Dibenzo[: C, g] indolo[3,2,1- jk] carbazole as a new chromophore for blue organic light-emitting diodes

Article abstract of DOI:10.1039/c9tc04783b

A novel sky-blue fluorescent emitter was developed from a novel dibenzo[c,g]indolo[3,2,1-jk]carbazole chromophore. A photoluminescence quantum yield of 94.1%, maximum quantum efficiency of 6.2%, high luminance over 80000 cd m^-2, and 4.8 times extension of the operational lifetime compared to that of a pyrene derived blue emitter were realized from the fabricated device.

Full text of DOI:10.1039/c9tc04783b

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Materials Chemistry C
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Dibenzo[c,g]indolo[3,2,1-jk]carbazole as a new
chromophore for blue organic light-emitting
diodes†
Cite this: J. Mater. Chem. C, 2019,
7, 14301
Vilas Venunath Patil, Kyung Hyung Lee and Jun Yeob Lee
*
Received 30th August 2019,
Accepted 16th October 2019
A novel sky-blue fluorescent emitter was developed from a novel dibenzo[c,g]indolo[3,2,1-jk]carbazole
chromophore. A photoluminescence quantum yield of 94.1%, maximum quantum efficiency of 6.2%,
high luminance over 80 000 cd mꢀ2, and 4.8 times extension of the operational lifetime compared to
that of a pyrene derived blue emitter were realized from the fabricated device.
DOI: 10.1039/c9tc04783b
rsc.li/materials-c
The development of highly efficient blue emitters has remained used which promoted the structural relaxation in the excited
a daunting task in the realm of organic light-emitting diodes state thereby increasing the Stokes shift. This, in turn, resulted
(OLEDs). Since the emission of blue fluorescent emitters is due in a higher value for the half-width at full maxima (FWHM) due
to the radiative decay of singlet excitons, the internal quantum to broadening of the electroluminescence (EL) peak and thus
efficiency (IQE) of blue fluorescent emitters is limited to 25%.1,2 lowering the color purity of blue TADF emitters.17,21,22
On the other hand, high efficiency can be achieved from blue
Therefore, the development of highly efficient and stable
phosphorescent and thermally activated delayed fluorescent blue fluorescent emitters has gained more importance in recent
(TADF) emitters. Both phosphorescent and TADF emitters can years. Blue fluorescent emitters show low-efficiency roll-off at
harvest all the generated excitons by means of the heavy atom high brightness, small full width at half maximum (FWHM),
effect3,4 and reverse intersystem crossing (RISC) process to offer and good device lifetime compared to blue phosphorescent and
100% IQE,5,6 respectively. However, the long excited-state life- TADF emitters.22 Moreover, the recent advancements in the new
time of blue phosphors and TADF emitters in a triplet excited molecular designs for blue fluorescent emitters have demonstrated
state gives rise to degradation by triplet–triplet annihilation promising results. The low-efficiency issue of conventional fluores-
(TTA) and triplet–polaron annihilation (TPA) processes.7–11 In cent emitters has been improved considerably by hybrid local-charge
order to improve the device lifetime of blue phosphorescent transfer (HLCT), triplet–triplet annihilation (TTA), and ‘‘hot exciton
and TADF emitters, great effort is still needed to suppress the pathway’’ processes. The chromophore, auxo-chromophore,
degradation by triplet excitons. Moreover, scarcity and high cost molecular geometry, substituents, and the substituent position
of transition metals such as Ir, Pt, Os, and Rh are also one of the play a vital role in the development of blue fluorescent emitters.
limitations in the development of phosphorescent emitters.12
They all collectively govern the physical and optical properties
In view of this, third generation blue TADF emitters are of the materials. Among these, the chromophore is the most
promising alternatives as metal-free and highly efficient important factor governing the light-emitting properties of
candidates.5,6 Although, blue TADF emitters have exhibited blue fluorescent emitters. Therefore, an understanding of the
above 20% quantum efficiency with over 10 000 cd mꢀ2 luminance, structure–property relationship is of great importance while
similar to blue phosphorescent emitters, they suffer from a major designing a chromophore aimed for specific applications.
drawback of efficiency roll-off at high luminance.13–20 Moreover,
Blue emitters bearing blue chromophores such as pyrene,23–26
the small energy gap (DEST) between the first singlet (S1) and triplet anthracene,27–30 phenanthrene,31–34 and perylene35–37 have
(T1) states, which is essential for an efficient RISC process, can be demonstrated excellent outcomes in the last few years. Pyrene is
achieved by separating the highest occupied molecular orbital the most well-known chromophore of blue fluorescent emitters.
(HOMO) and the lowest unoccupied molecular orbital (LUMO). A pyrene based HLCT emitter reported by Fu et al. has a high
To achieve this, donor–acceptor type structural designs were quantum efficiency of 10.5% with a maximum luminance of
10480 cd mꢀ2. However, it suffered from severe roll-off as the
efficiency was reduced to 4.6% at a luminance of 1000 cd mꢀ2 38
A pyrene-based 3Me-1Bu-TPPDA emitter demonstrated a high
efficiency of 9.25% and a long device lifetime over 30 000 h,
confirming the long lifetime characteristics of pyrene derivatives.24
.
School of Chemical Engineering Sungkyunkwan University, 2066 Seobu-ro,
Jangan-gu, Suwon, Gyeonggi 440-746, Korea. E-mail: leej17@skku.edu
† Electronic Supplementary Information (ESI) available: Fluorescence spectra,
DFT studies, TGA analysis, and NMR spectral details of the compounds. See DOI:
10.1039/c9tc04783b
Anthracene has also been popular as
a chromophore.
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Anthracene based TTA emitters, 3CzAnBzt and p-CzAnBzt,
Furthermore, a novel blue fluorescent emitter, N,N-diphenyl-
reported by Liu et al. exhibited high efficiencies of 10.06 and dibenzo[c,g]indolo[3,2,1-jk]carbazol-14-amine (DBICz-DPA) was
9.2%, respectively, and showed low-efficiency roll-off at high synthesized by functionalizing the DBICz chromophore with
brightness.39 A phenanthroimidazole–anthracene based D–A type the diphenylamine donor. The theoretical, photophysical and
TTA emitter, PIAnCN, demonstrated excellent device performance. electrochemical properties of the DBICz-DPA emitter were
The device fabricated using the PIAnCN dopant showed a maximum thoroughly investigated. The devices fabricated by using the
EQE of 9.4% along with an exceptionally high luminance of DBICz-DPA emitter showed a high EQE of 6.2%, a high max-
57 787 cd mꢀ2 40
‘‘hot exciton path’’ to achieve 410% EQE using a phenanthro- roll-off at high brightness. The high quantum efficiency of the
imidazole–anthracene chromophore.41
DBICz-DPA based device was due to the relatively high photo-
.
Recently Ma et al. have demonstrated the imum luminance over 80 000 cd mꢀ2, and negligible efficiency
The current literature survey about blue fluorescent emitters luminescence quantum yield of 94.1%. The device results
clearly demonstrates that the efficiency of blue emitters can be obtained from the DBICz-DPA emitter demonstrated that DBICz
greatly improved by developing efficient chromophores. How- can serve as an efficient chromophore to develop high-efficiency
ever, further improvements in the device lifetime, efficiency blue emitters which are useful to formulate stable and efficient
roll-off, and maximum luminance are still desired. Therefore, devices for OLED applications.
the development of a novel chromophore is yet to be explored
The synthetic pathway for the synthesis of the DBICz-DPA
for further advancement of the device performance of blue emitter is shown in Scheme 1. The intermediate dibenzo-
OLEDs. In general, a rigid chromophore is an ideal candidate carbazole 1a was synthesized by acid-catalyzed cyclization of
for constructing highly efficient blue emitters because the rigid [1,10-binaphthalene]-2,20-diamine in moderate yield.47 The
molecular structure of a chromophore can effectively restrict intermediate 1a was reacted with 1-bromo-2-fluorobenzene to
free bond-rotations to reduce the vibronic coupling and thus give 2a in good yield. DBICz (3a) was synthesized in moderate
suppress the non-radiative decay process. Overall, it results in yield by palladium(II) acetate [Pd(OAc)2] catalyzed cyclization of
enhanced efficiency of the emitter.1,2,42
2a.48 Bromination of 3a was done using N-bromosuccinimide
These crucial findings encouraged us to develop a blue (NBS) at room temperature. The desired mono-brominated
fluorescent emitter containing a fused chromophore with high product 4a was obtained in good yield. Finally, the DBICZ-
rigidity. 7H-dibenzo[c,g]carbazole (DBCz), a weak and bulky DPA emitter was synthesized by treating 4a with diphenylamine
donor, was recently used as a donor for dye-sensitized solar using a bis(dibenzylideneacetone)palladium (0)-tri-tert-butyl-
cells (DSSC),43 TADF dendrimers44 and for the synthesis of phosphine [Pd(dba)3-P(tBu)3] system in moderate yield.49
BODIPY dyes.45 DBCz derivatives are known to possess good All the products were purified by column chromatographic
thermal stability.46 Based on the weak electron-donating prop- techniques. The final product DBICz-DPA was further purified
erties, bulky structure, and good thermal stability of DBCz by a vacuum sublimation method to give a highly pure material
based materials, we decided to modify it as a rigid chromo- for OLED application. The intermediates and final products
1
phore for OLED applications. The unshared pair of electrons were confirmed by mass spectroscopy and H and 13C-nuclear
on the sp2-N atom of DBCz was shared with an additional magnetic resonance spectroscopy (1H and 13C NMR).
phenyl ring followed by its fusion with DBCz. Thus, a novel
Density functional theory (DFT) calculations were performed
dibenzo[c,g]indolo[3,2,1-jk]carbazole (DBICz) skeleton was for DBICz and DBICz-DPA to understand the frontier molecular
developed as a rigid blue-light-emitting chromophore. Apart orbital (FMO) distribution and photophysical parameters. The
from a rigid molecular structure, DBICz displayed excellent DFT calculations were carried out by using B3LYP/6-311+G**
properties such as wide bandgap (Eg – 3.02 eV), good photo- basic sets of the Gaussian 09 software. The optimized struc-
luminance quantum yield (PLQY – 58%), and emission in the tures and the highest occupied molecular orbital (HOMO) and
deep blue region lPL – 425 nm (dilute THF solution) and lowest unoccupied molecular orbital (LUMO) distribution are
444 nm (film PL-DBICz doped at 3 wt% concentration in the shown in Fig. S1 (ESI†). DBICz had a fused molecular system
polystyrene film). These findings rank DBICz as a potential and showed a planar structure. The substitution of the DPA
chromophore candidate for the development of blue fluores- auxochromophore on DBICz gave a twisted structure to DBICz-
cent emitters for OLED applications.
DPA with a twist angle of 46.771 from the DBICz plane. The HOMO
Scheme 1 Synthetic route for the target molecule.
14302 | J. Mater. Chem. C, 2019, 7, 14301--14305
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of DBICz was distributed all over the core structure, while that red-shifted. On the other hand, in the film due to high viscosity,
of DBICz-DPA was distributed over the DPA auxochromophore such relaxation of the excited state is lower than in the solvent.
and partially extended to the DBICz chromophore. The HOMO/ This resulted in blue shifting of the emission in the solid PL
LUMO values for DBICz and DBICz-DPA were obtained as ꢀ5.60/ compared to in the solution. The ES of DBICz-DPA, estimated
ꢀ2.04 and ꢀ5.24/ꢀ2.06 eV, respectively. The shallow HOMO of from the onset of solid PL, was found to be 3.10 eV. To
DBICz-DPA is attributed to the strong electron-donating DPA investigate the ICT features of the DBICz-DPA emitter, its emis-
auxochromophore. The LUMO of DBICz was located on the sion profile was investigated in different solvents with varying
dibenzocarbazole part and that of DBICz-DPA was dispersed polarities. The solvatochromic study data are shown in Fig. 1b.
over the DBICz chromophore. Thus, the HOMO–LUMO distribu- The emission of DBICz-DPA was red-shifted by 37 nm as the solvent
tion in DBICz-DPA demonstrated a considerable overlap of the polarity increased from non-polar cyclohexane (CH – 457 nm) to
frontier orbitals. The theoretically calculated LUMO values were highly polar N,N-dimethyl formamide (DMF – 494 nm). Moreover,
found to be nearly the same for DBICz and DBICz-DPA owing to the FWHM also increased from 55 nm in CH to 73 nm in DMF,
the LUMO distribution on the DBICz core. The HOMO–LUMO indicating the broadening of the emission peak with the
gaps for DBICz and DBICz-DPA were obtained as 3.56 and increasing solvent polarity. The broad red-shifted emission
3.18 eV, respectively. The singlet energies (ES) for DBICz and upon changing the solvent polarity indicates the ICT feature
DBICz-DPA were found to be 3.20 and 2.72 eV, respectively.
of an excited state of the DBICz-DPA emitter. Thus, the photo-
The photophysical properties of the DBICz-DPA emitter were physical study of DBICz-DPA suggested that the origin of the
investigated by ultraviolet-visible (UV-vis) and fluorescence singlet emission was due to CT emission.
spectroscopy to comprehend the correlation between the mole-
The transient PL and photoluminescence quantum yield
cular structures and the absorption-emission properties. UV-vis (PLQY) measurements of the material were performed using
absorption spectra were recorded in dilute THF solution at 3 wt% emitter doped PANB film at room temperature.
room temperature. Photoluminance (PL) measurements were DBICz-DPA showed a strong prompt fluorescence component
carried out using dilute THF solution and the solid film was with a very short lifetime of 6.5 ns without any long decay
prepared by doping DBICz-DPA in the (2-(10-phenylanthracen- component, proposing that DBICz-DPA is a conventional fluo-
9-yl)naphtho[2,3-b]benzofuran) (PANB) host at 3 wt% concen- rescent emitter. The PLQY under a nitrogen atmosphere was
tration. The absorption and PL spectra are shown in Fig. 1a and found to be 94.1%. The high PLQY of the DBICz-DPA emitter
the data obtained are presented in Table S1 (ESI†).
supports the appropriateness of DBICz as the chromophore of
The strong absorption bands below 300 nm are assigned to blue light-emitting materials.
the p–p* transitions and the red-shifted shoulder peaks between
Cyclic voltammetry (CV) analysis of DBICz-DPA was performed
300 and 350 nm are due to the n–p* transitions. The long- in dilute dichloromethane solution at room temperature and the
wavelength weak absorption peaks above 350 nm correspond to results obtained are listed in Table S1 (ESI†). The HOMO was
the intramolecular charge transfer (ICT) absorption bands from evaluated from the oxidation potential while the LUMO was
DPA to the DBICz. The optical bandgaps (Eg) of DBICz-DPA were calculated from the HOMO and the optical bandgap. The HOMO
estimated to be 2.95 eV. The emission maximum of DBICz-DPA and LUMO values of DBICz-DPA were found to be ꢀ5.68 and
in dilute THF solution was 477 nm. The solid PL of DBICz-DPA ꢀ2.73 eV, respectively. The HOMO level of DBICz-DPA was
was blue-shifted to 469 nm with a FWHM of 60 nm. The possible dominated by the DPA unit, while the LUMO level was governed
justification for blue-shifted emission in solid PL could be the by the DBICz unit as described in the FMO calculation results.
high polarity of THF than that of the PANB host. DBICz-DPA is The heat generated during the operation of the OLED device may
a donor–acceptor type molecule with intramolecular charge alter the morphology of the emitter. This may result in the
transfer (ICT) in its S1 state. Due to the ICT, the dipole moment degradation of the emitter and thus negatively impact the device’s
of DBICz-DPA increases in the S1 state compared to the S0 state. lifetime. Therefore, the emitter with excellent thermal stability is
In the case of THF due to strong solvent relaxation, emission was highly desirable for the OLED application. In order to investigate
the thermal stability of DBICz-DPA, thermogravimetric analysis
(TGA) was performed. The TGA curve for DBICz-DPA is shown in
Fig. S2 (ESI†) and the data obtained are presented in Table S1
(ESI†). DBICz-DPA showed excellent thermal stability with a high
decomposition temperature (Td) of 408.2 1C at 5% weight loss.
The high Td value for DBICz-DPA evidently implies the high
stability of the material during the device fabrication process.
The glass transition temperature of the materials could not be
detected during the heating scan.
DBICz-DPA demonstrated outstanding fluorescence properties
to explore it as an emitter for OLED applications. To optimize the
device performances, three multilayer devices named devices A, B,
Fig. 1 (a) The UV-visible, solution, and solid PL spectra of DBICz-DPA and
and C with DBICz-DPA doping concentrations of 3, 5, and 7%,
(b) photoluminescence spectra of DBICz-DPA in different solvents with
varying polarities.
respectively, were fabricated. The optimized device configuration
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C were (0.13, 0.23)/472 nm/52 nm, (0.13, 0.25)/473 nm/50 nm,
and (0.13, 0.26)/475 nm/52 nm, respectively. All the three devices
exhibited sky-blue emission with narrow FWHM. The well match-
ing EL spectra for all the three devices with the film PL spectrum
revealed the efficient recombination of charge carriers (holes and
electrons) in the EML. Further, the operational lifetimes of all the
three devices A, B, and C were evaluated at an initial brightness of
1000 cd mꢀ2. The lifetime of DBICz-DPA devices was compared
with that of the reference device fabricated by using a pyrene
based 4,40-(pyrene-1,6-diylbis(phenylazanediyl)) dibenzonitrile
(PyCN) emitter (Fig. 2d). The lifetime values up to 90% of the
initial luminance were 32.9, 32.9, 26.9, and 6.8 h in the A, B, C,
and reference PyCN devices, respectively. Compared with the
reference device with the PyCN emitter, the lifetime was extended
by 79%, indicating the superiority of the DBICz-DPA emitter as
an efficiency and lifetime boosting blue fluorescent emitter.
Although the lifetime is shorter than the state of the art lifetime
of a blue emitter, the DBICz-DPA emitter performed as a stable
emitter. The high quantum efficiency and operational stability of
the DBICz-DPA device indicate that the DBICz-DPA emitter is a
promising candidate for practical applications in blue OLED.
In summary, a novel DBICz chromophore and a DBICz
Fig. 2 (a) current density–voltage–luminance (I–V–L) curve, (b) quantum
efficiency–luminance (EQE–L) curve, (c) El spectra of A, B, and C devices
and (d) the lifetime of devices A, B, C, and reference PyCN measured at
brightness 1000 cd mꢀ2
.
was, ITO (50 nm), BPBPA:HATCN (40 nm: 30%)/BPBPA (10 nm)/ based DBICz-DPA blue emitter were developed for blue OLEDs.
LBIL001 (10 nm)/PANB: emitter (3/5/7%) (30 nm)/TNPT (5 nm)/ The DBICz-DPA emitter demonstrated a high PLQY of 94.1%
ZADN (20 nm)/LiF (1.5 nm)/Al (200 nm). Where, ITO (indium with a maximum external quantum efficiency of 6.2% as a blue
4
4
4
4
0
0
tin oxide) is an anode, BPBPA:HATCN (N ,N ,N ,N -tetra([1,10- emitter. Moreover, the DBICz-DPA emitter not only showed an
biphenyl]-4-yl)-[1,10-biphenyl]-4,40-diamine: dipyrazino[2,3-f:20,30- exceptionally high luminance of above 80 000 cd mꢀ2 but also
h]quinoxaline-2,3,6,7,10,11-hexacarbonitrile) was used as a hole showed an extended lifetime compared with the conventional
injection layer, BPBPA was utilized as a hole transport layer, TNPA pyrene based blue emitter. Thus, the DBICz chromophore
(tris(4-(naphthalen-1-yl)phenyl)amine) was applied as an electron can serve as an efficient and high performing core for OLED
blocking layer, PANB (2-(10-phenylanthracen-9-yl)naphtho[2,3-b]- applications to develop exceptionally stable blue OLED devices.
benzofuran) was employed as a host, PANB: DBICz-DPA was used
as an emitting layer (EML), TNPT (2,4,6-tris(4-(naphthalen-1-
yl)phenyl)-1,3,5-triazine) was adopted as a hole blocking layer,
Conflicts of interest
and ZADN (2-(4-(9,10-di(naphthalen-2-yl)anthracen-2-yl)phenyl)-1-
phenyl-1H-benzo[d]imidazole) was chosen as an electron trans-
port layer.
There are no conflicts to declare.
The current density–voltage–luminance (I–V–L) curve, quantum Acknowledgements
efficiency–luminance (EQE–L) curve, EL spectra of the DBICz-DPA
This work was supported by NRF-2019M3D1A2104067 funded
devices, and the lifetime curves are shown in Fig. 2(a)–(d); the
by National Research Foundation of Korea.
schematic device diagram is shown in Fig. S3 (ESI†), the current
efficiency–luminance curve is shown in Fig. S4 (ESI†), and the data
obtained for devices A, B, and C are displayed in Table S2
(ESI†). All the three devices showed a low turn-on voltage of
References
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and device C (5.90%). Notably, devices A and B demonstrated
very low-efficiency loss of 0.13 and 0.20%, respectively, as the
luminance increased from 10 000 to 60 000 cd mꢀ2. As expected,
the device C displayed slightly high-efficiency loss of ca. 0.58%
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