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On macroscopic scale, solar cell efficiency can be increased with concentrating lenses, that focus the sunlight on the cell. An equivalent effect can be achieved with nanostructures that make the light emitted by the solar cell directive towards the sun, while eliminating some of the drawbacks of macroscopic concentration. We have developed an evolutionary algorithm to design dielectric nanostructures for directional emission, leading to structures with a directivity as high as 306. In our previous work[1] the experimentally achieved value was significantly lower than the modeled directivity, due to several limiting factors. In this work we overcome these limitations, by using a newly build measurement setup and a specially designed material system. By combining a Fourier microscope with an integrating sphere, we are able to measure emission into the full 4π solid angle, which allows us to measure full directivity. The predicted directivity is calculated for a point source at the center of the nanolens. In the experiment the point source is resembled by small clusters of emitting particles, which are placed in an ordered array to facilitate the proper alignment. This is achieved with a recently developed technique of direct patterning of CsPbBr3 perovskite nanocrystals by e-beam lithography, by which the ligands of the nanocrystals are directly crosslinked upon exposure. Subsequently the dielectric nanolenses are fabricated on top of the clusters by 2-photon lithography.
[1] Johlin, E.; Mann, S. A.; Kasture, S.; Koenderink, A. F.; Garnett, E. C. Broadband highly directive 3D nanophotonic lenses. Nature Communications 2018, 9, 1-7.
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