High triplet energy exciton blocking materials based on triphenylamine core for organic light-emitting diodes

Article abstract of DOI:10.1016/j.orgel.2016.12.002

A series of novel high triplet energy materials have been designed and synthesized from the simple starting compounds through a simple one-step Friedel–Crafts reaction by using triphenylamine and methoxy, fluoro substituted diphenylmethanoles and triphenylmethanol as the starting materials. The synthesized compounds exhibit the ionization potentials in an interval of 5.4–5.7?eV in the solid state, the wide bang-gaps of 3.6?eV and the high triplet energies of about 3.0?eV. The photophysical properties have been confirmed by DFT. The introduction of a material with the lowest ionization potential as the high triplet energy exciton blocking thin layer of the green organic light-emitting diode doubled the quantum efficiency of the device. The best fabricated green device exhibited the maximum current, power, and external quantum efficiencies of 80.1?Cd?A^?1 and 31.4?Lm?W^?1, 23.2%, respectively. The triplet-triplet annihilation and triplet-polaron quenching effects for the devices without and with exciton blocking layer have been analyzed.

Full text of DOI:10.1016/j.orgel.2016.12.002

Accepted Manuscript
High triplet energy exciton blocking materials based on triphenylamine core for
organic light-emitting diodes
Gintautas Bagdziunas, Mindaugas Surka, Khrystyna Ivaniuk
PII:
S1566-1199(16)30533-X
10.1016/j.orgel.2016.12.002
ORGELE 3851
DOI:
Reference:
To appear in: Organic Electronics
Received Date: 14 October 2016
Revised Date: 26 November 2016
Accepted Date: 1 December 2016
Please cite this article as: G. Bagdziunas, M. Surka, K. Ivaniuk, High triplet energy exciton blocking
materials based on triphenylamine core for organic light-emitting diodes, Organic Electronics (2017), doi:
10.1016/j.orgel.2016.12.002.
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ACCEPTED MANUSCRIPT

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High Triplet Energy Exciton Blocking Materials Based on Triphenylamine Core for
Organic Light-Emitting Diodes
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Gintautas Bagdziunasa,b*, Mindaugas Surkab, Khrystyna Ivaniukb,c
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a Department of Material Science and Electrical Engineering, Center for Physical Sciences and
Technology, Sauletekio av. 3, Vilnius, LT-10257, Lithuania
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Santaka Valley, Kaunas University of Technology, K. Barsausko str. 59, LT-51423, Kaunas,
Lithuania
c Lviv Polytechnic National University, S. Bandera 12, 79013 Lviv, Ukraine
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*Corresponding author:
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E-mail address: gintautas.bagdziunas@gmail.com (G. Bagdziunas)
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Abstract
A series of novel high triplet energy materials have been designed and synthesized from the
simple starting compounds through a simple one-step Friedel–Crafts reaction by using
triphenylamine and methoxy, fluoro substituted diphenylmethanoles and triphenylmethanol as
the starting materials. The synthesized compounds exhibit the ionization potentials in an interval
of 5.4-5.7 eV in the solid state, the wide bang-gaps of 3.6 eV and the high triplet energies of
about 3.0 eV. The photophysical properties have been confirmed by DFT. The introduction of a
material with the lowest ionization potential as the high triplet energy exciton blocking thin layer
of the green organic light-emitting diode doubled the quantum efficiency of the device. The best
fabricated green device exhibited the maximum current, power, and external quantum
efficiencies of 80.1 Cd A-1 and 31.4 Lm W-1, 23.2%, respectively. The triplet-triplet annihilation
and triplet-polaron quenching effects for the devices without and with exciton blocking layer
have been analyzed.
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Keywords: Triphenylamine, Exciton blocking, OLED, High triplet energy, Triplet-triplet
annihilation, Triplet-polaron quenching
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1. Introduction
An organic light-emitting diode (OLED) is one of the most promising low-cost full-color
technology for the thin displays and devices, as well as solid-state lighting
[1,2]. However, the external quantum efficiency (EQE) strongly depends on a current density
(described as “efficiency roll-off”) [3]. This phenomenon have been investigated in numerous
studies and the main mechanisms responsible for the decreasing in the efficiency at high
luminescence have been proposed such as triplet-triplet annihilation (TTA)[4], triplet-polaron
quenching (TPQ)[5] and dissociation of the excitons into free carriers[6]. Moreover, the excitons
can be quenched at an interface between the low triplet energy hole/electron transporting and the
emitting layers[7]. Thus, the problems of low EQE can be resolved by design of the hole or
electron transport type exciton blocking materials which could reflect the singlet and triplet
excitons back into the emitting layer.
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The stability and lifetime of OLEDs are most important criteria in the lighting technology.[8]
The charge balance and the position of a recombination zone are the critical internal degradation
factors of devices. Using the high triplet energy hole[9] and electron[10] transporting or charge
and exciton blocking[8] layers are the ways to prevent a degradation of emitting layer. On the
other hand, three important criteria should be considered in the design of the exciton blocking
materials for the high effectiveness OLEDs are a high first excited triplet energy (>2.7 eV) of the
material, the suitable values of the ionization and/or electron affinity levels for holes and/or
electrons transporting to the emitting layer and a bond photochemical dissociation energy of the
chemical moieties included in the molecular structure.[11] Therefore, the stable organic
materials based on the carbazole[12,13], dibenzothiophene/furan[14], acridane[15] moieties and
perfluorinated phenylenes[16] worked very well as the exciton blocking materials.
In this work, four novel high triplet energy materials have been designed and synthesized from
the simple starting compounds by using the one-step Friedel–Crafts reaction conditions. The
material with the lowest ionization potential has been used as an exciton blocking layer in the
green emitting OLED. This strongly blocking material significantly increases EQE of the device.
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2. Experimental
2.1. General information
Materials: Triphenylamine (TPA, Aldrich), 98%, boron trifluoride diethyl etherate (BF3ꢀOEt2,
Aldrich), benzophenone (Aldrich), 98%, 4,4'-difluorobenzophenone (TCI), 99%, 4,4'-
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dimetoxibenzophenone (Aldrich), 97%, triphenylmethanol (Reachim), 98%, sodium borohydride
(Aldrich), 98%, tris(4-carbazoyl-9-ylphenyl)amine (TCTA, Aldrich), 97%, 4,4′,4′′-
tris[phenyl(m-tolyl)amino]triphenylamine (m-MTDATA, Aldrich), 99%, 4,7-diphenyl-1,10-
phenanthroline (Bphen, Aldrich), 97% and tris[2-phenylpyridinato-C2,N]iridium(III) (Ir(ppy)3,
Aldrich), 99% were used without further purification in this work. The methanol and
dichloromethane (DCM) solvents were distilled over sodium and calcium hydride under the
argon atmosphere, respectively. All reactions and a purity of the synthesized compounds were
monitored by a thin layer chromatography (TLC) using the Silica gel 60 F 254 aluminum plates
(Merck).
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1H and 13C NMR spectroscopy was carried out on a Bruker Avance 400 NMR spectrometer. The
chemical shifts are reported in units ppm (parts per million). The residue signals of the solvents
were used as the internal standards (1H NMR: internal standard CDCl3, 7.26 ppm, 13C NMR:
77.16 ppm). The mass spectrometry (MS) data were recorded on UPLC-MS Acquity Waters SQ
Detector 2. The UV/Vis absorption spectra were recorded with Avantes AvaSpec-2048XL
spectrometer. The photoluminescence spectra were recorded by an Edinburgh Instruments
FLS980 spectrometer. The cyclic voltammetry (CV) measurements were carried out with a
glassy carbon working electrode in a three electrode cell in the dry DCM solutions containing 0.1
M tetrabutylammonium hexafluorophosphate as an electrolyte at the room temperature under
argon atmosphere at a 100 mV s-1 potential rate. The electrochemical cell comprised platinum
wire with 1 mm diameter of working area as a working electrode, Ag wire calibrated versus
ferrocene/ferrocinium redox couple as a quasi-reference electrode and platinum coil as an
auxiliary electrode. The electron photoemission spectra were recorded from the thin solid films
of the materials on the indium tin oxide (ITO) coated glass substrates under negative voltage of
300 V with illumination by a deep UV deuterium light source ASBN-D130-CM and CM110
1/8m monochromator and A 6517B Keithley electrometer connected to the counter-electrode.
Thermogravimetric analysis (TGA) was performed on a Metter TGA/SDTA851e/LF/1100
apparatus at a heating rate of 20 °C min-1 under the nitrogen atmosphere. The differential
scanning calorimetry (DSC) measurements were done on a DSC Q 100 TA Instrument at a
heating rate of 10 °C min-1 under the nitrogen atmosphere.
2.2. Organic synthesis
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Diphenylmethanoles were synthesized via a standard reduction of ketones procedure with
sodium borohydride in methanol at 0oC. The purity of the products was checked using TLC
method.
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General procedure for the synthesis of tris(4-benzhydrylphenyl)amines (1-3 and 4): Under argon
atmosphere, a corresponding diphenylmethanole (3.5 mmol) and triphenylamine (1.0 mmol)
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were dissolved in 10 ml dry dichloromethane. After cooling the solution to 0 C, boron
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trifluoride diethyl etherate (3.5 mmol) was added into the mixture. After stirring for 24 hours at
room temperature, it was quenched with a saturated solution of sodium hydrogen carbonate
(about 20 ml) and the solution was extracted using dichloromethane (3x10 ml). The organic layer
was dried with sodium sulphate, and separated by filtration. Celite® was added in this solution
and evaporated. The mixture was purified by a column chromatography on silica gel using a
mixture of dichloromethane and hexane as eluent. A product was crystallized from an
ethylacetate and methanol mixture to afford 1-4 as white solids.
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Tris(4-benzhydrylphenyl)amine (1): Yield 81 %.
1H NMR (400 MHz, CDCl3): 5.49 (s, 3H, C-H), 6.69-6.89 (m, 6 H, Ar-H), 6.97-7.00 (m, 6H, Ar-
H), 7.15-7.34 (m, 30H, Ar-H). 13C NMR (101 MHz, CDCl3): 56.4, 123.9, 126.4, 128.4, 128.5,
129.5, 130.2, 144.2, 146.0. MS (ES+), m/z: 743.8 (M+), calc. 743.4 (C57H45N).
Tris(4-(bis(4-methoxyphenyl)methyl)phenyl)amine (2): Yield 72 %.
1H NMR (400 MHz, CDCl3): 3.79 (s, 18 H, OMe), 5.39 (s, 3H, C-H), 6.82-6.86 (m, 12 H, Ar-H),
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6.94-7.00 (m, 12H, Ar-H), 7.03-7.07 (m, 12H, Ar-H). C NMR (101 MHz, CDCl3): 54.7, 55.3,
113.7, 123.8, 130.0, 130.3, 136.8, 138.8, 145.9, 158.1. MS (ES+), m/z: 923.8 (M+), calc. 923.4
(C63H57NO6).
Tris(4-(bis(4-fluorophenyl)methyl)phenyl)amine (3): Yield 82 %.
1H NMR (400 MHz, CDCl3): 5.45 (s, 3H, C-H), 6.67-6.76 (m, 6H, Ar-H), 6.80-6.84 (m, 6H, Ar-
H), 6.92-7.09 (m, 24H, Ar-H). 13C BMR (101 MHz, CDCl3): 54.9, 115.4, 124.0, 130.1, 130.9,
137.9, 139.7, 146.1, 161.6 (d, J= 246 Hz). 19F NMR (376 MHz, CDCl3): -116.5. MS (ES+), m/z:
851.3 (M+), calc. 851.3 (C57H39F6N).
N-Phenyl-4-trityl-N-(4-tritylphenyl)aniline (4): Yield 44 %.
1H NMR (400 MHz, CDCl3): 6.95-6.99 (m, 4H, Ar-H), 7.00-7.03 (m, 2H, Ar-H), 7.04-7.08 (m,
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5H, Ar-H), 7.21-7.26 (m, 32H, Ar-H). C BMR (101 MHz, CDCl3): 64.6, 122.4, 122.8, 124.60,
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125.1, 126.0, 127.5, 131.3, 132.0, 140.8, 141.0, 145.4, 147.0. MS (ES+), m/z: 729.4 (M+), calc.
729.3 (C56H43N).
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2.3. Details of DFT calculations
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The geometries of 1-4 and TPA were optimized by the B3LYP functional and 6-31G(d,p) basis
set in vacuum at room temperature followed by calculations of their harmonic vibrational
frequencies to verify their stability. The energies of the lowest singlet (S1) and triplet (T1) excited
states were obtained by TD-DFT with Tamm-Dancoff approximation (TDA)[17] using the
B3LYP/6-31+G(d) method in vacuum. All DFT calculations were done with a Spartan’14
program [18].
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2.4. Device fabrication and measurements
The devices were fabricated employing a physical vapor deposition (PVD) process under the
vacuum higher than of 3·10−6 Bar. The host:guest emission layers were deposited by co-
deposition of host (m/m 90%) and guest (m/m 10%) materials from two different sources.
Deposition rate of Ir(ppy)3 was adjusted to 0.15 Å s-1, while the deposition rate of TCTA was
adjusted to 1.5 Å s-1, respectively. The deposition rates for calcium and aluminum were kept at
around 5 Å s-1. The indium tin oxide (ITO) with a sheet resistance of 70-100ꢁ/sq was used as the
anode. The active area of the obtained devices was of 3×6 mm2. The current density–voltage
characteristics of the devices were recorded employing 2400 series SourceMeter (Keithley). The
current density–luminance characteristics were estimated using a calibrated silicon photodiode
with the 6517B electrometer (Keithley) in air without passivation immediately after the
formation of the device. The electroluminescence (EL) spectra were recorded employing an
AvaSpec-2048XL spectrophotometer (Aventes). The external quantum, current and power
efficiencies were calculated utilizing the luminance, current density, and EL spectra as reported
earlier [19].
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3. Results and discussion
3.1. Synthesis and characterization
As well as it is known that TPA has a high triplet energy of about 3.0 eV and a low-lying
ionization potential (IP) level[20]. Therefore, a series of novel star shaped 1-4 compounds was
synthesized from tiphenylamine and diphenylmethanoles as the starting compounds through the
one pot Friedel–Crafts reaction conditions by employing a BF3ꢀOEt2 reagent as Lewis acid in
good yields of 70-80 %. (Scheme 1) The molecules 1-3 have the triphenylamine (TPA) core and
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diphenylmethane moieties with methoxy and fluoro electron donor and acceptor groups,
respectively. However, compound 4 was obtained with two trityl groups due to a low solubility
of an intermediate with one trityl group in DCM. All final products were purified by
crystallization to obtain the pure materials for the subsequent measurements and devices
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fabrication. The chemical structures of 1–4 were confirmed by H, 13C and 19F (for 3) NMR
spectroscopy and mass spectrometry.
< Insertion Scheme 1>
Scheme 1.
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3.2. Thermal properties
The thermal transitions of the compounds 1-4 were studied by differential scanning calorimetry
(DSC) (Fig. 1a) and thermogravimetric analysis (TGA) under the nitrogen atmosphere (Fig. 1b).
Among these novel compounds, 2 and 3 possessed the highest thermal stability with a
decomposition temperature (Td5%, corresponding to 5% weight loss) of 438 and 407 °C,
respectively. Such Td5% values were higher than those of 1 and 4 (Td5%= 382 and 377 °C,
respectively). The compounds with the methoxy and fluoro groups were found to exhibit better
thermal stability than that with the diphenylmethane and trityl moieties due to the intermolecular
interactions between -F (-OMe) and the aromatic ring hydrogens[21,22]. Additionally, from DSC
thermograms, the glass transition temperatures (Tg) of 1-4 compounds were found to be from 68
to 110 °C (Fig. 1b). All data of the thermal properties are summarized in Table 1. Generally, all
materials show good thermal stabilities for the OLED applications.
< Insertion Fig1 >
Fig. 1. Thermal properties of 1-4 (a) DSC and (b) TGA thermograms.
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Table 1. Summary of properties of compounds 1-4.
Compd Td5%/Tg
[oC]
λem
Eg
-HOMO
[eV]c
-LUMO
[eV]c
IP
[eV]d (exp/calc) (exp/calc)
ES [eV]a,b ET [eV]a,b
[nm]a [eV]b
(exp/calc) (exp/calc)
382/84
438/75
407/68
377/110
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3.64 5.17/4.93 1.53/0.50 5.42 3.56/3.58 3.03/3.25
3.64 5.10/4.84 1.46/0.35 5.36 3.54/3.57 3.02/3.19
3.64 5.22/5.09 1.58/0.74 5.74 3.54/3.57 3.05/3.25
3.55 5.18/4.98 1.65/0.57 5.40 3.55/3.61 3.06/3.17
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a) Measured using the tetrahydrofuran (THF) solutions (~10-5 M),
b) Estimated from the onset of spectra using formula E = 1240/λonset
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c) Estimated from the onset potentials
E
(against Fc/Fc+ redox couple) in the CV
onset
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experiments by following equation: HOMO = -1.40×(oxEonset) – 4.60 [23] and LUMO=
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Eg+HOMO,
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d) Estimated from the onsets of electron photoemission spectra.
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3.3. Photophysical properties
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The UV–vis absorption and photoluminescence (PL) spectra (Fig. 2a) in the diluted solutions
were used for an examination of the photophysical properties of 1-4 compounds and for
comparison with TPA. Their relevant photophysical data are summarized in Table 1. All these
compounds exhibit a similar spectral feature of UV-vis spectra which involves two absorption
bands at about 220 and 300 nm. Moreover, the compounds 1-4 have very similar photophysical
properties compared with TPA. Therefore, an absorption band at about 220 should be ascribed to
π-π* transition and band at about 300 nm to n-π* transition of a same TPA unit [24]. The wide
bang-gaps (Eg) from absorption onset of 1-4 were estimated to be about 3.6 eV. The first excited
singlet (S1) and triplet (T1) states energies (ES and ET, respectively) were experimentally
determined from the onsets of the fluorescence (at RT) and phosphorescence (at 77 K) spectra to
be in a range of 3.5-3.6 eV and 3.0-3.1 eV, respectively. Moreover, the theoretical calculations of
1-4 structures were done using the time-dependent density functional theory (TD-DFT) with
Tamm-Dancoff approximation (TDA), a restricted hybrid B3LYP functional and a 6-31+G(d)
basis set. The theoretical ES and ET values are very similar to values from PL spectra (see Table
1). Taking into account these data, a triplet spin density of all molecules is delocalized on only
the TPA core surface (see Fig. 4). These photophysical properties of 1-4 allow them to be
suitable for fabricating exciton blocking material for OLEDs.
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< Insertion Fig2 >
Fig. 2. Normalized UV-vis and PL emission spectra of diluted solutions of 1-4 and TPA in THF
at room temperature (a) and PL at 77 K (b) (wavelength of excitation is 310 nm).
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3.4. Electrochemical properties and energy level
The estimation of the highest occupied molecular orbital (HOMO) and lowest unoccupied
molecular orbital (LUMO) levels is essential to study the applicability of 1-4 as the exciton
blocking layers. The electrochemical properties of 1-4 were investigated by cyclic voltammetry
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(CV) in DCM solution to determine their HOMO and LUMO energy levels. The quasi-reversible
CV oxidation signals of 1-4 were observed. (Fig. 3a) The HOMO levels for 1-4 and TPA were
calculated to be -5.17, -5.10, -5.22, -5.18, and -5.26 eV, respectively. Thus, the diphenylmethane
groups have a significant impact on the electrochemical properties not only in terms of the
potential but also in the stability of the cationic species. In addition, the ionization potentials in
the solid state layers of the compounds 1-4 were measured to be 5.42, 5.36, 5.74 and 5.40 eV,
respectively, by an electron photoemission method in air. The electron photoemission spectra
and CV curves are shown in Fig. 3. However, the values of the ionization levels in the solution
and solid state are slightly different because the DCM solvent molecules (dielectric constant, ɛ
=8.9) are more polarizing the target molecules than the molecules themselves (ɛ =2-4) in the
solid state. Moreover, the very similar HOMO levels were estimated by DFT (see Table 1).
Thus, the lowest ionization energy level of 5.10 and 5.36 eV of compound 2 with the methoxy as
donor groups were determined in the solution and solid state, respectively. The reduction signals
up to -2.5 V versus Fc were not observed in CV curves of 1-4. Therefore, the experimental
LUMO energy values were estimated from the optical band gaps and the experimental HOMO
values of 1-4 and found to be -1.53, -1.46, -1.58, -1.65 eV, respectively. However, the theoretical
LUMO energy values are very different than the values from DFT theory (see Table 1). This
mismatch can be explained by the employment of the optical band gap for the estimation of the
experimental LUMO energy. However, the optical band gap is not identical to the
electrochemical band gap because an energy difference between the optical and electrochemical
band gaps corresponds to exciton dissociation energy [25].
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< Insertion Fig3 >
Fig. 3. Cyclic voltamperograms (with the internal ferrocene standard) (a) and electron
photoemission spectra (b) of 1-4.
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Fig. 4 shows that the bonding HOMO and unbonding LUMO frontier orbital densities of 2 are
mostly distributed on a TPA core moiety because the TPA core and the substituted aromatic
moieties are isolated by a sp3 hybridized methyne group. This fact is additionally supported by
the similar photophysical and electrochemical properties of 1-4 and TPA. Besides, the HOMO
and LUMO distributions are a very similar than the orbitals of 2.
< Insertion Fig4 >
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Fig. 4. Frontier HOMO and LUMO distributions and triplet spin density of 2 at the B3LYP/6-
31G(d,p) level (isovalue is 0.032).
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3.5. Fabrication and Characterization of OLEDs
Two types of the electroluminescent devices without (A) and with (B) an exciton blocking layer
(EBL) were fabricated by means of the vacuum deposition of the organic semiconductor layers
and metal electrodes onto pre-cleaned ITO coated glass substrate under vacuum. The devices
were fabricated by step-by-step deposition or co-deposition of the different organic layers. m-
MTDATA was used for the preparation of the hole-transporting layer (HTL) [26]. TCTA and
Ir(ppy)3 were applied for the fabrication of the emitting layer (EML) as a host and emitter,
respectively. As it is well known, TCTA has been widely used as a host for blue, green and
orange OLEDs because it exhibits a high thermal stability and a reasonable T1 level of 2.9 eV
[27]. However, m-MTDATA exhibits much lower T1 level of 2.6 eV [28] and TCTA only
behave as a hole-transporting type host material with a low capability to transport electrons, as a
result, an exciton recombination zone in EML is delocalized near HTL [29]. Therefore, the EBL
is required to reflect the triplet excitons back into the emitting layer. Bphen was used for a
preparation of the electron-transporting layer (ETL) [30]. Since Ca is highly reactive and
corrodes quickly in the ambient atmosphere, therefore, Ca layer topped with 200 nm aluminum
(Al) layer and it was used as a cathode. A material 2 as the exciton blocking layer was employed
and inserted between the HTL and EML layers because this material has similar IP level as
Ir(ppy)3. The energy-band diagrams of the devices A and B are shown in Fig. 5 and the
structures are as following:
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(A) ITO/ m-MTDATA (10 nm)/TCTA:Ir(ppy)3 (10%, 60 nm)/Bphen (10nm)/Ca/Al
(B) ITO/ m-MTDATA(7nm)/2 (3 nm)/TCTA:Ir(ppy)3 (10 %, 60 nm)/Bphen (10nm)/Ca/Al
< Insertion Fig5 >
Fig. 5. Energy-band diagrams of the devices A and B and chemical structures of the materials
used in OLEDs.
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As shown in Fig. 6a b, the electroluminescence (EL) spectra and the current density–voltage
characteristics of the devices A and B are very similar. However, a switch-on voltage of the
device B with EBL at a luminance of 1 Cd/m2 is higher than of the device A (i.e. 3.5 V and 4.0
V, respectively). Based on these results, EBL does not affect the transport of holes, however, it
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affects the injection of holes in EML. The luminance-voltage, current density-voltage,
efficiency-luminance and EL spectra of the devices A and B are shown in Fig. 6.
< Insertion Fig6 >
Fig. 6. Normalized electroluminescence spectra (a), luminance-voltage and current density-
voltage (b), current and power efficiency-luminance (c) and quantum-luminance characteristics
(d).
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All characteristics of the devices A and B are summarized in Table 2. The maximums current
and power efficiencies of the device B with EBL were estimated to be 80.1 Cd A-1 and 31.4 Lm
W-1, respectively, corresponding to about 50% increase compared to the standard device A
without EBL (41.8 Cd A-1 and 16.7 Lm W-1). A maximum external quantum efficiency (EQE) of
the device B reaches of 23.2% which is much higher than the referencing device B (12.1%). It is
important to mention that the EQE maximum of the devices A and B reach at high and practical
brightness of 4100 and 5500 Cd/m2, respectively, and the low current densities of 10-20
mA/cm2. More importantly, both OLEDs also showed the low efficiency roll-offs. For example,
at 10000 Cd/m2, the EQE values of the devices still reach of 10.1 and 20.7%, respectively.
(Table 2) It is only a decline of about 10% from maximum EQEs of both devices. However, EBL
does not affect the efficiency roll-offs at the low current densities because a triplet-polaron
quenching effect dominates in these conditions (vide infra). Taking into account these results and
also that our device using Ir(ppy)3-doped TCTA host are already between the most efficient
devices in their type. On the other hand, the blue PhOLEDs with a bis[2-(4,6-
difluorophenyl)pyridinato-C2,N](picolinato)iridium(III) (FIrpic) emitter and all our synthesized
materials as EBL using the same PhOLED structure were prepared and tested. Unfortunately, the
devices were not worked or worked very poorly. It can be explained that the energy levels of
Firpic, TCTA host and our EBL are not compatible in this structure devises.
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Table 2. Electroluminescence characteristics of the fabricated devices A and B.
Device
At 10000 Cd/m2
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A
B
3.5 38000
4.0 65000
41.8
80.1
16.7
31.4
12.1
23.2
36.0
73.8
11.1
24.7
10.1
20.7
1
2
3
4
5
6
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3.6. Roll-off modeling of OLEDs
An efficiency roll-off of OLEDs is primarily attributed to the long-lived excited states of the T1
excitons, which undergo exciton deactivation processes such as the triplet-triplet annihilation
(TTA) and triplet-polaron quenching (TPQ) mechanisms. The TTA and TPQ models are here
used to analyze the efficiency roll-off for the fabricated devices with (A) and without (B) exciton
blocking layer. Under the TTA process conditions, the EQE values of OLEDs can be calculated
from Eq (1) [4]
ꢀꢁꢀ
ꢃ
8
=
ꢂ ꢅ 1 + ꢇꢃ − 1ꢈ
(1)
ꢆ
ꢀꢁꢀ
ꢄꢃ
ꢃ
ꢂ
ꢂ
9
where EQE0 is an external quantum efficiency in absence of TTA and J0 is the critical current
10
density at EQE/EQE0 = ½. This current density J0 can be estimated from Eq (2) [5]
ꢄ ꢋ ꢌ
11
ꢉꢊ =
(2)
ꢎ
ꢍ
ꢏ
ꢐꢐꢑ
12
13
14
where e is an electron charge (1.6×10-19 C), w is a width of the exciton formation zone, τ is a
triplet exciton lifetime and kTTA is a rate constant of the TTA process. Besides, under the TPQ
process conditions, the EQE can be calculated from Eq (3) [5]
ꢚꢛ
ꢖ
ꢀꢁꢀ
15
= ꢒ1 + ꢓꢃꢃ ꢕꢗꢘꢖꢙ
(3)
ꢀꢁꢀ
ꢂ
ꢔ
16
17
18
19
20
where EQE0 is an external quantum efficiency in absence of TPQ and Je is a critical current
density at EQE/EQE0 = ½, m = 1, 2, 3 etc. For a nonlinear least-squares data fitting of EQE, the
Solver software in Excel [31] and the same method as in our previous work were employed.[26]
Thus, to obtain the starting points of fitting, the maxima current densities J0 were calculated from
Eq 2 and data from the literature [32]. A triplet exciton lifetime of 1.6 ꢂs and a rate constant kTTA
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1
2
3
4
5
6
of 3.0×10-12 cm3 s-1 were used for the Ir(ppy)3 emitter in TCTA host. This value was estimated to
be 500 mA cm-2 for all devices.
< Insertion Fig7 >
Fig. 7. Measured (scatter) and fitted EQE (green and black curves) versus the current density
characteristics of the devices without (a) and with (b) EBL. The red curves represent EQE0 in
absence of TPQ and the blue curves represent EQE0 in absence of TTA.
7
8
The fitted curves based on the TPQ and TTA model agreed well with the experimental data for
all devices (i.e. correlation coefficients, R2, are 0.999). The value of m=2 was estimated for all
devices. Fig. 7a b show EQE0 in the absence of the TPQ and TTA exciton deactivation effects of
the devices without and with exciton bloking layer. Therefore, the efficiency roll-off at low
current density for all devices was primarily caused by the TPQ deactivation mechanism because
the calculated EQE0 curves in the absence of TPQ are above in the graphs than EQE0 in the
absence of TTA. However, the device with EBL showed a higher Je value of 104 than that of 88
mA cm-2 without EBL, which implies that the device without EBL suffered from more severe the
TPQ effect. The values Jo were estimated to be 107 and 153 mA cm-2 for the devices without and
with EBL, respectively. Taking into account the Je and Jo values, the EBL much more inhibits the
triplet-triplet annihilation (TTA) than the triplet-polaron quenching effects in our OLEDs at high
current densities. Moreover, for a fitting, a maximum value of width (w) of the exciton formation
zone was used to be equal to the thickness of the emitting layer of both devices to 60 nm. Thus,
the widths of the exciton formation zones (w) were estimated to be 13 and 18 nm for the devices
without and with EBL, respectively, using the calculated J0 by Eq (2). For the device B with
EBL, it corresponds to 30% of the total emitting layer compared to the standard device A (20%).
In addition, Jeon and Lee showed that an insertion of high triplet energy EBL in the blue
phosphorescent OLEDs influenced to the width and stability of an exciton recombination zone
[13]. The application of the exciton blocking compounds 1-4 for blue thermally activated
delayed-fluorescence (TADF) OLEDs and an investigation of the devices lifetimes are in the
progress in our laboratory.
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25
26
27
28
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30
4. Conclusions
In summary, four novel derivatives based on a triphenylamine core have been synthesized as the
exciton bloking layer for OLEDs. The compounds showed the high triplet energies of 3.0 eV and
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2
3
4
5
high thermal stabilities. The efficiency and low efficiency roll-off of a constructed green OLED
with exciton bloking layer have been achieved to be 23.2% corresponding to about 50% increase
compared to the referencing device. From the roll-off modeling of OLEDs, the exciton bloking
layer much more inhibits the triplet-triplet annihilation than the triplet-polaron quenching effects
at high current densities.
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Acknowledgements
K. Ivaniuk thanks for the support from “AmbiPOD” project under grant agreement No 612670
(IRSES-GA-2013-612670). We are grateful to D. Gudeika for MS, L. Peciulyte for TGA and
DSC, D. Volyniuk for the electron photoemission measurements and G. Grybauskaite for the
valuable comments on the manuscript.
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Highlights
•
•
•
The simple novel materials based on triphenylamine core through a one-step Friedel–
Crafts reaction have been synthesized.
The synthesized compounds exhibit the ionization potentials in an interval of 5.4-5.7 eV
in the solid state and the high triplet energies of about 3.0 eV.
The introduction of a material with the lowest ionization potential as the high triplet
energy exciton blocking thin layer of the green OLED doubled the quantum efficiency of
the device.
•
The triplet-triplet annihilation and triplet-polaron quenching effects for the devices
without and with exciton blocking layer have been analyzed and discussed.