Synthesis and photovoltaic properties of silafluorene copolymers substituted by carbazole and triphenylamine pendants

Dyes and Pigments 149 (2018) 133e140

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Synthesis and photovoltaic properties of silafluorene copolymers substituted by carbazole and triphenylamine pendants Shaoshan Lian a, 1, Wenhao Zheng b, 1, Guangrong Jin a, Biao Xiao c, Zhe Liu b, Xinyuan Li a, Yajuan Pan a, Jinchang Huang a, Lintao Hou b, *, Yueqi Mo a, **, Hongbin Wu c, *** a

College of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, China Siyuan Laboratory, Guangzhou Key Laboratory of Vacuum Coating Technologies and New Energy Materials, Guangdong Provincial Key Laboratory of Optical Fiber Sensing and Communications, Department of Physics, Jinan University, Guangzhou 510632, China c Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou, 510640, China b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 15 April 2017 Received in revised form 23 June 2017 Accepted 7 July 2017 Available online 8 July 2017

In this paper carbazole and triphenylamine substituted 2,7-silafluorene-based copolymers with 5,6bis(octyloxy)-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole (DBT), namely Cz-PSF-DBT and TPA-PSFDBT, respectively, were synthesized and their photovoltaic device performance was investigated with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as the acceptor. These two polymers show nearly identical absorption spectra in the long wavelength range, which can be ascribed to the identical backbones consisting of silafluorene and DBT moieties. Cz-PSF-DBT shows a higher open circuit voltage of 1.0 V due to its higher oxidation onset coming from the s conjugation between the silica atoms at 9position and carbazole substituents compared to TPA-PSF-DBT of 0.93 V. However, the power conversion efficiency of TPA-PSF-DBT is much higher than that of Cz-PSF-DBT. The AFM images show the finer morphology of TPA-PSF-DBT:PC71BM film than that of Cz-PSF-DBT:PC71BM film with a rough surface and a large phase separation, which greatly influence the exciton separation at the donor/acceptor interface. The PL spectra indicate that the smoother morphology and smaller domain sizes can inhibit the radiative loss, which is in accordance with the corresponding device performance. It is demonstrated that different substituents in silafluorene-based polymers play a key role in determining the photovoltaic performance. © 2017 Elsevier Ltd. All rights reserved.

1. Introduction Silica containing conjugated polymer solar cells (PSCs) are of special interest to people due to their excellent optoelectronic properties. For example, 9,90 -dioctylsilafluorene-based copolymers with 4,7-dithienyl-2,1,3-benzothiadiazole (PSiFC8-DBT) is one of the earliest reported organic photovoltaic (OPV) materials with power conversion efficiency (PCE) exceeding 5% [1]. Dithienosilole is another famous silica-containing building block with a PCE of over 5% [2e6]. Recently, PCE of 7.56% was obtained by tuning the endcaps [7]. Silica atoms can lower the lowest unoccupied

* Corresponding author. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (L. Hou), [email protected] (Y. Mo), hbwu@ scut.edu.cn (H. Wu). 1 Shaoshan Lian and Wenhao Zheng contribute equally to this work. https://doi.org/10.1016/j.dyepig.2017.07.011 0143-7208/© 2017 Elsevier Ltd. All rights reserved.

molecular orbitals (LUMO) and the highest occupied molecular orbital (HOMO) of the final polymers, which effectively enhance the ambient stability against O2 oxidation. It was reported by Helgesen et al. [8] that dithienosilole-based copolymer with DBT was proved to be about 5 times more stable than its carbon analog [9,10]. We firstly reported the synthesis of poly(3,6- silafluorene)s in 2005 [11,12], nearly at the same time as poly(2,7-silafluorene)s by Holmes group [13]. Subsequently, we reported the photovoltaic performance of 2,7-silafluorene-based copolymer with DBT [1]. This work received wide attention and was followed by many groups. For example, Bo's group synthesized a 2,7-silafluorenebased copolymer with dialkyloxy-substituted DBT (PSiFC8-DBTC8) and got high PCE of 6.05% [14] while with 5,6Ddifluorobenzothiadiazole for organic photovoltaic cells and got a PCE of 4.03% was achieved [15]. Zhan's group synthesized 2,7diketopyrrolopyrrole silafluorene molecule(SiFC6-2DPP) and got a PCE of 2.05% [16]. Huang ’s group [17] also reported poly(2,7silafluorene)s with pendent acceptor groups and got a maximum

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PCE of 3.15%. Jin et al. [18] synthesized 3,6-silafluorene-based copolymer with DBT and got a PCE of 4.05%. More recently, Erlik et al. [19] reported 2,7-silafluorene-based copolymers with benzotriazole derivatives and got a maximum PCE of 2.57%. Marchiori et al. [20] investigated the annealing effect on the device performance and Garcia-Basabe et al. [21] studied the annealing effect on the charge transfer dynamics and molecular orientation of the typical PSiF-DBT. All of these silafluorene-based copolymers mentioned above have alkyl substituents at 9,9-positions. We have been working on the synthesis of aryl substituted poly(2,7-silafluorene)s in these year for their potential application as blue emitters in polymer light-emitting diodes (PLED). Previously, PLEDs from alkyl substituted poly(2,7-silafluorene)s always showed quite low efficiencies [13,22e24]. The fundamental advance occurred in 2011 after we synthesized a 9,9-diphenyl substituted polysilafluorene [25] with a luminous efficiency (LE) of 2.3 cd/A, which is superior to its corresponding polyfluorene (ca. 1.9 cd/A) [26]. Recently, we synthesized polysilafluorenes with carbazole substituents and triphenylamine pendants [27]. These polysilafluorenes were found to have higher efficiencies than the corresponding polyfluorene analogs. Remarkably, PSF-Cz was found to be a new promising deep blue-light emitter with a LE of ca. 3.28 cd/A, which is among the best deep blue emitters for PLED applications until now. After inserting a triazine [28] layer between PEDOT:PSS and PVK, a higher LE of 4.50 cd/A can be obtained. So we can see the significant influence of the hole-transporting pendants on the PLEDs' performance. As 2,7-silafluorene-based copolymers are never reported to date, we are curious about the effect of the hole-transporting pendants on their OPV performance. In this paper, we synthesized carbazole and triphenylamine substituted 2,7-silafluorene-based copolymers with 5,6-bis(octyloxy)-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole (DBT), namely Cz-PSF-DBT and TPA-PSF-DBT, and investigated their OPV performance using [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as the acceptor. It is found TPA-PSF-DBT shows a PCE of 3.48%, much higher than Cz-PSF-DBT with a PCE of 1.71%, which is adverse to results in blue PLEDs that PSF-Cz has much higher electroluminescence performance than PSF-TPA. Based on the measurements of energy level, morphology and donor/acceptor (D/ A) charge transfer PL spectra, etc. the effect of different substituents on silafluorene-based polymer OPV performance is demonstrated, shedding new light on developing silafluorene-based PSCs. 2. Experimental section 2.1. Materials All chemicals and reagents were used as received from Aldrich, TCI and Acros Chemical Co. unless specified otherwise. All solvents were carefully dried and purified before use. All manipulations involving air-sensitive reagents were performed under a dry argon atmosphere. 5,6-bis(octyloxy)-4,7-di(thiophen-2-yl)benzo[c] [1,2,5]thiadiazole (DBT) [29], 9,9-bis(N-(20 -ethylhexyl)-carbazole3-yl)-2,7-dibromosilafluorene (2) and 9,9-bis(4-(10-(diphenylamine-40 -phenylmethyloxy)-decyloxy)-phenyl)-2,7dibromosilafluorene (3) were synthesized according to the reported procedure [27]. 2.2. Characterization NMR (600 MHz) spectra were obtained using a Bruker 600 M Hz spectrometer with tetramethylsilane as an internal standard. C, H, N, S elemental analyses were performed on a Vario EL elemental analysis instrument (Elementar Co.). Gel Permeation

Chromatography (GPC) analysis was conducted with a Waters GPC 2410 in tetrahydrofuran (THF) using a calibration curve of polystyrene standards. UV-visible absorption spectra were recorded on a HP 8453 UV-vis spectrophotometer. Cyclic voltammetry (CV) was carried out on a Potentiostat/Galvanostat Model 283 (Princeton Applied Research Co.) in a solution of tetrabutylammonium hexafluorophosphate (Bu4NPF6) (0.1 M) in acetonitrile at a scanning rate of 100 mV/s at a room temperature under the protection of argon. Atomic force microscopy (AFM) experiments were carried out under ambient conditions by using a Veeco Multi Mode Nanoscope Шa system operating in tapping mode. Silicon cantilevers with a resonance frequency of around 300 kHz were used. 2.3. 5,6-bis(octyloxy)-4,7-bis[5-(tributylstannyl)-2-thienyl]-2,1,3Benzothiadiazole (1) 1 mL of Lithium diisopropylamide (2 M in pentane, 2 mmol) was added over 40 min to a solution 150 mg of 5,6-bis(octyloxy)-4,7di(thiophen-2-yl)benzo[c][1,2,5] thiadiazole (0.5 mmol) in 20 mL of dry THF at 78  C under argon atmosphere. The mixture was stirred for a further 2 h at 78  C. 0.55 mL of tributyltinchloride (2.0 mmol) was added dropwise, eventually allowed to rise to room temperature. After being stirred at room temperature for 16 h, the mixture was poured into water and extracted with dichloromethane. The combined organic layers were dried over magnesium sulfate, and concentrated. The crude product was purified by gel chromatography (silica gel, toluene:hexane ¼ 1:100e1:30 containing 0.5% triethylamine as gradient eluent) to afford 83 mg of 1 as a red solid (yield: 76%). Anal. Calcd for C46H92N2O9S3Sn2 (%): C, 48.01; H, 8.06; N, 2.43; O, 12.51; S, 8.36; Sn, 20.63. Found: C, 47.95; H, 8.08; N, 2.42; S, 8.34. 1H NMR (600 MHz, CDCl3) d 8.57 (d, J ¼ 3.4 Hz, 5H), 7.30 (d, J ¼ 3.4 Hz, 5H), 4.12 (m, 4H), 2.00e1.88 (m, 4H), 1.69e1.55 (m, 12H), 1.50e1.41 (m, 4H), 1.41e1.24 (m, 32H), 1.21e1.13 (m, 12H), 0.99e0.83 (m, 24H). 13C NMR (150 MHz, CDCl3) d 151.8, 151.0, 139.8, 139.5, 135.1, 131.3, 117.6, 74.3, 31.9, 30.5, 29.7, 29.4, 29.0, 27.3, 26.1, 22.7, 14.1, 13.7, 10.9. 2.4. Poly[9,9-di(N-(20 -ethylhexyl)-carbazole-3-yl)silafluorene-2,7diyl-alt-5,6-bis(octyloxy)-2,1,3-benzothiadiazole-4,7-diyl] (2, CzPSF-DBT) 1 (230 mg, 0.2 mmol), 2 (179 mg, 0.2 mmol) were dissolved in 4 mL of toluene. The solution was degassed with argon for 1 h, then Tris(dibenzylideneacetone)dipalladium(0) (6 mg, 0.006 mmol) and Tri(o-tolyl)phosphine (12 mg, 0.036 mmol) was added. The mixture was heated to reflux in argon atmosphere for two days. 60 mg of 2tributylstannanethiophene in 1 mL of toluene was added and the mixture was stirred for another 12 h. At last, 120 mg of 4bromothiophene in 1 mL of toluene was added and further reacted for 12 h to end up the reaction. After cooling down, the mixture was dilute with 20 mL of toluene, washed with H2O for 3 times. The solvent was removed under reduced pressure, and the crude polymer was refluxed with 50 mg trithiocyanuric acid (TMT) and 5 g carbon black in 100 mL of THF for 24 h, filtered, concentrated and Soxhlet extracted with methanol, acetone, hexane and toluene. The solvent was stripped off and the residue was purified by column chromatography (silica gel, toluene as eluent). After concentration followed by reprecipitation twice from methanol, 162 mg of Cz-PSF-DBT (2) can be obtained as a red fiber (yield 62%). Anal. Calcd for Cz-PSF-DBT (%):C82H94N4O2S3Si, C, 76.23; H, 7.33; N, 4.34; O, 2.48; S, 7.45; Si, 2.17. found: C, 76.18; H, 7.34; N, 4.32; S, 7.43. 1H NMR (600 MHz, CDCl3) d: 8.58 (2H), 8.48 (2H), 8.27 (2H), 8.07 (2H), 7.99 (2H), 7.86 (m, 4H), 7.51 (2H), 7.47e7.40 (m, 4H), 7.38 (2H), 7.17 (2H), 4.16 (m, 8H), 2.1e1.9 (m, 6H), 1.6e1.1 (m, 36H), 0.85 (m, 18H).

S. Lian et al. / Dyes and Pigments 149 (2018) 133e140

Mn: 28.9 kDa, polydispersity (PDI): 2.3. 2.5. Poly[9,9-di(4-(10-(diphenylamine-40 -phenylmethyloxy)decyloxy)- phenyl) silafluorene-2,7-diyl-alt-5,6-bis(octyloxy)-2,1,3benzothiadiazole-4,7-diyl] (3, TPA-PSF-DBT) TPA-PSF-DBT was synthesized according to the similar procedure used for preparation of Cz-PSF-DBT (2) in a yield of 85%. Anal. Calcd for TPA-PSF-DBT (%):C112H126N4O6S3Si, C, 76.93; H, 7.26; N, 3.20; O, 5.49; S, 5.50; Si, 1.61. found: C, 76.88; H, 7.28; N, 3.19; S, 5.49. 1H NMR (600 MHz, CDCl3) d: 8.52 (2H), 8.09 (2H), 7.93 (2H), 7.85 (2H), 7.65 (4H), 7.49 (2H), 7.23e7.17 (m, 12H), 7.07e7.02 (m, 12H), 6.99e6.92 (m, 8H), 4.41 (s, 4H), 4.18 (br, 4H), 3.96 (m, 4H), 3.46 (m, 4H), 1.98 (m, 4H), 1.76 (m, 4H), 1.6 (m, 4H), 1.5 (m, 4H), 1.43 (m, 4H), 1.4e1.2 (m, 36H), 0.87 (m, 6H). Mn: 14.5 kDa, polydispersity (PDI): 1.8. 2.6. OPV fabrication and characterization Patterned indium tin oxide (ITO)-coated glass with a sheet resistance of 15e20 ohm/square were cleaned by a surfactant scrub, then underwent a wet-cleaning process inside an ultrasonic bath, beginning with deionized water, followed by acetone and isopropanol. After oxygen plasma cleaning for 5 min, a 40 nm-thick poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) (Bayer Baytron 4083) anode buffer layer was spin-cast onto the ITO substrate and then dried by baking in a vacuum oven at 80  C for overnight. The active layer, with a thickness in the 70e80 nm range, was then deposited on top of the PEDOT:PSS layer, by casting from a chlorobenzene solution with a concentration of 25 mg mL1. The PFN solution in methanol was spin-coated on the top of the obtained active layer at 2000 rpm for 30 s to form a thin interlayer of 5 nm. Finally, a 100 nm aluminum layer was thermally evaporated with a shadow mask at pressure of 3  104 Pa. The thickness of spin-coated films was verified by a surface profilometer (Tencor, Alpha-500). The overlapping area between the cathode and anode defined a pixel size of 0.15 cm2. The thickness of the evaporated cathodes was monitored by a quartz crystal thickness/ratio monitor (Model: STM-100/MF, Sycon). UVVisible absorption spectra were recorded on a HP 8453 UV-Vis spectrophotometer. The PL spectra was detected by IHR550 spectrometer equipped with Synapse CCD. The power conversion efficiencies of the resulting polymer solar cells were measured under 1 sun, AM 1.5G (air mass 1.5 global) spectrum from a solar simulator (Oriel model 91192) (100 mW/cm2). The current densityevoltage (JeV) characteristics were recorded with a Keithley 2410 source unit. The luminance (L) was calibrated by a spectrophotometer (SpectraScan PR-705, Photo Research). The quantum efficiency was amended by measuring the entire light output in all directions in an integrating sphere (ISO-080, Labsphere). 3. Results and discussion 3.1. Synthesis and NMR analysis As polysilafluorene is a typical wide bandgap polymer, we have to combine silafluorene as the donor part with another acceptor part to form an alternative copolymer so that it exhibits lower band gap. This is a widely used strategy to get efficient polymeric donor materials for OPV [30e34]. In this study, we used two silafluorene monomers with hole-transporting substituents as the donor part, i.e. 9,9-bis(N-(20 -ethylhexyl)-carbazole-3-yl)-2,7dibromosilafluorene (2) and 9,9-bis(4-(10-(diphenylamine-40 -

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phenylmethyloxy)-decyloxy)-phenyl)-2,7-dibromo silafluorene (3) [27] which were synthesized according to our reported procedure (as shown in Scheme 1). For acceptor moieties, we chose DBT [29] reported by Bo's group for its high efficiency and good solubility. There are mainly two ways to synthesize silafluorene-based copolymers, Suzuki coupling or Stille coupling reaction [35]. We prefer Stille coupling reaction because we found that the CSi bonds were readily to hydrolyze at high temperature during polymerization [25]. So, 5,6-bis(octyloxy)-4,7-bis[5-(tributylstannyl)-2thienyl]-2,1,3-benzothiadiazole (1) was prepared using lithium diisopropylamide to strip the two H atoms of DBT followed by the addition of tributyltinchloride as depicted in Scheme 1. Although many tributylstannyl derivatives were reported to be unstable and always used without further purification, we found 1 was tolerant of purification by silica-gel chromatography. Both its NMR spectra and the yield are satisfying, which are enclosed in Figs. S1eS2. The polymerization was conducted according to the typical Stille polycondensation procedure [35], using tris(dibenzylideneacetone) dipalladium (0) and tri(o-tolyl)phosphine as catalysts and toluene as solvent. These monomers show high reaction activity in Stille polycondensation, affording Cz-PSF-DBT and TPA-PSF-DBT in high yields of 62% and 85%, respectively. The molecular weight of CzPSF-DBT (28.9/2.3 kDa) is higher than TPA-PSF-DBT (14.5/1.8 kDa). These polymers are readily soluble in chlorobenzene for OPV device fabrication. 3.2. Optical properties To investigate the photophysical properties of the polymers, the absorption spectra of their chloroform solution with a concentration of 0.01 mg mL1 and thin films were recorded as shown in Fig. 1. Surprisingly, these polymers show close spectra especially for the long wavelength bands, which means the 9,90 -substituents have little effect on the absorption range and band gaps. In solution, these polymers exhibit two similar distinct absorption bands, where the first absorption band at 385~391 nm are mainly contributed by p-p* transition of silafluorene moieties and the second band at 513e516 nm should be closely relative to DBT moieties [36]. It is supported by the facts that the silafluorenebased homopolymer exhibits an absorption maximum around 380 nm [25] and DBT-based homopolymer exhibits an absorption maximum around 540e580 nm [29]. On the other hand, we can still find some difference between these polymers on the range below 360 nm, which can be attributed to the 9,90 -substituents of silafluorene. The absorption peak at 306 nm for TPA-PSF-DBT should be ascribed to triphenylamine moieties. The absorption spectra in the solid state of these polymers show obvious red shift of ca. 30e40 nm compared with the corresponding spectra in dilute solution as shown in Fig. 1. Furthermore, the absorption band above 500 nm also becomes stronger and wider. All of these facts indicate the intermolecular interactions increase in the solid state. Based on the absorption onset of the polymer films at ca. 640 nm, we can get the optical band gaps (ca. 1.93 eV) for these two polymers, which is close to reported two 9,90 alkyl PSF-DBTs with the optical band gaps of 1.95e1.98 eV [14]. 3.3. Electrochemical properties It is necessary to determine the energy levels of the HOMO and LUMO of the conjugated polymers in order to provide key parameters for the design of PSCs. Thus, the electrochemical characteristics of the polymers films on a Pt electrode were investigated by cyclic voltammetry (CV) with 0.1 M Bu4NPF6 in acetonitrile as the

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Scheme 1. Synthesis of Cz-PSF-DBT and TPA-PSF-DBT.

Fig. 1. Absorption spectra of Cz-PSF-DBT and TPA-PSF-DBT solution at 0.01 mg/mL and thin films.

electrolyte and Hg/Hg2Cl2 as reference electrode at a scan rate of 100 mV s1. The results are summarized in Fig. 2 and Table 1. Contrary to the identical absorption spectra, the cyclic voltammograms of these two polymers films are obviously different from each other. As the HOMO levels of the polymers were obtained from the oxidation onsets according to an empirical formula, EHOMO ¼ -(Eox,onset þ 4.4) [37], we can get HOMO level of 5.50 eV for Cz-PSF-DBT (1.1 V) and 5.37 eV for TPA-PSF-DBT (0.97 V). As presented in our previous study [27], silica atom at 9-position of silafluorene showed strong s conjugation effect connecting substituents with backbone so that closer carbazole substituents may maintan higher oxidation potential of the final polymer compared

Fig. 2. Cyclic voltammogram of Cz-PSF-DBT and TPA-PSF-DBT.

to triphenylamine substituents. The LUMO levels were calculated according to the optical band gap and HOMO levels and summarized in Table 1. 3.4. Photovoltaic properties To investigate the photovoltaic properties of the polymers, solar cells with a sandwich configuration of Glass/ITO/PEDOT:PSS/CzPSF-DBT:PC71BM or TPA-PSF-DBT:PC71BM/PFN/Al were fabricated. The active layers of the solar cells were spin-coated from polymer: PC71BM dichlorobenzene solutions with the weight ratio of 1:2. PC71BM was chosen as the electron acceptor because it has a

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Table 1 Experimental results and optical properties of Cz-PSF-DBT and TPA-PSF-DBT. polymer

Yields (%)

Mn kDa/PDI

Abs

Cz-PSF-DBT TPA-PSF-DBT

62 85

28.9/2.3 14.5/1.8

545 545

max

(nm)

Abs

onset

640 640

(nm)

Eg (eV)

Eox,

1.93 1.93

1.10 0.97

onset

(V)

HOMO (eV)

LUMO (eV)

5.50 5.37

3.57 3.44

Table 2 Photovoltaic parameters of two polymers solar cells. Device

Thickness (nm)

Voc (V)

Jsc (mA cm2)

FF

PCE (%)

Cz-PSF-DBT:PC71BM ¼ 1:2 TPA-PSF-DBT:PC71BM ¼ 1:2

90 90

1.00 0.93

5.00 7.23

35 52

1.71 3.48

are beneficial in achieving higher PCE. This observation may be of importance in the designing of PSF-based polymers for highefficiency PSCs. To validate the measurements, the corresponding external quantum efficiencies (EQEs) were measured under illumination of monochromatic light (Fig. 3b). The current density values of 4.97 mA/cm2 for Cz-PSF-DBT and 7.20 mA/cm2 for TPAPSF-DBT calculated from integration of the EQEs of devices with an AM 1.5G reference spectrum, agree well with the Jsc values obtained from the JeV measurements. 3.5. Morphology

Fig. 3. a) J-V and b) EQE curves of TPA-PSF-DBT:PC71BM ¼ 1:2 and Cz-PSFDBT:PC71BM ¼ 1:2 devices.

considerably higher absorption-coefficient in the visible region than PCBM. PFN is chosen as the electron interfacial layer because it facilitates charge collection and extraction by means of the work function modification and the interfacial charge redistribution [38]. The photovoltaic performances of these devices are listed in Table 2. The corresponding current density-voltage (J-V) curves of devices, under illumination of AM 1.5G simulated solar light (100 mW cm2), are shown in Fig. 3a. Cz-PSF-DBT exhibited a PCE of 1.71% with Voc of 1.0 V, Jsc of 5.0 mA cm2 and fill factor of 0.35 while TPA-PSF-DBT exhibited a higher PCE of 3.48% with Voc of 0.93 V, Jsc of 7.23 mA cm2 and fill factor of 0.52. The devices from Cz-PSF-DBT exhibited 0.07 V higher Voc than devices from TPAPSF-DBT due to its deeper HOMO level (as listed in Table 1). It's surprising to find that TPA pendants on the PSF-co-DBT backbones

The donor/acceptor phase separation in the active layer can greatly influence photoproduced exciton separation and charge transport, which play a key role in determining the device performance. To look into the reason of the different device performance, the morphologies of the active layers were investigated by atomic force microscopy (AFM) and transmission electron microscopy (TEM). As shown in Fig. 4a, the roughness height of Cz-PSF-DBT:PC71BM film is higher with a root mean square (RMS) surface roughness of 0.442 nm compared to TPAPSF-DBT:PC71BM film (Fig. 4c) with a RMS value of 0.331 nm. At the same time, the domain size of Cz-PSF-DBT:PC71BM film is bigger than TPA-PSF-DBT:PC71BM film. The AFM phase contrast images in Fig. 4b,d are well in accordance with their AFM height images, illustrating the accuracy of film morphologies. The big phase separation in Cz-PSF-DBT:PC71BM increases the exciton diffusion distance, resulting in the decrease of charge separation. The observed variation in surface morphology by AFM is correlated with the 3D nanostructures of blends. In Fig. 4eef, the internal morphological structures of Cz-PSF-DBT:PC71BM and TPAPSF-DBT:PC71BM films were characterized using TEM images. The bulk size of TPA-PSF-DBT:PC71BM is smaller than that of CzPSF-DBT:PC71BM, which is consistent with AFM images of the corresponding film surface. Thus Cz-PSF-DBT:PC71BM device shows a lower photocurrent than TPA-PSF-DBT:PC71BM device. It can be concluded that polymers with different side chains show different phase separations, which greatly influence the photovoltaic performances. We are currently attempting to achieve optimal morphology and gain higher efficiency by varying the mixing solvents and/or processing additives. It should be pointed out here that unlike rigid Cz-PSF-DBT, TPA-PSF-DBT is actually a phenyl-substituted polysilafuorene at 9,90 -position and its triphenylamines are at the end of the long alkyloxy pendants linked with phenyl-rings. Triphenylamine groups render lower melting point and higher solubility in most of organic solvents, which endow it a good miscibility with aromatic compounds, resulting in a finer morphology.

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Fig. 4. AFM height images of (a) Cz-PSF-DBT:PC71BM ¼ 1:2 and (c) TPA-PSF-DBT:PC71BM ¼ 1:2 films. The corresponding AFM phase contrast images of (b) Cz-PSFDBT:PC71BM ¼ 1:2 and (d) TPA-PSF-DBT:PC71BM ¼ 1:2 films (size ¼ 5 mm  5 mm). TEM images of (e) Cz-PSF-DBT:PC71BM ¼ 1:2 and (f) Cz-PSF-DBT:PC71BM ¼ 1:2 films.

3.6. PL spectra of polymer:PC71BM blends In order to further study the potential limitations on exciton diffusion influenced by film morphology, we recorded a series of photoluminescence (PL) spectra of the pristine polymers and polymer:PC71BM blends in Fig. 5. For both Cz-PSF-DBT:PC71BM and TPA-PSF-DBT:PC71BM films, we observed efficient PL quenching of more than 95% in comparison to pristine polymer films, which implies that the efficiency of exciton quenching at the donor/ acceptor interface is effective in two polymers. For Cz-PSFDBT:PC71BM film, the first PL peak at 710 nm is ascribed to the PC71BM acceptor emission and the second PL peak at 780 nm should come from exciton charge transfer (CT) state emission. The CT emission intensity at about 780 nm for TPA-PSF-DBT:PC71BM is decreased by 235% than that for Cz-PSF-DBT:PC71BM film. The smaller PL intensity for TPA-PSF-DBT:PC71BM indicates that donor and acceptor moieties are in close contact and the exciton separation is more effective compared to Cz-PSF-DBT:PC71BM, which is consistent with their corresponding Jsc values and morphologies.

The EL-QE-voltage characteristics of pure polymers are also shown in Fig. 5b and c. The pure Cz-PSF-DBT device shows about three times higher light-emitting outputs and QE than pure TPA-PSF-DBT. The tendency is in accordance with the PL intensities of mixutures with PC71BM, indicating Cz-PSF-DBT is suitable to be used as a light-emitting polymer, not as a PV polymer. Thus it can be concluded that the increase in photocurrent generation for TPAPSF-DBT:PC71BM is more likely to result from an improved dissociation efficiency of the photoproduced polaron pairs from excitons into free charges.

4. Conclusions In this paper, two kinds of PSF-DBTs were synthesized and their OPV properties were investigated. These polymers show nearly identical absorption spectra but different HOMO levels, which influence their device Voc. The carbazole-substituted Cz-PSF-DBT polymer has a lower PCE than TPA-PSF-DBT polymer due to the inferior D/A phase separation morphology, which decreases the

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Acknowledgements The authors are grateful to the NSFC Projects (21174042, 61274062 and 11204106) and the Fundamental Research Funds for the Central Universities for financial support. Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.dyepig.2017.07.011. References

Fig. 5. (a) PL spectra of pristine Cz-PSF-DBT, Cz-PSF-DBT and their blends with PC71BM (polymer: PC71BM ¼ 1:2). (b) electroluminescence-voltage and (c) quantum efficiency-voltage curves of pristine Cz-PSF-DBT and TPA-PSF-DBT devices.

exciton separation. PL quenching results reveal TPA-PSFDBT:PC71BM has the less radiative loss than Cz-PSF-DBT:PC71BM, which is accordance with the corresponding AFM/TEM images and OPV performance. These results demonstrate flexible triphenylamine end groups can effectively improve the 2,7 silafluorenebased conjugated polymer device performance compared to the rigid carbazole substituents.

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