Proof-of-Concept of Dual-Junction (InGaP/GaAs) Solar Cells on Engineered Ge substrates for Affordable and Sustainable Space Applications

The space industry has transitioned from large geostationary satellites to constellations of small satellites, driven by ongoing efforts to reduce launch costs since the early 2010s. This shift has redirected solar cell development from a focus on maximizing performance to prioritizing cost-effective solutions. Silicon (Si) solar cells, though cost-efficient, are limited by their efficiency and long-term stability. Conversely, multijunction solar cells (MJSCs), known for their high efficiency and radiation resistance, are constrained by high costs and material requirements. To overcome these challenges, thin and flexible MJSCs on engineered germanium (Ge) substrates have emerged as a promising alternative. Umicore has developed two innovative processes to produce detachable Ge membranes: the single-porous PEELER process and the bi-porous ELLA process. The bi-porous method is particularly suited for high-volume, low-cost applications. In this approach, a conventional Ge substrate is porosified to form a bi-porous structure. A subsequent annealing step reorganizes the bi-porous structure into a crystalline foil supported by pillars, which serves as a growth template for the III-V layers. In collaboration with Universidad Politécnica de Madrid (IES-UPM), we present the first dual-junction, lattice-matched solar cell (InGaP/GaAs) fabricated using the bi-porous approach. III-V layers grown on bi-porous Ge (bi-PGe) substrates exhibit comparable quality to those on bulk Ge. Additionally, the external quantum efficiency (EQE) is similar for both substrate types. While a slightly lower open-circuit voltage (Voc) of 60 mV is observed on bi-PGe, likely due to surface defects, the I-V curves show similar performance for both bulk and porous substrates. This innovation enables the development of lightweight, flexible, and cost-efficient solar arrays, perfectly suited to the demands of satellite constellations.