We investigated the electrical properties and charge transport mechanisms of a rubidium-carbonate (Rb2CO3)-doped 4,7-diphenyl-1,10-phenanthroline (Bphen) electron transporting layer (ETL). The electron-only devices and photoemission spectroscopy analysis revealed that the formation of doping-induced gap states dominantly contributes to the improvement of carrier transport characteristics of the doped system. High-efficiency green phosphorescent p-doping/intrinsic/n-doping (p-i-n) organic light emitting diodes were demonstrated using the Rb2CO3-doped Bphen ETL and rhenium oxide (ReO3)-doped N,N-'-diphenyl-N,N-'-bis(1,1(')-biphenyl)-4,4(')-diamine hole transporting layer, exhibiting an external quantum efficiency of 19.2%, power efficiency of 76 lm/W, and low operation voltage of 3.6 V at 1000 cd/m(2).