REASONS AND IMPLICATIONS OF ASYMMETRIES IN THIN-FILM EL DEVICES

Thin film electroluminescent devices often exhibit asymmetric waves, i.e. different light sums are emitted in the two half-periods of the addressing bipolar voltage. In most cases differences in the tunnel emission threshold fields of the two interfaces are the reason. In spite of the amounts of transferred charge being equal in the stationary case, the energies deposited per half-period then in general differ from each other and from the simple ansatz, which is based on the back-to-back Zener model. Special complications with alkaline chalcogenide based samples, which exhibit trailing edge emissions and currents, are discussed: because of excitation energy storage one has to be very carefully with every kind of efficiency concepts relating to fractions of periods.