16 Luminescent Materials for Organic Light-Emitting Diodes
587
performances are comparable to those obtained from the red phosphorescent OLEDs
based on the organoiridium(III) emitters (Kim et al. 2011). Li and coworkers reported
excellent pure blue phosphorescent emitters by using unsymmetrical tetradentate
ligands, namely, PON1-tBu, PtON6-tBu, and PtON7-dtb (Fleetham et al. 2014).
These complexes exhibit blue emission at 444–448 nm with extremely narrow half
bandwidths pf 19–20 nm. It is also interesting that they are still emissive in a solid
matrix: the Φ PL s of PON1-tBu, PtON6-tBu, and PtON7-dtb in PMMA films) are
more than 0.7. The PL characteristics were reflected in the device performance, and
OLEDs employing these complexes exhibited pure blue EL with spectral half bandwidths less than 30 nm. Optimizing the device structure, the OLED with PtON7-dtb
recorded the maximum η ext of 24.8%, comparable to the theoretical limit. Interestingly, the CIE coordinate of (0.148, 0.079) was very close to the NTSC standard for
blue, defined as (0.14, 0.08).
16.4 TADF Materials: Third Generation Emitters
16.4.1 Advantage of TADF Emitters in OLED Applications
TADF is the exciton generation and light-emitting process in which the fluorescent
emission is obtained through up-conversion from the low-lying triplet state to the
high-lying singlet excited state, as shown in Fig. 16.3. This up-conversion process
can theoretically convert the whole of triplet exciton into the singlet, different from
the triplet–triplet annihilation process where one singlet exciton is produced from
two triplet excitons. In this context, OLEDs utilizing TADF emitters allow us to overcome the problem of intrinsically low internal quantum efficiencies of fluorescent
OLEDs: in TADF OLEDs, the whole of the first generated singlet and triplet excitons
can be used for fluorescent emission, and thus, they are comparable to phosphorescent OLEDs in terms of η ext of EL. Since TADF emitters are obtained as pure organic
molecules, TADF OLEDs are also beneficial in terms of reducing the device fabrication cost and protecting the resource of precious metals such as iridium and platinum
used for production of phosphorescent emitters. Thus, TADF emitting materials are
often called the third generation emitters.
At the early stage of developing TADF OLEDs, eosin and tin(IV) porphyrins
were known as TADF emitters. Using the latter compounds, the first TADF OLED
was demonstrated by Adachi and coworkers in 2009 (Endo et al. 2009). EL from the
device with SnF 2 -OEP (Fig. 16.24) was exclusively based on the prompt and delayed
fluorescence at relatively high temperatures (350 and 400 K), although phosphorescent emission was also included at 300 K. Anyway, they demonstrated utilization
of both of singlet and triplet excitons to obtain EL. Just after this report, TADF
from a cupper(I) dinuclear complex {Cu(PNP-tBu)} 2 (Fig. 16.24) was reported by
Deaton and coworkers (Deaton et al. 2010). The OLED including this complex as
an emitting dopant exhibited green EL. Although the red-shifted phosphorescent EL
587
performances are comparable to those obtained from the red phosphorescent OLEDs
based on the organoiridium(III) emitters (Kim et al. 2011). Li and coworkers reported
excellent pure blue phosphorescent emitters by using unsymmetrical tetradentate
ligands, namely, PON1-tBu, PtON6-tBu, and PtON7-dtb (Fleetham et al. 2014).
These complexes exhibit blue emission at 444–448 nm with extremely narrow half
bandwidths pf 19–20 nm. It is also interesting that they are still emissive in a solid
matrix: the Φ PL s of PON1-tBu, PtON6-tBu, and PtON7-dtb in PMMA films) are
more than 0.7. The PL characteristics were reflected in the device performance, and
OLEDs employing these complexes exhibited pure blue EL with spectral half bandwidths less than 30 nm. Optimizing the device structure, the OLED with PtON7-dtb
recorded the maximum η ext of 24.8%, comparable to the theoretical limit. Interestingly, the CIE coordinate of (0.148, 0.079) was very close to the NTSC standard for
blue, defined as (0.14, 0.08).
16.4 TADF Materials: Third Generation Emitters
16.4.1 Advantage of TADF Emitters in OLED Applications
TADF is the exciton generation and light-emitting process in which the fluorescent
emission is obtained through up-conversion from the low-lying triplet state to the
high-lying singlet excited state, as shown in Fig. 16.3. This up-conversion process
can theoretically convert the whole of triplet exciton into the singlet, different from
the triplet–triplet annihilation process where one singlet exciton is produced from
two triplet excitons. In this context, OLEDs utilizing TADF emitters allow us to overcome the problem of intrinsically low internal quantum efficiencies of fluorescent
OLEDs: in TADF OLEDs, the whole of the first generated singlet and triplet excitons
can be used for fluorescent emission, and thus, they are comparable to phosphorescent OLEDs in terms of η ext of EL. Since TADF emitters are obtained as pure organic
molecules, TADF OLEDs are also beneficial in terms of reducing the device fabrication cost and protecting the resource of precious metals such as iridium and platinum
used for production of phosphorescent emitters. Thus, TADF emitting materials are
often called the third generation emitters.
At the early stage of developing TADF OLEDs, eosin and tin(IV) porphyrins
were known as TADF emitters. Using the latter compounds, the first TADF OLED
was demonstrated by Adachi and coworkers in 2009 (Endo et al. 2009). EL from the
device with SnF 2 -OEP (Fig. 16.24) was exclusively based on the prompt and delayed
fluorescence at relatively high temperatures (350 and 400 K), although phosphorescent emission was also included at 300 K. Anyway, they demonstrated utilization
of both of singlet and triplet excitons to obtain EL. Just after this report, TADF
from a cupper(I) dinuclear complex {Cu(PNP-tBu)} 2 (Fig. 16.24) was reported by
Deaton and coworkers (Deaton et al. 2010). The OLED including this complex as
an emitting dopant exhibited green EL. Although the red-shifted phosphorescent EL