Vinylene- and ethynylene-bridged perylene diimide dimers as nonfullerene acceptors for polymer solar cells

Dyes and Pigments 146 (2017) 143e150

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Vinylene- and ethynylene-bridged perylene diimide dimers as nonfullerene acceptors for polymer solar cells Sufei Xie a, 1, Jicheng Zhang a, 1, Liangliang Wu a, Jianqi Zhang b, Cuihong Li a, *, Xuebo Chen a, ***, Zhixiang Wei b, Zhishan Bo a, ** a b

Beijing Key Laboratory of Energy Conversion and Storage Materials, College of Chemistry, Beijing Normal University, Beijing 100875, China National Center for Nanoscience and Technology, Beijing 100190, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 9 May 2017 Received in revised form 7 June 2017 Accepted 20 June 2017 Available online 23 June 2017

Vinylene (V)- and ethynylene (E)-bridged perylene diimide dimers (PDI-V and PDI-E) were designed, synthesized and used as nonfullerene acceptors for polymer solar cells. Our researches revealed that the linkage between two PDI units has a great impact on the molecular geometry, the optical properties, the blend film morphology, the molecular packing orientation, and the photovoltaic performance. Computational calculations via density functional theory (DFT) showed that PDI-E and PDI-V possessed planar and twisted geometric structures, respectively. TEM investigations showed that PTB7-Th:PDI-V based blend film exhibited a uniform morphology with small domain size and PTB7-Th:PDI-E based one showed apparent phase separation with large domain size. GIWAXS results revealed that the PDI-V can influence PTB7-Th to take on a face-on orientation, which is beneficial for vertical charge transport to increase Jsc. A PCE of 4.51% with a Voc of 0.76 V, a Jsc of 10.03 mA cm
2, and an FF of 0.59 was obtained for PSCs based on PTB7-Th:PDI-V, which is almost two times higher than that of PTB7-Th:PDI-E based devices, which showed a PCE of 2.66%, a Voc of 0.66 V, a Jsc of 7.33 mA cm
2, and an FF of 0.55. These results help to gain deeper insight into the design of new nonfullerene small molecular acceptors for high efficiency PSCs. © 2017 Elsevier Ltd. All rights reserved.

Keywords: Perylene diimide dimers Linkage Nonfullerene acceptors Polymer solar cells

1. Introduction Bulk heterojunction polymer solar cells (BHJ PSCs) with the active layer composed of a blend of a semiconducting polymer donor and a fullerene derivative acceptor (PC61BM, PC71BM, etc.) have attracted considerable attention due to their solutionprocessability, low-cost and light-weight and flexibility [1e6]. Fullerene derivatives have been widely used as the dominant electron acceptors in BHJ PSCs due to their high electron mobility, high electron affinity and isotropic charge transport properties [7,8]. Power conversion efficiencies (PCEs) of fullerene-based BHJ PSCs have reached 10% in single-junction devices [9,10]. However, the intrinsic drawbacks of fullerene derivatives such as expensive, difficult chemical modification, weak absorption in the visible * Corresponding author. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (C. (X. Chen), [email protected] (Z. Bo). 1 S. X. and J. Z. contributed equally to this work. http://dx.doi.org/10.1016/j.dyepig.2017.06.049 0143-7208/© 2017 Elsevier Ltd. All rights reserved.

Li),

[email protected]

spectral region and limited energy level variability could not be ignored [11e13]. Thus, the newly emerging nonfullerene (NF) acceptors become more and more attractive due to their advantages such as tunable molecular energy levels, excellent optical absorption properties, easy purification and low cost production processes [14,15]. Recently, some outstanding nonfullerene acceptors, such as ITIC [15e21], naphthalene diimide (NDI) derivatives [22e26], perylene diimide (PDI) derivatives [27e45], etc. have been developed and PCEs over 12% have been achieved [46e48]. Perylene diimide (PDI) derivatives have captured particular attention because of their several appealing merits including excellent photochemical stability, broad optical absorption range, high electron mobility, and tunable electronic structures and properties [29,43]. However, due to their highly planar and rigid structure, most mono-PDI derivatives are prone to self-aggregate in the solid state to form large domains that are detrimental to exciton separation and give low efficiency [30,41]. Linking two PDI units at the bay position with an appropriate spacer such as phenylene, thiophene, spirobifluorene, etc. has been proved to be an effective way to obtain dimers with nonplanar conformations, which can

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suppress the strong p-p aggregation of PDI units and prevent them from forming large aggregates in the blend films [30e34]. To investigate the influence of linker between two PDI units, we designed and synthesized two PDI based acceptors, vinylene (V) and ethynylene (E) -bridged PDI dimers as presented in Fig. 1, and used them as electron acceptors for the fabrication of nonfullerene PSCs. Our studies have revealed that the linker has a great influence on the optical properties, molecular geometry, molecular packing, blend film morphology, transport properties of blend film, and photovoltaic properties of devices. PDI-V exhibits a slightly twisted conformation; whereas PDI-E takes on a planar conformation. PDIV both in solution and as film shows a featureless broad absorption with higher molar extinction efficiency; whereas PDI-E presents a broad absorption with fine structures and lower molar extinction efficiency. To investigate the photovoltaic performance, PTB7-Th (Poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b; 4,5-b'] dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno [3,4-b] thiophene-)-2-carboxylate-2-6-diyl)]) was chosed as donor. Due to its slightly twisted conformation, the PTB7-Th:PDI-V based blend films showed a smooth morphology without large phase separation; whereas for the planar PDI-E, its blend films with PTB7-Th showed an apparent phase separation with a large domain size. In addition, PTB7-Th:PDI-V based blend films also exhibit higher and more balanced charge carrier mobility. Finally, PTB7-Th:PDI-V based PSCs gave a PCE of 4.51% with a Voc of 0.76 V, a Jsc of 10.03 mA cm
2, and an FF of 0.59; whereas PTB7-Th:PDI-E based PSCs only gave a PCE of 2.66% with a Voc of 0.66 V, a Jsc of 7.33 mA cm
2, and an FF of 0.55. 2. Results and discussion 2.1. Material synthesis and characterization The synthetic routes of PDI-V and PDI-E are outlined in Scheme 1. Compound 1 was prepared according to the previous C8H17 C8H17 C6H13 O

N

C6H13 O

C8H17

C8H17 N

O

C6H13 O

O

N

O

C6H13 O

N

O

literature procedure [42]. Pd-catalyzed Stille coupling of 1 with trans-1,2-bis(tri-n-butylstannyl)ethylene and bis(trimethylstannyl) acetylene afforded PDI-V and PDI-E in yields of 77% and 70%, respectively. PDI-V and PDI-E are of good solubility in common organic solvents, such as dichloromethane, chloroform, o-dichlorobenzene (DCB), chlorobenzene (CB), etc. at room temperature. Thermograviemetric analysis (TGA) indicated that the two small acceptors have excellent thermal stability with the 5%-decomposition temperature up to 433  C for PDI-V, and 442  C for PDI-E (Fig. S1). In the range of 50e250  C, no obvious melting peak or glass transition was observed for PDI-V in differential scanning calorimetry (DSC) curves under N2 atmosphere at a heating rate of 10  C/min; while a clear melting peak at about 200  C and a crystallization peak at about 150  C were detected for PDI-E (Fig. S2). The above results indicated that PDI-V could form amorphous film; whereas PDI-E are prone to form crystalline one. 2.2. Optical properties UVevis absorption spectra of PDI-V, PDI-E and PTB7-Th in DCB solutions and as films are depicted in Fig. 2. Different from monoPDI derivatives, PDI-V in DCB solutions displayed a broad absorption ranging from 350 to 650 nm with three peaks at 448, 517 and 554 nm; whereas PDI-E in DCB solutions exhibited a well-resolved absorption spectrum in the same range with four peaks located at 450, 515, 546 and 590 nm. The molar extinction efficiency for the main absorption peak of each molecule is 5.46  104 M
1cm
1 for PDI-V and 5.08  104 M
1cm
1 for PDI-E. In going from solutions to films, the absorption spectrum of PDI-V became broader and the maximum absorption peak slightly red-shifted from 554 to 558 nm, indicating the formation of weak aggregation in the solid state. As film, PDI-E exhibited a broad and well resolved absorption spectrum with all peaks red-shifted in comparison with their solution ones. The red-shifted and enhanced 0-0 absorption peak in the film spectrum indicated that PDI-E formed stronger aggregation in the solid state due to its much planar molecular geometry (vide infra). The absorption spectra of PTB7-Th:PDI-V and PTB7-Th:PDI-E blend films are showed in Fig. S3, PDI-V and PDI-E have complement absorption spectra with that of PTB7-Th, which will be beneficial for more efficient use of the solar radiation. Determined from the onset absorption of films, band gaps of PDI-V and PDI-E were calculated to be 1.88 and 1.85 eV, respectively. The related data are also summarized in Table 1. 2.3. DFT calculations

O O

N

N

O C6H13 C8H17

O C6H13

O

N

O C6H13

C8H17

C8 H17

PDI-V

O S

O

N

O C6H13 C8H17

PDI-E

O F

S S S

S

n

S

PTB7-Th Fig. 1. Molecular structures of PDI-V, PDI-E and donor PTB7-Th.

To investigate the structural properties of two compounds, density functional theory (DFT) calculations at the B3LYP/6-31G(d) level were performed to evaluate the molecular geometries of PDIV and PDI-E. The alkyl chains were simplified as methyl groups to facilitate the calculation. The optimized molecular geometries of PDI-V and PDI-E are shown in Table 2. The dihedral angles between the vinylene unit and two PDI units are 18.1 and 15.5 , resulting in a twisted geometry for PDI-V. The slightly twisted geometry of PDIV might be attributed to the steric hindrance of the hydrogen atoms at the bay region of PDI and at the vinylene group. Unlike PDI-V, PDI-E displays a completely planar conformation in the optimized geometry. To better understand the absorption properties of PDI-V and PDI-E, the peaks were assigned by calculations at the CASPT2// CASSCF(10e/8o)/6-31G level and the results are summarized in Table 3. The schematic frontier molecular orbitals are shown in Fig. S4. The computational results showed good agreement with the experimental data. Transitions of HOMO/LUMO, HOMO
1/LUMO and HOMO
2/LUMOþ1 of PDI-V and PDI-E

S. Xie et al. / Dyes and Pigments 146 (2017) 143e150

145

C8H17 C8H17 C6H13 O

N

C8 H17 C6H13 O

O

Br Bu3Sn

O

N

1

O C6H13

O

C8 H17

N

N

1

O

O

N

O C6H13

O

N

O C6H13 C8H17

O

Me3Sn

SnMe3

C8H17

PDI-V C8H17

C8H17

Br

O

N

(i)

SnBu3

C8H17 C6H13 O

N

C6H13 O

C6H13 O

N

O

O

N

O C6H13

C6H13 O

N

O

O

N

O C6H13

(i)

O C6H13 C8H17

C8H17

C8H17

PDI-E

Scheme 1. Synthetic routes of PDI-E and PDI-V. Reagents and conditions: (i) Pd(PPh3)4, toluene/DMF (5:1 by volume), reflux.

Fig. 2. UVevis absorption spectra of PDI-V, PDI-E and PTB7-Th in DCB solutions (a) and as films (b).

Table 1 Optical and electrochemical properties of PDI-V, PDI-E and PTB7-Th. Materials

labs [nm] solution

labs [nm] film

lonset [nm] film

Eg,opta [eV]

HOMOb [eV]

LUMOb [eV]

PDI-V PDI-E PTB7-Th [42]

448, 517, 554 450, 515, 546, 590 696

454, 558 452, 572, 620 705

658 670 777

1.88 1.85 1.58


5.88
5.90
5.22


3.83
3.90
3.64

a b

Calculated using the equation Eg,opt ¼ 1240/lonset. Measured by cyclic voltammetry.

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S. Xie et al. / Dyes and Pigments 146 (2017) 143e150

Table 2 Molecular structures of PDI-V and PDI-E and their corresponding molecular conformations from top view and side view by the computational modeling.

give rise to the major absorption peaks. PDI-V has one main absorption peak (HOMO/LUMO); whereas PDI-E has two main absorption peaks (HOMO/LUMO and HOMO
1/LUMO). The transition from HOMO to LUMOþ1 has a large oscillator strength (f) for PDI-E, but that has a negligible strength (<10
3) for PDI-V. This large oscillator strength (f) for PDI-E is mainly due to its planar structure with extended p conjugation. The twisted structure of PDI-V makes the transition from HOMO to LUMOþ1 more difficult. Although PDI-E has two main absorption peaks (HOMO/LUMO and HOMO
1/LUMO), the increased oscillator strength of HOMO
1/LUMO transition is at the expense of the oscillator strength of its dominating HOMO/LUMO transition. Therefore, the light absorbing efficiency hl of PDI-E is still lower than that of PDI-V as evidenced by our experimental data, which may be responsible for the lower Jsc in OPV devices [49]. Table 3 Assignments of optical absorption bands of PDI-V and PDI-E calculated at the CASPT2//CASSCF(10e/8o)/6-31G level of theory. The experimental values labs (nm) are given for comparison. Transitions

The electrochemical behaviors of PDI-V and PDI-E were investigated by cyclic voltammetry (CV), and the CV curves are shown in Fig. 3. The HOMO and LUMO energy levels of PDI-V and PDI-E were calculated from the cyclic voltammetric (CV) measurement results. The onsets of the oxidation and reduction potentials for PDI-V and PDI-E were determined against Fc/Fcþ as the internal standard. Assuming the absolute energy level of Fc/Fcþ to be 4.8 eV below vacuum, the HOMO levels were calculated to be
5.88 eV for PDI-V and
5.90 eV for PDI-E according to the equation: EHOMO ¼ -e [Eoxþ4.80-E(Fc/Fcþ)]. The ELUMO of PDI-V and PDI-E were calculated to be
3.83 eV and
3.90 eV based on the equation: ELUMO ¼ -e [Ereþ4.80-E(Fc/Fcþ)] [50]. The LUMO offsets between acceptor and PTB7-Th are 0.19 eV and 0.26 eV for PDI-V and PDI-E, respectively. Recent studies have revealed that in nonfullerene PSCs a LUMO offset of 0.18 eV is still sufficient for efficient charge separation [51]. Therefore, the low energy offset for PDI-V is beneficial for reducing the energy loss and achieving higher Voc in devices.

DEcal (eV) lcal (nm) labs(nm) solution f

PDI-V HOMO/LUMO 2.116 HOMO
1/LUMO 2.489 HOMO
2/LUMOþ1 2.814

586 498 441

554 517 448

2.574 0.994 0.181

PDI-E

594 538 481 440

590 546 515 450

2.009 0.219 2.304 0.154

HOMO/LUMO HOMO/LUMOþ1 HOMO
1/LUMO HOMO
2/LUMOþ1

2.4. Electrochemical properties

2.087 2.306 2.578 2.816

2.5. Photovoltaic properties To investigate the photovoltaic performance of PDI-V and PDI-E as nonfullerene acceptors, PTB7-Th was chosen as the donor [52] and inverted devices with a structure of ITO/ZnO/active layer/ MoO3/Al were fabricated. After optimization (Table S1), PDI-V and PDI-E based devices exhibited the best photovoltaic performance

S. Xie et al. / Dyes and Pigments 146 (2017) 143e150

147

Fig. 3. (a) Cyclic voltammetry curves of PDI-V and PDI-E; (b) Energy diagram of PDI-V, PDI-E and PTB7-Th.

with a donoreacceptor weight ratio of 1:3 and a thickness of about 80 nm. The current density-voltage (J-V) curves of optimized PSCs are shown in Fig. 4a, and the corresponding photovoltaic characteristics are summarized in Table 4. The optimized devices based on the blends of PTB7-Th:PDI-V exhibited a PCE of 4.51% with a Voc of 0.76 V, a Jsc of 10.03 mA cm
2 and an FF of 0.59. However, for devices based on the blends of PTB7-Th:PDI-E, a maximum PCE of 2.66% with a Voc of 0.66 V, a Jsc of 7.33 mA cm
2, and an FF of 0.55 was achieved. Obviously, PDI-V and PDI-E as the acceptors exhibited a dramatically different device performance. PDI-V based solar cells possess higher Voc, Jsc and FF. Higher Voc can be attributed to higher LUMO energy level and lower energy loss for PDI-V than that of PDI-E. The higher Jsc for PDI-V based devices is probably due to its broad absorption and higher molar extinction efficiency. The lower Jsc of PDI-E based devices is related to the low molar extinction efficiency and the obvious absorption valley around 500 nm for PDI-E (vide supra). The lower Jsc and FF of PDI-E based devices are also related to the poor film morphology. External quantum efficiency (EQE) experiments were also performed to verify the accuracy of the Jsc values obtained from the J-V measurement. The results indicated that Jsc values calculated from the integration of EQE curves agreed well with that obtained from J-V measurement. 2.6. Transport properties To shed light on the charge transport properties, hole-only devices with a structure of ITO/PEDOT:PSS/active layer/Au and electron-only devices with a configuration of FTO/active layer/Al were fabricated. The hole mobility (mh) and electron mobility (me) of the devices were estimated from dark J-V curves by the space charge limited current (SCLC) method. As shown in Fig. 5 and Table 4, PTB7-Th:PDI-V and PTB7-Th:PDI-E based plain devices

exhibited me of 5.39  10
5 and 7.43  10
6 cm2 V
1s
1, respectively, whereas the mh of PTB7-Th:PDI-V and PTB7-Th:PDI-E based plain devices were 5.99  10
5 and 3.86  10
5 cm2 V
1s
1, respectively. Higher me and more balanced charge transport properties might contribute to higher Jsc and FF for PTB7-Th:PDI-V than that of PTB7-Th:PDI-E. 2.7. Grazing-incidence wide-angle X-ray scattering (GIWAXS) and transmission electron microscopy (TEM) Grazing-incidence wide-angle X-ray scattering (GIWAXS) was used to investigate the molecular packing of blend films. The outof-plane packing appears nominally along the qz axis and the inplane one along the qxy. As shown in Fig. 6 and Fig. S5, stronger peak (100) can be observed, indicating PDI-E has higher crystallinity in PTB7-Th:PDI-E blend films. For the PTB7-Th:PDI-V blend films, the out of plane (qz) diffraction peak (010) at about 1.51 Å
1, corresponding to a pep distance of 4.16 Å for PTB7-Th [40]. Obviously, the peak (010) of PTB7-Th:PDI-V film is more intense and pronounced in the qz direction in contrast to that of PTB7Th:PDI-E film, indicating PTB7-Th is more likely to form a faceon orientation in the PTB7-Th:PDI-V blend film. Additionally, the pep distance of PTB7-Th in the blend film is slightly larger than that in the neat polymer film (4.02 Å, Fig. S5), indicating that PDI-V can influence the packing of polymer chains. The face-on orientation of polymer chains in the blend films is beneficial for vertical charge transport to achieve higher Jsc, coinciding well with the results from SCLC and J-V measurement. Transmission electron microscopy (TEM) also provides useful information about the morphology of blend films. As shown in Fig. 7, the PTB7-Th:PDI-V blend films exhibited a uniform and smooth morphology, whereas large domains can be observed in the PTB7-Th:PDI-E blend films. The planar molecular conformation of PDI-E may be responsible for

Fig. 4. JeV curves (a) and EQE (b) for the optimized devices based on the blends of PTB7-Th:PDI-V and PTB7-Th:PDI-E.

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Table 4 Summary of the photovoltaic and transport properties of PTB7-Th:PDI-V and PTB7-Th:PDI-E based devices. Acceptor

Voc (V)

Jsc (mA cm
2)

FF

PCE (%)best/avea

D (nm)

me (cm2 V
1s
1)

mh (cm2 V
1s
1)

PDI-V PDI-E

0.76 0.66

10.03 7.33

0.59 0.55

4.51/4.32 2.66/2.34

85 83

5.39  10
5 7.43  10
6

5.99  10
5 3.86  10
5

a

The average PCE were achieved from 10 devices.

Fig. 5. J1/2eV curves of PTB7-Th:PDI-V and PTB7-Th:PDI-E based devices for me (a) and mh (b) by SCLC.

the large phase separation in the PTB7-Th:PDI-E blend films. 3. Conclusions

Fig. 6. (a) GIWAXS 2D scattering patterns and (b) line profiles of PTB7-Th:PDI-V and PTB7-Th:PDI-E based blend films.

Recently, nonfullerene small molecules have emerged as very promising electron acceptors in organic solar cells, and PCEs of over 12% have been recorded [46e48]. It is of great importance to further improve the efficiency of nonfullerene organic solar cells via fine molecular design strategies. Therefore, the further study of their structure-performance relationship is highly meaningful. Here, we have designed and synthesized vinylene (V) and ethynylene-linked (E) perylene diimide dimers (PDI-V and PDI-E), and applied them as nonfullerene acceptors for polymer solar cells. As films, these two acceptors exhibited a broad absorption ranging from 350 to 650 nm. PDI-V exhibits a slightly twisted conformation, which can prevent the formation of large aggregates in the blend film; whereas PDI-E exhibited a planar conformation, resulting in large aggregates in the blend film. In addition, PDI-V has broader absorption and larger molar extinction efficiency than PDI-E. The blend film of PTB7-Th:PDI-V exhibited higher and more balanced charge carrier mobility than that of PTB7-Th:PDI-E. PTB7-Th:PDIV based PSCs gave a PCE of 4.51%; whereas PTB7-Th:PDI-E based ones only furnished a PCE of 2.66%. Our results have demonstrated

Fig. 7. TEM images of the blend films for PTB7-Th:PDI-V (a) and PTB7-Th:PDI-E (b) (1:3 by weight).

S. Xie et al. / Dyes and Pigments 146 (2017) 143e150

that the linkage between two PDI units played a crucial role to the properties of acceptors. It provides deeper insights into the further design of new nonfullerene small molecular acceptors for high efficiency PSCs. 4. Experimental section The material synthesis, characterization and the devices fabrication details are exhibited in supporting information. The structures of PDI-V and PDI-E were characterized by 1H, 13C NMR spectroscopy and elemental analyses. Acknowledgements S. X. and J. Z. contributed equally to this work. Financial support from the NSF of China (91233205, 21574013 and 51003006), the Beijing Natural Science Foundation (2132042), the Program for Changjiang Scholars and Innovative Research Team in University and the Fundamental Research Funds for the Central Universities is gratefully acknowledged. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.dyepig.2017.06.049. References [1] Sariciftci NS, Smilowitz L, Heeger AJ, Wudl F. Photoinduced electron-transfer from a conducting polymer to buckminsterfullerene. Science 1992;258(5087):1474e6. [2] Heeger AJ. 25th anniversary article: bulk heterojunction solar cells: understanding the mechanism of operation. Adv Mater 2014;26(1):10e28. [3] Brabec CJ, Gowrisanker S, Halls JJM, Laird D, Jia S, Williams SP. Polymerfullerene bulk-heterojunction solar cells. Adv Mater 2010;22(34):3839e56. [4] Cheng Y-J, Yang S-H, Hsu C-S. Synthesis of conjugated polymers for organic solar cell applications. Chem Rev 2009;109(11):5868e923. [5] Wei H, Lu H, Fang T, Bo Z. Evaluating the photovoltaic properties of two conjugated polymers synthesized by Suzuki polycondensation and direct C-H activation. Sci China Chem 2015;58(2):286e93. [6] Lu Z, Li C, Du C, Gong X, Bo Z. 6,7-dialkoxy-2,3-diphenylquinoxaline based conjugated polymers for solar cells with high open-circuit voltage. Chin J Polym Sci 2013;31(6):901e11. [7] Tumbleston JR, Collins BA, Yang L, Stuart AC, Gann E, Ma W, et al. The influence of molecular orientation on organic bulk heterojunction solar cells. Nat Photonics 2014;8(5):385e91. [8] Zhang S, Ye L, Zhao W, Yang B, Wang Q, Hou J. Realizing over 10% efficiency in polymer solar cell by device optimization. Sci China Chem 2015;58(2): 248e56. [9] Wu Z, Sun C, Dong S, Jiang X-F, Wu S, Wu H, et al. n-Type water/alcoholsoluble naphthalene diimide-based conjugated polymers for highperformance polymer solar cells. J Am Chem Soc 2016;138(6):2004e13. [10] Vohra V, Kawashima K, Kakara T, Koganezawa T, Osaka I, Takimiya K, et al. Efficient inverted polymer solar cells employing favourable molecular orientation. Nat Photonics 2015;9(6):403e8. [11] Eftaiha A, Sun J-P, Hill IG, Welch GC. Recent advances of non-fullerene, small molecular acceptors for solution processed bulk heterojunction solar cells. J Mater Chem A 2014;2(5):1201e13. [12] Zhao J, Li Y, Lin H, Liu Y, Jiang K, Mu C, et al. High-efficiency non-fullerene organic solar cells enabled by a difluorobenzothiadiazole-based donor polymer combined with a properly matched small molecule acceptor. Energy Environ Sci 2015;8(2):520e5. [13] Hwang Y-J, Li H, Courtright BAE, Subramaniyan S, Jenekhe SA. Nonfullerene polymer solar cells with 8.5% efficiency enabled by a new highly twisted electron acceptor dimer. Adv Mater 2016;28(1):124e31. [14] Cnops K, Rand BP, Cheyns D, Verreet B, Empl MA, Heremans P. 8.4% efficient fullerene-free organic solar cells exploiting long-range exciton energy transfer. Nat Commun 2014;5:3406. [15] Kang H, Kim G, Kim J, Kwon S, Kim H, Lee K. Bulk-heterojunction organic solar cells: five core technologies for their commercialization. Adv Mater 2016;28(36):7821e61. [16] Bin H, Zhong L, Zhang Z-G, Gao L, Yang Y, Xue L, et al. Alkoxy substituted benzodithiophene-alt-fluorobenzotriazole copolymer as donor in nonfullerene polymer solar cells. Sci China Chem 2016;59(10):1317e22. [17] Li Z, Jiang K, Yang G, Lai JYL, Ma T, Zhao J, et al. Donor polymer design enables efficient non-fullerene organic solar cells. Nat Commun 2016;7:13094.

149

[18] Yang Y, Zhang Z-G, Bin H, Chen S, Gao L, Xue L, et al. Side-Chain isomerization on n-type organic semiconductor ITIC acceptor make 11.77% high efficiency polymer solar cells. J Am Chem Soc 2016;138(45):15011e8. [19] Gao L, Zhang Z-G, Bin H, Xue L, Yang Y, Wang C, et al. High-efficiency nonfullerene polymer solar cells with medium bandgap polymer donor and narrow bandgap organic semiconductor acceptor. Adv Mater 2016;28(37): 8288e95. [20] Lin Y, Wang J, Zhang Z-G, Bai H, Li Y, Zhu D, et al. An electron acceptor challenging fullerenes for efficient polymer solar cells. Adv Mater 2015;27(7): 1170e4. [21] Yuan J, Qiu L, Zhang Z-G, Li Y, Chen Y, Zou Y. Tetrafluoroquinoxaline based polymers for non-fullerene polymer solar cells with efficiency over 9%. Nano Energy 2016;30:312e20. [22] Hou R, Feng S, Gong X, Liu Y, Zhang J, Li C, et al. Side chain effect of nonfullerene acceptors on the photovoltaic performance of wide band gap polymer solar cells. Synth Met 2016;220:578e84. [23] Zhang J, Zhang X, Xiao H, Li G, Liu Y, Li C, et al. 1,8-Naphthalimide-Based planar small molecular acceptor for organic solar cells. Acs Appl Mater Inter 2016;8(8):5475e83. [24] Zhang J, Xiao H, Zhang X, Wu Y, Li G, Li C, et al. 1,8-Naphthalimide-based nonfullerene acceptors for wide optical band gap polymer solar cells with an ultrathin active layer thickness of 35 nm. J Mater Chem C 2016;4(24): 5656e63. [25] Zhang J, Zhang X, Li G, Xiao H, Li W, Xie S, et al. A nonfullerene acceptor for wide band gap polymer based organic solar cells. Chem Commun 2016;52(3): 469e72. [26] Liu Y, Zhang L, Lee H, Wang H-W, Santala A, Liu F, et al. NDI-based small molecule as promising nonfullerene acceptor for solution-processed organic photovoltaics. Adv Energy Mater 2015;5(12):1500195. [27] Wu Q, Zhao D, Schneider AM, Chen W, Yu L. Covalently bound clusters of alpha-substituted PDI-Rival electron acceptors to fullerene for organic solar cells. J Am Chem Soc 2016;138(23):7248e51. [28] Meng D, Sun D, Zhong C, Liu T, Fan B, Huo L, et al. High-performance solutionprocessed non-fullerene organic solar cells based on selenophene-containing perylene bisimide acceptor. J Am Chem Soc 2016;138(1):375e80. [29] Zhong Y, Trinh MT, Chen R, Wang W, Khlyabich PP, Kumar B, et al. Efficient organic solar cells with helical perylene diimide electron acceptors. J Am Chem Soc 2014;136(43):15215e21. [30] Jiang W, Ye L, Li X, Xiao C, Tan F, Zhao W, et al. Bay-linked perylene bisimides as promising non-fullerene acceptors for organic solar cells. Chem Commun 2014;50(8):1024e6. [31] Yan Q, Zhou Y, Zheng Y-Q, Pei J, Zhao D. Towards rational design of organic electron acceptors for photovoltaics: a study based on perylenediimide derivatives. Chem Sci 2013;4(12):4389e94. [32] Yu Y, Yang F, Ji Y, Wu Y, Zhang A, Li C, et al. A perylene bisimide derivative with a LUMO level of-4.56 eV for non-fullerene solar cells. J Mater Chem C 2016;4(19):4134e7. [33] Sun D, Meng D, Cai Y, Fan B, Li Y, Jiang W, et al. Non-fullerene-acceptor-based bulk-heterojunction organic solar cells with efficiency over 7%. J Am Chem Soc 2015;137(34):11156e62. [34] Hartnett PE, Matte HSSR, Eastham ND, Jackson NE, Wu Y, Chen LX, et al. Ringfusion as a perylenediimide dimer design concept for high-performance nonfullerene organic photovoltaic acceptors. Chem Sci 2016;7(6):3543e55. [35] Chen Y, Zhang X, Zhan C, Yao J. In-depth understanding of photocurrent enhancement in solution-processed small-molecule:perylene diimide nonfullerene organic solar cells. Phys Status Solidi A 2015;212(9):1961e8. [36] Hartnett PE, Margulies EA, Matte HSSR, Hersam MC, Marks TJ, Wasielewski MR. Effects of crystalline perylenediimide acceptor morphology on optoelectronic properties and device performance. Chem Mater 2016;28(11):3928e36. [37] Singh R, Aluicio-Sarduy E, Kan Z, Ye T, MacKenzie RCI, Keivanidis PE. Fullerene-free organic solar cells with an efficiency of 3.7% based on a lowcost geometrically planar perylene diimide monomer. J Mater Chem A 2014;2(35):14348e53. [38] Zhong Y, Trinh MT, Chen R, Wang W, Khlyabich PP, Kumar B, et al. Efficient organic solar cells with helical perylene diimide electron acceptors. J Am Chem Soc 2014;136(43):15215e21. [39] Adhikari T, Rahami ZG, Nunzi J-M, Lebel O. Synthesis, characterization and photovoltaic performance of novel glass-forming perylenediimide derivatives. Org Electron 2016;34:146e56. [40] Yang L, Chen Y, Chen S, Dong T, Deng W, Lv L, et al. Achieving high performance non-fullerene organic solar cells through tuning the numbers of electron deficient building blocks of molecular acceptors. J Power Sources 2016;324:538e46. [41] Zhao D, Wu Q, Cai Z, Zheng T, Chen W, Lu J, et al. Electron acceptors based on alpha-substituted perylene diimide (PDI) for organic solar cells. Chem Mater 2016;28(4):1139e46. [42] Hadmojo WT, Nam SY, Shin TJ, Yoon SC, Jang S-Y, Jung IH. Geometrically controlled organic small molecule acceptors for efficient fullerene-free organic photovoltaic devices. J Mater Chem A 2016;4(31):12308e18. [43] Wu C-H, Chueh C-C, Xi Y-Y, Zhong H-L, Gao G-P, Wang Z-H, et al. Influence of molecular geometry of perylene diimide dimers and polymers on bulk heterojunction morphology toward high-performance nonfullerene polymer solar cells. Adv Funct Mater 2015;25(33):5326e32. [44] Sun J-P, Hendsbee AD, Dobson AJ, Welch GC, Hill IG. Perylene diimide based

150

[45]

[46]

[47]

[48]

S. Xie et al. / Dyes and Pigments 146 (2017) 143e150 all small-molecule organic solar cells: impact of branched-alkyl side chains on solubility, photophysics, self-assembly, and photovoltaic parameters. Org Electron 2016;35:151e7. Guo Y, Li Y, Awartani O, Zhao J, Han H, Ade H, et al. A vinylene-bridged perylenediimide-based polymeric acceptor enabling efficient all-polymer solar cells processed under ambient conditions. Adv Mater 2016;28(38): 8483e9. Li S, Ye L, Zhao W, Zhang S, Mukherjee S, Ade H, et al. Energy-level modulation of small-molecule electron acceptors to achieve over 12% efficiency in polymer solar cells. Adv Mater 2016;28(42):9423e9. Zhao F, Dai S, Wu Y, Zhang Q, Wang J, Jiang L, et al. Single-junction binaryblend nonfullerene polymer solar cells with 12.1% efficiency. Adv Mater 2017;29(18):1700144. Zhao W, Li S, Yao H, Zhang S, Zhang Y, Yang B, et al. Molecular optimization

[49]

[50]

[51]

[52]

enables over 13% efficiency in organic solar cells. J Am Chem Soc 2017;139(21):7148e51. Cui Y, Li P, Song C, Zhang H. Terminal modulation of d-pi-a small molecule for organic photovoltaic materials: a theoretical molecular design. J Phys Chem C 2016;120(51):28939e50. Lee TH, Choi MH, Jeon SJ, Moon DK. Correlation of intermolecular packing distance and crystallinity of D-A polymers according to pi-spacer for polymer solar cells. Polymer 2016;99:756e66. Li Y, Liu X, Wu F-P, Zhou Y, Jiang Z-Q, Song B, et al. Non-fullerene acceptor with low energy loss and high external quantum efficiency: towards high performance polymer solar cells. J Mater Chem A 2016;4(16):5890e7. Liao S-H, Jhuo H-J, Cheng Y-S, Chen S-A. Fullerene derivative-doped zinc oxide nanofilm as the cathode of inverted polymer solar cells with low-bandgap polymer (PTB7-Th) for high performance. Adv Mater 2013;25(34):4766e71.