Excited-state intramolecular proton transfer derivatives as self-absorption free luminophores for luminescent solar concentrators†

Luminescent solar concentrators (LSCs) have garnered considerable attention for their potential to enhance solar energy harvesting in photovoltaic (PV) systems. However, self-absorption often hinders their efficiency, caused by the overlap between the absorption and emission spectra. Herein, we design, synthesize, and study a series of novel excited-state intramolecular proton transfer (ESIPT) dyes as a new class of self-absorption-free luminophores for efficient transparent LSC-PV devices. HBTM, HBTPM, and HBTBP dyes comprise 2-(benzo[d]thiazol-2-yl)phenol as an electron-donating ESIPT unit functionalized with different π–acceptor moieties of ((3-hexylthiophen-2-yl)methylene)malononitrile, (4-(3-hexylthiophen-2-yl)benzylidene)malononitrile, and (4-(3-hexylthiophen-2-yl)phenyl)(phenyl)methanone, respectively. Theoretical and photophysical analyses confirm the ESIPT nature of these dyes. They show absorption in the UV-blue region and orange-red emissions with large Stokes shifts (4388–10269 cm⁻¹) and decent fluorescence quantum yields (28–47%). Their LSC samples are well prepared by dispersion in a transparent polymethyl methacrylate (PMMA) matrix. The LSC slabs possess good photophysical properties of the dyes with minimal overlap integrals (OI*) of 0.28–1.56% and edge emission efficiencies (η_edge) of 47–57%. Photovoltaic performance assessments reveal power conversion efficiencies (PCE) of 0.46% to 0.68% with external photon efficiencies (η_ext) of 7.69% for HBTM, 6.91% for HBTPM, and 2.98% for HBTBP. Particularly, HBTBP-based LSC exhibits excellent transparency (AVT = 93%; CRI = 97) suitable for window applications. This work represents a significant step toward reducing self-absorption in LSCs while improving photovoltaic performance, paving the way for scalable solar concentrator technologies based on organic materials.