Constructing host-σ-guest structures to optimize the efficiency of non-doped solution-processed OLEDs

Article abstract of DOI:10.1039/d0tc04959j

Two novel host-σ-guest emitters based on a blue emissive thermally activated delayed fluorescence (TADF) unit were designed and synthesized. By grafting 4TCzBN onto common host materials, these neat films (4TCzBN-o-PhCz and 4TCzBN-o-mCP) emit strong fluor

Full text of DOI:10.1039/d0tc04959j

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Materials Chemistry C
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Constructing host-r-guest structures to optimize
the efficiency of non-doped solution-processed
OLEDs†
Cite this: J. Mater. Chem. C, 2021,
9, 1221
Dan Liu,‡a Meng Zhang,‡a Haowen Chen,a Daiyu Ma,a Wenwen Tian,a
Kaiyong Sun,ab Wei Jiang *a and Yueming Sun*a
Two novel host-s-guest emitters based on a blue emissive thermally activated delayed fluorescence
(TADF) unit were designed and synthesized. By grafting 4TCzBN onto common host materials, these
neat films (4TCzBN-o-PhCz and 4TCzBN-o-mCP) emit strong fluorescence with 39.4% and 60.8%
photoluminescence quantum yields (PLQYs), respectively. And the oxygen-bridge linked host can not
only suppress the triplet exciton quenching between guest units, but also improve the carrier transport
ability. Non-doped OLEDs based on emitters formed by a solution process exhibit a maximum external
quantum efficiency (EQE) of 8.0%, 24.9 cd AÀ1 current efficiency (CE) and low efficiency roll off. These
results indicate that the device performance can be improved by constructing a host-s-guest structure
with the introduction of an o-bridge linked bipolar host. The strategy proposed in this paper may
provide inspiration for the design of more efficient blue solution-processable TADF emitters.
Received 19th October 2020,
Accepted 7th December 2020
DOI: 10.1039/d0tc04959j
rsc.li/materials-c
the doping concentration when mixing two materials by
vacuum deposition.10–12 Therefore, there is a great demand to
Introduction
The development of high-performance non-doped organic find new strategies to design high efficient non-doped emitters
light-emitting diodes (OLEDs) has been a great challenge in with strong emission.
the research of OLEDs because most luminescent materials are
To achieve such a goal, the molecular structure should be
unable to function efficiently in non-doped OLEDs because of rationally designed. First of all, to achieve theoretically 100%
severe concentration quenching and exciton annihilation.1–5 exciton utilization, that is, to design thermally activated delayed
For example, the conventional fluorescent and phosphorescent fluorescent molecules with a small DEST value, it is necessary to
materials almost exhibit the aggregation-caused quenching design DA-type molecules with twisted structures to reduce the
(ACQ) phenomenon due to p–p* interactions of conjugated overlap of the electron cloud of HOMO and LUMO.13–16 The
aromatic molecules or collision annihilation of long-lived tri- molecular design strategies related to small DEST values have
plet excitons of phosphors.6 TADF molecules are no exception, been reported and summarized by many researchers.17–19 The
and they also inevitably undergo triplet–triplet annihilation repetitious details will not be given here. Secondly, as the light-
(TTA) and singlet–triplet annihilation (STA) owing to the low emitting layer material in OLED devices, these emitters should
reverse intersystem crossing (RISC) rate.7 So the emitters generally fluoresce strongly in the thin film state with high PLQYs. To
have to be dispersed into host matrices to alleviate concentration achieve this, the first thing that naturally comes to mind is the
quenching.8 Correspondingly, this doping process also derives aggregation-induced emission (AIE) concept proposed by Tang
other problems such as phase separation and complex preparation et al.20–22 Correspondingly, they also proposed many methods to
process.9 Furthermore, it is very difficult to accurately control enhance the luminescence in the solid state, such as limiting the
vibration and rotation of molecules in the aggregate state.6,23–27
Combining the above two aspects, what we have to do is to design
high-performance luminescent materials with TADF and AIE
properties.28,29 However, most TADF molecules, like traditional
fluorescent phosphorescent materials, inevitably undergo an
aggregation quenching process through the TTA or STA
pathway.41 According to some literature reports, the fluorescence
quenching in the TADF molecular system mainly occurs in
triplet excitons, and its lifetime is in the ms range. These triplet
a Jiangsu Province Hi-Tech Key Laboratory for Bio-Medical Research,
Jiangsu Engineering Laboratory of Smart Carbon-Rich Materials and Device,
School of Chemistry and Chemical Engineering, Southeast University, Nanjing,
211189, China. E-mail: jiangw@seu.edu.cn, sun@seu.edu.cn
b School of Chemistry and Chemical Engineering, Yancheng Institute of Technology,
Yancheng, Jiangsu 224051, China
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
d0tc04959j
‡ Dan Liu and Meng Zhang contributed equally to this work.
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excitons undergo energy transfer through Dexter energy transfer
(DET).7 It is an intermolecular electron–exchange interaction,
and the interaction distance is about 10–15 Å.8,30 Therefore, the
short-range nature of DET prompts us to make appropriate
molecular modifications to inhibit the DET quenching process.
Based on the above ideas, we propose a new strategy to design
novel solution processable TADF molecules 4TCzBN-o-PhCz and
4TCzBN-o-mCP, which using o-bridges to connect fluorescent
guest fragments with host fragments. From the design perspec-
tive, we believe that the introduced host unit and peripheral
t-butyl group will increase the intermolecular distance, further to
suppress the collision quenching of triplet excitons.40 Moreover,
the host-s-guest co-system not only avoids the phase separation
phenomenon that can easily occur in physical doping, but also
improves the film-forming properties and the stability. At the
same time, the energy transfer from the intramolecular host to the
guest will also limit the exciton quenching between the guest
units, which is also confirmed by the experimental results.
4TCzBN-o-mCP shows excellent blue emission with the TADF
phenomenon and higher PLQY. The non-doped solution-
processed OLEDs based on 4TCzBN-o-mCP demonstrate improved
device performance with 24.9 cd AÀ1 current efficiency (CE), 8.0%
maximum external quantum efficiency (EQE), and 19.5 lm WÀ1
luminance efficiency (LE). Obviously, they are more prominent
than devices based on traditional TADF small molecule 4TCzBN.
These results prove the feasibility of host-s-guest molecules and
shed light on the development of new TADF molecules for
efficient non-doped solution processed OLEDs.
Fig. 1 Optimized geometries and electron cloud distribution maps of
4TCzBN-o-PhCz and 4TCzBN-o-mCP.
As shown in Fig. 1, 4TCzBN-o-PhCz and 4TCzBN-o-mCP have
separated distribution of frontier orbitals. The highest occu-
pied molecular orbitals (HOMOs) are predominantly centered
on tert-butyl carbazole, while the lowest unoccupied molecular
orbitals (LUMOs) are located on cyanobenzene. The s-bonded
phenylcarbazole (PhCz) and 3,5-diphenylcarbazole (mCP) units
hardly contribute to the HOMO and LUMO, which indicates
that o-bridge linked host fragments do not change the emission
wavelength of the guest unit, but play a role in promoting the
electron and hole transport. Similar spatial distributions of
HOMOs and LUMOs are also observed for 4TCzBN.8 The
HOMO and LUMO of these compounds are spatially well
separated and small DEST can be anticipated. The calculated
HOMO, LUMO, S1 and T1 values are listed in Table S1 (ESI†).
According to Table S1 (ESI†), the o-bridge PhCz and mCP units
can slightly reduce the values of DEST, which are beneficial to
achieve efficient thermally activated delayed fluorescence.31
Results and discussion
1. Materials and synthesis
Detailed synthesis of 4TCzBN-o-PhCz and 4TCzBN-o-mCP is
demonstrated in the ESI.† As shown in Scheme 1, 4TCzBN-o-
PhCz and 4TCzBN-o-mCP were synthesized via the nucleophilic
reaction between the 4TCzBN and PhCz (mCP) derivate
moieties. The chemical structures were confirmed by nuclear
magnetic resonance (NMR) and mass spectroscopy.
2. Theoretical calculations
Theoretical calculations were first carried out to simulate the
molecular structures and electronic transition properties.
3. Photophysical properties
The UV-Vis absorption and photoluminescence (PL) spectra of
both new TADF emitters were investigated at room tempera-
ture. In toluene, both 4TCzBN-o-PhCz and 4TCzBN-o-mCP
showed absorption peaks at around B340 nm and B410 nm.
The strong absorption peaks at B340 nm are ascribed to the
p–p* transition. The relatively low-energy absorption peaks at
400–420 nm are derived from the charge-transfer (CT) transi-
tion from carbazole to cyanobenzene. The energy gaps (Eg) were
calculated using maximum absorption edges and the values
matched well with the simulation results. The maximum PL
peak of 4TCzBN-o-PhCz and 4TCzBN-o-mCP in 10À5 M toluene
Scheme 1 Synthetic routes of 4TCzBN-o-PhCz and 4TCzBN-o-mCP.
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Fig. 2 The UV-Vis, fluorescence and phosphorescence spectra of 4TCzBN-o-PhCz (a) and 4TCzBN-o-mCP (b), where the Phos spectra were recorded
in toluene at 77 K with 10 ms delayed time. (c) Plots of Stokes shift versus orientation polarization (Df) of the solvent. (d) Transient PL decay spectra in neat
films measured at room temperature for 4TCzBN-o-PhCz and 4TCzBN-o-mCP.
was observed at 456 nm, which is the same as that of 4TCzBN. 474 nm and 471 nm for 4TCzBN, 4TCzBN-o-PhCz and 4TCzBN-
Fig. 2 and Fig. S1 (ESI†) also show the Phos spectra in a frozen o-mCP. 4TCzBN-o-mCP showed the same emission wavelength
toluene matrix at 77 K. The phosphorescence (Phos) spectra of as 4TCzBN even if a polar host matrix was introduced. It is
4TCzBN, 4TCzBN-o-PhCz and 4TCzBN-o-mCP are well resolved mainly because the sterically shielded fragments decrease the
and show characteristic vibrational structures, and the peaks intermolecular interaction and p–p stacking.33 The red-shifted
are at 473 nm, 477 nm, and 473 nm, respectively. The values of emission and increased DEST value of 4TCzBN-o-PhCz can be
the S1 and T1 energy levels of these compounds measured from interpreted as the rotation and the polarity of the introduced
the onset of the steady PL/Phos spectra are all 2.90/2.80 eV, PhCz unit, and it was further confirmed by the solvatochromic
2.94/2.83 eV, and 2.90/2.82 eV, respectively.32 Thus, the experi- effect measurements.
mental DEST values are 0.10 eV, 0.11 eV and 0.08 eV for 4TCzBN,
Furthermore, to confirm the better structural stability in the
4TCzBN-o-PhCz and 4TCzBN-o-mCP, respectively (Table 1). The host-s-guest structure, we recorded the absorption and emission
smaller DEST value of 4TCzBN-o-mCP can lead to fast spin–flip spectra in solvents with different polarities. As can be seen from
and efficient RISC due to the stronger steric hindrance intro- Fig. S1 and S2 (ESI†), no significant solvent polarity dependent
duced by mCP. The emission and PLQY of their neat films also spectral shifts in the absorption spectra were observed for these
support the opinion; the maximum wavelengths are 471 nm, emitters. This indicates that, in their ground states, these
Table 1 Basic photophysical and electrochemical parameters of 4TCzBN, 4TCzBN-o-PhCz and 4TCzBN-o-mCP
HOMOj [eV]
LUMOk [eV]
a
bcde
f
gh
i
Materials
Td, Tg
labs [nm]
lem
[nm]
Eg [eV]
S1/T1 [eV]
DEST [eV]
4TCzBN
4TCzBN-o-PhCz
4TCzBN-o-mCP
448, —
421, —
462, 223
341, 416
339, 409
338, 414
456, 472, 486, 471
456, 473, 486, 474
456, 470, 483, 471
2.80
2.84
2.81
2.90/2.80
2.94/2.83
2.90/2.82
0.10
0.11
0.08
À5.24
À5.20
À5.21
À2.44
À2.36
À2.40
a
b
c
d
Measured in toluene solution at 300 K. Measured in toluene solution. Measured in CH2Cl2 solution. Measured in ethanol solution.
Measured in films at 300 K. Estimated from the absorption edges in toluene. Calculated from the onset of the fluorescence spectra in toluene
e
f
g
h
i
at 300 K. Estimated from the onset of the phosphorescence spectra in toluene at 77 K. The difference between S1 energy and T1 energy.
j
k
Determined by the onset of the oxidation curves. EHOMO = Eref À Eox
.
Calculated from the energy gap and HOMO.
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chromophores are not significantly stabilized by solvation due
In order to research the photophysical variation trend of
to a relatively small dipole moment. Conversely, as shown in these compounds from the dispersed state to the aggregated
Fig. S1 and S2 (ESI†), strong solvatochromic red-shifts of the state, we studied the emission spectra of 4TCzBN, 4TCzBN-o-
emission spectra were observed for compounds 4TCzBN, PhCz and 4TCzBN-o-mCP under mixing volume ratios of solu-
4TCzBN-o-PhCz and 4TCzBN-o-mCP with increasing solvent tions containing water and tetrahydrofuran (THF); the fluores-
polarity. This demonstrates that the emission originates from cence intensities of these compounds showed interesting
the charge transfer (CT) states. Therefore, the maximum red change (Fig. 3). The PL intensity gradually decreased as the
shifts observed in the emission spectra when increasing the volume fractions of water (fw) increased until fw = 50% (4TCzBN)
polarity were 44 nm, 44 nm and 40 nm for 4TCzBN, 4TCzBN-o- or 60% (4TCzBN-o-PhCz and 4TCzBN-o-mCP). Subsequently, the
PhCz and 4TCzBN-o-mCP, respectively. The Stokes shift (na À nf) PL intensity sharply increased with the formation of nano-
can be calculated using the biggest absorption peaks and the particles owing to the continuing increased water ratios. The
emission peaks of the compounds, as summarized in Table S2 trend can be interpreted by the twist intramolecular charge
(ESI†). According to the plots of (na À nf) vs. Df, all emitters obey transfer (TICT) effect.35,36 Before the generation of aggregates,
a linear relationship with notable positive slopes (Fig. 2c).34 solvatochromism accounts for the red-shift of the emission peak
Compared to 4TCzBN and 4TCzBN-o-PhCz (Fig. S1, ESI†), the with increasing solvent polarity. In contrast, after the generation
slopes of 4TCzBN-o-mCP is even smaller; these observations of nano-aggregates, the intramolecular rotation is restricted and
clearly indicated that these compounds exhibited smaller dipole the local environment becomes less polar, thereby resulting in a
moment change (Dm) between the ground states and the blueshift in the emission color. It is worth mentioning that the
excited states. And then the dipole moments of the three maximum-wavelength variation of 4TCzBN-o-mCP in different
compounds were calculated using the Lippert–Mataga equation, water fractions is not obvious. It also indicated that introducing
and 4TCzBN-o-mCP shows the smallest change in dipole the mCP host unit can stabilize the molecular configuration in
moment, followed by 4TCzBN, and 4TCzBN-o-PhCz shows the excited states and this twist molecular structure can suppress the
largest change, which is consistent with the simulation calcu- p–p* stacking further to inhibit concentration quenching.
lated data. It is because the introduced large steric hindrance
unit mCP can stabilize the excited state configuration to a measured. As can be seen from Fig. 2d, prompt and delayed
certain extent. components were clearly observed from the transient PL decay
The transient PL characteristics of these emitters were
Fig. 3 PL spectra and emission images of 4TCzBN (a), 4TCzBN-o-PhCz (b) and 4TCzBN-o-mCP (c) in THF–water mixtures (10À5 M) with different
fractions of water, respectively, under 365 nm UV irradiation. (d) The plots of the fluorescence ratio (I/I0) and peak wavelength versus the water volume
fraction, where I0 is the initial fluorescence intensity.
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curves of both emitters. The behaviors confirmed that these stability of these emitters was investigated (Fig. S3, ESI†) and
compounds were TADF materials. The PLQY also was measured these compounds show good thermal stability with decomposi-
to evaluate the superiority of the molecular design. 4TCzBN-o- tion temperatures in the range of 421–462 1C and a high glass
mCP has a higher PLQY with 60.8% than 4TCzBN-o-PhCz transition temperature (Tg) in the range of 223–322 1C, in which
(39.4%) in the neat film state. The fluo decay rate (kF) and the the Tg of 4TCzBN is reported at 322 1C.8 Such excellent thermal
rate of RISC (kRISC) were also calculated and are summarized in properties readily endow OLEDs based on them with high
Table S3 (ESI†). The introduction of mCP units induces lower morphological stability, favorable for the OLED stability during
knr, and higher kRISC and fRISC. Therefore, the introduction of operation. Moreover, the AFM picture of these neat films is
the mCP host increases the luminescence intensity owing to shown in Fig. S4 (ESI†), and the small root mean square (RMS)
smaller DEST and suppressed non-radiative deactivation.
roughness show good film-forming properties and no phase
separation, which are beneficial to fabricate non-doped
solution-processed OLEDs.37–39 It also indicated that the solu-
bilized tert-butyl and nonconjugated oxygen bridges can
improve the morphological stability of neat films.
4. The electrochemical and thermal properties
The practical energy levels of the luminogens are evaluated by
cyclic voltammetry (CV) measurements. As can be seen from
Fig. S3 (ESI†), all the compounds show similar oxidation
processes. The HOMO energy levels of 4TCzBN, 4TCzBN-o-
5. The electroluminescence properties
PhCz and 4TCzBN-o-mCP are estimated to be À5.24, À5.20 Inspired by the high thermal stability and good film forming
and À5.21 eV, respectively. The shallower HOMO levels in the ability of the host-s-guest structure, simple double-layer non-
host-s-guest structure are attributed to the stronger donor doped solution-processed OLEDs were fabricated with an archi-
ability owing to the introduction of o-bridge host units PhCz tecture of ITO/PEDOT:PSS (40 nm)/4TCzBN (Device A), 4TCzBN-
and mCP. The LUMO values were calculated as À2.44, À2.36 o-PhCz (Device B) or 4TCzBN-o-mCP (Device C) (50 nm)/PO-T2T
and À2.40 eV for 4TCzBN, 4TCzBN-o-PhCz and 4TCzBN-o-mCP (40 nm)/Cs2CO3 (2 nm)/Al (Fig. 4a). where poly(3,4-ethyl-
by the equation ELUMO = EHOMO + Eg, where the energy gap (Eg) enedioxythiophene) doped with poly-(styrenesulfonate) (PEDOT:
is obtained using the absorption edges. Overall speaking, the PSS) served as a hole-injecting layer and 2,4,6-tris[3-(diphenyl-
HOMO and LUMO levels of these compounds are similar and phosphinyl)phenyl]-1,3,5-triazine (PO-T2T) acted as an electron
the introduction of host fragments hardly affects the energy transporting layer (ETL). Fig. 4 shows the current density–
level of the guest emitters. This is in line with the original voltage–luminance (J–V–L) characteristics, the EL spectra, and
intention of molecular design. Furthermore, the thermal the EQEs versus luminance curves of Devices A–C. The
Fig. 4 (a) Energy-level arrangement of the organic materials used in the OLED devices; (b) current density–voltage–luminance (J–V–L) characteristics
of devices A–C; (c) current efficiencies versus brightness and the external quantum efficiency versus brightness; and (d) electroluminescence spectra at
10 V.
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Table 2 Electroluminescent performances of non-doped solution-processed OLEDs based on 4TCzBN, 4TCzBN-o-PhCz and 4TCzBN-o-mCP
emitters
CEe
Lmax
CIE (x,y)g
a
b
c
d
f
Device
Emitter
Von
EL (nm)
CEmax
PEmax
EQEmax
A
B
C
4TCzBN
4TCzBN-o-PhCz
4TCzBN-o-mCP
3.9
3.7
3.5
484
496
484
6.7
18.4
24.9
4.7
11.6
19.5
2.1
5.9
8.0
6.1/5.0
16.7/12.1
24.2/14.3
2000
4300
7000
(0.19,0.30)
(0.22,0.39)
(0.18,0.31)
a
b
c
d
Von = turn-on voltage at 1 cd mÀ2
.
CEmax = maximum current efficiency. PEmax = maximum power efficiency. EQEmax = maximum external
quantum efficiency. Current efficiency at luminances of 100 cd mÀ2 and 1000 cd mÀ2
. Lmax = maximum luminance. CIE = the Commission
e
f
g
Internationale de L’Eclairage coordinates.
characteristic data for all devices are summarized in Table 2. The feasibility and versatility of the proposed molecular design, and
turn-on voltages of Devices B (3.7 V) and C (3.5 V) are signifi- it will be of high significance for the advancement of solution-
cantly lower than the corresponding Device A (3.9 V). The processed OLEDs.
superior electroluminescent performance of 4TCzBN-o-mCP is
attributed to the better matched HOMO and LUMO energy levels
that reduce the hole and electron injection barriers of Device C.
Conflicts of interest
Device C exhibits considerably high luminance and current
There are no conflicts to declare.
density at the same bias. A maximum current efficiency (Zcmax
)
of 24.9 cd AÀ1, a maximum power efficiency (Zp,max) of
19.5 lm WÀ1, and a maximum external quantum efficiency
(Zext,max) of 8.0% were achieved. It is worth mentioning that
these devices exhibit very high color stability at the elevated
operating temperatures which is one of the prerequisites for
practical application. Moreover, these devices show rather low
efficiency roll-off values than that based on 4TCzBN. The non-
doped devices show blue emission with emission peaks at
484 nm, 496 nm, and 484 nm for 4TCzBN, 4TCzBN-o-PhCz
and 4TCzBN-o-mCP, respectively. This is mainly because the
introduction of PhCz fragments increased the intramolecular
movement. It has been confirmed by the solvation effect
measurement and theoretical simulation. The results with the
same emission but higher device efficiency of 4TCzBN-o-mCP
further illustrates that the introduction of the host can suppress
the collision quenching between excitons. Therefore, it is
believed that the molecular design can optimize non-doped
solution-processed OLEDs.
Acknowledgements
We are grateful for the grants from the National Natural Science
Foundation of China (21875036 and 21801216), the Fundamental
Research Funds for the Central Universities (2242019K40150).
We are also thankful for the support from the Scientific Research
Foundation of Graduate School of Southeast University
(YBPY1975), the Priority Academic Program Development of
Jiangsu Higher Education Institutions, and Nanjing Science and
Technology Committee.
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