Quantum Dot-Sensitized Solar Cells - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 102

continuously on the grounded FTO substrate for 20 minutes. Subsequently, the
films were sintered at 450 C for 30 minutes in air. The diameter of the fibrous
membranes can be easily modified by adjusting the Taylor cone size through (a)
sol concentration (b) flow rate, and (c) applied potential.
Most fibrous electrodes have been fabricated using a two-step method. After fiber
synthesis, substrates are coated with the synthesized fibers, but in this case poor
particle interconnectivity is detected [ 96 , 103 ]. Continuous electrospun fibers can
solve this problem but peel off limitations has to be overcome. Various approaches
have been demonstrated for this purpose. Song et al. utilized a hot press method to
enhance the adhesion of the nanofiber to the substrate [ 95 ]. Similarly, chemical
treatment was also employed to relax the nanofiber to yield an improved adhesion of
the nanofiber to the substrate [ 104 ]. Onozuka et al. [ 105 ] applied a dimethyl
formamide (DMF) treatment to TiO 2 fibrous electrodes, where DMF induced the
swelling of the polymeric substances in the composite film and reinforced the
adhesion between the substrate and fibrous membranes. This DMF treatment has
improved the charge collection efficiency of DSCs by about 20 % compared to
untreated electrodes. Analogous DMF-treated electrospun spheroidal electrodes
were tested in QDSSCs, where the efficiencies increased from 0.85 to 1.2 % through
improving the electrical contact between the fibers and the Transparent Conducting
Oxide (TCO) substrate [ 106 ]. Despite the good electrical contact achieved by
chemical treatment, it could affect the fibrous morphology thus lower the pore
volume of the electrode, which severely affect the QDs loading and electrolyte
penetration. In view of maintaining electrode pore volume as well as good electrical
contact between TCO and fibrous network concurrently, it would be a better choice
to choose chemical vapor treatment instead of direct chemical treatment.
Recently, we studied the above said hypothesis of surface treatment (chemical
treatment and chemical vapor treatment) in fibrous electrodes and provide the
detailed insights in controlling recombination rate at QDSCs [ 63 ]. Figure 9 a-c
compares the SEM images of different surface-treated TiO 2 fibrous electrodes. It
clearly indicates that the untreated fibers (Fig. 9 a), show a smooth surface with
70-100 nm diameters and several micrometers length. After surface pretreatment
with DMF (Fig. 9 b), the fibrous channels were intertwined and became a rather
compact structure. This may be due to the fact that DMF treatment swells the
polymer content of the fibers, thus resulting in a coagulated fiber structure. This
coagulated fiber structure reduces the interpore distance between each fiber channel,
reducing consequently the effective surface area of the electrode for electrolyte
penetration. However, it could improve the electrical contact between the FTO
substrate and the fibrous TiO 2 layer. In the case of tetrahydrofuran (THF) vapor
treatment (Fig. 9 c), the fibrous surface seems etched, which could consequently
improve fibers inter-junction points. Also, the interpore distance between fibrous
channels is partially retained after the THF pretreatment. As a consequence, THF
treatment produces electrodes to half way between UT and DMF electrodes.
From Fig. 9 d THF-QDSSCs result high open-circuit voltage V oc = 0.57 V with
slight improved short circuit photocurrent J sc = 9.74 mA cm -2 compares to
untreated fibrous electrode based QDSSCs, while the DMF cell presents a lower