Benzofurocarbazole and benzothienocarbazole as donors for improved quantum efficiency in blue thermally activated delayed fluorescent devices

Article abstract of DOI:10.1039/c5cc01940k

Benzofurocarbazole and benzothienocarbazole were used as electron donors of thermally activated delayed fluorescence (TADF) emitters and the performances of the TADF devices were examined. The benzofurocarbazole and benzothienocarbazole donor moieties wer

Full text of DOI:10.1039/c5cc01940k

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Benzofurocarbazole and benzothienocarbazole as
donors for improved quantum efficiency in blue
thermally activated delayed fluorescent devices†
Cite this:DOI: 10.1039/c5cc01940k
Received 7th March 2015,
Accepted 28th March 2015
Dong Ryun Lee,a Seok-Ho Hwang,a Sang Kyu Jeon,b Chil Won Lee*a and
Jun Yeob Lee*b
DOI: 10.1039/c5cc01940k
www.rsc.org/chemcomm
Benzofurocarbazole and benzothienocarbazole were used as electron
In this work, two electron donors, benzofurocarbazole and
donors of thermally activated delayed fluorescence (TADF) emitters and benzothienocarbazole, were introduced as electron donating
the performances of the TADF devices were examined. The benzo- moieties of the TADF emitters and they were compared with a
furocarbazole and benzothienocarbazole donor moieties were better common carbazole moiety. It was found that the benzofurocarbazole
than carbazole as the electron donors of the TADF emitters.
and benzothienocarbazole work better than carbazole as the
electron donors to improve the quantum efficiency of the TADF
Thermally activated delayed fluorescent (TADF) materials are devices. The replacement of carbazole with benzofurocarbazole
attractive as third generation organic light-emitting materials and benzothienocarbazole more than doubled the quantum
because they have the merits of both 1st generation fluorescent efficiency of the TADF devices.
emitters and 2nd generation phosphorescent emitters. The
A synthetic scheme of BFCz-2CN and BTCz-2CN is shown in
TADF materials are similar to fluorescent emitting materials Scheme 1. The benzofurocarbazole and benzothienocarbazole
in that single excitons are involved in the light emission and donors were synthesized by Suzuki coupling and ring closing
similar to phosphorescent emitters in that both singlet and reactions. The two donor moieties were attached to 4,5-difluoro-
triplet excitons are harvested for light emission by intersystem phthalonitrile by NaH mediated coupling reaction to produce
crossing process although reverse intersystem crossing process 4,5-bis(benzofuro[3,2-c]carbazol-5-yl)phthalonitrile (BFCz-2CN) and
is used in the TADF devices.1
4,5-bis(benzo[4,5]thieno[3,2-c]carbazol-5-yl)phthalonitrile (BTCz-
The triplet exciton harvesting by reverse intersystem crossing 2CN) at a synthetic yield of about 50% after purification by column
of the TADF devices was dominated by thermal activation of the chromatography and vacuum train sublimation. Chemical analysis
triplet excitons for light emission in the singlet state, which was
induced by small singlet and triplet energy gap of the TADF
emitters.2 Therefore, the TADF material design was based on the
donor–acceptor structure to reduce the singlet energy of the
emitters, and strong donors and acceptors were introduced in
the molecular design of the TADF emitters.1 Carbazole,1,4–6
acridine3,4 and triphenylamine4,5 were representative donors of
the TADF emitters and the singlet–triplet energy gap of the TADF
emitters was determined by the electron donating capability of
the donors. However, only a few donors were considered as the
donor moieties of the TADF emitters and further study about the
effect of donor moieties on the photophysical properties and
device performances of the TADF emitters is required.
a Department of Polymer Science and Engineering, Dankook University, Yongin,
Gyeonggi, 448-701, Republic of Korea. E-mail: chili@dankook.ac.kr
b School of Chemical Engineering, Sungkyunkwan University, Suwon, Gyeonggi,
440-746, Republic of Korea. E-mail: leej17@skku.edu; Tel: +82 31 299 4716
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
Scheme 1 Synthetic scheme of BFCz-2CN and BTCz-2CN.
c5cc01940k
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Fig. 2 UV-vis and PL spectra of BFCz-2CN and BTCz-2CN.
emission peak in frozen tetrahydrofuran solution measured at
77 k after applying a delay time of 100 ms. The singlet–triplet
energy gaps were 0.13 eV and 0.17 eV for BFCz-2CN and BTCz-
2CN, respectively. The donor–acceptor structure of the BFCz-
2CN and BTCz-2CN emitters reduced the singlet–triplet energy
Fig. 1 HOMO and LUMO distribution of BFCz-2CN and BTCz-2CN.
of the final compounds identified BFCz-2CN and BTCz-2CN as gap for TADF emission and induced a solvatochromic effect in
products. The detailed procedure for the synthesis of BFCz-2CN different solvents. Solution PL spectra in different solvents are
and BTCz-2CN is presented in the ESI.†
presented in Fig. S2 (ESI†) and the bathochromic shift of the
As the molecular orbital distribution affects the singlet–triplet PL emission spectra in polar solvents was observed, which
energy gap and oscillation strength of the TADF emitters, the highest suggests the charge transfer emission of the TADF emitters.
occupied molecular orbital (HOMO) and the lowest unoccupied PL quantum yields (PLQY) of BFCz-2CN and BTCz-2CN in
molecular orbital (LUMO) of the synthesized compounds were toluene were 24.6 and 28.9% before N2 bubbling, and 94.6
calculated using the B3LYP/6-31G* basis set of Gaussian 09 program and 94.0% after N2 bubbling for 10 min from absolute PL
(Fig. 1). A small singlet–triplet energy gap is expected from spatial measurements using an integrating sphere. The PLQY of BFCz-
separation of the HOMO and LUMO, and good emission properties 2CN and BTCz-2CN in the mCP host was 85%, which was
are anticipated from a weak HOMO–LUMO overlap in BFCz-2CN higher than that of 2CzPN doped mCP film.1,7 This result
and BTCz-2CN. The donor structure had little effect on the HOMO confirms that the PL emission of BFCz-2CN and BTCz-2CN is
and LUMO distribution. However, the oscillation strength of BFCz- caused by the TADF process and the quantum yields of BFCz-
2CN (0.13) and BTCz-2CN (0.13) was higher than that of 2CzPN 2CN and BTCz-2CN were higher than that of 2CzPN.1
(0.09) with the carbazole moiety in the molecular structure. The use
Delayed fluorescence emission of BFCz-2CN and BTCz-2CN
of the benzofurocarbazole and benzothienocarbazole increased was investigated using transient PL measurement of BFCz-2CN
the oscillation strength of the TADF emitters, which may enhance and BTCz-2CN dispersed in 1,3-bis(N-carbazolyl)benzene (mCP).
light-emitting properties of the emitters. The HOMO/LUMOs of Transient PL decay curves of BFCz-2CN and BTCz-2CN at 297 K
BFCz-2CN and BTCz-2CN were ꢀ6.19/ꢀ3.58 eV and ꢀ6.17/ are shown in Fig. 3. Both prompt and delayed emissions of BFCz-
ꢀ3.58 eV, respectively, from cyclic voltammetry measurements. 2CN and BTCz-2CN were observed and emission spectra are
Light absorption and light emission of the TADF emitters shown in Fig. S3 (ESI†). The excited state lifetimes for the delayed
were analyzed using ultraviolet-visible (UV-vis) and photo- emission were 2.60 and 1.98 ms, respectively. Compared with 2CzPN
luminescence (PL) measurements. Fig. 2 presents the UV-vis which had an excited state lifetime of 166 ms,1 the excited state
absorption and PL emission spectra of BFCz-2CN and BTCz- lifetime of BFCz-2CN and BTCz-2CN was greatly shortened by the
2CN. The two host materials absorbed UV-vis light strongly at a benzofurocarbazole and benzothienocarbazole, which restrict the
short wavelength below 310 nm by p–p* transition and weakly molecular motion of the molecules and reduce the singlet–triplet
at a long wavelength between 310 nm and 430 nm by n–p* energy gap, resulting in a short lifetime. Fig. S4 (ESI†) shows
absorption. The introduction of benzofurocarbazole and benzo- transient decay curves at different temperatures. From 100 K to
thienocarbazole instead of carbazole extended the UV-vis 297 K, the delayed component was increased at high temperature.
absorption to 430 nm, which is good for energy transfer from
Thermal properties of BFCz-2CN and BTCz-2CN were also
host materials to the dopant materials. UV-vis absorption of studied and the glass transition temperatures of BFCz-2CN and
BFCz-2CN and BTCz-2CN was well matched with PL emission of BTCz-2CN were 204 and 219 1C, respectively. The rigid molecular
1,3-bis(N-carbazolyl)benzene (mCP) film (Fig. S1, ESI†). Singlet structure increased the glass transition temperatures of the BFCz-
energies of BFCz-2CN and BTCz-2CN were 2.59 eV and 2.63 eV 2CN and BTCz-2CN emitters. Thermal decomposition temperatures
from PL emission wavelength in polystyrene. Triplet energy of were 397 and 434 1C for BFCz-2CN and BTCz-2CN, respectively.
BFCz-2CN and BTCz-2CN was 2.46 eV from the phosphorescent Thermal analysis data are presented in Fig. S5 in the ESI.†
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The device performances of the BFCz-2CN and BTCz-2CN
emitters were collected by doping the emitters in mCP host material.
The mCP material was chosen as a matrix material because the high
triplet energy of mCP (2.9 eV) can suppress singlet and triplet exciton
quenching of the emitters. Device architecture was indium tin oxide
(ITO, 50 nm)/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)
(PEDOT:PSS, 60 nm)/4,40-cyclohexylidenebis[N,N-bis(4-methylphenyl)
aniline] (TAPC, 20 nm)/mCP (10 nm)/mCP:BFCz-2CN or BTCz-
2CN (25 nm, 1%)/diphenylphosphine oxide-4-(triphenylsilyl)-
phenyl (TSPO1, 35 nm)/LiF(1 nm)/Al(200 nm). TSPO1 was an
electron transport type exciton blocking material and mCP was
a hole transport type exciton blocking layer to avoid singlet and
triplet exciton quenching of emitters. Device performances at
1% doping concentration were compared since the driving
voltage and quantum efficiency were optimized at 1% doping
concentration. Fig. 4 displays device characteristics of the
BFCz-2CN and BTCz-2CN emitters and device performances
at other doping concentrations are summarized in the ESI.†
Maximum quantum efficiencies of the BFCz-2CN and BTCz-2CN
devices were 12.1% and 11.8%, which were higher than 5.0% of the
2CzPN emitter. The higher quantum efficiency of the BFCz-2CN
and BTCz-2CN devices than that of the 2CzPN device is caused by
high PL quantum yield and short excited state lifetime of the
emitters. As explained in the photophysical data, the quantum yield
of BFCz-2CN and BTCz-2CN was higher than that of 2CzPN, which
more than doubled the quantum efficiency of the BFCz-2CN and
BTCz-2CN devices compared to the 2CzPN device. Additionally,
efficient up-conversion by a short excited state lifetime for delayed
emission contributed to the high quantum efficiency of BFCz-2CN
and BTCz-2CN. Electroluminescence (EL) spectra of the BFCz-2CN
and BTCz-2CN devices were also similar and a peak wavelength of
486 nm and a full width at half maximum of 73 nm were exhibited
by the two emitters.
Fig. 3 Transient decay curves of BFCz-2CN and BTCz-2CN.
In conclusion, BFCz-2CN and BTCz-2CN were synthesized as
blue TADF emitters and showed high quantum efficiency about
12% at an optimum doping concentration of 1%. The intro-
duction of benzofurocarbazole and benzothienocarbazole
increased the PL quantum efficiency and shortened the excited
state lifetime of the TADF emitters.
This work was supported by Basic Science Research Program
through the National Research Foundation of Korea (NRF)
funded by the Ministry of Education, Science and Technology
(NRF-2013R1A1A2011560, 2013R1A1A2007991) and Ministry of
Science, ICT and future Planning (2013R1A2A2A010674).
Notes and references
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Fig. 4 (a) Current density–voltage–luminance curves and (b) quantum
efficiency–current density curves of BFCz-2CN, BTCz-2CN and 2CzPN
devices. (c) EL spectra of BFCz-2CN and BTCz-2CN devices.
7 K. Masui, H. Nakanotani and C. Adachi, Org. Electron., 2013, 14, 2721.
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