Light-Emitting Electrochemical Cells and Solution-Processed Organic Light-Emitting Diodes Using Small Molecule Organic Thermally Activated Delayed Fluorescence Emitters

Article abstract of DOI:10.1021/acs.chemmater.5b03245

Two novel charged organic thermally activated delayed fluorescence (TADF) emitters, 1 and 2, have been synthesized. Their TADF behavior is well-supported by the multiexponential decay of their emission (nanosecond and microsecond components) and the oxygen dependence of the photoluminescence quantum yields. Spin-coated electroluminescent devices have been fabricated to make light-emitting electrochemical cells (LEECs) and organic light-emitting diodes (OLEDs). The first example of a non-doped charged small organic molecule LEEC is reported and exhibited an external quantum efficiency (EQE) of 0.39% using 2. With a multilayer architecture, a solution-processed OLED device using neat 2 as the emitting layer gave an EQE of 5.1%, the highest reported to date for a nondoped solution-processed small molecule organic TADF OLED. These promising results open up a new area in light-emitting materials for the development of low-cost TADF LEECs.

Full text of DOI:10.1021/acs.chemmater.5b03245

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Article
Light-Emitting Electrochemical Cells and Solution-Processed OLEDs Using
Small Molecule Organic Thermally Activated Delayed Fluorescence Emitters
Michael Y. Wong, Gordon J Hedley, Guohua Xie, Lisa Kolln, Ifor D.
W. Samuel, Antonio Pertegás, Henk J Bolink, and Eli Zysman-Colman
Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b03245 • Publication Date (Web): 14 Sep 2015
Downloaded from http://pubs.acs.org on September 16, 2015
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LightꢀEmitting Electrochemical Cells and
SolutionꢀProcessed OLEDs Using Small
Molecule Organic Thermally Activated Delayed
Fluorescence Emitters
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Michael Y. Wong,a Gordon J. Hedley,b Guohua Xie,b Lisa Kolln,b Ifor D. W. Samuel,b Antonio
Pertegásc, Henk J. Bolinkc, and Eli Zysman-Colman*a
a
Organic Semiconductor Centre, EaStCHEM School of Chemistry, University of St
Andrews, St Andrews, Fife, UK, KY16 9ST, Fax: +44ꢀ1334 463808; Tel: +44ꢀ1334
463826; Eꢀmail: eli.zysmanꢀcolman@stꢀandrews.ac.uk
b Organic Semiconductor Centre, SUPA, School of Physics and Astronomy, University of
St Andrews, North Haugh, St Andrews, Fife, KY16 9SS, UK, Eꢀmail: idws@stꢀ
andrews.ac.uk
c Instituto de Ciencia Molecular, Universidad de Valencia, Catedrático José Beltrán 2,
Paterna, Eꢀ46980, Spain; Eꢀmail: henk.bolink@uv.es
URL: http://www.zysmanꢀcolman.com
Abstract
Two novel charged organic thermally activated delayed fluorescence (TADF) emitters, 1
and 2 have been synthesized. Their TADF behavior is well supported by multiꢀ
exponential decay of their emission (ns and µs components) and oxygenꢀdependence of
the photoluminescence quantum yields. Spinꢀcoated electroluminescent devices have
been fabricated to make lightꢀemitting electrochemical cells (LEECs) and organic light
emitting diodes (OLEDs). The first example of a charged small organic molecule LEEC
is reported and exhibited an external quantum efficiency (EQE) of 0.39% using 2. With a
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multilayer architecture, a solutionꢀprocessed OLED device using neat 2 as the emitting
layer gave an EQE of 5.1%, the highest reported to date for a nonꢀdoped solutionꢀ
processed small molecule organic TADF OLED. These promising results open up a new
area in lightꢀemitting materials for the development of lowꢀcost TADFꢀLEECs.
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Introduction
Organic Light Emitting Diodes (OLEDs) have come to the fore as stateꢀofꢀtheꢀart
technology for visual displays and lighting.1ꢀ4 OLEDs are desirable as they are light
weight, flexible, provide better contrast and possess large viewing angle.4 OLEDs are
also more power efficient than traditional lighting sources and thus their wide adoption
can alleviate significantly the strain on current energy demand because lighting alone
constitutes about 20% of electricity consumption worldwide.5 The “first generation”
OLEDs were based on organic fluorescent emitters whose efficiency was intrinsically
capped at 25% due to only being able to recruit singlet excitons.6 The “second
generation” OLEDs employed organometallic phosphorescent emitters, which harvest
both singlet and triplet excitons for emission due to the enhanced intersystem crossing
(ISC) mediated by the large spinꢀorbit coupling of the heavy metals such as iridium(III)
and platinum(II).7ꢀ9 Despite their highly desirable performance characteristics, the rarity
of these metals, their high cost and their toxicity are important detracting features that
inhibit largeꢀscale, worldwide adoption of OLED technology, particularly for lighting
applications. The “third generation” OLEDs were recently reported by Adachi and coꢀ
workers. His group demonstrated how small organic molecules, emitting via a thermally
activated delayed fluorescence (TADF) mechanism,10 could be integrated into OLEDs
and exhibit very high efficiencies as, like with phosphorescent emitters, both singlet and
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triplet excitons are recruited for emission.11ꢀ14 Thus, TADFꢀbased OLEDs address the key
detracting features endemic to “second generation” OLEDs while retaining their
advantages.15
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The principle of TADF relies on a small energy gap between the singlet and
triplet excited states (ꢁEST). Under these conditions, the electrons in the triplet state can
return to the singlet state by reverse intersystem crossing (RISC) using thermal energy,
followed by radiative fluorescence.11 The small ꢁEST is realized by spatial separation
between HOMO and LUMO to minimize the exchange integrals between the two
orbitals.16 A large number of organic TADF emitters have been reported to date and the
majority of them are based on a twisted intramolecular charge transfer (TICT) design in
which the donor and acceptor moieties are designed to be nearly orthogonal to each
other.11,13,17ꢀ18
Despite this important paradigm shift in emitter design, current OLEDs, including
TADFꢀOLEDs, still employ air sensitive electrodes requiring rigorous encapsulation, are
usually vacuum deposited and possess a complex multiꢀlayer architecture that adds to the
cost of fabrication. Singleꢀlayer solidꢀstate lightꢀemitting electrochemical cells (LEECs)
have received much recent attention for their potential to address these negative features
of OLEDs.19ꢀ22 The emitters in LEECs are frequently ionic transition metal complexes
(iTMCs), the most popular and highest performing class of which are cationic iridium(III)
complexes.23ꢀ24 Without a similar paradigm shift in emitter design coupled with improved
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device performance characteristics, LEECs will suffer a worse fate than 2nd generation
OLEDs and this promising technology will be relegated to academic inquiry. Recently,
Bolink and coꢀworkers demonstrated the first example of an operational LEEC with a
smallꢀmolecule charged organic cyanineꢀbased fluorophore25 while Edman showed how a
smallꢀmolecule neutral organic fluorophore can be used as a dopant in LEECs.26 We now
report the first example of an organic TADF–based LEEC wherein the TADF material
serves the dual role of charge transporter and emitter. Additionally, we also report doped
and singleꢀcomponent spinꢀcoated fabricated OLEDs, whose external quantum
efficiencies are superior to those few previous reports of nonꢀdoped TADF OLEDs in the
literature,17,27ꢀ28 and contrast their performance to the LEEC.
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Small molecule emitters must be charged in order to mediate charge transport in a
single layer LEEC. We thus set about incorporating tethered imidazolium units onto a Nꢀ
carbazoyldicyanobenzene scaffold first reported by Adachi11 to build charged analogues,
1 and 2, whose synthesis is illustrated in Scheme 1.
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Scheme 1. Synthesis of the charged TADF emitters for LEEC 1 and 2. a. NBS, ACN, rt,
o
1 h. b. i) NaH, THF, rt, 30 min, ii) TBDMSCl c. i) nꢀBuLi, THF, ꢀ78 C, 30 min, ii)
excess 1,4ꢀdiiodobutane. d. NaH, imidazole, THF, reflux, 4 h. e. i) NaH, 6, THF, rt, 30
min, ii) 2 equiv. 4,5ꢀdifluorophthalonitrile, rt, 4 h. f. i) NaH, 6, THF, rt, 30 min, ii) 0.6
o
equiv. 4,5ꢀdifluorophthalonitrile, rt, 4 h. g. i) MeI, ACN, 40 C, 2h ii) sat. NH4PF6 (aq).
(NBS = Nꢀbromosuccinimide; TBDMSCl = tertꢀbutyldimethylsilyl chloride)
The preparation of 3ꢀbromocarbazole 3 using Nꢀbromosuccinimide29 was
contaminated by some starting material and 3,6ꢀdibromocarbazole, both of which could
be removed by fractional recrystallization from toluene. The preparation of 5 was
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accomplished in good yield by dropwise introduction of the lithiated TBDMSꢀprotected
3ꢀbromocarbazole 4 intermediate to excess 1,4ꢀdiiodobutane. Compound 6 was obtained
by SN2 reaction of sodium imidazolate with 5 followed by silyl deprotection using
sodium hydride in a oneꢀpot fashion.30 Compounds 7 and 8 were synthesized by reacting
6 with subꢀstoichiometric or stoichiometric 4,5ꢀdifluorophthalonitrile, respectively, under
basic conditions via nucleophilic aromatic substitution. The targeted charged TADF
emitters 1 and 2 were obtained following methylation with iodomethane and anion
metathesis with saturated aqueous NH4PF6 solution in 33% and 27% yield, respectively,
over six steps. The solubilities of 1 and 2 in DCM were greatly improved after the anion
metathesis.
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Both 1 and 2 exhibit irreversible oxidation and reversible oxidation waves in
MeCN solution by cyclic voltammetry (Table S1, Figure S20). The HOMO of 1 (ꢀ5.93
eV) is slightly lower than that of 2 (ꢀ5.87 eV) due to the presence of the electronꢀ
withdrawing fluorine atom. The LUMO of 1 (ꢀ2.92 eV) is slightly higher than that of 2 (ꢀ
2.99 eV), which is due to the increased conjugation imparted by the second carbazole
moiety that lowers the LUMO in 2. These structureꢀproperty relationships are mirrored in
the absorption spectra wherein the absorption profile of 2 is slightly redꢀshifted compared
with in 1 (Figure 1).
Solution photoluminescence of the two materials was investigated in MeCN. The primary
photophysical parameters in solution are detailed in Table 1 (absorption data reported in
Table S2).
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Table 1. Photoluminescence of 1 and 2 in MeCN at 298 K.
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b,d
c,d
λem (nm)a,b
556
λem (nm)a,c
558
ΦPL
ΦPL
τe (ns)b,e
2.90
τe (ns)c,e
3.26
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0.014
0.029
0.026
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574
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4.97
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c
a λexc: 360 nm. Measured in aerated solvent. Measured in degassed solvent. d Quinine sulfate
used as the reference (ΦPL = 0.546 in 1N H2SO4 at 298 K).31 e λexc: 375 nm.
Figure 1. Normalized absorption spectra of 1 and 2 in aerated MeCN at 298 K and
normalized emission spectra in deaerated MeCN at 298 K and in pristine thin film under
air.
The emission in MeCN solution and in film for both 1 and 2 (Figure 1) is broad
and unstructured, which is characteristic of CT emission. The excitation and absorption
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spectra match, corroborating the high level of purity determined by the compound
characterization. The solution state emission for 1 and 2 are redꢀshifted by about 30 and
40 nm, respectively, compared to emission in film (vide infra). In solution, the emission
for both 1 and 2 is weak and the observed emission lifetimes are in the nanosecond
regime. There is little change in the photophysical properties upon removal of O2,
suggesting that in MeCN 1 and 2 act as fluorophores with no observed TADF. This result,
coupled with the observation that PL is redꢀshifted in solution compared to film, indicates
that solvent interactions may lower the energy of triplet (T1) with respect to the singlet
(S1), resulting in an increased ꢁEST and thus TADF would be switched off. By point of
comparison, dicarbazolyldicyanobenzene, which does not contain the tethered
imidazolium units, was shown by Adachi to emit at 473 nm in toluene solution via TADF
with a photoluminescence quantum yield, ΦPL, of 47%.11
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Photoluminescence in film is the process of interest when considering these
materials as TADF emitters. In spinꢀcoated neat film, 1 and 2 show significantly stronger
emission than in solution, with ΦPL values of 0.093 and 0.207, respectively, under aerated
conditions. As interactions between chromophores often lead to quenching of PL, we also
investigated the emission when the materials were diluted in a PMMA host. In samples
with 10 wt% of 1 and 2 in PMMA, ΦPL improved substantially with ΦPL values of 0.447
and 0.730 for 1 and 2, respectively. Table 2 provides full details of the measured
photophysical properties of the films.
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Table 2. Photoluminescence of 1 and 2 in neat and doped thin films.
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Film (neat)
Film
(10% doped in PMMA)
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a
λem (nm)
530
ΦPL
τave (µs)
0.26
λem (nm)
ΦPL
τave (µs)
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1
2
Aerated
Degassed
Aerated
0.09
0.10
0.21
0.21
490
0.44
2.54
0.96
0.97
2.73
0.56
0.73
0.90
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1.08
6.34
536
500
Degassed
a PLQY measured using Hamamatsu Absolute PL quantum yield measurement system,
with an integrating sphere and nitrogen purge where appropriate, estimated errors are ~
1% on the reported values.
The long (ꢀs) average lifetime and the shortening of the lifetime in air suggest a
tripletꢀderived TADF processes may be occurring, in contrast to the solution data. The
PL decay is observed to be complex, spanning four orders of magnitude of time and
requiring 7ꢀ9 exponential functions to adequately describe it. These exponentials are,
however, simply parameterization as the decay is derived from a mixture of initial
fluorescence from the singlet (often denoted as prompt fluorescence), and delayed
fluorescence mediated by intersystem crossing (ISC), followed by slow reverse
intersystem crossing (RISC) populated emission from the singlet, subject to Boltzmann
statistics. Consequently to enable a fair comparison of the two materials, and between
aerated and vacuum conditions, an average lifetime can be calculated that is the weighted
average of the individual components with their preꢀexponential factors. Full details of all
the decay components along with an experimental description is provided in Table S3
and Figures S21 and S22. We observe that in 10 wt% of 1 in PMMA the ΦPL increases
from 0.447 to 0.557 when going from air to degassed conditions, while the emission
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lifetime increases from 2.54 to 6.14 µs. In 10 wt% 2 in PMMA the ΦPL increases from
0.730 to 0.900, while the emission lifetime increases from 1.08 to 6.34 µs. The increase
in lifetimes and PLQY upon removal of oxygen allows us to assign emission in films of 1
and 2 to TADF. Absence of complete quenching of longꢀlived emission in the films is
likely due to the fact that both in neat films and when doped in PMMA, some of the
chromophores will be partially encapsulated, hampering the ability of oxygen to diffuse
into the film.
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Electroluminescent devices: LEECs
LEECs were prepared on top of a patterned indium tin oxide (ITO) coated glass
substrate. Prior to the deposition of the emitting layer, 80 nm of PEDOT:PSS was coated
in order to increase the reproducibility of the cells. The emitting layer (100 nm) was
prepared by spinꢀcoating of an MeCN solution consisting of the emitting compound alone
or with the addition of an ionic liquid (IL) 1ꢀbutylꢀ3ꢀmethylimidazolium
hexafluorophosphate [Bmim][PF6] at a molar ratio of 4:1 that is typically used in iTMC
LEECs. To complete the devices, a layer of 70 nm of aluminum that serves as the top
electrode was thermally evaporated onto the devices in a high vacuum chamber
integrated in the inert atmosphere glovebox.
In order to determine the performance of the LEECs, the devices were operated
using a blockꢀwave pulsed current driving method (1000 Hz and 50% of duty cycle) at
different average current densities of 10, 25 and 50 A·mꢀ2. This operational mode was
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selected over constant voltage mode as it decreases the turnꢀon time and leads to a more
sustained behavior versus time.32ꢀ33 Using this method but also using constant voltage
driving we were unable to observe lightꢀemission from the device employing 1 as the
emitting component in single layer devices. The driving voltage did reduce upon
application of the pulsed current indicating that electronic charges were able to inject and
move through the film. However, the reason why no lightꢀemission was observed remains
unclear at this moment. When 2 was employed as the emitting layer the situation changed
drastically as with these devices we did observe lightꢀemission. The luminance and
average voltage are depicted in Figure 2 for LEECs using 2 and 2:IL 4:1 as the
component(s) for the lightꢀemitting layer. The figures of merit for the different devices
studied are summarized in Table 3. Initially, for both LEECs, the average voltage applied
drops rapidly over the first seconds (as also observed for the device employing 1, not
shown). Coinciding with the decrease in driving voltage, an increase in the luminance
was observed. This reached a maximum value and then slowly decreased versus time.
This is typical for LEECs, as the injection barrier for electrons and holes reduces due to
the migration of ions to the electrode interfaces and the subsequent formation of doped
regions.34ꢀ36 The reduction of the injection barrier leads to the reduced driving voltage to
sustain the set current density. With increasing operating time the doped regions expand
leading to a slowly increasing quenching of the excitons, and as a result a (nonꢀ
permanent) luminance reduction.37
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Figure 2. Luminance and average voltage (inset) for LEECs using 2 as the emissive
material measured at different average current densities (50 A·mꢀ2: red squares, 25 A·mꢀ2:
green circles, 10 A·mꢀ2: orange triangles). Top: ITO/PEDOT:PSS/2:[Bmim][PF6] 4:1/Al.
Bottom: ITO/PEDOT:PSS/2/Al.
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We found that the LEECs containing additional ions in the emitting layer have a
lower luminance (and efficiency) than the LEECs with only the emitting material 2
(Figure 2). This was initially surprising as generally an increased luminance is observed
upon the addition of IL to the emitting layer in LEECs employing iTMCs.38ꢀ39 This
increased luminance is accompanied by an increase in ΦPL of the emitting layers due to
an increase in the interꢀemitter distance, which leads to a reduction in the concentration
quenching. The results, here obtained, indicate that concentration quenching effects are
less important in the TADF materials then they are in iTMCꢀbased LEECs. The addition
of the IL, similar to what was observed in MeCN solution, generates a more polar
environment, widening the singletꢀtriplet energy gap and arresting the TADF mechanism
in the device.
In Figure 2 it is also clearly seen that the luminance decreases with decreasing current
density, albeit this relationship is far from linear. A similar behavior has been observed
for iTMCꢀbased LEECs.40ꢀ42 This effect was attributed to a reduced quenching of the
excitons due to either a reduction of charge carriers, excited states or both. As the devices
are operated at a fixed average current density, the efficiency of the devices is directly
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proportional to the luminance. With a fivefold decrease in current density, the luminance
of the 2 only (no IL, device L1) device drops only from approximately 26 to 13 cd m−2.
Hence, the device power efficiency strongly increases to 0.7 Lm W−1 with an external
quantum efficiency (EQE) of 0.39 % (assuming Lambertian emission).
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The electroluminescence (EL) spectra are shown in Figure 6. All cells emit
homogenously from the active area. The spectra are rather similar to those obtained by
photoꢀexcitation, featuring an unstructured green emission centered at 538 nm (CIE
coordinates: 0.35, 0.57).
Table 3. Performance of LEEC devices
LEEC Devicesa
Electroluminescence
Avg. Current
Density
Lummax
PCEmax
EQEmax
(%)c
(cd·mꢀ2)a
(lm·Wꢀ1)b
(A·mꢀ2)
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0.39
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0.12
0.1
L1
L2
a. Device L1: ITO/PEDOT:PSS/2/Al; Device L2:
ITO/PEDOT:PSS/2:[Bmim][PF6] 4:1/Al biased with a block waveꢀ
pulsed current at a frequency of 1000Hz and a duty cycle 50% at
different current densities.
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Figure 3. Schematic diagram of the device structure: The emissive layer is either pristine
1/2 or doped as mCP:OXDꢀ7:1/2 (70:20:10). PEDOT:PSS = poly(3,4ꢀ
ethylenedioxythiophene): poly(styrenesulfonate); PVK = Poly(Nꢀvinylcarbazole);
B3PYMPM = bis[4,6ꢀ(3,5ꢀdiꢀ3ꢀpyridylphenyl)ꢀ2ꢀmethylpyrimidine]; mCP = 3,5′ꢀN,N′ꢀ
dicarbazoleꢀbenzene; OXDꢀ7 = 1,3ꢀbis[(4ꢀtertꢀbutylphenyl)ꢀ1,3,4ꢀoxadiazolyl] phenylene
We also explored 1 and 2 as the emitter layer (EML, ~ 20 nm) in solutionꢀ
processed multiꢀlayer architecture OLEDs shown in Figure 3, in which PEDOT:PSS (~
30 nm) acts as the hole injection layer (HIL), and PVK (~ 30 nm) behaves as the hole
transporting layer (HTL) and electron/exciton blocking layer (EBL) due to its high lowest
unoccupied molecular orbital (LUMO) level (2.2 eV) and triplet level (2.5ꢀ2.9 eV).43ꢀ44
The PVK was coated with the emissive layer and B3PYMPM deposited as the electron
transporting layer (ETL) because of its high electron mobility and excellent hole blocking
ability45 associated with its deep highest occupied molecular orbital (HOMO) level (6.8
eV). Calcium coated with aluminum was used as the cathode. For devices 1 and 2 the
emissive layer (EML) is consisted of, respectively, 1 or 2 doped into mCP (T1 = 2.9 eV)
and OXDꢀ7 (T1 = 2.7 eV), which are coꢀhosts for the emitters.46 Generally, the multiꢀ
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layer structure can either balance hole/electron in the EML or confine the excitons inside
the EML to enhance radiative recombination rate. As we found that neat 2 produced a
viable LEEC, we also made devices with neat films of the emitters (1 or 2, 10 nm thick,
devices 3 and 4, respectively).
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Figure 4. (a) Current density and (b) luminance vs. voltage characteristics of the devices.
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Figure 4a displays the current densityꢀvoltage characteristic curves of the four devices
based on emitters 1 and 2. With the similar architectures, the devices exhibited similar
current densities in the region of [ꢀ3V, 3V] where no emission was observed. For the
doped EML, device 2 showed higher current density above 5 V and also higher
luminance. Device 2 also showed much higher current efficiency (CE), power efficiency
(PE), and external quantum efficiency (EQE) than device 1 with emitter 1 (see Figure 5).
From Figure 6, we observe that the peak wavelength of device 2 was redꢀshifted by 99
nm compared to device 1. Thus, the Commission Internationale de L’Eclairage (CIE)
coordinates were changed from (0.42, 0.49) to (0.25, 0.31). Surprisingly but analogous to
what was observed with the LEECs, both the nonꢀdoped devices 3 and 4 exhibited better
performance (Table 4) compared to the doped devices 1 and 2, respectively. For the
devices with emitter 2, lower driving voltages were realized, especially for device 4,
which required only 3.6 V to turn on. It is worth mentioning that the maximum CE, PE,
and EQE were approximately double those of device 2. In contrast, devices 1 and 3
achieved similar luminous efficiencies, even though the EL spectra are significantly
different (shown in Figure 6, 7a and 7c). Unlike the other devices, the device 2 showed
current densityꢀdependent EL spectra as the component from the coꢀhost increased with
the current density mainly due to incomplete hostꢀguest energy transfer efficiency and
exciplex emission.47 Devices 2 and 4 not only showed better charge injection/transport
but also higher luminance. Given that emitters 1 and 2 have rather similar HOMO and
LUMO energy levels, which are likely not the origin of the difference in charge
injection/transport, it is proposed that the C2 symmetry of emitter 2 may induce better
packing in the solid state, giving better charge transport. The higher luminance is
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attributed to the enhanced film photoluminescence quantum yield observed in emitter 2.
From the maximum EQE of 5.1% of device 4, we can estimate that the delayed
fluorescent component of the quantum yield of 2 is around 18.4% according to Equation
2,48 which is consistent with that of 21.4% (Table 2) calculated from the transient
measurement.
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

1
3
ΦDF
PL −ΦDF )
EQE = γηoc ηPL
+
(2)



4
4 1−(
η

where is the ratio of holeꢀelectron recombination rate and considered to be 1 here,
is the
ηoc
γ
outcoupling efficiency (~20%) ,
the quantum yield.
is the quantum yield, and
is the delayed component of
Φ DF
ηPL
Table 4. Comparison of the performance of the four OLEDs.
b
c
e
.f
g
OLED Devices a
(emitter)
1 (doped – 1)
2 (doped – 2)
3 (neat – 1)
Von
λpeak
FWHM d EQEmax
.
CEmax
PEmax.
(lm/W)
0.2
4.0
0.3
CIE h
(V)
9.5
5.8
7.5
3.6
(nm)
467
566
532
546
(nm)
148
137
132
128
(%)
0.3
2.7
0.3
5.1
(cd/A)
0.5
(0.25, 0.31)
(0.42, 0.49)
(0.38, 0.52)
(0.41, 0.53)
7.4
0.7
14.9
4 (neat – 2)
10.3
a For Device architecture see Figure 3 and text. b Turnꢀon voltage @1 cd/m2. c Peak
wavelength at 1 mA/cm2. d Full width at half maximum of the EL spectrum at 1 mA/cm2.
e Maximum external quantum efficiency. f Maximum current efficiency. g Maximum
current efficiency. h The Commission Internationale de L’Eclairage coordinates at 1
mA/cm2.
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Figure 5. Current density vs current efficiency (a), power efficiency (b), and external
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quantum efficiency (c) curves of the four devices.
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Figure 6. Comparison of the normalized EL spectra at 1 mA/cm2 of the devices.
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Figure 7. Normalized EL spectra as a function of the current density of (a) device 1, (b)
device 2, (c) device 3, (d) device 4.
Conclusions
In summary, we synthesized in good yield and characterized the first charged
organic TADF emitters. Using 2, green lightꢀemitting devices that operate as LEECs and
OLEDs were prepared. For the LEECs, the operating voltages are very low with an
average of 2.7 volts. The efficiency increases strongly with decreasing carrier density and
reaches a maximum external quantum efficiency of around 0.4%.
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We demonstrated that the solutionꢀprocessed and colorꢀstable OLEDs with
pristine films outperformed the devices with the doped EML, which is rarely realized in
the structure with TADF characteristics (EQE up to 5.1%). Device 4 exhibited the highest
EQE of any singleꢀcomponent TADF OLED to date. Further optimization of the
molecular structure for high quantum yield will pave the way for orthogonally spinꢀ
coated OLEDs with high luminous efficiency, which would lead to all solutionꢀprocessed
device and practical application.
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Acknowledgements. EZꢀC thanks the University of St Andrews for support. We are
grateful to the EPSRC for financial support (grants EP/J01771X and EP/J00916). IDWS
is a Royal Society Wolfson Research Merit Award Holder. We thank the EPSRC UK
National Mass Spectrometry Facility at Swansea University for analytical services.
Supporting Information. Experimental section. NMR spectra for all compounds.
Supplementary electrochemical data for 1-2. Supplementary absorption data for 1-2.
Supplementary timeꢀresolved luminescence data for 1-2. Supplementary power intensity
as a function of excitation power data for 1-2.
Competing Financial Interests. The authors declare no competing financial interests.
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