Construction of photoconductivity measurement system and studies on excited states and electronic structure of phosphors

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In order to clarify the nonradiative process that dominates luminescence quantum efficiency of inorganic phosphor materials, we have developed a photoconductivity measurement system as functions of temperature and irradiating wavelength. Attention was paid on rare-earth doped phosphor materials for white LED, where 4f-5d electronic transition plays important role. By measurement of temperature dependence of photocurrent excitation spectra, the energy relation of Ce3+:5d level and conduction band of host was investigated for garnet ceramic phosphors of various compositions. A photo-ionization process of excited 5d electrons was found to be a quenching mechanisms at higher temperatures.