A tandem OLED includes an anode, a cathode, and at least two electroluminescent units disposed between the anode and the cathode, wherein each of the electroluminescent units includes at least one hole-injecting layer, one hole-transporting layer, one organic light-emitting layer, one electron-transporting layer, and one electron-injecting layer. The OLED also includes at least one intermediate connector, wherein each of the intermediate connectors includes at least one layer, and wherein each of the intermediate connectors is disposed between electroluminescent units, wherein the thickness of each layer in each of the electroluminescent units and the intermediate connectors is selected to satisfy the test condition that the voltage drop from the anode to the cathode is less than 4.0 V.times.N (the number of electroluminescent units) at 20 mA/cm.sup.2.