CAREER: Photonic spectrum splitting with nanocrystal luminophores

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The sun represents the most abundant potential source of sustainable energy on earth. Solar cells that capture the sun?s rays and convert this energy into electricity can potentially be improved through the science of photonics, which uses advanced materials to guide the light pathway through the device. This project will develop photonic structures which contain nanocrystal-polymer composite materials that split and concentrate the solar spectrum into its component wavelengths, and then direct the concentrated specific wavelengths to semiconductor materials that are best able to convert these wavelength energies into electricity. Through the combination of these processes, the overall efficiency for solar energy conversion can be increased. The proposed research targets designs that can be readily implemented with existing solar cell fabrication processes and could also be integrated into architectural features for buildings. These photonic materials may also find other applications in biological imaging for medical diagnostics, solid-state lighting, and display technology. This project also has broad educational goals coupled to the research activities, aimed at encouraging women and underrepresented minorities into STEM careers through project-based learning about solar energy.This project will address a fundamental challenge in solar photovoltaic conversion of sunlight to electricity - the mismatch between the broad solar spectrum and semiconductors that only efficiently convert light at wavelengths near their bandgap energy. In conventional multi-junction solar cells, the light must transfer through a stack of semiconductor layers designed to capture a portion of the solar spectrum. The transfer of light through this stack results in absorption losses that lower the theoretically-attainable solar energy conversion efficiency. The splitting of the solar spectrum into component wavelengths and the direction of these wavelengths to adjacent semiconductors, as opposed to within a stack, can reduce losses and improve solar energy conversion efficiency. The overall goal of the proposed research is to develop an efficient spectrum splitting system for enhancing solar energy harvesting using optical materials that combine photonic structures with luminescent nanocrystal-polymer composite materials. Towards this end, the proposed research seeks to gain a fundamental understanding of the interaction of these nanocomposite materials with various photonic structures for harvesting of light. The research plan has three primary objectives. The first objective is to consider photonic routes for enhancing the efficiency of solar spectrum splitting based on nanocomposite luminescent concentrators. For example, by incorporating light trapping elements such as Bragg mirrors into luminescent concentrators, luminophore quantum yield can be increased. The second objective will address loss mechanisms by tuning the properties of luminescent nanocrystal-polymer composite materials. For example, a photonic crystal consisting of alternating high index and low index nanocrystal-polymer composites could reduce losses within the luminescent concentrator. In the third objective, photonic nanocrystal-polymer composite materials will be integrated into single-junction solar photovoltaic cells to serve as luminescent concentrators that provide controlled angles of emission, reduced absorption loss, and enhanced radiative decay rate. The design principles established through the single-junction device will be used to develop a spectrum splitting and concentrating system for multi-junction solar cells that harvest a larger fraction of the solar spectrum using nanocrystal luminophores engineered with different bandgaps. Theoretical approaches will link nanoscale properties and photonic structures to their performance as waveguides that accept diffuse sunlight and direct it to adjacent solar cells. This information will used to predict the upper limits of efficiency for single and multi-junction solar cells. Experimental work will include synthesis of nanocrystal-polymer composites, correlation of the composite nanostructure to optical response, and fabrication of photonic structures. Optical spectroscopy and electron microscopy will be used to characterize the light propagation inside the concentrators. The proposed research is integrated with educational activities that focus on project-based learning about solar energy across educational and outreach platforms, including the development of introductory lecture and laboratory coursework materials, the creation of a program for undergraduate women to learn about undergraduate research opportunities in solar and related sustainable energy systems, and outreach to K-12 students that emphasizes the role of engineering in addressing societal goals.