256
M. Hiramoto
10.1.1 Bipolar Band-Conductive Organic
Semiconductor-Exciton Dissociation Using Single
Organic Semiconductor
Carrier generation by doping and light excitation is considered as standard since
the liberation process between positive and negative charges can be regarded as
identical. In the case of doping, a free carrier is generated by the dissociation of a
positive charge of a hole from a negatively ionized dopant ion, which is spatially
fixed, and vice versa. In the case of photocarrier generation, free electron and hole
are generated by the dissociation of an exciton, i.e., bound electron and hole, which
are not spatially fixed.
In the case of acceptor doping to the band-conductive rubrene single crystal, a
positive charge of a hole loosely bound around the negatively ionized dopant ion
can be liberated by the thermal energy of room temperature; i.e., the Wannier excitonic nature is observed in the band-conductive single crystal (Fig. 10.1a) (Chap. 9,
Sect. 9.6.1). A negatively ionized dopant is spatially fixed in the crystal lattice.
Therefore, for acceptor and donor doping processes, since the liberated carrier is
either a hole or an electron, only hole band-conductive nature or only electron
band-conductive nature are required, respectively.
On the other hand, in the case of photocarrier generation, both a hole and an
electron should be liberated at room temperature by the Wannier excitonic nature.
Therefore, the band-conductive nature is required both for the hole and the electron. When two kinds of organic crystals with hole band conduction and electron
band conduction are combined (Fig. 10.1b), exciton automatically dissociates to free
electron and hole by the thermal energy of room temperature. When single organic
crystal possesses both hole band-conductive and electron band-conductive nature,
i.e., possesses bipolar band-conductive nature, exciton automatically dissociates to
free electron and hole by thermal energy of room temperature in the single organic
crystal (Fig. 10.1c). We presume that the bipolar band-conductive organic semiconductor would dissociate exciton and generate photocurrent without the assistance of
donor/acceptor (D/A) sensitization.
Concerning holes, a significant number of band-conductive organic semiconductor crystals, such as rubrene [1] and films such as C8-BTBT [2], has been
reported in this decade. However, for electrons, only one organic semiconductor
(PDIF-CN 2 ) has been reported [3]. There has been no report on bipolar bandconductive organic semiconductor. Initially, a sign of bipolar band conduction under
very low temperature was reported in naphthalene and perylene single crystals in
1985 by Karl [4] (Fig. 10.2). The mobilities for both hole and electron increased
with decreasing temperature, suggesting the bipolar band-conductive nature. The
bipolar band-conductive organic semiconductor at room temperature would represent
a breakthrough in the field of organic solar cells.
M. Hiramoto
10.1.1 Bipolar Band-Conductive Organic
Semiconductor-Exciton Dissociation Using Single
Organic Semiconductor
Carrier generation by doping and light excitation is considered as standard since
the liberation process between positive and negative charges can be regarded as
identical. In the case of doping, a free carrier is generated by the dissociation of a
positive charge of a hole from a negatively ionized dopant ion, which is spatially
fixed, and vice versa. In the case of photocarrier generation, free electron and hole
are generated by the dissociation of an exciton, i.e., bound electron and hole, which
are not spatially fixed.
In the case of acceptor doping to the band-conductive rubrene single crystal, a
positive charge of a hole loosely bound around the negatively ionized dopant ion
can be liberated by the thermal energy of room temperature; i.e., the Wannier excitonic nature is observed in the band-conductive single crystal (Fig. 10.1a) (Chap. 9,
Sect. 9.6.1). A negatively ionized dopant is spatially fixed in the crystal lattice.
Therefore, for acceptor and donor doping processes, since the liberated carrier is
either a hole or an electron, only hole band-conductive nature or only electron
band-conductive nature are required, respectively.
On the other hand, in the case of photocarrier generation, both a hole and an
electron should be liberated at room temperature by the Wannier excitonic nature.
Therefore, the band-conductive nature is required both for the hole and the electron. When two kinds of organic crystals with hole band conduction and electron
band conduction are combined (Fig. 10.1b), exciton automatically dissociates to free
electron and hole by the thermal energy of room temperature. When single organic
crystal possesses both hole band-conductive and electron band-conductive nature,
i.e., possesses bipolar band-conductive nature, exciton automatically dissociates to
free electron and hole by thermal energy of room temperature in the single organic
crystal (Fig. 10.1c). We presume that the bipolar band-conductive organic semiconductor would dissociate exciton and generate photocurrent without the assistance of
donor/acceptor (D/A) sensitization.
Concerning holes, a significant number of band-conductive organic semiconductor crystals, such as rubrene [1] and films such as C8-BTBT [2], has been
reported in this decade. However, for electrons, only one organic semiconductor
(PDIF-CN 2 ) has been reported [3]. There has been no report on bipolar bandconductive organic semiconductor. Initially, a sign of bipolar band conduction under
very low temperature was reported in naphthalene and perylene single crystals in
1985 by Karl [4] (Fig. 10.2). The mobilities for both hole and electron increased
with decreasing temperature, suggesting the bipolar band-conductive nature. The
bipolar band-conductive organic semiconductor at room temperature would represent
a breakthrough in the field of organic solar cells.