*Thin Films Processed by SILAR Method DOI: http://dx.doi.org/10.5772/intechopen.106476*

**Table 6.**

*SILAR growth films demonstrating the properties of photoelectrochemical water splitting at 1.23 V vs. RHE.*

of highly pristine poly-pyrrole flexible electrodes, it was shown that among 0.025 M, 0.05 M, and 0.1 M pyrrole, the 0.1 M pyrrole exhibited excellent performance with specific capacitance as high as 899.14 Fg<sup>1</sup> at 5 mVs<sup>1</sup> in 0.2 M Na2SO4 showing retention stability of 61.5% even after 2000 cycles [107]. The SILAR synthesis, in such case, was performed on the stainless steel strips, which were firstly immersed in pyrrole precursor, followed by 30% H2O2 for 10 s each [107]. Further, studies on the use of the optimum precursor concentration of different Mn dopant (0.04 M, 0.075 M and 0.1 M) in CdS QDSSCs reveal that 0.075 M Mn-doped CdS can strongly enhance the incident photon to charge carrier efficiency (IPCE), due to the improved light harvesting, electron injection as well as charge collection efficiencies. As a result, the PCE of SILAR-grown Mn/CdS QDSC is up to 3.29%, which is much higher than that of QDSC without doping (2.01%) as well as other used concentration of Mn dopant under standard simulated AM 1.5 G, 100 mW cm<sup>2</sup> [157].

#### **5.2 Effect of precursors**

ZnO is one of the most investigated materials performed by SILAR technique. During fabrication, among the other properties precursor selection is one of the key requirements. In this study, the role of the precursor materials such as Zn(CH₃COO)₂, ZnSO₄, and ZnCl2 on the properties of SILAR-deposited ZnO films were examined and the outcomes showed that the films fabricated by utilizing Zn(CH₃COO)₂ and ZnSO₄ precursors exhibited better optical properties than ZnCl2. Besides, the crystallite sizes of all the fabricated samples were increased upon annealing [158]. On the other hand, the effect of four different precursors of Zn(NO3)2, Zn(CH3COO)2, ZnSO4, and ZnCl2 on structural, morphological, electrical and optical properties of AZO thin films using SILAR method was examined. After varying the different precursors, the significant effects on film crystallization, surface morphology, optical nature, and electrical resistivity of the deposited films were studied, in which chloride precursor demonstrated the best performance [159].

Sfaelou and co-workers studied the effect of the nature of three cadmium precursors such as Cd(NO3)2, CdSO4, and Cd(Ac)2 on the effectiveness of CdS SILAR deposition and measures the performance of sensitized solar cells and photo fuels. The CdS reflection spectra, load, and the size of CdS nanoparticles varied a lot from one precursor to the other as shown in **Figure 7**(a-e). The highest load and the largest nanoparticles were obtained in the case of Cd(Ac)2, and the smallest in the case of Cd (NO3)2. And acetate-derived photoanodes provide more effective outcomes in the case of QDSSCs, while nitrate-derived precursors were more effective in the case of photo fuel cells as in **Figure 7**(f, g) [11]. In a similar but detailed study, Zhou and coworker showed almost similar results showing a better performance of Cd(Ac)2 over Cd(NO3)2 during the study of another CdS QDSCs as shown in **Figure 7**(h) [119]. Another recent investigation on the effect of different precursors such as Mn (CH₃CO₂)₂, MnCl2, and MnSO4 on electrochemical properties of Mn3O4 thin films prepared by SILAR method using 1 M Na2SO4 aqueous electrolyte exhibited the specific capacitance of 222, 375, and 248 Fg<sup>1</sup> , respectively, at 5 mVs<sup>1</sup> scan rate. Hence, the MnCl2-derived Mn3O4 electrode showed a good electrochemical with maximum energy density of 17 Whkg<sup>1</sup> and power density of 999 Wkg<sup>1</sup> at 0.5 mAcm<sup>2</sup> current density showing retention stability of 94% after 4500 CV cycles [160]. Besides, a study of SILAR-deposited SnO2 films showed improvement of the crystallite with solution molarity performed by using different precursor concentration of both cations and anions [161].

#### **Figure 7.**

*TEM and HRTEM photographs of (a) pure titania and titania loaded with CdS deposited by using the three precursors: (b) Cd(NO3)2, (c) CdSO4, and (d) Cd(Ac)2. (e) Reflection spectra using Cd(NO3)2, CdSO4, Cd (Ac)2, and titania film without CdS. J V curves recorded with a (f) QDSSC and (g) photo fuel cell employing 1 cm<sup>2</sup> active photoanode and 2.25 cm2 active cathode electrode [11]. (h) J V characteristics of CdS QDSCs fabricated by using acetate and nitrate precursors and measured under the illumination of one sun (AM 1.5, 100 mW/cm<sup>2</sup> ) [119].*

#### **5.3 Number of deposition cycles**

SILAR technique involves the successive immersion of the substrate in anionic and cationic precursors following the substrate rinsing procedures in between. The deposition rate and the thickness of the required films can be simply controlled over a wide range by varying the deposition cycle and there are no boundaries on the substrate material, dimensions, or surface profile to be used, which in turn influence the properties such as crystallite size, surface morphology, and possibly light absorption. Nevertheless, overloading may consequence in delamination and fragmentation of the films owing to undesirable mechanical stress. The number of cycles optimization is therefore requisite to all SILAR system for the anticipated utilization.

Recently, lily flower-like ZnO structures were demonstrated by a group of researchers deposited by SILAR method [162]. In the study, lily flower-like morphologies were obtained when the deposition cycle number increases from 1 to 10 as shown in **Figure 8**(a-d). Another group, while studying the growth of porous Fe2V4O13 films for photoelectrochemical water oxidation, the deposition cycle had directly altered the current density as shown in **Figure 8**(e). The highest photocurrent was achieved at the potential of 1.23 V *vs.* RHE for a Fe2V4O13 film attained through 20 deposition cycles, which was chosen to improve the performance of the material further [38]. Further, Das and coauthors studied the influence of dipping cycle on SILAR-synthesized NiO thin film and observed that 40 cycle dipping NiO electrode provides highest specific energy of 64.38 WhKg<sup>1</sup> with the highest specific power 2305 WKg<sup>1</sup> , by retaining fast electron transfer as well as admission of electrolyte ions much easily due to porous nanostructure of fabricated electrode [78]. Moreover, other efforts including the effect of immersion cycles on structural, morphology, and optoelectronic properties such as Ag2S [163], CdO [164], ZnS [165] thin films were studied extensively, which make them desirable for optical coating as well as other optoelectronic applications.

#### **Figure 8.**

*(a-d) FE-SEM photographs of ZnO lily flower-like structures deposited by varying number of deposition cycles via SILAR method [162]. (e) Chopped LSVs of Fe2V4O13 films with different number of cycles annealed at 500°C for 1 h in a buffer solution of pH 9.2 [38].*

#### **5.4 Impact of pH**

By altering the pH of both cationic and anionic precursors, it is possible to tune the bandgap of thin films over a wide range for optoelectronic device applications. Preetha and co-workers investigated the effect of cationic precursor pH on optical as well as transport properties of SILAR-fabricated nanocrystalline PbS thin films. They successfully showed that the pH of the cationic precursor and in turn the size of the crystallites affect the optical and electrical properties of PbS thin films [166]. Besides, Sakthivelu and coauthors demonstrated a similar effect on ZnO thin films that the grain size of ZnO increased with the increase in pH of the precursor solution as represented in the SEM micrograms in **Figure 9**(a-e). The film deposited at pH =8.5 shows aggregated and non-uniform grains, while flower-like appearance appeared at pH =9. Later, at pH =9.5, bigger grains with hexagonal nanorods structure appeared and finally, at pH =10, the size of the nanorods increased further with the well

#### **Figure 9.**

*SEM micrograms of ZnO thin films prepared at (a) pH = 8.5, (b) 9.0, (c) 9.5, (d) 10.0, and (e) 10.5 [167].*

elongated nanorods sticking with each other. They also showed a decrease of bandgap from 3.29 to 3.09 eV with the increase of pH [167].

Moreover, both acidic and basic mediums can be used based on the requirement to deposit films as listed on **Table 7**. Farhad and co-author worked on Cu2O thin film under pH range from 2 to 8 [169, 170], while Gençyılmaz [171] and Visalakshi et al. [172] on CuO thin film under pH range from 9 to 12 by showing promising electrical and optical properties. In another research, CdO was deposited using Cd(CH3COO)2 as a cationic precursor and thickness as well as bandgap tuning was effectively observed in the pH range of 11.3 to 12.5, in addition to NH3 solution [173]. However, not only a wide range of study to understand the influence of pH on the thin films fabricated by SILAR, but also more device fabrications were performed under diverse pH conditions with excellent performance, as described in the application section.

#### **5.5 Annealing**

SILAR is currently in demand to maintain the high quality of films with high growth rate. Despite extensive efforts, the adsorption of complex agents from precursors, drop of rinsing time, and slow production rate for almost all kinds of films has still been a main disadvantage, which restricts its usage in semiconductor industry. New deposited films may have defects, for instance, oxide vacancies and hydroxide phase since the presence of hydroxide phase is unavoidable owing to an aqueous alkaline medium for fabrication [174]. However, annealing of films can minimize such defects and eliminates the hydroxide phase along with the recrystallization. As hightemperature annealing mostly induces a rise in crystallite size, and possibly alters in morphology, which is steady with thermally induced grain growth. Still, the bandgap does not allow a steady trend with annealing due to numerous factors coming into ground outlining the absorption onset: size of particles, existence of defects, stoichiometry as well as existence of oxygen vacancies, etc. [175].

Putri and co-authors studied annealing temperature effect on the photovoltaic performance of BiOI-based materials and showed that at 300°C temperature, the role of the device which consisted of Bi7O9I3 attained three times higher efficiency than the annealed parent BiOI at 100°C. Hence, the structural tuning due to the addition of oxygen *via* annealing to BiOI structure had an influence on the photoelectrochemical cell [69]. Besides, Ashith and co-worker studied the effect of post-deposition annealing on the properties of ZnO films and demonstrated that the crystallite size of the films increased significantly after annealing. The annealed films further showed very high absorption in the UV region with marginal modification in bandgap. Both the crystallite size and optical absorbance were observed to rise proportionately with the annealing temperature [176]. In a separate study of annealing and light effect on structural, optical and electrical properties of CuS, and Cu0.6Zn0.4S thin films grown by the SILAR demonstrated that the current increase with increasing light intensity and increasing rate in illuminated 500 Wcm<sup>2</sup> films were greater than the others that have annealed at 400°C [43]. Further studies are reported showing grain size increase after annealing or bandgap tuning are listed in the **Table 8**, including Cu2O, ZnO, CuO, CdO, MgO, NiO etc.

#### **5.6 Impact of doping**

In order to have a maximum number of carriers to take part in the functioning, a material with low activation energy is necessary so that electrons can easily jump from


**Table 7.** *Properties of thin films deposited by varying solution pH* via *SILAR method [168].*

*Thin Films - Deposition Methods and Applications*

**64**


#### *Thin Films Processed by SILAR Method DOI: http://dx.doi.org/10.5772/intechopen.106476*


**Table 8.**

*A*

 *list of thin films grown* via *SILAR and annealed further for better film quality.*


### *Thin Films Processed by SILAR Method DOI: http://dx.doi.org/10.5772/intechopen.106476*


*Thin Films - Deposition Methods and Applications*


**Table 9.**

*A*

 *list of the doped films with some of their properties, fabricated by using SILAR techniques.*

*Thin Films Processed by SILAR Method DOI: http://dx.doi.org/10.5772/intechopen.106476* valence band to conduction band and doping is one of the best options in such structure tuning [181, 182]. In many cases, the incorporation of cation doping is an effective way to improve the electrical conductivity [183, 184]. By decreasing the bandgap, electron transfer between the valence and conduction bands will increase, and thus in case of energy storage device, electrode capacitance will increase [185]. For example, Y-doping in Sr.(OH)2 improves both electronic conductivity as well as electrochemical performance of the electrode for energy storage device [89].

Again, electrochemical performance of In3+-doped WO3/[Cu7Te4/Bi2Te3] electrodes for similar applications enhanced the capacitance to a great extent [90]. The study showed the specific capacity of undoped WO3 was around 64 mAhg<sup>1</sup> whereas it was increased to 90.2 mAhg<sup>1</sup> for the same scan rate of 10 mVs<sup>1</sup> for In3+-doped films, with the high-power density of 1.7 kWkg<sup>1</sup> at the highest energy density of 18.85 Whkg<sup>1</sup> . Inside the WO3 lattice, the doped In3+ cation diffused and was connected in the insertion-removal exchange method of electrons, with further electrochemical S2 insertion or extraction striking at the Cu7Te4/Bi2Te3 and polysulfide electrolyte surfaces.

Besides, Zhu and coauthors prepared Cu-doped CdS on QDSCs and investigated the effect of Cu doping on several cells based on the doping concentration. When the doping ratio of Cu decreased successively from 1:10, all the parameters such as Jsc, Voc, and PEC increased and reached at the maximum value when the ratio became 1: 500 [186]. In a separate report of Mn-doped-CdS/CdSe deposited on mesoscopic TiO2 film as photoanode using Cu2S/graphene oxide composite electrode, in the presence of sulfide/polysulfide electrolyte provide PEC of 5.4%, higher than undoped sample [118], whereas Hg2+-doped PbS QDSC having unprecedentedly high photocurrent delivered PEC of 5.6% at one sun illumination over the undoped PbS QD cell [120]. Moreover, Abel and co-workers developed an improved photoelectrochemical water splitting device *via* SILAR-fabricated Ti-doped α-Fe2O3 thin films. Ti, acting as water oxidation intermediates, enhanced interfacial hole transfer efficiency from less than 3–80% by increasing the concentration of surface-trapped holes, which is then triggered by FeOOH to amplify hole transfer efficiency to 100%. Both Ti doping and FeOOH overlayer resulted in photocurrents of 0.85 mAcm<sup>2</sup> at 1.23 V vs. RHE [138]. However, a lot of work has been done by several authors to get better film quality through doping. A list of such initiatives of the doped films with the starting materials of growth and other properties is shown in **Table 9**.

### **6. Conclusion**

This chapter represented detailed discussions on the methods and techniques of the fabrication process of thin films by utilizing SILAR for the optoelectronic device applications. Among the diverse fabrication techniques both physically and chemically, SILAR is the simplest to fabricate thin films having remarkable quality. It is widely fit for the fabrication of thin films of metal chalcogenides, hydroxides, peroxides, as well as complex and composite nanostructures with innovative functionalities. The role of experimental conditions on the structural, optical, and electrical properties of the thin films as well as device performances is reviewed in this chapter mainly for the advanced utilization of both the generation and storage of energy such as solar cells, photoelectrochemical water splitting, supercapacitors, and so on. The technological advancement of a fabrication technique is deeply reliant on the opportunity of controlling the experimental factors involved. In this chapter, a brief advantage of SILAR technique is highlighted, including flexibility of the film growth, thickness control, composition control, and low

*Thin Films Processed by SILAR Method DOI: http://dx.doi.org/10.5772/intechopen.106476*

temperature management, along with a broad range of applications. From this point of view, a deep knowledge of the connections between processing, structure, specific characteristics, and performances is the foundation for accurate and rational engineering of such optoelectronic devices. Moreover, a comprehensive profile of recent status is required to focus on further prospects. This work will therefore deliver a strong contribution to move ahead with future research goals on SILAR technique, by utilizing lowcost deposition of high-quality thin films and associated optoelectronic devices.
