Efficiency Enhancement in Polycrystalline CdS / CdTe Solar Cell via Diffraction Grating and Engineering Absorber and Back Surface Field Layers

In this paper, the effect of diffraction grating and engineering absorber and back surface field (BSF) layers on performance of a single-­‐junction polycrystalline cadmium sulfide/cadmium telluride (CdS/CdTe) solar cell have investigated. At first, the electrical characteristics of reference CdS/CdTe solar cell is simulated and validated with experimental data of fabricated CdS/CdTe solar cell. In order to improve the maximum efficiency, a new structure with diffraction grating and engineering absorber and back surface field layers is presented. Furthermore, the effect of carrier lifetime variation in the absorber layer on the conversion efficiency of solar cell was investigated. It is found that diffraction grating and engineering absorber and back surface field layers can increase the conversion efficiency of the solar cell by about 1.02% and 6% compared with reference cell, respectively. Under global AM 1.5 conditions, the open circuit voltage, short circuit current density, fill factor and conversion efficiency of optimized solar cell structure are 1114 mV, 25.35 mA/cm, 0.8856 and 25.022%, respectively.


INTRODUCTION
Solar energy can be converted using photovoltaics that can be made using crystalline materials, multiple--junctions, or polycrystalline thin films.Although, polycrystalline thin films such as CIGS or cadmium telluride (CdTe) have several potential advantages.Polycrystalline thin films reduce cost and materials needs because the materials usage is much lower.On the other hands, less material and lower purity requirements cause lower embodied energy.Therefore, the energy payback time can be well under a year (Marjani, et al., 2016;Sabaghi, et al., 2015).The CdTe cells have underperformed crystalline Silicon cells but the CdTe is also easily manufacturable by various methods including the closed space sublimation, vapor transport deposition, or molecular beam epitaxy (Khosroabadi, et al., 2014).In recent years, the efficiency of these cells have increased but further gains in efficiency will continue to be beneficial.Since the same area or number of panels can provide more energy, improved efficiency helps to reduce the cost of the cells and panels and balance of system costs.Therefore, solar photovoltaics better able to supply the world's energy needs.
The thickness of CdS is one of important factor in the conversion efficiency of cadmium sulfide/cadmium telluride (CdS/CdTe) solar cell.Reducing the CdS layer thickness causes high short circuit current densities (Krishnakumar, et al., 2011).Generally, CdTe films are usually deposited by the close--spaced sublimation (CSS), electrodeposition, spraying, or screen printing techniques and by Chemical bath deposition (CBD), vacuum evaporation, or CSS methods.On the other hands, because of limited availability and the rising price of telluride with regards to very high volume photovoltaic module manufacture in the future, reducing the CdTe absorber layer thickness is important prospect.In order to form an ohmic contact, Amin proposed a ZnTe buffer layer as a back surface reflector (Amin, et al., 2007).
In this paper, in order to further improve the performance of single--junction polycrystalline CdS/CdTe solar cell, we used diffraction grating and engineering absorber and back surface field (BSF) layers.All of the simulation results is compared with already--fabricated and simulated CdTe/CdS cell structure as a reference cell.In addition, optimization of the carrier lifetime is made for achieving the highest efficiency.The rest of this paper is described as follows: The structure, model and its validation are described in section 2. Section 3 presents the results and discussion.Finally, we conclude in section 4.

DEVICE STRUCTURE, MODEL and ITS VALIDATION
It is essential to take into account the interaction of optical and electrical that occur during the solar cells operation when modeling thin film solar cells.The fundamental equations are electrostatic potential and carrier densities, which link together.The primary equations are Poisson's Equation, the continuity equations and transport equations that derived from An experimentally--fabricated CdS/CdTe solar cell has been used as a reference cell and our simulation results are very close to the previous experimental results (Amin, et al., 2007).Therefore, the results can be basically valid for an actual cell with the proposed configuration.Figure . 1 shows the reference cell structure where glass is used as the substrate.Structure was consisted of 200 nm indium--tin--oxide (ITO), 50nm CdS, 3.5μm CdTe.All parameters used for simulation CdS/CdTe solar cell have been listed in Table 1.Under AM 1.5 conditions, simulated structure shows an open circuit voltage of 988 mV, a short circuit current density of 22.14 mA/cm 2 , a fill factor of 0.823, and a conversion efficiency of 18%.Table 2 compares the experimental results of open circuit voltage (V OC ), short circuit current density (J SC ), fill factor, and cell efficiency with the simulated ones.As can be seen, the simulated results are quite close to the actual experimental data (Amin, et al., 2007).

RESULTS AND DISCUSSION
At first, we investigated the effect of step doping grading of absorption layer by creating a built--in electrical field in the CdTe layer.In this way, the absorption layer is divided into two layers with different doping concentration of 10 14 --10 17 cm --3 and 10 18 cm --3 for top and bottom layers, respectively.The thickness of layers were considered 150 nm. Figure .2 shows the proposed structure with step doping grading of absorption layer for optimization of the cell performance.
Figure .3 shows the performance of the proposed structure with step doping grading of absorption layer as a function of doping concentration of top CdTe layer.A built--in electric field in the CdTe layer has been created by the step doping grading of absorption layer that assists carrier migration.Therefore, it reduces the series resistance and charge storage time.As can be seen, the performance is maximized with doping concentration of 7×10 17 cm --3 and 2×10 18 cm --3 for top and bottom CdTe layers.Under global AM 1.5 conditions, the designed cell had an open In this stage, in order to increase the efficiency of the polycrystalline CdS/CdTe/ZnTe cell, a built--in electrical field was formed in the ZnTe layer by step doping grading.In this way, the ZnTe layer was divided into two layers with different doping levels.Top and bottom layers were 150 nm thick with 10 14 --10 18 cm --3 doping concentration and 150 nm thick with 10 18 cm --3 doping concentration.Figure .4 shows this structure with step doping grading in the ZnTe layer that doping of top ZnTe layer was varied from 10 14 cm --3 to 10 18 cm --3 .
The variation of the doping concentration of the top ZnTe layer for different doping of bottom ZnTe layer are shown in figure.5. Ass seen, the efficiency decreases when the doping Figure 4: The schematic structure with stepped doping grading of the ZnTe layer.concentration of top ZnTe layer increases.Due to both high built--in electrical field and high absorption level, the efficiency was highest for doping concentration of 10 15 cm --3 .However built-in electrical field and absorption are constant before doping concentration of 10 15 cm --3 .On other hands, the built--in electrical field is low and the absorption is high after doping concentration of 10 15 cm --3 .
In order to achieve best device performance, various designs using different length of CdTe and ZnTe layers of diffraction grating were investigated.The charge storage time was decreased considerably to improve performance of the polycrystalline CdS/CdTe/ZnTe cell structure by increasing the length of CdTe of diffraction grating.Therefore, length of CdTe and ZnTe layers of diffraction grating were both optimized, however the doping concentration of the layers remained unchanged.The structure of polycrystalline CdS/CdTe/ZnTe cell with diffraction grating including CdTe and ZnTe layers was shown in figure.6. Figure .7 shows the cell efficiency of the new structure as a function of different length of CdTe and ZnTe layers of diffraction grating.As can be seen from figure.7, the efficiency is maximum for 200 and 50 nm of CdTe and ZnTe layers in diffraction grating, respectively.As compared to the reference cell, polycrystalline CdS/CdTe/ZnTe cell with diffraction grating exhibited higher efficiencies.The best diffraction grating structure shows performance of 24.47% and a fill factor of 0.885, which is of particular importance in the solar cell design.
Figures. 8 and Since the carrier lifetime of CdTe has a strong influence on the solar cell fill--factor and the voltage calculation, we investigate the impact of carrier lifetime in the CdTe layer on the J SC , V OC , FF and Efficiency of the proposed cell to improve the further efficiency.The variations of carrier lifetime can be modeled by variation of defect density.Increased defect density and purity of the CdTe alters the electron lifetime as well as the hole lifetime.Physically, a reduction in the defect density could be the key to improvement in increased lifetime through a smaller number of recombination centers.All carriers generated in the depletion region will be collected at higher lifetimes.Therefore, the V OC and conversion efficiency improve with increasing the carrier lifetime.The V OC is defined as: where n is the ideally factor.k, T, q and J SC are Boltzmann's constant, absolute temperature, electrical charge and short current density.J 0 is dark current density that is directly linked to the defect density and purity of the material.As can be seen, the V OC can be increased with decreasing dark current density (Jensen, et al., 2016).Figure .10 shows the J SC , V OC , FF and Efficiency of cell as a function of carrier lifetime.As seen, longer carrier lifetimes resulted in a higher V OC .Therefore, the conversion efficiency increases with increasing carrier lifetime.data (Amin, et al., 2007).

CONCLUSION
In this paper, a new structure is proposed which is achieved by diffraction grating and engineering absorber and back surface field layers in order to achieve the maximum efficiency.Also, the effect of carrier lifetime variation in the absorber layer on the conversion efficiency of solar cell was investigated.The results show the maximum conversion efficiency of 25.022% with the open circuit voltage, short circuit current density and fill factor of 1114 mV, 25.35 mA/cm 2 and 0.8856, respectively under global AM 1.5 conditions.

Figure 2 :Figure 3 :
Figure 2: The configuration of the polycrystalline CdS/CdTe cell with step doping in the CdTe layer.
9 show polycrystalline CdS/CdTe/ZnTe cell with tandem diffraction grating including CdTe and ZnTe layers and the current and power curves as a function of voltage, respectively.The results show the maximum conversion efficiency of 25.022% with the open circuit voltage, short circuit current density and fill factor of 1114 mV, 25.35 mA/cm 2 and 0.8856, respectively under global AM 1.5 conditions.

Figure 5 :
Figure 5: The efficiency as a function of the top ZnTe layer doping concentration.

Figure 6 :Figure 7 :
Figure 6: The schematic structure with diffraction grating including CdTe and ZnTe layers.

Figure 10 :
Figure 10: The variation of J SC , V OC , FF and Efficiency of the proposed cell as a function of the CdTe layer's carrier lifetime.