A high-efficiency and low-operating-voltage green electrophosphorescent device employing a pure-hydrocarbon host material

Article abstract of DOI:10.1039/b904795f

A suitable triplet energy level and high morphological stability and hole mobility make indenofluorene an effective host material for green phosphorescent OLEDs.

Full text of DOI:10.1039/b904795f

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A high-efficiency and low-operating-voltage green electrophosphorescent
device employing a pure-hydrocarbon host materialw
Liang-Chen Chi,a Wen-Yi Hung,*b Hao-Chih Chiub and Ken-Tsung Wong*a
Received (in Cambridge, UK) 12th March 2009, Accepted 24th April 2009
First published as an Advance Article on the web 26th May 2009
DOI: 10.1039/b904795f
A suitable triplet energy level and high morphological stability
and hole mobility make indenofluorene an effective host material
for green phosphorescent OLEDs.
and a value of Zext of 19% at 100 cd mꢀ2 (2.75 V) after
p-doping the hole transporting layer (HTL), n-doping the
electron transporting layer (ETL) and employing a double
emission layer.7 Using a similar strategy, the Kido group
recently reported ultrahigh-efficiency green PhOLEDs using
Ir(ppy)3 as an emitter, realizing high values of Zp and Zext
(133 lm Wꢀ1 and 29% at 100 cd mꢀ2, respectively).8 As this
approach increases the complexity of the device structure, the
required fabrication processes would add considerably to the
overall cost for mass production. Therefore, the challenge
remains to develop new host materials to reduce the driving
voltage while maintaining high quantum efficiencies at high
luminance without the need for p–i–n-type OLED structures.
Several materials have been reported as hosts for highly
efficient green electrophosphorescent devices, including
4,40,400-tris(N-carbazolyl)triphenylamine (TCTA), 4,40-bis-
(9-carbazolyl)biphenyl (CBP), 2,9-dimethyl-4,7-diphenyl-
phenanthroline (BCP), 1,3-bis(N,N-tert-butylphenyl)-1,3,
4-oxadiazole (OXD7), and 3-phenyl-4-(10-naphthyl)-5-phenyl-
1,2,4-triazole (TAZ).9 In a previous study, we employed spiro-
configured bi(9,9-diaryfluorene)s (triplet energy: ET1 = 2.3 eV)
as effective host materials in green and red PhOLEDs.10 To
better match the triplet energy of the green complexes, the
conjugation length of the host must be selected carefully to
achieve a wide energy gap and a high value of ET1. Electron
delocalization in p-phenylene linkages is extended along the
elongated molecular axis, with ET1 decreasing with the number
of phenylene rings. The Yagi group reported that p-terphenyl has
a value of ET1 of 2.55 eV, making it suitable for use as a host for
a green phosphorescent dopant.11 In this paper, we report a
unique amorphous host material, SInF3, featuring a coplanar
pure-hydrocarbon p-terphenylene (indenofluorene) backbone
and exhibiting a high triplet energy, excellent thermal stability,
and high hole mobility, making it suitable for use in green
PhOLEDs. A device incorporating SInF3 as the host, accommo-
dating bis(2-phenylpyridinato-N,C20)iridium(acetylacetonate)
ppy2Ir(acac)12 as the green emitter, achieved a value of Zext of
15.8% (60 cd Aꢀ1) and a value of Zp of 63 lm Wꢀ1 at a
practical brightness of 50 cd mꢀ2 at 3 V. Furthermore, the
device provided a brightness of 10 000 cd mꢀ2 at an extremely
low operating voltage (5 V); the efficiencies remained at 14%
(53.5 cd Aꢀ1) and 33.6 lm Wꢀ1—the highest values ever
reported for green PhOLEDs lacking a p–i–n junction. To
the best of our knowledge, this is the first report of an
indenofluorene derivative being used as an efficient host
material for PhOLEDs. Scheme 1 displays our synthetic route
toward SInF3. Suzuki coupling of the spirobifluorene-based
boronic ester 1 and 2-iodoethyl benzoate gave the ester 2 in
92% yield. Double nucleophilic addition of p-tolyllithium
The development of phosphorescent organic light-emitting
devices (PhOLEDs) has attracted a great deal of attention
because the incorporation of heavy metal-containing com-
plexes into appropriate host materials can allow the harvesting
of both singlet and triplet excitons, making it possible
to render devices with close to 100% internal quantum
efficiency.1 To obtain high quantum efficiency in a PhOLED,
the host materials should possess (i) triplet energies higher
than those of the guest molecules (to prevent exothermic
reverse energy transfer),2 (ii) good charge balance properties
(to effectively confine triplet excitons within the emitting
layer), and (iii) good charge carrier transport properties
(to achieve a low operating voltage). In this regard, Adachi
et al.3 demonstrated efficient green PhOLEDs exhibiting
external quantum efficiencies (Zext) close to 20% and power
efficiencies (Zp) of 60 lm Wꢀ1 at low current densities (o2 mA
cmꢀ2) and low brightness. The values of Zext and Zp for these
devices decreased significantly, however, upon increasing the
current, due to triplet–triplet annihilation of the phosphors;4
indeed, it is generally difficult to obtain high quantum efficiencies
at high currents. Recently, much higher efficiencies have been
obtained for modified PhOLED structures, such as those
featuring hole- and exciton-blocking layers to confine holes
and excitons within the emitting layer;5 nevertheless, the
voltage drop across these layers requires more power to drive
the devices, leading to high operating voltages and lower
power efficiencies. This problem can be solved by increasing
the conductivity of the organic layers, e.g., through doping
with donors and acceptors that can significantly reduce the
voltage drop across these films. By introducing a p–i–n struc-
ture into an OLED, Pfeiffer et al. demonstrated highly efficient
green-emitting PhOLEDs using the hole transport material
TCTA as a host, obtaining high power efficiencies of
52 lm Wꢀ1 (Zext = ca. 13.7%) with 100 cd mꢀ2 at 2.75 V
and 45 lm Wꢀ1 (Zext = ca. 12.6%) with 1000 cd mꢀ2 at 3.1 V.6
Subsequently, they obtained a power efficiency of 77 lm Wꢀ1
a Department of Chemistry, National Taiwan University, Taipei 106,
Taiwan. E-mail: kenwong@ntu.edu.tw; Fax: 886 2 33661667;
Tel: +886 2 33661665
b Institute of Optoelectronic Sciences, National Taiwan Ocean
University, Keelung 202. E-mail: wenhung@mail.ntou.edu.tw;
Fax: 886 2 24634360; Tel: +886 2 24622192 ext. 6718
w Electronic Supplementary Information (ESI) available: Synthetic
procedure and spectroscopic characterizations, TGA, DSC, CV, and
TOF hole mobility measurements. See DOI: 10.1039/b904795f/
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3892 | Chem. Commun., 2009, 3892–3894

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redox couple of ferrocene, we calculated the energy level of the
highest occupied molecular orbital (HOMO) to be ꢀ5.83 eV.
Subtracting the optical band gap (3.49 eV, determined from
the cutoff of the optical absorption spectrum) from the
HOMO energy level, we estimated the energy level of the
lowest unoccupied molecular orbital (LUMO) to be ꢀ2.34 eV.
From the energy alignments, we anticipated efficient exo-
thermic energy transfer from the SInF3 triplet state to the
ppy2Ir(acac) triplet state, with excellent triplet energy confine-
ment on the ppy2Ir(acac) molecules.
We performed a time-of-flight (TOF) measurement to
evaluate the carrier mobility of SInF3. The TOF transient
displays non-dispersive hole-transport characteristics, imply-
ing that the carriers moved with constant velocity across
the SInF3 film (ESI,w Fig. S3). In the double-logarithmic
representation (inset to Fig. S3w), the transit time (tT) can be
determined from the intersection point of the two asymptotes.
The hole mobilities of SInF3 as a function of the electric field
(ESI,w Fig. S4) ranged from 3.9 ꢂ 10ꢀ3 to 5.6 ꢂ 10ꢀ3 cm2 Vꢀ1 sꢀ1
Scheme 1 Synthetic route toward SInF3.
onto the ester group of 2 and subsequent intramolecular
Friedel–Crafts cyclization gave SInF3 in an isolated yield
of 74%.
We selected indenofluorene as the core structure of our
green host because its derivatives possess rigid coplanar
structures that enhance the conjugation, carrier mobility,
and fluorescence quantum efficiency of the blue emission.13
The molecular design in SInF3 preserves the conjugated
p-terphenyl backbone, which plays a prominent role in deter-
mining the overall electronic, electrochemical, and photo-
physical properties. The spiro configuration14 and peripheral
p-tolyl substituent groups desymmetrize the molecule, thereby
improving its thermal and morphological stability. SInF3
exhibits a high thermal tolerance [5% weight loss occurred
at 354 1C, determined using thermogravimetric analysis
(TGA)] and a distinct glass transition temperature (Tg) of
ca. 171 1C [determined using differential scanning calorimetry
(DSC)] (ESI,w Fig. S1). We observed completely different
morphological behavior for SInF3 relative to that of
its symmetric unsubstituted15 and tert-butyl-substituted16
dispirofluorene–indenofluorene (DSF-IF) derivatives, which
are highly crystalline.
for fields varying from 6 ꢂ 104 to 4.8 ꢂ 105 V cmꢀ1
,
respectively. We attribute the high hole mobility of SInF3
to the spiro configuration (orthogonal arrangement) of its
indenofluorene and peripheral fluorene units, which bridge
and enhance the intermolecular charge transport of holes.17
Employing SInF3 as a host material, we fabricated a multi-
layer device having the configuration ITO/PEDOT–PSS
(30 nm)/NPB (15 nm)/TCTA (5 nm)/10 wt% ppy2Ir(acac)-
doped SInF3 (25 nm)/TPBI (50 nm)/LiF (0.5 nm)/Al (100 nm).
In this device, PEDOT–PSS functioned as the hole injection
layer; 4,40-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (NPB)
was the hole transport layer. To decrease the hole-injection
barrier between NPB and the emitting layer, we incorporated a
layer of 4,40,400-tris(N-carbazolyl)triphenylamine (TCTA)
because of its appropriately matching HOMO energy level.
1,3,5-Tris(N-phenylbenzimidizol-2-yl)benzene (TPBI) func-
tioned as both the hole-blocking and ETL. Lithium fluoride
(LiF) was used as the electron-injection layer; aluminium (Al)
was the cathode.
The small Stokes shift (5 nm) and the mirror image of the
absorption and fluorescence spectra of SInF3 in CHCl3
(Fig. 1) confirm its molecular coplanarity and rigidity. The
value of ET1 of SInF3, which we estimated from the phospho-
rescence peak (492 nm) at 77 K, was 2.52 eV; this value is
close to that of the parent p-terphenylene (ET1 = 2.55 eV)
and higher than that of the green phosphorescent dopant
ppy2Ir(acac) (ET1 = 2.41 eV). We used cyclic voltammetry
to investigate the electrochemical properties of SInF3, obser-
ving one reversible oxidation process at 1.5 eV (vs. Ag/AgCl)
(ESI,w Fig. S2). From the redox potential referenced to the
Fig. 2(a) presents the current density–voltage–luminance
(I–V–L) characteristics of this device. The I–V and L–V curves
both feature steep increases after the turn-on voltage reached
2.5 V. The operating voltage for a luminance of 1000 cd mꢀ2
was 3.7 V; a luminance of 10 000 cd mꢀ2 was achieved at
voltages as low as 5 V. The device exhibited an unusually high
current of ca. 2700 mA cmꢀ2 at 9.5 V, giving a maximum
brightness of 255 000 cd mꢀ2. The electroluminescence (EL)
spectrum [inset to Fig. 2(a)] reveals that the emission [CIE
coordinates: (0.32, 0.64)] arose purely from ppy2Ir(acac), i.e.,
no emission occurred from the neighboring materials. Fig. 2(b)
presents the values of Zext and Zp as functions of the luminance.
The maximum values of Zext [15.8% (60 cd Aꢀ1)] and Zp
(63 lm Wꢀ1) were achieved at a practical brightness of
50 cd mꢀ2 at 3 V. It is noteworthy that even when the luminance
reached 10 000 cd mꢀ2 (injection current density: 18.7 mA cmꢀ2),
the value of Zext remained at 14% (54 cd Aꢀ1). These results
clearly indicate that appropriately designed host materials
enable PhOLEDs to function with extremely low operating
voltages without the need for a p–i–n junction. We observed
only a limited roll-off in the values of Zext, even when the
Fig. 1 Normalized UV-Vis absorption and fluorescence spectra
(CHCl3, room temperature) and phosphorescence spectrum (EtOH,
77 K) for SInF3.
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Fig. 2 (a) L–V and I–V characteristics. Inset: EL spectrum of the
device. (b) Quantum efficiency–luminance and power efficiency–
luminance characteristics.
luminance reached a value as high as 10 000 cd mꢀ2. We
attribute the excellent performance of this device to the
balanced injection of electrons and holes, and to the efficient
confinement of triplet excitons generated within the
emitting layer.
In conclusion, we have synthesized a pure-hydrocarbon host
material, comprising a coplanar and rigid indenofluorene back-
bone presenting different peripheral aryl substituents, for use in
green PhOLEDs. A device incorporating this tailor-made host
material exhibited a brightness of 10 000 cd mꢀ2 at an extremely
low operating voltage (5 V), with efficiencies of 14% (53.5 cd Aꢀ1
)
and 33.6 lm Wꢀ1. Thus, PhOLEDs exhibiting high quantum
efficiencies and low operating voltages at high brightness can be
prepared without using complicated p–i–n junction techniques.
Notes and references
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3894 | Chem. Commun., 2009, 3892–3894