The study of the excitation efficiency &eegr;excon thin‐film electroluminescent devices with special semiconductor layer—a very thin probe‐doped layer located at different part of the pure ZnS layer—has proved that &eegr;excis not homogeneous across the active layer. Moreover, this variation of &eegr;excdepends on the amount of the transferred charge. At high excitation range, &eegr;excdecreases continuously from the cathode toward the anode of the active layer. This behavior of &eegr;excis related to a space charge located in some part of the ZnS layer and generated by the energetic electrons. This space charge reduces the total efficiency of a conventional device when operating at high excitation range.