16 Luminescent Materials for Organic Light-Emitting Diodes
589
and LUMO and maximization of the transition dipole moment of the HOMO–LUMO
transition. The molecule of PIC-TRZ consists of a (biphenyl-4-yl)-1,3,5-triazine unit
as an acceptor and two 12-phenyl-11,12-dihydroindolo[2,3-a]carbazol-11-yl units as
donors. The donor units are heavily distorted from the acceptor unit through steric
hindrance, and thus the donor–acceptor π-conjugation system is partially broken.
From the theoretical calculations, HOMO and LUMO are localized at the donor and
acceptor units, respectively, with a small overlap at the triazine unit. As a result,
a small E ST is achieved to obtain TADF. Indeed, although a toluene solution of
PIC-TRZ without nitrogen gas bubbling showed a Φ PL of 0.10, the solution filled
with nitrogen gas did an improved Φ PL of 0.35 with an increased TADF component.
Using PIC-TRZ as an emitting dopant, an OLED was fabricated, and the maximum
η ext reached 5.3%, comparable to the theoretical limit. It is worthy to note that η ext
of 14% over the theoretical limit for fluorescent EL was achieved in 2013, where
PIC-TRZ2 (Fig. 16.25), having an analogous structure to that of PIC-TRZ, was used
as a TADF emitter (Sato et al. 2013). In the case of PIC-TRZ2, the HOMO and
LUMO are completely separated, and indeed, a quite small E ST of 0.02 eV was
achieved.
Starting with the development of PIC-TRZ, lots of TADF emitters having donor–
acceptor ICT-type structures were developed to improve the device efficiencies of
OLEDs. In 2012, Adachi et al. reported a series of TADF emitters based on a
cyano-substituted benzene acceptor and carbazole donors, as shown in Fig. 16.26a
(Uoyama et al. 2012). Various combination of cyano and carbazolyl groups on the
benzene core allowed for a wide range of color variation of PL from sky blue
to orange (Fig. 16.26b). Among the TADF emitters, 4CzIPN, 4CzTPN-Ph, and
2CzPN were employed to fabricate OLEDs. The device structure for 4CzIPN and
4CzTPN-Ph is ITO/α-NPD (35 nm)/5 wt% emitter-doped 4,4
-bis(N-carbazolyl)1,1
-biphenyl (CBP, 15 nm)/TPBi (65 nm)/LiF (0.8 nm)/Al, and that for 2CzPN is
ITO (100 nm)/α-NPD (40 nm)/1,3-bis(9-carbazolyl)benzene (mCP, 10 nm)/5 wt%
emitter-doped 2,8-bis(diphenylphosphoryl)dibenzo[b,d]thiophene (PPT, 40 nm)/LiF
(0.8 nm)/Al. In the case of the device with the green emitter 4CzIPN, an extremely
high maximum η ext of 19.3% was obtained, much higher than the theoretical limit
for emitters exhibiting prompt fluorescent emission. This value is comparable to
the internal quantum efficiency of 64.3–96.5% when light out-coupling efficiency
of 20–30% is assumed. Thereafter, using TCz-TRZ (Hirata et al. 2015) and DACTII (Kaji et al. 2015) (Fig. 16.27) as TADF emitters, the Adachi’s group achieved
η ext more than 20%. In the case of the device with TCz-TRZ, the maximum η ext of
20.6% was obtained when the device structure was ITO/α-NPD (30 nm)/3,3
-di(9Hcarbazol-9-yl)-1,1
-biphenyl (m-CBP, 10 nm)/6 wt% emitter-doped (oxybis(2,1phenylene))bis(diphenylphosphine oxide) (DPEPO, 15 nm)/DPEPO (10 nm)/TPBi
(30 nm)/LiF (0.8 nm)/Al. The CIE chromaticity coordinate of the obtained EL was
(0.19, 0.35), corresponding to sky blue. In the case of the device with the green
emitter DACT-II, the maximum η ext of 29.6% was achieved when the following
device structure was employed; ITO/1,1-bis[4-(di-4-tolylamino)phenyl]cyclohexane
(TAPC, 100 nm)/9 wt% emitter-doped CBP (40 nm)/([1,1
-biphenyl]-4-yloxy)bis((2methylquinolin-8-yl)oxy)aluminum (BAlq, 30 nm)/LiF/Al. The extremely high η ext
589
and LUMO and maximization of the transition dipole moment of the HOMO–LUMO
transition. The molecule of PIC-TRZ consists of a (biphenyl-4-yl)-1,3,5-triazine unit
as an acceptor and two 12-phenyl-11,12-dihydroindolo[2,3-a]carbazol-11-yl units as
donors. The donor units are heavily distorted from the acceptor unit through steric
hindrance, and thus the donor–acceptor π-conjugation system is partially broken.
From the theoretical calculations, HOMO and LUMO are localized at the donor and
acceptor units, respectively, with a small overlap at the triazine unit. As a result,
a small E ST is achieved to obtain TADF. Indeed, although a toluene solution of
PIC-TRZ without nitrogen gas bubbling showed a Φ PL of 0.10, the solution filled
with nitrogen gas did an improved Φ PL of 0.35 with an increased TADF component.
Using PIC-TRZ as an emitting dopant, an OLED was fabricated, and the maximum
η ext reached 5.3%, comparable to the theoretical limit. It is worthy to note that η ext
of 14% over the theoretical limit for fluorescent EL was achieved in 2013, where
PIC-TRZ2 (Fig. 16.25), having an analogous structure to that of PIC-TRZ, was used
as a TADF emitter (Sato et al. 2013). In the case of PIC-TRZ2, the HOMO and
LUMO are completely separated, and indeed, a quite small E ST of 0.02 eV was
achieved.
Starting with the development of PIC-TRZ, lots of TADF emitters having donor–
acceptor ICT-type structures were developed to improve the device efficiencies of
OLEDs. In 2012, Adachi et al. reported a series of TADF emitters based on a
cyano-substituted benzene acceptor and carbazole donors, as shown in Fig. 16.26a
(Uoyama et al. 2012). Various combination of cyano and carbazolyl groups on the
benzene core allowed for a wide range of color variation of PL from sky blue
to orange (Fig. 16.26b). Among the TADF emitters, 4CzIPN, 4CzTPN-Ph, and
2CzPN were employed to fabricate OLEDs. The device structure for 4CzIPN and
4CzTPN-Ph is ITO/α-NPD (35 nm)/5 wt% emitter-doped 4,4
-bis(N-carbazolyl)1,1
-biphenyl (CBP, 15 nm)/TPBi (65 nm)/LiF (0.8 nm)/Al, and that for 2CzPN is
ITO (100 nm)/α-NPD (40 nm)/1,3-bis(9-carbazolyl)benzene (mCP, 10 nm)/5 wt%
emitter-doped 2,8-bis(diphenylphosphoryl)dibenzo[b,d]thiophene (PPT, 40 nm)/LiF
(0.8 nm)/Al. In the case of the device with the green emitter 4CzIPN, an extremely
high maximum η ext of 19.3% was obtained, much higher than the theoretical limit
for emitters exhibiting prompt fluorescent emission. This value is comparable to
the internal quantum efficiency of 64.3–96.5% when light out-coupling efficiency
of 20–30% is assumed. Thereafter, using TCz-TRZ (Hirata et al. 2015) and DACTII (Kaji et al. 2015) (Fig. 16.27) as TADF emitters, the Adachi’s group achieved
η ext more than 20%. In the case of the device with TCz-TRZ, the maximum η ext of
20.6% was obtained when the device structure was ITO/α-NPD (30 nm)/3,3
-di(9Hcarbazol-9-yl)-1,1
-biphenyl (m-CBP, 10 nm)/6 wt% emitter-doped (oxybis(2,1phenylene))bis(diphenylphosphine oxide) (DPEPO, 15 nm)/DPEPO (10 nm)/TPBi
(30 nm)/LiF (0.8 nm)/Al. The CIE chromaticity coordinate of the obtained EL was
(0.19, 0.35), corresponding to sky blue. In the case of the device with the green
emitter DACT-II, the maximum η ext of 29.6% was achieved when the following
device structure was employed; ITO/1,1-bis[4-(di-4-tolylamino)phenyl]cyclohexane
(TAPC, 100 nm)/9 wt% emitter-doped CBP (40 nm)/([1,1
-biphenyl]-4-yloxy)bis((2methylquinolin-8-yl)oxy)aluminum (BAlq, 30 nm)/LiF/Al. The extremely high η ext