Organic light emitting diode

An organic light emitting diode (OLED) comprising a substrate bearing a light emitting layer between an electrically conducting anode and an electrically conducting cathode, the diode being configured for light emission through said cathode, the cathode being transmissive at a light emission wavelength of the diode, the cathode comprising an electron injecting layer for injecting electrons into said light emitting layer, an optical interference structure, and an electrically conducting layer, said electron injecting layer being closest to the light emitting layer and said optical interference structure being disposed between said electron injecting and electrically conducting layers, wherein said optical interference structure is configured to enhance light transmission through said cathode at said emission wavelength and comprises an optical interference layer disposed between first and third layers of different refractive indices such that reflections from front and back surfaces of said optical interference layer interfere to enhance light transmission through said cathode at said emission wavelength, said first layer comprising the electron injecting layer for injecting electrons into said light emitting layer, said third layer comprising the electrically conducting layer, and said optical interference layer having an optical thickness of between a third of said emission wavelength and a fifth of said emission wavelength.