color shift associated with the intense charge-trapping effect of the
doped R species with the increased voltages is observed. To
eliminate the color shift, another multi-EML WOLED with RG-B
structure where R- and G- emitters are combined into one region
were proposed. Fig. 7b and d show the device structures and EL
performances of the RG-B WOLEDs. The resulting device efficiency
remains the same high level, but the present EL spectra are rather
stable. In a wide range of operational voltages, the CIE coordinates
of (0.39, 0.42) are not variable, and the CRIs remain at 80. The color
shift is generally closely related with the change of the exciton
recombination zone and the charge trapping effect of dopant
emitters. Here, there is a certain charge trapping effect of the red
emitter molecules in R-G-B WOLEDs, while a very low red doping
concentration in the RG-B WOLEDs reduces this charge trapping
effect, and hence improves the color stability, evidencing that by
the precise manipulation of the charges and generated excitons
high-performance mutil-EML WOLEDs with superior efficiency/
CRI/color stability can be realized. Similar to the most conventional
all-phosphor WOLEDs, the present device shows severe efficiency
roll-off, the EQE of the RG-B device decreases to 13.6% at the
practical brightness of 1000 cd m2, which should be further
improved in the future.