9 Parts-Per-Million-Level Doping Effects …
241
to 8 × 10
−5 caused the rapid shift in the E F to 1.29 eV, which corresponds to the
trap filling region. When the concentration of holes (N A ) created by acceptor doping
is smaller than that of traps (N T ), all created holes are captured by traps (N A < N T ,
Fig. 9.11b). This situation is maintained until all traps are filled by the created holes
(N A = N T , Fig. 9.11b). After trap filling (MR > 10
−4 ), i.e., when the concentration
of holes (N A ) exceeds that of traps (N T ), free holes appear (N A > N T , Fig. 9.11b).
This situation corresponds to the dopant saturation region, i.e., the appearance of
majority carriers, and the doping efficiency at MR = 2 × 10
−4 reaches 0.98 [10,
66]. Recently, Tieze et al. reported that an amorphous hole transporting film (MeOTPD) showed systematically lower activation energies (9.1 meV) than crystalline
ZnPc (21 meV) or pentacene (19 meV) for F 6 -TCNNQ doping [67]. The energetic
disorder is essential for the dissociation of integer charge transfer complexes with
low activation energy.
Trap Filling by Donor Doping
Trap filling was further confirmed by donor doping [23]. C 60 films can be doped with a
Ru-complex [16] acting as a donor dopant. By increasing the doping concentration,
the work function measured by UPS decreases (black dots) (Fig. 9.11c), i.e., the
Fermi level (E F ) negatively shifts and approaches the LUMO. At a high doping ratio
from 6 × 10
–3 to 10
–1 , a slow negative shift is observed. In contrast, at a low doping
ratio from 10
–4 to 6 × 10
–3 , a very rapid negative shift is observed, which is attributed
to the filling of deep traps by electrons created by n-doping. Simultaneously, a very
rapid increase in both conductivity and mobility (Fig. 9.11d) is observed at the same
doping ratio from 10
–4 to 6 × 10
–3 . The electron mobility clearly increases after
filling the traps by electrons created by donor dopants.
9.8.1.2 Majority Carrier, Homojunction, Mobility Decrease
In this section, the effects of 1 to 1,000 ppm doping on practical organic photovoltaic
devices are described.
The formation of a typical junction, an abrupt and one-sided n
+ p-homojunction
formed in a photovoltaic composed of 6 T:C 60 co-deposited films was investigated
(Fig. 9.12a, b) [23]. The acceptor dopant (Fe 2 Cl 6 ) concentration in the p-layer, which
was in contact with the heavily doped n
+ -layer (10,000 ppm Cs 2 CO 3 ), was varied
from 0, 1, 10, 100 to 1,000 ppm using a rotary shutter (Fig. 9.2b, c); 1 ppm is
equivalent to a MR of 3.7 × 10
–6 .
Both the short-circuit photocurrent (J sc ) and fill factor (FF) significantly increased
upon direct ppm-level doping in the bulk of the photocarrier-generating co-deposited
layer (Fig. 9.12a). Even at 1 ppm doping, slight effects can be seen (green curves).
Clear effects appeared at 10 ppm doping (orange curves). The photocurrent reached
its maximum at 100 ppm. However, a further increase to 1,000 ppm reduced both
the photocurrent (blue solid curve) and forward current (blue-dashed curve).
241
to 8 × 10
−5 caused the rapid shift in the E F to 1.29 eV, which corresponds to the
trap filling region. When the concentration of holes (N A ) created by acceptor doping
is smaller than that of traps (N T ), all created holes are captured by traps (N A < N T ,
Fig. 9.11b). This situation is maintained until all traps are filled by the created holes
(N A = N T , Fig. 9.11b). After trap filling (MR > 10
−4 ), i.e., when the concentration
of holes (N A ) exceeds that of traps (N T ), free holes appear (N A > N T , Fig. 9.11b).
This situation corresponds to the dopant saturation region, i.e., the appearance of
majority carriers, and the doping efficiency at MR = 2 × 10
−4 reaches 0.98 [10,
66]. Recently, Tieze et al. reported that an amorphous hole transporting film (MeOTPD) showed systematically lower activation energies (9.1 meV) than crystalline
ZnPc (21 meV) or pentacene (19 meV) for F 6 -TCNNQ doping [67]. The energetic
disorder is essential for the dissociation of integer charge transfer complexes with
low activation energy.
Trap Filling by Donor Doping
Trap filling was further confirmed by donor doping [23]. C 60 films can be doped with a
Ru-complex [16] acting as a donor dopant. By increasing the doping concentration,
the work function measured by UPS decreases (black dots) (Fig. 9.11c), i.e., the
Fermi level (E F ) negatively shifts and approaches the LUMO. At a high doping ratio
from 6 × 10
–3 to 10
–1 , a slow negative shift is observed. In contrast, at a low doping
ratio from 10
–4 to 6 × 10
–3 , a very rapid negative shift is observed, which is attributed
to the filling of deep traps by electrons created by n-doping. Simultaneously, a very
rapid increase in both conductivity and mobility (Fig. 9.11d) is observed at the same
doping ratio from 10
–4 to 6 × 10
–3 . The electron mobility clearly increases after
filling the traps by electrons created by donor dopants.
9.8.1.2 Majority Carrier, Homojunction, Mobility Decrease
In this section, the effects of 1 to 1,000 ppm doping on practical organic photovoltaic
devices are described.
The formation of a typical junction, an abrupt and one-sided n
+ p-homojunction
formed in a photovoltaic composed of 6 T:C 60 co-deposited films was investigated
(Fig. 9.12a, b) [23]. The acceptor dopant (Fe 2 Cl 6 ) concentration in the p-layer, which
was in contact with the heavily doped n
+ -layer (10,000 ppm Cs 2 CO 3 ), was varied
from 0, 1, 10, 100 to 1,000 ppm using a rotary shutter (Fig. 9.2b, c); 1 ppm is
equivalent to a MR of 3.7 × 10
–6 .
Both the short-circuit photocurrent (J sc ) and fill factor (FF) significantly increased
upon direct ppm-level doping in the bulk of the photocarrier-generating co-deposited
layer (Fig. 9.12a). Even at 1 ppm doping, slight effects can be seen (green curves).
Clear effects appeared at 10 ppm doping (orange curves). The photocurrent reached
its maximum at 100 ppm. However, a further increase to 1,000 ppm reduced both
the photocurrent (blue solid curve) and forward current (blue-dashed curve).