. Phase
separation at other PCPDTBT:MEH-PPV ratios might contribute to
the reduced interfacial area between PCPDTBT and MEH-PPV, thus
resulting in lower photocurrent. The inset in Fig. 4(b) depicts the
photo-generated current vs illumination intensity relationship for
the PCPDTBT and MEH-PPV independently. It is observed that the
photo current magnitude of both organic polymers PCPDTBT and
MEH-PPV are individually less than the blend. This might be due to
a broader absorption range of the PCPDTBT:MEH-PPV blend and
more uniform phase distribution of the molecules of both
polymers in the specific volumetric ratio of the bulk heterojunction
blend.
The response/recovery time and temporal photo sensitivity are
important parameters in order to evaluate the behavior of the
photo sensor for practical application. Response and recovery
times (tr and tc) are generally defined as the time required for the
photo current to increase the final settled value or vice versa [26].
The photo response characteristics of the fabricated DSPSs
demonstrate a sharp change in its magnitude of photo current
as a function of periodic pulsed stimulated solar light as shown in
Fig. 5. In order to record the response and recovery times,100 mW/
cm2 illumination was employed by an Oriel solar simulator (model
67005). The pulse width was 5 s. The average response time (tr)
for the independent PCPDTBT and MEH-PPV DSPS are given in
Fig. 5(a), and (b) and were found to be (tr = 937 ms and 685 ms),
respectively. The average response time for the PCPDTBT:MEH-PPV
volumetric ratio 1:0.6, is 382 ms. The time-resolved response/
recovery time is regular, steady and repeatable as shown by three
consecutive cycles of abrupt switching between dark and
illumination states. Typically the response times for bulk heterojunction
based photo sensors are in the range of (108 to 103
) s
[27] which is comparable with inorganic photo sensors. It is worth
noting, that the binary blend based photo sensor exhibits good
photoconductive performance in terms of sensitivity, response and
reset times as compared to the individual organic polymer based
devices. However, it can be further reduced by using a ternary
blend system [18]. It is obvious from Fig. 4, the higher intensity of
the incident illumination gives noticeable upsurge in the
photocurrent. Indeed, there are a few bottlenecks which limits
the response of the photo sensor: (a) limited absorption; (b)
restricted generation and splitting of excitons and (c) incompetent
transport of charges toward the collection electrodes. The bulk
heterojunction blend technique supports the improvement in light
absorption and exciton dissociation only. However, the movement
of splitted exciton toward electrodes is external field assisted.
The reproducibility and repeatability of the response-reset time
measurements was investigated for all devices and the devices are
found to be very stable and reproducible. Furthermore, the photoconductivity
sensitivity and the responsivity of the DSPS have also
been investigated. Comparison of the key sensing parameters of
the DSPSs such as photo-conductivity sensitivity, responsivity,
and response time is given in Table 1. The photo-conductivity
sensitivity and the responsivity of the DSPSs have been described
in detail in our previous research [11]. Fig. 5(d) depicted the
external quantum efficiency (EQE) spectra of MEH-PPV and
PCPDTBT and their blend films. The EQE spectra are in good
agreement with the absorption spectra of the individual and blend
films.