9 Parts-Per-Million-Level Doping Effects …
247
Holes created by doping were compensated by the doping-induced traps. Methods
for suppressing trap formation, such as post-annealing and organic-crystal-friendly
dopants, could be introduced.
9.9.6 Future Prospects
9.9.6.1 Inherent High Doping Efficiency
An ionization rate of 24% (E A = 72 meV) (Fig. 9.14a) corresponds to a Bohr’s
radius of 2.8 nm (Fig. 9.14b). This resembles the acceptor doping (B) of Si, which
has an activation energy of 45 mV [31] and a Wannier excitonic nature (Fig. 9.1c).
This model predicts that the observed high ionization rate can be considered the
inherent nature of the rubrene single crystal itself and essentially independent of the
dopant characteristics. However, our preliminary results suggest that the ionization
rates of inorganic and organic dopants are not very different, despite their relatively different dielectric constants (ε). We presume that due to the nature of band
conduction, the orbital of a hole is delocalized over the crystal lattice of the rubrene
molecules, especially for an ab-crystal plane with strong π–π stacking (Fig. 9.14b).
Such anisotropy may affect the dopant ionization. This model also predicts that
various band-conductive organic single crystals [81–85] would show a doping efficiency close to 100%, similar to that of Si, which would be beneficial for creating
new types of organic electronic devices with a variety of junctions, e.g., pn-, pin-,
and pnp-junctions, in the organic bulk single crystal.
9.9.6.2 Defect Science of Organic Single Crystals
The decrease in the ionization rate above 100 ppm can be explained by dopinginduced defects, such as vacancies (Fig. 9.14c) and interstitials (Fig. 9.14d), that
may be able to act as carrier traps. Carriers created by doping are consumed by these
traps. A vacancy-dopant pair (V Molecule
+ -A
− ), i.e., a hole trapped at a molecular
vacancy and a negatively ionized acceptor dopant (Fig. 9.14c), may compensate for
the doping. In the history of inorganic single-crystal electronics, for example, for
oxide semiconductors such as ZnO, many kinds of traps, such as oxygen vacancies
(V o ), metal vacancies (V M ), and metal interstitials (M i ), have been identified in terms
of their microscopic structures and energetic level in the bandgap [86–94]. In the
present stage of organic semiconductors, however, the microscopic spatial structures,
energetic levels, and electrical behaviors of defects in organic single crystals are either
unknown or poorly understood.
Therefore, we now stand at the entrance of a vast uncultivated field of defect
science for organic single crystals, which is comparable to that of inorganic single
crystals such as silicon cultivated during the past half century. If we succeed in
247
Holes created by doping were compensated by the doping-induced traps. Methods
for suppressing trap formation, such as post-annealing and organic-crystal-friendly
dopants, could be introduced.
9.9.6 Future Prospects
9.9.6.1 Inherent High Doping Efficiency
An ionization rate of 24% (E A = 72 meV) (Fig. 9.14a) corresponds to a Bohr’s
radius of 2.8 nm (Fig. 9.14b). This resembles the acceptor doping (B) of Si, which
has an activation energy of 45 mV [31] and a Wannier excitonic nature (Fig. 9.1c).
This model predicts that the observed high ionization rate can be considered the
inherent nature of the rubrene single crystal itself and essentially independent of the
dopant characteristics. However, our preliminary results suggest that the ionization
rates of inorganic and organic dopants are not very different, despite their relatively different dielectric constants (ε). We presume that due to the nature of band
conduction, the orbital of a hole is delocalized over the crystal lattice of the rubrene
molecules, especially for an ab-crystal plane with strong π–π stacking (Fig. 9.14b).
Such anisotropy may affect the dopant ionization. This model also predicts that
various band-conductive organic single crystals [81–85] would show a doping efficiency close to 100%, similar to that of Si, which would be beneficial for creating
new types of organic electronic devices with a variety of junctions, e.g., pn-, pin-,
and pnp-junctions, in the organic bulk single crystal.
9.9.6.2 Defect Science of Organic Single Crystals
The decrease in the ionization rate above 100 ppm can be explained by dopinginduced defects, such as vacancies (Fig. 9.14c) and interstitials (Fig. 9.14d), that
may be able to act as carrier traps. Carriers created by doping are consumed by these
traps. A vacancy-dopant pair (V Molecule
+ -A
− ), i.e., a hole trapped at a molecular
vacancy and a negatively ionized acceptor dopant (Fig. 9.14c), may compensate for
the doping. In the history of inorganic single-crystal electronics, for example, for
oxide semiconductors such as ZnO, many kinds of traps, such as oxygen vacancies
(V o ), metal vacancies (V M ), and metal interstitials (M i ), have been identified in terms
of their microscopic structures and energetic level in the bandgap [86–94]. In the
present stage of organic semiconductors, however, the microscopic spatial structures,
energetic levels, and electrical behaviors of defects in organic single crystals are either
unknown or poorly understood.
Therefore, we now stand at the entrance of a vast uncultivated field of defect
science for organic single crystals, which is comparable to that of inorganic single
crystals such as silicon cultivated during the past half century. If we succeed in