EAGER: Enhanced Solar Energy Conversion by Ultra-slow Photon Sub-diffusion in Aperiodic Media

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EAGER: Enhancing Solar Energy Conversion by Slowing Diffusion in Photonic NanostructuresNontechnical Description: Solar Photovoltaics (PV) is the fastest growing sector among renewable energy technologies. Most modern solar cells utilized in deployed systems are made from either crystalline silicon (Si) or thin-film semiconductor materials. Crystalline Si cells are more efficient at converting sunlight to electricity, but generally feature higher manufacturing costs. On the other hand, thin-film materials typically have lower conversion efficiencies but are simpler and less costly to manufacture. The increasing materials cost and the growing demands of the Si solar industry have driven a dramatic reduction of the absorbing cell thickness. However, due to their small absorption length, thin-film solar cells have a limited conversion efficiency that needs to be substantially increased in order to become competitive on the global market. This EAGER research will address the fundamental challenges of thin-film PV solar energy conversion by developing a novel photonics-led approach that boosts the optical absorption in Si layers with thickness less than 100nm. Our ambitious goal will be accomplished by designing, fabricating and testing a novel class of photonic nanostructures that are capable of trapping solar radiation by slowing down optical diffusion by design. The PI anticipates that the impact of this EAGER program will be distributed across the solar energy industry and in the field of integrated silicon optoelectronics. In particular, the research will create novel device principles that can also be used to improve the performances of photodetectors and optical sensors integrated on planar chips. The educational activities of this project are designed to integrate sustainable energy topics and nanophotonics research into the education of graduate, undergraduate, and high school students, as well as high school teachers. Technical Description: The specific objective of this exploratory EAGER project is to create, design and engineer a novel nanophotonic-driven approach to efficiently couple incident radiation into sub-wavelength semiconductors films and dramatically enhance their photon absorption rate, irrespective of polarization, over a broad range of wavelengths and incident angles. Differently from well-established photonic crystals, randomly textured surfaces or grating-enhanced solar cells, which suffer from polarization sensitivity, require a small range of incident directions, and rely on limited increase of the photon path length within the absorbing medium, this novel strategy builds on a fundamentally different photon transport mechanism, known as logarithmic photon sub-diffusion. This approach combines for the first time ultraslow (i.e., logarithmic) photon transport phenomena in dielectric nanostructures with isotropic scattering in aperiodic systems in order to achieve efficient PV solar energy conversion without relying on the excitation of photonic narrowband resonances. The ultraslow anomalous photon diffusion regime will be engineered using Si and SiN nanostructures fabricated by magnetron sputtering and standard nanolithography over large areas covered by designed aperiodic patterns that create correlated photon walks across the active regions of thin-film poly-Si and a-Si cells. While focusing the efforts on Si for ease of integration, cost-effectiveness and technological scalability, this concept can similarly be applied to alternative semiconductor material platforms as well, such as third-generation solar cells materials. Leveraging sub-diffusive photon anomaly in engineered aperiodic photonic media offers unique performance advantages, such as non-resonant operation, tunable photon diffusion length, insensitivity to incident angles and polarization. This research is interdisciplinary and potentially transformative as it creates sub-diffusive solar energy converters based on a completely novel and tunable photonic mechanism for ultra-thin film PV devices.