Simultaneous harvesting of triplet excitons in OLEDs by both guest and host materials with an intramolecular charge-transfer feature via triplet-triplet annihilation

Article abstract of DOI:10.1039/c5tc00779h

A red naphthalimide derivative with an intramolecular charge-transfer (ICT) feature, namely (E)-2-(4-(t-butyl)phenyl)-6-(2-(6-(diphenylamino)naphthalen-2-yl)vinyl)-1H-benzo[de]isoquinoline-1,3(2H)-dione (NA-TNA), was designed and synthesized. Photophysical and magneto-electroluminescence (MEL) characterization results revealed that NA-TNA could harvest triplet excitons via a triplet-triplet annihilation (TTA) process in organic light-emitting diodes (OLEDs) due to the presence of a lower-lying triplet excited state with ³ππ character. Consequently, using NA-TNA as a guest compound and CzPhONI, another ICT-featured naphthalimide derivative with triplet fusion delayed fluorescence (TFDF) character as host material, a high-performance orange OLED with 6 wt% NA-TNA doped CzPhONI film as the emitting layer was acquired. The maximum current efficiency (LE_max), brightness (L_max), and external quantum yield (EQE_max) of this OLED is 7.73 cd A^-1, 31940 cd m^-2 and 5.83%, respectively, while the theoretical EQE_max of this device should not exceed 3.34%. On the contrary, the reference device with a NA-TNA doping level of 1.4 wt% showed much inferior performance, with a LE_max, a L_max, and an EQE_max of 3.19 cd A^-1, 24 900 cd m^-2 and 2.49%, respectively. The high performance of the 6 wt% NA-TNA doped device was attributed to the efficient harvesting of triplet excitons by both the guest and host materials.

Full text of DOI:10.1039/c5tc00779h

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DOI: 10.1039/C5TC00779H
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ARTICLE TYPE
Simultaneous Harvesting of Triplet Excitons in OLEDs by both Guest
and Host Materials with Intramolecular Charge-transfer Feature via
Triplet-Triplet Annihilation†
Xujun Zheng,a‡ Qiming Peng,b‡ Jie Lin,c‡ Yi Wang,a Jie Zhou,a Yan Jiao,a Yuefeng Bai,d Yan Huang,a
5
Feng Li,*b Xingyuan Liu,*c Xuemei Pu,a Zhiyun Lu*a
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
A red naphthalimide derivative with intramolecular chargeꢀtransfer (ICT) feature, namely (E)ꢀ2ꢀ(4ꢀ(tꢀ
butyl)phenyl)ꢀ6ꢀ(2ꢀ(6ꢀ(diphenylamino)naphthalenꢀ2ꢀyl)vinyl)ꢀ1Hꢀbenzo[de]isoquinolineꢀ1,3(2H)ꢀdione
10 (NA-TNA), was designed and synthesized. Photophysical and magnetoꢀelectroluminescence (MEL) charꢀ
acterization results revealed that NA-TNA could harvest triplet excitons via tripletꢀtriplet annihilation
(TTA) process in organic lightꢀemitting diodes (OLEDs) due to the presence of a lowerꢀlying triplet exꢀ
cited state with 3ππ* character. Consequently, using NA-TNA as guest compound and CzPhONI, another
ICTꢀfeatured naphthalimide derivative with triplet fusion delayed fluorescence (TFDF) character as host
15 material, a highꢀperformance orange OLED with 6 wt% NAꢀTNA doped CzPhONI film as the emitting
layer was acquired. The maximum current efficiency (LEmax), brightness (Lmax), and external quantum
yield (EQEmax) of this OLED is 7.73 cd/A, 31940 cd/m2 and 5.83%, respectively, while the theoretical
EQEmax of this device should not exceed 3.34%. On the contrary, the reference device with NA-TNA
doping level of 1.4 wt% showed much inferior performance, with LEmax, Lmax, and EQEmax of 3.19 cd/A,
20 24900 cd/m2 and 2.49%, respectively. The high performance of the 6 wt% NA-TNA doped device was
attributed to the efficient harvesting of triplet excitons by both the guest and host materials.
materials,4,12ꢀ14 although high photoluminescent (PL) efficiency
could be achieved, these compounds should disobey the Kasha's
rule, so that the internal conversion from Tn to Tm states (m<n)
could be blocked effectively.14,15 Therefore, the rational molecuꢀ
50 lar design of HLCT materials is still a great challenge.3 However,
for OLEDs based on TFDF materials, their IQEmax could reach
62.5% if triplet excitons could be upꢀconverted efficiently into
singlet ones through tripletꢀtriplet annihilation (TTA) process.8,16ꢀ
Introduction
In the past two decades, OLEDs have been regarded as the “third
generation of flat panel display” and the “next generation of lightꢀ
25 ing technology”, and have attracted much attention in both scienꢀ
tific and industrial communities.1–4 According to electroluminesꢀ
cence (EL) mechanism, OLEDs could be classified into two main
categories, namely fluorescent (FOLEDs) and phosphoresꢀ
cent ones (PhOLEDs). Although high reliability has been
30 achieved in FOLEDs, the maximum internal quantum efficiency
(IQEmax) of FOLEDs is generally limited to 25% according to
spin statistics.5 While for PhOLEDs, although high IQEmax of
100% could be achieved,6 their disadvantage lies in the relatively
high cost of the rare metal complexes.4,7
Recently, FOLEDs capable of harvesting both singlet and triꢀ
plet excitons have aroused great research interest due to their
combined high efficiency and low cost.3,4,8 Nowadays, three posꢀ
sible mechanisms have been reported for such FOLEDs, namely
thermallyꢀactivated delayed fluorescence (TADF), hybridized
24
More importantly, the harvesting of triplet excitons could be
55 realized not only by the nonꢀdoped emitters21,22 and doping guests
at relative high doping levels,17,23 but also by the host compounds
with TFDF character.24 Because the prerequisite for TFDF mateꢀ
rials is 2ET ≥ ES, which could be fulfilled by many conjugated
systems whose S1 and T1 states are both of ππ* character,25ꢀ29 the
60 rational molecular design of TFDF materials should be easier
than that of TADF and HLCT compounds.
35
Currently, most of the research works related to TFDFꢀOLEDs
are focused on the elucidation of the role TTA process plays in
the enhancement of EL efficiency;19ꢀ20 while the TFDF comꢀ
65 pounds used are generally limited to those with ππ* character.25ꢀ29
However, our recent study has revealed that ICTꢀfeatured comꢀ
pounds could also act as promising TFDF materials if they posꢀ
40 local and chargeꢀtransfer excited state (HLCT), and tripletꢀfusion
delayed fluorescence (TFDF). For TADFꢀOLEDs, although their
potential IQEmax could reach 100%,7,9–11 the conflict between
small singletꢀtriplet energy gap (ꢁEST) and high radiative rate
within TADF luminogens makes it difficult to design highꢀ
45 performance TADF materials rationally.4,11 While for HLCT
3
sess a lowest triplet excited state with ππ* character.30 Because
most of the lowerꢀbandgap fluorophores like orange and red ones
70 are DꢀπꢀA compounds with ICT feature,31ꢀ33 and the harvesting of
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triplet excitons could be actualized by TFDFꢀguests if their dopꢀ 50 PerkinꢀElmer LS55 fluorescence spectrophotometer at 298 K.
ing levels are relatively high,17,23 the development of novel lowerꢀ
bandgap ICTꢀTFDF guest fluorophores with alleviated concentraꢀ
tion quenching may give access to highꢀperformance OLEDs
with desirable batchꢀtoꢀbatch reproducibility for massive producꢀ
Phosphorescent spectra were determined on a Horiba Jobin Yvon
Fluoromaxꢀ4 spectrometer at 77 K. The transient photoluminesꢀ
cence decay characteristics of the predegassed solution samples
were recorded on a single photon counting controller FluoroHubꢀ
5
tion. Moreover, if the host compound also displays TFDF characꢀ 55 B (Horiba Jobin Yvon). The solution samples of NAꢀTNA in
ter, triplet excitons might be utilized not only by the TFDF guest
dopant, but also by the TFDFꢀhost compound through TTA proꢀ
cess followed by efficient energy transfer (ET) from host to guest
toluene and dimethylsulfoxide (5 × 10ꢀ6 mol/L) at room temperaꢀ
ture were excited at 450 nm using a NanoLEDꢀ450 as the excitaꢀ
tion light source, and the sample of NAꢀTNA in dimeꢀ
thylsulfoxide (5 × 10ꢀ6 mol/L) at 77 K was excited at 460 nm
10 materials.30 Yet to the best of our knowledge, so far no reports
could be found using TFDFꢀcharactered hosts and guests to harꢀ 60 using a xenon flashlamp as the excitation light source, and the
vest triplet excitons simultaneously in OLEDs.
emitted photons were detected by a TBX detector connected to a
TBXꢀPS power supply. Photoluminescence quantum yields
(PLQYs) of the dilute solution samples were determined using
Rhodamine B (75% C2H5OH solution, φPL = 0.69) as the standard
Inspired by our recent discovery that the ICT compound
CzPhONI (structure shown in Fig. 1) is a promising TFDFꢀhost
15 material for highꢀperformance OLED,30 and fluorophores with
good structural similarity could form efficient ET pair,34 herein, 65 under excitation of 460 nm; while the absolute PLQYs of the film
we report the design and synthesis of an ICTꢀfeatured red DꢀπꢀA
fluorophore NAꢀTNA (Fig. 1), in which diphenylamine, viꢀ
nylnaphthalene and 1,8ꢀnaphthalimide acts as electron donor (D),
20 πꢀbridge and electron acceptor (A), respectively. As both
samples were determined on a FLS 980 fluorometer (Edinburgh
Instruments Ltd.) equipped with an integrating sphere and digital
photometer. The UVꢀVis absorption spectra were measured on a
PerkinꢀElmer Lambda 950 scanning spectrophotometer. The NAꢀ
CzPhONI and NAꢀTNA are 1,8ꢀnaphthalimide derivatives, they 70 TNA/PMMA composite thin film samples were spinꢀcoated from
could form an efficient host/guest ET pair; and the presence of a
bulky diphenylamine D segment in NAꢀTNA could endow it with
suppressed intermolecular interactions. In addition, similar to
25 CzPhONI, NA-TNA also shows TFDF property. Using NAꢀTNA
their corresponding chloroform solutions with concentration of
25 mg/mL at a speed of 2000 rpm on quartz substrates; while the
NAꢀTNA/CzPhONI composite thin film samples were fabricated
via thermoꢀevaporation in vacuum, and the film thickness is 50
as guest and CzPhONI as host materials, a 6 wt% NAꢀTNA 75 nm. The PL emission spectrum comprising both the fluorescence
doped OLED with LEmax of 7.73 cd/A, Lmax of 31940 cd/m2, and
EQEmax of 5.83% was achieved, and the high performance of this
device was attributed to the concurrent harvesting of triplet exciꢀ
30 tons by the host and guest materials via TTA process.
and phosphorescence bands of NA-TNA (77 K, after 2 ꢂs delay)
was deconvoluted on the basis of the assumption that both the
two emission bands are Gaussian. Cyclic voltammetry (CV)
measurement was carried out on a PARSTAT 2273 electrochemiꢀ
80 cal workstation at room temperature in anhydrous dichloroꢀ
methane solution using tetrabutylammonium perchlorate (0.1
mol/L) as the supporting electrolyte at a scanning rate of 50
mV/s. The CV system was constructed using a threeꢀelectrode
electrochemical cell consisted of a Ptꢀwire working electrode, a
85 Ptꢀwire counter electrode, and a Ag/AgNO3 reference electrode
(0.1 mol/L in acetonitrile) under protection of nitrogen, and each
measurement was calibrated with
a ferrocene/ferrocenium
Fig. 1 Molecular structure of NA-TNA and CzPhONI.
Experimental section
(Fc/Fc+) redox system as the internal standard. Melting points
were determined on a Xꢀ6 microscopic melting point apparatus.
90 Computational method
General Information
Quantum chemical optimizations on the energy levels of NA-
TNA in the ground state and the lowest singlet and triplet excited
states were performed using the Gaussian 09 package.35 The
ground state and the lowest triplet excited state were optimized
95 using the density functional theory (DFT) with restricted BP86
and unrestricted BP86 (UBP86) at the basis set level of 6ꢀ
31G(d,p), respectively, while the lowest singlet state geometry
optimization was done by means of time dependent DFT
(TDDFT) at BP86/6ꢀ31G(d,p) level. Full geometry optimizations
100 were performed without any symmetry constraint. Also, all geꢀ
ometries were confirmed as stationary structures by the presence
of only real frequencies at the same level of theory. Transition
energies, oscillator strength and natural transition orbitals
(NTOs)36 were evaluated using TDDFT calculations at the levels
105 of M06ꢀ2X/6ꢀ311G(d,p) for the S0 and T1 states, and B3LYP/6ꢀ
311G(d,p) for the S1 state, based on their corresponding optiꢀ
35 All the commercially available chemicals were directly used
without further purification unless otherwise stated. All the solꢀ
vents were of analytical grade and were freshly distilled prior to
use. 1H NMR and 13C NMR spectra were measured using a
Bruker Avance AV ΙΙꢀ400 MHz spectrometer, and the chemical
40 shifts were recorded in units of ppm with TMS as the internal
standard. Coupling constants (J) were reported in Hertz. FTꢀIR
spectra were recorded on a PerkinꢀElmer 2000 infrared spectromꢀ
eter with KBr pellets under an ambient atmosphere. High resoluꢀ
tion MS spectra were measured with a QꢀTOF Premier ESI mass
45 spectrometer (Micromass, Manchester, UK). Thermogravimetric
analysis (TGA) and differential scanning calorimetry (DSC) were
performed on TGA Q500 and DSC Q100 instruments under niꢀ
trogen atmosphere at a heating rate of 10 ºC/min. PL emission
spectra of both solution and thinꢀfilm samples were recorded on a
2
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DOI: 10.1039/C5TC00779H
mized geometries. Considering the solvent effects, all the calculaꢀ
tions above were carried out in DMSO media within the frameꢀ
work of the polarized continuum model.37,38
To validate the ICT character of NA-TNA, UVꢀVis absorption
and PL emission spectra of NA-TNA in solvents with different
55 polarity were measured (10−6 mol/L). As shown in Fig. 2 and
Table 1, although no significant solvent effect could be observed
in its absorption spectra, the PL emission spectra of NA-TNA
display distinct positive solvatochromism. In nonpolar cyclohexꢀ
ane (CHX), NA-TNA shows a fluorescence band with vibrational
60 structure (λPLmax = 505 nm), which could be chiefly assigned to
OLED fabrication and measurements
5
Indiumꢀtin oxide (ITO) coated glass substrates were cleaned with
alcohol, acetone, and deionized water sequentially before used
and then treated by oxygen plasma for 2 min. Organic functional
layers were thermoꢀevaporated in vacuum (3 × 10–4 Pa) with a
deposition rate of 2~3 Å/s, then metallic cathode was deposited
1
the ππ* transition of NA-TNA; with increasing solvent polarity,
the emission spectrum of NA-TNA becomes broadened and
bathochromicꢀshifted (e.g., λPLmax = 540 nm in Tol; λPLmax = 608
nm in THF; λPLmax = 634 nm in DCM; and λPLmax = 687 nm in
65 DMSO), indicative of the strong ICT character of its lowest sinꢀ
glet excited state. Additionally, according to the LippertꢀMataga
plot of NA-TNA (Fig. 3), a good linear correlation between solꢀ
vent polarity parameter and emission maxima (ῦmf ax ) could be
achieved, confirming the ICT nature of its S1 state in more polar
70 environments.
10 with a rate of 2~3 Å/s at 3 × 10–3 Pa. The active area of the
OLEDs was 1 × 1 mm2. Immediately after the sample preparaꢀ
tion, the current densityꢀvoltageꢀluminance (JꢀVꢀL) characteristics
of the OLEDs were measured with a Keithley 2611 Source Meter
and PR705 Spectra Colorimeter, which can also record EL specꢀ
15 tra and Commission Internationale de l’Eclairage (CIE) coordiꢀ
nates accurately. All of the measurements were conducted in air
at room temperature. The EQEmax of the OLEDs were calculated
with a computer program on the basis of previous reported literaꢀ
ture.39
20 Transient electroluminescence decay measurement
For the transient EL experiments, a pulse generator (Agilent
8114A, 100 V/2 A) was used to apply rectangular pulse voltage
of 9 V to our devices. The pulse repetition rate was 500 Hz with
width of 20 ꢂs. The emission of the OLEDs was collected by an
25 optic fiber connected to a Hamamatsu photomultiplier (PMT)
(H10721–20) with the time resolution of 0.57 ns. The PMT was
connected to one of the channel of a digital oscilloscope (Tektroꢀ
nix DPO7104, sampling rate: 5 GS/s; resolution: 100 ꢂV) with 50
ꢃ input resistance. In order to extract the remnant and trapped
30 carriers in the OLEDs (which would recombine and produce deꢀ
layed fluorescence), an offset voltage of ꢀ4 V was inserted beꢀ
tween the voltage pulses.
Fig. 2 (a) Normalized absorption and (b) normalized fluorescence
spectra of NA-TNA in solvents with different polarity under exꢀ
citation of 460 nm. Here, CHX denotes cyclohexane; Tol denotes
75 toluene; EA denotes ethyl acetate; THF denotes tetrahydrofuran;
DCM denotes dichloromethane; and DMSO denotes dimethylꢀ
sulfoxide.
Table 1 Photophysical properties of NA-TNA in solvents with
different polarity.
Magneto-electroluminescence (MEL) measurements
NA-TNA
a
b
For the MEL measurements, the magnetic field with maximum
35 strength of 200 mT was applied parallel to the device surface
(perpendicular to the current direction). A Keithley 2612 Source
Meter was used to provide the voltage bias from one channel and
simultaneously recorded the current signals. Another channel of
Keithley 2612 was used to record the EL intensity of the OLEDs
40 collected by an optic fiber connected to a highly sensitive Hamaꢀ
matsu photomultiplier (H10721ꢀ20). During the test, the photoꢀ
multiplier was placed far away from the electromagnet to make
sure there is no magnetic field dependence on its output. The
resolution of MEL response has been tested to be 0.01%. In this
45 method, the EL signals before and after the subjection to the
magnetic field were recorded to calculate the average value of the
zeroꢀfield signals. The magnetic field effects in EL were obtained
by MEL=ꢁEL/EL=(EL(B)–EL(aver,0))/EL(aver,0). All the
measurements were carried out in ambient atmosphere without
50 encapsulation.
Solvent
f(ε)–
λabsmax
(nm)
λPLmax
(nm)
ῦmf ax
c
φPL
1/2f(n2)
(cmꢀ1)
CHX
Tol
0.110
0.127
0.282
0.308
0.319
0.374
442
443
445
446
453
505
540
605
608
634
687
19802
18519
16529
16447
15773
14556
0.46
0.34
0.28
0.26
0.06
0.00
EA
THF
DCM
DMSO
455
b
a
80 λabs: absorption maximum; λem: fluorescence maximum (λex
=
460 nm); cφPL: PL quantum yield.
Results and discussion
Excited-state properties of NA-TNA
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Fig. 3 LippertꢀMataga plot of the wave number of the emission
Fig. 5 Transient PL characteristics of NA-TNA in DMSO soluꢀ
maxima ῦmf ax of NA-TNA versus solvent polarity parameter f(εꢀn) 45 tion (5 × 10–6 M) at 77 K (λex = 460 nm).
= f(ε) – 1/2f(n2).40
Theoretical calculations
5
In Tol solution, NA-TNA displays a biexponential decay (λem
= 540 nm, τ1 = 0.22 ns, 25.0%; τ2 = 2.26 ns, 75.0%); while in
DMSO solution, its fluorescence decay could only be wellꢀfitted
by a triꢀexponential function (λem = 687 nm, τ1 = 0.08 ns, 97.9%;
τ2 = 0.45 ns, 2.0%; τ3 = 1.82 ns, 0.1%) (Fig. 4a and Table S1).
10 Taking into account that NA-TNA shows much redꢀshifted emisꢀ
sion band in DMSO (λPLmax = 687 nm), its ultraꢀlow PLQY in
DMSO (φPL~0) and much shortened lifetime could be ascribed to
its much lowered energy gap that favors the nonꢀradiative internal
conversion process.41 Nevertheless, after being cooled to 77 K,
15 the PL intensity of NA-TNA was observed to increase signifiꢀ
To better understand the properties of the excited states of NA-
TNA, quantum chemical calculations on the ground state, the
lowest singlet and triplet excited states of NA-TNA were perꢀ
50 formed using the Gaussian 09 package. As depicted in Fig. 6, for
the S0→S1 transition in the ground state of NA-TNA, the hole is
mainly delocalized on the whole DꢀπꢀA molecular skeleton of
NA-TNA, while the particle is mainly localized on the πꢀA moieꢀ
ty. Hence the S0→S1 excitation of NA-TNA should contain a
55 major part of ππ*ꢀfeatured transition with some contribution of
CT transition from D to A subunits. While in the case of the lowꢀ
est singlet state (S1), the hole and particle are predominantly loꢀ
calized on the Dꢀπ and A moieties of NA-TNA, respectively,
implying that the radiation process from S1 to S0 should be preꢀ
60 vailed with CT character. Differed from the S1 state, the hole and
particle NTOs of the lowest triplet state were both found to be
localized mainly on the πꢀA moiety, indicating that the radiation
process of NA-TNA from T1 to S0 should be predominated with
ππ* character. The calculated transition energies of both the abꢀ
65 sorption and fluorescence emission bands of NA-TNA (2.79 eV
and 1.87 eV, respectively, vide Table S2) well reproduce their
corresponding experimental values (2.73 eV and 1.80 eV), sugꢀ
gesting that these computational results are reliable.
cantly, together with drastically increased decay lifetime (λem
=
620 nm, τ1 = 0.530 µs, 62.0%; τ2 = 1.554 µs, 24.0%; τ3 = 6.369
µs, 14.0%) (Fig. 4b and Table S1). After being delayed for 2 µs, a
newly emerged emission band at longer wavelength with λPLmax
=
20 775 nm was observed (Fig. 5), which could be safely assigned to
the phosphorescence of NA-TNA. It is noteworthy that with proꢀ
longed delay time of 30 µs, although the phosphorescence band
disappeared, the emission band with λPLmax of 640 nm still could
be observed, indicative of its delayed fluorescence (DF) feature.
25 Taking into consideration that the concentration of NA-TNA is as
low as 5 × 10–6 M, the TTA process of NA-TNA in this frozen
sample should be inefficient, hence the DF should not be of Pꢀ
type, i.e., TFDF.
Accordingly, the energy levels of the 1ππ* (2.55 eV) and
30 1CT* (2.20 eV) states of NA-TNA were determined from the
onset of its fluorescence bands in nonpolar CHX and polar
DMSO, respectively;42 while the T1 energy level of NA-TNA
was estimated to be 1.96 eV from the onset of its phosphoresꢀ
cence spectrum42 through spectral deconvolution (Fig. S1). Conꢀ
35 sequently, the ∆Est of NA-TNA was calculated to be 0.24 eV.
Transition
Hole
Particle
96.2%
S0→S1
99.8%
98.2%
S0←S1
S0←T1
Fig. 6 The natural transition orbitals (isovalue surface 0.02 a.u.)
70 for S0→S1, S0←S1 and S0←T1 transitions for NA-TNA in
DMSO, derived from TDDFT calculations.
Consequently, according to these calculation results, the T1
state of NA-TNA is of 3ππ* character, while the HOMO and
LUMO of NA-TNA in its S1 state still have some overlap, indicaꢀ
Fig. 4 (a) Fluorescence decay curves of NA-TNA in N2ꢀsaturated 75 tive of the presence of a 3CT* energy level between the 3ππ* and
1CT* energy levels. Because the ꢁEST of NA-TNA is as low as
Tol solution (monitored at 540 nm) and DMSO solution (moniꢀ
3
tored at 687 nm) at room temperature (RT) (λex = 450 nm); and
40 (b) the PL decay curve of NA-TNA in DMSO solution (moniꢀ
tored at 620 nm) at 77 K (λex = 460 nm). Concentration: 5 × 10–6
M.
0.24 eV, the ππ* excitons of NA-TNA may undergo a reverse
3
internal conversion to its CT* state, followed by a reverse interꢀ
3
1
system crossing process from the CT* state to the CT* state.43
80 Hence the DF of NA-TNA in DMSO solution at 77 K might be
4
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assigned to TADF. Yet at room temperature, the T1 state of NA-
be an aggregationꢀinducedꢀemissionꢀenhancement (AIEE) luꢀ
TNA should suffer from serious nonꢀradiative relaxation due to 50 minogen. Consequently, the PL behavior of NAꢀTNA in
the relative low bandgap of NA-TNA, hence no DF was obꢀ
served.
CH3CN/water mixtures with different water fraction (fw) was
investigated (Fig. S3). The results indicated that NA-TNA is
AIEEꢀactive in CH3CN, hence the much enhanced PLQYs of
NA-TNA:CzPhONI film samples may arise from the restriction
55 of intramolecular rotation of NA-TNA in condensed state.
5
Energy transfer between NA-TNA and CzPhONI
According to these photophysical and theoretical studies, NA-
TNA should be an ICTꢀfeatured compound possessing a T1 state
Thermal and electrochemical properties
3
with ππ* character, hence analogous to CzPhONI,30 NA-TNA
may be a TFDF compound capable of utilizing triplet excitons
10 through TTA process. However, the ultraꢀlow PLQY of NA-TNA
in its thin solid film state (0.068, vide Table 2) would limit its
The thermal stability of NA-TNA was investigated by thermoꢀ
gravimetric analysis (TGA) and differential scanning calorimetry
(DSC) (diagrams shown in Fig. S4, S5). NA-TNA has a high
potential application as highꢀperformance nonꢀdoped OLED 60 decomposition temperature of 423 ºC (Td corresponds to the temꢀ
lightꢀemitter. As CzPhONI has been demonstrated to be a promisꢀ
ing TFDF host material,30 to evaluate whether CzPhONI and NA-
15 TNA could act as an efficient host/guest ET pair, so that triplet
excitons could be utilized concurrently by the host and guest
materials in OLEDs, the absorption spectrum of NA-TNA and PL
emission spectrum of CzPhONI in solid film states were recordꢀ
ed. As illustrated in Fig. 7, good spectral overlap between the
20 emission band of CzPhONI and absorption band of NA-TNA
could be observed, implying effective ET may take place between
them. Further PL characterizations on NA-TNA doped PMMA
films (Fig. S2) and NA-TNA doped CzPhONI films at different
doping levels (Fig. 7) indicated that even in the 1.4 wt% NA-
25 TNA doped CzPhONI film, no emission from CzPhONI could be
observed, implying that the ET between NA-TNA and CzPhONI
is quite efficient. The 1.4 wt% NA-TNA doped CzPhONI film
displays a high PLQY of 0.802; but with increasing NA-TNA
composition (6.0 wt%, 10.0 wt% and 15.0 wt%), the PLQY of the
30 blending films drops gradually (0.668, 0.605, and 0.336 in seꢀ
quence), which may be ascribed to the concentration quenching
of NA-TNA. Yet even at a high doping level of 10 wt%, a satisꢀ
factory PLQY of 0.605 could be achieved, indicating that the
molecular interactions in NA-TNA is not much serious, and highꢀ
35 performance heavily doped OLEDs may be achieved using NA-
TNA as guest compound.
perature at 5 wt% loss). According to DSC measurement, altꢀ
hough no distinct Tg could be identified, NA-TNA shows a high
melting point of 278 ºC. Thus NA-TNA displays good thermoꢀ
stability, which is highly desirable for the OLED applications.
65
To estimate the energy levels of the frontier orbitals of NA-
TNA, the electrochemical properties of NA-TNA has been invesꢀ
tigated by cyclic voltammetry in degassed DCM solution (5 ×
10−4 mol/L) with Fc/Fc+ redox couple as the internal standard. As
shown in Fig. S6, during the anodic scan from 0 to 0.7 V, a reꢀ
70 versible oxidation process could be observed in NA-TNA, and its
ox
1/2
E
was determined to be 0.52 V relative to Fc/Fc+. Hence by
comparison with the Fc/Fc+ redox couple whose energy level is –
4.80 eV in vacuum, the highest occupied molecular orbital (HOꢀ
MO) energy level of NA-TNA was calculated to be –5.32 eV. As
75 no reduction wave could be detected due to the limited range
available in DCM, the lowest unoccupied molecular orbital
(LUMO) energy level of –3.00 eV was deduced from the HOMO
energy level and optical bandgap of NA-TNA according to the
equation of LUMO = HOMO + Eg. As the HOMO and LUMO
80 energy levels of CzPhONI are –5.64 eV and –3.14 eV, respecꢀ
tively,30 CzPhONI could act as a suitable host for NA-TNA.
Electroluminescence properties
To evaluate whether NA-TNA is a TFDF material capable of
harvesting triplet excitons via TTA process, firstly, we fabricated
85 device I with NA-TNA as nonꢀdoped lightꢀemitting material, and
the device structure is ITO/MoO3 (1 nm)/TCTA (40 nm)/CBP (2
nm)/NA-TNA (20 nm)/ TPBI (45 nm)/ LiF (1 nm)/Al (80 nm),
where TCTA (4,4’,4’’ꢀtri(Nꢀcarbazolyl)triphenylamine) acts as
holeꢀtransporting material, TPBI (1,3,5ꢀtris(1ꢀphenylꢀ1Hꢀ
90 benzo[d]imidazolꢀ2ꢀyl)benzene) acts as electronꢀtransporting
material (energy level diagram shown in Fig. 8).
Fig. 7 Normalized fluorescence spectrum of CzPhONI, absorpꢀ
tion spectrum of NA-TNA, and fluorescence spectra of NA-TNA
40 in CzPhONI at different doping levels in solid film state (λex
=
380 nm).
It should be pointed out that at relatively low doping levels (≤
10 wt%), the PLQYs of NA-TNA doped CzPhONI films are even
higher than that of the NA-TNA solution (~46%), which should
45 be an abnormal phenomenon. After careful structural analysis, we
conjectured that in solution state, the presence of a diphenylaꢀ
mino group in NA-TNA may trigger intramolecular rotation that
exhausts the energy of the excited states,44 i.e., NA-TNA might
Fig. 8 Device configuration and energy level diagram of devices
I, II, III, and IV.
95
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Table 2 EL performance of devices I, II, III, and IV and PL maximum and PLQY of the corresponding active films.
a
b
f
Von
(V)
Lmax
LEmax
λELmax
(nm)
λPLmax
c
Device
EQEmax
CIE (x, y)d
PLQYe
(cd/m2)
(cd/A)
(nm)
635
566
575
592
I
4.2
2.6
2.8
3.2
5099
24900
31940
27640
0.53 (124, 5.6)
3.19 (5, 3.2)
7.73 (12, 3.2)
5.41 (8, 3.2)
639 (6)
570 (10)
583 (10)
608 (10)
0.59% (124)
2.49% (5)
5.83% (12)
4.14% (8)
(0.64, 0.36) (6)
(0.47, 0.50) (10)
(0.51, 0.48) (10)
(0.57, 0.43) (10)
0.068
0.802
0.668
0.336
II
III
IV
a Data in parentheses are the corresponding EL brightness (cd/m2) and driving voltages (V); b Data in parentheses are the corresponding
c
driving voltages; Data in parentheses are the corresponding EL brightness (cd/m2); d Data in parentheses are the corresponding driving
5
voltages (V); e PLQY data of the corresponding active films; f PL maximum of the corresponding active films.
The current densityꢀvoltageꢀluminance (J–V–L) and the current 45 a quite promising guest candidate. More importantly, as NA-
efficiency–current density characteristics of device I are shown in
Fig. 9a and Fig. S7, respectively. The EL spectra of device I unꢀ
der different driving voltages are shown in Fig. S8, and some
TNA and CzPhONI could form an efficient ET pair, triplet exciꢀ
tons could be harvested by both NA-TNA and CzPhONI via TTA
process in OLEDs if heavily doped NA-TNA/CzPhONI films are
used as the lightꢀemissive layer. To validate this hypothesis, we
10 representative data are summarized in Table 2. At positive bias of
6 V, device I could emit red EL with λELmax of 639 nm and CIE 50 fabricated two OLEDs with device configuration of ITO/MoO3 (1
coordinates of (0.64, 0.36). The maximum brightness (Lmax) and
current efficiency (LEmax) of device I is 5100 cd/m2 and 0.53
cd/A, respectively. Since the PLQY of the neat film of NA-TNA
15 is as low as 0.068, the EL performance of this nonꢀdoped OLED
is quite satisfactory. In fact, the EQEmax of device I is 0.59%,
which is much higher than that predicted from the 25% singlet
production limit (0.34%), indicative of the effective utilization of
triplet excitons in this device. Because magnetoꢀ
20 electroluminescence (MEL) measurement has been verified to be
an effective method to identify TTA process,45,46 here, we emꢀ
ployed MEL measurement to testify if the triplet excitons were
harvested through TTA process in device I. As shown in Fig. 9b,
at low driving voltage of 4 V (around the turnꢀon voltage), the
25 MEL of device I increases sharply within the lowꢀfield hyperfine
regime (< 20 mT), then becomes saturated with increasing magꢀ
netic field, suggesting that there is no significant TTA process in
device I. Yet with increasing applied bias hence increasing curꢀ
rent density, the MEL of device I shows a rapid increase within
30 the lowꢀfield regime (<20 mT) and a down trend in the highꢀfield
regime (>20 mT).45,46 Taking into consideration that the charge
injection within device I is ambipolar and balanced, this negative
MEL effect could be safely assigned to TTA rather than tripletꢀ
polaron interactions (TPI).47ꢀ49 Accordingly, the TTA process
35 becomes more remarkable in this device at higher bias, and NA-
TNA should be a TFDFꢀfeatured lightꢀemitting material.
nm)/TCTA (40 nm)/CBP (2 nm)/CzPhONI:NA-TNA (20 nm, x
wt%)/TPBI (45 nm)/LiF (1 nm)/Al (80 nm), where the composiꢀ
tion of NA-TNA is 1.4 wt% (device II) or 6.0 wt% (device III).
55 Fig. 10 (a) Current density–voltage–luminance (J–V–L) characꢀ
teristics of devices II, III and IV; and (b) EL spectra of devices II,
III and IV (at 10 V).
As shown in Fig. 10 and Table 2, at positive bias of 10 V, the
1.4 wt% NA-TNA-doped device II exhibits yellow emission with
60 λELmax of 570 nm and CIE coordinates of (0.47, 0.50), and the
Lmax, LEmax and EQEmax of this device is 24900 cd/m2, 3.19 cd/A
and 2.49%, respectively. While for device III with higher NA-
TNA doping level of 6.0 wt%, its λELmax is redꢀshifted to 583 nm
with CIE coordinates of (0.51, 0.48) at 10 V, which is consistent
65 with the PL spectral characteristics of the active layers (Fig. 7).
Moreover, in comparison with that of device II, the EL perforꢀ
mance of device III is drastically enhanced, with Lmax, LEmax and
EQEmax of 31940 cd/m2, 7.73 cd/A and 5.83%, respectively. It is
noteworthy that although the PLQY of 1.4 wt% NA-
70 TNA/CzPhONI doped film is higher than that of the 6.0 wt%
doped one (φPL: 0.802 vs 0.668), the EQEmax of device II is much
inferior to that of the device III (2.49% vs 5.83%). In fact, the
EQEmax of 5.83% has broken through the 25% singlet production
limit of device III (3.34%), hence more efficient triplet exciton
75 harvesting should occur in device III than device II.
Fig. 9 (a) Current density–voltage–luminance (J–V–L) characterꢀ
istics of device I (inset: the EL spectrum of device I at 6 V); and
40 (b) the MEL of device I as a function of external magnetic field
(MEL = (EL(B) – EL(0))/EL(0)).
Further MEL characterization results indicated that although
analogous to device I, devices II and III also show distinct TTAꢀ
featured MEL response, they both exhibit more apparent downꢀ
ward trend in MEL relative to device I at higher driving voltage,
80 and device III shows the highest TTA portion value among the
three devices (Fig. 9b, Fig. 11). For the nonꢀdoped device I, the
Although the low φPL of NA-TNA in neat film limits its appliꢀ
cation as a nonꢀdoped OLED fluorophore, the high PLQYs of its
heavilyꢀdoped films and the TFDF character of NA-TNA make it
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TTA process of the NA-TNA triplet excitons should be a competꢀ
itive process with the radiative and nonꢀradiative decay processes.
secondꢀscaled DF, the relative DF intensity in device III is much
higher than that in device I or II, confirming the more efficient
Since the relative low bandgap of NA-TNA would favor the nonꢀ 40 TTA process in device III.
radiative decay process, the TTA process in device I might be
less efficient. For device II, as the composition of NA-TNA is as
low as 1.4 wt%, most of the triplet excitons should be formed on
CzPhONI upon charge injection, which might undergo radiaꢀ
tive/nonꢀradiative decay, TTA, and Dexter ET process to NA-
TNA. Taking into consideration that the Dexter ET process
5
10 should be inefficient in this low dopingꢀlevel device,50 and the
higher bandgap of CzPhONI relative to NA-TNA should be adꢀ
verse to the nonꢀradiative decay of triplet excitons, the TTA proꢀ
cess for CzPhONI excitons may be more efficient, hence higher
TTA portion value was observed in device II than in device I. Yet
15 it should be pointed out that the triplet excitons of NA-TNA
formed through direct charge injection or Dexter ET in device II
could not be utilized effectively due to the low composition of
NA-TNA. In the case of device III, however, as the guest compoꢀ
sition is as high as 6 wt%, although the triplet excitons should be
20 formed mainly on CzPhONI upon direct charge injection, some
triplet excitons could also be formed on NA-TNA through charge
trapping or efficient Dexter ET.51,52 Since the composition of
CzPhONI is still as high as 94 wt%, similar to device II,
CzPhONI should also contribute to the harvesting of triplet exciꢀ
25 tons via TTA process in device III; but the triplet excitons of NA-
TNA could also be utilized via TTA process owing to the relaꢀ
tively high doping level of NA-TNA. Consequently, in device III,
both the host and the guest compounds could contribute to the
harvesting of triplet excitons, which may account for the higher
30 TTA portion value in this device.
Fig. 12 Transient EL response of devices IꢀIII (driven under recꢀ
tangular pulsed voltages of 9 V with an offset of ꢀ4 V).
In principle, at higher guest dopingꢀlevel, the triplet excitons of
45 NA-TNA formed through Dexter ET and charge trapping
processes could be utilized more efficiently via TTA process, yet
the dropped PLQY stemming from the concentration quenching
of NA-TNA should be adverse to the device efficiency. To
confirm this conjecture, we fabricated device IV with NA-TNA
50 dopingꢀlevel of 15 wt%. Despite the fact that the PLQY of the
active layer is as low as 0.336, device IV shows a relatively high
EQEmax of 4.14%, note that the 25% singlet production limit of
device IV should just be 1.68%. Hence the triplet exciton
harvesting efficiency is indeed higher in device IV than device
55 III. Nevertheless, the EQEmax of device IV is still inferior to that
of device III (4.14% vs 5.83%), which should be attributed to the
much lower PLQY of the active layer of device IV (0.336 vs
0.668). Therefore, to get OLEDs with high EQE, a tradeꢀoff
between triplet exciton harvesting efficiency and concentration
60 quenching should be achieved.
According to the energy level diagram, a possible mechanism
of the TTA and ET processes within device III is tentatively
proposed (Fig. 13). Upon charge injection, most of the singlet and
triplet excitons are formed on CzPhONI with 1CT* and 3CT*
65 character, respectively. As efficient ET could take place between
1
CzPhONI and NA-TNA, most of the CT* excitons of the host
could be transformed into the 1CT* excitons of NA-TNA through
Förster ET process. But the majority of the 3CT* triplets of
CzPhONI would be quenched by its lowerꢀlying 3ππ* state.30
Fig. 11 The MEL of device II(a) and III(b) as a function of exꢀ
ternal magnetic field (MEL = (EL(B) – EL(0))/EL(0)).
70
Figure 13. Schematic diagram for the proposed TTA and energy transfer processes in device III.
35
To gain further insights into the origin of the high efficiency of
device III, we measured the transient EL of devices IꢀIII. As
shown in Fig. 12, although all these three devices show microꢀ
Taking into consideration that the concentration of NA-TNA in
device III is 6 wt%, some ππ* excitons of CzPhONI could be
transformed into the triplet excitons of NA-TNA via Dexter ET
3
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1
process,51ꢀ52 which could be converted into ππ* excitons of NA-
Platform of Specialized Laboratory, College of Chemistry,
Sichuan University for providing NMR data for the intermediates
and objective molecules.
TNA through TTA process. While the rest 3ππ* excitons of
CzPhONI could undergo a TTA process to generate 1ππ* excitons
of CzPhONI, which could be converted to 1CT* via internal
conversion. Consequently, in device III with relatively high guest
concentration, the triplet excitons could be effectively harvested
not only by the host compound, but also by the guest compound,
resulting in high EL performance.
5
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110
a
Key Laboratory of Green Chemistry and Technology (Ministry of Eduꢀ
cation), College of Chemistry, Sichuan University, Chengdu 610064, P.
R. China. Eꢀmail: luzhiyun@scu.edu.cn;
b State Key Laboratory of Supramolecular Structure and Materials, Jilin
40 University, Changchun 130012, P. R. China. Eꢀmail:lifeng01@jlu.edu.cn;
c
State Key Laboratory of Luminescence and Applications, Changchun
Institute of Optics, Fine Mechanics and Physics, Chinese Academy of
Sciences, Changchun 130033, P. R. China. Eꢀmail: liuxy@ciomp.ac.cn;
d College of Chemistry and Materials Science, Sichuan Normal Universiꢀ
45 ty, Chengdu 610068, P. R. China.
† Electronic Supplementary Information (ESI) available: synthetic
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Journal of Materials Chemistry C
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DOI: 10.1039/C5TC00779H
Table of contents
Triplet excitons could be harvested simultaneously by both guest and host materials with intramolecular charge-transfer feature via tri-
plet-triplet annihilation in heavily–doped OLED.
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