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Organic light-emitting devices (OLEDs) are studied, containing polyvinylcarbazole and bathocuproine layers, with or without insertion of a polymethylmethacrylate layer at the organic interface. The latter efficiently blocks electrons but not holes. Large currents flowing in the diodes are limited by the interplay of transmission of holes through the organic heterojunction and space charge limitation in the anodic part of the diode. Minority electron currents are always small. An illustrative analytical model is presented, showing the evolution of fields in the anodic and cathodic compartments and the possibility of two physically acceptable solutions. Electroluminescence is observed only in the diodes without polymethylmethacrylate above a threshold V ∼ 7 V. Quantum yields of both phosphorescence and fluorescence and the triplet lifetime of BCP have also been determined.