Achieving efficient violet-blue electroluminescence with CIE:Y 6% from naphthyl-linked phenanthroimidazole-carbazole hybrid fluorophores

Article abstract of DOI:10.1039/c6sc05619a

In this work, we revealed a new approach for the development of efficient violet-blue emitting materials featuring a hybrid local and charge transfer (HLCT) excited state through the incorporation of naphthyl group(s) as a weak n-type π spacer in a donor-

Full text of DOI:10.1039/c6sc05619a

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Achieving Efficient VioletꢀBlue Electroluminescence with CIEy < 0.06
and EQE > 6% from NaphthylꢀLinked PhenanthroimidazoleꢀCarbazole
Hybrid Fluorophores
b
WenꢀCheng Chen,a Yi Yuan,ab ShaoꢀFei Ni,c QingꢀXiao Tong,
*
FuꢀLung Wong,a ChunꢀSing
a
Lee
*
a Center of SuperꢀDiamond and Advanced Films (COSDAF) and Department of Biology and Chemistry, City University of
Hong Kong, Hong Kong SAR, PR China
Eꢀmail: apcslee@cityu.edu.hk
b
Department of Chemistry and Key Laboratory for Preparation and Application of Ordered Structural Materials of
Guangdong Province, Shantou University, 243 University Road, Shantou, Guangdong, 515063, PR China
Eꢀmail: qxtong@stu.edu.cn
c Department of Chemistry, Southern University of Science and Technology, Shenzhen, 518055, PR China
Abstract:
In this work, we revealed a new approach for developing efficient violetꢀblueꢀemitting
materials featured with hybrid local and charge transfer (HLCT) excited state through
incorporation of naphthyl group(s) as a weak nꢀtype π spacer in a donorꢀπꢀacceptor (DꢀπꢀA)
system. The resulting materials (TPINCz and TPIBNCz) show improved intramolecular
charge transfer properties and highly efficient violetꢀblue fluorescence. It is demonstrated that
the pattern of the π spacers has significant influences on photophysical properties. The
incorporation of a naphthyl/binaphthyl spacer between donor and acceptor moieties can
alleviate a common dilemma that enhancing device performance by increasing charge transfer
excited properties often leads to redꢀshifted emissions. A device using TPINCz as an emissive
dopant shows a violetꢀblue emission with CIE coordinates of (0.153, 0.059) and a record high
EQE of 6.56 ± 0.11% at a brightness of 1,000 cd mꢀ2. To the best of our knowledge, this
performance is the highest among the reported devices with CIEy ≤ 0.08. Our study provides a
new pathway for designing highꢀperformance violetꢀblue emitters with a DꢀπꢀA architecture
in organic electroluminescence application.
Introduction
The dramatic progress in the area of organic optoelectronics, observed over the last couple of
decades, has been largely realized by the successful advance in numerous conjugated organic
1

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materials, showing a wide range of tunable electrical/photo physical characteristics.1–7
Understanding the structureꢀproperty relationship and mastering the ways to manipulate these
unique properties significantly leads to a rapid development in organic semiconductor based
applications, such as organic fieldꢀeffect transistors, photovoltaic and electroluminescence
(EL) devices etc. In the area of light generation, considerable interest in organic lightꢀemitting
devices (OLEDs) is derived from their attractive prospects as a new generation of fullꢀcolor
flatꢀplat display and solidꢀstate lighting technologies.8–10 Efficient violetꢀblue EL is very
important in OLEDs since violetꢀblue fluorophores can be employed as energy donors to
generate light covering the entire visible region and white light emission.11–13 Additionally,
efficient shortꢀwavelength EL plays a key role in reducing power consumption and expanding
the color gamut in fullꢀcolor displays.14,15 OLEDs with shortꢀwavelength emission also has
promising applications in other fields, such as chemical and biological sensing,16
phototherapy,17 photocopying,18 highꢀdensity information storage,19 sterilization20 etc.
Therefore, development of highꢀperformance violetꢀblue lightꢀemitting materials is crucial for
the burgeoning OLED technology.
However, unlike their red, green and skyꢀblue counterparts with desirable performances,
to date, reports on efficient organic fluorophores with deepꢀblue or violetꢀblue emission in the
Commission Internationale de L’Eclairage (CIE) map matching or surpassing the
highꢀdefinition television (HDTV) standard blue index (CIE coordinates of (0.15, 0.06)) are
rare.21,22 In principle, violetꢀblue emission requires materials with wide optical gap, which
implies large HOMO (highest occupied molecular orbital)ꢀLUMO (lowest unoccupied
molecular orbital) offset, leading to inferior carrier injections. Moreover, the corresponding
molecular conjugation must be strictly controlled. This often in turn results in drop of
photoluminescence quantum yield (Φf) and poor electrical properties. These also cause
difficulty in molecular design and synthesis for better thermal and morphological stability.23,24
In the past decade, much effort has been devoted to exploit a wide range of blue
fluorescent dyes. For instance, Gao et al. developed a phenanthro[9,10ꢀd]imidazole (PI)
derivative and applied it in a violetꢀblue OLED exhibiting CIE coordinates of (0.166, 0.056),
but the maximum external quantum efficiency (EQE) was only 3.02%.25 Zhang et al. reported
a blue emitter TBPMCN that can utilize ~100% excitons in a nonꢀdoped device.26 This OLED
2

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showed high performance with an EQEmax of 7.8% and a skyꢀblue EL emission (CIE
coordinates of (0.16, 0.16)). An effective violetꢀblue OLED based on a silicon containing
compound SiPIM was reported, which realized an EQE of 6.29% and a superior color index
of (0.163, 0.040).27 Unfortunately, the nonꢀconducting silicon molecular framework of SiPIM
leads to a high turnꢀon voltage (Von, 4.2 V) and low power efficiency (PE). So far, the most
effective violetꢀblue OLED was from a donorꢀacceptor (DꢀA) molecule with bisanthracene as
an emitting core, showing an EQE up to 12% and a color purity of (0.15, 0.06).28 It is
suggested that triplet fusion (TF) is responsible for such a high EL performance. Another
efficient deepꢀblue OLED reported by Adachi’s group employed a sulfone/carbazole hybrid
emitter that can emit thermally activated delayed fluorescence (TADF) with a maximum EQE
of 9.9% and CIE coordinates of (0.15, 0.07).29 Very recently, Hatakeyama and coꢀworkers
designed an ultrapure blue TADFꢀbased emitter DABNAꢀ1; its HOMO/LUMO can be
separated interatomically, which endows the corresponding device with a high EQE of 13.5%
and narrow blue EL spectra having CIE coordinates of (0.13, 0.09).30 Nevertheless, such high
efficiencies from the aboveꢀmentioned devices suffered from serious efficiency rollꢀoff at
high luminances (e.g. 1,000 cd mꢀ2), resulting in compromised performance at applicable
brightness. To date, few reports for highly efficient deepꢀblue OLEDs matching or
approaching the HDTV standard with slow efficiency rollꢀoff are available, except a handful
of recent literatures.31–33
Generally, common fluorescent dyes mainly rely on πꢀπ* transition upon excitation, and
emit highly efficient fluorescence from their locally excited (LE) states.34 However, spin flip
does not exist in OLEDs based on LEꢀbased emitters due to strong holeꢀelectron pairs with a
high exciton binding energy (~ 1 eV),35 causing relatively low electronꢀtoꢀphoton conversion
efficiency. On the other hand, charge transfer (CT) exciton, another type of excited species
with loosely bounded nature (~ 10 meV),35 typically in DꢀA based molecular structures, can
facilitate spin flip upon electrical charge injection and promote the exciton utilization
efficiency (ηexc) significantly.36 Nevertheless, CT emitters with strong DꢀA structures often
show redꢀshifted emissions accompanied with broad fullꢀwidths at haftꢀmaximum (FWHM),
which is detrimental for deepꢀblue color purity.23 Furthermore, low Φf is another drawback
because of the lack of overlap between frontier molecular orbitals.37 High Φf and ηexc are two
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prerequisites to obtain a decent EQE, but both of LE and CT emitters fail to meet these
requirements separately. To take full advantage of LE and CT excitons simultaneously, Ma
and colleagues have established a series of highly emissive DꢀA type fluorescent materials
that can employ more than 25% ηexc without using longꢀlived excitons.26,36,38 It is reported that
moderate DꢀA pairs can lead to hybrid local and charge transfer (HLCT) excited state, which
is responsible for achieving high Φf and large ηexc. In this excited system, loosely bounded
excitons from upper CTꢀlike triplet state can be converted to singlets via a “hot exciton”
channel, then produce photons from LEꢀlike singlet state.39 With these advantages, it is highly
promising to design deepꢀblue DꢀA fluorescent materials employing the concept of HLCT
excited states. However, up till now OLEDs using emitters featured with HLCT excited states
having CIEy coordinates below 0.06 and EQEs over 5% are still rarely reported.
Scheme 1 Chemical structures of target DꢀπꢀA based fluorophores.
In this work, we report three DꢀπꢀA based PIꢀcarbazole hybrid fluorophores—TPIBCz,
TPINCz and TPIBNCz—with different πꢀlinking moieties, and applied them as emitting core
in violetꢀblue OLEDs (Scheme 1). To obtain HLCTꢀbased shortꢀwavelength emitting
molecules, D and A have to be carefully selected to obtain appropriate electron pushꢀpull
strength such that both LE and CT excited states can be simultaneously achieved. In this
molecular design, PI is chosen as
a mild electron withdrawing group, while
Nꢀphenylꢀ9Hꢀcarbazole serves as a moderate electron donating segment. PI and carbazole are
both rigid planar groups, which are beneficial to achieving high Φf. We also systematically
studied the influence of π spacers, with different length and size between the D and the A
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moieties, on their photophysical properties and EL performances. All the materials emit
efficient violetꢀblue light and show excellent EL performances. It is worth noting that the
OLED based on TPINCz as an emissive dopant exhibits a violetꢀblue color index of (0.153,
0.059) and a high efficiency with EQE up to 6.96 ± 0.08%. In a practical luminance of 1,000
cd mꢀ2, the EQE is still up to 6.56 ± 0.11%, which is the highest value among the blue OLEDs
with CIEy coordinates below 0.08.
Results and discussion
Synthesis and characterization
Synthesis of the three new fluorescence materials is shown in Scheme S1 in the Electronic
Supplementary Information. First, the key intermediates AdhNꢀBr and AdhBNꢀBr were
prepared by asymmetrical Suzuki reactions between bibromo naphthyl starting materials and
(4ꢀformylphenyl)boronic acid under mild conditions.40 Then bromated PI derivatives TPIꢀBr,
TPINꢀBr and TPIBNꢀBr were synthesized via “oneꢀpot” reactions.41 The mixture of an
aromatic aldehyde (4ꢀbromobenzaldehye, AdhNꢀBr or AdhBNꢀBr), phenanthreneꢀ9,10ꢀdione,
4ꢀ(tertꢀbutyl)aniline, and ammonium acetate were reacted in refluxing CH3COOH under Ar
for 10 h. At last, target compounds were obtained by Suzuki reactions from bromated PI
intermediates (TPIꢀBr, TPINꢀBr and TPIBNꢀBr) and (Nꢀphenylꢀ9Hꢀcarbazolꢀ3ꢀyl)boronic
1
acid with good yields. The new compounds were characterized and determined by H/13C
NMR and mass spectroscopy.
Thermal properties
Thermal properties of the new materials were characterized by thermogravimetric analysis
(TGA) and differential scanning calorimetry (DSC) in a nitrogen atmosphere. The relevant
data are listed in Table 1. Joule heat, which results from carrier injection among interfacial
heterojunctions and nonꢀradiative energy loss, is common in operating devices.42 Thus good
thermal stability is of high significance for an OLED. Bulky and rigid structures of the new
compounds are derived from the bulky conjugated backbone of PI and carbazole. The
naphthylꢀbased compounds are more heatꢀresisting, with higher Tds (5% weight loss
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temperatures) of 480 and 510 °C (for TPINCz and TPIBNCz, respectively) over 439 °C of
TPIBCz (Fig. 1). Moreover, TPINCz and TPIBNCz have high ash values upon heating to
800 °C, nevertheless TPIBCz experiences completely weight loss, demonstrating that
adoption of naphthyl group is good for thermal stability.43 High glass transition temperatures
(Tg, inset of Fig. 1) also indicate superior thermal stability of the asꢀdesigned materials, which
are competent for general device fabrication and operation.
Fig. 1 Thermal properties of the new materials.
Table 1. Physical properties of TPIBCz, TPINCz and TPIBNCz.
Compd.
Tda (°C)
Tgb (°C)
Egc (eV)
HOMOd (eV)
LUMOe (eV)
λabsf (nm)
λemf (nm)
Φf f (%)
TPIBCz
439
144
3.04
ꢀ5.33
ꢀ2.29
338, 367 /
344, 370
335, 364 /
344, 369
334, 364 /
336, 368
395, 417, 428 / 414
85.2 / 64.2
TPINCz
480
510
165
192
3.06
3.14
ꢀ5.50
ꢀ5.54
ꢀ2.44
428 / 450
425 / 433
~100 / 90.5
~100 / 96.8
TPIBNCz
a
ꢀ2.40
b
c
Decomposition temperature (5% weight loss). Glass transition temperature. Optical
energy gap estimated from absorption onset in solid film. d Measured by cyclic voltammetry. e
Calculated from LUMO = HOMO + Eg. Measured in THF solution (10ꢀ6 mol Lꢀ1) and solid
f
film (30 nm), respectively.
Theoretical calculations
Theoretical calculations using density functional theory (DFT) method on B3LYP/6ꢀ31g(d,p)
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level are performed to study the structureꢀproperty relationship of the new compounds. All the
three compounds show similar molecular configurations apart from the central πꢀconjugated
segments (Fig. S1). Because the naphthyl group can provide a large steric hindrance, TPINCz
and TPIBNCz have more twisted configurations than TPIBCz.43 This indicates that TPINCz
and TPIBNCz can generate comparable emission to TPIBCz, although adoption of naphthyl
group seems to extend the whole molecular conjugation to some degree.
Fig. 2 Spatial distributions of the molecular orbitals and S0→S1 natural transition orbitals.
Fig. 2 shows spatial distributions of the molecular orbitals of the new compounds.
Except for TPIBNCz, HOMOs of the other two molecules delocalize over the whole
molecular frameworks with little contribution from the 4ꢀ(tertꢀbutyl)phenyl group. The
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HOMO energy levels of the three compounds have slight differences, while those of
HOMOꢀ1 gradually shift from ꢀ5.41 eV of TPIBCz to ꢀ5.25 eV of TPIBNCz. In addition, we
note that the energy offsets between the HOMO and the HOMOꢀ1 energy level become
narrower from TPIBCz to TPIBNCz, leading to a pseudoꢀdegenerate orbital pair in TPIBNCz.
On the other hand, because of the incorporation of the naphthyl group with relatively large
electron affinity,44 energy levels of the LUMO and the LUMO+1 are deepened progressively,
which is good for electron injection. Differences are also reflected in the orbital distribution,
with naphthyl/binaphthyl group dominates the electron density of LUMOs in TPINCz and
TPIBNCz. The naphthyl/binaphthyl group acts a week nꢀtype πꢀlinker in the DꢀπꢀA molecular
design, and plays a key role in manipulating photophysical property, which will be discussed
later. Unlike the completely separated HOMO/LUMO distributions in the strong DꢀA based
molecules, the relatively weak electron pushing and pulling strengths of the D and the A lead
to partially HOMOs/LUMOs overlaps in the present new compounds. TPINCz and TPIBNCz
display more CTꢀlike properties compared with TPIBCz due to the weak
electronꢀwithdrawing naphthyl group.
Natural transition orbital (NTO)45 analysis was implemented by timeꢀdependent DFT
(TDꢀDFT) calculations (based on TDꢀM062X/6ꢀ31G(d,p)) to further investigate the excited
state properties. As shown in Fig. 2, both “hole” and “particle” are distributed on the PI and
the central πꢀlinking moieties for S0 (singlet ground state)→S1 (first singlet excited state)
transitions in the three molecules, with little contribution from the Nꢀphenylꢀ9Hꢀcarbazole. It
indicates that PI may be an active emitting core, and the central πꢀlinking pattern can
influence photophysical properties significantly. For TPIBCz, “hole” and “particle” seem to
be located on the same part of the molecule, but π spacer contributes more in “hole”
distribution. In comparison, “hole” and “particle” distributions have certain superposition in
moiety from PI to naphthalene, but separated in the phenyl group between PI and naphthalene
in TPINCz and TPIBNCz. The NTO analysis indicates that the three molecules may have LE
characters and show partial CT features for S0→S1 transition. In addition, introduction of
naphthalene also enhances the oscillator strength (f), which helps to maintain high Φfs.46
Photophysical properties
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UVꢀvis absorption and photoluminescence (PL) spectra of the new compounds are shown in
Fig. 3. The relative data are summarized in Table 1. In diluted THF solution, TPIBCz,
TPINCz and TPIBNCz exhibit similar absorption bands, where the peaks between 330 and
340 nm are ascribed to the πꢀπ* transition of the 2ꢀsubstituent of imidazole to the PI group,
while the subꢀbands at longer wavelength region (~ 365 nm) are originated from PI’s πꢀπ*
transition.47 There are no evident broad absorptions, implying that the S0s are of negligible CT
feature. The absorptions in solid film on quartz show slight redꢀshifted spectra to those of
solutions. From the absorption onsets, optical energy gaps (Egs) are estimated to be 3.04, 3.06
and 3.14 eV, for TPIBCz, TPINCz and TPIBNCz, respectively. More twisted configurations
induced by the naphthyl spacers may be responsible for the wider Egs in TPINCz and
TPIBNCz. Combined with the results from cyclic voltammetry (Fig. S2), in which HOMO
levels are measured to be ꢀ5.33, ꢀ5.50 and ꢀ5.54 eV respectively, the LUMO levels are
calculated to be ꢀ2.29, ꢀ2.44 and ꢀ2.40 eV for TPIBCz, TPINCz and TPIBNCz, respectively.
All the materials can emit intense violetꢀtoꢀblue fluorescence in diluted THF solution, but
show quite different PL spectra. TPIBCz displays the highest energy emission in violetꢀblue
region with fine structural peaks maximized at 417 nm, which is similar to those observed in
isolated phenylꢀsubstituted PI.48,49 The relative Φf was measured to be 85.2%. Conversely,
adding a naphthyl or binaphthyl to the π spacer results in structureless, redꢀshifted and
relatively broad emissive bands stemmed from the effects of conjugated extension and
improved CT processes in transitions. Fluorescences of TPINCz and TPIBNCz are very
efficient, with Φfs approaching unity in THF solution. Interestingly, TPIBNCz with longer
conjugated dimension has shorterꢀwavelength emission than TPINCz. It indicates that effect
of conjugated confinement induced by the large dihedral angle in the twoꢀfoldꢀnaphthyl
spacer dominates the influence of conjugated extension after inserting another naphthyl. This
nicely meets the demand of molecular design for violetꢀblue emitters. In vacuumꢀevaporated
solid films prepared on clean quartz, although slight red shifts in emission are observed, the
influence of close packing on emission is not significant, as evidenced by the high Φfs (see
Table 1) and relatively narrow emission bands different from the typical excimer’s PL
spectra.50 TPIBCz’s film also exhibits LEꢀlike vibrational fine emission bands with main peak
at 414 nm. TPINCz shows the largest red shift in emission (peak at 450 nm), which results
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from polarization effect in solid.51 It implies that excited state of TPINCz is more sensitive to
polarity of medium than TPIBCz.
Fig. 3 Absorption and PL spectra of TPIBCz, TPINCz and TPIBNCz; solid line and dash line
are for THF solution (~10ꢀ6 mol Lꢀ1) and vacuumꢀevaporated film (30 nm), respectively.
To further study the excited state properties, solvation effects on photophysical properties
were investigated in solvents with different polarities (Fig. 4a and Fig. S3ꢀ5). Upon increasing
the solvent polarities gradually from nꢀhexane to acetonitrile (ACN), TPIBCz constantly
displays a sharp emission band, and exhibits a slight red shift of 8 nm, which is responsible
for polarityꢀinsensitive LE transition. For TPINCz and TPIBNCz, on the contrary, their PL
spectra not only exhibit significant red shifts, but also progressively broadened structureless
bands as orientation polarization (ꢁf) of solvent increases. Compared with TPIBCz, such
solvatochromic PL behavior in TPINCz and TPIBNCz implies CT properties in excited
states.52 The distinction of solvatochromic effect in TPIBCz and TPINCz/TPIBNCz is also in
accord with the hypothesis by calculated results that the different properties of excited state
are induced by the naphthyl spacers. Based on the LippetꢀMataga model,53,54 we further
estimated the dipole moments of excited state (ꢀe) from the linear fitting analysis of the
Stokes shift (vaꢀvf) and ꢁf. As shown in Fig. 4b, all the three materials display twoꢀsection
linear relations in low and high ꢁf regions, but the slopes of individual fittings are different.
The two slopes for TPIBCz’s fittings are estimated to be 8.2 D and 13.9 D, for low and high
ꢁf regions, respectively. Based on the small ꢀes, TPIBCz shows a LE excited state in the
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region of small ꢁf and a LEꢀdominating HLCT excited state in the region of large ꢁf. In
contrast, although relatively small ꢀes are obtained in TPINCz and TPIBNCz (12.6 and 9.6 D,
respectively) in solvents with small ꢁf values, steep slopes are evident in solvents of high ꢁf
values, corresponding to ꢀes of 18.0 and 20.1 D, respectively. The large ꢀes can be treated as
CTꢀlike characters. TPINCz and TPIBNCz show a mixture of LE and CT excited properties,
namely HLCT excited states. On the other hand, the two fitted lines for TPINCz show smaller
difference in their slopes comparing to those for TPIBNCz. With this observation, we
speculate that LE and CT excited states mix better in the former. In the previous publications,
efficient HLCT excited states in blue emitters are generally achieved by the means of
connecting different donor and acceptor moieties with single bond,55 different πꢀlinking
modes or tuning the strength of electron donating/withdrawing in D/A pairs.26 In contrast, we
can easily obtain accessible deepꢀblue emitters featured with HLCT excited states by
incorporation of a naphthyl/binaphthyl spacer with weak nꢀtype nature and high twisting
angles in π linking moiety, and this strategy can be applied to modify the common DꢀπꢀA to
invoke HLCT excited state.
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Fig. 4 (a) Solvatochromic PL spectra of TPIBCz, TPINCz and TPIBNCz in solvents with
increasing polarities; (b) Linear fitting based on LippertꢀMataga model in various solvents.
Electrical properties
Electrical properties of the three fluorophores were studied in single carrierꢀonly type devices.
The
structure
of
holeꢀonly
devices
is
indium
tin
oxide
(ITO)/
N,N'ꢀbisꢀ(1ꢀnaphthalenyl)ꢀN,N'ꢀbisꢀphenylꢀ(1,1'ꢀbiphenyl)ꢀ4,4'ꢀdiamine (NPB, 10 nm)/ one of
the new compounds (70 nm)/ NPB (10 nm)/ Al (150 nm), and the electronꢀonly devices have
a structure of ITO/ 2,9ꢀdimethylꢀ4,7ꢀdiphenylꢀ1,10ꢀphenanthroline (BCP, 10 nm)/ one of the
new compounds (70 nm)/ BCP (10 nm)/ LiF (1 nm)/ Al (150 nm). The current densityꢀvoltage
(JꢀV) curves are shown in Fig. S6. Obviously, similar to most of the common organic
semiconductors, the compounds have much better conductivity for hole than for electron. The
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relatively low hole transporting property of TPIBNCz may be caused by the highly twisting
molecular configuration, as such geometry is not beneficial to charge flowing
intramolecularly. On the other hand, the planar molecule TPIBCz has the highest electron
mobility. The TPINCzꢀbased electronꢀonly device has lower current density than its TPIBNCz
counterpart. This can be attributed to the incorporation of the binaphthyl spacer can enhance
LUMO overlap in solid, although the highly twisting geometry more or less hinders charge
transport.43
Electroluminescence performances
To evaluate the EL performances of the new materials as emitting cores, we initially
fabricated three nonꢀdoped OLEDs with a structure of ITO/ NPB (70 nm for the TPIBCz and
the TPIBNCz based devices, 50 nm for the TPINCz based OLED)/ TcTa
(tris(4ꢀcarbazoylꢀ9ꢀylphenyl)amine,
5
nm)/ one of the emitters (30 nm)/
1,3,5ꢀtris(1ꢀphenylꢀ1Hꢀbenzimidazolꢀ2ꢀyl)benzene (TPBi, 30 nm)/ LiF (1 nm)/ Al (100 nm),
in which ITO is the transparent anode, NPB is used as a hole transporting layer, TcTa as a
buffer and exciton confining layer,24 TPBi as an electron transporting and hole blocking layer,
LiF as an electron injection layer and Al is the cathode. Although the emitters used here have
better hole than electron transporting properties, the unfavorable effects due to the unbalanced
electron and hole currents are partly remedied by the exciton confining structures. The thin
TcTa layer can hinder the hole flow in the device due to additional interfacial heterojunction,
and confine the exciton in emissive layer original from its high Eg (~ 3.4 eV). Besides, the
deep HOMO level (ꢀ6.30 eV) of TPBi can also block the hole in the devices. Fig. 5 shows the
device performances, and key performance data are summarized in Table 2. The OLED using
TPINCz displays the highest EQE of 5.95 ± 0.10% and deepꢀblue EL emission with CIE
coordinates of (0.157, 0.084) (at 1,000 cd mꢀ2). It is worth noting that this device does not
show serious efficiency rollꢀoff. At a practical luminance of 1,000 cd mꢀ2, a highꢀlevel EQE of
5.83 ± 0.06% is still maintained. The maximum current efficiency (CE) and PE are up to 5.00
± 0.14 cd Aꢀ1 and 5.15 ± 0.15 lm Wꢀ1, respectively. The devices based on TPIBCz and
TPIBNCz show slightly lower performances, having maximum EQEs of 3.38 ± 0.10% and
5.09 ± 0.13%, respectively. But their EL spectra shift to the shorterꢀwavelength region with
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peaks at 435 and 436 nm, and the corresponding CIE coordinates are (0.154, 0.063) and
(0.157, 0.074) at 1,000 cd mꢀ2, respectively. Fig S7 shows the EL and PL spectra of the
nonꢀdoped devices and solid thin films respectively. Two types of spectra have slight
difference in bandwidth which might be due to weak microꢀcavity effects in the devices. The
EL spectra are comparable to the solid film PL spectra, indicating that the devices emit
intrinsic emission from the title fluorophores. It is suggested that the highest performances
attained in TPINCzꢀbased could be ascribed to the high CT component in the excited state
and better hybridization of LE and CT states. Fig. S8 shows the calculated firstꢀten singlet and
triplet energy levels of the three molecules. Some upper triplet states are close to their
corresponding S1s, like T5/T6, T7 and T9 for TPIBCz, TPINCz and TPIBNCz respectively. In
addition, the NTOs of these highꢀlying triplet states exhibit complicated transitions with
similar contributions and HLCT excited properties (Fig. S9ꢀ11). This implies that this high
performance may be due to the potential “hot exciton” channels through reverse interꢀsystem
crossing (RISC) from upper triplet states to S1 (red arrows in Fig. S8). Contribution to these
high efficiencies from TF and TADF are considered to be insignificant. Linear relation of the
luminanceꢀcurrent density (LꢁJ) curves (Fig. S12) implies that few emissive triplet excitons
via TF,56 and large S1 and T1 energy gaps (Fig. S8) as well as certain HOMOs/LUMOs
overlap (Fig. 2) indicate that the new molecules may not emit TADF.7
Fig. 5 (a) Current densityꢀvoltageꢀluminance characteristics, (b) EQEꢀluminance curves, (c)
EL spectra of the TPIBCz, the TPINCz and the TPIBNCz based nonꢀdoped OLEDs.
Table 2 EL performances of the OLEDs based on TPIBCz, TPINCz and TPIBNCz.
Emissive layer
TPIBCz
Vona (V)
λEL (nm)
CIEb (x, y)
CEmaxc (cd Aꢀ1)
PEmaxd (lm Wꢀ1)
EQEmaxe (%)
3.0
435
0.154, 0.063
1.70 ± 0.02
1.44 ± 0.05
3.38 ± 0.10
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TPINCz
3.1
3.2
3.7
3.3
3.2
3.3
3.1
3.1
3.4
3.2
3.1
448
436
428
432
432
436
440
440
424
428
428
0.157, 0.084
5.00 ± 0.14
3.29 ± 0.08
1.91 ± 0.02
1.96 ± 0.02
2.10 ± 0.04
3.02 ± 0.15
3.71 ± 0.11
3.75 ± 0.06
1.89 ± 0.05
2.34 ± 0.04
2.55 ± 0.05
5.15 ± 0.15
2.80 ± 0.09
1.72 ± 0.02
1.68 ± 0.04
2.20 ± 0.08
2.53 ± 0.20
3.71 ± 0.15
3.36 ± 0.05
1.51 ± 0.10
2.14 ± 0.04
2.24 ± 0.10
5.95 ± 0.10
TPIBNCz
0.157, 0.074
0.156, 0.043
0.156, 0.046
0.156, 0.047
0.155, 0.054
0.153, 0.059
0.153, 0.062
0.158, 0.044
0.157, 0.048
0.157, 0.051
5.09 ± 0.13
5.06 ± 0.05
5.46 ± 0.07
5.47 ± 0.10
6.60 ± 0.20
6.96 ± 0.08
6.71 ± 0.12
5.48 ± 0.15
5.99 ± 0.05
6.03 ± 0.14
10 wt% TPIBCz
20 wt% TPIBCz
30 wt% TPIBCz
10 wt% TPINCz
20 wt% TPINCz
30 wt% TPINCz
10 wt% TPIBNCz
20 wt% TPIBNCz
30 wt% TPIBNCz
a Voltage at 1 cd mꢀ2. b Detected at 1,000 cd mꢀ2. c Current efficiency, d power efficiency and e
external quantum efficiency at maximum.
To further improve the efficiency and color purity, we also fabricated doped devices with
a structure of ITO/ NPB (70 nm)/ TcTa (5 nm)/ CBP (4,4'ꢀbis(Nꢀcarbazolyl)ꢀ1,1'ꢀbiphenyl):
one of emitters (30 nm)/ TPBi (30 nm)/ LiF (1 nm)/ Al (100 nm). Influence of different
doping levels (10, 20 and 30 wt%) on the device performances was investigated. CPB, a
highꢀenergyꢀgap compound, was chosen as a host material. In the hostꢀguest system,
efficiency can improved significantly due to efficient Förster energy transfer.57 Besides, in the
host matrix, better color purity, wider recombination zone and less nonꢀradiative relaxation
can be expected. Fig. 6aꢀc show the EL spectra of the devices. All the doped devices show
obviously blueꢀshifted EL spectra compared with the corresponding nonꢀdoped devices. This
can be mainly attributed to the decrease of the medium polarity, since CBP is a nonꢀpolar host
material.58 As the doping concentration decreases, finer structural EL spectra are observed in
the TPIBCzꢀbased devices, which is in accord with the LEꢀlike photophysical behavior of
TPIBCz. The corresponding CIEy coordinate shifts from 0.063 (nonꢀdoped) to 0.043 (10 wt%
in CBP). On the other hand, the EL spectra of the TPINCz and the TPIBNCzꢀbased OLEDs
are relatively more sensitive to doping ratios, which may be responsible for CT excited
properties. For example, the EL spectra of the TPINCzꢀbased doped devices show an
evidently hypsochromic shift from deepꢀblue region (λEL = 448 nm, CIE: (0.152, 0.084)) in
the nonꢀdoped device to violetꢀblue region (λEL = 436 nm, CIE: (0.155, 0.054)). Similar
changes are observed in the TPIBNCzꢀbased OLEDs. The doped devices show superior
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violetꢀblue EL emission, their corresponding CIEy coordinates range from ~ 0.04 to 0.06,
even surpassing the standard blue for National Television System Committee (NTSC) of (0.14,
0.08) and HDTV of (0.15, 0.06), as shown in Fig. S13. Fig. 6dꢀf show the EQEꢀLꢀPE curves
for the doped devices, and their JꢀVꢀL characteristics are shown in Fig. S14. The doped
OLEDs show improved efficiency. The maximum EQE of the TPIBCzꢀbased 10 wt% doped
OLEDs reaches 5.06 ± 0.05%, comparing to the value of 3.38 ± 0.10% in the nonꢀdoped
device. However, the Von is raised to 3.7 V, mainly due to the larger energy gap of CBP. Upon
increasing the doping concentration from 10 to 30 wt%, the device efficiency first increases
and then nearly saturates at 30 wt%. The electrical property also followed this trend: current
density is the highest and the Von decreases to 3.2 V in the 30 wt% TPIBCzꢀbased device.
Similar changes are observed in the TPINCz and the TPIBNCzꢀbased OLEDs. However, the
EL spectra gradually shift to longerꢀwavelength region as the dopant concentration increases.
Comprehensively speaking, we suggest that the optimized doping level is 20 wt%. Among all
the device fabricated, the TPINCzꢀbased doped device (20 wt%) exhibited the highest
performance, with an EQE up to 6.96 ± 0.08% and a decent maximum PE of 3.71 ± 0.15 lm
Wꢀ1. The highest performance of the TPINCzꢀbased device may be mainly ascribed to the
better mixed LE and CT components, resulting in a more efficient HLCT excited. EL
performances of the reported highꢀefficiency deepꢀblue OLEDs are summarized in Table S2,
23,24,27,28,31–33,59–69
and illustrated in Fig. 7.
It can be seen that few OLEDs can emit efficient
violetꢀblue light with EQE over 5% at a luminance of 1,000 cd mꢀ2. The OLEDs based on our
new emitters exhibit superior efficiencies over most of the reported devices with CIEy < 0.08
(NTSC standard) at a luminance of 1,000 cd mꢀ2. More importantly, the TPINCzꢀbased doped
OLED (red solid star) shows a champion performance with an EQE of 6.56 ± 0.11% at 1,000
cd mꢀ2.
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Fig. 6 EL spectra of the CBPꢀdoped devices based on a) TPIBCz, b) TPINCz and c)
TPIBNCz, respectively. EQEꢀluminance curves for the CBPꢀdoped devices based on e)
TPIBCz, f) TPINCz and g) TPIBNCz, respectively.
Fig. 7 A plot of EQE at 1,000 cd mꢀ2 against CIEy for the stateꢀofꢀtheꢀart deepꢀblue OLEDs
with CIEy ≤ 0.08. Data points marked with empty circles and reference number are taken
from published devices. Details of these devices are listed in the Electronic Supplementary
Information (Table S2). Data point marked with empty and solid stars are devices in this work
by using the three emitters in nonꢀdoped and doped emitting layers respectively.
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Conclusions
We have designed and synthesized three DꢀπꢀA type PIꢀcarbazole hybrid fluorophores:
TPIBCz, TPINCz and TPIBNCz. It is found that the pattern of the π spacers have significant
influences on photophysical properties and device performances. The incorporation of a
naphthyl/binaphthyl
group
between
and
1ꢀ(4ꢀ(tertꢀbutyl)phenyl)ꢀ2ꢀphenylꢀ1Hꢀphenanthro[9,10ꢀd]imidazole
Nꢀphenylꢀ9Hꢀcarbazole can alleviate a common dilemma that enhancing device efficiency by
increasing CT excited properties often leads to redꢀshifted emission. Quantum calculations
and photophysical experiments demonstrated that naphthylꢀlinked TPINCz and TPIBNCz
show remarkable HLCT excited states, while TPIBCz exhibits a LEꢀdominating character.
Our materials can emit violetꢀblue emission with high Φfs in solid, indicating that they are
promising candidates for highꢀperformance violetꢀblue emitters in OLED applications. The
doped device of TPINCz shows violetꢀblue EL with CIE coordinates of (0.153, 0.059), which
are very close to the HDTV blue color standard of (0.15, 0.06). Its maximum EQE can reach
6.96 ± 0.08%. Additionally, this value retains 94% of the peak value at a practical brightness
of 1,000 cd mꢀ2, showing a very low efficiency rollꢀoff. The performances of the OLEDs
reported here are among the best value in the devices with CIEy ≤ 0.08. Our study may
provide a new pathway for designing highꢀperformance violetꢀblue emitters in OLED
application.
Acknowledgement
CSL would like to acknowledge the supports from the Ministry of Science and Technology of
China (Project No. 2016YFB0401002) and Guangdong Innovative and Entrepreneurial
Research Team Program (Project Nos. 2013C090 and KYPT20141013150545116); QXT
acknowledges the supports from the Natural Science Foundation of China (Project Nos.
51273108 and 51673113) and the National Basic Research Program of China (973 Program
Nos. 2013CB834803).
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Graphical Abstracts
Naphthylꢀlinked donorꢀπꢀacceptor fluorophores were utilized to achieve high performances
and good color purity violetꢀblue emission in organic lightꢀemitting devices (OLEDs).
8.11 × 3.79 cm
22