Blue Electroluminescence in SrSO4:Ce-Based Thin-Films

A blue electroluminescence (EL) device was fabricated using SrSO4:Ce-based phosphor layers deposited by rf sputtering in Ar–H2S atmosphere. Peak emission wavelength was observed at 420 nm corresponding to the energy transition from the T2g(5d) level to the F5/2(4f) level of Ce ions. This wavelength is shorter than any previously reported, which suggests that the energy state of the excited 5d electrons is sensitive to the crystal field surrounding Ce ions. A high-resolution two-crystal X-ray fluorescence spectrometer (HRXRF) was used to examine the composition of the SrSO4-based phosphor layers and chemical state of Ce ions in them. The chemical state of Ce ions was primarily 3+ in the blue-green emitting phosphor layers, which were mainly composed of SrS. An increase in SrSO4 concentration in the phosphor layers leads to a change in the chemical state of Ce ions, resulting in a blue shift in EL emission. A constant high luminance of 5 cd/m2 was obtained under 5 KHz sinusoidal driving. The transferred charge density of the SrSO4:Ce-based EL device was found out to be fairly low compared to the conventional SrS:Ce EL device.