Investigation of Symmetric and Nonsymmetric Dithienothiophene-dioxide Derivatives as Fluorescent Emitters for Luminescent Solar Concentrators

Four conjugated donor–acceptor compounds featuring a central dithieno[3,2-b:2′,3′-d]thiophene-4,4′-dioxide (do-DTT) core, with either a symmetric (DTT-H2, O2, S2) or nonsymmetric (DTT-H1) structure, have been designed based on DFT computational investigations and prepared using direct arylation reactions as the key C–C bond-forming steps. Spectroscopic analysis of the compounds in solution, carried out with both stationary and time-resolved techniques, confirmed that they have properties compatible with application as fluorescent emitters in Luminescent Solar Concentrators (LSCs). Accordingly, their performances were initially screened in thin-film LSCs employing poly(methyl methacrylate) (PMMA) as host matrix. The devices fabricated with the emitter DTT-S2, featuring thiomethyl-substituted donor groups, appeared very promising, with a good external photon efficiency (η_ext) of up to 5.6%, accompanied by a notable internal photon efficiency (η_int) of up to 43%. Due to these favorable characteristics, this compound was selected as the emitter for PMMA-based slab LSC devices (5 × 5 × 0.3 cm³) fabricated using regenerated MMA. Remarkably, a η_ext of 6.7% was reached together with a fluorescence quantum yield (Φ_fl) of 90%, which resulted in a device efficiency of 0.74% once the LSC was coupled with a Si–PV cell. In addition, a preliminary stability assessment of the doped slabs, conducted by an accelerated protocol, provided encouraging results, with the nonsymmetric emitter DTT-H1 being able to retain >90% of its initial emission intensity after 960 h of simulated time.