Photoluminescent Effects on the Temperature Coefficient of Multi-Junction Solar Cells
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Photoluminescent Effects on the Temperature Coefficient of Multi-Junction Solar Cells 1
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Alaeddine Mokri , Mahieddine Emziane
Solar Energy Materials and Devices Laboratory, Masdar Institute of Science and Technology Masdar City, PO Box 54224, Abu Dhabi, United Arab Emirates Corresponding authors: 1 [email protected], 2 [email protected] ABSTRACT In this work, we re-evaluate the temperature coefficient in multi-junction solar cells by including the effects of radiative coupling (i.e. re-absorption of emitted photons within the cell) and eliminating radiative losses towards the substrate (i.e. use of a back mirror). The model developed is for two-junction devices, and it takes into account: the number of terminals, the energy bandgaps of the sub-cells, the light concentration, and whether a back mirror is used or not. The temperature coefficients obtained are compared with the case where no luminescent effects are considered. The results show that, in two-terminal and four-terminal devices, the sensitivity to temperature is almost the same whether luminescent effects are taken into account or not. However, these effects are most significant in three-terminal devices. In four-terminal devices, the results show that these effects depend to a large extent on the materials used, the design of the system, i.e. on the effectiveness of radiative exchange between the cells involved. INTRODUCTION The effect of temperature on the performance of multi-junction solar cells has been attributed to the fact that (1) the rate of recombination increases and (2) the energy bandgap decreases with temperature, without taking into account the radiative thermal exchange between the junctions, and between the junctions and the substrate.
Figure 1. The mechanisms of radiation exchange within two-junction devices.
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In this investigation, we construct a model to evaluate the temperature coefficient in twojunction solar cells by including the effects of radiative coupling (i.e. re-absorption of emitted photons within the cell) and eliminating radiative losses towards the substrate (i.e. use of a back mirror). Mechanisms of radiation exchange between the top and bottom junctions, and between the junctions and the substrate are shown in figure 1. The model developed is for two-terminal, three-terminal and four-terminal (beam-splitting) devices, and it takes into account: the energy bandgaps of the top and bottom cells, the variation of light concentration, and whether a back mirror is used or not. To achieve this, we discretize the solar spectrum into small bands of widths in the range 0.5 nm – 5 nm, and solve the diode equation for each band. In order to take the luminescent effects into consideration, we modify the diode equation as explained in the rest of this paper. TWO TERMINAL SOLAR CELLS The two-terminal photovoltaic technology consists in combining two junctions in a single system, and subjecting them to specific parts of the solar spectrum. The spectral response, light concentration and the cutoff wavelengths are all cri
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