An electron-transporting host material compatible with diverse triplet emitters used for highly efficient red- and green-electrophosphorescent devices

Article abstract of DOI:10.1039/b807954d

A bifluorene analogue, T2N, containing a pyridyl moiety serves as both a host and an efficient electron-transporting material that is compatible with various heavy metal-containing red (Ir, Ru, Os, and Pt) and green (Ir) phosphors for highly efficient phosphorescent OLEDs possessing simple device architectures. The Royal Society of Chemistry.

Full text of DOI:10.1039/b807954d

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An electron-transporting host material compatible with diverse triplet
emitters used for highly efficient red- and green-electrophosphorescent
devicesw
Tsyr-Yuan Hwu,a Tsung-Cheng Tsai,b Wen-Yi Hung,*b Sheng-Yuan Chang,c
Yun Chi,*c Mei-Hsin Chen,d Chih-I Wu,d Ken-Tsung Wong*a and
Liang-Chen Chia
Received (in Cambridge, UK) 9th May 2008, Accepted 14th July 2008
First published as an Advance Article on the web 2nd September 2008
DOI: 10.1039/b807954d
A bifluorene analogue, T2N, containing a pyridyl moiety serves
as both a host and an efficient electron-transporting material
that is compatible with various heavy metal-containing red (Ir,
Ru, Os, and Pt) and green (Ir) phosphors for highly efficient
phosphorescent OLEDs possessing simple device architectures.
4,40-bis(9-carbazolyl)-2,20-biphenyl (CBP).2a,3a We suspected
that if we implanted a p-electron-deficient heteroarene into the
bifluorene backbone, the resulting molecule would have a
relatively higher electron affinity, which would facilitate injec-
tion and transport of electrons from the cathode. In this paper,
we report a new 4-azafluorene-based host, T2N (Scheme 1), in
which a nitrogen atom has been implanted into the backbone
of a bifluorene unit, exhibiting modified electronic properties.
This new material serves as a suitable ET host for a range of
distinctive green and red emitters possessing wide structural
features, giving electrophosphorescent devices with low turn-
on voltages and remarkable efficiencies.
Dispersing transition metal-containing phosphors into suita-
ble host materials can reduce the quenching processes asso-
ciated with the relatively long lifetimes and triplet–triplet
annihilations of triplet excited states. Using this approach,
both singlet and triplet excitons can be harvested efficiently,
making it possible to prepare organic light-emitting devices
(OLEDs) exhibiting 100% internal efficiency.1 Most of the
efficient host materials reported were hole-transporting (HT)
compounds—in particular, carbazole-based organoaryls;2
electron-transporting (ET) host materials are relatively rare.3
Electrophosphorescent devices incorporating HT-type host
materials normally require sophisticated configurations, in-
corporating various functional materials, including transport-
ing and blocking layers, to facilitate efficient exciton
confinement on the phosphors. Such devices are not suffi-
ciently cost-effective to compete with other contemporary flat-
panel display technologies. In contrast, ET-type host materials
are capable of being fabricated into efficient OLEDs with
simplified device structures. Thus, the development of efficient
ET-type host materials is in high demand for providing extra
flexibility in designing device configurations. In previous stu-
dies, we used spiro-configured bifluorenes as hosts for red
electrophosphorescent devices;4 the maximum electrolumines-
cence (EL) quantum efficiencies observed were substantially
higher than that obtained when using the carbazole-based host
Scheme 1 depicts our synthetic pathway toward T2N. Suzuki
coupling of the pyridine-containing amide 1 and the fluorene
boronic ester 2 afforded the amide 3 in 68% isolated yield. The
azafluorenone 4 was obtained in 77% yield through intramole-
cular cyclization upon treating 3 with LDA. The Grignard
reaction of 4 with p-tolylmagnesium bromide and subsequent
Friedel–Crafts alkylation provided T2N in 42% yield.
Scheme 1 Synthesis of T2N.
a Department of Chemistry, National Taiwan University, Taipei,
Taiwan 106. E-mail: kenwong@ntu.edu.tw; Fax: +886 2 33661667;
Tel: +886 2 33661665
The introduction of a pyridyl moiety as a constituent of the p-
conjugated backbone had an interesting effect on the physical
properties of T2N relative to those of its pure-hydrocarbon
counterpart, T2.5 The fluorescence and absorption spectra of
T2N in CH2Cl2 solution (Fig. 1) exhibited limited red shifting
relative to those of the parent bifluorene T2. The phosphores-
cence of T2N was also measured at 77 K (in EtOH). The
difference between the fluorescence (377 nm) and phosphores-
cence (506 nm) spectra corresponds to an exchange energy of ca.
1.2 eV between the singlet and triplet states of T2N. The triplet
energy of 2.45 eV (defined as the 0–0 transition in the
b Institute of Optoelectronic Sciences, National Taiwan Ocean
University, Keelung, Taiwan 202.
E-mail: wenhung@mail.ntou.edu.tw
c Department of Chemistry, National Tsing Hua University, Hsinchu,
Taiwan 300. E-mail: ychi@mx.nthu.edu.tw
d Department of Electrical Engineering and Graduate Institute of
Electro-optical Engineering, National Taiwan University, Taipei,
Taiwan 106
w Electronic supplementary information (ESI) available: Synthesis
and characterization of new compounds; UPS traces of T2 and
T2N. See DOI: 10.1039/b807954d
ꢀc
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4956 | Chem. Commun., 2008, 4956–4958

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Scheme 2 Structures of the heavy metal complexes used in this study.
Fig. 1 UV-Vis, fluorescence (room temperature in CH2Cl2 solution),
and phosphorescence (77 K, EtOH) spectra of T2N (black) and T2
(red).
polystyrene sulfonate (PEDOT:PSS, 30 nm)/4,40,400-tri(N-
carbazolyl)triphenylamine (TCTA, 40 nm)/T2N:dopant (25
nm)/T2N (35 nm)/LiF (0.5 nm)/Al; TCTA functioned as the
HT layer, while T2N functioned as both host and ET materi-
als. The low barrier between the HOMO energy levels of
TCTA (ꢁ5.6 eV) and T2N (ꢁ5.75 eV) allowed direct hole
injection from TCTA into T2N. The electrons injected from
the T2N layer were trapped by the dopants dispersed within it,
recombining with the holes to generate excitons directly,
allowing efficient confinement of the emissive excitons within
the emissive layer. We used the heavy metal complexes6
(Scheme 2) Os(fptz)2(PPh2Me)2, Os(fppz)2(PPhMe2)2, Btp2Ir-
(acac), Mpq2Ir(acac), Pt(iqdz)2, and Ru(ifpz)2(PPh2Me)2 as
red dopants and PPy2Ir(acac) as a green dopant for the T2N
layer. Table 1 summarizes the electroluminescence charac-
teristics of the resulting devices.
phosphorescent spectrum) of T2N is similar to that of T2 (2.48
eV).4 These results indicate that the photophysical properties are
slightly altered upon introducing the pyridyl moiety onto the
conjugated chromophore.
The highest occupied molecular orbital (HOMO) energy
level (ꢁ5.75 eV) of T2N, taken from ultraviolet photoelectron
spectroscopy (UPS) measurements (Fig. S-1, ESIw), and the
corresponding lowest unoccupied molecular orbital (LUMO)
energy (ꢁ2.50 eV) derived by subtracting the HOMO energy
from the UV-Vis absorption edge (3.25 eV). The energy band
diagrams of T2 and T2N can be obtained from the UPS data
(ESIw). The main features of occupied molecular orbitals of
T2N at 4.6 eV, 7 eV, and 9 eV below the Fermi level all shift
toward lower binding energy with respect to that of T2. The
data indicate that the energy level of HOMO, as well as
LUMO, of the T2N is lower than those of T2 (HOMO:
ꢁ5.65 eV, LUMO: ꢁ2.35 eV). Thus, the electronic structure
of the parent bifluorene T2 was perturbed after embedding the
nitrogen atom into the parent hydrocarbon backbone. This
modification decreased both the HOMO and LUMO energy
levels; in particular, the low LUMO energy level of T2N is a
crucial feature for reducing the energy barrier for electron
injection from the cathode. In addition, the presence of the C9
p-tolyl substituents provided high thermal and morphological
stability to the thin films of T2N, as indicated by their high
decomposition (5% weight loss) temperatures (Td = 340 1C)
As indicated in Fig. 2(a), these devices turned on at low
voltages (2.9–5.7 V at a brightness of 0.1 cd mꢁ2), probably
because the HOMO and LUMO energy levels of T2N matched
well with those of TCTA and cathode, leading devices to have
high current density (Fig. 2(b)). All of the devices exhibited
high external quantum efficiencies (Zext 410%; Fig. 3(a)), and
the EL spectra revealed pure dopant emissions (Fig. 3(b)).
These results suggest that our present device structure, incor-
porating T2N as an ET-type host, generally provides high-
efficiency OLEDs for red phosphors featuring diverse metal
centres, and T2N can also accommodate a green emitter
[PPy2Ir(acac)] efficiently.
Among these devices, the highest external quantum effi-
ciency (15.5% at 0.1 mA cmꢁ2) and highest brightness (29 700
cd mꢁ2 at 19 V) with saturated red emission of CIE1931
coordinates (0.68, 0.32) were obtained when using 9 wt%
Os(fptz)2(PPh2Me)2 as the dopant. To the best of our knowl-
edge, this is the highest efficiency reported for a red
and moderately high glass transition temperatures (Tg
=
144 1C). The relatively high triplet energy of T2N suggests
that it might be a suitable host material for yellow to red
phosphors. We employed a relatively simple configuration for
fabricating OLED devices: ITO/polyethylenedioxythiophene/
Table 1 Electroluminescence characteristics of devices incorporating various red and green dopants
Dopant (wt%)
Vona/V
Lmax/cd mꢁ2
Imax/mA cmꢁ2
Zext max (%, cd Aꢁ1
)
Z
p max/lm Wꢁ1
CIE (x,y)
Btp2Ir(acac) (5%)
Mpq2Ir(acac) (9%)
Os(fptz)2(PPh2Me)2 (9%)
Os(fppz)2(PPhMe2)2 (9%)
Pt(iqdz)2 (9%)
3.2
4.7
5.5
5.7
3.8
5.6
2.9
8600 (18 V)
1330
1180
1150
1250
1200
800
10.4, 8.1
13.7, 13.3
15.5, 11.4
13, 9.7
10.2, 12.3
11.2, 7.9
12.3, 45
6.8
9.3
6.9
6.8
9.4
6.3
38
0.68; 0.32
0.66; 0.34
0.68; 0.32
0.68; 0.32
0.64; 0.36
0.68; 0.32
0.48; 0.52
23 300 (19.5 V)
29 700 (19 V)
22 000 (19.5 V)
15 000 (19.5 V)
4200 (19.5 V)
69 000 (17 V)
Ru(ifpz)2(PPh2Me)2 (6%)
PPy2Ir(acac) (10%)
1600
a
At a brightness of 0.1 cd mꢁ2
.
ꢀc
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In summary, we have synthesized a novel host material,
T2N, featuring an electron-deficient pyridyl moiety embedded
into a bifluorene backbone. The electronic behaviour of this
chromophore was modulated significantly relative to that of
its parent compound, rendering T2N suitable for use as an
electron-transporting host material compatible with various
heavy metal-containing red and green phosphors. Most im-
portantly, taking advantage of the dual functions of T2N,
high-efficiency red- and green-phosphorescent OLEDs posses-
sing simple device architectures can be fabricated.
This study was supported financially by the National
Science Council and Ministry of Economic Affairs of Taiwan.
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Fig. 3 (a) External quantum efficiencies (EQEs) and (b) EL spectra of
devices incorporating various dopants.
phosphorescent OLED employing an ET-type host material.
We note that although the efficiency dropped upon increasing
the brightness,7 this device retained its high efficiency (10% at
18 mA cmꢁ2) at a brightness of 1000 cd mꢁ2
.
ꢀc
This journal is The Royal Society of Chemistry 2008
4958 | Chem. Commun., 2008, 4956–4958