Influence of the Graded Index Cholesteric Liquid Crystal Reflectors on the Luminescent Solar Concentrator Efficiency

This article examines the impact of graded-index reflectors (GIRs) constructed from cholesteric liquid crystals (CLCs) on the modification of escape-cone loss and the improvement of luminescent solar concentrator (LSC) efficiency. GIRs are comprised of a structure in which the refractive index changes gradually with a spatial function. In this study, the LSC is a planar optical waveguide made of Poly (methyl methacrylate) (PMMA) with dimensions of 5×5×0.5 cm³ and contains CH₃NH₃PbBr₃ perovskite material as a fluorophore. Two types of GIRs are employed at the bottom of the LSC: periodic and quasi-periodic GIRs. Periodic reflectors (PRs) have a refractive index matrix that gradually changes with a periodic sinusoidal function, while the refractive index matrix of the quasi-periodic reflectors (QPRs) gradually changes with a chirped sinusoidal function. To analyze the models of the study, Monte-Carlo and Finite-Difference Time-Domain (FDTD) methods were utilized in conjunction with experimental results. The results indicated that the reflection band of a PR exhibited the maximum overlap (lowest escape cone) with the dye emission spectrum when the mirror’s reflection band underwent a redshift of 20 nm compared to the dye emission spectrum. On the other hand, the QPR generated a broader reflection band, resulting in complete overlap and higher efficiency. Moreover, GIRs enhanced sunlight absorption in the LSC by reflecting transmitted solar photons through it. The optical efficiency (OE) of the LSC increased by 12% (33%) once a periodic (quasi-periodic) reflector is utilized. Furthermore, GIRs reduced escape cone loss, thereby increasing reabsorption, and subsequently, the system selected a lower optimal concentration to minimize reabsorption losses.