This invention pertains generally to the field of light-emitting diodes (LEDs). More particularly, this invention relates to polymer LEDs which offer high brightness, high efficiency and extended operating life, and which comprise (a) a transparent hole-injecting anode layer; (b) an emissive layer comprising an electroluminescent polymer; and, (c) an electron-injecting cathode layer; wherein said cathode layer comprises an ultra-thin layer of alkaline earth metal (such as calcium, strontium and barium) having a thickness of about 100 .ANG. or less, typically from about 15 to about 100 .ANG..