Intermolecular locking design of red thermally activated delayed fluorescence molecules for high-performance solution-processed organic light-emitting diodes

Article abstract of DOI:10.1039/d0tc05624c

Design of high-performance red thermally activated delayed fluorescence (TADF) materials remains a great challenge owing to their small energy bandgaps with severe nonradiative decay for low luminous efficiency. Introducing rigid and fused moieties is an

Full text of DOI:10.1039/d0tc05624c

Journal of
Materials Chemistry C
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PAPER
Intermolecular locking design of red thermally
activated delayed fluorescence molecules for
high-performance solution-processed organic
light-emitting diodes†
Cite this: J. Mater. Chem. C, 2021,
, 2291
9
a
a
a
a
a
a
Jibiao Jin, Wuji Wang, Peiran Xue, Qingqing Yang, He Jiang, Ye Tao,
a
b
ac
a
Chao Zheng,* Guohua Xie, * Wei Huang
and Runfeng Chen
*
Design of high-performance red thermally activated delayed fluorescence (TADF) materials remains a
great challenge owing to their small energy bandgaps with severe nonradiative decay for low luminous
efficiency. Introducing rigid and fused moieties is an effective way to enhance the luminescence of red
emitters, but the solubility is also significantly reduced, prohibiting inevitably their applications in
solution-processed TADF organic light-emitting diodes (OLEDs). Herein, we propose an intermolecular
locking strategy to improve both the solution processibility and photoluminescence efficiency of red
TADF emitters by using a highly-soluble flexible difluoroboron b-diketonate unit with exposed and easily
reachable fluorines that can form hydrogen bonds to induce strong intermolecular locking in the solid
state for high luminescent efficiency. The thus designed emitters show excellent device performance in
solution-processed TADF OLEDs with a low turn-on voltage of 4.4 V, red electroluminescence around
Received 30th November 2020,
Accepted 13th January 2021
600 nm, a high external quantum efficiency up to 8.2% and a small efficiency roll-off of 9.0% at
À2
1
000 cd m , which are among the best results of solution-processed red OLEDs. These results
demonstrate that the intermolecular locking strategy by directly addressing the internal conflicts
between solubility and luminescent efficiency provides important clues in developing highly efficient and
solution-processible red emitters for high-performance OLEDs.
DOI: 10.1039/d0tc05624c
rsc.li/materials-c
4–7
efficiency (EQE). To overcome the limitation imposed by the
spin-forbidden triplet exciton emission, many efforts have been
Introduction
Organic light-emitting diodes (OLEDs) have received considerable made by using noble-metal complexes with spin-allowed triplet
8,9
attention in recent years because of their promising potential in exciton emission for phosphorescent OLEDs, thermally acti-
1
–3
flexible flat-panel displays and solid-state lighting.
performance OLEDs rely on the efficient conversion of both excitons to singlet ones for TADF OLEDs,
electronically excited singlet (25%) and triplet (75%) excitons into organic radicals with extraordinary doublet emission for radical-
High- vated delayed fluorescence (TADF) molecules to transform triplet
10–13
and even stable
14,15
photons for electroluminescence with high external quantum based OLEDs.
Among them, metal-free purely organic TADF
materials with efficient reverse intersystem crossing (RISC) under
a
a significantly reduced singlet–triplet splitting energy (DE ) to
ST
Key Laboratory for Organic Electronics and Information Displays & Jiangsu Key
Laboratory for Biosensors, Institute of Advanced Materials (IAM), Jiangsu National
Synergistic Innovation Center for Advanced Materials (SICAM), Nanjing University
of Posts & Telecommunications, 9 Wenyuan Road, Nanjing 210023, China.
E-mail: iamczheng@njupt.edu.cn, iamrfchen@njupt.edu.cn
harvest both singlet and triplet excitons for 100% internal quan-
tum efficiency of the device are the most attractive since their
16
report in 2009, and great progress has been achieved in both
17–19
blue and green TADF OLEDs with EQEs over 30%.
Unfortu-
b
c
Sauvage Center for Molecular Sciences, Hubei Key Lab on Organic and Polymeric
Optoelectronic Materials, Department of Chemistry, Wuhan University, Wuhan,
nately, red TADF emitters lag far behind in both investigations
and device performance, mainly owing to their narrow bandgaps
with substantial energy loss via nonradiative decay as governed by
4
30072, China. E-mail: guohua.xie@whu.edu.cn
Frontiers Science Center for Flexible Electronics (FSCFE), Shaanxi Institute of
Flexible Electronics (SIFE) & Shaanxi Institute of Biomedi-cal Materials and
Engineering (SIBME), Northwestern Polytechnical University (NPU),
20,21
the energy gap law.
Generally, rigid and fused moieties have been proved to be
effective to suppress nonradiative transitions in designing
1
27 West Youyi Road, Xi’an 710072, China
†
Electronic supplementary information (ESI) available. CCDC 2021616. For ESI
2
2,23
high-efficiency red TADF materials (Fig. 1a).
Red TADF
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
d0tc05624c
OLEDs fabricated by vacuum thermal deposition technology
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Results and discussion
Molecular design and synthesis
To construct the TADF molecules with intermolecular inter-
locking, difluoroboron b-diketonate with strong electron-
withdrawing ability is selected as an acceptor unit and the
widely used phenylcarbazole or triphenylamine as a donor unit
in a donor–acceptor–donor (D–A–D) architecture, typically
shown in DPhCzB and DTPAB (Fig. 1b). The thus designed
compounds were facilely synthesized through a conventional
three-step reaction of Claisen condensation, C–N coupling, and
a borylation reaction with high total yields up to 39% (Scheme
1
13
S2, ESI†), and were fully characterized by H and C NMR
spectroscopy, high resolution mass spectrometry, and single-
crystal X-ray diffraction (Fig. S1–S6 and Table S2, ESI†).
Thermogravimetric analysis (TGA) and differential scanning
calorimetry (DSC) reveal that these D–A–D molecules have excel-
Fig. 1 (a) Design of highly efficient red TADF materials for solution
processing by the intermolecular locking strategy. (b) Molecular structures
of the designed red TADF emitters.
2
4–26
lent thermal stability with high decomposition temperatures (T )
d
show high EQEs over 20%.
But, the rigid molecular frame-
over 380 1C, glass-transition temperatures (T ) over 130 1C and
g
work usually leads to poor solubility, which significantly prohibits
their application in solution-processed OLEDs, although solution-
processing has been widely recognized to have significant
advantages, especially in low-cost and large-area production for
commercial manufacture. A few solution-processed red TADF
OLEDs with EQE around 10% have been achieved by introducing
the widely used tert-butyl group to improve the solubility of
melting temperatures (T ) over 200 1C (Fig. S7, ESI†), which are
m
among the highest thermal stabilities of organic boron-containing
optoelectronic materials. Also, the compounds exhibit excellent
solubility in common solvents (Table S3, ESI†). Especially, DTPAB
27
À1
has superior solubility (4100 mg mL ) and a smooth film
morphology with a small root-mean-square (RMS) roughness of
0.206 nm when spin-coated on an ITO substrate owing to its
flexible diphenylamine compared to the rigid carbazole in
DPhCzB (Fig. S8, ESI†). Therefore, promising applications of these
molecules in solution-processed OLEDs can be highly expected.
28–31
organic molecules.
However, the turn-on voltages of these
solution-processed devices are up to 7.0 V with large efficiency
roll-off, owing to the steric hindrance for inferior carrier trans-
portation and non-conductivity of the introduced tert-butyl
group.
Herein, we propose an intermolecular locking strategy to
29
Photophysical properties
solve the above dilemma by combining a flexible molecular The photophysical properties of DPhCzB and DTPAB were studied
architecture for good solubility in solution and strong intermo- using ultraviolet-visible (UV-vis) absorption and photolumines-
À5
À1
lecular interactions in aggregation to improve the emission cence (PL) spectra in dilute toluene solution (10 mol L ) and
efficiency for highly efficient solution-processed red TADF solid films. Both emitters show similar UV-vis absorption spectra
OLEDs (Fig. 1a). Strong intramolecular charge transfer (CT) in toluene solution (Fig. 2a), exhibiting one distinct absorption
characteristics for red emission and typical TADF features with band peaking at around 295 nm ascribed to the carbazole or
small DEST and efficient RISC processes were facilely realized by diphenylamine-centered p–p* transition, and a weaker absorption
3
2
using a strong acceptor of difluoroboron b-diketonate. This at around 340 nm that can be assigned to the n–p* transition of
33
rarely used acceptor unit is rather flexible to enable high the entire molecule. The strongest absorption bands of DPhCzB
solubility of the TADF materials in common organic solvents. and DTPAB were at 468 and 502 nm owing to the facile CT
But, more importantly, this unit with an exposed acceptor core of transition from the donor of carbazole or diphenylamine to the
difluoroboron enables abundant and multiple intermolecular acceptor of the difluoroboron b-diketonate group. DTPAB shows a
hydrogen bond interactions between the easily reachable fluor- more red-shifted CT absorption peak because of the stronger
ine on the difluoroboron group and the surrounding hydrogen electron-donating ability of the diphenylamine unit than carba-
atoms of the aromatic donor units on the nearby molecules to zole for a stronger CT transition. This CT state also shows a broad
suppress nonradiative transitions. Thus, the photoluminescence and structureless CT emission band peaking at 538 and 560 nm
quantum yield (PLQY) increases significantly from 22% in for DPhCzB and DTPAB in toluene solution, and was further
dichloromethane (CH
7% when doped (5 wt%) into 1,3-bis(N-carbazolyl)benzene solvents with different polarities (Fig. S9, ESI†). When turned into
mCP). The solution-processed red (B600 nm) TADF OLEDs the solid thin film state, DPhCzB and DTPAB exhibit significantly
2 2
Cl ) to 54% in the pure film, and to confirmed by the large variations of their emission bands in
9
(
based on these red emitters exhibit excellent performance with red-shifted (up to 33 nm) CT absorption bands to 501 and
a low turn-on voltage of 4.4 V, an EQE up to 8.2% and small 528 nm compared to those in solution owing to the extended
À2
efficiency roll-offs of 3.7% and 9.0% at 100 and 1000 cd m
,
p-conjunction and strong intermolecular interaction (Fig. 2b). The
respectively. These performances are comparable to the best red-shift (up to 100 nm) of their PL spectra is even larger in the
results of red TADF OLEDs (Table S1 and Scheme S1, ESI†).
film state for typical red emission at 637 and 650 nm, indicating
2292 | J. Mater. Chem. C, 2021, 9, 2291À2297
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Fig. 2 (a and b) UV-vis absorption and photoluminescence spectra in (a) toluene solution and (b) solid films, (c) fluorescence and phosphorescence
delay 5 ms) spectra in toluene at 77 K and (d) transient PL decay curves of these intermolecular locking molecules (5 wt%) doped in mCP films at room
temperature of DPhCzB (black) and DTPAB (red).
(
2 2
again strong intermolecular interactions of these molecules in the CH Cl owing to the flexible molecular structures with severe
solid state. molecular vibrations in the single-molecule state, but exhibit
From the onset thresholds of the absorption spectra in solid much increased values to 54% and 56% in pure films, illustrating
films, the optical band gaps (E s) of DPhCzB and DTPAB were obvious aggregation enhanced emission (AEE) behavior owing to
g
identified to be as narrow as 2.03 and 1.96 eV, respectively the suppressed nonradiative decays in solid films. When the red
(Table 1). From the PL and time-resolved phosphorescence spec- emitters were doped (5 wt%) into mCP, the PLQYs further increase
tra measured at 77 K in toluene, a small DEST of 0.15 and 0.17 eV to 87% and 97% for DPhCzB and DTPAB, respectively. The high
can be evaluated for DPhCzB and DTPAB, respectively (Fig. 2c). PLQYs of these red TADF emitters in either pure or doped films
With such a small DEST, effective RISC for TADF can be expected. should be highly attractive for high-efficiency OLEDs.
Indeed, the transient PL decay curves of the two D–A–D molecules
Intra- and inter-molecular interaction investigation
doped into mCP films (5 wt%) exhibit a prompt decay followed by
a delayed decay with corresponding lifetimes of 15.7 ns and 19.5 To understand the significantly red-shifted UV absorption and PL
ms for DPhCzB and 3.7 ns and 55.7 ms for DTPAB, respectively emission and increased PLQYs of the red TADF molecules in the
(Fig. 2d and Fig. S10, ESI†), providing direct evidence of their solid state, the intra- and intermolecular interactions were inves-
TADF characteristics. Moreover, the two new red TADF emitters of tigated. The reduced density gradient (RDG) isosurface calcula-
DPhCzB and DTPAB show acceptable PLQYs of 22% and 44% in tions of DTPAB and DPhCzB in the single molecule state based on
Table 1 Photophysical and electrochemical properties of DPhCzB and DTPAB
d
l
abs (nm)
l
em (nm)
t
t
CV (eV)
HOMO
a
e
Compound
Tol.
Film
E
g
(eV)
Tol.
Film
DEST (eV)
PF (ns)
tDF (ms)
PLQY
LUMO
b
c
DPhCzB
DTPAB
292, 468
300, 501
294, 501
301, 528
2.03
1.96
538
560
637
650
0.15
0.17
15.7
3.7
19.8
55.7
0.54b
0.87c
À5.41
À5.36
À3.38
À3.40
0.56
0.97
a
b
c
E
g
was estimated from the absorption onset in neat films. Absolute PLQY measured in pure films at room temperature. Absolute PLQY
measured in doped films at room temperature. Measured in doped films. LUMO energy level estimated by adding E to the HOMO energy level.
g
d
e
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Fig. 3 (a) Molecular packing and inter/intra-molecular interactions in the DPhCzB crystal. (b) PLQY values of mCP blend films doped with DPhCzB and DTPAB
at various doping concentrations (in wt%). (c) Reduced density gradient (RDG) versus sign(l )r with the view of the RDG isosurface of the DPhCzB dimer.
2
6
their optimized ground molecular structures (Fig. S11, ESI†) show ammonium hexafluorophosphate (TBAPF ) as the supporting elec-
heavy intramolecular interactions, especially in DPhCzB. From the trolyte and ferrocene as the internal standard (Fig. S12, ESI†). From
single crystal structure analyses, abundant intra/intermolecular the onset of the oxidation wave, the highest occupied molecular
interactions with short distances can be observed with strong orbital (HOMO) energy levels of DPhCzB and DTPAB were measured
intermolecular hydrogen bonds of C–HÁÁÁF and C–HÁÁÁO, owing to be À5.41 and À5.36 eV, respectively. With the aid of the optical
to the exposed difluoroboron b-diketonate group with multiple band gaps, the lowest unoccupied molecular orbital (LUMO) energy
fluorine and oxygen atoms in forming hydrogen bonds (Fig. 3a). levels were estimated to be À3.38 and À3.40 eV. It should be noted
The heavy intra- and intermolecular interactions in the solid state that these HOMO and LUMO energy levels are very close to those of
offer a rigid environment to dramatically suppress the nonradia- widely used hole-transporting and electron-transporting materials,
tive decay for high luminescent efficiency, which explains the suggesting their high potential in optoelectronic device applica-
bathochromic shift of the PL peaks and much improved PLQY in tions. To evaluate their frontier molecular orbital distributions and
film states. Further, the PLQYs of the doped films with different electronic structures, density functional theory (DFT) computations
concentrations were measured (Fig. 3b). The increased PLQY at were performed (Fig. 4). Similar to typical TADF molecules, the
reduced doping concentration indicates the strong intermolecular HOMOs of these molecules are dominantly located on the donor
interactions between the emitters and mCP to well disperse segments of phenylcarbazole or triphenylamine, while the LUMOs
the emitters in mCP owing to the easily reachable fluorine on are mainly distributed on the acceptor segments of difluoroboron
the difluoroboron group for hydrogen bonding. Furthermore, the b-diketonate, leading to partly separated frontier orbital distribu-
intermolecular interaction was theoretically verified by RDG iso- tions with moderate overlap extents (IH/L) of 37.34% and 48.52%
surface calculations based on the single crystal structure of between the HOMO and LUMO for small DEST and high PLQYs.
34
DPhCzB. Strong intermolecular attractive and repulsive interac- Moreover, by comparing to the theoretically optimized single
tions with denser interaction regions were theoretically revealed molecule geometry in the ground state, the geometry in the crystal
(Fig. 3c). Therefore, the intermolecular locking strategy solves the becomes more planar with a reduced dihedral angle between
intrinsic competition between solubility and luminescent effi- difluoroboron b-diketonate and phenyl from 12.71 to 7.531 (Scheme S3,
ciency, resulting in extraordinary excellent solubility, good film ESI†), indicating that the intermolecular hydrogen bonding
morphology, and high luminescent efficiency for high- interaction can also conformationally turn the molecules more
performance solution-processed red OLEDs.
planar for a higher luminescent efficiency and AEE phenomenon.
Electrochemical and computational investigations
Solution-processed red TADF OLEDs
The electrochemical properties of the red TADF emitters were In light of the excellent solubility and high PLQYs of the red
investigated by cyclic voltammetry (CV) using tetrabutyl- TADF molecules designed by the intermolecular locking
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the cathode, respectively; poly(3,4-ethylenedioxythiophene):poly
(styrenesulfonic acid) (PEDOT:PSS) and 8-hydroxyquinolinolato-
lithium (Liq) serve as the hole- and electron-injecting layer,
respectively; and mCP was selected as the host material, bis
[
2-(diphenylphosphino)phenyl]ether oxide (DPEPO) as the
exciton-blocking layer, and 1,3,5-tri[(3-pyridyl)-phen-3-yl]
benzene (TmPyPB) as the hole-blocking and electron-transporting
material.
Efficient red electroluminance (EL) peaking at 587 nm and
605 nm was observed in the OLEDs, suggesting the exothermic
energy transfer from mCP to the guest molecules (Fig. 5b), and
only the emission bands of the TADF emitters were observed in
light of the consistent EL emission with the PL spectra of their
doped (5 wt%) mCP films (Fig. S14, ESI†). Excitingly, these
devices exhibit impressive high performance with low turn-on
voltages (Von) of 4.7 and 4.4 V, maximum EQEs of 6.7%
Fig. 4 Experimental frontier molecular orbital energy levels of DPhCzB
and DTPAB as well as their simulated frontier orbital distributions and
overlap extents (IH/L).
and 8.2%, maximum current efficiencies (CEs) of 17.0 and
À1
À1
1
7
6.4 cd A , and maximum power efficiencies (PEs) of 5.9 and
.4 lm W for OLEDs based on DPhCzB and DTPAB, respec-
tively (Fig. 5c and d). Moreover, both devices have low efficiency
À2
strategy, solution-processed OLEDs using these red TADF roll-offs, which are as low as 3.7% and 9.0% at 100 and 1000 cd m
,
materials as emitters were fabricated in a device configura- respectively. The low efficiency roll-off could be related to the
tion of ITO/PEDOT:PSS (40 nm)/mCP:5 wt% red TADF short lifetime of excitons to avoid triplet excited state-related
emitter (50 nm)/DPEPO (10 nm)/TmPyPB (50 nm)/Liq quenching effects; DPhCzB exhibits a short lifetime of 19.5 ms,
(1 nm)/Al (100 nm) (Fig. 5a and Fig. S13, ESI†). In these while DTPAB has a lifetime of 55.7 ms (Table 1). In addition,
devices, indium tin oxide (ITO) and Al act as the anode and the strong intermolecular interactions can also facilitate the
Fig. 5 (a) Device configuration, (b) electroluminescence spectra at 7.0 V, (c) current density–voltage (open symbols) and luminance–voltage (solid
symbols) curves and (d) efficiency–current density curves of solution-processed red TADF OLEDs.
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transportation/injection of charge carriers and improve the
film stability of the emission layer, resulting in good device
stability. The maximum EQEs of the OLEDs based on the two
TADF emitters are significantly higher than the fluorescence
efficiency limit of 5%, indicating that the 75% electronically
excited triplet excitons are successfully harvested for EL by
their facile RISC in the TADF mechanism with an exciton
utilization efficiency of 39.0% and 57.7% in the DPhCzB- and
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Conclusions
In summary, we propose a new strategy, namely inter-molecular
locking, to design highly efficient red TADF emitters with
excellent solubility for high-performance solution-processed
OLEDs. Based on flexible difluoroboron b-diketonate with
exposed and easily reachable fluorine in forming hydrogen
bonds with the surrounding hydrogen atoms, two novel red
TADF molecules of DPhCzB and DTPAB were facilely prepared,
exhibiting excellent solubility in common organic solvents and
red emission beyond 630 nm with high PLQYs over 55% in neat
films due to the rigidified molecules in the solid state by
intermolecular hydrogen bonding to suppress the nonradiative
decay significantly. Using DPhCzB and DTPAB as emitters in a
simple solution-processed device structure, highly efficient red
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