Copper oxide nanostructured thin films processed by SILAR for optoelectronic applications

Abstract

The lack of high-functioning p-type semiconductor oxide material is one of the critical challenges that face the widespread performance of transparent and flexible electronics. Cu_xO nanostructured thin films are potentially appealing materials for such applications because of their innate p-type semi-conductivity, transparency, non-toxicity, abundant availability, and low-cost fabrication. This review summarizes current research on Cu_xO nanostructured thin films deposited by the SILAR technique. After a brief introduction to the advantages of Cu_xO semiconductor material, diverse approaches for depositing and growing such thin films are discussed. SILAR is one of the simplest deposition techniques in terms of better flexibility of the substrate choice, the capability of large-area fabrication, budget-friendly, deposition of stable and adherent film, low processing temperature for the film fabrication as well as reproducibility. In addition, various fabrication parameters such as types of copper salts, pH of precursors, number of cycles during immersion, annealing of as-deposited films, doping by diverse dopants, and growth temperature affect the rate of fabrication with the structural, electrical, and optical properties of Cu_xO nanostructured thin films, which led the technique unique to study extensively. This review will include the recent progress that has recently been made in different aspects of Cu_xO processed by the SILAR. It will describe the theory, mechanism, and factors affecting SILAR-deposited Cu_xO. Finally, conclusions and perspectives concerning the use of Cu_xO materials in optoelectronic devices will be visualized.

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1. Introduction

1.1 Background

Copper (Cu) and copper oxide (Cu_xO) thin films have been studied extensively due to their potential application in semiconductor technology long before the Ge and Si era started, and researchers have faced much more difficult to work with this oldest material ever. The n-type window layer semiconductors such as ZnO, ITO, FTO, and GaN with large bandgap energies have already achieved outstanding optical as well as electronic transport properties. Consequently, the effort of detecting new, prospective p-type absorber layers for optoelectronics devices has led to intensive research.

Cu_xO semiconductors are very attractive and have been broadly studied in both theoretical analysis and investigations into applied executions of nano or optoelectronic devices due to their chemically stable nature, nontoxicity, relative abundance, potential particle size effects, excellent performance as a catalyst, and fulfill all the requirements for low-cost manufacturing at ambient conditions, which have high potential usage in energy storage, conversion, and next-generation rechargeable lithium-ion batteries.^1–6 Furthermore, Cu_xO nanostructures are extensively used in other diverse applications, including photovoltaics,⁷ photodetectors,⁸ nanofluid,^9,10 energetic materials,¹¹ field emissions,¹² supercapacitors,^13,14 biosensors,^15,16 gas sensors,^17,18 photocatalysis,^19,20 removal of inorganic pollutants,^21,22 and magnetic storage media.^23,24

Both the Cu₂O and CuO show direct transition nature with a direct band gap of around 2.1 and 1.5 respectively, having a high extension coefficient of above 10⁵ cm⁻¹. Since the theoretical limit of the energy conversion efficiency of Cu₂O and CuO is as high as 20 and 29%, respectively under air mass (AM) 1.5 solar illumination, numerous efforts were done to increase the efficiency of Cu_xO solar cells, but the performance remains very poor.²⁵ In the case of Cu₂O solar cells, it is not more than 8.1%,⁷ whereas in the case of CuO it is lower and still about 3%.²⁶ Toward the large area fabrication, it is crucial to establish the thin film growth technique for Cu_xO. Thus, the research of Cu_xO thin films has both high-tech and scientific consequences.

Cu_xO nanostructured thin films have been synthesized by various approaches like electrodeposition,²⁷ electron beam evaporation,²⁸ magnetron sputtering,^29–31 molecular beam epitaxy,³² sol–gel,³³ solution growth,³⁴ spin coating,³⁵ successive ionic layer adsorption and reaction (SILAR),^36,37 thermal evaporation,³⁸ and vapor deposition.³⁹ Among all the deposition methods, SILAR is one of the simplest methods in terms of better flexibility on substrate choice, the capability of large area fabrication and deposition of the stable and adherent film, low processing temperature for film fabrication as well as reproducibility.⁴⁰ This technique is very budget friendly since it does not require any sophisticated equipment. Moreover, various fabrication parameters such as pH, annealing temperature and time, doping elements, the concentration of precursor solutions, and temperature of the precursor solutions 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.

More than a few reviews of different aspects of Cu_xO-based optoelectronics have been published based on the fabrication technique but still no such report for the SILAR technique. This paper concerns the progress that has recently been made in diverse aspects of Cu_xO-based thin films processed by the SILAR method, following the introduction in section one, several deposition techniques are reviewed in section two. The third section of this paper describes the theory and mechanism of Cu_xO-based thin films fabricated by the SILAR method. The fourth section, which incorporated the core focus of this review, leads to the factors that affect SILAR-based Cu_xO deposition which is followed by the application of Cu_xO in section five. Finally, conclusions and perspectives concerning the use of Cu_xO in optoelectronic devices are presented.

1.2 Properties of copper oxides (Cu_xO)

Cu₂O exists as a simple cubic Bravais lattice^8–10 with a space group of (Pn3m) or (O4_h). Each unit cell consists of six atoms, the four Cu atoms are in a face-centered cubic lattice while the two O atoms are at the tetrahedral positions creating a body-centered cubic sublattice. Thus, O atoms are fourfold coordinated with Cu atoms as closest neighbors, and Cu atoms are linearly coordinated with two O atoms as closest neighbors as shown in Table 1. On the other hand, the unit cell of CuO fits into a monoclinic structure with the space group C2/c and the lattice parameters are represented in the table (PDF No. 89-5898). In each CuO unit, there exist four Cu–O bonds. As demonstrated in the table, in a unit, each Cu atom is surrounded by the four closest coplanar O atoms. The four O atoms are positioned at the angles of an almost rectangular parallelogram, which then unites another two O atoms to shape a highly distorted octahedron. The O atom is enclosed by the four closest Cu atoms positioned at the angle of a tetrahedron.

Table 1 Crystallographic properties of Cu₂O and CuO⁴¹

Parameters		Cu2O	CuO
Structure: Cu(I)-yellowish, Cu(II)-greyish black, O-red
Unit cell		a = b = c = 4.26 Å	a = 4.6837 Å, b = 3.4226 Å, c = 5.1288 Å
		α = β = γ = 90°	α, γ = 90°, β = 99.54°
Space group		Pn3̄m (224)	C2/c (15)
Bond length, Å	Cu–O	1.849	1.96
	O–O	3.68	2.62
	Cu–Cu	3.012	2.90
Cell volume, Å^3		77.83	81.08
Formula weight		143.14	79.57
Density, g cm^−3		5.749–6.140	6.515
Melting point, °C		1235	1201

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1.3 Band-structure calculation

Ab initio calculations are mandatory to understand the optical and electronic properties of the Cu_xO systems. But there is a challenge for standard ab initio investigations based on DFT for both Cu₂O and CuO. The exchange–correlation function is the crucial ingredient in the theoretical description. Fig. 1 and 2 represent the band structures, density of states (DOS), and partial density of states (PDOS) of the Cu₂O and CuO compounds. The results were simulated for both Cu₂O and CuO unit cells using CASTEP software within the LDA + U and the calculated bandgaps were found as 1.647 and 1.52 eV respectively.⁴²

Fig. 1 Band structures of Cu₂O and CuO unit cells are drawn by CASTEP using LDA + U.

Fig. 2 DOS and PDOS of Cu₂O and CuO unit cells are drawn by CASTEP using LDA + U.

2. Thin film deposition process

2.1 Physical deposition methods

The physical deposition methods have diverse techniques to attain thin films with good quality. It can be summarized with the raw materials, deposition conditions as well as cost of production as shown in Table 2.

Table 2 Physical deposition techniques with the requirements during deposition

Physical deposition
Techniques	Raw materials	Conditions	Substrate	Film quality	Budget	Ref.
Thermal evaporation	Red Cu2O powder	Base pressure: 5 × 10^−4 Pa	Glass tantalum SiO2	—	High	43
		Temperature: 300 °C
		Annealing temperature: 500 °C
Electron beam evaporation	Cu2O pellets	Deposition time: 5–16 min	Glass	Moderate	High	44
		Substrate temperature: 200 °C
		Accelerating voltage: 2, 4 and 6 kV
		Evaporation pressure: 3 × 10^−2 Pa
		Ultimate pressure: 4 × 10^−4 Pa
		Filament current: 30 mA
Pulsed laser deposition	Cu, O2	Substrate temperature: 25–400 °C	Quartz ITO NaCl	Excellent	High	45
		pO2: 0–10 mTorr
		Vacuum chamber pressure: Torr
Molecular beam epitaxy	Cu, O2	Incident O^+ beam energy: 50 eV	MgO	Excellent	High	46
		Substrate temperature: 100–400 °C
		Cu flux: 2.5 × 10^13 to 1.6 × 10^14 atoms per cm^2 s
		Base pressure: 3 × 10^−10 torr
		Total pressure: 3 × 10^−9 to 2 × 10^−8
		O^+ flux: 2.7 × 10^14 atoms per cm^2 s
Ion plating evaporation	Cu, O2	Base pressure: 10^−4 Pa	Glass	Excellent	High	47
		O2 flow rate: 15–45 sccm
		N2 flow rate: 0–6 sccm
		RF power: 300 W
		Annealing temperature: 300 °C
		Substrate temperature: 25 °C
Direct current (DC) sputtering	Cu, O2	Deposition pressure: 6.3 × 10^−3 torr	Glass	Excellent	High	48 and 49
		Base pressure: 6 × 10^−6 torr
		Sputtering power: 60 W
		Ar gas pressure: 20 sccm
		pO2: 8.0 × 10^−4 to 1.8 × 10^−3 torr
	Cu, O2	Sputtering power: 10–40 W	Glass stainless steel
		Ar flow rate: 15 sccm
		O2 flow rate: 10 sccm
		Substrate temperature: 300 °C
		Ambient gas pressure: 0.045 Pa
Radio frequency (RF) sputtering	Cu, O2	Substrate temperature: 300 °C	Glass	Excellent	High	50
		Ar flow rate: 10 sccm
		O2 flow rate: 0–2 sccm
		Deposition time: 60 min
		Background pressure: <3 × 10^−4 Pa
		Working pressure: 1.7–1.8 Pa

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2.2 Chemical deposition methods

Likewise, diverse chemical deposition techniques with the deposition condition, raw materials, cost of production, the usual use of substrate etc. are discussed in Table 3 as shown below:

Table 3 Chemical deposition techniques with the requirements during deposition

Techniques	Raw materials	Chemical deposition	Substrate	Film quality	Budget	Ref.
		Conditions
Sol–gel	CuCl2·2H2O, CH3OH, (CH2CH2OH)NH, glucopone, ethylene glycol	Rotating speed: 2000 rpm	TiO2	Good	Low	51 and 52
		Annealing temperature: 200–400 °C
	Cu(II) acetate, H2N(CH2)2OH, 2-methoxyethanol, poly (ethylene glycol)	Rotating speed: 1000 and 1500 rpm	FTO Si
		Annealing temperature: 350, 500 °C
Chemical bath deposition	Cu(NO3)2·3H2O, triethanolamine, hydrazine hydrate	Temperature: 30 °C	Glass	Excellent	Low	53 and 54
	Cu, HNO3, HF, C2H5OH, Na2SO4, CH3COCH3	Temperature: 30 °C	Cu
		Molar ratio: HNO3 : HF-10 : 1 to 135 : 1
		HNO3 conc: 0–1.2 mmol L^−1
		Water bath time: 2–6 days
		Water-bath temperature: 10–45 °C
SILAR	CuSO4·5H2O, NaOH, Na2S2O3·5H2O	Temperature: 70 °C	Glass FTO	Excellent	Low	36 and 37
		pH: 2.35–7.33
	CuSO4·5H2O, NaOH, Na2S2O3·5H2O	Temperature: 70 °C	Glass FTO
		pH: 4.50–7.95
		No. of cycles: 20–80
		Annealing temperature: 75–350 °C
Spray pyrolysis	Cu(CH3COO)2·H2O, (CH3)2CHOH, C6H12O6	Temperature: 200–350 °C	Glass	Excellent	Low	55 and 56
		Deposition time: 45 min
		No. of cycles: 450
	Cu(CH3COO)2·H2O, C6H12O6, (CH3)2CHOH	Temperature: 200–350 °C	Glass
		Glucose conc: 0–0.08 M
		Isopropanol volume (%): 0–100
		Cu salt conc: 0–0.08 M
Electrodeposition	CuSO4·5H2O, lactic acid, NaOH	Temperature: 30 and 60 °C	Ti	Excellent	Low	57–60
		pH: 9, 12
		Applied potential: −150 to −800 mV
	CuSO4·5H2O, tri-sodium citrate dehydrate, C6H5Na3O7; KOH	pH: 11	FTO
		Time: 20–60 min
		Applied potential: −0.4 V
		Temperature: 30 °C
	Cu(CH3COO)2·H2O, CH3COONa·3H2O	Temperature: 20–80 °C	ITO
		Time: 2–80 min
		NaCl conc: 1–10 mM
		Potential: −0.1 to −0.4 V
	Cupric acetate, sodium acetate, CH3COOH, NaOH	Temperature: 20, 55 °C	Ti ITO Pt
		Deposition time: 45 min
		pH: 5.4–7.49
		Applied potential: −200 to −400 mV
		Cu(II) acetate con.: 0–16 mM
Chemical vapor deposition (CVD)	Cu dipivaloylmethanate, Cu(C11H9O2)2; O2	O2 flow rate: 1 and 300 cm^3 min^−1	Borosilicate glass	Excellent	High	61–63
		pO2: 1.689 × 10^2 and 5.07 × 10^4 Pa
		Temperature: 300 and 500 °C
	N,N′-di-sec-butylacetamidinato)Cu(I), Cu-(sec-Bu-Me-amd)]2; DI H2O	Substrate temperature: 125–225 °C	Si waferGlassy carbon SiO2 glass
		Process pressure: 1–10 torr
		Vapor flow rate: 5 sccm
		N2 flow rate: 100 sccm
	Cu(II) acetylacetonate, Cu(C5H7O2)2; O2	Pressure: 5–200 sccm	Sapphire MgO
		Temperature: 350–500 °C
Atomic layer deposition (ALD)	Cu(CH3COO)2·H2O, Cu(CH3COO)2, H2O vapor	Temperature: 180–220 °C	Glass Si	Excellent	High	64 and 65
		Rector pressure: 10 mbar
		N2, H2O and O2 flow rate: 400 sccm
		Deposition cycles: 500–7000
	Cu(II)-bis-(dimethylamino-2-propoxide), O3	pO2: 34 Pa	SiO2/Si
		Substrate temperature: 112–165 °C
		Deposition cycles: 500–10000

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2.3 Advantages and disadvantages of deposition techniques

Till now, a lot of deposition techniques are available to fabricate high-quality thin films having diverse applications. For a better understanding, the advantages, and disadvantages of some of the chemical deposition techniques such as chemical bath deposition (CBD), atomic layer deposition (ALD) as well as spin coating are summarized to understand the potentiality of the SILAR method in Table 4.

Table 4 Involved advantages and disadvantages of the deposition techniques

Techniques	Merits	Demerits
CBD	Simple and cost effective	Precipitation occurs in the bath causing serious problems
	Stoichiometric deposition^66	Materials are lost^70
	Low fabrication temperature	Films are badly cohered onto the substrate
	Capability of depositing large area films (∼10 cm^2)^67	Produces powdery films
	Various types of substrates are used^68	Failed to control film thickness
	Tuning film qualities by controlling growth parameters	Deposited films are contaminated though organic additives
	Deposition of ternary and quaternary compounds^69	Opposite ions present in the reaction bath
SILAR	Facile and economical	Perfect adsorption of ions requires on the substrate surface
	Potentiality to grow large-surface films (∼10 cm^2)^71	Substrate surface must be balanced completely through precursor solution^73
	Reproducibility
	Any kind of substrate can be used
	No need to use sophisticated instrument or vacuum pump
	No precipitation occurs in the bath
	Synthesis of doped, ternary, and quaternary compounds
	Does not need premium quality target
	Controlling on film thickness
	Avoid unnecessary heating
	Minimization of dislocation density by controlling deposition parameters^72
	Fabricated stable and sticky films^36
ALD	Films thicknesses are under controlled	Sluggish deposition process
	Layer by layer film deposition	Highly refined substrate is required
	Deposition can be performed at relatively low temperature	Instrument and substrate are highly priced
	Soft substrates can be used	Several trials required to set optimize film growth condition
	Ability to use thermally unstable precursors due to slow degradation^74,75	Process is restricted to non-volatile compounds
		Unfavorable for heat sensitive biological substrates^76,77
Spin coating	Dominated over film thicknesses	Hindered to use multiple substrates at a time
	Easy and effortless	Restricted to utilize big substrate
	Affordable	Low material productivity
	Fabricated stable cohered films	Inexpensive with respect to photoresist and substrate size
	Deficit of coupled variables	95–98% materials are wasted^80
	Reproducibility^78,79	Less effective in nanotechnology due to quick drying^81
Solvothermal	Dominate over crystallinity of the deposited films	Requirement of expensive autoclave
	Produces intermediate to specific quality films	Safety issues
	Potentiality to synthesize solid-state materials	Impossible to observe in situ reaction process^82
		Less dominance over particle size^83

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3. Theory and mechanism of SILAR process

SILAR is an extensively applied technique to fabricate high-quality metal oxide or halide thin films.^84,85 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_xB_y is deposited onto the substrate surface by dint of the adsorbed cations, xA^p+ and anions, yB^q− due to the following heterogeneous chemical reaction:

x[A(L_n)]^p+ + pP^x− + qQ^y+ + yB^q− → A_xB_y + qQ^y+ + pP^x−

where, x, p, q, y and p⁺, x⁻, y⁺, q⁻ are the number and charges of the corresponding ions A (metal ions), P (cationic precursor), Q (anionic precursor) and B (anions) respectively.^85,86 Sometimes, the ligands, L_n are a necessity to complete the reaction.^87–90

In the case of Cu_xO film deposition mechanism, salts of Cu²⁺ are used to deposit copper oxide thin films. In most of the research on Cu₂O, firstly copper(I) thiosulfate complex is formed by the redox reaction between Cu²⁺ and S₂O₃²⁻ ions which results in a colorless solution. The corresponding reactions are:

Oxidation half-reaction:

2S₂O₃²⁻ → S₄O₆²⁻ + 2e⁻

Reduction half-reaction:

2Cu²⁺ + 2e⁻ → 2Cu⁺

Overall reaction:

2Cu²⁺ + 4S₂O₃²⁻ → 2[Cu(S₂O₃)]⁻ + S₄O₆²⁻

In the above reactions, [Cu(S₂O₃)]⁻ the complex solution is regarded as the cationic precursor solution (cold solution) while NaOH is the anionic precursor solution, which is being kept at 70 °C (hot solution).⁹¹ When the substrate is immersed in the hot solution, OH⁻ ions are adsorbed onto the substrate and subsequently dipping into the cold solution results in the adsorption of Cu⁺ ions. Thus, one SILAR cycle is completed and Cu₂O thin film is formed due to the reaction between Cu⁺ and OH⁻ ions. Rinsing is carried out after every immersion to exclude loosely adhered particles. The number of cycles as well as dipping time varies based on required film thicknesses. Corresponding reactions are given below ⁹² and the growth mechanism is schematically represented in Chart 1.

[Cu(S₂O₃)]⁻ → Cu⁺ + 2S₂O₃²⁻

2Cu⁺ + 2OH⁻ → Cu₂O(s) + H₂O

Chart 1 Representation of different steps during a SILAR cycle.

Therefore, a basic SILAR cycle comprises four different steps. The steps are represented in the following chart:

Consequently, a SILAR cycle covers four diverse steps on the surface, associating alternative immersion of the substrate into cationic and anionic precursor solution followed by rinsing in each immersion cycle to remove loosely adhered particles as demonstrated in Chart 2 and discussed below:

Chart 2 Schematic presentation of the deposited Cu₂O nanostructured films on the substrate surface during a SILAR cycle.

3.1 Adsorption

First SILAR stage forms the Helmholtz double layer owing to the initial adsorption of the cationic precursor such as Cu⁺ on the substrate surface. This layer is generally composed of two charged layers, the positively charged, Cu⁺, inner layer and the negatively charged, (S₂O₃)²⁻, outer layers.

3.2 Rinsing I

In the second stage, extra adsorbed ions, Cu⁺ and (S₂O₃)²⁻, are rinsed away from the diffusion layer towards the bulk solution and a hypothetical monolayer is formed, resulting in a saturated electrical double layer.

3.3 Reaction

In the reaction step, the anions, OH⁻, from the anionic precursor solution are entered into the scheme. A solid substance, Cu₂O, is synthesized on the interface due to the low stability of the material. This procedure pays the reaction of Cu⁺ species with the anionic precursor such as OH⁻.

3.4 Rinsing II

In the final SILAR cycle, the extra and unreacted species such as (S₂O₃)²⁻, Na⁺ as well as by-products of the reaction from the diffusion layer are removed leaving expected thin films.

The above deposition process involved alternate immersion of the substrate into cationic and anionic precursor solution followed by rinsing in every immersion cycle to eliminate loosely adhered particles.³⁶ Fig. 3 represents the synthesis of copper(I) oxide nanorod thin films in presence of NaCl using the SILAR deposition system.⁹³ Earlier to the film deposition, the colorless copper–thiosulfate complex was made ready by mixing 10 mL 1 M copper(II) sulfate and 40 mL 1 M sodium thiosulfate into a 100 mL volumetric flask. Then, in addition to DI water, the required amount of NaCl electrolyte was further added to the same flask and the produced complex solution was the cold solution. Meanwhile, 2 M NaOH solution was kept constant at 70 °C and treated as the hot solution. The substrate such as soda lime glass was then alternatively submerged in cold and hot solutions respectively for the required time interval and completed one SILAR cycle. To fabricate a thin film, this procedure was repeated for up to several immersion cycles.

Fig. 3 Synthesis of the copper(I) oxide nanorod thin films.⁹³

The formation of Cu₂O nanorod thin films in presence of a NaCl electrolyte at various concentrations were discussed by using SEM micrographs as shown in Fig. 4. The film fabricated with no NaCl electrolyte demonstrated pencil-thin, and crack-free nanorod with an overgrown cluster in some areas on the substrate surface, as also detected in our earlier study.³⁷ When 2 mmol of NaCl of the electrolyte was introduced into the solution, the crowded nanorods were developed, and the formation of nanorods enhanced with the increase in the concentration of NaCl to 4 mmol, showing a larger size and shape as observed in the Fig. 4(c). Very rough, tiny and dense spherical grains as well as some overgrown clusters were seen with an additional increase in the concentration of NaCl to 6 mmol. Such an overgrown cluster was produced due to the coalescence of the particles.⁹⁴ Characteristically distributed, clear, and larger-sized spherical grains were revealed with further addition of NaCl electrolyte of 8 mmol. Thus, the NaCl electrolyte has the potential impact to change the surface morphologies from nanorods to spherical grains. The growth of Cu₂O nanorod thin films was sensitive to the concentration of salts added as also stated for CuO.⁹⁵ The growth of Cu₂O nanostructures was increased gradually with the rise of NaCl concentration but until a limit. Such phenomena signify that NaCl concentration will consequence in similar morphology of the product and perform key roles in governing the size and shape of the Cu₂O nanorods. Moreover, the steric hindrance caused by salt concentration might have affected the micelle aggregates, and these effects collected the assemblies of the products. More investigations are ongoing to elucidate the mechanisms for the growth process caused by the novel anticipated route.

Fig. 4 SEM images of the samples fabricated at (a) 0 mmol, (b) 2 mmol, (c) 4 mmol, (d) 6 mmol, and (e) 8 mmol of NaCl electrolyte.⁹³

4. Factors affecting SILAR deposition process

4.1 Types of copper salts

In the SILAR technique, Cu_xO thin films were studied and fabricated by using different copper salts as summarized in Table 5. Generally, most of the studies were done by using CuSO₄·5H₂O as a basic salt for the formation of Cu₂O thin films, whereas CuCl₂·2H₂O was used for the formation of CuO thin films. The mechanism of both salts was discussed elsewhere in this article.

Table 5 Raw materials are used for the formation of Cu_xO thin films by the SILAR method

Formation	Cationic precursors	Complexing agents	Anionic precursors	Ref.
Cu2O	Cu(CH3COO)2·H2O	Na2S2O3·5H2O	NaOH	96
	CuSO4·5H2O	Na2S2O3·5H2O	NaOH	96
	Cu(NO3)2·3H2O	Na2S2O3·5H2O	NaOH	96
	CuCl2·2H2O	Na2S2O3·5H2O	NaOH	96
	CuCl2·2H2O	NH3	H2O2	97
CuO	CuCl2·2H2O	NH3	H2O	98
	Cu(CH3COO)2·nH2O	NH4CH3COO	H2O	99

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Altindemir et al. examined CuSO₄·5H₂O with the other three different salts such as (CH₃COO)₂·H₂O, Cu(NO₃)₂·3H₂O and CuCl₂·2H₂O to fabricate Cu₂O thin films. Field emission-scanning electron microscope (FE-SEM) photographs of the deposited Cu₂O thin films were demonstrated in Fig. 5(a)–(d). The deposited Cu₂O thin film using the (CH₃COO)₂Cu·H₂O salt demonstrated the cauliflower-like pattern having zero voids between the grains as seen from the images whereas, in the case of the CuSO₄·5H₂O salt, the grain size of the film showed the spherical form having no voids between the grains as in Fig. 5(a) and (b). The grain of the Cu₂O thin film deposited using the Cu(NO₃)₂·3H₂O salt revealed both cauliflower-like and rod shapes as in Fig. 5(c) whereas by means of the CuCl₂·2H₂O salt showed the cauliflower-like shape with discrete spaces between the grains as in Fig. 5(d).

Fig. 5 The FE-SEM images of Cu₂O nanostructured thin films deposited from four different Cu salts⁹⁶ (a) (CH₃COO)₂Cu·H₂O, (b) CuSO₄·5H₂O, (c) Cu(NO₃)₂·3H₂O, and (d) CuCl₂·2H₂O.

On the other hand, CuCl₂·2H₂O salt was employed to fabricate CuO thin films using NH₃ as a complexing agent. But Chatterjee and co-workers utilized H₂O₂ as an oxidizing agent with CuCl₂·2H₂O and NH₃ solution to fabricate Cu₂O thin films instead of CuO thin films.⁹⁵ Similarly, Cu(CH₃COO)₂ nH₂O and NH₄CH₃COO solution could be the potential choice to fabricate CuO thin films.⁹⁹

4.2 pH of the precursor solution

Impact of solution pH on the properties of Cu_xO nanostructured thin films deposited by SILAR was studied in the pH scale range from 2.35 to 12 as shown in Table 6. The study was accomplished by controlling the pH of cationic and anionic precursor solutions by adjusting the additional acid and/or bases such as H₂SO₄, CH₃COOH, NaOH and NH₄OH.

Table 6 Properties of Cu_xO thin films deposited by varying solution pH applying SILAR method

pH	Precursors		To alter pH	Phase formed	Thickness (nm)	Crystallite (nm)	Band gap (eV)	Resistivity × 10^3 (Ω cm)	Ref.
	Cationic	Anionic
a OP = optimized precursor (2 M NaOH).
2.35	[Cu(S2O3)]^−	2 M NaOH^a	H2SO4	Cu2O	340	17	2.05	0.21	36
3.45		2 M NaOH^a	CH3COOH		729	21	2.10	0.18
4.50		2 M NaOH	CH3COOH		800	15–22	2.30	72	37
5.10		2 M NaOH	CH3COOH		1000		2.28	103
6.20		2 M NaOH	CH3COOH		1800		2.43	742
7.33		2 M NaOH^a	—		1130	18	2.15	0.37	36
7.33		1 M NaOH	—		336	13	2.16	0.18
7.95		2 M NaOH	—		1477	15–22	2.42	21.9	37
9.0	[Cu(NH3)4]^2+	H2O	—	CuO	42	37.4	1.61	—	105
9.5		H2O	—		67	22.4	1.49	—
10.0		H2O	—		85	22.9	1.49	—
10.0		H2O	H2SO4		520	14	2.17	6.5	94
10.5		H2O	H2SO4		590	21	2.07	5.5
11.0		H2O	H2SO4		680	27	2.02	4.0
11.5		H2O	H2SO4		770	30	1.99	4.25
12.0		H2O	H2SO4		820	36	1.89	4.5

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To optimize the growth condition to fabricate the FTO/Cu₂O/ZnO heterojunction solar cell, Farhad and co-workers extensively studied the effect of pH in between 2.35 and 7.95.³⁶ During the study, the Cu₂O thin films have grown by slightly modifying the original SILAR method,¹⁰⁰ just by eliminating step 2 as shown in Chart 1 and named the technique as modified SILAR method. 10% H₂SO₄ and CH₃COOH were added dropwise into cationic precursor solution to adjust solution pH, as well as the concentration of anionic precursor (NaOH), was also varied (1–2 M). The addition of CH₃COOH into the optimized precursor solution (OP + CH₃COOH) improved the quality of the crystal having a larger crystallite size of 21 nm whereas H₂SO₄ played the opposite role. Strong H₂SO₄ etches film thickness and it decreased with decreasing pH of the cationic precursor solution. From the SEM micrograph it is observed that the optimized solution (OP ∼ 2 M NaOH) showed compacted and larger spherical grains (D ∼ 231–259 nm) while the non-optimized solution (1 M NaOH) revealed irregular surface morphology with tiny grains (D ∼ 164 nm) which is as shown in Fig. 6. Grain size and band gap decreased with decreasing pH of cationic precursor solution such as 259–164 nm and 2.16–2.05 eV respectively. The electrical resistivity varies in the range of 0.18–0.38 kΩ cm and among optimized solutions OP + CH₃COOH showed the lowest resistivity. The resistivity of the modified SILAR grown samples had (1–5) order magnitudes less than those deposited by Nair and Ristov's SILAR, and electrodeposition method.³⁶

Fig. 6 SEM images representing {Cu₂O: (a)–(d) and CuO: (e)–(g)} thin films deposited by varying pH.^36,105

To justify further the effects of CH₃COOH and rinsing steps, Farhad and co-workers again deposited thin films at pH 4.50–7.95 by adding CH₃COOH into cationic precursor solutions.^37,101 It is seen that film deposited without any rinsing step showed the lowest band gap due to the high film thickness and vice versa which is shown in Fig. 7(a). pH 5.10 which was maintained by adding CH₃COOH exhibited a larger and densely packed grain size (∼300–530 nm) compared to pH 7.95 where no use of CH₃COOH. Band gap and resistivity values were 2.41–2.30 eV and 742–72 kΩ cm respectively and decreased with decreasing pH of cationic precursor solution.³⁷ The activation energy was 0.004–0.19 and 0.01–0.68 eV respectively in the temperature range 40–90 and 100–250 °C. In these studies, FTO/Cu₂O/ZnO heterojunction shows diode-like characteristics¹⁰¹ at pH 7.95 as shown in Fig. 7(b) with maximum power (P) of ∼45 mW cm⁻² under LED illumination with Knee voltage −0.5 V. These results indicate that CH₃COOH has a significant influence on the controlling of the properties of Cu₂O thin films.

Fig. 7 (a) Variation of band gap with rinsing steps. (b) I–V characteristic curve for FTO/Cu₂O/ZnO heterojunction cell grown on FTO substrate at pH-7.95.¹⁰¹

Umeri and coworkers described the effect of pH and growth temperature during the deposition of Cu_xO thin films⁹⁴ at RT and 70 °C in the pH range from 8 to 11. At RT, the pure Cu₂O phase exists with pH 8, while mixed phases of Cu_xO appeared at pH 11. Whereas, at 70 °C, only the pure Cu₂O phase was deposited in the pH region of 8–11, which indicates that 70 °C is the optimum temperature for the growing phase of pure Cu₂O thin films as also supported by other reported results.¹⁰² At 70 °C, the optical band gap (E_g) rises from 1.85 to 2.0 eV with the rise of pH from 8 to 11, while the trend showed the opposite at RT and declines from 2.0 to 1.6 eV with the rise of pH. This might be because of the change in the composition from Cu₂O to CuO. From SEM micrographs, it is seen that at RT, the compact thin film was produced with pH 8 with an overgrown cluster in some spaces and when the pH increased to 11, overgrown cluster formation was diminished, and network-like nanofibers were observed. Conversely, at 70 °C and pH 9, a fiber-like nanostructure was formed, that looked like the morphology of films grown at RT with pH 11. Consequently, uniform, overgrown clusters free of close-packed and interconnected nanofibers of Cu₂O were observed at 70 °C and pH 11 with a band gap of 2.0 eV. Thus, temperature-dependent pH has a significant controlling overgrowth and properties of the deposited films.

Likewise, Cu₂O, the influence of pH on the physical properties of CuO thin films was investigated by Visalakshi et al.¹⁰³ The pH (∼10–12) of the cationic precursor solution was maintained by adding concentrated NH₄OH. Film thickness, crystallite size, and texture coefficient rise with the rising pH of cationic precursor solution but dislocation density and strain decreases. SEM images concluded that pH 10 and 10.5 exhibited cluster-like surface morphologies due to the coalescence of the grains but when it reached pH 11, uniformly distributed spherical grains were observed. At pH > 11 the agglomeration of the grains occurs which outcomes in larger grain size. The optical transmittance and band gap (2.17–1.89 eV) reduces with increasing pH. The resistivity decreases initially from 6.5 × 10³ to 4.0 ×10³ Ω cm with increasing pH from 10 to 11, then further increases with increasing pH. As represented in Fig. 8(a), with the increase of solution pH from 11 to 12, the carrier concentration decreases from 7.1 × 10¹⁴ to 4.8 × 10¹⁴ cm⁻³ which is in good agreement with the obtained result by Saravanakannan et al.¹⁰⁴ Conversely, the mobility is first declined to pH 11; then, it is raised for further growth in pH. The decrease in mobility may be owing to the scattering formed at grain boundaries. The decrease in resistivity of the film synthesized at high pH may be attributable to the growth in film thickness without voids, whereas the rise in resistivity and decrease in carrier concentration and mobility detected at low pH may be owing to the existence of bulky voids. However, almost different properties were exhibited when the sample was annealed at 400 °C for 2 hours after deposition reported by Gençyılmaz and co-workers.¹⁰⁵

Fig. 8 (a) Electrical resistivity, (b) mobility and carrier concentration variation of SILAR-coated CuO thin films with pH.^94,103

4.3 Number of cycles during deposition

The film characteristic is closely related to the number of immersions of the substrates into the precursor solution. In almost all studies, the number of cycles is generally kept constant to understand the other properties of the Cu_xO films. There are few investigations where the effect of the number of cycles on the fabrication of Cu_xO films^106–110 is discussed, as summarized in Table 7. It is obvious that with the increase of immersion cycles thickness of the films increase.

Table 7 Effect of the number of cycles on the fabrication of SILAR grown Cu_xO films

Product	Cycles	Thickness (nm)	Crystallite (nm)	Dislocation density (δ × 10^−3, nm^−2)	Strain (ε ×10^−3)	Bandgap (eV)	Ref.
Cu2O	50	550	16	3.72	6.82	2.11	106
	75	830	19	2.90	6.01	2.06
	100	1050	20	1.51	4.35	1.84
	10	120	2.46	—	—	2.01	107
	30	350	5.23			2.48
	40	520	7.15			2.53
	20	654	—	—	—	2.48	37
	40	1130				2.45
	60	1200				2.41
	80	1477				2.38
CuO	30	500	11	8	3.12	1.56	108
	40	850	13	6	2.66	1.52
	50	950	18	3	1.95	1.48
	20	87	7	2.04	3.62	2.48	109
	30	179	8	1.56	3.73	2.41
	40	298	9	1.23	3.83	2.37
	50	415	11	0.83	4.87	2.31
	30	—	10	10	3.43	1.92	110
	40		15	4.44	2.27	1.89
	50		24	1.74	1.42	1.69

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The surface SEM morphologies of m-SILAR deposited Cu₂O films in the top of the FTO substrates using 40, 60, and 80 immersion cycles were shown in Fig. 9(a)–(c). Throughout the area investigated, the surface morphology of all films was seen to be compact as well as coherently carpets. However, the Cu₂O film grown with 40 immersion cycles demonstrated fiber-like microstructures with small grains of around 200 nm. Instead, thin films having 60 and 80 immersion cycles exhibited bigger spherically shaped grains of size around (200–550) and (350–650) nm respectively as shown in Fig. 9(b) and (c). This reflection recommends that grain size develops as the thickness increases with the increase of immersion cycles.³⁷ As can be seen from Table 7 and Fig. 9(d), the optical bandgap of the Cu₂O thin films deposited at pH ∼ 7.95 using 20 cycles (film thickness ∼ 654 nm), 40 cycles (film thickness ∼ 1130 nm), 60 cycles (film thickness ∼ 1200 nm) as well as 80 cycles (film thickness ∼ 1477 nm) were calculated to be ∼2.48, ∼2.45, ∼2.41 and ∼2.38 eV respectively. Obviously, there is a decreasing trend of optical bandgap with increasing film as represented in Fig. 9(d), probably due to the bigger grains usually existing in the thicker films, which verifies the results stated by Nair et al.¹¹¹

Fig. 9 SEM micrographs of the films grown with different immersion cycles at pH: 7.95 for (a) 40 cycles (b) 60 cycles (c) 80 cycles and (d) film thickness of Cu₂O thin films using different immersion cycles grown by m-SILAR.³⁷

Fig. 10(a)–(c) demonstrates the Cu₂O nanostructured films fabricated with different dipping cycles of the nanorods spread homogeneously on the substrate surface, showing a large number of grains with fine particle edges. As seen from morphological studies and Table 7, 50 cycles grown sample has a smaller crystallite size and higher dislocation density with good nanorod morphology. For light absorption, although it has an opportunity for a larger surface area to the photoelectrode, due to the presence of considerable grain boundaries, it creates recombination problems in the film. So, the electron trapping at the surface and in the intergrain boundaries lowered the efficiency value of the film grown through 50 cycles. The samples deposited by 75 and 100 cycles have comparatively better crystallite size and lower dislocation density, which leads to reduce grain boundaries.¹⁰⁶ Due to the drop in grain boundary resistance, the photogenerated charge carriers can significantly reduce the recombination losses. The cell was fabricated as ITO/ZnO NRs/Cu₂O/Al with varied efficiency mainly due to the number of cycles of the films shown in Fig. 11. Even though the attained efficiency of the ZnO/Cu₂O heterojunction was lower, the efficiency was high in the samples deposited at high cycles such as 100. Hence, the effect of the film thickness on cell performance was evidenced by the enhancement of efficiency due to the substantial development of crystallinity and absorbance of Cu₂O films.

Fig. 10 SEM image of the Cu₂O nanostructured films deposited at (a) 50 (b) 75 and (c) 100 dipping cycles.¹⁰⁶

Fig. 11 Current–voltage curve of ITO/ZnO NRs/Cu₂O/Al cells (a) 50 (b) 75 and (c) 100 dip Cu₂O films.¹⁰⁶

4.4 Effect of bath temperature

Fig. 12(a) illustrates the deposition of the thin films grown by varying bath temperatures of anionic precursors as a function of the immersion cycles. The figure demonstrated that the fabrication rate reduced as the immersion cycle proceeds characteristically at 10 nm per cycle. In the case of fabrication using the alkali solution at 90 °C, the production was faster, and the film thickness was >0.3 μm with 20 cycles of immersions, whereas, at 70 °C, the film growth slightly falls after 20 immersions.¹¹¹ For the films fabricated with NaOH solution temperature of 50–90 °C, the photo response curves were given for a range of thicknesses as demonstrated in Fig. 12(b). Irrespective of the solution temperature, the dark current and the photocurrent logged in the films were comparable for the films with thicknesses smaller than 0.1 mm. The values were higher in samples fabricated using NaOH solution at 70 °C having films of higher thickness. The measured electrical conductivity of a 0.15 μm film is about 5 × 10⁻⁴ Ω⁻¹ cm⁻¹. And it was found that the increase of film thickness of two orders increases the conductivity by nearly two orders.

Fig. 12 Samples in NaOH solution (a) at different temperatures as a function of film thickness and immersions. (b) At the designated temperatures to record the photocurrent response of the Cu₂O thin films of different thicknesses.¹¹¹ [Light source: tungsten–halogen, intensity of illumination: 1 kW m⁻², time length: 60–180 s, bias: 1 V applied across Ag print electrodes 5 mm (long) × 5 mm (separation)].

The structural parameters, elemental composition, and optical band gap for different bath temperature of Cu₂O films are given in Table 8, studied by Baig et al.¹¹² It is seen from the table that when the temperature climbed from 40 to 80 °C, the size of grain increased from 16.78 to 18.84 nm whereas strain in the crystal lattice was reduced. The fall in strain signifies that the imperfection in the crystal lattice with the rising temperature was decreased. The SEM images of Cu₂O thin films deposited on ITO substrate with different anionic bath temperatures are demonstrated in Fig. 13. From the figures, with the rise of anionic bath temperature the structure of the wire became compact compared to that at 40 °C. Likewise, the oxygen concentration was decreased with an increase in temperature as observed in Energy-dispersive X-ray spectroscopy (EDS) value of Cu₂O film in Table 8. Further, the photocatalytic activity for water splitting by the deposited Cu₂O thin films at different temperatures was studied in a photochemical cell and the result revealed that film grown at 80 °C had a higher current ratio with respect to the other two samples and the photocurrent produced by that sample is relatively steady (figure in 5.2 section).

Table 8 Structural parameters, elemental composition (EDS), and optical band gap for different bath temperatures of Cu₂O films

Bath temperature (°C)	D (nm)	Dislocation density (δ) × 10^14 m^−2	Micro strain (ε)	Atomic (%)		Band gap (eV)
				Cu	O
40	16.78	35.51	0.007025	50.75	49.25	2.25
60	17.10	34.19	0.006892	56.45	43.55	2.14
80	18.84	28.17	0.006193	61.76	38.24	2.07

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Fig. 13 SEM images of Cu₂O nanostructured thin films deposited by SILAR at different bath temperatures (a) 40 °C, (b) 60 °C, and (c) 80 °C.¹¹²

4.5 Addition of additives

The influence of the different additives on the surface morphological characteristics of CuO films was studied by using SEM. Fig. 14 illustrates the SEM images of the CuO thin films fabricated in the solution containing additives such as coumarin, saccharin, and sodium dodecyl sulfate (SDS) having different concentrations. In the first step without coumarin, Fig. 14(a) there were plate-like CuO nanostructures that homogenously cover the entire surface. Then, the nanostructures start to change their shapes with the increase of coumarin concentration, form some clusters on the surface and lose their homogeneity. From Fig. 14(b), in the case of saccharin, it was observed that all the samples have nearly the same morphology that is they are all composed of plate-like nanostructures as in ref. 113. On the other hand, the homogeneity of the films deteriorates with increasing saccharin concentration. In the case of SDS, Fig. 14(c) demonstrates that the fabricated CuO thin films were adhered and spread well onto the substrate surface without SDS. With the addition of SDS and its concentration, the surface morphology of the CuO film was changed dramatically. From this viewpoint, the discrepancy of SDS molar concentration impressed the morphology of the surface of all the fabricated thin films. This variation in morphology may be owing to the electrostatic interaction between Cu²⁺ and CH₃(CH₂)11OSO₃⁻. SDS may affect particle growth as well as morphology after nucleation. Thus, during the crystallization process, the SDS can affect nucleation.¹¹⁴

Fig. 14 SEM images of CuO nanostructured thin films as a function of different additive concentrations, such as (a) coumarin,¹¹⁵ (b) saccharin¹¹⁶ and (c) SDS.¹¹⁷

Through UV-Vis's spectrophotometer study, it was clear that both the optical band gap and the transmission spectra were affected by the additive concentration. The band gap, as well as spectral transmittance values of the films, were decreased for the higher content of both coumarin and saccharin,^115,116 while showing the opposite tendencies in the case of SDS. The optical bandgap energy of both organic (coumarin and saccharin) additives decreased from around 1.50 to 1.27 eV, while increased from 1.32 to 1.49 eV for inorganic SDS, with the increasing concentration of the additives.¹¹⁷

4.6 Complexing agent

Cavusoglu and co-workers studied the role of the complexing agent such as triethanolamine (TEA) mediated fabrication of nanocrystalline CuO thin films via SILAR technique at room temperature and the results are summarized in Table 9. As a function of increasing TEA concentration, the optical band gap energy of the fabricated CuO thin films was increased from 1.33 to 2.00 eV while the average transmittance of all the films increased from 2.5 to 42.5%. A minimum resistivity of 3.74 × 10⁵ Ω cm was found with zero TEA in CuO thin films whereas, with a TEA concentration of 1.0 M%, the resistivity subsequently increased to 509 × 10⁵ Ω cm. Surface morphology on the film surfaces demonstrated the homogeneous distribution of the nanostructured CuO as demonstrated in Fig. 15(a)–(d) whereas, the figure of merit (FOM) was represented as a function of TEA concentration as shown in Fig. 15(e). TEA concentration of 0.25 M% in CuO thin film provided the high FOM values of 786 × 10⁻¹² Ω⁻¹ at distinct wavelengths of between 600 and 900 nm.¹¹⁸ Therefore, the range of optical and electrical properties developed by such a study having a different complexing agent is favorable for the applications of numerous optoelectronic devices.

Table 9 Properties of CuO thin films as a function of TEA concentrations

TEA concentration M%	Crystallite (nm)	Thickness (nm)	Bandgap (eV)	Conductivity (σ) ×10^−6 (Ω cm)^−1	Resistivity (ρ) ×10^5 Ω cm	FOM × 10^−12 Ω^−1
0	19.95	797	1.33	2.67	3.74	149
0.25	19.80	387	1.57	1.05	9.50	786
0.50	18.92	199	1.67	0.07	149	37.2
1.00	17.47	101	2.00	0.02	509	1.70

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Fig. 15 SEM images of CuO thin films at (a) 0 M%, (b) 0.25 M%, (c) 0.50 M%, (d) 1.00 M% as well as (e) FOM with TEA at different molar concentrations.¹¹⁸

4.7 Annealing of as-deposited films

Annealing is a vital parameter to control the phases of the deposited thin films. Both the phases of Cu_xO could be synthesized by changing the atmospheric condition (air, vacuum) and temperature of annealing as summarized in Table 10. Here, air or oxygen^{37,101,119–124} annealing of the SILAR-grown films has been studied more extensively than vacuum annealing.¹²⁵ Recently, SILAR deposited Cu_xO films are mainly studied between 75 to 500 °C^{37,101,119–124} in presence of air or N₂. The study revealed that Cu₂O phase was stable until 250 °C,^120,122 though Farhad and co-workers showed a mixed phase of both Cu₂O and CuO at 250 °C³⁷ due to the pH effect, while Ozaslan et al. showed a mixed phase even at 500 °C.¹²³ The CuO phase could be found at 300 °C¹²⁰ by annealing of Cu₂O or even could be deposited by using NH₃ solution (pH = 10) with the reaction of CuCl₂ at ambient temperature.¹²⁴

Table 10 Annealing of the SILAR grown Cu_xO nanostructured thin films

Anionic salt, NaOH (M)	Cycles	Time		Temperature		Crystal phase	Crystallite (nm)	Band gap (eV)	Ref.
		Dipping (s)	Annealing (min)	Growth (°C)	Annealing (°C)
CuSO4·5H2O + NaOH + Na2S2O3·5H2O
0.5	20	20	—	70	200–400	As deposited: Cu2O	27.76	—	119
						200 °C: Cu2O	49.95
						300 °C: Cu2O + CuO	40.88
						400 °C: CuO	62.32
1	10	20	60	70	200–350	As deposited: Cu2O	14	2.20	120
						200 °C: Cu2O	14	2.20
						250 °C: Cu2O	14	2.20
						300 °C: CuO	14–26	1.35
						350 °C: CuO	—	1.35
1	30	20	—	70	200–400	As deposited: Cu2O	14–26	2.40	121
						200 °C: Cu2O		2.40
						300 °C: Cu2O + CuO		2.06
						400 °C: CuO		1.73
1	30	20	—	50–90	250–400 (air, N2)	As deposited: Cu2O	∼18	2.10	122
						250 °C: Cu2O		2.10
						300 °C: Cu2O + CuO		—
						350 °C: CuO		1.75
						400 °C: CuO		1.75
2	40	2	60	70	100–500	As deposited: Cu2O	—	2.57	123
						100 °C: Cu2O		2.52
						300 °C: Cu2O (2.27%) + CuO (97.73%)		2.45
						500 °C: Cu2O (1%) + CuO (99%)		1.91
2	40–80	—	60–180	70	75–350	As deposited: Cu2O	15–22	2.42	37
						75 °C: Cu2O		2.02
						150 °C: Cu2O		1.98
						200 °C: Cu2O		1.94
						250 °C: Cu2O + CuO		1.62
						350 °C: Cu2O + CuO (1 h)		—
						350 °C: CuO (3 h)		1.44
2	60	5	60	70	350	As deposited: Cu2O		2.06–2.16	101
						350 °C: Cu2O + CuO (1 h)		1.43–1.51
CuCl2 + NH3OH + H2O2
—	2–10	30	30	RT	20–500	As deposited: Cu2O	14	2.17	124
						100 °C: Cu2O	14	2.22
						150 °C: Cu2O	14	2.17
						450 °C: CuO	16	1.43
						500 °C: CuO	16	1.44
—	400	>30	60	RT	27–600 (air, vacuum)	As deposited: Cu2O	14–21	2.30	125
						100 °C: Cu2O		2.40
						400 °C: CuO		1.85
						600 °C: CuO		1.70
CuCl2 + NH3 solution (pH = 10)
—	80	30	30	—	200–400	As deposited: CuO	11.09	1.17	126
						200 °C: CuO	12.05	1.29
						300 °C: CuO	13.86	1.30
						400 °C: CuO	14.88	1.36

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Amudhavalli and co-workers successfully showed the increasing trend of the crystallite size of copper oxides with the increase of annealing temperature while depositing the films at 0.5 M NaOH. Fig. 16 demonstrated the change of resistivity, mobility, and carrier concentration of copper oxide (Cu₂O and CuO) phases with annealing temperature as shown by Ozaslan et al.¹²³ It is found that the carrier concentration was decreased from 3.07 × 10¹⁷ to 6.61 × 10¹⁵ cm⁻³ with increasing annealing temperature from 70 to 500 °C respectively. The hole mobility of the films was increased from 4.20 to 31.87 cm² V⁻¹ s⁻¹ with decreasing the carrier concentration, while the electrical resistivity of the films decreased with annealing temperature, inducing the increment in the conductivity of the films. Nair et al. observed the dark conductivity of the CuO film produced by air annealing of a 0.15 μm Cu₂O film at 400 °C is high about 7.2 × 10⁻³ Ω⁻¹ cm⁻¹.

Fig. 16 Representation of the change of resistivity, mobility, and carrier concentration of copper oxide (Cu₂O and CuO) films with annealing temperature.¹²³

4.8 Doping by diverse dopants

Tuning of the structural, electrical, and optical properties of SILAR-deposited Cu_xO films through Fe, Eu, Zn, Co, B, Mg, Ni, and Pb doping has been reported extensively by several authors. The summary of the effect of doping on SILAR-deposited Cu_xO films is represented in Table 11. Interestingly, in the case of Cu₂O film fabrication, the reactants were the same except for the doping materials such as Fe, Eu, Zn, and Co.^127–130 Similarly, during Co, B, Mg, and Pb doping into CuO films,^131–134 the reactants were also the same except in the case of Ni doping.¹³⁵

Table 11 Cu_xO nanostructured materials grown in presence of different dopants with their properties

Product	Dopant		Cycle	Crystallite (nm)	Grain (nm)	Bandgap (eV)	Ref.
	Material	Amount
Reactants: CuSO4·5H2O + NaOH + Na2S2O3·5H2O
Fe: Cu2O	FeSO4 (wt%)	0	30	62.83	—	1.80	128
		1		59.80		2.10
		2		41.83		2.36
		5		36.40		2.45
Eu: Cu2O	Eu(NO3)3·5H2O (at%)	1	100	27	—	2.08	129
		3		24		2.26
		5		21		2.41
Zn: Cu2O	ZnSO4 (wt%)	0	50	18	—	2.34	130
		1		30		2.35
		2		39		2.37
		3		52		2.38
		5		69		2.41
		10		44		2.39
Co: Cu2O	CoSO4 (wt%)	0	30	62.83	—	1.94	127
		1		53.30		2.03
		2		48.47		2.12
		5		39.24		2.18
		10		28.44		2.47
Reactants: CuCl2·2H2O + H2O + NH3
Co: CuO	CoCl2·6H2O (at%)	0	10	22.7	70	1.53	132
		0.5		15.7	44	1.47
		1		13.6	42	1.45
		2		13.1	36	1.41
		3		12.6	32	1.38
		4		12.2	38	1.36
B: CuO	H3BO4 (at%)	0	10	12.9	45	1.52	131
		1		13.1	42	1.48
		2		14.2	38	1.43
		3		15.9	30	1.39
Mn: CuO	Mn(NO3)2 (at%)	0	10	—	9.94	1.42	133
		1			7.83	1.98
		3			8.21	2.08
		5			9.76	2.20
Pb: CuO	Pb(NO3)2 (at%)	0	10	—	9.94	1.43	135
		1			17.22	1.80
		2			16.21	1.76
		4			15.79	1.72
		8			13.07	1.68
		16			8.98	1.65
Reactants: Cu(CH3COO)2·nH2O + H2O + NH4CH3COO
Ni: CuO	Ni(CH3COO)2·nH2O		30	—	10–15	—	134

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In the case of Co or Fe doping, the crystallite size of the films of Cu₂O decreased between 62.83 and 28.44 nm when the concentration of the doped material increased gradually, whereas it showed an opposite trend in the case of Zn doping. Interestingly, Fe, Eu, Zn and Co doping into Cu₂O rises the bandgap of the material a little in every case. On the other hand, in the case of Co and B doping into CuO films, the bandgap deceased while it was increased for Mg and Pb doping.

The films prepared at high doped Cu₂O thin films such as 5% Eu showed a low resistivity value of 1 × 10³ Ω cm as shown in Fig. 17. The Hall mobility and carrier concentration values in such cases are 0.52 cm² V⁻¹ s⁻¹ and 13.8 × 10¹⁵ cm⁻³, respectively.

Fig. 17 Resistivity, carrier concentration and mobility for Eu doped Cu₂O films.¹²⁹

Fig. 18(a) shows the current density–voltage (J–V) characteristics of the ZnO/Cu₂O heterojunction solar cells prepared using the Eu-doped Cu₂O thin films. The V_oc was increased with increasing Eu content from 265 mV (1% Eu) up to 332 mV (5% Eu). The conversion efficiency can be enhanced by dropping recombination centers avoiding lattice-mismatch defects, and by reducing the resistance of Cu₂O. The ionic radius of Eu³⁺ ion was 0.109 nm whereas, Cu⁺ is 0.077 nm. Therefore, Eu³⁺ ion could not be incorporated by substitution rather it was incorporated as an interstitial creating getter center. It overwhelms the recombination losses and thus advances current levels and improved Eu doping levels.¹³⁶

Fig. 18 (a) 1%, 3% and 5% Eu doped thin films having current–voltage characteristics of ITO/ZnO NRs/Eu: Cu₂O/Al cell, (b) carrier transport and band structure of p–n junction.¹²⁹

Fig. 18(b) illustrates the band structure as well as carrier transport of the deposited p–n junction. As there was much difference between conduction and valence band off-sets triggering effective separation of charge carriers, a built-in potential barrier was developed. When the light was absorbed onto the device photocarriers were generated and drifted to the respective electrodes depending upon the applied potential causing current conduction. As an acceptor dopant, impurity levels of Eu were adjacent to the valence band edge. In the case of ZnO, the green luminescence at 535 nm could be produced by the diffused Cu ion and replacing Zn. The V_O center was atop the valence band whereas the Zn vacancy was (V_Zn) in an acceptor level, which occurred at 0.8 eV. Nevertheless, the ZnO coated over Eu: Cu₂O performed as a passivation layer improving the V_oc and declining the consequence of impurity center-mediated recombination loss.^137,138

Magnetic measurements were performed by employing a vibration sample magnetometer (VSM) at ambient temperature for both Fe and Co-doped Cu₂O. Undoped Cu₂O has a diamagnetic property.¹³⁹ The outcome agrees with Fig. 19(a) and (b) which demonstrate the change of magnetization against the applied magnetic field (M–H). In the case of Co-doped Cu₂O, undoped and minimum doped such as 1 and 2 wt% films showed diamagnetic (high magnetization) behavior whereas, at the maximum doped such as 10 wt%, the films showed ferromagnetic (low magnetization) properties.¹⁴⁰ The diamagnetic order was Cu_2−xCo_xO (x = 0 > 1 > 2 > 5 > 10 wt%). The ferromagnetic behavior was possibly due to the intrinsic coupling (Co–Co) between the atoms of doped material. Similarly, in the case of 1% Fe doped Cu₂O at 305 K, the film showed diamagnetic properties. An increase of the Fe-doping (2 wt%), slightly altered the diamagnetic property because of the increased hole concentrations and further doping of Fe ions (5 wt%), the film showed anti-ferromagnetic behavior. With the increase in the concentration of Fe, both the number of Fe³⁺–Cu²⁺ pairs and the hole concentrations increased and consequently, the crystallite size reduced.

Fig. 19 Magnetic behavior of (a) Co-doped¹²⁷ (b) Fe-doped Cu₂O thin films.¹²⁸

Lobinsky and co-workers studied the cyclic voltammograms of the nickel foam electrode with Ni-doped CuO nanolayers in a potential space between 0 and 550 mV vs. Ag/AgCl electrode at the scanning rates of 5, 10, 15 and 20 mV s⁻¹ as shown in Fig. 20. Two of the redox reactions on the anodic curve took place in the layer, including the Cu⁺ → Cu²⁺ transformation at 310 mV while the Ni²⁺ → Ni³⁺ at 390 mV at a scan rate of 5 mV s⁻¹. The proportionality of currents to scan rate delivers data that the film is sufficiently thick, and the charge transfer rate was restricted by the diffusion of charge carriers in the film.¹³⁴

Fig. 20 CVA curves for nickel foam electrode with Ni-doped CuO nanolayers at a scan rate of 5, 10, 15 and 20 mV s⁻¹.¹³⁵

Inset of Fig. 21 demonstrates the specific capacitance of the Ni-doped CuO nickel foam electrode, which was found from charge–discharge curves, and it was 154 mA h g⁻¹ (1240 F g⁻¹) at the current densities of 1 A g⁻¹.¹³⁵ The high value of the specific capacitance of the sample can be explained based on the good conductivity of CuO and the substantial role of Ni atoms in pseudo-capacity. The capacity retention of the Ni foam electrode with Ni-doped CuO nanolayers after 1000 charge–discharge cycles at a current density of 2 A g⁻¹ was retained at 92%, showing good cycling stability of the material as presented in Fig. 21. High cycling stability could be described by the feature morphology of ultrathin nanocrystals of CuO which deliver fast diffusion of ions on the electrode surface and while not being ruined in the charge–discharge process.

Fig. 21 The cycling stability for the Ni Foam electrode with Ni-doped CuO nanolayers at 2 A g⁻¹ whereas the inset graph represents galvanostatic charge–discharge curves of the electrode with Ni-doped CuO nanolayers.¹³⁵

5. Applications

The optoelectronic properties of SILAR synthesized thin films have shown outstanding performance in diverse applications, for instance, photovoltaics,¹⁴¹ supercapacitors,^142,143 photoelectrochemical water splitting,¹⁴⁴ gas sensors^143,145 and many more. The method appears to be easier and represents an efficient way to manufacture devices. Some of the potential applications such as antibacterial activities, supercapacitors, surface wettability and photoelectrochemical water splitting in presence of Cu_xO nanostructured thin films will be discussed in the following section.

5.1. Antibacterial activities

To control pathogens, nanoparticles are in great demand due to their huge applications in the health industries. Results achieved from nanocrystalline Cu₂O nano-thin films fabricated by Dhanabalan et al. possessed substantial antimicrobial activity against the experienced human pathogen at a maximum inhibition zone of 16 mm against Gram-positive Staphylococcus aureus.¹⁴⁶ The surface morphological studies exhibited that the needle-shaped grains which play a crucial role in the antibacterial activity of the fabricated Cu₂O films by SILAR technique as shown in Fig. 22(a) and (b). The synthesized Cu2O thin film can exhibit antibacterial activity from 18 to 24 hours of incubation time. The bacterial growth will decrease with the increase in the concentrations of nanoparticles, which may be the cause of the reduction of voids affording space for the growth of bacteria that remains resistant to the pathogenic bacterial strain.

Fig. 22 (a) TEM (b) SEM images of the Cu₂O grains showing needle-shaped uniformity on the surface.¹⁴⁶

5.2. Water splitting

Cu_xO was considered a good candidate for photoelectrochemical (PEC) water splitting due to its abundance, low price, and high stability in aqueous solution.^147,148 Baig et al. fabricated Cu₂O at a high bath temperature of 80 °C by SILAR which showed high photocurrent and good stability¹¹¹ as discussed earlier. The photocatalytic activity for water splitting of the Cu₂O thin film was studied with the photochemical system containing Pt (counter), Ag/AgCl (reference), Cu₂O (working electrode) and KCl (pH = 13.6) as electrolytes. PCE data shown in Fig. 23 revealed that samples synthesized at 80 °C have a higher current ratio and produced a stable photocurrent compared to the other samples by using a 300 W Xenon lamp (PLSSXE300/300UV).

Fig. 23 PEC measurement of Cu₂O thin films at different bath temperatures.¹¹¹

5.3. Surface wettability study

The surface wettability study of films determines its capability to interact with ions when immersed into electrolyte by measuring the contact angle with liquid electrolyte as shown in Fig. 24.¹⁴⁹ If the contact angle is >90°, then the film surface is hydrophobic, while for <90°, it is hydrophilic. For better interaction of electrolyte ions, the contact angle must be as low as possible with the electroactive site on the thin film surface.^150,151 Fig. 24 (A1, A2, A3, and A4) signifies the image of the contact angle with the surface of the film. The observed angles of CuO thin films with 50, 60, 70 and 80 SILAR cycles were 65°, 58°, 50°, and 43°, respectively. The observed CuO films were hydrophilic in nature, as the contact angles for CuO decline with the rise in SILAR cycles, which will allow more interaction of electroactive sites of the CuO on the film surface.

Fig. 24 Study of surface wettability of CuO thin films,¹⁴⁹ (A1) 50 SILAR cycles, (A2) 60 SILAR cycles, (A3) 70 SILAR cycles and (A4) 80 SILAR cycles.

5.4. Super capacitive behavior

The electrochemical impedance, as well as super capacitive properties, of SILAR synthesized CuO thin films are studied by Patil et al.¹⁴⁹ The synthesized CuO thin film showed the lowest charge transfer resistance of 41.45 Ω cm⁻² with the highest specific capacitance of 184 F g⁻¹ at the scan rate of 50 mV s⁻¹ and demonstrated 83% capacitive retention after 5000 cycles. Super capacitive performance of the film was verified using cyclic voltammetry (CV) in 1 M KOH electrolytes in a three-electrode cell equipped with CuO (working electrode), Pt (counter electrode) and saturated Ag/AgCl (reference electrode). As shown in Fig. 25, the CVs were studied with a potential window of 0 to 0.6 V/Ag/AgCl at several scan rates such as 10, 20, 50 and 100 mV s⁻¹.

Fig. 25 Cyclic voltagramms of CuO with various scan rates of 1 M KOH electrolyte.¹⁴⁹

To examine the charge–discharge properties of CuO, the chronoamperometry technique was applied. Fig. 26(a) demonstrates galvanostatic charge–discharge curves at various current densities for CuO and signify a good capacitive behavior of CuO electrode as ref. 152. In Fig. 26(b), the difference of specific capacitance with various scan rates was displayed, which enhanced exponentially with decreasing scan rate.¹⁵³ The electrochemical stability of CuO film electrode was examined by applying CVs at a scan rate of 100 mV s⁻¹ for 5000 cycles. Fig. 26(c) demonstrated the cyclic voltammetry scan of CuO film electrode after the 1st to 5000th cycles and confirmed cyclic stability of 83% after 5000 cycles. By using the Ragone plot, the highest values of specific power and specific energy were measured as 3 and 14.1 W h kg⁻¹, respectively, using the GCD technique at a current density of 1 mA cm⁻² for CuO electrode attained in the potential range from 0 to 0.5 V as shown in Fig. 26(d).

Fig. 26 (a) Representation of galvanostatic charge–discharge curves of CuO at diverse current densities. (b) Change of specific capacitance with scan rate (c) study of stability of CuO thin films (d) specific energy versus specific power of Ragone plot.¹⁴⁹

Moreover, the electrochemical investigation of Ni-doped CuO nanolayers modified with Ni foam electrodes synthesized by Lobinsky et al. revealed the specific capacitance of 154 mA h g⁻¹ (1240 F g⁻¹) at a current density of 1 A g⁻¹,¹³⁵ as already discussed in the doping section. Thus, SILAR-grown CuO material can be potential usage as an electroactive resource for alkaline batteries and pseudo-capacitors.

5.5. Photoelectrochemical characterization

The photo-responsive performance of m-SILAR grown Cu₂O/FTO electrodes was studied by Farhad and co-workers, through transient surface photovoltage under periodic illumination of a green LED by a HITACHI VG-4429 generator with ∼0.1 Hz–square wave for ‘5 s ON and 5 s OFF’ cycle.³⁶ The generated surface photovoltage of the Cu₂O/FTO electrode was observed by a Keithley SMU 2450 by employing Cu₂O/FTO thin films as a working electrode, a graphite rod as a counter electrode and 0.1 M Na₂SO₄ aqueous solution as an electrolyte as established in Fig. 27(a) and (b). In the presence of an aqueous electrolyte, upon 2500 s LED exposure of the photocathode, the estimated V_oc for the samples grown with the non-optimized precursor, optimized precursor, and optimized precursor with CH₃COOH precursor solutions, were observed as 247 ± 38 μV, 36.0 ± 2.0 mV and 47 ± 8 μV respectively. The large V_oc value projected for optimized precursor film revealed a better Schottky junction produced at Cu₂O/electrolyte interface, consequently, advocating a better optoelectronic quality of Cu₂O thin film. The transient surface photovoltage and V_oc retention for 5000 s, advocating better stability of the SILAR fabricated Cu₂O thin films in aqueous electrolyte.

Fig. 27 (a) A representation of the transient surface photovoltage measurement system; (b) measurement of a typical Cu₂O/FTO electrode in a PEC with 0.1 M Na₂SO₄ electrolyte followed by real-time measurement in SMU 2450 demonstrating the shape of the LED modulated transient surface photovoltage.³⁶

6. Conclusion

In summary, Cu_xO thin films have been extensively studied and are receiving profound attention because of their fascinating properties and promising uses in a variety of fields. In this article, an inclusive review of the state-of-the-art research activities of diverse Cu_xO thin films was represented based on the SILAR method. This technique has fascinated substantial attention because of its simplicity and low cost, demands less time, and is fit for the large-scale growth of Cu_xO. The morphology, as well as diverse properties of Cu_xO, can be monitored by altering the number of SILAR cycles, the pH of precursor solutions, types of salt, bath temperature, annealing, doping, and the dipping time allowed for reactions. However, the technique does not yet allow for precise control of Cu_xO particle sizes, which can affect the power conversion efficiency in optoelectronic devices. The main limitation of this technique is the high rate of surface roughness as well as less study of the defects in the deposited sample which is very important to control the optical as well as electrical properties in optoelectronics. Having the optimum amount of the deposited Cu_xO is a very significant factor in improving optoelectronic performance. Thus, the inclusion of ligands, complexing agents and surfactants in the precursor solution employed during the SILAR growth could advance the stability of Cu_xO. Precise control of Cu_xO fabrication could accelerate multiple exciton generation effects, leading to a development of overall efficiency.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is funded by Comilla University and University Grants Commission, Bangladesh. The authors like to acknowledge the assistance and scientific contribution from the Department of Chemistry, Comilla University, Cumilla 3506, Bangladesh, as well as the collaborative support from the Department of Electrical and Electronics Engineering, Saga University, Japan and Center for Nanotechnology, the Department of Natural Sciences, Coppin State University, Baltimore, MD, USA.

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