Orange-to-Red Emission from Bi2+and Alkaline Earth Codoped Strontium Borate Phosphors for White Light Emitting Diodes

SrB4O7 and SrB6O10 were considered as host materials for Bi2+ dopants with luminescence in the orange-to-red spectral range. Based on the predominance of tetrahedral anion groups, both compounds are highly efficient for stabilizing the Bi2+-ion over its trivalent form. SrB4O7: Bi2+ exhibits a broad blue excitation band while SrB6O10: Bi2+ can be excited in the yellow spectral range. These bands suggest high interest for application in light emitting diode devices such as InGaN (SrB4O7:Bi2+) and YAG:Ce3+ (SrB6O10:Bi2+) with improved color rendering index. Absorption due to strongly allowed 2P(1/2)-> 2S(1/2) occurs at 245 and 286 nm, respectively. By substituting Sr2+ with other alkaline earth species, the parity selection rule for transitions to 2P(3/2) (blue and yellow excitation) can be lifted to a large extent, in this way increasing the transition probability from both bands and, thus, improving quantum yield for visible excitation. Analyses of the temperature dependence of luminescence reveal superior performance for SrB4O7:Bi2+.