Enhanced Band Gap Energy and Electrochromic Behaviour of Selenium Incorporated Copper Thin Film

Selenium incorporated copper (CuSe) thin films are deposited on indium tin oxide (ITO) substrate by cyclic voltammogram of 5.0 ml of 0.1 M CuSO4 and 2 ml of 0.05 M selenium dioxide at potential range from –0.8 V to 0.7 V vs. Ag/AgCl in pH 1.0. The adherent nature, electroactivity and stability of the CuSe thin film were found to be good. The transmittance and absorption of thin films was analysed through UV-Vis spectra. An energy gap of 1.78 eV is obtained for CuSe thin films by extrapolating the linear portion of the curves hυ versus (αhυ). X-ray diffraction patterns revealed that the deposited films possess cubic structure with lattice constant (a) value 80.892Å. Surface morphology of thin film shows uniform granular mixed texture-like structure. The sizes of the grains are found to be in the range between 53.33 nm and 93.33 nm. The film composition was investigated using an energy dispersive X-ray (EDX) micro analytic unit attached with scanning electron microscope. The in-situ spectroelectrochemical behaviour of CuSe thin film at various applied potentials was observed in 0.1 M H2SO4. CuSe thin film exhibits dual colour chromic behaviour from contrast red colour to contrast yellows colour. The electrochromic device shows good optical contrast, coloration efficiency, response time and stability.


INTRODUCTION
7][8][9] Thin films of CuSe are normally crystallised in hexagonal structure and orthorhombic structure.Numerous techniques have been used to obtain CuSe thin films viz., thermal evaporation, 10 vacuum evaporation, 11 solution growth technique, [6][7][8][9][10][11][12] chemical bath deposition, [13][14][15][16][17][18][19] colloidal hot injection method, 20 vapour deposition, 21 and electrophoretic deposition. 22Among them, electrodeposition technique provides many advantages over vacuum and other processes, such as low temperature growth, control of film thickness and morphology, potentially low capital cost 1,23 and others.The Cu 2-x Se films are typically p-type, highly conductive, semitransparent semiconductors with a band gap varying between 1.1 and 1.4 eV, suitable for solar energy conversion and, as semitransparent layers, in high speed detectors working within the visible range. 24,25Cu 2 Se is used as a window layer in solar cells, in photovoltaic cells of Schottky diodes 26,27 and as radiation filters. 28 this paper, thin films of CuSe have been prepared on indium doped tin oxide (ITO) coated glass substrates using electrodeposition technique.Deposited films were subjected to electrochemical studies, X-ray diffraction, scanning electron microscopy, optical analysis and spectroelectrochemical analysis.

Materials
Reagents of copper sulphate, selenium dioxide, H 2 SO 4 were analytical grade and ITO coated glass plate was obtained from Merck (India) Ltd.Chemicals were used as received without further purification.All the solutions were prepared by ultra-pure water.

Methods
UV-visible spectroscopy analysis was carried out by a UV-visible spectrophotometer Jasco V-530 between 300 and 1100 nm, possessing a scanning speed of 400 nm min -1 .Cyclic voltammetric studies were done through computer controlled CH Instruments, and performed using a single-compartment cell with three electrodes, at room temperature.The surface morphology was studied by computer-controlled JEOL JSM-5600 LV.The computer controlled XRD system JEOL IDX 8030 was used to record the X-ray diffraction of samples.

Growth of Copper Thin Film
Figure 1 presented the growth of copper thin film on ITO surface, which was carried out in 5 ml of 0.1 M CuSO 4 in 0.1 M H 2 SO 4 electrolyte, with potential range from -800 to 600 mV at a scan rate of 50 mV s -1 through cyclic voltammetry.
Copper thin film shows one well-defined oxidation peak around 125 mV and reduction peak around -450 mV in the first cycle.After completion of six successive cycles, a contrasting red colour thin film was observed on the ITO surface.

Effect of Scan Rate
The CuSe thin film was washed well to remove copper, selenium and sulphate ions.The film was then subjected to cyclic voltammetry between -700 and 600 mV in pH 1.0 aqueous supporting electrolyte solution.Figure 3 shows the cyclic voltammetric behaviour of CuSe thin film studied at different scan rates.As the scan rate was increased from 50 to 500 mV s-1, both the anodic and cathodic peak current also increased linearly.This suggests good adherence and the electroactive nature of the formed thin film.

Stability of CuSe Thin Film
In order to examine the stability of the CuSe modified ITO plate, it was stored in an open atmosphere and the response current was monitored by employing cyclic voltammetry in 24 h interval to study the film stability.The potential cycling between -1000 mV and 1000 mV at the scan rate of 50 mV s -1 was carried out and the changes were observed in the redox responses.The thin film exhibited no significant change in the redox behaviour during cycling up to 200 cycles in pH 1.0.Figure 4 illustrates the plot of peak current vs. time of modified electrode, where good stability is shown towards the atmospheric conditions, while responsibility of current is almost constant up to seventh day.
Figure 4: Plot of peak current vs. time.

UV-Vis Spectroscopy and Optical Properties of CuSe Thin Films
The UV-Vis photo spectra of CuSe thin films were recorded with respect to the bare substrate placed in the reference beam using double beam spectrophotometer in the range 300-1100 nm. Figure 5 shows the absorbance and transmission data of the films versus wavelength.The spectrum shows a gradually increasing absorbance and a band at 500 nm.Transmission throughout the visible region was good; cut-off transmission of 52% was observed at 300 nm.where A is the optical constant, α is the absorption coefficient, Eg is the optical band gap and m is an index which assumes the values 1/2, 3/2, 2 and 3 depending on the nature of electronic transition responsible for the reflection.
The band gap energy of thin film was determined by tauc plot.Tauc plot has the photon energy (hν) on the X-axis and a quantity (αhν) 2 on the Y-axis, and extrapolating the linear portion of the curve to the X-axis yields the band gap energy of the material.Figure 6 shows the tauc plot of CuSe thin film coated with ITO plates in air medium.The band gap energy obtained for CuSe thin film is 1.78 eV which is an improved value with the electrochemical deposition.

SEM Analysis of CuSe Thin Films
The scanning electron microscopy image of CuSe thin films prepared at room temperature is shown in Figure 8.The image clearly demonstrates that the surface is well covered with CuSe thin films.The grains are much smaller in size, distributed evenly over the entire surface of the film and look like densely packed microcrystals.The average grain size continues decreasing with increasing amount of copper in the film composition.The larger the concentrations of the copper, the grains produced become smaller.For films with higher concentration of copper ions, the growth occurs with multiple nucleation centres, resulting in a lower grain size.On the other hand, for lower concentration of Cu ions, comparatively lower nucleation centres give higher grain size.The amount of feed material available in the reaction vessel is constant for a particular reaction.
If the same material is divided into a larger number of nucleation centres, the grain would not grow larger but remain smaller. 13The grain shows uniform granular mixed texture-like structure.The sizes are found to be in the range between 53.33 nm and 93.33 nm.
Figure 8: SEM behaviour of CuSe thin film.

EDX Analysis of CuSe Thin Films
The CuSe thin film composition was investigated using an EDX micro analytic unit attached with scanning electron microscopy and shown in Figure 9.The figure shows the presence of silica, indium, calcium, copper, selenium.The presence of emission lines in the investigated energy range indicates the formation of selenium incorporated copper thin films.

Spectroelectrochemical Analysis of CuSe Thin Film
Figure 10 shows the in-situ spectra of CuSe thin film at various applied potentials in 0.1 M H 2 SO 4 .Within the applied potential range, two absorption bands were observed at 300 and 900 nm, while the absorbance increased linearly with the increasing potential from -800 mV to 800 mV.When the applied potentials were varied from -800 mV to -100 mV, the thin film exhibited yellowish red colour.The band at 300 nm is associated with the  - * transitions; one weaker band in visible region around 900 nm usually appeared during the doping process of thin films and was ascribed to formation of alloys subgap state.The thin film exhibited contrasting yellow colour within 0 and 800 mV potential ranges.The importance in electrochromic devices based on thin film is due to their good optical contrast and stability.The CuSe thin film was stepped between its reduced and oxidised states.While the film was switched, the percentage transmittance at  max (300 nm) was monitored as a function of time.The contrast is given as the difference between the reduced and oxidised states and reported as %T.The results are presented in Table 1.The controlled potential coulometry was employed to evaluate the coloration efficiency and response time.

CONCLUSION
The cyclic voltammogram of 0.1 M of CuSO 4 and 0.05 M selenium dioxide at potential range from -0.8 to 0.8 V vs. Ag/AgCl in pH 1.0 formed yellowish red colour CuSe thin film on ITO surface.Formed thin film possesses good adherent nature on electrode surface, good electroactivity and stability.The transmittance and absorption of thin films was analysed through UV-Vis spectra.An energy gap of 1.78 eV is obtained for electrochemically synthesised CuSe thin films.X-ray diffraction patterns revealed that the deposited films possess cubic structure with lattice constant a = 80.892 Å. Surface morphology of thin film shows uniform granular mixed texture like structure.The sizes of the grains are found to be in the range between 53.33 nm and 93.33 nm.CuSe thin film exhibits dual colour chromic behaviour from contrasting yellowish red to contrasting yellow.The electrochromic device shows good optical contrast, coloration efficiency and response time.

Figure 1 :
Figure 1: Cyclic voltammetric behaviour of copper thin film growth on ITO in pH 1.0 scan rate at 50 mV s -1 .

Figure 2
Figure 2 illustrates the result of preparing CuSe thin film on ITO surface studied in 5 ml of 0.1 M CuSO4 in 0.1 M H2SO4 and 2 ml of 0.05 M SeO2 electrolyte, with potential range from -800 to 700 mV at a scan rate of 50 mV s-1 vs. Ag/AgCl and through cyclic voltammetry.Voltammogram of thin film shows one oxidation and one reduction peak at 180 mV and -425 mV for first cycle.After completion of the first cycle of voltammogram, five oxidation and one reduction peaks appear.Compared to voltammogram of copper thin film deposition, the additional number of oxidation peak indicates incorporation of selenium during deposition of thin films.As the number of cycle increased, the peak current of anodic peak and cathodic peak also increased.After completion of twelve cycles, a yellowish, red colour thin film formed on ITO surface.

Figure 2 :
Figure 2: Cyclic voltammetric behaviour of CuSe thin film growth on ITO in pH 1.0: scan rate at 50 mV s -1 .

Figure 3 :
Figure 3: Cyclic voltammetric behaviour of CuSe thin film at different scan rate at 50-500 mV s -1 .

Figure 5 :
Figure 5: UV-Vis absorption and transmittance spectrum of CuSe thin film.The band gap energy can be determined using the Tauc relation.It is a convenient way of studying the optical absorption spectrum of a material.According to the Tauc relation, the absorption coefficient α for direct band gap material is given by: αhν = A(hν -Eg) m
Figure 11 represents the electrochromic behaviour of CuSe (a) before oxidation; and (b) complete oxidation.

Table 1 :
Electrochromic parameters of CuSe thin films.