N14A-T003 TITLE: Light-Weight, Solar Cells with High Specific Power and Conversion Efficiency

TECHNOLOGY AREAS: Air Platform, Battlespace

OBJECTIVE: Develop a light-weight, high-performance solar cell with specific power (>1.2 kW/kg) and high conversion efficiency (>37%).

DESCRIPTION: There is an ever growing Navy interest in the development of Small Unmanned Aerial Vehicles (SUAVs) to provide persistent Intelligence, Surveillance, Reconnaissance and Communication (ISRC) capabilities in extended and uninterrupted flight missions. To achieve the goals of persistent surveillance, it is highly desirable and usually necessary to have the SUAVs stay deployed at operating altitude in a continuous, long-endurance 24-hour, 7-day-a-week (24/7) operation.

The long-endurance mission requirements pose a very difficult challenge to large and small UAVs with either internal combustion engine-based or electrical motor-based propulsions. SUAVs with combustion engines typically cannot carry enough fuel to meet long endurance requirements without refueling. Likewise, SUAVs with electrical motors also need recharging as the energy density of on-board batteries is lower than that of chemical or fossil fuels. That requires the SUAVs returning to the base every few hours for recharging and hence drastically limits their field ISRC capabilities.

Current state-of-the-art solar power and energy systems for SUAVs are far from being adequate for any continuous, persistent ISRC applications. In spite of the recent advances for efficient electrical propulsion systems and light-weight composite materials for air frames, there is still a critical need for efficient, light-weight, deployable and renewable energy source, in the form of solar cells that can be integrated with the UAVs� exterior surfaces, to meet the SUAVs� 24/7 operational challenges. While it is intuitive that the performance of a solar cell scales with the light-to-electric conversion efficiency, another lesser known but even a more important performance figure of merit of solar cells that are critical for integration with SUAVs is their specific power in terms of the amount of power generated per unit weight of the cells.

State-of-the-art multi-junction solar cells grown on single crystal have been demonstrated with efficiencies approaching 37%, but with the specific power of crystalline-based solar arrays limited to no more than 0.2 kW/kg due to the single-crystal cells� requirement of stiff and heavy support structures. On the other hand, thin-film photovoltaic solar cells on light-weight flexible substrates have been demonstrated with specific power close to 1kW/kg. However, the thin-film solar cells typically have low conversion efficiencies that are less than 10%. Therefore, the Navy is seeking the innovative technology development of solar cells that have the combined characteristics of high conversion efficiency (>37%) and specific power (>1.2 kW/kg) to power SUAVs for future Naval 24/7 persistent ISRC, and autonomous operations.

PHASE I: Develop innovative technological design approach for solar cells with the combined performance characteristics of over 37% conversion efficiency and over 1.2 kW/kg specific power. Identify technological and reliability challenges, propose viable risk mitigation strategies, and demonstrate proof-of-concept for the proposed technology. A viable design path forward for scaling up the output power of the solar cell array should be proposed and included as part of the deliverable for Phase I.

PHASE II: Design, fabricate, characterize and deliver a solar cell array prototype that produces over 1.2kW/kg specific power at over 37% conversion efficiency. It is also critical to incorporate manufacturing cost reduction as part of the design criteria for the scalable solar cell array throughout all the phases of this program.

PHASE III: Commercialize the solar cell technology and leverage the advantages of scalable manufacturing process to develop a very cost-effective manufacturing process for technology transition to various system integrations for both DoD and civilian applications.

PRIVATE SECTOR COMMERCIAL POTENTIAL/DUAL-USE APPLICATIONS: This high-performance, light-weight solar cell technology has a very vast private sector commercial potential. The private sector can significantly benefit from this technology development in any commercial areas that can utilize highly efficient, portable, renewable energy source that has virtually zero carbon foot-print.

REFERENCES:
1. King, R. R. et al., 2009, Proceeding of 24th European photovoltaic solar energy conference, p. 55. http://www.spectrolab.com/pv/support/Band-Gap-Engineered_Architectures_for_High-Efficiency_Multijunction_Concentrator_Solar_Cells-Presentation.pdf.

2. Takamoto, T., Agui T., Washio H., Takahashi N., Nakamura K., Anzawa O., Kaneiwa M., Kamimura K., Okamoto K., & Yamaguchi M., 2005, January, "Future development of InGaP/(In)GaAs based multijunction solar cells," 31th IEEE PVSC, p. 519, Orlando, FL.http://www.dodtechmatch.com/DOD/Opportunities/
PrintSBIR.aspx?id=AF073-098.

3. Wanlass, M., Geisz J., Kurtz S., Wehrer R., Wernsman B., Ahrenkiel S., Ahrenkiel R., Albin D., Carapella J., Duba A., & Moriaty T., "Lattice-mismatched approaches for high performance III-V photovoltaic energy," 2005, January. 31th IEEE PVSC, p. 530, Orlando, FL. http://www.researchgate.net/publication/4154101_Lattice-mismatched_approaches_for_high-performance_IIIV_photovoltaic_energy_converters/file/72e7e52cb418b3a554.pdf