Quantum Dot-Sensitized Solar Cells - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 111

4.3 Inverse Opal Architecture Photoanodes
The disordered geometrical structure of conventional nanoparticle-based pho-
toanode is often limiting the performance of the sensitized-type solar cells due to
interfacial interference for electron transport. Mainly, trap-limited diffusion pro-
cess in randomly connected networks can be affected by recombination with the
oxidizing species in the electrolyte during trapping process. Therefore, designing
the anode frameworks with highly interconnected morphology is a promising
approach in achieving superior charge transport and high penetration of both
sensitizers and redox couples. In this context, a nanostructure which contains
bottom-up 3D host-passivation-guest (H-P-G) electrode has been realized as
promising candidate in DSSCs [ 129 , 130 ]. Since, 3-D H-P-G electrode offer good
structural control in the electron extraction and the recombination dynamics, this
new type of H-P-G electrode has significantly promoted the photocurrent, fill
factor, and most importantly the photovoltage of DSSCs [ 131 ].
The H-P-G electrode is basically developed by micromolding in inverse opals
(IO) structures using colloidal crystals [ 132 ]. It is well reported that inverse opal
TiO 2 has large interconnected pores that lead to a better infiltration, also it exhibits
a photonic bandgap (photonic crystal), which depends on the filling fraction of
TiO 2 in the inverse opal structure. The preparation procedure of IO electrodes has
been demonstrated in two stages. First, the host layer is self-assembled on TCO
substrates, subsequently secondary coating of guest (TiO 2 ) layer coated on the
host. The preparation methods of 3-D TiO 2 IO electrodes were schematically
explained in the Fig. 16 a. The host layer may be assembled by either chemical or
physical technique. It is widely demonstrated that the polymer microspheres are
utilized as host layer (polystyrene, co-polymers, etc.) since it is easily removable
without altering the final TiO 2 morphology. Figure 16 a shows the deposition of
TiO 2 layer by chemical method, which support for large-scale fabrication at low
cost. Besides, it is difficult to control the thickness of the TiO 2 layer in this
approach. The undesired thick TiO 2 coating could clog the mesoscopic pores of
the host layer, which would inhibit sensitization and electrolyte infiltration. This
can be overcome by physical coating like ''atomic layer deposition'' (ALD), where
the thin guest layer can be conformably coated on the entire polymer microspheres
(Fig. 16 b) without clogging the pore structure [ 129 , 133 - 135 ]. Finally, polymer
beads were removed (Fig. 16 c) by high temperature annealing or with solvents at
room temperature, we obtained a 3D host backbone that is well connected to the
underlying TCO substrate. The resultant direct electronic connection with TCO
facilitates the charge extraction throughout the interconnected 3D H-P-G elec-
trode. The IO photonic crystal are playing crucial roles in DSSCs as a (a) dielectric
mirror for wavelengths corresponding to the stop band and (b) medium for
enhancing light absorption on the long-wavelength side of the stop band [ 136 , 137 ].
The advantages of light interaction in these structures ultimately enhance the
backscattering of the device through localization of heavy photons near the edges of
a photonic gap. This scattered light increases the probability of light absorption