Dibenzothiophene-Based Phosphine Oxide Host and Electron-Transporting Materials for Efficient Blue Thermally Activated Delayed Fluorescence Diodes through Compatibility Optimization

Article abstract of DOI:10.1021/acs.chemmater.5b02012

Thermally activated delayed fluorescence (TADF) organic light-emitting diodes arise from the development of high-performance host materials and carrier transporting materials fitting for TADF dyes with optimized respective properties and interplays, making simultaneous performance improvement and device structure simplification feasible. In this work, a highly efficient blue TADF diode with simplified four-layer structure was successfully achieved by utilizing bis[4-(9,9-dimethyl-9,10-dihydroacridine)phenyl]sulfone (DMAC-DPS) as blue emitter, 4,6-bis(diphenylphosphoryl)dibenzothiophene (DBTDPO) as host, and 4,6-bis(diphenylphosphoryl)dibenzothiophene sulfone (46DBSODPO) as electron-transporting layer. The compatibilities between DBTDPO and DMAC-DPS and DBTDPO and 46DBSODPO were optimized with respect to configuration, polarity, energy level, and interfacial interaction, resulting in the unchanged roughness of ～0.25 nm before and after doping, high photoluminescence quantum yield over 85%, and reduced interfacial exciplex emissions. With the similar triplet excited energy (T₁) of ～3.0 eV but inferior electrical properties compared to its analogues 28DBSODPO and 37DBSODPO, besides the homogeneity with DBTDPO, 46DBSODPO suppressed the formation of interfacial exciplex and dipole for efficient exciton confinement and electron injection and transportation, in virtue of the steric effects of its ortho-substituted phosphine oxide groups. Consequently, DBTDPO and 46DBSODPO endowed their DMAC-DPS based four-layer devices with the state-of-the-art performance, for example, the maximum external quantum efficiency over 16%, which was more than two-fold of those of conventional electron-transporting material 1,3,5-tri[(3-pyridyl)-phen-3-yl]benzene (TmPyPB). This design strategy about material compatibility could pave a way for developing high-performance blue TADF diodes with simplified configurations.

Full text of DOI:10.1021/acs.chemmater.5b02012

Article
pubs.acs.org/cm
Dibenzothiophene-Based Phosphine Oxide Host and Electron-
Transporting Materials for Efficient Blue Thermally Activated
Delayed Fluorescence Diodes through Compatibility Optimization
Chaochao Fan,†,§ Chunbo Duan,†,§ Ying Wei,†,‡ Dongxue Ding,† Hui Xu,*,†,‡ and Wei Huang*,‡
†Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education, Heilongjiang University, 74 Xuefu Road, Harbin
150080, P. R. China
‡Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials (IAM), Jiangsu National Synergistic Innovation
Center for Advanced Materials (SICAM), Nanjing Tech University (NanjingTech), 30 South Puzhu Road, Nanjing 211816, P.R.
China
S
* Supporting Information
ABSTRACT: Thermally activated delayed fluorescence (TADF) organic light-
emitting diodes arise from the development of high-performance host materials
and carrier transporting materials fitting for TADF dyes with optimized
respective properties and interplays, making simultaneous performance
improvement and device structure simplification feasible. In this work, a
highly efficient blue TADF diode with simplified four-layer structure was
successfully achieved by utilizing bis[4-(9,9-dimethyl-9,10-dihydroacridine)-
phenyl]sulfone (DMAC-DPS) as blue emitter, 4,6-bis(diphenylphosphoryl)-
dibenzothiophene (DBTDPO) as host, and 4,6-bis(diphenylphosphoryl)-
dibenzothiophene sulfone (46DBSODPO) as electron-transporting layer. The
compatibilities between DBTDPO and DMAC-DPS and DBTDPO and
46DBSODPO were optimized with respect to configuration, polarity, energy
level, and interfacial interaction, resulting in the unchanged roughness of ∼0.25
nm before and after doping, high photoluminescence quantum yield over 85%, and reduced interfacial exciplex emissions. With
the similar triplet excited energy (T1) of ∼3.0 eV but inferior electrical properties compared to its analogues 28DBSODPO and
37DBSODPO, besides the homogeneity with DBTDPO, 46DBSODPO suppressed the formation of interfacial exciplex and
dipole for efficient exciton confinement and electron injection and transportation, in virtue of the steric effects of its ortho-
substituted phosphine oxide groups. Consequently, DBTDPO and 46DBSODPO endowed their DMAC-DPS based four-layer
devices with the state-of-the-art performance, for example, the maximum external quantum efficiency over 16%, which was more
than two-fold of those of conventional electron-transporting material 1,3,5-tri[(3-pyridyl)-phen-3-yl]benzene (TmPyPB). This
design strategy about material compatibility could pave a way for developing high-performance blue TADF diodes with simplified
configurations.
(TTA),24 crucial to realize high EL efficiencies. With this
fundamental concern, EML compositions and interfacial effects
1. INTRODUCTION
After the success of phosphorescence organic light-emitting
of TADF devices should be rationally optimized through
diodes (PHOLEDs) in harvesting both singlet and triplet
excitons for 100% internal quantum efficiency,1−3 recently,
organic light-emitting diodes (OLEDs) employing pure-organic
thermally activated delayed fluorescence (TADF) dyes achieve
the same effect, in virtue of reverse intersystem crossing (RISC)
enhancing host-dopant and EML-carrier transporting layer
(CTL) compatibility, making the development of high-
performance host materials and carrier transporting materials
(CTMs) fitting for TADF dyes imperative.25−29
induced triplet−singlet transformation.4,5 Since efficient RISC
A substantial challenge is addressed that the first triplet
energy levels (T1) of blue TADF dyes,20,30−38 for example,
requires small triplet−singlet energy splitting (ΔEST),6,7
bis[4-(9,9-dimethyl-9,10-dihydroacridine)phenyl]sulfone
donor−acceptor (D−A) structure is mostly adopted to
(DMAC-DPS, T1 = 2.9 eV),35 are remarkably higher than those
construct TADF dyes on account of the relatively low energy
for the singlet charge transfer state (1CT*).8−17 In this case, the
of their phosphorescent counterparts, such as bis(4,6-
(difluorophenyl) pyridinato-N,C2)picolinate iridium(III)
large polarity of CT states18,19 and the involvement of triplet
exciton in electroluminescent (EL) process make suppression
of emissive layer (EML)-inside20,21 and interfacial quenching
effects,22 for example, concentration quenching (CQ),23 triplet-
polaron quenching (TPQ), and triplet−triplet annihilation
Received: May 29, 2015
Revised: July 1, 2015
© XXXX American Chemical Society
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Figure 1. Molecular structures and packing diagrams of single crystals of DBTDPO and mDBSODPO and their dipole moments in comparison with
DMAC-DPS.
(FIrpic, T1 = 2.7 eV)39 even though their EL spectra are
similar. Therefore, to confine exciton on DMAC-DPS, the
requirement of optical properties for host materials and EML-
adjacent CTMs is so severe that their T1 values should be
higher than or at least equivalent to 2.9 eV at the expense of
electrical performance,34,35 consequently forcing the employ-
ment of exciton-blocking and CTLs to accordingly suppress
exciton diffusion and balance charge flux. As the result, the
configuration of blue TADF devices was much more
complicated than those of their green, yellow, and red
analogues,35 making it become one of bottlenecks of practical
applications for TADF technology. Obviously, structural
simplification of blue TADF devices can be established through
taking both the own performance of each material and,
especially, the intralayer and interlayer compatibilities into
account.
Herein, we report a simplified highly efficient four-layer blue
TADF diode utilizing 4,6-bis(diphenylphosphoryl) dibenzo-
thiophene (DBTDPO) as host, its derivative 4,6-bis-
(diphenylphosphoryl) dibenzothiophene sulfone (46DBSOD-
PO) as electron-transporting layer (ETL), and DMAC-DPS as
dopant. Low driving voltages of <3 V for onset and <5.5 V at
150 cd m−2 and the state-of-the-art EQE of 16.1% for maximum
and 14.7% and 12.4% at 150 and 1000 cd m−2 were realized.
The consistent V-shape configuration and polarity of
DBTDPO and DMAC-DPS and their well-matched optical
properties rendered the unchanged morphology and stabilized
T1 states of DMAC-DPS-doped thin films with root−mean−
square (RMS) roughness of ∼0.25 nm, long delay-fluorescence
lifetime of 11.5 μs, and high photoluminescence quantum yield
(PLQY) more than 85%. In contrast to its constitutional
isomers 28DBSODPO and 37DBSODPO (Figure 1 and
Scheme 1), 46DBSODPO is comparable in optical properties,
for example, high T1 of 2.97 eV for exciton diffusion
suppression, but inferior in electron injection and trans-
portation. Nevertheless, owing to the optimized compatibilities
between DBTDPO and 46DBSODPO in polarity, config-
uration, energy level, and interfacial interaction, the device
efficiencies of 46DBSODPO were more than 1.5-fold those of
28DBSODPO and 37DBSODPO and 2-fold those of a
conventional high-T1 electron-transporting material (ETM)
1,3,5-tri[(3-pyridyl)-phen-3-yl]benzene (TmPyPB). This con-
tribution rationally demonstrated the significance of material
compatibility for blue TADF diodes, especially when
simplifying configurations without loss of performance.
2. EXPERIMENTAL SECTION
Materials and Instruments. All the reagents and solvents used
for the synthesis were purchased from Aldrich and Acros companies
and used without further purification. DBTDPO was prepared
according to our previous report.40
1H NMR spectra were recorded using a Varian Mercury plus 400NB
spectrometer relative to tetramethylsilane (TMS) as internal standard.
Molecular masses were determined by FINNIGAN LCQ electrospray
ionization−mass spectrometry (ESI−MS) or matrix-assisted laser
desorption ionization time-of-flight MS (MALDI-TOF-MS). Elemen-
tal 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
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Scheme 1. Synthetic Procedure of mDBSODPO
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 dichloro-
methane. 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 time-correlated
single photon counting (TCSPC) method with a microsecond pulsed
Xenon light source for 10 μs−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.
was removed in vacuo. The residue was purified by flash column
chromatography to afford 0.35 g of white powder with a yield of 50%.
1H NMR (TMS, CDCl3, 400 M Hz): δ = 8.476 (d, J = 12.4 Hz, 2H),
7.969 (d, J = 6.8 Hz, 2H), 7.781 (t, J = 9.6 Hz, 2H), 7.714 (q, J1 = 7.6,
J2 = 12.0 Hz, 8H), 7.581 (t, J = 7.2 Hz, 4H), 7.490 ppm (t, J = 6.2 Hz,
8H). LDI-TOF: m/z (%): 584 (100) [M+]; elemental analysis (%) for
C36H26O2P2S: C 73.96, H 4.48, S 5.48; found: C 73.94, H 4.50, S 5.53.
General Procedure of Oxidation for Sulphone. A sample of
0.584 g of DBT phosphine oxide derivative (1 mmol) was dissolved
into 10 mL of dichloromethane at room temperature, and then 0.86 g
of 3-chlorobenzoperoxoic acid (5 mmol) was added to the mixture and
stirred for 6 h. The reaction was quenched by addition of water (5
mL) and extracted by dichloromethane (3 × 2 mL). The organic layer
was dried with anhydride Na2SO4. The solvent was removed in vacuo.
The residue was purified by flash column chromatography.
2,8-Bis(diphenylphosphoryl)dibenzothiophene Sulfone
(28DBSODPO). A sample of 0.35 g of white powder with a yield of
57%. 1H NMR (TMS, CDCl3, 400 M Hz): δ = 8.262 (d, J = 11.6 Hz,
2H), 7.912 (d, J = 8.0 Hz, 2H), 7.795 (t, J = 9.4 Hz, 2H), 7.612 (m,
12H), 7.517 ppm (t, J = 7.4 Hz, 8H). LDI-TOF: m/z (%): 616 (100)
[M+]; elemental analysis (%) for C36H26O4P2S: C 70.12, H 4.25, S
5.20; C 70.15, H 4.27, S 5.25.
Synthesis of 2,8-Dibromodibenzothiophene (28DBTBr2). In
Ar2, 1.84 g of dibenzothiophene (DBT, 10 mmol) was dissolved in 20
mL of chloroform, and then 1.54 mL of Br2 (30 mmol) was added.
The mixture was stirred for 24 h at room temperature. The reaction
was quenched by addition of aq. NaHSO3 (4 mL) and extracted by
dichloromethane (3 × 2 mL). The organic layer was combined and
dried with anhydride Na2SO4. The solvent was removed in vacuo. The
residue was purified by flash column chromatography to afford 1.4 g of
1
white powder with a yield of 40%. H NMR (TMS, CDCl3, 400 M
Hz): δ = 8.233 (s, 2H), 7.699 (d, J = 8.4 Hz, 2H), 7.591 ppm (d, J =
8.4 Hz, 2H). LDI-TOF: m/z (%): 342 (100) [M+]; elemental analysis
(%) for C12H6Br2S: C 42.14, H 1.77, S 9.37; found: C 42.15, H 1.79, S
9.42.
2,8-Bis(diphenylphosphoryl)dibenzothiophene
(28DBTDPO). In Ar2, 0.34 g of 28DBTBr2 (1 mmol) and 1.3 mL of n-
BuLi (2.5M, 3.25 mmol) in 15 mL of THF were mixed and stirred at
−78 °C for 2 h. Then, 0.9 mL of Ph2PCl (4.77 mmol) was added to
the system at −78 °C and stirred for 8 h. The reaction was quenched
by addition of water (4 mL) and extracted by dichloromethane (3 × 2
mL). The organic layer was dried with anhydride Na2SO4. The solvent
4,6-Bis(diphenylphosphoryl)dibenzothiophene Sulfone
(46DBSODPO). A sample of 0.25 g of white powder with a yield of
1
40%. H NMR (TMS, CDCl3, 400 M Hz): δ = 8.015 (d, J = 7.6 Hz,
2H), 7.697 (m, 10H), 7.614 (t, J = 7.0 Hz, 2H), 7.493 (t, J = 7.0 Hz,
4H), 7.396 ppm (t, J = 7.4 Hz, 8H). LDI-TOF: m/z (%): 616 (100)
[M+]; elemental analysis (%) for C36H26O4P2S: C 70.12, H 4.25, S
5.20; C 70.13, H 4.25, S 5.23.
3,7-Dibromodibenzothiophene Sulfone (37DBSOBr2). At 0
°C, 0.216 g of dibenzothiophene S,S-dioxide (DBSO, 1 mmol) was
C
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dissolved in 7.0 mL of H2SO4, and then 0.36 g (2 mmol) of NBS was
added to the mixture and stirred for 10 h. The reaction was quenched
by addition of water (4 mL) and extracted by dichloromethane (3 × 2
mL). The organic layer was dried with anhydride Na2SO4. The solvent
was removed in vacuo. The residue was purified by flash column
chromatography to afford 0.2 g of white powder with a yield of 45%.
1H NMR (TMS, CDCl3, 400 M Hz): δ = 7.943 (s, 2H), 7.786 (d, J =
DMAC-DPS in DBTDPO uniform during evaporation and
deposition (Figure 1). Direct evidence was established by atom
force microscopy (AFM) images of vacuum-evaporated thin
films of DBTDPO and DBTDPO:DMAC-DPS (10 wt %),
which revealed the identical surface morphologies and RMS
roughness as small as ∼0.25 nm (Figure 2). In contrast, the
8.4 Hz, 2H), 7.634 ppm (d, J = 8.0 Hz, 2H). LDI-TOF: m/z (%): 374
(100) [M+]; elemental analysis (%) for C12H6Br2O2S: C 38.53, H 1.62,
S 8.57; C 38.55, H 1.61, S 8.64.
3,7-Bis(diphenylphosphoryl)dibenzothiophene Sulfone
(37DBSODPO). The procedure was similar to that of 28DBTDPO
except for the use of 37DBSOBr2 (0.374 g, 1 mmol) instead of
1
28DBTBr2. Yield: 0.3 g of white powder (49%). H NMR (TMS,
CDCl3, 400 M Hz): δ = 8.183 (t, J = 9.4 Hz, 2H), 7.966 (t, J = 9.8 Hz,
4H), 7.672 (q, J1 = 7.0, J2 = 12.2 Hz, 8H), 7.612 (t, J = 7.4 Hz, 4H),
7.521 ppm (t, J = 7.4 Hz, 8H). LDI-TOF: m/z (%): 616 (100) [M+];
elemental analysis (%) for C36H26O4P2S: C 70.12, H 4.25, S 5.20; C
70.11, H 4.27, S 5.28.
DFT Calculations. DFT computations were carried out with
different parameters for structure optimizations and vibration analyses.
The ground states and triplet states of molecules in vacuum were
optimized according to single-crystal structures by the restricted and
unrestricted formalism of Beck’s three-parameter hybrid exchange
functional41 and Lee, and Yang and Parr correlation functional42
(B3LYP)/ 6-31G(d), respectively. The charged triplet states with spin
multiplicity of 4 were chosen to simplify the calculation on the basis of
optimized triplet state configurations. The fully optimized stationary
points were further characterized by harmonic vibrational frequency
analysis to ensure that real local minima had been found without
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.43
Device Fabrication and Testing. Before being loaded 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 oxygen plasma for 3 min. Devices were fabricated by evaporating
organic layers at a rate of 0.1−0.2 nm s−1 onto the ITO substrate
sequentially at a pressure below 4 × 10−4 Pa. Onto the electron-
transporting 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 0.09 cm2, as determined by the
overlap area of the anode and the cathode. After fabrication, the
devices were immediately transferred to a glovebox for encapsulation
with glass coverslips using epoxy glue. 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 4200 source meter and a calibrated
silicon photodiode. All the easurements were carried out at room
temperature under ambient conditions. For each structure, five devices
were fabricated to confirm the performance repeatability. To make
conclusions reliable, the data reported herein were most close to the
average results.
Figure 2. AFM images of 100 nm vacuum-evaporated thin films of
neat (a) DBTDPO and (b) DBTDPO:DMAC-DPS (10 wt %).
neat DMAC-DPS film showed the remarkably larger roughness
of ∼10 nm due to the strong intermolecular interaction induced
crystallization and aggregation (Figure S1, Supporting
Information).
Oxidation of sulfur atom in electron-donating dibenzothio-
phene (DBT) can generate strong electron-withdrawing
dibenzothiophene sulfone (DBSO) without changing molec-
ular configuration, which inspired us to utilize the S,S-dioxide of
DBTDPO, namely 46DBSODPO, as ETM, in virtue of their
compatibility for modifying EML|ETL interface. As expected,
the single-crystal structures of DBTDPO and 46DBSODPO
show their identical V-shaped molecular configurations and
monoclinic crystal systems, providing the similar packing
modes (Figure 1). Obviously, the high consistency of
3. RESULTS AND DISCUSSIONS
3.1. Design and Structural Compatibility. When T1
energy of host was equivalent or only slightly higher than
that of dopant, the uniform distribution of dopant in host
matrix at molecular level would shorten the average distance
and time for host−dopant triplet energy transfer, thereby
improving the utilization of triplet exciton, besides reducing
CQ and TTA to the greatest extent. In this regard, although T1
value of DBTDPO is only 2.91 eV,40 according to similarity−
intermiscibility theory, it was employed as host for DMAC-
DPS on account of their similar V-shaped configurations and
molecular polarities of ∼4.3 D, which can make dispersion of
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DBTDPO and 46DBSODPO in crystallography is beneficial to
improve their interface during device fabrication. In contrast,
37DBSODPO has the same crystal system but different
molecular configuration, while 28DBSODPO shows the similar
V-shape configuration but distinct orthorhombic crystal system.
Therefore, it is rational that the order of structural compatibility
between mDBSODPO (collective name of 28DBSODPO,
37DBSODPO, and 46DBSODPO) and DBTDPO is
46DBSODPO > 37DBSODPO > 28DBSODPO.
3.2. Optical Properties. The PL spectrum of the vacuum-
evaporated thin film for DBTDPO:DMAC-DPS (10 wt %)
consists of the pure emission from DMAC-DPS with peak at
482 nm and high PLQY of 87%, which is attributed to the large
overlap between PL emission of DBTDPO and absorption of
DMAC-DPS for efficient positive energy transfer (Figure 3a).
system. mDBSODPO reveals similar optical properties in dilute
solutions (10−6 mol L−1 in CH2Cl2, Figure 3b and Table 1).
DBSO-originated bands are preserved in the absorption spectra
of mDBSODPO with bathochromic shifts around 5−15 nm
accompanied by diphenylphospohine oxide (DPPO)-attributed
peaks at 228 nm. The first singlet excited energy levels (S1) of
mDBSODPO are identical as 3.41 eV, estimated by their
absorption edges at 363 nm. Their fluorescence spectra in
solution are also slightly red-shifted in comparison with that of
DBSO, while the phosphorescence spectra of mDBSODPO
and DBSO are identical in both profile and range, in accord
with the same locations of their T1 states on DBSO (Figure
S1). Their first phosphorescent peaks of 0−0 transitions at 418
nm correspond to higher T1 values of 2.97 eV. In this case,
triplet exciton diffusion from EML of DBTDPO:DMAC-DPS
to ETL of mDBSODPO can be blocked.
The intermolecular charge transfer (ICT) between
DBTDPO and strongly electron-withdrawing mDBSOPO at
the interface between EML and ETL should be considered,
which can give rise to the formation of low-energy exciplex,46
thereby worsening the interfacial quenching of triplet exciton.
The exciplex formation between DBTDPO and mDBSODPO
was experimentally manifested by the almost identical bath-
ochromic emissions in the range of 350−550 nm in PL spectra
of their mixtures in CH2Cl2 (Figure 4a). Nevertheless, since
ICT on the basis of donor−acceptor collision, is different with
the larger molecular degree of freedom in solution for more
sufficient intermolecular interactions, in solid states, the steric
effects of peripheral groups on intermolecular interactions
become predominant. With this consideration, monolayer and
bilayer thin films of mDBSODPO and mDBSODPO|
DBTDPO were prepared through vacuum deposition on
quartz substrates to simulate the interfacial interaction in the
devices. Compared to PL spectrum of the monolayer film for
28DBSODPO, a distinct long tail in the spectrum of its bilayer
film in the range of 425−550 nm is consistent with exciplex
emission of 28DBSODPO:DBTDPO in solution, verifying the
formation of interfacial exciplex (Figure 4b). Contrarily, the
tailing peaks in PL spectra of bilayer films for 37DBSODPO
and 46DBSODPO are either reduced or negligible. Since
electron is mainly captured by DBSO cores of mDBSOPO, it is
rational that the suppression of exciplex formation is directly
proportional to the encapsulation degree of their DBSO cores,
namely 46DBSODPO > 37DBSODPO > 28DBSODPO, just
contrary to the order of their electron mobility.
3.3. Electrical Properties. The electrochemical properties
of mDBSODPO were investigated with CV analysis to figure
out the influence of DPPO-substitution on frontier molecular
orbital (FMO) energy levels of DBSO (Figure 5a and Table 1).
The onset potentials of the irreversible oxidation peaks for
mDBSODPO were identical to that of DBSO as 1.78 V,
corresponding to their highest occupied molecular orbital
(HOMO) levels of −6.56 eV. The reduction curves of
28DBSODPO and 37DBSODPO consisted of one reversible
and one irreversible cathodic peak, while 46DBSODPO
revealed two irreversible reduction peaks, in contrast with
one irreversible peak of DBSO. Therefore, the lowest
unoccupied molecular orbital (LUMO) energy levels of
28DBSODPO, 37DBSODPO, and 46DBSODPO are reduced
to −3.46, −3.54, and −3.22 eV, respectively, indicating the
order of their electron injection ability as 37DBSODPO >
28DBSODPO > 46DBSODPO, which is consistent with the
involvement degrees of their DPPO-substituted C atoms in the
Figure 3. (a) UV absorption spectrum of DMAC-DPS, PL spectrum
of DBTDPO, and steady-state and time-resolved (inset) emission
spectra of DMAC-DPS in DBTDPO:DMAC-DPS (10 wt %) thin
film. (b) Electronic absorption and fluorescence (FL) spectra of
mDBSODPO in CH2Cl2 (10−6 mol L−1) and low-temperature
phosphorescence (PH) spectra (inset) by time-resolved technology
after a delay of 300 μs, in contrast to DBSO.
The transient PL spectrum of the emission at 482 nm shows
the remarkable delayed fluorescence component with a long
lifetime (τDF) of 11.5 μs, which is almost four-fold that of
DMAC-DPS in a conventional host 1,3-bis(9H-carbazol-9-
yl)benzene (mCP, τDF = 3.1 μs, T1 = 2.9 eV). Although the T1
energies of DBTDPO, mCP, and DMAC-DPS are almost
equivalent, the T1 state of DMAC-DPS can be effectively
stabilized when doped in DBTDPO. The contribution of
delayed fluorescence to PLQY is consequently improved to
66%, which is about 10% larger than that of mCP:DMAC-DPS
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Table 1. Physical Properties of mDBSODPO
c
g
h
emission
(nm)
Tg/Tm/Td
(°C)
HOMO
(eV)
LUMO
(eV)
RE
μe
compound
DBSO
absorption (nm)
S1 (eV) T1 (eV)
(eV)
(cm2/V/s)
a
c
e
f
f
324, 291, 278, 269, 242, 234,
358
3.60
3.00
−/−/207
−6.56
−3.08
0.4261
a
228
b
b
a
b
a
b
a
b
d
c
d
e
d
d
337, 283, 205
402
371
4.85
3.41
4.68
3.41
4.58
3.41
4.72
2.86
2.97
2.79
2.97
2.76
2.97
2.85
−6.67
−6.56
−6.84
−6.56
−6.80
−6.56
−6.52
−1.82
−3.46
−2.16
−3.54
−2.22
−3.22
a
f
f
28DBSODPO 333, 284, 249, 240, 227
−/311/456
−/−/481
0.5159
0.5603
0.5736
7.02 × 10−4
1.56 × 10−4
3.65 × 10−5
b
d
c
d
e
d
d
337, 295, 286, 276, 258
375
a
f
f
37DBSODPO 329, 304, 289, 253, 245, 227
362
b
d
c
d
e
d
d
331, 307, 294, 251, 249
385, 368
372
a
f
f
46DBSODPO 330, 295, 282, 248, 228
−/353/459
b
d
d
d
d
337, 297, 281, 253
372
−1.80
e
a
b
c
d
In CH2Cl2 (10−6 mol L−1). In film. Estimated according to the absorption edges. DFT calculated results. Calculated according to the 0−0
transitions of the phosphorescence spectra. Calculated according to the equation, HOMO/LUMO = 4.78 + onset voltage.44 Reorganization energy
f
g
h
of electron. Electron mobility estimated by I−V characteristics of electron-only devices according filed-dependent SCLC model.45
Figure 4. (a) PL spectra of mDBSODPO (10−3 M) before (hollow
Figure 5. (a) Cyclic voltammograms of mDBSODPO and DBSO
measured in CH2Cl2 at room temperature with tetrabutylammonium
symbols) and after (solid symbols) addition of DBTDPO (0.1 M); (b)
hexafluorophosphate (0.1 M) as electrolyte under the scanning rate of
PL spectra of vacuum-evaporated monolayer and bilayer thin films of
100 mV s−1; (b) volt−ampere characteristics of mDBSODPO-based
mDBSODPO (lines) and mDBSODPO|DBTDPO (symbols). The
thickness for each layer of double-layer films was 30 nm, making the
electron-only devices in comparison with those of TmPyPB.
whole film thickness equivalent to those of single-layer neat films. ICT
refers to intermolecular charge transfer.
single-layer electron-only devices (Figure 5b and Table 1). The
electron mobilities of 28DBSODPO, 37DBSODPO, and
LUMOs (Figure S2). The electrochemical stability of
mDBSODPO was further manifested by their stable peaks
during 10 reduction cycles (Figure S3), which reflected their
device stability. The electron transporting abilities of
mDBSODPO were evaluated by I−V characteristics of their
46DBSODPO were estimated as 7.02 × 10−4, 1.56 × 10−4,
and 3.65 × 10−5 cm2 V−1 s−1 according to field-dependent
space charge limited current (SCLC) model,45 which was lower
than that of TmPyPB as 2.93 × 10−3 cm2 V−1 s−1. DBSO cores
form the main channel for intermolecular electron hopping
F
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Article
since they make the major contributions to the unoccupied
molecular orbitals of mDBSODPO (Figure S2). In this case, it
is rational that the electron mobility of mDBSODPO is in
direct proportion to the exposure degree of their DBSO cores,
namely 28DBSODPO > 37DBSODPO > 46DBSODPO.
3.4. EL Performance of Blue TADF Diodes. To figure out
the significance of the intralayer and interlayer compatibilities
on device performance, DMAC-DPS-based blue TADF diodes
using DBTDPO as host and mDBSOPO as ETL, respectively,
were fabricated with a four-layer configuration of ITO|MoO3 (6
nm)|TAPC (70 nm)|DBTDPO:DMAC-DPS (10%, 20 nm)|
DBTDPO (10 nm)|mDBSOPO (30 nm)|LiF (1 nm)|Al, in
which TAPC is 1,1-bis(di-4-tolylaminophenyl) cyclohexane as
hole transporting layer (HTL) (Figure 6). Adoption of TAPC
Figure 6. Device configuration of four-layer blue TADF devices and
energy level diagram of FMOs and excited states of the employed
materials.
Figure 7. (a) Luminance−current density (J)-voltage characteristics
and (b) efficiencies−luminance curves of mDBSODPO-based blue
TADF devices in comparison to TmPyPB-based controls.
is because of its high hole mobility, T1 value of 2.97 eV, and
especially its V-shape configuration to improve interlayer
compatibility with EML. If the interfacial effects are not
considered, the singlet and triplet excitons can be confined and
utilized in EML, in virtue of the higher S1 and T1 values of
TAPC and mDBSOPO than those of DMAC-DPS (Figure 6).
Although T1 values of DBTDPO and DMAC-DPS are almost
equivalent, the deeper LUMO and shallower HOMO of
DMAC-DPS formed electron and hole traps with the depths of
0.42 and 0.13 eV for direct charge and exciton capture.
Furthermore, the EL spectra of these devices consisted of pure
blue-greenish emissions from DMAC-DPS with peak wave-
length of 476 nm and Commission Internationale de l’Eclairage
(CIE) coordinates of (0.16, 0.26), indicating the effective
exciton confinement on the dopant (inset of Figure 7a).
Because of the low efficiency of DBTDPO:mDBSODPO
exciplex emissions, which were overlapped with that of DMAC-
DPS, no distinct exciplex emissions can be observed in EL
spectra. However, it can be noticed that the profile of EL
spectrum for 28DBSODPO-based devices was the broadest,
which might be attributed to the strongest exciplex emission
from DBTDPO|28DBSODPO interface in accord with PL
results.
according to CV analysis and I−V characteristics of electron-
only devices, the electron injecting and transporting ability of
46DBSODPO is the worst among mDBSODPO. Furthermore,
it is noteworthy that the current density (J) of 46DBSODPO-
based devices was bigger than those of 28DBSODPO and
37DBSODPO-based analogues, reflecting the superior electron
injection and transportation in the former. Since 46DBSODPO
is not dominant in intrinsic electrical performance, the reversed
I−V characteristics of these blue TADF diodes should be
mainly attributed to interfacial effect and EML−ETL
compatibility. The ICT-induced exciplexes of DBTDPO and
28DBSODPO gave rise to the interfacial dipole47 with positive
and negative charges localized on them, respectively, actually
forming a built-in field inverted to applied field, which blocked
the subsequent electron injection and transportation from
cathode (Figure 4b). The situation of 37DBSODPO-based
devices was similar; merely the formation of interfacial exciplex
was reduced. Therefore, the superiority of 46DBSODPO in
interfacial exciplex suppression and compatibility with
DBTDPO should be the main reason for the highest luminance
and lowest driving voltage of its devices, which was further
verified by the approximate brightness and operation voltages
of TmPyPB-based devices, in spite of its electron mobility two
orders of magnitude larger than that of 46DBSODPO.
As expected, 46DBSODPO endowed its devices with the
lowest turn-on voltage less than 3 V at 1 cd m−2, which was
even 0.7 V lower than that of DMAC-DPS-based six-layer
devices (Figure 7a and Table 2). At 150 and 1000 cd m−2 for
practical applications, the driving voltages were less than 5.5
and 8 V, respectively, which were 1−3 V lower than those of
28DBSODPO and 37DBSODPO-based devices; even through
46DBSODPO-based devices showed the state-of-the-art
efficiencies with the maxima of 31.5 cd A−1 for current
efficiency (CE), 32.9 lm W1− for power efficiency (PE), and
16.1% for EQE, which were among the best results for blue
TADF devices to date (Figure 7b and Table 2). At 150 cd m−2
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Table 2. EL Performance of the Representative Blue TADF Devices
c
roll-offs (%)
a
max. luminance
operating voltage
(V)
b
device structure
(cd m−2
)
max. efficiencies
25.5, −, 14.3
CE
PE
EQE
ITO|α-NPD|mCP|DPEPO: CC2BP|DPEPO|TPBi|LiF|Al31
ITO|α-NPD|mCP|mCP: CzTPN|PPT|LiF|Al32
3900
4.4
16500
4.8, −, −
27.1, 16.7, 11.9
ITO|α-NPD|mCP|mCP: 2CzPN|PPT|TPBi|LiF|Al20
−, −, 13.6
−, −, 16.4
−, −, 9.9
−, −, 19.5
ITO|PEDOT:PSS|mCP|mCP: DCzIPN|TSPO1|LiF|Al33
<5000
3.5, −, −
−, ∼54
−, 18
ITO|α-NPD|TCTA|CzSi|DPEPO: DtBCz-DPS|DPEPO|TPBi|LiF|Al34
ITO|α-NPD|TCTA|CzSi|DPEPO: DMAC-DPS|DPEPO|TPBi|LiF|Al35
ITO|MoO3|mCP|DMAC-DPS|DPEPO|LiF|Al36
>10000
5970
7754
7872
9144
6529
3.7, −, −
4.3, −, −
5.0, <7.5, <11.0
4.5, <7.5, <11.0
3, <5.5, <8.0
−, −, 19.5
d
d
d
ITO|MoO3|TAPC|DBTDPO:DMAC-DPS|DBTDPO|28DBSOPO|LiF|Al
ITO|MoO3|TAPC|DBTDPO:DMAC-DPS|DBTDPO|37DBSOPO|LiF|Al
ITO|MoO3|TAPC|DBTDPO:DMAC-DPS|DBTDPO|46DBSOPO|LiF|Al
12.8, 8.0, 6.5
19.8, 13.8, 10.1
31.5, 32.9, 16.1
15, 32
18, 37
9, 23
46, 69
51, 74
50, 71
44, 75
14, 32
18, 37
9, 23
d
ITO|MoO3|TAPC|DBTDPO:DMAC-DPS|DBTDPO|TmPyPB|LiF|Al
3.5, <5.5, <8.5
15.5, 13.9, 7.9
12, 39
11, 39
a
b
c
d
In the order of onset, 100, and 1000 cd m−2. In the order of CE (cd A−1), PE (lm W1−), and EQE (%). In the order of 100 and 1000 cd m−2. In
this work.
for display and 1000 cd m−2 for lighting, their EQE values
remained as 14.7 and 12.4%, corresponding to reduced
efficiency roll-offs of 9 and 23%, respectively. In contrast, the
maximum efficiencies of 37DBSODPO-based devices were
remarkably decreased to 19.8 cd A−1, 13.8 lm W1−, and 10.1%,
accompanied by worse EQE roll-offs of 18 and 37% at 150 and
1000 cd m−2, respectively. Both 46DBSODPO and
37DBSODPO endowed their devices with the efficiencies
higher than those of TmPyPB-based controls, which should be
ascribed to the stronger compatibility of these DBSO
phosphine oxide derivatives with DBTDPO. Furthermore,
compared to DPEPO-based counterparts with the simplified
four-layer structure (Figure S4), DBTDPO and 46DBSODPO
supported their devices with remarkably higher luminance and
efficiencies, manifesting the significance of intralayer and
interlayer compatibility optimization when device simplification
for blue TADF diodes. 28DBSODPO-based devices achieved
the maximum efficiencies of only 12.8 cd A−1, 8.0 lm W1−, and
6.5%, which were less than a half of those of 46DBSODPO-
based devices. Consequently, the order of device performances
for mDBSODPO was 46DBSODPO > 37DBSODPO >
28DBSODPO, identical to the order of their compatibility with
DBTDPO and suppression effect of interfacial exciplex, and
contrary to the order of their electrical properties. Actually, in
multilayer devices, carrier injection and transportation are not
only dependent on intrinsic electrical performances of each
involved materials, but also dramatically determined by
interfacial effects. Therefore, the optimization of EML/CTL
compatibilities is crucial to achieve high EL performance
through simple device structures.
positions of DPPO groups in mDBSODPO. Although it had
inferior in intrinsic electron injecting and transporting ability,
46DBSODPO endowed its DMAC-DPS-based blue TADF
devices with the state-of-the-art EQE of 16.1% as maximum and
reduced roll-offs of 9 and 23% at 150 and 1000 cd m−2,
respectively, owing to its highest compatibility with DBTDPO
host and the strongest suppression effect on interfacial exciplex.
This work shows that besides the intrinsic optoelectronic
properties, material compatibility and interfacial optimization
should be also concerned for achieving high-performance blue
TADF diodes.
ASSOCIATED CONTENT
■
S
* Supporting Information
AFM image of neat DMAC-DPS film; energy levels and
contours of FMOs and spin-density distributions of the T1
states of mDBSODPO; multicyclic reduction curves of
mDBSODPO; and EL performance of DPEPO-based control
devices. The Supporting Information is available free of charge
on the ACS Publications website at DOI: 10.1021/
acs.chemmater.5b02012.
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: hxu@hlju.edu.cn or iamhxu@njtech.edu.cn.
*E-mail: wei-huang@njtech.edu.cn.
Author Contributions
§These authors contributed equally.
Notes
4. CONCLUSIONS
The authors declare no competing financial interest.
In summary, a series of dibenzothiophene-based phosphine
oxide materials DBTDPO and mDBSODPO were utilized as
host and ETM, respectively, to construct high-efficiency blue
TADF devices with simple four-layer configurations. The
consistent V-shape configuration and polarity of DBTDPO and
DMAC-DPS and their well-matched optical properties
rendered the unchanged morphology and stabilized T1 states
of DMAC-DPS-doped thin films with RMS roughness of ∼0.25
nm, long τDF of 11.5 μs, and high PLQY more than 85%. The
compatibility between DBTDPO and mDBSODPO was
optimized with respect to configuration, packing mode, and
interfacial interaction through adjusting the substitution
ACKNOWLEDGMENTS
■
This project was financially supported by NSFC (61176020
and 51373050), New Century Excellent Talents Supporting
Program of MOE (NCET-12-0706), Program for Innovative
Research Team in University (MOE) (IRT-1237), Science and
Technology Bureau of Heilongjiang Province (ZD201402 and
JC2015002), Education Bureau of Heilongjiang Province
(2014CJHB005), and the Fok Ying-Tong Education Founda-
tion for Young Teachers in the Higher Education Institutions
of China (141012).
H
DOI: 10.1021/acs.chemmater.5b02012
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Chemistry of Materials
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