One of the most power full alternatives energy sources for future large-scale electricity production is photovoltaics that is the conversion of sunlight directly into electricity. The use of photovoltaic cells (PVC) utilizing thin films semiconductor materials, as a direct route to harness solar energy has become a subject of intensive research due to its overriding advantages over the other conventional devices. The photovoltaic cells are much cheaper, simple to construct and need no complicated processing steps. As the PVC utilizing a polycrystalline aggregate can achieve 67% efficiency of the same material in a single crystal form . The efficiency and stability of PVC depend strongly on the preparation condition of the photoelectrodes, electrolytes and the experimental conditions set during the experimentation . Unlike semiconductor-semiconductor junction in solar cells, the semiconductor-electrolyte junction in PVC is readily established so that the study of junction energetics becomes easy   . The binary semiconductor materials are known to function effectively in conversion of solar energy into electrical energy  .
Zinc telluride is a wide band gap material at room temperature. It is efficiently used in gray, and light gray light-emitting diodes, photovoltaics, laser screens, thin films transistor and photovoltaic cells   . In photovoltaic cells (PVC), the use is made of the interface which forms on mere dipping the semiconductor into an electrolyte solution and the liquid junction potential barrier can be easily established  . Thus, PVC cells provide an economic chemical route for trapping solar energy . Along with PVC, the semiconductor electrolyte interface may be used for photoelectrolysis, photocatalysis and photovoltaic power generation .
The properties of such systems are mainly dependent on the interface formed between the semiconductor electrode and electrolyte hence from material point of view the microstructure of the photoelectrode surface is of main importance . We reported the successful deposition of crystalline zinc telluride thin films by the chemical bath deposition technique. Growth mechanism, structural, morphological, optical, electrical and thermoelectrically properties are studied . This paper revels the study of photovoltaic performance of chemically deposited Zinc telluride photo electrode which include I–V, C–V characteristics, barrier height measurements, power out curves, spectral response and photo response. Series and shunt resistance in solar cells are parasitic parameters, which affect the illumination current-voltage (I-V) characteristics and efficiency of cells. Very high values of series resistance (Rs) and very low values of shunt resistance (Rsh) reduce short-circuit current (Isc) and open-circuit voltage (Voc) respectively. In this study, the analysis of (Rs) and (Rsh) is for the zinc telluride solar cells.
2. Experimental Details
2.1. Construction and Working of Photovoltaic Cell
The Photovoltaic cell is the semiconductor device that converts 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 1. A simple photovoltaic solar cell is formed a p-n junction diode. The schematic of the p-n junction device is shown in Figure 2. The n-type region is heavily doped and thin so that the light can penetrate through it easily. The p-type region is lighted doped so that most of the depletion region lies in the p-type region side. 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
Figure 1. Construction and working of PV solar cell.
Figure 2. Formation of the p-n junction diode in PV solar cell.
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 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.
2.2. Synthesis of ZnTe Thin Films
The chemicals used for the preparation of zinc telluride thin films were used as the chemical is AR grade zinc sulphate, tartaric acid, liquor ammonia, sodium sulphite and selenium. Sodium telenosulphate was prepared by the following method reported earlier . In actual experimentation, 10 mL (0.2 M) zinc sulphate was taken in 250 mL beaker. 2.5 mL (1 M) tartaric acid, 10 mL (1%) liquor ammonia and 10 mL (0.25 M) sodium telenosulphate were added in the reaction bath. The total volume of the reactive mixture was made up to 100 mL by adding double distilled water. The beaker containing the reactive solution was transferred to an oil bath at 278 K temperature. The pH of the resulting solution was found to be 10.05 ± 0.05. To obtain the film, four stainless steel plate substrates were positioned vertically on a specially designed substrate holder and rotated in a reactive solution with a speed of 52 ± 2 rpm. The temperature of the solution was then allowed to rise slowly to 359 ± 5 K. The substrates were subsequently removed from the beaker after 6 h of deposition. Figure 3 is shown by the experimental deposition of the solar thin film.
2.3. Fabrication of PVC Cell
A calomel electrode is used as a reference electrode and sulphide-polysulphide as an electrolyte is shown in Figure 4. Photovoltaic cell is consisting of three-electrode configurations are used in experiment. Zinc telluride as photoanode, CoS-treated graphite rod as a counter electrode. This electrode acts as a photocathode.
Figure 3. Experimental setup of CBD method to deposition of thin film.
Figure 4. A schematic diagram of photovoltaic solar cell electrode. L: Light; A: Anode; P.E.: Photoelectrode; C.E.: Counter Electrode; S.E.: Standard Calomel Electrode; S: Stirrer; W: Water; H: “H” shaped cell, E: Electrolyte.
2.4. Characterization of PVC Cell
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 type of conductivity exhibited by the film is determined by nothing the polarity of the emf developed in PVC 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 are calculated from photovoltaic power output characteristics. The power output characteristic has been obtained for a PEC cell at a constant illumination of 30 mW/cm2.
3. Results and Discussion
3.1. Conductivity Type
A photovoltaic cell with configuration n-ZnTe|NaOH (0.1M) + S (0.1M) + Na2S (0.1M)|C(graphite) was formed. PVC 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 photovoltage gives the conductivity type of ZnTe. This suggests that ZnTe is an n-type conductor which has also been proved from TEP measurement studies .
3.2. I-V, C-V Characteristics in Dark
Photovoltaic cell of zinc telluride thin films current-voltage (I-V) characteristics in dark have been studied at 300 K and shown in Figure 5. The characteristics
Figure 5. I-V characteristics of ZnTe photovoltaic solar cell (in dark).
are symmetrical indicating the formation of rectifying type junction . Using famous ideal Schottky diode equation junction ideality factor is calculated from the equation  .
where, I, is the forward current in dark, I0, reverse saturation current, v, applied forward bias voltage and nd is the junction ideality factor. The value of junction ideality factor (nd) can be determined of the linear regions of the log(I) versus volt (V) are shown in Figure 6. The ideality factor was found to be 2.27. The higher value of nd 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 .
The PVC 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 (Vfb). The flat band potential of a semiconductor gives information of the relative position of the fermi levels in photoelectrode as well as the influence of electrolyte and charge transfer process across the junction. This is also useful to measure the maximum open circuit voltage (Voc) 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 . Under such circumstances, Vfb can be obtained using Mott-Schottky relation by standardizing with saturated calomel electrode (SCE).
where the terms involved have meaning, C−2 is space charge capacitance per unit area, q the electronic charge, εs is the dielectric constant of the semiconductor
Figure 6. Determination of junction ideality factor of ZnTe photovoltaic solar cell.
electrode, ε0 is the permittivity of the free space, Nd the donor density, k the Boltzmann constant, T the absolute temperature, V the applied potential and Vfb is the flat band potential. The 1/C−2 versus voltage (mV) plots (Mott-Schottky plot) were constructed for the sample and 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 was found to be −0.652 (SCE) for ZnTe-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 ZnTe and polysulphide electrolyte may be possible reasons for deviation from linearity in C-V plot.
3.3. Barrier Height Measurements
The photovoltaic solar cell zinc telluride thin films barrier height was determined by measuring the reverse saturation current (I0) through the junction at different temperature from 370 to 315 K. The reverse saturation current flowing through junction is related to temperature as .
where, A is Richardson constant, k is the Boltzmann constant, Φβ is the barrier height in eV. To determine the barrier height of the photoelectrode, a graph of log(I0/T2) with 1000/T was plotted. The plot of log(I0/T2) with 1000/T for the representative sample 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 0.583 eV.
3.4. Power Output Characteristics
Solar cell may operate over a wide range of voltages and currents. The photovoltaic
Figure 7. C-V characteristics of ZnTe photovoltaic solar cell.
Figure 8. Determination of barrier height of ZnTe photovoltaic solar cell.
solar cell is illuminated with the light constant intensity of the current voltage characteristics shift in the four quadrants, this behavior is in accordance with the theory of solar cells acting as an electricity generator. By applying the resistive load on an irradiated cell continuously from a short circuit to a very high value of open circuit it is possible to determine the maximum power point (Pm = Vm × Im), 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.
where E is power of input light (mW/cm2) and Ac 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 (Vm × Im) divided by the short circuit current (Isc) and open circuit voltage (Voc) in light current voltage (I-V) characteristics of the cells can be measured by the following equation.
The open circuit voltage and short circuit current are found to be 150 mV and 25.6 μA, respectively. The power efficiency conversion factor can be studied by the following equation.
where Vfb is the flat band potential, Vredox the electrolyte redox potential and Eg is the energy band gap. It is important to note here that Voc and η depend on Vfb and Eg. The photovoltaic power output characteristics for a cell under illumination of 30 mW/cm2 shows Figure 9. The calculation shows the fill factor is 24.86%. The power conversion efficiency is found to be 0.49%. The low efficiency may be due to high series resistance and interface states which are responsible for recombination mechanism   . The value of series resistance and shunt resistance were found to be 263 (Ω) and 689 (Ω), respectively.
3.5. Study of Photo and Spectral Response
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 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 photoelectrode-electrolyte interface modeled as a Schottky
Figure 9. Power output curve for ZnTe photovoltaic solar cell.
Figure 10. Photo response as a function of Isc for ZnTe photoelectrode.
Figure 11. Photo response as a function of Voc for ZnTe photoelectrode.
barrier solar cell it is possible to represent the current-voltage relationship  .
where, Iph the photocurrent densities, I is the net current density, I0 the reverse saturation current density, Id the dark current density, V the applied bias voltage and nd is the junction ideality factor. In bias voltage condition V > 3kT/q and at equilibrium open circuit conditions
Iph = Id and V = Voc thus,
where, Voc is the open circuit voltage and Isc is the short circuit current. As Isc I0, a plot of log Isc against Voc 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 log Isc with Voc for ZnTe photoelectrode. The lighted ideality factor was calculated and found to be 2.87.
Spectral response 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 - 1050 nm wavelength range. The photocurrent action spectra were examined and are shown in Figure 13. It is seen that spectra attain
Figure 12. Determination of lighted ideality factor for ZnTe photoelectrode.
Figure 13. Determination of spectral response for ZnTe photoelectrode.
maximum value of current at λ = 560 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 = 560 nm.
The photovoltaic solar cell of zinc telluride thin films can be successfully deposited by using zinc sulphate tartaric acid, and liquor ammonia and sodium telenosulphate onto a stainless steel plate. The photovoltaic solar cell can be easily fabricated using ZnTe photoanode, sulphide-polysulphide as the electrolyte, CoS-treated graphite rod as a counter electrode. A saturated calomel electrode was used as a reference electrode. Through photovoltaic solar cell has some disadvantage associated it, but the disadvantages are expected to overcome as the technology advances, since the technology is advancing, the cost of solar plates, as well as the installation cost, will decrease down so that everybody can effort to install the system. Furthermore, the government is laying much emphasis on solar energy so after some years we may expect that every household and also every electrical system is powered by solar or renewable energy.
One of the author (KCR) thankfully acknowledge to Dr. V. M. Patil, the Principal, The New college, Kolhapur for providing all facilities to do this work.
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