Thin Films Processed by SILAR Method

Abstract

SILAR is one of the simplest techniques in terms of the better flexibility of the substrate choice, capability of large-area fabrication, deposition of stable and adherent film, low processing temperature for the film fabrication as well as reproducibility. This technique is very budget friendly since it does not require any sophisticated equipment. Moreover, various fabrication parameters such as solution concentration, precursors, the number of cycles during immersion, pH, annealing, doping, and growth temperature affect the rate of fabrication as well as the structural, optical, and electrical properties of the fabricated thin films led the technique unique to study in an extensive manner. A chapter regarding different aspects of semiconductors-based optoelectronics by SILAR has yet to be published. This chapter will concern the recent progress that has recently been made in different aspects of materials processed by the SILAR. It will describe the theory, mechanism, and factors affecting SILAR deposition as well as recent advancements in the field. Finally, conclusions and perspectives concerning the use of materials in optoelectronic devices will be represented.

1. Introduction

The successive ionic layer adsorption and reaction (SILAR) technique was first introduced in 1985 by Nicolau for the deposition of ZnS and CdS [1], and Ristov et al. for the deposition of Cu₂O thin films [2]. Nicolau applied the adsorption technique of film preparation, relating two sources—a metal source of aqueous solutions of CdSO₄ or ZnSO₄, and a sulfide source of Na₂S, maintaining ambient temperature. The substrate was dipped in each of the solutions following consecutive cycles, applying rinsing steps in between to eliminate excessive precursors. In the case of Ristov et al., the method was employed using clean glass substrates, which were sequentially immersed into a solution of NaOH and a Cu-complex such as [Cu(S₂O₃)]⁻ to prepare Cu₂O thin films. The complex [Cu(S₂O₃)]⁻ was well-maintained at ambient temperature, while the NaOH solution was kept at a temperature range from 60 to 80°C. The outstanding structural, electronic, and optical properties of the thin films produced in the two investigations stimulated the persistence of the practices. Hence, the technique was functional to the fabrication of a large variety of thin film semiconductors till to date [3, 4, 5]. Consequently, SILAR appeals huge scientific attention and has been well known as a facile technique for the fabrication of thin films of metal oxides, sulfides, selenides, peroxides, and hydroxides, as well as more complex hetero-structured thin films [6, 7, 8, 9, 10, 11]. Compared with the other popular deposition techniques, SILAR is unique due to multiple reasons [3, 12, 13, 14] as mentioned below:

1. The thin film is generally fabricated on a simply cleaned and rigid planar substrate having no dimensional limits. There is no restriction on the surface of the coating materials, so films can be fabricated on temperature-sensitive substrates like plastics. Even oxidation or corrosion of metal backing substrates can be used to deposit films by choosing suitable precursors.

2. The deposition rates and film thickness can be well controlled by monitoring reactant precursors, which are generally the desired cationic and anionic salts dissolved in solvents, while the anticipated stoichiometry can be attained through changing their type, concentration, or other involved dipping parameters.

3. By controlling the number of deposition cycles and concentration of species, the thicknesses of the thin films can be simply tuned over a wide range such as from nm scale to μm.

4. SILAR fabrications are very convenient and energy efficient as the technique is mostly controlled at room or low temperature. If required, the as-deposited coating materials can be annealed post-deposition to activate grain growth, crystallization, etc.

5. Besides, the fabricated thin films can be reformed to show preferential crystallographic orientation as well as grain assembly due to the controllability of ionic reactions performed at the substrate solution interface.

6. The SILAR method supports film development only on the surface of the substrate that is immersed into the solution, hence diminishing unnecessary consumption of the used reactants. If required, precursor solutions could also be reloaded and reprocessed.

Therefore, SILAR is a vastly multipurpose and influential process for the fabrication of numerous thin film materials having huge technological attention and, hence, unlocked a wide window in optoelectronic device applications.

2. Theory and process mechanism

SILAR is widely used, simple technique to fabricate high-quality thin films [3, 15]. During deposition, successive ionic layer adsorption and reaction of the ions take place at the solid-solution interface of the substrate. Thus, the thin film of the compound, A_pB_y is deposited on to the substrate surface by dint of the adsorbed cations, xA^y+ and anions, qB^p− due to the following heterogeneous chemical reactions:

$${\mathit{A}}_{\mathit{x}}{\mathrm{Q}}_{\mathit{y}}\left(\mathrm{s}\right)\to \mathit{x}{\mathrm{A}}^{\mathit{y}+}\left(\mathrm{aq}\right)+\mathit{y}{\mathrm{Q}}^{\mathit{x}-}\left(\mathrm{aq}\right)$$E1

$${\mathrm{P}}_{\mathrm{p}}{\mathrm{B}}_{\mathrm{q}}\left(\mathrm{s}\right)\to \mathrm{p}{\mathrm{P}}^{\mathrm{q}+}\left(\mathrm{aq}\right)+\mathrm{q}{\mathrm{B}}^{\mathit{p}-}\left(\mathrm{aq}\right)$$E2

$${\mathrm{A}}^{\mathit{y}+}\left(\mathrm{aq}\right)+{\mathrm{B}}^{\mathit{p}-}\left(\mathrm{aq}\right)\to {\mathrm{A}}_{\mathit{p}}{\mathrm{B}}_{\mathit{y}}\left(\mathrm{s}\right)\downarrow$$E3

where x, y, p, q and y⁺, q⁺, x⁻, p⁻ are the number and charges of the corresponding ions A (metal ions), P (cationic precursor), Q (anionic precursor), and B (anions) respectively [2, 16]. Sometimes, the ligands L_n are a necessity to complete the reaction [17, 18, 19, 20]. The solution having the first element containing the final target material can be thought as the compound A_xQ_y fully dissociated in the chosen solvent such as in water (Reaction 3). Usually, A_xQ_y is a metal salt where A^y+ represents cations such as Zn²⁺, Cu²⁺, Mn²⁺, Cd²⁺, Bi³⁺, and B^p− represents anions such as NO³⁻, Cl⁻, SO₄²⁻.

Hence, a basic SILAR cycle comprises four different steps, correlating alternate immersion of the substrate into cationic and anionic precursor solution followed by rinsing in every immersion cycle to eliminate loosely adhered particles as shown in Figure 1 and described below:

Figure 1.

Representation of different steps during a SILAR cycle.

2.1 Adsorption

First step of the SILAR process is the formation of the Helmholtz double layer, which is due to the initial adsorption of cationic precursor, xA^y+, on the surface of the substrate. This layer is generally composed of two charged layers, the positively charged inner layer and negatively charged outer layers. The positive (+ve) layer consists of the cations, xA^y+, while the negative (−ve) layer, yQ^x−, is the counter ions of the cations.

2.2 Rinsing I

In the second step, excessive adsorbed ions, xA^y+ and yQ^x−, are rinsed away from the diffusion layer toward the bulk solution and a hypothetical monolayer is formed. This results in a saturated electrical double layer showing an ideal scenario of the process.

2.3 Reaction

In the reaction stage, the anions, qB^p−, from anionic precursor solution are introduced into the system. A solid substance, A_pB_y, is formed on the interface due to the low stability of the material. This process employs the reaction of xA^{y +} surface species with the anionic precursor, qB^p−.

2.4 Rinsing II

In the final step of a SILAR cycle, the excess and unreacted species (yQ^x−, pP^q+) and the reaction by product from the diffusion layer are removed leaving expected films.

A schematic presentation of a single cycle for the fabrication of Cu₂SnS₃ film is shown in Figure 2 [21]. In the case of Cu₂SnS₃ film fabrication, ion-by-ion type of deposition takes place through nucleation spots of the adsorbed surfaces [22]. Nucleation occurs due to the surface condensation of the ions and outcomes of, that is, an dense adherent thin film [23]. The substrate was firstly dipped into the cationic precursor containing mixed CuCl₂ and SnCl₂ solutions, where Cu²⁺ and Sn²⁺ species were available. Sn²⁺ ion in solution is good reducing agents, and thus, Cu²⁺ reduces to Cu⁺ and Sn²⁺ is oxidized to Sn⁴⁺ in cationic solution as shown by the following reaction:

Figure 2.

Schematic representation of Cu₂SnS₃ thin film fabrication by SILAR technique [21].

$${\text{2CuCl}}_2+{\text{SnCl}}_2+{\text{6H}}_2\mathrm{O}={\text{2Cu}}^{+}+{\mathrm{Sn}}^{4+}+\text{6HCl}+{\text{6OH}}^{-}$$E4

The substrate was then rinsed off with DI H₂O to eliminate the loosely bounded reactants. Then, it was dipped into an anionic precursor containing Na₂S.xH₂O solution, which gave sulfide ions (S²⁻) to react with the cations Cu⁺ and Sn⁴⁺. Finally, the reaction occurred between the pre-adsorbed Cu⁺, Sn⁴⁺ cations, and the S²⁻anion to form a solid Cu₂SnS₃ thin film as,

$${\mathrm{Na}}_2\mathrm{S}+{\mathrm{H}}_2\mathrm{O}\to {\text{2Na}}^{+}+{\mathrm{HS}}^{-}+{\mathrm{OH}}^{-}$$E5

$${\mathrm{HS}}^{-}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}_3{\mathrm{O}}^{+}+{\mathrm{S}}^{2-}$$E6

$${\text{2Cu}}^{+}+{\mathrm{Sn}}^{4+}+{\text{3S}}^{2-}\to {\mathrm{Cu}}_2{\mathrm{SnS}}_3$$E7

In the last step of the process, the substrate was again dipped into the DI H₂O to remove the unwanted excessive particles to provide a uniform surface containing Cu₂SnS₃ thin film.

3. SILAR-facilitated material deposition: A summary

The deposition of the series of chalcogenide mainly metal oxides, sulfides, selenides, and tellurides films has been always on numerous attentions in the advancement of the SILAR since its launch. Currently, SILAR has become a broadly functional technique in the deposition of a huge variety of semiconductor thin films. For the simplicity of discussion, we have summarized most of the metals still synthesized as metal compounds by SILAR in Table 1.

Oxides	Sulfides	Selenides	Tellurides
Mg, Ti, V, Mn, Fe, Ni, Cu, Zn, Mo, Ag, Cd, W, Bi, Sn	Cu, Zn, Cd, Ni, Sn, Pd, Mo, Ag, As, Bi, Mn, Sb, Fe, La	Cu, Sb, Cd, Bi	Cu, Cd, La

Table 1.

List of the metals still grown by SILAR technique.

A list of materials deposited using SILAR technique with their growth conditions with the required raw materials for the growth is summarized in Table 2. For the simplicity, the discussion is divided into four parts as of Table 1, as specified below:

Material	Precursors		pH; (C, A)	Temperature, ^0C		Cycles	Rinsing	Dipping time, s (C, A)	Refs.
	Cationic	Anionic		Growth	Anneal
				OXIDES
Cu2O	Cu(S2O3)^−	NaOH	—	70	—	10, 15, 20, 25, 30	No	5, 5	[24]
CuO	CuSO4	NH4OH	C:10	80	—	10, 20, 30, 40	H2O	10, 15	[25]
ZnO	ZnSO4	NH4OH	—	100	200	100	No	2, 2	[26]
TiO2	TiCl3	NaOH	C: 3		400	100, 150	H2O	20, 10	[27]
CdO	CdCl3	NH4OH	C:12	90	400	60, 70, 80, 90, 100, 110	H2O	10, 20	[28]
FexOy	FeCl3	NaOH	—	—	400	120	H2O	10, 10	[29]
Mn3O4	Mn2SO4	NaOH	—	60		50	H2O	10, 10	[30]
NiO	Ni (NH3)4	H2O	—	90	200, 300, 400	100	H2O	30, 7	[31]
WO3	H2O4W	NaOH	—	80	400	25, 50, 75, 100, 125	H2O	20, 10	[32]
Bi2O3	Bi(NO3)2+ N(CH₂CH₃)₃	H2O	—	70	—	80	H2O	10, 10	[33]
MgO	Mg(NO3)2	NH4OH	A:11.5	90	400	—	No	30, 30	[34]
Cu2V2O7	0.025 M Cu(NO3)2	0.025 M NH4VO3	—	75	—	5, 10, 20, 30, 60, 70, 80, 100	No	20, 20	[35]
Ag3VO4	AgNO3	Na3VO4	—	70	100 ∼ 350	5, 10, 15, 20	H2O	20, 20	[36]
BiVO4	Bi(NO3)3	NH4VO3	—	75	450	5, 10, 20, 30	H2O	20, 20	[37]
Fe2V4O13	Fe(NO3)3	NH4VO3	—	75	400, 500	20	H2O	20, 20	[38]
ZnFe2O4	ZnCl2, FeCl2	Aq. NH3	—	—	550	—	H2O	30, 30	[39]
BiOI	Bi(NO3)3	KI	—	—	—	5, 15, 30, 45	H2O	10, 10	[40]
NiMoO4	NiCl2 + NH4OH	Na2MoO4	C:12.2	70	300	30	H2O	20, 20	[41]
FePO4	FeCl3	NaH2PO4	—	—	—	25	H2O	10, 10	[42]
				SULFIDES
CuS	CuCl2	Na2S	C: 3, A:12	RT	200, 300, 400	40	H2O	30, 30	[43]
Ag2S/Ag2S3	AgNO3	Na2S.xH2O	—	55	—	12	CH3OH	30, 45	[44]
Ag2S	AgNO3	SC(NH2)2	C:8, A:6	—	—	30	DD, H2O	12, 10	[45]
ZnS	ZnCl2	Na2S	C:5, A:12	RT	200, 300, 400	40	H2O	20, 20	[43]
CdS	Cd(NO3)2	Na2S		—	—	9,12,15,18	H2O	60, 60	[46]
NiS	NiSO4	Na2S	C:8, A:10	RT	—	150	H2O	60, 60	[47]
SnS	SnCl2	Na2S	C:1.8, A:9.5	7	—	—	H2O	20, 20	[48]
PbS	Pb(NO3)2	Na2S	—	10, 30, 50	—	—	CH3OH	60, 60	[49]
MoS2	(NH4)6Mo7O2	Na2S	C: 3, A:13.5	27	—	100	H2O	25, 25	[50]
As2S3	As2O3 + EDTA	Na2S2O3	C: 1, A: 6	80	—	10, 15, 20, 25, 30, 35, 40, 45, 50	DI H2O	15, 15	[51]
Bi2S3	Bi(NO3)3	Na2S	—	75	200	30	2-CH3OC2H5 & CH3OH	15, 15	[52]
MnS	MnCl2	Na2S	C:5.5, A:12	RT	—	60, 80, 100, 120, 140	H2O	30, 30	[53]
Sb2S3	SbCl3	Na2S2O3	—	50	—	25, 30, 33	H2O	20, 20	[54]
FeS	FeSO4	Na2S	—	27	—	40	H2O		[55]
FexSx	Fe(NO3)2	Na2S	—	27	200	20	H2O	30, 30	[56]
CoS	CoSO4	Na2S	C:8–10 A: 12	27	—	25	H2O	50, 50	[57]
La2S3	La2O3	Na2S2O3	2 ± 0.1	RT	300	90	H2O	30, 30	[58]
				SELENIDES
Cu3Se2	CuSO4	Na2SeSO3	C:2	RT	300	60	H2O	20, 20	[59]
Sb2Se3	SbCl3	Se^2−		RT	80	10	C2H5OH		[60]
CdSe	CdCl2. H2O	Na2SeSO3	C: 8, A:11.3	RT		30, 40, 50 60	H2O	60, 60	[61]
Bi2Se3	Bi(NO3)3 + N(CH2CH2OH)3	Na2SeSO3	C: 8, A: 8.5	27	—	150	H2O	30, 30	[62]
Sb2Se3	K(SbO)C4H4O6	Na2SeSO3	C:3, A:8.5	27	—	350	H2O	40, 40	[62]
				TELLURIDES
CdTe	Cd(CH3CO2)2	Na2TeO3	C: 5, A: 11	27	—	910	H2O	30, 30	[63]
Cu2Te	CuSO4	Na2TeO3	C:5, A: 9	27	—	60	H2O	20, 20	[64]
La2Te3	LaCl3+ CH3COOH	Te pow. + HCl + HNO3	C:2, A:1	RT	180	—	H2O	20, 20	[65]

Table 2.

A list of materials deposited using SILAR technique with their growth conditions. (C, a: Cationic, anionic).

3.1 Metal oxides

An increasing number of oxide materials deposited by SILAR have demonstrated high chemical, thermal, and expected stability that is one of the reasons to increase the popularity of oxide synthesis by SILAR. However, the technique of oxide synthesis is somehow difficult compared to sulfides, selenides, and tellurides due to the unavailability of the anionic precursors, which is the direct source of O²⁻ to form oxides. For example, in case of the synthesis of most of the binary metal oxides, H₂O, NaOH, and NH₄OH are used as anionic precursors with a mild thermal treatment of around (70 ∼ 90) ⁰C to activate the precipitation of hydroxides. On the other hand, the most common cationic precursors are mainly of metal thiosulfates, sulfates, chlorides, nitrates, etc., to provide metal ion adsorption on the substrate surface. Until today, Cu_xO, ZnO, TiO₂, and CdO are the most examined materials by SILAR. The investigation of Mn₃O₄, NiO, and Bi₂O₃ is also increasing [31, 32, 33, 34]. Recently, both nanostructured Fe₂O₃ and Fe₃O₄ have been fabricated applying sulfate and chloride salts using NaOH as the anionic precursor via SILAR [30, 66]. But research on WO₃ [32], MgO [34], and SnO [67] fabrication is still rare. In case of ternary metal oxides, the SILAR deposition has been widely increased due to their ability to the additional modulate characteristics by controlling the composition of the materials. The synthesis of CST is discussed in the theory and mechanism section, which can be again done by two ways—a combined solution of both the deposited metal cations or, an alternating (one by one) fabrication of the two cations. A good technique to produce ternary metal oxides with excessive control on stoichiometry is to react one of the two metal ions by its own oxyanion. For example, Bi(NO₃)₃ and NH₄VO₃ react to fabricate BiVO₄ [37], as ammonium vanadates are extremely soluble in water, while the anticipated metal vanadates are not. Consequently, they precipitate out of solution as the expected phase on the substrate surface. The other oxides such as Ag₃VO₄ [36], BiVO₄ [37, 68], Cu₂V₂O₇ [35], and Fe₂V₄O₁₃ [38] follow similar trends. Though bismuth oxyhalides, for instance, BiOI [69, 70] is not a pure oxide but have been synthesized by the SILAR using cationic precursor of bismuth nitrate and anionic precursor of KI [71].

3.2 Metal sulfides

The characteristic easiness of the procedure and wide-ranging obtainability of the anionic precursors afford metal sulfides the most fabricated materials by employing SILAR technique. The easiest way of sulfide thin film deposition is to use metal salts as cationic precursors and H₂O-soluble sodium sulfides or thiosulfates as anionic precursors. For example, NiS thin films could be fabricated using NiSO₄ (pH: 8) as cationic precursors and Na₂S (pH: 10) as anionic precursors even at room temperature [47]. Generally, the solubility product constants of used sulfide materials in water are higher than 10⁻²⁰, for instance, CuS: ≈10⁻³⁶ and CdS: ≈10⁻²⁷, which is the key force of the deposition of the expected metal sulfides. Among the sulfides, CuS [72], ZnS [43, 73], CdS [46], Ag₂S [44, 45], SnS [48], and PbS [49] have been broadly investigated mostly depending on the usage of chlorides and nitrates as the metal precursors and Na₂S as anionic precursors for the S²⁻ source. Moreover, the investigation on NiS [47], Bi₂S₃ [52] and MoS₂ [13, 50], As₂S₃ [13, 51], MnS [53] are growing fast, while in the case of CoS [57], La₂S₃ [58] studies are still infrequent. Moreover, core@shell-like SnS₂@Co₃S_4, ternary (NiCo₂S₄)_, and quaternary (Cu₂BaSnS₄) films as well as nanocomposites of CdS and Bi₂S₃ have been reported with their potential applications.

3.3 Metal selenides

In most of the cases, SILAR fabrication of the metal selenides has been directed through a solution of chloride, nitrate, or sulfate functioned as cationic precursor consisting of the anticipated metal, and a solution of Na₂SeSO₃ worked as the anionic precursor consisting of the source of Se²⁻ to form selenides. For example, 0.2 M CdCl₂. H₂O (pH: 8) reacts with 0.1 M Na₂SeSO₃ (pH: 11.3) to fabricate CdSe thin film [61]. Based on requirements, sometimes NaHSe, ethanolic NaBH₄, or Na₂Se can also be used as an anionic precursor to fabricate metal selenides. The SILAR deposition of Cu₃Se₂, Sb₂Se₃, Bi₂Se₃, and CdSe were studied and investigated mainly at room temperature avoiding thermal treatment during the sample growth, which was always maintained at (70–90) ⁰C in the traditional cases.

3.4 Metal tellurides

The minimum studied materials among the chalcogen members via SILAR technique are metal tellurides because of the unavailability of the appropriate anionic precursors. Na₂TeO₃ or ethanolic Te or TeO₂ with NaBH₄ is the mostly used anionic precursor performed as the source of Te²⁻ to form tellurides. For example, 0.1 M CuSO₄. 5H₂O (pH: 5) and 0.05 M Na₂TeO₃ (pH: 9) react at an ambient temperature to synthesize Cu₂Te film. Till now, CdTe, Cu₂Te, La₂Te₃, Cu₇Te₄, and Bi₂Te₃ thin films were fabricated and investigated via SILAR technique having potential uses in case of radiation detectors, photovoltaics, and thermo-electric devices [63, 64, 65]. More scientific research is expected to understand and control the characteristics of such fabricated films to build outstanding optoelectronic devices.

4. Recent advances by SILAR

The optoelectronic properties of SILAR grown thin films have been demonstrated in many more applications, for example, supercapacitors, photovoltaics, photoelectrochemical water splitting, gas sensors, and many more. The technique seems to be simpler and represents an efficient way to fabricate devices. Three potential applications such as supercapacitors, photovoltaics, and photoelectrochemical water splitting will be discussed in the following section.

4.1 Supercapacitors

The rapid progress in state-of-the-art tools has guided to a profound reliance on energy storage devices. Satellites, electric vehicles, laptops, cellphones, and sensors need some species of energy storage to function properly. The lead-acid battery was the first device, discovered around the 1800s, and most common storage energy till today. Supercapacitors, another promising energy storage device, well known as electrochemical capacitor or ultracapacitor creates a gap bridging role between conventional capacitors and batteries [74]. They can offer 1 ∼ 2 orders of higher magnitude of power density than rechargeable batteries as well as supply much more energy than traditional dielectric capacitors.

A supercapacitor works following two-charge storage mechanisms: (i) surface ion adsorption such as electric double-layer capacitance (EDLC) and (ii) redox reactions such as pseudo capacitance. Supercapacitors reveal an extraordinary set of features in comparison with batteries, for instance, high-power density, low maintenance cost, reliable cycling life, fast rates of charge or discharge, and safe operation as well as offer versatile powering solutions to many appeals ranging from portable consumer electronic appliance and electric automobiles to large-scale smart utility grids. Nevertheless, carbon-based EDLC supercapacitors show very low energy densities, which are limited through the finite electrical charge separation at the interface of electrolyte and electrode materials, as well as the approachability of surface area. Consequently, efforts to surge the energy densities of supercapacitors have involved the application of better pseudo-capacitance electrode supplies, equipped by conducting polymers and nanostructured metal oxides, bearing the low cost of high-power density as well as chemical stability, which have the significance of phase changes and faradaic reactions in it [74].

Several types of metal oxides, sulfides, and tellurides have been used in supercapacitor device fabrication so far, by utilizing the ever-fast-growing technique SILAR as summarized in Tables 3 and 4. Initially the single metal oxides or sulfides such as CuO, NiO, NiMoO₄, WO₃, Bi₂O₃, Mn₃O₄, or MnS have been prepared by following SILAR technique and then tested for the supercapacitor behaviors to acquire the results of specific capacitance with their retention stability using cyclic voltammetry (CV) with the assistance of 3-electrode measurement system. Higher capacitance was attained at the lower scan rate and/or lower current density during such measurements and usually a relatively small quantity of electrochemical active material was developed atop of the working electrode. Moreover, the performance found using the 3-electrode system is higher than 2-electrode test cells, and the latter can be either a symmetric (S) or asymmetric (A) cell. Generally, in a symmetric cell both positive and negative electrodes are alike, whereas they are different active materials in an asymmetric cell.

Electrode	Scan voltage (V)	Electrolyte	Specific capacitance, Fg^−1	Current density	Capacitance retention		Ref
MnO2	−0.1 to +0.9	1 M Na2SO4	314	5 mVs^−1	90%	10,000	[75]
Mn3O4	−0.2 to +1.2	1 M Na2SO4	786.2	5 mVs^−1	71%	1000	[76]
MnS	−0.15 to +0.55	6 M KOH	828	5 mVs^−1	85%	5000	[77]
NiO	−1.0 to +0.45	1 M KOH	1341	2 mVs^−1	90%	1000	[78]
CuO	−0.3 to +0.5	1 M Na2SO3	476	1 mAcm^−2	89%	1500	[79]
CuO	−0.8 to +0.4	1 M Na2SO4	554	1 Ag^−1	92%	4000	[25]
Bi2O3	−0.8 to +0.3	1 M Na2SO4	329.6	5 mVs^−1	72%	3000	[33]
WO3	−0.7 to +0.4	1 M Na2SO4	266	10 mVs^−1	81%	1000	[32]
Fe2O3	−0.6 to +0.1	1 M NaOH	178	5 mVs^−1	—	—	[24]
Fe3O4	−1.4 to −0.3	1 M KOH	488	1 mAcm^−1	—	—	[29]
NiCo2O4	−0.1 to +0.5	3 M KOH	1936	5 mVs^−1	95%	1000	[80]
NiMoO4	−0.1 to +0.4	2 M KOH	1180	1 Ag^−1	27%	1000	[81]
NiMoO4	−0.6 to +0.6	2 M KOH	1853	1 Ag^−1	65%	2500	[41]
SnO2-RuO2	−0.2 to +0.6	0.5 M H2SO4	1010	1 mAcm^−2	85%	2500	[82]
SnS2@Co3S4	0 to +4.5	—	1580	1 mVs^−1	95%	3000	[83]
NiO/MWCNTs	−0.1 to +0.5	2 M KOH	1727	5 mAcm^−2	91%	2000	[84]
Ni-Co-Ox/C-black	−0.2 to +0.4	1 M KOH	1811	0.5 mAcm^−2	92%	8000	[85]
Bi2S3: PbS/MWCNTs	−0.1 to +0.7	0.5 M Na2SO4	864.2	2 mVs^−1	68%	4000	[86]

Table 3.

Properties of various electrode materials deposited by SILAR for electrochemical capacitors by 3-electrode system.

Electrode	S/A	Scan Voltage	Electrolyte	Power density	Energy density	Capacitance		Ref
Materials		(V)		(WKg^−1)	(WhKg^−1)	retention
MnO2	S	−0.1 to +0.9	1 M Na2SO4	1100	4.2	Stable	10,000	[30]
La2S3	S	−1.2 to 0	1 M Na2SO4	1260	35	78%	1000	[58]
La2Te3	S	−1.2 to 0	1 M LiClO4/PC	7220	60	82%	1000	[65]
ZnCo2O4	S	0 to 1	PVA-KOH	1451	9.7	69%	2000	[87]
ZnFe2O4	S	0 to 1	PVA-LiClO4	277	4.5	66%	1000	[88]
ZnFe2O4	A	0 to 1.6	6 M KOH	7970	28	74%	3000	[39]
Bi2S3: PbS/MWCNTs	S	0.6 to 1.2	0.5 M Na2SO4	830	13.36	68%	4000	[86]
MnS@NF/rGO@NF	A	0 to 1.6	PVA–KOH	1280	34.1	87%	2000	[77]
Y-doped Sr.(OH)2	S	0 to 1.5	PVA-Na2SO4	2270	3.7	61%	1000	[89]
In: WO3/Cu7Te4/Bi2Te3	S	0 to 1	Polysulfide	1700	18.85	—	—	[90]

Table 4.

Supercapacitors performance of SILAR grown films measured by 2-electrode system.

Not only binary, but also ternary or even doped metal oxides, sulfides or tellurides were synthesized via SILAR for supercapacitor device application. For example, ZnCo₂O₄ and ZnFe₂O₄ were synthesized via SILAR technique from binary cationic solutions in the presence of Zn and Co (or Fe) precursors and demonstrate high energy density of 9.67 and 28 Wh kg⁻¹ as well as power density of 1451 and 7970 W kg⁻¹, respectively. La₂S₃ and La₂Te₃ with mesoporous pine-leaf structure prepared with SILAR showed 35 and 60 WhKg⁻¹ energy density and power density of 1260 and 7220 WKg⁻¹, respectively. A flexible La₂Te₃│LiClO₄-PVA│La₂Te₃ supercapacitor cell was further fabricated and is represented as in Figure 3.

Figure 3.

Schematic diagrams of La₂Te₃│LiClO₄-PVA│La₂Te₃ supercapacitor device [65].

Hybrid supercapacitors, EDLC and pseudo-capacitance, build of charge storage mechanisms reduce the superior features of the device. On the other hand, among the other EDLC electrode materials multiwalled carbon nanotubes (MWCNTs) fascinated major interest due to their favorable features such as high surface area and mesoporous network, good mechanical strength and flexibility, excellent electrical conductivity, and chemical stability. The facile synthesis of composites of metal oxides with carbon materials was facilitated by SILAR as well. For instance, the fabrication of NiO/MWCNTs nanohybrid thin films via SILAR and the specific capacitance was as high as 1727 Fg⁻¹ and current density 5 mAcm⁻² with 91% retention ability after 2000 cycles as demonstrated in the Figure 4 [84]. Moreover, an analogous synthesis style was employed to NiCoOx/Carbon-black hybrid thin films accomplishing coatings with a high specific capacitance of 1811 Fg⁻¹ at 0.5 mAcm⁻² [85].

Figure 4.

Assembly of highly flexible symmetric NiO/MWCNTs-NiO/MWCNTs nanohybrid device: (a) image of flexible NiO/MWCNTs thin film deposited on the stainless steel, (b) NiO/MWCNTs nanohybrid thin film electrodes with closed ends (2 × 3 cm² area), (c) coating of the electrode by PVA/LiClO₄ gel electrolyte, (d) flexible supercapacitor built under the ∼1 ton pressure through sandwiching the two-gel electrolyte coated electrodes, (e) two flexible supercapacitors in series can successfully light a LED [84].

Therefore, SILAR is a unique as well as multipurpose technique to fabricate thin films for supercapacitor device application with superior power and energy densities in comparison with other available and more conventional deposition techniques, justifying the quality of the SILAR growth thin films.

4.2. Solar cells

Photovoltaic (PV) is a simple device, which promotes the direct conversion of light radiation into electrical energy by following the photovoltaic effect [91]. The discovery of such a device for the conversion of sunlight radiation directly into electricity was first carried out during the late 1800s. C. Fritts first demonstrated the solid-state PV by fabricating a thin layer of Au on Se semiconductor material [92]. At American Telephone and Telegraph Bell Laboratory the modern PV was discovered by Ohl in 1946 [93] but demonstrated by Chapin, Fuller and Pearson in 1954 [94]. The cell was fabricated by single-crystal Si wafer having an efficiency of ∼5%.

At present for feasible use, extensive research is going for efficiency enhancement of solar cells, as the efficiency of solar cells is one of the very vibrant parameters to promote this technology. Over the years, the efficiency of single crystal-Si solar cells has shown a sound development. In 1950s, it was only 15% and nowadays it is improved to around 26.7% [95]. The commercial efficiency of Si solar cell is approaching in between 12% and 15%, while the theoretical Shockley-Queisser (SQ) limit energy conversion efficiency is of around 28% [96]. The PV cell and module market was mostly occupied on first-generation Si-based cells until 2004, for example, sc- and poly-Si cells, which covered about 85% of the overall international PV modules market. In the meantime, thin film cells or second-generation PV have exhibited great advantages, for instance, the ease of large area fabrication and usage of minimum materials, though their market share was much smaller in comparison with the Si-cells [97]. After 2005, the developments were spurred by the sharp increase in the country’s implementation of solar energy due to the rapid advancement of the PV production industry in China. The price of PVs is generally supplemented by the strict requirement for fabricating high-purity materials such as GaAs and Si, or the rare-earth elements such as CIGS. The element, In, is rare and can be certainly exhausted, which might affect the prospect of such PVs. Later, CdTe thin-film PVs have increased longing in the market of South-Eastern countries. But Cd has a serious environmental distress, which is due to its high toxicity [98, 99]. For example, chronic Cd exposure breeds an extensive acute and chronic effects in humans [100, 101]. Moreover, Cd is a rare earth element and it will also generate a higher cost within the demand in future. Further, a third generation of devices has newly developed in addition to the thin-film solar cells, based on fresh organic materials such as dye-sensitized solar cells (DSSC) [102], quantum dot solar cells (QDSC), perovskites, bulk heterojunctions, having innovative device architectures with the usage of multiple exciton generation, upconverting layers, and others. Though, organic materials-built PVs have small life spans as the nature of the materials, for example, thermal stability [103] or concerns of electrolyte-based variability [104]. Inherently, a mostly striking new field of PV devices using metal oxide (MO) semiconductors has performed [105]. Atop of the MO thin films, favorable next-generation PV cells such as exciting thin absorber cells [106], DSSC [107], and QDSC [46, 108] are built as they are promising applicants for being stable, eco-friendly, and ultra-low-cost PV materials.

SILAR accounts itself directly into the third generation, by affording ultra-thin, compositionally fabricated by layers of several semiconductors that could be subjugated in a diversity of device architectures. Besides Si, most of the absorbers in PVs are conventionally II–VI, III–V semiconductors, or organic polymers, or small molecules, or perovskites. Still, the number of metal oxides is not adequate, which can be effectively used as absorber layers. Consequently, research into SILAR-grown light absorber layers for PV device applications has been aimed mainly on selective transition metal oxides, sulfides, and selenides. These materials have drawn incredible interest in technological and scientific research due to their unique optical, electrical, and mechanical properties in the past few decades [109, 110, 111]. Nevertheless, there are some examples of SILAR grown metal oxides, sulfides, and selenides applied in different types of solar cells such as thin film, DSSC, perovskite, and QDSC as summarized in Table 5. The layers were used not only to achieve high efficiency, but also served diverse roles inside the PVs such as light absorber, selective charge transport (electrons or holes), and passivation.

Absorber	Cell	Type	Current density	Circuit voltage	Fill factor	η; %	Refs.
			Jsc, mAcm^−2	Voc, mV	FF, %
Cu2O	Cu2O/Pt: FTO	Thin film	3.29	0.35	30	0.35	[24]
Cu2O	Cu2O/MAPbI3/PCBM	Perovskite	16.52	0.89	56	8.23	[112]
ZnO	ZnO/Pt: FTO	DSSC	4.04	0.44	33	0.59	[102]
ZnO	ZnO/Pt: FTO	DSSC	5.43	0.42	32	0.70	[113]
TiO2	FTO/TiO2/N3/I^− - I3^−/Pt: FTO	DSSC	8.17	0.42	34	1.16	[27]
TiO2	FTO/TiO2/spiro OMeTAD/Au	Perovskite	18.05	0.81	66.3	∼10	[114]
CuS, Ag2S	CuS/Sn: Ag2S	Thin film	8.3	0.66	52	2.85	[44]
CdS, PbS	ITO/CdS/PbS/C	Thin film	0.87	0.14	31.4	0.13	[19]
Bi2Se3	n-Bi2Se3/0.1 M polysulfide/C	Thin film	0.52	0.12	43	0.03	[115]
Sb2Se3	FTO/TiO2/Sb2Se3/electrolyte/Pt	SSC	4.19	0.57	51.66	1.22	[60]
CdS	FTO/TiO2/CdS QD/I^−-I3^−/Pt: FTO	QDSC	3.81	0.76	61.9	1.80	[46]
SnS	Gr/TiO2/SnS QD/I^−-I3^−/Pt: FTO	QDSC	1.35	0.52	42	0.30	[108]
CdSe	ZnO/CdS/CdSe QD/MnS/electro. /Cu2S	QDSC	13.74	0.60	44	3.70	[116]
CdSe	FTO/TiO2/CdSe/ Na2S: S/CoS2	QDSC	16.86	0.54	39	3.53	[117]
CdS: Mn	Mn-d-CdS/CdSe/S^2−-S/RGO/Cu2S	QDSC	20.7	0.56	47	5.42	[118]
CdSe	FTO/TiO2/CdS/CdSe/S^2−-Sn^2−/Cu2S	QDSC	13.64	0.56	51	3.89	[119]
PbS: Hg	FTO/TiO2/PbS: Hg QD/S^2−-Sn^2−/Cu2S/Brass	QDSC	30	0.40	46.8	5.6	[120]
PbS	FTO/TiO2/PbS QD/ S^2−-Sn^2−: CH3OH/Cu2S	QDSC	18.34	0.43	50.86	4.01	[121]
SnSe	FTO/TiO2/SnSe QD/S^2−-Sn^2−/Cu2S /Brass	QDSC	6.94	0.57	19	0.78	[122]

Table 5.

SILAR growth PV cells demonstrating with the cell properties.

N.B. N3: ethanolic 0.3 mM cis-Bis (isothiocyanato) bis (2,2′-bipyridyl-4,4′-dicarboxylato ruthenium (II).

In many cases, the core light-absorbing layers, within the solar cells, fabricated via SILAR have been investigated. For example, in ITO/CdS/PbS/C heterojunction solar cell, n-type layer, CdS thin films were deposited by CBD on transparent conductive oxide (ITO) substrates, whereas PbS film by SILAR using different deposition cycles, 15, 20, 30, 40 and 60 to obtain different thicknesses, showed that 40 cycles PbS film has a greater photovoltaic conversion efficiency [115]. In another study, p-type CuO was utilized as photo-absorber in the p-CuO/n-Si heterojunction cell [123], where a vibrant role of the SILAR deposition was observed in the overall device performance, depending mainly on the concentration of the copper precursor solution. In an all-oxide solar cell, NiO/Cu₂O or CuO/ZnO/SnO₂, both Cu₂O and CuO fabricated were examined as light absorber fabricated by SILAR and the hole transporting layer (p-type NiO), buffer layer (ZnO) as well as n-type SnO₂ were deposited by sol-gel method [124]. The cell having Cu₂O showed better performance than CuO, which is due to the reduced conductivity, mobility, and carrier concentration of CuO. However, the study showed an overall efficiency over 1%. In a different study, heterojunction solar cells have been fabricated between layers of p-type CuS and n-type Ag₂S deposited via SILAR method and Sn²⁺ and Al³⁺ heterovalent dopants are introduced in Ag₂S so that Fermi energy of the semiconductor can be modified to alter the band diagram of pn junctions. The Sn²⁺-doped Ag₂S resulted in better solar cell parameters with an efficiency of 2.85% as compared to that based on Al³⁺-doped Ag₂S, which consists of many defect states due to mismatch in ionic radii of the cation and the dopant ions [44].

Further, metal oxides were worked in charge transportation in between different layers in solar cells, and both electron transport layer (ETL) and hole transport layer (HTL) can enhance the performance of PVs. Since the early 1990s, TiO₂ is one of the key materials used as ETL owing to its wide popularity in DSSC [120]. In a recent study, TiO₂ nanocrystalline film was directly deposited using SILAR at 90°C for perovskite solar cell applications and used as an ETL [114]. Due to the fast charge transport, kinetics and slow charge recombination process of the TiO₂ ETL synthesized from the solutions of TiCl₄ and hot K₂S₂O₈, with subsequent annealing at 450°C, advances the efficiency of the cell to around 10%. Further, a couple of studies showed the deposition of TiO₂ layers from solutions of TiCl₃ and NaOH [27, 125] followed by annealing at 400°C, as ETLs in DSSC with the modest efficiency of just over 1%. Other SILAR-fabricated layers used as ETLs in PVs consist of ZnO and ZrO₂ as interfacial layer attached to porous TiO₂, both demonstrated performance in DSSC [102, 126].

In this study, Cu₂O thin films were introduced as a HTL in a planar perovskite solar cell and successfully enhanced the efficiency of the cell to around 8.23%, as shown in Figure 5(a-c). The Cu₂O films were deposited via SILAR by followed the complexation reaction of copper and ammonia with H₂O₂ [112]. The methylammonium lead triiodide (MAPbI₃) perovskite layer is sandwiched between a p-type Cu₂O HTL layer and another n-type PCBM (phenyl-C61-butyric acid methyl ester) ETL layer, respectively. The Cu₂O films demonstrated suitable band structure after annealing at 170°C and boosted device performances better than conventional sol-gel-deposited NiO and Cu-doped NiO hole transport layers, confirming the quality of the SILAR-Cu₂O.

Figure 5.

(a) Cell structure, (b) schematic energy level diagram; the dashed line represents the Fermi energy after contact (c) current-voltage characteristics of under dark and a white light illumination condition of Cu₂O/MAPbI₃/PCBM heterojunction [112].

In a report of 2009, Lee et al. showed a novel technique for preparing selenide (Se²⁻) by the SILAR process in pursuit of efficient QD-sensitized solar cells atop of mesoporous TiO₂ photoanodes. After several optimization of the QD-sensitized TiO₂ films via regenerative photoelectrochemical cells in presence of a cobalt redox couple [Co(o-phen)₃^2+/3+], with a final layer of CdTe, the overall efficiencies of the was recorded around 4.2% at 100 Wm⁻² [127]. To find the answer to a question, “How does a SILAR CdSe film grow?” Becker et al. tuned the deposition steps to suppress interfacial charge recombination in FTO/TiO₂/CdSe/Na₂S: S/CoS₂ cell by showing an efficiency of 3.53% [117]. Recently, in another report, SILAR- and CBD-grown CdSe-sensitized TiO₂ solar cells were examined concentrating on the influences of two commonly used QD deposition techniques [119], and atop of pre-accumulated CdS seed layers, a successful CdSe deposition was performed. The PEC of both the cells has been recorded as 4.85%, for CBD grown CdS/CdSe cell, whereas for SILAR grown cell the value was 3.89%. One research group enhanced the PCE of CdS/CdSe/S²⁻-S/RGO/Cu₂S cell to 5.4% by employing Mn²⁺ doping of CdS via SILAR method [118], whereas another group reported on a PbS: Hg QD-sensitized solar cell by Hg²⁺ doping into PbS employing similar deposition technique and showed an unprecedentedly high J_SC of 30 mA/cm² with the PEC of 5.6% [120]. More studies are ongoing with great efforts to find new alternative, clean, and environment-friendly energy resources due to the increasing demands.

4.3. Photoelectrochemical water splitting

Hydrogen energy is a key issue to cope with the present global energy crisis and environmental complication exploiting clean and inexhaustible energy [128]. Photoelectrochemical (PEC) water splitting is a promising technique to create hydrogen fuel by utilizing solar energy. Within the nonstop efforts in developing efficient photoelectrodes, the major challenge researchers presently face is to explore cost effective, nontoxic, and earth-abundant photoelectrodes with high efficiency [129]. In a recent study, Ag/Ag₂WO₄ was fabricated on ZnO nanorods using 0.05 M AgNO₃ and 0.05 M Na₂WO₄ as the cationic and anionic precursors, respectively, by following SILAR technique and the composite material demonstrated outstanding performances in PEC water splitting with 3 mAcm⁻² at 1.23 V versus RHE in the presence of 0.1 M Na₂SO₄ electrolyte. Based on these results, a brief possible updated mechanism of the PEC activity was demonstrated by Adam et al. for the better understanding of the technique with Figure 6 [130]. The development of PEC activity of the semiconductors was principally attributed to electrons and hole transfer at the interfaces of the photoelectrodes. The band edge potentials of the Ag/Ag₂WO₄ and the ZnO materials showed a significant role in the efficiency of growth and separation technique of the electron (e⁻) and hole (h⁺) pairs. The energy of valence band (E_VB) of ZnO and Ag₂WO₄ is calculated as +2.86 and + 3.03 eV, whereas energy of conduction band (E_CB) of them is projected as −0.34 and − 0.07 eV, respectively [131].

Figure 6.

(a) and (b) FM-SEM images of the ZnO NRs and the ZnO/Ag/Ag₂WO₄ heterostructure. (c) Curves of the ZnO NRs, and the ZnO/Ag/Ag₂WO₄ photo-electrodes under light and dark conditions using linear sweep voltammetry. Schematic diagram presenting the energy band structure and probable electron-hole separation as well as transportation in ZnO/Ag/Ag₂WO₄ heterostructure with the SPR effect [130].

In sunlight, both the semiconductors absorb light and the electrons in the VB become excited up to a higher potential of −0.34 and − 0.07 eV for the ZnO and Ag₂WO₄, respectively. Consequently, due to high photon energy, within the semiconductor, the effective charge transfer process proceeds. Ag⁰ nanoparticles (NPs) cause active separation of h⁺ or e⁻ pairs upon the absorption of light owing to the surface plasmon resonance (SPR) effect. Electrons from the Ag NPs are transported to the CB of the Ag₂WO₄ and the ZnO, while holes persist in the Ag NPs. In the meantime, to occupy the vacant holes created by the plasmonic absorption, the photogenerated electrons in the CB of ZnO will be transported to the Ag NPs [132]. The photogenerated charge carriers can be proficiently separated to enhance the PEC performance by following this mechanism. Further, the photogenerated electrons will eventually reach at the Pt electrode (counter) and contribute to H₂ generation. Also, the photogenerated holes in the VB of Ag₂WO₄ and ZnO will contribute on O₂ production via H₂O oxidation. Hence, these outcomes validate the modification via Ag/Ag₂WO₄, which is an active technique to attain a high PEC activity by means of ZnO NRs arrays.

Like the above example, many studies based on SILAR had been devoted toward exploring the potential of semiconductor thin films as photoelectrodes for water splitting as shown in Table 6 with their potential applications. In terms of low-cost, simplicity, and theoretically high solar to H₂ efficiency, PEC water splitting is much more favorable than solar photobiological, photochemical, and thermochemical generation of hydrogen [146]. The most investigated semiconductor materials include BiVO₄, Fe₂O₃, CuCoO₂, WO₃, and TiO₂ [147]. Other semiconductor materials such as Cu₂O [24, 148, 149], ZnO [150, 151], TiO₂ [152, 153], and CdO [28] were also produced using SILAR method but the PEC performances are quite low under visible light due to their wide bandgap.

Films	Electrolyte	pH	Precursors			Rinsing	Current density	Light source	Refs.
			Cationic 1	Cationic 2	Anionic		mAcm^−2	lamp
Ag3VO4	Na2CO3	11.4	Ag3VO4	—	Na3VO4	DI H2O	0.07	Xe arc	[36]
BiVO4	Na2SO3	7	Bi(NO3)3	—	NH4VO3	DI H2O, H^+	∼1.2	Xe	[37]
BiVO4	0.1 M KPi	7	Bi(NO3)3	—	NH4VO3	HNO3	1.5	Thorlabs	[68]
Fe2V4O13	Na2SO3	9.2	Fe(NO3)3	(NH4)6Mo7O24	NH4VO3	—	0.028	300 mWcm^−2	[38]
TiO2 NR/BiOI	Na2SO4		Bi(NO3)3	—	KI	—	0·40	Xe	[133]
SILAR-BiVO4	Co-Pi	7	Bi(NO3)3	HNO3	NH4VO3	—	2.3	Plasma	[134]
Bi2Mo2O9@Bi2MoO6	K3PO4	7	Bi(NO3)3	(NH4)6Mo7O24	—	DI H2O	0.16	Xe arc	[135]
WO3/BiVO4	H2SO4	—	BiNO3	—	NH4VO3	DI H2O	1.9	Xe	[136]
Fe2O3/TiO2/FTO	NaOH	13.6	FeCl3	—	NaOH	DI H2O	0.4	Xe arc	[137]
Ti: α-Fe2O3	NaOH	13.6	FeCl3	Ti{OCH(CH₃)₂}₄	NaOH	DI H2O	0.85	Xe arc	[138]
CuCoO2/BiVO4	Na2SO3	6.86	Bi(NO3)3	—	KI	—	3.3	Xe lamp	[139]
ZnFe2O4/BiVO4	Na2SO4	7	Zn(NH3)4^+2	FeSO4	NaOH	H2O2	1.46	Sim. Sun.	[140]
CuWO4/BiOI	K3PO4	7	Bi(NO3)3	—	KI	(CH₂OH)₂	2.10	AM1.5 G	[141]
Ag/Ag2WO4/ZnO	Na2SO4	—	Ag(NO)3	—	Na2WO4	DI H2O	3	Xe	[130]
Fe2O3/BiVO4	KH2PO4	7	Fe(NO3)3	—	NaOH	—	1.63	Xe	[142]
TiO2/BiVO4/NiFe-LDH	Na2SO4	—	Bi(NO3)3	—	NH4VO3	DI H2O	0.018	Xe	[143]
BiVO4/ZnO	Na2SO5	—	Bi(NO3)3	—	NH4VO3	—	1.72	Xe	[144]
AgVO3/Mo: BiVO4	Na2SO4	6.4	AgNO3	—	NH4VO3	DI H2O	1.93	Xe arc	[145]

Table 6.

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

5. Factors affecting SILAR deposition

A lot of research work has been done on the deposition and optimization of the SILAR thin films for optoelectronic device applications. Solution concentration, composition of precursors, the number of SILAR cycles, pH, annealing, and doping will absolutely affect the quality and quantity of thin films, which directly influence the cell performance. The effect of different parameters used in SILAR deposition on the performance of thin films is reviewed based on the contemporary research work.

5.1 Solution concentration

Solution concentration of the used precursor is one of the key factors in governing the properties as well as the performances of SILAR grown thin films. From a general viewpoint, depositing through a more concentrated solution results with bigger grain size and higher surface roughness during deposition. Consequently, thinner, smoother, and probably pinhole-free deposition can be attained using multiple SILAR cycles with a lower concentrated precursor solution.

With the increase of molar concentration (0.03, 0.05, and 0.1 M) of the cationic solution prepared by Cd(CH₃COO)₂ and H₂O₂, the surface morphology of the SILAR-deposited nanostructured CdO thin films was improved toward the crack free and homogeneous nature [154]. On the other hand, the structural change such as nanorods, nanoflowers, and nanoflakes morphologies was observed by altering only the concentration of anionic precursors NaOH (high, 0.05 M; moderate, 0.01 M; and low, 0.001 M) with fixed Zn precursor concentration (0.005 M) [155]. A comparative study of CdS films deposited by SILAR and CBD techniques revealed that the S/Cd ratio in the sample increases (0.83 to 1.04) for SILAR deposited films with the molar concentration of sulfur (1:1, 3:1, 5:1 and 7:1) in the starting solution increases, while it was almost constant (∼0.80) for CBD films [156]. During the investigation of the effect of the molar concentration of pyrrole monomer on the electrochemical behavior 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⁻¹ at 5 mVs⁻¹ 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⁻² [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 co-worker 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⁻¹, respectively, at 5 mVs⁻¹ scan rate. Hence, the MnCl₂-derived Mn₃O₄ electrode showed a good electrochemical with maximum energy density of 17 Whkg⁻¹ and power density of 999 Wkg⁻¹ at 0.5 mAcm⁻² 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 cm² 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/cm²) [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⁻¹ with the highest specific power 2305 WKg⁻¹, 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 opto-electronic 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 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].

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].

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.

Film	pH	Precursors		To Adjust pH	Thickness (nm)	Crystallite size (nm)	Dislocation density (δ × 10^15 lines m^−2)	Strain (ε × 10^−3lines^−2 m^−4)	Texture coefficient TC (hkl)	Bandgap (eV)	Resistivity ×10^3(Ωcm)	Refs.
		Cationic	Anionic
Cu2O	2.35	[Cu(S2O3)]^−	*2 M NaOH	SA	340	17	3.460	2.28	0.744	2.05	0.21	[169]
	3.45		*2 M NaOH	AA	729	21	2.268	2.11	0.756	2.10	0.18
	4.50		2 M NaOH	AA	800	15–22	—	—	—	2.30	72	[170]
	5.10		2 M NaOH	AA	1000		—	—	—	2.28	103
	6.20		2 M NaOH	AA	1800		—	—	—	2.43	742
	7.33		*2 M NaOH	∼	1130	18	3.086	2.20	0.768	2.15	0.37	[169]
	7.33		^#1 M NaOH	∼	336	13	5.917	2.90	0.561	2.16	0.18
	7.95		2 M NaOH	∼	1477	15–22	—	—	—	2.42	21.9	[170]
CuO	9.0	[Cu(NH3)4]^2+	H2O	∼	42	37.4	0.714	9.26	1.21	1.61	—	[171]
	9.5		H2O	∼	67	22.4	1.992	15.4	1.17	1.49	—
	10.0		H2O	∼	85	22.9	1.906	15.1	1.07	1.49	—
	10.0		H2O	SA	520	14	5.102	2.31	1.05	2.17	6.5	[172]
	10.5		H2O	SA	590	21	2.268	1.63	1.34	2.07	5.5
	11.0		H2O	SA	680	27	1.372	1.48	1.65	2.02	4.0
	11.5		H2O	SA	770	30	1.111	1.33	1.71	1.99	4.25
	12.0		H2O	SA	820	36	0.716	1.24	1.83	1.89	4.5
CdO	11.3	Cd(CH3COO)2	H2O	NH3	520	14.59	5.44	2.48	2.52	3.26	—	[173]
	11.7		H2O	NH3	1390	16.86	4.09	2.15	1.89	3.21	—
	12.1		H2O	NH3	1820	17.22	3.91	2.10	1.90	2.67	—
	12.5		H2O	NH3	2360	17.38	3.84	2.08	1.71	2.35	—

Table 7.

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

*

OP = Optimized precursor (2 M NaOH), NOP = Non optimized precursor, SA = Sulfuric acid and AA = Acetic acid.

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 high-temperature 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 Cu_0.6Zn_0.4S thin films grown by the SILAR demonstrated that the current increase with increasing light intensity and increasing rate in illuminated 500 Wcm⁻² 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.

Temperature, ^0C		Precursors		Cycle	Time		Crystallite	Bandgap	Ref
Annealing	Growth	Cationic	Anionic		Dipping (sec)	Annealing (min)	(nm)	(eV)
As deposited: Cu2O; 200°C: Cu2O; 250°C: Cu2O; 300 °C: CuO; 350°C: CuO	70	Cu(S2O3)^−	NaOH	10	20	60	14; 14; 14; 14–26; --	2.20; 2.20; 2.20; 1.35; 1.35	[177]
As deposited: ZnO; 200°C: ZnO; 300°C: ZnO; 400°C: ZnO	90	ZnCl2	H2O	150	10	60	18.4; 23.1; 26.9; 27.6	3.1–3.2	[176]
As deposited: CuO; 200°C: CuO; 300°C: CuO; 400°C: CuO	—	CuCl2	NH3	80	30	30	11.09; 12.05; 13.86; 14.88	1.17; 1.29; 1.30; 1.36	[178]
As deposited: CuS; 200°C: CuS; 300°C: CuS; 400°C: CuS	RT	CuCl2	Na2S	40	30	3 (N2)	16.4; 13.1; 33.6; 37.6	2.03; 1.99; 1.97; 1.87	[43]
As deposited: Cu0.6Zn0.4S; 200°C: Cu0.6Zn0.4S; 300°C: Cu0.6Zn0.4S; 400°C: Cu0.6Zn0.4S	RT	CuCl2 + ZnCl2	Na2S	40	30	3 (N2)	38.6; 29.9; 30.8; 27.9	2.14; 2.12; 2.05; 1.85	[43]
As deposited: ZnS; 200°C: ZnS; 300°C: ZnS; 400°C: ZnS	RT	ZnCl2	Na2S	40	30	3 (N2)	2.7; 1.9; 3.7; 6.3	3.92; 3.84; 3.67; 3.47	[43]
250°C: CdO; 350°C: CdO; 450°C: CdO	70	Cd(NO3)2	H2O2	40	20	120	6.11; 8.18; 9.48	2.44; 2.37; 2.24	[179]
As deposited: MgO; 100°C: MgO; 200°C: MgO; 300°C: MgO; 400°C: MgO; 500°C: MgO	90	MgSO4	H2O	30	30	60	12; 10; 10; 10; 6; 6	4.18; 4.18; 4.22; 4.25; 4.27; 4.33	[180]
As deposited: NiO; 200°C: NiO; 300°C: NiO; 400°C: NiO	90	NiCl2	H2O	100	30	30	10.03; 11.04; 8.24; 13.32	3.3; 3.25; 3.21; 3.11	[31]

Table 8.

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

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 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⁻¹ whereas it was increased to 90.2 mAhg⁻¹ for the same scan rate of 10 mVs⁻¹ for In³⁺-doped films, with the high-power density of 1.7 kWkg⁻¹ at the highest energy density of 18.85 Whkg⁻¹. 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 J_sc, V_oc, 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⁻² 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.

Product	Precursors		Dopant		No of	Crystallite	Grain	Bandgap	Ref
	Cationic	Anionic	Compound	Amount	Cycle	(nm)	(nm)	(eV)
Mn: Fe2O3	FeCl2	NaOH	MnCl2 (wt %)	0; 2; 4; 6; 8	50	28; 26; 23; 21; 20	—	3.02; 3.01; 3.00; 2.97; 2.95	[187]
Cu: Mn3O4	Mn(NO₃)₂	NH4OH	Cu(NO3)2 (at %)	0; 0.1; 0.2; 0.3	20	29; 25.7; 28.4; 30	—	2.09; 2.06; 1.91; 1.71	[188]
Co: CuO	CuCl2	NH4OH	CoCl2 (at %)	0; 0.5; 1; 2; 3; 4	10	22.7; 15.7; 13.6; 13.1; 12.6; 12.2	70; 44; 42; 36; 32; 38	1.53; 1.47; 1.45; 1.41; 1.38; 1.36	[189]
B: CuO	CuCl2	NH4OH	H3BO4 (at %)	0; 1; 2; 3	10	12.9; 13.1; 14.2; 15.9	45; 42; 38; 30	1.52; 1.48; 1.43; 1.39	[190]
Co: Cu2O	Cu(S2O3)^−	NaOH	CoSO4 (wt %)	0; 1; 2; 5; 10	30	62.83; 53.30; 48.47; 39.24; 28.44	—	1.94; 2.03; 2.12; 2.18; 2.47	[191]
Fe: Cu2O	Cu(S2O3)^−	NaOH	FeSO4 (wt %)	0; 1; 2; 5	30	62.83; 59.80; 41.83; 36.40	—	1.80; 2.10; 2.36; 2.45	[192]
Cr: CdO	Cd(NO3)2	NH4OH	Cr(NO₃)₃ (M %)	0; 0.1; 0.3; 0.5	20	129; 121; 129; 131	1000; 1000; 1000; 1000	2.50; 2.63; 2.78; 2.87	[193]
Mn: CdO	Cd(CH3COO)2	NH4OH	Mn solution (M %)	0; 1; 2	15	33.40; 39.26; 38.15	50; to; 35	2.23; 2.29; 2.39	[194]
Ag: CdO	Cd(CH3COO)2	NH4OH	Ag^+ solution (M %)	0; 1; 2; 3	10	21; 15; 17; 19	—	2.12; 2.34; 2.27; 2.18	[195]
Mo: CdS	Cd(CH3COO2)	SC(NH2)2	(NH4)6Mo7O24 (at %)	0; 5; 10; 15	70	—	~50; ~50; ~50; ~50	2.45; 2.46; 2.47; 2.48	[196]
Sn: CdS	Cd(CH3COO)2	CH4N2S	SnCl2 (at %)	0; 2; 4; 6; 8	70	27.20; 32.79; 24.01; 18.87; 12.96	—	2.48; 2.50; 2.51; 2.51; 2.43	[197]
Ba: PbS	Pb(CH3COO)2	CH3CSNH2	BaCl2 (at %)	0; 1; 2; 4; 6; 8	200	18.6; 24.8; 22.4; 26.8; 27.6; 22.3	480; 570; 650; 475; 840; 930	1.77; 1.86; 1.98; 2.00; 2.08; 2.12	[198]
Co: ZnS	Zn(NO3)2	Na2S	Undoped; Cu(NO3)2; Co(NO3)2; AgNO3	0; 3%; 3%; 3%	15	34; 41; 49; 57	60; to; 90	4.12; 3.99; 3.90; 3.78	[199]
F: ZnO	Zn(CH3COO)2	NaOH	NH4F (at %)	0; 5; 10; 15	50	49.67; 83.22; 52.01; 48.98	—	3.21; 3.24; 3.23; 3.20	[200]
Mg: ZnO	ZnSO4	NaOH	Cu(CH3COO)2 (wt %)	0; 1; 3; 5	50	37.6; 36.6; 36.2; 35.1	—	3.09; 3.16; 3.24; 3.35	[201]

Table 9.

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

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 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 low-cost deposition of high-quality thin films and associated optoelectronic devices.

Acknowledgments

The author thanks the Department of Chemistry, Comilla University and UGC, Bangladesh, for supporting this work. Also, thanks to Professor Dr. Jamal Uddin, Center for Nanotechnology, Department of Natural Sciences, Coppin State University, Baltimore, MD, USA, for his insightful discussion and support.

Conflict of interest

The authors declare no conflict of interest.