InGaN/bivalent fluorescein salt luminescence conversion light-emitting diode: Stability and photochemical reaction

A light-emitting diode (LED) was fabricated using a commercial InGaN LED (460 nm) and bivalent fluorescein salt dispersed in an epoxy matrix, as a primary light source and a luminescence conversion (LUCO) material, respectively. The LUCO LED emitted white light, which faded almost completely within 1 h when a current of 20 mA was continuously supplied. Based on the FT-IR, UV-visible and XPS spectral data, a large portion of bivalent fluorescein salt in the epoxy matrix underwent photochemical reactions, generating neutral fluorescein on irradiation with blue light. However, when bivalent fluorescein salt was dispersed in poly(methyl methacrylate) instead of epoxy, the luminous output of the resulting LUCO LED decreased much more slowly during continuous operation. Furthermore, the chemical structure of bivalent fluorescein salt remained unaltered during irradiation with blue light when the freeze-dried salt was kept under high vacuum. It was concluded that bivalent fluorescein salt should be almost completely protected from the environment for application as a LUCO material when coupled with a blue LED.