Upconversion (UC) presents a possibility to exploit sub-bandgap photons for current generation in solar cells by creating
one high-energy photon out of at least two lower-energy photons. Photonic structures can enhance UC by two effects: a
locally increased irradiance and a modified local density of photon states (LDOS). Bragg stacks are promising photonic
structures for this application, because they are straightforward to optimize and overall absorption can be increased by
adding more layers. In this work, we present a comprehensive simulation-based analysis of the photonic effects of a
Bragg stack on UC luminescence. The investigated organic-inorganic hybrid Bragg stack consists of alternating layers of
Poly(methylmethacrylate) (PMMA), containing purpose-built β-NaYF4:25% Er<sup>3+</sup> core-shell nanoparticles and titanium
dioxide (TiO<sub>2</sub>). From optical characterization of single thin layers, input parameters for simulations of the photonic
effects are generated. The local irradiance enhancement and modulated LDOS are first simulated separately.
Subsequently they are coupled in a rate equation model of the upconversion dynamics. Using the integrated model, UC
luminescence is maximized by adapting the Bragg stack design. For a Bragg stack of only 5 bilayers, UC luminescence
is enhanced by a factor of 3.8 at an incident irradiance of 2000 W/m2. Our results identify the Bragg stack as promising
for enhancing UC, especially in the low-irradiance regime, relevant for the application in photovoltaics. Therefore, we
experimentally realized optimized Bragg stack designs. The PMMA layers, containing UC nanoparticles, are produced
via spin-coating from a toluene based solution. The TiO<sub>2</sub> layers are produced by atomic layer deposition from molecular
precursors. The reflectance measurements show that the realized Bragg stacks are in good agreement with predictions
Upconversion of sub-band-gap photons is a promising approach to increase the efficiency of solar cells. In this paper, we review the recent progress in upconverter material development and realization of efficient upconverter silicon solar cell devices. Current published record values for the increase in the short-circuit current density due to upconversion are 13.1 mA/cm<sup>2</sup> at a solar concentration of 210 suns determined in a sun simulator measurement. This increase is equivalent to a relative efficiency enhancement of 0.19% for the silicon solar cell. Although this is a considerable enhancement by more than one order of magnitude from values published only a few years ago, further enhancement of the upconversion performance is necessary. To this end, we investigate theoretically the application of resonant cavity and grating photonic structures. Our simulation based analysis considers irradiance enhancement and modified density of photon states due to the photonic structures and their impact on the upconversion dynamics in <i>β</i>-NaYF<sub>4</sub>: 20%Er<sup>3+</sup>. It shows that an optimized grating can increase upconversion luminescence by a factor of 3 averaged over the whole structure in comparison to an unstructured reference with the same amount of upconverter material.
For high band gap solar cells, organic molecule based upconverter materials are promising to reduce transmission losses of photons with energies below the absorption threshold. We investigate the approach of embedding the organic upconverter DPA:PtOEP directly into each second layer of a Bragg stack to achieve an enhancement of upconversion performance. The two major effects that influence the upconversion process within the Bragg stack are simulated based on experimentally determined input parameters. The locally increased irradiance is simulated using the scattering matrix method. The variation of the density of photon states is obtained from calculations of the eigenmodes of the photonic crystal using the plane wave expansion method. A relative irradiance enhancement of 3.23 has been found for a Bragg stack of 31 layers including λ/8-layers on both sides. For suppressing the loss mechanism of direct sensitizer triplet decay via variations of the density of photon states, a different design of the Bragg stack is necessary than for maximum irradiance enhancement. In order to find the optimum design to increase upconversion quantum yield, both simulation results need to be coupled in a rate-equation model. The irradiance enhancement found in our simulation is significantly higher than the one found in the simulation of a grating-waveguide structure, which achieved an increase of upconversion quantum yield by a factor of 1.8. Thus, the Bragg structure is very promising for upconversion quantum yield enhancement.