Efficiency of carrier recombination in bilayer organic light emitting diodes is described using numerical simulations and analytical model. It is shown that the most important factor that determines efficiency is the ratio of hole and electron mobility in the electron transport layer. When this ratio is much less than unity, recombination efficiency is 100% even without the presence of either an electron or hole barrier at the organic-organic interface. However, if the hole mobility is much higher than electron mobility, then a hole barrier at the organic interface is necessary to achieve a high recombination efficiency. The minimum interfacial hole barrier height required for high efficiency increases with the ratio of hole and electron mobilities in electron transport layer. (C) 2004 Elsevier Ltd. All rights reserved.