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This paper presents a performance study of silicon-based multi-junction (MJ) solar cell which is produced by Spectrolab is the most efficient solar cell in the world, with a record efficiency of over 40%. The I-V characteristics of solar cells were investigated for different environmental conditions (both at indoor and outdoor). Photovoltaic (PV) parameters like short circuit current (I
_{SC}
), open circuit voltage (V
_{OC}
), fill factor (FF) and efficiency (?
) were evaluated for the fabricated cell. The fabricated
cell provides I-V characteristics curve which shows that the maximum efficiency of the cell is obtained when the cell temperature is lower. It was also observed that, efficiencies of the cells are about 17% to 24% at indoor and 7% to 20% at outdoor
and fill factors are 0.40 to 0.71. From the result
,
it can also be found that fill factor is directly affected by the values of the cells series and shunt resistance.

Energy plays a crucial role in human activities and has gradually become the foundation of modern economy all over the world. Due to prevalently utilizing fossil fuels, global warming and energy crisis are getting increasingly serious, resulting in an urgent research and development of solar energy [

In this work, a digital pyranometer has been used to measure the solar radiation. The resulting equivalent circuit of a solar cell is shown in

The I-V characteristics of such a device are given by,

I = I s { exp [ q V K T ] − 1 } − I L (1)

where, I L is photo generated current (amperes), q is elementary charge, 1.6 × 10 − 19 Coulombs , k is Boltzmann’s constant, 1.38 × 10 − 23 J / K , T is the cell temperature in Kelvin. At 25˚C, kT/q » 0.0259 volts.

Fill factor is the ratio of the maximum power point divided by the open circuit voltage ( V o c ) and the short circuit current ( I s c ):

F F = P m V o c × I s c = η × A c × E V o c × I s c (2)

where, ŋ is the solar cell conversion efficiency, A_{c} is the surface area of the solar cell, E is the input light irradiance.

FF can also be interpreted graphically as the ratio of the rectangular areas depicted in

F F = P MAX P T = I m V m I s c V c (3)

Efficiency is the ratio of the electrical power output P_{out} compared to the solar power input, P_{in}, into the PV cell. P_{out} can be taken to be P_{MAX} since the solar cell can be operated up to its maximum power output to get the maximum efficiency

η = P out P in , η MAX = P MAX P in (4)

P_{in} is taken as the product of the irradiance of the incident light, measured in W/m^{2}, with the surface area of the solar cell which is measured in m^{2}.

STC specifies a temperature of 25˚C and an irradiance of 1000 W/m^{2} with an air mass 1.5 (AMI 1.5) spectrum.

The current-voltage characteristics of the solar cell are measured at different indoor and outdoor conditions. The solar cell has been characterized by using the experimental set and also characterized both in the indoor and outdoor conditions. Figures 3(a)-(d) shows the variation of current with voltage when the cell is illuminated with the light source and the sun. The shapes of the curves are shown in Figures 3(a)-(d). In the I-V characteristics curve, it was observed that the current remains constant at the beginning and there is small variation of current with voltage in short time. At the end of the graph, it shows that current changes rapidly with small change in voltage. Figures 3(a)-(d) shows the current-voltage characteristics curves of solar cell at different light intensities. The V_{oc} and I_{sc} can be directly determined from graph, the maximum voltage V_{m} and current I_{m} at the maximum power point can be determined by computing the IV product at the turning points on the curve and selecting the points where the product is maximum. Thus the maximum power and fill-factor can be obtained from the relations P_{m} = I_{m}V_{m} and FF = I_{m}V_{m}/VocIsc, respectively. The efficiency of the cell can also be obtained from this graph, if it compare the maximum output power with the incident input power. By analyzing the various kinds of situation of the solar cell, it was observed that from this characteristics curve the maximum efficiency of the cell is obtained when the cell temperature is lower. From this I-V characteristic curves it was concluded that, the curve which becomes more square shape gives high efficiency.

The ideal characteristics of solar cells is obtained by plotting corresponding fill factor (FF) of normalized open-circuit voltage [Voc/(KT/q)] and its logarithmic are shown in

The graph shown in

In this work, the efficiency of MJSC is evaluated at different light intensities on various times of the day. At the more insulation, more current and more voltage are produced. Variation of efficiency, open circuit voltage (V_{oc}), short circuit current (J_{sc}) and fill factor (FF) with the variation of different light intensity have been investigated. When the cell temperature becomes high, its efficiency decreases. Due to wind effect, cell temperature becomes low and more efficiency is obtained. As the temperature increases, the diffusion length increase, because the diffusion constant stays the same or increase with temperature, and the minority life time increase with temperature. The increase in minority-carrier diffusion length causes an increase in J_{L} (photo-generated current density). This effect causes a reduction of efficiency as the temperature increases. The main impact of series resistance is to reduce the fill factor, although excessively high values may also reduce the short -circuit current. A large fill factor is desirable and corresponds to an I-V sweep that is more square-like. Typical fill factors range from 0.5 to 0.82. The fill factor is directly affected by the values of the cells series and shunt resistance. Increasing the shunt resistance (R_{sh}) and decreasing the series resistance (R_{s}) will lead to higher fill factor, thus resulting in greater efficiency, and pushing the cells output power closer towards its theoretical maximum.

Authors thank Dr. Md. Nurul Islam, Dept. of Physics, University of Chittagong for helpful discussions.

The authors declare no conflicts of interest regarding the publication of this paper.

Kali, K., Islam, M.A., Yasmin, M.A., Das, S. and Sharker, K.K. (2019) Performance Study of Silicon-Based Multi-Junction Solar Cell. Computational Water, Energy, and Environmental Engineering, 8, 91-98. https://doi.org/10.4236/cweee.2019.84006