16 Luminescent Materials for Organic Light-Emitting Diodes
595
(Kondo et al. 2019). This TADF emitter exhibits blue emission at 457 nm with a
quite narrow half bandwidth of 14 nm in toluene, the Φ PL of which is fairly excellent (0.74). Thus, through the multiple resonance effect of the constituent boron and
nitrogen atoms in the fused DABNA chromophore, the suppressed vibronic coupling
between S 0 and S 1 caused the narrow half bandwidth of the PL spectrum. The OLED
with ν-DABNA as an emitting dopant afforded efficient blue electroluminescence
at 469 nm with a half bandwidth of 18 nm (Fig. 16.30b). The CIE coordinate was
(0.12, 0.11), close to the NTSC standard for blue. The fabricated device showed the
extremely high maximum η ext of 34.4% at 15 cd m
–2 , accompanied by a small device
efficiency roll-off (η ext ; 26.0% at 1000 cd m
–2 ).
16.5 Summary
In this chapter, emitting materials for OLED were introduced, focusing on the exciton
generation mechanism. First, fluorescent materials were reviewed according to their
emission colors. For low-mass fluorescent emitters, various types of structural frameworks are available, allowing for tuning the luminescence properties such as the
emission color and the quantum efficiencies. Fluorescent emitters based on polymer
and dendrimer structures are applied to fabrication of solution-processed OLEDs,
and their excellent film-forming properties are beneficial to fabricate flexible lightemitting devices. The η ext of a fluorescent OLED is fatally low to be ca. 5% at most,
because electric current excitation of an emitter provides the singlet and triplet excitons according to spin statics. However, fluorescent materials are still popular at the
level of industrial implementation, especially as blue emitters.
Second, phosphorescent materials were reviewed, especially focusing on
organoiridium(III) and organoplatinum(II) complexes. In these complexes, strong
spin–orbit interactions induced by the constituent heavy metals facilitate effective
mixing of the singlet and triplet states, and thus the triplet excitons are efficiently
obtained even at ambient temperature. In OLEDs, triplet excitons are electrically
generated directly and via the intersystem crossing from S 1 to T 1 , and thus the η ext of
a phosphorescent OLED is as high as 100%, four times more than that of a fluorescent
OLED. Thus, phosphorescent materials are widely used to fabricate high-efficiency
devices, especially as red and green emitters. At this point, blue phosphorescent
OLEDs do not meet the demands for industrial implementation, in terms of color
purity and device lifetime.
In the last part, a new type of fluorescent material is introduced, that is, a TADF
emitter. This emitter utilizes both singlet and triplet excitons to produce photons.
The key process to achieve TADF is the reverse intersystem crossing from T 1 to
S 1 . To obtain a small E ST , a minimized HOMO–LUMO overlap and a miximized
transition dipole moment are required. So, the molecular design for a TADF emitter
has been based on construction of a distorted donor–acceptor π-conjugation system
showing ICT electronic transition. So far, lots of TADF emitters achieving η ext s over
the theoretical upper limit of conventional fluorescent emitters have been reported.
595
(Kondo et al. 2019). This TADF emitter exhibits blue emission at 457 nm with a
quite narrow half bandwidth of 14 nm in toluene, the Φ PL of which is fairly excellent (0.74). Thus, through the multiple resonance effect of the constituent boron and
nitrogen atoms in the fused DABNA chromophore, the suppressed vibronic coupling
between S 0 and S 1 caused the narrow half bandwidth of the PL spectrum. The OLED
with ν-DABNA as an emitting dopant afforded efficient blue electroluminescence
at 469 nm with a half bandwidth of 18 nm (Fig. 16.30b). The CIE coordinate was
(0.12, 0.11), close to the NTSC standard for blue. The fabricated device showed the
extremely high maximum η ext of 34.4% at 15 cd m
–2 , accompanied by a small device
efficiency roll-off (η ext ; 26.0% at 1000 cd m
–2 ).
16.5 Summary
In this chapter, emitting materials for OLED were introduced, focusing on the exciton
generation mechanism. First, fluorescent materials were reviewed according to their
emission colors. For low-mass fluorescent emitters, various types of structural frameworks are available, allowing for tuning the luminescence properties such as the
emission color and the quantum efficiencies. Fluorescent emitters based on polymer
and dendrimer structures are applied to fabrication of solution-processed OLEDs,
and their excellent film-forming properties are beneficial to fabricate flexible lightemitting devices. The η ext of a fluorescent OLED is fatally low to be ca. 5% at most,
because electric current excitation of an emitter provides the singlet and triplet excitons according to spin statics. However, fluorescent materials are still popular at the
level of industrial implementation, especially as blue emitters.
Second, phosphorescent materials were reviewed, especially focusing on
organoiridium(III) and organoplatinum(II) complexes. In these complexes, strong
spin–orbit interactions induced by the constituent heavy metals facilitate effective
mixing of the singlet and triplet states, and thus the triplet excitons are efficiently
obtained even at ambient temperature. In OLEDs, triplet excitons are electrically
generated directly and via the intersystem crossing from S 1 to T 1 , and thus the η ext of
a phosphorescent OLED is as high as 100%, four times more than that of a fluorescent
OLED. Thus, phosphorescent materials are widely used to fabricate high-efficiency
devices, especially as red and green emitters. At this point, blue phosphorescent
OLEDs do not meet the demands for industrial implementation, in terms of color
purity and device lifetime.
In the last part, a new type of fluorescent material is introduced, that is, a TADF
emitter. This emitter utilizes both singlet and triplet excitons to produce photons.
The key process to achieve TADF is the reverse intersystem crossing from T 1 to
S 1 . To obtain a small E ST , a minimized HOMO–LUMO overlap and a miximized
transition dipole moment are required. So, the molecular design for a TADF emitter
has been based on construction of a distorted donor–acceptor π-conjugation system
showing ICT electronic transition. So far, lots of TADF emitters achieving η ext s over
the theoretical upper limit of conventional fluorescent emitters have been reported.