Enhanced Lifetime of Cyanine Salts in Dilute Matrix Luminescent Solar Concentrators via Counterion Tuning

Organic luminophores offer great potential for energy harvesting and light emission due to tunable spectral properties, strong luminescence, high solubility, and excellent wavelength selectivity. To realize their full potential, the lifetimes of luminophores must extend to many years under illumination. Many organic luminophores, however, have a tendency to degrade and undergo rapid photobleaching, leading to the perception of intrinsic instability of organic molecules. In this work, we demonstrate that by exchanging the counterion of a heptamethine cyanine salt the photostability and corresponding lifetime of dilute cyanine salts can be enhanced by orders of magnitude from 10 h to an extrapolated lifetime of greater than 65,000 h under illumination. To help correlate and comprehend the underlying mechanism behind this phenomenon, the water contact angle and binding energy of each pairing were measured and calculated. We find that increased water contact angle, and therefore increasing hydrophobicity, generally correlates to improved lifetimes. Similarly, a lower absolute binding energy between cation and anion correlates to increased lifetimes. Utilizing the binding energy formalism, we predict the stability of a new anion and experimentally verify it with good consistency. Moving forward, these factors could be used to rapidly screen and identify highly photostable organic luminophore salt systems for a range of energy harvesting and light-emitting applications.