Luminescent solar concentrators (LSC) are an attractive emerging concept for semi-transparent, building-integrated photovoltaics at low cost and weight. Hereby, large-area, luminescent wave-guide foils collect solar radiation, being harvested at the wave-guide’s edge by a small-size solar module. To further improve the performance of LSCs, emitter materials with photoluminescence quantum yields (PLQY) near-unity are required, as this minimizes losses due to photon re-absorption events. This presentation show-cases a highly sensitive approach to experimentally determine the PLQY of emitters, combining spectroscopic and photothermal techniques. Screening the PLQY of six emitter molecules in solution, we are able to measure a maximum value of 99.4% with an unprecedented precision down to ±0.3% – which is about ten times better than established techniques. This newly developed method will therefore contribute to the development of future highly efficient LSCs, which require emitters with extremely high PLQYs, well above 99%. We further show that such emitters can perform thermally assisted photon upconversion, illustrating their potential for optical refrigeration.
Several emerging applications relying on absorption and re-emission of light by molecular species require a photo-luminescence quantum yield (PLQY) near unity. However, when the PLQY approaches unity and the optical density is low, it is surprisingly difficult to measure accurately, as the most commonly used techniques rely either on exact knowledge of material parameters or on a comparison to a measurement of a non-fluorescent material, introducing errors in the measurement, further complicated by possible non-linearities of the detection system.
In this work, we propose a new setup which circumvents these issues. It involves a simultaneous relative measurement of the produced heat, the amount of re-emitted light and the average energy of the emitted photons, allowing us to determine the fraction of absorbed energy which is converted to heat. Crucial to the set-up is a tunable continuous-wave laser, allowing to perform these measurements at varying excitation energies. The photon energy dependent part of the fraction of absorbed energy converted to heat is due to an increased thermalization energy with increasing incident photon energy. The remaining heat produced by the fully thermalized excitation determines the fraction of absorbed energy not converted to photons and thus the deviation of the PLQY from unity. Using this new method, we determine the PLQY of seven different luminescent molecules in low concentration solutions, and use a rigorous error analysis to achieving PLQY measurements with an unprecedented accuracy down to ±0.3% within 95% confidence intervals. Perylene orange is found to have the highest PLQY of the molecules studied, being 99.0±0.3%.
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