Warm-white light emitting diodes require a red emitting phosphor to provide sufficient emission in the red spectral region. Currently, Eu²⁺ activated nitrides and Mn⁴⁺ activated fluorides are state-of-the-art compounds [1,2]. Both materials have significant disadvantages: Divalent europium in nitrides suffers from a broad emission band that reaches into the NIR which strongly decreases the luminous efficacy. The synthesis of fluoride hosts for Mn⁴⁺ involves toxic hydrofluoric acid and their chemical stability is limited due to the oxidative properties of Mn⁴⁺ towards F^-. Thus, Eu³⁺ presents an interesting alternative. Very high luminous efficacy and excellent thermal and chemical stability can be achieved in easily accessible oxide hosts [3]. However, the low absorption cross section in the UV-B to visible spectral range hinders the application of Eu³⁺ in warm-white LEDs. Approaches to overcome this limitation include, among others, excitation via charge transfer bands [4], sensitization with Tb³⁺ [5], or the use of Ce/Tb comprising core/shell materials [6]. Nonetheless, there have been no reports of successful implementation of a Eu³⁺ activated phosphor on a blue emitting LED chip so far.

As a novel approach we chose the uranyl cation to sensitize Eu³⁺. While the energy transfer from uranyl to Eu³⁺ is well known, it has not been utilized thus far to enable Eu³⁺ activated red emitting LED phosphors. One reason might be that uranium is usually perceived as a health hazard for its toxicity and radioactivity. However, the dose received from ingestion of 10 mg of depleted uranium (DU) amounts to merely 1 µSv (ICRP 72), which is lower than the dose received daily through natural radiation. Consequently, to date it has not been conclusively shown if DU is a carcinogen [7,8]. With an assumed LD₅₀ of 5 g in humans, the chemical toxicity is comparable to that of cadmium which is used in quantum dots [9]. DU would be present on an LED in minute amounts of approximately 0.1 mg per chip.

The number of suitable hosts, i.e. those exhibiting both a doping site for Eu³⁺ and comprising the uranyl cation, is comparatively low. We studied two different compounds, Eu(UO₂)₃(PO₄)₂O(OH)∙xH₂O and K₄Eu₂(UO₂)(Ge₂O₇)₂. The phosphate suffers from quenching due to the presence of H₂O and OH moieties. Exchange of H⁺ by D⁺ was successfully attempted to alleviate this problem [10]. The germanate possesses excellent properties, allowing us to manufacture a warm-white LED employing K₄Eu₂(UO₂)(Ge₂O₇)₂ as the red emitter. The LED shows an excellent luminous efficacy and color rendering index at a correlated color temperature of 2700 K. To the best of our knowledge this is the first LED comprising a blue-emitting chip and a red emitting Eu³⁺ phosphor [11].

References

[1] Chem. Rev. 118 (2018) 1951
[2] https://www.ge.com/reports/spreading-light-ge-ventures-finding-new-partners-ge-intellectual-property/
[3] J. Mater. Chem. C 3 (2015) 2054
[4] ECS J. Solid State Sci. Technol. 2 (2013) R3153
[5] J. Lumin. 189 (2017) 71
[6] J. Lumin. 203 (2018) 467
[7] Int. J. Environ. Res. Public Health 7 (2010) 303
[8] Environ. Res. 156 (2017) 665
[9] Health Phys. 94 (2008) 170
[10] J. Lumin. 196 (2018) 431
[11] J. Mater. Chem. C 6 (2018) 6966