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

must go forward to the front surface of the cell as well as diffracted beam are internal reflected back into the solar cell when the angle of critical angle overcome

311 nm inverse opal sphere that shows the main reflection peak at 526 nm

**4.2 Measurements and modeling of DSSCs with 3D PhC**

**Figure 14(c)** shows the reflection spectra of Ag film and 3D PC with 198, 311, 375, and 410 nm diameters. The reflectivity of the Ag film is more than 80% in wavelengths ranging from 400 to 800 nm; clearly, this Ag film has better reflectivity than the 3D PC. The red line represents the spectrum from the 198 nm inverse opal sphere; a reflection peak can be observed at 410 nm corresponding to the lowest photonic band gap (**PBG**). The orange line represents the spectrum from the

corresponding to the lowest PBG and additional reflection spectra peaks at 382 and 365 nm. The blue line shows the reflection spectrum of the 375 nm inverse opal sphere: it comprises a reflection peak at 661 nm corresponding to the fundamental PBG and additional reflection spectra peaks at 415 and 390 nm. The green line shows the reflection spectrum of the 410 nm inverse opal sphere that has a main reflection peak at 715 nm corresponding to the lowest PBG and additional reflection spectra peaks at 442 and 411 nm. The 3D PC show reflectivity peaks amplitudes of around 73% at the lowest PBG and around 25% at the high order PBG. This implies that we can recycle the photons back into the DSSC for further absorption and processing. The peak positions can be related to the sphere diameter and the effective refractive index of the medium using Bragg's law, *λ max = 2neff d111*, where *d*<sup>111</sup> is the 111 lattice spacing and *neff* is the effective refractive index of the medium. Furthermore, the differences in the frequency of the reflection peaks are a result of the different sizes of the inverse opal spheres because the same effective refractive indices (ZnO) of the medium were used in all the experiments. Thus, the 3D PC can be devised to present a Bragg peak that matches the absorption band of the ruthenium dye but has a significant effect on longer wavelengths, thereby increasing the

The effects of selective light trapping, which was caused by the reflection and diffraction by 3D PhCs, on the solar-to-electric conversion efficiencies and AC impedance measurements of the cell, are analyzed by measuring the photocurrentdensity-voltage (*J-V*) curves under simulated sunlight radiations of 1000 Wm<sup>2</sup> intensity. From a series of *J V* curves, samples coupling with Ag and PhC reflector display the increased photocurrent (seen in **Figure 15(a)**) The corresponding impedance measurements are plotted in **Figure 15(b)**, and the solid curves

*(a) J* V *characteristics and (b) Nyquist plots of DSSC attached Ag metal and different sized PhC reflection*

the diffracted beam [86].

*Solar Cells - Theory, Materials and Recent Advances*

absorption efficiency.

**Figure 15.**

**208**

*(film thickness 11 μm).*

represent the best fits from our modeling calculations. The fitting parameters are tabulated in **Table 3**. (the photoanode comprised 11 μm thick TiO2 film and approximately 0.332 cm2 active area with mask). The DSSC with the Ag film and 3D PCs showed higher short-circuit current densities and solar-to-electric conversion efficiencies than the traditional DSSC. For the Ag film, the short-circuit current densities were increased by approximately 0.84 mA/cm<sup>2</sup> ; the open-circuit voltage and fill factor were almost identical; and the solar-to-electric conversion efficiency was enhanced by 5% .

In the case of the DSSCs with 3D PhCs, the solar-to-electric conversion efficiencies were enhanced by 1.94, 7.78, 10.7, 11.3 and 14.4% for the 198 nm, 311 nm, 375 nm, 410 nm, and double layer (375/410 nm), respectively, as compared to the traditional typed DSSC. The 3D PC with 198 nm sized PhC showed the lowest efficiency enhancements because the traditional DSSC absorbed most of the light, this corresponds to the reflection peak of this PhC, as shown in **Figures 13(c)** and **14(c)**. The 375 nm and 410 nm sized PhC showed high enhancements in the solarto-electric conversion efficiencies because the traditional DSSC transmitted more than 50% of the light; this corresponds to the reflection peaks of these PhC. This enhancement in the solar-to-electric conversion efficiencies is higher than that in the Ag film even though the Ag film has a considerably higher reflection intensity; this can be explained by the diffraction effect of the PhC as shown in **Figure 14(b)**. The PhC with 375/410 nm double layers showed highest enhancements in the solarto-electric conversion efficiencies because the double layer PhC has a significant overlap with the quantum efficiency spectra of the ruthenium dye; moreover, the diffraction effect of the PhC is also present. From **Table 3**, the multilayer whose PBG has a large overlap with the quantum efficiency spectra of the ruthenium dye leads to larger enhancements in the photocurrent. Furthermore, the PhC -based DSSC exhibits considerably higher short circuit photocurrents than the traditional DSSC and is much more effective than the conventional Ag film.

to obtain different morphologies in photoanode materials. In this study, a variety of nanostructures from zero dimensional (0D) to three dimensional (3D) has been tested. Electro-spinning and spraying technique has been adopted to prepare the the different dimensional metal oxide semiconductor film by controlling the polymer host and solvents. The detailed results can be seen in followed section (**Figure 16**).

Various typed metal oxide semiconductors such as TiO2, ZrO2, SnO2, ZnO, Nb2O5, Fe2O3, Al2O3, (binary compounds) and and ternary compounds such as SrTiO3 and Zn2SnO4 can all act as photoelectrodes in DSSC due to their electronic structures which are characterized by a filled valence band and an empty conduction band [7, 92]. Among these heterogenous semiconductors, TiO2 is the most widely used photoelectrode material because of the large amount of grain boundary and the large interface between the TiO2 surface, dye and the electrolyte in the solar cell, further defects in the crystal structure are expected. In TiO2, the Ti ions are in a

uration. The different obital hybridized structure of valence band (VB) and conduction bands (CB) at Ti 3d states occur the decreased the transition probability of electrons to the VB when the electron–hole recombination probability is reduced [92]. Therefore, in view of the electronic configuration and recombination probability TiO2 are the best choice as photoelectrodes among 3d transition metal oxides. TiO2 exsist naturally four commonly known crystalline polymorphs, i.e., anatase (tetragonal, Eg = 3.23 eV), rutile (tetragonal, Eg = 3.05 eV), brookite (orthorhombic, Eg = 3.26 eV) and metastable forms (monoclinic, orthrhombic, cotunnute) [93]. These structures can be described in terms of chains of TiO6 octahedra. The crystal structures differ in the distortion of each octahedron and by the assembly pattern of

the octahedra chains [94]. The crystalline strucuture of TiO2 can be seen in **Figure 17** [95]. Both rutile and anatase have tetragonal structure with a = 0.46 nm and c = 0.29 nm (rutile); a = 0.3782 nm and c = 0.9502 nm (anatase). Brookite has orthorhombic structure with a = 0.5456 nm, b = 0.9182 nm, and c = 0.5143 nm and it is very hard to synthesize in the laboratory, while the rutile and anatase can be easily prepared. For solar cell application, anatase structure is more prferred because of 0.1 eV higher the Fermi level, lower recombination rate of

electron–hole pairs and lower formation temperature [96].

) electronic config-

**5.1 Titanium dioxide (TiO2) nanomaterials as a photoelectrode**

*SEM images of different types of 0D 1D 2D 3D- TiO2. Produced in our lab.*

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

**Figure 16.**

**211**

distorted octahedral environment and formally have a Ti4+(3d0

The generation of photocurrent is primarily influenced by the light absorption of the dye. Therefore, when coupling PhC with the back surface of the DSSC, light absorption is increased attributed to the reflection and diffraction of light. This directly reflect the enhancement of the short-circuit current densities and the overall efficiency, while the open-circuit voltage, fill factor is unaffected. The DSSC with the PhC of 375, 410 nm and double layer diameters showed higher conversion efficiency than that with the Ag film; this can be explained not only by the reflection but also the diffraction effect in the DSSCs. As a result, we demonstrated here that recycling of photons by PhC is an effective way to increase the cell efficiency.
