**4. Photon management using three-dimensional photonic crystals**

Besides the interfacial modification, cell performance can be improved by external source (such as anti-reflection film and photonic crystal etc.) as reflecting passed photons back into the absorbing film [26, 70]. For the photon trapping, two approaches for geometrical or wave optics are employed [71]. In this book, photon management concept using 3-dimensional photonic crystals (**PhC**) is introduced. Photonic crystals are highly ordered materials with a periodically modulated dielectric constant. The presence of photonic band gap can be confining and controlling the propagation of light propagation of light, a band of frequencies in which light propagation in the photonic crystal is forbidden [72]. Therefore, we can manage to reflect, transmit, and diffract light for specific wavelengths by appropriate designing the crystal structure. In fact, a diffuse scattering layer of large TiO2 colloids [73] is typically introduced for this specific purpose [74]. There are successful demonstrations for enhancement in performance of silicon photovoltaic cells based on the realization of coherent scattering processes [75–81]. However, these concepts cannot be easily realized in DSSC. Herein, this book takes the most efficient cells and measure how much of the income light is actually being absorbed by the N719 dye as photons transit across the solar cell.

#### **4.1 Photon management effect on DSSCs**

For 3D PhC layer, the vertical deposition technique for polystyrene (PS) opal templates is employed [55] (see in **Figure 13(a)**). The experimental procedures are reported in earlier work [82, 83]. **Figure 13(b)** is a SEM micrograph of a ZnO PhC (or inverse opal) layer showing both the side view (cross sectional, top) and top view (bottom).

The optical property can be estimated by using UV- vis- NIR spectrophotometer in a wavelength range of 185 3300 nm. The quantum efficiency and transmission of the N719 cell with a thickness of about 11.5 μm as a function of the wavelength is shown in **Figure 13(c)** [76]. N719 dye displays a broad absorption spectral coverage in the region 400 700 nm, but a large proportion of infrared (IR) spectral range, which constitutes almost half the energy of the sun's radiation, cannot be utilized.

To make up for insufficient photon absorption, a reflector by either using a Ag mirror or a stack of the PhC was simply attached to the bottom cell or cathode electrode (**Figure 14(a)** and **(b)**). As seen in this illustration, there are two physical mechanisms by which wave optics approaches (based on reflector and photonic crystals) can improve light-trapping: reflection and diffraction. Firstly, Ag film can provide high reflection property like a mirror. Distributed Bragg reflectors exhibit high index contrast and they can reflect light over a extensive wavelengths region and incident light angles [84]. Likewise, a PhC can reflect incident light from broad

range of angles for frequencies and polarizations within the photonic crystal in that

*Schematic of a DSSC (a) with a typical geometric optic concept of reflection to trap light. And (b) with wave optics (3D PhC) to trap light with reflection and diffraction (c) reflection spectra of Ag film and 3D PC with*

*(a) Fabrication procedure of the 3D PhC, (b) SEM micrograph of a typical ZnO PhC (or inverse opal) layer, showing both the side (cross sectional) view (top) and the top view (bottom) (c) quantum efficiency measurement of the N719 dye and the transmittance measurement of DSSCs using the same dye.*

Secondly, wave optics-based devices was developed for diffracting incoming beams into indirect angles according to Bragg's law [85]. The diffraction originated from an interface improve light trapping with the distance increment that light

it can reflect light within the bandgap incident from any angle or medium.

*different sphere diameters (198, 311, 375, and 410 nm).*

**Figure 14.**

**207**

**Figure 13.**

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

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

#### **Figure 13.**

treated with TiCl4 coating with same treatment as cell (c), followed by fluorine etching with same treatment as cell (c), and cell (f) with the TiO2 NP film etched by fluorine first and followed by TiCl4 treatment and then high-temperature. As we expected, cell (f) has the lowest series resistance (i.e., *R*total = *R*<sup>0</sup> + *R*<sup>1</sup> + *R*<sup>2</sup> + *R*3) and therefore the highest efficiency. The contribution to this increase of cell efficiency comes primarily from the 27.7% increase in *J*sc, no loss in the *Voc* and *FF* value. It also notice from our model that the effective electron diffusion coefficient is a significant improved by as much as 59%, the interfacial recombination (*keff*) is decreased by 52.7%, which leads to large *Rk* compared with the cell (b). These changes are quite significant, and as a result, the cell efficiency was increased nearly 26%.

*Solar Cells - Theory, Materials and Recent Advances*

**4. Photon management using three-dimensional photonic crystals**

by the N719 dye as photons transit across the solar cell.

**4.1 Photon management effect on DSSCs**

view (bottom).

**206**

Besides the interfacial modification, cell performance can be improved by external source (such as anti-reflection film and photonic crystal etc.) as reflecting passed photons back into the absorbing film [26, 70]. For the photon trapping, two approaches for geometrical or wave optics are employed [71]. In this book, photon management concept using 3-dimensional photonic crystals (**PhC**) is introduced. Photonic crystals are highly ordered materials with a periodically modulated dielectric constant. The presence of photonic band gap can be confining and controlling the propagation of light propagation of light, a band of frequencies in which light propagation in the photonic crystal is forbidden [72]. Therefore, we can manage to reflect, transmit, and diffract light for specific wavelengths by appropriate designing the crystal structure. In fact, a diffuse scattering layer of large TiO2 colloids [73] is typically introduced for this specific purpose [74]. There are successful demonstrations for enhancement in performance of silicon photovoltaic cells based on the realization of coherent scattering processes [75–81]. However, these concepts cannot be easily realized in DSSC. Herein, this book takes the most efficient cells and measure how much of the income light is actually being absorbed

For 3D PhC layer, the vertical deposition technique for polystyrene (PS) opal templates is employed [55] (see in **Figure 13(a)**). The experimental procedures are reported in earlier work [82, 83]. **Figure 13(b)** is a SEM micrograph of a ZnO PhC (or inverse opal) layer showing both the side view (cross sectional, top) and top

The optical property can be estimated by using UV- vis- NIR spectrophotometer in a wavelength range of 185 3300 nm. The quantum efficiency and transmission of the N719 cell with a thickness of about 11.5 μm as a function of the wavelength is shown in **Figure 13(c)** [76]. N719 dye displays a broad absorption spectral coverage in the region 400 700 nm, but a large proportion of infrared (IR) spectral range, which constitutes almost half the energy of the sun's radiation, cannot be utilized. To make up for insufficient photon absorption, a reflector by either using a Ag mirror or a stack of the PhC was simply attached to the bottom cell or cathode electrode (**Figure 14(a)** and **(b)**). As seen in this illustration, there are two physical mechanisms by which wave optics approaches (based on reflector and photonic crystals) can improve light-trapping: reflection and diffraction. Firstly, Ag film can provide high reflection property like a mirror. Distributed Bragg reflectors exhibit high index contrast and they can reflect light over a extensive wavelengths region and incident light angles [84]. Likewise, a PhC can reflect incident light from broad

*(a) Fabrication procedure of the 3D PhC, (b) SEM micrograph of a typical ZnO PhC (or inverse opal) layer, showing both the side (cross sectional) view (top) and the top view (bottom) (c) quantum efficiency measurement of the N719 dye and the transmittance measurement of DSSCs using the same dye.*

#### **Figure 14.**

*Schematic of a DSSC (a) with a typical geometric optic concept of reflection to trap light. And (b) with wave optics (3D PhC) to trap light with reflection and diffraction (c) reflection spectra of Ag film and 3D PC with different sphere diameters (198, 311, 375, and 410 nm).*

range of angles for frequencies and polarizations within the photonic crystal in that it can reflect light within the bandgap incident from any angle or medium.

Secondly, wave optics-based devices was developed for diffracting incoming beams into indirect angles according to Bragg's law [85]. The diffraction originated from an interface improve light trapping with the distance increment that light

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 the diffracted beam [86].

**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 311 nm inverse opal sphere that shows the main reflection peak at 526 nm 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 absorption efficiency.

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

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

*D***eff (105**

**209**

*k***eff**

**τeff**

**Rk/**

*Con*

*R***d**

*ns*

*D***1 (106**

*V***OC**

*J***sc (mA/**

*FF*

**EFF**

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

**cm2**

non

w Metal

w 3D

198 311 375 410 Double

2.647

 7.94

 125

 2.97

 0.068

 3.7

 11.1

(375/410)

**Table 3.** *Parameters*

 *for the best fit of the impedance*

 *data and Photovotaic*

 *for the different sized 3D PhC attached DSSC film. Measured in Figure 15.*

2.807

 7.94

 125

 3.15

 0.068

 4.3

 10.8

2.725

 7.94

 125

 3.06

 0.068

 3.0

 10.9

2.506

 7.52

 132

 2.97

 0.065

 3.5

 10.8

2.175

 7.52

 132

 2.60

 0.064

 3.0

 10.5

PhC

(nm)

**s**

2.153

2.273

 7.52

 132

 2.54

 0.069

 3.0

 10.6

 7.52

 132

 2.48

 0.067

 3.3

 10.4

**)**

**(Hz)**

**(ms)**

**R**

**(Ωcms1**

**)**

**(Ω)**

**(1018 cm3**

**)**

**cm2**

**s**

0.48

0.06

0.06

0.01

0.12

0.01

0.12

 0.812

 18.10

 71.4

 10.5

 0.813

 17.52

 71.8

 10.2

 0.818

 17.51

 70.9

 10.1

 0.813

 17.16

 70.9

 9.89

 0.815

 16.00

 71.7

 9.36

 0.818

 16.58

 71.2

 9.64

 0.815

 15.74

 71.5

 9.18

**)**

**(V)**

**cm2**

**)**

**(%)**

**(%)**

**1**

**w**

**1**

**Figure 15.** *(a) J* V *characteristics and (b) Nyquist plots of DSSC attached Ag metal and different sized PhC reflection (film thickness 11 μm).*



**Table**
