Organic Letters
Letter
Table 2. EL Characteristics of Various PhOLEDs Devices
a
b
b
b
device
host
Von (V)
Lmax (cd/m2)
CE (cd/A)
PE (lm/W)
EQE (%)
c
A: 12% FIrpic
o-CbzBz
3.5
3.7
3.8
3.7
14 850@11 V
11 160@10.5 V
22 790@12 V
34 600@12 V
49.5, 47.3
57.5, 55.4
78.4, 60.6
60.3, 52.0
43.6, 26.4
48.9, 30.0
66.5, 29.4
54.1, 29.1
22.2, 21.3
27.0, 26.0
20.4, 14.3
20.1, 17.3
c
B: 6% FIrpic
o-DiCbzBz
o-DiCbzBz
o-DiCbzBz
d
C: 6% Ir(ppy)3
c
D: 0.5% (Bt)2Ir(acac)
a
b
c
Turn-on voltage at 10 cd/m2. The maximum value and the values at 1000 cd/m2. ITO glass/TAPC (50 nm)/mCP (10 nm)/hosts: X% dopant
d
(30 nm)/DPPS (35 nm for o-CbzBz and 45 nm for o-DiCbzBz)/LiF (0.9 nm)/Al (120 nm). 50 nm for DPPS.
DiCbzBz (2.3 eV) and FIrpic (2.9 eV), the electron-trapping
mechanism is expected. In contrast, FIrpic should act as a
shallow hole trap because of a small difference in HOMO levels
between FIrpic (5.8 eV) and o-DiCbzBz (5.8 eV).12 The
observation of the hole-mobility drop in the FIrpic doped o-
DiCbzBz is quite unexpected. Nevertheless, the relatively high
hole mobility in comparison to the electron mobility secures
the position of the recombination zone nearby the cathode.
This is important for the high EL efficiency because the exciton
quenching phenomenon is usually significant at the mCP
interface.
In the (Bt)2Ir(acac) doped o-DiCbzBz, due to the higher
HOMO of (Bt)2Ir(acac), the hole current drops significantly in
comparison to that of the FIrpic doped device. Nevertheless,
the apparent hole mobility is still higher than the electron
mobility to secure the recombination zone nearby the cathode.
On the other hand, for the Ir(ppy)3 doped devices, although
the hole current is higher than the electron current in the low
voltage region, the curves cross over at 2 V and the electron
current in the EOD becomes slightly higher than that of the
HOD at the high voltage region, indicating that the
recombination zone may migrate toward the anode under
high electrical voltage conditions. This reflects on the relatively
large roll-off of the device efficiency.
155-028-MY3, 104-2221-E-002-156-MY3, and NSC 102-2221-
E-002-182-MY3). Prof. Yu-Tai Tao, Institute of Chemistry,
Academia Sinica, for discussions and AC-II measurements.
REFERENCES
■
(1) (a) Baldo, M. A.; O’Brien, D. F.; You, Y.; Shoustikov, A.; Sibley,
S.; Thompson, M. E.; Forrest, S. R. Nature 1998, 395, 151−154.
(b) Baldo, M. A.; Thompson, M. E.; Forrest, S. R. Nature 2000, 403,
750−753.
(2) (a) Holmes, R. J.; Forrest, S. R.; Tung, Y.-J.; Kwong, R. C.;
Brown, J. J.; Garon, S.; Thompson, M. E. Appl. Phys. Lett. 2003, 82,
2422−2424. (b) Zhou, G.-J.; Wong, W.-Y.; Yao, B.; Xie, Z.; Wang, L. J.
Mater. Chem. 2008, 18, 1799−1809.
(3) Sun, D.; Ren, Z.; Bryce, M. R.; Yan, S. J. Mater. Chem. C 2015, 3,
9496−9508.
(4) (a) Lai, C.-C.; Huang, M.-J.; Chou, H.-H.; Liao, C.-Y.; Rajamalli,
P.; Cheng, C.-H. Adv. Funct. Mater. 2015, 25, 5548−5556. (b) Ho, C.-
L.; Chi, L.-C.; Hung, W.-Y.; Chen, W.-J.; Lin, Y.-C.; Wu, H.; Mondal,
E.; Zhou, G.-J.; Wong, K.-T.; Wong, W.-Y. J. Mater. Chem. 2012, 22,
215−224. (c) Yang, X.; Zhou, G.; Wong, W.-Y. Chem. Soc. Rev. 2015,
44, 8484−8575.
(5) (a) Wagner, D.; Hoffmann, S. T.; Heinemeyer, U.; Munster, I.;
̈
Kohler, A.; Strohriegl, P. Chem. Mater. 2013, 25, 3758−3765.
̈
(b) Chaskar, A.; Chen, H.-F.; Wong, K.-T. Adv. Mater. 2011, 23,
3876−3895. (c) Choi, W.-H.; Tan, G.; Sit, W.-Y.; Ho, C.-L.; Chan, C.
Y.-H.; Xu, W.; Wong, W.-Y.; So, S.-K. Org. Electron. 2015, 24, 7−11.
(6) (a) Leung, M.-k.; Yang, W.-H.; Chuang, C.-N.; Lee, J.-H.; Lin, C.-
F.; Wei, M.-K.; Liu, Y.-H. Org. Lett. 2012, 14, 4986−4989. (b) Liu, D.;
Du, M.; Chen, D.; Ye, K.; Zhang, Z.; Liu, Y.; Wang, Y. J. Mater. Chem.
C 2015, 3, 4394−4401.
In conclusion, we demonstrate the use of the ortho-
substituent steric effect to control orthogonal alignments that
successfully interrupt the π-conjugation. High triplet state
energy can therefore be maintained that would be beneficial as
universal hosts for highly efficient PhOLEDs.
(7) Ding, L.; Dong, S.-C.; Jiang, Z.-Q.; Chen, H.; Liao, L.-S. Adv.
Funct. Mater. 2015, 25, 645−650.
(8) (a) Pan, B.; Wang, B.; Wang, Y.; Xu, P.; Wang, L.; Chen, J.; Ma,
D. J. Mater. Chem. C 2014, 2, 2466−2469. (b) Lin, M.-S.; Yang, S.-J.;
Chang, H.-W.; Huang, Y.-H.; Tsai, Y.-T.; Wu, C.-C.; Chou, S.-H.;
Mondal, E.; Wong, K.-T. J. Mater. Chem. 2012, 22, 16114−16120.
(c) Lee, C. W.; Lee, J. Y. Adv. Mater. 2013, 25, 5450−5454. (d) Leung,
M.-k.; Hsieh, Y.-H.; Kuo, T.-Y.; Chou, P.-T.; Lee, J.-H.; Chiu, T.-L.;
Chen, H.-J. Org. Lett. 2013, 15, 4694−4697.
(9) (a) Qin, W.; Yang, Z.; Jiang, Y.; Lam, J. W. Y.; Liang, G.; Kwok,
H. S.; Tang, B. Z. Chem. Mater. 2015, 27, 3892−3901. (b) Huang, J.-J.;
Leung, M.-k.; Chiu, T.-L.; Chuang, Y.-T.; Chou, P.-T.; Hung, Y.-H.
Org. Lett. 2014, 16, 5398−5401. (c) Liu, M.; Li, X.-L.; Chen, D. C.;
Xie, Z.; Cai, X.; Xie, G.; Liu, K.; Tang, J.; Su, S.-J.; Cao, Y. Adv. Funct.
Mater. 2015, 25, 5190−5198.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
■
S
Synthetic procedures, spectral data, AC-II measurements,
energy diagram, device performance (PDF)
X-ray data for o-CbzBz (CIF)
X-ray data for o-DiCbzBz (CIF)
AUTHOR INFORMATION
Corresponding Authors
(10) (a) D’Andrade, B. W.; Datta, S.; Forrest, S. R.; Djurovich, P.;
Polikarpov, E.; Thompson, M. E. Org. Electron. 2005, 6, 11−20.
(b) Djurovich, P. I.; Mayo, E. I.; Forrest, S. R.; Thompson, M. E. Org.
Electron. 2009, 10, 515−520.
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Notes
(11) Mamada, M.; Ergun, S.; Per
Phys. Lett. 2011, 98, 073305.
́
ez-Bolívar, C.; Anzenbacher, P. Appl.
(12) (a) Li, C.; Duan, L.; Li, H.; Qiu, Y. J. Phys. Chem. C 2014, 118,
10651−10660. (b) Matsusue, N.; Ikame, S.; Suzuki, Y.; Naito, H. J.
Appl. Phys. 2005, 97, 123512.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Ministry of Science and Technology, R.O.C. (MOST 104-
2119-M-002-023, 101-2113-M-002-010-MY3, 103-3113-E-155-
001, 104-3113-E-155-001, 105-3113-E-155-001, 103-2221-E-
D
Org. Lett. XXXX, XXX, XXX−XXX