Novel blue exciplex comprising acridine and sulfone derivatives as a host material for high-efficiency blue phosphorescent OLEDs

Article abstract of DOI:10.1246/cl.151094

Awide-energy-gap exciplex comprising bis[4-(9,9-dimethyl- 9,10-dihydroacridine-10-yl)phenyl]diphenylsilane (SiDMAC) and 9,9-bis[4-(phenylsulfonyl)phenyl]fluorene (BPSPF) was developed as a host material for blue phosphorescent organic light-emitting devices (OLEDs). Using a combination of this exciplex as a host material and blue phosphorescent emitter iridium(III) bis[(2′,6′-difluoro-2,3′-bipyridinato-N,C4′)]acetylacetonate [(dfpypy)2Ir(acac)], we developed OLEDs that realized a low driving voltage of 3V and a high power efficiency (ν_p) of 32 lmW^-1 at 100 cdm^-2.

Full text of DOI:10.1246/cl.151094

Received: November 26, 2015 | Accepted: December 23, 2015 | Web Released: December 29, 2015
CL-151094
Novel Blue Exciplex Comprising Acridine and Sulfone Derivatives as a Host Material
for High-efficiency Blue Phosphorescent OLEDs
Yuki Seino,1,2 Hisahiro Sasabe,*1,2 Masato Kimura,1,2 Susumu Inomata,1,2 Kohei Nakao,1,2 and Junji Kido*1,2
1
Department of Organic Device Engineering, Yamagata University, 4-3-16 Jonan, Yonezawa, Yamagata 992-8510
2
Research Center for Organic Electronics, Yamagata University, 4-3-16 Jonan, Yonezawa, Yamagata 992-8510
(
E-mail: kid@yz.yamagata-u.ac.jp, h-sasabe@yz.yamagata-u.ac.jp)
Awide-energy-gap exciplex comprising bis[4-(9,9-dimethyl-
,10-dihydroacridine-10-yl)phenyl]diphenylsilane (SiDMAC)
9
O
O
S
S
and 9,9-bis[4-(phenylsulfonyl)phenyl]fluorene (BPSPF) was
developed as a host material for blue phosphorescent organic
light-emitting devices (OLEDs). Using a combination of this
exciplex as a host material and blue phosphorescent emitter
O
O
N
Si
N
4¤
iridium(III) bis[(2¤,6¤-difluoro-2,3¤-bipyridinato-N,C )]acetyl-
acetonate [(dfpypy)2Ir(acac)], we developed OLEDs that realized
a low driving voltage of 3 V and a high power efficiency (©p) of
SiDMAC
BPSPF
¹
1
¹2
Figure 1. Chemical structures of SiDMAC and BPSPF.
3
2 lm W at 100 cd m .
Keywords: Organic light-emitting device (OLED)
Wide-band-gap exciplex
Blue phosphorescent OLED
|
First, we conducted density functional theory (DFT)
calculations of the electron-donor material SiDMAC and the
electron-acceptor material BPSPF to estimate their photophys-
ical properties. The optimized ground-state structures were
calculated at the RB3LYP 6-31G(d) level of theory. The single-
point energies were calculated at the corresponding RB3LYP
6-311+G(d,p) levels. The highest occupied molecular orbital
(HOMO) energy of SiDMAC was estimated to be 5.39 eV, and
the lowest unoccupied molecular orbital (LUMO) energy was
1.23 eV. The corresponding HOMO/LUMO energies of BPSPF
were estimated to be 6.59 and 2.01 eV, respectively. Thus, the
energy difference (¦EDA) between the HOMO level of SiDMAC
and the LUMO level of BPSPF was evaluated to be 3.37 eV.
Because the triplet energy of an exciplex is close to the ¦EDA
value, this combination of SiDMAC and BPSFP is promising as
a host material for blue OLEDs.
|
High-efficiency organic light-emitting devices (OLEDs)
incorporated with phosphorescent and/or thermally activated
delayed fluorescence emitters have attracted considerable atten-
tion for next-generation lighting and eco-friendly flat panel
display applications because they can harvest all the molecular
excitons of triplets and singlets.1 For OLEDs using triplet
excitons, the host material plays a key role in determining OLED
­5
6
performance. Recently, high-efficiency and low-driving-voltage
phosphorescent OLEDs using exciplexes as host materials have
7
attracted considerable attention. An exciplex host comprises
electron-donor and electron-acceptor molecules, creating a
bipolar transport property that leads to superior carrier balance
and a small difference in energy (¦EST) between the excited
singlet (ES) and triplet (ET) states. Consequently, a low driving
voltage is realized. Recently, we reported high-performance sky
blue OLEDs based on iridium(III) bis[(4,6-difluorophenyl)pyr-
The synthetic routes to SiDMAC and BPSPF are shown
in Scheme S-1. SiDMAC was synthesized via the Buchwald­
Hartwig reaction of 9,9-dimethyl-9,10-dihydroacridine and
bis(4-bromophenyl)diphenylsilane in 50% yield. BPSPF was
prepared from 9,9-bis(4-aminophenyl)fluorene via a multistep
reaction in reasonable yield (see SI in detail). SiDMAC and
2¤
idinate-N,C ]picolinate (FIrpic) that utilize an exciplex formed
at the interface between bis[4-(N,N-ditolylamino)phenyl]cyclo-
1
hexane (TAPC) and 5¤,5¤¤¤¤-sulfonyldi-1,1¤:3¤,1¤¤-terphenyl
BPSPF were characterized by H NMR, mass spectrometry,
8
(
BTPS). The optimized device exhibited a low driving voltage
and elemental analyses. The products were purified by train
sublimation before device fabrication.
¹
2
¹1
¹2
of 2.9 V at 100 cd m , an © of 46 lm W at 100 cd m , and
p
Commission Internationale de l’Eclairage chromaticity coordi-
nates (CIEx,y) of (0.16, 0.38). However, this exciplex cannot be
used as a host material in OLEDs with deeper blue emitters than
FIrpic, because the ET of this exciplex is around 2.8 eV.
The thermal properties of SiDMAC and BPSPF were
measured by differential scanning calorimetry (DSC) and
thermogravimetric analysis (TGA). The glass-transition temper-
atures (Tg) of SiDMAC and BPSPF were observed to be 116 and
129 °C, respectively, and their respective melting points (Tm)
were 308 and 279 °C. Weight losses of only 5% (Td5) were
observed for SiDMAC and BPSPF at temperatures above
400 °C, indicating high thermal stabilities. The ionization
potentials (Ip) were determined by photoelectron yield spec-
troscopy (PYS) to be 5.60 eV for SiDMAC and 6.60 eV for
BPSPF. The electron affinities (Ea) were estimated to be 1.97 eV
for SiDMAC and 2.69 eV for BPSPF by subtracting the optical
energy gap (E ) from I . Phosphorescent spectra were measured
In this work, we developed a novel exciplex host com-
prising bis[4-(9,9-dimethyl-9,10-dihydroacridine-10-yl)phenyl]-
diphenylsilane (SiDMAC) and 9,9-bis[4-(phenylsulfonyl)phen-
yl]fluorene (BPSPF) for blue OLEDs (Figure 1). We used a blue
phosphorescent emitter, iridium(III) bis[(2¤,6¤-difluoro-2,3¤-
4¤
9
bipyridinato-N,C )]acetylacetonate [(dfpypy) Ir(acac)], which
2
shows bluer emission than the sky blue emitter FIrpic.
Consequently, we realized a blue device with a low driving
¹2
¹1
¹2
voltage of 3 V at 100 cd m , an © of 32 lm W at 100 cd m
,
p
g
p
and CIEx,y of (0.15, 0.27).
at 5 K using a streak camera. Onset phosphorescence was
Chem. Lett. 2016, 45, 283–285 | doi:10.1246/cl.151094
© 2016 The Chemical Society of Japan | 283

Table 1. Physical properties of SiDMAC and BPSPF
(
a)
0 wt%
8 wt%
12 wt%
a
T /°Ca
I /eVb
E /eVc
d
Comp.
Tg/°C
Ea/eV
m
p
g
16 wt%
SiDMAC
BPSPF
a
116
129
308
279
5.60
6.60
b
3.63
3.91
1.97
2.69
Determined by a DSC measurement. Measured by a photo-
c
electron yield spectrometer. Taken as the point of intersection
of the normalized UV­vis absorption spectra. Calculated
d
using Ip and Eg values.
300
400
500
600
700
SiDMAC
BPSPF
Wavelength/nm
SiDMAC:BPSPF
250
200
150
100
50
0
4
3
2
1
0
-1
(b)
1
0
0
8
wt%
wt%
12 wt%
10
16 wt%
10
10
10
10
3
00 350 400 450 500 550
Wavelength/nm
-2
10
0
1
2
3
4
5
6
7
8
Figure 2. Normalized PL spectra of SiDMAC, BPSPF, and
SiDMAC/BPSPF (1:1) co-deposited films.
Voltage/V
4
0
(c)
0 wt%
8 wt%
observed at 2.99 eV for SiDMAC. Unfortunately, the phosphor-
escent spectrum of BPSPF could not be obtained. However,
based on a calculated ET of 3.05 eV, we assume that the ET
of BPSPF is similar to that of SiDMAC. All the determined
physical properties are summarized in Table 1.
35
12 wt%
6 wt%
1
3
0
5
2
20
To confirm exciplex formation between SiDMAC and
BPSPF, the PL spectra of SiDMAC, BPSPF, and 1:1 SiDMAC:
BPSPF co-deposited films were collected (Figure 2). The PL
emission peaks of SiDMAC and BPSPF were located at 390 and
1
5
0
5
0
1
3
55 nm, respectively, while the emission peak of the SiDMAC/
BPSPF film was observed at a much longer wavelength
437 nm). The corresponding photon energy of the exciplex
was evaluated to be 2.84 eV, which is almost equivalent to the
1
10
100
1000
–2
(
Luminance/cd m
¦
EDA of 2.91 eV.
Prior to device fabrication, the photophysical properties
Figure 3. (a) EL spectra, (b) J­V­L characteristics, and (c) ©p­
L characteristics of blue OLEDs at various doping concentration.
of 16 wt % [(dfpypy)2Ir(acac)]-doped host films (30 nm) were
evaluated. The photoluminescent quantum efficiency (©PL) was
istics are shown in Figure S-1. The OLED using the exciplex
host showed a much lower driving voltage (2.67 V at 1 cd m )
¹
2
measured under N flow using an integrating sphere with ex-
2
citation at 330 nm and a multichannel spectrometer as the optical
detector. The ©PL’s values of the [(dfpypy)2Ir(acac)]-doped
SiDMAC, BPSPF, and exciplex (1:1 SiDMAC:BPSPF) films
were similar (51 « 1%, 52 « 1%, and 54 « 1%, respectively).
To investigate the function of host materials, blue OLEDs
with the following structures were fabricated: [ITO (130 nm)/
than the SiDMAC-based (3.00 V) and BPSPF-based (2.76 V)
OLEDs, clearly demonstrating the advantage of the exciplex
host. Although the © ’s values of the three [(dfpypy) Ir(acac)]-
PL
2
doped host films were similar, the exciplex host-based OLED
¹
1
¹1
achieved higher ©ext values of 13.7% (22.2 lm W , 25.2 cd A
)
¹
2
¹1
¹1
at 100 cd m
and 12.7% (15.8 lm W , 23.3 cd A ) at
¹
2
TAPC (30 nm)/SiDMAC (5 nm)/16 wt % [(dfpypy) Ir(acac)]-
1000 cd m . Therefore, the use of exciplex as the host material
in OLEDs can improve the charge-carrier balance, leading to
higher efficiency.
2
doped host material (30 nm)/BPSPF (5 nm)/3,5,3¤¤,5¤¤-tetra-
3-pyridyl-[1,1¤;3¤,1¤¤]terphenyl (B3PyPB) (50 nm)/Liq (1 nm)/
Al (100 nm)]. The device performances are summarized in
Table S-1. The EL spectra of all devices exhibited emissions
only from [(dfpypy)2Ir(acac)] with no emissions from the neigh-
boring materials. The luminance­voltage (L­V) characteristics
and external quantum efficiency­luminance (©ext­L) character-
We also investigated the effect of doping concentration
on device performance. Blue OLEDs with structures of [ITO
(130 nm)/TAPC (40 nm)/SiDMAC (5 nm)/x wt % [(dfpypy)2-
Ir(acac)]-doped exciplex host (1:1) (30 nm, x = 0, 8, 12,
16 wt %)/BPSPF (5 nm)/B3PyPB (40 nm)/LiF (0.8 nm)/Al
284 | Chem. Lett. 2016, 45, 283–285 | doi:10.1246/cl.151094
© 2016 The Chemical Society of Japan

(100 nm)] were fabricated (see SI Figure S-2 for details). The
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¹
1
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¹
2 9b
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8
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1
Chem. Lett. 2016, 45, 283–285 | doi:10.1246/cl.151094
© 2016 The Chemical Society of Japan | 285