We present a comprehensive study of enhancement mechanisms in electroluminescence devices where the active layer is doped with fluorescent dye. We demonstrate two typical systems where the emission mechanism is based on either energy transfer or direct carrier recombination. For the energy transfer mechanism, we find that the maximum quantum efficiency of multilayered devices depends on the nature of the host in addition to the luminescent properties of the guest, by comparing electroluminescence and absolute photoluminescence quantum yields in various guest-host systems. For the direct carrier recombination mechanism using rubrene as the dopant and TPD as the host, we find that the dual electronic nature of rubrene acting as a hole trap and an electron hopping site is responsible for maximizing the carrier recombination on the dopant molecule.