Anomalous current-voltage characteristics in organic light-emitting devices

Quasi-reversible current maxima at low voltage, leading to N-type current-v oltage characteristics with negative differential resistance, have been obs erved in different types of organic light-emitting, devices including conju gated polymer LEDs, dye-doped polymeric LEDs and LEDs from evaporated small molecules. We have investigated the dependence of this phenomenon on diffe rent external parameters, like layer thickness, temperature and time. We fo und that the usual explanations, e.g. by tunneling, cannot satisfactory exp lain our observations. Instead, our experiments indicate that spatially loc al effects are responsible for the anomalous high current flow at low volta ge. The implications for device operation and lifetime are discussed.