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

Abstract

Virtual substrates based on thin Ge layers on Si substrates by direct deposition have recently achieved high quality. In this work, their application as low cost, removable substrates for the growth of high efficiency, lightweight and flexible multijunction solar cells for space applications is analyzed. Experimental Ge single-junction solar cells and GaInP/Ga(In)As/Ge triple-junction solar cells using the Ge/Si virtual substrate as an active bottom junction (being the Si inactive), are implemented using medium quality Ge/Si virtual substrates with a 5 urn Ge layer thickness. A lower quality in the Ge material, as compared to standard substrates, but enough carrier collection efficiency for a standard triple-junction, are shown. The expected formation of cracks during growth, due to the large thermal expansion coefficient mismatch with the Si substrate, is confirmed, and is found to be a major limiting factor for the performance of the solar cells. Strategies such as thinning the Ge + III-V structure and minimizing the thermal cycling during growth are discussed. Using an embedded porous Si layer to serve as buffer for the strain is being investigated. This porous layer could also serve as sacrificial layer for high throughput mechanical epitaxial lift-off in the manufacturing of lightweight and flexible multijunction cells. These embedded porous Si layers need to be engineered for optimum performance and compatibility with the Ge and III-V deposition processes.

Thinned GaInP/GaInAs/Ge solar cells grown with reduced cracking on Ge|Si virtual substrates

“…Figure 8 shows the EQE of the top and middle cells in the 3JSC, compared to the case of a typical design on standard Ge substrates. The Ge bottom cell is not shown because it was not possible to measure it due to its leaky behaviour similarly as already reported for Ge|Si substrates with 5 µm Ge layer [19], [20]. As compared to the standard design, the GaInP top cell shows the expected shift in absorption edge in both the absorber material and window layer as a result of the higher In content.…”

“…The Ge layers were deposited by CVD as explained elsewhere [19]. In this work, virtual substrates with a 3 µm thick Ge layer are used, as compared to our previous reports for 5 µm thick Ge layers [19], [20]. These thinner Ge subcells were experimentally confirmed to achieve the required J sc for the 3JSC application.…”

“…Conversely, the approach that we investigate in this work pursues limiting the formation of cracks by redesigning the solar cell structure, in the context of GaInP/GaInAs/Ge 3JSC. In our previous works, we showed that standard 3JSC structures (over 6 micron thick) could be grown on Ge|Si templates with a 5 µm thick Ge layer, but the formation of cracks contributed to the low performance of the solar cells fabricated [19], [20]. In this work we analyze the thinning of the 3JSC structure to reduce the formation of cracks.…”

“…Nevertheless, this voltage drop represents a performance loss in comparison to the standard 3JSC. The tradeoff between the advantages of using Ge|Si substrates (lower usage of Ge, potential lower cost, possibility of substrate detachment using embedded porous layers [19], [20], etc) and the lower efficiency will determine the economic viability of this approach.…”

Design of Multijunction Solar Cells Based Potable Water Purification System using Cationic Polyacrylamide Alum Lime and Ozone to Remove Covid-19

“…Purification purpose added alum and chlorine at the end of purifier cycles and obtained pure water which is useful [16], [17]. The gallium layer is used on silicon for the direct deposition to get the highquality solar cells, application low in cost, high efficiency, low weight, and flexible III-V multijunction solar cells produced [18]. Various metamorphic tunnel junction designs, to target maximum application, of single-junction, double junction solar photovoltaic cells, grown via both molecular beam epitaxy (MBE); and metal-organic chemical vapor deposition (MOCVD), have been demonstrated.…”