Structure, Electrical and Optical Properties of ITO Thin Films and their Influence on Performance of CdS/CdTe Thin-Film Solar Cells

In terms of mixing graded TiO2 and SnO2 powders by solid-state reaction method, ITO was prepared. Using electron beam gun technology, ITO films with different thicknesses were prepared. The influence of film thickness on structure, electrical and optical properties was studied. The XRD patterns were utilized to determine the structural parameters (lattice strain and crystallite size) of ITO with different thicknesses. It is observed that the average crystallite size increases as the film thickness increases, but the lattice strain decreases. SEM shows that as the film thickness increases, the grain size of ITO increases and improves. The electrical properties of ITO films with different thicknesses were measured by the standard four-point probe method. It can be seen that as the thickness of the ITO film increases from 75 nm to 325 nm, the resistivity decreases from 29x10^-4 Ohm/cm to 1.65x10^-4 Ohm/cm. This means that ITO films with lower electrical properties will be more suitable for high-efficiency CdTe solar cells. Three optical layer models (adhesive layer of the substrate/B-spline layer of ITO film/surface roughness layer) are used to calculate the film thickness with high-precision ellipsometry. In the higher T(lambda) and R(lambda) absorption regions, the absorption coefficient is determined to calculate the optical energy gap, which increases from 3.56 eV to 3.69 eV. Finally, the effects of ITO layers of various thicknesses on the performance of CdS/CdTe solar cells are also studied. When the thickness of the ITO window layer is 325 nm, Voc = 0.82 V, Jsc = 17 mA/cm2, and FF = 57.4%, the highest power conversion efficiency (PCE) is 8.6%.


1-Introduction
High optical transparency at the same time (90%) in the visible light region and with high electricity conductivity needs to produce electronic degeneracy introduction of nonstoichiometric method in wide gap (3 eV) oxide or suitable dopants. These setting can be achieved by a variety of indium, tin, cadmium, and zinc oxides and its combination.
Mixed oxides of TiO2 and SnO2 have attracted much consideration in the field of gas sense. Compared with pure binary oxides [1][2][3], their response to H2 and CO is enhanced.
In the field of photocatalysis, activity is obtained under ultraviolet and visible light [4][5][6][7][8][9][10][11]. In terms of gas sensing and photocatalysis, it has been initiated that adding a little amount of SnO2 to TiO2 can achieve the most useful outcome. The electronic structure of the material is significant to the efficient performance. Although the lattice parameters of SnO2 (a = 4.594A°, c = 2.959A°) are slightly larger than that of TiO2 (a = 4.737 A°, c = 3.360 A°). By reacting between SnO2 and TiO2 at high temperature, an alternative solid solution SnxTi1-xO2 with retained bottom red stone structure is obtained. [12,13] Above 1450 °C, a solid solution with a composition range of 0.0 <x <1.0 is thermodynamically stable [14]. Lower than 1450 °C, there is a nearly symmetric miscibility gap in which the solid solution decomposes spinodal into phases rich in Sn and Ti [15][16]. However, such decomposition is very slow at room temperature, and rapid quenching of the composition near the end members (x <0.2, x> 0.8) gives long-term stable single-phase samples; [17,18] or, stable can make the composition close to room temperature under kinetic control [19]. Although the individual electronic structure of TiO2 and SnO2 has been studied for many years [20][21][22], but the electronic structure of SnxTi1-xO2 solid solution has not studied until recent [23][24]. The direct band gaps of rutile TiO2 and SnO2 are 3.062 and 3.596 eV, respectively [25][26][27]. The target of the present work divided into two folds the first is to study the effects of film thickness on the micro structural parameters (crystallize size and lattice strain) and electrical resistivity of ITO window layer. The second is calculation of film thickness with high precision in terms of spectroscopic ellipsometry. The third is studying the film thickness effect on optical properties via measuring T(λ) and R(λ) of ITO window layer. The fourth is checking the impact of ITO layers with a variety of thicknesses on the performance of CdS/CdTe solar cells. The fifth is the interpretation on the change in optical parameters and the performance of CdS/CdTe solar cells in terms of microstructural parameters and electrical resistivity.

Experimental procedures
Using ball milling technology, high purity (99.9%) analytical grade powders purchased from Aldrich in stoichiometric quantities are mixed in a ball mill for about one hour. The mixed powder is then pressed into disc-shaped particles. The powder was compressed into pellets by uniaxial compression (20 MPa), and then pressed at 210 MPa. Then the pellets were sintered at 1200 °C at a heating rate of 20 °C /min in a 2-cap ambient atmosphere, and then cooled to space temperature at a rate of 20 °C /min. Such ITO particles are used as the initial material (after gridding), and the electron beam gun (Denton Vacuum DV 502 A) was used to deposit the powdered sample presence inside the quartz glass crucible at a pressure of about 10 -6 Pa onto clean the glass substrate.
FTM6 thickness monitor was used to monitor both the film thickness and rate of deposition. During the deposition process, the substrates were kept at temperature 100 °C and the deposition rate was adjusted at 2 nm/sec. X-ray diffraction (XRD) of powder

Structural analysis
Rietveld refining is a method used to characterize crystalline materials [29]. The XRD pattern of the ITO powder sample result is characterized by reflection (peak intensity) at convinced positions. The position, height and width of this reflection can be used to determine many aspects of the structure of the material. The Rietveld treatment utilizes the least squares method to treat the theoretical line contour until it matches the measured contour. Fig.1 illustrates the sample grinding of Rietveld powder to ITO. Fig. 2 shows the diffraction peaks of ITO films with different thicknesses in the XRD pattern belonging to the ITO (JCPDS data file: 39-1058-cubic), which is better oriented along the (222) plane.
The main characteristics of these trends are the same, but only small differences are noticed during the peak duration. For the (222), (400), and (411) orientation planes, the diffraction angle 2θ is 30.27, 35.17, and 50.98, respectively, and suitable sharp diffraction peaks are observed. Also, Fig. 3 shows that as the thickness of the ITO film increases, the diffraction intensity of the (222) plane increases, and the increase in thickness significantly improves the crystallization efficiency of the deposited film.
The peak broadening is attributed to instrumental factors and structural factors t (that depend on lattice strain and crystal size). The crystal size (D) and lattice strain (e) are calculated by Scherrer and Wilson equations [30,31] as follows: Where β is the width of the peak, which is equal to the difference between the width of the film and the width of standard silicon.   Figure 2 increases, the crystal quality gradually improves. It should also be noted that the film is fully crystalline for higher thicknesses, which can lead to enhanced charge carrier transport and collection and further enhance device performance.
Therefore, it can be concluded that the higher thickness (325 nm) of ITO film shows better crystal feature, which is also in good harmony with the earlier results of XRD. The SEM average grain size value is significantly larger than the grain size calculated by the XRD study, because the grain is composed of many grains [32].

Electric properties
The electrical properties of ITO layer films with different thicknesses were measured by a standard four-point probe method. The necessary formula for sheet resistance measurement is: Rs = 4.53 . V/I [Ω/ sq], where: V is the voltage in volts, I is the current in amperes, and the value 4.53 is the correction constant. If the film thickness is d, the relationship between resistivity ρ (in ohm cm) and Rs is given by Rs = ρ/d [33].
The relationship between the resistivity of the ITO film and the film thickness is shown in Fig. 7. It can be seen from Fig. 7 that as the ITO film thickness increases from 75 nm to 325 nm, the resistivity decreases from 29 x 10 -4 Ω/cm to1.65 x 10 -4 Ω/cm, respectively. The decrease in resistivity leads to a relatively high charge carrier density, which in turn causes mobility, which may be attributed to the relatively high crystal quality and larger grain size and excess cations (In or Sn), all of which both lead to an increase in charge carriers and a decrease in grain boundary scattering. This means that ITO films with lower electrical properties will be more appropriate for high-efficiency CdTe solar cells.

Optical properties
For ITO films of different thicknesses, a dual beam spectrophotometer can be used to obtain the measured values of optical transmittance (T) and reflectance (R) relative to wavelength. Fig. 8 shows T(λ) and R(λ) with respect to the wavelength λ. The transmittance is found to be decreased with increasing the film thickness particulary in NIR region. In the near-infrared region, due to the large number of free electrons in the film, the interaction between free electrons and incident light occurs. This interaction may cause the polarization of the light in the film, which causes a remarkable decreasing in the transmission spectra, thereby affecting the dielectric constant. Transmittance is highly dependent on thickness of prepared thin films. But the reflectance spectra of the same set of films. It shows that for wavelengths above 1900 nm, the reflectivity will increase slightly. However, this increase in reflectance does not coincide with the decrease in transmittance in the same area. Therefore, as the thickness of the nearinfrared region increases, the decrease in transmittance is attributed to free carrier absorption, which is common in all transparent conductors with high carrier concentration [34].
The film thickness is calculated by the spectroscopic ellipsometry parameters (ψ and Δ), which are measured in the wavelength range of 380-880 nm using a rotating compensator instrument (J.A. Woollam, M-2000). All the details of this method have been clearly seen in references [35][36][37]. The data was acquired at a 70° incident angle.
According to the developed WVASE32 program, as shown in Figure 9, three optical layer models ((Cauchy layer of substrate /B-spline layer of ITO film/surface roughness layer.)) are used to determine the film thickness with high accuracy.
In a higher absorption region for both T(λ) and R(λ), the absorption coefficient, α can be derived from the following expression [38,39]: where d is the thickness of the film. Fig. 10 shows the reliance of α (hν) on photon energy, hν as a function of film thickness. Pure semiconducting compounds are known to have a sharp absorption edge [38,39]. The edge of absorption was sharper and moved to higher wavelengths, as the film thickness rose from 75 nm to 325 nm. It is known that the α value is described in the higher neighborhood of the fundamental absorption edge (higher 10 4 cm -1 ), for allowed direct transition from valance band to conduction band.
But the value of α at energy rang extended from 1 to 2 eV represent a transparent visible region. The energy gap values can be calculated using Tauc relation [40] as follows Where K is a parameter independent of hν for the individual transitions [35], increase was may attributed to reasons the first is a reduction in the resistivity of the films, implying an enhancement of the carrier density as in Fig. 7, and this change is well known as the Burstein-Moss shift [44] and the second is due to the increasing in crystallites size i.e increasing of the crystallinity of the film with increasing the thickness.

Impact of thickness of ITO window layer in the performance CdS/CdTe solar cells
The schematic diagram of the configuration of the CdS/CdTe solar cell is shown in Fig. 11. Thus, the device configuration is a simple n-p heterojunction with an ohmic contact at the p-CdTe/metal interface. In order to show the influence of ITO layer thickness on the performance of CdS/CdTe solar cells, CdS/CdTe cells based on varies ITO window layer thicknesses were equipped. Figure 12  Beyond solar cells specifically, such material characterization for light-matter interactions bears a high-relevance for photo conversion applications such as solar cells.
In fact, recent work on window-integrated PC concentrators [45,46] has shown to deliver both renewable power generation whilst allowing for spectral-tuning of the optical structure to allow for natural room lighting and reduced air conditioning load on a dwelling by designing photonic structures (e.g., high contrast gratings) to reflect the infrared spectrum (reducing heat load) yet transmitting the visible portion. Indeed, in this study we found that interplay between material quality to solar cell performance can be extended, in principle, to high absorption atomically flat van-der Waals materials such as those belonging to the class of transition metal dichalcogenides (TMD) for light emission [47] and light detection [48,49]. In fact, recent work points to some interesting scaling laws in photodetectors combining such emerging highly-absorptive materials with scaling-length-theory known from transistor short-channel devices, enabling a new class of high-gain-bandwidth product photodetectors [50].
Furthermore, controlling the ITO material properties during the deposition process is critical given plurality of control parameters of ITO such as activating the Sb dopants via annealing or adding/subtracting oxygen [51,52]; both impact the complex optical index, and hence the transport properties such as the resistivity and mobility. Our material property findings for ITO in this study are indeed far-ranging beyond photoabsorption but can further be used to electro-optic devices as well [53][54][55][56]. For instance, the optical index can be tuned electrostatically via chancing the free carrier concentration leading to electro-optic modulators, both for electro-absorption [57][58][59], but also used in interferometric schemes [60][61][62] to demonstrate high-performance modulators.This tunability of ITO thin films can be uniquely utilized also for 2x2 switches in integrated photonics [63], as electro-optic nonlinear activation function in photonic neural networks [64], or as a phase-shifter for optical phase array applications for LiDAR systems [65].
Interestingly, the unique properties of ITO have recently inspired applications for photonic and nano-optic analog computing schemes such as for solving partial differential equations [66,67].

Conclusions
In