Increased Spectrum Utilization with GaAsP/SiGe Solar Cells Grown on Silicon Substrates
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Increased Spectrum Utilization with GaAsP/SiGe Solar Cells Grown on Silicon Substrates Anastasia H. Soeriyadi1, Brianna Conrad1, Xin Zhao1, Dun Li1, Li Wang1, Anthony Lochtefeld2, Andrew Gerger3, Ivan Perez-Wurfl1, and Allen Barnett1 1 School of Photovoltaic and Renewable Energy Engineering, UNSW Australia (The University of New South Wales), Sydney 2052 Australia 2 AmberWave Inc., Salem, NH 03079 USA 3 SolAero Technologies Corp, Albuquerque, NM 87123, USA ABSTRACT World-record solar-to-electricity energy conversion efficiency has been previously achieved by photovoltaic devices that maximize the use of the solar spectrum, such as multijunction tandem solar cells. These cells are made of III-V materials whose high cost is a strong limitation on their widespread commercial application. One solution to suppress the cost of these types of devices is to grow III-V solar cells on low-cost carrier materials such as silicon. We will discuss the material, structure and analysis of GaAsP/SiGe-on-silicon multi-junction tandem solar cells. A low threading dislocation density is realized by effective lattice-matching of the top and bottom cells which demonstrate a device that achieves high open-circuit voltage in the top solar cell. The GaAsP/SiGe solar cells have reached a measured efficiency of 20.6% under one sun concentration. Analysis of these results based on the product of the best parameters shows efficiency potential of 26% under one sun, 30.8% at 20× and 35.1% at 400×. INTRODUCTION The highest solar cell efficiencies of more than 40% have been achieved through an optimised solar spectrum utilization [1]. This is typically achieved by the use of III-V materials in a multi-junction system where different wavelength bands are absorbed in different layers. However, the commercialization for terrestrial applications of such solar cells is difficult due to the high cost of the lattice-matched substrates and III-V materials. To tackle this, one of the options is to grow III-V materials on silicon (Si) substrates, which demonstrate superior mechanical properties at a much lower cost than III-V wafers. Many studies have pursued this approach, producing some notable achievements [2]–[4]. In this paper, the material, structure and analysis of GaAsP/SiGe-on-silicon solar cells are discussed. Theoretical calculations have shown a limiting efficiency of 45% under one sun for an unrestricted bandgap material for a dual-junction tandem cell [5] while a GaAsP/SiGe structure has a theoretical efficiency of 39.4% under AM1.5 [6]. The design is simultaneously optimized for lattice and current matching [6]. Graded SiGe was grown on Si to minimize the threading dislocation density (TDD) [7]; a low TDD results in a small band gap-voltage offset (Woc), which is critical for high-performance multi-junction devices [8]. The GaAsP/SiGe solar cells have reached a measured efficiency of 20.6% under one sun [9], while the product of the best parameters obtained imply that a one-sun efficiency of 22.5% is presently achievable [10]. The material was
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