On the Photoluminescence Linearity of Eu²⁺Based LED Phosphors upon High Excitation Density

This work deals with the saturation behavior of Eu2+ doped luminescent materials. Eu2+ activated phosphors, excited by high radiance pump sources, like high power LEDs or Laser Diodes, offer considerable potential for high radiance conversion in Solid State Lighting. Remarkably, theoretical arguments suggest that the radiance of the luminescent spot should increase linearly with the excitation density of the incoming light source up to 1 kW/mm2. In practice, however, thermal quenching and (non-thermal) optical saturation limit the maximum attainable radiance of the luminescent material. We present experimental data of the widely applied red emitting LED phosphors (Ca,Ba)2SiO4:Eu2+ (OSE), Ba2Si5N8:Eu2+ (258:Eu), and CaAlSiN3:Eu2+ (CASN:Eu) in which these limits have been investigated. High excitation fluxes are achieved using laser pumping at 405 nm up to 800 W/mm2. In case of 258:Eu and CASN:Eu, optical pumping intensities have been shown to produce a significant efficiency depreciation already at low pump powers, which does not correlate with the relatively short Eu2+ luminescence lifetime (∼1 μs). We assume a de-excitation mechanism that depletes the excited state [Xe]4f 65d1 to the conduction band of the host structure, which bottlenecks the limit maximum pump intensities. We find that the investigated Eu2+ phosphors tend to show different efficiency depreciation. Finally, we use a rate equation model to simulate this nonlinear optical pumping.