80
T. Kaji
assumed film thickness of 10 μm. This is a normal epi-illumination optical microscope image, the actual film thickness is about 12 μm, and rod-shaped domains are
seen in the vertical direction (direction perpendicular to the substrate surface). This
rod can be identified as a ZnPc crystal by comparing the bulk shape and color (purple
red) of a general ZnPc acicular single crystal, and the crystal grains surrounding the
rod can be assigned to the C 60 crystal. The polarized microscope images suggest that
many ZnPc rods have the same crystallographic orientation, and scanning electron
microscopy (SEM) shows that the rod diameter is about 100 nm and the surrounding
C 60 crystal grains of similar size. Since the crystal growth of this film was optimized
for higher J sc by co-evaporant molecules, it is considered that it was optimized on a
scale equivalent to the exciton diffusion length of ZnPc and C 60 crystals.
Thus, it was found that the 10-μm active layer of the fabricated OPV consisted of
ZnPc rod-shaped crystals surrounded by C 60 crystal grains. Since the rod and granular
crystal shapes of these crystals are very similar to those of their bulk crystals, other
crystalline pigments with a needlelike or platelike crystal morphology in bulk, similar
to ZnPc:C 60 , are suggested to have the possibility of achieving an ultra-thick film
that maintains the photoelectric conversion characteristics.
4.2.4 Relationship Between Active Layer Thickness
and Absorption/Solar Cell Characteristics
Light absorption by the active layer of the OPV cell depends on several factors
such as the film thickness, the standing wave due to the reflection of the upper metal
electrode, and the roughness of the substrate, in addition to the absorption coefficient
and spectrum of the material of the active layer. The solid line (-) and broken line
(---) plots in Fig. 4.4 show the percentage of light absorbed by the ZnPc:C 60 active
layer deposited on the flat ITO glass substrate for organic EL while crystallizing with
co-evaporant molecules. It is plotted against the film thickness of the active layer. In
other words, these lines are corresponding to the optical efficiency of this solar cell
for the standard solar spectrum in the 350–850 nm range, ignoring the optical losses
used in the measurements and the optical losses of the electrical contacts. (Since it
is not a practical module cell, it is strictly different from the definition of optical
efficiency.)
Both the solid line (-) and the broken line (---) were calculated from the actual
measurement data of the absorption spectrum of the actually prepared active layer
based on the standard sunlight spectrum of AM1.5G. The optical efficiency was
obtained from the “measurement result of actual OPV element containing Ag”: solid
line (-) and from the “measurement result of transmittance of the same active layer
not containing Ag”: broken line (---), respectively. Therefore, the solid line (-) is
wavy reflecting the standing wave actually generated by the reflection of the Ag
electrode, and the effect is canceled from the broken line (---). The broken line at
the bottom of the graph shows the amplification of the standing wave due to the
T. Kaji
assumed film thickness of 10 μm. This is a normal epi-illumination optical microscope image, the actual film thickness is about 12 μm, and rod-shaped domains are
seen in the vertical direction (direction perpendicular to the substrate surface). This
rod can be identified as a ZnPc crystal by comparing the bulk shape and color (purple
red) of a general ZnPc acicular single crystal, and the crystal grains surrounding the
rod can be assigned to the C 60 crystal. The polarized microscope images suggest that
many ZnPc rods have the same crystallographic orientation, and scanning electron
microscopy (SEM) shows that the rod diameter is about 100 nm and the surrounding
C 60 crystal grains of similar size. Since the crystal growth of this film was optimized
for higher J sc by co-evaporant molecules, it is considered that it was optimized on a
scale equivalent to the exciton diffusion length of ZnPc and C 60 crystals.
Thus, it was found that the 10-μm active layer of the fabricated OPV consisted of
ZnPc rod-shaped crystals surrounded by C 60 crystal grains. Since the rod and granular
crystal shapes of these crystals are very similar to those of their bulk crystals, other
crystalline pigments with a needlelike or platelike crystal morphology in bulk, similar
to ZnPc:C 60 , are suggested to have the possibility of achieving an ultra-thick film
that maintains the photoelectric conversion characteristics.
4.2.4 Relationship Between Active Layer Thickness
and Absorption/Solar Cell Characteristics
Light absorption by the active layer of the OPV cell depends on several factors
such as the film thickness, the standing wave due to the reflection of the upper metal
electrode, and the roughness of the substrate, in addition to the absorption coefficient
and spectrum of the material of the active layer. The solid line (-) and broken line
(---) plots in Fig. 4.4 show the percentage of light absorbed by the ZnPc:C 60 active
layer deposited on the flat ITO glass substrate for organic EL while crystallizing with
co-evaporant molecules. It is plotted against the film thickness of the active layer. In
other words, these lines are corresponding to the optical efficiency of this solar cell
for the standard solar spectrum in the 350–850 nm range, ignoring the optical losses
used in the measurements and the optical losses of the electrical contacts. (Since it
is not a practical module cell, it is strictly different from the definition of optical
efficiency.)
Both the solid line (-) and the broken line (---) were calculated from the actual
measurement data of the absorption spectrum of the actually prepared active layer
based on the standard sunlight spectrum of AM1.5G. The optical efficiency was
obtained from the “measurement result of actual OPV element containing Ag”: solid
line (-) and from the “measurement result of transmittance of the same active layer
not containing Ag”: broken line (---), respectively. Therefore, the solid line (-) is
wavy reflecting the standing wave actually generated by the reflection of the Ag
electrode, and the effect is canceled from the broken line (---). The broken line at
the bottom of the graph shows the amplification of the standing wave due to the