16 Luminescent Materials for Organic Light-Emitting Diodes
567
devices, closer to the National Television Standards Committee (NTSC) standard
for red (defined as (0.67, 0.33)) than those obtained for DCJTI and DCJTB (Yu and
Shirota 2002). Especially, the ACY-based device showed relatively efficient performance; a maximum luminance of 6400 cd m
–2 at 15 V and η ext of 0.8%. As other
examples, 2,1,3-benzothiadiazole and fumaronitrile skeletons were incorporated to
obtain BTZA (λ EL , 626 nm; CIE (0.64, 0.36)) (Thomas et al. 2004) and NPAFN (λ EL ,
636 nm; CIE (0.64, 0.36)) (Yeh et al. 2003), respectively (Fig. 16.5c). Especially,
the latter is highly emissive in the solid state, suitable for preparation of a non-doped
EML.
For green fluorescent emitters, Alq3 and coumarin were used at the early stage
(Tang and VanSlyke 1989). Then, lots of green fluorophores have been reported, as
shown in Fig. 16.6. C545T is one of the commercially available green fluorescent
emitters, which has been used in prototypical OLEDs. Chen and coworkers modified
the structure of this dye to improve the device performance as well as thermal stability
and photostability, namely C-545P (PL maximum λ PL , 514 nm; Φ PL , 0.99 in 1,2dichloroethane) (Lee et al. 2004). Strategically placing methyl groups on the julolidyl
rings, concentration quenching was more suppressed in comparison with C545, and
the device efficiency of the C545P-based OLED was notably improved. Modification
of C540 (or C6) and C545 series dyes with bulky substituents is also effective to
improve the device efficiency of OLED as well as thermal stability, as seen in C6DB
and C-545TB (Chen and Tang 2001). Quinacridone (QAD) and its derivatives are also
typical green fluorescent emitters showing high thermal stability and photostability.
Due to intermolecular strong hydrogen bonds, quinacridone is insoluble itself in
any solvents. So, alkyl substituents are often introduced to the nitrogen atoms, as
seen in DMQA (λ EL , 544 nm; CIE (0.39, 0.59)) (Shi and Tang 1997). As other
green fluorescent emitters, an anthracene-9,10-diamine derivative (TpAD) (Yu et al.
2011) and an oligoquinoxaline derivative (TPA-2PQx) (Hancock et al. 2006) have
been reported. The former exhibits efficient green emission in dichloromethane (λ PL ,
546 nm; Φ PL , 0.94), and λ EL of its device is blue-shifted by 12 nm, showing the CIE
coordinate of (0.35, 0.61). The latter exhibits an ambipolar character and allows for
fabrication of an OLED with a non-doped EML showing η ext of 3.0%. Green emitters
for efficient non-doped OLEDs were also developed by Tang and coworkers, where
they utilized aggregation-induced emission (AIE) behavior of 2,5-difuluororenylsubstituted silole derivatives (Chen et al. 2013). AIE is observed for certain dyes,
where their emissions are enhanced by aggregation to suppress free bond rotations
facilitating the non-radiative decay. So high Φ PL s are obtained for AIE luminophores
in the solid state rather than in solution. For example, in the case of MFMPS, the
Φ PL of the drop-cast film is 0.88, although that of the solution is just 0.026. The nondoped device with an MFMPS-based EML afforded green EL (CIE (0.36, 0.57))
with a high η ext of 5.5% corresponding to the limit value for fluorescent OLEDs.
Blue fluorescent emitters are very important in OLED applications because they
are still used as practical emitting materials for displays and illumination devices:
as for blue OLEDs, the devices with fluorescent emitters are superior to those with
phosphorescent and TADF emitters in duration and color purity, although η ext is
567
devices, closer to the National Television Standards Committee (NTSC) standard
for red (defined as (0.67, 0.33)) than those obtained for DCJTI and DCJTB (Yu and
Shirota 2002). Especially, the ACY-based device showed relatively efficient performance; a maximum luminance of 6400 cd m
–2 at 15 V and η ext of 0.8%. As other
examples, 2,1,3-benzothiadiazole and fumaronitrile skeletons were incorporated to
obtain BTZA (λ EL , 626 nm; CIE (0.64, 0.36)) (Thomas et al. 2004) and NPAFN (λ EL ,
636 nm; CIE (0.64, 0.36)) (Yeh et al. 2003), respectively (Fig. 16.5c). Especially,
the latter is highly emissive in the solid state, suitable for preparation of a non-doped
EML.
For green fluorescent emitters, Alq3 and coumarin were used at the early stage
(Tang and VanSlyke 1989). Then, lots of green fluorophores have been reported, as
shown in Fig. 16.6. C545T is one of the commercially available green fluorescent
emitters, which has been used in prototypical OLEDs. Chen and coworkers modified
the structure of this dye to improve the device performance as well as thermal stability
and photostability, namely C-545P (PL maximum λ PL , 514 nm; Φ PL , 0.99 in 1,2dichloroethane) (Lee et al. 2004). Strategically placing methyl groups on the julolidyl
rings, concentration quenching was more suppressed in comparison with C545, and
the device efficiency of the C545P-based OLED was notably improved. Modification
of C540 (or C6) and C545 series dyes with bulky substituents is also effective to
improve the device efficiency of OLED as well as thermal stability, as seen in C6DB
and C-545TB (Chen and Tang 2001). Quinacridone (QAD) and its derivatives are also
typical green fluorescent emitters showing high thermal stability and photostability.
Due to intermolecular strong hydrogen bonds, quinacridone is insoluble itself in
any solvents. So, alkyl substituents are often introduced to the nitrogen atoms, as
seen in DMQA (λ EL , 544 nm; CIE (0.39, 0.59)) (Shi and Tang 1997). As other
green fluorescent emitters, an anthracene-9,10-diamine derivative (TpAD) (Yu et al.
2011) and an oligoquinoxaline derivative (TPA-2PQx) (Hancock et al. 2006) have
been reported. The former exhibits efficient green emission in dichloromethane (λ PL ,
546 nm; Φ PL , 0.94), and λ EL of its device is blue-shifted by 12 nm, showing the CIE
coordinate of (0.35, 0.61). The latter exhibits an ambipolar character and allows for
fabrication of an OLED with a non-doped EML showing η ext of 3.0%. Green emitters
for efficient non-doped OLEDs were also developed by Tang and coworkers, where
they utilized aggregation-induced emission (AIE) behavior of 2,5-difuluororenylsubstituted silole derivatives (Chen et al. 2013). AIE is observed for certain dyes,
where their emissions are enhanced by aggregation to suppress free bond rotations
facilitating the non-radiative decay. So high Φ PL s are obtained for AIE luminophores
in the solid state rather than in solution. For example, in the case of MFMPS, the
Φ PL of the drop-cast film is 0.88, although that of the solution is just 0.026. The nondoped device with an MFMPS-based EML afforded green EL (CIE (0.36, 0.57))
with a high η ext of 5.5% corresponding to the limit value for fluorescent OLEDs.
Blue fluorescent emitters are very important in OLED applications because they
are still used as practical emitting materials for displays and illumination devices:
as for blue OLEDs, the devices with fluorescent emitters are superior to those with
phosphorescent and TADF emitters in duration and color purity, although η ext is