(775g) Photonic Mirrors for Enhanced Optical Transport in Luminescent Solar Concentrators

Authors: 
Connell, R., University of Minnesota
Ferry, V. E., University of Minnesota
Puri, M., University of Minnesota, Twin Cities
Luminescent solar concentrators (LSCs) increase the performance of solar cells by concentrating both direct and diffuse sunlight onto small photovoltaic devices. A typical LSC consists of a large plastic sheet embedded with luminophores such as dye molecules or luminescent nanocrystals. Incident sunlight is absorbed by the luminophore and emitted at a longer wavelength. Emitted light propagates to the edges of the plastic sheet via total internal reflection, and a small photovoltaic cell is mounted at the concentrator edge to collect the guided light. The solar cell design can then be optimized for a spectrally narrow and focused light source. Moreover, these colorful LSCs form components of architectural materials and may serve as window replacements.

Nanocrystal luminophores have recently emerged as candidate luminophores for LSCs, offering improved tunability, the ability to engineer the Stokes shift, narrow emission bandwidths, and high quantum yields. Nevertheless, challenges remain for using these luminophores in LSCs: the nanocrystals aggregate or suffer from reduced quantum yields when incorporated into polymer matrices, and non-Cd based materials with broader emission bands are needed for architectural applications.

In this work we have created LSCs based on the combination of large Stokes shift nanocrystals with various photonic light-guiding designs. We study how the combination of different types of photonic mirrors, integrated onto the front, back, or throughout the LSC structure, influences the overall concentration factor. By using different types of photonic mirrors, we show how these designs can overcome luminophore non-idealities such as reduced quantum yields or reabsorption losses. We use a combination of ray-tracing Monte Carlo simulations and full-field electromagnetic calculations to design the mirrors in these LSCs and to separately track losses to the escape cone and reabsorption.

To integrate photonic mirrors throughout the LSC structure we design a photonic LSC consisting of a periodic stack of polymer and titania layers with semiconductor nanocrystals embedded in one set of the layers. This creates a photonic bandgap, or a spectral region where light is forbidden. The photonic bandgap can be tuned via the thickness of the layers to match the emission of the luminophore. Using this design, we predict a decrease in escape cone loss from 41% to 5% for luminophores embedded in the center polymer layer that emit at 1.11 times the photonic band gap of the LSC. A decrease in escape cone loss from 20% to 6% is also predicted for luminophores embedded in the center titania layer that emit at 1.15 times the photonic band gap of the LSC. We find that it is preferential to embed nanocrystals with significant reabsorption losses in the high index layer, whereas nanocrystals with low reabsorption losses should be embedded in the low index layer. This is due to the high concentration of emitted radiation that propagates through the polymer layer when the luminophore is embedded in the polymer or the titania layers. Although these systems were designed using the optical properties of CdSe/CdS nanocrystals, the results are extendable to other types of nanocrystals with broader emission spectra.

These theoretical predictions are also corroborated by experimental measurements of optical transport inside LSCs. CdSe/CdS nanocrystals with varying shell thickness are incorporated into different polymers, including poly (lauryl methacrylate). Optical spectroscopy measurements of the radiative lifetime indicate that the materials are incorporated without significant changes in the optical quality. Transport measurements indicate good agreement with the predictions from simulation.

This work provides a pathway to high performance LSCs through enhanced optical transport. By controlling the emission angle we work to experimentally design LSCs with large collection areas that efficiently guide light to the edges, thus increasing the optical efficiency and the concentration factor of the LSC.