Theoretical energy yield of GaAs-on-Si tandem solar cells
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Theoretical energy yield of GaAs-on-Si tandem solar cells Haohui Liu1,2, Zekun Ren3, Zhe Liu1, Riley E. Brandt4, Jonathan P. Mailoa4, Sin Cheng Siah4, Armin G. Aberle1, Tonio Buonassisi3,4, Ian Marius Peters1 1 Solar Energy Research Institute of Singapore (SERIS), National University of Singapore, 7 Engineering Drive 1, Singapore (117574) 2 NUS Graduate School for Integrative Sciences & Engineering (NGS), 28 Medical Drive, Singapore (117456) 3 Singapore-MIT Alliance for Research and Technology (SMART), 1 CREATE Way, Singapore (138602) 4 Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, U.S.A ABSTRACT III-V on Si multijunction solar cells represent an alternative to traditional compound IIIV multijunction cells as a promising way to achieve high efficiencies. A theoretical study on the energy yield of GaAs/Si tandem solar cells is performed to assess the performance potential and sensitivity to spectral variations. Recorded time-dependent spectral irradiance data in two locations (Singapore and Denver) were used. We found that a 4-terminal contact scheme with thick top cell confers distinctive advantages over a 2-terminal scheme, giving a yield potential 21% higher than the 2-terminal scheme in Singapore and 17% higher in Denver. The theoretical energy yield benefit of a 4-terminal device emphasizes the need for further technology development in this design space. INTRODUCTION Multijunction solar cells represent a promising approach that can surpass efficiency limits of single junction cells. Currently, III-V multijunction cells reach efficiencies above 40% under direct beam concentration 1. These multijunction solar cells, made of III-V compound semiconductors, represent a significant advancement in solar cell technology and have found widespread application in space as well as in terrestrial concentrating photovoltaic systems. Most III-V multijunction solar cells use germanium (Ge) as the substrate and the bottom cell. The Ge bottom cell, which has a low bandgap of only 0.66 eV, tends to produce more current than the upper cells. At the same time, Ge is an expensive material, rendering it less economical in largescale applications. In comparison, silicon (Si) is cheaper and more widely available. Furthermore, replacing Ge with Si confers many other potential advantages, such as lighter weight, higher thermal conductivity, stronger mechanical strength, and better bandgap combination 2, 3. Therefore, considerable research effort has been spent on incorporating Si in IIIV multijunction solar cells recently 3-6. The actual energy yield potential of this type of cell is of particular interest. It addresses the questions of whether to employ a 2-terminal (2T) or 4-terminal (4T) contact scheme, as well as how much additional complication and cost in fabrication can be justified. Theoretical efficiency limits for multijunction solar cells were investigated by several authors 7-9. However, it was found that spectral variations under real operation conditions have significant influence on
