We have seen the effects of buffer layer in organic light-emitting diodes using poly(vinyl carbazole)(PVK), copper phthalocyanine(CuPc) and poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate)(PEDOT:PSS) in a device structure of ITO/buffer/TPD/Alq 3 /Al. We have obtained an improvement of external quantum efficiency by a factor of four when the PVK layer is used. The PEDOT:PSS layer not only gives an improvement of efficiency by a factor two, but reduces an operating voltage as well.