Prof. Dr. D. Poelman, Ghent University, Belgium
Stretching the wave: the quest for long-wavelength phosphors for displays, lighting and medical imaging
Most of today’s high efficiency white LEDs are based on a blue-emitting LED chip, combined with one or more phosphor materials. In order to achieve the high colour quality necessary for home lighting, new phosphors should be developed having suitable emission in the red part of the spectrum, thus decreasing the CCT (correlated colour temperature) and increasing the CRI (colour rendering index) of the white light. We will discuss the requirements for this class of materials for both displays and lighting: they have to combine a sufficiently long dominant wavelength with a minimal fraction of the emission beyond 650 nm, where the eye sensitivity is low. Materials include both Eu-doped and Mn-doped compounds. Using Eu, the challenge is to stretch the emission to a sufficiently long wavelength, while using Mn, efforts are needed to tune the emission to a sufficiently short wavelength.
For some applications, such as in vivo medical imaging, even longer wavelengths are needed, typically in the range between 650 and 950 nm, in the so-called first tissue transparency window. We will evaluate the possible use of Cr3+ or Mn4+ as dopants for this application. In addition, we will try to turn these phosphors into persistent luminescent emitters, showing emission up to hours after the excitation has ended. Such long afterglow materials open op exciting possibilities for medical imaging, avoiding many of the drawbacks of commonly used imaging methods.