**2. Conventional electrodeposition**

A conventional three-electrode electrolytic cell connected to potentiostat equipment is used for material synthesis investigation in solar cell applications by electrodeposition. A direct current (DC) potential is applied to the working electrode (WE) with respect to the reference electrode (RE). The DC potential in the WE is regulated in a closed-loop control system by adjusting the current at an auxiliary electrode (AE). According to the electrolytic solution, different materials can be deposited on the WE. Synthesizing a material by the electrodeposition technique consists of finding the values of the variables that produce a film growth with the desired characteristics. Thin films of materials with homogeneous growth and compact morphology are desirable. If the material to be formed consists of two or more elements, there are diverse electrodeposition alternatives, principally (1) electrodeposition of elements in layers, followed by annealing, to get the desired structure, and (2) electrodeposition of elements in a simultaneous way, also followed by annealing, to increase crystallinity. The latter alternative is known as a one-step electrodeposition in which the electrochemical conditions must be met in such a way that the material composition is homogeneous from the first instant of formation. The advantage of the one-step electrodeposition alternative is that the film that is obtained is electro-crystallized. In addition, a single electrolytic solution with ions of the precursor elements is used, while in the electrodeposition by layers, an electrolytic solution is used for each layer.

The basic modes of crystal growth on a WE under electrochemical conditions have been established; the electrodeposition takes place at the electrode-electrolyte interface under the influence of an electric field [8]. A scheme of load distribution as well as a simple electrical model of a three-electrode electrolytic cell is represented in **Figure 1**. Charge distribution in the electrode-electrolyte interface is analogous to charge distribution in a capacitor which is called the electrical double layer. The electric field lines are defined only at the interface, where CWE and CAE are the equivalent capacitances between the WE and AE and the solution, respectively. RSOL is the solution electric resistance. In a three-electrode-electrolytic cell, the electrode-solution interface is principally important in the WE and AE. Other detailed electric models [9] for the electrode-solution interface suggest that the interface is affected by the capacitance of the double layer, the resistance in the charge transfer zone, and the impedance due to adsorption and mass transport, and the parameter of the electrical model, mentioned earlier, can be obtained by electrochemical impedance [10].

materials synthesis have been widely investigated for solar energy conversion. These methods can be classified into physical and chemical. Electrodeposition is a chemical method that has been used to obtain metallic or semiconducting films on substrates, with the aim of protecting the surface against the oxidation and corrosion, giving a better esthetic appearance, and providing some mechanical and electrical characteristics, different to the base material, to improve its physical properties. Electrodeposition has been considered for solar cell applications for a long time [1], with a considerable potential for the fabrication of a low-cost thin film for solar cells [2]. It is simple, versatile, and economical as compared to physical methods, such as high vacuum processes. Some of its advantages are requiring less capital investment, saving raw material, application on irregular surfaces, and industry scalability potential. It

(CIGS) solar cell [5]. It is also used for nanoparticle synthesis for solar collectors [6] and to

The electrodeposition method is considered difficult; perhaps because the electrodeposition process is affected by many variables. Some of them are concentration of the solution, solution temperature, pH, working electrode potential, working electrode resistivity, and distance between electrodes. During the electrodeposition process, there are phenomena that affect the film growth, among them, the diffusion layer, the depletion region, and the natural flow by convection. For such reasons, the electrodeposition method is still under investigation to

A conventional three-electrode electrolytic cell connected to potentiostat equipment is used for material synthesis investigation in solar cell applications by electrodeposition. A direct current (DC) potential is applied to the working electrode (WE) with respect to the reference electrode (RE). The DC potential in the WE is regulated in a closed-loop control system by adjusting the current at an auxiliary electrode (AE). According to the electrolytic solution, different materials can be deposited on the WE. Synthesizing a material by the electrodeposition technique consists of finding the values of the variables that produce a film growth with the desired characteristics. Thin films of materials with homogeneous growth and compact morphology are desirable. If the material to be formed consists of two or more elements, there are diverse electrodeposition alternatives, principally (1) electrodeposition of elements in layers, followed by annealing, to get the desired structure, and (2) electrodeposition of elements in a simultaneous way, also followed by annealing, to increase crystallinity. The latter alternative is known as a one-step electrodeposition in which the electrochemical conditions must be met in such a way that the material composition is homogeneous from the first instant of formation. The advantage of the one-step electrodeposition alternative is that the film that is obtained is electro-crystallized. In addition, a single electrolytic solution with ions of the precursor elements is used, while in the electrodeposition by layers, an electrolytic solution is

The basic modes of crystal growth on a WE under electrochemical conditions have been established; the electrodeposition takes place at the electrode-electrolyte interface under the

ZnSnS4

[4], and CuInx

Ga(1-x)Se2

has been used for materials synthesis for perovskite [3], Cu2

develop technology in the energy storage [7].

100 Perturbation Methods with Applications in Science and Engineering

**2. Conventional electrodeposition**

improve the material quality.

used for each layer.

In an electrodeposition process, the charge, which consists of electrons, is considered to be evenly distributed on the WE surface. When M2+ ions are present in the solution, the electrochemical of M2+ + 2e → M is carrier out on the WE. When more than one type of ions must be reduced, the transport mechanism of the ions until the load transfer zone plays an important role. It is assumed that the ratio at which the ions are consumed by the charge transfer reaction is equal to the ratio at which the ions arrive at the charge transfer zone.

At the beginning of the electrodeposition process, the first nuclei grow on the WE surface. At that time, the solution around the nuclei is depleted of ionic species, and the only factor that influences the ion movement is diffusion [11], through which the depleted zones around the

**Figure 1.** A diagram of a simple electrical model of a three-electrode electrolytic cell.

nuclei start to propagate throughout the WE surface; as the electrodeposition time elapses, these zones begin to overlap [12]. **Figure 2** shows a schematic representation of the ion movement distribution on the WE surface at the initial stage of the film formation. The arrow lines

show the ion movement from the bulk solution to the WE vicinity. In this region, the ions concentration is different from their value in the bulk solution; this region is known as the

Mechanical Perturbations at the Working Electrode to Materials Synthesis by Electrodeposition

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When the DC potential is applied at the WE, the current density versus time shows a transitory stage evolution, followed by a stage where the current variation is lower than the transitory stage. Also, the potential has a transitory stage; after that, the WE potential reaches the set value. With acquisition data software, the voltage and current signals can be plotted as time function, as it is shown in the graph in **Figure 3**. According to the above, the electric model of the three-electrode-electrolytic cell represents an average model of the electrical phenomena that take place in the electrode-solution interface during the electrodeposition

The semiconductor CIGS is a promising absorber in thin film solar cells, due to their direct band gap and large optical absorption coefficient. Small-area CIGS solar cells with efficiencies reaching 22.6% have been built with this semiconductor synthesized by the high vacuum deposition method [14]. A compact absorber morphology is a quality indicator to obtain high performance CIGS solar cells. To improve solar cell efficiency, the CIGS absorber should have, in cross view, large grains extending from the back to the front [15]. The CIGS absorber has the highest potential to develop large-scale solar cells. Employing high vacuum deposition method, high efficiencies have been achieved; however, economical deposition methods with the possibility of implementing in large area still need to be developed [16, 17]. It is considered that the CIGS absorber can be synthesized in a large area and with compact morphology employing the electrodeposition method. However, when CIGS solar cells have been built by synthesizing the absorber by electrodeposition, solar cell efficiencies of 11.3% for a one-step electrodeposition [18] and 14.17% for layers electrodeposition [19] have been achieved. The low efficiency is attributed to lack of absorber quality when it is obtained by the electrodeposition method, usually associated with the morphology. Microcracks have been identified in the CIGS film obtained by a one-step electrodeposition [20], which is one reason why relatively

To develop the CIGS solar cells on a large scale by electrodeposition, there are still aspects that must be investigated. The major challenges and required strategies have been identified. Among them, (1) the precise control of film stoichiometry (optimization of Ga content and Ga distribution), (2) novel deposition strategies, (3) understanding on the mechanism of Ga incorporation, and (4) establishing the strategy that allows electrodepositing the semiconductor with homogeneous composition and uniform morphology throughout the film [16, 21].

In several works, electrodeposition strategies for thin films synthesis have been used. It has been established that by using a pH-regulating solution, stability is provided to the electrodeposition process. No oxides or hydroxides are obtained in the solution, and it is possible to incorporate a higher percentage of gallium in the film [20]. In the first stages of Cu-In-Se on Mo-coated glass by electrochemical deposition, the first nuclei are made of a copper-rich Cu-Se without indium and the nucleation is developed by a quasi-instantaneous three-dimensional

low efficiencies are obtained in the CIGS solar cell by electrodeposition.

diffusion layer.

process.

**3. CIGS thin film by electrodeposition**

**Figure 2.** A schematic representation of the mass transport distribution at the initial stage of the film formation by electrodeposition (modified from [12]).

**Figure 3.** A graphic representation of electrodeposition signals versus time obtained during the electrodeposition process: (a) WE potential and (b) WE current density [13].

show the ion movement from the bulk solution to the WE vicinity. In this region, the ions concentration is different from their value in the bulk solution; this region is known as the diffusion layer.

When the DC potential is applied at the WE, the current density versus time shows a transitory stage evolution, followed by a stage where the current variation is lower than the transitory stage. Also, the potential has a transitory stage; after that, the WE potential reaches the set value. With acquisition data software, the voltage and current signals can be plotted as time function, as it is shown in the graph in **Figure 3**. According to the above, the electric model of the three-electrode-electrolytic cell represents an average model of the electrical phenomena that take place in the electrode-solution interface during the electrodeposition process.
