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Lightweight ultrathin solar cells with high efficiency and reliability serve as a convenient untethered power source for new types of electronic devices, such as attachable or implantable electronics, small‐scale robots, and many others. However, the extreme mechanical properties of high‐performance solar cells and ultrathin films present challenges when handling and processing them to realize ultrathin solar cell arrays. In this paper, a highly efficient GaAs photovoltaic array integrated on an ultrathin polymer film (1.4 µm thick) is presented. Full processes, including framing, cold‐welding, epitaxial lift‐off (ELO), and microfabrication, are used to realize ultra‐flexible and lightweight GaAs photovoltaic arrays. The mechanical characteristics are analyzed via numerical and experimental methods along with demonstrations with electrically functional devices. The power‐to‐weight ratio (specific power: 5.44 W g−1) is in the highest range, even with single‐junction solar cells. [ABSTRACT FROM AUTHOR]