Bisperylene Bisimide Based Conjugated Polymer as Electron ...

Chinese Journal of Polymer Science Vol. 35, No. 2, (2017), 239−248

Chinese Journal of Polymer Science © Chinese Chemical Society Institute of Chemistry, CAS Springer-Verlag Berlin Heidelberg 2017

Bisperylene Bisimide Based Conjugated Polymer as Electron Acceptor for Polymer-Polymer Solar Cells* Fan Yang, Cheng Li**, Gui-tao Feng, Xu-dong Jiang, An-dong Zhang and Wei-wei Li** Beijing National Laboratory for Molecular Sciences, Chinese Academy of Sciences Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China Abstract Perylene bisimide (PBI) unit has been widely used to design conjugated materials, which can be used as electron acceptor in organic solar cells due to its strong electron-deficient ability. In this work, a conjugated polymer based on PBI dimer as monomer was designed, synthesized, and compared to the conjugated polymer containing single PBI as repeating units. The two conjugated polymers were found to have similar molecular weight, absorption spectra and energy levels. Density functional theory calculation revealed that the PBI dimer-based polymer exhibited highly twisted conjugated backbone due to the large dihedral angle between the two PBI units. The PBI-based polymers as electron acceptor were applied into polymer-polymer solar cells, in which PBI dimer-based polymer solar cells were found to show a high short circuit current density (Jsc = 11.2 mA⋅cm−2) and a high power conversion efficiency (PCE) of 4.5%. In comparison, the solar cells based on PBI-based polymer acceptor only provided a Jsc of 7.2 mA⋅cm−2 and PCE of 2.5%. The significantly enhanced PCE in PBI dimer-based solar cells was attributed to the mixed phase in blended thin films, as revealed by atom force microscopy. This study demonstrates that PBI dimer can be used to design polymer acceptors for high performance polymerpolymer solar cells. Keywords Polymer solar cells; Conjugated polymers; Electron acceptor; Perylene bisimide

INTRODUCTION Since firstly being reported in 1995[1−4], conjugated polymers as electron acceptors for application in polymer solar cells (PSCs) have been intensively studied in the past two decades. Although buckminsterfullerene derivatives are the most popular acceptors in PSCs[5], modification of the chemical structures of fullerene materials is not easy to provide wide absorption spectra, tunable energy levels and better charge transport properties. The drawback can be overcome in non-fullerene conjugated polymers as electron acceptors, so that polymer-polymer solar cells have the great potential to realize high power conversion efficiencies (PCEs) in PSCs, even exceeding the performance of fullerene-based solar cells[6−11]. Currently, polymer-polymer solar cells have achieved PCEs above 8%[12], which is still lag behind fullerene-based PSCs with PCEs close to 12%[13]. Developing new polymer acceptors and deep understanding between the chemical structures and device performance are still required to further enhance the device performance.

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This work was financially supported by the Recruitment Program of Global Youth Experts of China, the National Natural Science Foundation of China (Nos. 51603209 and 21574138) and the Strategic Priority Research Program (No. XDB12030200) of the Chinese Academy of Sciences. ** Corresponding authors: Cheng Li (李诚), E-mail: [email protected] Wei-wei Li (李韦伟), E-mail: [email protected] Invited paper for special issue of “Opto-electromic Functional Polymer” Received August 18, 2016; Revised September 7, 2016; Accepted September 12, 2016 doi: 10.1007/s10118-017-1870-4

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Perylene bisimide (PBI)-based conjugated small molecules and polymers have shown their promising application in organic electronics, such as field-effect transistors (FETs)[14−17] and PSCs[4, 18−28]. Electrondeficient PBI compounds always desire deep frontier orbital levels, such as with lowest unoccupied molecular orbital (LUMO) levels below −4.0 eV. Combining with high crystalline properties, PBI compounds have shown high electron mobility in FET devices. This also indicates that PBI-based materials can be potentially used as electron transporting materials for application in non-fullerene solar cells. Recently, PBI-based small molecules as electron acceptor have performed high PCEs above 9% in PSCs[29], while PBI-based polymer acceptors show PCEs below 7%[27]. Notably, most of the PBI-based small molecules for high performance solar cells were designed as twisted structures, such as PBI dimer[18−20]. Single PBI unit has strong tendency to form crystal structures with large domain in bulk-heterojunction thin films, which will significantly prevent exciton diffusion into the interface of donor and acceptor[30]. In order to improve the micro-phase separation in photo-active layers, twisted PBI compounds with reduced crystal property have been developed as electron acceptor, which also represent the successful strategies toward high performance solar cells[31−34]. Therefore, it will be interesting to explore the conjugated polymer acceptors containing PBI dimer for polymer-polymer solar cells. In this work, we intend to explore PBI-based polymers as electron acceptor for application in PSCs. The new polymer acceptor, PSdiPBI2T containing PBI dimer (SdiPBI) alternating with bithiophene (2T), was designed and synthesized (Fig. 1b). For comparison, we also synthesized polymer acceptor PPBI2T with single PBI as repeating unit (Fig. 1a). The two polymers were found to have similar molecular weight, absorption spectra and frontier orbital levels. Density functional theory (DFT) calculation reveals that PSdiPBI2T has highly twisted conjugated backbone due to the large dihedral angle of PBI dimer. The two polymers as electron acceptor were further applied into solar cells, in which a conjugated polymer PTB7-Th (Fig. 1c) was used as electron donor. We found that PTB7-Th:PSdiPBI2T cells showed a significantly high photocurrent and PCEs of 4.5% compared to that of PTB7-Th:PPBI2T cells (2.5%). Further investigation of the nano-morphology in blended thin films by atom force microscopy (AFM) reveals that PSdiPBI2T-based thin films have better mixed morphology with small domain size, which is helpful for charge separation. Our results demonstrate that PBI dimer-based conjugated polymers as electron acceptor can be potentially used for high performance solar cells.

Fig. 1 Chemical structures of perylene bisimide-based polymer acceptors (a, b) and PTB7-Th as electron donor (c)

EXPERIMENTAL All syntheses were performed under argon atmosphere. Commercial chemicals were used as received. THF and toluene were distilled from sodium under an N2 atmosphere. The monomers Br-PBI[35] and Br-SdiPBI[36] were synthesized according to literature procedures. 5,5'-Bis(trimethylstannyl)-2,2'-bithiophene was purchased from SunaTech Inc. The polymer donor PTB7-Th was purchased from Solarmer Materials Inc. Molecular weight was determined with gel permeation chromatography (GPC) at 140 °C on a PL-GPC 220 system using a PL-GEL 10 μm MIXED-B column, with o-DCB as the eluent and polystyrene as the standard. Low concentration of

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0.1 mg⋅mL−1 polymer in o-DCB was used to reduce aggregation. Thermogravimetric analysis (TGA) measurement was performed on a Perkin-Elmer TGA-7 apparatus. Optical absorption spectra were recorded on a JASCO V-570 spectrometer with a slit width of 2.0 nm and a scan rate of 1000 nm·min−1. Cyclic voltammetry (CV) was performed under an inert atmosphere at a scan rate of 0.1 V⋅s−1 using tetrabutylammonium hexafluorophosphate in acetonitrile (1 mol·L−1) as the electrolyte, a glassy-carbon working electrode coated with samples, a platinum-wire auxiliary electrode, and an Ag/AgCl as a reference electrode. Atomic force microscopy (AFM) images were recorded using a Digital Instruments Nanoscope IIIa multimode atomic force microscope in tapping mode under ambient conditions. Photovoltaic devices with an inverted configuration were made by spin coating a ZnO sol-gel[37] at 4000 r⋅min−1 for 60 s onto pre-cleaned, patterned ITO substrates. The photoactive layer was deposited by spin coating a chlorobenzene (CB) solution containing PTB7-Th and PBI-polymer and the appropriate amount of processing additive such as DIO or 1-CN in air. MoO3 (10 nm) and Ag (100 nm) were deposited by vacuum evaporation at ca. 4 × 10−5 Pa as the back electrode. The active area of the cells was 0.04 cm2. The J-V characteristics were measured by a Keithley 2400 source meter unit under AM1.5G spectrum from a solar simulator (Enlitech model SS-F5-3A). Solar simulator illumination intensity was determined at 100 mW⋅cm−2 using a monocrystal silicon reference cell with KG5 filter. Short circuit currents (Jsc) under AM1.5G conditions were estimated from the spectral response and convolution with the solar spectrum. The external quantum efficiency (EQE) was measured by a Solar Cell Spectral Response Measurement System QE-R3011 (Enli Technology Co., Ltd.). The thickness of the active layers in the photovoltaic devices was measured on a Veeco Dektak XT profilometer. PPBI2T To a degassed solution of the monomer Br-PBI (103.26 mg, 0.121 mmol) and 5,5'-bis(trimethylstannyl)-2,2'bithiophene (59.28 mg, 0.121 mmol) in toluene (2 mL) and DMF (0.2 mL), tris(dibenzylideneacetone)dipalladium(0) (3.31 mg, 3.6 µmol) and triphenylphosphine (3.79 mg, 14.5 µmol) were added. The mixture was stirred at 115 °C for 24 h, after which the reaction mixture was precipitated in methanol and filtered through a Soxhlet thimble. The polymer was extracted with acetone, hexane and then dissolved in chloroform (60 mL) at 90 °C, which was then precipitated into acetone. The polymer was collected by filtering over a 0.45 µm PTFE membrane and dried in a vacuum oven to yield PPBI2T (103 mg, 99%) as a dark solid. GPC (o-DCB, 140 °C): Mn = 29.1 kg⋅mol−1, PDI = 2.97. PSdiPBI2T To a degassed solution of Br-SdiPBI (75.34 mg, 0.048 mmol) and 5,5'-bis(trimethylstannyl)-2,2'-bithiophene (23.85 mg, 0.048 mmol) in toluene (2 mL) and DMF (0.2 mL), tris(dibenzylidene-acetone)dipalladium(0) (1.33 mg, 1.5 µmol) and triphenylphosphine (1.53 mg, 5.8 µmol) were added. The mixture was stirred at 115 °C for 24 h, after which the reaction mixture was precipitated in methanol and filtered through a Soxhlet thimble. The polymer was extracted with acetone, hexane and then dissolved in chloroform (60 mL) at 90 °C, which was then precipitated into acetone. The polymer was collected by filtering through a PTFE membrane (0.45 µm) and dried in a vacuum oven to yield PSdiPBI2T (63 mg, 83%) as a dark solid. GPC (o-DCB, 140 °C): Mn = 37.2 kg⋅mol−1, PDI = 2.91. RESULTS AND DISCUSSION Synthesis of the Conjugated Polymers The synthetic procedures for the PBI-based polymers are presented in Scheme 1. The key monomers Br-PBI and Br-SdiPBI were prepared according to the literatures, in which the brominated isomers were always detected. The PBI-based polymers were synthesized via Stille polymerization, by using a standard procedure containing Pd2(dba)3/PPh3 as catalyst system and toluene/DFM as reaction solvent. After purification by Soxhlet exaction, PPBI2T and PSdiPBI2T were obtained with high yield of 99% and 83%. The two polymers were of good solubility in polar solvents such as THF, chloroform and aromatic solvent, ensuring their good processability for

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device fabrication. The molecular weight of the polymers was determined by GPC with o-DCB as eluent at 140 °C, as summarized in Table 1. The polymer PPBI2T showed a number-average molecular weight (Mn) of 29.1 kg⋅mol−1 with polydispersity (PDI) of 2.97. In comparison, PSdiPBI2T exhibited relatively high Mn of 37.2 kg⋅mol−1 with a PDI of 2.91. The similar molecular weight of the two polymers was beneficial for comparing their photovoltaic performance. The stability of the polymers was tested by TGA, showing that both the polymers had good stability with a weight loss below 5% at 350 °C (Fig. 2).

Scheme 1 Synthetic procedures via Stille polymerization by using Pd2(dba)3/PPh3 in toluene/DMF (10/1, V/V) at 115 °C Table 1 Molecular weight, optical, and electrochemical properties of the polymer acceptors Polymer

Mn a (kg·mol−1)

Mw a (kg·mol−1)

PDI

EgCHCl3 (eV)

Egfilm (eV)

EHOMO b (eV)

ELUMO c (eV)

PPBI2T 29.1 86.6 2.97 1.68 1.65 −5.76 −4.11 PSdiPBI2T 37.2 108.2 2.91 1.68 1.70 −5.77 −4.07 a Determined with GPC at 140 °C using o-DCB as the eluent; b Determined using a work function value of −4.8 eV for Fc/Fc+; c Determined as EHOMO + Egfilm

Absorption Spectra and Energy Levels Absorption spectra of the PBI-based polymers in chloroform solution and thin films are present in Fig. 3(a) and the data are summarized at Table 1. Interestingly, PPBI2T and PSdiPBI2T exhibited similar absorption spectra in solution, although their chemical structures were different. Three absorption regions, 300–400 nm, 400–550 nm and 550–700 nm, were found as the typical absorptions of PBI-polymers. The absorption spectra of the two polymers in thin films were slightly blue-shifted, indicating their small tendency to form H-aggregation in thin films. In summary, PPBI2T showed an optical band gap (Eg) of 1.65 eV in thin films, while PSdiPBI2T demonstrated slightly high Eg of 1.70 eV. The frontier energy levels of the two polymers were determined by CV measurement, as shown in Fig. 3(b)


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and summarized in Table 1. PPBI2T had highest occupied molecular orbital (HOMO) level of −5.76 eV and LUMO level of −4.11 eV, while PSdiPBI2T had similar HOMO and LUMO levels of −5.77 and −4.07 eV. Both polymers showed LUMO level below −4.0 eV, which was close to that of fullerene derivatives, indicating that they might be used as universal electron acceptor for solar cells.

Fig. 2 TGA plots of the PBI polymers with a heating rate of 10 K/min under N2 atmosphere Temperatures with 5% weight loss for PPBI2T and PSdiPBI2T are 386 and 364 °C.

Fig. 3 (a) Absorption spectra (solid symbol: in CHCl3 solution; open symbol: in thin films) and (b) cyclic voltammograms of the polymer acceptors (potential versus Fc/Fc+)

Molecular Geometries of the Polymers It is interesting to study the molecular configuration in these PBI-based polymers. We used DFT calculations at B3LYP/6-31G level to analyze the oligomers based on the two polymers. In the DFT calculations, methyl units were used to replace branched alkyl chains in order to reduce the calculation time. Oligomer based on PPBI2T was found to have a large dihedral angle of 53.3° between PBI and 2T units (Fig. 4), which was slightly reduced to 48.7° in PSdiPBI2T-based oligomer. In addition, we found a dihedral angle of 57.1° in PBI dimer, which could significantly induce the highly twisted conjugated backbone compared to PPBI2T. The frontier orbital levels of the oligomer were also calculated, as shown in Fig. 4. HOMO level of the oligomer was distributed along the conjugated backbone in the two polymers, while the LUMO level was mainly located at PBI units. This indicated that the LUMO levels of conjugated polymers were mainly determined by the PBI units.

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Fig. 4 DFT calculations of the PBI-based polymer segments and their frontier molecular orbitals (The dihedral angle is also included. 57.1° is the dihedral angle between two PBI units. 53.3° and 48.7° are the dihedral angle between PBI and thiophene unit.)

Solar Cells Performance The two polymers as electron acceptor were then applied into polymer solar cells with PTB7-Th as electron donor. Inverted devices configuration with ITO/ZnO was used in the solar cells as electron transporting and collecting electrode and MoO3/Ag as hole transporting and collecting electrode. The photo-active layers containing PTB7-Th:PBI-based polymer were carefully optimized, including the processing solution with additive and the thickness of active layers. The device performance of the cells is summarized in Table 2 and the optimal J-V characteristic is shown in Fig. 5(a). PTB7-Th:PPBI2T cells showed a best PCE of 2.5% with Jsc of 7.2 mA⋅cm−2, open-circuit voltage (Voc) of 0.85 V and a small fill factor (FF) of 0.42. The PCE was greatly enhanced to 4.5% for PTB7-Th:PSdiPBI2T cells, combining with enhancing Jsc of 11.2 mA⋅cm−2, a slightly reduced Voc of 0.75 V and high FF of 0.54. The different Voc in the two cells may be originated from the different morphology in blended thin films, although similar LUMO levels were observed for PPBI2T and PSdiPBI2T. In addition, CV measurement may also cause the derivation when determining the HOMO and LUMO levels of the polymers, resulting in inconsistent Voc observed in solar cells. The enhanced photocurrent can also be reflected by the EQE of the cells (Fig. 5b). PPBI2T-based cells performed photo-response from 300 nm to 800 nm with EQE below 0.40, while EQE was


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greatly increased to 0.57 for PSdiPBI2T-based cells. The charge transport properties of the cells were studied by space charge limited current (SCLC) measurement, as shown in Fig. 6 and Table 3. Both of cells perform hole (μh) and electron (μe) mobilities around 10−4 cm2⋅V−1⋅s−1 with the ratio of μh/μe close to 1. This indicated that both of the cells had similar good charge transport into the electrodes. Therefore, we believe that the different Jsc in these cells may be originated from the exciton diffusion and separation process. Table 2 Solar cell parameters of optimized solar cells of PTB7-Th:PBI-based acceptors Voc Jsc b PCE Active layer a Solvent FF (mA·cm−2) (V) (%) PTB7-Th:PPBI2T CB 5.6 0.88 0.34 1.7 CB/DIO (1%) 5.7 0.88 0.35 1.7 CB/DIO (2.5%) 4.3 0.89 0.31 1.2 CB/1-CN (3%) 7.2 (6.7) c 0.85 (0.84) c 0.42 (0.41) c 2.5 (2.3) c PTB7-Th:PSdiPBI2T CB 11.2 (10.9) d 0.75 (0.74) d 0.54 (0.53) d 4.5 (4.3) d CB/DIO (1%) 10.0 0.74 0.50 3.7 CB/DIO (2.5%) 8.9 0.75 0.46 3.1 CB/1-CN (3%) 9.1 0.73 0.50 3.3 a Ratio of donor to acceptor is 1:1; b J was calculated by integrating the EQE spectrum with the AM1.5G spectrum. The sc thickness of active layers is 60 nm for PTB7-Th:PPBI2T cells, 70–80 nm for PTB7-Th:PSdiPBI2T cells; c Average data was calculated from seven devices in the parentheses; d Average data was calculated from 14 devices in the parentheses.

Fig. 5 (a) J-V characteristics in the dark (open symbol) and under white light illumination (solid symbol); (b) EQE of the optimized solar cells of PTB7-Th:PBI-based acceptors PPBI2T-based thin films were fabricated from CB/1-CN (3%) with the thickness of 60 nm. PSdiPBI2T-based thin films were fabricated from CB with the thickness of 70 nm.

Fig. 6 J-V characteristics under dark for (a) hole-only devices and (b) electron-only devices

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Table 3 Hole and electron mobilities of optimized solar cells of PTB7-Th:PBI-based acceptors from SCLC measurements

Active layer

Solvent

PTB7-Th:PPBI2T PTB7-Th:PSdiPBI2T

CB/1-CN (3%) CB

μh

(cm2⋅V−1⋅s−1) 1.33 × 10−4 1.41 × 10−4

μe

(cm2⋅V−1⋅s−1) 1.83 × 10−4 1.60 × 10−4

μh/μe 0.73 0.88

Morphology Investigation We further analyzed the morphology of the blended thin films using AFM, as shown in Fig. 7. Micro-phase separation with large domain size (> 100 nm) was observed in PTB7-Th:PPBI2T systems. It has been reported that in organic solar cells the short lifetime of photo-generated exciton made the diffusion length within 10 nm, so that the large domain of polymers in the thin films would greatly reduce the exciton diffusion efficiency and hence result in low photocurrent[38]. The large domain in PTB7-Th:PSdiPBI2T blended thin films disappeared, and the roughness of the surface was also reduced to 2.09 nm compared to 5.16 nm in PTB7-Th:PPBI2T thin films, indicating better mixed phase between PTB7-Th and PSdiPBI2T. This was also further confirmed by AFM phase images, as shown in Figs. 7(c) and 7(d). Large domain phase was found in PTB7-Th:PPBI2T thin films, while small domain with fibrillar structure was observed in PTB7-Th:PSdiPBI2T film. We speculate that, PSdiPBI2T with highly twisted backbone had poor crystalline tendency so as to improve the miscibility with PTB7-Th. The blended thin films would improve the exciton diffusion efficiency, explaining the better charge generation and high Jsc in PTB7-Th:PSdiPBI2T cells. In addition, it is noteworthy that the PSdiPBI2T-based cells owned higher FF compared to the PPBI2T-based cells, which might be ascribed to the different morphology. But the detailed reason was required to be further investigated.

Fig. 7 AFM height (a, b) and phase (c, d) images (3 μm × 3 μm) of the photo-active layers: (a, c) PTB7Th:PPBI2T; (b, d) PTB7-Th:PSdiPBI2T The fabrication condition is referred to Table 2; the root mean square (RMS) roughness is also included.

CONCLUSIONS In conclusion, two conjugated polymers containing single and binary PBI units were designed for application in polymer solar cells. These two polymers exhibit similar absorption spectra and energy levels, while the PBI dimer-based conjugated polymers are found to have highly twisted conjugated backbone. Solar cells based on these two polymers as electron acceptor show that, PSdiPBI2T with PBI dimer can provide better photocurrent


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and PCEs of 4.5%, while PPBI2T with single PBI as repeating units only performs a low PCE of 2.5%. The improved PCEs in the PSdiPBI2T-based cells is found to relate to the better mixed blended thin films between donor and acceptors, which can help exciton diffuse into the interface of donor and acceptor. We believe that the PCEs based on PSdiPBI2T as acceptor can be further improved, such as by combining with other high efficient conjugated polymers as donor or by optimizing the interface layers in solar cells. This work demonstrates that the designation by using binary PBI units in the conjugated backbone is an efficient route to explore polymer acceptors toward high performance polymer-polymer solar cells. ACKNOWLEDGMENTS We thank Mr. Ralf Bovee at Eindhoven University of Technology for GPC analysis.

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