Figure 9.

(a) Bragg stack (multilayered material comprised of TiO₂ layers/PMMA+NPs) for charge generation in a solar cell by a photonic upconverter. SEM image of the (b) Bragg stack and (c) of upconverter nanoparticles. (d) Scheme of core–shell upconverter NPs, converting near-infrared (NIR) to visible (VIS) photons in the core. (e) Energy levels in the upconverter Er³⁺ and the upconversion process [from [57] this work by C.L.M. Hofman is licensed under a Creative Commons Attribution (CC BY 4.0)].