15 Yet to Be Challenged: TiO 2 as the Photo-Anode Material …
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shown that by reducing the disorder that is typically present at the top of the NT
layers, an increase in efficiency can be obtained as the dye uptake is increased, while
the internal light scattering effect is seen to be strongly (Zhu et al. 2006, 2007).
It was later discovered that some of these advanced morphologies can be directly
grown on the conductive substrate such as wires (Feng et al. 2008), templated rods
(Liu and Aydil 2009) or self-organized anodic structures (Macak and Schmuki 2006;
Stergiopoulos et al. 2008) that can be obtained as an oxide layer firmly attached to
the Ti-(metal) substrate.
In the aim of allowing a longer, uninterrupted path for the photogenerated electrons, Kim et al. synthesized a bamboo type morphology, by controlled anodization
of a Ti plate, for the increase in the photovoltaic efficiency. However, it was observed
that there was no change in the transport properties or the recombination kinetics of
bamboo type NTs and normal NTs. The only improvement observed was the dye
loading capacity, as the bamboo type NTs allow the exterior as well as the interior
coverage which enhanced the generated current, giving a maximum efficiency of
2.98%, which is greater than that obtained for normal NTs (Kim et al. 2008). But
later on, several more studies have been done using this bamboo type NTs, in order to
further increase the dye loading capacity. Luan et al., studied a two-step method, to
form ridges on the surface of the bamboo type NTs and to obtain evenly lengthened
NTs. It was noted that the ridge density and the length of the NT had a significant
impact on the overall efficiency. Highest efficiency of 6.80% was obtained with the
bamboo type NTs synthesized under a lower water content, which led to a longer tube
length (Luan et al. 2012). Further studies were done by Ji et al. where they synthesized double walled bamboo type NTs. The dye loading was found to be significantly
larger for the DSCs fabricated with NTs of higher ring density, leading to a better
conversion efficiency (Ji et al. 2012). Wang et al. too studied a similar morphology
which led to a reduced interfacial resistance and increased interfacial capacitance,
compared to that of smooth walled NTs, resulting in a higher dye loading capability
(Wang et al. 2014).
More recently, in 2019, research has been carried out to construct a hierarchical
hetero-structured TiO 2 photoanode material, where anatase nano branches and rutile
nanorods were used, aiming for a higher surface area (Jin et al. 2019). Also, research
has been conducted to include TiO 2 hollow spheres as well as nanorods as a composite
powder, to be applied as the light scattering layer in the photoanode of DSC. The
increase in efficiency, up-to 9.58%, can be attributed to the two different scattering
structures incorporated (Marandi et al. 2019) (Table 15.1).
15.3 TiO 2 Composites/Hybrid Materials
The VB of TiO 2 is composed of hybridized 2p orbitals of oxygen and the 3d orbitals
of titanium, while the CB is purely 3d orbitals of titanium, which causes a decrease
in the transition of electrons back to the VB, minimizing recombination reactions.
293
shown that by reducing the disorder that is typically present at the top of the NT
layers, an increase in efficiency can be obtained as the dye uptake is increased, while
the internal light scattering effect is seen to be strongly (Zhu et al. 2006, 2007).
It was later discovered that some of these advanced morphologies can be directly
grown on the conductive substrate such as wires (Feng et al. 2008), templated rods
(Liu and Aydil 2009) or self-organized anodic structures (Macak and Schmuki 2006;
Stergiopoulos et al. 2008) that can be obtained as an oxide layer firmly attached to
the Ti-(metal) substrate.
In the aim of allowing a longer, uninterrupted path for the photogenerated electrons, Kim et al. synthesized a bamboo type morphology, by controlled anodization
of a Ti plate, for the increase in the photovoltaic efficiency. However, it was observed
that there was no change in the transport properties or the recombination kinetics of
bamboo type NTs and normal NTs. The only improvement observed was the dye
loading capacity, as the bamboo type NTs allow the exterior as well as the interior
coverage which enhanced the generated current, giving a maximum efficiency of
2.98%, which is greater than that obtained for normal NTs (Kim et al. 2008). But
later on, several more studies have been done using this bamboo type NTs, in order to
further increase the dye loading capacity. Luan et al., studied a two-step method, to
form ridges on the surface of the bamboo type NTs and to obtain evenly lengthened
NTs. It was noted that the ridge density and the length of the NT had a significant
impact on the overall efficiency. Highest efficiency of 6.80% was obtained with the
bamboo type NTs synthesized under a lower water content, which led to a longer tube
length (Luan et al. 2012). Further studies were done by Ji et al. where they synthesized double walled bamboo type NTs. The dye loading was found to be significantly
larger for the DSCs fabricated with NTs of higher ring density, leading to a better
conversion efficiency (Ji et al. 2012). Wang et al. too studied a similar morphology
which led to a reduced interfacial resistance and increased interfacial capacitance,
compared to that of smooth walled NTs, resulting in a higher dye loading capability
(Wang et al. 2014).
More recently, in 2019, research has been carried out to construct a hierarchical
hetero-structured TiO 2 photoanode material, where anatase nano branches and rutile
nanorods were used, aiming for a higher surface area (Jin et al. 2019). Also, research
has been conducted to include TiO 2 hollow spheres as well as nanorods as a composite
powder, to be applied as the light scattering layer in the photoanode of DSC. The
increase in efficiency, up-to 9.58%, can be attributed to the two different scattering
structures incorporated (Marandi et al. 2019) (Table 15.1).
15.3 TiO 2 Composites/Hybrid Materials
The VB of TiO 2 is composed of hybridized 2p orbitals of oxygen and the 3d orbitals
of titanium, while the CB is purely 3d orbitals of titanium, which causes a decrease
in the transition of electrons back to the VB, minimizing recombination reactions.