Through SEM imaging, we anticipate that these highdensity CdS nanotube arrays will provide novel ways
of manipulating light absorption, carrier generation,
and collection. As observed from the SEM images, the
nanotubes with a 35-nm inner diameter and a 35-nm
intertube space will enable carriers to be effectively
collected before they are lost by recombination, and
hence will significantly improve carrier collection for
absorbers with short (less than 100 nm) minority carrier
diffusion lengths. Copper phthalocyanine (CuPc) is
an example of such an absorber.
The fundamental absorption and transmittance of
CdS nanotubes are especially helpful in understanding
their unique light manipulation characteristics. UVvisible
spectroscopy measurements were conducted
over a wavelength range of 300 to 900 nm, which covers
most sunlight photons with an energy higher than
the band gap of the CdTe absorber. Figure 4(a) shows
the transmittance spectrum of the CdS nanotubes
embedded in the AAO template. This transmittance
spectrum was acquired before the partial removal of
the AAO membrane. Of course, the AAO membrane
would subsequently be partially removed before the
CdTe layer was deposited.
From Fig. 4(a), it can be seen that the CdS nanotubes
exhibit nearly zero absorption and strong transmission
from 450 to 900 nm. Only a weak absorption peak is
observed at 600 nm and the corresponding transmittance
falls to 0.88. A long absorption tail extends
from 450 to 296 nm, and the corresponding transmittance
is reduced from 0.925 to 0.10 between 450
and 296 nm, where the transmittance of the CdS
nanotubes is reduced to 0.8 at 430 nm. It is thought
that such unique optical absorption and transmittance
properties are related to the transparent AAO membrane
and the geometry of the CdS nanotubes. First, the
absorption-negligible AAO membrane facilitates the
transmission of sunlight through the substrate. The
length of the CdS nanotubes is about 200 nm, which is
significantly smaller than the wavelength of incident
light, thus making any light scattering negligible. This
phenomenon differs from the enhanced light scattering
in long silicon nanowire structures [1, 6].
This unique light absorption behavior exemplifies
the beneficial properties of the 200-nm CdS nanotube
geometry, which is designed to enhance light absorption
and carrier conversion in the CdTe absorber layer.