15 Yet to Be Challenged: TiO 2 as the Photo-Anode Material …
287
Fig. 15.1 Working diagram
of DSC
In DSCs, the semiconductor layer functions as the support for the sensitizer, and
as the transporter of photo-excited electrons from sensitizer to the external circuit.
The semiconductor/sensitizer interface provides the platform for charges to separate
after the photoexcitation of the dye, and consequently the injected charge carriers
diffuse to the external circuit. Therefore, an ideal semiconductor should have a large
surface area, for high dye loading; and fast charge transport, for efficient electron
collection (Ye et al. 2015). Furthermore, strong binding with the dye keeps the dye
in close proximity to the semiconductor; while suitable energy band potentials make
the electron transfer process more efficient. Therefore, these characteristics can be
considered qualifications for potential semiconductor materials (Gong et al. 2012).
The semiconductor is usually not fabricated as a monolith and is deposited as
separate dense and mesoporous layers (see Fig. 15.2). The dense layer, a fine layer
of the semiconductor material of about 100–200 nm thickness, prevents recombination reactions. The mesoporous layer is deposited on the dense layer, and is
several micrometers thick. The mesoporous layer can be further divided into the
transparent layer and the scattering layer. The transparent layer, having a thickness
of about 1–15 μm, consists of particles of size 20–30 nm. A typical transparent layer
ensures a large surface area of the nanocrystalline semiconductor, which allows
higher dye adsorption. This leads to efficient light harvesting, especially for wavelengths under 600 nm. The scattering layer consist of much larger particles of a size
about 300–400 nm and is coated on the transparent conductive oxide layer. This size
of particles scatter light of longer wavelengths (red) back to the transparent layer, and
enhances the absorption of light by the sensitizer (Ghadiri et al. 2010). The thickness
287
Fig. 15.1 Working diagram
of DSC
In DSCs, the semiconductor layer functions as the support for the sensitizer, and
as the transporter of photo-excited electrons from sensitizer to the external circuit.
The semiconductor/sensitizer interface provides the platform for charges to separate
after the photoexcitation of the dye, and consequently the injected charge carriers
diffuse to the external circuit. Therefore, an ideal semiconductor should have a large
surface area, for high dye loading; and fast charge transport, for efficient electron
collection (Ye et al. 2015). Furthermore, strong binding with the dye keeps the dye
in close proximity to the semiconductor; while suitable energy band potentials make
the electron transfer process more efficient. Therefore, these characteristics can be
considered qualifications for potential semiconductor materials (Gong et al. 2012).
The semiconductor is usually not fabricated as a monolith and is deposited as
separate dense and mesoporous layers (see Fig. 15.2). The dense layer, a fine layer
of the semiconductor material of about 100–200 nm thickness, prevents recombination reactions. The mesoporous layer is deposited on the dense layer, and is
several micrometers thick. The mesoporous layer can be further divided into the
transparent layer and the scattering layer. The transparent layer, having a thickness
of about 1–15 μm, consists of particles of size 20–30 nm. A typical transparent layer
ensures a large surface area of the nanocrystalline semiconductor, which allows
higher dye adsorption. This leads to efficient light harvesting, especially for wavelengths under 600 nm. The scattering layer consist of much larger particles of a size
about 300–400 nm and is coated on the transparent conductive oxide layer. This size
of particles scatter light of longer wavelengths (red) back to the transparent layer, and
enhances the absorption of light by the sensitizer (Ghadiri et al. 2010). The thickness