292
J. B. Liyanage et al.
Another nanostructure that was considered for the development of TiO 2 , as a better semiconductor material in DSCs, would be hollow nanoparticles. In 2008, Yang
et al. synthesized quasi-ordered TiO 2 hollow hemispheres, using a colloidal template
and radio frequency-sputtering, in order to obtain fast electron transport and high
surface area in a thin photoelectrode, and obtained a photoconversion efficiency of
3.49%. The fact that the diameter, wall thickness and the height of these unique
microstructures could be easily changed and the enhanced surface activity, encourages its application in DSCs (Yang et al. 2008). Koo et al. studied nano-embossed
hollow spherical TiO 2 particles to obtain bifunctionality: light scattering and photoexcited electron generation, to obtain an efficiency of 10.34%. It has been confirmed
that the nano-embossed hollow spheres containing layer was able to adsorb 5 times
more of the dye while acting as the scattering layer (Koo et al. 2008). Similarly, Yu
et al., studied hollow structured TiO 2 due to its low density, high surface area and
its porous structure which affects the increase in light harvesting efficiency and fast
motion of charge carriers. Under the optimal conditions, they were able to generate
an efficiency of 4.82% (Yu and Zhang 2010).
Nanofibers have also been used as a structural modification implemented on to
TiO 2 , in the aim of improving the photovoltaic properties. Ghadiri et al. used cellulose
fibers as a template material to synthesize the hollow TiO 2 nanofibers, by a stepwise
hydrolysis and dehydration of a saturated Ti
4+ solution. The unoriented nanostructure showed enhanced electron transport properties compared to the mesoporous
layer made of spherical nanoparticles, leading to 7.15% efficiency under optimal
conditions. The retardation of surface recombination, between the conduction band
electrons and oxidized species in the electrolyte, observed in nanostructured fiberbased cells was able to balance out the adverse shift of the band edge toward positive
potential and produce an open-circuit photovoltage of 760 mV, which is a gain compared to that of the DSC containing nanoparticles (Ghadiri et al. 2010), which is
seen with the decrease of the photoactive layer. This shift of the conduction band
edge towards the positive potential, has been generally observed for DSCs based
on nanotubes and nanoparticles (Kuang et al. 2008). TiO 2 nanorods have been further studied by Pandanga et al. to optimize the morphology and dye uptake and to
encourage fast electron transport avoiding recombination reactions (Pandanga et al.
2019).
Self-organized porous metal oxides with a nanotubular structure have attracted
significant interest in recent years (Fraoucene et al. 2019). It possible to apply anodically formed TiO 2 nanotubes to photovoltaic devices, yielding semiconductor material with a large interconnected internal surface area (O’Regan and Grätzel 1991).
Although considerable research has been concentrated on the development of nanotubular TiO 2 layers, there is no simple synthesis route to obtain a uniformly stretched
nanotubular structure, other than by an electrochemical anodization process. However, electrons in the nanotube can also meet with many grain boundaries, defects,
and trap sites, becoming a factor that retards the electron transport time (Kang et al.
2007).
Kai Zhu et al. reported that the light-harvesting efficiencies of nanotube (NT)based DSCs were higher than those of nanoparticle (NP)-based DSCs. It was also
J. B. Liyanage et al.
Another nanostructure that was considered for the development of TiO 2 , as a better semiconductor material in DSCs, would be hollow nanoparticles. In 2008, Yang
et al. synthesized quasi-ordered TiO 2 hollow hemispheres, using a colloidal template
and radio frequency-sputtering, in order to obtain fast electron transport and high
surface area in a thin photoelectrode, and obtained a photoconversion efficiency of
3.49%. The fact that the diameter, wall thickness and the height of these unique
microstructures could be easily changed and the enhanced surface activity, encourages its application in DSCs (Yang et al. 2008). Koo et al. studied nano-embossed
hollow spherical TiO 2 particles to obtain bifunctionality: light scattering and photoexcited electron generation, to obtain an efficiency of 10.34%. It has been confirmed
that the nano-embossed hollow spheres containing layer was able to adsorb 5 times
more of the dye while acting as the scattering layer (Koo et al. 2008). Similarly, Yu
et al., studied hollow structured TiO 2 due to its low density, high surface area and
its porous structure which affects the increase in light harvesting efficiency and fast
motion of charge carriers. Under the optimal conditions, they were able to generate
an efficiency of 4.82% (Yu and Zhang 2010).
Nanofibers have also been used as a structural modification implemented on to
TiO 2 , in the aim of improving the photovoltaic properties. Ghadiri et al. used cellulose
fibers as a template material to synthesize the hollow TiO 2 nanofibers, by a stepwise
hydrolysis and dehydration of a saturated Ti
4+ solution. The unoriented nanostructure showed enhanced electron transport properties compared to the mesoporous
layer made of spherical nanoparticles, leading to 7.15% efficiency under optimal
conditions. The retardation of surface recombination, between the conduction band
electrons and oxidized species in the electrolyte, observed in nanostructured fiberbased cells was able to balance out the adverse shift of the band edge toward positive
potential and produce an open-circuit photovoltage of 760 mV, which is a gain compared to that of the DSC containing nanoparticles (Ghadiri et al. 2010), which is
seen with the decrease of the photoactive layer. This shift of the conduction band
edge towards the positive potential, has been generally observed for DSCs based
on nanotubes and nanoparticles (Kuang et al. 2008). TiO 2 nanorods have been further studied by Pandanga et al. to optimize the morphology and dye uptake and to
encourage fast electron transport avoiding recombination reactions (Pandanga et al.
2019).
Self-organized porous metal oxides with a nanotubular structure have attracted
significant interest in recent years (Fraoucene et al. 2019). It possible to apply anodically formed TiO 2 nanotubes to photovoltaic devices, yielding semiconductor material with a large interconnected internal surface area (O’Regan and Grätzel 1991).
Although considerable research has been concentrated on the development of nanotubular TiO 2 layers, there is no simple synthesis route to obtain a uniformly stretched
nanotubular structure, other than by an electrochemical anodization process. However, electrons in the nanotube can also meet with many grain boundaries, defects,
and trap sites, becoming a factor that retards the electron transport time (Kang et al.
2007).
Kai Zhu et al. reported that the light-harvesting efficiencies of nanotube (NT)based DSCs were higher than those of nanoparticle (NP)-based DSCs. It was also