Reducing reabsorption in luminescent solar concentrators with a self-assembling polymer matrix

Abstract

Reabsorption in luminescent solar concentrators (LSC) is a major factor hindering their commercialisation. This issue is caused by the inherently small Stokes shift of luminophores which creates a large spectral overlap between absorption and emission, leading to reabsorption of waveguided photons. As reabsorption events amplify loss mechanisms such as escape cone losses and non-radiative emission, they are the most detrimental to photon output. Reducing reabsorption can be achieved by decreasing the amount of luminophore-doped waveguide compared to the overall LSC, resulting in an increase in research of both thin film LSCs, and new methods of templating or etching polymer thin films to reduce lateral reabsorption. Here we use self-assembly to alter the structure of a common host matrix, poly (methylmethacrylate) (PMMA) by the addition of a co-polymer, polystyrene-block-poly (acrylic acid) (PS-b-PAA); when in a 1:1 PMMA:PS-b-PAA ratio this creates holes of ~1.6 μm circumference within the polymer matrix, thus providing a simple way to reduce the volume of the luminophore doped host matrix. While quantifying the trade-offs associated with different LSC fabrications and surface effects, we show that the alteration of the host matrix with the PS-b-PAA results in a close to halving of the total number of reabsorption events (PS-b-PAA = 4.4; PMMA = 4.0; 1:1 = 1.8). Device photoluminescence efficiency increases by 10% compared to single component polymer matrices (PS-b-PAA = 76%; PMMA = 77%; 1:1 = 87%), though there is an increase in detrimental outcoupling also linked to this. We also investigated the following polymer ratios of PMMA:PS-b 1:1, 1:2, 1:3, 3:1, 2:1 and found that the ideal ratio to maximise LSC performance is the 1:1 ratio. These trade-offs and new design strategy will aid future efforts in device optimisation.