**5.5 3-dimensional (3D) titanium dioxide (TiO2)**

In the section of "*Photon Management using Three-Dimensional Photonic Crystals*", the effective use of three-dimensional photonic crystals (3D PhC) for photon management in our cells was demonstrated. Here, 3D PhC is used as a photoelectrode for DSSCs. An important approach to enhance DSSC efficiency is to increase the path length of light by enhancing light scattering in TiO2 films by

#### **Figure 22.**

*2D TiO2 film (a) TiO2 nanofiber (NF) film prepared by E-spraying technique (b) TiO2 nanotube (NT) film by hydrothermal treatment. (c) JV and BET analysis of each cells.*

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

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

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

In the section of "*Photon Management using Three-Dimensional Photonic Crystals*", the effective use of three-dimensional photonic crystals (3D PhC) for photon management in our cells was demonstrated. Here, 3D PhC is used as a photoelectrode for DSSCs. An important approach to enhance DSSC efficiency is to increase the path length of light by enhancing light scattering in TiO2 films by

*2D TiO2 film (a) TiO2 nanofiber (NF) film prepared by E-spraying technique (b) TiO2 nanotube (NT) film*

"*DSSC Fabrication for Conventional typed Cell*."

*Solar Cells - Theory, Materials and Recent Advances*

**5.5 3-dimensional (3D) titanium dioxide (TiO2)**

*by hydrothermal treatment. (c) JV and BET analysis of each cells.*

cell efficiency.

**Figure 22.**

**218**

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

**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> ) and 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 spectrum.

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 of 3.7 <sup>10</sup><sup>6</sup> and 2.2 <sup>10</sup><sup>6</sup> cm<sup>2</sup> /Vs, respectively. The observed fast ion transport 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

#### **Figure 23.**

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

temperature, a waxy solid was obtained. **Figure 23(c)** shows the JV curve for solid state of 311 nm 3D PhC bilayer. Interestingly, no obvious efficiency change could be seen in all typed solid state electrolyte. The PCE of the solid state system is found to be 2% of that of the liquid electrolyte system. In conclusion, the design of 3D PhC bilayer film enables effective dye sensitization, electrolyte infiltration and charge collection because the layers are in direct physical and electronic contact, light harvesting in specific spectral regions was significantly increased by the 3D PhC effect and PC-induced resonances. This approach should be useful in solid-state devices where pore infiltration is a limiting factor as well as in weakly absorbing photovoltaic devices.

and the Ru center has three thiocyanato ligands and one terpyridine ligand substituted with three carboxyl groups, (also called the "**black dye**") are widely used as reference and high efficiency sensitizers for DSSCs. The amphiphilic heteroleptic ruthenium sensitizer, known as **Z907**, reported noticeable thermal stability with stable 7% energy conversion efficiency [114]. For optimized DSSC with N3 or N719, the certified IPCE values are 80% for wavelengths 650 nm. However, the IPCE increases only gradually from the absorption onset to shorter wavelengths due to relatively low extinction coefficients (1.40 <sup>10</sup><sup>4</sup> <sup>M</sup><sup>1</sup> cm<sup>1</sup>

[7, 47]. In addition, this class of compounds contains expensive ruthenium metal and requires careful synthesis and tricky purification steps. Therefore, efforts in the synthesis of sensitizers for DSSCs step forward to the metal-free organic donor–

The idea for mimicking the light harvesting processes based on chlorophyll occurring photosynthetic reaction centres inspire the research for porphyrins based sensitizer. The porphyrin-based dyes exhibit large absorption coefficients in the visible and infrared region as well as their rigid molecular structures consisted of four meso and eight β reaction sites, which can control their properties [122–124]. The designed porphyrin dyes with a π-conjugated link at the β-position of the porphyrin ring enhance the electronic coupling of the dye with the surface of TiO2, reaching η =7.1% of a PCE [125]. Numerous series of porphyrins are reported for the DSSC application as an effective sensitizer. of DSSC synthetized. Among them, a class of sensitizer consisting of a push–pull porphyrins with an electron-donating diarylamino group and an electron-withdrawing carboxyphenylethynyl anchoring group shows the outstanding solar properties. For example, advances in optimization of the device performance for a zinc porphyrin sensitizer (YD2-oC8) cosensitized with an organic dye (Y123) using a cobalt-based electrolyte to enhance photovoltage of the device attained an unprecedented power conversion efficiency of *η* = 12.3% [126]. (see **Figure 25(a)**) Further improvement can be found at

*(a) Typical structure of a porphyrin showing the four meso- and the eight β-positions to be functionalized for porphyrin-sensitized solar cells. Reprinted from [122, 123] (b) solar performance for the different series of*

acceptor (D–A) dyes system.

**Figure 25.**

**221**

*porphyrin based DSSC at liquid electrolyte and TiO2 NSs.*

**6.2 Porphyrins based sensitizer**

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

)
