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Lei Chen , Pengcheng Li , Yanxiang Cheng , Zhiyuan Xie , * Lixiang Wang , * Xiabin Jing , and Fosong Wang

White Electroluminescence from Star-like Single Polymer Systems: 2,1,3-Benzothiadiazole Derivatives Dopant as Orange Cores and Polyfl uorene Host as Six Blue Arms

White polymer light-emitting diodes (WPLEDs) have attracted wide research interest for their potential applications in next-generation full color fl at-panel displays, as a backlight for liquid-crystal displays, and as light sources. [ 1–4 ] Recently, we reported white emissive single-polymer systems through cova-lently attaching a small amount of donor–acceptor–donor (D-A-D)-type orange dopants to a polyfl uorene (PF) backbone. [ 5–7 ] Intrinsic phase separation and voltage-dependent electrolumi-nescent (EL) spectra in polymer blend systems [ 8–10 ] and multi-layer devices [ 11 , 12 ] will be avoided because the dopant can be chemically distributed in the polymer host.

Compared with linear polymers, star-shaped polymers usu-ally show unique properties because of their highly branched molecular structure, especially those of effectively suppressed molecular interaction and enhanced solid-state luminescence. [ 13 ] Our group reported star-like single-polymer systems through incorporating four PF arms into an orange core to realize white electroluminescence with a current effi ciency of more than 7 cd A − 1 . [ 7 ] However, their performance in terms of current effi -ciency and power effi ciency is still not comparable with that of single-layer WPLEDs based on linear single-polymer systems. [ 14 ]

In this communication, we reported two kinds of star-like single-polymer systems through incorporating six blue PF arms onto star-shaped D-A type orange cores (see Scheme 1 ). The six blue PF chains as the branching arms are expected to prevent the orange cores from aggregation and suppress their concentra-tion quenching effect more effectively than previous linear and star-shaped single-polymers, which results in a higher WPLED effi ciency. White electroluminescence was obtained from these star-like single-polymer systems with simultaneous blue emis-sion (436/460 nm) and orange emission (560 nm). Their single-layer devices achieved a high current effi ciency of 18.01 cd A − 1 and an external quantum effi ciency (EQE) of 6.36% with CIE

© 2011 WILEY-VCH Verlag wileyonlinelibrary.com

Dr. L. Chen , Dr. P. Li , Prof. Y. Cheng , Prof. Z. Xie , Prof. L. Wang , Prof. X. Jing , Prof. F. Wang State Key Laboratory of Polymer Physics and ChemistryChangchun Institute of Applied ChemistryChinese Academy of SciencesChangchun 130022, P. R. China E-mail: xiezy_n@ciac.jl.cn; lixiang@ciac.jl.cn L. Chen , P. Li Graduate School of the Chinese Academy of SciencesBeijing 100039, P. R. China

DOI: 10.1002/adma.201100297

coordinates of (0.33, 0.35). To the best of our knowledge, this is the highest reported current effi ciency of single-layer WPLEDs based on single-polymer systems.

The chemical structures of the star-shaped orange cores and a schematic illustration of the polymers are shown in Scheme 1 . The 2,1,3-benzothiadiazole unit was selected as the electron withdrawing unit of the D-A type orange cores, [ 15 ] while triphenylamine and 1,3,5-triphenylbenzene were chosen to be the central units, respectively. As a result, we designed and synthesized two kinds of star-shaped orange model com-pounds (S-OMCs: TPA6 and TPB6) with fi ne-tuned absorp-tion and emission wavelengths as well as high Φ f (TPA6 75%, TPB6 78%). PF was utilized as the branching arms because of its blue emission, high Φ f in the solid state, and good charge transport properties. [ 16 ] Bromination of the S-OMCs with n -(C 4 H 9 ) 4 NBr 3 afforded A 6 -type monomers. Star-shaped poly-mers were synthesized by Suzuki polycondensation with an AB-type 9,9- dioctylfl uorene monomer and A 6 -type orange dopant monomers. The doping concentration of TPA6 and TPB6 was controlled to be 0.01%, 0.02%, and 0.03% to tune the relative intensity of the blue and orange emission. The polymers are denoted as S-WP- x TPA6 and S-WP- x TPB6, where x denotes the contents of TPA6 or TPB6.

The photophysical, electrochemical, and thermal proper-ties of the two S-OMCs are listed in Table 1 . The absorption and emission peaks of TPB6 are blue-shifted by 25 and 16 nm, respectively, compared to TPA6, because the electron-donating ability of 1,3,5-triphenylbenzene is weaker than triphenylamine. The absorption of TPB6 (around 449 nm) overlaps better with the photoluminesence (PL) spectrum of PF (around 420 nm) than that of TPA6 (around 474 nm), suggesting a more effi cient Förster energy transfer for TPB6. Moreover, the human eye is most sensitive to a yellowish-green color at 555 nm, so the blue-shifted orange emission of TPB6 (around 566 nm) will be more helpful for improving WPLED effi ciency than TPA6 (around 582 nm). At the same time, both of the highest occupied molecu lar orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of TPA6 and TPB6 are located between the HOMO/LUMO energy levels of PF, promising effi cient charge trapping of S-OMCs in the EL process. [ 5–7 , 17 , 18 ] Their high glass transition temperature ( T g ) ( > 150 ° C) and thermal decomposition temperature ( T d ) ( > 500 ° C) further guarantee their thermal stability in EL devices.

All these polymers exhibit similar absorption spectra to PF with an absorption peak around at 393 nm, ascribed to

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Scheme 1 . Chemical structures of the star-shaped orange model compounds (S-OMCs) and schematic illustration of the star-like polymers.

the π – π * transition of the PF backbone. The absorption of orange dopants at about 474/449 nm is negligible because of the low doping concentration. However, the PL spectra of all these polymers consists of not only of the blue emission from the PF backbone but also the orange emission from the cores ( Figure 1 ). The orange emission can be attributed to the effi cient Förster energy transfer from the PF host to the orange dopants as mentioned above. The relative intensity of the orange emis-sion increases successively with the doping concentration. As expected, the orange emissions of S-WP- x TPB6 are higher than that of S-WP- x TPA6 at the same doping level because of the better spectral overlap.

To investigate the EL properties of the resulting poly mers, single-layer devices with a confi guration of indium-tin oxide (ITO)/poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT: PSS) (40 nm)/polymer (100 nm)/Ca (10 nm)/Al (150 nm) were fabricated. The EL spectra of the pristine devices of S-WP- x TPA6 and S-WP- x TPB6 are

© 2011 WILEY-VCH Verlag GmAdv. Mater. 2011, 23, 2986–2990

Table 1. Photophysical, electrochemical, and thermal properties of the S-OM

S-OMC λ abs [nm] λ emi [nm] Φ f [%] E HO

TPA6 474 582 75

TPB6 449 566 78

shown in Figure 2 , their peak emission wavelengths and CIE coordinates are shown in Table 2 . The EL spectra of all these polymers exhibit simultaneous blue emission at 424–440 nm from the PF host and orange emission at 576/560 nm from the TPA6/TPB6 cores. For these polymers, the relative inten-sity of the orange emission band is much stronger in the EL spectra compared to those in the PL spectra because of the charge-trapping effect as was expected. [ 23 , 24 ] The relative inten-sity of the orange emission in the EL spectra of these polymers is also enhanced successively with increasing dopant con-centration. Unlike previously reported white emissive single-polymer systems, the orange emission bands of these star-like polymers do not red-shift when the dopant content increases, indicating that the six PF arms can shield the orange cores from aggregation. However, as shown in Table 2 , only S-WP-001TPA6 exhibits white emission with a CIE coordinate at (0.34, 0.31), and the EL spectra of all the other fi ve poly-mers are away from standard white light (0.33, 0.33) because

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Cs.

MO [eV] E LUMO [eV] E g [eV] T g [ ° C] T d [ ° C]

–5.17 –3.11 2.06 165 550

–5.25 –3.13 2.12 155 621

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Figure 1 . Absorption (a,b) and PL (c,d) spectra of the star-like polymers in solid fi lms.

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the blue emission is relatively too weak. For these two kinds of polymers, all the current effi ciency, maximum brightness, and EQE values increase successively with doping concentration,

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Figure 2 . EL spectra of a,b) the pristine devices without thermal annealing

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which also demonstrates that the concentration quenching effect of the dopant units has been effi ciently suppressed in the doping concentration range. For example, as the TPA6

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Table 2. EL performances of the pristine devices without thermal annealing.

Turn-on voltage [V]

Current effi ciency [cd A − 1 ]

Power effi ciency [lm W − 1 ]

Maximum brightness [cd m − 2 ]

EQE [%]

λ max [nm]

CIE ( x , y )

S-WP-001TPA6 4.0 6.10 3.83 7189 2.74 424/448/576 (0.34, 0.31)

S-WP-002TPA6 4.0 8.25 5.45 11 900 3.55 424/448/576 (0.44, 0.41)

S-WP-003TPA6 4.0 9.39 5.56 19 200 3.58 424/448/576 (0.46, 0.43)

S-WP-001TPB6 4.0 11.33 8.33 11 000 3.78 424/448/560 (0.40, 0.46)

S-WP-002TPB6 5.0 14.98 8.96 15 130 4.96 424/448/560 (0.42, 0.46)

S-WP-003TPB6 5.0 18.11 9.48 17 370 5.85 424/448/560 (0.45, 0.51)

concentration increases from 0.01% to 0.03%, the EL performance is improved from a current effi ciency of 6.12 cd A − 1 , a maximum brightness of 9146 cd m − 2 , and a maximum EQE of 2.74% for S-WP-001TPA6 to a current effi ciency of 9.39 cd A − 1 , a maximum brightness of 19 200 cd m − 2 , and a maximum EQE of 3.58% for S-WP-003TPA6. The EL perform-ances of the devices of S-WP- x TPB6 follow the same trend but with even higher EL effi ciencies, perhaps because of the better energy transfer from PF to TPB6. The best device of S-WP-003TPB6 shows a current effi ciency of 18.11 cd A − 1 , a maximum brightness of 17 370 cd m − 2 , and a maximum EQE of 5.85%. It is clear that the pristine device of S-WP-001TPA6 realizes white electroluminescence with only moderate effi -ciency; while the highly effi cient pristine devices of S-WP- x TPB6 do not realize white electroluminescence.

In order to realize highly effi cient white electrolumines-cence, the devices were thermally annealed at 120 ° C for 0.5 h in a vacuum box before the Ca/Al cathode was evaporated. The relative intensity of the blue emission of the annealed device is enhanced obviously with a red-shifted peak emis-sion at 436–460 nm as shown in Figure 2 c and d, which is attributed to the formation of crystalline α -phase PF. [ 19–23 ] As a result, EL performances of the annealed devices of the poly-mers ( Table 3 ) show almost standard white electrolumines-cence for S-WP-003TPA6, S-WP-003TPA6, and S-WP-002TPB6 with the corresponding CIE coordinates of (0.35, 0.31), (0.37, 0.33), and (0.33, 0.35), respectively. The absorption and PL spectra of an annealed and pristine fi lm of S-WP-003TPA6 are shown in Figure 3 . It illustrates that about 20% of α -phase

© 2011 WILEY-VCH Verlag GAdv. Mater. 2011, 23, 2986–2990

Table 3. EL performances of the devices with thermal annealing at 120 ° C

Turn-on voltage [V]

Current effi ciency [cd A − 1 ]

Power effi ciency [lm W − 1 ]

Ma

S-WP-001TPA6 5.0 7.03 4.02

S-WP-002TPA6 5.0 9.49 5.41

S-WP-003TPA6 5.0 11.17 5.61

S-WP-001TPB6 5.0 12.49 7.13

S-WP-002TPB6 5.8 18.01 8.38

S-WP-003TPB6 5.8 20.04 9.85

PF was formed with a peak absorption wavelength around 422 nm after thermal annealing. The generated crystalline α -phase PF can act as a self-dopant [ 24 ] in the amorphous PF host, leading to effi cient energy transfer from the amorphous PF host to the crystalline α -phase, so the blue emission of the annealed devices can be reinforced in both PL and EL proce-sses. In addition, the formation of crystalline α -phase PF will balance the hole and electron transport of the PF host, and fur-ther improve the EL effi ciency. [ 19 , 20 ] In fact, the EL effi ciencies of all the annealed PLEDs are higher than the corresponding pristine ones, especially for the EQE values, which improved around 30%. Among them, the WPLED of S-WP-002TPB6 realized almost pure white EL emission with a high current effi ciency of 18.01 cd A − 1 and an EQE of 6.36% at the CIE coordinate (0.33, 0.35).

In conclusion, we have succeeded in developing star-like white EL single-polymer systems by incorporating six blue PF arms into star-shaped orange cores. Excellent energy transfer from the PF host to the orange dopants was observed, and the concentration quenching effect was effectively suppressed in their PLEDs. Furthermore, we achieved even better white elec-troluminescence with reinforced and red-shifted blue emission through optimizing the devices with thermal annealing treat-ment to generate self-doping α -phase PF. As a result, a white emissive single-layer device was realized with a current effi -ciency of 18.01 cd A − 1 , an EQE of 6.36%, and CIE coordinates of (0.33, 0.35), which, to the best of our knowledge, is among the most highly effi cient single-layer WPLEDs based on single-polymer systems. What’s more, the EL effi ciency can be further

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ximum brightness [cd m − 2 ]

EQE [%]

λ max [nm]

CIE ( x , y )

4852 4.28 436/460/576 (0.25, 0.20)

8226 4.36 436/460/576 (0.35, 0.31)

12 540 4.71 436/460/576 (0.37, 0.33)

7923 5.03 436/460/560 (0.29, 0.29)

12 040 6.36 436/460/560 (0.33, 0.35)

17 440 6.77 436/460/560 (0.39, 0.45)

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Figure 3 . Absorption spectra (a) and PL spectra (b) of pristine and annealed fi lms of S-WP-003TPA6.

improved through device optimization, [ 20 , 25 ] which is under investigation.

Acknowledgements This work was supported by the Changchun Institute of Applied Chemistry, Chinese Academy of Sciences (CX07QZJC-24), the National Natural Science Foundation of China (No.50803062, 60977026), the Science Fund for Creative Research Groups (No.20921061), and the 973 Project (2009CB623601).

Received: January 25, 2011 Revised: March 7, 2011

Published online: May 20, 2011

[ 1 ] B. W. D. Andrade , S. R. Forrest , Adv. Mater. 2004 , 16 , 1585 . [ 2 ] I. U. H. Raja , J. Y. Lee , I. T. Kim , S. H. Lee , Monatsh. Chem 2008 ,

139 , 725 . [ 3 ] H. B. Wu , L. Ying , W. Yang , Y. Cao , Chem. Soc. Rev. 2009 , 38 ,

3391 . [ 4 ] K. T. Kamtekar , A. P. Monkman , M. R. Bryce , Adv. Mater. 2010 , 22 ,

572 . [ 5 ] G. L. Tu , C. Y. Mei , Q. G. Zhou , Y. X. Cheng , Y. H. Geng , L. X. Wang ,

D. G. Ma , X. B. Jing , F. S. Wang , Adv. Funct. Mater. 2006 , 16 , 101 . [ 6 ] J. Liu , Q. G. Zhou , Y. X. Cheng , Y. H. Geng , L. X. Wang , D. G. Ma ,

X. B. Jing , F. S. Wang , Adv. Funct. Mater. 2006 , 16 , 957 . [ 7 ] J. Liu , Y. X. Cheng , Z. Y. Xie , Y. H. Geng , L. X. Wang , X. B. Jing ,

F. S. Wang , Adv. Mater. 2008 , 20 , 1357 . [ 8 ] J. H. Kim , P. Herguth , M.-S. Kang , A. K.-Y. Jen , Y. H. Tseng ,

C. F. Shu , Appl. Phys. Lett. 2004 , 85 , 1116 .

© 2011 WILEY-VCH Verlag Gmwileyonlinelibrary.com

[ 9 ] X. Gong , S. Wang , D. Moses , G. C. Bazan , A. J. Heeger , Adv. Mater. 2005 , 17 , 2053 .

[ 10 ] H. A. Al Attar, A. P. Monkman , M. Tavasli , S. Bettington , M. R. Bryce , Appl. Phys. Lett. 2005 , 86 , 121 101 .

[ 11 ] C. C. Huang , H. F. Meng , G. K. Ho , C. H. Chen , Appl. Phys. Lett. 2004 , 84 , 1195 .

[ 12 ] Y. H. Xu , J. B. Peng , Y. Q. Mo , Q. Hou , Y. Cao , Appl. Phys.Lett. 2005 , 86 , 163 502 .

[ 13 ] W. Y. Lai , R. D. Xia , Q. Y. He , P. A. Levermore , W. Huang , D. D. C. Bradley , Adv. Mater. 2009 , 21 , 355 .

[ 14 ] J. Liu , S. Y. Shao , L. Chen , Z. Y. Xie , Y. X. Cheng , Y. H. Geng , L. X. Wang , X. B. Jing , F. S. Wang , Adv. Mater. 2007 , 19 , 1859 .

[ 15 ] K. R. J. Thomas , J. T. Lin , M. Velusamy , Y. T. Tao , C. H. Chuen , Adv. Funct. Mater. 2004 , 14 , 83 .

[ 16 ] M Leclerc , J. Polym. Sci., Part A: Polym. Chem. 2001 , 39 , 2867 . [ 17 ] K. Utsugi , S. Yakano , J. Electrochem. Soc. 1992 , 139 , 3610 . [ 18 ] M. Uchida , C. Adachi , T. Koyama , Y. Taniguchi , J. App. Phys. 1999 ,

86 , 1680 . [ 19 ] X. D. Niu , B. H. Zhang , Z. Y. Xie , Y. X. Cheng , L. X. Wang , Appl. Phys.

Lett. 2010 , 96 , 073 303 . [ 20 ] B. H. Zhang , C. J. Qin , J. Q. Ding , L. Chen , Z. Y. Xie , Y. X. Cheng ,

L. X. Wang , Adv. Funct. Mater. 2010 , 20 , 2951 . [ 21 ] S. H. Chen , A. C. Su , C. H. Su , S. A. Chen , Macromolecules 2005 , 38 ,

379 . [ 22 ] S. H. Chen , H. L. Chou , A. C. Su , S. A. Chen , Macromolecules 2004 ,

37 , 6833 . [ 23 ] A. L. T. Khan , P. Sreearunothai , L. M. Herz , M. J. Banach , A. Köhler ,

Phys. Rev. B 2004 , 69 , 085 201 . [ 24 ] H. H. Lu , C. Y. Liu , C. H. Chang , S. A. Chen , Adv. Mater. 2007 , 19 ,

2574 . [ 25 ] X. D. Niu , C. J. Qin , B. H. Zhang , J. W. Yang , Z. Y. Xie , L. X. Wang ,

X. B. Jing , F. S. Wang , Appl. Phys. Lett. 2007 , 90 , 203 513 .

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