Luminescent solar concentrators (LSCs) improve the performance of solar cells by concentrating sunlight onto small photovoltaic devices. A LSC consists of a polymer slab embedded with luminophores such as dyes or luminescent nanocrystals. Incident sunlight is absorbed by the luminophore and emitted at a longer wavelength, which propagates to solar cells on the edges of the concentrator via total internal reflection. This allows the solar cell to be optimized for a spectrally narrow light source. Moreover, the luminophores in the LSC can be designed to form colorful architectural components, making them well-suited for the building integrated photovoltaic industry.
To reach the thermodynamic performance limit for a LSC, a photonic mirror that both transmits high energy light and traps luminescent light is necessary (Rau 2005). However, due to the overlap between the absorption and emission spectrum of most luminophores, it is challenging to design ideal mirrors. Here we design photonic mirrors for LSCs that can be placed on the front or back, and show how the design choices change for different luminophores or concentrator geometries.
The photonic mirrors on the front of the LSC consist of one-dimensional, aperiodic layers of HfO2 and SiO2. CdSe/CdS nanocrystals are used as the luminophore. To examine the tradeoff between incident light transmission and trapping of luminescent light, an optimization algorithm was used to design the mirrors. Using Monte Carlo simulations we predict that mirrors designed to trap luminescent light are preferred for quantum yield (QY) greater than 85%, collection areas greater than 10 cm, loading fraction less than an optical density of 1.4 at 450 nm, and low absorption and emission spectral overlap. For QY less than 85%, mirrors that are optimized for transmission and trapping are comparable. For the remaining cases the mirror weighted toward incident light transmission is preferred. These mirrors all significantly outperform open top concentrators. We analyze the mirrors by tracking the loss processes inside the concentrator, revealing how changes in the mirror design influence the escape cone losses and reabsorption fractions during propagation.
We also design metasurface-based mirrors for the back of the LSC to be combined with the top mirror. These metasurface mirrors are designed to change the angle of a propagating photon upon reflection, guiding it more effectively toward the edge of the concentrator and away from steep angles that require multiple passes to reach the edge. We show that LSCs incorporating metamirrors can achieve high efficiency with lower QY nanocrystals.
This work provides a pathway to high performance LSCs through enhanced optical transport and improved trapping efficiency. By controlling the emission angle or trapping emitted photons we work to design LSCs that efficiently guide light to the edges, an important property needed for future commercialization.