Although a very high electroluminescence (EL) efficiency has been demonstrated with organic light emitting diodes (OLEDs) using electrophosphorescence, enhancement of energy conversion efficiency, that is decreasing driving voltage, has been required from the standpoint of practical device applications. In this paper, we report on a decrease in operation voltage by doping carrier transport layers and discuss the possible mechanisms. In particular, we demonstrate an OLED with very low driving voltage (2.9 V at J = 100 mA/cm²) with a p-i-n structure.