**4. Recent research advances**

The approach to progress further is to increase the efficiency as well as decrease the cost of the solar cells. Therefore new concepts and new cell structures should be brought in the development of the film solar cells. One of the plausible solutions is to implant nanostructures in the conventional thin film photovoltaic devices. Zinc oxide (ZnO) nanorod arrays are one of the nanostructures that can be implanted in the solar cells. ZnO nanostructures can be grown on top of the CIGS solar cells' window layers as an antireflective coating layer or implanted into the solar cells. On the one hand, the implanted nanostructures will decrease the reflection and increase the light path due to light coupling effects. On the other hand, the ZnO nanostructures will put the electrode close to the photoinduced carrier generation area with larger carrier collection function. It will assist in boosting the solar cells' efficiency by carrier collection enhancement.

#### **4.1. Photon management**

Thin film photovoltaic device technology relies on light management to enhance light absorption in thin absorber layers. One of the plausible solutions is to implant nanostructures in the conventional thin film photovoltaic devices. For example, the zinc oxide (ZnO) nanorod arrays can be implanted in the CIGS solar cells. The use of the ZnO nanorods in the thin film solar cells is an effective way to decrease the reflection. The variation of the geometrical parameters of the ZnO nanorods, such as the diameter, the height and the density can lead to an optimum which results in the maximal absorption in the absorber. An approach of a rigorous threedimensional (3D) modeling based on the finite element method (FEM) can be used to simulate and optimize the light absorption in the Cu(In,Ga)Se2 absorbers with nanostructures.

Modeling the optical properties of the Cu(In,Ga)Se2 absorbers with nanostructures starts by defining the characteristics of the incident light. In the stationary case, the electric and magnetic field can be expressed as follows.

$$E\left(\mathbf{x}, \mathbf{y}, z, t\right) = \bar{E}\left(\mathbf{x}, \mathbf{y}, z\right) e^{-i\alpha t} \tag{1}$$

$$H\left(\mathbf{x}, \mathbf{y}, z, t\right) = \tilde{H}\left(\mathbf{x}, \mathbf{y}, z\right) e^{-i\alpha t} \tag{2}$$

where *E* is the electric field, *H* is the magnetic field and *ω* is the angular frequency related by *ω* = 2π*f* to the frequency *f* of light. The mathematical model of the light propagation is Maxwell's equation:

$$\mathbf{E}\left(\mathbf{x},\mathbf{y},z,t\right) = \tilde{\mathbf{E}}\left(\mathbf{x},\mathbf{y},z\right)e^{-i\alpha t} \tag{3}$$

Copper Indium Gallium Selenide Thin Film Solar Cells http://dx.doi.org/10.5772/65291 191

$$
\nabla \cdot \boldsymbol{H} = 0 \tag{4}
$$

$$
\nabla \times \mathbf{E} = -\frac{\partial \mu \mathbf{H}}{\partial t} \tag{5}
$$

$$
\nabla \times \mathbf{H} = \mathbf{J} + \frac{\partial \epsilon \mathbf{E}}{\partial t} \tag{6}
$$

where *E* is the electric field, *H* is the magnetic field, *ρ* is the electric charge density, *ε* is the permittivity, *μ* is the magnetic permeability and *J* is the current density. In the time harmonic case the magnetic field can be determined by the electric field and vice versa. From Maxwell's equations we can find

**4. Recent research advances**

enhancement.

190 Nanostructured Solar Cells

**4.1. Photon management**

field can be expressed as follows.

equation:

The approach to progress further is to increase the efficiency as well as decrease the cost of the solar cells. Therefore new concepts and new cell structures should be brought in the development of the film solar cells. One of the plausible solutions is to implant nanostructures in the conventional thin film photovoltaic devices. Zinc oxide (ZnO) nanorod arrays are one of the nanostructures that can be implanted in the solar cells. ZnO nanostructures can be grown on top of the CIGS solar cells' window layers as an antireflective coating layer or implanted into the solar cells. On the one hand, the implanted nanostructures will decrease the reflection and increase the light path due to light coupling effects. On the other hand, the ZnO nanostructures will put the electrode close to the photoinduced carrier generation area with larger carrier collection function. It will assist in boosting the solar cells' efficiency by carrier collection

Thin film photovoltaic device technology relies on light management to enhance light absorption in thin absorber layers. One of the plausible solutions is to implant nanostructures in the conventional thin film photovoltaic devices. For example, the zinc oxide (ZnO) nanorod arrays can be implanted in the CIGS solar cells. The use of the ZnO nanorods in the thin film solar cells is an effective way to decrease the reflection. The variation of the geometrical parameters of the ZnO nanorods, such as the diameter, the height and the density can lead to an optimum which results in the maximal absorption in the absorber. An approach of a rigorous threedimensional (3D) modeling based on the finite element method (FEM) can be used to simulate

and optimize the light absorption in the Cu(In,Ga)Se2 absorbers with nanostructures.

( ,,, ,, ) ( ) *i t xyzt xyz e*-

( ,,, ,, ) ( ) *i t xyzt xyz e*-

( ,,, ,, ) ( ) *i t xyzt xyz e*-

where *E* is the electric field, *H* is the magnetic field and *ω* is the angular frequency related by *ω* = 2π*f* to the frequency *f* of light. The mathematical model of the light propagation is Maxwell's

Modeling the optical properties of the Cu(In,Ga)Se2 absorbers with nanostructures starts by defining the characteristics of the incident light. In the stationary case, the electric and magnetic

w

w

w

*E E* = % (1)

*H H*= % (2)

*E E* = % (3)

$$H = -\frac{1}{i\rho\sigma} \nabla \times \mathbf{E} \tag{7}$$

$$\mathbf{E} = \frac{\mathbf{l}}{\dot{\iota}\dot{\alpha}} \nabla \times \mathbf{H} \tag{8}$$

Thus the electric field in the electromagnetic will be taken into account in the simulation. The electric field distribution of the incident light is described by unpolarized stationary plane waves.

$$
\tilde{E} = A e^{i(\mathbf{k} \cdot \mathbf{r} + \boldsymbol{\varphi})} \tag{9}
$$

where *A* is the constant, *k* is the wave vector, *r* is the position vector and *φ* is the phase angle. For simplifying the simulation, the wave vector of the incident light is perpendicular to the surface of the Cu(In,Ga)Se2 absorbers and *ϕ* is taken as 0.

The nanorod arrays as a light coupling component can be incorporated into two types of solar cells, i.e., the bifacial solar cells and the superstrate solar cells. As shown in **Figure 7(a)**, both sides of the bifacial solar cells are illuminated and the ZnO nanorods work as a light-coupling component and a nanocontact electrode. The intrinsic ZnO nanorods in the superstrate solar cells play a buffer role. Optionally a buffer layer can be inserted between the ZnO nanorods and the absorber in the superstrate solar cells. Since both the bifacial and superstrate solar cells possess the same components which are glass/transparent conductive oxides/ZnO nanorods/ Cu(In,Ga)Se2 absorber, the same components are regarded as the simulated structure. As illustrated in **Figure 7(c)**, the simulated structure can be divided into two parts: the glass and the transparent conductive oxides/ZnO nanorods/Cu(In,Ga)Se2 absorber (TNA) structure.

**Figure 7.** (a) The schematic structure of the bifacial Cu(In,Ga)Se2 solar cell. (b) The schematic structure of the superstrate Cu(In,Ga)Se2 solar cell. (c) The schematic drawing of the simulated structure. The TNA is the abbreviation for the transparent conductive oxides/nanorods/absorber.

As shown in **Figure 8**, the total reflection consists of the specular reflectance and the scattered reflectance:

**Figure 8.** The schematic structure of the simulated structure. The structure is illuminated through the glass side. The figure defines the incident light intensity *I*0, the specular intensity *ITNA* illuminating the glass/TNA interface, the specular reflectance *R*<sup>s</sup> the scattered reflectance , *R*sc the specular reflectance of the *R*<sup>s</sup> TNA structure and the scattered reflectance of the *R*sc TNA structure.

where *RS* and *RSC* are the specular reflectance and the scattered reflectance, respectively. The specular reflectance is given as:

$$R^S = R\_{\text{glass}} + \left(1 - R\_{\text{glass}}\right)^2 \cdot \mathbf{r}\_{\text{glass}}^2 \cdot R\_{\text{TM}}^S \cdot \sum\_{\nu=0}^{\kappa} \left[R\_{\text{glass}} \cdot \mathbf{r}\_{\text{glass}}^2 \cdot R\_{\text{TM}}^S\right]^\nu \tag{11}$$

where *Rglass* is the reflectance at the air/glass interface with the incident of light illuminating the interface, is theabsorptance of the thick glassand is the reflectance of the TNA structure illuminated from the glass. The *Rglass* is calculated from the Fresnel's equation:

$$R\_{g\_{\rm glass}} = \left| \frac{\mathbf{n}\_{g\_{\rm glass}} - 1}{\mathbf{n}\_{g\_{\rm glass}} + 1} \right|^2 \tag{12}$$

$$
\mathfrak{m}\_{g\_{\text{class}}} = n + i \cdot k \tag{13}
$$

where *nglass* is the complex refractive index of the glass, *n* is the refractive index of the glass and *k* is extinction coefficient of the glass. The *τglass* is calculated as:

$$
\sigma\_{\text{glass}} = \exp\left(-\alpha\_{\text{glass}} \cdot d\_{\text{glass}}\right) \tag{14}
$$

$$a\_{\text{glass}} = \frac{4\pi k}{\lambda 0} \tag{15}$$

where *αglass* is the optical absorption coefficient of the glass and *λ*<sup>0</sup> is the vacuum wavelength of light.

The reflection, transmission and absorption in TNA structure can be calculated using the finite element method (FEM) method. The FEM is a numerical technique used in finding solutions of Maxwell equations. The volume of the simulated structure is meshed and electromagnetic field components are computed. Once the simulation of the design (structure, boundary conditions, light sources and frequency range) is set up, FEM process operates through three steps: meshing, solving and postprocessing. In the first step, for a given wavelength, the volume of the designed structure is discretized. During the second step, the resulting system of the equations is solved. In the third step, the reflection, transmission and absorption of the simulated structure is computed. The reflectance *RTNA* is given by:

$$R\_{\text{TNA}} = R\_{\text{TNA}}^S + R\_{\text{TNA}}^{\text{SC}} \tag{16}$$

where is the scattered reflectance of the TNA structure. Since tracing the scattered reflected light of the TNA structure in the thick glass and air/glass interface dramatically boosts the computation amount, it is not possible to get the results using FEM. The total reflectance is approximately calculated as:

$$R^{Total} = R\_{g\_{\text{class}}} + R\_{\text{TNA}} \tag{17}$$

#### **4.2. Nanostructures**

**Figure 7.** (a) The schematic structure of the bifacial Cu(In,Ga)Se2 solar cell. (b) The schematic structure of the superstrate Cu(In,Ga)Se2 solar cell. (c) The schematic drawing of the simulated structure. The TNA is the abbreviation for

As shown in **Figure 8**, the total reflection consists of the specular reflectance and the scattered

**Figure 8.** The schematic structure of the simulated structure. The structure is illuminated through the glass side. The figure defines the incident light intensity *I*0, the specular intensity *ITNA* illuminating the glass/TNA interface, the specu-

where *RS* and *RSC* are the specular reflectance and the scattered reflectance, respectively. The

<sup>2</sup> 2 2 0

where *Rglass* is the reflectance at the air/glass interface with the incident of light illuminating

µ =

 t

<sup>=</sup> <sup>+</sup> é ù ×- <sup>ë</sup> <sup>×</sup> å <sup>û</sup> (11)

1 ··· *<sup>u</sup> <sup>S</sup> S S glass glass glass TNA glass glass TNA u*

structure illuminated from the glass. The *Rglass* is calculated from the Fresnel's equation:

*RR R R R R* t

the scattered reflectance , *R*sc the specular reflectance of the *R*<sup>s</sup>

( )

the interface, is theabsorptance of the thick glassand

*Total S SC R RR* = + (10)

TNA structure and the scattered reflec-

is the reflectance of the TNA

the transparent conductive oxides/nanorods/absorber.

reflectance:

192 Nanostructured Solar Cells

lar reflectance *R*<sup>s</sup>

TNA structure.

specular reflectance is given as:

tance of the *R*sc

An antireflective or antireflection coating (ARC) is a type of optical coating applied to the surface of lenses and other optical devices to reduce reflection. This improves the efficiency of the system since less light is lost. The ARC is generally made of a dielectric layer, e.g., MgF2, SiN, TiO2 or ZnS, with a thickness of a quarter-wavelength [22–25]. Another approach of

boosting light coupling is to structure the surface of the solar cells by means of the moth-eye effect. Moths' eyes have an unusual property: their surfaces are covered with a natural nanostructured film which eliminates reflections. The ZnO nanorods have an appropriate refractive index of ~2. Coating an absorber surface leads to continuously varying refractive index profiles in the tapered ZnO nanorods. Consequently they suppress the surface reflection via a subwavelength structure. Therefore the tapered ZnO nanorods are a promising light coupling layer for ARC of solar cells as well as solar thermal selective surfaces.

**Figure 9.** The ZnO nanowire arrays prepared by the hydrothermal method.

**Figure 2** has shown the cross section scanning electron microscopy (SEM) images of ZnO nanorods on CIGS. The ZnO nanorod arrays serving as an ARC were electrodeposited on thin film Cu(In,Ga)Se2 solar cells. According to the research, the weighted reflectance was reduced from 8.6 to 3.5%. Highest increases in both the saturation-current of 5.7% and the solar cells efficiency of 7.2% were achieved [26]. In addition, the ZnO nanorod arrays have been incorporated into a superstrate or a bifacial cell structure of the other thin film photovoltaic devices such as DSSCs [27], QDSCs [28] and organic solar cells [29].

ZnO nanostructures have been prepared by various methods [30–46]. The solution-based fabrication routes including hydrothermal method and electrochemical deposition (ECD) method are the only ways to grow ZnO nanostructures at a low temperature down to the range between 60°C and 90°C [35–46]. Meanwhile the growth can be achieved over large areas up to 10 cm × 10 cm [44–46]. **Figure 9** shows the SEM image of the ZnO nanowire arrays prepared by the hydrothermal method.

The ECD technique consists of an electrochemical cell and accessories for providing a galvanic current which flows through the electrochemical cell. The cell usually contains electrolyte and electrodes. The first of these electrodes has been named the anode. At an anode, electrons go away from the electrolyte to the anode. Hence, an anodic reaction must generate electrons. The second has been named the cathode. The cathode supplies electrons to the positively charged cations which flow to it from the electrolyte. Within the electrolyte, the current flow is always from the anode to the cathode, which means the electron transports from the cathode to the anode. In the external parts of the closed circuit ("external" relative to the electrolyte), the current flow is from cathode to anode, which equates the electron transport from the anode to the cathode [47].

**Figure 10.** (a) Two-electrode system and (b) three-electrode system.

boosting light coupling is to structure the surface of the solar cells by means of the moth-eye effect. Moths' eyes have an unusual property: their surfaces are covered with a natural nanostructured film which eliminates reflections. The ZnO nanorods have an appropriate refractive index of ~2. Coating an absorber surface leads to continuously varying refractive index profiles in the tapered ZnO nanorods. Consequently they suppress the surface reflection via a subwavelength structure. Therefore the tapered ZnO nanorods are a promising light

**Figure 2** has shown the cross section scanning electron microscopy (SEM) images of ZnO nanorods on CIGS. The ZnO nanorod arrays serving as an ARC were electrodeposited on thin film Cu(In,Ga)Se2 solar cells. According to the research, the weighted reflectance was reduced from 8.6 to 3.5%. Highest increases in both the saturation-current of 5.7% and the solar cells efficiency of 7.2% were achieved [26]. In addition, the ZnO nanorod arrays have been incorporated into a superstrate or a bifacial cell structure of the other thin film photovoltaic devices

ZnO nanostructures have been prepared by various methods [30–46]. The solution-based fabrication routes including hydrothermal method and electrochemical deposition (ECD) method are the only ways to grow ZnO nanostructures at a low temperature down to the range between 60°C and 90°C [35–46]. Meanwhile the growth can be achieved over large areas up to 10 cm × 10 cm [44–46]. **Figure 9** shows the SEM image of the ZnO nanowire arrays prepared

The ECD technique consists of an electrochemical cell and accessories for providing a galvanic current which flows through the electrochemical cell. The cell usually contains electrolyte and electrodes. The first of these electrodes has been named the anode. At an anode, electrons go away from the electrolyte to the anode. Hence, an anodic reaction must generate electrons. The second has been named the cathode. The cathode supplies electrons to the positively charged cations which flow to it from the electrolyte. Within the electrolyte, the current flow is always from the anode to the cathode, which means the electron transports from the cathode to the

coupling layer for ARC of solar cells as well as solar thermal selective surfaces.

**Figure 9.** The ZnO nanowire arrays prepared by the hydrothermal method.

such as DSSCs [27], QDSCs [28] and organic solar cells [29].

by the hydrothermal method.

194 Nanostructured Solar Cells

During the ECD process, the electrodeposited products are deposited on one of the electrodes. The electrode is the working electrode (WE). However, the WE is not enough for the ECD process. At least another electrode is necessary for allowing current to flow. In the simplest case a two-electrode cell is used for ECD (**Figure 10(a)**). The second electrode is used both as the reference electrode (RE) to measure the WE potential and as a counter electrode (CE) to allow current to flow. However, the potential of the second electrode changes accordingly when there is a current flow, which has been termed electrode polarization. In order to measure the accurate WE potential, a three-electrode cell containing a WE, a CE and a RE is more common (**Figure 10(b)**). A current flows between the WE and CE, while the potential of the WE is measured against the RE. No current flows in the circuit of the RE/WE, which therefore is not polarized [48]. The glass substrates coated by FTO and AZO transparent conductive oxides are used as the WE. The electrochemical cell was placed in a thermoregulated bath. The liquid electrolyte contains zinc salts and additives. During the experiments the liquid electrolyte in which there is a magnetic stirring bar is agitated. A schematic illustration of the setup for the ECD process is shown in **Figure 11**. The electrochemical process is controlled and recorded by a potentiostat/galvanostat.

**Figure 11.** Schematic description of the electrochemical deposition setup.
