The electronic transport properties of 1, 3, 5-tri(1-phenyl-1H-benzo[d]imidazol-2-yl)phenyl (TPBI) electron transporting layers (ETLs) have been investigated as a function of cesium carbonate () doping for organic light-emitting diodes (OLEDs). The current density-voltage and light emission characteristics were measured as a function of the ETL thickness. TPBI decreased OLED operating voltage by 26% and increased device luminance by 17% in a wide concentration range (3.5% to 10.5%) compared to undoped devices. The effects of 7% ETL thickness indicated that the operating voltage continuously decreased to 37% when the ETL thickness increased to 600 Å and luminance output continued to increase to 21% at ETL thickness 525 Å. The blue OLED can be optimized by adjusting the thicknesses of TPBI ETL to balance the electron and hole injection.