**5.4 2-dimensional (2D) titanium dioxide (TiO2) nanosheets**

Two-dimensional (2D) anisotropic nanostructures of metal oxides and semiconductor possess pronounced quantum surface effects and dramatic changes in electronic structures and thus in the physical and chemical properties, which are



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

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

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

One-dimensional (1D) nanostructures (e.g., nanowires, nanorods, nanotubes, nanobelts, cauliflower-shaped structures) have been widely considered to have superior electron transport characteristics compared to conventional nanorod (NP) -based systems. Eariler reported research have demonstrated that highly efficient TiO2 nanorod-based NR DSSCs and compare charge transport properties to those of typical TiO2 nanoparticle-based (NP, commercially available P-25) NP-DSSCs [35]. The electrospun TiO2 NR based photoelectrodes exhibited about 2 times larger the pore volume and 2.5 times more dye coverages at the same weight than those of TiO2 NP. Therefore, NR based cell indicated about 40% higher efficiencies than NP-DSSCs attribied to 8 times slower electron–hole recombination in NR-DSSCs. With the TiCl4 post-treatment, further enhancement of electron diffusion coefficient, a charge collection efficiency and the efficiency can be demonstrated [35].

Two-dimensional (2D) anisotropic nanostructures of metal oxides and semicon-

ductor possess pronounced quantum surface effects and dramatic changes in electronic structures and thus in the physical and chemical properties, which are

than originates from the higher dye loading.

*Solar Cells - Theory, Materials and Recent Advances*

**Figure 21.**

**216**

*TiO2 NSs films.*

**5.3 1-dimensional (1D) titanium dioxide (TiO2) nanorod**

**5.4 2-dimensional (2D) titanium dioxide (TiO2) nanosheets**

anticipated to inspire new fundamental and technological research [107]. In my earlier research, TiO2 or ZnO nanofiber mat is used for employing DSSC. [108, 109] For empolying 2D TiO2 film for DSSCs, two different technique are used for TiO2 photoelectrode: (1) Nanofiber (NF) mats produced by electrospinning: (2) Nanotubes (NT) film from 2-step hydrothermal method. As see in **Figure 22(a)**, TiO2/poly(vinly acetate, PVAc) composite fiber mats are directly electrospun onto a FTO glass and then polymer binders are removed by two step process: (i) a THF solvent tratement for melting PVAc polmyer, (ii) calcination this mat at 500°C for 30 min. With the post treatment, the adhesion issues between TiO2 NFs or NFs and FTO glass can be overcome. The TiO2 NTs film is prepared from adjusting pH values [110]. Following this method, the well-growed NT poweders are deposited by mixing a mixture of PEO/PEG polymer binder as described in a section of "*DSSC Fabrication for Conventional typed Cell*."

coating the large particles onto the small sized of TiO2 NPs [111]. To understand the effect on the PhC coupled DSSCs that feature enhanced photocurrent over a large spectral region, a double layered photoanode integrates a high surface mesoporous underlayer TiO2 with an optically active TiO2 PhC overlayer is fabricated as illustrated in **Figure 23(a)**. For the FTO/3D TiO2 PhCs structure, the polystyrene (PS) opal template of sphere size 370 nm was prepared using a vertical deposition technique onto the nc-TiO2 film and then TiO2 is coated by ALD. The inverse opal

**Figure 23(b)** shows the J-V curves for the different sized 3D PhC at liquid electrolyte. (as seen above) When the size of hole is increased from 198 nm to 410 nm, the *Voc* values increased from 0.787 V to 0.849 V. The 311 nm 3D PhC bilayer exhibits the best PCE, which is a 46.1% increase in *J*sc (8.657 mA/cm<sup>2</sup>

36.2% increase in PCE (5.102%) compared to the NPs layers (7.064 mA/cm<sup>2</sup> of *J*sc, 3.258% of PCE) The significant improved *PCE* of 3D PhC indicates that the 3D PhC top layer is electrically connected and contributes to light harvesting over the entire

Moreover, large porosity structures of the inverted 3D TiO2 PhCs have enabled good infiltration for high-viscosity electrolytes. For solid state cell fabrication, we have chosen (P1,4I)-doped succinonitrile plastic crystal electrolyte [112, 113]. This electrolyte has the best performance for solid state DSSC as reported in the literature [114]. At a room temperature, this compound electrolyte has a high conductivity of 3.3 mS/cm and fast ion transport of iodine and triiodine in its plastic phase

in this solid material can be seen as a decoupling of diffusion and shear relaxation times, which probably originates from local defect rotations in the succinonitrile plastic crystal [115–117]. Thus, this plastic electrolyte showed best high cell efficiency among other competing electrolyte materials. **Figure 23(c)** top shows the molecular structure of the compound. For better cell performance, we firstly injected the liquid state electrolyte into TiO2 NP film and evaporated this electrolyte by putting it in the dry oven at 80°C for 12 hour. Next, the plastic electrolyte was heated until it became a liquid (>70°C) and injected into the warmed sandwiched cells. After the cells included the plastic electrolyte was cooled down to room

*3D TiO2 film (a) schematic design (b) JV characteristics of 3D PhC TiO2 at the different size (c) solid state*

/Vs, respectively. The observed fast ion transport

) and

structure can be made by ion etching (CF4 5 min).

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

spectrum.

**Figure 23.**

*results.*

**219**

of 3.7 <sup>10</sup><sup>6</sup> and 2.2 <sup>10</sup><sup>6</sup> cm<sup>2</sup>

The detailed surface and cross sectional images can be confirmed by the SEM images (1.22(b)). as can bmodified 2-step hydrothermal process. **Figure 22(c)** show the nitrogen adsorption–desorption isotherm and Barret–Joyner–Halenda (BJH) pore size distribution plot of the NF mats and NT film, respectively. The specific surface area and pore volume of both samples is very small and it is difficult to obtain a sufficient photocurrent density. In agreement with the BET analysis, the JV curve of all cells display an obviously low photocurrent density, leading to the poor cell efficiency.
