Update on Thin Film space race.
THIN-FILM SOLAR CELLS
Thin-film solar cells , comprised of polycrystalline or amorphous materials deposited on foreign, flexible substrates, are being developed for next-generation space solar arrays for their potential to enable large (>30 kW) power systems. Thin-film solar cell technologies, originally developed as a high production volume, low-cost terrestrial power generation source, offer advantages in terms of cost, specific power (W/kg), stowability, and radiation resistance for space solar arrays. Terrestrial thin-film solar cells have been significantly modified for space use, and goals for conversion efficiency in large-area cells have been set at 15%. The major issues addressed for adaptation of terrestrial solar cells to space use has involved the transition of thin-film solar cells to flexible substrates, the development of inorganic transparent protective coatings, characterization of radiation-induced damage, and characterization of thermal and light-induced radiation damage recovery.
Thin-film solar cells are fabricated on metal foil and polyimide substrates and provide favorable metrics of specific power (W/kg), stowage volume during launch (W/m^sup 3^), cost ($/W), and radiation-induced degradation for space solar arrays.
The thin-film solar cell types chosen for space solar arrays are amorphous silicon (a-Si)13 and polycrystalline Cu(In,Ga)Se^sub 2^ (or CIGS) and its alloys. Amorphous silicon solar cells are produced commercially for terrestrial use on flexible substrates, 5-mil- thick stainless steel from United Solar Ovonics (Auburn Hills, MI) and 2-mil-thick polyimide from Iowa Thin Film Technologies (Boone, IA). Ongoing work to adapt these solar cells to space use involves the thinning of the existing substrates to 1-mil-thick stainless steel and polyimide films, transition from stainless steel foil to polymer substrates, the modification of the active device layers to optimize the solar cells for the AMO solar spectrum, increases in conversion efficiency through readjustment of cost/performance trade- offs, and modification of electrical contacts to withstand space thermal cycling conditions. Large area a-Si solar cells optimized for space have demonstrated 9% efficiency on polyimide substrates.
CIGS solar cell manufacturing technology is less mature than that of amorphous silicon solar cells, but CIGS solar cells offer the promise of higher efficiencies. The CIGS solar cells at 1 cm^sup 2^ in size have been demonstrated at 15% efficiency on Ti foil substrates. The GIGS solar cells were originally developed for terrestrial uses on Na-containing glass substrates, and the cells generally demonstrate lower efficiency when deposited on flexible metal-foil substrates. Cell efficiency has been improved somewhat by the incorporation of Na into the solar cell by the introduction of NaF during growth. Other factors that seem to decrease cell efficiency are the increased roughness of metal foils; poor cell adhesion, sometimes due to thermal expansion coefficient mismatches; and differences in maintaining uniform temperatures during processing. The highest CIGS cell efficiency of greater than 19% AMI.5 has been demonstrated by the National Renewable Energy Lab for 0.1 cm^sup 2^ cells on glass using a three-step evaporation process. Achieving these high solar cell efficiencies on larger samples and on flexible substrates in a high-volume production process is a significant hurdle to be overcome.
Transitioning CIGS solar cells to lightweight polymer substrates has been more difficult to achieve than for a-Si cells due to the higher processing temperatures required, normally greater than 500C. Reduction of the processing temperatures typically results in reductions in efficiency. Current approaches to the deposition of CIGS on polymers include NASA Glenn in-house and directed efforts to lower the process temperature of CIGS by a modification of precursor materials and AFRL-directed efforts to deposit CIGS on polymers capable of withstanding temperatures over 500C. Success of these efforts will not only decrease the mass of CIGS solar cells
but will enable the solar cell material to be scribed into individual cells and series interconnected while remaining on the polymer film. This monolithic integration process18 has the potential to reduce solar array costs further through the reduction of touch labor while increasing device yield.
Protective coatings for the thin-film solar cells compatible with the space environment are being developed under efforts directed by AFRL. The coatings are required to have high transparency to maintain cell performance, high emissivity for passive thermal control, and good mechanical properties to resist cracking during flexure. The coatings incorporate a thin conductive outer layer to prevent high voltage arcing in the space plasma and are inorganic to resist attack by atomic oxygen. An integrated antireflection coating increases cell performance. Initial depositions of lowtemperature, vacuum-deposited coatings of alumina and silica have met the essential requirements for the protective coatings. Non-vacuum- deposited coatings are also being investigated for potentially favorable cost/performance trade-offs.
Transition of thin-film solar cells to space ultimately rests on the ability of the technology to withstand the space environment. Individual and interconnected thin-film solar cells are being subjected to a battery of environmental space qualification testing. This testing is, in part, in preparation for a space flight experiment called the Deployed Structures Experiment. Further details on the qualification testing and the flight experiment are found in a paper by Winter. |
