Ge virtual substrates for high efficiency III-V solar cells: applications, potential and challenges

TL;DR

This paper explores the use of Ge/Si virtual substrates for high-efficiency III-V solar cells, analyzing their potential, challenges, and initial experimental results with thin Ge layers on Si.

Contribution

It demonstrates the feasibility of replacing traditional Ge substrates with Ge/Si virtual substrates in high-efficiency solar cells and reports initial experimental progress.

Findings

Photocurrent in Ge bottom cell is nearly sufficient for current matching.

Overall device performance is limited by low voltages and structural cracks.

Reducing Ge layer thickness and improving substrate quality are future directions.

Abstract

Virtual substrates based on thin Ge layers on Si by direct deposition have achieved high quality recently. Their application to high efficiency III-V solar cells is analyzed in this work. Replacing traditional Ge substrates with Ge/Si virtual substrates in standard lattice-matched and upright metamorphic GaInP/Ga(In)As/Ge solar cells is feasible according to our calculations using realistic parameters of state-of-the-art Ge solar cells but with thin bases (< 5um). The first experimental steps are tackled by implementing Ge single-junction and full GaInP/Ga(In)As/Ge triple-junction solar cells on medium quality Ge/Si virtual substrates with 5um thick Ge layers. The results show that the photocurrent in the Ge bottom cell is barely enough to achieve current matching with the upper subcells, but the overall performance is poor due to low voltages in the junctions. Moreover, observed cracks in the triple-junction structure point to the need to reduce the thickness of the Ge + III-V structure or using other advanced approaches to mitigate the thermal expansion coefficient mismatch effects, such as using embedded porous silicon. Next experimental work will pursue this objective and use more advanced Ge/Si virtual substrates available with lower threading dislocation densities and different Ge thicknesses.