We investigated the current-voltage (I-V) and electroluminescence (EL) characteristic of metal/polymer and metal/insulator/polymer (MIP) LEDs based on poly (1,4-phenylene vinylene) (PPV). The I-V- and EL characteristics of the MIP structures display a pronounced dependence of the insulator thickness and we measure an increase of the quantum efficiency of more than a factor of 40 at an A1O(x) layer thickness of 3-5nm The device characteristic is qualitatively understood within inorganic metal/insulatos/semiconductor (MIS) theory and can be explained by a voltage dependent barrier for minority carrier injection in connection with a hole blocking barrier at the PPV/insulator interface. With MIP structures we reveal external quantum efficiencies up to 0.01%, comparable to values achieved on monolayer Ca LEDs. The MIP structures, however have the advantage, that a more stable device performance is obtained.