Porous-lift-off germanium membranes as lightweight, flexible, and cost-effective template for III-V space solar cells: a review.

The growing demand for low-cost, lightweight, and high-efficiency space technologies has driven the search for photovoltaic solutions which minimize material use without sacrificing performance. III-V multijunction solar cells remain the state of the art for space power applications due to their exceptional efficiency and radiation resistance. Among III-V solar cells architectures, Germanium (Ge)-based structure remains widely used; however, their broader deployment is limited by reliance on thick and expensive Ge substrates. Germanium membranes, however, offer a promising path to address these limitations through substrate reuse strategies. This review highlights progress in the development of Ge membrane as a transformative platform for space photovoltaics, with a focus on porous Ge-based lift-off approaches while reusing Ge substrate. These methods enable the fabrication of detachable, monocrystalline Ge membrane through pore etching, epitaxial growth, porous transformation, layer lift-off and substrate reuse, significantly reducing Ge consumption and device weight. Two main fabrication routes for porous Ge formation are discussed: lithography-based dry etching using Ge-On-Nothing approach and electrochemical etching. The fundamentals of pore formation, thermal reorganization of porous, and subsequent III-V epitaxy are reviewed. Key challenges related to process uniformity, detachment scalability, and industrial integration are examined, and potential strategies for scalable manufacturing are outlined.