Spiro-configured bipolar hosts incorporating 4,5-diazafluroene as the electron transport moiety for highly efficient red and green phosphorescent OLEDs

Article abstract of DOI:10.1039/c2jm30749a

We designed and synthesized a series of spiro-configured bipolar hosts (CSC, DSD, TST, and NSN) comprising electron-transporting 4,5-diazafluorene and hole-transporting carbazole or diarylamine with thorough characterizations of their thermal, photophysical, electrochemical properties, and carrier mobilities. Time-of-flight (TOF) measurements and single-carrier devices revealed these compounds had good hole and electron mobilities (10^-5 - 10^-7 cm² V^-1 s^-1) together with promising physical properties, rendering them good host materials for efficient electrophosphorescence. The green PhOLED with (PPy)₂Ir(acac) as the emitter incorporating CSC as the host exhibited a remarkable high brightness of 260000 cd m^-2 at 2400 mA cm^-2 (11 V) with the maximum external quantum efficiency (η_ext) of 17.1%, the η_ext remained as high as 15.4% under a high brightness of 10000 cd m^-2 at a low operating voltage of 5.1 V even without p-i-n device configuration. The NSN-based PhOLED with the red dopant Os(bpftz) ₂(PPhMe₂)₂ has a maximum brightness of 77900 cd m^-2 at 1100 mA cm^-2 (10 V) with the maximum η_ext of 19.5%. The η_ext remained as high as 19.1% under a practical brightness of 1000 cd m^-2 (4.6 V). These results suggest that the spiro-configured D-A hosts are well-suited for achieving highly efficient PhOLED with limited efficiencies roll-off.

Full text of DOI:10.1039/c2jm30749a

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PAPER
Spiro-configured bipolar hosts incorporating 4,5-diazafluroene as the electron
transport moiety for highly efficient red and green phosphorescent OLEDs
a
Hsiao-Fan Chen, Ting-Chih Wang, Wen-Yi Hung, Hao-Chih Chiu, Chi Yun and Ken-Tsung Wong
a
b
b
c
a
*
*
Received 8th February 2012, Accepted 15th March 2012
DOI: 10.1039/c2jm30749a
We designed and synthesized a series of spiro-configured bipolar hosts (CSC, DSD, TST, and NSN)
comprising electron-transporting 4,5-diazafluorene and hole-transporting carbazole or diarylamine
with thorough characterizations of their thermal, photophysical, electrochemical properties, and
carrier mobilities. Time-of-flight (TOF) measurements and single-carrier devices revealed these
compounds had good hole and electron mobilities (10ꢀ5 ꢀ 10ꢀ7 cm2 Vꢀ1 sꢀ1) together with promising
physical properties, rendering them good host materials for efficient electrophosphorescence. The green
PhOLED with (PPy)2Ir(acac) as the emitter incorporating CSC as the host exhibited a remarkable high
brightness of 260 000 cd mꢀ2 at 2400 mA cmꢀ2 (11 V) with the maximum external quantum efficiency
(hext) of 17.1%, the hext remained as high as 15.4% under a high brightness of 10 000 cd mꢀ2 at a low
operating voltage of 5.1 V even without p–i–n device configuration. The NSN-based PhOLED with the
red dopant Os(bpftz)2(PPhMe2)2 has a maximum brightness of 77 900 cd mꢀ2 at 1100 mA cmꢀ2 (10 V)
with the maximum hext of 19.5%. The hext remained as high as 19.1% under a practical brightness of
1000 cd mꢀ2 (4.6 V). These results suggest that the spiro-configured D–A hosts are well-suited for
achieving highly efficient PhOLED with limited efficiencies roll-off.
and (iii) high triplet energy for ensuring exothermic host-to-guest
energy transfer and preventing reverse energy transfer to confine
phosphorescence emission from the metal-centered phosphor.
Recently, PhOLED-related research has significantly drifted to
the development of host materials possessing bipolar properties.
The incorporation of hole- and electron-transporting moieties into
the host material provides more balanced charge fluxes and better
charge transport properties in the emitting layer, potentially
simplifying the device structure (i.e., free of using an HTL and/or
ETL). In this regard, developing host materials with efficient hole-
and electron-transporting properties (ambipolarity) is highly
demanded to achieve highly efficient and cost-effective PhOLEDs.
Our recent review classified some representative bipolar host
materials into several categories according to their structural
features as well as the molecular design strategies. The structure–
property–performance relationships of various classes of bipolar
host materials were also highlighted.3 In general, carbazole or
arylamine derivatives are usually adopted as the hole-transport
moieties in bipolar materials, whereas various electron-deficient
moieties such as nitrogen-heterocycles, 1,10-phenanthroline, 1,3,4-
oxadiazole, benzimidazole, phosphine oxide, and phenylsulfonyl
are suitable for incorporating as electron-transport moieties. For
example, Tao et al. reported an ortho-linked oxadiazole/amine-
based bipolar compound as the host for green and red dopants,
giving maximum hext as high as 20.2% for green and 18.5% for deep
red electrophosphorescence.4 Hsu et al. achieved peak hext of 20.6%
Introduction
Phosphorescent organic light-emitting diodes (PhOLEDs) have
attracted much attention because they effectively harvest elec-
trogenerated singlet and triplet excitons to accomplish internal
quantum efficiency close to 100% [external quantum efficiency
(hext) ¼ ca. 20% (ph/el)].1 A massive amount of research effort
has been devoted to the development of novel materials and
sophisticated device architectures for the prospective uses of
PhOLEDs in display and lighting technologies. To realize highly
efficient PhOLEDs, a suitable host material is usually employed
to suppress the intrinsic detrimental effects, such as aggregation
quenching and/or triplet–triplet annihilation, of transition metal-
centered phosphors.2 The molecular design of such host material
is rather challenging as it requires optimized physical properties
including (i) appropriate HOMO–LUMO levels to effectively
allow smooth charge carriers (hole and electron) injection from
the neighboring active layers into the emitting layer for achieving
low operating voltages; (ii) balanced charge carrier mobilities to
give maximum charge recombination within the emitting layer;
aDepartment of Chemistry, National Taiwan University, Taipei 10617,
Taiwan. E-mail: kenwong@ntu.edu.tw; Fax: +886 2 33661667; Tel:
+886 2 33661665
bInstitute of Optoelectronic Sciences, National Taiwan Ocean University,
Keelung, 20224, Taiwan. E-mail: wenhung@mail.ntou.edu.tw
cDepartment of Chemistry, National Tsing Hua University, Hsinchu 30071,
Taiwan
with
a
phosphine-oxide/amine-based bipolar host for blue
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phosphorescent emission.5 Most recently,
a
blue-emitting
Results and discussion
PhOLED reported by Jeon et al. incorporating the bipolar host
comprising of hole-transporting carbazole and electron-trans-
porting diphenylphosphoryl oxide achieved the highest hext of
25.1%.6 These bipolar host materials explicitly manifested
their importance and competence for realizing highly efficient
PhOLEDs. The introduction of twisted conformation and/or
saturated system of designing bipolar hosts efficiently diminishes
the electronic communication between the donor and the acceptor,
which therefore leads to high triplet energies.5b,7 Accordingly, in line
with the molecular design strategy, the non-conjugated spiro-
bridged donor (D)–acceptor (A) molecular scaffold can effectively
impede p-orbital interactions between the individual chromophore
branches and thus afford the flexibility of independent fine-tuning
of the electronic properties of donor and acceptor. Moreover, the
orthogonal molecular configuration is beneficial to the thermal
and emission stability due to suppressed intermolecular interaction
in the solid state. Our previous report revealed the successful
application of bipolar hosts comprising of a 9,90-spirobifluorene
core, achieving the highest hext of 20.4% for red-emitting
PhOLEDs.8 However, these hosts are generally engaged with
low triplet energies which substantially limit their compatibility
with various shorter wavelength-emitting phosphors. This
shortcoming can be circumvented by replacing the 2,7-dicyano-
fluorene (acceptor) with a less-conjugated 4,5-diazafluorene as
the electron-transport moiety of a spiro-bridged bipolar host
system.
Synthesis
Herein, we designed and synthesized a series of bipolar materials
with a spiro-bridged D–A configuration, 4,5-diaza-20,70-bis(car-
bazol-N-yl)spirobifluorene (CSC), 4,5-diaza-20,70-bis(diphenyla-
mino)spirobifluorene (DSD), 4,5-diaza-20,70-bis(di-p-tolylamino)
spirobifluorene
(TST),
and
4,5-diaza-20,70-bis(1-naph-
thylphenylamino)spirobifluorene (NSN) (Chart 1). It has been
mentioned that the 4,5-diazafluorene derivatives possess high
electron affinity, so are capable of being electron injecting/trans-
porting moieties. The different donors on 20,70-position of fluo-
rene moiety have considerable effect on the photo-induced
electron transfer (PET) process which results in different energy
levels, triplet energies, and hole injecting/transporting proper-
ties.11 The perpendicular spiro-linkage minimizes the p-orbital
interactions between the individual D and A chromophore
branches, which retains the electron-transport behavior in
each bipolar material. The synthesis of CSC and DSD was
previously reported by our group.10 TST and NSN were
synthesized according to the synthetic procedure of DSD with the
use of di(p-tolyl)amine and 2-naphthylphenylamine, respectively
(Scheme 1).
Thermal properties
The morphological properties and thermal stabilities of spiro-
bridged bipolar hosts CSC, DSD, TST, and NSN were charac-
terized by differential scanning calorimetry (DSC) and ther-
mogravimetric analysis (TGA) (Table 1), respectively. We
observed distinct glass transition temperature (Tg) for CSC
The heterocyclic 4,5-diazafluorene was demonstrated in our
previous work as an efficient electron-transport moiety in
a
spiro-configured D–A bipolar host material, 4,5-diaza-
20,70-bis(carbazol-9-yl)-9,90-spirobifluorene (CSC), for highly
efficient single-layer green PhOLEDs with an external quantum
ꢁ
ꢁ
ꢁ
ꢁ
(179 C), DSD (137 C), TST (148 C), andꢁNSN (190 C)ꢁfol-
lowed by crystallization peaks of CSC (255 C), DSD (226 C),
and TST (238 ꢁC), no crystallization of NSN during the DSC
scan. Compounds CSC, DSD, TST, and NSN exhibited good
thermal stabilities with decomposition temperatures (Td, corre-
sponding to 5% weight loss) ranging from 348 to 416 ꢁC. A high
glass transition temperature is desirable for host material in
PhOLEDs because it suppresses the formation of aggregates and
prevents morphological changes upon heating. Such an orthog-
onal molecular configuration of spiro-configured bipolar host
material is very beneficial to the thermal stability and thin
film homogeneity, which are critical criteria for PhOLED
devices.
efficiency and a power efficiency of 8.3% and 12.3 lm Wꢀ1
,
respectively, at a practical brightness of 1000 cd mꢀ2 (8.0 V).9 The
bipolar characteristic of CSC is imparted by the electron-trans-
porting 4,5-diazafluorene10 and the hole-transporting carbazoles.
Facile synthetic modification on the donor branch is readily
provided by the reactive 20 and 70 positions of 4,5-diazaspirobi-
fluorene. As a result, we adopted the advantages of spiro-bridged
D–A system and substantially extended our previous work on
bipolar host materials by introducing various arylamine donors
with different electron-donating ability such as diphenylamine,
di-p-tolylamine, and 1-naphthylphenylamine. We anticipate that
the triplet energies, energy levels, and carrier-transport proper-
ties of these bipolar materials can be tailored via judicious
molecular design on the donor branch. As a consequence, the
structural-property-performance relationships in the corre-
sponding PhOLEDs will be systematically established after
detailed characterizations of the bipolar hosts. On the other
hand, in the consideration of device structure, the single-layer
devices generally attract much attention due to their cost-effec-
tiveness and simple fabrication process. However, the extraor-
dinary high demand in their molecular design imposes significant
difficulties on the development of single-layer device. Compar-
atively, several important merits of multiple-layer device archi-
tecture such as energy-barrier reduction, carrier balance, and
exciton confinement ensure a maximum device performance,
rendering this type of device structure suitable for characterizing
the properties of new materials.
Chart 1 Chemical structures of the bipolar hosts.
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Scheme 1 Synthesis of TST and NSN.
Photophysical properties
Fig. 1 depicts the UV-visible absorption and photoluminescence
(PL) spectra of spiro-bridged bipolar molecules CSC, DSD, TST,
and NSN in solution (CH2Cl2) and solid thin films. The photo-
physical data are summarized in Table 1. All molecules exhibit two
sets of absorption peaks which are assigned to the electronic
transitions of 4,5-diazafluorene (shorter wavelength) and the donor
branch (longer wavelength). The pronounced red-shifted emission
maxima with Stokes shift of ca. 140 nm of DSD, TST, and NSN
which are due to a mechanism involving rapid photoinduced
electron transfer (PET).12 The emission generated from the charge-
separated excited state is highly sensitive to the dielectric environ-
ment because of its high polarity. Therefore, the longer emission
wavelengths in solution than that in thin films are expected due to
higher dielectric environment of CH2Cl2 and suppressed intermo-
lecular interaction. In contrast, with less electron-donating carba-
zoles for CSC, the PET process becomes less efficient as observed
for the smaller Stokes shift. The phosphorescence spectra of CSC,
DSD, TST, and NSN in EtOH at 77 K show the highest-energy 0–0
phosphorescence emission at 468, 515, 519, and 521 nm corre-
sponding to the triplet energy (ET) of 2.65, 2.41, 2.39, and 2.38 eV,
respectively. The higher ET of CSC is consistent with the shorter
emission wavelength in solution and film, attributed to the less
donating ability of carbazole moieties. The triplet energy of CSC is
sufficiently high for hosting green emitter whereas the others are
appropriate for red phosphorescent emitters as a result of efficient
prevention of reverse triplet energy transfer.
Fig. 1 Room-temperature absorption and emission (PL) spectra of
CSC, DSD, TST, and NSN in CH2Cl2 solutions and in neat films and
corresponding phosphorescence (Phos) spectra recorded from their
EtOH solutions at 77 K.
Fig. 2 Cyclic voltammograms of CSC, DSD, TST, and NSN; 0.1 M
TBAP (reduction) in THF and 0.1 M TBAPF6 (oxidation) in CH2Cl2
were used as supporting electrolytes. A glassy carbon electrode was used
as the working electrode; scan rate 100 mV sꢀ1
.
Cyclic voltammetry
(Fig. 2). The irreversible oxidation during the first CV scan of
CSC is due to electropolymerization of carbazoles occurring at
the C3 and C6 positions. Two reversible oxidations in a range
We used cyclic voltammetry (CV) to probe the electrochemical
properties of the bipolar compounds CSC, DSD, TST, and NSN
Table 1 Physical properties of CSC, DSD, TST, and NSN
lPL/nm,
sol./film
Tg/ꢁC
Tc/ꢁC
Tm/ꢁC
Td/ꢁC
E1/2ox/Va
E1/2red/Vb
(HOMOc/LUMO/Egd)/eV
labs/nm, sol./film
ET/eV
CSC
DSD
TST
NSN
179
137
148
190
255
226
238
n.d.
>400
291
285
416
348
360
397
Irr.
ꢀ1.93
ꢀ2.01
ꢀ2.12
ꢀ2.01
ꢀ5.63/ꢀ2.36/3.27
ꢀ5.20/ꢀ2.20/3.00
ꢀ5.00/ꢀ2.00/3.00
ꢀ5.07/ꢀ2.07/3.00
293, 344/297, 347
307, 380/320, 382
308, 385/320, 389
320, 382/322, 385
377/413
518/470
538/478
503/460
2.65
2.41
2.39
2.38
0.70, 0.99
0.60, 0.91
0.71, 1.02
308
Oxidation potential. b Reduction potential. c HOMO energy level determined using photoelectron yield spectroscopy (AC-2). LUMO ¼ HOMO + Eg.
a
d
Calculated from the absorption onset of the solid film.
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NSN indicates that the efficient resonance delocalization of the
radical cation occurs on the donor branch. Four bipolar
compounds underwent one reversible reduction at a reduction
potential close to that of 4,5-diaza-9,90-spirobifluorene. There-
fore, we unambiguously assign the reduction site as being located
on the acceptor branch, 4,5-diazafluorene. In line with the trend
of electron-donating capability, the largest reduction potential
(ꢀ2.12 V) was found for TST. For the determination of HOMO
energy levels, we used photoelectron yield spectroscopy (Riken
AC-2), and then estimated the LUMO energy levels from the
HOMO values and the optical band gaps (Eg) by using the
equation of LUMO ¼ HOMO + Eg. The measured redox
potentials and calculated HOMO/LUMO energies are summa-
rized in Table 1.
Charge carrier mobility and electron only devices
Fig. 3 Typical transient photocurrent signals for holes: (a) DSD (1.55
mm thick) at E ¼ 5.8 ꢃ 10ꢀ5 V cmꢀ1, (b) TST (1.8 mm thick) at E ¼ 5.0 ꢃ
10ꢀ5 V cmꢀ1, (c) NSN (1.55 mm thick) at E ¼ 7.0 ꢃ 10ꢀ5 V cmꢀ1. Insets are
the double logarithmic plots of (a)–(c). (d) Electron and hole mobilities of
The charge-transport characteristics of the bipolar compounds
can be investigated by their charge carrier mobility using the
conventional time-of-flight (TOF) technique.13 The typical
transient photocurrent signals (holes for DSD, TST, and NSN)
revealed their dispersive transport behaviors and presented the
charge mobilities plotted as a function of the square root of the
electric field in Fig. 3. For electric field strengths ranging from
3.6 ꢃ 105 to 6 ꢃ 105 V cmꢀ1, the electron and hole mobilities of
CSC are within the range from 5 ꢃ 10ꢀ6 to 8 ꢃ 10ꢀ6 cm2 Vꢀ1 sꢀ1
and from 4.1 ꢃ 10ꢀ6 to 5.2 ꢃ 10ꢀ6 cm2 Vꢀ1 sꢀ1, respectively. The
same order of magnitude of the hole and electron mobilities
suggests an effective ambipolarity and balanced carrier trans-
port behavior of CSC.8 The hole mobilities of bipolar
compounds with amino-type donors (DSD, TST, and NSN)
recorded by TOF are less than that of CSC, in the range of 5 ꢃ
10ꢀ7 and 6 ꢃ 10ꢀ6 cm2 Vꢀ1 sꢀ1. The hole mobilities follow the
trend of CSC > DSD > NSN > TST, which does not correlate
to their electron-donating capability estimated from HOMO
energy levels (TST > NSN > DSD > CSC). Since charge carrier
mobility is related to the intermolecular interaction such as the
mode of molecular aggregation in the solid state, electron rich
structure moiety does not necessary to enhance the hole
mobility.
CSC, DSD, TST, and NSN plotted with respect to E1/2
.
Fig. 4 Current–voltage (I–V) characteristics of electron-only devices.
Inset: the relative energy level diagram of the electron-only devices.
For DSD, TST, and NSN, no photocurrents were observed
for electrons. Therefore, we fabricated electron-only devices
with a device structure of ITO/BCP (300 nm)/bipolar host
(500 nm)/TPBI (200 nm)/LiF/Al to evaluate the electron trans-
port behavior. Fig. 4 depicts the current vs. voltage character-
istics of electron-only devices. The introduction of BCP with
contact of ITO is for limiting the injection of hole carrier, as
shown in the inset of Fig. 4. The magnitude of current follows
the order of NSN > TST > DSD > CSC. Although the
assessment of the electron mobility via the current density of the
electron-only device does not reflect the same trend of electron
mobility obtained from TOF measurement due to low electron
mobilities of BCP and TPBI which serve as the limiting factor of
the current density in electron-only devices,14 the higher
currents obtained from NSN, TST, and DSD compared
with that of CSC still imply comparable electron mobilities,
which should lie in the region of 105–106 cm2 Vꢀ1 sꢀ1
(the electron mobility of CSC is in the range of 5 ꢃ 10ꢀ6 to 8 ꢃ
10ꢀ6 cm2 Vꢀ1 sꢀ1).
Scheme 2 Structures of the heavy metal complexes used in this study.
between 1.2 and 0.5 V for DSD, TST, and NSN are assigned to
the oxidations of arylamine moieties. The lowest oxidation
potential is observed for TST (0.60 V), as expected for the
strongest electron-donating capability of di-p-tolylamine among
the four donors. The pronounced potential difference (DE ꢂ 0.3
V) between the first and second oxidations of DSD, TST, and
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Table 2 EL Performance of devices as a function of the host and emitter
Device Host: 8 wt% dopanta Vonb/V L at 1000 nitc/V (%) Lmax/cd mꢀ2
Imax/mA cmꢀ2 hext max/%, cd Aꢀ1 hp max/lm Wꢀ1 CIE1931 (x, y)
A1
A2
A3
A4
B1
B2
C1
C2
D1
D2
CSC: (PPy)2Ir(acac)
CSC: Ir(PPy)3
CSC: (PBi)2Ir(acac)
CSC: (TPm)2Ir(acac)
DSD: OS1
DSD: OS2
TST: OS1
TST: OS2
NSN: OS1
NSN: OS2
2
2
2
2
2
2
2
2
2
2
3.7, 16.9
4.1, 15
3.8, 17.2
4.7, 17.1
5.0, 19
4.6, 17.2
4.9, 19.2
4.7, 15.6
4.6, 19.1
4.3, 17.8
260,000 (11V)
208,000 (10.5V) 2600
241,400 (10.5V) 2500
268,200 (12V)
47,700 (11V)
63,700 (11V)
41,800 (11V)
42,500 (10.5V) 1200
61,300 (10.5V) 1310
2410
17.1, 64
15.5, 56
17.4, 60.5
17.1, 64
19.4, 20.6
17.3, 28.5
19.4, 20.3
16.0, 26
19.5, 21.5
18.0, 30.2
65.4
58.4
58.7
60.6
22.2
27.3
20.2
24.1
22
0.36, 0.61
0.33, 0.62
0.39, 0.58
0.40, 0.58
0.65, 0.35
0.62, 0.38
0.66, 0.34
0.62, 0.38
0.65, 0.35
0.62, 0.38
2200
1160
1710
1020
77,900 (10V)
1110
30
a
Device configuration: ITO/PEDOT:PSS (30 nm)/NPB (20 nm)/TCTA (5 nm)/host:dopant (25 nm)/TPBI (50 nm)/LiF (0.5 nm)/Al (100 nm). b Turn-on
voltage at which emission became detectable. c The values of driving voltage and hext of device at 1000 cd mꢀ2
.
Fig. 5 (a) J–V–L characteristics, (b) plots of EL efficiency versus
brightness and (c) EL spectra for devices incorporating CSC doped with
Fig. 6 (a) J–V–L characteristics and (b) plots of EL efficiency versus
different green dopants.
brightness for red devices.
Electroluminescence
To study the device performance, we fabricated multi-layer devices
using those spiro-bridged D–A bipolar hosts doped with green or
red dopant as emitter in the structure: ITO/PEDOT:PSS (30 nm)/
NPB (20 nm)/TCTA (5 nm)/emitter (25 nm)/TPBI (50 nm)/LiF
(0.5 nm)/Al (100 nm). Here, we used CSC as the host material with
a series of common Ir-based green dopants, bis(2-phenyl-
pyridinato)iridium(III) acetylacetonate (PPy)2Ir(acac),12 tris(2-phe-
nylpyridinato)iridium(III) Ir(PPy)3,15 bis(2,N-diphenylbenzimida-
zolito)iridium(III) acetylacetonate (PBi)2Ir(acac),16 and bis(2-o-tol-
ylpyrimidine)iridium(III) acetylacetonate (TPm)2Ir(acac)9 for device
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A. The DSD, TST, and NSN hosts doped with osmium(II)
bis(3-(trifluoromethyl)-5-(4-tert-butylpyridyl)-1,2,4-triazolate)dime-
thylphenylphosphine Os(bpftz)2(PPhMe2)2 (OS1)17 and osmiu-
hext of 19.5% (21.5 cd Aꢀ1) with saturated red emission of
CIE1931 coordinates (0.65, 0.35) were obtained in device D1
when using NSN as host and OS1 as dopant. Even at a practical
brightness of 1000 cd mꢀ2, the operating voltage was only 4.6 V
and the efficiency remained high at 19.1%. The EL spectra
revealed pure dopant emissions without any other emission
originating from the adjacent organic layer, indicating the
excellent confinement of carriers within the emitting layer
(Fig. 6c).
m(II)
bis(3-(trifluoromethyl)-5-(4-tert-butylpyridyl)-1,2,4-tri-
azolate)diphenylmethylphosphine Os(bpftz)2(PPh2Me)2 (OS2)18
as red phosphors for devices B, C, and D, respectively. The
conducting polymer polyethylene dioxythiophene:polystyrene
sulfonate (PEDOT:PSS) is used as a hole injection layer (HIL). A
thin layer of 4,40,400-tri(N-carbazolyl)triphenylamine (TCTA)
with a high triplet energy (ET ꢂ 2.76 eV) as an exciton-blocking
layer19 is inserted at 4,40-bis(N-(1-naphthyl)-N-phenyl)biphe-
nyldiamine (NPB)/emitter interface, which can prevent the
exciton diffusion, giving higher device efficiency. The low
We attribute this behaviour to the relatively balanced bipolar
transport of these spiro-bridged D–A host materials, thereby
increasing the opportunities for carrier recombination. The
suitable values of ET of 2.38–2.41 eV effectively suppress the
reverse energy transfer from the red triplet dopant to the host.21
Furthermore, it is worth mentioning that the roll-off character-
istic of the quantum efficiency was suppressed at higher bright-
ness. The performance of these devices was outstanding as
compared to those of previously reported results for deep-red
OLEDs.5b,22
HOMO/LUMO
energy
levels
of
1,3,5-tris(N-phenyl-
benzimidizol-2-yl)benzene (TPBI) are beneficial for blocking
holes and facilitating electron injection/transport.20 LiF and Al
served as the electron-injection layer and cathode, respectively.
Such multi-layer structure possesses efficient confinement of
holes and electrons in the emitting layer and prevents the exciton
diffusion, resulting in higher device efficiency. Scheme 2 shows
chemical structures of the heavy metal complexes used in this
study; Table 2 summarizes the electroluminescence data of the
devices.
Conclusions
In summary, we have realized highly efficient red- and green
electrophosphorescent devices incorporating new spiro-config-
ured bipolar host CSC, DSD, TST, and NSN, in which 4,5-
diazafluorene was introduced as electron-transporting moiety.
The promising physical properties of these bipolar hosts such as
high morphological and thermal stabilities, suitable energy
levels, and balanced electron/hole mobilities make them
compatible with various Ir-green and Os-red phosphors. In order
to confine triplet excitons within the emitting layer, we adopted
corresponding dopant having triplet energy less than that of
host, a series of devices with CSC:green dopant and (DSD or
TST or NSN):red dopant were fabricated. The green-phospho-
rescent device doped with (PBi)2Ir(acac) achieved an external
quantum efficiency, maximal luminance, and power efficiency of
17.4%, 241400 cd mꢀ2, and 58.7 lm Wꢀ1, respectively. Particu-
larly, the device A1 incorporating (PPy)2Ir(acac) as the dopant
exhibited maximum efficiencies of 17.1%, 64 cd Aꢀ1 and 65.4
ml Wꢀ1, and showed a slow quantum efficiency roll-off as the
brightness reaches to 10 000 cd mꢀ2, hext is still as high as 15.4%.
Among the red PhOLEDs, the one employing bipolar host
material NSN doped with Os(bpftz)2(PPhMe2)2 (device D1)
achieved an external quantum efficiency, a maximal luminance,
Fig.
5 presents the current density–voltage–luminance
(J–V–L) characteristics, device efficiencies and EL spectra of the
devices with CSC host and different green dopants (device
A1–A4). The device A1 incorporating (PPy)2Ir(acac) as the
dopant exhibited the best performance – a relatively low turn-on
voltage of 2 V (defined as the voltage at which the EL increased
rapidly) and its driving voltage was merely 5.1 V at a practical
brightness of 10000 cd mꢀ2. The turn-on voltage is much lower
than that of the signal-layer device (1000 cd mꢀ2 at 8 V).9 We
ascribe the reduced driving voltage of the multi-layer structure to
facile hole/electron-injections. Fig. 5b presents the external
quantum and power efficiency profiles plotted with respect to the
brightness. The device exhibited maximum efficiencies of 17.1%,
64 cd Aꢀ1 and 65.4 ml Wꢀ1 greater than that of its signal-layer
counterpart (8.3%, 31.4 cd Aꢀ1 and 16.1 ml Wꢀ1). Even as the
brightness reaches 10 000 cd mꢀ2, hext is still as high as 15.4% and
shows a slow quantum efficiency roll-off. We attribute this large
enhancement in CSC-based device performance to the well-
matched energy levels, charge balance and triplet exciton
confinement provided by suitable hole- and electron-transport
layers.
Fig. 6 presents the J–V–L characteristics and EL efficiencies
of devices incorporating DSD, TST, and NSN as host materials
doped with OS1 and OS2 as red phosphors. Table 2 summa-
rizes the electroluminescence characteristics of the resulting
devices. All J–V curves exhibited almost the same profiles due to
similar energy levels (HOMO/LUMO), triplet energies (ET), and
carrier mobilities (see Table 1). The devices exhibited signifi-
cantly lower operating voltages, presumably because of the
smaller energy barriers when using DSD, TST, and NSN as the
host materials. As a result, these devices turned on sharply at
low voltages of 2 V and exhibited a maximum brightness of
63 700 cd mꢀ2 at 1710 mA cmꢀ2 (device B2). Fig. 6b shows these
devices exhibited high hext (16–19.5%) and a slow quantum
efficiency roll-off, when the brightness reaches 1000 cd mꢀ2, hext
is still as high as 15.6–19.2%. Among these devices, the highest
and a power efficiency of 19.5%, 61 300 cd mꢀ2, and 22 lm Wꢀ1
,
respectively. More importantly, these highly efficient PhOLEDs
showed limited efficiencies roll-off due to the balanced charge
carriers flux within the emitting layer. Our results indicate that
spiro-configured bipolar hosts with electron/hole transport
moiety and spiro-configured architecture are promising molec-
ular scaffold for achieving highly efficient electrophosphorescent
devices.
Acknowledgements
We greatly appreciate the financial support from National
Science Council of Taiwan (NSC 98-2119-M-002-007-MY3 and
100-2112-M-019-002-MY3) and Ministry of Economic Affairs
(100-EC-17-A-08-S1-042).
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 9658–9664 | 9663

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