54
M. Hiramoto
3.3.3 Layer Width
When only the C 60 or the H 2 Pc were selectively excited using monochromatic lights
of 500 and 600 nm, respectively, under a relatively small applied electric field,
the photocurrent generated in the vertical superlattice was reproducibly observed.
Figure 3.10a shows the dependence of the photocurrent generation quantum efficiency on the layer width (x; Fig. 3.8a) of the vertical superlattice. The layer width
(x) was varied down to 2.5 nm. A decrease in x means an increase in the number of
interfaces between C 60 and H 2 Pc. At x = 2.5 nm, the number of layers would be 4
million layers per 1 cm. The quantum efficiency rapidly increased as x decreased, and
the maximum, 56% for C 60 , was observed at 5–10 nm. However, it was also found
that the efficiency decreased when the layer width was decreased to 2.5 nm. Thus, we
could experimentally determine the optimal width (x) to be 5–10 nm, at which the
performance of photoelectric conversion in the C 60 /H 2 Pc system was maximized.
The increase in quantum efficiency by decreasing the layer width (x) is related
to the exciton diffusion length. A photocurrent is generated when the photogenerated excitons dissociate into free electrons and holes when they reach the C 60 /H 2 Pc
interface (Fig. 3.10c). The excitons that could not reach the boundary surface are
deactivated. The range where the excitons can reach the interface is the active area
in which the photocurrent is generated (shaded area), and the remainder where the
excitons are deactivated is the dead area (non-shaded area). The increase in the
photocurrent quantum efficiency occurs due to an increase in the active area and
a decrease in the dead area (Fig. 3.10b). In other words, when x is thick, i.e., the
number of interfaces is small, the majority of the region will be the dead area, and
only a small photocurrent is generated. When x becomes thin, meaning the number
of interfaces increases, the photocurrent gradually increases due to the increase in
the active area. Further, at x = 5–10 nm, the dead area in C 60 is eliminated, and the
maximum photocurrent is generated (Fig. 3.10b).
3.3.4 Exciton Diffusion Length
The exciton diffusion length could be obtained quantitatively. A semilogarithmic
plot of the photocurrent quantum efficiency as a function of x (Fig. 3.10d) showed
a clear linear relationship. This signifies that the concentration of the excitons could
contribute to the photocurrent decreased exponentially by increasing the distance
from the interface. The distance at which 90% of the excitons could reach the interface
for C 60 was precisely determined to be 1.7 nm, and similarly, that for H 2 Pc was
determined to be 4.6 nm by the selective excitation of H 2 Pc. As given in Table 3.1,
by adopting the C 60 /H 2 Pc system, the exciton diffusion lengths for C 60 and H 2 Pc
were determined to be 1.7 and 4.6 nm, respectively. Furthermore, by adopting the
Me-PTC/H 2 Pc system, the exciton diffusion lengths for Me-PTC and H 2 Pc were
determined to be 4.4 and 5.0 nm, respectively. Similar values of the exciton diffusion
M. Hiramoto
3.3.3 Layer Width
When only the C 60 or the H 2 Pc were selectively excited using monochromatic lights
of 500 and 600 nm, respectively, under a relatively small applied electric field,
the photocurrent generated in the vertical superlattice was reproducibly observed.
Figure 3.10a shows the dependence of the photocurrent generation quantum efficiency on the layer width (x; Fig. 3.8a) of the vertical superlattice. The layer width
(x) was varied down to 2.5 nm. A decrease in x means an increase in the number of
interfaces between C 60 and H 2 Pc. At x = 2.5 nm, the number of layers would be 4
million layers per 1 cm. The quantum efficiency rapidly increased as x decreased, and
the maximum, 56% for C 60 , was observed at 5–10 nm. However, it was also found
that the efficiency decreased when the layer width was decreased to 2.5 nm. Thus, we
could experimentally determine the optimal width (x) to be 5–10 nm, at which the
performance of photoelectric conversion in the C 60 /H 2 Pc system was maximized.
The increase in quantum efficiency by decreasing the layer width (x) is related
to the exciton diffusion length. A photocurrent is generated when the photogenerated excitons dissociate into free electrons and holes when they reach the C 60 /H 2 Pc
interface (Fig. 3.10c). The excitons that could not reach the boundary surface are
deactivated. The range where the excitons can reach the interface is the active area
in which the photocurrent is generated (shaded area), and the remainder where the
excitons are deactivated is the dead area (non-shaded area). The increase in the
photocurrent quantum efficiency occurs due to an increase in the active area and
a decrease in the dead area (Fig. 3.10b). In other words, when x is thick, i.e., the
number of interfaces is small, the majority of the region will be the dead area, and
only a small photocurrent is generated. When x becomes thin, meaning the number
of interfaces increases, the photocurrent gradually increases due to the increase in
the active area. Further, at x = 5–10 nm, the dead area in C 60 is eliminated, and the
maximum photocurrent is generated (Fig. 3.10b).
3.3.4 Exciton Diffusion Length
The exciton diffusion length could be obtained quantitatively. A semilogarithmic
plot of the photocurrent quantum efficiency as a function of x (Fig. 3.10d) showed
a clear linear relationship. This signifies that the concentration of the excitons could
contribute to the photocurrent decreased exponentially by increasing the distance
from the interface. The distance at which 90% of the excitons could reach the interface
for C 60 was precisely determined to be 1.7 nm, and similarly, that for H 2 Pc was
determined to be 4.6 nm by the selective excitation of H 2 Pc. As given in Table 3.1,
by adopting the C 60 /H 2 Pc system, the exciton diffusion lengths for C 60 and H 2 Pc
were determined to be 1.7 and 4.6 nm, respectively. Furthermore, by adopting the
Me-PTC/H 2 Pc system, the exciton diffusion lengths for Me-PTC and H 2 Pc were
determined to be 4.4 and 5.0 nm, respectively. Similar values of the exciton diffusion