Annealing Effect on One Step Electrodeposited CuSbSe₂ Thin Films

Abstract

The purpose of this work is to study the influence of the annealing temperature on the structural, morphological, compositional and optical properties of CuSbSe₂ thin films electrodeposited in a single step. CuSbSe₂ thin films were grown on fluorine-doped tin oxide (FTO)/glass substrates using the aqueous electrodeposition technique, then annealed in a tube furnace under nitrogen at temperatures spanning from 250 to 500 °C. The resulting films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray analysis, Raman spectroscopy and UV-Vis spectrophotometer. The annealing temperature plays a fundamental role on the films structural properties; in the range 250–350 °C the formation of pure CuSbSe₂ phase from electrodeposited binary selenides occurs. From 400 to 500 °C, CuSbSe₂ undergoes a preferential phase orientation change, as well as the increasing formation of copper-rich phases such as Cu₃SbSe₃ and Cu₃SbSe₄ due to the partial decomposition of CuSbSe₂ and to the antimony losses.

1. Introduction

The increasing global demand for energy and the urgent need to move from fossil fuels to renewable sources impose the rapid increase of photovoltaic (PV) installations. Besides the mature silicon-based PV technologies, the thin film PV is gaining more and more interest for the possible integration in buildings and products (BIPV and PIPV, Building and Product Integrated PV).

Nowadays, the most used absorber materials for thin film solar cells are CdTe and Cu(In, Ga)Se₂ (CIGS) [1], but due to the scarcity and high cost of In/Ga and the toxicity of Cd there is a strong need to develop new materials for thin film solar cells. To date, several potential absorber materials, such as Cu₂ZnSn(S, Se)₄ [2], Sb₂(S, Se)₃ [3] and CuSb(S, Se)₂ [4] have been studied.

The focus of this work is on CuSbSe₂ thin films, a promising candidate for infrared detectors [5], thermoelectric [6], photovoltaic and photoelectrochemical applications [7]. CuSbSe₂ is a chalcogenide semiconductor that meets the main properties required for thin film solar cells absorber: direct and ideal band gap (at 1.1 eV) and high absorption coefficient (α > 10⁴ cm⁻¹) [8]. According to a theoretical study by Goyal et al., the high absorption in CuSbSe₂ is due to the lone pair 5s² electron present in trivalent “Sb” [9]. Another interesting property is the 2D structure of CuSbSe₂ creating a chemically inert surfaces with fewer bonds attached compared to 3D structures, thus reducing the loss of carrier recombination and improving the efficiency of thin film solar cells [4].

Both physical and chemical processes have been used to synthesize CuSbSe₂ thin films. So far, the photovoltaic conversion efficiency of CuSbSe₂ based solar cells are still <5%, slightly depending on the technique applied. Using a hydrazine solution process, Yang et al. reported a CuSbSe₂ thin film solar cell with an efficiency of 2.7% [10], Wang et al. 3.04% by reactive close-spaced sublimation (RCSS) method [11] while Rampino et al. achieved an efficiency of 4.0% by using the low-temperature pulsed electron deposition (LTPED) technique [12]. Similarly, a high throughput experimental combinatorial synthesis (HTE) was used by Welch et al. to achieve a CuSbSe₂ thin film solar cell device with an efficiency of 4.7% [13].

Among the various techniques, electrodeposition is very appealing for its simplicity, low cost and scalability. However, only few works have been devoted to the elaboration of CuSbSe₂ thin layers by single step electrodeposition [14,15,16].

Based mainly on the work of Tang et al. [14,15], in this study we report the synthesis of CuSbSe₂ thin films by one-step electrodeposition, followed by a detailed study of the effect of annealing on the electrodeposited films.

The heat treatment is known to significantly affect the properties of CuSbSe₂ thin film. Karup-Moller [17] studied the complex ternary phase relations in the Cu–Sb–Se system at temperatures between 350 and 700 °C; he identified several binary phases (Cu₂Se, CuSe, γCuSe₂, Cu₂Sb, and Sb₂Se₃) and a total of five solid ternary compounds (Cu₃SbSe₄, Cu₃SbSe₃, CuSbSe₂, Cu_10.53Sb_33.78Se_55.68 and Cu₆SbSe₉). Xue et al. [18] explored the temperature dependence of the band gap energy of CuSbSe₂ films deposited by hydrazine solution process; Yan et al. [19] found that five intermetallic compounds (CuSbSe₂, Sb₂Se₃, CuSe₂, Cu₂Se and Cu₃SbSe₃) were generated after annealing of Sb₂Se₃/Cu multilayers deposited by pulsed laser deposition (PLD). However, no study has yet been carried out on the effect of the annealing process on electrodeposited CuSbSe₂ thin films.

In this work, the evolution of CuSbSe₂ thin films grown on FTO and glass substrates is investigated through a detailed structural, morphological and compositional characteri-zation of the film conditions and the chemical reactions involved and thus the optimal electrodeposition conditions are find by combining electrochemical technique (voltamme-try and chronoamperometry).

2. Materials and Methods

The electrochemical experiments were performed in a three-electrode cell configuration at room temperature. The cell contains a fluorine-doped tin oxide (FTO) coated glass substrate (7 Ω/sq) as the working electrode, a platinum grid as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode. This system was controlled by a Potentiostat VoltaLab 40 (PGZ301 and VoltaMaster 4) (Figure 1). The substrates were ultrasonically cleaned with acetone and ethanol for 10 min each, rinsed with deonized water and dried.

The electrolyte solution contains 100 mM ammonium chloride (NH₄Cl) as supporting electrolyte, 2 mM copper chloride (CuCl₂), 2.5 mM selenious acid (H₂SeO₃) and 2 mM of antimony potassium tartrate (K₂[Sb₂(C₄H₂O₆)₂]·3H₂O) as prescursors of Cu, Se and Sb respectively. The pH of the solution was adjusted to 2.00 ± 0.01 using concentrated HCl [14]. Linear sweep voltammetry (LSV) method was carried out to determinate the oxidation/reduction potentials of electrolyte solution. It is also used for a comparative study between Cu–Se, Sb–Se binary solutions and Cu–Sb–Se ternary solution. All voltammograms were measured at a scan rate of 5 mV/s over a potential range of −0.8 to +0.3 V/SCE. The chronoamperometry, an electrochemical technique in which the potential of the working electrode is fixed and the resulting current is monitored as a function of time, was then performed at the potential of −0.4 V/SCE without stirring for 30 min at room temperature to deposit the layers. The obtained samples were annealed in a quartz tube furnace at temperature of 250, 300, 350, 400, 450 and 500 °C for 15 min under nitrogen flow.

The structure of CuSbSe₂ thin films was investigated using an X-Ray diffractometer (D8 Advance Twin, Bruker, Billerica, MA, USA), with Cu Kα1 radiation wavelength 1.5418 Å, with a sweep of 5° to 80° and a step of 0.02°. The surface morphology and chemical composition were studied by scanning electron microscope (SEM, Format JEOL/EO/Version 1.00, JEOL, Tokyo, Japan) and energy dispersive X-ray (EDX) detector. The optical properties of electrochemically grown films were tested using an UV/Vis double-beam spectrophotometer (V730, Jasco, Tokyo, Japan) with the light source of deuterium lamp (190 to 350 nm) and halogen lamp (330 to 1100 nm). Raman scattering measurements were performed using Bruker VERTEX 70 Raman spectroscopy, using a 532 nm excitation laser with a 0.95/100× objective; focused in a spot of ~1 μm in diameter. Operating at the maximum power of 30 mW and an accumulation time of 30 s.

3. Results and Discussion

3.1. Linear Sweep Voltammetry

Figure 2 shows the voltammogram of FTO-coated electrode in the Cu–Sb–Se solution; the voltammograms of the Cu–Se and Sb–Se solutions are shown for comparison. A small increase in cathodic current was observed from the beginning of the sweep (+0.07 V/SCE), followed by a plateau between −0.03 V and −0.25 V/SCE. Compared to the Cu–Se solution, the voltammogram of the Cu–Sb–Se solution has no visible peaks corresponding to the respective reduction of Cu²⁺ and Cu⁺. This could be explained by their competition with other precursors for adsorption sites at the substrate [14]. After that, the cathodic current becomes larger at −0.3 V/SCE, which refers to a simultaneous deposition of Cu and Se as in the Equation (1) [20]. The second large cathodic peak of Cu–Sb–Se solution at −0.5 V/SCE corresponds to Sb–Se deposition as in Equation (2) [21]. It is interesting to note that the reduction peaks of the Cu–Sb–Se solution show a positive shift compared to the Cu-Se solution and the Sb–Se solution. This may be due to the fact that the presence of the three compounds together promotes the reduction process [14].

H₂SeO₃ + Cu²⁺ + 4H⁺ + 6e⁻ ↔ CuSe + 3H₂O

(1)

3H₂SeO₃ + 2SbO⁺ + 16H⁺ + 18e⁻ ↔ Sb₂Se₃ + 11H₂O

(2)

Since the electrodeposited films are very sensitive to potential variation, potentials between −0.30 and −0.48 V/SCE were tested by chronoamperometry [16] The deposition potential of −0.40 V/SCE was found to be the most suitable for the preparation of CuSbSe₂ thin films yielding deposited films with a nearly stoichiometric composition.

3.2. Morphological Characterization

The as-deposited Cu–Sb–Se thin film (Figure 3) exhibits a homogeneous and compact surface with spherical grains of 250 nm in size. The EDX analysis (

Table 1. EDX measurement of annealed samples at different temperature: atomic % and ratios.

	Formula	Atom% Cu	Atom% Sb	Atom% Se	Atom% O	Atom% Sn	Atom% Si	Cu/Sb	Sb/Se
T °C
As-deposited		13.51	22.13	34.91	3.25	21.74	ND	0.61	0.63
250 °C		17.31	22.86	36.7	9.38	13.75	ND	0.76	0.62
300 °C		16.78	22.58	36.08	10.83	13.72	ND	0.74	0.62
350 °C		19.34	22.39	39.97	3.45	14.86	ND	0.86	0.56
400 °C		18.58	14.34	35.54	15.19	16.39	ND	1.29	0.4
450 °C		12.84	3.32	17.61	24.37	30.22	11.62	3.86	0.19
500 °C		8.75	3	21.64	18.5	37.56	11.05	2.9	0.14

) shows that the as-deposited samples are poor in copper and selenium. The annealing process changes the samples morphology depending on the temperature, in a drastic way (Figure 3). At 250 °C, the spherical shapes become disordered and thus expand to a diameter of 1250 nm, turning rough and cracking into small crystal grains with an average size of 125 nm. At 300 °C, the crystal grains continue to grow up to 400 nm in size. They then uniformly spread out on the surface of the substrate. At this temperature, some octahedrons, identified as Sb₂O₃ by EDX and XRD, start to appear. At 350 °C, the morphology of the deposit was significantly improved, more compact and uniform. From the SEM image, the grains size is reduced to 250 nm, which might be due to the compression effect during the layer growth. Few Sb₂O₃ crystals are present, even though their sizes increases. EDX analysis shows a sharp decrease in oxygen and a Cu/Sb ratio = 0.86, which increase to 0.91 if we consider the atomic % of Cu, Sb and Se only, close to the stoichiometric CuSbSe₂ (Table 1). At 400 °C, the sample became porous with an even distribution of cavities, which could be attributed to the evaporated Sb₂Se₃ phase [22], resulting from the decomposition of CuSbSe₂ as shown in Equation (3) [19]:

3CuSbSe₂ (s) ↔ Cu₃SbSe₃ (s) + Sb₂Se₃ (g)↑

(3)

At 450 °C the layer shrinks and forms big agglomerates exceeding 10 microns in size, copper-rich and antimony-poor as shown by the EDS/mapping measurements. These agglomerates are surrounded by a thin layer of a deposit of Cu–Se and triangular grains with sizes from about 100 nm to ≥1 μm, likely Sb₂O₃, given their composition with ratio of Sb/O ~ 0.64. This strange morphology has been explained by Guo et al., suggesting that the annealing temperature approaches the melting point of CuSbSe₂ (480 °C), the grain size increases progressively, and the small particles clustered together to form large block particles [23]. Finally, at 500 °C, the agglomerates diffuse into a fairly flat structure with a rail-like appearance and the grain boundaries became unclear in the SEM image. Visually, the layer is very thin. EDX analysis (Table 1) supports that the increase in Sn and Si content at 450 and 500 °C comes from the substrate, due to their exposure following the shrinkage and thinning of the deposits. Generally, from 400 to 500 °C, the Cu/Sb ratio was found to increase up to 3.86, suggesting that a part of the Sb might have been lost due to Sb₂Se₃ sublimation. However, the Sb/Se ration is less than 0.14, which could be explained by the fact that the selenium released from the dissociation of Sb₂Se₃ is recovered by other reactions to form Cu₂Se contributing to ternary compounds formation, as in equations below [10,19]:

Sb₂Se₃ (s) ↔ 1/4Sb₄ (g) + SbSe (g) + Se₂ (g)

(4)

2Cu + 1/2Se₂ ↔ Cu₂Se (s)

(5)

3Cu₂Se (s) + Sb₂Se₃ (s) ↔ 2Cu₃SbSe₃

(6)

SEM-EDS analysis is unable to separate if the detected oxygen comes from the substrate or from the remaining oxygen of the precursors or of the tube during annealing. However, it has been reported that crystalline Sb₂O₃ is formed during the reaction of Sb with O at temperatures in the range 270–360 °C [24]. Therefore, Sb₂O₃ octahedrons are not present on the as-deposited sample and on that annealed at 250 °C.

3.3. Structural Characterization

3.3.1. XRD Measurements

As demonstrated by the study of linear voltammetry, the electrodeposition of Cu–Sb–Se takes place in the form of binary compounds such as CuSe and Sb₂Se₃ (Equations (1) and (2)). Before the annealing process, the as-deposited sample have an amorphous nature with XRD diffraction peaks indexing only the substrate SnO₂ (JCPDS 46-1088) as shown in Figure 4. After annealing CuSbSe₂ thin films XRD patterns show the presence of mixed ternary and binary phases. CuSbSe₂, Sb₂Se₃, Cu₃SbSe₃, Cu₃SbSe₄ and Sb₂O₃ compounds are mainly found in the prepared films.

At 250 °C, the weak intensity of peaks indicates a poor crystallinity of deposited films; it can also be noticed that the binary phases (CuSe and Sb₂Se₃) are predominant and that CuSbSe₂ starts to form (peak at 27.8°). From 300 to 350 °C, Cu becomes involved in the diffusion processes and reacts, both with the selenium liberated by a partial decomposition of Sb₂Se₃ as expressed previously in Equations (4) and (5) and with CuSe (Equation (7)), which lead to the formation of Cu₂Se (s).

Cu + CuSe ↔ Cu₂Se (s)

(7)

The generated Cu₂Se would further react with Sb₂Se₃ phase to form CuSbSe₂ as expressed in Equation (8) [17]:

Cu₂Se (s) + Sb₂Se₃ (s) ↔ 2CuSbSe₂ (s)

(8)

At 350 °C, the two strong peaks centered at 28.8° and 27.8° are indexed to the (112) and (200) planes of CuSbSe₂ phase (JCPDS 75-0992). In addition, the weak diffraction peaks indexed to (013) plane of CuSbSe₂ phase, Cu₃SbSe₃ (JCPDS 086-1751) and Sb₂O₃ (JCPDS 75-1565) can also be observed. At 400 °C, Sb₂Se₃ sublimates to gradually leave the films, explaining the absence of the corresponding peaks in the XRD spectra [10]. However, there is a dominant presence of CuSbSe₂ and Sb₂O₃ phases with a more sharper peaks centered at 27.8° and 27.5° corresponding respectively to the (200) plane of CuSbSe₂ phase and the (222) of Sb₂O₃ phase. Besides a slight emergence of the Cu₃SbSe₃ phase at 23.87°, diffraction peaks at 27.3°, 45.5° and 54° corresponding to the Cu₃SbSe₄ phase (JCPDS 85-0003) also appeared at 400 °C. From 450 to 500 °C, the peaks at 23.93° and 48.93° attributable respectively to the (121) and (133) planes of Cu₃SbSe₃ phase become considerably intense and sharp. It can also be noticed that the peak at 11.9° belonging to the (002) plan of CuSbSe₂ phase increased significantly.

As far as the crystallographic orientations vs. temperature concerns, the CuSbSe₂ phase changes from the (112), (013) and (200) directions at 350 °C to a predominance of the (200) direction at 400 °C. At temperature of 450–500 °C, the CuSbSe₂ phase changes predominantly to the (002) direction. These results are in agreement with the observations reported by Yang et al. [10], which also demonstrated that CuSbSe₂-based solar cells with preferred (013)- and (112)-orientations show better efficiency, due to the easier carriers transport within the layers and to the lower recombination loss.

3.3.2. Raman Measurements

In order to further investigate the structural properties and to analyse the phase formation at different temperatures, Raman spectroscopy was performed on CuSbSe₂ thin film, using 532 nm laser excitation sources in the wavenumber range of 80–400 cm⁻¹.

The Raman spectrum of the as-deposited sample shows a very weak and broad peak, as shown in Figure 5 (Figure S4 in Supplementary Materials), which confirms that the electrodeposited and unannealed CuSbSe₂ thin film is amorphous. However, the Raman spectra of the samples annealed at 250 and 300 °C show two strong peaks centered at about 217 and 264 cm⁻¹ and four weak peaks at 89, 105, 144 and 154 cm⁻¹. The Raman peaks at 217 (Ag), 154 (Ag), 144 (B_2g), 105 (A_g) and 89 cm⁻¹ (A_g) are assigned to CuSbSe₂ phase [18,25,26]. The Raman peak at 264 cm⁻¹ is ascribed to A_1g of Cu₂Se phase [27]. The peaks representing the Sb₂Se₃ phase are not distinguishable, but it appears that the peak at 189 cm⁻¹ (A_g) due to Se–Sb–Se bonds of Sb₂Se₃ phase overlaps with the peak at 217 cm⁻¹ (A_g) of the CuSbSe₂ phase, resulting in an asymmetric peak broadning [9,25]. At 350 °C, the Raman spectrum is distinguished by two overlapping high peaks at 217 and 208 cm⁻¹, corresponding respectively to A_g and B_2g modes of CuSbSe₂ phase [18,25]. This overlap can be attributed to a broadening of the peaks probably resulting from the high power of the laser used. Indeed, Kumar et al. [28], had recently studied the effect of the annealing temperature and the excitation power on the Raman spectrum of Sb₂Se₃. They mention that the laser heating effect always results in anharmonic effects in solids, evidenced by the broadening and redshift of the Raman bands. Besides, a weak peak corresponding to B_1g mode of Cu₃SbSe₃ phase is also observed at about 167 cm⁻¹ [25]. However, the peaks representing the binary phases of Cu₂Se (264 cm⁻¹) and Sb₂Se₃ (189 cm⁻¹) disappeared, confirming their transformation into ternary phases at annealing temperature of 350 °C.

As previously demonstrated by SEM and XRD measurements, the annealing temperatures above 350 °C promote the formation of Cu-rich phases due to the loss of Sb associated to the sublimation of Sb₂Se₃. At 400 °C, the main peak at 187 cm⁻¹ can be attributed to CuSe₄/SbSe₄ stretching vibrations of the Cu₃SbSe₄ phase as reported in [29,30] or to the A_g mode of the Cu₃SbSe₃ phase [9,25]; the peaks at 230 and 169 cm⁻¹ correspond to Cu₃SbSe₄ [29]. It can also be seen that the minor peaks (83, 106 and 144 cm⁻¹) corresponding to the CuSbSe₂ phase reappear next to the main peak at 217 cm⁻¹ which seems to decrease. The peak at 259 cm⁻¹ corresponds to A_1g mode of Sb₂O₃ phase [31]. While, the peak at 320 is unknown so far. At 450 °C, the highest peak at 259 cm⁻¹ has been assigned to the Se–Se stretch vibration of the Se²⁻ ions of Cu₂Se phase [27]. However, the majority of peaks at 89, 194, 258 and 376 cm⁻¹ belong to cubic Sb₂O₃, in agreement with the reported Raman modes [31]. The presence of Cu₃SbSe₃ and CuSbSe₂ phases has not been identified, due to the poor intensity of their bands compared to other phases. At 500 °C, the peaks representing Cu₃SbSe₃ and CuSbSe₂ phases reappear, respectively at 107, 144 and 217 cm⁻¹ for CuSbSe₂ and at 188 cm⁻¹ for Cu₃SbSe₃ phase. The observed Raman modes of all samples are tabulated in

Table 2. Raman modes for CuSbSe₂ annealed at different temperature.

Annealed Samples	Raman Shift (cm−1)	Assigned Symmetry	Corresponding Phase
250 and 300 °C	89	Ag	CuSbSe2
	105	Ag
	144	B2g
	154	Ag
	217	Ag
	264	A1g	Cu2Se
	189	B2g	Sb2Se3
350 °C	208	B2g	CuSbSe2
	217	Ag
	167	B1g	Cu3SbSe3
400 °C	187	-	Cu3SbSe4/Cu3SbSe3
	169	-	Cu3SbSe4
	230	-
	83	Ag	CuSbSe2
	106	Ag
	144	B2g
	217	Ag
	259	A1g	Sb2O3
450 °C	259	Se-Se stretch	Cu2Se
	89	-	Sb2O3
	194	-
	258	A1g
	376	-
500 °C	107	Ag	CuSbSe2
	144	B2g
	217	Ag
	188		Cu3SbSe3

.

3.4. Optical Characterization

The optical transmittance spectra of all CuSbSe₂ thin films were measured in the wavelength range 400–1100 nm as shown in Figure 6a and, as expected, they largely differ depending on the thermal treatment. All the annealed samples show high absorption in the visible to near-infrared (NIR) spectral region 400–900 nm with an optical transmittance ≤6%. However, the sample annealed at 450 °C has a transmittance reaching 30%, that could be due to the shrinkage of the films that uncovers the transparent FTO substrate, as shown by SEM/EDS measurements.

The variation of the optical direct band gap of CuSbSe₂ thin films vs. annealing temperature is estimated by using the Tauc’s equation (Equation (9)), as shown in Figure 6b:

(αh𝜈)ⁿ = A(h𝜈E_g)

(9)

where α, h𝜈, A and Eg are the linear absorption coefficient, photon energy, a constant which related to effective mass and the band gap, respectively; n assumes the values of 2 for direct and 1/2 indirect transition. The optical band gap (Eg) can be derived from the extrapolation of the straight line part of the curve (αh𝜈)ⁿ vs. h𝜈 to (αh𝜈)ⁿ = 0.

It was observed that for all the samples, the best straight line is obtained for a value of n equal to 2, which is typical of direct allowed transitions [32,33].

The bandgap values of all the samples were higher than the expected value for CuSbSe₂ (~1.1 eV [8]) and affected by the presence of other phases as indicated by the compositional analyses. For the samples annealed at 250 °C to 400 °C, that could be due to the existence of higher band-gap Sb₂O₃ (~3.5 eV) [34]. For the 450 and 500 °C annealed samples, it is due to morphological transformations. The smallest bandgap value of 1.28 eV belongs to the sample annealed at 350 °C, which is in agreement with the results reported by Colombara et al. [7].

4. Conclusions

The effect of the heat treatment on the evolution of electrodeposited CuSbSe₂ thin films has been investigated by varying the annealing temperature from 250 to 500 °C. We showed that the phases, composition, morphology, optical properties and crystalline orientations of the Cu–Sb–Se films, were strongly dependent by the annealing temperature as shown by X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive spectrometer (EDX) and Raman spectroscopy.

During the electrodeposition process, defined by voltammetry and chronoamperometry, single compounds such as Cu(s) and Sb(s) and binary compounds such as CuSe and Sb₂Se₃ are deposited on the substrate. The different annealing conditions lead to the assembly of these compounds and the formation of new products including CuSbSe₂ phase and other ternary phases as well as antimony oxide.

These film characterizations indicate that the growth of single-phase CuSbSe₂ thin films is feasible at temperature of 350 °C; the formation of Sb₂O₃ secondary phases, which might be detrimental to the performance of CuSbSe₂-based solar cells, can be avoided by preventing oxygen sources during the electrodeposition and the annealing process or easily removed with diluted HCl acid (Supplementary Materials). The good optical properties of the 350 °C annealed film, combined to the simple, cheap and easily scalable deposition technique, make the electrodeposition a promising solution, in particular for the realization of CuSbSe₂-based thin film solar cells.