Stacked OLEDs emitting

The fifth of the color methods places the three emitting structures in a stack one on top of the other, rather than side by side ]20l ]. Clearly there is a requirement here that the two electrodes in the middle of the structure must be transparent. The advantages are that the display can be made much brighter with up to three times the luminance from each pixel, and the requirements for high resolution patterning are relaxed by a factor of three. The disadvantages are that three times as many layers must be coated (without defects) over the area of the display and electrical driving circuitry must make contact with four sets of elec- trades. It will be extremely difficult to incorporate a stacked OLED into a active matrix array. 

The fluorescence color converter technique [32] can, in principle, overcome much of this power loss by replacing the white light emitter with a blue-emitting organic stack, and the absorbing filters with green and red fluorescent dyes. Thus when a green pixel is desired, the OLED underneath is turned on and the blui... 

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. 

Although stability could be obtained by the tandem structures where each EL stack is white-emitting, it is difficult to obtain high efficiency. The primary reason for the low efficiency is the fact that it is nearly impossible to optimally position both of the identical emitting zones within the OLED structure with respect to the reflective cathode. Taking this into consideration, the following section describes the fabrication of an efficient and highly stable tandem white device. 

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

The tandem structure demonstrates an operational stability of 1000 h, whereas the stabilized blue stack and the G-R-emitting stack have stabilities of 1300 and 1200 h, respectively, at SOmA/cm. The slightly lower stability of the tandem at the same current density is primarily due to the p-n connecting layer. The estimated lifetimes at 1000 cd/m in Table 14.3 show that a high-efficiency tandem OLED with a very high operational... 

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