WEB Efficient Transparent nanophosphor films for visible light generationTuesday (22.09.2020) 11:50 - 12:05 P: Processing and Synthesis 1 Part of:
Light-emitting coatings based on rare-earth (RE)-doped phosphors are the key ingredients in recent light-emitting devices. Their robustness lies in the high chemical and thermal stability of their optical performance, which makes them almost insensitive to temperature or environmental changes. The canonical configuration found in most luminaries of this kind consists of an electroluminescent blue-light-emitting diode (LED) used to photoexcite a thick coating of cerium-doped yttrium aluminum garnet (Ce-YAG) crystals, which emits a broad band in the green-to-red region of the visible spectrum. Another approach to prepare transparent phosphor layers is based on the use of RE nanocrystals or nanophosphors. However, transparent nanophosphor films generally feature low brightness and low quantum yield (QY) because of the small absorption cross section of RE cations and the large surface-to-volume ratio of as-synthesized nanocrystals, which represent the main barriers for practical applications. To improve the emission of these materials, a judicious choice of the inorganic host material along with its crystal structure is central. Indeed, tungstate, molybdate, and vanadate compounds are of special interest in this regard as they enable the excitation of RE cations through energy transfer from the host that is more efficient than the direct excitation, which results in a higher brightness. However, a comparative study of the QY of such compounds is still missing. Herein, we perform a combined structural and optical analysis, which reveals that the tetragonal GdVO4 matrix gives rise to the highest efficiency among the different transparent nanophosphor films compared. Then, we demonstrate that by a sequential stacking of optical quality layers made of Eu3+- and Dy3+-doped nanocrystals,  it is possible to attain highly transparent white-light-emitting coatings of tunable shade with photoluminescence quantum yields above 35%. Layering provides a precise dynamic tuning of the chromaticity based on the photoexcitation wavelength dependence of the emission of the nanophosphor ensemble without altering the chemical composition of the emitters or degrading their efficiency. The total extinction of the incoming radiation along with the high quantum yields achieved makes these thin-layered phosphors one of the most efficient transparent white converter coatings ever developed.