482
18 Organic Semiconductors
(a)
(b)
Fig. 18.15 a Schematic term diagram of host (T H ) and guest (T G ) triplet states. Straight arrows denote energy transfer
between triplet states, wiggly arrows denote radiative transitions to the (singlet) ground state. b Electroluminescence
transients (at room temperature, detected in the range 500–560 nm [cmp. Fig. 18.14b]) of CBP:6% Ir(ppy) 3 (τ ≈ 1 µs)
and TDP:6% Ir(ppy) 3 (τ ≈ 15 µs). Insets: term schemes with arrow denoting the rate limiting step. Based on data
from [1578]
(a)
(b)
Fig. 18.16 a Schematic term scheme of CBP:Ir(ppy) 3 /DCM2 and energy transfer and recombination paths (cmp.
spectrum in Fig. 18.14). The rate constants are shown for various processes, the rate limiting step is shown with a bold
arrow. b Electroluminescence transients after 100 ns excitation pulse (grey area) of Ir(ppy) 3 and DCM2 luminescence
from CBP:10% Ir(ppy) 3 /1% DCM2. Based on data from [1586]
In the case of N,N’-diphenyl-N,N’-bis(3-methylphenyl)-[1,1’-biphenyl]-4,4’-diamine [TDP]
(Fig. 18.14c) host, the triplet energy transfer to the phosphorent Ir(ppy) 3 guest is endotherm ( ≈
+0.1 eV) and represents the rate limiting step [1578]. In this case the recombination of Ir(ppy) 3 has
a decay constant of about 15 µs (Fig. 18.15b). The thermal activation character is confirmed by even
longer decay times at low temperatures (τ ≈ 80 µs at T = 200 K) [1578]. Endothermic transfer allows
to pump a blue guest phosphor without a blue host material.
Further, subsequent Förster energy transfer [1587] from the guest triplet state to a fast and efficient
singlet state (S D ) of a fluorescent dye is possible, e.g. from CBP:Ir(ppy) 3 to DCM2 [1586]. The transient
lifetime of pure DCM2 is about 1 ns. In a mixture of CBP:10% Ir(ppy) 3 /1% DCM2 the luminescence
of DCM2 appears with the same 100 ns decay constant as that of Ir(ppy) 3 (Fig. 18.16b). This decay
constant (rate limiting step, see Fig. 18.16a) corresponds to the energy transfer depleting the triplet
state of Ir(ppy) 3 to DCM2 and is much faster than the pure Ir(ppy) 3 radiative lifetime.
18 Organic Semiconductors
(a)
(b)
Fig. 18.15 a Schematic term diagram of host (T H ) and guest (T G ) triplet states. Straight arrows denote energy transfer
between triplet states, wiggly arrows denote radiative transitions to the (singlet) ground state. b Electroluminescence
transients (at room temperature, detected in the range 500–560 nm [cmp. Fig. 18.14b]) of CBP:6% Ir(ppy) 3 (τ ≈ 1 µs)
and TDP:6% Ir(ppy) 3 (τ ≈ 15 µs). Insets: term schemes with arrow denoting the rate limiting step. Based on data
from [1578]
(a)
(b)
Fig. 18.16 a Schematic term scheme of CBP:Ir(ppy) 3 /DCM2 and energy transfer and recombination paths (cmp.
spectrum in Fig. 18.14). The rate constants are shown for various processes, the rate limiting step is shown with a bold
arrow. b Electroluminescence transients after 100 ns excitation pulse (grey area) of Ir(ppy) 3 and DCM2 luminescence
from CBP:10% Ir(ppy) 3 /1% DCM2. Based on data from [1586]
In the case of N,N’-diphenyl-N,N’-bis(3-methylphenyl)-[1,1’-biphenyl]-4,4’-diamine [TDP]
(Fig. 18.14c) host, the triplet energy transfer to the phosphorent Ir(ppy) 3 guest is endotherm ( ≈
+0.1 eV) and represents the rate limiting step [1578]. In this case the recombination of Ir(ppy) 3 has
a decay constant of about 15 µs (Fig. 18.15b). The thermal activation character is confirmed by even
longer decay times at low temperatures (τ ≈ 80 µs at T = 200 K) [1578]. Endothermic transfer allows
to pump a blue guest phosphor without a blue host material.
Further, subsequent Förster energy transfer [1587] from the guest triplet state to a fast and efficient
singlet state (S D ) of a fluorescent dye is possible, e.g. from CBP:Ir(ppy) 3 to DCM2 [1586]. The transient
lifetime of pure DCM2 is about 1 ns. In a mixture of CBP:10% Ir(ppy) 3 /1% DCM2 the luminescence
of DCM2 appears with the same 100 ns decay constant as that of Ir(ppy) 3 (Fig. 18.16b). This decay
constant (rate limiting step, see Fig. 18.16a) corresponds to the energy transfer depleting the triplet
state of Ir(ppy) 3 to DCM2 and is much faster than the pure Ir(ppy) 3 radiative lifetime.