The proposed research focuses on developing novel energy harvesting devices that can be integrated with loadbearing structures in an air vehicle (e.g. a UAV). Several ambient energy sources are available on a UAV: light, heat, and vibration. The amount of energy available from light and heat exceeds that in vibration, so this work focuses on the first two modes of harvesting.

In the Photovoltaic Fiber Structure, very thin active organic layers and metallic electrodes are deposited concentrically around a fiber core, and light is coupled in through the outer electrode.
The approach is to create energy harvesting devices in the form of long fibers that eventually could be woven into lightweight, high-strength, multifunctional textiles for seamless integration with aerospace structural composites. The fiber form factor is a powerful paradigm for these energy conversion devices, since it can lead to improved light trapping in the organic photovoltaic (PV) cells, and allow for a high density of thermocouple junctions without the use of costly patterning techniques, significantly enhancing the cost-benefit performance.

The initial focus was on modeling and experimentally demonstrating prototype devices consisting of single fibers capable of the thermoelectric (TE) and PV modes of energy conversion. The results obtained were highly encouraging, and have opened up several exciting new research directions. In a solar cell geometry, the active organic layers and metallic electrodes are formed concentrically around a fiber core, and light is coupled in through the outer electrode. This structure is quite different from the conventional planar PV cells, and requires special considerations in its design and for predicting its optoelectronic performance.

Fresh advances in modeling OPV devices on fibers include the application of multilayer dielectric coatings to fiber bundles. This architecture maximizes light in-coupling in individual fibers, and takes advantage of photon recycling in multi-fiber arrays. The modeling combines ray-tracing and transfer-matrix simulations at multiple length scales. Each component of the model has been independently validated by experiments.

Improved power conversion efficiency of planar OPV cells was demonstrated using a metal-organic-metal layer structure. Importantly, these devices now match the efficiency of conventional ITO-based cells, which were improved. The ITO-free device exhibits a slightly lower short circuit current density (JSC), but compensates with a higher open circuit voltage (VOC). Further analysis of how JSC varies with anode thickness reveals that the device performs unexpectedly better than the far-field transmittance of the anode would suggest. The enhanced performance is due to the microcavity effects dominating the thin-film OPV cell, in which the far-field optical transmission of the electrode is less important than its ability to place the antinode of the optical field close to the donor-acceptor junction in the organic layers. Detailed optical modeling enables mapping of the performance of a wide range of electrode materials, and predicts that silver is not far from the conventionally employed ITO with respect to the JSC values it can allow.

Conversion of heat to electricity (thermoelectric generation) can be accomplished by connecting two dissimilar materials (metals or semiconductors) in a series of junctions, and sandwiching the junctions between a hot source and a cold sink. The voltage produced by the junction is proportional to the temperature gradient between the hot and cold sides. The conventional series-connected junction geometry can be reproduced in the form of thin-film segments deposited along fibers. Weaving these fibers can position the junctions as required for power generation. The TE generator is optimized by maximizing the temperature gradient, minimizing the thermal conductivity, and maximizing the Seebeck coefficient and electrical conductivity.

Woven thermoelectric generators have been demonstrated utilizing several TE fibers at once. Several fiber diameters have been explored, varying also the TE segment length and weave density, and spanning square inches. For smaller fibers, increased weave density, and greater temperature gradients, the power density increases dramatically. The thinness and flexibility of these mats suggests that multilayer TE fabrics can be used to efficiently span temperature gradients using individual layers tuned to work at their maximum ZT point.

This work was done by Max Shtein and Kevin Pipe of the University of Michigan, and Peter Peumans of Stanford University for the Air Force Office of Scientific Research. AFOSR-0004

This Brief includes a Technical Support Package (TSP).
Solar and Thermal Energy Harvesting Textile Composites for Aerospace Applications

(reference AFOSR-0004) is currently available for download from the TSP library.

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