220
M. Hiramoto
molecular orbital (HOMO) of a donor dopant should be less than that of the lowest
unoccupied molecular orbital (LUMO) of the OSC molecule. Upon electron transfer
(ET) from the donor molecule to the OSC molecule, a charge transfer (CT) state
is formed (Fig. 9.1d, left). The negative electric charge on the OSC molecule is
thermally released at RT. Consequently, the OSC shows n-type behavior. In contrast,
as shown in Fig. 9.1a (right), the energy of the LUMO of an acceptor dopant molecule
should be higher than the energy of the HOMO of the host OSC molecule. Upon ET
from the OSC molecule to the acceptor molecule, a CT state is produced (Fig. 9.1d,
right). The positive electric charge on the OSC molecule is released thermally at RT.
Consequently, the OSC shows p-type behavior.
9.2.2 Ionization
The ionization process of a dopant is analogous to the dissociation process of photogenerated electron–hole pairs (excitons). Here, the charge generation process by
doping is analogously discussed based on the charge photogeneration process in an
organic photovoltaic cell. Coulomb’s law dictates that the dissociation of an exciton
depends on the dielectric constant of the solid (ε):
F = (1/4pεε 0 )
q 1 q 2 /r
2
(9.1)
where ε 0 , q 1 , q 2 , and r are the absolute dielectric constant, the elementary charges,
and the distance between elementary charges, respectively. For a small ε value, the
opposite charge pair feels a comparatively strong attraction. In contrast, for a large
ε value, the opposite charge pair feels a comparatively weak attraction. A large ε
corresponds to inorganic semiconductors such as Si (ε = 11.9). In this case, an
exciton delocalizing approximately 10
4 Si atoms has a large diameter of 9.0 nm
(Fig. 9.1c) and is a so-called Wannier-type exciton, which thermally dissociates into
a free electron and a hole at RT and generates photocarriers [31].
A pair of positive and negative charges in Si can also be considered a negatively
ionized dopant atom (B
− ) and weakly bound hole (Fig. 9.1c). Because the attractive
force between the negative and positive charges in the Si crystal is essentially the
same, a hole weakly bound to the negatively ionized dopant atom can thermally
dissociate into a free hole at RT, thereby creating the majority carrier. Unlike an
exciton, the negative charge of the ionized acceptor atom (B
− ) cannot move in the
crystal lattice of Si.
In contrast, a small ε corresponds to OSCs such as fullerenes (C 60 ) (ε = 4.4) [32]. In
this case, an exciton localizing on one molecule has a small diameter of 0.50 nm (not
shown) and is a so-called Frenkel-type exciton, which hardly dissociates thermally
into a free electron and hole at RT and generates few photocarriers.
To dissociate Frenkel excitons, donor/acceptor (D/A) sensitization is utilized.
Electron-donating (D) and electron-accepting (A) molecules are blended (Fig. 9.3,
M. Hiramoto
molecular orbital (HOMO) of a donor dopant should be less than that of the lowest
unoccupied molecular orbital (LUMO) of the OSC molecule. Upon electron transfer
(ET) from the donor molecule to the OSC molecule, a charge transfer (CT) state
is formed (Fig. 9.1d, left). The negative electric charge on the OSC molecule is
thermally released at RT. Consequently, the OSC shows n-type behavior. In contrast,
as shown in Fig. 9.1a (right), the energy of the LUMO of an acceptor dopant molecule
should be higher than the energy of the HOMO of the host OSC molecule. Upon ET
from the OSC molecule to the acceptor molecule, a CT state is produced (Fig. 9.1d,
right). The positive electric charge on the OSC molecule is released thermally at RT.
Consequently, the OSC shows p-type behavior.
9.2.2 Ionization
The ionization process of a dopant is analogous to the dissociation process of photogenerated electron–hole pairs (excitons). Here, the charge generation process by
doping is analogously discussed based on the charge photogeneration process in an
organic photovoltaic cell. Coulomb’s law dictates that the dissociation of an exciton
depends on the dielectric constant of the solid (ε):
F = (1/4pεε 0 )
q 1 q 2 /r
2
(9.1)
where ε 0 , q 1 , q 2 , and r are the absolute dielectric constant, the elementary charges,
and the distance between elementary charges, respectively. For a small ε value, the
opposite charge pair feels a comparatively strong attraction. In contrast, for a large
ε value, the opposite charge pair feels a comparatively weak attraction. A large ε
corresponds to inorganic semiconductors such as Si (ε = 11.9). In this case, an
exciton delocalizing approximately 10
4 Si atoms has a large diameter of 9.0 nm
(Fig. 9.1c) and is a so-called Wannier-type exciton, which thermally dissociates into
a free electron and a hole at RT and generates photocarriers [31].
A pair of positive and negative charges in Si can also be considered a negatively
ionized dopant atom (B
− ) and weakly bound hole (Fig. 9.1c). Because the attractive
force between the negative and positive charges in the Si crystal is essentially the
same, a hole weakly bound to the negatively ionized dopant atom can thermally
dissociate into a free hole at RT, thereby creating the majority carrier. Unlike an
exciton, the negative charge of the ionized acceptor atom (B
− ) cannot move in the
crystal lattice of Si.
In contrast, a small ε corresponds to OSCs such as fullerenes (C 60 ) (ε = 4.4) [32]. In
this case, an exciton localizing on one molecule has a small diameter of 0.50 nm (not
shown) and is a so-called Frenkel-type exciton, which hardly dissociates thermally
into a free electron and hole at RT and generates few photocarriers.
To dissociate Frenkel excitons, donor/acceptor (D/A) sensitization is utilized.
Electron-donating (D) and electron-accepting (A) molecules are blended (Fig. 9.3,