The quantum efficiency of organic light emitting devices depends on the exciton emission efficiency. Exciton quenching at the metal cathode can be responsible for lowering the device performance. We consider exciton quenching by the metal cathode, taking into account both exciton diffusion and radiationless energy transfer to the metal. The quenching effect in tris(8-hydroxyquinoline)aluminum is analyzed, and ways to improve the device efficiency by suppressing the interaction with the electrode are discussed.