Copper oxide nanostructured thin films processed by SILAR for optoelectronic applications

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. CuxO 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 CuxO nanostructured thin films deposited by the SILAR technique. After a brief introduction to the advantages of CuxO 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 CuxO 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 CuxO processed by the SILAR. It will describe the theory, mechanism, and factors affecting SILAR-deposited CuxO. Finally, conclusions and perspectives concerning the use of CuxO materials in optoelectronic devices will be visualized.


Background
Copper (Cu) and copper oxide (Cu x O) thin lms 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 x O 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 fulll 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][2][3][4][5][6] Furthermore, Cu x O nanostructures are extensively used in other diverse applications, including photovoltaics, 7 photodetectors, 8 nanouid, 9,10 energetic materials, 11 eld 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 Cu 2 O 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 10 5 cm −1 . Since the theoretical limit of the energy conversion efficiency of Cu 2 O 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 x O solar cells, but the performance remains very poor. 25 In the case of Cu 2 O 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 lm growth technique for Cu x O. Thus, the research of Cu x O thin lms has both high-tech and scientic consequences. Cu x O nanostructured thin lms 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 exibility on substrate choice, the capability of large area fabrication and deposition of the stable and adherent lm, low processing temperature for lm 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 lms led the technique unique to study in an extensive manner.
More than a few reviews of different aspects of Cu x O-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 x O-based thin lms 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 x O-based thin lms 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 x O deposition which is followed by the application of Cu x O in section ve. Finally, conclusions and perspectives concerning the use of Cu x O in optoelectronic devices are presented.

Properties of copper oxides (Cu x O)
Cu 2 O 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 bodycentered 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 ts into a monoclinic structure with the space group C2/c and the lattice parameters are represented in the table (PDF No. . 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.

Band-structure calculation
Ab initio calculations are mandatory to understand the optical and electronic properties of the Cu x O systems. But there is a challenge for standard ab initio investigations based on DFT for both Cu 2 O and CuO. The exchange-correlation function is the crucial ingredient in the theoretical description. Fig. 1  2. Thin film deposition process

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

Advantages and disadvantages of deposition techniques
Till now, a lot of deposition techniques are available to fabricate high-quality thin lms 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.

Theory and mechanism of SILAR process
SILAR is an extensively applied technique to fabricate highquality metal oxide or halide thin lms. 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 lm of the compound, A x B 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: 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][88][89][90] In the case of Cu x O lm deposition mechanism, salts of Cu 2+ are used to deposit copper oxide thin lms. In most of the research on Cu 2 O, rstly copper(I) thiosulfate complex is formed by the redox reaction between Cu 2+ and S 2 O 3 2− ions which results in a colorless solution. The corresponding reactions are: Oxidation half-reaction: Reduction half-reaction:

Produces powdery lms
Various types of substrates are used 68 Failed to control lm thickness Tuning lm qualities by controlling growth parameters Deposited lms 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 lms (∼10 cm 2 ) 71 Substrate surface must be balanced completely through precursor solution 73 78,79 Less effective in nanotechnology due to quick drying 81 2Cu 2+ + 2e − / 2Cu + Overall reaction: In the above reactions, [Cu(S 2 O 3 )] − 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 Cu 2 O thin lm is formed due to the reaction between Cu + and OH − ions. Rinsing is carried out aer every immersion to exclude loosely adhered particles. The number of cycles as well as dipping time varies based on required lm thicknesses. Corresponding reactions are given below 92 and the growth mechanism is schematically represented in Chart 1.
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:

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, (S 2 O 3 ) 2− , outer layers.

Rinsing I
In the second stage, extra adsorbed ions, Cu + and (S 2 O 3 ) 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.

Reaction
In the reaction step, the anions, OH − , from the anionic precursor solution are entered into the scheme. A solid substance, Cu 2 O, 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 − .

Rinsing II
In the nal SILAR cycle, the extra and unreacted species such as (S 2 O 3 ) 2− , Na + as well as by-products of the reaction from the diffusion layer are removed leaving expected thin lms.
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 lms in presence of NaCl using the SILAR deposition system. 93 Earlier to the lm 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 ask. Then, in addition to DI water, the required amount of NaCl electrolyte was further added to the same ask 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 lm, this procedure was repeated for up to several immersion cycles.
The formation of Cu 2 O nanorod thin lms in presence of a NaCl electrolyte at various concentrations were discussed by using SEM micrographs as shown in Fig. 4. The lm fabricated with no NaCl electrolyte demonstrated pencil-thin, and crackfree 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 Chart 2 Schematic presentation of the deposited Cu 2 O nanostructured films on the substrate surface during a SILAR cycle. electrolyte has the potential impact to change the surface morphologies from nanorods to spherical grains. The growth of Cu 2 O nanorod thin lms was sensitive to the concentration of salts added as also stated for CuO. 95 The growth of Cu 2 O 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 Cu 2 O 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.

Types of copper salts
In the SILAR technique, Cu x O thin lms were studied and fabricated by using different copper salts as summarized in    showed the cauliower-like shape with discrete spaces between the grains as in Fig. 5(d).
On the other hand, CuCl 2 $2H 2 O salt was employed to fabricate CuO thin lms using NH 3 as a complexing agent. But Chatterjee and co-workers utilized H 2 O 2 as an oxidizing agent with CuCl 2 $2H 2 O and NH 3 solution to fabricate Cu 2 O thin lms instead of CuO thin lms. 95 Similarly, Cu(CH 3 COO) 2 nH 2 O and NH 4 CH 3 COO solution could be the potential choice to fabricate CuO thin lms. 99

pH of the precursor solution
Impact of solution pH on the properties of Cu x O nanostructured thin lms 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 H 2 SO 4 , CH 3 COOH, NaOH and NH 4 OH.
To optimize the growth condition to fabricate the FTO/Cu 2 O/ 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 Cu 2 O thin lms 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 modi-ed SILAR method. 10% H 2 SO 4 and CH 3 COOH 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 CH 3 COOH into the optimized precursor solution (OP + CH 3 COOH) improved the quality of the crystal having a larger crystallite size of 21 nm whereas H 2 SO 4 played the opposite role. Strong H 2 SO 4 etches lm 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 kU cm and among optimized solutions OP + CH 3 COOH showed the lowest resistivity. The resistivity of the modied SILAR grown samples had (1-5) order magnitudes less than those deposited by Nair and Ristov's SILAR, and electrodeposition method. 36 To justify further the effects of CH 3 COOH and rinsing steps, Farhad and co-workers again deposited thin lms at pH 4.50-7.95 by adding CH 3 COOH into cationic precursor solutions. 37,101 It is seen that lm deposited without any rinsing step showed the lowest band gap due to the high lm thickness and vice versa which is shown in Fig. 7(a). pH 5.10 which was maintained by adding CH 3 COOH exhibited a larger and densely packed grain size (∼300-530 nm) compared to pH 7.95 where no use of Umeri and coworkers described the effect of pH and growth temperature during the deposition of Cu x O thin lms 94 at RT and 70°C in the pH range from 8 to 11. At RT, the pure Cu 2 O phase exists with pH 8, while mixed phases of Cu x O appeared at pH 11. Whereas, at 70°C, only the pure Cu 2 O 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 Cu 2 O thin lms 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 Cu 2 O to CuO. From SEM micrographs, it is seen that at RT, the compact thin lm 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 nanobers were observed. Conversely, at 70°C and pH 9, a ber-like nanostructure was formed, that looked like the morphology of lms grown at RT with pH 11. Consequently, uniform, overgrown clusters free of close-packed and interconnected nanobers of Cu 2 O were observed at 70°C and pH 11 with a band gap of 2.0 eV. Thus, temperature-dependent pH has a signicant controlling overgrowth and properties of the deposited lms.
Likewise, Cu 2 O, the inuence of pH on the physical properties of CuO thin lms was investigated by Visalakshi et al. 103 The pH (∼10-12) of the cationic precursor solution was maintained by adding concentrated NH 4 OH. 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 × 10 3 to 4.0 ×10 3 U 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 × 10 14 to 4.8 × 10 14 cm −3 which is in good agreement with the obtained result by Saravanakannan et al. 104 Conversely, the mobility is rst 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 lm synthesized at high pH may be attributable to the growth in lm 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 aer deposition reported by Gençyılmaz and coworkers. 105

Number of cycles during deposition
The lm characteristic is closely related to the number of immersions of the substrates into the precursor solution. In  Table 7. It is obvious that with the increase of immersion cycles thickness of the lms increase. The surface SEM morphologies of m-SILAR deposited Cu 2 O lms 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 lms was seen to be compact as well as coherently carpets. However, the Cu 2 O lm grown with 40 immersion cycles demonstrated ber-like microstructures with small grains of around 200 nm. Instead, thin lms 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 reection 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 Cu 2 O thin lms deposited at pH ∼ 7.95 using 20 cycles (lm thickness ∼ 654 nm), 40 cycles (lm thickness ∼ 1130 nm), 60 cycles (lm thickness ∼ 1200 nm) as well as 80 cycles (lm 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 lm as represented in Fig. 9(d), probably due to the bigger grains usually existing in the thicker lms, which veries the results stated by Nair et al. 111 Fig . 10(a)-(c) demonstrates the Cu 2 O nanostructured lms fabricated with different dipping cycles of the nanorods spread homogeneously on the substrate surface, showing a large number of grains with ne 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 lm. So, the electron trapping at the surface and in the intergrain boundaries lowered the efficiency value of the lm 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  Fig. 11. Even though the attained efficiency of the ZnO/Cu 2 O heterojunction was lower, the efficiency was high in the samples deposited at high cycles such as 100. Hence, the effect of the lm thickness on cell performance was evidenced by the enhancement of efficiency due to the substantial development of crystallinity and absorbance of Cu 2 O lms. Fig. 12(a) illustrates the deposition of the thin lms grown by varying bath temperatures of anionic precursors as a function of the immersion cycles. The gure 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 lm thickness was >0.3 mm with 20 cycles of immersions, whereas, at 70°C, the lm growth slightly falls aer 20 immersions. 111 For the lms 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 lms were comparable for the lms with thicknesses smaller than 0.1 mm. The values were higher in samples fabricated using NaOH solution at 70°C having lms of higher thickness. The measured electrical conductivity of a 0.15 mm lm is about 5 × 10 −4 U −1 cm −1 . And it was found that the increase of lm thickness of two orders increases the conductivity by nearly two orders.

Effect of bath temperature
The structural parameters, elemental composition, and optical band gap for different bath temperature of Cu 2 O lms 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 signies that the imperfection in the crystal lattice with the rising temperature was decreased. The SEM images of Cu 2 O thin lms deposited on ITO substrate with different anionic bath temperatures are demonstrated in Fig. 13. From the gures, 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 Cu 2 O lm in Table 8. Further, the photocatalytic activity for water splitting by the deposited Cu 2 O thin lms at different temperatures was studied in a photochemical cell and the result revealed that lm 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 (gure in 5.2 section).

Addition of additives
The inuence of the different additives on the surface morphological characteristics of CuO lms was studied by using SEM. Fig. 14 illustrates the SEM images of the CuO thin lms fabricated in the solution containing additives such as coumarin, saccharin, and sodium dodecyl sulfate (SDS) having different concentrations. In the rst 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  . SDS may affect particle growth as well as morphology aer nucleation. Thus, during the crystallization process, the SDS can affect nucleation. 114 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 lms, 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

Complexing agent
Cavusoglu and co-workers studied the role of the complexing agent such as triethanolamine (TEA) mediated fabrication of nanocrystalline CuO thin lms 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 lms was increased from 1.33 to 2.00 eV while the average transmittance of all the lms increased from 2.5 to 42.5%. A minimum resistivity of 3.74 × 10 5 U cm was found with zero TEA in CuO thin lms whereas, with a TEA concentration of 1.0 M%, the resistivity subsequently increased to 509 × 10 5 U cm. Surface morphology on the lm surfaces demonstrated the homogeneous distribution of the nanostructured CuO as demonstrated in Fig. 15(a)-(d) whereas, the gure 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 lm provided the high FOM values of 786 × 10 −12 U −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.

Annealing of as-deposited lms
Annealing is a vital parameter to control the phases of the deposited thin lms. Both the phases of Cu x O 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 lms has been studied more extensively than vacuum annealing. 125 Recently, SILAR deposited Cu x O lms are mainly studied between 75 to 500°C 37,101,119-124 in presence of air or N 2 . The study revealed that Cu 2 O phase was stable until 250°C, 120,122 though Farhad and co-workers showed a mixed phase of both Cu 2 O and CuO at 250°C 37 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°C 120 by annealing of Cu 2 O or even could be deposited by using NH 3 solution (pH = 10) with the reaction of CuCl 2 at ambient temperature. 124 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 lms at 0.5 M NaOH. Fig. 16 demonstrated the change of resistivity, mobility, and carrier concentration of copper oxide (Cu 2 O 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 × 10 17 to 6.61 × 10 15 cm −3 with increasing annealing temperature from 70 to 500°C respectively. The hole mobility of the lms was increased from 4.20 to 31.87 cm 2 V −1 s −1 with decreasing the carrier concentration, while the electrical resistivity of the lms decreased with annealing temperature, inducing the increment in the conductivity of the lms. Nair   Table 11. Interestingly, in the case of  The lms prepared at high doped Cu 2 O thin lms such as 5% Eu showed a low resistivity value of 1 × 10 3 U cm as shown in Fig. 17. The Hall mobility and carrier concentration values in such cases are 0.52 cm 2 V −1 s −1 and 13.8 × 10 15 cm −3 , respectively. Fig. 18(a) shows the current density-voltage (J-V) characteristics of the ZnO/Cu 2 O heterojunction solar cells prepared using the Eu-doped Cu 2 O thin lms. 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 Cu 2 O. The ionic radius of Eu 3+ ion was 0.109 nm whereas, Cu + is 0.077 nm. Therefore, Eu 3+ 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(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 dried 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: Cu 2 O 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 Cu 2 O. Undoped Cu 2 O has a diamagnetic property. 139 The outcome agrees with Fig. 19(a) and (b) which demonstrate the change of magnetization against the applied magnetic eld (M-H). In the case of Co-doped Cu 2 O, undoped and minimum doped such as 1 and 2 wt% lms showed diamagnetic (high magnetization) behavior whereas, at the maximum doped such as 10 wt%, the lms showed ferromagnetic (low magnetization) properties. 140 The diamagnetic order was Cu 2−x Co x O (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 Cu 2 O at 305 K, the lm 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 lm showed anti-ferromagnetic behavior. With the increase in the concentration of Fe, both the number of Fe 3+ -Cu 2+ pairs and the hole concentrations increased and consequently, the crystallite size reduced.
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 + / Cu 2+ transformation at 310 mV while the Ni 2+ / Ni 3+ at 390 mV at a scan rate of 5 mV s −1 . The proportionality of currents to scan rate delivers data that the lm is sufficiently thick, and the charge transfer rate was restricted by the diffusion of charge carriers in the lm. 134 Inset of Fig. 21 demonstrates the specic 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 specic capacitance of the sample can be explained based on the good conductivity of CuO and the substantial role of Ni atoms 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.

Applications
The optoelectronic properties of SILAR synthesized thin lms 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 x O nanostructured thin lms will be discussed in the following section.

Antibacterial activities
To control pathogens, nanoparticles are in great demand due to their huge applications in the health industries. Results achieved from nanocrystalline Cu 2 O nano-thin lms 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 Cu 2 O lms by SILAR technique as shown in Fig. 22(a) and (b). The synthesized Cu2O thin lm 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.

Water splitting
Cu x O was considered a good candidate for photoelectrochemical (PEC) water splitting due to its abundance, low price, and high stability in aqueous solution. 147 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).

Surface wettability study
The surface wettability study of lms 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 lm 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 lm surface. 150,151 Fig. 24 (A1, A2, A3, and A4) signies the image of the contact angle with the surface of the lm. The observed angles of CuO thin lms with 50, 60, 70 and 80 SILAR cycles were 65°, 58°, 50°, and 43°, respectively. The observed CuO lms 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 lm surface.

Super capacitive behavior
The electrochemical impedance, as well as super capacitive properties, of SILAR synthesized CuO thin lms are studied by Patil et al. 149 The synthesized CuO thin lm showed the lowest charge transfer resistance of 41.45 U cm −2 with the highest specic capacitance of 184 F g −1 at the scan rate of 50 mV s −1 and demonstrated 83% capacitive retention aer 5000 cycles. Super capacitive performance of the lm was veried using cyclic voltammetry (CV) in 1 M KOH electrolytes in a threeelectrode 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 .
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 specic capacitance with various scan rates was displayed, which enhanced exponentially with decreasing scan rate. 153 The electrochemical stability of CuO lm 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 lm electrode aer the 1st to 5000th cycles and conrmed cyclic stability of 83% aer 5000 cycles. By using the Ragone plot, the highest values of specic power and specic 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).
Moreover, the electrochemical investigation of Ni-doped CuO nanolayers modied with Ni foam electrodes synthesized by Lobinsky et al. revealed the specic 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.

Photoelectrochemical characterization
The photo-responsive performance of m-SILAR grown Cu 2 O/ 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 Cu 2 O/FTO electrode was observed by a Keithley SMU 2450 by employing Cu 2 O/FTO thin lms as a working electrode, a graphite rod as a counter electrode and 0.1 M Na 2 SO 4 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 CH 3 COOH precursor solutions, were observed as 247 ± 38 mV, 36.0 ± 2.0 mV and 47 ± 8 mV respectively. The large V oc value projected for optimized precursor lm revealed a better Schottky junction produced at Cu 2 O/electrolyte interface, consequently, advocating a better optoelectronic quality of Cu 2 O thin lm. The transient surface photovoltage and V oc retention for 5000 s, advocating better stability of the SILAR fabricated Cu 2 O thin lms in aqueous electrolyte.

Conclusion
In summary, Cu x O thin lms have been extensively studied and are receiving profound attention because of their fascinating properties and promising uses in a variety of elds. In this article, an inclusive review of the state-of-the-art research activities of diverse Cu x O thin lms 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 t for the large-scale growth of Cu x O. The morphology, as well as diverse properties of Cu x O, 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 x O 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 x O is a very signicant 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 x O. Precise control of Cu x O fabrication could accelerate multiple exciton generation effects, leading to a development of overall efficiency.

Conflicts of interest
There are no conicts to declare.