Expanding the hole delocalization range in excited molecules for stable organic light-emitting diodes employing thermally activated delayed fluorescence

Article abstract of DOI:10.1039/d0tc01897j

Metal-free, thermally activated delayed fluorescent (TADF) emitters have emerged as a promising new generation of organic light-emitting diode (OLED) materials. Donor-acceptor (D-A) structures are widely used in TADF molecular design to ensure a small energy splitting between the singlet and triplet excitons. Here, a series of efficient bluish-green TADF emitters are constructed using one or two phenyltriazine acceptors and one tercarbazole, bicarbazole or indolo[2,3-b]carbazole donor through an ortho-linkage. The impact of the D/A ratio on the photoluminescence and electroluminescence stability of these emitters in doped films is thoroughly investigated. According to the two-exciton dynamics and the degradation products, device degradation is deduced to be a result of electrophilic substitution between two charge-transfer excitons. Within a limited molecular weight range, increasing the number of acceptor moieties leads to a decrease in the hole delocalization range in the excited state, which facilitates the substitution reaction. Based on an optimized device structure, the device containing an emitter with bulk a tercarbazole donor achieves a long half-life of 1512 hours with an initial luminescence of 1000 cd m-2. Our findings reveal a possible mechanism for exciton-exciton and exciton-polaron annihilation-induced device degradation and provide new approaches for achieving stable OLEDs employing TADF.

Full text of DOI:10.1039/d0tc01897j

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DOI: 10.1039/D0TC01897J
ARTICLE
Expanding the Hole Delocalization Range in Excited Molecules for
Stable Organic Light-Emitting Diodes Employing Thermally
Activated Delayed Fluorescence
Received 00th January 20xx,
Accepted 00th January 20xx
a
a,b
c
a
a
d
a
DOI: 10.1039/x0xx00000x
Xinyuan Zheng,‡ Fangyi Cao,‡ Chao Wang, Taiju Tsuboi, Yunhui Zhu, Qi Ai, Chao Deng, Dan
a
a
a
a
Wang,* Liwu Su, Zhang Liu, Qisheng Zhang*
Metal-free, thermally activated delayed fluorescent (TADF) emitters have emerged as a promising new generation of
organic light-emitting diode (OLED) materials. Donor-acceptor (D-A) structures are widely used in TADF molecular design
to ensure a small energy splitting between the singlet and triplet excitons. Here, a series of efficient bluish-green TADF
emitters are constructed using one or two phenyltriazine acceptors and one tercarbazole, bicarbazole or indolo[2,3-
b]carbazole donor through an ortho-linkage. The impact of the D/A ratio on the photoluminescence and
electroluminescence stability of these emitters in doped films is thoroughly investigated. According to the two-exciton
dynamics and the degradation products, device degradation is deduced to be a result of electrophilic substitution between
two charge-transfer excitons. Within a limited molecular weight range, increasing the number of acceptor moieties leads
to a decrease in the hole delocalization range in the excited state, which facilitates the substitution reaction. Based on an
optimized device structure, the device containing an emitter with bulk a tercarbazole donor achieves a long half-life of
2
1
512 hours with an initial luminescence of 1000 cd/m . Our findings reveal a possible mechanism for exciton-exciton and
exciton-polaron annihilation-induced device degradation and provide new approaches for achieving stable OLEDs
employing TADF.
environmental thermal energy and then undergo radiative
1
. Introduction
As a new paradigm in flat-panel display technologies,
decay from S
fluorophore via a dipole-dipole interaction. A small S
energy gap ( ST) is key for the efficient harvesting of triplet
energy in OLEDs for TADF emitters.
Currently, the most efficient TADF emitters are mainly
intramolecular charge transfer (CT) molecules because a small
overlap between the highest occupied molecular orbital
1
or energy transfer to a lower-energy acceptor
1
−T
1

E
organic light-emitting diodes (OLEDs) have received extensive
attention from industry and academia. Currently,
commercialized OLED displays offer highly efficient green and
red electroluminescence (EL) by doping noble metal-based
phosphorescent dyes into their emitting layers (EMLs).1-4 In the
past two decades, numerous efforts have been made to
develop low-cost and highly efficient emitters that can replace
2
6-31
(
HOMO) and lowest unoccupied molecular orbital (LUMO) is
2
6-31
necessary to achieve a small

E
ST
.
Most reported TADF
molecules contain one acceptor (A) and one or two donor (D)
moieties with a sequence of D-A, D-A-D, or D-D-A.1
TADF molecules containing multiple acceptors have rarely
5
-10
phosphorescent materials in OLED applications.
OLEDs employing thermally activated delayed fluorescence
Recently,
6-23, 26-33
11-25
(
TADF) have attracted great attention.
the EML can upconvert from its lowest triplet excited state (T
to its lowest singlet excited state (S ) by absorbing
A TADF emitter in
3
4-36
been investigated.
In particular, the influence of the D/A
1
)
ratio in the emitter on device stability is poorly understood.
Generally, it is believed that balanced carrier injection and
transport in the EML improves the stability of the device,37
because the interactions between excess carriers and excitons
1
a. MOE Key Laboratory of Macromolecular Synthesis and Functionalization,
Department of Polymer Science and Engineering, Zhejiang University, Hangzhou
might lead to degradation, such as the so-called triplet-polaron
3
10027, P. R. China. E-mail: qishengzhang@zju.edu.cn; 0016720@zju.edu.cn.
b.
c.
38-41
Yungu (Gu'an) Technology Co. Ltd., Hebei, 065500, China.
Singapore University of Technology and Design, 8 Somapah Road, 487372,
Singapore.
State Key Laboratory Breeding Base of Green Chemistry-Synthesis Technology,
College of Chemical Engineering, Zhejiang University of Technology, Hangzhou,
annihilation (TPA) process.
When the doping concentration
of the emitters in the EML is higher than 30 wt%, electrons and
holes are mainly transported through the dopants. Because
bipolar transport materials generally have higher hole mobility
than electron mobility, increasing the proportion of acceptor
d.
3
10014, China.
‡
These authors contributed equally.
Electronic Supplementary Information (ESI) available: Supplemental tables and moieties in the TADF molecule might enhance the electron
figures; detailed synthetic procedures; measurement and quantum chemical
mobility and balance the electron and hole transport in the
calculation methods; investigation of photodegradation kinetics. See
3
7
DOI: 10.1039/x0xx00000x
EML, thereby improving the device stability.
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To understand the impact of the D/A ratio based on moieties are close to 66°. The q-OHF method baVsieewdAortincletOimnliene-
molecular weight on the device stability, we selected carbazole dependent DFT (TDDFT) was used to caDlcOuIl:a1t0e.10t3h9e/Dz0eTrCo0–1z8e9r7oJ
(
Cz) and 5,7-dihydroindolo[2,3-b]carbazole (BBCz) as the donor energies (E0-0) of these molecules in their S
0
state (Table 1 and
42
moieties and 2,4,6-triphenyl-1,3,5-triazine (TRZ) as the S1). The E0-0(S
1
) values for these molecules are in a small
acceptor moiety to construct five D-A type TADF molecules range (2.92−3.02 eV). Owing to the large D-A twist angles, the
with ortho-linkages and different D/A ratios. Our results do not orbitals involved in the intramolecular CT transition are well
demonstrate a simple dependence between the device separated, leading to a very small energy difference between
1
3
lifetime and the D/A ratio in the emitter. In contrast, the their CT and CT states (0.04−0.05 eV). The energy of the
3
delocalized range of the hole in the CT state has a greater lowest local triplet excited state ( LE) for both of the
impact on the photoluminescence (PL) and EL stability of the compounds with BBCz as the donor is approximately 2.76 eV,
emitters in doped films. In this regard, we speculate that the which is lower than that (~2.90 eV) of the compounds with TCz
photochemical reaction between a radical anion and a radical and BCz donors.
cation may be the main cause for the degradation in blue and
green TADF OLEDs.
The S
at the TDDFT/CAM-B3LYP/Def2-SVP level in toluene (Figure
b). Relative to their ground state geometries, all the TRZ
moieties exhibit better planarity in S . Additionally, the
dihedral angles between the phenyl bridges and the triazine
rings decrease to ~12°, while those between the phenyl
bridges and the donor moieties increase to ~80°. The
1
geometry of the five TRZ molecules was optimized
1
1
2
. Results and Discussion
The structures of five molecules containing one or two
acceptors (TRZ) and one donor (9,9''-diphenyl-9H,9'H,9''H-
,3':6',3''-tercarbazole (TCz), 3,3'-bicarbazole (BCz) or BBCz)
1
distribution of the hole and electron in S is shown in Figure
3
1
b. The hole is distributed over the whole donor moiety, while
are shown in Figure 1a. The N atom in the donor is connected
to the triazine ring via a phenyl bridge with an ortho-linkage.
The ground state structures of these molecules were
optimized using density functional theory (DFT) at the
B3LYP/6-31G(d) level (Figure S1). Unlike para-linked
the electron is localized on one TRZ acceptor with little overlap
with the hole. Although the electrons of the dual-acceptor
molecules are distributed on both TRZ acceptors in S
0
(see
Figure S1 in SI), they are only localized on one of the acceptors
4
3
17
1
in S due to symmetry breaking, implying that there will be
compounds having a planar TRZ moiety, the dihedral angles
between the phenyl bridges and the triazine rings are
approximately 38° in these five ortho-linked compounds due
to the large steric hindrance. Moreover, the dihedral angles
between the phenyl bridges and the adjacent Cz or BBCz
little difference in photophysical properties between the
single- and dual-acceptor molecules.
The five investigated compounds were synthesized
according to the methods described in the Experimental
1
Figure 1. Structures of the five investigated molecules, (a) and the hole and electron of these molecules in their S state in toluene (b). The molecules are optimized at the
TDDFT/CAM-B3LYP/Def2-SVP level. The localization distance of the hole (RLOL) calculated at the LOL-tuned LC-ωPBELOL/Def2-SVP level is used to measure the localization of hole
density.
2
| J. Name., 2012, 00, 1-3
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DOI: 10.1039/D0TC01897J
Figure 2. (a) UV−vis absorption and emission spectra measured in toluene at RT. (b) Transient decay spectra of the TADF emitters in 10 wt %-doped PYD2 films at RT. Inset:
Emission spectra of the doped films at RT. (c) Cyclic voltammograms for the reduction and oxidation processes in acetonitrile and dichloromethane, respectively. (d) Current
density–voltage characteristics of the electron-only devices (EODs) and hole-only devices (HODs).
section of the SI. All these molecules exhibit good thermal (PLQYs) of 0.40−0.53. No TADF component was found in their
stability. Their differential scanning calorimetry (DSC) and transient PL spectra (Figure S3), indicating a LE nature for the
31
thermogravimetric analysis (TGA) data are shown in Figure S2.
The glass transition temperatures (T values) and 5% weight- calculated based on PLQYs and fluorescence lifetimes (τ
loss temperatures of the five compounds are in the ranges of in a small range of 2.2−2.8×10 s . The quite similar k
30−179 and 395−505 , respectively. The T of the dual- for single- and dual-acceptor molecules confirm that the
acceptor compound is higher than that of the single-acceptor LUMO (one electron) in S is localized mainly on an individual
compound with the same donor owing to the enhanced acceptor in the dual-acceptor molecules, in accordance with
molecular weight. the theoretical prediction. According to the E0-0(S ) and E0-0(T
T
1
of these compounds.
The fluorescence rates (k
) are
values
F
)
g
F
7
-1
F
1
℃
℃
g
1
1
1
)
The absorption spectra of the five compounds in toluene determined from the fluorescence and phosphorescence
are shown in Figure 2a. The wide and weak absorption spectra at 77 K (Figure S4), the ΔEST values of the three
shoulders at 370−450 nm could be attributed to the S
transition with CT character. The molar extinction coefficients estimated to be 0.04−0.06 eV and 0.12-0.17 eV, respectively,
) for the S absorption bands of the single-acceptor which are in good agreement with the theoretical values.
molecules are approximately 2.5 mM cm , which are
comparable to those of TADF molecules having large D-A twist wt%-doped 2,6-di(carbazol-9-yl) pyridine (PYD2) films were
1
carbazole-based molecules and two BBCz-based molecules are
(

1
-1
-1
The photophysical properties of the five compounds in 10
28,43
angles.
1
The coefficients  of the S absorption bands for measured. When in the film, the emission maximum of the
the dual-acceptor molecules are approximately twice that of compound blueshifts by 5−10 nm and the PLQY (0.75-0.86)
the single-acceptor molecules because of the higher CT significantly improved compared with that in toluene (Table 1)
transition probability in dual-acceptor molecules.4 In oxygen- owing to the suppression of molecule collision-induced
free toluene at room temperature (RT), these molecules nonradiative inactivation in the solid matrix (page 288 in ref
exhibit broad and unstructured emission bands with a 44). TADF components with lifetimes as short as a few
maximum at 502−510 nm (Figure 2a) and PL quantum yields Φ microseconds can be observed in the transient decay spectra
3
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
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ARTICLE
Journal Name
of the doped films (Figure 2b). The TADF lifetimes are
positively correlated with the ΔEST values measured in toluene compounds, electron-only devices (EODsD)OwI: 1it0h.10a39s/tDr0uTcCtu0r1e89o7Jf
at 77 K. The transient decay spectra of the emitters in different ITO/Li CO (2 nm)/TADF compound (60 nm)/Li CO (2 nm)/Al
hosts are shown in Figure S5. Because the optimal doping (100 nm) and hole-only devices (HODs) with a structure of
concentration of CT-type TADF emitters in devices is often ITO/MoO (10 nm)/TADF compound (60 nm)/MoO (10 nm)/Al
To compare the carrier transport propertiesVieowfArttihclee Ofnilvinee
2
3
2
3
3
3
approximately 30 wt%, the PLQYs of the 30 wt%-doped films (120 nm) were fabricated. According to the voltage-current
were also measured. Except for TCz-o-TRZ, each emitter density (V-I) characteristics of these devices (Figure 2d), the
exhibits similar PLQYs in the 30 wt%- and 10 wt%-doped films electron mobilities in the two dual-acceptor molecules are
(
Table 1), indicating that the twisted structure can effectively significantly higher than that of the single-acceptor molecules
inhibit self-aggregation-induced quenching in the condensed except for TCz-o-TRZ. The TCz-o-TRZ-based EOD exhibits V-I
state (Figure S6). characteristics comparable to those of devices based on dual-
The oxidation and reduction potentials of these five acceptor molecules, even in doped EODs (Figure S7), which can
compounds were measured by cyclic voltammetry (CV) in be attributed to the relatively strong π-π interaction between
dichloromethane and acetonitrile, respectively (Figure 2c). All TCz-o-TRZ molecules as indicated in the PLQY measurement.
the compounds show a similar reduction peak with an onset at The hole mobilities of the three carbazole-based compounds
+
-
2.2 V (vs. Fc /Fc), which corresponds to the reduction of the are in the order TCz-o-TRZ> BCz-o-TRZ> BCz-o-2TRZ, which is in
TRZ moiety. The compounds based on BCz and BBCz donors accordance with the proportion of the donor moiety in terms
can undergo a two-electron oxidation, while TCz-o-TRZ exhibits of molecular weight. Because the BBCz moiety is more planar
multiple oxidation peaks in the test range. The oxidation and therefore has stronger conjugation than the BCz moiety,
potentials (vs. Fc/Fc*) determined from the onset of the first the hole mobilities in the BBCz compounds are higher than
oxidation peak are 0.65 V for TCz-o-TRZ, 0.69 V for the two BCz those of the corresponding BCz compounds.
compounds and 0.74 V for the two BBCz compounds.
Table 1. Theoretical and experimental photophysical data for the investigated molecules.
TCz-o-TRZ
BCz-o-TRZ
Theoretical data in toluene a
BCz-o-2TRZ
BBCz-o-TRZ
BBCz-o-2TRZ
1
3
E
E
0-0( CT/ CT) (eV)
0-0(3LE) (eV)
2.92/2.88
2.89
3.01/2.98
2.91
2.96/2.92
2.91
2.94/2.91
2.76
2.93/2.89
2.76
ΔEST (eV)
0.04
0.10
0.05
0.18
0.17
Experimental data in toluene
λ
em (nm) b
510
0.53
19.0
2.8
503
0.47
17.0
2.8
506
0.40
16.8
2.4
505
0.46
20.5
2.2
2.89
2.72
0.17
502
0.41
19.0
2.2
2.85
2.73
0.12
Φ b, d
b, e
F
τ (ns)
7
-1 f
k
F
(×10 s )
E
E
0-0(S
0-0(T
1
) (eV) c
) (eV) c
2.83
2.79
0.04
2.90
2.84
0.06
2.83
2.79
0.04
1
ΔEST (eV) c
Experimental data of 10 wt%-doped PYD2 film b
λ
em (nm)
498
0.86 (0.69)
28
491
0.82 (0.80)
25
493
0.75 (0.75)
25
497
0.83 (0.81)
31
498
0.86 (0.85)
27
Φ d
τ
τ
F
(ns) e
TADF (μs) g
2.8
3.2
2.6
7.2
4.8
a
geometry optimized by DFT/B3LYP/6-31G(d). Measured at RT. Measured at 77 K. d PLQY of the 30 wt%-doped PYD2 films are
b
c
Calculated by the q-OHF method on the S
given in parentheses. e Fluorescence lifetime determined from the PL decay curve in the nanosecond range. f
0
F F
k = Φ/τ
. g TADF lifetime determined from the PL decay curve in
the microsecond range.
4
| J. Name., 2012, 00, 1-3
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Figure 3. (a) Structure of the devices employing PYD2 or PPT as the host. The doping concentration in the EML is 30 wt%. (b) EL spectra at 10 mA/cm2, (c) luminance−current
density−voltage characteristics, and (d) EQE−current density characteristics of the OLEDs employing different emitters and hosts.
The EL properties of all five TADF emitters were examined the EL performance (Figure 3). However, the relatively high
using the following device structure: ITO/HAT-CN (10 nm)/NPB polarity of the PPT host seems to promote self-aggregation of
(
30 nm)/TCTA (10 nm)/PYD2 (10 nm)/EML (30 nm)/PPT (5 TCz-TRZ, resulting in a decrease in EQE and an 8 nm redshift in
nm)/Bepp :Li CO (5 wt%, 30 nm)/Li CO (1 nm)/Al (100 nm), the EL spectrum relative to the corresponding devices based
where HAT-CN, NPB, TCTA, and Bepp2 are 1,4,5,8,9,11- on the PYD2 host. In contrast, the EQE roll-off is significantly
2
2
3
2
3
hexaazatriphenylene
hexacarbonitrile,
1,4-bis[(1- reduced for BBCz-based devices by replacing the PYD2 host
naphthylphenyl)amino]-biphenyl,
tris[4-(9H-carbazol-9-yl)- with PPT because the highly polar host can effectively reduce
phenyl]amine, and bis[2-(2-hydroxyphenyl)-pyridine]beryllium, the ΔEST of BBCz molecules through the solid-state solvation
respectively (Figure 3a). In EML, the TADF emitter is doped effect.31
into hole-transporting PYD2 or electron-transporting 2,8-
bis(diphenylphosphoryl)-dibenzo[b,d]thiophene (PPT) at
Although the half-life (LT50) of the device employing the
PYD2 host is higher than that employing the PPT host, the
a
concentration of 30 wt%. The EL spectra of the five devices orders of the EL stabilities of the five TADF emitters are the
based on the PYD2 host are similar to each other (Figure 3b) as same regardless of the host (Figure 4a and Figure S8), i.e., TCz-
are
their
voltage-current
density-luminance
(V-I-L) o-TRZ > BCz-o-TRZ > BCz-o-2TRZ > BBCz-o-TRZ > BBCz-o-2TRZ.
characteristics (Figure 3c) despite the differences in redox The device based on the TCz-o-TRZ emitter and the PYD2 host
potentials and carrier mobilities for the doped emitters. These achieves an LT50 of 1512 hours with an initial luminescence
2
devices turn on at 3 V and can reach a maximum luminance up (L
0
) of 1000 cd/m , which is among the best reported for
5
2
21-24,37,45-48
to 1.2×10 cd/m at a voltage of approximately 14 V. The blueish-green TADF OLEDs.
The above results show
maximum external quantum efficiencies (EQEs) for these that increasing the number of acceptors in the TADF molecule
devices are in the range of 14.0−16.6% (Figure 3d), and the does not improve the stability of the device, although it does
order is the same as that of the PLQYs of the EMLs (30 wt%- improve the balance between the hole- and electron-transfer
doped PYD2 films). The EQE roll-off in BBCz-based devices is in the EML. Using an electron-transporting material as a host
larger than that of the other three devices because of the can also improve the balance of hole- and electron-transfer in
larger adiabatic ΔEST in BBCz molecules. For the BCz molecule- EML but does not improve the device stability.
based devices, replacing the PYD2 host with PPT slightly affects
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Table 2. EL performance of the devices employing different hosts
View Article Online
DOI: 10.1039/D0TC01897J
a
EQE0.1 b
(%)
EQE10 c
(%)
LT50 d
(h)
Host
Emitters
λ
max
V
on
L
max
4
2
(
nm)
(V)
3.0
3.0
3.0
3.0
3.0
2.9
2.9
2.9
2.9
2.9
(10 cd/m )
11.3
7.5
TCz-o-TRZ
BCz-o-TRZ
BCz-o-2TRZ
BBCz-o-TRZ
BBCz-o-2TRZ
TCz-o-TRZ
501
495
497
499
498
509
497
500
499
498
14.0
16.2
15.3
15.3
16.6
12.7
16.4
15.4
16.5
17.5
12.1
14.5
14.0
11.5
12.6
11.2
14.5
14.0
13.2
14.6
1512
606
495
345
262
840
335
318
262
147
PYD2
7.9
9.1
11.5
11.9
11.9
11.8
8.7
BCz-o-TRZ
PPT
BCz-o-2TRZ
BBCz-o-TRZ
BBCz-o-2TRZ
8.5
a
V
o
l
t
a
g
e
a
t
a
l
u
m
i
n
a
n
c
e
o
f
1
c
d
/
m
2
.
b
E
Q
E
a
t
0
.
1
m
A
/
c
m
2
.
c
E
Q
E
a
t
1
0
m
A
/
c
m
2
.
d
= 1000 cd/m2.
0
L
in both PL and EL processes. Because the photodegradation
rate was found to be highly dependent on the doping
concentration of the TADF emitters in the films, 4
7,48
the
degradation may be governed by an exciton-exciton
interaction process. On the basis of nuclear magnetic
resonance (NMR) and MALDI-TOF mass spectrometry, we
deduced that the photodegradation might be due to the
formation of a small amount of coupling products with low-
lying energy levels, which can effectively quench the emission
3
8,48,49
from the TADF emitters doped into the films.
According
to this quenching hypothesis, the PL intensity of the film
should be proportional to the exciton concentration and
inversely proportional to the quenching center concentration.
The luminescence decay curves shown in Figure 4b
demonstrate second-order reaction kinetics (see section 3 in
SI), which supports a coupling-induced quenching mechanism.
The coupling products having narrow band gaps might be
responsible for the newly emerged emission band detected in
the long wavelength region in an operated device (Figure S9),
and this band was previously attributed to the transfer of the
electron-hole recombination region from the center of the
5
0,51
EML to the interface.
Because of the relatively low EL stability, BBCz-o-TRZ was
selected as a model for analyzing the photodegradation
pathways. The neat film of BBCz-o-TRZ was irradiated under
3
65 nm light in a glove box for 30 min, during which the
luminescence of the neat film decreased to half of its initial
value. NMR analysis of the degradation products showed that
there is little difference between the spectra of the
compounds before and after irradiation (Figure S10), indicating
that a small amount of the degradation product can effectively
3
8,48,49
quench the luminescence.
The MALDI-TOF mass and
2
Figure 4. (a) LT50 at a fixed current density with an L
0
of 1000 cd/m . The TADF
liquid chromatography mass spectra (LC-MS) show that the
degradation products include a diphenyltriazine fragment and
the coupling product of BBCz-o-TRZ and diphenyltriazine
emitters were doped into a PYD2 host in device structure shown in Figure 3a. (b)
Photostability of the compounds in 30 wt%-doped PYD2 films under 350 nm light
excitation.
(
Figure 5 and S8). Because the C-C δ-bond in the TRZ moiety is
rather stable both in the ground and excited states, the bond
would not be the preferential cleavage site during UV
The photodegradation of these five TADF emitters was
investigated in 30 wt%-doped PYD2 films under 350 nm light
excitation. As shown in Figure 4b, the relative PL stabilities of
these films are consistent with the EL stabilities of the devices
containing them, indicating a similar degradation mechanism
52-54
photolysis.
Instead, the C-C bond breaking and forming can
be a result of electrophilic substitution between the radical
cation/radical anion couples formed in the excited state (page
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ARTICLE
View Article Online
DOI: 10.1039/D0TC01897J
Figure 5. Proposed decomposition reaction between excited BBCz-TRZ molecules (upper panel) and MALDI-TOF-MS spectrum of BBCz-o-TRZ after 365 nm UV irradiation for 30
min (lower panel). A component with a molecular weight in the ranges of 233.5−234.5 Da was detected by LC-MS (Figure S11).
8
99 in ref 44),55,56 taking into account two-exciton dynamics in Multiwfn 3.4.0 based on the kinetic energy density (see the
57,58
the photodegradation. In a CT state, the BBCz donor loses an experimental details in the SI)
electron to form a radical cation (BBCz ), while the TRZ device lifetimes. As shown in Figure 6, the RLOL of the hole
acceptor acquires an electron to form a radical anion (TRZ ). distribution has a strong positive correlation with the LT50
The excited-state orbital analysis shows that the contributions value on the log–log scale. For the excited single-acceptor
of the triazine ring and the C3 atom to the LUMO (one molecules, the hole delocalizes on both their donor moieties
electron) are as high as 50.5% and 16.7%, respectively (Figure
and compared with the
●+
●
−
1
b). When the BBCz●+ group attacks the negatively charged C3
atom of another excited molecule, the BBCz group on the
other molecule will be substituted by the entire BBCz-o-TRZ
unit, forming a coupling product with a molecular weight of
8
70 Da. Alternatively, the intermediate product in the coupling
reaction can be decomposed into a BBCz-o-TRZ molecule and a
diphenyltriazine fragment. In addition to attacking the C3 atom
of another excited molecule, the BBCz●+ group can also attack
the C1 and C2 atoms, forming a coupling product with a
molecular weight of 1201 Da and a fragment with a molecular
weight of 563 Da. These two products were also detected by
MALDI-TOF MS, as shown in Figure 5.
Based on the mechanism of electrophilic substitution, we
can infer that smaller the hole delocalization ranges
●+
correspond to higher electrophilicity of the reagent BBCz .
The mean localization distance of the hole (RLOL in unit of Bohr)
Figure 6. Dependence of the device lifetime (LT50) on the hole localization distance of
1
for the five TRZ molecules in their S were calculated using the excited emitter (RLOL).
This journal is © The Royal Society of Chemistry 20xx
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Journal Name
and the isolated phenyl substituents. The hole delocalization (Figure 4). Because these five compounds have nVoiewabArstioclrepOtniloinne
range in the dual-acceptor molecule is smaller than that in the at 532 nm, the possibility of exciton-DpOoIl:a1r0o.1n03a9/nDn0iThCil0a1t8io9n7J-
single-acceptor molecule having the same donor because the induced degradation can be ruled out. We found that the
additional acceptor moiety inhibits the delocalization of the driving voltage of
a constant-current device suddenly
hole to the phenyl bridge by increasing the twisting angle decreased after exposure to 532 nm light and returned to
between the bridge and the donor. Consequently, the dual- normal after stopping irradiation (Figure S13). The electrons in
acceptor molecule is more reactive than the analogous single- the aluminum cathode seemed to be excited to higher energy
acceptor molecule in their excited states, and the stability of levels by the 532 nm light and then injected into the organic
6
1
the device based on the former is worse than that based on layer to form excitons near the cathode. The narrow-band-
the latter. Previous works have shown that the introduction of gap product formed by the exciton-exciton reaction might trap
a benzene ring substituent on the donor of blue TADF carriers in the constant-current device and consequently
molecules can significantly improve the stability of the increase the driving voltage with time.
2
4,45
device.
expanding the hole delocalization range to improve the polaron prevents the polaron from reacting with an anti-
chemical stability of the exciton with diradical character. charged polaron or an exciton. However, if a device suffers
This phenomenon can also be explained by
In a well-designed OLED, the high hopping rate for a
Theoretically, the electrophilic substitution reaction from charge trapping in the EML or charge blocking at the
mentioned above can also occur between an electron polaron interface, 40,62 the substitution reaction involving a polaron
and a hole polaron and between a polaron and an exciton. does occur. This explains why the device employing the PPT
However, we do not think these processes govern the host has a relatively short LT50 compared with the analogous
degradation in our devices because the charge hopping rate device employing the PYD2 host. Because the hole mobility in
11
14 -1 59,60
can be up to 10 −10 s ,
which is much higher than the the EML with electron-transporting host PPT must be lower
chemical reaction rate, which highly depends on the rate of than that with hole-transporting host PYD2, the hole polarons
7
9
-1
polarization orientation (10 −10 s ) and thermal motion (< in the former are more likely to react with the electron
5
-1
31
1
0 s ) in molecular thin films. When the polarons recombine polarons or excitons. Analogously, increasing the doping
to form excitons, the carriers are constrained and can exist on concentration of TADF molecules from 10 wt% to 30 wt% often
the nanosecond time scale, which is on the same order of improves the stability of the device,46,47 because the
magnitude as the polarization orientation and thermal motion continuous hole channels between the guests at high doping
in thin films. During this period, the possibility of chemical concentrations prevent hole polaron trapping in the EML. In
reactions is much higher. We previously used UV light to addition, removing the hole blocking layer (HBL) adjacent to
irradiate HODs to confirm that hole polarons can interact with the EML can prevent the accumulation of hole polarons in the
excitons to cause device degradation.22 However, we found electron-hole recombination region, which can improve device
herein that even if the energy of the irradiation light is lower stability.47,63 Obviously, the most effective way to inhibit the
than the excitation energy of the emitter, the light can accumulation of hole polarons at the EML/HBL interface is to
accelerate the degradation. When the five HODs mentioned increase the electron mobility in the electron-transporting
above were operated under a constant current density of 1.0 layer (ETL), allowing it to transport more electrons to the
2
mA/cm for 20 hr, the driving voltages remained almost interface to quickly neutralize excess holes. In comparison to
2 3
constant (Figure S12). However, if the operating devices were the device with a neat ETL, the device with a Li CO -doped ETL
irradiated with 532 nm light at the same time, the driving shows a decreased driving voltage as well as an increased
voltage of these devices increased rapidly. As shown in Figure operating lifetime (Figure S14).
7
, the magnitude of the voltage growth rates is consistent with
that of the PL and EL degradation rates for these compounds
Conclusions
Efficient bluish-green TADF emitters with phenyltriazine
acceptors and carbazole or indolo[2,3-b]carbazole donors
were constructed with ortho-linkages and different D/A ratios.
These five compounds exhibit comparable emission maxima
(
(
491−498 nm), PLQYs (0.75−0.86) and fluorescence lifetimes
25−31 ns) in doped films but rather different ST (0.04−0.17

E
eV), TADF lifetimes (2.6−7.2 μs) and charge mobilities. The
devices based on these emitters demonstrate high EQEs, up to
5
2
1
7.4%, and very high luminances, up to 1.2×10 cd/m . The
relative EL stabilities of these five emitters doped into hole-
transporting material PYD2 were consistent with those doped
into electron-transporting material PPT and with the relative
PL stabilities of these emitters in films. It is difficult to explain
the different EL stabilities of these emitters by different TADF
lifetimes or different charge balances in the EML. Instead, the
Figure 7. Time-dependent voltage difference between the hole-only devices with and
without illumination (532 nm) under a constant current density of 1.0 mA/cm . Inset: A
2
possible degradation mechanism for the HODs with both illumination and current.
8
| J. Name., 2012, 00, 1-3
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Journal of Materials Chemistry C
Journal Name
ARTICLE
1
1
5 H. Nakanotani, T. Higuchi, T. Furukawa, KV.iewMAraticslue iO,nliKne.
DOI: 10.1039/D0TC01897J
C. Adachi, Nat. Commun., 2014, 5, 4016.
stability differences are highly dependent on the delocalized
range of the hole in the excited emitters and are thus related
to the D/A ratio in these emitters. The device based on TCz-o-
Morimoto, M. Numata, H. Tanaka, Y. Sagara, T. Yasuda and
6 S. Hirata, Y. Sakai, K. Masui, H. Tanaka, S. Y. Lee, H. Nomura,
N. Nakamura, M. Yasumatsu, H. Nakanotani, Q. Zhang, K.
Shizu, H. Miyazaki and C. Adachi, Nat. Mater., 2015, 14, 330–
TRZ containing a TRZ acceptor and a tercarbazole donor
2
achieved an LT50 of 1512 hours with an L
0
of 1000 cd/m ,
3
36.
7 K. Shizu, H. Noda, H. Tanaka, M. Taneda, M. Uejima, T. Sato,
K. Tanaka, H. Kaji and C. Adachi, J. Phys. Chem. C, 2015, 119
6283–26289.
which is one of the best reported to date. Based on the two-
exciton dynamics and charge density dependence of the
degradation, we proposed that the radical electrophilic
substitution occurring between excitons generates narrow-
band-gap products to quench the EL emission, which was
demonstrated by analysis of the degradation products.
Because the degradation reaction can also occur between an
exciton and a trapped polaron, increasing the hole mobility in
the EML and the electron mobility in the ETL was found to
1
,
2
1
1
8 R. Wang, Y.-L. Wang, N. Lin, R. Zhang, L. Duan and J. Qiao,
Chem. Mater., 2018, 30, 8771−8781.
9 T.-L. Wu, M.-J. Huang; C.-C. Lin, P.-Y. Huang, T.-Y. Chou, R. -
W. Chen-Cheng, H.-W. Lin, R.-S. Liu and C.-H. Cheng, Nat.
Photonics, 2018, 12, 235−240.
0 D. H. Ahn, S. W. Kim, H. Lee, I. J. Ko, D. Karthik, J. Y. Lee and J.
H. Kwon, Nat. Photonics, 2019, 13, 540–546.
2
effectively improve device stability. Further studies on 21 D. R. Lee, J. M. Choi, C. W. Lee and J. Y. Lee, ACS Appl. Mater.
Interfaces, 2016, 8, 23190−23196.
2 L.-S. Cui, Y.-L. Deng, D. P. Tsang, Z.-Q. Jiang, Q. Zhang, L.-S.
Liao and C. Adachi, Adv. Mater., 2016, 28, 7620−7625.
3 J. G. Yu, S. H. Han, H. L. Lee, W. P. Hong and J. Y. Lee, J.
Mater. Chem. C, 2019, 7, 2919–2926.
suppressing the reaction-associated degradation are
underway.
2
2
2
Conflicts of interest
There are no conflicts to declare.
4 M. Cai, D. Zhang, J. Xu, X. Hong, C. Zhao, X. Song, Y. Qiu, H.
Kaji and L. Duan, ACS Appl. Mater. Interfaces, 2019, 11
096–1108.
,
1
2
5 S. Shao, J. Hu, X. Wang, L. Wang, X. Jing and F. Wang, J. Am.
Chem. Soc., 2017, 139, 17739−17742.
Acknowledgements
This work is supported by the National Key R&D Program of
China (Grant No. 2016YFB0401004) and the National Natural
26 Y. Liu, C. Li, Z. Ren, S. Yan and M. R. Bryce, Nature Rev.
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7 A. Endo, K. Sato, K. Yoshimura, T. Kai, A. Kawada, H. Miyazaki
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