248
M. Hiramoto
controlling doping-induced defects, a doping efficiency close to 100% will no longer
be a dream.
9.9.6.3 Scattering Science of Organic Single Crystals
The Hall mobility (μ H ) was found to decrease above 10 ppm. The gradual μ H reduction is explained by carrier scattering due to the ionized dopant molecule, neutral
dopant molecule, and crystal lattice deformation (Fig. 9.14e). Carrier scattering at
the ionized dopant has been observed previously in hopping-conductive n-doped C 60
thin films [69]. We believe that the physical meaning of carrier scattering during the
hopping conduction of carrier should be clarified. Carrier scattering by dopants in
band-conductive organic single crystals is a new issue. For organic semiconductors,
the carrier scattering causes not only intermolecular vibrations but also intramolecular vibrations, which have many modes, such as C-H stretching and bending [95–97].
The physics of carrier scattering is significantly different from that of inorganic semiconductor crystals. Thus, we also stand at the entrance of the immense undeveloped
field of scattering science for organic single crystals, which is comparable to that of
inorganic single crystals.
9.9.6.4 Organic Single-Crystal Electronics
The quality of organic single crystals is still low. The observed carrier concentration
of an undoped organic single crystal of approximately 10
15 cm
−3 (Fig. 9.13d) is
significantly higher than that for undoped Si of 10
10 cm
−3 . Many carriers of unknown
origins exist at the surface and in the bulk. If the science underlying the single-crystal
growth of organic semiconductors is developed, defect-minimized organic single
crystals could be achieved.
In the future, bulk-doped organic single-crystal wafers, similar to Si wafers,
could be introduced. Organic single-crystal electronics could be constructed utilizing
not only bulk-doped homoepitaxy but also bulk-doped heteroepitaxy on doped
organic single-crystal substrates. Organic single-crystal solar cells with pn- and
pin-homojunctions, organic single-crystal bipolar transistors with pnp- and npnhomojunctions, and organic single-crystal lasers with double heterojunctions could
be developed.
9.9.7 Conclusion
Historically, the true nature of organic semiconductors had been veiled by external
impurities, and they had been unresponsive to intentional doping. Progress over the
past decade has lifted the veil, and organic semiconductors have become responsive
to even 1-ppm-level doping. This progress has led to the creation of the field of
M. Hiramoto
controlling doping-induced defects, a doping efficiency close to 100% will no longer
be a dream.
9.9.6.3 Scattering Science of Organic Single Crystals
The Hall mobility (μ H ) was found to decrease above 10 ppm. The gradual μ H reduction is explained by carrier scattering due to the ionized dopant molecule, neutral
dopant molecule, and crystal lattice deformation (Fig. 9.14e). Carrier scattering at
the ionized dopant has been observed previously in hopping-conductive n-doped C 60
thin films [69]. We believe that the physical meaning of carrier scattering during the
hopping conduction of carrier should be clarified. Carrier scattering by dopants in
band-conductive organic single crystals is a new issue. For organic semiconductors,
the carrier scattering causes not only intermolecular vibrations but also intramolecular vibrations, which have many modes, such as C-H stretching and bending [95–97].
The physics of carrier scattering is significantly different from that of inorganic semiconductor crystals. Thus, we also stand at the entrance of the immense undeveloped
field of scattering science for organic single crystals, which is comparable to that of
inorganic single crystals.
9.9.6.4 Organic Single-Crystal Electronics
The quality of organic single crystals is still low. The observed carrier concentration
of an undoped organic single crystal of approximately 10
15 cm
−3 (Fig. 9.13d) is
significantly higher than that for undoped Si of 10
10 cm
−3 . Many carriers of unknown
origins exist at the surface and in the bulk. If the science underlying the single-crystal
growth of organic semiconductors is developed, defect-minimized organic single
crystals could be achieved.
In the future, bulk-doped organic single-crystal wafers, similar to Si wafers,
could be introduced. Organic single-crystal electronics could be constructed utilizing
not only bulk-doped homoepitaxy but also bulk-doped heteroepitaxy on doped
organic single-crystal substrates. Organic single-crystal solar cells with pn- and
pin-homojunctions, organic single-crystal bipolar transistors with pnp- and npnhomojunctions, and organic single-crystal lasers with double heterojunctions could
be developed.
9.9.7 Conclusion
Historically, the true nature of organic semiconductors had been veiled by external
impurities, and they had been unresponsive to intentional doping. Progress over the
past decade has lifted the veil, and organic semiconductors have become responsive
to even 1-ppm-level doping. This progress has led to the creation of the field of