Impact of Air Gap Length and Waveguide Refractive Index on Luminescent Solar Concentrator Optical Behavior

Abstract

The optical properties of luminescent solar concentrators (LSCs) containing CdSe/CdS core/shell nanocrystal luminophores in a matrix with a high index of refraction were characterized using UV-Vis spectroscopy, photoluminescence (PL), and PL lifetime measurements. The high refractive index matrix was realized by spin-coating solutions containing polyvinylpyrrolidone (PVP), titania, butanol, and luminophores onto glass substrates. The proportion of titania in the solutions was varied in order to change the refractive indices of the resulting samples. The transmission and reflection spectra of the samples showed evidence of thin film oscillations, known as Fabry-Perot modes, that increased in magnitude with the amount of titania in the sample, suggesting that increasing the amount of titania in the matrix of a sample likely increased its refractive index. The PL peaks of the luminophore blue-shifted from 630 to 627 nm as the concentration of titania increased. The PL lifetime of these samples was found to be 1.8-2.7 ns, with an outlier of 0.5 ns for one sample. Complementary Monte Carlo ray tracing simulations were used to explore the impact of a variable thickness air gap on the optical efficiency of an LSC system. It was found that an air gap of 0.05 mm reduced the peak optical efficiency by 14.6% as compared to a system with no air gap. This decrease is attributed to escape cone loss pathway as photons couple out the air gap as opposed to another loss pathway. Therefore, it is suggested the thickness of an adhesive layer between the waveguide and solar cells should be no greater than 0.05 mm to preserve the optical performance of the device.