Excimer Formation Promoted by Steric Hindrance in Dual Core Chromophore for OLED Emitters
Lee et al.
ꢀ
hole injection layer (HIL) and N ,N -bis(naphthalen-1-yl)-
ꢀ
N ,N -bis(phenyl) benzidine (NPB) as the hole transporting
layer (HTL). The synthesized materials were used as the
EML, 8-hydroxyquinoline aluminum (Alq ꢃ as the electron
3
transporting layer (ETL), lithium fluoride (LiF) as the
electron injection layer (EIL), and ITO as the anode, and
Al as the cathode. All organic layers were deposited under
−6
−1
1
0
Torr, at a rate of deposition of 1 Å s , to give an
2
emitting area of 4 mm . The LiF and aluminum layers
were continuously deposited under the same vacuum con-
ditions. The luminance efficiency data of the fabricated EL
devices were obtained using a Keithley 2400 electrometer.
Light intensities were obtained using a Minolta CS-1000A.
The operational stabilities of the devices were measured
under encapsulation in a glove box.
3
. RESULTS AND DISCUSSION
The optical properties of the synthesized materials are
summarized in Table I. Figure 1 exhibits UV-Visible
(
3
UV-Vis.) absorption and PL spectra of TP-AP-TP and
P-AP-3P in THF solution and film state. UV-Vis. absorp-
tion of the two materials in solution and film state shows
the typical anthracene absorption band in the range of
3
3
3
50∼400 nm. Major absorption peaks of TP-AP-TP and
P-AP-3P in THF solution state were exhibited at 360,
79, 400 nm and 360, 378, 399 nm, respectively (see
Fig. 1(a)). Compared t oD seo l li vu et i ro en ds bt a yt e I, n Ug Ve n- Vt ai s t. oa:b Ss ot ar pt et i oUn niversity of New York at Binghamton
in film state was slightly red shifted, which indicated an
increase in conjugation length in solid state. The increase
in conjugation length might be due to the more planar con-
formation of ꢀ-stacked compounds in solid state. The PL
IP: 5.101.220.147 On: Wed, 07 Jun 2017 10:35:36
Copyright: American Scientific Publishers
Figure 1. UV-Vis. absorption (solid line) and PL spectra (open line) of
TP-AP-TP (square) and 3P-AP-TP (triangle): (a) solution in THF (1.0×
−5
8
10 M), (b) evaporated film.
maximum wavelengths of TP-AP-TP and 3P-AP-3P were
also measured at 446 nm and 444 nm in solution state.
As shown by PL spectra in Figure 1(b), unlike TP-AP-TP,
ITO/2-TNATA (60 nm)/NPB (15 nm)/the synthesized
materials (35 nm)/Alq (20 nm)/LiF (1 nm)/Al (200 nm).
3
3
P-AP-3P exhibited an excimer emission at 602 nm by
The results are summarized in Table II.
As shown in Figure 2(a), the EL maximum value of TP-
AP-TP was 455 nm. 3P-AP-3P showed blue emission at
introducing pyridine groups despite its highly twisted
core chromophore. Pyridine group is known to increase
molecular orientation in evaporated film because of inter-
4
63 nm, and excimer emission was observed at 601 nm
9
molecular hydrogen bond between C–H and N. There-
similar to PL measurement in film state. Figure 2(b)
shows the luminance (cd/A) and power (lm/W) efficiencies
versus current density of 3P-AP-3P. OLED devices based
on TP-AP-TP and 3P-AP-3P showed luminance efficien-
fore, molecular orientation of 3P-AP-3P would be more
increased in comparison to TP-AP-TP, which can promote
molecular orientation suitable for excimer formation in
film state.6
2
cies of 5.34 cd/A and 3.45 cd/A at 10 mA/cm , respec-
OLED devices were fabricated by using the synthe-
sized materials as the EML layer, and the EL char-
acteristics were examined with device configuration of
tively. Power efficiencies (lm/W) were 2.58 lm/W and
Table II. EL performance of the devices: ITO/2-TNATA (60 nm)/NPB
(
15 nm)/the synthesized materials (35 nm)/Alq (20 nm)/LiF (1 nm)/Al
Table I. Optical properties of the synthesized materials.
3
2
(200 nm) at 10 mA/cm .
a
b
Solution
Film
UVmax (nm)
a
b
c
L. E. (cd/A) P. E. (lm/W)
CIE(x, y)
ELmax (nm)
UVmax (nm)
PLmax (nm)
PLmax (nm)
TP-AP-TP
3P-AP-TP
5.34
3.45
2.58
1.67
(0.150, 0.130)
(0.192, 0.203)
455
463, 601
TP-AP-TP 360, 379, 400
446
444
361, 382, 403
365, 383, 404
458
463, 602
3P-AP-TP 360, 378, 399
Notes: aL. E.: luminance efficiency, bP. E.: power efficiency, cCIE: Commission
Internationale de l’Eclairage.
Notes: aTHF solution (1×10−5 M), bevaporated film.
8856
J. Nanosci. Nanotechnol. 16, 8854–8857, 2016