18.6 Optical Properties
481
(a)
(b)
(c)
Fig. 18.13 a Band structure of poly-thiophene (‘I’: naked chain, ‘II’: chain in a dielectric medium ( = 3)), b singleparticle energies and band gap, c exciton levels (’E’: experimental values). Reprinted with permission from [1581],
©2002 APS
(a)
(b)
(c)
(d)
Fig. 18.14 Molecular structure of a Ir(ppy) 3 , b CBP and c TDP (see text). d Electroluminescence spectra (at room
temperature) of CBP:6% Ir(ppy) 3 , CBP:10% Ir(ppy) 3 /1%DCM2 and CBP:2%DCM2. Based on data from [1585, 1586]
A successful route is the use of a phosphorescent guest material. Radiative transitions from triplet
states become partially allowed when the excited singlet and triplet states are mixed. This is typically
achieved in metalorganic molecules with heavy metal atoms, providing large spin-orbit interaction
effects [1583, 1584]. Most prominently Ru-, Pt- and Ir-containing compounds are used, e.g. fac tris(2phenylpyridine) iridium [Ir(ppy) 3 ] in 4,4’-N,N’-dicarbazole-biphenyl [CBP] (Fig. 18.14a,b) [1585].
The luminescence spectrum of Ir(ppy) 3 is shown in Fig. 18.14d. The radiative decay constant of the
Ir(ppy) 3 triplet state is about 800 ns and observable if energy transfer from the host triplet state is
exotherm ( = G G − G H < 0 [1578], see Fig. 18.15a) and fast. This is the case for CBP:Ir(ppy) 3
(Fig. 18.15b), ≈ −0.2 eV. Actually, reverse transfer from Ir(ppy) 3 to CBP seems responsible for
some loss in luminescence efficiency and the decrease in phosphorescent lifetime from 800 ns to 400 ns.
481
(a)
(b)
(c)
Fig. 18.13 a Band structure of poly-thiophene (‘I’: naked chain, ‘II’: chain in a dielectric medium ( = 3)), b singleparticle energies and band gap, c exciton levels (’E’: experimental values). Reprinted with permission from [1581],
©2002 APS
(a)
(b)
(c)
(d)
Fig. 18.14 Molecular structure of a Ir(ppy) 3 , b CBP and c TDP (see text). d Electroluminescence spectra (at room
temperature) of CBP:6% Ir(ppy) 3 , CBP:10% Ir(ppy) 3 /1%DCM2 and CBP:2%DCM2. Based on data from [1585, 1586]
A successful route is the use of a phosphorescent guest material. Radiative transitions from triplet
states become partially allowed when the excited singlet and triplet states are mixed. This is typically
achieved in metalorganic molecules with heavy metal atoms, providing large spin-orbit interaction
effects [1583, 1584]. Most prominently Ru-, Pt- and Ir-containing compounds are used, e.g. fac tris(2phenylpyridine) iridium [Ir(ppy) 3 ] in 4,4’-N,N’-dicarbazole-biphenyl [CBP] (Fig. 18.14a,b) [1585].
The luminescence spectrum of Ir(ppy) 3 is shown in Fig. 18.14d. The radiative decay constant of the
Ir(ppy) 3 triplet state is about 800 ns and observable if energy transfer from the host triplet state is
exotherm ( = G G − G H < 0 [1578], see Fig. 18.15a) and fast. This is the case for CBP:Ir(ppy) 3
(Fig. 18.15b), ≈ −0.2 eV. Actually, reverse transfer from Ir(ppy) 3 to CBP seems responsible for
some loss in luminescence efficiency and the decrease in phosphorescent lifetime from 800 ns to 400 ns.