We studied solution-processed single-layered phosphorescent organic light-emitting diodes (PHOLEDs), doped with ionic salt and treated with simultaneous electrical and thermal annealing. Because the simultaneous annealing causes the accumulation of salt ions at the electrode surfaces, the energy levels of the organic molecules are bent by the electric fields due to the adsorbed ions, i.e., the simultaneous annealing can induce the proper formation of an ionic p-i-n structure. As a result, an ionic p-i-n PHOLED with a peak luminescence of over ${sim}35,000;cd/m^2$ and efficiency of 27 cd/A was achieved through increased and balanced carrier-injections.