Towards Monolithically Integrated GaAs on Si Tandem Solar Cell

GaAs and Si solar cells are the technologies that achieve the highest single-junction efficiencies. Due to the nonideal bandgap combination, incorporating those two materials into a monolithic integrated tandem is, however, not straightforward. In this paper, we discuss the potential of GaAs/GaAs/Si triple-junction architecture. As the reachable short-circuit current in a GaAs solar cell is about two thirds of that reachable with Si, current matching can be achieved by combining a double-junction GaAs/GaAs top cell with a Si bottom cell. As the first result, we fabricated a prototype of GaAs/GaAs dual-junction top cell with an efficiency of 17.8% and demonstrated a fourterminal tandem solar cell with an efficiency of 20.4%. However, with state-of-the-art material properties, the integrated GaAs/GaAs/Si concept is envisioned to reach a standard-testingcondition efficiency of 33.0% under AM1.5G.

I. Introduction

Single junction solar cells made from Si and III-V compound semiconductors have each achieved very high efficiencies and have demonstrated reliable long-term performance during outdoor operation. Hence, combining these two technologies to realize a high-efficiency one-sun tandem solar cell is an attractive perspective [1]–[3]. The efficiency potential of GaAs on Si solar cells is among the highest achievable for any Si based tandem solar cell with technology available today [4]–[6]. However, the ideal bandgap partner for silicon is in the range of 1.75 to 1.8 eV, which can be realized by . GaAs, on the other hand, has a band gap of 1.42 eV, making it a far less ideal choice. Correspondingly, the radiative efficiency limit for the combination is 45%, and only 41.7% for GaAs/Si, as shown in Fig. 1.