High-efficiency red and green light-emitting polymers based on a novel wide bandgap poly(2,7-silafluorene)

Abstract

A new type of high-efficiency red and green light-emitting polymer was synthesized via the Suzuki coupling reaction by incorporating narrow bandgap (NBG) comonomers 4,7-di(4-hexyl-2-thienyl)-2,1,3-benzothiadiazole (DHTBT) and 2,1,3-benzothiadiazole (BT), respectively, into the backbone of poly(2,7-silafluorene) (PSiF). The thermal, photophysical, electrochemical and electroluminescent properties of the PSiF copolymers were investigated and compared with those of the corresponding polyfluorene (PF)-based polymers. The advantages of polymers with PSiF as the main chain over PFs were confirmed by comparison of the electroluminescent performances of PSiF-DHTBT10 and PSiF-BT10 with those of the PF-based copolymers with the same NBG content. Preliminary results showed that the device efficiencies of the emitters containing the same NBG units in the PSiF main chain are higher than those based on PFs. The devices with the configuration of ITO/PEDOT : PSS/PVK/polymer/Ba/Al showed the maximum external quantum efficiency (EQE) of 2.89% and current efficiency (CE) of 2.0 cd A⁻¹ with CIE coordinates of (0.67, 0.33) for PSiF-DHTBT10 and the maximum EQE of 3.81% and CE of 10.6 cd A⁻¹ with CIE coordinates of (0.38, 0.57) for PSiF-BT10, respectively, which are among the best results of fluorescent red or green light-emitting polymers reported so far, indicating PSiF derivatives are a promising class of light-emitting polymers.

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Introduction

Polymer light-emitting diodes (PLEDs) have attracted significant attention in both academic and industrial laboratories because of their potential applications in large-area, flat-panel displays which can operate at relatively low driving voltages.¹ For full-color display applications, the most common accepted method is to print individual red, green and blue (RGB) pixels from light-emitting polymers of high efficiency, color purity and stability.^1,2 A large quantity of conjugated polymers such as polyfluorenes (PFs),^3,4 poly(p-phenylenevinylene),⁵ poly(p-phenylene),⁶carbazole-containing polymers,⁷ and polyalkylthiophenes⁸ have been investigated. Among the conjugated polymers investigated so far, PFs and their derivatives have evolved as a major class of polymers for commercial applications because of their high photoluminescent quantum yield, good solubility and acceptable charge carrier mobility.^2–4 One of the most effective methods to realize red and green emitters is incorporation of suitable narrow bandgap (NBG) chromophores into the side chain or main chain of wide bandgap polymer hosts. The resulting copolymers emit exclusively red or green emission via Förster-type energy transfer from the polymer hosts to the NBG segments.^1–4 Owing to their wide bandgap, PFs can be not only a good blue emitter, but also an excellent wide bandgap host material for preparation of red- and green-emitting polymers.^2–4 However, one of the disadvantages of PF-based polymers is associated with spectral instability due to an undesired emission appearing in the long-wavelength side from 500 to 600 nm in both photoluminescence (PL) and electroluminescence (EL).⁹ Recently, a stable blue light-emitting polymer poly(2,7-silafluorene) (PSiF) was reported by Holmes’ group and it was shown that the replacement of the 9-carbon by silicon in the fluorene ring significantly raises the spectral stability of PSiF over PFs.¹⁰ Later, we showed that both PSiF and poly(2,7-silafluorene-co-3,6-silafluorene) as blue emitters exhibited higher device efficiency than poly(2,7-dioctylfluorene) (PFO) homopolymers.^11,12

Like most π-conjugated polymers, the hole mobility of PFs is much higher than their electron mobility, which results in the imbalance of the charge transport.^2a Moreover, it has been reported that silole-containing compounds exhibit excellent electron-transporting properties originating from the σ*–π* conjugation.¹³ This indicates that PSiF may possess a better balance of hole and electron currents as a result of the improved electron mobility, leading to higher device efficiency than PFs. Owing to the wide bandgap, spectral stability at thermal annealing and possibly better balance of charge transport of PSiF, it could serve as the main chain unit in energy-transfer type copolymers. There is no report on red- or green-emitting PSiF-based polymers so far, although the copolymer poly[2,7-silafluorene-co-5,5-(4,7-di(2′-thienyl)-2,1,3-benzothiadiazole)] was synthesized and reported just very recently as a good donor phase for polymer hetero-junction solar cells by Leclerc’s group.¹⁴

In this paper, we report a new type of high-efficiency red and green light-emitting polymer synthesized by incorporating the NBG comonomers 4,7-di(4-hexyl-2-thienyl)-2,1,3-benzothiadiazole (DHTBT) and 2,1,3-benzothiadiazole (BT), respectively, into the PSiF backbone via the Suzuki coupling reaction. The differences in chemical and photophysical properties between the PSiF- and PF-based copolymers are discussed. The EL performance showed that the efficiencies of the emitters based on PSiF are superior to those based on PFs. The efficiencies of PSiF-based copolymers are among the best results so far reported for red or green fluorescent light-emitting polymers.

Experimental

General information

¹H NMR spectra were recorded on a Bruker AV-300 (300 MHz) in deuterated chloroform solution. Thermal gravimetric analysis (TGA) was conducted on a TGA2050 (TA instruments) thermal analysis system under a heating rate of 30 °C min⁻¹ and a nitrogen flow rate of 20 mL min⁻¹. Differential scanning calorimetry (DSC) was run on a DSC 204 F1 (NETZSCH) thermal analysis system. Number-average (M_n) and weight-average (M_w) molecular weights were determined by a Waters GPC 2410 in tetrahydrofuran (THF) using a calibration curve with standard polystyrene as a reference. The elemental analysis was performed on a Vario EL elemental analysis instrument (ELEMENTAR Co.). UV-vis absorption spectra were recorded on an HP 8453 spectrophotometer. PL spectra were measured using a Jobin-Yvon spectrofluorometer. The PL quantum yields were determined on an integrating sphere IS080 (Labsphere) with 325 nm excitation from a HeCd laser (Mells Griod). Cyclic voltammetry (CV) was performed on a Potentiostat/Galvanostat Model 283 (Princeton Applied Research) electrochemical workstation with platinum working electrodes at a scan rate of 50 mV s⁻¹ against a saturated calomel electrode (SCE) as the reference electrode with a nitrogen-saturated anhydrous solution of 0.1 mol L⁻¹ tetrabutylammonium hexafluorophosphate (Bu₄NPF₆) in acetonitrile (CH₃CN). A thin polymer layer was drop-cast onto the working electrode from a dilute chloroform solution.

EL device fabrication

The LED was fabricated on pre-patterned indium–tin oxide (ITO) with a sheet resistance of 10–20 Ω square⁻¹. The substrate was ultrasonically cleaned with acetone, detergent, deionized water, and 2-propanol subsequently. Onto the ITO glass a layer of polyethylenedioxythiophene–polystyrene sulfonic acid (PEDOT : PSS) film with a thickness of 40 nm was spin-coated from its aqueous dispersion (Baytron P 4083, Bayer AG). The PEDOT : PSS film was dried at 80 °C for 8 h in a vacuum oven. Then polyvinylcarbazole (PVK) from chlorobenzene solution was spin-coated onto PEDOT : PSS and dried at 80 °C for 2 h. The solution of PSiF-DHTBT10 or PSiF-BT10 copolymers in toluene was spin-coated onto the ITO/PEDOT : PSS/PVK surface. We note that since PVK has poor solubility in toluene, PVK remains almost untouched during spinning the top layer from toluene solution. Then, a thin layer of barium as an electron injection cathode and a subsequent 150 nm thick aluminium capping layer were thermally deposited by vacuum evaporation through a mask at a base pressure below 2 × 10⁻⁴ Pa. The deposition speed and thickness of the barium and aluminium layers were monitored by a thickness/rate meter (model STM-100, Sycon Instrument, Inc.). The spin coating of the EL layer and the device performance tests were carried out within a glove box (Vacuum Atmosphere Co.) with nitrogen circulation. Current–voltage (I–V) characteristics were measured with a computerized Keithley 236 Source Measure Unit. The luminance of the device was measured with a photodiode calibrated by using a PR-705 SpectraScan Spectrophotometer (Photo Research). External quantum efficiency was verified by measurement in the integrating sphere (IS080, Labsphere) after encapsulation of the devices with UV-cured epoxy and thin cover glass. EL spectra were recorded on a Instaspec 4 CCD spectrophotometer (Oriel Co.).

Materials

All chemicals and reagents were used as received from Aldrich, ABCR, and Acros Chemical Co. unless otherwise specified. All solvents were carefully dried and purified under nitrogen. All manipulations involving air-sensitive reagents were performed under a dry argon atmosphere. 2,7-Dibromo-9,9-dioctylsilafluorene (1),^10,112,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-dioctylsilafluorene (2),^10,114,7-bis(5-bromo-4-hexyl-2-thienyl)-2,1,3-benzothiadiazole (3)¹⁵ and 4,7-dibromo-2,1,3-benzothiadiazole (4)¹⁶ were prepared according to the published procedures.

Copolymer synthesis

Copolymer PSiF-DHTBT10. Monomer 1 (226 mg, 0.4 mmol), 2 (329 mg, 0.5 mmol), 3 (63 mg, 0.1 mmol), tricyclohexylphosphine (10 mg), and palladium(II) acetate (5 mg) were dissolved in a mixture of toluene (6 mL) and aqueous tetraethylammonium hydroxide (20 wt%, 2 mL) under argon. The resulting mixture was refluxed with vigorous stirring for 48 h, and then phenylboronic acid and bromobenzene were added in order to end-cap the polymer chain. After cooling to room temperature, the resulting mixture was poured into methanol. The precipitated material was collected by filtration through a funnel, and then purified by washing with acetone for 24 h to remove catalyst residues to yield the corresponding copolymer (278 mg, yield: 70%). ¹H NMR (300 MHz, CDCl₃) δ (ppm) 7.42–7.92 (brm, ArH), 0.83–1.41 (brm, CH₂ + CH₃); Anal. calcd: C, 81.26; H, 9.57. Found: C, 80.53; H, 9.74%.

Copolymer PSiF-BT10. Monomer 1 (226 mg, 0.4 mmol), 2 (329 mg, 0.5 mmol) and 4 (29 mg, 0.1 mmol) were used in this polymerization following the similar procedure to copolymer PSiF-DHTBT10 (283 mg, yield: 75%). ¹H NMR (300 MHz, CDCl₃) δ (ppm) 7.45–7.98 (brm, ArH), 0.86–1.26 (brm, CH₂ + CH₃); Anal. calcd: C, 82.05; H, 9.66. Found: C, 81.51; H, 9.84%.

Results and discussion

Synthesis and characterization

The synthesis routes to the copolymers are shown in Scheme 1. PSiF-DHTBT10 and PSiF-BT10 were synthesized by incorporating 10 mol% of DHTBT or BT, respectively, into the PSiF main chain via a modified Suzuki polycondensation. The copolymers are readily soluble in common organic solvents, such as toluene, xylene, THF and chloroform. The chemical structures of the polymers were verified by ¹H NMR and elemental analyses. The M_n of PSiF-DHTBT10 and PSiF-BT10 were 40 000 and 17 000 with polydispersity indices of 2.8 and 2.5, respectively, which were determined by gel permeation chromatography (GPC) using polystyrene as the standard. To investigate the thermal stability of the copolymers, DSC and TGA were performed. Both of the copolymers exhibited good thermal stability with degradation temperatures (T_d) of 408 °C (for PSiF-DHTBT10) and 454 °C (for PSiF-BT10) (5% weight loss). DSC showed both PSiF-DHTBT10 and PSiF-BT10 possess high glass transition temperatures (T_g) of 93 °C and 72 °C, respectively.

Scheme 1 Synthesis routes to PSiF-DHTBT10 and PSiF-BT10.

Optical and photoluminescence properties

The absorption and PL spectra of the polymers in THF solution and films spin-coated from toluene solution are shown in Fig. 1a and b, respectively. The optical bandgaps deduced from the absorption onset and the wavelengths at the maximum of the absorption band of the polymers are summarized in Table 1. The absorption spectra of the polymers in the solid state are similar to (slightly broader than) those of the corresponding copolymers in THF solution, indicating that there are no noticeable molecular conformation changes from solution to the solid state. The absorption spectra of the copolymers show two separate peaks both in solution and in the solid state. The absorption peak appearing at ca. 370 nm in films can be attributed to the SiF segments in the copolymers, for it is consistent with those reported for the PSiF homopolymer (ca. 386 nm).^10,11 A side peak of about 455 nm (for PSiF-BT10) and 515 nm (for PSiF-DHTBT10) can be respectively assigned to the absorption of the BT or DHTBT unit (Fig. 1). The absorption spectra of PSiF copolymers are similar to those of the corresponding PF-based copolymers, which also show two separate absorption peaks at almost the same peak position (Table 1).^15–17 The PL spectrum of PSiF-DHTBT10 in solution under 375 nm excitation shows two emission peaks at 416 and 622 nm, which can be assigned to the emissions from the SiF segments and the DHTBT unit, respectively, judging from the PL spectra of the PSiF homopolymer^10,11 and the PFO-DHTBT copolymer.¹⁵ Unlike in solution, the PL spectrum of PSiF-DHTBT10 in the solid state shows almost only one emission peak at 629 nm indicating that a more effective energy transfer from the SiF segments to the DHTBT unit occurs in the solid state. We note that the PL spectrum of PSiF-DHTBT10 in the solid state peaked at 629 nm showing a blue shift compared with that of its analogous PFO-DHTBT10 (peaked at 647 nm).¹⁵ However, there is almost no such blue shift for the PL peaks of BT emissions in PSiF-BT10 (530 nm) compared with PFO-BT10 (533 nm) (see Table 1 and Fig. 1). Optical properties of the PSiF-based copolymers and the corresponding PF-based copolymers are summarized in Table 1 for comparison. The absolute PL efficiencies of the copolymer films measured in the integrating sphere are included in Table 1, which are comparable for both PSiF-based emitters and PF-based emitters.

Table 1 Physical properties of the copolymers

Polymer	E ox /eV^a	HOMO/eV	LUMO/eV	Bandgap^b/eV	THF solution λmax/nm		Film λmax/nm
					Abs.	PL	Abs.	PL	Φ PL
a Against a SCE. b Deduced from the onset of absorption spectra of the BT and DHTBT units. c Data from reference 15.
PSiF-DHTBT10	0.97, 1.34	−5.74	−3.60	2.14	371, 506	416, 622	372, 515	629	0.53
PSiF-BT10	0.83, 1.43	−5.83	−3.39	2.44	369, 450	414, 532	370, 455	530	0.52
PFO-DHTBT10^c	1.28, 1.38	−5.68	−3.61	2.07	383, 504	418, 654	381, 520	647	0.68
PFO-BT10	1.33	−5.73	−3.34	2.39	379, 450	420, 533	373, 444	533	0.46

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Fig. 1 The absorption and PL spectra of the polymers (a) in THF solution and (b) in the solid state.

Electrochemical properties

To optimize the configuration of the EL devices, it is necessary to determine the energy levels of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the polymers. Thus, the electrochemical behaviors of the polymers were investigated by CV. Both of the polymers displayed two p-doping processes. The first onset of the p-doping processes (0.97 V) of PSiF-DHTBT10 can be attributed to p-doping of the DHTBT unit and the second onset (1.34 V) due to p-doping of the SiF segments, for the value is consistent with that of the PSiF homopolymer (1.45 V).^10,11 Similar oxidation waves can be observed for PSiF-BT10. The HOMO levels of the polymers were calculated according to the empirical formula E_HOMO = −(E_ox + 4.40) (eV),¹⁸ the LUMO levels of the copolymers estimated by subtraction of the optical bandgap (E_g) (eV) from the HOMO values. The electrochemical data are summarized in Table 1. The two well-defined oxidation processes in the CV with the HOMO values of the SiF segments in the copolymers close to those of PSiF homopolymers indicate that the electronic configurations of the SiF segments are not changed significantly by incorporation of the DHTBT or BT unit in the copolymers. This is quite consistent with the two separate absorption peaks observed in the UV absorption spectra (Fig. 1). It was observed that the electrochemical behaviors of the two PSiF-based copolymers were similar to those of the corresponding PF-based copolymers, which are listed in the Table 1 for comparison.

Electroluminescence properties

To investigate the EL properties of the copolymers, double-layer devices were fabricated with a configuration of ITO/PEDOT : PSS(40 nm)/PVK(40 nm)/polymer(80 nm)/Ba(4 nm)/Al(150 nm). Considering a significant barrier for hole injection between the HOMO values of PEDOT (4.8–5.2 eV)¹⁹ and PSiF-DHTBT10 (5.74 eV), a PVK interlayer (with a HOMO value of 5.6–5.8 eV)²⁰ was inserted to improve hole injection in such devices. In fact, inserting a thin film of PVK between the PEDOT : PSS layer and the emitting layer resulting in a general improvement of brightness and external efficiencies has been reported for the PF-based emitters.^15,17Fig. 2 shows the EL spectra of the devices from the copolymers, and the EL spectrum of P27SiF homopolymer synthesized in our previous work¹¹ is also included for comparison. The EL emission peaks of the PSiF-DHTBT10 and PSiF-BT10 copolymers were slightly red-shifted compared to those of the corresponding PL emission in the solid state (Fig. 1b), which indicates both of them originated from the same excited state. The EL emission of the SiF segments was completely quenched for the copolymers and only one peak responsible for the NBG emission appeared in the EL spectra of the copolymers indicating the effective energy transfer occurred from the SiF segments to the NBG units. For direct comparison with PSiF-BT10, PFO-BT10 copolymer containing 10 mol% of 2,1,3-benzothiadiazole and 90 mol% of fluorene was synthesized following the similar procedure to the PSiF copolymers,¹⁶ and the device performance from PFO-BT10 in the same device configuration was investigated. Current efficiency–current density (CE–J) curves of the three copolymer-based devices are plotted in Fig. 3 and the device data are summarized in Table 2. The maximum external quantum efficiency (EQE) of 3.81% and CE of 10.6 cd A⁻¹ with CIE coordinates of (0.38, 0.57) for devices based on PSiF-BT10 were achieved at 585 cd m⁻², which are much higher than those for devices from PFO-BT10 in the same device configuration with the EQE of 2.28% and CE of 5.1 cd A⁻¹, respectively. We note that the device performance of PFO-BT10 in our experiments is comparable to that reported by other groups.¹⁶ This indicates that the performance of PSiF-BT10 as an emitting polymer is superior to that of PFO-BT10, which is among the highest reported so far for green-emitting polymers.¹⁶ A slight enhancement was also observed for devices from PSiF-DHTBT10 with the maximum EQE of 2.89% and CE of 2.0 cd A⁻¹ compared to devices from PFO-DHTBT10 with the maximum EQE of 2.54% and CE of 1.5 cd A⁻¹ reported in the previous work.¹⁵ To the best of our knowledge, the performance of the saturated red-emission with CIE coordinates of (0.67, 0.33) from PSiF-DHTBT copolymers is among the highest reported so far for saturated red-emitting fluorescent polymers.^15,17,21 These results indicate that both PSiF-DHTBT10 and PSiF-BT10 could be promising candidates for high-efficiency red- and green-emitting polymers, respectively. Combining with our previous work¹¹ all three primary colors (RGB) light-emitting polymers with PSiF as the main chain have been realized, indicating a potential for PSiF-based polymers to be used for full-color display applications.

Table 2 EL performance of the copolymers

Polymer	λ max/nm	EQE (%)	CE/cd A^−1	Brightness^b/cd m^−2	1931 CIE (x, y)
a Data from reference 15. b Achieved at 100 mA cm^−2.
PSiF-DHTBT10	643	2.89	2.0	973	(0.67, 0.33)
PFO-DHTBT10^a	638	2.54	1.5	787	(0.66, 0.34)
PSiF-BT10	544	3.81	10.6	6410	(0.38, 0.57)
PFO-BT10	541	2.28	5.1	4033	(0.39, 0.57)

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Fig. 2 EL spectra of the devices based on P27SiF, PSiF-DHTBT10 and PSiF-BT10.

Fig. 3 CE–J characteristics of the three copolymers.

Conclusion

In summary, we synthesized novel red (PSiF-DHTBT10) and green (PSiF-BT10) light-emitting polymers with PSiF as the main chain via the Suzuki coupling reaction by incorporating DHTBT and BT, respectively, into the PSiF backbone. To the best of our knowledge, this is the first report on red and green light-emitting polymers with PSiF as the main chain. The advantages of polymers with PSiF as the main chain over PFs were confirmed by comparison of the EL performances of PSiF-DHTBT10 and PSiF-BT10 with those of the corresponding PF-based copolymers. Preliminary results showed that the device efficiencies of the emitters containing the same NBG units in the PSiF main chain are higher than those based on PFs. The performances of PSiF-DHTBT10 with the maximum EQE of 2.89 % and CE of 2.0 cd A⁻¹ and PSiF-BT10 with the maximum EQE of 3.81% and CE of 10.6 cd A⁻¹ are among the best results of fluorescent red or green light-emitting polymers reported so far, indicating PSiF derivatives are a promising class of light-emitting polymers.

Acknowledgements

This work was supported by research grants from the Ministry of Science and Technology, China (MOST) Project (#2002CB613402) and Natural Science Foundation of China (NSFC) project (#50433030).

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Footnote

† Electronic supplementary information (ESI) available: CV, DSC and TGA characteristics. See DOI: 10.1039/b716607a

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