Novel host material for highly efficient blue phosphorescent OLEDs

Article abstract of DOI:10.1039/b616043c

We report the synthesis and characterization of a novel silane-fluorene hybrid, triphenyl-(4-(9-phenyl-9H-fluoren-9-yl)phenyl)silane (TPSi-F), used as the host material for blue phosphorescent devices. TPSi-F is constructed by linking p-substituted tetrap

Full text of DOI:10.1039/b616043c

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Novel host material for highly efficient blue phosphorescent OLEDs
Ping-I Shih,a Chen-Han Chien,a Chu-Ying Chuang,a Ching-Fong Shu,*a Cheng-Han Yang,b Jian-Hong Chenb
and Yun Chi*b
Received 3rd November 2006, Accepted 19th January 2007
First published as an Advance Article on the web 2nd February 2007
DOI: 10.1039/b616043c
We report the synthesis and characterization of a novel silane–fluorene hybrid, triphenyl-(4-(9-
phenyl-9H-fluoren-9-yl)phenyl)silane (TPSi-F), used as the host material for blue phosphorescent
devices. TPSi-F is constructed by linking p-substituted tetraphenylsilane to the fluorene
framework through a non-conjugated, sp3-hybridized carbon atom (C-9) to enhance thermal and
morphological stabilities, while maintaining the much needed higher singlet and triplet energy
gap. Highly efficient sky-blue phosphorescent OLEDs were obtained when employing TPSi-F as
the host and iridium(III) bis[(4,6-difluorophenyl)pyridinato-N,C29] (FIrpic) as the guest. The
maximum external quantum efficiency (max. E.Q.E.) of this device reached as high as 15%
(30.6 cd A21). Furthermore, upon switching the guest from FIrpic to a new blue phosphor FIrfpy,
better blue-emitting OLEDs were produced with a max. E.Q.E. of 9.4% (15.1 cd A21). These
TPSi-F based blue phosphorescent devices show a 2-fold enhancement in device efficiency
compared with reference devices based on the conventional host material 1,3-bis(9-
carbazolyl)benzene (mCP).
lower than those of the general blue triplet emitters (.2.62 eV),
resulting in an inefficient energy transfer from host to guest.14
Introduction
The destination of organic light emitting diodes (OLEDs) is
To surmount this constraint, many host materials for blue-
obviously linked to the future commercialization of full-color
emitting phosphorescent devices have been developed, but the
flat-panel displays.1 In this sense, finding electrochemically
chromaticity and performance of these devices still lag behind
those of the green- or red-emitting counterparts.12–23 Among
stable and highly efficient emitting materials that can generate
all blue–green–red colors is an important and challenging goal.
them, 1,3-bis(9-carbazolyl)benzene (mCP) has been typically
For improving the OLED efficiency, there is a continuous
utilized for fabricating blue-emitting OLEDs, while iridium(III)
trend of shifting research directions from fluorescent2 to
phosphorescent materials.3 This shift is based on the con-
bis[(4,6-difluorophenyl)pyridinato-N,C29] (FIrpic) is the most
popular choice to serve as a dopant.14 Nevertheless, these blue-
sideration that phosphorescent emitters can harvest both
emitting phosphorescent devices are still unable to achieve the
singlet and triplet excitons, and thus, their internal quantum
desired efficiency and saturated blue color. In order to realize
efficiency can reach a theoretical level as high as 100%.4–6
efficient and saturated blue-emitting devices, some tetraphenyl-
During the past five years, many highly efficient green- and
silane functionalized ultrahigh energy gap hosts (UGHs) were
red-emitting phosphorescent OLEDs have been fabricated
successfully used to replace mCP in fabricating deep blue
phosphorescent OLEDs.15–17 However, tetraphenylsilane and
with electroluminescence (EL) efficiencies reported in the
literature as high as 19% (or 73 cd A21) for green and 15%
related UGHs were reported to have an undesired low glass
(or 21 cd A21) for red light-emitting devices.7–11 However,
transition temperature (Tg) in the range of 26–53 uC. Moreover,
the OLED devices constructed with these hosts exhibit peak
authentic blue-emitting phosphorescent OLED devices remain
rare, which is attributed to the failure in designing suitable
efficiencies at a relatively low current density of less than
0.1 mA cm22, which may pose severe limitations during the
true-blue phosphorescent dyes and accompanying host mate-
rials. In general, a good host material should possess two
fabrication of high performance phosphorescent OLEDs.
intrinsic criteria: (i) its triplet energy must be higher than that
In this paper, we report the synthesis and characterization of
of the guest molecule to prevent backward energy transfer
an effective host material, triphenyl-(4-(9-phenyl-9H-fluoren-
during operation;12–14 (ii) it must possess good morphological
9-yl)phenyl)silane (TPSi-F), for FIrpic as well as for a new
and chemical stabilities to extend the operational lifetime of
triplet blue emitter FIrfpy as components of phosphorescent
the device.
blue-light emitters in OLEDs. The design concept of TPSi-F
Although 4,49-bis(9-carbazolyl)-2,29-biphenyl (CBP) has
is based on the fact that both tetraphenylsilane and fluorene
been commonly used as a host material in green and red
possess large triplet energy gaps of 3.5 and 2.95 eV, respec-
phosphorescent devices, the triplet energy of CBP (2.56 eV) is
tively.17,24,25 Given that the tetraphenylsilane is connected to
the sp3 carbon at the C-9 position of fluorene which serves as a
spacer to block extended p-conjugation,26 the conjugation
aDepartment of Applied Chemistry, National Chiao Tung University,
300 Hsinchu, Taiwan
bDepartment of Chemistry, National Tsing Hua University, 300,
Hsinchu, Taiwan
length and triplet energy of each individual building block in
the resulting composite should remain unperturbed. Moreover,
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the 3-D cardo structure{ of substituted fluorene would
improve rigidity and hinder unwanted aromatic p-stacking
interactions among all phenyl substituents, resulting in
materials with an enhanced morphological stability.
precipitation. The precipitate was collected by filtration and
washed with diethyl ether (10 mL). Yellow crystals of
[(dfppy)2Ir(fpy)] (FIrfpy) were obtained by cooling the
mixed solution of CH2Cl2 and methanol at room temperature
(125 mg, 146 mmol, 89%). 1H NMR (400 MHz, CD2Cl2): d
8.28 (d, J = 8.4 Hz, 1H), 8.23 (d, J = 8.8 Hz, 1H), 8.10 (d, J =
8.4 Hz, 1H), 7.78–7.73 (m, 4H), 7.66 (d, J = 5.6 Hz, 1H), 7.50
(d, J = 4.8 Hz, 1H), 7.03 (td, J = 7.0, 1.6 Hz, 1H), 6.96–6.92
(m, 2H), 6.88 (s, 1H), 6.51 (ddd, J = 12.4, 9.4, 2.4 Hz, 1H), 6.41
(ddd, J = 12.4, 9.4, 2.0 Hz, 1H), 5.68 (dd, J = 8.4, 1.2 Hz, 1H),
5.62 (dd, J = 8.4, 1.2 Hz, 1H). 19F{1H} NMR (470 MHz,
CD2Cl2): d 2112.1 (s, 1F), 2110.1 (s, 1F), 2109.8 (s, 1F),
2107.4 (s, 1F), 259.2 (s, 3F), 255.1 (s, 3F). MS (FAB, 192Ir):
observed m/z [assignment]: 851 [M+], 572 [M+ 2 fpy]. Anal.
Calcd. for C33H17F10IrN4: N, 6.58; C, 46.54; H, 2.01. Found:
N, 6.62; C, 46.72; H, 1.95%.
Experimental
Materials
The CF3 substituted pyridyl pyrrole ligand (fpyH) was
prepared from the reaction of hexafluoroacetylacetone and
2-(aminomethyl)pyridine in the presence of a catalytic amount
of sulfuric acid.27 The iridium complex [(dfppy)2Ir(m-Cl)]2 was
synthesized using IrCl3?nH2O and 4,6-difluorophenyl pyridine
in 2-ethoxyethanol according to the literature method. The
solvents were dried using standard procedures. All other
reagents were used as received from commercial sources, unless
otherwise stated.
Selected X-ray crystal data of FIrfpy: the asymmetric unit
contains half of a CH2Cl2 molecule which is disordered about
an inversion centre, formula: C33.5H18ClF10IrN4, M = 894.17,
Characterization
¯
triclinic, space group P1, a = 10.0361(1), b = 11.1106(1), c =
1H and 13C NMR spectra were recorded on Varian UNITY
INOVA 500 MHz, Varian Unity 300 MHz and Bruker-DRX
300 MHz spectrometers. Mass spectra were obtained by using
a JEOL JMS-HX 110 mass spectrometer. Differential scanning
˚
14.0054(1) A, a = 96.0383(7), b = 90.7590(6), c = 107.0188(5)u,
V = 1483.43(2) A , Z = 2, rcalcd = 2.002 g cm21, F(000) = 862,
3
˚
˚
crystal size 0.18 6 0.18 6 0.10 mm, l(Mo-Ka) = 0.7107 A,
T = 150 K, m = 4.687 mm21, 28743 reflections collected,
6797 with R(int) = 0.0438, final wR2(all data) = 0.0696.
R1[I . 2s(I)] = 0.0275. CCDC reference number 626385. For
crystallographic data in CIF or other electronic format see
DOI: 10.1039/b616043c
calorimetry (DSC) was performed by using
a SEIKO
EXSTAR 6000DSC unit at a heating rate of 20 uC min21
and a cooling rate of 50 uC min21. Samples were scanned from
30 to 280 uC, cooled to 0 uC, and then scanned again from
30 to 280 uC. The glass transition temperatures (Tg) were
determined from the second heating scan. UV-vis spectra were
measured using an HP 8453 diode-array spectrophotometer.
PL spectra were obtained using a Hitachi F-4500 luminescence
spectrometer. Cyclic voltammetry (CV) spectra were measured
using a BAS 100 B/W electrochemical analyzer operated at a
scan rate of 50 mV s21 in anhydrous CH2Cl2 solution with
0.1 M of supporting electrolyte tetrabutylammonium hexa-
fluorophosphate (TBAPF6). The potentials were measured
against an Ag/Ag+ (0.01 M AgNO3) reference electrode using
ferrocene as an internal standard. The HOMO energies of
organic thin films were measured using the Riken-Keili AC-2
atmospheric low-energy photoelectron spectrometer. The
LUMO energies of materials were estimated by subtracting
the optical energy gap from the measured HOMO. The low-
temperature phosphorescence spectrum of TPSi-F was
obtained using a composite spectrometer containing a mono-
chromator (Jobin Yvon, Triax 190) coupled with a liquid
nitrogen-cooled charge-coupled device (CCD) detector (Jobin
Yvon, CCD-10246256-open-1LS).
Preparation of 9-(4-bromophenyl)-9-phenyl-9H-fluorene
A mixture of 9-(4-bromophenyl)-9H-fluoren-9-ol (2.00 g,
5.95 mmol),28 benzene (8.49 g, 109 mmol), and CF3SO3H
(0.89 g, 5.93 mmol) was stirred and heated at reflux for 4 h
under N2. After cooling, the mixture was treated with a
saturated NaHCO3 solution and extracted with ethyl acetate.
The organic extract was dried over MgSO4 and the solvent was
evaporated in vacuo. The crude product was purified by
column chromatography (hexane–CH2Cl2) to give 9-(4-bro-
mophenyl)-9-phenyl-9H-fluorene (0.67 g, 28%). 1H NMR
(300 MHz, CDCl3): d 7.78–7.75 (m, 2 H), 7.39–7.35 (m, 4 H),
7.33 (dt, J = 8.7, 2.1 Hz, 2 H), 7.30–7.27 (m, 2H), 7.25–7.16 (m,
5 H), 7.06 (dt, J = 8.7, 2.1 Hz, 2 H). 13C NMR (75 MHz,
CDCl3): d 150.6, 145.3, 145.2, 140.1, 131.3, 129.9, 128.3, 128.0,
127.8, 127.7, 126.8, 126.0, 120.7, 120.3, 65.0. Anal. Calcd. for
C25H17Br: C, 75.58; H, 4.31. Found: C, 75.52; H, 4.64%.
Preparation of TPSi-F
n-Butyllithium in hexane (2.5 M, 0.60 mL) was added slowly
under N2 to a stirred solution of 9-(4-bromophenyl)-9-phenyl-
9H-fluorene (600 mg, 1.51 mmol) in anhydrous ether (50 mL)
at 278 uC. The mixture was warmed to 0 uC. A solution of
chlorotriphenylsilane (445 mg, 1.56 mmol) in ether (50 mL)
was added, and the resulting mixture was heated at reflux for
4 hours. After stopping the reaction, the precipitate was
collected by filtration, washed with water followed by ether
and dried under vacuum. Finally, the product was purified by
Preparation of FIrfpy
A mixture of [(dfpz)2IrCl]2 (101 mg, 82 mmol), fpyH (50 mg,
0.17 mmol) and Na2CO3 (87 mg, 0.82 mmol) in 2-ethoxy-
ethanol (20 mL) was heated to reflux for 4 hours. After cooling
to room temperature, excess water was added to induce
{ The term ‘cardio structure’ is defined as a structure containing at
least one element of the constitutive unit, which carries a lateral ring
connected to the main framework of a molecule by a quaternary
carbon atom.
1
high vacuum sublimation to yield TPSi-F (970 mg, 74%). H
NMR (300 MHz, CDCl3): d 7.77 (dd, J = 6.9, 0.6 Hz, 2 H),
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7.54 (td, J = 6.1, 1.8 Hz, 6 H), 7.43 (d, J = 8.4 Hz, 6 H), 7.39–
7.32 (m, 10 H), 7.27 (dt, J = 7.4, 1.2 Hz, 2 H), 7.23–7.20 (m,
6 H). 13C NMR (75 MHz, CDCl3): d 150.9, 147.3, 145.7, 140.1,
136.4, 136.3, 134.2, 132.0, 129.5, 128.2, 128.1, 127.8, 127.7,
127.6, 127.5, 126.6, 126.3, 120.1, 65.5. HREI-MS (m/z): [M+]
calcd. for C43H32Si, 576.2273; found 576.2269. Anal. Calcd.
for C43H32Si: C, 89.54; H, 5.59. Found: C, 89.67; H, 5.78%.
Fabrication of light-emitting devices
The EL devices were fabricated by vacuum deposition of the
materials at 1026 Torr onto a clean glass that was pre-
coated with a layer of indium tin oxide with a sheet resistance
of 25 V square21. All of the organic layers were deposited at a
rate of 1–2 A s21. A layer of Mg/Ag alloy (10 : 1, 100 nm) was
˚
deposited as the cathode by the co-evaporation of magnesium
21
˚
and silver metals, with deposition rates of 4 and 0.4 A s
,
respectively. The cathode was then capped with silver metal
(100 nm) by thermal evaporation at a rate of 3 A s21. The
˚
active area of the emitting diode was 9.00 mm2. The current–
voltage–luminance relationships of the devices were measured
using a Keithley 2400 Source meter and a Newport 1835C
Optical meter equipped with an 818ST silicon photodiode. The
EL spectrum was obtained using a Hitachi F4500 spectro-
fluorimeter. The external quantum efficiency (E.Q.E.) was
estimated based on luminance, EL spectra, current densities,
and the eye-sensitivity curve.29
Scheme 1 Reagents: (i) p-dibromobenzene, Mg–Et2O; (ii) CF3SO3H–
benzene; (iii) n-BuLi, Ph3SiCl–Et2O.
Result and discussion
Synthesis and characterization
Scheme 1 illustrates the synthetic procedures used to prepare
the fluorene-based silane hybrid, triphenyl-(4-(9-phenyl-9H-
fluoren-9-yl)phenyl)silane (TPSi-F). The Grignard reaction of
fluorenone with p-bromophenylmagnesium bromide gave the
corresponding alcohol, 9-(4-bromophenyl)-9H-fluoren-9-ol,28
which on acid promoted Friedel–Crafts-type substitution
reaction with benzene yielded 9-(4-bromophenyl)-9-phenyl-
9H-fluorene. Treatment of the bromo compound with n-BuLi
and subsequent quenching of the lithiated intermediate with
chlorotriphenylsilane gave the desired TPSi-F. We also synthe-
sized a new blue-emitting, iridium-based phosphor FIrfpy,
which can be obtained in high yield from a reaction of
[(dfpz)2IrCl]2 with a pyridyl pyrrole ligand (fpyH) in the
presence of Na2CO3. The molecular structure of FIrfpy is
shown in Scheme 1 along with the structural drawing of its
parent FIrpic, while the ORTEP diagram of FIrfpy is depicted
in Fig. 1.
Fig. 1 ORTEP diagram of FIrfpy with thermal ellipsoids shown at
50% probability level; bond distances: Ir–N(1) = 2.046(3), Ir–N(2) =
2.053(3), Ir–N(3) = 2.144(3), Ir–N(4) = 2.155(3), Ir–C(1) = 2.006(3),
˚
Ir–C(12) = 2.004(4) A.
distances of the dfppy ligands. This lengthening of the Ir–N
distances is attributed to the stronger Ir–C bonding interaction
of the dfppy ligands, which consequently weakens the Ir–N
bonds at their trans-disposition.30
FIrfpy exhibits a distorted octahedral geometry around the
Ir atom with two cyclometalated dfppy ligands and one
anionic 2-pyridyl pyrrolide (fpyro) ligand. The dfppy ligands
adopt a mutually eclipsed configuration with the nitrogen
atoms N(1) and N(2) residing at the trans locations with
To date, 2-(2,4-difluorophenyl)pyridine has been one of the
most potent ligands in synthesizing heteroleptic iridium com-
plexes with blue emission. As a result, the blue-shifting power
of the ancillary ligand becomes the decisive factor in improving
the blue color of FIrpic.22,31,32 We chose the trifluoromethyl-
substituted pyrrolate as the ancillary ligand for our new blue
phosphor FIrfpy, because it has a blue-shifting power better
˚
distances Ir–N(1) = 2.046(3) and Ir–N(2) = 2.053(3) A. The
substituted phenyl groups are arranged cis to each other
with the shorter distances Ir–C(1) = 2.006(3) and Ir–C(12) =
˚
2.004(4) A. The third, anionic fpyro ligand displays notably
elongated Ir–N distances (Ir–N(3) = 2.144(3) and Ir–N(4) =
˚
2.155(3) A) compared with those of the trans-orientated Ir–N
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than the known picolate ligand of FIrpic. In addition, it
appears that the electron deficient CF3 substituents on the
pyrrolide ancillary ligand would stabilize the complex and
prevent the unwanted intermolecular interaction that quenches
emission. As anticipated, in CH2Cl2 solution FIrfpy showed a
noticeably bluer photoluminescent emission with the first peak
maximum located at 461 nm, about 9 nm blue-shifted from the
470 nm emission of FIrpic. FIrfpy also displayed much reduced
intensities for the accompanying vibronic bands in the longer
wavelength region (Fig. 2).
The thermal properties of TPSi-F were investigated through
differential scanning calorimetry (DSC). In the DSC measure-
ments,
a distinct glass transition temperature (Tg) was
observed at 100 uC, showing a much higher value than those
of previously discussed host materials such as mCP (65 uC) or
other silane based UGHs (26–53 uC).17,33 Consequently, TPSi-
F forms a more stable amorphous glassy state and, therefore, is
more promising in terms of its thermal properties for applica-
tion in OLEDs. We attribute the enhanced morphological
stability of TPSi-F to its rigid 3D cardo structures hindering
the crystallization process.
To confirm the photophysical properties, the absorption
and photoluminescent spectra (PL) of TPSi-F, 9,9-diphenyl-
fluorene and tetraphenylsilane were measured in CHCl3
solution (Fig. 3). Tetraphenylsilane exhibits absorption and
PL signals at the highest energy region of all three samples,
while the absorption and emission signals of TPSi-F coincide
with those of its parent fluorene, confirming the rationale
mentioned earlier. Fig. 3(c) displays the phosphorescence of
TPSi-F measured in a frozen 2-methyltetrahydrofuran matrix
at 77 K. The highest-energy 0–0 phosphorescent emission
located at 2.89 eV is taken for calculating the triplet energy gap
of TPSi-F, giving a value higher than that reported for the
common triplet blue-emitter FIrpic (2.62 eV) as well as our new
blue emitting complex, FIrfpy (2.68 eV). Higher triplet energy
of host materials is a provision for effective confinement of
the triplet excitons on the guest and for the consequential
prevention of back energy transfer from the dopant to the host
material.12–14 In this case, the triplet energy of TPSi-F is high
Fig. 3 (a) Absorption spectra of TPSi-F, 9,9-diphenylfluorene, and
tetraphenylsilane in CHCl3 solution at room-temperature, (b) fluores-
cence spectra of TPSi-F, 9,9-diphenylfluorene, and tetraphenylsilane in
CHCl3 solution at room temperature, and (c) phosphorescence
spectrum of TPSi-F in 2-methyltetrahydrofuran at 77 K.
enough to serve as a decent host for short wavelength dopants
such as FIrpic and FIrfpy.
Electroluminescence properties of OLEDs
Blue phosphorescent devices (I and II) were fabricated using
either FIrfpy or FIrpic emitters co-evaporated with the host
material TPSi-F. The typical multi-layer architecture consists
of indium-tin-oxide (ITO)/4,49-bis[N-(1-naphthyl)-N-phenyl-
amino]biphenyl (NPB) (30 nm)/mCP (10 nm)/TPSi-F : 7 wt%
of dopant (40 nm)/2,9-dimethyl-4,7-diphenyl-1,10-phenanthro-
line (BCP) (10 nm)/1,3,5-tris(N-phenylbenzimidazol-2-yl)ben-
zene (TPBI) (30 nm)/Mg : Ag (100 nm)/Ag (100 nm), for which
the dopants are FIrfpy for device I, or FIrpic for device II. As
shown in Fig. 4, NPB and TPBI were employed as the hole-
transporting layer (HTL) and the electron-transporting layer
Fig. 2 Photoluminescent spectra of FIrpic and FIrfpy in CH2Cl2
Fig. 4 Energy level diagram for the OLED devices.
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(ETL), respectively. Moreover, a thin layer (ca. 10 nm) of mCP
was inserted between NPB and the emitting layer (EML) to
reduce the excessive energy barrier between NPB and the
TPSi-F host material, thereby facilitating resonant hole
injection.16,17 On the other hand, a layer of BCP with the
highest occupied molecular orbital (HOMO) energy level of
6.5 eV was inserted between EML and TPBI to bolster the
hole-blocking capability of TPBI (6.2 eV) and to confine the
excitons within the emissive layer. For comparison, conven-
tional mCP-based devices (III and IV) were also fabricated,
consisting of the configuration ITO/NPB (30 nm)/mCP : 7 wt%
of dopant (40 nm)/TPBI (40 nm)/Mg : Ag (100 nm)/Ag
(100 nm), with the dopants FIrfpy and FIrpic for devices III
and IV, respectively. We also prepared two controlled devices
by inserting a BCP layer between the EML and TPBI
layer in devices III and IV in order to investigate the effect
of the additional BCP layer on the performance of the
mCP-based devices. It was found that the efficiencies of the
controlled devices were inferior even to those of the original
mCP-based devices. This result suggests that TPBI may
perform adequately as a hole-blocking and confinement
layer in the mCP-based devices. Therefore, the additional
BCP layer would not further improve the device perfor-
mance and thus is unnecessary for fabrication of mCP-based
devices.
According to the EL spectra shown in Fig. 5(a), the FIrfpy-
based devices (I and III) exhibit blue-shifted emissions
compared with the FIrpic-based devices (II and IV), while
the insert depicts the corresponding CIE coordinates. The
extent of blue-shifting in the EL spectra of FIrfpy doped
devices I and III (ca. 10 nm each) is similar to that observed in
the PL spectra recorded in room temperature solutions.
Fig. 5(b) displays the current density–luminescence (I–L)
characteristics of each device; device I exhibits a maximum
luminance as high as 25200 cd m22 at 16 V (500 mA cm22
)
with CIE coordinates at (0.13, 0.23), which represents, so far,
the brightest and best true-blue phosphorescent device to
our knowledge.16,17,34 According to the plots of external
quantum efficiency and luminance efficiency vs. current
density shown in Fig. 6(a and b), the maximum external
quantum efficiency (max. gext) of devices I and II are
calculated to be 9.4% (15.1 cd A21, 12.2 mA cm22) and
15.0% (30.6 cd A21, 18.9 mA cm22) respectively. Both have
quantum efficiencies 2.2 times higher than those of the
reference devices III and IV (mCP-based), which show
max. gext of 4.3% (6.3 cd A21, 25.4 mA cm22) and 6.70%
(12.4 cd A21, 23.7 mA cm22), respectively. It is worth
mentioning that the max. efficiencies in our devices
appeared to occur in the more useful current density range
of 10–20 mA cm22, in contrast to those reported for other
UGH-based devices (,0.1 mA cm22). Key characteristics of
Fig. 5 (a) EL spectra of devices I–IV. Inset: the respective CIE
coordinates. (b) Electroluminescence intensity vs. current density for
devices I- IV.
Fig. 6 (a) External quantum efficiency vs. current density for devices
I–IV, (b) luminance efficiency vs. current density for devices I–IV.
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Table 1 Performance of devices I–IV
I
II
III
IV
Voltage/Va
9.9
9.1
9.2
9.3
Brightness/cd m22 a,b
E.Q.E. (%)a,b
2900 (10600)
9.0 (6.6)
14.6 (10.6)
25200 (@ 16 V)
9.4
6100 (22200)
14.9 (10.9)
30.4 (22.2)
35700 (@ 15 V)
15.0
30.6
474
0.14 and 0.34
c
1300 (5800)
4.2 (4.0)
6.3 (5.8)
17800 (@ 16 V)
4.3
6.3
462
0.13 and 0.21
2500 (10800)
6.7 (5.8)
12.4 (10.8)
35900 (@ 16 V)
6.7
12.4
472
0.13 and 0.30
L.E./cd A21a,b
Max. brightness
Max. E.Q.E. (%)
Max. L.E./cd A21
EL lmax/nmc
15.1
464
CIE, x and yc
0.13 and 0.23
The data in the parentheses were taken at 100 mA cm22
. At 9 V.
a
b
At 20 mA cm22
.
these blue phosphorescent devices are listed in Table 1. In
addition, our TPSi-F based devices exhibit a much reduced
degree of efficiency roll-off at high current densities;
the external quantum efficiencies of devices I and II at
100 mA cm22 were 6.6% and 10.9%, respectively. The high-
current decline in efficiency is typical in phosphorescent
OLEDs due to triplet–triplet annihilation. Apparently, the
rigid cardo structure of TPSi-F provides an ideal packing
environment for dispersing and isolating the phosphorescent
emitters, yielding an improved efficiency via reduction of
triplet–triplet annihilation of the blue dopant.
(y3.0 eV),14,34,35 it is still high enough to prevent the
backward energy transfer from dopant to host and is suitable
to serve as a host material for phosphorescent dopants such as
FIrpic and FIrfpy in fabrication of blue-emitting OLEDs. The
OLED device fabricated using our fluorene-based host and the
more traditional dopant emitter FIrpic showed an E.Q.E. of
14.9% at 20 mA cm22, while switching the dopant from FIrpic
to FIrfpy gave a blue-shifted emission with a CIE value of
0.13 and 0.23 and an E.Q.E. of 9.0% at 20 mA cm22. These
TPSi-F based blue phosphorescent devices exhibited a 2-fold
enhancement in the device efficiency compared with reference
devices based on the mCP host material. Finally, in addition to
the higher operation efficiencies, our TPSi-F based devices
exhibit a less pronounced efficiency roll-off at high current
densities, which is in sharp contrast to that of the UGH-based
and 3,6-bis(triphenylsilyl)carbazole-based blue-emitting phos-
phorescent devices.14–17,35
Moreover, the utilization of TPSi-F in devices I and II
significantly improved their efficiencies compared with those
of the mCP-based OLED devices III and IV. We attribute
this to a notable enhancement in charge trapping and
recombination, which is a result of the lowered HOMO energy
gap for TPSi-F. It is noted that, for the typical phosphorescent
OLED, it is desirable to have HOMO and LUMO energy
levels of the host lying below and above those of the dopant,
respectively, to ensure efficient carrier collection and recombi-
nation.17 Nevertheless, the HOMO level of mCP (5.9 eV) is too
close to that of the sky-blue phosphor FIrpic (5.9 eV), and is
unavoidably higher than that of the blue-emitting phosphor
FIrfpy (6.2 eV). Notably, the cyclic voltammetry (CV) of
TPSi-F reveals an estimated HOMO energy level at 6.3 eV,
which is 0.4 eV lower than that of the mCP host. Conse-
quently, this lower HOMO energy of TPSi-F facilitates the
hole trapping at the phosphor sites, followed by recombination
of opposite charges (electrons) to form excitons. Finally, it has
been reported that the substituted tetraphenylsilane moiety
of TPSi-F may bring a more balanced hole and electron
combination process and lead to effective exciton generation in
the emissive layer.34
Acknowledgements
We thank the National Science Council of Taiwan for funding.
We also thank Professor C.-H. Cheng for his generosity and
permission in letting us use his OLED fabrication and
measurement systems.
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