236
M. Hiramoto
p
+ -layer (green broken double-headed arrow) should be added together as shown in
n
+ -region in Fig. 9.9. Here, green broken curve is obtained by inverting and shrinking
in width from x p to x n of green solid curve. The calculated work function (Fig. 9.8a,
black solid curve) clearly reproduces the observed work function (Fig. 9.8a, black
squares).
Oppositely, when the p
+ -type layer was deposited step-wise on the n
+ -type layer,
band is bent not only in the p
+ -type layer but also in the underlying n
+ -type layer. In
order to calculate the work function change, potential change occurred in p
+ -layer
(green solid double-headed arrow) and that occurred in n
+ -layer (red broken doubleheaded arrow) should be added together as shown in p
+ -region in Fig. 9.8d. Here,
red broken curve is obtained by inverting and extending in width from x n to x p of
red solid curve. Again, the calculated work function (Fig. 9.8b, black solid curve)
clearly reproduces the observed work function (Fig. 9.8b, black squares). Irrespective
of whether the n
+ - is deposited on the p
+ -layer or the p
+ - is deposited on the n
+ -layer,
the work function change can be reproduced by the calculation. This supports the
validity of the values of W p , N A
– , W n , and N D
+ obtained and potential distribution
in Fig. 9.8b (solid curve).
The energy band diagram for the p
+ n
+ -homojunction can be illustrated precisely
by turning the potential distribution shown in Fig. 9.8d (solid curve) upside down
(Fig. 9.8c). Since this p
+ n
+ -homojunction has an extremely narrow depletion layer
width (W ) of 18 nm, it confirms that it behaves as an organic/organic ohmic junction
due to tunneling (Sect. 6.1.2., Fig. 9.7).
For the lightly doped pn-homojunction (ii), as was the case for the first sample,
the simulated curve (Fig. 9.9a, b, solid curves) reproduces the observed changes in
work function (Fig. 9.9a, b, black squares), and the values of W n and W p obtained
were each 22 nm. As a result, N D
+ and N A
– were each determined to be 5.0 ×
10
17 (Fig. 9.9a, b). These values of N D
+ and N A
– almost accord with those obtained
from band-bending measurements made in Schottky junctions, i.e., ITO/n-6 T:C 60
(6.0 × 10
17 cm
−3 )[63] or p-6 T:C 60 (3.0 × 10
17 cm
−3 ). The consistency between
the parameters in the pn-homojunction and the corresponding Schottky junctions
supports the validity of the obtained ionized dopant concentrations.
For the pn
+ -homojunction (iii), since the doping concentration of the n
+ -region is
considerably larger than that of the p-region, it is assumed that in the n
+ p-junction,
which forms a one-sided abrupt junction, the SCL spreads predominantly into the
p-region. As shown in Fig. 9.9d, e, as expected, the band-bending mainly occurred
in the p-region (Fig. 9.9e). On the other hand, a sharp decrease in work function
was observed to a depth of 2 nm in the n
+ -region. This means that positive charge
within 2 nm of the interface is sufficient to compensate for the negative charge in the
depletion layer of the p-region. Conversely, deposition of an n
+ -layer of only 2 nm
thick on the p-layer can induce band-bending of 92 nm width in the underlying player. The values of W p and W n obtained were 92 nm and 2 nm (see inset in Fig. 9.9d),
respectively. As a result, N D
+ and N A
– were determined to be 1.9 × 10
18 and 4.2
× 10
16 cm
−3 , respectively (Fig. 9.9d, e). The values of N D
+ and N A
– accord with
those obtained from band-bending measurements made on a Schottky junction, i.e.,
M. Hiramoto
p
+ -layer (green broken double-headed arrow) should be added together as shown in
n
+ -region in Fig. 9.9. Here, green broken curve is obtained by inverting and shrinking
in width from x p to x n of green solid curve. The calculated work function (Fig. 9.8a,
black solid curve) clearly reproduces the observed work function (Fig. 9.8a, black
squares).
Oppositely, when the p
+ -type layer was deposited step-wise on the n
+ -type layer,
band is bent not only in the p
+ -type layer but also in the underlying n
+ -type layer. In
order to calculate the work function change, potential change occurred in p
+ -layer
(green solid double-headed arrow) and that occurred in n
+ -layer (red broken doubleheaded arrow) should be added together as shown in p
+ -region in Fig. 9.8d. Here,
red broken curve is obtained by inverting and extending in width from x n to x p of
red solid curve. Again, the calculated work function (Fig. 9.8b, black solid curve)
clearly reproduces the observed work function (Fig. 9.8b, black squares). Irrespective
of whether the n
+ - is deposited on the p
+ -layer or the p
+ - is deposited on the n
+ -layer,
the work function change can be reproduced by the calculation. This supports the
validity of the values of W p , N A
– , W n , and N D
+ obtained and potential distribution
in Fig. 9.8b (solid curve).
The energy band diagram for the p
+ n
+ -homojunction can be illustrated precisely
by turning the potential distribution shown in Fig. 9.8d (solid curve) upside down
(Fig. 9.8c). Since this p
+ n
+ -homojunction has an extremely narrow depletion layer
width (W ) of 18 nm, it confirms that it behaves as an organic/organic ohmic junction
due to tunneling (Sect. 6.1.2., Fig. 9.7).
For the lightly doped pn-homojunction (ii), as was the case for the first sample,
the simulated curve (Fig. 9.9a, b, solid curves) reproduces the observed changes in
work function (Fig. 9.9a, b, black squares), and the values of W n and W p obtained
were each 22 nm. As a result, N D
+ and N A
– were each determined to be 5.0 ×
10
17 (Fig. 9.9a, b). These values of N D
+ and N A
– almost accord with those obtained
from band-bending measurements made in Schottky junctions, i.e., ITO/n-6 T:C 60
(6.0 × 10
17 cm
−3 )[63] or p-6 T:C 60 (3.0 × 10
17 cm
−3 ). The consistency between
the parameters in the pn-homojunction and the corresponding Schottky junctions
supports the validity of the obtained ionized dopant concentrations.
For the pn
+ -homojunction (iii), since the doping concentration of the n
+ -region is
considerably larger than that of the p-region, it is assumed that in the n
+ p-junction,
which forms a one-sided abrupt junction, the SCL spreads predominantly into the
p-region. As shown in Fig. 9.9d, e, as expected, the band-bending mainly occurred
in the p-region (Fig. 9.9e). On the other hand, a sharp decrease in work function
was observed to a depth of 2 nm in the n
+ -region. This means that positive charge
within 2 nm of the interface is sufficient to compensate for the negative charge in the
depletion layer of the p-region. Conversely, deposition of an n
+ -layer of only 2 nm
thick on the p-layer can induce band-bending of 92 nm width in the underlying player. The values of W p and W n obtained were 92 nm and 2 nm (see inset in Fig. 9.9d),
respectively. As a result, N D
+ and N A
– were determined to be 1.9 × 10
18 and 4.2
× 10
16 cm
−3 , respectively (Fig. 9.9d, e). The values of N D
+ and N A
– accord with
those obtained from band-bending measurements made on a Schottky junction, i.e.,