On the excitation efficiency in ZnS:Mn thin-film electroluminescent devices

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Abstract
In ac-coupled ZnS:Mn thin-film electroluminescent devices, the excitation efficiency ηexc depends on the insulator/ZnS interfaces and the excitation level. For optimized devices which are used in industrial production, ηexc does not depend on the transferred charge as long as the excitation level is below the saturation range. In the saturation range, ηexc decreases with increasing charge transfer. Hence the decrease of the total efficiency η at high excitation levels is partly due to the behavior of the excitation efficiency and not entirely to the decrease of the radiative efficiency ηlum. In devices with shallow insulator/ZnS interface states, however, charge injection starts at lower fields and ηexc increases strongly with the transferred charge and the applied voltage.