Phosphors for solid-state lighting: New systems, deeper understanding

Author(s): Denault, Kristin Ashley | Advisor(s): Seshadri, Ram | Abstract: Phosphor materials are a crucial component in energy efficient lighting with the ability to affect overall device performance. The structure and crystal chemistry of these materials greatly dictate the resulting optical properties. Understanding the relationship between structural and optical properties in phosphors allows for insights into the methods for developing materials with targeted properties. Here, we explore the structure-composition-property relationships in phosphor materials using a multitude of structural and optical characterization methods including high resolution synchrotron X-ray and neutron powder diffraction and total scattering, low-temperature heat capacity, temperature- and time-resolved photoluminescence, and density functional theory calculations. We describe the development of several new phosphor compositions and provide an in-depth description of the structural and optical properties. We show structural origins of improved thermal performance of photoluminescence and methods for determining structural rigidity in phosphor hosts that may lead to improved luminescent properties. New white light generation strategies are also explored including quantum dots for improved color rendition and laser-based lighting for efficient and thermally stable high-power lighting.We begin by presenting the development of a green-yellow emitting oxyfluoride solid-solution phosphor Sr2Ba(AlO4F)1-x(SiO5)x:Ce3+. An examination of the host lattice, and the local structure around the Ce3+ activator ions points to how chemical substitutions play a crucial role in tuning the optical properties of the phosphor. The emission wavelength can be tuned from green to yellow by tuning the composition, x. Photoluminescent quantum yield is determined to be 70±5% for some of the examples in the series with excellent thermal properties. Phosphor-converted LED devices are fabricated using an InGaN LED and are shown to exhibit high color rendering white light. Next, we identify two new phosphor solid-solution systems, (Ba1-xSrx)9Sc2Si6O24:Ce3+,Li+ and Ba9(Y1-yScy)2Si6O24:Ce3+. The substitution of Sr for Ba in (Ba1-xSrx)9Sc2Si6O24:Ce3+,Li+results in a decrease of the alkaline earth--oxygen bond distances at all three crystallographic sites, leading to changes in optical properties. The room temperature photoluminescent measurements show the structure has three excitation peaks corresponding to Ce3+ occupying the three independent alkaline earth sites. The emission of (Ba1-xSrx)9Sc2Si6O24:Ce3+,Li+ is red-shifted from the near-UV to blue with compositional changes. The red-shifted photoluminescent quantum yield also increases when Sr is substituted for Ba in these compounds. The end member Ba9Y2Si6O24:Ce3+ was identified as an efficient blue-green phosphor with high thermal stability of the luminescence, viable for near-UV LED excitation. An efficient emission, with a quantum yield of 60%, covers a broad portion of the visible spectrum leading to the observed blue-green color. The emission of this compound can be red-shifted via the solid- Ba9(Y1-yScy)2Si6O24:Ce3+ allowing for tunable color properties when device integration is considered.We then explore the structure--composition relationships and optical properties in newly developed cerium-substituted (Sr,Ba)3(Y,La)(BO3)3 borate phosphors. Examination of the coordination environment of the Ce3+ active site polyhedra coupled with low-temperature photoluminescence reveals three distinct excitation bands corresponding to Ce3+ located on three distinct crystallographic sites. Comparing the position of these excitation bands with crystal field splitting effects due to changes in polyhedral volumes and distortions suggests an assignment of the three excitation bands. These compounds are efficiently excited by UV light with blue emission, the most efficient compound determined to be Sr3La(BO3)3:Ce3+,Na+ with a quantum yield of 50%. A data-driven discovery of energy materials then reveals the efficient BaM2Si3O10:Eu2+ (M = Sc, Lu) phosphors with UV-to-blue and UV-to-blue-green phosphors. Interestingly, substituting Eu2+ in the Lu3+ containing material produces two emission peaks, at low temperature, as allowed by two substitution sites. The photoluminescence of the Sc3+ compound is robust at high temperature, while the Lu-analogue has a large decrease of its room temperature intensity. The decrease in emission intensity is explained as stemming from charge transfer quenching due to the short distances separating the luminescent centers on the Lu3+ substitution site. The correlation between structure and optical response in these two compounds indicates that even though the structures are three-dimensionally connected, high symmetry is required to prevent structural distortions that could impact photoluminescence.Next, the consequences of optimal bond valence on structural rigidity are explored and linked to the improved luminescence properties in SrxBa2-xSiO4:Eu2+ orthosilicate phosphors. We observe that in the intermediate compositions, the two cation sites in the crystal structure are optimally bonded as determined from bond valence sum calculations. Optimal bonding results in a more rigid crystal, as established by the intermediate compositions possessing the highest Debye temperature, determined experimentally from low-temperature heat capacity measurements. Greater rigidity, in turn, results in high luminescence efficiency for intermediate compositions at elevated temperatures.We then conduct an in-depth analysis of the average and local structure, Debye temperature, and structural rigidity in oxide phosphor host materials. The average and local structure of the oxides Ba2SiO4, BaAl2O4, SrAl2O4, and Y2SiO5 are examined in order to evaluate crystal rigidity in light of recent studies suggesting that highly connected and rigid structures yield the best phosphor hosts. Simultaneous momentum-space refinements of synchrotron X-ray and neutron scattering yield accurate average crystal structures, with reliable atomic displacement parameters. The Debye temperature, which has proven to be a useful proxy for structural rigidity, is extracted from the experimental atomic displacement parameters and compared with predictions from density functional theory calculations and experimental low-temperature heat capacity measurements. The role of static disorder on the measured displacement parameters, and the resulting Debye temperatures, are also analyzed using pair distribution function analysis of total neutron scattering, as refined over varying distance ranges of the pair distribution function. The interplay between optimal bonding in the structure, structural rigidity, and correlated motion in these structures is examined, and the different contributions are delineated.Finally, new light generation strategies including quantum dots and laser-based lighting are explored.