*A New Generation of Energy Harvesting Devices DOI: http://dx.doi.org/10.5772/intechopen.94291*

photoelectrode because of its structural advantages such as high surface area, submicro- or meso-porous structure for light scattering function and better infiltration of electrolyte. There are several different efforts to produce spheres [101, 102]. Nevertheless, there are limitations for the quality control, large-scale production and flexibility. In this book, a simple electro-spraying technique is introduced [103, 104]. As a first step, well-dispersed TiO2 suspensions is very important to make a continuous fluid jet. **Figure 18** show the three phase statues during E-sparying process: (i) TiO2 suspension zone, which strong electric field extracts droplets from Taylor cone (the non-sphere formation), (ii) mixed zone of TiO2 suspension and solid, which non-spheres and spheres are formed by solvent evaporates, leading to the droplet shrink, and (iii) solidified TiO2 zone, depositing the TiO2 NSs onto the conductive glass.

In this systmem, the formation of tight cluster sphere filled sphere is caused by the ultrafast evaporation of alcholic solvent under an electric field. For the desireable sphere typed TiO2 film, it is necessary to controll several parameters such as electric field, feed rate, a size of tip and a distance between the nozzle and FTO substrate. It is noted that the the sphere size is controlled by changing the TiO2 concentration and the mixture of solvent in the dispersion solution. In order to find the optimazed condition, TiO2 SPs with the different concentration are tested at 6 μm thick film. **Figure 19(a)** show the SEM results for E-sprayed film prepared from the different concentration at 1 wt%, 3 wt%, 5 wt% and 10 wt%. With increasing the concentration, the diameters of sphere are almost linearly increased, while the number of molecules calculated from UV–vis absorption spectra of desorbed sensitizers reveal that a TiO2 NSs with 5 wt% have about 11.6% and 13.9% higher value than that of 1 wt% and 10 wt% TiO2 NSs film, respectively (see **Figure 19(b)**).

Experiemntally, E-spraying at the low weight percent of TiO2 make it hard to achieve the competely formed sphere over 8 μm thick film. Therefore, in this reaserch, about 5 wt% TiO2 suspension in EtOH is used as the best condition. The detailed phase diagram accroding to the electrospraying parameters can be seen in the literature [105]. **Figure 19(c)** displays the thickness profiler on the various

#### **Figure 18.**

*The schematic diagram of the formation of hierarchically structured 0D TiO2 Nanosphere (NS).*

Thermodynamic calculations based on calorimetric data predict that rutile is the stablest phase at all temperatures, exhibiting lower total lower total free energy than metastable phases of anatase and brookite. The small differences in the Gibbs free energy (4 20 kJ/mole) between the three phases suggest that the metastable polymorphs are almost as stable as rutile at normal pressures and temperatures [97]. The physical and chemical properties of TiO2 nanocrystals are affected not only by the intrinsic electronic structure, but also by their size, shape, organization, and surface properties. For example, if the particle sizes of the three crystalline phases are equal, anatase is most thermodynamically stable phase below 10 15 nm, brookite is most stable between 11 35 nm, and rutile is most stable at sizes greater than 35 nm [98]. Herein, interesting morphologies and properties have recently attracted considerable attention and many nanostructured TiO2 materials, such as nanotubes, nanorods, nanofibers, nanosheets, and interconnected architectures, mesoporus material such as inverse opal and photonic crystal have been fabricated and applied in PV devices [35, 83, 99, 100]. In order to be effective photoelectode, several parameters such as morphologies, pore volume, and the crysltalinity of TiO2

*The schematic TiO2 structure and the DOS calculated band structure for (a) anatase, (b) rutile and (c) brookite TiO2. The big green spheres represent Ti atoms and the small red spheres represent O atoms.*

Particles which are more or less spherical in shape like fullerenes, quantum dots, nano-onions, nanoparticles, etc. are considered as 0D. These 0D nanomaterials are sized at nanoscale level in all three dimensions and must be amorphours or crystalline; single or polycrystalline; and composed of single- or multichemical element. Within 0D materials, all electrons are fully confined and their length equals the width. Due to confinement of both electrons and holes, the lowest energy optical transition from the valence to conduction band will increase in energy, effectively increasing the band gap. A 0D TiO2 nanosphere (**NS**) film is noticed as a effective

influence the charge transport and recombination processes.

**Figure 17.**

**212**

*Reprinted from [95].*

*Solar Cells - Theory, Materials and Recent Advances*

**5.2 0-dimensional (0D) titanium dioxide (TiO2) Nanosphere**

**Figure 19.**

*(a) SEM images and (b) absorbance curve of aqueous dye solutions after desorption and (c) the different thickness of E-sprayed TiO2 spheres at the different concentration a. 1 wt%, b. 3 wt%, c. 5 wt% and d. 10 wt%.*

thickness of TiO2 NSs photoelectrode film. In an E-spraying process, the film can control the feeding volume. **Figure 19(c)** shows the cross-sectional image of an E-sprayed TiO2 SPs film that demonstrates the uniform shape of the spheres from bottom to top. However, with increasing film thickness, TiO2 NSs photoelectrode tends to increase the film roughness since intrinsic insulator properties of TiO2 can influence on an electric field film during depositing process.

intrinsic surface area of each TiO2 NSs surface as well as the crack free surface morphology. In general, casting TiO2 NPs from a polymer supporter for the thick TiO2 layer onto substrates is by far the preferred method of depositing electrodes for DSSCs. Therefore, heating process at temperature ranging from 450–500°C is necessary to burn out the residual organic binder and to enhance the inter-particle connection between NPs. In this process, cracks are formed at the interface with TiO2 NPs, leading to deteriorate dye absorption. (see **Figure 21(a)**). In the case of E-sprayed TiO2 SPs from binder-free, the surface of the e-sprayed TiO2 layer is uniform and without cracks, unlike the surfaces produced using paste methods as

*SEM images of E-sprayed (a) TiO2 NSs and (b) TiCl4 treated TiO2 NSs (c) TEM images of the TiO2 NSs, which themselves are formed from crystallized 10 nm particle. (d) N2 adsorption–desorption isotherms of*

The *J–V* curves, EIS analysis and IPCE data and detailed information of dye-

light intensity are shown in **Figure 21(c)** and **(d)** and **Table 4**, respectively. Their

solar cell (0.831 V) is only about 2% higher than that of the anatase-based cell (0.815 V). The respective overall energy conversion efficiencies of the NPs and SPs -based cells are a 7.28% and 7.46%. The predominant increase of cell efficiency at the TiO2 SPs film can be obtained by TiCl4 treatment because of the enhancement of the interconnection between TiO2 SPs, which illustrated in **Figure 20(b)**. The TiCl4 treated sample represents a 21.0% increase in the energy conversion over the non-treated SPs. This increment of SPs films is about twice higher than that of NPs because the presence of intrinsic microporous SPs structure, which make its more

models, I confirm the electron diffusion coefficient rate of triiodide *D*<sup>1</sup> for TiCl4

increase of *V*oc and *FF* observed in **Figure 21(c)**. Hence the SPs based cell plays a

S<sup>1</sup>

S<sup>1</sup>

key role in attaining the higher cell efficiency with solid state electrolyte.

) at 1 sun

), and *Voc* of the SP-based

a in the pores. From the suggested impedance

, 53% higher than that obtained from TiCl4

. This increase is in good agreement with the

sensitized NPs and SPs film of the same thickness with mask (0.220 cm<sup>2</sup>

photocurrent density is almost the same (10.6 mA/cm<sup>2</sup>

shown in **Figure 21(b)**.

**Figure 20.**

*TiO2 NPs and NSs.*

*A New Generation of Energy Harvesting Devices DOI: http://dx.doi.org/10.5772/intechopen.94291*

open structure increase amount of I3

treated SP film is 4.3 <sup>10</sup><sup>7</sup> cm<sup>2</sup>

treated SP film, at 2.1 <sup>10</sup><sup>7</sup> cm<sup>2</sup>

**215**

**Figure 20(a)** and **(b)** show SEM images of a distribution of mesoporous TiO2 NSs in the anode electrode of E-sprayed TiO2 NSs film. This film exhibits a range of diameters between 100 800 nm, formed from nucleation and crystallization of 10 nm TiO2 particles. Also, the existence of such mesopores improved the electrolyte penetration. With the TiCl4 treatement, a filled sphere with a relatively rough surface is generated. (seen in **Figure 20(b)**) This is benefical for improving the interconnection between primary particles inside the TiO2 NSs. **Figure 20(c)** shows a TEM image of the spheres with the crystalline nano particles. The surface area and porosity of both TiO2 SPs and NPs film can be estimated by N2 adsorption–desorption isotherm at 77 K (see in **Figure 20(d)**). The SPs films show a type-IV isotherm as well as an increase in the adsorbed amount at high relative pressure, indicating the existence of mesopores in the sample [106]. The measured specific surface area of TiO2 SPs is 188.47 m<sup>2</sup> g<sup>1</sup> , 2.9 higher than that of similar amount of TiO2 NPs electrode (65.33 m<sup>2</sup> g<sup>1</sup> ). Furthermore, the cumulative BJH mesopore volume and maximum pore radius is 1.194 cm<sup>3</sup> g<sup>1</sup> and 10.05 nm, respectively, while hydrothermal nanoparticle film shows the pore volume of 0.607 cm3 g<sup>1</sup> and maximum pore radius of 7.50 nm. In the case of TiCl4 treated SP, the surface area and the cumulative pore volume is decreased by as much as 65.3% and 65.2%, respectively compared with the non-treated SPs film. The amounts of dye adsorbed onto each of the pristine- and TiCl4 treated TiO2 SP layers at 11 <sup>μ</sup>m were 5.80 <sup>10</sup><sup>6</sup> and 8.19 <sup>10</sup><sup>5</sup> , respectively.

It shows about 2.1 times increased dye absorption properties compared with the NP. This high density of dye molecules onto TiO2 NS can be explained by the high

*A New Generation of Energy Harvesting Devices DOI: http://dx.doi.org/10.5772/intechopen.94291*

#### **Figure 20.**

thickness of TiO2 NSs photoelectrode film. In an E-spraying process, the film can control the feeding volume. **Figure 19(c)** shows the cross-sectional image of an E-sprayed TiO2 SPs film that demonstrates the uniform shape of the spheres from bottom to top. However, with increasing film thickness, TiO2 NSs photoelectrode tends to increase the film roughness since intrinsic insulator properties of TiO2 can

*(a) SEM images and (b) absorbance curve of aqueous dye solutions after desorption and (c) the different thickness of E-sprayed TiO2 spheres at the different concentration a. 1 wt%, b. 3 wt%, c. 5 wt% and d. 10 wt%.*

**Figure 20(a)** and **(b)** show SEM images of a distribution of mesoporous TiO2 NSs in the anode electrode of E-sprayed TiO2 NSs film. This film exhibits a range of diameters between 100 800 nm, formed from nucleation and crystallization of 10 nm TiO2 particles. Also, the existence of such mesopores improved the electrolyte penetration. With the TiCl4 treatement, a filled sphere with a relatively rough surface is generated. (seen in **Figure 20(b)**) This is benefical for improving the interconnection between primary particles inside the TiO2 NSs. **Figure 20(c)** shows a TEM image of the spheres with the crystalline nano particles. The surface area and porosity of both TiO2 SPs and NPs film can be estimated by N2 adsorption–desorption isotherm at 77 K (see in **Figure 20(d)**). The SPs films show a type-IV isotherm as well as an increase in the adsorbed amount at high relative pressure, indicating the existence of mesopores in the sample [106]. The measured specific surface area

, 2.9 higher than that of similar amount of TiO2 NPs

g<sup>1</sup> and 10.05 nm, respectively, while hydro-

g<sup>1</sup> and maximum

). Furthermore, the cumulative BJH mesopore volume and

influence on an electric field film during depositing process.

*Solar Cells - Theory, Materials and Recent Advances*

g<sup>1</sup>

thermal nanoparticle film shows the pore volume of 0.607 cm3

pore radius of 7.50 nm. In the case of TiCl4 treated SP, the surface area and the cumulative pore volume is decreased by as much as 65.3% and 65.2%, respectively compared with the non-treated SPs film. The amounts of dye adsorbed onto each of the pristine- and TiCl4 treated TiO2 SP layers at 11 <sup>μ</sup>m were 5.80 <sup>10</sup><sup>6</sup> and

It shows about 2.1 times increased dye absorption properties compared with the NP. This high density of dye molecules onto TiO2 NS can be explained by the high

g<sup>1</sup>

maximum pore radius is 1.194 cm<sup>3</sup>

, respectively.

of TiO2 SPs is 188.47 m<sup>2</sup>

electrode (65.33 m<sup>2</sup>

**Figure 19.**

8.19 <sup>10</sup><sup>5</sup>

**214**

*SEM images of E-sprayed (a) TiO2 NSs and (b) TiCl4 treated TiO2 NSs (c) TEM images of the TiO2 NSs, which themselves are formed from crystallized 10 nm particle. (d) N2 adsorption–desorption isotherms of TiO2 NPs and NSs.*

intrinsic surface area of each TiO2 NSs surface as well as the crack free surface morphology. In general, casting TiO2 NPs from a polymer supporter for the thick TiO2 layer onto substrates is by far the preferred method of depositing electrodes for DSSCs. Therefore, heating process at temperature ranging from 450–500°C is necessary to burn out the residual organic binder and to enhance the inter-particle connection between NPs. In this process, cracks are formed at the interface with TiO2 NPs, leading to deteriorate dye absorption. (see **Figure 21(a)**). In the case of E-sprayed TiO2 SPs from binder-free, the surface of the e-sprayed TiO2 layer is uniform and without cracks, unlike the surfaces produced using paste methods as shown in **Figure 21(b)**.

The *J–V* curves, EIS analysis and IPCE data and detailed information of dyesensitized NPs and SPs film of the same thickness with mask (0.220 cm<sup>2</sup> ) at 1 sun light intensity are shown in **Figure 21(c)** and **(d)** and **Table 4**, respectively. Their photocurrent density is almost the same (10.6 mA/cm<sup>2</sup> ), and *Voc* of the SP-based solar cell (0.831 V) is only about 2% higher than that of the anatase-based cell (0.815 V). The respective overall energy conversion efficiencies of the NPs and SPs -based cells are a 7.28% and 7.46%. The predominant increase of cell efficiency at the TiO2 SPs film can be obtained by TiCl4 treatment because of the enhancement of the interconnection between TiO2 SPs, which illustrated in **Figure 20(b)**. The TiCl4 treated sample represents a 21.0% increase in the energy conversion over the non-treated SPs. This increment of SPs films is about twice higher than that of NPs because the presence of intrinsic microporous SPs structure, which make its more open structure increase amount of I3 a in the pores. From the suggested impedance models, I confirm the electron diffusion coefficient rate of triiodide *D*<sup>1</sup> for TiCl4 treated SP film is 4.3 <sup>10</sup><sup>7</sup> cm<sup>2</sup> S<sup>1</sup> , 53% higher than that obtained from TiCl4 treated SP film, at 2.1 <sup>10</sup><sup>7</sup> cm<sup>2</sup> S<sup>1</sup> . This increase is in good agreement with the increase of *V*oc and *FF* observed in **Figure 21(c)**. Hence the SPs based cell plays a key role in attaining the higher cell efficiency with solid state electrolyte.

**Figure 21.**

*SEM images of TiO2 film (a) doctor bladed and (b) E-sprayed TiO2 film. (c) and (d) JV and IPCE data for TiO2 NSs films.*

Interestingly, despite of the decreased charge density of SPs film, the photocurrent density is nearly the same. This reason can be explained by scattering effect on the SPs film. **Figure 20(d)** shows the IPCE curves for each sample range from 350 to 800 nm. The films with SPs layer the entire IPCE curve is slightly shifted upward in the region between 550 and 800 nm. With TiCl4 treatment, the increase is more pronounced, which is good agreement with those reported by Sommeling [56]. The effect of scattering clearly results in a much larger improvement of the red response than originates from the higher dye loading.
