We have demonstrated that the exemplary red fluorescent organic light-emitting diodes (OLEDs) gain as much as half of their electroluminescence from annihilation of triplet states generated by recombining charge carriers. The magnitude of triplet-triplet annihilation (TTA) contribution in combination with the remarkably high total efficiencies [>11% external quantum efficiency (EQE)] indicates that the absolute amount of electroluminescence attributable to TTA substantially exceeds the limit imposed by spin statistics, which was independently confirmed by studying magnetic field effects on delayed luminescence. We determined the value of 1.3 for the ratio of the rate constants of singlet and triplet channels of annihilation, which is indeed substantially higher than the value of 0.33 expected for a purely statistical annihilation process. It is, however, in an excellent quantitative agreement with the extent of the experimental contribution of delayed luminescence to steady-state electroluminescence. The nonstatistical branching ratio of the two annihilation channels is attributed to the favorable relationship between the energies of the excited singlet and triplet states of rubrene—emissive layer host. We surmise that, with the appropriate emissive layer materials, the fluorescent OLED devices are capable of using a considerably larger fraction of triplet states than was previously believed. In principle, the upper limit for the singlet excited state yield in the TTA process is 0.5, which makes the maximum internal quantum efficiency of fluorescent OLEDs to be 25%+0.5×75%=62.5%. The estimates of maximum EQE of the fluorescent OLEDs should be revised to at least 0.2×62.5%=12.5% and, likely, even higher to account for optical outcoupling exceeding 0.2.

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....

In October 1999, Eastman Kodak and Sanyo Electric jointly announced the development of a high quality, 2.4 inch diagonal Full Color active matrix Organic Light Emitting Diode (OLED) display. This technology demonstration resulted from the successful integration of Kodak's organic electroluminescence display technology and Sanyo's low temperature polysilicon TFT technology. Commercial samples are expected to reach the market in 2001. The active matrix OLED displays feature a wider viewing angle and a faster response speed than conventional LCDs. With its low power consumption, high brightness and thin design, these OLED displays when incorporated in digital cameras, personal digital assistants (PDA), videophones and other portable imaging devices, will offer a superior value proposition to consumer electronics products. Several OLED display designs are under development now to serve various market segments. This paper will summarize the status of active matrix full color OLED display development, the key technical challenges, and the path ahead.

Abstract— A full‐color AMOLED display with an RGBW color filter pattern has been fabricated. Displays with this format require about one‐half the power of analogous RGB displays. RGBW and RGB 2.16‐in.‐diagonal displays with average power consumptions of 180 and 340 mW, respectively, were characterized for a set of standard digital still camera images at a luminance of 100 cd/m2. In both cases, a white‐emitting AMOLED was used as the light source, and standard LCD filters were used to provide the R, G, and B emission. The color gamuts of these displays were identical and the higher overall efficiency of the RGBW format results from two factors. First, a large fraction of a typical image is near neutral in color and can be reproduced using the white sub‐pixel. Second, the white sub‐pixel in an RGBW AMOLED display is highly efficient because of the absence of any color filter. The efficiency of these displays can be further enhanced by choosing a white emitter optimized to the target display white point (in this case D65). A two‐emission layer configuration based upon separate yellow and blue‐emitting regions is shown to be well suited for both the RGBW and RGB formats.

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