Polymer Electron Acceptors with Conjugated Side Chains for

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Polymer Electron Acceptors with Conjugated Side Chains for Improved Photovoltaic Performance Ruyan Zhao,†,§ Zhaozhao Bi,‡ Chuandong Dou,*,† Wei Ma,*,‡ Yanchun Han,† Jun Liu,*,† and Lixiang Wang† †

State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China ‡ State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an 710049, P. R. China § University of Science and Technology of China, Hefei 230026, P. R. China S Supporting Information *

ABSTRACT: The development of polymer electron acceptors lags far behind that of polymer electron donors. A general approach to improve photovoltaic performance of polymer electron donors is to incorporate conjugated side chains to the electron-rich unit. In this article, we introduce the “conjugated side chain” strategy to molecular design of polymer electron acceptors by incorporating conjugated side chains to the electron-deficient unit. The polymer backbones consist of alternating electron-deficient double B←N bridged bipyridine (BNBP) unit and electron-rich thiophene or selenophene unit. Polymer electron acceptors are developed by incorporating conjugated alkoxyphenyl side chains to the BNBP unit. Compared with conventional alkyl side chains, the conjugated alkoxyphenyl side chains endow the polymer electron acceptors with low-lying LUMO energy levels, enhanced π−π stacking, and high electron mobilities, which are very desirable for electron acceptors. The resulting all-PSCs exhibit an enhanced power conversion efficiency (PCE) of 4.46% with a small photon energy loss (Eloss) of 0.51 eV or a PCE of 3.77% with an extremely small Eloss of 0.47 eV. This Eloss is among the smallest values reported for organic solar cells. These results demonstrate that the “conjugated side chain” strategy can be used not only for high-efficiency polymer electron donors but also for high-performance polymer electron acceptors.

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INTRODUCTION Conjugated polymers have received tremendous attention because of their tunable optoelectronic properties and applications in organic optoelectronic devices.1−5 In the past two decades, great progress have been made in the development of polymer electron donors for polymer solar cells (PSCs). High-performance polymer electron donors are always D−A type conjugated polymers, in which electron-rich unit (D) and electron-deficient unit (A) are alternatively linked.6−10 As reported by Li et al. and Hou et al., a widely used molecular design strategy to enhance the PSC device performance is to use conjugated side chains. Conjugated side chains are always attached to the D unit of D−A type polymer electron donors.11−14 They can enhance hole mobility, improve spectral coverage, and tune energy levels of polymer electron donors.15−20 Moreover, in all-polymer solar cells (all-PSCs), conjugated side chains can make polymer electron donors more isotropic and consequently facilitate charge separation and reduce bimolecular recombination.21,22 A well-known example is poly[[4,8-bis[2-(2-ethylhexyl)thienyl-5-yl]benzo[1,2-b:4,5b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexy)carbonyl]thieno[3,4-b]-thiophenediyl]] (PTB7-Th or PCE10) and © XXXX American Chemical Society

poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexy)carbonyl]thieno[3,4-b]-thiophenediyl]] (PTB7). PTB7-Th with conjugated alkylthienyl side chains always exhibits better PSC device performance than that of PTB7 with the same main chain and alkyl side chains.15,23,24 Compared with the intensively studied polymer electron donors, the development of polymer electron acceptors is indeed in infancy. Most of polymer electron acceptors are D−A type conjugated polymers with the strong electron-deficient naphthalene diimide (NDI) or perylene diimide (PDI) unit as the A unit.25−38 Owing to the synthesis difficulty to functionalize NDI or PDI unit, no polymer electron acceptors with conjugated side chains attached to the A unit have been reported. To circumvent this problem, scientists have attempted to develop polymer electron acceptors with conjugated side chains attached to the D unit.12−14 However, all-PSC device performance of the resulting polymer electron Received: February 21, 2017 Revised: April 7, 2017

A

DOI: 10.1021/acs.macromol.7b00386 Macromolecules XXXX, XXX, XXX−XXX

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BNBPP-Se, in which conjugated alkoxyphenyl side chains are attached to the electron-deficient BNBP unit. The dihedral angle between the alkoxyphenyl group and the BNBP unit is 68° in the optimized structure (Figure S1 in the Supporting Information). This nonperpendicular configuration indicates effective conjugation between them. The corresponding polymers with conventional alkyl side chains, P-BNBP-T and P-BNBP-Se, are also shown for comparison. Scheme 2 shows the synthetic routes of P-BNBPP-T and P-BNBPP-Se. The starting material 1 was synthesized according to our previous method.42 Coupling of 1 and 1-((2-hexyldecyl)oxy)-4-iodobenzene with Pd2(dba)3 as catalyst afforded 2. Treatment of 2 with BF3·Et2O readily produced the monomer 3 as a red solid. 3 was very stable in air and could be purified by column chromatography on silica gel. P-BNBPP-T and P-BNBPP-Se were synthesized by Stille polymerization of the monomer 3 and bis(trimethyl)stannyl thiophene/selenophene monomers, respectively. The chemical structures of P-BNBPP-T and PBNBPP-Se were confirmed by 1H NMR and 13C NMR spectroscopy as well as elemental analysis. According to gel permeation chromatography (GPC) at 150 °C with 1,2,4trichlorobenzene as the eluent, the number-average molecular weight (Mn) and the polydispersity index (PDI) are 29.7 kDa and 2.35 for P-BNBPP-T and 107.2 kDa and 2.35 for PBNBPP-Se, respectively.

acceptors is not satisfactory. Therefore, it is important but challenging to develop high-performance polymer electron acceptors via attaching conjugated side chains to the A unit. We have recently reported polymer electron acceptors containing boron−nitrogen coordination bond (B←N).39−41 In particular, we have developed a new kind of electrondeficient building block, double B←N bridged bipyridine (BNBP), to design polymer electron acceptors with good allPSC device performance.42−44 In this article, we report polymer electron acceptors with conjugated side chains attached to the A unit by incorporating alkoxyphenyl side chains to the BNBP unit (Scheme 1). We find that the conjugated side chains Scheme 1. Chemical Structures of P-BNBPP-T, P-BNBPPSe, P-BNBP-T, and P-BNBP-Se

Table 1. Molecular Weights and Thermal Decomposition Temperatures (Td) of P-BNBPP-T, P-BNBPP-Se, P-BNBPT, and P-BNBP-Se44 endow the resulting polymer acceptors with low-lying LUMO energy levels, enhanced π−π stacking, and high electron mobilities, which are very desirable for electron acceptors. The resulting all-PSCs exhibit an enhanced power conversion efficiency (PCE) of 4.46% at a small photon energy loss (Eloss) of 0.51 eV or a PCE of 3.77% at an extremely small Eloss of 0.47 eV. To the best of our knowledge, this value is among the smallest Eloss reported for organic solar cells.45−50 These results demonstrate that the “conjugated side chain” strategy can be used not only for high-efficiency polymer electron donors but also for high-performance polymer electron acceptors.

polymer

Mna (kDa)

PDIa

Tdb (°C)

P-BNBP-T P-BNBPP-T P-BNBP-Se P-BNBPP-Se

46.0 29.7 26.3 107.2

2.01 2.35 1.93 2.35

357 393 400 402

Determined by GPC at 150 °C in 1,2,4-trichlorobenzene. bEstimated by TGA under a N2 atmosphere.

a

P-BNBPP-T and P-BNBPP-Se both show good solubility in common organic solvents, such as chloroform, toluene, and chlorobenzene. According to thermogravimetric analysis (TGA) (Figure S2), P-BNBPP-T and P-BNBPP-Se both show good thermal stability with thermal decomposition temperatures (Td) at 5% weight loss of 393 and 402 °C, respectively.

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RESULTS AND DISCUSSION Synthesis and Characterization. Scheme 1 shows the chemical structures of the two polymers, P-BNBPP-T and PScheme 2. Synthesis of P-BNBPP-T and P-BNBPP-Sea

Reagents and conditions: (a) 1-((2-hexyldecyl)oxy)-4-iodobenzene, Pd2(dba)3, dppf, t-BuONa, toluene, 120 °C; (b) BF3·Et2O, Et3N, CH2Cl2, 50 °C; (c) 2,5-bis(trimethylstannyl)thiophene or 2,5-bis(trimethylstannyl)selenophene, Pd2(dba)3·CHCl3, P(o-Tolyl)3, toluene, 120 °C, then bromobenzene. a

B

DOI: 10.1021/acs.macromol.7b00386 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Photophysical and Electrochemical Properties. Figure 1 shows the UV/vis absorption spectra of P-BNBPP-T and P-

Figure 1. UV/vis absorption spectra of P-BNBPP-T and P-BNBPP-Se in CHCl3 solutions and in thin films.

BNBPP-Se in dilute chloroform solutions and in thin films. In solutions, P-BNBPP-T and P-BNBPP-Se show the maximum absorption peaks (λmax) at 581 and 587 nm, respectively. In thin films, the absorption peaks of P-BNBPP-T and P-BNBPP-Se are red-shifted to 603 and 632 nm, respectively, because of the interactions of conjugated polymers in solid states. According to the onset absorption wavelength in thin films, the optical band gaps (Eg) of P-BNBPP-T and P-BNBPP-Se are estimated to be 1.97 and 1.87 eV, respectively. As listed in Table 2, the polymers with alkoxyphenyl side chains exhibit slightly blueshifted absorption spectra compared to those of the polymers with alkyl side chains (Figure S3). To estimate the LUMO/HOMO energy levels (ELUMO/ EHOMO) of P-BNBPP-T and P-BNBPP-Se, cyclic voltammetry (CV) measurements were carried out using the thin films. The cyclic voltammograms are shown in Figure 2a. The reduction and oxidation onset potentials (Eonsetred/Eonsetox) are −1.08 V/ +0.94 V for P-BNBPP-T and −1.07 V/+0.95 V for P-BNBPPSe, respectively. Accordingly, the ELUMO/EHOMO of P-BNBPP-T and P-BNBPP-Se are calculated to be −3.72 eV/−5.74 eV and −3.73 eV/−5.75 eV, respectively. Figure 2b displays the ELUMO and EHOMO alignments of a traditional donor, PTB7-Th, and the four polymer acceptors based on BNBP unit. The ELUMO of P-BNBPP-T and P-BNBPP-Se with conjugated alkoxyphenyl side chains are obviously lower than that of P-BNBP-T and PBNBP-Se with alkyl side chains (Figure S4).44 The downshift of ELUMO is ascribed to the weaker electron donating capability of alkoxyphenylamine than that of alkylamine because of the conjugation between the extra phenyl group and the nitrogen atom in alkoxyphenylamine. The lower-lying ELUMO of P-

Figure 2. (a) Cyclic voltammograms of P-BNBPP-T and P-BNBPPSe; Fc = ferrocene. (b) LUMO/HOMO energy level alignments of PBNBPP-T, P-BNBPP-Se, P-BNBP-T, P-BNBP-Se, and PTB7-Th.

BNBPP-T and P-BNBPP-Se is very desirable for effective charge separation between polymer donor and polymer acceptor in the active layer. Molecular Stacking and Electron Transporting Properties. Figure 3 shows the grazing incident X-ray diffraction (GI-XRD) patterns of the drop-cast films of the four polymers based on the BNBP unit. P-BNBPP-T and P-BNBPP-Se show strong diffraction peaks at 24.5° and 24.1°, respectively, corresponding to the π−π stacking distances (dπ−π) of polymer backbones of 3.63 and 3.69 Å, respectively. In comparison, PBNBP-T and P-BNBP-Se exhibit diffraction peaks at 23.3° and 23.6°, respectively, corresponding to the dπ−π of 3.81 and 3.77 Å, respectively. The polymers with conjugated side chains display smaller dπ−π than those with alkyl side chains, indicating that the conjugated side chains can promote π−π stacking of polymers. While alkyl side chains act as steric hindrance and prevent close π−π stacking, the alkoxyphenyl side chains alleviate the steric hindrance effect and consequently improve π−π stacking of conjugated polymer.22 As can be seen later, the decreased dπ−π leads to enhanced electron mobilities of the resulting polymers. On the other hand, in contrast to the sharp

Table 2. Photophysical and Electrochemical Properties, π−π Stacking Distances, and Electron Mobilities of the Four Polymers polymer

λmaxa (nm)

P-BNBP-Te P-BNBPP-T P-BNBP-See P-BNBPP-Se

593 581 600 587

εmaxa (M−1 cm−1) 1.53 8.90 1.49 8.80

× × × ×

105 104 105 104

εmaxb (cm−1)

λmaxb (nm)

Egopt b (eV)

Eonsetox c (V)

Eonsetred c (V)

EHOMOd (eV)

ELUMOd (eV)

dπ−π (Å)

× × × ×

622 603 635 632

1.92 1.97 1.87 1.87

+0.97 +0.94 +1.04 +0.95

−1.30 −1.08 −1.14 −1.07

−5.77 −5.74 −5.84 −5.75

−3.50 −3.72 −3.66 −3.73

3.81 3.63 3.77 3.69

1.20 9.43 1.83 9.61

105 104 105 104

μe (cm2 V−1 s−1) 7.16 3.24 2.07 4.60

× × × ×

10−5 10−4 10−4 10−4

a Measured in CHCl3 solution. bMeasured in thin film. cOnset potential vs Fc/Fc+. dCalculated using the equation of EHOMO/LUMO = −(4.80 + Eonsetox/Eonsetred) eV. eThe data of this reference polymer are from ref 44.

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Figure 3. Grazing-incidence X-ray diffraction patterns of P-BNBPP-T, P-BNBPP-Se, and the reference polymers of P-BNBP-T and P-BNBPSe.

lamellar stacking diffraction peaks at about 4.5° of P-BNBP-T and P-BNBP-Se, no lamellar stacking diffraction peaks are observed in the GI-XRD patterns of P-BNBPP-T and PBNBPP-Se. These results indicate that the alkoxyphenyl side chains disturb the lamellar stacking of conjugated backbones, and P-BNBPP-T and P-BNBPP-Se are amorphous in the solid states. For polymer donor/polymer acceptor blend in all-PSCs, crystallization is an important driving force for phase separation. The amorphous feather of P-BNBPP-T and PBNBPP-Se with alkoxyphenyl side chains is expected to affect the active layer morphology of all-PSCs. The electron mobilities (μe) of the four polymers were estimated using space-charge-limited current (SCLC) method with the current density−voltage (J−V) curves of the electrononly devices (Figures S6 and S7). As listed in Table 2, the electron mobility of P-BNBPP-T (μe = 3.24 × 10−4 cm2 V−1 s−1) is higher than that of P-BNBP-T (μe = 7.16 × 10−5 cm2 V−1 s−1) by about 5 times, and the electron mobility of PBNBPP-Se (μe = 4.60 × 10−4 cm2 V−1 s−1) is higher than that of P-BNBP-Se (μe = 2.07 × 10−4 cm2 V−1 s−1) by about 2 times. These results indicate that the conjugated alkoxyphenyl side chains on the electron-deficient BNBP unit improve the electron mobilities, which is very important for efficient polymer electron acceptors. As reported in the literature, for polymer electron donors, conjugated side chains attached to electron-rich unit always lead to increased hole mobilities due to the improved intermolecular π-orbital overlap.16 Here, we also attribute the increased electron mobilities of P-BNBPP-T and P-BNBPP-Se to the improved intermolecular π-orbital overlap with π-conjugated side chains. Moreover, the decreased π−π stacking distances of polymers with conjugated side chains also contribute to the improved electron mobility. All-PSC Devices. To investigate the photovoltaic properties of P-BNBPP-T and P-BNBPP-Se as polymer electron acceptors, we selected a widely used polymer electron donor, PTB7Th, to blend with them to fabricate all-PSCs. The device configuration was ITO/PEDOT:PSS/PTB7-Th:P-BNBPP-T or P-BNBPP-Se/Ca/Al. The active layer was spin-coated from the blend of the polymer donor and polymer acceptor in chloroform solution with (w) or without (w/o) 0.5 vol % 1chloronaphthalene (CN) as solvent additive. Figure 4 shows the current density−voltage (J−V) curves under AM 1.5G illumination (100 mW cm−2) and the external quantum

Figure 4. (a) J−V curves and (b) EQE spectra of all-PSCs devices based on the PTB7-Th:P-BNBPP-T and PTB7-Th:P-BNBPP-Se active layers spin-coated with or without 0.5 vol % CN.

efficiency (EQE) spectra of the devices. The device characteristics are summarized in Table 3. The P-BNBPP-Se-based device processed with 0.5 vol % CN solvent additive exhibits an open-circuit voltage (Voc) of 1.07 V, a short-circuit current (Jsc) of 9.21 mA cm−2, and a fill factor (FF) of 0.45, corresponding to a PCE of 4.46%. The P-BNBPP-T-based device with solvent additive shows a Voc of 1.11 V, a Jsc of 7.58 mA cm−2, and a FF of 0.45, leading to a PCE of 3.77%. The device performances of P-BNBPP-T and P-BNBPP-Se with conjugated alkoxyphenyl side chains are somewhat better than those of the counterparts with conventional alkyl side chains (Table 3),42 suggesting that conjugated side chains lead to improved all-PSCs device performance of polymer electron acceptors. In addition, the molecular weights of the polymer electron acceptors may affect the device performance. For P-BNBPP-Se-based devices, the application of 0.5 vol % CN solvent additive leads to much improved Jsc and FF and results in PCE enhancement from 3.26% to 4.46%. Similarly, the solvent additive in P-BNBPP-Tbased devices leads to PCE enhancement from 2.58% to 3.77%. This is due to the optimized film morphology with the small amount of CN, which will be discussed later. The EQE spectra display a broad photoresponse from 300 to 780 nm. The maximum EQE value is 0.43 at 630 nm for the PTB7-Th:PBNBPP-Se blend and 0.37 at 620 nm for the PTB7-Th:PBNBPP-T blend. The Jsc values calculated from the integration of EQE spectra with AM1.5G spectrum are consistent with the Jsc values obtained from the J−V scans within 10% error. It is worthy to note the remarkably small photon energy losses (Eloss) of these all-PSCs. Eloss (Eloss = Eg − eVoc) is defined as the difference between the lowest optical bandgap Eg of donor/acceptor and the eVoc of photovoltaic device.46 Most of efficient PSCs have Eloss in the range of 0.7−1.0 eV, and few PSCs exhibit Eloss below 0.6 eV.47−49 Recently, a very small Eloss D

DOI: 10.1021/acs.macromol.7b00386 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Table 3. Summary of the All-PSC Device Performance active layer

CN additive

Voc (V)

Jsc (mA cm−2)

FF

PTB7-Th:P-BNBPP-T

w/o w w/o w w/o w/o

1.10 1.11 1.07 1.07 1.12 1.02

6.14 7.58 7.43 9.21 5.24 10.02

0.38 0.45 0.41 0.45 0.39 0.42

PTB7-Th:P-BNBPP-Se PTB7-Th:P-BNBP-T PTB7-Th:P-BNBP-Se a

PCEmax/avea (%) 2.58 3.77 3.26 4.46 2.27 4.26

(2.53) (3.72) (3.22) (4.40) (2.08) (4.11)

Eloss (eV)

thickness (nm)

0.48 0.47 0.51 0.51 0.47 0.55

78 83 80 85 80 80

The average PCE value is calculated from eight devices.

Figure 5. 2D-GIWAXS images of (a, b) PTB7-Th:P-BNBPP-T and (c, d) PTB7-Th:P-BNBPP-Se blend films spin-coated with or without 0.5 vol % CN. (e) In-plane and (f) out-of-plane cuts of the corresponding 2D-GIWAXS patterns.

Table 4. Crystallographic Parameters of the Active Layers with or without 0.5 vol % CN IPa (100) active layer

CN additive

d-spacing (nm)

CL (nm)

d-spacing (Å)

CL (nm)

domain size (nm)

domain purityd

PTB7-Th:P-BNBPP-T

w/o w w/o w

2.64 2.67 2.60 2.66

6.49 6.64 5.88 6.29

0.39 0.40 0.39 0.39

1.95 1.81 1.66 1.63

21 38 88 50

0.85 1.00 0.82 0.90

PTB7-Th:P-BNBPP-Se a

OOPb (010) c

IP: in-plane. bOOP: out-of-plane. cCL: coherence length. dRelative domain purity: compared after deducting the mass−thickness.

of 0.46 eV have been reported by Hou et al.50 The active layer was composed of polymer donor and nonfullerene molecular acceptor. Large Eloss is one of the main reasons for the relatively low PCE upper limit of organic solar cells. For the all-PSC devices based on the PTB7-Th:P-BNBPP-Se blend, the Eg of PTB7-Th is 1.58 eV and the Voc is 1.07 V, corresponding to the Eloss of 0.51 eV. For the PTB7-Th:P-BNBPP-T-based device, the Eloss is as small as 0.47 eV. To the best of our knowledge, this value is among the smallest Eloss reported for organic solar cells. The two polymers bearing conjugated side chains exhibit the PCE of 4.46% with the small Eloss of 0.51 eV or the PCE of 3.77% with the extremely small Eloss of 0.47 eV. The reason for the small Eloss is unknown at present. Charge Transporting Properties of Active Layers. The electron and hole mobilities of the blends are measured using SCLC method with the electron-only devices (ITO/PEIE/

active layer/Ca/Al) and the hole-only devices (ITO/ PEDOT:PSS/active layer/MoO3/Ag), respectively (Figure S9). The results are listed in Table S1. For the PTB7-Th:PBNBPP-Se film spin-coated from CHCl3 solution, the μe and hole mobility (μh) are 2.15 × 10−5 cm2 V−1 s−1 and 4.39 × 10−4 cm2 V−1 s−1, respectively. With the addition of 0.5 vol % CN solvent additive, the μe and μh are increased to 4.02 × 10−5 cm2 V−1 s−1 and 5.75 × 10−4 cm2 V−1 s−1, respectively. The increased and balanced electron and hole mobilities are in accordance with the enhanced Jsc and FF values of the devices with the solvent additive. A similar effect of solvent additive on charge carrier mobilities is also observed in the PTB7-Th:PBNBPP-T device. Blend Morphology Analysis. As discussed before, CN solvent additive leads to increased Jsc and FF, improved and balanced electron/hole mobilities of the resulting all-PSC E

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and a decreased average domain size from 88 to 50 nm. The increase of domain purity suppresses the bimolecular recombination and the decrease of domain size is beneficial for charge separation by enlarging the donor:acceptor interface. These results are consistent well with the increased FF and Jsc with CN solvent additive. The PTB7-Th:P-BNBPP-T blend film without CN exhibits “small and mixed” morphology, which could lead to efficient charge separation but high carrier recombination in the active layer due to the lack of bicontinuous phase. After adding CN additive into the CHCl3 solvent, the average domain size increases from 21 to 38 nm and the relative phase purity increases from 0.85 to 1.00. These changes contribute to the enhancements of Jsc and FF by balanced charge separation, transport and exciton diffusion, and improved device performance.

devices. This motivates us to investigate the effects of CN on the blend morphology using grazing incidence wide-angle X-ray scattering (GIWAXS) and resonant soft X-ray scattering (RSoXS).51−57 Figure 5a−d shows the GIWAXS results of the PTB7-Th:P-BNBPP-T and PTB7-Th:P-BNBPP-Se films spincoated from CHCl3 with or without CN. The line-cut profiles in the in-plane (IP) direction and in the out-of-plane (OOP) direction are compared in Figure 5e,f. Table 4 summarizes the characteristics. Comparison of the line profiles of the pure polymer donor/polymer acceptor film and the blend films suggests that the diffraction peaks of the blend films are predominantly assigned to PTB7-Th. Therefore, the two polymer acceptors are amorphous in the blend films, and they have few impacts on the molecular packing of PTB7-Th. The PTB7-Th:P-BNBPP-Se films show pronounced (010) π−π stacking peaks at 1.61 Å−1 along the out-of-plane direction but weak (010) peaks along the in-plane direction, indicating overall preferential face-on orientation of PTB7-Th. The faceon orientation of PTB7-Th is favorable for the hole transporting in the photovoltaic devices.58 As shown in Figure 5 and Table 4, there are few changes about the coherence length of (010) peak after introducing CN solvent additive. Therefore, the enhancement of device efficiency with CN solvent additive is not due to the improvement of molecular packing, crystallinity, or orientation of the polymer backbones. For the PTB7-Th:P-BNBPP-T blend films, similar molecular packing and similar effects of solvent additive are also observed. Resonant soft X-ray scattering (R-SoXS) was utilized to further study the effects of CN solvent additive on phaseseparated domains of blend films. X-rays with photon energy of 286.8 eV were used to enhance the contrast between PTB7-Th and P-BNBPP-T (or P-BNBPP-Se). Figure 6 shows the R-

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CONCLUSIONS In summary, we have developed two polymer electron acceptors, P-BNBPP-T and P-BNBPP-Se, by introducing conjugated alkoxyphenyl side groups into the electron-deficient building block of BNBP. Compared with the corresponding polymers with conventional alkyl side chains, P-BNBPP-T and P-BNBPP-Se show lower-lying LUMO energy levels, enhanced π−π stacking, higher electron mobilities, and improved all-PSC device performance. We also find that solvent additive can enhance the Jsc and FF of the all-PSCs by tuning the domain size and improving the domain purity of the active layers. The resulting all-PSCs exhibit a PCE of 4.46% with a small Eloss of 0.51 eV or a PCE of 3.77% with an extremely small Eloss of 0.47 eV. These results demonstrate that the “conjugated side chain” strategy can be used not only for high-efficiency polymer electron donors but also for high-performance polymer electron acceptors.

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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.7b00386. Synthesis and 1H and 13C NMR characterizations, TGA measurements, photophysical properties of polymers, allPSC device and SCLC device fabrications and measurements (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected]. *E-mail [email protected]. *E-mail [email protected]. ORCID

Jun Liu: 0000-0003-1487-0069 Figure 6. R-SoXS scattering profiles at 286.8 eV of the PTB7-Th:PBNBPP-T and PTB7-Th:P-BNBPP-Se active layers processed with or without 0.5 vol % CN.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for the financial support by the National Key Basic Research and Development Program of China (973 program, Grant No. 2014CB643504, 2015CB655001) Founded by MOST, the Nature Science Foundation of China (No. 51373165, 21625403, No. 21574129, No. 21404099), the Strategic Priority Research

SoXS scattering profiles of the PTB7-Th:P-BNBPP-T and PTB7-Th:P-BNBPP-Se films spin-coated from CHCl3 with or without CN. Table 4 summarizes the relative domain purity and the domain size calculated at 286.8 eV. For the PTB7Th:P-BNBPP-Se blend film, the application of CN solvent additive leads to an increased domain purity from 0.82 to 0.90 F

DOI: 10.1021/acs.macromol.7b00386 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

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Program of Chinese Academy of Sciences (No. XDB12010200), the Youth Innovation Promotion Association of the Chinese Academy of Sciences (No. 2017265), and the State Key Laboratory of Supramolecular Structure and Materials in Jilin University (No. sklssm201704).

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DOI: 10.1021/acs.macromol.7b00386 Macromolecules XXXX, XXX, XXX−XXX