Organic light-emitting diodes have been attracting a great deal of attention because they are useful for applications as next-generation displays and for solid-state lighting. [1,2] An OLED is actually a current-driven device, and its luminance increases with increasing current density. However, the operational lifetime of an OLED decreases with increasing current density. [3] In order to achieve high brightness, an OLED must be operated at a relatively high current density, but this will result in a short lifetime. Thus, it is crucial to improve the luminous efficiency of an OLED while operating at the lowest possible current density consistent with the intended luminance requirement to increase the operational lifetime. In order to dramatically improve the luminous efficiency and to increase the lifetimes of OLEDs, a tandem (or stacked) OLED structure can be fabricated. This is accomplished by vertically stacking several individual electroluminescent (EL) units, each with a hole-transporting layer (HTL)/light-emitting layer (LEL)/electron-transporting layer (ETL) structure, with the entire device driven by a single power source. [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] In a tandem OLED with N EL units (N > 1), the luminous efficiency can be about N times as high as that of a conventional OLED that contains only one EL unit (the drive voltage will also be about N times as high as that of the conventional OLED). Therefore, the tandem OLED needs only about 1/N times the current density used in the conventional OLED to obtain the same luminance, which results in an operational lifetime N times that of the conventional OLED. Alternatively, the tandem OLED can achieve a luminance N times higher than that of the conventional OLED while maintaining about the same lifetime. In a tandem OLED, all of the EL units are electrically connected in series by inserting an intermediate connector (or connecting unit) between adjacent EL units. The intermediate connector plays an important role in the function of tandem OLEDs. The intermediate connector can be formed using a metal-metal (or metal oxide) bilayer, [4,11] an organic-metal (or metal oxide) bilayer, [5,6,7,9,10,[12][13][14]16,17] or an organic-organic bilayer. [6,8,15,[18][19][20] Using a metal or a metal oxide in an intermediate connector can introduce fabrication complexity. Many kinds of metals and metal oxides cannot be deposited by thermal evaporation at a temperature below 300°C; hence, methods for their deposition are not usually compatible with the underlying organic layers. Additionally, metals and some metal oxides can cause pixel crosstalk in dot-matrix displays and low optical transparency. In contrast, organic-organic intermediate connectors can be formed using thermal evaporation methods with relatively low evaporation temperatures, and they do not cause pixel crosstalk or low optical transparency.In the organic-organic intermediate connector, the first organic layer is an n-type doped organic layer, such as an alkali metal-doped ETL....