Constructing low-triplet-energy hosts for highly efficient blue PHOLEDs: Controlling charge and exciton capture in doping systems

Article abstract of DOI:10.1021/cm403160p

With the aim to rationally figure out the significance of charge and exciton capture in the electrophosphorescent processes in low-triplet-energy hosts involved doping systems, the frontier molecular orbital (FMO) energy levels of a series low-triplet-energy hosts with diphenylphosphine oxide (DPPO) and triphenylamine (TPA), collectively named DPE_xPOTPA_n, were successfully and gradually tuned on the basis of their constant triplet energy levels (T₁) of 2.63 eV to get rid of interference from host-dopant energy transfer. It was showed that device efficiencies were directly proportional to the depths of carrier traps formed on the dopants and inversely proportional to the exciton capture ability of the hosts, which were evidenced by the highest external quantum efficiency of ～15% from FIrpic-based PHOLED of DPESPOTPA with the deepest hole and electron traps and the weakest exciton capture ability among DPE_xPOTPA_n. This work not only demonstrated the great advantages and potential of this kind of host materials for low-driving-voltage application but also clarified the determinants of highly efficient low-triplet-energy hosts for blue PHOLEDs, which are consequentially referable for purposeful molecular design.

Full text of DOI:10.1021/cm403160p

Article
pubs.acs.org/cm
Constructing Low-Triplet-Energy Hosts for Highly Efficient Blue
PHOLEDs: Controlling Charge and Exciton Capture in Doping
Systems
Chunmiao Han,†,‡ Fangchao Zhao,†,§ Zhen Zhang,‡ Liping Zhu,§ Hui Xu,*,‡ Jing Li,‡ Dongge Ma,*,§
and Pengfei Yan‡
‡Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education, Heilongjiang University, 74 Xuefu Road, Harbin
150080, P. R. China
§State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences,
Changchun 130022, P. R. China
S
* Supporting Information
ABSTRACT: With the aim to rationally figure out the significance of charge
and exciton capture in the electrophosphorescent processes in low-triplet-
energy hosts involved doping systems, the frontier molecular orbital (FMO)
energy levels of a series low-triplet-energy hosts with diphenylphosphine oxide
(DPPO) and triphenylamine (TPA), collectively named DPExPOTPAn, were
successfully and gradually tuned on the basis of their constant triplet energy
levels (T1) of 2.63 eV to get rid of interference from host-dopant energy
transfer. It was showed that device efficiencies were directly proportional to the
depths of carrier traps formed on the dopants and inversely proportional to the
exciton capture ability of the hosts, which were evidenced by the highest
external quantum efficiency of ∼15% from FIrpic-based PHOLED of
DPESPOTPA with the deepest hole and electron traps and the weakest
exciton capture ability among DPExPOTPAn. This work not only
demonstrated the great advantages and potential of this kind of host materials
for low-driving-voltage application but also clarified the determinants of highly efficient low-triplet-energy hosts for blue
PHOLEDs, which are consequentially referable for purposeful molecular design.
KEYWORDS: blue electrophosphorescence, low triplet energy host, charge capture, exciton capture, phosphine oxide
the harmony and unity between low driving voltage and high
EL efficiency for portable and low-power applications.28−47
Under the conventional strategy, two prerequisites for
molecular design should be considered that (i) only groups
with high-enough T1 can be involved,48−55 otherwise, the T1
excited state should be localized on the groups with the high
enough T1;56,57 (ii) the intermolecular interaction should be
carefully restrained to suppress low-energy charge transfer
(CT) states and other negative effects.32,58−64 Recently, our
group developed several novel linkage styles, such as multi-
insulating,65 indirect and short-axis linkages,66,67 to construct a
series of high-performance carrier-transporting phosphine oxide
(PO) hosts with the focus on achieving high T1 for low-voltage-
driving efficient blue PHOLEDs.68 Nevertheless, along with
increased complexity of molecular structures, function enhance-
ment and property adjustment are more and more difficult due
to the limited utilizable functional groups and linkage styles.
This forced us to reconsider the determinants for the molecular
1. INTRODUCTION
Efficient utilization of high energy excitons is one of the most
important issues for electroluminescence (EL) because of the
high sensitivity of these excitons to the various quenching
effects.1−11 Intuitively, the efficient blue organic light-emitting
diodes (OLED) can be achieved through the well-managed
material characteristics and device configurations for balanced
carrier injection and transportation to emitting layers (EML)
and the confinement of excitons on emitters.12−16 This arouses
the vibrant development of high energy-gap organic semi-
conductors with both improved electroactivity and optical
properties to suppress the nonradiative energy losses during the
EL process.17−24 Among these materials, hosts for blue
phosphorescent OLEDs (PHOLED) are highly focused in
recent years because of their typical optoelectronic character-
istics for fundamental investigation of the structure−property
correlations and the corresponding feasible approaches for
controllable and selective modifications.4,25,26
For the sake of breaking through the intrinsic inferiority of
blue electrophosphorescence in carrier injection and trans-
portation,27 ambipolar host materials with the high enough first
triplet energy levels (T1) are paid much attention in pursuit of
Received: October 6, 2013
Revised: November 24, 2013
© XXXX American Chemical Society
A
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design of blue electrophosphorescent host materials. When T1
of host is equivalent or slightly lower than that of dopant, two
main procedures will determine the ultimate exciton availability,
viz., (i) energy transfer from host to guest with the thermal
assistance69−73 and (ii) direct charge capture and exciton
recombination on dopant, which are corresponding to optical
and electrical procedures, respectively (Scheme 1).16,74 Forrest
these two procedures is dominant. In this case, the explicit
insight of one procedure, such as the effects of the electrical
characteristics of the hosts on the device performance, without
any interference from the other one becomes urgent and
imperative. Actually, the deep understanding of the effects and
restraints of each single procedure is crucial to guide the future
molecular design. In our opinion, the essential condition for
achieving high efficiencies through low-triplet-energy hosts
should be the exciton confinement on phosphors. In this sense,
the charge capture and exciton recombination by the host
materials should be suppressed to, in turn, facilitate the similar
procedure on dopants.
Scheme 1. Two Key EL Mechanisms for PHOLEDs
a
Involving Low-Triplet-Energy Hosts
In this contribution, we rationally utilized the inductive effect
of PO for electrical property adjustment, accompanied with
its negligible influence on T1, to illustrate the influence of the
charge capture and exciton recombination behaviors of low-
triplet-energy hosts on the EL performance of FIrpic-based
PHOLEDs. A series diphenylphosphine oxide (DPPO)-
triphenylamine (TPA) hybrids with low T1 of 2.63 eV slightly
lower than that of FIrpic were prepared, namely 4′-(2-
(diphenylphosphoryl)phenoxy)-N,N-diphenylbiphenyl-4-
amine (DPESPOTPA), 4′-(4′-(diphenylamino)biphenyl-4-
yloxy)-3′-(diphenylphosphoryl)-N,N-diphenylbiphenyl-4-
amine (DPESPOTPA2), 1-diphenylamino-4-(2,4-di-
(diphenylamino)-phenoxy)-5-(diphenylphosphinoyl)-benzene
(DPESPOTPA3), 3′-(diphenylphosphoryl)-4′-(2-
(diphenylphosphoryl)phenoxy)-N,N-diphenylbiphenyl-4-
amine (DPEPOTPA) and 4′,4″-oxybis(3′-(diphenylphosphor-
yl)-N,N-diphenylbiphenyl-4-amine) (DPEPOTPA2), with the
collective name as DPExPOTPAn (Scheme 2), in which TPA
served as donor (D) to tune the hole injecting/transporting
ability and DPPO was involved as acceptor (A) to adjust the
electron injecting/transporting ability, accompanied with
diphenylether as the bridge. The frontier molecular orbital
(FMO) energy levels were successfully modulated through
varying functional group ratio, while the negative effects on T1
were mostly suppressed to result in precisely the same
phosphorescence spectra of DPExPOTPAn, which established
a feasible platform for the investigation about the influences of
electrical characteristics of the hosts on their EL performance
without the interference from energy transfer procedure. It
indicated that the EL efficiency was grosso modo in direct
proportion to the charge trap depth between the host and
FIrpic and simultaneously in reverse proportion to the exciton
recombination ability of the host. As expected, among these
compounds, DPESPOTPA with the deepest charge traps and
the weakest exciton recombination ability supported the best
blue PHOLEDs with the impressively high efficiencies as 14.8%
for E.Q.E. and 40.3 lm W−1 for power efficiency (P.E.), which
were about 5 times greater than those of the devices based on
DPEPOTPA2 with the shallowest electron trap and the much
stronger exciton recombination ability. This work clarified the
determinants for the low-triplet-energy hosts and thereby
opened the window for the rational molecular design of more
complicated and multifunctional host materials.
a
(a) EL mechanism based on thermal-assistant energy transfer
(TAET), in which exciton is firstly formed on host with low T1 and
then transferred to phosphor with the thermal assistance to overcome
the triplet energy gap. Because energy of thermal fluctuation is less
than 0.3 eV at room temperature, the host-dopant triplet energy gap
should be within 0.3 eV. (b) Charge and exciton capture (CEC)
afforded EL mechanism, in which charge is directly captured by
phosphorbecause of charge traps formed by the FMO energy level
differences between host and phosphor and then recombined to afford
EL.
et al. already demonstrated endothermic energy transfer from
4,4′-bis(N-carbazolyl)-1,1′-bipheny (CBP, T1 = 2.56 eV) to
bis(4,6-difluorophenylpyridinato-N,C2)picolinato Iridium(III)
(FIrpic, T1 = 2.65 eV),69 but they just achieved E.Q.E. of
only 5% through conventional device configuration. Illuminat-
ingly, the recent work by Padmaperuma et al. reported the
achievement of unexpected high E.Q.E. approaching to 14%
from CBP-based blue PHOLEDs with FIrpic as the phosphor,
ascribed to the well confinement of excitons in EML by the
high T1 of adjacent carrier transporting layers (CTL).75
Therefore, it is extremely significant to confirm which one of
2. EXPERIMENTAL SECTION
Materials and Instruments. All the reagents and solvents used
for synthesis were purchased from Aldrich and Acros companies and
used without further purification. 1-(4-Bromophenoxy)-1−2-(diphen-
yl-phosphinoyl)-phenoxy-benzene (DPESPOBr), 1-bromo-4-(2,4-di-
bromo-phenoxy)-5-(diphenylphosphinoyl)-benzene (DPESPOBr3),
4-bromo-2-(diphenyl-phosphinoyl)-1−2-(diphenyl-phosphinoyl)-phe-
B
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Scheme 2. Synthetic Procedure of DPExPOTPAn
noxy-benzene (DPEPOBr), and 1-bromo-4−4-bromo-2-(diphenyl-
phosphinoyl)-phenoxy-5-(diphenylphos-phinoyl)-benzene (DPE-
POBr2) were synthesized according to our previous reports.76
1H NMR spectra were recorded using a Varian Mercury plus 400NB
spectrometer relative to tetramethylsilane (TMS) as internal standard.
Molecular masses were determined by a FINNIGAN LCQ Electro-
Spraying Ionization-Mass Spectrometry (ESI-MS), or a MALDI-TOF-
MS. Elemental analyses were performed on a Vario EL III elemental
analyzer. Absorption and photoluminescence (PL) emission spectra of
the target compound were measured using a SHIMADZU UV-3150
spectrophotometer and a SHIMADZU RF-5301PC spectrophotom-
eter, respectively. Thermogravimetric analysis (TGA) and differential
scanning calorimetry (DSC) were performed on Shimadzu DSC-60A
and DTG-60A thermal analyzers under nitrogen atmosphere at a
heating rate of 10 °C min−1. Cyclic voltammetric (CV) studies were
conducted using an Eco Chemie B. V. AUTOLAB potentiostat in a
typical three-electrode cell with a platinum sheet working electrode, a
platinum wire counter electrode, and a silver/silver nitrate (Ag/Ag+)
reference electrode. All electrochemical experiments were carried out
under a nitrogen atmosphere at room temperature in CH2Cl2 for
oxidation and THF for reduction. Phosphorescence spectra were
measured in dichloromethane using an Edinburgh FPLS 920
fluorescence spectrophotometer at 77 K cooling by liquid nitrogen
with a delay of 300 μs using the time-correlated single photon
counting (TCSPC) method with a microsecond pulsed Xenon light
source for 10 μs to 10 s lifetime measurement, the synchronization
photomultiplier for signal collection and the Multi-Channel Scaling
Mode of the PCS900 fast counter PC plug-in card for data processing.
The thin films of the PO compounds were prepared through vacuum
evaporation on glass substrates under the same condition of device
fabrication. The morphological characteristics of these films were
measured with an atom force microscope (AFM) Agilent 5100 under
the tapping mode.
General Procedure of Suzuki Coupling. in Ar2, 6 equiv. of aq.
NaOH (2 M) was added to a stirred solution of 1 equiv. of the
bromide, 1.5 equiv. of N,N-diphenyl-4-(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)aniline, 0.1 equiv. of Pd(PPh3)4 and 0.1 equiv. of
TBAB in 10 mL of THF. The reaction mixture was heated to 80 °C
and stirred for 24 h. The reaction was quenched by 10 mL of aq.
NH4Cl, and extracted by dichloromethane (3 × 10 mL). The organic
layer was dried with anhydride Na2SO4. The solvent was removed in
vacuo, and then the residue was purified by flash column
chromatography.
4′-(2-(Diphenylphosphoryl)phenoxy)-N,N-diphenylbiphen-
yl-4-amine (DPESPOTPA). DPESPOTPA was purified by flash
column chromatography using petroleum ether/ethyl acetate (2:1−
0:1) as eluent to afford 307 mg of white powder with the yield of 50%.
1H NMR (TMS, CDCl3, 400 MHz): δ = 8.104 (qd, J1 = 7.6 Hz, J2 =
4′-(4′-(Diphenylamino)biphenyl-4-yloxy)-3′-(diphenylphos-
phoryl)-N,N-diphenylbiphenyl-4-amine (DPESPOTPA2). DPES-
POTPA2 was purified by flash column chromatography using
petroleum ether/ethyl acetate (2:1−0:1) as eluent to afford 86 mg
1
of white powder with the yield of 20%. H NMR (TMS, CDCl3, 400
M Hz): δ = 8.370 (dd, J = 2.4 Hz, 13.6 Hz, 1H); 7.845 (q, J = 7.0 Hz,
12.6 Hz, 4H); 7.672 (dd, J = 2.4 Hz, 8.4 Hz, 1H);7.541−7.474 (m,
4H); 7.477−7.381 (m, 8H); 7.336−7.258 (m, 8H); 7.196−7.114 (m,
12H); 7.064 (t, J = 7.4 Hz, 4H); 6.877 (q, J = 5.2 Hz, 8.4 Hz, 1H);
6.683 ppm (d, J = 8.4 Hz, 2H). LDI-TOF: m/z (%): 856 (100) M+.
Elemental anal. Calcd for C60H45N2O2P (%): C, 84.09; H, 5.29; N,
3.27; O, 3.73. Found: C, 84.13; H, 5.30; N, 3.85; O, 3.84.
1-Diphenylamino-4-(2,4-di(diphenylamino)-phenoxy)-5-(di-
phenylphosphinoyl)-benzene (DPESPOTPA3). DPESPOTPA3
was purified by flash column chromatography using petroleum
ether/ethyl acetate (2:1−0:1) as eluent to afford 220 mg of white
powder with the yield of 40%. 1H NMR (TMS, CDCl3, 400 M Hz): δ
= 8.374 (dd, J = 2.4 Hz, 13.2 Hz, 1H); 7.766 (q, J = 7.6 Hz, 12.0 Hz,
4H); 7.634 (dd, J = 2.2 Hz, 8.6 Hz, 1H); 7.537 (d, J = 2.4 Hz, 1H);
7.483 (d, J = 8.4 Hz, 2H); 7.434 (d, J = 8.4 Hz, 2H); 6.838−6.781 (m,
2H); 7.369−7.261 (m, 16H); 7.214 (dd, J1 = 2.4 Hz, J2 = 8.4 Hz, 1H);
7.677−7.617 (m, 18H); 7.092 (t, J = 7.2 Hz, 2H); 7.060 (t, 7.2 Hz,
4H); 6.947 (d, J = 8.87 Hz, 2H); 6.791 (q, J1 = 1.2 Hz, J2 = 8.4 Hz,
1H); 6.307 ppm (d, J = 8.8 Hz, 1H); LDI-TOF: m/z (%): 1099 (100)
M+; elemental analysis (%) for C78H58N3O2P: C 85.14, H 5.31, N
3.82, O 2.91; found: C 85.11, H 5.33, N 3.90, O 3.05.
3′-(Diphenylphosphoryl)-4′-(2-(diphenylphosphoryl)-
phenoxy)-N,N-diphenylbiphenyl-4-amine (DPEPOTPA). purified
by flash column chromatography using petroleum ether/ethyl acetate
(2:1−0:1) as eluent to afford 456 mg of white powder with the yield of
56%. 1H NMR (TMS, CDCl3, 400 M Hz): δ = 7.995 (d, J = 13.2 Hz,
1H); 7.819−7.615 (m, 10H); 7.591−7.202 (m, 20H); 7.149−7.073
(m, 6H); 7.066−7.009 (m, 2H); 6.171−6.097 (m, 1H); 6.097−6.006
ppm (m, 1H). LDI-TOF: m/z (%): 813 (100) M+. Elemental anal.
Calcd for C54H41NO3P2 (%): C, 79.69; H, 5.08; N, 1.72; O, 5.90.
Found: C, 79.72; H, 5.09; N, 1.88; O, 6.04.
4′,4″-Oxybis(3′-(diphenylphosphoryl)-N,N-diphenylbiphen-
yl-4-amine) (DPEPOTPA2). DPEPOTPA2 was purified by flash
column chromatography using petroleum ether/ethyl acetate (2:1−
0:1) as eluent to afford 529 mg of white powder with the yield of 50%.
1H NMR (TMS, CDCl3, 400 M Hz): δ = 7.945 (d, J = 13.2 Hz 2H);
7.855−7.615 (m, 8H); 7.562−7.315 (m, 14H); 7.318−7.240 (m,
12H); 7.158−7.111 (m, 8H); 7.111−7.081 (m, 4H); 7.079−7.018 (m,
4H); 6.193−6.078 ppm (m, 2H). LDI-TOF: m/z (%): 1056 (100)
M+. Elemental anal. Calcd for C78H58N3O2P (%): C, 81.80; H, 5.15;
N, 2.65; O, 4.54. Found: C, 81.79; H, 5.12; N, 2.77; O, 4.61.
DFT Calculations. The DFT simulations were carried out with
different parameters for structure optimizations and vibration analyses.
The ground states and triplet states of molecules in vacuum were
optimized without any assistance of experimental data by the restricted
and unrestricted formalism of Beck’s three-parameter hybrid exchange
functional77 and Lee, Yang, and Parr correlation functional78
(B3LYP)/ 6-31G(d), respectively. The fully optimized stationary
points were further characterized by harmonic vibrational frequency
analysis to ensure that real local minima had been found without
12.8 Hz, J3 = 1.6 Hz, 1H), 7.810 (q, J1 = 7.0 Hz, J2 = 12.6 Hz, 4H),
7.542−7.368 (m, 11H), 7.332−7.234 (m, 5H), 7.188−7.098 (m, 6H),
7.056 (t, J = 7.4 Hz, 2H), 6.818 (q, J1 = 5.2 Hz, J2 = 8.0 Hz, 1H), 6.659
ppm (d, J = 8.8 Hz, 2H). LDI-TOF: m/z (%): 613 (100) M+.
Elemental anal. Calcd for C42H32NO2P (%): C, 82.20; H, 5.26; N,
2.28; O, 5.21. Found: C, 82.22; H, 5.25; N, 2.47; O, 5.36.
C
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imaginary vibrational frequency. The total energies were also corrected
by zero-point energy both for the ground state and triplet state. The
spin density distributions were visualized with Gaussview 3.0. All
computations were performed using the Gaussian 03 package.79
Device Fabrication and Testing. Before loading into a
deposition chamber, the ITO substrate was cleaned with detergents
and deionized water, dried in an oven at 120 °C for 4 h, and treated
with UV-ozone for 20 min. Devices were fabricated by evaporating
organic layers at a rate of 0.1−0.3 nm s−1 onto the ITO substrate
sequentially at a pressure below 1 × 10−6 mbar. Onto the TPBI layer, a
layer of LiF with 1 nm thickness was deposited at a rate of 0.1 nm s−1
to improve electron injection. Finally, a 100 nm thick layer of Al was
deposited at a rate of 0.6 nm s−1 as the cathode. The emission area of
the devices was 0.14 cm2 as determined by the overlap area of the
anode and the cathode. The EL spectra and CIE coordinates were
measured using a PR650 spectra colorimeter. The current−density−
voltage and brightness−voltage curves of the devices were measured
using a Keithley 2400/2000 source meter and a calibrated silicon
photodiode. All the experiments and measurements were carried out at
room temperature under ambient conditions. For each host, five
devices with the same configurations were fabricated to confirm the
performance repeatability. The data reported herein were most close
to the average results. The average data are shown in Figure S4 in the
Supporting Information with the error bars corresponding to the range
of the results.
function of PO moieties in DPExPOTPAn was tuning their
electroactivity without changing T1 energy levels, which
accordingly established the platform to confirm the effects of
CEC on the device performance.
DPExPOTPAn were conveniently prepared from the bro-
mide precursors DPExPOBrn through Suzuki coupling with
N,N-diphenyl-4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-
aniline with moderate yields around 50% (Scheme 2). The
structure characterization was established on the basis of mass
spectrometry, NMR spectroscopy and elemental analysis.
3.2. Optical Properties. As mentioned above, the similar
optical properties of these hosts are the basis for rationally
investigating the effect of CEC on the EL performance.
Therefore, the UV/vis absorption and photoluminescent (PL)
spectra of DPExPOTPAn in CH2Cl2 (× 10−6 mol L−1) were
measured to confirm the selective adjustment of excited state
energy by structural variation (Figure 1 and Table 1).
3. RESULTS AND DISCUSSIONS
3.1. Design and Synthesis. The success of CBP in FIrpic-
based PHOLEDs is due to its high endothermic energy transfer
efficiency and the deep charge and exciton traps on FIrpic
formed by the much lower highest occupied molecular orbital
(HOMO, −6.2 eV) and the much higher lowest unoccupied
molecular orbital (LUMO, −2.5 eV) of CBP. Therefore, it can
hardly reflect independent effects of thermal assistant energy
transfer (TAET) and charge and exciton capture (CEC).
Theoretically, direct charge recombination on phosphors is
more efficient than host-dopant energy transfer. This is
evidenced by E.Q.E. of 14% from CEC-involved PHOLEDs,75
three times higher than TAET afforded E.Q.E.69 Adachi et al.
recently also report the remarkably improved E.Q.E. of deep-
blue PHOLEDs by the direct charge recombination on dopants
through dropping the HOMO of the host to form the hole trap
with a depth of 0.1 eV.80 Nevertheless, the direct evidence
about the significance of CEC on EL performance, especially
for low-triplet-energy hosts, is still absent because of the
simultaneous involvement of both TAET and CEC in
conventional devices. Therefore, with the purpose of
independently investigating the effects of CEC, the ideal
hosts should be designed with the exactly same TAET to FIrpic
and the much different charge capture and recombination
abilities. In our previous works, the selective adjustment of
electrical properties without changing T1 was successfully
realized.31,45,59,76 This made this study probable, since the host-
dopant triplet energy gap is one of the key determinants for
TAET. Different from our previous works about PO hosts, in
which PO moieties mainly served as the electroactive insulating
groups to achieve high T1 and polarize molecules, herein, with
no regards to the achievement of high T1 by PO groups, the
insulating feature of PO moieties was utilized to suppress the
influence of structural variations on T1 with the assistance of
the meta linkage and ether bridge. TPA was involved to
enhance hole transporting ability and achieve the relative low
T1 through forming diphenylbiphenylamine segment. The ratio
of TPA and electron transporting DPPO was gradually changed
to purposefully adjust molecular electrical properties. The
Figure 1. Absorption and emission spectra of DPExPOTPAn in
CH2Cl2 (× 10−6 mol L−1). The phosphorescence (PH) spectra were
recorded at 77 K with a delay of 300 μs.
DPExPOTPAn have three main absorption bands around
320−350, 250−300 and 230 nm, corresponding to n-π*
transitions from TPA to diphenylether, n−π* transitions from
TPA to DPPO and π−π* transitions between TPA and
diphenylether. It is showed that the extinction coefficients are
exactly in accord with the number of corresponding functional
groups, which indicated the strong sensitization ability and
antenna effect of TPA and DPPO. PL quantum yields (PLQY)
of DPExPOTPAn were directly proportional to TPA number
and in reverse proportion to DPPO number, ascribed to strong
electron-donating effect of TPA as chromophore and electron-
withdrawing effect of DPPO, respectively (Table 1). Accord-
ingly, the probabilities of radiative transition for the mono-
DPPO derivatives are rather high under irradiative excitation,
exhibiting their greater potential as electrofluorophores in
OLEDs with efficient charge recombination. Estimated from
the absorption edges, the first singlet excited energy levels (S1)
of DPExPOTPAn are gradually reduced due to the conjugation
extension with more TPA and/or DPPO groups (Table 1). It is
showed that one additional TPA and DPPO can result in the S1
reductions of ∼0.1 and 0.15 eV, respectively. Because S1 is
approximately equivalent with the HOMO−LUMO energy
gap, obviously, through adjusting the functional group ratios,
the FMO energy levels of the hosts can be purposively tuned to
fit with those of the adjacent CTLs for improving carrier
injection and transportation and form the charge traps with
suitable depths. In solid state, absorption and PL spectra of
D
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Table 1. Physical Properties of DPExPOTPAn
c
d
d
emission
(nm)
PLQY
(%)
S1
(eV)
T1
(eV)
Tg/Tm/T
(°C)
RMS
(nm)
HOMO
(eV)
compd
absorption (nm)
LUMO (eV)
a
a
b
a
b
a
b
a
b
a
e
g
h
h
i
f
DPESPOTPA
324, 255, 226
408(54)
407(50)
416(58)
414(53)
416(59)
413(48)
422(59)
422(58)
428(65)
90
3.38
2.63
−/ 119/416
123/−/ 492
146/−/ 509
−/239/393
206/254/466
1.14
1.19
1.50
0.41
0.75
−5.91
−5.01
−5.91
−4.84
−5.75
−4.84
−5.90
−4.90
−2.44 / −2.53 −0.93
b
f
f
f
336, 262, 225
4.08
2.65
a
e
g
h
h
i
f
f
f
f
DPESPOTPA2 339, 308, 231
98
3.26
2.63
−2.62 / −2.65 −0.98
b
f
f
f
345, 222
3.86
2.63
a
e
g
h
h
i
DPESPOTPA3 337, 300, 274, 238
100
40
3.16
2.63
−2.56 / −2.59 −1.06
b
f
f
f
343, 305, 233
3.78
2.69
a
e
g
h
h
i
DPEPOTPA
DPEPOTPA2
342,304,227
3.22
2.63
−2.63 / −2.68 −0.87
b
a
b
f
f
f
343, 264, 221
345, 304, 233
343, 302, 230
4.03
2.67
e
g
h
h
i
48
3.11
3.95
d
2.63
−5.88
−4.87
−2.70 / −2.77 −0.92
b
f
f
f
467(142)
2.65
a
b
c
e
In CH2Cl2 (× 10−6 mol L−1). In film. fwhm was in parentheses. Calculated by using 9,10-diphenylanthracene as standard. Estimated according
f
g
h
to the absorption edges. DFT calculated results. Calculated according to the 0−0 transitions of the phosphorescence spectra. Calculated according
to the equation EHOMO/LUMO = 4.78 + onset voltage. Estimated according to the optical energy gaps and the experimental HOMOs.
i
Figure 2. Contours of FMOs for DPExPOTPAn and the spin density distributions of their T1 values by DFT calculation.
DPExPOTPAn were almost the same as those in dilute solution
with the preserved fine structures and the similar absorption
band wavelengths, which indicated the suppressed aggregation
and restrained intermolecular interactions, except for DPE-
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POTPA2, whose symmetrical structure resulted in the
aggregation-induced bathochromic shifts (Table 1).
T1 of these molecules was much concerned because of its
direct relevancy to triplet energy transfer process, which was
estimated from the 0−0 transitions in their time-resolved low-
temperature phosphorescent (PH) spectra (inset in Figure 1).
As demonstrated in our previous works about mixed linkage
strategy, the combined meta and multi-insulating linkage
endows DPExPOTPAn with the almost same PH spectra in
shape and peak wavelength, resulting in their same T1 of 2.63
eV. Low-triplet energy diphenylbiphenylamine segments as
chromophore in these molecules should make the main
contributions to their T1 excited state, which was further
evidenced by the same spin density distributions (SDD) on
TPA and partial diphenylether according to density function
theory (DFT) calculation (Figure 2 below),81,82 whereas
insulating linkages almost have no contributions to T1 state.
Therefore, in the systems like DPExPOTPAn, T1 states can be
feasibly controlled at the same time of adjusting the molecular
structure and functional group ratio for tuning electrical
properties. The slight lower T1 of DPExPOTPAn than 2.65
eV of FIrpic gives rise to the similar TAET processes.
According to optical analysis, through varying the number
and ratio of the functional groups, much different S1 and the
radiative transition characteristics of DPExPOTPAn were
successfully realized to establish the different charge traps and
charge recombination possibilities in PHOLEDs, which
provides a flexible platform for rationally investigating the
effect of CEC on device performance.
3.3. Electrical Properties. The competition on charge
capture between the hosts and dopants in EMLs is mainly
determined by the differences of their FMO energy levels.
Therefore, we first performed the structural DFT optimization
of the ground-state DPExPOTPAn in vacuum (Figure 2 above).
It is showed that electron cloud densities of their HOMOs are
mainly localized on the electron-donating groups, especially
TPA.52,82 Furthermore, DPPOs are almost not involved in the
frontier OMOs. Contrarily, although the LUMOs are also
mainly localized on TPA and diphenylether groups because of
their bigger conjugation, DPPOs make considerable contribu-
tions to the frontier unoccupied molecular orbitals (UMOs).
Especially for DPEPOTPAn, their LUMO+1 and/or LUMO+2
are mainly localized on DPPO groups. Therefore, the
incorporation of DPPO moieties can facilitate the electron
injection and transportation.81 Furthermore, because of the
strong electron-donating ability of TPA, the incorporation of
one more DPPO in DPEPOTPAn does not dramatically
weaken their hole injecting/transporting ability. Although
conjugation, molecular configuration and number and ratio of
functional groups are three main determinants for FMO energy
levels of these molecules, number and ratio of functional groups
is predominant because the former two are also actually related
to it.
The experimental HOMO and LUMO energy levels were
estimated with cyclic voltammetrical (CV) analysis (Figure 3a).
According to the oxidation curves of DPEPOTPAn, all of the
compounds exhibited one reversible oxidation peak at ∼1.35 V
and two irreversible oxidation peaks at ∼1.75 and 2.0 V, which
can be ascribed to TPA, diphenylether and DPPO, respectively.
Except for DPESPOTPA3, the onset voltages of other four
compounds are very similar as around 1.1 V, corresponding to
the HOMO energy levels about −5.9 eV (Table 1). The
highest proportion of TPA moieties in DPESPOTPA3 results
Figure 3. (a) Cyclic voltammograms of DPExPOTPAn measured in
CH2Cl2 for oxidation and THF for reduction at room temperature
with the scanning rate of 100 mV S1− and tetra-n-butylammonium
hexafluorophosphate as supporting electrolyte (0.1 mol L−1); (b) IV
characteristics of nominal single-carrier transporting devices based on
DPExPOTPAn.
in much smaller onset voltage at ∼0.95 V, corresponding to the
HOMO of approximately −5.75 eV. Taking account of the
HOMO of FIrpic as −5.6 eV, the depth of the hole trap formed
between DPESPOTPA3 and FIrpic is only 0.15 eV, much
shallower than ∼0.3 eV for the other hosts. As shown in
Scheme 1, the efficient charge capture requires the trap depth at
least 0.2 eV to overcome the disturbance from thermal
fluctuations. Therefore, when doped in the matrixes of
DPExPOTPAn (n = 1 and 2), FIrpic can efficiently trap the
hole injected from cathode, whereas for the DPESPOTPA3-
FIrpic doping system, isotropic hole migration is possible with
the heat assistance. It is indicated that the LUMO energy levels
of DPExPOTPAn are dependent on the functional groups
number and ratio and the molecular conjugation degree (Table
1). Accordingly, DPEPOTPA2 with two DPPO groups and
symmetrical configuration achieves the deepest LUMO energy
levels of −2.70 eV, which is about 0.05 eV lower than those of
DPEPOTPA and DPESPOTPA2, and about 0.15 and 0.25 eV
lower than those of DPESPOTPA3 and DPESPOTPA,
respectively. Thus, the electron trap formed between FIrpic
and DPExPOTPAn (n = 1 and 2) is too shallow with a depth of
only 0.1−0.15 eV to hardly confine electron on the dopants.
However, when doped in DPESPOTPA and DPESPOTPA3,
FIrpic can realize the efficient electron capture with a suitable
electron trap depth of about 0.2−0.4 eV. Therefore, in these
hosts, DPESPOTPA can endow FIrpic with the strongest
capability for both hole and electron capture, while in
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Figure 4. (a) Energy level scheme of the PHOLEDs; (b) EL spectra of phosphorescent (left) and fluorescent (right) devices with the corresponding
CIE coordinates.
DPEPOTPA2 and DPESPOTPA3, the electron and hole
capture ability of FIrpic is the weakest, respectively.
actions. Furthermore, these ternary hybrids showed the
uniform feature of hole-dominant volt-ampere characteristics.
The exciton stability directly determines the luminescent
performance of the phosphors in the devices, so it is desired if
electron and hole were both localized on FIrpic to form the
most stable Frenkel excitons like the situation of DPES-
POTPA. However, DPESPOTPA2 and DPEPOTPAn (n = 1
and 2) and DPESPOTPA3 can only efficiently confine the hole
or electron on FIrpic, respectively, to afford the formation of
Wannier−Mott excitons with the weaker stability. Because the
electron would be the minority carrier in the devices of these
hole-transporting predominant hosts, the weaker capture ability
for electron is more inferior to the number and stability of
excitons. In this sense, DPEPOTPA2 might be the worst host
for FIrpic among these materials.
3.4. EL Performance of DPExPOTPAn and the Influence
of Charge and Exciton Capture. It can be confirmed that
DPExPOTPAn provide a platform to selectively investigate
CEC effect on the basis of the probably same energy transfer
process and the much different electrical properties. Accord-
ingly, their FIrpic-based PHOLEDs with the configuration of
ITO|MoO3 (8 nm)|NPB (70 nm)|TCTA (5 nm)| Host: 15%
FIrpic (20 nm)|TPBi (30 nm)|LiF (1 nm)|Al were fabricated as
PA-PE, corresponding to DPESPOTPAn (n = 1, 2 and 3) and
DPEPOTPAn (n = 1 and 2), respectively, where TCTA is
tris(4-(9H-carbazol-9-yl)phenyl)amine as exciton-blocking
layer (Figure 4a). It is showed that since the HOMO and
The carrier transporting ability of DPExPOTPAn was further
assessed with the IV characteristics of their nominal single-
carrier transporting devices with the structures of ITO|MoO3
(8 nm)|NPB (20 nm)|Host (40 nm)|NPB (30 nm)|MoO3 (8
nm)|Al for hole-only and ITO|LiF (1 nm)|TPBi (30 nm)|Host
(40 nm)|TPBi (20 nm)|LiF (1 nm)|Al for electron-only,
respectively, in which MoO3 and LiF served as hole- and
electron-injecting layers, NPB is N,N-bis(naphthylphenyl)-4,4′-
biphenyldiamine as the hole-transporting layer (HTL), TPBI is
1,3,5-tris(1-phenyl-1H-benzodimidazol-2-yl)benzene as elec-
tron-transporting hole-blocking layer, respectively (Figure
3b). NPB and TPBI were involved to modify the interfaces
and simulate the environment in emitting devices. Thicknesses
of hole-only and electron-only devices were the same to make
comparison between them rational. The hole-only current
density (J) of DPESPOTPA2 and DPEPOTPA2 were the
highest and lowest among these derivatives, respectively, while
those of other materials were similar. The electron-only J of
DPESPOTPA2 was also almost the biggest, comparable with
that of DPESPOTPA. It is noticed that the electron-only J of
DPESPOTPA3 and DPEPOTPA were very approximate,
which were lower than that of DPEPOTPA2. It indicated
that the carrier transporting properties were the integrated
results of molecular compositions and intermolecular inter-
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Figure 5. Brightness−current density (J)−voltage curves and efficiency curves of (a, b) phosphorescent PA-PE and (c, d) fluorescent FA-FE based
on DPExPOTPAn.
LUMO of FIrpic are shallower and deeper than those of
DPExPOTPAn, the hole and electron traps can be formed on
FIrpic with the different depths. The similar energy transfer
process from hosts to FIrpic was further evidenced by EL
emissions definitely from FIrpic without any host emissions
observed (Figure 4b). Therefore, it can be expected to figure
out CEC effect in the low-triplet-energy-host-involved electro-
phosphorescence process according to EL performance of PA-
PE.
Table 2. EL Performance of DPExPOTPAn
maximum
operating
device voltage (V)
brightness (cd
C.E. (cd P.E. (lm E.Q.E.
a
b
c
c
c
m−2
)
A−1
)
W1−
)
(%)
PA
PB
PC
PD
PE
FA
FB
FC
FD
FE
2.7, <3.5,
<6.1
5979 (12.1, 489) 34.6,
40.3,
20.0
14.8,
9.5
22.3
2.7, <3.9,
<7.7
4090 (12.5, 562) 14.6, 8.6 16.9, 6.9 5.8, 3.4
2.7, <4.1,
<7.1
4242 (10.9, 604) 7.3, 5.7
5500 (12.3, 633) 7.4, 1.9
1877 (11.9, 627) 5.1, 1.1
8.2, 4.4
5.9, 0.7
5.5, 0.5
3.1, 2.4
3.2, 0.8
2.4, 0.5
Operating voltages of these devices were rather low that the
onset voltages for all of the PHOLEDs were lower than 3 V
(Figure 5a). PA showed the lowest driving voltages as 2.7 V for
onset, <3.5 V at 100 cd m−2 and <6.1 V at 1000 cd m−2,
respectively. The latter two were about 0.4 and 1.6 V lower
than those of PB. PC and PD had the equal driving voltages for
onset, 100 and 1000 cd m−2, which were 0.2, 2.0, and 3.6 V
lower than those of PE. Taking account of the difference less
than 1.0 eV between the work functions of electrodes, it is
believed that the reduction of S1 is beneficial to the balanced
carrier injection. As mentioned above, through the conjugation
extension by tuning the functional group number and ratio, S1
of DPExPOTPAn was gradually reduced without changing their
T1, in which S1 of DPESPOTPA is the highest and that of
DPEPOTPA2 is the lowest. However, the tendency of the
device operating voltages was just inverted that the driving
voltage seemed to be inversely proportional to S1 of the hosts
(Table 2), which was also inconsistent with the results of CV
analysis and IV characteristics of the single-carrier transporting
devices. Actually, since the driving voltage is defined regarding
to the specific brightness, luminescent performance of the
device should be one of the determinants for operating voltage,
which reflects the utilization of injected carriers and the
confinement efficiency of exciton on phosphors. This was
further testified by much increased differences between the
2.7, <4.1,
<7.1
2.9, <6.1,
<10.7
3.1, <6.9,
<10.7
1976 (12.5, 626) 0.75,
0.56
0.76,
0.26
1.10,
0.95
3.1, <6.1,
<9.9
2444 (11.9, 554) 1.00,
0.79
1.01,
0.41
1.70,
1.56
3.1, <6.3,
<10.5
1253 (11.3, 542) 1.92,
1.51
0.73,
0.25
1.45,
1.06
3.1, <5.3,
<8.7
2283 (11.7, 466) 1.92,
1.97
1.79,
0.99
1.97,
1.73
3.5, <6.1,
<9.7
2389 (11.9, 621) 1.03,
0.97
0.75,
0.50
1.48,
1.43
a
b
In the order of onset, 100 and 1000 cd m−2. The corresponding
c
voltage (V) and J (mA cm−2) were listed in the parentheses. In the
order of maximum and 100 cd m−2.
driving voltages of PA-PE at higher brightness, which should be
ascribed to the more stringent demand for efficient utilization
of carriers and exciton at higher concentrations.
EL efficiencies of PA-PE showed more direct information
about the relationship between the host properties and the
utilization efficiency of carriers and exciton (Figure 5a and
Table 2). It is showed that DPESPOTPA endowed PA with
the highest maximum efficiencies among these PHOLEDs as
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34.6 cd A−1 for current efficiency (C.E.), 40.3 lm W−1 for P.E.
and 14.8% for E.Q.E., which were favorable among the most
efficient FIrpic-based PHOLEDs and comparable to the devices
involving high-triplet-energy host materials. At the practical
luminance of 100 cd m−2, its efficiencies remained as 22.3 cd
A−1, 20.0 lm W−1, and 9.5%, indicating the suppressed
quenching processes at the high carrier and exciton
concentrations. However, the maximum efficiencies of PB
were sharply reduced to be less than an half of those of PA,
which were further decreased at 100 cd m−2 with a bigger roll-
off more than 40% for E.Q.E. The involvement of
DPESPOTPA3 and DPEPOTPA in PC and PD resulted in
their efficiency losses for about 50% compared with PB. The
difference between PC and PD was that the efficiencies of the
former was much more stable than those of the latter, which
should be ascribed to the more stable FIrpic-localized excitons
in DPESPOTPA3 matrix. Finally, PE, using DPEPOTPA2 as
host, revealed the lowest efficiencies and the most serious
efficiency roll-offs. Because T1 values of DPExPOTPAn are
equivalent to afford the similar host-dopant energy transfer
process, the striking difference between EL performances of
these DPPO-TPA hybrids should be mainly ascribed to their
various electrical properties, especially the capture ability of
carriers and exciton.
conjugated fluophors. γ is equal to 1 when the injected carriers
were completely recombined. γ of FA-FE are calculated
according to their maximum E.Q.E. as 0.11, 0.17, 0.15, 0.49,
and 0.31, respectively. Therefore, the exciton recombination
ability of DPESPOTPA is the weakest among these materials,
whereas DPEPOTPAn are much stronger in this aspect
compared with their mono-DPPO analogues.
As a summary, the orders of the HOMO, LUMO energy
levels and exciton recombination abilities for these hosts were
DPESPOTPA = DPESPOTPA2 < DPEPOTPA < DPEPOT-
PA2 < DPESPOTPA3, DPEPOTPA2 < DPEPOTPA <
DPESPOTPA2 < DPESPOTPA3 < DPESPOTPA and
DPESPOTPA < DPESPOTPA3 < DPESPOTPA2 <
DPEPOTPA2 < DPEPOTPA, respectively. The relationship
between the carrier and exciton capture ability of the hosts and
their EL performance was illustrated in Figure 6 by establishing
It is believed that the direct exciton recombination on dopant
is more efficient than the typical two-step EL process involving
exciton recombination on host and sequential host-dopant
energy transfer, which is more reasonable for low-triplet-energy
hosts because of inefficient host-to-dopant energy transfer. In
this sense, the exciton capture by low-triplet-energy hosts is
inferior in achieving high EL efficiencies. Taking account of the
direct proportionality between exciton capture ability and
exciton recombination efficiency, we fabricated OLEDs using
DPExPOTPAn as EMLs (20 nm) with the same configurations
as their PHOLEDs, corresponding to FA-FE, to evaluate
exciton recombination efficiencies of these host materials. It is
showed that all of the fluorescent OLEDs gave out deep-blue
lights similar to the solid-state PL emissions of the
corresponding DPPO-TPA hybrids (Figure 4c). Since hole is
the majority carrier in these devices, DPEPOTPA2 with the
weakest hole-transporting ability still resulted in the highest
onset voltage of FE (Figure 5c and Table 2). However, at 100
and 1000 cd m−2, its driving voltages were 0.8 and 1.0 V lower
than those of FA based on DPESPOTPA. The driving voltages
of FB and FC were close to those of FE, while DPEPOTPA
supported FD with the lowest operating voltages, which were
0.8 and 1.0 V lower than those of FE. More importantly, FD
achieved the highest maximum efficiencies among these devices
as 1.92 cd A−1, 1.79 lm W−1, and 1.97%, accompanied with the
well-controlled roll-off as 12% for E.Q.E. at 100 cd m−2, which
were comparable with the reported best results about the
nondoped deep-blue fluorescent OLEDs (Figure 5d and Table
2). Contrarily, the maximum efficiencies of FA became the
lowest as only 0.75 cd A−1, 0.76 lm W−1 and 1.10%, which
implied the remarkably weaker exciton recombination ability of
DPESPOTPA. On the basis of E.Q.E., the exciton recombi-
nation efficiency (γ) can be derived as
Figure 6. Correlation between the charge trap depths in PA-PE and
the exciton recombination efficiencies of DPExPOTPAn and the
maximum E.Q.E. of the corresponding PHOLEDs.
the correspondence between E.Q.E. and the carrier trap depths
on FIrpic and γ of the hosts. It was convincible that the highest
efficiencies of PA were the combined results of the deepest hole
and electron traps on FIrpic and the weakest exciton
recombination ability of DPESPOTPA, which gave rise to
the thoroughly direct carrier and exciton capture by FIrpic so as
to avoid the efficiency loss through irreversible energy transfer
to nonradiative T1 of DPESPOTPA. For PB, where the hole
trap depth on FIrpic was equivalent to that for PA, the higher γ
of DPESPOTPA2 and the remarkable reduced electron trap
depth made the efficiencies declined sharply, in which the
shallow electron trap (<0.2 eV) on FIrpic in PB should be the
main reason because of the unstable electron capture. The
depth of the hole trap formed between FIrpic and
DPESPOTPA3 was the smallest as 0.15 eV, accompanied
with the electron trap of just 0.24 eV. Therefore, although γ of
DPESPOTPA3 was only 0.15, the inefficient hole and electron
capture by FIrpic still leaded to the further efficiency reduction
of PC. The exciton recombination abilities of DPEPOTPAn
were significantly enhanced, especially for DPEPOTPA with γ
about 0.5; meanwhile the electron traps between them and
FIrpic are only 0.17 and 0.1 eV, even if the hole traps are deep
enough to confine the hole on FIrpic. Therefore, PD and PE
revealed the much lower efficiencies compared with other
ΦEL
ΦPLχηOC
γ =
(1)
Where ΦEL is E.Q.E., ΦPL is PLQY, ηoc is outcoupling factor as
0.25 for organic solid, and χ is the fraction of excitons as 0.4 for
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Notes
PHOLEDs. It is noticeable that the carrier capture seems more
dominant than exciton capture because the latter actually is
dependent on the former. Furthermore, because electron is
minority carrier, the electron capture ability becomes more
important to determine the final exciton location, which is
consistent with the higher efficiencies of PD based on
DPEPOTPA with the highest γ than those of PE based on
DPEPOTPA2 with the shallowest electron trap.
Therefore, the effective confinement of carrier and exciton
on phosphorescent dopants is the most important determinant
for low-triplet-energy host involved PHOLEDs. Because the
low-energy-gap hosts are theoretically advantageous in reducing
operating voltages, the achievement of high efficiencies makes
this kind of host materials promising in portable applications.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This project was financially supported by NSFC (50903028,
61176020 and 51373050), New Century Excellent Talents
Supporting Program of MOE (NCET-12-0706), Program for
Innovative Research Team in University (MOE) (IRT-1237),
New Key Project of MOE (212039), New Century Excellent
Talents Developing Program of Heilongjiang Province (1252-
NCET-005), Education Bureau of Heilongjiang Province
(10td03), and the Supporting Program of High-Level Talents
of HLJU (2010hdtd08).
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4. CONCLUSIONS
■
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ASSOCIATED CONTENT
* Supporting Information
Morphological and thermal properties of the films, current
density−efficiency curves of the devices and device repeatability
data.This material is available free of charge via the Internet at
http://pubs.acs.org.
■
S
(22) Wang, L.; Jiang, Y.; Luo, J.; Zhou, Y.; Zhou, J.; Wang, J.; Pei, J.;
Cao, Y. Adv. Mater. 2009, 21, 4854.
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AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: hxu@hlju.edu.cn.
*E-mail: mdg1014@ciac.jl.cn.
Author Contributions
†Authors C.H. and F.Z. contributed equally.
Adv. Mater. 2011, 23, 926.
J
dx.doi.org/10.1021/cm403160p | Chem. Mater. XXXX, XXX, XXX−XXX

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