Highly efficient polymer solar cells based on poly(carbazole-alt-thiophene-benzofurazan)

Abstract

An octyloxy substituted benzofurazan based poly(carbazole-alt-thiophene-benzofurazan) (PCzDTBF) was synthesized by Suzuki polycondensation. The absorption spectra of the polymer were peaked at 385 and 540 nm, and the HOMO and LUMO energy levels were −5.34 and −3.46 eV, respectively. The hole mobility was found to be 4.8 × 10⁻⁴ cm² V⁻¹ S⁻¹ by the space charge limited current (SCLC) method, and the surface energy (E_s) was calculated to be 35.8 mJ m⁻² from the contact-angle measurement. Polymer solar cells (PSCs) based on the blend of PCzDTBF and PCBM with a weight ratio of 1 : 2 were fabricated. The devices were found to have a power conversion efficiency (PCE) of 4.2% in the device configuration of ITO/PSS:PEDOT/PCzDTBF:PC₇₀BM/Al. By utilizing poly[(9,9-dioctyl-2,7-fluorene)-alt-(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)] (PFN) as the cathode interfacial layer, the PCE was enhanced to 5.48%, with an open-circuit voltage (V_oc) of 0.90 V, a short-circuit current density (J_sc) of 7.73 mA cm⁻², and a fill factor (FF) of 67%. The high efficiencies could be ascribed to the good morphology, resulting from the matched surface energies of the PCzDTBF and PCBM components, and the distinct enhancement on V_oc and FF by the PFN layer, where it is possible that a built-in field potential exists between the active layer and cathode.

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Introduction

Polymer solar cells (PSCs) are becoming increasingly more and more attractive as renewable energy resources because of their versatile merits, such as low cost, flexibility, light weight, and large-scale fabrication.^1,2 Some PSCs with a high power conversion efficiency (PCE), overwhelming 8%, were achieved, which were potentially applicable in commercial power generation.³

Bulk-heterojunction (BHJ) is one kind of the widely utilized device structure, based on a blend of electron-donating materials (donor) and electron-accepting moieties (acceptor). So far, fullerene derivatives have been the best electron acceptor candidates, such as [6,6]-phenyl-C₆₁-butyric acid methyl ester (PC₆₀BM) and [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₀BM). Therefore, many studies have been focused on the development of electron donors, for instance, polythiophenes (PThs),^4–7 polybenzodithiophenes (PBDTs),^8,9 polyfluorenes (PFs),¹⁰ and polysilafluorenes (PSiFs),¹¹ In addition, polycarbazoles (PCzs) were star polymers for PSCs, because of the fantastic characteristics of carbazoles, such as high hole mobility, deep highest occupied molecular orbit (HOMO) energy levels, high thermal stability and feasible molecular structure tuning.^12,13 In the donor polymers, the mostly used molecular architecture was donor–acceptor–donor (D–A–D), in which the donor was an electron-rich unit (such as thiophene and furan), and the acceptor refers to the electron-deficient unit (such as, benzothiadiazole (BT),^12,13 benzoselenodizole¹⁴ and quinoxaline¹⁵). Benzofurazan (BF),^16–19 contains a higher electronegative oxygen, compared with the sulfur atom in the BT unit, which can induce the deeper HOMO energy level. However, the solubility of BT and BF based D–A–D monomers is poor in organic solvents (e.g., toluene) due to their high polarity and planarity. Therefore, the side chain substituted BT²⁰ and BF^21–23 units can improve the solubility tremendously in common solvents, and make it more feasible to synthesize the polymers with high molecular weights and good solubility.

Previously, the alcohol soluble poly[(9,9-dioctyl-2,7-fluorene)-alt-(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)] (PFN) was utilized as the cathode interfacial layer in photovoltaic devices, where the photovoltaic performances were improved, possibly because of the existence of built-in electric fields between the active layer and the cathode.^24,25 Therefore, the insertion of an alcohol soluble PFN layer as the cathode interfacial layer is an attractive strategy to achieve highly efficient PSCs.

Here, an alternating conjugated polymer containing an octyloxy substituted BF unit and 9-octyl-2,7-carbazole was synthesized, and used as a donor material in BHJ solar cells. Moreover, the PFN was used as the cathode interfacial layer, which ultimately contributed to the large enhancement in the photovoltaic efficiency because of the possible distinct improvements in FF and V_oc.

Experimental section

Characterization and instrumentation

Both ¹H nuclear magnetic resonance (NMR) and ¹³C NMR measurements were carried out on a Bruker 300 MHz DRX spectrometer with tetramethylsilane (TMS) as the internal reference. Mass spectrometry was performed on a Bruker Esquire HCT PLUS. The number-average molecular weight (M_n) and weight-average molecular weight (M_w) were determined on a Waters gel permeation chromatography (GPC) system with linear polystyrene as the standard and chloroform containing 0.25 v/v% triethylamine as eluent. Cyclic voltammetry (CV) was characterized on a CHI600D electrochemical workstation with a standard three electrode cell based on a platinum (Pt) working electrode and a Pt wire counter electrode, against a saturated calomel electrode (SCE) as the reference electrode, at a scan rate of 50 mV s⁻¹ within a nitrogen-saturated anhydrous solution of 0.1 mol L⁻¹ tetrabutylammonium hexafluorophosphates (Bu₄NPF₆) in acetonitrile, versus ferrocene/ferrocenium (Fc/Fc⁺) as the internal reference. UV-vis absorption spectra were performed on a HP 8453 spectrophotometer. Tapping-mode atomic force microscopy (AFM) was carried out for the identification of topography and blend film phase images on a Veeco Nanoscope V scanning probe microscope. Surface energy was measured on Dataphysics OCA40 Micro by calculations from the surface tension data of diiodomethane and water.

Device fabrication

All devices were fabricated on ITO-coated glass substrates. A thin-layer (40 nm) of PEDOT:PSS (Clevios 4083) was spin-coated onto UV-ozone treated ITO substrates at 4000 rpm for 40 s and then baked at 140 °C for 15 min in air. The solution containing the polymer (10 mg ml⁻¹) and fullerene (20 mg ml⁻¹) (polymer : fullerene = 1 : 2, wt/wt) in o-dichlorobenzene (ODCB) were spin-coated inside a glove-box with nitrogen at 1200 rpm for 40 s to form the active layers (∼80 nm). For the device structure of ITO/PEDOT-PSS/active layer/PFN/Al, the PFN layer (5 nm) was prepared by spin-casting a mixed solution (0.2 mg ml⁻¹) with a methanol to acetic acid volume ratio of 100 : 1 before the cathode evaporation. The aluminum cathode (80 nm) was then thermally evaporated under vacuum (∼10⁻⁶ Torr) through a shadow mask defining an active device area of 0.16 cm². The current–voltage (J–V) curves were measured using a Keithley 2400 multimeter under AM 1.5 G solar illumination at 87 mW cm⁻² and a Thermal-Oriel 150 W solar simulator. Incident photon-to-current conversion efficiency (IPCE) values were obtained with a monochromator and calibrated with a silicon photodiode. Hole mobility data of neat polymer and PCzDTBF:PCBM blend films were measured by the space charge limited current (SCLC) method with the device configuration of ITO/PEDOT:PSS (40 nm)/active layer (80–100 nm)/MoO₃ (10 nm)/Al (90 nm). Preliminarily, PEDOT:PSS was spin casted on the ITO coated glass substrate, then, active layers were prepared from the ODCB solution with a 1% concentration by spin coating. Finally, MoO₃ and Al layers were deposited by thermal evaporation under high vacuum, respectively.

Synthesis of monomers and polymer

The synthetic routes were displayed in Scheme 1. 1,2-Bis(octyloxy) benzene,²⁶ 1,2-dinitro-4,5-bis(octyloxy)benzene and 2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxa borolan-2-yl)-9-octyl-carbazole²⁰ were synthesized according to the published procedures. All the other monomers and the polymer were obtained by the following synthetic methodologies.

Scheme 1 Synthetic routes of monomers and polymer.

5,6-Bis(octyloxy)benzofurazan 3. 1,2-Dinitro-4,5-bis(octyloxy)benzene 2 (8.49 g, 20 mmol), NaN₃ (6.5 g, 100 mmol) and TBAB (1.29 g, 4 mmol) were dissolved in 100 ml toluene and refluxed for 5 h under the argon protection. Then, PPh₃ (6.29 g, 24 mmol) was added in small portions; when the PPh₃ was completely added, the reaction continued to reflux for an additional 2 h. The reaction system was cooled to room temperature (r.t.) and filtered through a short silica plug; the solvent was removed by low pressure evaporation and a gray solid was obtained. The final solid was recrystallized in ethanol, and yielded 3.33 g of the target compound. ¹H NMR (300 MHz, CDCl₃, δ): 6.80 (s, 2H, ArH), 4.07 (t, 4H, CH₂), 1.89 (m, 4H, CH₂), 1.51 (m, 4H, CH₂), 1.40–1.30 (br, 16H, CH₂), 0.89 (t, 6H, CH₃). ¹³C NMR (75 MHz, CDCl₃, δ): 155.25, 146.86, 90.74, 69.41, 31.80, 29.28, 29.24, 28.63, 26.00, 22.67, 14.08. MS (APCI): Calcd, 376.5; found (M + 1)⁺, 377.5.

4,7-Dibromo-5,6-bis(octyloxy)benzofurazan 4. 5,6-Bis(octyloxy)benzofurazan 3 (3.33 g, 9.67 mmol) was dissolved in 100 ml dichloromethane and 100 ml AcOH at r.t. Br₂ (10.83 g, 67.66 mmol) was added dropwise, and then reacted at r.t. for 40 h. The excess Br₂ was quenched by NaHSO₃ and extracted by dichloromethane three times, washed by NaCl aqueous solution and water respectively, and dried over MgSO₄. The solvent was removed by evaporation under low pressure, and the resulting product was recrystallized in methanol, and yielded 2.78 g of the target compound as a white solid. ¹H NMR (300 MHz, CDCl₃, δ): 4.14 (t, 4H, CH₂), 1.86 (m, 4H, CH₂), 1.51 (m, 4H, CH₂), 1.38–1.29 (br, 16H, CH₂), 0.89 (t, 6H, CH₃). ¹³C NMR (75 MHz, CDCl₃, δ): 155.67, 147.45, 99.57, 58.48, 31.80, 30.19, 29.33, 25.89, 22.65, 18.43, 14.09. MS (APCI): Calcd, 534.3; found (M + 1)⁺, 535.3.

4,7-Di(thiophen-2-yl)-5,6-bis(octyloxy)benzofurazan 5. 4,7-Dibromo-5,6-bis(octyloxy)benzofurazan 4 (2.06 g, 4.1 mmol), tributyl(thiophen-2-yl)stannane (4.6 g, 12.3 mmol) and Pd(PPh₃)₂Cl₂ (287.7 mg, 0.41 mmol) were dissolved in 41 ml THF and refluxed overnight. THF was evaporated, and the resulting mixture was purified by column chromatography in petroleum ether (b.p.: 60–90 °C) : DCM = 10 : 1 as mixed eluent, and yielded 2.0 g of the target compound as a yellow solid. ¹H NMR (300 MHz, CDCl₃, δ): 8.46 (dd, J = 3.84 Hz, 2H, ArH), 7.50 (dd, J = 5.16 Hz, 2H, ArH), 7.22 (t, 2H, ArH), 4.15 (t, 4H, CH₂), 1.98 (m, 4H, CH₂), 1.47–1.30 (br, 20H, CH₂), 0.90 (t, 6H, CH₃). ¹³C NMR (75 MHz, CDCl₃, δ): 151.73, 146.77, 132.92, 130.85, 127.98, 127.12, 113.06, 74.51, 31.79, 30.26, 29.48, 29.23, 25.87, 22.63, 14.05. MS (APCI): Calcd, 540.8; found (M + 1)⁺, 541.8.

4,7-Bis(5-bromothiophen-2-yl)-5,6-bis(octyloxy)benzofurazan 6. To a solution of 4,7-di(thiophen-2-yl)-5,6-bis(octyloxy) benzofurazan 5 (2 g, 3.7 mmol) in 42 ml chloroform and 42 ml AcOH in the absence of light, NBS (1.45 g, 8.14 mmol) was added in small portions. Then, the reaction continued for 48 h. Water was added and extracted by DCM three times, washed by NaCl aqueous solution and water respectively, and dried over MgSO₄. The solvent was removed by evaporation, and the resulting product was purified by column chromatography in petroleum ether as eluent, and yielded 2.2 g of the target compound as an orange solid. ¹H NMR (300 MHz, CDCl₃, δ): 8.24(d, J = 4.14 Hz, 2H, ArH), 7.17(d, J = 4.14 Hz, 2H, ArH), 4.15(t, 4H, CH₂), 1.99(m, 4H, CH₂), 1.55–1.31(br, 20H, CH₂), 0.91(t, 6H, CH₃). ¹³C NMR (75 MHz, CDCl₃, δ): 151.27, 146.23, 134.29, 131.17, 130.05, 116.33, 112.69, 74.81, 31.78, 30.16, 29.42, 29.23, 25.81, 22.65, 14.07. MS (APCI): Calcd, 698.6; found (M + 1)⁺, 699.6.

Poly(2,7-(9-octyl-carbazole)-alt-4,7-di(thiophen-2-yl)-5,6-bis (octyloxy)benzofurazan) PCzDTBF. 2,7-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9-octyl-carbazole (0.266 g, 0.5 mmol), 4,7-bis(5-bromothiophen-2-yl)-5,6-bis(octyloxy)benzofurazan 6 (0.349 g, 0.5 mmol and Pd(PPh₃)₄ (11.6 mg, 0.01 mmol) were dissolved in 10 ml toluene under argon protection. Then, tetraethylammonium hydroxide (2 ml, 20 wt/wt% in water) was added in one portion at r.t. The reaction was then heated to 85 °C and reacted at this temperature for 48 h. Phenylboronic acid (0.609 g, 5 mmol) was added and reacted for 12 h at 85 °C; then, bromobenzene (1.570 g, 10 mmol) was added in one portion and reacted for additional 12 h. The reaction mixture was cooled to room temperature and precipitated in methanol. The crude polymer was purified by Soxhlet extraction in methanol, acetone, hexane and chloroform respectively; the final chloroform solution was precipitated in methanol, filtered off and dried at 50 °C in the vacuum oven for 10 h, yielding 0.183 g of the target compound as a red solid. ¹H NMR (300 MHz, d-ODCB, δ), 8.53–8.19 (br, 2H, ArH), 7.98–7.79 (br, 4H, ArH), 7.50–7.29 (br, 4H, ArH), 4.51–3.72 (br, 6H, CH₂), 2.48–1.83 (br, 6H, CH₂), 1.48–0.89 (br, 39H, CH₂, CH₃). GPC (CHCl₃): M_n = 6000, M_w = 12 500, PDI = 2.1.

Results and discussion

Synthesis

Detailed synthetic routes of the monomers and polymer are shown in Scheme 1. The polymer PCzDTBF was synthesized by Suzuki polycondesation in toluene, and purified by Soxhlet extraction in methanol, acetone, hexane and chloroform with 100 ml for 12 h each, and was then precipitated in methanol. The solubility of the polymer is good in organic solvents (such as tetrahydrofuran, chloroform, chlorobenzene, dichlorobenzene), because of the octyloxy substituted on the BF unit. The gel permeation chromatography (GPC) characterization shows that the number-average molecular weight (M_n) and weight-average molecular weight (M_w) are 6000 and 12 500, respectively, with a polydispersity index (PDI) of 2.1 (Table 1). The PCzDTBF does not show a higher molecular weight, which is possibly because the octyl side chain in the carbazole unit makes the polymer solubility limited and ultimately inhibits the increment of the molecular weight during polymerization.¹⁹

Table 1 Physical properties of PCzDTBF

Polymer	M n	PDI	λ max ^a (nm)	E g ^b (eV)	E ox (V)	E re (V)	E HOMO (eV)	E LUMO (eV)	E s (mJ m^−2)
a Maximal absorption in solution. b Optical band gap.
PCzDTBF	6000	2.1	385, 540	1.93	0.93	−0.95	−5.34	−3.46	35.8

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Absorption and electrochemical analysis

The UV-vis absorption spectra of PCzDTBF in solution and in film are presented in Fig. 1a. The absorption spectra peaks at 385 and 540 nm are attributed to the localized π–π* transition and the internal charge transfer (ICT) interaction between the donor and acceptor units, respectively.²² Compared with the absorption in solution, the absorption spectrum in film displays peaks that are a little broadened, which implicates the amorphous film and no further molecular stacking.²² The onset of the absorption spectrum in film is 644 nm, which corresponds to the optical band gap of 1.93 eV (Table 1).

Fig. 1 UV-vis absorption spectra of PCzDTBF in chloroform and in film (a); cyclic voltammogram of PCzDTBF in film (b) (inset: graph of the ferrocene oxidative curve).

The cyclic voltammogram (CV) characterization (Fig. 1b) presents the reversibly oxidative and reductive curves, in which the onsets of the oxidation and reduction potentials are located at 0.93 and −0.95 V versus SCE, respectively. The redox potential of the Fc/Fc⁺ internal reference is 0.39 V vs. SCE. The highest occupied molecular orbit (HOMO) and the lowest unoccupied molecular orbit (LUMO) energy levels, determined by calculating the empirical formula of E_HOMO = −e(E_ox + 4.8 − E_{1/2, (Fc/Fc⁺)}) and E_LUMO = −e(E_re + 4.8 − E_{1/2, (Fc/Fc⁺)}), are −5.34 and −3.46 eV, respectively.⁵ The energy band gap of PCzDTBF from the CV data is 1.88 eV, which is approximate to the optical band gap of 1.93 eV (Table 1).

Photovoltaic properties

PSCs were fabricated with the configurations of ITO/PEDOT:PSS/PCzDTBF:PCBM/Al and ITO/PEDOT:PSS/ PCzDTBF:PCBM/PFN/Al, respectively. The current density–voltage (J–V) curves are shown in Fig. 2a. We can see that the short circuit current density (J_sc) and the open circuit voltage (V_oc) of PC₇₀BM based device are larger than the ones of PC₆₀BM as acceptor. Furthermore, the insertion of PFN as the cathode interfacial layer made the V_oc and fill factor (FF) enhanced distinctly, and therefore, resulted in the power conversion efficiency (PCE) increased ultimately. In Fig. 2b, the incident photon-to-current conversion efficiency (IPCE) of PC₇₀BM based devices are broadened and exhibits better photo-to-current conversion in the visible region with approximate 10% improvement compared with the PC₆₀BM device, which is possibly ascribed to the intensified and broadened absorption of the PCzDTBF:PC₇₀BM based blend film.

Fig. 2 Current density-voltage (J–V) characteristics of PCzDTBF: PCBM under AM 1.5 G, 87 mW cm⁻² (a); incident photon-to-current conversion efficiency (IPCE) and UV-vis absorption spectra of PCzDTBF:PCBM blend films (b).

Table 2 shows the distinctive increment in J_sc values and PCEs from 6.61 mA cm⁻² and 3.63% for PC₆₀BM to 7.36 mA cm⁻² and 4.20% for the PC₇₀BM device, respectively. Compared with the devices without a PFN layer, the performances with PFN as a cathode interfacial layer were enhanced dramatically. Even though the J_sc increased only a little, the V_oc and FF were distinctly enhanced, where the V_oc increased with maximal 0.1 V, and FF realized a 10% enhancement. Ultimately, the PCE reached 5.48%, which had more than a 30% increment both in PC₆₀BM and PC₇₀BM based photovoltaic devices.

Table 2 Photovoltaic performances under AM 1.5 G, 87 mW cm⁻²

Active layer (1 : 2, w/w)	Cathode	J sc (mA cm^−2)	V oc (V)	FF (%)	PCE (%)
PCzDTBF:PC60BM	Al	6.61	0.80	60	3.63
	PFN/Al	6.67	0.90	69	4.73
PCzDTBF:PC70BM	Al	7.36	0.85	57	4.20
	PFN/Al	7.73	0.90	67	5.48

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Hole mobility and film morphology

The hole mobility, both in neat polymer and blend films, of PCzDTBF:PCBM were measured by the space charge limited current (SCLC) method with the device configuration of ITO/PEDOT:PSS/active layer/MoO₃/Al. Fig. 3 shows the J–V curves of the PCzDTBF and blend films from SCLC devices. The hole mobilities (Table 3) were 4.8 × 10⁻⁴, 2.5 × 10⁻⁴ and 2.1 × 10⁻⁴ cm² V⁻¹ s⁻¹ for PCzDTBF, PCzDTBF:PC₆₀BM and PCzDTBF:PC₇₀BM, respectively, which were comparable to that of the polycarbazole based polymers.^16,20 Compared with the neat polymer, the hole mobilities of the blend films decreased by about half, but still sustained the same order of magnitude, which would provide a sufficient hole transporting ability in the photovoltaic devices and pay extensive contribution to the PCE.²⁷

Fig. 3 J–V characteristics of PCzDTBF and blend films from SCLC devices.

Table 3 Hole mobilities of the PCzDTBF and blend films

Active layer	Thickness (nm)	μ (×10^−4 cm^2 V^−1 s^−1)
PCzDTBF	100	4.8
PCzDTBF:PC60BM	92	2.5
PCzDTBF:PC70BM	82	2.1

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AFM images of the blend films are shown in Fig. 4. The root-mean-square (RMS) roughness of the PC₆₀BM and PC₇₀BM based films are 0.58 and 0.72 nm, respectively, which shows that both films are very smooth. Moreover, the phase images (Fig. 4c–d) display the nano-scale phase separation,²⁸ which can support the effective holes and electron transporting channels and inhibit the charge carriers recombination, and ultimately affect the photovoltaic properties. On the other hand, the miscibility decides the film morphology in blend systems directly. From Table 1, we can see that the surface energy (E_s) of PCzDTBF is 35.8 mJ m⁻², which is similar to that of the PC₆₀BM of 34.2 mJ m⁻².²⁹ The similar E_s in the blend components caused well-mixed films, resulting from optimal phase separation of the PCBM domains. This scenario can also depict the evidences for the formation of nano-scale phase separation between PCzDTBF and PC₆₀BM or PC₇₀BM greatly.

Fig. 4 AFM images of blend films (1 : 2 weight ratio), topography of PCzDTBF:PC₆₀BM (a) and PCzDTBF:PC₇₀BM (b); phase images of PCzDTBF:PC₆₀BM (c) and PCzDTBF:PC₇₀BM (d).

Compared with the decyloxy substituted BF unit based polycarbazole (PC-DTBX),²¹PCzDTBF had a shorter octyloxy side chain and higher molecular weight, which may lead to the better device performances. On the other hand, the high FF and nano-scale phase separation could also be effectively contributable to the high photovoltaic properties.

Nevertheless, the weaker absorption of PCzDTBF after 650 nm resulted in the smaller J_sc value compared with other PSCs, in which the J_sc value reached more than 12 mA cm⁻².⁷ Therefore, if the absorption of this polymer can be extended to the longer wavelength, the photovoltaic performances will be escalated to a higher level. Prospective research work is in progress.

Conclusions

An octyloxy substituted BF based polycarbazole (PCzDTBF) was synthesized by Suzuki polycondensation and utilized as a donor material in PSCs. The photovoltaic performance gave a maximal PCE of 4.2% under the device configuration of ITO/PSS:PEDOT/PCzDTBF:PC₇₀BM/Al. By using PFN as the cathode interfacial layer, the PCE was enhanced to 5.48%, with a V_oc of 0.90 V, a J_sc of 7.73 mA cm⁻², and a FF of 67%. The high efficiencies were possibly attributed to the good nano-scale phase separation between PCzDTBF and PCBM components, resulting from their matchable surface energies, and the distinct improvement of V_oc and FF by the PFN layer, where a built-in field potential possibly exists between the active layer and cathode.

Acknowledgements

This work was financially supported by the National Nature Science Foundation of China (nos. 50990065, 51010003 and 21074038), and the State Key Basic Research Project of China (nos. 2009CB623600 and 2009CB930604).

Notes and references

1. Y. F. Li, Acc. Chem. Res., 2012, 45, 723.
2. P. M. Beaujuge and J. M. J. Fréchet, J. Am. Chem. Soc., 2011, 133, 20009.
3. Z. C. He, C. M. Zhong, X. Huang, W. Y. Wong, H. B. Wu, L. W. Chen, S. J. Su and Y. Cao, Adv. Mater., 2011, 23, 4636.
4. Y. Y. Liang, Y. Wu, D. Q. Feng, S. T. Tsai, H. J. Son, G. Li and L. P. Yu, J. Am. Chem. Soc., 2009, 131, 56.
5. Y. Y. Liang, D. Q. Feng, Y. Wu, S. T. Tsai, G. Li, C. Ray and L. P. Yu, J. Am. Chem. Soc., 2009, 131, 7792.
6. J. H. Hou, H. Y. Chen, S. Q. Zhang, R. I. Chen, Y. Yang, Y. Wu and G. Li, J. Am. Chem. Soc., 2009, 131, 15586.
7. H. Y. Chen, J. H. Hou, S. Q. Zhang, Y. Y. Liang, G. W. Yang, Y. Yang, L. P. Yu, Y. Wu and G. Li, Nat. Photonics, 2009, 3, 649.
8. L. J. Huo and J. H. Hou, Polym. Chem., 2011, 2, 2453.
9. J. H. Hou, M. H. Park, S. Q. Zhang, Y. Yao, L. M. Chen, J. H. Li and Y. Yang, Macromolecules, 2008, 41, 6012.
10. O. Inganäs, F. L. Zhang, K. Tvingstedt, L. M. Andersson, S. Hellstrom and M. R. Andersson, Adv. Mater., 2010, 22, E100.
11. E. G. Wang, L. Wang, L. F. Lan, C. Luo, W. L. Zhuang, J. B. Peng and Y. Cao, Appl. Phys. Lett., 2008, 92, 033307.
12. N. Blouin, A. Michaud and M. Leclerc, Adv. Mater., 2007, 19, 2295.
13. S. H. Park, A. Roy, S. Beaupré, S. Cho, N. Coates, J. S. Moon, D. Moses, M. Leclerc, K. Lee and A. J. Heeger, Nat. Photonics, 2009, 3, 297.
14. W. Zhao, W. Z. Cai, R. X. Xu, W. Yang, X. Gong, H. B. Wu and Y. Cao, Polymer, 2010, 51, 3196.
15. E. G. Wang, L. T. Hou, Z. Q. Wang, S. Hellström, F. L. Zhang, O. Inganäs and M. R. Andersson, Adv. Mater., 2010, 22, 5240.
16. N. Blouin, A. Michaud, D. Gendron, S. Wakim, E. Blair, R. Neagu-Plesu, M. Belletete, G. Durocher, Y. Tao and M. Leclerc, J. Am. Chem. Soc., 2008, 130, 732.
17. J. C. Bijleveld, M. Shahid, J. Gilot, M. M. Wienk and R. A. J. Janssen, Adv. Funct. Mater., 2009, 19, 3262.
18. C. V. Hoven, X. D. Dang, R. C. Con, J. Peet, T. Q. Nguyen and G. C. Bazan, Adv. Mater., 2010, 22, E63.
19. W. Y. Nie, C. M. MacNeill, Y. Li, R. E. Noftle, D. L. Carroll and R. C. Coffin, Macromol. Rapid Commun., 2011, 32, 1163.
20. R. P. Qin, W. W. Li, C. H. Li, C. Du, C. Veit, H. F. Schleiermacher, M. Andersson, Z. S. Bo, Z. P. Liu, O. Inganäs, U. Wuerfel and F. L. Zhang, J. Am. Chem. Soc., 2009, 131, 14612.
21. P. Ding, C. M. Zhong, Y. P. Zou, C. Y. Pan, H. B. Wu and Y. Cao, J. Phys. Chem. C, 2011, 115, 16211.
22. J. M. Jiang, P. A. Yang, H. C. Chen and K. H. Wei, Chem. Commun., 2011, 47, 8877.
23. J. M. Jiang, P. A. Yang, T. H. Hsieh and K. H. Wei, Macromolecules, 2011, 44, 9155.
24. J. Luo, H. B. Wu, C. He, A. Y. Li, W. Yang and Y. Cao, Appl. Phys. Lett., 2009, 95, 043301.
25. C. He, C. M. Zhong, H. B. Wu, R. Q. Yang, W. Yang, F. Huang, G. C. Bazan and Y. Cao, J. Mater. Chem., 2010, 20, 2617.
26. J. L. Sessler, W. B. Callaway, S. P. Dudek, R. W. Date and D. W. Bruce, Inorg. Chem., 2004, 43, 6650.
27. J. D. Kotlarski, D. J. D. Moet and P. W. M. Blom, J. Polym. Sci., Part B: Polym. Phys., 2011, 49, 708.
28. G. J. Zhao, Y. J. He and Y. F. Li, Adv. Mater., 2010, 22, 4355.
29. J. S. Kim, Y. M. Lee, J. H. Lee, J. H. Park, J. K. Kim and K. Cho, Adv. Mater., 2010, 22, 1355.

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