480
18 Organic Semiconductors
(a)
(b)
Fig. 18.11 a Projected density of states (for C, N, and O) of excited states in an Alq 3 molecule. The origin of the energy
axis is the HOMO level. b Orbitals for the four states labeled I–IV in (a). Reprinted with permission from [1556], ©1998
AIP
(a)
Fig. 18.12 Photoluminescence (PL) spectra at T = 20 and 300 K and absorption spectrum (green line) of poly-thiophene.
The vertical dashed line denotes the detection energy (E det = 1.83 eV) of the PL excitation (PLE) spectrum (blue circles)
(T = 20 K). Adapted from [1580]
several phonon replica whose separation of 180 meV corresponds to the C–C stretching mode. The PL
excitation (PLE) spectrum of poly-thiophene demonstrates that the PL at 1.83 eV can be excited via
the exciton level.
The theoretical band structure of poly-thiophene is shown in Fig. 18.13a. The Brillouin zone is onedimensional. The situation I corresponds to a single molecular chain, the situation II pertains to the
chain embedded in a medium with a dielectric constant = 3. The predicted band gaps are 3.6 eV and
2.5 eV, respectively. The exciton binding energy is about 0.5 eV. The exciton is a Frenkel exciton that
has a small extension and is localized. The high binding energy is favorable for radiative recombination
since the exciton is stable at room temperature. For photovoltaic applications, it is unfavorable since
it has to be overcome in order to separate electrons and holes (after absorption). Generally, intrachain
excitons (as here) and interchain excitons, where electron and the hole are localized on different chains,
are distinguished.
Collection of carriers in the ‘dark’ triplet state poses a problem limiting the quantum efficiency to
25% in a simple model [1582]. Harvesting luminescence from all exciton states would yield significantly higher efficiencies than is possible in purely fluorescent materials (or devices).
18 Organic Semiconductors
(a)
(b)
Fig. 18.11 a Projected density of states (for C, N, and O) of excited states in an Alq 3 molecule. The origin of the energy
axis is the HOMO level. b Orbitals for the four states labeled I–IV in (a). Reprinted with permission from [1556], ©1998
AIP
(a)
Fig. 18.12 Photoluminescence (PL) spectra at T = 20 and 300 K and absorption spectrum (green line) of poly-thiophene.
The vertical dashed line denotes the detection energy (E det = 1.83 eV) of the PL excitation (PLE) spectrum (blue circles)
(T = 20 K). Adapted from [1580]
several phonon replica whose separation of 180 meV corresponds to the C–C stretching mode. The PL
excitation (PLE) spectrum of poly-thiophene demonstrates that the PL at 1.83 eV can be excited via
the exciton level.
The theoretical band structure of poly-thiophene is shown in Fig. 18.13a. The Brillouin zone is onedimensional. The situation I corresponds to a single molecular chain, the situation II pertains to the
chain embedded in a medium with a dielectric constant = 3. The predicted band gaps are 3.6 eV and
2.5 eV, respectively. The exciton binding energy is about 0.5 eV. The exciton is a Frenkel exciton that
has a small extension and is localized. The high binding energy is favorable for radiative recombination
since the exciton is stable at room temperature. For photovoltaic applications, it is unfavorable since
it has to be overcome in order to separate electrons and holes (after absorption). Generally, intrachain
excitons (as here) and interchain excitons, where electron and the hole are localized on different chains,
are distinguished.
Collection of carriers in the ‘dark’ triplet state poses a problem limiting the quantum efficiency to
25% in a simple model [1582]. Harvesting luminescence from all exciton states would yield significantly higher efficiencies than is possible in purely fluorescent materials (or devices).