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Nano-Inspired Broadband Photovoltaics Sheets


OBJECTIVE: Design and develop proof-of-concept light-weight, flexible and rugged photovoltaic sheets capable of converting solar radiation into usable electric power that can meet or exceed the Soldier"s requirements for low to moderate power and be scalable for use in higher power mobile/field applications, thereby extending the lifetime of batteries for off-base missions, power telecommunication equipment and support Army logistics systems. DESCRIPTION: Photovoltaics show immense promise for supporting Soldier and mobile/field power. However, current technologies (silicon) that are dominating the commercial market are unable to provide the high power per weight required for the dismounted Soldier, and the best prospects require substantial improvements to meet the insatiable demand (high power, light-weight, low cost) for Soldier power. The maximum theoretical efficiency for the photovoltaic conversion of unconcentrated solar radiation that can be achieved in conventional single-junction solar cell is given by the Shockley-Queisser limit of 33.3%. By employing photovoltaic nanomaterials this limit can be exceeded due to multi-step absorption and/or multi-exciton generation. Taking into account this challenge and the vast opportunities nanotechnology provides, the main goal of this program is to develop photovoltaic nano-materials for demonstrating prototypes of single-junction devices that exceed the Shockley-Queisser limit, and to integrate the technology into flexible and light weight photovoltaic sheets with power requirements range in the range of 4W to 20W and scalable for use in mobile/field power (200W to 1kW). This can potentially extend the lifetime of batteries for ~72 hour missions, power telecommunication equipment and support Army logistics systems. Significant improvements in photovoltaic devices can be achieved due to the following nano-inspired technologies: (i) Nano-patterned coatings for advanced light trapping schemes;(iii) Nano-enhanced absorbers in IR range; (iv) Advanced windows based on novel transparent conductors; (iv) Bandstructure nano-engineering for high conversion performance; (v) Nano-engineered electron processes for suppression of thermalization and recombination losses. These technologies can potentially lead to the broadband operation with strong harvesting and conversion of below-bandgap photons. However, one potential technical barrier associated with implementing nano-inspired technologies is that small bandgap materials will increase the dark current and decrease the open circuit voltage. Therefore, the novel photovoltaic devices should utilize materials that allow for flexibility in bandgap tunability and must be based on reliable fabrication technologies that are potentially lower in cost compared with the state of the art multi-junction solar cells. The photovoltaic devices are expected to be scalable to facilitate integration into large photovoltaic sheets. The developing novel technologies should be universal enough for all-weather photovoltaics conversion as well as for use in thermo-photovoltaic applications. This program has the potential to feed CERDEC"s Soldier wearable nano-grid energy harvesting program. PHASE I: Efforts should include modeling and analysis of the type of nanomaterial, thicknesses, and compositions as well as investigate the relation between bandstructure, wavelength and structure to determine the feasibility of the proposed innovation. Therefore, at the end of Phase I, the technical merit of the approach proposed for exceeding the Shockley-Queisser efficiency limit should be presented in terms photocurrent (>25mA/cm2), external quantum efficiency (>85%)and open circuit voltage (>0.9V). PHASE II: Knowledge gained from models developed in Phase I should be applied for designing and developing a representative proof-of-concept solar cell device incorporating novel nanomaterials and advanced nanophotonic technologies. At the end of Phase II, the proof-of-concept device should demonstrate at least 25% photovoltaic efficiency at AM 1.5G. The overall success will be evaluated by direct comparison with performance and functionality of the state-of-the-art multi-junction solar cells. Demonstration of proof of concept light-weight (~6.5W/g) and flexible (curvature of radius of at least 3 inches) solar cells at the 4W level. PHASE III: Development of modular PV sheet designs that allow scalability to the high power mobile/field application is expected to be aggressively pursued by the offerer. Commercial benefits include increased competitiveness for more providers of high efficiency, light weight PV sheets for use by the fielded soldier on extended missions. REFERENCES: 1. K.A. Sablon et al., Nano Lett., 2011, 11, pp 23112317; 2. V.E. Ferry et al., Nano Lett., 2008, 8, pp 43914397; 3. J. Li et al., Appl. Phys. Lett. 2009, 95, 033102.; 4. O.E. Semonin et al., Science 2011, 334 pp. 1530-1533; 5. P.D. Cunningham et al., Nano Letters 2011,11, pp. 3476-3481.
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