**4.1 Binary transition metal oxides**

The binary transition metal oxide materials play an important role in the DSSCs and PSCs (**Figure 4a**). Titanium dioxide (TiO2) deserves special attention since its cheap, non-toxicity, abundant, biocompatible, facile preparation with diverse morphologies, stability in both acidic and alkaline media features. The TiO2 exists naturally in three crystalline polymorphs, namely, rutile (Eg = 3.05 eV), anatase (Eg = 3.23 eV), and brookite (Eg = 3.26 eV), and the uniqueness of each lattice structure leads to multifaceted physicochemical and optoelectronic properties [22]. These interesting properties reveal different functionalities, thus influencing their performances in various applications. For instance, rutile phase of TiO2 exhibits a high refractive index and UV absorptivity and is thus capable of being applied in optical communication devices (isolators, modulators, switches, etc.). Meanwhile, anatase is largely preferred in photovoltaics and photocatalysis because of its superior electron mobility, surface chemistry, potentially higher conduction band edge energy, and catalytic activity compared with the other two phases [23]. The problems associated

**71**

*Nanostructures in Dye-Sensitized and Perovskite Solar Cells*

with TiO2 metal oxide such as high surface state and fast electron recombination rate contribute to adverse effects on the electron mobility and charge transport kinetics. Zinc oxide (ZnO), an important II–VI semiconductor with a wide bandgap of 3.37 eV,

exciton binding energy of 60 meV [24]. Moreover, the electron injection efficiency of ZnO is almost equivalent to that of TiO2. The electron lifetime of ZnO is significantly higher, and the recombination rate is lower than TiO2. Nevertheless, ZnObased DSSCs also have suffered from chemical instability in acidic electrolytes and thereby showed slow electron injection kinetics from dye to ZnO photoanode due to the formation of an insulating surface agglomeration layer. Alternatively, tin oxide (SnO2) can be a good choice as it is an *n*-type and wide bandgap semiconductor with excellent optical and electrical properties compared to TiO2. The electron mobil-

electrons. Secondly, SnO2 has a higher bandgap (3.4 eV) than anatase TiO2 (3.2 eV), which creates fewer oxidative holes in the valence band (fewer oxidative holes facilitate long-term stability and higher stability under long-term UV irradiation) [25]. The Niobium oxide (Nb2O5) is another wide bandgap semiconductor with 3.49 eV bandgap energy, which is nearly 0.29 eV larger than that of TiO2 (anatase). Because of its larger bandgap and higher conduction band edge compared with anatase TiO2 it is used to achieve relatively higher *Voc* than anatase TiO2 [26]. Recently, Tungsten oxide (WO3) has attracted immense attention due to its 2.8 eV bandgap energy, which would theoretically utilize ∼12% of incident solar light of the visible region. In comparison with TiO2, WO3 possesses a higher mobility and has its conduction band edge at a more-positive location (∼0.5 V). Therefore, it is speculated that the *Voc* in WO3 nanostructured electrode is limited due to the lower difference between its conduction band and redox potential of electrolytes [27]. Bismuth oxide (Bi2O3) has several advantages due to its unique electrical, optical, and mechanical properties. It exists in four crystal phases, i.e., monoclinic α-Bi2O3, tetragonal β-Bi2O3, cubic γ-Bi2O3, and cubic δ-Bi2O3. The α-Bi2O3 phase is most stable at low temperatures up to 730°C, while δ-Bi2O3 phase is stable when the temperature is above 1000 K. The β-Bi2O3 and *γ*-Bi2O3 phases are high-temperature metastable phases. The Bi2O3 also exhibits a high refractive index, dielectric permittivity, high oxygen ion conductivity, and remarkable photoconductivity and photoluminescence [28]. Its bandgap energy of 2.5–3.1 eV mostly depends on the crystal phase type. The narrow bandgap of Bi2O3 makes it suitable for a large range of applications including optical coatings, photo-

 V<sup>−</sup><sup>1</sup> s<sup>−</sup><sup>1</sup>

) is two orders of magnitude higher than TiO2

), suggesting a faster diffusion transport of the photo-induced

) and a large

similar to TiO2, has high electron mobility (~155–1000 cm2

 V<sup>−</sup><sup>1</sup> s<sup>−</sup><sup>1</sup>

voltaics, microwave-integrated circuits, superconductor, etc.

ity in SnO2 (∼100–250 cm2

 V<sup>−</sup><sup>1</sup> s<sup>−</sup><sup>1</sup>

(∼0.1–1.0 cm2

**Figure 4.**

*DOI: http://dx.doi.org/10.5772/intechopen.83803*

*(a and b) Materials used for the DSSCs and PSCs.*

*Nanostructures in Dye-Sensitized and Perovskite Solar Cells DOI: http://dx.doi.org/10.5772/intechopen.83803*

*Nanostructures*

**3.3 Two-dimensional nanostructures**

**3.4 Three-dimensional nanostructures**

**4. Material selection**

**4.1 Binary transition metal oxides**

etc. are also forms of two-dimensional nanostructures [20].

applications. It is generally accepted that one-dimensional nanostructures are ideal for exploring a large number of novel phenomena at the nanoscale level and corroborating the size and dimensionality dependence of functional properties. In **Figure 3b**, onedimensional nanostructures are endowed with typical spherical, pseudo-spherical, dodecahedral, tetrahedral, octahedral, cubic, and corresponding hollow shapes. One-dimensional nanostructures/morphologies also include nanotubes, nano-needles, nano-rods or nano-wires, nano-shuttles, nano-capsules, hollow structures, etc. [19].

Two-dimensional nanostructures have two dimensions outside of the nanometric size range. In recent years, a synthesis of two-dimensional nanostructures with certain geometries exhibits unique shape-dependent characteristics and their subsequent utilization as building blocks for the key components of nano-devices. In **Figure 3c**, two-dimensional nanostructures, such as junctions (continuous islands), branched structures, nano-prisms, nano-plates, nano-sheets, nano-walls, nanodisks, etc., are confirmed in the literature. Round disks, hexagonal/triangular/ quadrangular plates or sheets, belts, mesoporous hollow nanospheres, hollow rings,

Owing to the large specific surface area and other superior properties over the bulk counterparts arising from the quantum size effect, three-dimensional nanostructures have attracted considerable research interest, and many threedimensional nanostructures have been synthesized in the past decade (**Figure 3d**). It is well-known that the surface area, shape, size, dimensionality, and morphologies of the nanostructures are key factors to obtain better performance of the device when they are envisaged. As these materials offer higher surface area, they can supply enough absorption sites for all involved molecules in a small space. On the other hand, such materials with higher porosity can lead to a better transportation of dye molecules. A typical three-dimensional nanostructured such as nanocoils, nanocones, nanoflowers, and nanoballs (dendritic structures) are on a great demand [21].

The binary transition metal oxide materials play an important role in the DSSCs and PSCs (**Figure 4a**). Titanium dioxide (TiO2) deserves special attention since its cheap, non-toxicity, abundant, biocompatible, facile preparation with diverse morphologies, stability in both acidic and alkaline media features. The TiO2 exists naturally in three crystalline polymorphs, namely, rutile (Eg = 3.05 eV), anatase (Eg = 3.23 eV), and brookite (Eg = 3.26 eV), and the uniqueness of each lattice structure leads to multifaceted physicochemical and optoelectronic properties [22]. These interesting properties reveal different functionalities, thus influencing their performances in various applications. For instance, rutile phase of TiO2 exhibits a high refractive index and UV absorptivity and is thus capable of being applied in optical communication devices (isolators, modulators, switches, etc.). Meanwhile, anatase is largely preferred in photovoltaics and photocatalysis because of its superior electron mobility, surface chemistry, potentially higher conduction band edge energy, and catalytic activity compared with the other two phases [23]. The problems associated

**70**

**Figure 4.** *(a and b) Materials used for the DSSCs and PSCs.*

with TiO2 metal oxide such as high surface state and fast electron recombination rate contribute to adverse effects on the electron mobility and charge transport kinetics. Zinc oxide (ZnO), an important II–VI semiconductor with a wide bandgap of 3.37 eV, similar to TiO2, has high electron mobility (~155–1000 cm2 V<sup>−</sup><sup>1</sup> s<sup>−</sup><sup>1</sup> ) and a large exciton binding energy of 60 meV [24]. Moreover, the electron injection efficiency of ZnO is almost equivalent to that of TiO2. The electron lifetime of ZnO is significantly higher, and the recombination rate is lower than TiO2. Nevertheless, ZnObased DSSCs also have suffered from chemical instability in acidic electrolytes and thereby showed slow electron injection kinetics from dye to ZnO photoanode due to the formation of an insulating surface agglomeration layer. Alternatively, tin oxide (SnO2) can be a good choice as it is an *n*-type and wide bandgap semiconductor with excellent optical and electrical properties compared to TiO2. The electron mobility in SnO2 (∼100–250 cm2 V<sup>−</sup><sup>1</sup> s<sup>−</sup><sup>1</sup> ) is two orders of magnitude higher than TiO2 (∼0.1–1.0 cm2 V<sup>−</sup><sup>1</sup> s<sup>−</sup><sup>1</sup> ), suggesting a faster diffusion transport of the photo-induced electrons. Secondly, SnO2 has a higher bandgap (3.4 eV) than anatase TiO2 (3.2 eV), which creates fewer oxidative holes in the valence band (fewer oxidative holes facilitate long-term stability and higher stability under long-term UV irradiation) [25]. The Niobium oxide (Nb2O5) is another wide bandgap semiconductor with 3.49 eV bandgap energy, which is nearly 0.29 eV larger than that of TiO2 (anatase). Because of its larger bandgap and higher conduction band edge compared with anatase TiO2 it is used to achieve relatively higher *Voc* than anatase TiO2 [26]. Recently, Tungsten oxide (WO3) has attracted immense attention due to its 2.8 eV bandgap energy, which would theoretically utilize ∼12% of incident solar light of the visible region. In comparison with TiO2, WO3 possesses a higher mobility and has its conduction band edge at a more-positive location (∼0.5 V). Therefore, it is speculated that the *Voc* in WO3 nanostructured electrode is limited due to the lower difference between its conduction band and redox potential of electrolytes [27]. Bismuth oxide (Bi2O3) has several advantages due to its unique electrical, optical, and mechanical properties. It exists in four crystal phases, i.e., monoclinic α-Bi2O3, tetragonal β-Bi2O3, cubic γ-Bi2O3, and cubic δ-Bi2O3. The α-Bi2O3 phase is most stable at low temperatures up to 730°C, while δ-Bi2O3 phase is stable when the temperature is above 1000 K. The β-Bi2O3 and *γ*-Bi2O3 phases are high-temperature metastable phases. The Bi2O3 also exhibits a high refractive index, dielectric permittivity, high oxygen ion conductivity, and remarkable photoconductivity and photoluminescence [28]. Its bandgap energy of 2.5–3.1 eV mostly depends on the crystal phase type. The narrow bandgap of Bi2O3 makes it suitable for a large range of applications including optical coatings, photovoltaics, microwave-integrated circuits, superconductor, etc.

## **4.2 Ternary transition metal oxides**

Besides the simple binary metal oxide systems, ternary metal oxide systems such as Strontium titanate (SrTiO3), Zinc Stannate (Zn2SnO4), and Barium Stannate (BaSnO3) have also been considered as photoanode materials in the DSSCs and PSCs (**Figure 4b**). The SrTiO3 is a semiconductor with bandgap similar of 3.2 eV. However, its conduction band is relatively at higher position than that of TiO2, which results in a higher *Voc* [29]. A high dielectric constant makes SrTiO3 as electrically mesoporous even with a large particle size of ~80 nm [30]. In addition, Zn2SnO4 is particularly interesting because of its physical and electrical properties. The 3.6 eV bandgap and 10–15 cm<sup>2</sup> V<sup>−</sup><sup>1</sup> s<sup>−</sup><sup>1</sup> electron mobility of Zn2SnO4 have made it stable against UV light with high, electrical conductivity, and low visible absorption over TiO2 [31]. The ternary BaSnO3 is an *n*-type semiconductor with a wide bandgap of 3.1 eV, and its band structure and electrical properties can be controlled easily by atomic substitution or doping into the Ba or Sn site for better performance when used in DSSCs' application [32]. In this sense, as the electrode materials in DSSCs, the ternary oxides are better than the binary.
