The study introduces a novel laminated structure for luminescent solar concentrators (LSCs) that enhances their efficiency by using a patterned low-refractive-index medium as an optical 'guard rail' to guide photons with minimal loss. The design rules for LSC dimensions and fluorophore spectral characteristics are proposed, leading to record-high external quantum efficiencies (EQEs) of 45% for a 100 cm² area and 32% for an LSC with an edge aspect ratio of 7:1. The device efficiency reaches 7.6%, the highest reported value to date. This improvement is attributed to the reduction in reabsorption and scattering, achieved through the use of a nearly lossless propagation pathway. The study also highlights the importance of selecting appropriate fluorophores and dimensions, along with the use of a low-refractive-index medium, to ensure reliable optical performance for building-integrated photovoltaic solar windows. The findings have significant industrial implications and could accelerate the commercialization of LSCs.The study introduces a novel laminated structure for luminescent solar concentrators (LSCs) that enhances their efficiency by using a patterned low-refractive-index medium as an optical 'guard rail' to guide photons with minimal loss. The design rules for LSC dimensions and fluorophore spectral characteristics are proposed, leading to record-high external quantum efficiencies (EQEs) of 45% for a 100 cm² area and 32% for an LSC with an edge aspect ratio of 7:1. The device efficiency reaches 7.6%, the highest reported value to date. This improvement is attributed to the reduction in reabsorption and scattering, achieved through the use of a nearly lossless propagation pathway. The study also highlights the importance of selecting appropriate fluorophores and dimensions, along with the use of a low-refractive-index medium, to ensure reliable optical performance for building-integrated photovoltaic solar windows. The findings have significant industrial implications and could accelerate the commercialization of LSCs.