**3. CIGS thin film by electrodeposition**

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

**Figure 2.** A schematic representation of the mass transport distribution at the initial stage of the film formation by

**Figure 3.** A graphic representation of electrodeposition signals versus time obtained during the electrodeposition

process: (a) WE potential and (b) WE current density [13].

electrodeposition (modified from [12]).

102 Perturbation Methods with Applications in Science and Engineering

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

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 nucleation [22]. In CuInSe<sup>2</sup> (CIS) one-step electrodeposition, it has been established that the Cu-Se phase is formed at a low potential, and a reaction path has been established as a function of the potential. The Cu-Se phase acts as a nucleation site for indium incorporation [23, 24]. The CIS film morphology deposited at various potentials has been analyzed [23]. At low polarizations between −0.4 and − 0.5 V, platelets characteristic of the Cu-Se were observed; when the polarization increased, the morphology was nodular. The mechanisms of Ga to CIS incorporation also have been established. It is incorporated as gallium selenide and GaO<sup>3</sup> [25]. The CIGS film morphology obtained by the one-step electrodeposition with potentiostat mode has been described as nodules with a cauliflower-like growth [22, 26]. The as-electrodeposited CIGS film morphology is strongly influenced by the bath composition. Microcracks in the films have been observed when the films were deposited at low concentrations of CuCl<sup>3</sup> , InCl3 , and GaCl3 salts and at high concentrations of H2 SeO3 [27].

transport of the chemical species from the bulk solution to the charge transfer zone. By increasing the electrodeposition time, the film obtained is more rugged and darker in color; this is due to the lack of ions near to WE and to the increase of the diffusion layer thickness. The stirring of the solution is desirable since it enhances ion transport to the substrate and decreases the thickness of the diffusion layer [34]. However, the agitation method of stirring for a laboratory-scale

Mechanical Perturbations at the Working Electrode to Materials Synthesis by Electrodeposition

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The CIGS films characterized by scanning electron microscope (SEM) are shown in **Figure 5**. **Figure 5(a, b)** shows micrographs of surface and cross section. In both cases, the morphology consists of vertical nodules with a well-defined boundary between them. Some nodules are larger than others, which apparently have stopped growing. The stunted nodules increase the boundary between the nodules that have a greater growth. **Figure 5(c)** shows the surface morphology of the vertical nodules with a cauliflower-like growth. The surface morphology among the nodule boundaries is shown in **Figure 5(d)**. The film morphology in the nodule has differences with the one that exists in the boundary. Apparently, the film formed between boundaries is less compact than those formed in the nodule. In general, the CIGS films that were obtained through the one-step electrodeposition are not very compact and have a low crystalline structure, so that they do not have the properties to be used in solar cells. The principal morphology consisted in groups of atoms forming the cauliflowerlike growth. The annealing process in a selenium atmosphere is necessary to transform the as-electrodeposited film into a more crystalline, with large grains and with compact

The CIGS thin films that were subjected to an annealing process in a selenium atmosphere are shown in **Figure 6**. The selenization temperature was 550°C for 180 min. **Figure 6(a,b)** shows the surface and cross-section micrographs. In the micrographs, there is evidence that the nodules are of different length. On increasing the deposition time, some nodules continue

**Figure 4.** A diagram of an electrolytic cell with three horizontal electrodes.

deposition leads to gradients of thickness in the flow direction of the electrolyte [35].

morphology.

Many studies have examined ways of improving the CIGS film morphology by a one-step electrodeposition. The effect of sodium sulfamate as a complexing agent on the film morphology was evaluated [28]. An improvement on CIGS thin film morphology was obtained when a short electrode pretreatment of a 1-min deposition at −0.5 V was carried out prior to deposition of the film [29]. The pulse electrodeposition process can produce a CIGS film that is more smooth, compact, and homogeneous than the one deposited by the DC potential electrodeposition [30]. Electrochemical studies in CIGS electrodeposition, generally, use an electrochemical cell with electrodes suspended vertically. However, an electrochemical cell with electrodes in a horizontal position has advantages over a cell with vertical electrodes, principally because the ion transport mechanism as well as the natural flow by convection allows a better uniformity on the WE surface; in this way, the composition is homogeneous through the film [31].
