The silicon p-n junction is the most successful solar energy technology to date, yet it accounts for a marginal percentage of worldwide energy production. To change the status quo, a disruptive technological breakthrough is needed. In this Perspective, we discuss the potential for complex silicon nanowires to serve as a platform for next-generation photovoltaic devices. We review the synthesis, electrical characteristics, and optical properties of core/shell silicon nanowires that are subwavelength in diameter and contain radial p-n junctions. We highlight the unique features of these nanowires, such as optical antenna effects that concentrate light and intense built-in electric fields that enable ultrafast charge-carrier separation. We advocate a paradigm in which nanowires are arranged in periodic horizontal arrays to form ultrathin devices. Unlike conventional planar silicon, nanowire structures provide the flexibility to incorporate multiple semiconductor, dielectric, and metallic materials in a single system, providing the foundation for a disruptive, unconventional solar energy technology.