  We investigate the time-resolved photocurrent response of CdSe quantum dot
(QD) thin films sensitized with zinc beta-tetraaminophthalocyanine (Zn4APc) on
three different substrates, namely silicon with 230 nm SiO2 dielectric, glass
as well as polyimide. While Si/SiO2(230 nm) is not suitable for any transient
photocurrent characterization due to an interfering photocurrent response of
the buried silicon, we find that polyimide substrates invoke the larger optical
bandwidth with 85 kHz vs. 67 kHz for the same quantum dot thin film on glass.
Upon evaluation of the transient photocurrent, we find that the photoresponse
of the CdSe quantum dot films can be described as a combination of carrier
recombination and fast trapping within 2.7 ns, followed by slower multiple
trapping events. The latter are less pronounced on polyimide, which leads to
the higher bandwidth. We show that all devices are RC-time limited and that
improvements of the photoresistance are the key to further increasing the
bandwidth.
