Output characteristics and optical efficiency of SrS:Ce and ZnS:Mn thin-film electroluminescent devices

The output characteristics and the optical efficiency of SrS:Ce and ZnS:Mn thin-film electroluminescent devices are studied by measuring and evaluating light either directly emitted from the active area or indirectly emitted from its surroundings, including substrate edge. A special preparation of the devices allows access to edge emission and emission between pixel area and sample edge caused by scattering. The measuring method is optimized for registration of the entire output into the far field, exploiting the azimuthal symmetry of the pixel emission. In this study the optical efficiency is defined as the ratio of the directly emitted luminous flux to the total flux emitted from the segment within the sandwiched phosphor layer, which is activated. Optical efficiencies ranging from 0.16 for a smooth ZnS:Mn to 0.26 for a rougher SrS:Ce specimen are found. Theoretical limitations of the measuring method are discussed. A new quantity called scattering gain is introduced for characterizing the coupling of the output into the front hemisphere. Differential scattering gains ranging from a few percent to nearly 20% are observed. The optical characterization of SrS:Ce and ZnS:Mn samples also allows for an estimate of the optical efficiency of future inverted electroluminescent structures.