The deposition was completed on commercially available ITO coating glass substrate size is 25 × 75 × 2 mm dimensions. This ITO coating glass substrate was cleaned by acetone followed by rinsing in alcohol and lastly stored in double distilled water before use.

Analytical grade chemicals cadmium sulphate, triethanolamine and sodium telenosulphite (Na₂TeSO₃) were used. The solutions were prepared in double distilled water.

The deposition of cadmium telluride thin film on ITO coating glass substrate is used in a reactive solution. Cadmium sulphate solution, 10 ml (0.25 M), complexed with 10 ml of (1 M) triethanolamine (TEA), 10% 10 ml ammonium solution were mixed with 10 ml of (0.25 M) sodium telenosulphite solutions in 100 ml capacity beaker at high temperature 347 K. The total volume of the reaction mixture was made to 80 ml by adding double distilled water [23]. The reaction mixture was reserved kept in an oil bath. The substrate was mounted on a particularly planned substrate holder and rotated in reaction mixture with a rate of 65 ± 5 rpm, at high temperature, maintained the pH constant of the reaction mixture. After 7 h, the glass substrate covered with films were removed, washed with distilled water, dried naturally and kept in dark desiccator. Figure 1 is shown by the experimental deposition of the CdTe solar thin film.

Solar radiation is the electromagnetic radiation emitted by the sun. Solar radiation interacts with the earth’s atmosphere to create three types of irradiances at ground level. Solar cell is the semiconductor device that converts the light into electrical energy. The electrons of the semiconductor material are joined together by the covalent bond. The electromagnetic radiations are made of small energy particles called photons. When the photons are incident on the semiconductor material, then the electrons become energised and starts emitting

solar energy is shown in Figure 2. A simple photovoltaic solar cell is formed a p-n junction diode. The schematic of the p-n junction device is shown in Figure 3. The penetration depends on the deposition thickness of the thin films. Electron hole pairs are mainly created in the depletion region and due to the built in potential and electric field because, electrons move to the n-type region and the holes move to the p-type region. When the external load is applied, the excess electrons travel through the load to recombine with the excess holes. The electrons and holes are also generated with the p-type and n-type regions. The thinner thin films absorb the higher light in the n-type region and thicker thin films absorb the lower light in the p-type region. Electron hole pairs are generated in these regions can also contribute to the current. Generally, these electron-hole pairs that are generated within the minority current carrier diffusion length of thin films for electrons in the p-type region and holes in the n-type region.

A calomel electrode is used as reference electrode and sulphide-polysulphide as electrolyte is shown in Figure 4. Photovoltaic cell is consisting in three electrode configurations are used in experiment. Cadmium telluride as photoanode,

CoS-treated graphite rod as a counter electrode. This electrode is acts as a photocathode. A small amount of tellurium was added in electrolyte to stabilize the dissolution of photoelectrode [24].

The Mott-Schottky plot is used to determine the flat band potential One-kilohertz frequency is used to determine the flat band potential. The illuminated area of electrode was 3.0 cm². The illumination intensity was measured with a (Meco) Lux meter. The type of conductivity exhibited by the film is determined by nothing the polarity of the emf developed in PV cell under illumination. The current-voltage (I-V) characteristic in dark has been plotted. The junction ideality factor has been determined by plotting the graph of log I versus V. The fill factor and power conversion efficiency of the cell is calculated from photovoltaic power output characteristics. The fill factor and power conversion efficiency of the cell is calculated from photovoltaic power output characteristics. The power output characteristic has been obtained for a PV cell at a constant illumination of 30 mW/cm².

Photovoltaic solar thin film cell shows dark voltage and dark current even in the dark. The polarity of this dark voltage was negative towards semiconductor electrode. The sign of the photo voltage gives the conductivity type of CdTe. This suggests that CdTe is an n-type conductor which has also been proved from TEP measurement studies [22] [25]. A solar cell of CdTe with configuration n-CdTe/NaOH (0.15 M) + S (0.15 M) + Na₂S (0.15 M)/C_(graphite) was formed.

The photovoltaic effect is the direct conversion of light into electricity in solar cells. When solar cells are exposed to sunlight, electron excite from the valence band to the conduction band creating charged particles called electrons. Solar cell of cadmium telluride thin films current-voltage (I-V) characteristics in dark have been studied in ITO coating glass materials at CT-RT, CT-318 K, and 328 K and shown in Figure 5. The characteristics are non-symmetrical indicating the formation of rectifying type junction [26]. Using famous ideal Schottky diode equation junction ideality factor is calculated from the equation [27] [28].

I = I 0 e e v n d k T (1)

where, I, is the forward current in dark, I₀, reverse saturation current, v, applied forward bias voltage and n_d is the junction ideality factor. The value of junction ideality factor (n_d) can be determined of the linear regions of the logI versus volt (V) at temperature CT-RT K, CT-318 K, and CT-328 K is shown in Figure 6. The ideality factor was found to be 2.63, 2.13, and 1.89. The higher value of n ′ d suggests the dominance of series resistance as well as structural imperfection. It

also suggests that average transfer across the semiconductor electrolyte interface with significant contribution from surface states and deep traps [29].

The solar cells of capacitance are measurements as a function of applied voltage provided useful information such as type of conductivity, depletion layer width and flat band potential (V_fb). The flat band potential of a semiconductor gives information of the relative position of the fermi levels in photo electrode as well as the influence of electrolyte and charge transfer process across the junction [5]. This is also useful to measure the maximum open circuit voltage (V_oc) that can be obtained from a cell. Measured capacitance is the sum of the capacitance due to depletion layers and Helmholtz layer in electrolyte which is neglected by assuming high ionic concentration [24]. Under such circumstances, V_fb can be obtained using Mott-Schottky relation by standardizing with saturated calomel electrode (SCE).

c − 2 = 2 q ε s ε 0 N d × V − V f b − k T q (2)

where the terms involved have meaning, C⁻² is space charge capacitance per unit area, q the electronic charge, ε_s is the dielectric constant of the semiconductor electrode, ε₀ is the permittivity of the free space, N_d the donor density, k the Boltzmann constant, T the absolute temperature, V the applied potential and V_fb is the flat band potential. The 1/C⁻² versus voltage (mV) plots (Mott-Schottky plot) were constructed for the samples ITO coating glass material CT-RT K, CT-318 K, and CT-328 K are shown in Figure 7. Intercepts of plots on voltage axis determine the flat band potential value of the junction. The flat band potential value found to be CT-RT −0.401, CT-318 K −0.450, and CT-328 K −0.501 (SCE) for CdTe-polysulphide redox electrolyte, which is a measure of electrode potential at which band bending is zero. The non-linear nature of the graph is an indication of graded junction formation between CdTe and polysulphide electrolyte may be possible reasons for deviation from linearity in C-Vplot.

The barrier height, Φb is the energy difference between the edge of the conduction band band and the redox Fermi level of the electrolyte. This potential prevents most of the photo generated charge carriers (hole and electron) from passing from one to the other. However, these carriers under a biasing potential, can get enough to cross the barrier. The photovoltaic solar cell cadmium telluride thin films barrier height was determined by measuring the reverse saturation current (I_o) through the junction at different temperature from 370 to 315 K. The reverse saturation current flowing through junction is related to temperature as [30].

I 0 = A T 2 exp [ φ β k T ] (3)

where, A is Richardson constant, k the Boltzmann constant, Φ_β is the barrier height in eV. To determine the barrier height of the photo electrode, a graph of log(I₀/T²) with 1000/T was plotted. The plot of log(I₀/T²) with 1000/T for representative sample CT-RT K, CT-318 K, and CT-328 K is shown in Figure 8. From the slope of the linear region of plots, the barrier height was determined. The barrier height value is found to be 0.523, 0.487, and 0.436 eV.

Solar cell is illuminated with the light of constant intensity, the current voltage characteristics shift in the four quadrants; this behavior is in accordance with the theory of solar cells acting as electricity generator Photovoltaic solar cell may operate over a wide range of voltages and currents. By applying the resistive load on an irradiated cell continuously from a short circuit (from zero) to a very high value of open circuit it is possible to determine the maximum power point (P_m = V_m × I_m), that is the load for which the cell can deliver maximum electrical power. The Energy conversion efficiency (η “eta”) of a cell is the percentage of power converted of the observed light to electrical energy, and collected and are shown by the following equation.

η = P m E × A c (4)

where E is power of input light (mW/cm²) and A_c is the surface area of the cell in cm². Another measuring term in the overall behavior of a cell is the fill factor (ff), which is the ratio of the maximum power (V_m × I_m) divided by the short circuit current (I_sc) and open circuit voltage (V_oc) in light current voltage (I-V)

characteristics of the cells can be measured by the following equation.

f f = P m V o c × I s c = η × E × A c V o c × I s c (5)

The open circuit voltage and short circuit current are found to be 120 to 80 mV and 23.6 to 18 μA, respectively. The power efficiency conversion factor can be studied by the following equation.

η max = [ V r e d o x − V f b ] × e E g (6)

where V_fb is the flat band potential, V_redox the electrolyte redox potential and E_g is the energy band gap. It is important to note here that V_oc and η depends on V_fb and E_g. The photovoltaic power output characteristics for a cell under illumination of 30 mW/cm² shows Figure 9 CT-RT K, CT-318 K and CT-328 K. The calculation shows the fill factor is 32.68%, 26.18%, and 19.73%. The power conversion efficiency is found to be 0.63%, 0.37%, and 0.23%. The low efficiency may be due high series resistance and interface states which are responsible for recombination mechanism [7] [21] [24]. The value of series resistance and shunt resistance were found to be 800 (Ω), 1000, and 1200 (Ω), respectively.

The solar cell photo response is studied by the logarithmic variation of open circuit voltage with incident light intensity was observed from photo response measurements whereas short circuit current follows almost a straight-line path. Figure 10 shows variation of short circuit current and open circuit voltage of sample CT-RT K, CT-318 K and CT-328 K which were measured as function of light intensity whereas, variation of open circuit voltage as a function of light intensity was shown in Figure 11. The photo electrode-electrolyte interface

modeled as a Schottky barrier solar cell it is possible to represent the current-voltage relationship [20] [21].

I = I p h − I d = I p h − I 0 exp ( q v n d k T ) − 1 (7)

where, I_ph the photocurrent densities, I is the net current density, I₀ the reverse saturation current density, I_d the dark current density, V the applied bias voltage and n_d is the junction ideality factor. In bias voltage condition V > 3kT/q and at equilibrium open circuit conditions

I_ph = I_d and V = V_oc thus,

V o c = n L k T q × I n I s c I 0 (8)

where, V_oc is the open circuit voltage and I_sc is the short circuit current. As I s c ≫ I 0 , a plot of logI_sc against V_oc should give a straight line and from the slope of the line the lighted ideality factor can be determined. Figure 12 shows the plot of logI_sc with V_oc for CdTe photoelectrode sample CT-RT K, CT-318 K and CT-328 K. The lighted ideality factor was calculated and found to be 3.26, 1.87, and 1.17 respectively.

Photovoltaic solar cell temperature increases absorption of current is decreases with increasing the wavelength. Spectral response of CdTe solar cell is studied by the current verse’s wavelength the shorter wavelength is high current absorption and higher the current value and higher wavelength are low current absorption and lower the current value. Photovoltaic cell is one of the most powerful techniques to measure the performance of the spectral response cell qualitatively. Therefore, the spectral response of a cell has been recorded in the 400 - 1000 nm wavelength range. The photocurrent action spectra were examined and are shown in Figure 13. It is seen that spectra attain maximum value of current at λ = 580 nm and decreases with increase in wavelength. The decrease in current on longer wavelength side may be attributed to non-optimized thickness and transition between defect levels. The maximum current is obtained corresponding to λ = 580 nm.