Stacked OLEDs units

To date, a variety of connecting units have been introduced for stacked OLEDs, which include Cs BCP/ITO [65], Cs BCP, LirBCP/VzOs [66,67], LirAlqg, Li l,3,5-tris(N-phenylbenzimidazol-2-yl) benzene (TPBIj/FeClgrNPB [68], MgrAlqg/WOg, V2O5, 2,3,5,6-tetrafluoro-7,7,8,8,-tetracyanoquinodimethane... 

These results also have certain implications on tandem OLEDs, which have multiple OLED units stacked vertically in series [24-26], In the simplest viewpoint, a proportional increase in the luminance efficiency could be expected (i.e., cd A) with the number of emitting units. However, the results here suggest that the enhancement in cd/A efficiency could significantly exceed such proportional increase (extra efficiency gain from the tandem structure), since the emitting unit further away from the reflective metal electrode could contribute a larger cdA efficiency or total outcoupling. 

Recently, a tandem (or stacked) OLED structure was developed in order to improve luminous efficiency and to increase lifetime. This was accomplished by vertically stacking several individual electroluminescent (EL) units, each of which comprise a HTL EML ETL, and driving the entire device with a single power source. The schematic structure of the tandem device is shown in Figure 14.19. 

In a stacked OLED having N EL units N > 1), the luminous efficiency (cd/A) can be about N times as high as that of a conventional OLED containing only one EL unit (the drive voltage will also be about N times as high as that of the conventional OLED). Therefore, the stacked OLED needs only about 1/N... 

A two-stack tandem white OLED was fabricated using the above-described stabilized blue-emitting unit (HTL yellow EML buffer layer blue... 

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