42
M. Hiramoto
dissociates into free carriers, depending on the strength of the electric field [17, 18].
The author thought that the extremely high ability for the charge carrier generation
through an exciplex is clearly manifested in Fig. 2.4 (curve b) since the photocurrent
quantum efficiency increased drastically to 40% due to a small increase in the reverse
bias of only −0.4 V.
2.3.7.3 Donor–Acceptor Pair
In the 1980s, few researchers in the field of electrophotography investigated the
photocarrier generation mechanism in organic semiconductors since it was considered to be a very difficult and complicated issue. Popovic was the only researcher who
energetically tackled this problem, for example, by using field-induced fluorescence
quenching measurements [19]. He studied the carrier generation mechanism in electrophotographic device consisting of a photocarrier generation layer using Im-PTC
and a hole transporting layer using tetraphenyldiamine (TPD) [20]. He found that
the enhanced carrier generation efficiency in the Im-PTC/TPD system compared to
the Im-PTC alone is due to a sensitization process. He then proposed a mechanism
for this process as shown in Fig. 2.20b. His explanation was as follows. The generation mechanism involves exciton diffusion to the Im-PTC/TPD interface where the
excitons dissociate by injecting an electron into the pigment and a hole into the
TPD. In the absence of TPD (Fig. 2.20b, No TPD), the dominant mechanism for the
decay of the first excited singlet state is radiative or radiationless relaxation to the
ground state. A small fraction of excited states leads to photogenerated carriers. In
the presence of TPD (Fig. 2.20b, With TPD), the excited state decay is dominated by
a process in which charge exchange occurs between the excited Im-PTC molecule
and a TPD at the interface. This leads to fluorescence quenching and generation of
a geminate electron–hole pair that, aided by electric field, subsequently dissociates
into free carriers.
This mechanism is the origin of the donor (D)–acceptor (A)
sensitization (Chap. 1, Fig.1.7, and Chap. 9, Fig. 9.1b) which is a basic principle of the contemporary organic solar cells. TPD and Im-PTC can be regarded
as donor (D) and acceptor (A), respectively. The photocurrent enhancement in
the H 2 Pc/Me-PTC and CuPc/Im-PTC systems can be attributed to the replacement of TPD with phthalocyanines (H 2 Pc, CuPc). The offsets of the HOMO and
LUMO levels formed between the donor and acceptor are essential for the exciton
dissociation and thus the photocarrier generation.
2.3.7.4 Relationship to Recent Progress
Recent research on organic solar cells focused on the nature of the charge–transfer
(CT) state between the donor (D) and acceptor (A). This CT state can be regarded as
an exciplex, i.e., (A
− –D
+ )* shown in Fig. 2.20a. Energetically, the CT state can be
illustrated as shown in Fig. 2.20b (with TPD). Recently, the CT states were revealed
M. Hiramoto
dissociates into free carriers, depending on the strength of the electric field [17, 18].
The author thought that the extremely high ability for the charge carrier generation
through an exciplex is clearly manifested in Fig. 2.4 (curve b) since the photocurrent
quantum efficiency increased drastically to 40% due to a small increase in the reverse
bias of only −0.4 V.
2.3.7.3 Donor–Acceptor Pair
In the 1980s, few researchers in the field of electrophotography investigated the
photocarrier generation mechanism in organic semiconductors since it was considered to be a very difficult and complicated issue. Popovic was the only researcher who
energetically tackled this problem, for example, by using field-induced fluorescence
quenching measurements [19]. He studied the carrier generation mechanism in electrophotographic device consisting of a photocarrier generation layer using Im-PTC
and a hole transporting layer using tetraphenyldiamine (TPD) [20]. He found that
the enhanced carrier generation efficiency in the Im-PTC/TPD system compared to
the Im-PTC alone is due to a sensitization process. He then proposed a mechanism
for this process as shown in Fig. 2.20b. His explanation was as follows. The generation mechanism involves exciton diffusion to the Im-PTC/TPD interface where the
excitons dissociate by injecting an electron into the pigment and a hole into the
TPD. In the absence of TPD (Fig. 2.20b, No TPD), the dominant mechanism for the
decay of the first excited singlet state is radiative or radiationless relaxation to the
ground state. A small fraction of excited states leads to photogenerated carriers. In
the presence of TPD (Fig. 2.20b, With TPD), the excited state decay is dominated by
a process in which charge exchange occurs between the excited Im-PTC molecule
and a TPD at the interface. This leads to fluorescence quenching and generation of
a geminate electron–hole pair that, aided by electric field, subsequently dissociates
into free carriers.
This mechanism is the origin of the donor (D)–acceptor (A)
sensitization (Chap. 1, Fig.1.7, and Chap. 9, Fig. 9.1b) which is a basic principle of the contemporary organic solar cells. TPD and Im-PTC can be regarded
as donor (D) and acceptor (A), respectively. The photocurrent enhancement in
the H 2 Pc/Me-PTC and CuPc/Im-PTC systems can be attributed to the replacement of TPD with phthalocyanines (H 2 Pc, CuPc). The offsets of the HOMO and
LUMO levels formed between the donor and acceptor are essential for the exciton
dissociation and thus the photocarrier generation.
2.3.7.4 Relationship to Recent Progress
Recent research on organic solar cells focused on the nature of the charge–transfer
(CT) state between the donor (D) and acceptor (A). This CT state can be regarded as
an exciplex, i.e., (A
− –D
+ )* shown in Fig. 2.20a. Energetically, the CT state can be
illustrated as shown in Fig. 2.20b (with TPD). Recently, the CT states were revealed