564
S. Yagi
Fig. 16.3 Illustration of photophysical processes for emitting materials upon electric current excitation. S 0 , S 1 , and T 1 represent the singlet ground state, the lowest singlet excited state, and the
lowest triplet state, respectively. ISC and RISC represent the intersystem crossing from S 1 to T 1
and its reverse process, respectively. IC represents non-radiative internal conversion
crossing from S 1 to T 1 into consideration, the exciton formation efficiency η ph of
the triplet exciton reaches as high as 1, although the singlet exciton generates with
η ph of 0.25 at most. The external quantum efficiency η ext (%) of EL from OLED is
defined as the following equation
η ext = η out × η ph × Φ PL × γ
where η out , Φ PL , and γ represent a light-extraction efficiency, a photoluminescence
(PL) quantum yield, and a charge carrier balance factor, respectively. The typical
value of η out is ca. 0.2. According to this equation, OLEDs using phosphorescent
emitters theoretically provide η ext as high as 20%, whereas those using fluorescent
ones do η ext of 5% at most. Thus, phosphorescent OLEDs are superior in the device
efficiency to fluorescent ones.
From the historical viewpoint, fluorescent emitters have been widely used since
the early stage of development of the EL devices, because many of their basic
skeletons have been derived from traditional organic fluorescent dyes. Thereafter,
organometallic complexes emitting efficient room-temperature phosphorescence
were developed in 2000s (Lamansky et al. 2001). Since then, phosphorescent
organometallic complexes with platinum(II) and iridium(III) centers have been used
to fabricate high-efficiency OLEDs. It is noteworthy that achievement of nearly 100%
internal quantum efficiency was demonstrated by using a phosphorescent material
(Adachi et al. 2001a). More recently, organic molecules exhibiting thermally activated delayed fluorescence (TADF) have been attracting much attention because
the up-conversion process from the low-lying triplet state to the high-lying singlet
S. Yagi
Fig. 16.3 Illustration of photophysical processes for emitting materials upon electric current excitation. S 0 , S 1 , and T 1 represent the singlet ground state, the lowest singlet excited state, and the
lowest triplet state, respectively. ISC and RISC represent the intersystem crossing from S 1 to T 1
and its reverse process, respectively. IC represents non-radiative internal conversion
crossing from S 1 to T 1 into consideration, the exciton formation efficiency η ph of
the triplet exciton reaches as high as 1, although the singlet exciton generates with
η ph of 0.25 at most. The external quantum efficiency η ext (%) of EL from OLED is
defined as the following equation
η ext = η out × η ph × Φ PL × γ
where η out , Φ PL , and γ represent a light-extraction efficiency, a photoluminescence
(PL) quantum yield, and a charge carrier balance factor, respectively. The typical
value of η out is ca. 0.2. According to this equation, OLEDs using phosphorescent
emitters theoretically provide η ext as high as 20%, whereas those using fluorescent
ones do η ext of 5% at most. Thus, phosphorescent OLEDs are superior in the device
efficiency to fluorescent ones.
From the historical viewpoint, fluorescent emitters have been widely used since
the early stage of development of the EL devices, because many of their basic
skeletons have been derived from traditional organic fluorescent dyes. Thereafter,
organometallic complexes emitting efficient room-temperature phosphorescence
were developed in 2000s (Lamansky et al. 2001). Since then, phosphorescent
organometallic complexes with platinum(II) and iridium(III) centers have been used
to fabricate high-efficiency OLEDs. It is noteworthy that achievement of nearly 100%
internal quantum efficiency was demonstrated by using a phosphorescent material
(Adachi et al. 2001a). More recently, organic molecules exhibiting thermally activated delayed fluorescence (TADF) have been attracting much attention because
the up-conversion process from the low-lying triplet state to the high-lying singlet