290
J. B. Liyanage et al.
1. Morphology
2. Composites/hybrids
3. Doping.
In this chapter, we focus on changes in the structures and morphology, composites,
and doping, in terms of their influence on the properties of TiO 2 DSCs. These three
categories will be discussed separately. Apart from these surface modifications aim to
improve the properties roughness, hydrophobicity, biocompatibility, surface energy,
gas diffusion barrier etc. (De Jonge et al. 2008). In this chapter a separate space has
not been allocated for the surface modifications done on TiO 2 as these are discussed
under other modification techniques.
15.2 TiO 2 Morphology
The availability, ease of synthesis, and the intrinsic energy band structure are some
reasons TiO 2 remains the forerunner among semiconductor materials for n-type
DSCs. Apart from the material, the structure and morphology can have a considerable influence on the photogenerated electrons produced by a semiconductor. The
multi-layered structuring of TiO 2 in conventional DSCs has many advantages, as
described in the previous section. However, there are many facets of this architecture
which have been targeted for optimization.
Increasing total dye adsorbed on the TiO 2 has been an objective in tuning the
morphology. Raising the number of sensitizer molecules in contact with the semiconductor material enhances the light harvesting efficiency (O’Regan and Grätzel
1991). Either improving the binding of dye to the semiconductor or increasing the
surface area of the material could elevate dye adsorption. Several studies have investigated the change in efficiency of cells when the porosity and particle size of TiO 2
are varied.
O’Regan et al. studied colloidal TiO 2 , of average particle size of 15 nm, which
was then deposited on a conducting glass sheet (O’Regan and Grätzel 1991). After
the deposition a monolayer of the Ru based dye, the film turned into a deep reddish
brown colour, shifting the absorption onset to 750 nm and giving a light harvesting
efficiency of almost 100% below 550 nm, in the visible range (η = 7.9%, at 10%
sunlight; 7.12% at 100% sunlight).
After Grätzel and O’Regan’s initial work the highest efficiency obtained so far,
of 11.1% was attained by Chiba et al. They used the strategy of trapping incident
light by incorporating submicron (400 nm) sized TiO 2 nanoparticles together with
nanosized TiO 2 particles. The investigators optimized the haze, which is the portion
of diffused light in total transmittance, at 800 nm to achieve high J SC .
After decades, a similar study has been done on flexible substrates by Pichot et al.,
using a low temperature sintering method, without the use of an organic surfactant,
which resulted in a high dye loading capacity, with respect to the film thickness, that
compensated the lower efficiency of the DSC. The lower efficiency can be attributed
J. B. Liyanage et al.
1. Morphology
2. Composites/hybrids
3. Doping.
In this chapter, we focus on changes in the structures and morphology, composites,
and doping, in terms of their influence on the properties of TiO 2 DSCs. These three
categories will be discussed separately. Apart from these surface modifications aim to
improve the properties roughness, hydrophobicity, biocompatibility, surface energy,
gas diffusion barrier etc. (De Jonge et al. 2008). In this chapter a separate space has
not been allocated for the surface modifications done on TiO 2 as these are discussed
under other modification techniques.
15.2 TiO 2 Morphology
The availability, ease of synthesis, and the intrinsic energy band structure are some
reasons TiO 2 remains the forerunner among semiconductor materials for n-type
DSCs. Apart from the material, the structure and morphology can have a considerable influence on the photogenerated electrons produced by a semiconductor. The
multi-layered structuring of TiO 2 in conventional DSCs has many advantages, as
described in the previous section. However, there are many facets of this architecture
which have been targeted for optimization.
Increasing total dye adsorbed on the TiO 2 has been an objective in tuning the
morphology. Raising the number of sensitizer molecules in contact with the semiconductor material enhances the light harvesting efficiency (O’Regan and Grätzel
1991). Either improving the binding of dye to the semiconductor or increasing the
surface area of the material could elevate dye adsorption. Several studies have investigated the change in efficiency of cells when the porosity and particle size of TiO 2
are varied.
O’Regan et al. studied colloidal TiO 2 , of average particle size of 15 nm, which
was then deposited on a conducting glass sheet (O’Regan and Grätzel 1991). After
the deposition a monolayer of the Ru based dye, the film turned into a deep reddish
brown colour, shifting the absorption onset to 750 nm and giving a light harvesting
efficiency of almost 100% below 550 nm, in the visible range (η = 7.9%, at 10%
sunlight; 7.12% at 100% sunlight).
After Grätzel and O’Regan’s initial work the highest efficiency obtained so far,
of 11.1% was attained by Chiba et al. They used the strategy of trapping incident
light by incorporating submicron (400 nm) sized TiO 2 nanoparticles together with
nanosized TiO 2 particles. The investigators optimized the haze, which is the portion
of diffused light in total transmittance, at 800 nm to achieve high J SC .
After decades, a similar study has been done on flexible substrates by Pichot et al.,
using a low temperature sintering method, without the use of an organic surfactant,
which resulted in a high dye loading capacity, with respect to the film thickness, that
compensated the lower efficiency of the DSC. The lower efficiency can be attributed