9 Parts-Per-Million-Level Doping Effects …
245
9.9.2 Hall Effects
Fe 2 Cl 6 -doped rubrene single crystals show Hall voltage signals (Fig. 9.13c).
Although the voltage signals (V H ) were low for undoped crystals (0.059 mV) and
those doped at 1 ppm (0.065 mV), after a sharp increase to 2.5 mV at 10 ppm,
the voltage gradually decreased from 50 ppm (0.37 mV), 100 ppm (0.11 mV) to
1,000 ppm (0.073 mV). Holes were the dominant carrier because Fe 2 Cl 6 acts an
acceptor dopant.
9.9.3 High Ionization Rate
The hole concentration (N) rapidly increased from 1 × 10
15 to 3 × 10
17 cm
−3 upon
100 ppm doping (Fig. 9.13d, red curve). A surprising feature is the high ionization
rate, which reached 24% at 100 ppm doping (Fig. 9.13d, green curve) and corresponds
to the activation energy of the acceptor dopant (E A ) of 72 meV, which is close to
the E A of 45 mV for the acceptor dopant (B) in Si [31].
Recently, a very low activation energy (9.1 meV) was reported for an amorphous
film [67]. Energetic disorder is essential for the dissociation of integer charge transfer
complexes with low activation energy. Actually, an amorphous hole transporting film
(MeO-TPD) shows systematically lower activation energies than crystalline ZnPc or
pentacene. A systematic trend can be observed wherein smaller energetic disorder
causes a higher activation energy. Though the activation energy for a band-conductive
organic single crystal with low disorder should be large, a significantly low activation
energy of 72 meV was observed. Therefore, we supposed that a significantly small
activation energy obtained from the hole concentration, which was directly observed
by the Hall effect for a band-conductive single crystals, has a different origin.
9.9.4 Scattering
The Hall mobility (μ H ) decreased from 4.6 to 0.7 cm
2 V
−1 s
−1 upon increasing the
dopant concentration from 10 to 500 ppm (Fig. 9.13e, red curve). Because holes in
the traps cannot move due to the Lorentz force, holes captured in the traps did not
contribute to the Hall mobility (μ H ). Therefore, the observed μ H decrease is assigned
to hole scattering by the negatively ionized neutral dopant Fe 2 Cl 6 and the dopinginduced lattice disturbances. Research on the scattering mechanism in bulk-doped
organic single crystals is an unexplored field and should be clarified, especially the
temperature dependence of μ H [76–80].
245
9.9.2 Hall Effects
Fe 2 Cl 6 -doped rubrene single crystals show Hall voltage signals (Fig. 9.13c).
Although the voltage signals (V H ) were low for undoped crystals (0.059 mV) and
those doped at 1 ppm (0.065 mV), after a sharp increase to 2.5 mV at 10 ppm,
the voltage gradually decreased from 50 ppm (0.37 mV), 100 ppm (0.11 mV) to
1,000 ppm (0.073 mV). Holes were the dominant carrier because Fe 2 Cl 6 acts an
acceptor dopant.
9.9.3 High Ionization Rate
The hole concentration (N) rapidly increased from 1 × 10
15 to 3 × 10
17 cm
−3 upon
100 ppm doping (Fig. 9.13d, red curve). A surprising feature is the high ionization
rate, which reached 24% at 100 ppm doping (Fig. 9.13d, green curve) and corresponds
to the activation energy of the acceptor dopant (E A ) of 72 meV, which is close to
the E A of 45 mV for the acceptor dopant (B) in Si [31].
Recently, a very low activation energy (9.1 meV) was reported for an amorphous
film [67]. Energetic disorder is essential for the dissociation of integer charge transfer
complexes with low activation energy. Actually, an amorphous hole transporting film
(MeO-TPD) shows systematically lower activation energies than crystalline ZnPc or
pentacene. A systematic trend can be observed wherein smaller energetic disorder
causes a higher activation energy. Though the activation energy for a band-conductive
organic single crystal with low disorder should be large, a significantly low activation
energy of 72 meV was observed. Therefore, we supposed that a significantly small
activation energy obtained from the hole concentration, which was directly observed
by the Hall effect for a band-conductive single crystals, has a different origin.
9.9.4 Scattering
The Hall mobility (μ H ) decreased from 4.6 to 0.7 cm
2 V
−1 s
−1 upon increasing the
dopant concentration from 10 to 500 ppm (Fig. 9.13e, red curve). Because holes in
the traps cannot move due to the Lorentz force, holes captured in the traps did not
contribute to the Hall mobility (μ H ). Therefore, the observed μ H decrease is assigned
to hole scattering by the negatively ionized neutral dopant Fe 2 Cl 6 and the dopinginduced lattice disturbances. Research on the scattering mechanism in bulk-doped
organic single crystals is an unexplored field and should be clarified, especially the
temperature dependence of μ H [76–80].