Table 1 Characteristics of devices 1–3
100 cd mꢀ2
1000 cd mꢀ2
V (V) PE (lm Wꢀ1
)
CE (cd Aꢀ1
)
EQE (%) V (V) PE (lm Wꢀ1
)
CE (cd Aꢀ1
) EQE (%) lmax (nm) FWHM (nm) CIE
Device
1
2
3
3.4
3.3
3.1
6.0
8.1
22.4
6.5
8.4
22.2
3.0
3.6
6.2
4.3
4.0
3.6
4.8
6.7
19.1
6.6
8.6
22.0
3.0
3.7
6.2
487
492
512
101
94
57
0.191, 0.318
0.196, 0.366
0.265, 0.647
blue to green may be useful for white light applications achieving
a high color rendering index.
In summary, the bonding of two pyrenes to the 1,8-positions
of a naphthalene core creates a strong intramolecular excimer-
emitting compound. The device made with BPyN as a host for
a green fluorescent dopant showed high efficiencies: 22 cd Aꢀ1
,
22 lm Wꢀ1 and 6.2% EQE. The EL spectrum of the device with
BPyN as an emitter was very broad, with a FWHM of 101 nm
due to the intramolecular excimer emission of the pyrenes.
This study was supported by an Industrial Technology
Research Grant for Young Researchers from the New Energy
and Industrial Technology Development Organization (NEDO).
We would also like to thank the Japan Regional Innovation
Strategy Program by the Excellence (J-RISE) of Japan Science
and Technology Agency (JST) for partial support.
Fig. 3 EL spectra of BPyN in device 1 (dotted line) and C545T in
device 3 (solid line).
Notes and references
EML (Fig. S5, ESIw). TAPC also has a higher LUMO level,
efficiently blocking electrons from EML, and has high energy
levels in the S1 and T1 states, which can efficiently confine
the singlet and triplet excitons in the EML. EQE was 3.7% at
1000 cd mꢀ2 (Table 1), much higher than the theoretical upper
limit, which is 2.3% EQE when outcoupling efficiency is
assumed to be 20% and 3.5% EQE with 30% outcoupling
efficiency. Device 3 with BPyN as a host of the green emitting
¨
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dopant exhibited high efficiencies: 6.2% EQE, 22 cd Aꢀ1
,
22 lm Wꢀ1 at 100 cd mꢀ2 (0.45 mA cmꢀ2) and 6.2% EQE,
22 cd Aꢀ1, 19 lm Wꢀ1 at 1000 cd mꢀ2 (4.5 mA cmꢀ2). There
was almost no roll-off of the efficiency from 100 cd mꢀ2 to
1000 cd mꢀ2 and even at 6500 cd mꢀ2, 6.0% EQE was
obtained (Fig. 2(b)). This is a large advantage for applications
requiring high luminance. The PLQE of 1 wt% C545T in
BPyN was 65%. So, the theoretical upper limit of the EQE would
be in a range of 3.3–4.9%, depending on the outcoupling
efficiency. The much higher EQEs than the theoretically obtained
values could be due to increased singlet exciton from triplet–
triplet annihilation between BPyN and/or C545T molecules.
These results showed that BPyN should be a comparable
candidate as a host material for highly efficient fluorescence
OLEDs to widely-used diphenylanthracene derivatives. Device
3 with C545T green emitting dopant had a peak emission at
512 nm with a shoulder at 552 nm and a narrow FWHM of
57 nm. In contrast, device 1 with BPyN as an emitter had a
peak emission at 487 nm and a large FWHM of 101 nm,
resulting from the excimer emission of BPyN (see Fig. 3). The
EL spectrum of BPyN almost covered the entire green emission
spectrum of C545T and contained most of the deep blue
emission at approximately 450 nm. This broad spectrum from
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c
This journal is The Royal Society of Chemistry 2012
Chem. Commun.