Measurement of the photovoltaic properties as a
function of the illumination intensity (Fig. 4(b) and
Table 1) shows that the efficiency is varied by 8%,
indicating the stability of the nanotube solar cells
under light. The efficiency is substantially higher and
reaches 10.9% at a 0.5-sun illumination. The dependence
of Jsc and Voc on the light intensity is shown in Fig. 5(c)
and Table 1. The value of Jsc increases from 6.81 to
38.8 mA/cm2 while Voc increases from 675 to 770 mV
as the light intensity increases, suggesting that photogenerated
carriers are dominant and that the photon
flux intensity does not result in an overly large increase
in the reverse saturation current; this indicates a robust,
good-quality junction between the CdS nanotubes and
the CdTe absorber.