Microstrucral, Optical and Electrical Characteristics of Cu-doped CdTe Nanocrystalline Films for Desinging of Absorber Layer in Solar Cell Applications

This paper reports the microstrcure, optical and electrical characteristics of undoped and Cu doped CdTe nanostructured thin films prepared on glass substrates by electron beam evaporation technique. The Crystallographic study of X-ray diffraction shows that CdTe and Cu doped CdTe films crystallize in the form of a cubic zinc blende structure. Microstructure analysis reveals that as the Cu doping level increases, the average crystallite size increases, while the microstrian decreases due to the improvement of the crystallinty, thereby reducing defects. XRD and AFM investigations confirmed the nanostructure characteristic of undoped and Cu doped films. It was found that the optical band gap energy increases from 1.485 eV to 1.683 eV as the Cu concentration increases from 0 wt. % to 10 wt. %, which may be related to the Burstein-Moss effect. The refractive index is calculated from the Swanepoel envelope method and found to decrease with the increase of the Cu doping due to the decrease in the prolizability. Similarly, the extinction coefficient decreases with the increase of Cu in CdTe matrix. The dc electrical conductivity is found to increase with increasing Cu doping, which is attributed to the increase in the grain size, thereby reducing the scattering of the grain boundary. Furthermore, two conduction mechanisms of the carrier transport in nanostrcutured undoped and Cu doped CdTe films were observed. The low temperature dependence of the conductivity of undoped and Cu doped CdTe nanostructured films is explained based on Mott’s variable range hopping conduction mechanism model (VRH). Interestingly, the calculated values of hopping distance R , the hopping energy W and the the density of states at the Fermi level N(E F ) are consistent with the Mott's VRH. Finally, Hall effect measurements show that all the films have p-type conduction behavior. Besides, the results show that as Cu doping level increases, the carrier concentration and the Hall mobility increase due to the decrease in grain boundary scattering with the increase in grain size. Accordingly, it can be concluded that by increasing the Cu doping level in the CdTe film, the conductivity is increased, thereby improving the performance of the CdTe absorber layer in the solar cell structure.


Introduction
In recent years, the binary semiconductor compounds A II B VI are provided a great deal of interest due to their potential application in solar cells and photoconductive devices [1].Cadmium telluride (CdTe) is a member of the A II B VI semiconductor compound family, which is considered to be one of the most outstanding absorber layers for solar cells because of its unique characteristics such as, direct band gap (1.5 eV ) at room temperature located in the central spectrum of the solar spectrum, highly absorption coefficient in the visible region of the solar spectrum (α≈10 5 cm -1 ) [2], good transport performance, high average atomic number (50), high resistivity, and exhibits n-type and p-type conductivity, which allows the production of solar cells with heterojunction and homojunction structure [3].CdTe has a sharp absorption edge, which allows 90% of incident photons to be absorbed in a 2 m opaque CdTe layer, whereas the absorption of similar radiation intensity needs thick layer of 20 m in the case of Si [4] All these characteristics make CdTe an ideal candidate materials for various applications, e.g.photovoltaic conversion, solar cell structure, field effect transistors, and X-ray and gamma-ray detectors [role 5,6].Several deposition techniques have been used to fabricate CdTe thin films, such as rf sputtering.[7], chemical bath deposition, [8], pulsed laser deposition [9], close-range sublimation [10], electro deposition [11], thermal evaporation [12] and Electron beam evaporation [13].Nevertheless, the difficult duty is to obtain a stoichiometic CdTe films to better design the absorber layer in the solar cell structure, since defects and/or impurities generated during the deposition process will affect the optoelectronic properties of prepared thin films, thereby changing the efficiency of the solar cells [14,[15][16][17].Also, it is reported that the electrical and optical properties of CdTe thin films are tuned by doping with appropriate dopant (Bi, Co, Ag, Cu, In and Mo) into CdTe semiconductor lattice, which enhances the efficiency of solar cells [18][19][20][21][22][23].In this context, Cu was reported to be an amphoteric dopant type, that can involve in the CdTe lattice as an interstitial ion (Cui + ), producing a shallow donor level, or substituted by Cd atoms to form a deeper acceptor level (CuCd -), or complexes contains Cu + and Cd vacancies (Cui + +VCd -2 ) and (Cu + -CuCd), forming a shallow acceptors [24].One of the main advantages of Cu doping into CdTe is the increase in carrier concentration, which can improve the ohmic contact.
In this paper, electron beam evaporation technique is used to deposite high quality undoped and Cu doped CdTe nanocrystalline thin films with different Cu concentrations on a glass substrates.The detailed analysis of the microstructur, morphology, optical and electrical properties of CdTe and Cu doped CdTe nanocrystalline films is studied, which is very important for the design of the absorber layer in the solar cell structure.

Materials and Methods
CdTe and Cu doped CdTe ingots with different Cu concentrations (0, 2, 4, 6, 8, and 10 wt.%) have been synthesized using mechanical mailing method.Analytical grades with stoichiometric CdTe and Cu2Te powders (with a chemical purity of (99.999%,Aldrich) were mixed together and milled in a mechanical ball mill machine at 200 rpm for 6 hours.The mixture is made into disk-shaped to avoid splashing the mixture powders during the evaporation process.The prepared pure and Cu doped CdTe ingots were used as a source for thin film deposition.The CdTe and Cu doped CdTe thin films with various Cu concentrations were deposited by electron beam evaporation technique (Edward Auto 306) at room temperature.Amorphous glass with a size of (25 mm × 25 mm) is used as the substrate.To clean the substrate carefully, the substrate was immersed in acetone for 15 minutes, then washed with purified water for 15 minutes, and subsequently with alcohol for 10 minutes.At last, the substrate was ultrasonically cleaned in deionised water for 15 minutes, and then was dried in air at a temperature of 100°C.The substrates and ingots have been placed in the chamber, which was then evacuated at pressure of 5 ×10 -6 Pa.The pellet ingot was preheated for 5 minutes before evaporation to remove any pollutants and degas the pellets.The distance from the substrate to the source is kept at about 20 cm.
The thickness of the film was adjusted at 300 nm at a deposition rate of 2 nm/sec, which was controlled by a thickness monitor device (model; FTM6).More details of the deposition methodology are explained elsewhere [25].
X-ray diffractometer (XRD, Cu-Kα = 1.54056Å,Philips diffraction 1710) was used for crystallographic investigation.The ratio of the elemental composition of the film was checked by using energy dispersive X-ray spectroscopy (EDXS).The surface morphology of the film was performed using an atomic force microscope (AFM, model MLCT-MT-A).The transmission and reflection spectra were measured with a dual beam spectrophotometer (UV-VIS-NIR, Shimadzu model V-670).The electrical performance was studied by using the Hall measurement technique in van der Pauw configuration with a 0.6 T magnetic field (model Ecopia-HMS-3000).

Elemental composition analysis
The elemental composition analysis of Cu-doped films with different Cu doping levels has been performed by EDXS measurement.).In addition, the XRD pattern reveals a preferred oriented grain growth toward (111) plane due to the challenge between energy of surface and strain energy [26].It is worth noting that, the XRD spectra did not show any foreign peak related to copper phases such as copper oxide and or copper cluster, indicating a successful inclusion of Cu 2+ ions into the CdTe lattice without change of cubic structure of CdTe.It was found from Fig. 2(b) that the peak position of (111) plane is shifted towards higher diffraction angles due to the strain in the film by the incorporation of Cu ions in the CdTe structure of semiconductor matrix with remarkable shrinks in the cell volume.Fig. 3 shows the reduction of the lattice parameter (a) with increasing of  .The observed reduction of the lattice parameter is attributed the incorporation of Cu 2+ ions of smaller ionic radius (0.72 Å) by Cd 2+ ions of larger ionic radius (0.97 Å).The calculated value of the lattice parameter of Cu doped film with different Cu doping levels is tabulated in Table 1.As can be seen the obtained lattice parameter (a) value of undoped CdTe is consistent with the standard value of CdTe cubic structure (6.41 A˚, JCPDS No. 01-075-2086).Furthermore, the intensity of the preferred oriented peak (111) is improved and its full width at half maximum of diffraction peak (FWHM) decreases with the increase of the Cu doping, which are attributed to the improvement of the crystal growth of the film due to the occupation of Cd site by Cu ions.This behaviour was given in literature for CdTe doped Mo [24] and CdTe doped Cu thin films [27].In addition, the nanostructure Fig. 3 The lattice parameter versus Cu doping concentration in CdTe matrix.
nature of the films is examined by using Debye-Scherrer's equations from the calculations of the mean crystallite sizes, and lattice microstrain, where, , , k ' and  are the Bragg angle of most preferred oriented peak, radian FWHM, shape factor (≈ 0.9) and the wavelength of the CuKα radiation, respectively.The value of the average crystallite size of the Cu doped CdTe film is found to vary from 16.07 nm for CdTe to 28.87 nm for CdTe:10 wt.%, confirming the nanostructure characteristic of the film.Fig. 4 displays the dependence of the microstructure parameters with Cu doping, see also Table 1.It can be seen that as the Cu doping increases, the average crystallite size increases while the microstrian decreases which refers to improvement of the crystallinty and the reduction of the defects due to the completely integration of Cu 2+ ions into the CdTe lattice.Such increase in the average crystallite size and decrease in microstain is reported for films that have smaller ionic radius dopants [28].

Surface morphology analysis
The microscopic description of the surface morphology of the undoped and Cu doped CdTe films has been performed using AFM investigation.Fig. 5 shows the three dimensional (3D) AFM images of CdTe, CdTe:4 and CdTe:10 films.The images show that the surface of the films has a highly densely spherical elongated packing grain with uniform arrangement.The observed regular distribution of the elongated spherical grains with similar directions confirms the observed preferred oriented grain growth toward (111) plane.The micrographs of the CdTe and Cu doped films were analyzed in details in order to identify the microscopic surface morphology parameters, such as the grain size, surface roughness and root mean square (RMS) surface roughness, see Table 1.The data shows that the grain size increases with the increase of the Cu concentration into CdTe lattice.However, it was found that the surface roughness and RMS surface roughness are decreased with the increase of the Cu doping.The reduction in the surface roughness with the increase of Cu doping into CdTe films [27] and ZnO films [29] is reported in literature.It has to be mentioning that the grain size obtained from SEM is higher than the crystallite size calculated by XRD.This inconsistency can be ascribed to the fact that the crystallite size is a record of the size coherent scattering domain, whilst the grain size is a set of this coherently scattering domain separated by grain boundary.Besides, crystallite size reveals two distinct ranges when dislocations are located in the composition, while the difference between them is not visible in the SEM micrographs [30].

Optical properties
The optical band gap energy of the semiconductor films is calculated from absorption spectra of the films via their transmission and reflection measurements.
Fig. 6 shows the spectral dependence of transmittance measurement T of nanocrystalline Cu doped CdTe film with various Cu doping levels in wavelength range (200 -2500 nm).The average transmittance in the near infrared region was found to vary from 78% to 82%.Thus, the highly transparent Cu doped CdTe films can be used in n-type window layer for photovoltaic solar cells applications as reported for highly transmission CdS film [31,32] and CdO [33] thin film.It is which are evaluated from the transmission and reflection measurements in the strong absorption region by using the following relation [34]: where d is film thickness, and R1 , R2 , and R3 the power of Fresnel reflection coefficients for air-film interface, for film-substrate interface and for substrate-air interface, respectively.The films show a high absorption coefficient value (≈10 6 cm - 1 ), which can be used as an absorber layer in solar cells.The results reveal further that the absorption coefficient decreases with increasing copper concentration.
Moreover, the absorption value suddenly decreases at the absorption edge, which is observed to shift to higher energy as the Cu doping level increases.The optical band -gap energy of nanostructured Cu doped film with the variety of Cu contents is estimated using Tauc's model [35,36]: where αo is constant and the value of the exponent n represents the direct (n=1/2) [37].The Tauc's relation is represented in Fig. 8  are occupied.This effect is also reported for the increase of energy band gap in Mo doped CdTe films [23] and Mo doped CdO films [38].
It is worth mentioning that, the optical constants, such as the refractive index and extinction coefficient, of semiconducting materials are essential optoelectronic parameters for designing the solar cell and photovoltaic devices.The refractive index of semiconductor thin films was calculated with different methods [39][40][41][42][43][44].In this context, the refractive index of nanostructured Cu doped CdTe thin film is calculated by using Swanepoel method [45], then latter improved by Manifacier et al. [46].
This method is based on the suppression of the observed interference patterns in the transmittance spectra by constructing envelope curves around maxima and minima transmittance.The detailed process of this method has been described elsewhere [47,48].The envelope of Cu doped CdTe:10 film is displayed in Fig. 9.The    is attributed to the decrease in the scattering of the grain boundary, which is due to the increase in grain size originated with the rise in Cu doping level.This can be confirmed from AFM measurements, where the lack of any voids or carks is remarked due to the improved crystallinity and the absence of defects in the films.Thus, as a result of the enhanced mobility, the resistivity of undoped and Cu doped CdTe thin films at room temperature decreased with the increase of Cu doping level from 66.38 x 10 -4 (.cm) for CdTe:0 wt.% to 2.93 x 10 -4 (.cm) for CdTe:10 wt.%.This is in accordance with the reported huge reductions in the electrical resistivity of CdTe single crystals [55][56][57][58] and thin films [13,60] upon incorporated tiny amount of Cu.Thus, upon the increase of Cu 2+ ions concentration instead of the Cd 2+ ions concentration, more electrons (free carrier concentration) are promoted to the conduction band, resulting in a further decrease in resistivity.This explains the observed decrease in resistivity with increasing Cu content.In order to investigate the mechanism of conductivity, the temperature dependent dc conductivity measurements (in the form of Ln versus 1000/T representations) of nanocrystalline undoped and Cu doped CdTe films with different Cu concentrations were analysed and presented in Fig. 13.Obviously, a non-linear temperature-dependent conductivity behavior is observed.Further, it was observed that the electrical dc conductivity of nanocrystalline undoped and Cu doped CdTe films increases exponentially with increasing temperature.As a consequence, the results exhibit semiconducting like behavior for all Cu doped CdTe films over the entire measurement temperature region from 300 K to 450 K. Additionally, the dc conductivity increases with the increase of the Cu doping level in the CdTe film which is attributed to the increase in the grain size, thereby reducing grain boundary scattering.Accordingly, it can be concluded that by increasing the Cu doping level in the CdTe film, as the conductivity increases, the performance of the CdTe layer in the solar cell can be improved.Similar behaviour has been reported for Cu doped CdSe films [61], Cu doped CdTe films [27] and Ag-CdSe films [62].Table 2 illustrates the decreases in the resistivity and the increase of the electrical conductivity at room temperature as the Cu doping increases.
the Furthermore, according to the Arrhenius relationship  =o exp(-Ea/kT), where Ea is the thermal activation energy and k is the Boltzmann constant, the results reveal two different regions with two slops at low and high distinct temperature ranges, which indicates two conduction mechanisms for the carrier transport in nanostrcutured undoped and Cu doped CdTe films.1.The low temperature activation energy (Ea2) may be due to the low temperature conductivity by hopping of carriers between the localized states above the edge of the valence band to the extended states in the conduction band, while in the high temperature region, the normal type of band conduction in extended states can be considered to be the main mode of the conduction mechanism.
On the other hand, it is found that the low temperature activation energy Ea2 decreases as the Cu content increases from 58 meV (CdTe:0) to 25 meV (CdTe:10).Also, the high temperature activation energy Ea1 is found to decrease with the increase of the Cu doping level from 578 meV (CdTe:0) to 320 meV (CdTe:10).The decrease in activation energy is attributed to the observed increase in crystallinity and decrease in defects.Different groups similarly reported the decrease in activation energy with increasing dopant concentration [63,64].Besides, at all temperatures up to room temperature, the dependence of ln σ and T −1/4 in the undoped and Cu doped CdTe nanocrystalline film is checked, which is an excellent tool for examining the hopping conduction mechanism, particularly the Mott's variable range hopping conduction model (VRH).It is worth mentioning that VRH in semiconductors is expected to dominate at appropriate low temperatures [61].Therefore, the low temperature dependence of the conductivity of undoped and Cu doped CdTe nanostructured films can be analyzed based on Mott's variable range hopping conduction model (VRH) [65]: where Mott temperature To is given by [66] where N(EF) is the density of states at the Fermi level, α is the exponential decay coefficient of the localized states wave function (≈ 0.124 Å -1 [67]) and C0(≈24/ ) is the constant of proportionality.The observed linear low temperature dependence of the conductivity in ln (σT 1/2 ) and (T −1/4 ) representation of undoped and Cu doped CdTe nanocrystalline films (see Fig. 14), indicating a dominant variable range hopping (VRH) conduction mechanism where the low temperature conduction mechanism is ascribed to hopping of free carriers between localized states.Furthermore, the hopping distance R, the hopping energy W and the the density of states at the Fermi level N(EF) are calculated at room temperature (300 K) for undoped and Cu doped CdTe nanostructured films from the linear correlation slope in Fig. 12 and are summarized in Table 2 [61].Apparently, as Cu doping increases from CdTe:0 to CdTe:10 wt.%, the density of state increases from 1.178  10 17 eV -1 cm -3 to 14.28  10 17 eV -1 cm -3 .Also, it was found that the values of αR (734.51(CdTe:0)-60.58(CdTe:10)) are greater than 1 and the values of W (0.053 (CdTe:0) -0.029 (CdTe:10)) are greater than the thermal energy (0.0259 eV) which are essential conditions for the above mentioned factors to be consistent with the Mott's VRH [68,69].Finally, αR 1 indicates that the carries are more localized in the trap states.

Summary
To summarize, pure and Cu doped CdTe nanostructure thin films were deposited by electron beam evaporation technique on glass substrates.The XRD investigation shows that all the films are cubic zinc blende structure.It was found that the average crystallite size increases from 16 nm to 18 nm, while the microstrian decreases from       The spectral variation of the transmittance and refractance of pue and Cu doped CdTe lms with various Cu contents.
The spectral variation of the transmittance and refractance of pue and Cu doped CdTe lms with various Cu contents.The envelope of the transmittance curve of the CdTe:10 lm.
The spectral dependence of the refractive of pure and Cu doped CdTe lm with different Cu concentrations.
The spectral variation of k for Cu doped lms with various Cu doping level.
The Hall mobility μH , carrier concentrations nH and resistivity ρ at room temperature of nanostructured undoped and Cu doped CdTe lm with various Cu doping levels.

Fig. 1
represents the EDXS spectra CdTe:Cu 2 wt.% and CdTe:Cu 10 wt.%.The spectra of the Cu doped films confirmed the appearance of three peaks corresponding to Cd, Te, and Cu.The spectra further show that the peak intensity of Cu increases with the increase of Cu doping, which indicates the stoichiometry of the film and Cu ions have been successfully incorporated into the CdTe matrix.

3. 2
Fig. 2(a) shows the XRD spectra of undoped and Cu doped CdTe (CdTe:Cu) thin films deposited by electron beam evaporation method at room temperature on glass substrate with different Cu concentrations of 2, 4,6,8,10 wt.% .The results reveal that all films have a polycrystalline like structure with three reflection lines belonging to (111), (220) and (311) diffraction planes of cubic zinc blende structure suggested the existence of the cubic phase of CdTe structure; see (JCPDS No. 01-075-2086).In addition, the XRD pattern reveals a preferred oriented grain growth

Fig. 2 (
Fig. 2 (a) XRD diagrams of undoped and Cu doped thin films at Cu concentrations, (b) The magnification of diffraction peak at (111) plane.

Fig 4
Fig 4 The variation of the microstructural parameters with different Cu concentrations of undoped and Cu doped CdTe films.

Fig. 6
Fig. 6 The spectral variation of the transmittance and refractance of pue and Cu doped CdTe films with various Cu contents.

Fig. 7
Fig. 7 The absorption coefficient versus photon energy of pure and Cu doped CdTe films at different Cu dopants.
by (αhν) 2 versus (hν).The linear part of the Tauc's plot is extrapolated to (αhν) 2 = 0.The optical band gap energy is determined from the intersection of the extrapolated line with (αhν) 2 = 0.The obtained values for nanocrystalline undoped and Cu doped CdTe film is tabulated in Table 2. Obviously, the optical band gap energy of undoped CdTe film ( opt g E =1.485 eV) is in accordance with the published value ( opt g E ≈ 1.49 eV ) [1].Moreover, it is observed that the optical band gap energy increases with the increase of Cu doping level in CdTe lattice, which may be related to the Burstein-Moss effect.This effect is corresponding to degenerate doping semiconductors, in which the Fermi level lies inside the conduction band.According to the Burstein-Moss effect, as the Cu doping increases into CdTe, the electron carrier density of states of the conduction band edge increases.Therefore, the energy band gap increases with increasing the Cu doping level when the electrons are excited from the maximum edge of the valance band to the conduction band above the Fermi level since all the states below the Fermi level

Fig. 9
Fig.9The envelope of the transmittance curve of the CdTe:10 film.

Fig. 10
Fig. 10 The spectral dependence of the refractive of pure and Cu doped CdTe film with different Cu concentrations.

Fig. 11
Fig. 11 The spectral variation of k for Cu doped films with various Cu doping level.

3. 5
Fig. 12 The Hall mobility H , carrier concentrations nH and resistivity  at room temperature of nanostructured undoped and Cu doped CdTe film with various Cu doping levels.

Fig. 13
Fig. 13 ln σdc versus 1000/T of undoped and Cu doped CdTe nanostructure film with various Cu doping levels.

1. 2 10 - 3 (
Lin/m 2 ) to 0. 70 10 -3 (Lin/m 2 ) as the Cu doping level increases from 0 wt.% to 10 wt.% .The average transmittance of Cu doped CdTe films in the near infrared region was found to vary from 78% to 82%, which can be used as an n-type window layer for photovoltaic solar cell applications.The optical band gap energy of Cu doped CdTe films increases from 1.485 eV to 1.683 eV as the Cu concentration increases from 0 wt.% to 10 wt.%.The optical analysis of the spectral behavior of the optical constants of Cu doped CdTe films shows that the refractive index and extinction coefficient decrease with the increase of the Cu doping in CdTe matrix.It is found that the resistivity decreases and the dc electrical conductivity increases at room temperature with increasing Cu doping.Additionally, it is observed that the conductivity temperature dependent of nanostrcutured undoped and Cu doped CdTe films shows two conduction mechanisms of the carrier transport.Mott's variable range hopping conduction mechanism model (VRH) is used to interpret the low temperature dependence of the conductivity, the behaviour of hopping distance R, the hopping energy W and the the density of states at the Fermi level N(EF) of undoped and Cu doped CdTe nanostructured films.Finally, p-type conduction behavior is observed from Hall effect measurements for all the films.Besides, the carrier concentration and Hall mobility increase with increasing of Cu doping level.It can be concluded that as the Cu doping level increases, the conductivity increases, thereby improving the performance of the CdTe absorber layer in the solar cell structure.

Figure 3 The
Figure 3

Figure 4 The
Figure 4

Table 1
Structural, microstructural of undoped and Cu doped CdTe thin films

Table 2
The optical band gap energy and electrical properties of pure and Cu doped CdTe films.