18.6 Optical Properties
479
Fig. 18.9 Carrier velocity
in ultrapure and highly
perfect single crystals of
(n-conducting) perylene at
T = 30 K and
(p-conducting) naphtalene
at T = 4.3 K. For
comparison electron (hole)
velocity in silicon at room
temperature is shown as
solid (dashed) line.
Adapted from [1561]
18.6 Optical Properties
Organic molecules can emit light efficiently and are thus useful for light emitters. For the photo-physics
of organic materials it is essential to recall the molecular physics of singlet and triplet states. In the
singlet (triplet) state, the total spin quantum number of the unpaired electrons is S = 0 (S = 1). A
simple energy scheme includes a ground state S 0 and excited singlet (S 1 ) and triplet (T 1 ) states. The
recombination transition S 1 → S 0 is allowed and its lifetime short. Such luminescence is termend
‘fluorescence’. Recombination from the triplet state is forbidden or at least very slow (‘phosphorescence’).
As an example for a small organic molecule, the fluorescence lifetime of Alq 3 is about 12 ns [1577].
The triplet lifetime is in the 10 µs range [1578]. Luminescence and absorption spectra of Alq 3 are
shown in Fig. 18.10. The luminescence peak is redshifted with respect to the absorption edge due to
the Frank-Condon principle (Fig. 10.21). The density of excited (empty) states of the Alq 3 molecule
is shown in Fig. 18.11 together with the orbitals associated with the four prominent states. The lowest
orbital is the LUMO and leads to the visible luminescence of the Alq 3 in the red.
In Fig. 18.12, the photoluminescence (PL) and absorption of a polymer, poly-thiophene are shown.
The recombination is below the band gap of 2.1–2.3 eV on an excitonic level at 1.95 eV. There are
Fig. 18.10 Luminescence
and absorption spectra of
Alq 3 (vapor-deposited
150 nm thin film on a
quartz substrate) at room
temperature. Adapted
from [1579]
479
Fig. 18.9 Carrier velocity
in ultrapure and highly
perfect single crystals of
(n-conducting) perylene at
T = 30 K and
(p-conducting) naphtalene
at T = 4.3 K. For
comparison electron (hole)
velocity in silicon at room
temperature is shown as
solid (dashed) line.
Adapted from [1561]
18.6 Optical Properties
Organic molecules can emit light efficiently and are thus useful for light emitters. For the photo-physics
of organic materials it is essential to recall the molecular physics of singlet and triplet states. In the
singlet (triplet) state, the total spin quantum number of the unpaired electrons is S = 0 (S = 1). A
simple energy scheme includes a ground state S 0 and excited singlet (S 1 ) and triplet (T 1 ) states. The
recombination transition S 1 → S 0 is allowed and its lifetime short. Such luminescence is termend
‘fluorescence’. Recombination from the triplet state is forbidden or at least very slow (‘phosphorescence’).
As an example for a small organic molecule, the fluorescence lifetime of Alq 3 is about 12 ns [1577].
The triplet lifetime is in the 10 µs range [1578]. Luminescence and absorption spectra of Alq 3 are
shown in Fig. 18.10. The luminescence peak is redshifted with respect to the absorption edge due to
the Frank-Condon principle (Fig. 10.21). The density of excited (empty) states of the Alq 3 molecule
is shown in Fig. 18.11 together with the orbitals associated with the four prominent states. The lowest
orbital is the LUMO and leads to the visible luminescence of the Alq 3 in the red.
In Fig. 18.12, the photoluminescence (PL) and absorption of a polymer, poly-thiophene are shown.
The recombination is below the band gap of 2.1–2.3 eV on an excitonic level at 1.95 eV. There are
Fig. 18.10 Luminescence
and absorption spectra of
Alq 3 (vapor-deposited
150 nm thin film on a
quartz substrate) at room
temperature. Adapted
from [1579]