(Invited) Red Fluoride Phosphors: A Story of Reliability
Authors: Verstraete Reinert , Sijbom Heleen F. , Joos Jonas J , Korthout Katleen , Detavernier Christophe , Smet Philippe F , Poelman Dirk 
Published: February 2020

Ever since the introduction of the high-brightness blue LED in 1994 [1], research on photoluminescent materials has gained tremendous interest. The combined emission from a blue emitting LED chip and a yellow emitting phosphor yields white light. The first white LEDs (wLEDs) were based on an (In,Ga)N LED chip and the efficient, chemically stable yellow phosphor Y3Al5O12:Ce3+. Despite the high luminous efficacy of the radiation (LER) attainable with this single phosphor wLED, the performance in terms of color rendering is low due to a lack of red emission. Improvement of the color rendering index (CRI) requires an additional emitter in the red spectral region. Therefore, tremendous effort has been spent on the development of red emitting phosphor materials. Red sulfide phosphors such as (Ca,Sr)S:Eu2+ [2] were first investigated. However, the poor chemical stability and strong thermal quenching associated with sulfides led research to new phosphor classes. Due to their high chemical stability and high quantum efficiency, Eu2+ nitride phosphors [3] such as CaAlSiN3:Eu2+ are currently considered as the most suited candidate for wLED applications. Nevertheless, even the Eu2+ doped nitrides possess serious drawbacks. Apart from their higher production cost, they often suffer from broad excitation- and emission bands. Excitation bands extending beyond 500 nm cause reabsorption issues when used in a phosphor blend together with green or yellow phosphors. On the other hand, broad emission bands partially reaching above 650 nm are inefficient as the human eye sensitivity becomes negligible in this spectral region.
Several red fluoride phosphors show a saturated red emission band below 650 nm based on the photoluminescence of Mn4+ in an octahedral fluorine coordination. In addition to their optimal optical performance, the low-cost production by means of wet chemical synthesis is an additional benefit. However, trace impurities in the form of impurity phases and secondary Mn valences often occur after synthesis. The present study reveals a strong correlation between purity after synthesis and stability of K2SiF6:Mn4+, a benchmark red fluoride phosphor [4-6]. Recently a two-step precipitation method has gained interest to improve the Mn valence related issue. K2MnF6, is first synthesized as a precursor material, incorporating Mn4+ in a [MnF6]2- coordination complex. In a second step, these complexes are incorporated in a fluoride host by dropwise addition of K2MnF6/KF in 40% HF and SiO2 in 40% HF. Although the chemical yield is higher compared to other chemical routes, such as wet chemical etching, and some improvements in stabilizing the Mn4+ valence state can be expected, careful examination of the purity proves necessary.
Combined feedback from in- and ex-situ X-ray diffraction, diffuse reflection spectroscopy, X-ray absorption near edge spectroscopy, thermal gravimetric and differential thermal analysis revealed the presence of impurities after synthesis of K2SiF6:Mn4+. On the one hand, hydrated secondary crystalline phases, such as KMnF4•H2O, K2MnF5•H2O and similar hydrated structures, are easily formed during synthesis. As a result, parasitic absorption due to Mn3+ occurs which limits the attainable quantum efficiency of the as-synthesized impure phosphor. On the other hand, impurities such as KHF2 severely increase the hygroscopic behavior of the phosphor after synthesis. KHF2 is easily hydrolyzed, leading to hydrated KF, an impurity hardly detectable by XRD at room temperature. As a result of water absorption in the impure phosphor, a further reduction of Mn4+ to Mn3+ is observed when the phosphor is placed in high humidity. Nevertheless, detection, identification and elimination of these impurities lead to efficient and high purity K2SiF6:Mn4+, showing good chemical and thermal stability. To further enhance stability, thin protective Al2O3, TiO2 or SiO2 shells were coated using thermally assisted, plasma enhanced or ozone enhanced Atomic Layer Deposition (ALD) and subsequently evaluated.

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