Abstract
Persistent phosphors are a specific type of luminescent materials having the unique ability to emit light long after the excitation has ended. They are commonly used as emergency signage in near ideal, isothermal indoor situations. Recently, their energy storage capacity was relied on for outdoor situations, e.g. for glow-in-the-dark road marks and in combination with solar cells and photo catalytic processes. In this work the influence of temperature, illumination intensity and the duration of the night is critically evaluated on the performance of afterglow phosphors. The persistent luminescence of SrAl2O4:Eu,Dy green emitting phosphors is studied under realistic and idealized conditions. It is found that the light output profile is hardly influenced by the ambient temperature in a wide range. This is due to the presence of a broad trap depth distribution, which is beneficial to cover the longer and colder winter nights. Temperature drops during the night are however detrimental. For traffic applications, the total light output of glow-in-the-dark road marks at the end of the night is not sufficient for the studied compound, although re-charging by the car's headlamps partially alleviates this. For energy storage applications, the trap density should be improved and tunneling recombination processes might be needed to overcome overnight temperature drops.
Highlights
Luminescent materials, often called phosphors, have a long tradition in lighting, displays and imaging techniques
A specific type of luminescence is persistent luminescence, denoted as ‘glowin-the-dark’, where the emission can continue for hours after the excitation ends [6]
Powder X-ray diffraction (XRD) measurement confirmed that the powder is pure phase (ICSD No 26466) without traces of other crystalline products
Summary
Luminescent materials, often called phosphors, have a long tradition in lighting, displays and imaging techniques. The first reports in Europe date back to the early seventeenth century, describing a material presumed to be copper doped barium sulfide [7], the main applications emerged with the introduction of ZnS:Cu,Co as green persistent phosphor and especially the much brighter SrAl2O4:Eu2+,Dy3+ in 1994 [8]. These phosphors are nowadays commonly found in watch dials, toys and gadgets, their main application area is safety signage, e.g. as guiding strips in the aisle of an airplane and as emergency exit signage in ferries and buildings. The light intensity decreases quickly, initially following an exponential decay, the simultaneous dark adaptation of the human eye leads to an almost constant light perception in the first hour, with even an initial increase in perceived light intensity [9]
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