**2. Zinc oxide as thin film in solar cells**

In solar cells, ZnO plays important roles in collecting the energy from sunlight in various solar cells such as silicone-based solar cell (first-generation solar cell), thin film (secondgeneration), organic, multijunction, dye-sensitized (third-generation), hybrid and perovskite solar cells (fourth generation). In the first to the third generation of solar cell, ZnO plays a role as transparent conductive oxide (TCO), except in organic solar cell in which ZnO acts as a junction for exciton separation [3].

past the other layers to the top layer. On the other hand, the holes move to the back-contact as a hole summation in solar cells. Since the electrons and holes move to the opposite side, this makes a difference potential on either side and produces electricity. If the band gap gradation is not sequentially arranged, it will disturb electron and hole movement. As a consequence, the electricity will not appear. **Figure 1** shows illustrations of electrons and holes, movement

Besides its physical properties, ZnO is a common material as TCO for thin film solar cell because this material is non-toxic and abundant. However, ZnO still lacks in optical and electrical (opto-electrical) properties. Thus, some of elements or molecules have been successfully developed to improve its optoelectrical properties such as Al, Ga, B, In, Y, Sc, F, V, Si, Ge, Ti,

trical properties of ZnO. **Table 2** shows some changes of ZnO optical and electrical properties

ZnO thin film could be prepared by physical or vacuum methods such as radio frequency (RF) magnetron sputtering or direct current (DC) sputtering, pulsed laser deposition (PLD), and e-beam evaporation [12–15]. However, all of these methods need particular vacuum equipments and investment. Contrary from physical or vacuum methods, chemical methods are well known as a low-cost method because it does not require particular equipment or expensive investment. Thus, chemical process becomes a promising and low-cost process.

There are several processes for depositing ZnO by using chemical process, such as chemical bath deposition (CBD)/dip coating, chemical vapor deposition (CVD)/spray pyrolysis, sol-gel spin coating, doctor blade coating, printing deposition, and electrochemical deposition. Even though there are various deposition techniques, the steps of each technique are the same. The first step is preparing the precursor which can be in a solution, sol-gel, colloidal, or even in a dispersion precursor form. The second step is depositing using various techniques, and the last step is drying or heat treatment to remove the solvent and to develop the ZnO thin film.

Common steps in the preparation of ZnO by chemical process are shown in **Figure 2**.

**Figure 1.** A schematic layer structure of a classical CIGSe solar cell [4] and a schematic band profile of a typical CIGSe

[6]. Some of elements provide a significant change in optical and elec-

) solar cell.

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through several layers with different band gaps in the CIGSe (CuInGeSe<sup>4</sup>

Zr, Hf, In2

O3

and SnO<sup>2</sup>

solar cell under zero bias voltage condition [5].

as TCO after being doped by some elements.

Transparent conductive oxide (TCO) is very important for harvesting electricity from sunlight and can improve solar cell efficiency. This layer is used to place on top of thin film solar cells and has a high transmittance (greater than 80% in the visible spectrum area) but has a low resistivity (less than 10−4 Ωcm). These properties will minimize the probability of the received photon absorbed by the layers before the absorber layer or reflected back. **Table 1** shows several properties of ZnO as TCO on solar cells.

In addition, TCO should have a higher band gap than the absorber layer and the buffer layer, approximately 3.2 eV. If all of the layers are stacked together, they will make a gradation of band gap energy from low band gap energy (absorber layer) to the high energy (the top contact). This gradation can help electrons of the absorbed photons in the absorber layer to move


**Table 1.** Properties of ZnO [1].

past the other layers to the top layer. On the other hand, the holes move to the back-contact as a hole summation in solar cells. Since the electrons and holes move to the opposite side, this makes a difference potential on either side and produces electricity. If the band gap gradation is not sequentially arranged, it will disturb electron and hole movement. As a consequence, the electricity will not appear. **Figure 1** shows illustrations of electrons and holes, movement through several layers with different band gaps in the CIGSe (CuInGeSe<sup>4</sup> ) solar cell.

**2. Zinc oxide as thin film in solar cells**

a junction for exciton separation [3].

46 Emerging Solar Energy Materials

**Table 1.** Properties of ZnO [1].

eral properties of ZnO as TCO on solar cells.

In solar cells, ZnO plays important roles in collecting the energy from sunlight in various solar cells such as silicone-based solar cell (first-generation solar cell), thin film (secondgeneration), organic, multijunction, dye-sensitized (third-generation), hybrid and perovskite solar cells (fourth generation). In the first to the third generation of solar cell, ZnO plays a role as transparent conductive oxide (TCO), except in organic solar cell in which ZnO acts as

Transparent conductive oxide (TCO) is very important for harvesting electricity from sunlight and can improve solar cell efficiency. This layer is used to place on top of thin film solar cells and has a high transmittance (greater than 80% in the visible spectrum area) but has a low resistivity (less than 10−4 Ωcm). These properties will minimize the probability of the received photon absorbed by the layers before the absorber layer or reflected back. **Table 1** shows sev-

In addition, TCO should have a higher band gap than the absorber layer and the buffer layer, approximately 3.2 eV. If all of the layers are stacked together, they will make a gradation of band gap energy from low band gap energy (absorber layer) to the high energy (the top contact). This gradation can help electrons of the absorbed photons in the absorber layer to move Besides its physical properties, ZnO is a common material as TCO for thin film solar cell because this material is non-toxic and abundant. However, ZnO still lacks in optical and electrical (opto-electrical) properties. Thus, some of elements or molecules have been successfully developed to improve its optoelectrical properties such as Al, Ga, B, In, Y, Sc, F, V, Si, Ge, Ti, Zr, Hf, In2 O3 and SnO<sup>2</sup> [6]. Some of elements provide a significant change in optical and electrical properties of ZnO. **Table 2** shows some changes of ZnO optical and electrical properties as TCO after being doped by some elements.

ZnO thin film could be prepared by physical or vacuum methods such as radio frequency (RF) magnetron sputtering or direct current (DC) sputtering, pulsed laser deposition (PLD), and e-beam evaporation [12–15]. However, all of these methods need particular vacuum equipments and investment. Contrary from physical or vacuum methods, chemical methods are well known as a low-cost method because it does not require particular equipment or expensive investment. Thus, chemical process becomes a promising and low-cost process.

There are several processes for depositing ZnO by using chemical process, such as chemical bath deposition (CBD)/dip coating, chemical vapor deposition (CVD)/spray pyrolysis, sol-gel spin coating, doctor blade coating, printing deposition, and electrochemical deposition. Even though there are various deposition techniques, the steps of each technique are the same. The first step is preparing the precursor which can be in a solution, sol-gel, colloidal, or even in a dispersion precursor form. The second step is depositing using various techniques, and the last step is drying or heat treatment to remove the solvent and to develop the ZnO thin film. Common steps in the preparation of ZnO by chemical process are shown in **Figure 2**.

**Figure 1.** A schematic layer structure of a classical CIGSe solar cell [4] and a schematic band profile of a typical CIGSe solar cell under zero bias voltage condition [5].


**3. Chemical deposition methods**

**3.1. Chemical bath deposition/dip coating**

and other compounds as precursor mixture.

Zn (OH)

**Figure 3.** The arrangement of CBD equipment [16].

[

This is the reaction mechanism of ZnO thin film fabrication.

Zn (NH3)

Zn<sup>2</sup><sup>+</sup> + OH<sup>−</sup> → Zn (OH)

4]

Chemical bath deposition (CBD) or dip coating is the simplest and low-cost deposition method because this method does not need expensive and special equipment. In addition, it also has a

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At the precursor preparation, zinc oxide is prepared from zinc-salt compound such as zinc acetate dihydrate, zinc nitrate, zinc chloride, and zinc sulfate [17–21]. After these kinds of zinc salts were dissolved in the solution, zinc in the salt becomes a cation which can react with anion from basic compound, as well as form a seed of nuclei which adhere on the substrate. Besides that, zinc cation also reacts with other compounds in the precursor mixture such as surfactants which act as a binder and form zinc complex compound or in order to get desire properties of the mixture such as viscosity or homogeneity. **Table 3** shows various zinc salts

Further, this seed of nuclei will develop as zinc-salt precipitation which adheres on the substrate, and after drying or heat treatment under various atmospheres, it becomes oxidized and forms a zinc oxide as thin film. The schematic of this preparation is shown in **Figure 4**.

<sup>2</sup><sup>+</sup> + H<sup>2</sup> O → Zn<sup>2</sup><sup>+</sup> + NH4+ + OH<sup>−</sup>

<sup>2</sup> → ZnO + H<sup>2</sup> O

2

simple deposition principle. The arrangement of equipment is shown in **Figure 3**.

**Table 2.** Properties of ZnO films with different dopants.

**Figure 2.** A common preparation step to fabricate ZnO thin film by chemical processes.

In the chemical preparation process, to adjust ZnO thin films into p or n semiconductor, it is done by adding some particular element of salt such as aluminum salt or boron salt into the precursor mixture.

Some of experiments only need a single step of heat treatment to develop the ZnO thin film, but in spin-coating method, several repetition steps of coating and heat treatment are required in order to get the desired thickness, and this repetition step is called as preheat treatment. Contrary with the spin coating, chemical vapor deposition (CVD) or spray pyrolysis method may not have specific drying or heat treatment steps because in this method, the heat treatment is done simultaneously with deposition. To adjust the thickness in CVD or spray pyrolysis, it is adjusted by the deposition time.
