We found that hole diffusion in a centimeter-scale can be achieved in a PEDOT:PSS layer via composition and interface engineering. This ultralong distance hole diffusion enables substantially enhanced hole diffusion current in the lateral direction perpendicular to the applied electric field in typical organic optoelectronic devices. By introducing this lateral-holediffusion layer (LHDL) at the anode side of organic light-emitting diodes (OLEDs), both reduced efficiency roll-off and enhanced operation stability are demonstrated. In conventional OLEDs, balance in electron and hole currents is typically achieved by leakage of the major carrier through the devices or by accumulation of the major carrier inside the devices. Both of these are known to reduce performances leading to efficiency roll-off at high currents, reduction of operation stability due to exciton-polaron annihilation etc. The application of the LHDL provides a new strategy for current balancing with much reduced harmful effects from the previous two approaches. For example, by incorporating the diffusion layer in a white phosphorescent OLED, 94% of its maximum efficiency can be maintained even at a brightness of 10000 cd/cm2. At a high brightness of 30000 cd/cm2, the OLED maintains a record high 2 external quantum efficiency of 13.9% without using any optical photon extraction layer. The OLED also show 5.5 times improvements in operation lifetime over the device without the diffusion layer. This study shows that centimeter-scale hole diffusion can be achieved in organic semiconductors and generally applied for enhancing efficiency and stability of OLEDs.