(Invited) Investigation of Blue Excitation Flux Dependent Conversion Efficiency in Phosphors for High Luminance Applications

Abstract

Solid State Lighting (SSL) devices based on phosphor conversion have significantly evolved on a scale of output flux and white light luminance and extend now to the range where competition with the brightest arc discharge-based sources is ongoing. Such sources are unique in the SSL world in that they provide both an expanded emission spectrum and high luminance. The efficiency of high luminance semiconductor pumped phosphor sources depends not only on efficient generation of primary radiation from the semiconductor (LEDs or laser diode), but also strongly on the conversion material which is being pushed to optical and thermal limits. Such conversion materials are typically operating under heavy irradiance and thermal load and nearly always exhibit sublinear behavior in output at high input fluxes, due to a multitude of reasons. In the present study, we have separated contributions from thermal and non-thermal quenching mechanisms, in particular investigated phosphor saturation, or excitation quenching, in the converter light output as a function of blue pump intensity. It is important here to distinguish between the true loss mechanisms (for example via Auger-like or excited state absorption processes [1-6]) and the reduction in converted output due to increased transparency for the pump light (e.g. activator ground state depletion). We discuss the behavior of measured conversion efficiency as a function of excitation intensity (and parameters like temperature, activator type and concentration or host material). A model is proposed to explain the observed dependencies in different host-activator systems. [1] W.J. Miniscalco et al, J.Appl.Phys., 49, 6109 (1978)[2] D.S. Hamilton et al, Phys.Rev.B 39(13), 8807 (1989) [3] K. Wisniewski et al, Acta Phys. Polonica A, 95(3), 403 (1999)[4] K.C. Mishra et al, J.Electrochem.Soc. 151(5), H105 (2004)[5] A. Lenef et al, SPIE 884107 (2013) [6] O.B. Shchekin et al, Phys.Status Solidi RPL 1-5 (2016)