Non-Technical: Conventional state-of-the-art tandem junction photovoltaic solar cells, composed of multiple sub-cells of III-V compound semiconductors, are capable of converting incident radiation from the Sun to electricity with greater efficiency than all other types of solar cells. The high performance of these devices is enabled in part due to the use of high quality monocrystalline III-V materials and in part due to the coupling of multiple sub-cells that collectively allow for absorption of a broadband solar spectral range. However, the materials and manufacturing costs required for the development of III-V tandem junction devices is prohibitively high for use in wide-scale terrestrial consumer applications. As a consequence, the world's highest performance solar cells are limited to use in niche markets such as terrestrial high concentration and space power applications. A high-risk, high-payoff exploratory research path is proposed here that aims to provide an unconventional, yet potentially transformative nanotechnology-enabled solution to the above consumer market penetration challenges faced by state-of-the-art tandem junction solar cells. The project aims to dramatically reduce manufacturing costs by monolithically integrating III-V sub-cell composed of vertical nanowire arrays to a central silicon sub-cell, thereby simultaneously eliminating the primary III-V substrate cost-driver while cutting III-V crystal growth volumes by up to 95% compared to conventional technologies. The broader significance of this EAGER project lies in the potential realization of a low-cost and high-efficiency renewable energy innovation that provides greater national energy independence and clean power. This research also impacts and advances fundamental knowledge in science and engineering in the fields of physics, nanomaterials growth and characterization, nanoelectronics, and optoelectronics. Immediate anticipated societal impacts include outreach activities that promote science, technology, engineering, and mathematics concepts to the general public, training new members of a highly-skilled workforce, and direct inclusion of high school, undergraduate, and graduate students from under-represented communities.
The technical approach of this EAGER project relies on selective-area heteroepitaxy of a GaAsP (1.75 eV) nanowire array on the top surface of a thinned Si (1.1 eV) sub-cell by metal-organic chemical vapor deposition. A bifacial, three dissimilar materials, tandem junction device is formed via monolithic integration of a back-side InGaAs (0.5 eV) nanowire array. The vertical nanowires comprising the top- and back-surface arrays will contain radially-segmented p-i-n junctions and will be serially connected to the central Si sub-cell via epitaxial tunnel junctions. This design enables absorption of broadband incident solar energy as well as albedo radiation. Standard lattice-matching constraints are overcome via strain relaxation along nanowire free surfaces. Therefore, ideal spectral matching is realized without a need for graded buffer layers or dislocation mediation strategies. Use of vertical nanowire arrays with coaxial p-i-n junction geometries permits key advantages, including near-unity absorption of solar irradiance at normal and tilted incidence without the use of anti-reflection coatings, decoupling of photon absorption and carrier collection directions, and dramatic reduction of 95% in epitaxial volumes. Rigorous modeling of device parameters will be iteratively coupled with extensive materials characterization and property correlation experiments for optimization of III-V sub-cell structure on the single nanowire and ensemble array levels. The ultimate target of this work is demonstration of a functional bifacial, three dissimilar materials, nanowire-based tandem junction solar cell with one Sun power conversion efficiency of 30% or better.
