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Nanophotonic coatings and structures provide an attractive approach for enhancing light-matter interactions in advanced photovoltaics. A little-discussed aspect of many photonic architectures is the presence of strong anisotropies in the electromagnetic field enhancements. These anisotropies are of particular importance in relation to nanostructured materials that possess intrinsic structure-dependent optical anisotropies. In this talk, we describe a novel class of momentum-resolved spectroscopies that provide new insight into structure-dependent optical properties of thin-film material and discuss approaches to exploit these effects in photovoltaic devices that incorporated nanophotonic enhancements.
Specifically, we use Fourier imaging techniques to measure or control the momentum distribution of in-coming or out-going light rays respectively. These techniques are applied to test-case organic photovoltaic materials that can be deposited with distinct morphologies depending on processing conditions. Using momentum-resolved photoluminescence, we determine the morphology-dependent orientation of transmission and absorption dipoles. We subsequently demonstrate the use of momentum-resolved reflectometry to perform “model-free” measurements of optical constants. The approach provides precise and accurate optical constants with quantified error estimates, obviating the complications associated with highly model-dependent, multi-parameter spectral fitting procedures used in ellipsometry. We conclude by describing ongoing efforts to exploit optical anisotropies to enhance light absorption in nanostructured photovoltaics.
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