Inset in Fig. 2 depicts the optical absorption spectra of the each
independent
film. The combination of these two polymers would
have a significant influence on the optical properties of the DSPS
because of efficient photon captures by the blend in the visible part
of the solar spectrum. The absorption spectrum of PCPDTBT
exhibits the absorption in the visible region of the solar spectrum
from 500 nm onward whereas MEH-PPV absorbs in the range of
400–550 nm. The absorption spectrum of PCPDTBT exhibits two
maxima, the
first located at 415 and the second at 730 nm, whereas
the MEH-PPV exhibits a peak at 500 nm. Therefore, the binary
blend covers most of the visible light spectrum. The broader
absorption spectrum of the binary blend
film is expected to be
favorable for broad range visible spectrum sensitivity of the sensor.
It is observed that the ratios 1:0.4, 1:0.6 and 1:0.8 of PCPDTBT:
MEH-PPV blend are the optimum ratios in terms of
flat absorption
profile as compared to other ratios (it is noteworthy that several
different ratios of PCPDTBT:MEH-PPV blend have been investigat-
ed (1:1, 1:1.2, and 1:1.4); only the best three with more uniform
and even absorption height have been shown in Fig. 2). However, it
is found that there are two dips in the absorption spectrum of the
blend
films which are located around 450 and 575 nm, respective-
ly. Thus, the inlay of MEH-PPV into the PCPDTBT would
significantly broaden the absorption spectrum of the PCPDTBT:
MEH-PPV binary bulk heterojunction dye sensitizer.
Fig. 3 shows the photoluminescence (PL) spectra of PCPDTBT:
MEH-PPV blend
films at different volumetric ratios (1:0.4,
1:0.6 and 1:0.8). At 1:0.6 volumetric ratio of the blend, the PL
intensity is significantly quenched, indicating a clue of more
efficient charge transfer in the PCPDTBT:MEH-PPV blend at that
certain ratio. The inset in Fig. 3 shows that the PCPDTBT exhibit a
shoulder at 590 nm and a peak at 720 nm, whereas the MEH-PPV
gives a PL peak at 610 nm. In the case of PCPDTBT:MEH-PPV blend,
a wide PL band is observed in the range of 500–620 nm which is
contributed by the shoulder of PCPDTBT and the peak of MEH-PPV.
It is also observed that the quenching of the PL spectrum changes
with the change in the ratio of MEH-PPV, while the peak position
does not change. The decrease of PL intensity indicates the better
exciton disassociation between PCPDTBT and MEH-PPV complex.
For more accurate elaboration of the PL quenching, the absorption
intensity of the blend
film should be taken into account. The higher
PL emission of PCPDTBT:MEH-PPV for the case blend ratio (1:0.8)
might also be contributed to by the higher absorption of the blend
film