Modeling of AlGaAs on Si tandem PV cells for extended spectral conversion
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Modeling of AlGaAs on Si tandem PV cells for extended spectral conversion Mahieddine Emziane, Alaeddine Mokri Solar Energy Materials and Devices Lab. Masdar Institute of Science and Technology, Masdar City, PO Box 54224, Abu Dhabi, UAE. Email: [email protected] ABSTRACT The primary objective of this modeling investigation is to optimize a two-junction threeterminal device under the AM1.5G spectrum. Based on previous studies, AlGaAs and Si cells, because of their energy bandgaps, can be combined together to achieve high-efficiency doublejunction devices. In this study, the top cell is made of Al0.3Ga0.7As (1.817 eV) while the bottom cell is made of Si (1.124 eV). In order to avoid the losses and design constraints observed in twoterminal and four-terminal devices, the tandem cell AlGaAs/Si is designed with three-terminals. In order to determine the optimal structure of the device, the top and bottom junctions were investigated and optimized with regard to the thicknesses and doping level. The optimum configuration of the device shows an efficiency of 26.27% under the AM1.5G spectrum and one sun, which is higher than the efficiency of an optimized single-junction Si cell under the same illumination conditions. We also studied the effect of the optical concentration on the performance of the device and we found that the overall efficiency reaches over 31% under 50 suns. INTRODUCTION Single-junction photovoltaic devices have limitations in the ability to utilize efficiently the photons of the broad solar spectrum ranging from 300 nm to 2500 nm. For instance, in the case of Si solar cells, they cannot absorb photons with wavelengths longer than 1100 nm, which represents more than 20% of the standard terrestrial normal radiation at AM1.5 [1]. Short wavelengths in the ultraviolet region also are not converted efficiently by Si solar cells because of thermalization effects. Because solar cells only convert photons with specific wavelengths efficiently, stacking solar cells made of different materials (i.e. different energy bandgaps) together proved to be a good approach to increase the efficiency of photovoltaic devices, and many devices made of a stack of single-junction cells have been proposed by many research groups [2]. Indeed, efficiencies as high as 41.3% and 32.6% have been reported for triple-junction and double-junction devices under 343 and 1026 suns, respectively, using the AM1.5 spectrum [3]. Multi-junction cells are usually monolithic or mechanically stacked and can have either two, three or four terminals based on the number of external connections [2]. Compared with the monolithic two-terminal device configurations, the three-terminal configuration allows the two sub-cells to operate independently and avoid losses due to current mismatching between the two sub-cells. This three-terminal configuration also avoids losses originating from the tunnel junction between the top and bottom cells. Compared with the four-terminal configuration, threeterminal devices have the advantage of less complexity for connec
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