*3.2.2 Photosensitizer (purifed N719 dye)*

For photosensitization studies, the calcined TiO2 nanoparticle electrode were immersed in the ethanol solution containing purified 3x10�<sup>4</sup> M cis-di(thiocynato)- N,N<sup>0</sup> -bis(2,2<sup>0</sup> -bipyridyl-4-caboxylic acid-4<sup>0</sup> -tetrabutylammonium carboxy late) ruthenium (II) (N719, Solaronix) for 18 h at room temperature [46]. Commercial N719 dye may not produce high efficiency because of impurities. Therefore, purification is required. The N719 complex is firstly dissolved in water with

#### **Figure 4.**

*Scheme showing a typical procedure for highly crystalline TiO2 nanoparticle based DSSC. Reprinted from [26].*

tertrabutylammonium hydroxide and then purified three times with chromatography. (Sephadex LH-20) The pH values below 4.5 is controlled by 0.02 M HNO3. The titration is carried out slowly over a period of three hours. Then, the solution is kept at -20°C for 15 hours. After allowing the flask to warm to 25°C, the precipitated complex is collected on a glass frit and air dried.

## *3.2.3 Liquid electrolyte*

he liquid electrolyte was prepared by dissolving 0.6 M of 1- butyl-3-methylimidazolium iodide (**BMII**), 0.03 M of iodine, 0.1 M of guanidinium thiocyanate (**GuSCN**) and 0.5 M of 4-tert-butylpyridine (**tBP**) in acetonitrile and valeronitrile (85:15 v/v). Most additives are understood at a fairly phenomenological level, and their effects are often attributed to modification of redox couple potential, band shifts of the semiconducting electrode material, effects of surface blocking, or surface dye organization. The study for additives have been reviewed extensively elsewhere [47, 48]. For example, it was found that smaller size Li<sup>+</sup> ions cause a shift in the TiO2 conduction band edges toward more positive potentials than larger size 1,2-dimethyl-3-hexylimidazolium ions. As a result, an electrolyte containing Li<sup>+</sup> ions produced a lower photovoltage and at the same time higher photocurrent than that containing imidazolium cations because of an alteration of both the energy and number of excited state levels of the dye that participate in electron injection [46]. The effects of tBP were studied in more detail in DSSCs showing that both band edge shift and increased electron lifetime play a role [49].

the compositions of the samples at 200 kV. The NPs were also characterized by x-ray diffraction (D/Max-A, Rigaku) measurements. **Figure 5(a)** shows a X-ray scan of the Anatase TiO2 NPs before and after sintering as described below. The inset shows a typical SEM photo of the NPs used in the experiment. These results reveal broad diffraction peaks at 25.3°, 37.8°, 48.2°, 53.9° and 55.2° were observed, which can be indexed to the (101), (004), (200), (105) and (211) reflections of anatase TiO2. **Figure 5(b)** provides the high resolution transmission microscopic (TEM) images and selected area electron diffraction pattern of the TiO2 NPs. It provides the corresponding electron diffraction pattern taken from the TiO2 NPs showing that the NPs are crystalline Anatase TiO2. **Figure 5(b)** gives the high-resolution TEM (HRTEM) image of NPs indicating that the TiO2 NPs are single-crystalline. Lattice of 0.189 nm and 0.243 nm corresponding to (200) and (103) planes of the tetragonal TiO2, respectively, have been resolved. The inset is the corresponding fast Fourier transform (IFFT) image of selected area in **Figure 5(b)c**. In **Figure 5(b)d** the EDS taken from TiO2 NPs shows that the NPs are composed of Ti and O. The Cu peak comes from the grid. From these measurements, we concluded that our starting materials were pure Anatase.

*(a) X-ray diffraction patterns of hydrothermal TiO2 nanoparticle after calcination at 500oC. Insert shows the SEM images of nanopaticle after calcination. (b) a.TEM, b. SEAD pattern, c. HR-TEM bright field images and d. EDX analysis of TiO2 nanocrystals prepared by 2-step autoclaving technique scheme showing a typical*

To find the optimized condition for hydrothermal treated TiO2 film, the cell efficiency as a function of the TiO2 film thicknesses is studied. In **Figure 6** (top), I give plots of *V*oc, *J*sc, *FF*, and *η* as functions of film thicknesses. The efficiency per NP increases almost linear with decreasing film thickness until about 2 μm. From these measurements, the optimal TiO2 film thickness is around 11.5 μm with our cell architect and fabrication procedures. It should also be noted that the highest open circuit voltage and fill factor are achieved at the thinnest active layer thickness, while the short circuit current density increases with increasing film thickness (for thickness below 15 μm). From this thickness study, we then varied other cell parameters to optimize the overall cell efficiency. In **Figure 6** (bottom) we plot cell efficiency times the number TiO2 NP as a function of NP film thickness. it is shown that the efficiency per TiO2 NP increases dramatically as the active NP layer decreases. This shows the large inherent loss of charges in these DSSCs.

In **Figure 7**, AC impedance measurements with best-fit model curves of the cell is used for analyzing function on the different TiO2 NP film thicknesses. Form the Bode phase plots, the negative shift of the frequencies of the main peaks with an

*3.3.2 Study for the optimal TiO2 film thickness*

*procedure for highly crystalline TiO2 nanoparticle based DSSC.*

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

**Figure 5.**

**197**

Even if tBP and Li<sup>+</sup> ions play a different role for solar cell performance, both additives are widely used in liquid electrolyte. As another effective additive, adding GuSCN can be also good choice. It shifts the conduction band edge of TiO2 toward lower energies, suggesting adsorption of the cation onto the TiO2 surface. Coadsorbants, adsorbed onto the TiO2 during the dye adsorption procedure, can have similar effects as additives in the electrolytes [50].

#### *3.2.4 Catalytic layer*

The cathode-electrode was produced by coating F:SnO2 glass with a thin layer of a 5 mM solution of H2PtCl6 in isopropanol and was heated at 400°C for 20 min. Best performance and long-tern stability has been achieved with very low Pt-loadings (5 μg cm<sup>2</sup> ), so that the counter electrode remains transparent. Charge transfer resistances of less than 1 Ω cm<sup>2</sup> can be achieved.

#### **3.3 DSSC fabrication for conventional typed cell**

A paste of anatase hydrothermal TiO2 powder was made by stirring with the mixture 0.5 g of anatase-TiO2 NPs, 100 μl of Triton X-100, 0.2 g of polyethylene glycol (PEG, Fluka, Mw = 20,000) into 3 ml acetic acid (0.1 M). The TiO2 paste is coated on a FTO glass by doctor blade technique. The thickness of the TiO2 film was measured by a surface profiler (TENCOR. P-10) [26, 51]. **Figure 4(d)** and **(e)** gives an illustrative sequence of steps that were used in the fabrication of our solar cells. The top and bottom electrodes were sandwiched together with thermal melt polymer film (**Figure 4e**).
