R.M. McDonald, N.C. Scott, L. Micklow, S. Furst, C. Youtsey
Smart Material Solutions,
United States
Keywords: solar, nanopatterning, antifouling
Summary:
Lightweight, high-efficiency III–V photovoltaic cells critically power unmanned aerial vehicles, rovers, and spacecrafts. However, further improvements in cell efficiency are plateauing. Additional efficiency and power gains can be made by modifying the cover glass encapsulation to enhance the transmission and capture of incident light. Ceria-doped cover glass is the standard encapsulant for space-grade panels due to its proven durability against atomic oxygen and high-energy radiation. However, a portion of the incident light is lost to reflection or absorption before reaching the underlying cell. While conventional quarter-wave antireflective coatings can mitigate some reflection losses, their limited spectral bandwidth and sensitivity to the angle of incidence can restrict their effectiveness. Furthermore, particle fouling of rover or ground-based solar panels can result in additional losses. By texturing cover glass with subwavelength and light-trapping structures, Fresnel reflections can be broadly suppressed while the optical path length is increased to enhance light absorption by the cell. These surface textures can also passively mitigate dust accumulation on surfaces. This project explored various nanopatterned structures and materials to serve as large-area, cover glass alternatives. Roll-to-roll nanoimprint lithography (NIL) enabled large-area replication of micro- and nanostructures into polymer films which were tested on top of space-qualified triple-junction III-V solar cells. These patterns included nanoscale moth-eyes, microscale pyramids, and multiscale structures that demonstrated a 5% increase in current and efficiency at normal incidence over non patterned cover glass and films. Since traditional polymer materials used in NIL could degrade in space, we also explored scalable methods to pattern more robust materials including space-grade silicone, ETFE, colorless polyimide, and silica-based sol-gel. Ongoing work focuses on balancing the trade-offs among material robustness, solar cell performance, and manufacturability required to achieve higher-efficiency solar cells with patterned encapsulation.