Proceedings Article | 18 April 2016
Proc. SPIE. 9885, Photonic Crystal Materials and Devices XII
KEYWORDS: Photovoltaics, Refractive index, Energy, Polymethylmethacrylate, Titanium dioxide, Nanoparticles, Solar cells, Luminescence, Photons, Reflectivity, Photonic crystals, Photonic crystals, Atomic layer deposition, Upconversion, Energy conversion efficiency, Absorption
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% Er3+ core-shell nanoparticles and titanium
dioxide (TiO2). 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 TiO2 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
from simulation.
