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

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 (< 5µm). 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 5µm thick Ge layers. The resultsshow 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.Index Terms -III-V multijunction solar cell, germanium buffer, low cost substrate, virtual substrate. This is an accepted proceedings paper presented at IEEE PVSC 2019, in press.

Space III-V Multijunction Solar Cells on Ge/Si virtual substrates

“…The purpose of these preliminary experiments was to implement the first prototype solar cells and gauge the potential to achieve high efficiencies. No issue was found when growing III-V structures on these Ge/Si virtual substrates, which behave similarly as standard Ge substrates, as long as surface readiness for epitaxy and first nucleation steps are concerned, as observed during in-situ spectrometry measurements (details in [7]).…”

“…The details about the methods used for the theoretical and experimental work presented in this work can be found in our previous work on this topic [7]. Briefly, the modelling work was carried out using a numerical approach by means of the TCAD tool Silvaco Atlas.…”

“…This can compensate for possible losses, such as lower minority carrier properties in the Ge material of virtual substrates, as will be shown in next sections. Obviously, for other solar cell designs other than the standard 3JSC, such as the upright metamorphic, where the upper subcells bandgaps are lower and, therefore, less light is transmitted to the bottom cell, the minimum Ge thickness required increases, similarly as in the case of cells for terrestrial applications [7]. In the case of a typical 4JSC designed by inserting a leV subcell in the standard 3JSC, the minimum thickness for the Ge bottom cell is found to be too high (tens of microns) to be realizable using Ge/Si virtual substrates.…”

“…This provides a potential technique for high throughput ELO to manufacture lightweight solar cells. Moreover, the porous layer is expected to help accommodating the thermal expansion coefficient mismatch between the silicon substrate and the Ge and III-V layers deposited on top, mitigating the formation of cracks, which is currently a major issue in this technology [6], [7].…”

“…In any case, the development of high efficiency solar cells on Ge/Si virtual substrates poses a series of challenges that need to be tackled. The reader is referred to our previous introductory work on the use of these virtual substrates for high efficiency III-V cells, including a discussion of the potential applications to different solar cell architectures and preliminary experimental results in the context of terrestrial concentrator applications [7], In this work we examine the use of Ge/Si virtual substrates as an alternative to standard Ge for the manufacturing of lattice-matched GaInP/Ga(In)As/Ge triple junction solar cells for space, and we discuss the potential of virtual substrates with embedded porous Si layer as release layer for efficient substrate removal.…”