Covalent Functionalization of Polyhydroxyurethanes with Fluorescent Groups for Colourless Luminescent Solar Concentrators

Abstract

Luminescent solar concentrators (LSCs) are attractive for building-integrated photovoltaics but require host matrices combining high transparency, stability, and sustainability. Here, bio-based, isocyanate-free polyhydroxyurethanes (PHUs) are introduced as covalently functional host materials for transparent LSCs. Two aromatic fluorescent aldehydes based on fluorene and 8-hydroxycoumarin were synthesised and grafted onto a tailor-made PHU backbone via a facile post-synthetic modification strategy, yielding PHU1-Flu and PHU1-Coum with high degrees of functionalization (up to 80%). The resulting polymers were directly processed into thin-film LSCs by spin coating, acting simultaneously as host matrix and photoactive component. The devices exhibited excellent optical transparency and colour neutrality, with an average visible transmittance of ~90%, comparable to clear glass. Among the two systems, PHU1-Flu showed superior performance, achieving external and internal photon efficiencies of 2.19% and 37.27%, respectively, a device efficiency of 0.34%, and a light utilisation efficiency of 0.30%. By covalently immobilising the luminophores, PHU-based LSCs inherently suppress dye migration and phase separation, addressing key stability limitations of conventional systems. These results demonstrate that bio-based PHUs represent not only sustainable but also functionally superior host matrices for durable and transparent LSCs.