Copper selenide thin films from growth to applications

Solid State Sciences 100 (2020) 106101

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Copper selenide thin films from growth to applications Raja Azadar Hussain a, *, Iqtadar Hussain b a b

Department of Chemistry, Quaid-i-Azam University, 45320, Islamabad, Pakistan Department of Mathematics, Statistics and Physics, Qatar University, Doha, 2713, Qatar

A R T I C L E I N F O

A B S T R A C T

Keywords: Copper selenide Energy crises Solar cells Water splitting

This article deals with the synthetic methods (chemical bath deposition, electrodeposition, microwave synthesis, sol-gel method, dip coating, sputtering method, spin coating, colloidal method, solid state reaction, hydrothermal method and vapor based methods) and applications (electrocatalyst for water splitting, counter electrodes in solar cells, self-repairable electrodes, thermoelectric properties and flexible electronics) of copper selenide thin films.

1. Introduction Copper selenide is an important member of first row transition metal chalcogenides [1–6] and has different compositions, stoichiometric and nonstoichiometric forms. It exists as cubic Cu2Se (bellidoite), ortho­ rhombic Cu2Se, monoclinic Cu2Se, tetragonal Cu3Se2 (umangite), face centered cubic Cu2-xSe (berzelianite), hexagonal CuSe, Cu0.87Se (klockmannite), orthorhombic Cu5Se4 (athabascaite) and CuSe2 [7,8]. Phase diagram of Cu–Se system was developed way back in 1966 [9] but detailed investigation of Cu–Se system on thin films has been re­ ported by John O. Thompson et al. [10]. According to this investigation hexagonal α-CuSe formed at 60% Se is converted to hexagonal γ-CuSe at ~130 � C. Amorphous material is predominant above 60% Se and be­ comes crystalline as temperature increase with a competition between hexagonal γ-CuSe and cubic Cu2Se. Slow heating rate supports cubic Cu2Se and at ~66% Se, only cubic phase is observed even at 100 � C and 110 � C. This means that metastable cubic phase grows earlier than the thermodynamically stable orthorhombic Cu2Se. At melting point Se re­ acts with α-CuSe to form thermodynamically stable orthorhombic Cu2Se (Fig. 1) [11]. At 0.20 V relative to saturated calomel electrode cubic Cu2Se (bluish black) is converted to orthorhombic CuSe (grey) [12]. Another study reported the transformation of cubic phase to orthorhombic phase at 0.78 V [13]. Thermodynamic stability of Cu2Se has also been evalu­ ated with emf (electromotive force) between 852 K and 972 K by Muhsin IDER [14]. Gibbs energy of formation in this study is 71.82 KJmol-1 at 298 K which is in very close agreement with the value determined by Barin [15]. Standard enthalpy of formation determined by this method is 65.81 KJmol-1 and is in agreement with previously determined

65.27 KJmol-1 [15] and 65.7 KJmol-1 [16]. Bulk Cu2Se is a zero band gap material but nonstoichiometric Cu2-xSe is a p-type semiconductor with a direct band gap of 2.1–2.3 eV and in­ direct band gap of 1.2–1.4 eV. Composition of copper selenide along with arrangement of atoms in a crystalline structure alters its electronic, chemical and thermal properties. Different size regimes and variable morphologies are also used to tune the properties. To achieve the desired properties via morphological and structural changes, different synthetic routes have been adopted by the workers. These routes include chemical bath deposition (CBD), electrodeposition (ED), microwave synthesis, sol-gel method, dip coating, sputtering method, spin coating, colloidal method, solid state reaction, hydrothermal method and vapor based methods. In this review article we shall focus on the fabrication of copper selenide thin films with different methods and will discuss their applications as counter electrodes (CEs) in different type of solar cells, electrocatalyst for oxygen evolution reactions, self-repairable elec­ trodes, flexible electronic materials and thermoelectric materials. 2. Synthetic routes for copper selenide 2.1. Chemical bath deposition Chemical bath deposition or electroless deposition is a technique in which cations get dissolved in the solution of anions and then are reduced to form thin films via internal cell currents. This is a simple and instant method for thin films fabrication. Thin films fabricated with this method have good electrical and physical contact with the conducting substrate for different applications. Cu2Se thin films (black in color) were fabricated by dipping copper

* Corresponding author. E-mail address: [email protected] (R.A. Hussain). https://doi.org/10.1016/j.solidstatesciences.2019.106101 Received 25 October 2019; Received in revised form 20 December 2019; Accepted 21 December 2019 Available online 23 December 2019 1293-2558/© 2019 Elsevier Masson SAS. All rights reserved.

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Solid State Sciences 100 (2020) 106101

Cu2þ þ 4NH3 → [Cu(NH3)4]2þ

(6)

2[Cu(NH3)4]2þ þ SeSO23 þ 2OH → Cu2Se þ 8NH3 þ SO4 2 þ H2O

(7)

In the basic media sodium selenosulfate decomposes to form selenide ion which then reduce Cu2þ to Cuþ and produces Cu2Se via following reaction Na2SeO3 þ OH → HSe þ Na2SO4

(8)

2 þ H2O þ 8NH3 2[Cu(NH3)4]2þ þ HSe þ OH → Cu2þ 2 þ Se

(9)

Cu2þ 2

þ Se

2

(10)

→ Cu2Se

Thickness of film can be controlled in this process by controlling the deposition time at a particular temperature and concentration of re­ actants. Thinner films can be made thicker by re-dipping them in the dipping solution. Surface resistivity of these films varies from 50 to 500 Ω per square on the basis of Cu to Se ratio. At mole ratio (Cu/Se) 1:1, least conductive films were obtained but if this mole ratio is 1:4, CuSe is precipitated instead of Cu2Se. Films do not have good attachment with polystyrene substrates in this method [18]. Same group has coated Cu2-xSe thin films on Pt substrates by reacting solution A and B. Solution A was the mixture of 30 mL of 3 M copper acetate, 9 mL of 98% triethanolamine (TEA), 7 mL of ammonia (25%) and 45 mL of distilled water whereas solution B was prepared by mixing 5 g of Se with 95% sodium sulfite in 25 mL distilled water. Following are the main reactions of this process [13]; 2Cu(TEA)2þ þ 2SeO23 þ 4OH → Cu2Se þ 2TEA þ 2SO24 þ Se þ 2H2O (11)

Fig. 1. Phase diagram of Cu–Se system between 48% and 85% Se. Red triangles represent the compositions (determined via electron probe microanalysis) which were prepared to draw the phase diagram. Reprinted with permission from Ref. [11].

Another possibility of kinetically slow reaction is

After these initial attempts different other groups have synthesized copper selenide for their requirements. Details of different Se sources, Cu sources, bath compositions and materials obtained have been given in Table 1. V.M Garcia et al. [8] have used N,N dimethyl selenourea as a source of Se and copper sulfate and copper chloride as sources of copper with composition given at Sr 1 and Sr 2 of Table 1 with following major reactions.

plates in a solution of selenous acid (10 mM) and 0.3% sulfuric acid at pH 1. Concentration of selenous acid has to be optimized for required growth rate and thickness of films. At a concentration higher than 10 mM, thick films are produced but they are not adherent with the sub­ strate. Films of 5 μm thickness showed good contact with substrate. Main reactions between Cu electrodes and selenous acid are as under; In the acidic media, Cu is oxidized to form Cu2þ which reacts with selenous acid 2þ

2(Cu → Cu

þ 2e )

2Cu þ H2SeO3 þ 4Hþ þ 4e → Cu2Se þ 3H2O

(1)

2þ

[Cu(C4H4O6)] ↔ Cu

(2)

SO24 þ H2O þ 2e ↔ SO23 þ 2OH (E ¼ 2þ

Byproducts of hydrogen selenide and elemental Se are not 100% impossible but at pH ¼ 1 and low concentration of selenous acid following reactions may not occur; (4)

2H2Se þ H2SeO3 þ → 3Se þ 3H2O

(5)

þ

(14)

C4H4O26

In the bath which contains sodium sulfite (Sr.2) there is a possibility for the reduction of Cu(II) to Cu(I) via following reactions

(3)

H2SeO3 þ 6Hþ þ 6e → H2Se þ 3H2O

(13)

[Cu(NH3)4]2þ ↔ Cu2þ þ 4NH3

By the addition of 1 and 2 we get the overall reaction as 4Cu þ H2SeO3 þ 4Hþ→ 2Cu2þ þ Cu2Se þ 3H2O

(12)

3Cu2Se þ SeSO23 → Cu3Se2 þ SO23

Cu

0.93V)

þ

þ e ↔ Cu (E ¼ 0.153V)

(15) (16)

Sodium selenosulfate and dimethyl selenourea liberate selenide ions in the bath via following reactions;

Films prepared via this method are polycrystalline, copper deficient and orthorhombic type Cu2-xSe [12,17]. Cu2Se films have also been deposited in the basic media (pH ¼ 10–10.5) by mixing 10 mL of 0.5 M ammonical copper sulfate solution (1:3) with 10 mL of 1 M selenosulfate. Total volume of reaction mixture is maintained between 80 and 90 mL. Polystyrene sheets (50 mm � 25 mm) are dipped in the above mentioned solution to have yellow brown coatings of Cu2Se on them. This method can be used for coating almost every type of substrate with polycrystalline (cubic, monoclinic, tetrag­ onal) Cu2Se. Chemistry of this process is as under;

SeSO23 þ 2OH ↔ Se2 þ SO24 þ H2O

(17)

(CH3)2NNH2CSe þ 3H2O ↔ (CH3)2NNH2CO þ Se2 þ 2H3Oþ

(18)

Reaction of copper ions and selenide ions produces copper selenide on the substrates [8]. M. Lakshmi et al. [19] reported that at Cu/Se ratio of 1:1, pH 6.9 and 27 � C, Cu2-xSe is the final product. At Cu/Se ratio 1:2 at pH ~7.8 and 5 � C Cu3Se2 is the predominant phase. By keeping Cu/Se ratio 2:1 at pH ~6.2 and 27 � C Cu2-xSe is the composition of final product. Above mentioned synthesis of copper selenide thin films with different com­ positions and phases is controlled by differential release of copper ions from trisodium citrate complex at varying reactant ratios and pH. At 1:1 (Cu/Se) there exists a possibility for the presence of copper in þ1 2

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Solid State Sciences 100 (2020) 106101

Table 1 Different Cu/Se precursors and bath compositions for copper selenides via chemical bath deposition. S. #

Cu Source (A)

Se Source (B)

Bath composition

Product

Ref.

1

CuSO4.5H2O

Na2SeSO3

10 mL of 0.5 M A þ 1.5 mL of 30% ammonia þ30 mL of 0.18 M B to make volume 100 mL

Cu2-xSe (cubic)

2

CuCl2.2H2O

0.5 M A þ 20 mL 0.8 M sodium tartrate þ 0.07 M B in 0.01 M sodium sulfite þ 57 mL water

CuSe (hexagonal)

3

CuSO4.5H2O

Dimethyl Selenourea Na2SeSO3

[8, 64] [7]

10 mL of 0.2 M A þ 10 mL of 0.3 M trisodium citrate þ 10 mL of 0.2 M Na2SeO3

Cu2-xSe (cubic)

4 5 6 7 8 9 10 11 12

CuSO4.5H2O CuSO4.5H2O CuSO4.5H2O CuCl2.2H2O CuSe powder CuSO4.5H2O CuSO4.5H2O CuCl2.2H2O CuSO4.5H2O

Na2SeSO3 Na2SeSO3 Na2SeSO3 Na2SeSO3 CuSe powder Na2SeSO3 Na2SeSO3 Na2SeSO3 Na2SeSO3

10 mL of 0.1 M A þ 10 mL of 0.3 M trisodium citrate þ 10 mL of 0.2 M Na2SeO3 10 mL of 0.4 M A þ 10 mL of 0.5 M trisodium citrate þ 10 mL of 0.2 M Na2SeSO3 (0.1 M A þ tataric acid to maintain the pH at 2) þ (0.05 M B at pH 12) A þ B þ 10 mL of 0.1 M triethanol amine þ desired quantity of 30% ammonium hydroxide A þ B in polyethyleneglucol þ ethanol þ diethanol amine 50 mL of 0.2 M A þ 50 mL of 0.3 M trisodium acetate þ required 0.25 N B 50 mL of 0.2 M A þ 50 mL of 0.3 M trisodium acetate þ required 0.25 N B 10 mL of 0.1 M A þ 0.8 mL of 30% ammonia þ10 mL of B wih overall volume of 50 mL 10 mL of 0.5 M A þ 5 mL of monoethanolamine þ 2.5–7 mL of buffer (ammonium chloride/ ammonium hydroxide, pH ¼ 10) þ 15 mL of 0.1 M B with a make up volume of 50 mL

Cu3Se2 (tetragonal) Cu2-xSe (cubic) Cu2Se (monoclininc) Cu2-xSe (cubic) CuSe (hexagonal) Cu2-xSe (cubic) Cu3Se2 (tetragonal) CuSe (hexagonal) Cu2Se (orthorhombic, Cubic)

oxidation state due to excessive sulfite ions. Temperature governs the final phase of product in this case. At comparatively basic pH (7.8) Cu3Se2 is produced. However, higher pH values make the precipitation reaction too fast so it was carried out at 5 � C. Later on same group reported that Cu2-xSe phase is unstable at room temperature and pressure and is converted to Cu3Se2 by the formation of copper oxide impurity. Cu3Se2 is stable at ambient conditions but con­ verts back to Cu2-xSe above 140 � C [20]. Exclusively monoclinic Cu2Se thin films were fabricated [21] by using the conditions at Sr.6 of Table 1. For the formation of 330 nm thick films, substrate has to be dipped in bath solution for 65 times. Cubic Cu2-xSe (x ¼ 0.2, spherulite) with poor crystallinity were reported by Al Mamun et al. Crystallinity was improved by temperature treatment at 250 � C [22]. Hexagonal CuSe thin films were reported by Zulkernain Zainal et al. [23] by combination of dip coating and chemical bath deposition. In this modified method, product CuSe powder was first precipitated by the reaction of cupric ion with elemental Se using 12 M sodium hydroxide. Dipping medium was prepared by mixing polyethylene glycol, ethanol and diethanol amine. CuSe powder was mixed in the dipping solution and ITO (Indium Tin Oxide) glass was used as substrate. After dip coating ITO glass was thermally treated at 100 � C for 30 min and then at 500 � C for 30 min. It has been reported that same plating solution (Sr.9, Sr.10 in Table 1) can produce different phases i.e. Cu2-xSe and Cu3Se2 with only a change of 0.1 units in the pH. Cu2-xSe is formed at pH 6.2 and Cu3Se2 is yielded at pH 6.3 [24]. M. G. Sandoval et al. [25] have determined the effect of pH change on the phase of synthesized Cu2Se (Sr.12 Table 1). Ten different com­ positions of ammonium chloride/ammonium hydroxide buffer with volumes in mL of 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, 6.0, 6.5 and 7 provided the pH of deposition baths as 9.79, 9.9, 10.05, 10.14, 10.28, 10.35, 10.45 and 10.5 respectively. With these variations two types of crys­ talline phases i.e. orthorhombic and cubic were obtained (Fig. 2). Orthorhombic phase was produced from the solution which contained metal ions and hydroxyl ions together in a way that hydroxide cluster mechanism was adopted. Cubic phase was formed in the absence of hydroxide ions via ion by ion growth [25].

[19, 65] [19] [19] [21] [22] [23] [24] [24] [66] [25]

Fig. 2. SEM images of copper (I) selenide deposited at, a) pH 3, b) pH 5. Reprinted with permission from Ref. [25]. Copyright Elsevier 2016.

acidic Cu2þ/SeO23 solution using Au as working electrode (Fig. 3) two oxidation maxima A1 and A2 and two reduction maxima C1 and C2 are observed. Cathodic peak C1 is attributed to CuxSe deposition of varying composition owing to following reactions;

2.2. Electrodeposition Electrodeposition is one of the pioneer techniques for the growth of metal chalcogenide thin films and extensive efforts have been carried out [26–29] to optimized the conditions for structure and composition of Cu2Se using different solution temperatures and Cu2þ/SeO23 ratios with this technique. If we have a close look at cyclic voltammogram of an

Cu2þ þ SeO23 þ 6Hþ þ 6e ↔ CuSe þ 3H2O

3

(19)

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Solid State Sciences 100 (2020) 106101

[Cu2þ]/[Se2 ] ratio of 2 between a potential of 0.0–0.2 V vs. SHE (standard hydrogen electrode). Slight change of ratio positively or negatively introduces the impurity phase (elemental Cu and Se) in the films. If deposition is carried out at room temperature and 0.0 V, Cu3Se2 is the product but at 0.2 V mixture of Cu3Se2 and Cu2Se are formed. Annealing of thin films above 150 � C produces Cu2Se. If temperature of deposition solution is more than 70 � C then Cu2Se is the predominant product at 0.0 and 0.2 V. Thin films at 0.2 V are however more stable than the films synthesized at 0.0 V. Effect of deposition time was not observed (no effect) between 2 and 30 min [28]. Rui Yu et al. [30] have determined the effect of CTAB (cetyl ammoniumbromide) on the morphology of Cu2-xSe thin films synthe­ sized via cathodic electrodeposition. According to this study morphology of Cu2-xSe crystals can be controlled by CTAþ hydrocarbon chain and bromide ion rest on the facet of Cu2-xSe crystals. Morphology and phase of copper selenide changes with different concentrations of surfactants (Fig. 4), temperature and Cu/Se ratios. Low temperature electrophoretic deposition of Cu1.95Se (synthesized via chemical method) NCs has been used for their thin films fabrication. Electrophoresis has been carried out at a zeta potential of 1.38 mV. Higher values of zeta potential forbid the coagulation and settling of NCs which is necessary for smooth thin film fabrication. Electrophoretic deposition was carried out at 70 V and 30 V. Films fabricated at 30 V are smooth and compact relative to films fabricated at 70 V. Thickness of films at 30 V is ~30 μm and at 70 V is ~50 μm (Fig. 5). Electrical properties of Cu2-xSe are dependent on the holes produced by Cu va­ cancies. Greater is the number of holes greater will be the conductivity. In this study holes in the system were constant due to the use of same precursor (Cu1.95Se). So resistivity of films increased at high voltage. This means that higher resistivity is because of high number of pores and low packing density of films [31]. Irradiation of CuSe thin films with electron beam increases the crystallinity mainly because of heating effect. Drastic changes in surface morphology and increase in the band gap also occurs with electron beam irradiation [32]. Same group has electrodeposited CuSe on stainless steel substrate (cathode) using graphite as anode and SCE (saturated calomel electrode) as a reference electrode. A close look on the CV of solution containing 0.05 M CuSO4.5H2O and 0.025 M SeO2 at 20 mVs 1 reveals that there is a reduction in the peak current at two successive cycles. This shows that the deposition is occurring on electrode surface. Deposition potential of CuSe is 0.65 V/SCE. So, CuSe was deposited at this potential using a current density of 1.5–2.0 mAcm 2 for various bath compositions keeping the deposition time same. At higher con­ centration of reactants, crystallite sizes of films increase irrespective of Cu/Se ratio and size of nanorods increase by increasing the concentra­ tion in the bath (Fig. 6) [32,33]. After potentiostatic deposition next step in electrodeposition is the pulsed laser deposition for thin film fabrication of copper selenide. In this regard M. Bouroushian [34] and coworkers have synthesized different copper selenides at room temperature on Ti substrate from acidic solutions of cupric nitrate and selenium dioxide in one step by constant pulsed potentials. They have carried out pulsed electrodepo­ sition of solutions containing different Cu(II)/Se(IV) ratios with linear sweep voltammetry. pH of solution was managed with nitric acid at 1.4 and rectangular pulsed potentials were employed involving a forward scan at 0.85 V or 0.65 V/MSE (Hg/Hg2SO4) for 0.1s and reverse scan at 0.1 V/MSE for 0.9s. Voltammetric data at Q ¼ 0.5–10 where Q ¼ [Cu (II)]/Se(IV)], shows that a reduction peak finishes at ~ -0.4 V owing to the formation of metallic Cu at higher Q values. This peak may contain complications by Se(IV) diffusion limited synthesis of Cu–Se deposits. At lower Q values this cathodic peak shifts towards positive potential (Fig. 7a) possibly because of forced co-deposition effects and due to decrease of nucleation overpotential for reaction of Cu(II) in the pres­ ence of Se. For low Q values there is an additional reduction maxima owing to six electron reduction of Se to hydrogen selenide, reduction of already deposited copper selenide and two electron reduction of

Fig. 3. Cyclic voltammogram of cupric cations (10 mM) and selenite (5 mM) anions at 25 � C using gold as working electrode, 0.05 M sulfuric acid as sup­ porting electrolyte and a scan rate of 10 mVs 1. Arrows 1 and 2 present the range of potential applied for potentiostatic deposition. Reproduced with permission from Ref. [28]. Copyright Pergamon 1998.

Cu2þ þ CuSe þ 2e ↔ Cu2Se

(20)

2Cu2þ þ SeO23 þ 6Hþ þ 8e ↔ Cu2Se þ 3H2O

(21)

3Cu2þ þ 2SeO23 þ 12Hþ þ 14e ↔ Cu3Se2 þ 6H2O

(22)

Cathodic peak C2 corresponds to the formation of H2Se via following reactions; 2CuSe þ 2Hþ þ 2e ↔ Cu2Se þ H2Se

(23)

þ

(24)

Se þ 2H þ 2e ↔ H2Se

(25)

Cu2Se þ 2H þ 2e ↔ 2Cu þ H2Se þ

Another peak A1 represents the oxidation of Cu2Se via following possible routes; Cu2þ ↔ Cu þ 2e

(26)

Cu2Se ↔ 2Cu2þ þ Se þ 4e

(27)

þ Se þ 2e

(28)

2þ

(29)

2þ

CuSe ↔ Cu

Cu3Se2 ↔ 3Cu

þ 2Se þ 6e

And an anodic maximum A2 corresponds to the oxidation of elemental Se at the surface of electrode via following equation; Se þ 3H2O ↔ SeO23 þ 6Hþ þ 4e

(30)

However it is worth mentioning that Cu3Se2 is a pure compound and is not a mixture of Cu2Se and CuSe. Some other workers [28,29] have attributed following reactions to anodic peak A1. 2Cu2Se ↔ Cu3Se2 þ Cu2þ þ 2e 2þ

Cu3Se2 ↔ 2CuSe þ Cu

þ 2e

(31) (32)

Ideally speaking, every oxidation and reduction reaction should be available in the CV (cyclic voltammetry) profile (Fig. 3) in the form of a peak however, changes of scan rates between 10 mVs 1 to 0.5 mVs 1 does not split peak A1. Form this experimental observation it can be deduced that the observed phases Cu2Se, Cu3Se2 and CuSe are formed possibly by one step reactions 19, 21 and 22. Best depositions with pure CuxSe phases were obtained with a 4

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Solid State Sciences 100 (2020) 106101

Fig. 4. SEM micrographs of Cu2-xS films prepared with cathodic electrodeposition at different concentrations of CTAB a) 0 mM, Cu2Se3 b) 1 mM, c) 3 mM, d) 5 mM, e) 8 mM and f) Cu2þ 0.005 M, SeO3 2 0.003 M and CTAB 0.008 M. Reprinted with permission from Ref. [30]. Copyright ACS 2009.

and placed in microwave irradiation for 10 min. After this the reaction mixture was cooled to room temperature and film in each case was washed with absolute ethanol. Absolute ethanol, isopropyl alcohol, cyclohexyl alcohol and benzyl alcohol were used as solvents. Absolute ethanol (80 � C) and isopropyl alcohol (83 � C) provided no film growth even when the irradiation time was prolonged to 40 min. This may be because of low boiling point of these solvent which does not allow the reaction mixture to attain the growth temperature. Cyclohexyl alcohol (100 � C provided cubic (Cu2� xSe) dendride like morphology whereas benzyl alcohol (150 C) gives hexagonal (CuSe) flaky crystals. Fig. 8 presents different morphologies under different parameters, grown with this method [35].

Fig. 5. Cu1.95Se thin films synthesized by low temperature electrophoretic deposition at 30 V and 70 V. Reprinted with permission from Ref. [31].

elemental Se. All Q values in this study provide mixture of all the phases with copper rich Cu2Se phase at higher Q values. Morphological changes were also evident with change of Q values and DC voltage as depicted in Fig. 7 [34].

2.4. Sol-gel method Sol-gel method is an important method for matrix based synthesis of inorganic materials. V. S. Gurin et al. [36] have used silica sol-gel films for synthesis of copper selenide by selenization of metallic copper NPs (nanoparticles). Fig. 9 presents the details of this process. Silica glass was selected as substrate due to its optical transparency but other sub­ strates can also be used. Increase in the selenization time introduces impurity phases in the films. If we have a close look at the XPS spectra of copper selenide thin films on silica substrate it is evident that Si2P spectra undergo splitting

2.3. Microwave synthesis Microwave irradiation for copper selenide is comparatively a rarely used technique. Cu foil (1.0 � 1.5 cm2, thickness 0.2 mm) and 0.015 g of Se powder were placed in a three necked round bottom flask with 25 mL of solvent. This setup was stirred for 10 min and then heated to 160 � C

Fig. 6. a) CV on stainless steel electrode containing 0.05 M CuSO4. 5H2O and 0.025 M SeO2 at 20 mVs 1. b) SEM image of hexagonal CuSe thin film electrodeposited with bath composition of 0.1 M CuSO4. 5H2O and 0.05 M SeO2. Modified from Ref. [33]. Copyright Elsevier 2015. 5

R.A. Hussain and I. Hussain

Solid State Sciences 100 (2020) 106101

Fig. 7. a) Polarization curves of Ti electrode at different Q values for [Cu(II)] 10 mM, pH 1.4 and varying concentrations of Se. SEM images of samples deposited at DC –0.65 V (b, d) and PP –0.65 V (c, e) from Q ¼ 0.5 (b, c) and 1.0 (d, e) baths. Reproduced with permission from Ref. [34]. Copyright Elsevier 2015.

Fig. 8. SEM images of films produced via microwave irradiation a) using cyclohexyl alcohol as solvent, b) benzyl alcohol as solvent, c) using cyclohexyl alcohol as solvent and 140 � C, d) using cyclohexyl alcohol as solvent and 160 � C, e) benzyl alcohol as solvent at 150 � C, f) benzyl alcohol as solvent at 160 � C, g) Cu2-xSe films with cyclohexyl alcohol and microwave irradiation of 5 min, h) 10 min, i) 15 min. Reproduced with permission from Ref. [35]. Copyright Elsevier 2013.

and broadening (Fig. 9d) and these changes become more prominent with increase in selenization time. This shows that copper selenide particles are not 100% inert and there is an interaction between sub­ strate and deposited thin films. Most plausibly, appearance of some new states is evident and not the new compounds because selenization temperature is lower than the reactivity range of Si-substrate. However confirmation to this hypothesis requires more studies, may be via Rutherford Backscattering [36].

2.5. Sputtering method Sputtering is also a rarely used technique for the fabrication of copper selenide thin films. Less adoptability of this technique is because of less control of stoichiometry and high cost. Curiosity of Y. Z. Li and fellows [37] forced them to explore the effect of sputtering power on the properties of copper selenide thin films prepared by magnetic sputter­ ing. Two important factors in magnetic sputtering are the sputtering power and chamber pressure. Target copper selenide was prepared by solid state reaction of 1:1 copper and Se powders at 500 � C in a sealed quartz tube. As prepared 6

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Solid State Sciences 100 (2020) 106101

Fig. 9. a) Schematic diagram presenting sol-gel process for the production of copper selenides. XPS spectra of the Cu-doped silica films b) Cu2p3/2 core levels; c) Si2p core levels; d) Se3d3/2 core levels; e) CuL3M45M45 Auger line. Reproduced with permission from Ref. [36]. Copyright Elsevier 2008.

copper selenide powder was pressure molded into a pallet and was sintered at 900 � C in argon atmosphere. This target was deposited on glass substrate at room temperature via magnetic sputtering. The chamber was vacuumed at 3 � 10 4 Pa and then the chamber pressure was maintained with argon at 0.8 Pa for 60 min during sputtering. Films were deposited at 50 W, 70 W, 90 W and 110 W separately to see the effect. It was observed that increase in sputtering power introduces impurity phases in the cubic Cu2-xSe films. There were small changes in the properties and no major changes in morphologies were observed [37]. In another attempt Yan den Li et al. [38] have used ion beam sput­ tering for copper selenide thin films. Ion beam sputtering is a sort of physical vapor deposition technique with high deposition rates. Versa­ tility in deposition can be achieved by changing the target composition and sputtering energy. In this study films were deposited on 30 mm � 30 mm x 2 mm slides of sodium calcium glass in argon atmosphere. Target area ratio of fan like Cu and Se was 15:100. Chamber was vac­ uumed to 8.0 � 10 4 Pa and working pressure was maintained at 6 � 10 2 Pa with 8.0 sccm (standard cubic centimeters per minute) of working gas. Ion source parameters were 0.6 KeV (energy of plasma), 220 V (accelerating voltage), 75 V (anode voltage), 12 mA (beam cur­ rent) and 5A neutral current. All the films were deposited at room temperature and were annealed for 1 h at 250 � C, 300 � C and 350 � C. Thin films produced via this method are discontinuous and have island structures. At 250 � C, phase of material is hexagonal which changes to cubic at 300 � C and at 350 � C, Cu2-xSe phase was observed [38]. Jaime A. et al. [39] have modified magnetron sputtering to pulsed hybrid reactive magnetron sputtering (PHRMS). This technique is based on reactive sputtering and a vacuum technique which is extensively used in industry for thin films. Conventional reactive sputtering has draw­ backs of poor control of composition and crystallinity of product, poisoning of target and vacuum system by Se overpotentials and film non-uniformity by chalcogens (Te, O, S, Se). Ions in sputtering media are at energy of ~100 eV. Alternatively, introduction of chalcogens in the system by thermal vapor source solves the problem but no effect has been reported to control the negative ions, substrate temperature and chalcogens overpressure. These problems have been tackled by creating a periodic pulse of Se in the system which controls the reactivity of film surface by modifying the kinetics of cation/anion reactions. This is a single step methodology which does not involve annealing and post temperature treatment. Instead of metallic and glass substrates

polymeric substrates can also be used in this technique. Growth rate of films in PHRMs is better than other sputtering techniques. Fig. 10 shows the images of films fabricated at different Cu/Se ratios with PHRMS [39]. 2.6. Vapor deposition methods Paul O Brein and his group has been working on chemical vapor deposition (CVD) [40] and aerosol assisted chemical vapor deposition (AACVD) [41]. AACVD does not require the volatility of precursor and depends on the solubility of precursor. Volatility is the requirement for low pressure CVD, atmospheric pressure CVD, liquid injection CVD, plasma enhanced CVD and molecular beam epitaxial growth [42–47]. Cu2Se thin films were deposited with AACVD by using copper ace­ tylacetonate as a source of copper and trioctylphosphine selenide as a source of selenium in dichloromethane. All these things were placed in a two necked round bottom flask which has been connected to argon cylinder on one side and carbolite tube furnace on the other side. Cu2Se was deposited on glass substrates of 2 � 1 cm size at 120 sccm flow of carrier gas. Reactor temperature was maintained at 300 � C, 350 � C, 400 � C and 450 � C separately for comparative studies. Fig. 11 shows the changes of morphologies at different reactor temperatures of fabricated thin films [41]. Another vapor based technique is the evaporation technique which require plating of the sample in elemental form (Cu and Se) in quartz ampule at 1326 K for three days in a rotary furnace for Cu2Se [48]. Recently thermal evaporation of Cu (II) selenide at different substrate temperatures (Room temperature, 100 � C, 150 � C, 200 � C, 250 � C) has been carried out with success. Phase transformation from orthorhombic Cu2Se to cubic Cu2-xSe and morphological changes were the outcome of this study [49]. 3. Application of copper selenide thin films 3.1. Electrocatalyst for O2 evolution reaction Fossil fuels are causing damage to the environment due to their toxic oxide by products. Environmental friendly sources of energy are need of the time. Water splitting to produce hydrogen as a fuel and oxygen as a byproduct is the aim of modern research. Catalysts (Pt, Ru) which are giving better results are too much costly and rare in earth crust. Copper 7

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Solid State Sciences 100 (2020) 106101

Fig. 10. Top and cross sectional SEM images of thin films fabricated at different Cu/Se ratios by pulsed hybrid reactive magnetron sputtering. Images taken from Ref. [39]. Copyright Wiley 2017.

Fig. 11. SEM images of tetragonal Cu2Se films fabricated with aerosol assisted chemical vapor deposition at a) 300 � C, b) 350 � C, c) 400 � C and d) 450 � C. Reproduced with permission from Ref. [41]. Copyright Elsevier 2016.

in the first row of transition metal is cheap and abundant. So, water splitting of its selenides has been evaluated by Jahangir Masud et al. [50]. This group synthesized Cu2Se via three different methods (CVD, hydrothermal (HD) and electrodeposition (ED)) and water splitting has been checked with copper selenide synthesized by all the three methods which suggests that this activity is the intrinsic property of the material. In basic media oxygen evolution reaction is kinetically sluggish due following to 4e reduction process; 4OH → O2 þ 2H2O þ 4e

RuO2 and IrO2 have shown best performances for improvement of the rate of above reaction but are not practically useful due to avail­ ability and economic concern. Chalcogenides of Cd, Ni and Fe have been extensively used as a reciprocal of RuO2 and IrO2 in the recent years however technological breakthrough is still a dream. Electrocatalytic activity was checked in 1 M KOH as supporting electrolyte with Cu2Se/Cu and/or GC (glassy carbon) substrate as working electrode, Ag/AgCl as reference electrode and GC plate as counter electrode. Fig. 12a and b presents OER polarization curves of different catalysts scanned at 10 mVs 1 in nitrogen saturated media with

33 8

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Solid State Sciences 100 (2020) 106101

linear scan voltammetry (LSV). It is evident that OER performance of Cu is poor but Cu2Se synthesized by all the three methods showed good performances. Onset potentials for Cu2Se (CVD), Cu2Se (ED-2), Cu2Se (HD) and Cu2Se (ED-1) were 1.45, 1.45, 1.50, 1.53 V vs. RHE (reversible hydrogen electrode) respectively. Electrodeposited Cu2Se showed better performance than hydrothermally deposited Cu2Se due to the fact that hydrothermal sample was prepared with Nafion binder whereas elec­ trodeposited sample was grown directly on electrode surface. So, contact between the catalyst and electrode was better in case of ED sample. Efficiency of OER catalyst is determined by measuring overpotential values required to get the geometric current density of 10 mAcm 2. It is believed that this current density achievement is equivalent to 10% solar energy conversion efficiency. Over potential of 270 mV (ED-2), 290 mV (HD), 300 mV (CVD) and 320 mV (ED-I) were observed to get 10 mAcm 2. Changes in ED and CVD values may be because of surface morphology, nanostructuring and minor impurities in these two types of films. These effects are prominent at higher current densities (50 mAcm 2) as well. Tafel plots of catalyst synthesized by all the three methods are pre­ sented in Fig. 12d and they are almost similar. Amorphous samples directly attached to the electrode with smaller grain size show enhanced activity even with low loadings. Better performance of Cu2Se relative to copper hydroxide and oxides (Table 2) are attributed to the covalency of metal selenium bond in selenide. Tafel slopes indicate catalyst kinetics on steady state current density. Lower values of Tafel slopes (determined with LSV) indicate better OER kinetics owing to porous structure and nanostructuring. OER performance of Cu2Se is even better than RuO2 (117.1 mVdec 1). Similarly, slopes for CVD and ED catalyst represent the similarity of mechanistic routes adopted by both films. ED catalyst which has an overpotential value of 270 mV and a Tafel slope of 48.1 mVdec 1 shows excellent stability for O2 evolution even after 6 h [50].

use will be the ease of disposal of materials because if they have selfrepairable capabilities then less material will be available for disposal. Subhash C. Singh et al. [51] have synthesized different copper selenide materials via solvothermal method and reported the self-repairable applications of α-Cu2Se and β-Cu1.3Se. CV with conven­ tional three electrode system (Pt counter, SCE as reference, copper selenide as working) in sodium sulfate (0.5 M) as supporting electrolyte was run at 20 mVs 1 between 1.0 and þ1.0 V in dark and simulated sunlight to determine the self-repairable capabilities. Six cycles (Fig. 13a) in dark were run for α-Cu2Se which provided oxidation maxima at 0.7 V for oxidation of Cu2Se to CuSe and cathodic maxima at 0.14 V for the reduction of CuSe to Cu2Se. This reaction is not 100% reversible which is evident from unequal currents of oxidation and reduction waves. With increasing number of cycles there is a decrease in the intensity of oxidation peak current with a shift of oxidation peak potential towards lower values whereas cathodic peak current remains the same. This infers that copper ions which are leaving the electrode surface on oxidation cycle are not coming back to the electrode when polarity of electrode changes during reverse cycle. This means they are being degraded. Same experiment is repeated 2 min after the 6th cycle under AM 1.5 G (air mass 1.5 global) solar illumination and CV behavior of electrode is retained i.e. oxidation peak current attains its original value. This means the electrodes have healed themselves in the presence of sunlight [51]. 3.3. Solar cell applications Photoconversion efficiency (PCE) of 30% or higher have been re­ ported recently by photovoltaic (PV) devices based on homojunctions tandem cells. Lowering the cost of PV system by simple manufacturing methods is however, a challenge in itself. One reciprocal to this costly pn junction type PV device may be thin films electrodes. Metal chalco­ genides of formulae MX (where M ¼ Cd, Ni, Zn, Cu and X ¼ S, Se, Te) can be easily molded to thin films but their thin films have drawbacks of low PCE and instability under operating conditions. Ahed Zyoud et al. [52] have recently reported copper selenide electrodes via combined electrochemical(EC)/chemical bath (CB) deposition method with PCE of ~14.6%. J-V plots during dark experi­ ment (Fig. 14a) in aqueous media using [Fe(CN)6]4-/[Fe(CN)6]3- redox couple show negative dark current values. These plots are typical for n-type semiconductors whereas pure Cu2Se is a p-type semiconductor. This anomaly seems to appear due to excess copper phase Cu2-xSe. Photocurrent experiments (Fig. 14b) show that ECD and CBD electrode show poor short current density (Jsc) and open circuit voltage (Voc) relative to ECD/BD electrodes. All electrodes show negative values of Voc and positive Jsc which are a characteristic of n-type semiconductor. CBD/ECD electrodes have Voc of 0.32 V, Jsc of 3.52 mAcm 2 and PCE of 14.6% which are better than ECD electrodes ( 0.11 V, 0.28 mAcm 2, 0.42%) and CBD electrodes ( 0.20 V, 0.76 mAcm 2 and 1.83%) [52]. For CdSe quantum dots (QDs) sensitized solar cells efficiency is limited by unsatisfactory conductivity of counter electrodes (CEs). In a quest to find out a suitable CE, Hua Zhang et al. [53] have coated copper selenide on fluorine doped tin oxide (FTO) for CdSe QDs solar cells. Cu/Se ratios, thickness of films, sintering temperatures and time were optimized to achieve PCE of 8.78% relative to 6.49% and 8.72% of CdSe. At Cu/Se ratio 1:2 (Fig. 15a) PCE is 5.92%, Jsc is 15.64 mAcm 2, Voc is 0.27 V which are inferior to Cu/Se ratio of 1:4 (PCE 6.49%, Jsc 1.08 mAcm 2 and Voc 0.643 V). Further increase of Se to 1:6 decreases the PV performance. Superior performance of 1:4 (Cu/Se) was verified by ICPE curves (Fig. 15b) which show a value of 80% in the region between 450 nm and 650 nm. Nyquist plots (Fig. 15c) show charge transfer resistance at solid-solid interface (RCT1), electrolyte-CE interface (RCT2) and sheet resistance of CE. These three values are determinative of catalyst’s electro­ conductivity, substrate contents in thin films and catalytic activity of catalyst. Intercept of horizontal coordinates and radius of two

3.2. Self-repairable electrodes In living organisms self-repair is a continuous phenomenon to heal the wounds and to maintain the physiological and metabolic functions for sustaining life. If this phenomenon is introduced in nonliving ma­ terials such as batteries then their usage time can be increased. Another

Fig. 12. a) Oxygen evolution reaction polarization curves of Cu2Se synthesized with a) electrodeposition relative to RuO2 and bare Cu, b) chemical vapor deposition (CVD) and hydrothermal method (HT), c) overpotentials to achieve 50 mAcm 2 and d) Tafel plots. Reprinted with permission from Ref. [50]. Copyright ACS 2018. 9

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Solid State Sciences 100 (2020) 106101

Table 2 Electrocatalytic performances of different Cu precursors for oxygen evolution reaction. Catalyst

Electrolyte

Onset potential/V vs PHE

Overpotential at 10 mAcm

Cu(OH)2 NWs/CF CuO NWs/CF CuOx NWs/CF Annealed CuO H2O2 treated CuO Cu0.3Ir0.7Oδ CuCO2O4-SSM CuRhO2 Cu3P/CF Cu2Se ED-1 Cu2Se CVD Cu2Se ED-2 Cu3Se2 HD

0.1 N NaOH 0.1 N NaOH 0.1 N NaOH 1 M KOH 0.1 M KOH 0.1 M KOH 1 M KOH 1 M KOH 0.1 M KOH 1 M KOH 1 M KOH 1 M KOH 1 M KOH

1.625 1.627 1.67 1.58 1.57

530 590 630 430 (1) 520 (2.5) 415 400 410 412 (50) 320 300 270 190

1.55 1.56 1.53 1.45 1.45 1.5

2

Catalyst loading mgcm2

Tafel slope mV/dec

Ref.

0.8 0.8 0.8

86 84 108 61.4

[67] [67] [67] [68] [69] [70] [71] [72] [73] [50] [50] [50] [50]

0.2 0.8 68.5 0.8 2 0.7 5

105 63 48.1 90.9 107.6 136.7

Fig. 13. CV profiles a, b) Cu2Se and c, d) Cu1.3Se under dark (a, c) and light (b, d) illumination. Inset are the images of electrodes. Reproduced with permission from Ref. [51]. Copyright Elsevier 2018.

Fig. 14. a) Dark experiments (J-V plots), b) photoexperiments (J–V) plots for 1) ECD/BD, 2) CBD and ED. Reproduced with permission from Ref. [52]. Copyright Elsevier 2017.

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Solid State Sciences 100 (2020) 106101

of electrodes, low charge carrier mobility of conjugated polymer and low chemical stability in comparison to inorganic semiconductors. Sergey Vikulov et al. [58] have fabricated CuSe nanosheets (NSs, 1 μm � 5 μm) and drop coated them into thin films using concentrated solutions on flexible substrates (polyvinyl chloride, polyvinylalcohol and paper See Fig. 16). All the films were prepared at room temperature without any temperature treatment. These films showed conductivities up to 645 Sm-1 which is maintained in air after several months. Appli­ cations of bending, stretching and folding do not effect the electrical properties of Cu2-xSe NSs and they are almost 100% retained when these operations are removed. In comparison to NSs, NPs based samples of Cu2-xSe produce 20–30% irreversible loss of conductivities. So, copper selenide may be a future horizon candidate of printable electronics on flexible substrates or may be in stress sensors [58]. 3.5. Thermoelectric properties of copper selenide Environmental pollution is endangering the existence of human being on earth. Among different environmental friendly technologies, thermoelectric power conversion is the one which is solid state and has its applications in power generation, cooling and waste heat recovery. If 100% industrialized, this technology is clean to environment, without noise and hazardous chemical emission, highly reliable, has safe elec­ trical output and long term usability. ZT determines the efficiency of thermoelectric device (ZT ¼ S2σT/κ where S is seebeck coefficient, σ is electrical conductivity, κ is thermal conductivity and T is absolute temperature [59]. Considerable efforts have been made to improve the ZT by modifying electron and phonon transport via nano-synthesis which can reduce the thermal conductivity of phonon while maintaining/improving the electronic transport via quantum confinement effect etc. High ZT have been observed in thin films and nano bulk samples. However commer­ cialization of thermoelectric technology is still a future horizon due to expensive raw materials (Bi, Te etc.) and difficulty in fabrication pro­ cesses [60,61]. Cu2Se has been explored for its thermoelectric properties with a dimensionless ZT value of 1.5 at 1000 K (bulk) and 1.6 at 973 K [62] mainly because of earth abundance of Cu and Se relative to Bi and Te [63]. Spin coated thin films of Cu2Se provided a power factor of 0.62 and 0.46 mW(mK2) 1 on rigid alumina and flexible polyimide substrates respectively. These results are superior than previously reported Cu2Se thin films (0.1–0.5 mW(mK2) 1). Structural perfection and excellent electrical performance was attributed to soluble precursor ink which is

Fig. 15. a) J-V curves at different Cu/Se ratios, b) IPCE spectra at different Cu/ Se ratios, c) EIS spectra at different Cu/Se ratios and d) Tafel plots at different Cu/Se ratios. Reproduced with permission from Ref. [53]. Copyright Elsev­ ier 2016.

semicircles are minimum for 1:4 (Cu/Se) which in turn present high conductivity, lowest charge transfer resistance and high catalytic ac­ tivity. Tafel plots (Fig. 15 d) also confirm the order of activity as 1:4 > 1:6 > 1:2 determined from anodic and cathodic slope of arms which indicate that exchange current densities (Jo) are in the same order. At Cu/Se 1:2, product is pure tetragonal Cu3Se2, whereas at 1:1 and 1:6 Cu3Se2 is contaminated with hexagonal CuSe. This impure crystalline phase has been indicated as the source of better PV performances of CEs. Electrical conductivity also increases by the defects imparted by higher Se contents. 450 � C was determined as the optimum sintering temper­ ature relative to 400 � C and 500 � C for best PV performance in this study. Previously ZnO based QD SSCs were evaluated with Cu2Se as CE with Jsc of 11.26 mAcm 2, Voc of 0.68 V and PCE of 2.28% which are better than CuS materials [54]. Copper indium selenide solar cells have shown PCE of 3% which are higher than ZnO based QDSSCs [55]. In comparison to copper indium selenide, CuInGaSe2 solar cells when populated with low Cu contents show an efficiency of 11%, Voc of 0.52 V, Jsc of 32 mAcm 2 and FF of 0.67. However, performance of Cu rich CuInGaSe2 is inferior even if it is treated with KCN. This implies that excessive Cu2-xSe have to be removed from film surface for best PV performance [56]. Hybrid semiconductors consisting of organic and inorganic combi­ nations are useful due to low cost, stability and shielding capability of inorganic materials. In this regard Adriano Cesar Rabelo et al. have used Cu/Cu2-xSe/MeH-PPV/Al (poly-[2methoxy-5-(2-ethylhexyloxy)-1,4phenylenevinylene) as hole injector for polymer light emitting diodes. Operational voltage for Cu2-xSe based device decrease two fold relative to conventional ITO/MeH-PPV/Al based device [57]. 3.4. Copper selenide in flexible electronics Flexible electronics is getting its place in commercial appliances such as solar cells, biomedical sensors and displays. Flexible properties of polymeric materials such as plastic, polystyrene and poly­ methylmethacrylate are combined with semiconducting properties of conjugated polymers in this technology. There are different polymers which are flexible but only a few (poly(3,4-ethylenedioxythiophene)poly(styrenesulfonate) (PEDOT:PSS) have good conductivities. So there is a limitation on the design of flexible devices in terms of work function

Fig. 16. Synthesis of copper selenide nanosheets and spherical nanocrystals and their conversion to films by drop casting on polyvinyl chloride (PVC), polyvinyl alcohol (PVC) and paper followed by electrical conductivities on folding, stretching and bending. Reproduced from Ref. [58]. Copyright Wiley 2016. 11

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Solid State Sciences 100 (2020) 106101

comparatively better than nanocrystals ink. Thermal conductivity of 1.3–1.5 W/(mK) was measured with bulk ultrafast optical spectroscopy and is better than bulk Cu2Se samples. This low cost and scalable process may lead to a road map for future generation devices and sensors [59].

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4. Conclusions Of all the methods reported in this article for the synthesis of copper selenide thin films, films are not adherent with the substrate at higher precursor concentration for chemical bath deposition (CBD). Thick films cannot be prepared at low concentrations and multiple dippings are required for thick films which make the process cumbersome. In com­ parison with CBD, electrodeposition (ED) provides adherent and uni­ form films on the substrates but flexible substrate (polymeric) cannot be used in ED. Microwave irradiation method and sputtering methods for copper selenide thin films are still in somophoric phase and more work is still required for their acceptance in research institutes and universities. Among vapor methods AACVD is the most advantageous because it does not require the solubility of precursor. Films fabricated with this method are adherent with the substrate and uniform. Electrocatalytic activity of copper selenides thin films for water splitting is better than copper hydroxides and oxides owing to more covalency of copper-selenium bond relative to copper-oxygen bond. Self-repairable properties and thermoelectric properties of copper sele­ nides thin films are interesting but more work is still required to reach a fruitful conclusions. In solar cells copper selenides thin films have been mainly used as counter electrodes and better performances have been achieved by copper rich impurity phases. More work is still in progress and to the best of our knowledge no product of pure copper selenide counter electrodes has been marketed yet. CuSe thin films on polyvinyl chloride, polyvinyl alcohol, and paper show conductivities up to 645 Sm-1. These conductivities are retained for several months and application of stretching, bending and folding does not effect them. These flexible copper selenide films may be a future horizon candidate for printable electronics and stress sensors. CRediT authorship contribution statement Raja Azadar Hussain: Writing - review & editing. Iqtadar Hussain: Writing - review & editing. References [1] M.D. Khan, M.A. Malik, N. Revaprasadu, Progress in selenium based metal-organic precursors for main group and transition metal selenide thin films and nanomaterials, Coord. Chem. Rev. 388 (2019) 24–47. [2] R.A. Hussain, I. Hussain, Fabrication and applications of nickel selenide, J. Solid State Chem. 277 (2019) 316–328. [3] R.A. Hussain, A. Badshah, B. Lal, Fabrication, characterization and applications of iron selenide, J. Solid State Chem. 243 (2016) 179–189. [4] U. Shamraiz, R.A. Hussain, A. Badshah, Fabrication and applications of copper sulfide (CuS) nanostructures, J. Solid State Chem. 238 (2016) 25–40. [5] U. Shamraiz, R.A. Hussain, A. Badshah, B. Raza, S. Saba, Functional metal sulfides and selenides for the removal of hazardous dyes from water, J. Photochem. Photobiol., B 159 (2016) 33–41. [6] Um-e-Habiba, A. Badshah, R.A. Hussain, Synthesis of iron chalcogenides from single source precursors, Appl. Organomet. Chem. 30 (2016) 783–795. [7] E. Andrade, V.M. Garcı ́a, P.K. Nair, M.T.S. Nair, E.P. Zavala, L. Huerta, M.F. Rocha, Ion beam analysis of copper selenide thin films prepared by chemical bath deposition, Nucl. Instrum. Methods Phys. Res. B 161–163 (2000) 635–640. [8] V.M. Garcı ́a, P.K. Nair, M.T.S. Nair, Copper selenide thin films by chemical bath deposition, J. Cryst. Growth 203 (1999) 113–124. [9] R.D. Heyding, The copper selenium system, Can. J. Chem. 44 (1966) 1233–1236. [10] J.O. Thompson, M.D. Anderson, T. Ngai, T. Allen, D.C. Johnson, Nucleation and growth kinetics of co-deposited copper and selenium precursors to form metastable copper selenides, J. Alloy. Comp. 509 (2011) 9631–9637. [11] T. Massalski, Binary Alloy Phase Diagrams, ASM Int, Materials Park, 1990. [12] S.K. Haram, K. Santhanam, Photoelectrochemical responses of orthorhombic and cubic copper selenides, J. Electroanal. Chem. 396 (1995) 63–68.

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