Recyclable Phosphor Films: Three Water-Soluble Binder Systems Enabling the Recovery of Phosphor Powders in White LEDs

Abstract

A recyclable luminescence down-conversion film as utilised in commercial white light-emitting diodes (LEDs) is introduced to avoid waste of valuable materials such as rare-earth metal-containing phosphors. As proof of principle, the commercial phosphor Y3Al5O12:Ce3+ is embedded in three easy soluble binders instead of the commonly utilised non-recyclable silicone binder. It will be demonstrated that these phosphor films allow for a highly efficient reuse of the phosphor. The investigated binders are, first, soluble sodium silicates (water glass) mixed with water in a ratio of 1:3, second, a 1 wt.%/vol.% solution of hydroxyethyl cellulose (HEC) in a 1:1 mixture of water and ethanol and, third, a 5 wt.%/vol.% solution of polyvinyl alcohol (PVA) in water. The phosphor-containing films show the same quality as comparable state-of-the-art phosphor converter films as demonstrated by preparation of fully functional white surface-mount device (SMD) LEDs based on commercially available blue SMD LED chips. It is demonstrated that the converter films can be recycled by dissolving the films in water at room temperature for HEC and PVA and at 60°C for the sodium silicates. Subsequently, the phosphor is reclaimed by sedimentation. The average recycling rates are 98.7 wt.% for sodium silicates, 95.6 wt.% for HEC and 98.0 wt.% for PVA. The phosphor does not suffer any losses of quality or functionality during this process as shown by fluorescence spectroscopy.