**4. Conventional electrodeposition plus periodical perturbations**

to grow and others stop growing. The cross-section film micrograph shows the Mo layer and over it, a CIGS film with a compact morphology with 300 nm of thickness; this layer is also evident from the as-electrodeposited film shown in **Figure 5(b)**. It was noticeable that the compact layer is due to the initial growth when the current density is in a transitory state and the diffusion layer is thin. Over the compact CIGS film, there are only formations of isolated nodules of different sizes with very large boundaries between them; in this stage of formation, the current density and the mechanism of mass transport are not locally uniform. In order to reduce the activation energy and grow large grains during the annealing process, a Cu-rich film was prepared. The film composition ratios were Ga/(In+Ga) = 0.29 and Cu/(In+Ga) = 1.18. The Cu content of the film determines the activation energy for grain boundary motion. It has been determined that by increasing the Cu content of the film from 17.9 to 25.7%, the activation energy decreases from 3.5 to 3.0 eV [36]. The micrographs of annealed films are shown in **Figure 6(c, d)**. From these micrographs, it can be noted that there is also a compact CIGS film over the Mo film, which shows that in the copper-poor and copper-rich films, the films are compact in the first stage of growth, up to a thickness of 300 nm. A nonuniform grain growth is identified. There is only a grain growth in the boundaries indicating that the kinetic of grain growth during selenization process in the nodule boundaries was different, which is believed to be caused by the non-homogeneity in the film composition originated by the nonuniformity of the current density during the one-step electrodeposition process. Copper-rich films were formed in the nodule boundaries. In this way, the atomic composition in the films is not uniform, the nodules are copper-poor, and the boundary nodules are copper-rich. That is, the

**Figure 5.** Micrographs of the CIGS film that has been electrodeposited in a conventional mode: (a) surface, (b) cross

section, (c) nodule with a cauliflower-like growth, and (d) morphology in the nodule boundary [13].

106 Perturbation Methods with Applications in Science and Engineering

film locally will have different electrical, structural, and optical characteristics.

In the electrodeposition theory, it is assumed that the WE surface is homogeneous so that the current density in the macroscopic level is uniformly distributed over the WE surface [8]. However, at the microscopic level, if the surface of the WE is considered as a surface with roughness, there will be a greater electric field strength in the peaks than in the surface valleys, as shown in **Figure 7(a)**, where the WE roughness has been amplified. Thus, the electrochemical kinetic is affected. For this reason, the electric load will be concentrated in the crests of the WE. With the formation of the first nodules, the WE roughness increases in such a way that the current density and therefore the mass transport mechanism are concentrated at the nodules. The foregoing has been observed in other studies, where it has been determined that the nonuniformity in the local current densities can exist even when the macroscopic current distribution over a given surface is completely uniform [34]. Assuming that there is a direct relationship between the load transfer ratio and the current density that is demanded during the electrodeposition process, the load transfer process can be analyzed using the current density. The points of greatest intensity of the electric field are the crests of the WE. In this way, they produce a greater current density during the ion reduction process, in such a way that, in the crests, the growth of the film originates grains that grow perpendicularly with respect to the WE. In the valleys, the current density is lower, and therefore, the density of ions is reduced and the growth speed of the film is slower. From the previous results, it can be established that as a consequence of nonuniformity in the local current densities through the WE, due to the diffusion layer growth, the CIGS morphology consists of isolated nodules with a cauliflower-like growth. It is evident after the transitory stage in the electrodeposition current density, and this is more noticeable when the electrodeposition time increases, a representation of isolated nodules is shown in **Figure 7(b)**.

The contour lines are used in topographic maps to represent points of the same elevation. Here, we use it to represent the CIGS film morphology as shown in **Figure 8**. The contour lines were traced according to the film morphology shown in **Figures 5** and **6**. The contour lines represent the CIGS film morphology of the same thickness. According to the micrographs, the CIGS films morphology is not uniform through the WE surface, being a function of the current density distribution during the electrodeposition process. Thus, the contour lines also represent the mass transport mechanism during the electrodeposition process. **Figure 8** shows the contour lines that represent the electrodeposition process evolution. The highest mass transport density and the nodule formation are represented in zones with a dark color while less mass transport density and the boundary layer formation between nodules are represented with a light gray color. When the electrodeposition process begins, a large number of nodules grow randomly distributed. The nodules distributions, which can be appreciated after the current transitory stage, are represented in **Figure 8(a)**. When the first nodule has a cauliflower-like growth, the mass transport mechanism is concentrated in them; however, not all nodules grow at the same rate and eventually some stop growing. When the deposition time elapses, the nodules become more and more isolated and the boundary between them grows, because the mass transport mechanism is concentrated at very isolated points; this is represented as contour lines in **Figure 8(b, c)**. The above causes the CIGS film morphology to appear as isolated nodules with a high roughness, and this is evident when the electrodeposition time increases. In previous studies, it has been shown that in the CIGS film synthesized by electrodeposition, the composition is related to the current density; if

the current density is high, the CIGS film composition is copper-poor. On the other hand, if the current density is low, the CIGS film composition is copper-rich [31]. Therefore, the CIGS film has a composition that varies according to the contour line; the zones with a dark color are cooper-poor while the zones with a light gray color are copper-rich. The above is evident in the surface morphology of CIGS films that were subjected to an annealing process as shown in **Figure 6(c, d)**. At the initial stage of the electrodeposition process, the ratio of ion reduction in the WE surface is much greater than the speed at which the ionic species arrive at the load transfer zone; for this reason, the diffusion layer increases up to maintain an electric load balance. In this way, the zone of the depletion varies according to the contour plot. At this microscopic level, the diffusion layer is not uniform along the working electrode. It acquired this principal shape after the transient of the electrodeposition current density. At the macroscopic level, experimental results [37] revealed that the electrochemical kinetic behavior of CIGS thin films is strongly influenced by the electrical double layer existing between the substrate. With an increase in the electrodeposition time, the kinetic behavior of this electrodeposition system was gradually dominated by the diffusion process rather than

**Figure 8.** Contour lines that represent the CIGS film morphology, mass transport, and current density distribution in

Mechanical Perturbations at the Working Electrode to Materials Synthesis by Electrodeposition

http://dx.doi.org/10.5772/intechopen.78544

109

**5. CIGS thin film by electrodeposition plus periodical mechanical** 

tion to the working electrode was applied every 0.066 C/cm<sup>2</sup>

The strategy of applying mechanical perturbations to the WE during the electrodeposition process was the result of analyzing the morphology of CIGS film produced by electrodeposition using DC potential at the WE; details of film preparation can be found in [13]. As it was shown with the cross-section micrographs, during the initial stage of CIGS film growth, a more compact CIGS layer is produced. This is evident in the as-electrodeposited film and in the annealed film. In order to promote this formation, a mechanical perturba-

processes. With the mechanical perturbation, the solution near the WE, producing perhaps turbulent flow, the diffusion layer tends to disappear for a moment and a new nucleation and growth center was originated, and if the perturbation is periodical, the film will be

during the electrodeposition

charge transfer process.

three different stages of the electrodeposition process [13].

**perturbations**

more compact.

**Figure 7.** Growth model of CIGS films, obtained by the one-step electrodeposition: (a) effect of the WE roughness on the distribution of the electric field and (b) effect of the distribution of the electric field on the film morphology.

with a cauliflower-like growth. It is evident after the transitory stage in the electrodeposition current density, and this is more noticeable when the electrodeposition time increases, a

The contour lines are used in topographic maps to represent points of the same elevation. Here, we use it to represent the CIGS film morphology as shown in **Figure 8**. The contour lines were traced according to the film morphology shown in **Figures 5** and **6**. The contour lines represent the CIGS film morphology of the same thickness. According to the micrographs, the CIGS films morphology is not uniform through the WE surface, being a function of the current density distribution during the electrodeposition process. Thus, the contour lines also represent the mass transport mechanism during the electrodeposition process. **Figure 8** shows the contour lines that represent the electrodeposition process evolution. The highest mass transport density and the nodule formation are represented in zones with a dark color while less mass transport density and the boundary layer formation between nodules are represented with a light gray color. When the electrodeposition process begins, a large number of nodules grow randomly distributed. The nodules distributions, which can be appreciated after the current transitory stage, are represented in **Figure 8(a)**. When the first nodule has a cauliflower-like growth, the mass transport mechanism is concentrated in them; however, not all nodules grow at the same rate and eventually some stop growing. When the deposition time elapses, the nodules become more and more isolated and the boundary between them grows, because the mass transport mechanism is concentrated at very isolated points; this is represented as contour lines in **Figure 8(b, c)**. The above causes the CIGS film morphology to appear as isolated nodules with a high roughness, and this is evident when the electrodeposition time increases. In previous studies, it has been shown that in the CIGS film synthesized by electrodeposition, the composition is related to the current density; if

**Figure 7.** Growth model of CIGS films, obtained by the one-step electrodeposition: (a) effect of the WE roughness on the

distribution of the electric field and (b) effect of the distribution of the electric field on the film morphology.

representation of isolated nodules is shown in **Figure 7(b)**.

108 Perturbation Methods with Applications in Science and Engineering

**Figure 8.** Contour lines that represent the CIGS film morphology, mass transport, and current density distribution in three different stages of the electrodeposition process [13].

the current density is high, the CIGS film composition is copper-poor. On the other hand, if the current density is low, the CIGS film composition is copper-rich [31]. Therefore, the CIGS film has a composition that varies according to the contour line; the zones with a dark color are cooper-poor while the zones with a light gray color are copper-rich. The above is evident in the surface morphology of CIGS films that were subjected to an annealing process as shown in **Figure 6(c, d)**. At the initial stage of the electrodeposition process, the ratio of ion reduction in the WE surface is much greater than the speed at which the ionic species arrive at the load transfer zone; for this reason, the diffusion layer increases up to maintain an electric load balance. In this way, the zone of the depletion varies according to the contour plot. At this microscopic level, the diffusion layer is not uniform along the working electrode. It acquired this principal shape after the transient of the electrodeposition current density. At the macroscopic level, experimental results [37] revealed that the electrochemical kinetic behavior of CIGS thin films is strongly influenced by the electrical double layer existing between the substrate. With an increase in the electrodeposition time, the kinetic behavior of this electrodeposition system was gradually dominated by the diffusion process rather than charge transfer process.
