB7-Th (Mw: 93 000; polydispersity index: 2.5) was purchased from 1-Materials Inc. All other chemicals were purchased from the commercial source and used without further purification unless otherwise indicated. Characterization. 1H NMR spectra were recorded on a Bruker AV400 NMR spectrometer and used tetramethylsilane (TMS) as an internal standard in CDCl3. The molecular weight was determined by GPC using THF as the eluent and monodispersed polystyrene as the standard. The UV−vis spectra of polymers were measured on with a TU-1601 spectrophotometer by using a 1 cm glass cuvette. Cyclic voltammetry (CV) was performed in 0.1 M tetrabutylammonium hexafluorophosphate in acetonitrile at a scan rate of 100 mV/s with ITO as the working electrode, Pt wire as the counter electrode, and Ag/Ag+ as the reference electrode. Atomic force microscopy (AFM) images were obtained using a NanoMan VS microscope in the tapping mode. The thickness of the active layer of the device was measured via a VeecoDektak 150 surface profiler. F

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Macromolecules 8.62%, N, 2.60%, S, 2.93%. Mn = 70 800, Mw = 145 200, polydispersity index = 2.05. Synthesis of PFPDI-2FT. The PFPDI-2FT was synthesized by following the above method in the yield of 77%. 1H NMR (400 MHz, CDCl3): δ (ppm) = 9.24 (12H, br), 4.32 (8H, br), 2.06 (4H, br), 1.25−0.82 (152H, br). Elemental Analysis: found, C, 78.20%, H, 9.01%, N, 2.48%, S, 2.63%. Mn = 85 600, Mw = 160 100, polydispersity index = 1.87.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b01539. Tables S1−S3 and Figures S1−S7 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

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

Yong Zhang: 0000-0002-9587-4039 Erjun Zhou: 0000-0003-1182-311X Author Contributions

M.L. and J.Y. contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by National Natural Science Foundation of China (21644006, 51403044, 51473040, 51673048, and 21602040), the National Natural Science Foundation of Beijing (2162045), and the Chinese Academy of Sciences (QYZDB-SSW-SLH033). Y. Zhang thanks the support from the Fundamental Research Funds for the Central Universities (Harbin Institute of Technology).

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REFERENCES

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DOI: 10.1021/acs.macromol.7b01539 Macromolecules XXXX, XXX, XXX−XXX