Effect of radiation on electrical characteristic measurement of the fabricated CdTe / P-Si heterojunction Solar Cell
Abstract
The electrical and photovoltaic properties of CdTe/p-Si heterojunction solar cells prepared by evaporation coating on single-crystal p-type silicon substrates were examined under an illumination intensity of 100 mW/cm2 at 25 °C. The best fabricated cell shows an open-circuit voltage (VOC) before γ-irradiation of 0.59 V, which decreases to 0.565 V after irradiation. The short-circuit current density (JSC) before irradiation is 35 mA/cm2, and after irradiation it is reduced to 30 mA/cm2. The fill factor (FF) before γ-irradiation is 54.5%, while after irradiation it decreases to 53%. The conversion efficiency (based on active area) before irradiation is 11.2%, and after irradiation it drops to 5.1%. These results were observed during a two-hour illumination test and after storing the cell in air for three months. Illumination was performed from the CdTe side (front wall). The solar cells were analyzed using I–V and P–V measurements, with particular focus on the influence of irradiation time, light intensity, and effective γ-radiation dose, all of which play crucial roles in determining solar cell efficiency. A γ-irradiation campaign with different doses was carried out on a series of solar cells. The deterioration of silicon solar cell parameters due to gamma irradiation is strongly supported by minority carrier lifetime measurements, which show a clear decrease in minority carrier lifetime as the radiation dose increases.
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Introduction
Using the clean and free energy from the sun, mono-crystalline silicon solar cells are still the best options for photovoltaic solar energy systems. The electrical characteristics of silicon solar cells are affected by environment condition. During operation of photovoltaic solar cells, they are exposed to radiation such as used in space systems and satellites. The irradiation of solar cells by high-energy levels of radiation in the form of gamma rays, neutrons, charged particles, etc. leads to radiation defects and electrical damage in the solar cells bulk and results a significant degradation of the electrical parameters of silicon solar cells [1, 2].The lifetime and performance of the solar cells is limited by the amount of radiation damage.
Crystalline silicon solar cells, however, exhibit a response to electromagnetic radiation having substantially shorter wavelengths such as gamma ray. When silicon solar cells irradiated with gamma rays, two types of radiation damage occur within it: displacement damage and ionization effects. Displacement damage is the movement of atoms from their initial location in the crystal lattice to another placement that results a defect in the crystal lattice of solar cells. Ionization effect is the generation of electron-hole pairs in the bulk of solar cell. The eject electrons from the atoms of the crystal results a track of ionized atoms in the solar cells crystal. These defects mostly act as recombination points that decreased the diffusion length and life time of minority carrier as well as increased internal parameters of cells. Output parameters of solar cell such as maximum output power, fill factor, efficiency, short circuit current, and open circuit voltage strongly depend on internal parameters of solar cells such as series resistance, RS, saturation current, I0 and ideal factor, n. it has been proved that increasing each of above internal parameters of solar cell causes that the output characteristics of solar cells decreased [3-5].
Conventional single-junction semiconductor solar cells only effectively convert photons of energy close to the semiconductor band gap Eg as a result of the mismatch between the incident solar spectrum and the spectral absorption properties of the material [6]. Photons with energy Eph smaller than the band gap is not absorbed .Photons with energy Eph larger than the band gap is absorbed, but the excess energy Eph- Eg is not used effectively due to thermalization of the electrons.
Conclusion
The fabrication of CdTe/p-Si Heterojunction by physical thermal evaporation method has been reported .We have investigated the influences of various conditions in the CdTe/p-Si solar cell performance. The effect of cell thick-ness was observed. The thin CdTe/p-Si cell .thickness leads to the typically reduced spectral response in the infrared, and the wave length which has the maximum spectral response was shifted towards the shorter wave length, as the time radiation decrease. In addition the radiation hardness of solar cell performance. The effects of time radiation were observed. The affects of radiation noted a decrease in electrical performance as the radiation dose increase. Parameters such as donor concentration in CdTe, as well as current – voltage, and capacity voltage are described. Efficiency of the best performance solar cell CdTe/ Si approached around 11.2 %..
A deterioration of the electric properties of solar cells under gamma irradiation was observed when the gamma dose was increased (1 to 20 KGy). Except the fill factor, which in some cases showed increased or relatively steady values, gamma radiation causes a significant Reduction in the Isc and η while the Voc is slightly reduced. The decrease in short circuit current and other fundamental parameters is mainly related to the minority carriers life time. The life time of minority carriers is sensitive to the radiation induced defects that mostly act as recombination points, and the decrease in the minority carrier life time reduced the solar cells parameters.
According to the spectral photo current results, after gamma irradiation, the most of the cells performance is lost in the low wavelength of the spectrum this means that production defects due to gamma radiation occurred near the cell surface.
Of this manuscript must on any scientific group to operate in this area. Must be put solar cells panel far from the radiation sources, particularly the development of gamma rays at each application of solar cells operations in the practical life.