Effect of luminescent nanocrystal alignment on fluorescence anisotropy and light guiding in polymer films

Elongated semiconductor nanocrystals with oriented transition dipoles, such as nanorods (NRs) and elongated nanoplatelets (NPLs), can exhibit emission anisotropy when confined in thin films. In this work, we report the effect of nanocrystal alignment on thin polymer film optical properties including absorbance, fluorescence polarisation and light guiding. We find that changes in absorbance after stretching films containing elongated nanocrystals are associated with changes in nanocrystal alignment. Stretching the NPL films induces fluorescence anisotropy, which follows an exponential trend that can be approximated as linear within a moderate range of true strain values. This empirical relationship provides a means to monitor strain in polymers undergoing deformation. The light guiding properties of polymer films containing luminescent nanocrystals are also investigated by applying experimental and Monte Carlo simulation methodologies developed for the luminescent solar concentrator field. We show that drop cast polymer films containing spherical quantum dots, with isotropic fluorescence, more effectively guide light to the film edges compared with elongated nanocrystals. The elongated nanocrystals are aligned in the film plane by both stretching and drop casting processes, resulting in increased fluorescence emission from the film faces due to escape cone losses. • Drop casting and stretching align nanocrystals towards the plane in polymer films. • Nanocrystals are more aligned in stretched films than in drop cast films. • Relationship between true strain and fluorescence anisotropy is exponential. • Alignment of nanocrystals in cast and stretched films reduces light guiding. • Effect of nanocrystal alignment on light guiding is confirmed by optical modelling.