  We have studied photocurrent generation in large carbon nanotube (CNT) films
using electrodes with different spacings. We observe that the photocurrent
depends strongly on the position of illumination, with maximum observed
response occurring upon illumination at the electrode edges. The rate of change
of the response decays exponentially, with the fastest response occurring for
samples with the smallest electrode spacing. We show that the time response is
due to charge carrier diffusion in low-mobility CNT films.
