9 Parts-Per-Million-Level Doping Effects …
237
ITO/p-H 2 Pc:C 60 (4.3 × 10
16 cm
−3 ), which has a similar energy structure to an n
+ pjunction (Fig. 9.9f). Again, the simulated curves (Fig. 9.9e, solid curves) precisely
reproduce the observed changes in work function (Fig. 9.9e, open squares).
The energy band diagram in the present n
+ p-homojunction based on the simulated
curve is shown in Fig. 9.9f. Since the ionized donor concentration is significantly
larger than the ionized acceptor concentration, a one-sided abrupt junction, in which
the SCL spreads predominantly into the p-type region, is formed.
We have demonstrated precise mapping of the band-bending in three cases, a p
+ n
+ -
homojunction (i), a pn-homojunction (ii), and a pn
+ -homojunction (iii), irrespective
of the doping concentrations of the n- and p-layers and the balance between the doping
concentrations of these layers. It was revealed that consideration of the change in
band-bending in the underlying n-(or p-) type layer when a p-(or n-) type layer is
gradually accumulated on it is necessary. The validity of the conventional theory for
the space charge layer suggests that the dopants are spatially fixed even in the organic
semiconductor films. The present results clearly show that the precise configuration
of the built-in electric field in organic photovoltaic cells is fundamentally identical
to that in inorganic ones.
9.7 Doping Sensitization in Blended Films
In this section, the doping sensitization observed for blended films (H 2 Pc:C 60 ) is
described [26]. The donor dopant (Cs 2 CO 3 ) donates an electron to the conduction
bands (LUMOs) of both H 2 Pc of 3.2 eV and C 60 of 3.9 eV [64, 65] (Fig. 9.10c),
giving them n-type character. The carrier concentration evaluated by band-mapping
(Fig. 9.4a) is about five times higher for the blended films (Fig. 9.10a, red dots)
than for the corresponding single component films (Fig. 9.10a, yellow diamonds,
blue triangles). Thus, a doping efficiency of approximately 50% was attained for the
blended films (Fig. 9.10b, red dots), while that of the corresponding single films was
approximately 10% (Fig. 9.10b, yellow diamonds, blue triangles). For H 2 Pc:C 60
blended films (Fig. 9.10a), we confirmed that the specific conductivity increased
2,000 times from 7.5 × 10
–6 (undoped) to 1.5 × 10
–2 Scm
−1 (MR = 0.05).
Doping sensitization can be explained by the charge separation superlattice model.
Before contact (Fig. 9.10c), the corresponding single films show ionization rates
of 10%, which correspond to E D (activation energy) = 0.12 eV. After contact
(blending) (Fig. 9.10d), ET from H 2 Pc to C 60 (Fig. 9.10c, red arrow) accelerates
electron release from the donor levels. As a result, the donor ionization rate increases
for only H 2 Pc. The ionization rates in the H 2 Pc region of 100% and in the C 60 region of
10% reproduce the observed total rate of 55%, i.e., 100% × 0.5 + 10% × 0.5 = 55%
(Table 9.2). The ionization rate increases from 10 to 100% indicates a sensitization
factor of 10.
Identical ET from donor (H 2 Pc) to acceptor (C 60 ) (Fig. 9.10c, red arrow) can be
utilized to cause exciton dissociation, i.e., D/A sensitization (Fig. 9.1b). Therefore,
237
ITO/p-H 2 Pc:C 60 (4.3 × 10
16 cm
−3 ), which has a similar energy structure to an n
+ pjunction (Fig. 9.9f). Again, the simulated curves (Fig. 9.9e, solid curves) precisely
reproduce the observed changes in work function (Fig. 9.9e, open squares).
The energy band diagram in the present n
+ p-homojunction based on the simulated
curve is shown in Fig. 9.9f. Since the ionized donor concentration is significantly
larger than the ionized acceptor concentration, a one-sided abrupt junction, in which
the SCL spreads predominantly into the p-type region, is formed.
We have demonstrated precise mapping of the band-bending in three cases, a p
+ n
+ -
homojunction (i), a pn-homojunction (ii), and a pn
+ -homojunction (iii), irrespective
of the doping concentrations of the n- and p-layers and the balance between the doping
concentrations of these layers. It was revealed that consideration of the change in
band-bending in the underlying n-(or p-) type layer when a p-(or n-) type layer is
gradually accumulated on it is necessary. The validity of the conventional theory for
the space charge layer suggests that the dopants are spatially fixed even in the organic
semiconductor films. The present results clearly show that the precise configuration
of the built-in electric field in organic photovoltaic cells is fundamentally identical
to that in inorganic ones.
9.7 Doping Sensitization in Blended Films
In this section, the doping sensitization observed for blended films (H 2 Pc:C 60 ) is
described [26]. The donor dopant (Cs 2 CO 3 ) donates an electron to the conduction
bands (LUMOs) of both H 2 Pc of 3.2 eV and C 60 of 3.9 eV [64, 65] (Fig. 9.10c),
giving them n-type character. The carrier concentration evaluated by band-mapping
(Fig. 9.4a) is about five times higher for the blended films (Fig. 9.10a, red dots)
than for the corresponding single component films (Fig. 9.10a, yellow diamonds,
blue triangles). Thus, a doping efficiency of approximately 50% was attained for the
blended films (Fig. 9.10b, red dots), while that of the corresponding single films was
approximately 10% (Fig. 9.10b, yellow diamonds, blue triangles). For H 2 Pc:C 60
blended films (Fig. 9.10a), we confirmed that the specific conductivity increased
2,000 times from 7.5 × 10
–6 (undoped) to 1.5 × 10
–2 Scm
−1 (MR = 0.05).
Doping sensitization can be explained by the charge separation superlattice model.
Before contact (Fig. 9.10c), the corresponding single films show ionization rates
of 10%, which correspond to E D (activation energy) = 0.12 eV. After contact
(blending) (Fig. 9.10d), ET from H 2 Pc to C 60 (Fig. 9.10c, red arrow) accelerates
electron release from the donor levels. As a result, the donor ionization rate increases
for only H 2 Pc. The ionization rates in the H 2 Pc region of 100% and in the C 60 region of
10% reproduce the observed total rate of 55%, i.e., 100% × 0.5 + 10% × 0.5 = 55%
(Table 9.2). The ionization rate increases from 10 to 100% indicates a sensitization
factor of 10.
Identical ET from donor (H 2 Pc) to acceptor (C 60 ) (Fig. 9.10c, red arrow) can be
utilized to cause exciton dissociation, i.e., D/A sensitization (Fig. 9.1b). Therefore,