Optical and dielectric properties of electrochemically deposited p-Cu2O films

A perfect crystalline phases of cuprous oxide were synthesized using electrochemical method at different duration ( 15, 30 and 60 min). The deposited samples were examined by XRD, SEM, UV–Vis absorption and Mott-Schottky measurements. The effect of the deposition time on the optical and dielectric properties of Cu2O was studied in detail. The x-ray diffraction indicated increasing of crystallinity and crystallite size with increasing of deposition time. SEM micrographs exhibited grains with three-faced pyramid shape and grains size increased with improvement of crystallinity. Optical study is performed to calculate optical band gap (Eg), absorption coefficient (α), extinction coefficient (k), refractive index (n), dielectric constants (ε), urbach energy (EU) and optical conductivity (σopt) using the transmittance and absorption spectra in the wavelength range of 400–1100 nm. Among all grown samples, the film deposited at 60 min shows interesting optical and dielectric properties. The Mott-Schottky analysis shows that the film deposited at 60 min has a low carrier density compared to samples deposited in other deposition times.


Introduction
Cuprous oxide (Cu 2 O) is a p-type semiconductor and a promising candidate for solar energy conversion thanks to its good absorption coefficient in the visible range and its direct band gap of 2.1 eV [1,2]. In addition, the Cu 2 O thin film has many advantages such as abundance and nontoxicity and low production cost [3]. Up to now, there have been several reports on various Cu 2 O-based solar cells [4,5]. The theoretical energy conversion efficiency of Cu 2 O was found to be 20% [6]. Several methods are usually used to prepare Cu 2 O thin films, including pulsed laser deposition [7], sol gel [8], hydrothermal [9,10], chemical bath deposition [11,12], chemical vapor deposition [13], electrodeposition [14,15] and spray pyrolysis [16]. Among these methods, electrodeposition is one of the most interesting methods for the synthesis of thin films of semiconductor oxides [14]. This technique allows to control the stoichiometry, thickness, and structure of Cu 2 O films by adjusting the deposition parameters such as temperature, pH, voltage and time deposition, thus the thin films could be highly ameliorated. According to the literature, Messaoudi et al [17] studied the effect of deposition time on Cu 2 O thin films in the range time of 5 min to 10 min, they observed a betterment in the crystallinity and grain size when the deposition time is increased, and they revealed that Cu 2 O films are suitable as photo-absorbers in the industrial applications. Hossain et al [18] have investigated the effect of deposition time on Cu 2 O thin films synthesized by electrodeposition method by using an aqueous precursor solution at room temperature. They found that the films are comprised of Cu 2 O nanoparticles with distinctive wavelike surface feature. Hanif [19] reported that deposition time affected the structural, morphological and topological properties of n-Cu 2 O thin films. From the above literature reviews, most of the works have reported on effect of deposition times on surface morphology, crystal quality properties of the Cu 2 O films. While in-depth studies of the effect of deposition time on optical and dielectric properties have rarely been reported. Therefore, the objective of this work is to study in detail the effect of the electrodeposition time on optical and dielectric properties of Cu 2 O thin films deposited at Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. −0.5 V. Moreover, this work aims to optimize the properties of Cu 2 O thin films as absorber to make a Cu 2 O/ZnO NRs junctions.

Experimental
All experiments electrodeposition were conducted in a standard three electrode cell connected to potentiostat/ galvanostat Autolab PGZ 301. In the electrochemical cell a working electrode consisting of FTO substrate with a surface area of 1 cm×1.5 cm, the counter-electrode was platinum mesh and saturated calomel electrode (SCE) as a reference electrode. Before using, all FTO substrates were ultrasonically cleaned in acetone for 10 min, ethanol for 10 min and deionized water. The bath temperature was 60°C at a potential of −0.5 V versus SCE. The Cu 2 O thin films were deposited from an aqueous solution containing 0.2 M copper (II) sulfate (CuSO 4 .4H 2 O) and 3 M lactic acid (C 3 H 6 O 3 ) as chelating agent, the pH of the bath was adjusted to 9 with sodium hydroxide (NaOH) in order to deposit p-type Cu 2 O [20]. The thin film deposition potential −0.5 V was determined from the results of the linear voltammetry measurement. The Cu 2 O thin films were analyzed through x-ray diffraction using a X'PERT-PRO diffractometer analysis with CuKα radiation (λ=1.5406 Å). The surface morphology of Cu 2 O samples were investigated by using scanning electron microscope (FEIQuanta200). The transmittance and UV-Visible absorption spectra were recorded by a Jasco V-730 spectrophotometer. The spectra were recorded in the wavelength range from 400 to 1100 nm. The conductivity type of Cu 2 O films was characterized through Mott-Schottky measurements, the potential sweeps were carried out from −0.2 V to 0.6 V, the AC frequency used in the impedance measurements was 1000 Hz. Figure 1(a) shows the linear sweep voltammetry (LSV) curve of FTO substrate in a solution containing 0.2 M copper (II) sulfate and 3 M lactic acid with scan rate of 20 mV s −1 , pH and temperature of the bath were maintained at values 9 and 60°C respectively. The potential range is between −1.3 and 0 V versus SCE. It is clearly shown in the figure that there are two reduction regions. We were interested in the first region that was detected between −0.23 V and −0.6 V in which there is the deposition of cuprous oxide Cu 2 O according to the following mechanism [21]:

Electrochemical characterization
Cu 2 O films were synthesized at the selected potential of −0.5 V. Figure 1(b) represents the shape of current transient of Cu 2 O thin films. The shape of the curve indicates clearly the nucleation-growth step [21], the curve exhibits three regions, the first region corresponding to the double-layer charging at the interface of FTO/ electrolyte, in which the current density increases until the maximum current (I max ) is reached at a maximum time t max . The second region corresponds to the nucleation and growth of Cu 2 O nuclei, in this step the cathodic current density decreases due to Cu 2 O film cover the surface. Finally, the cathodic current density value become steady with increase deposition time due to growth process of the Cu 2 O film on the FTO substrate.  Figure 2 shows the XRD pattern of Cu 2 O thin films electrodeposited on the conductive substrate FTO at different time 15, 30 and 60 min, respectively. The several peaks shown by the XRD pattern confirm polycrystalline nature of Cu 2 O thin film which has cubic structure according the JCPDS N°: 05-0667 [22]. Moreover, the XRD patterns show obviously no secondary phases. The films deposited at 15, 30 and 60 min are oriented preferentially along [111] direction with lattice parameter a=4.26 Å. When the deposition time is varied from 15 to 60 min, a clear increase of (111) peak intensity is observed indicating an improvement in the crystallinity of the films.

Structural characterization
The crystallite size of the samples deposited was estimated by using Scherrer formula (equation (3)) [23].
Where D, k, λ, θ and β are the crystallite size, a constant equal to 0.9, the wavelength of the x-ray, the diffraction angle and the full width at half maximum (FWHM) of the diffraction peak, respectively.
The dislocation density (δ), defined as the length of dislocation lines per unit volume, was evaluated from the formula (equation (4)) [24]: Strain (ε) of the films was calculated by the formula (equation (5)) [24]: The calculated values of the lattice parameter, the crystallite size, the dislocation density and the microstrain are summarized in table 1. The calculated crystallite size is increased from 62.83 nm to 66.53 nm when the deposition time is varied from 15 to 60 min. While, the δ and ε decrease from 2.533×10 −14 to 2.259×10 −14 lines m −2 and 0.551×10 −3 to 0.52×10 −3 , when increasing the deposition time from 15 to 60 min. This is due to a decrease in defect levels and grain boundaries as well as an increase in crystallite size [25].

Optical properties
Optical absorption studies are important to determine the optical band gap of semiconductor films, and for understanding the optoelectronic properties of our growing semiconductor. The absorbance spectra in the visible region of the deposited Cu 2 O thin films synthetized at different deposition times are displayed in figure 5(a). We observe from the showed spectra an absorption edge localized between 500 and 600 nm which corresponds to the excitonic band gap of Cu 2 O. Moreover, figure 5(b) illustrates that the sample electrochemically grown at 15 min exhibits a higher transmittance. We observe that the transmittance was significantly decreases with increasing of deposition time. The plots of (αhν) 2 versus (hν) obtained from optical transmittance spectra for all samples is shown in figure 5(c). From the straight lines of plots of (αhν) 2 versus (hν), we conclude that the deposited films have a direct forbidden band gaps.  [22,30,31]. The absorption coefficient α of Cu 2 O thin films was evaluated from the transmittance data using relation (equation (7)), where α, T and d are the absorption coefficient, the transmittance and the thickness of Cu 2 O films, respectively [32].   increases, an enhancement in the absorption coefficient of Cu 2 O thin films is observed. The absorption coefficient is used to define the Urbach parameter (E u ) corresponding to the transitions between the extended states of the valence band and the localized states of the conduction band. The absorption coefficient is related to Urbach parameter by the following formula [33]: a a n = exp h E 8 The Urbach energy E u is determined by plotting lnα versus photon energy (hυ) as shown in figure 6. The results are summarized in table 3, the Urbach energy decreases with increase of deposition time. E g optical band gap values change inversely with E u energy. The results of Urbach energy are comparable to those reported by authors interpreting it as the bandwidth of the states located within the band gap width.  The more critical optical constant of a semiconductor material is refractive index, its relationship with wavelength of incident light is known as dispersion name [34]. The refractive index is expressed as n=n * +ik, where n * is the complex refractive index, and k is imaginary refractive index also named as extinction coefficient as it indicates the total optical loss caused by transmittance and scattering. The n and k can be calculated from the following relations [34]: where the parameters α, R and k are absorption coefficient, reflectance and extinction coefficient. Figures 7(a), (b) shows the refractive index (n) and the extinction coefficient (k) as a function of wavelength of the Cu 2 O films synthetized with different deposition times. We conclude that with increasing of wavelength, the refractive index and extinction coefficient values decrease, this is attributed to normal semiconducting dispersion behavior of the material. In other hand the extinction coefficients and refractive index values are influenced by the deposition time, and both increase with it. The slight increase can be attributed to an increase in density of the films with increasing deposition time. Table 4 shows the values of 'n' and 'k' on wavelength 1100 nm for all films.
In semiconductors the fundamental electronic transition is allied to the wavelength dependent complex dielectric constant, expressed as: ε r +iε i . The real (ε r ) and imaginary parts (ε i ) of complex dielectric constant can be calculated from the extinction coefficient and refractive index using the following equations [38]:

Conclusion
In summary, Cu 2 O films were successfully elaborated on (FTO)-coated glass substrates at different deposition time using potentiostatic deposition. The electrodeposited films were analyzed for their structural, morphological, optical and dielectric properties. The XRD analysis revealed that Cu 2 O films crystalizes in a cubic structure with preferential orientation in the plane (111) and showed that the crystallinity and grain size were increased by increasing of deposition time. The SEM micrographs displayed three-sided pyramid-shaped grains, the uniformity and grains size increase with time deposition. The optical and dielectric properties of the films were calculated from the measurement optical of transmittance and absorbance. The optical energy band gap values were found to be 2.32, 2.22 and 2.17 eV at 15, 30 and 60 min respectively. Mott-Schottky measurements shows that all films are p-type semiconductors, the low carrier density attributed to the film Table 4. The refractive index (n), extinction coefficient (k), real dielectric constant (εr), imaginary dielectric constant (εi) and optical conductivity (σ opt ) in the wavelength (λ=1100 nm).  deposited at 60 min. From this study, it is evident that the deposition time is an important key to obtain thin films suitable for solar cell applications