AbstractGroup III–V semiconductor multi-junction solar cells are widely used in concentrated-sun and space photovoltaic applications due to their unsurpassed power conversion efficiency and radiation hardness. To further increase the efficiency, new device architectures rely on better bandgap combinations over the mature GaInP/InGaAs/Ge technology, with Ge preferably replaced by a 1.0 eV subcell. Herein, we present a thin-film triple-junction solar cell AlGaAs/GaAs/GaAsBi with 1.0 eV dilute bismide. A compositionally step-graded InGaAs buffer layer is used to integrate high crystalline quality GaAsBi absorber. The solar cells, grown by molecular-beam epitaxy, achieve 19.1% efficiency at AM1.5G spectrum, 2.51 V open-circuit voltage, and 9.86 mA/cm² short-circuit current density. Device analysis identifies several routes to significantly improve the performance of the GaAsBi subcell and of the overall solar cell. This study is the first to report on multi-junctions incorporating GaAsBi and is an addition to the research on the use of bismuth-containing III–V alloys in photonic device applications.