Importance of Evaluating the Intensity Dependency of the Quantum Efficiency: Impact on LEDs and Persistent Phosphors

Abstract

The quantum efficiency is a key metric in lighting technology and for the quantification of luminescent processes, indicating how many photons are emitted with respect to the number of absorbed photons. Ideally, this value should approach unity to reduce losses, for instance in the common phosphor converted white LEDs. In this work we demonstrate that in luminescent materials where energy can be stored at defect centers, like in the extreme case of persistent phosphors, the quantum efficiency depends on the excitation intensity. For the green emitting SrAl 2 O 4 :Eu 2+ ,Dy 3+ , which has been proposed for use in AC-LEDs, the internal quantum efficiency drops for increasing excitation intensity from 71% to 54%. At elevated excitation intensities, as encountered in LEDs, the trapped charge carriers can be optically detrapped by the excitation light, leading to this strong reduction of the overall quantum efficiency. Considering that the absorption cross section for this process is 6−29× larger than the absorption cross section for the luminescent ion, the efficiency of LED phosphors can be increased by avoiding the presence of defects acting as trapping centers. Finally, designing persistent phosphors with defects that show a limited optical response for the excitation light could strongly increase their energy storage capacity.

Spectroscopic Study of a Single Crystal of SrAl2O4:Eu2+:Dy3+

“…The later observations reveal that the presence of dysprosium has a strong effect not only in the duration of the phosphorescence but also in the QY of the material. Very recently, Van der Heggen et al 37 have reported that the "trapped charge carriers can be optically detrapped by the excitation light", which implies that these trapped charge carriers equally absorb very strongly around 350-450 nm. Such influence can be only understood considering a high proportion of europium and dysprosium sites participating in the long persistent phosphorescence mechanism, that is to say, a high number of europium sites being excited.…”

“…The highest QY values of the same compound have been previously stablished as 55 %, 39 although higher values have been recently observed in the case of cage-like microspheres of SrAl2O4:Eu 2+ up to 95% and 94% after excitation at 365 nm and 460 nm respectively, 40 and 28% and 71% in the case of SrAl2O4:Eu 2+ :Dy 3+ powder samples after excitation at 375 nm and 445 nm respectively at very low excitation intensities. 37 In order to verify the reliability of the values obtained with the integrating sphere, the internal QY of the SrAl2O4: Eu 2+ (1%) powder samples has been estimated through lifetimes measurements of the green site emission at 10 K and 300 K after irradiation at 370 nm. From reference 9 lifetimes of 1190 ns and 408 ns were obtained for the mentioned samples at 3 K and 300 K, respectively.…”

Insights into the complexity of the excited states of Eu-doped luminescent materials

“…), the origin of these so-called anomalous emissions is typically not explained, or only hypothesized, without valid experimental or theoretical justification. Also, there is a long-lasting question about the mechanism causing the persistent luminescence (afterglow) in many Eu-based phosphors, where it is very difficult to probe the exact charge-carrier dynamics or the presence and role of intrinsic and extrinsic crystal defects by existing experimental techniques [33][34][35] .…”

Microwave-assisted synthesis followed by a reduction step: making persistent phosphors with a large storage capacity

“…On the contrary, the absorption ( , ) is increasing considerably when the co-dopant concentration is increased. The reasons on the decrease in the quantum yield values in the MASS-obtained materials is not entirely clear, however, two main reasons can be cited: i) with the increase of the concentration of the Dy 3+ co-dopants a higher number of process can be active, such as optical stimulated luminescence, which can reduce the quantum efficiency because it's a two-forone process, 50 ii) microwave irradiation increase the diffusion of the ions that enhance the incorporation of Dy 3+ , which subsequently creates more defects via charge compensation effects; Hence, the defects can act as killing center after the absorption of the incident light, and iii) The presence of Eu 2+ -Eu 3+ pairs can act as quenching centers via intervalence charge transfer transition, as mentioned previously. The combination of these effects corroborate with the increase of the light absorption contribution and the hindering the quantum yield.…”