Buffer-modified n/p-type and p/n-type planar organic heterojunctions as charge generation layers for high performance tandem organic light-emitting diodes

•Tandem OLEDs were demonstrated using both n/p-type and p/n-type CGLs.•The influence of MoOx and LiF on the charge generation process was investigated.•A doping layer Ir(ppz)3:MoOx was introduced in n/p-type CGL.

High performance tandem organic light-emitting diodes (TOLEDs) were demonstrated for the first time using both buffer-modified n/p-type and p/n-type planar organic heterojunctions (OHJs) as charge generation layers (CGLs), and the optimized configurations of these two CGLs were “LiF/4,7-diphenyl-1,10-phenanthroline (Bphen)/tris(phenylpyrazole)iridium (Ir(ppz)3)/Ir(ppz)3:molybdenum oxide (MoOx)/MoOx” and “LiF/Ir(ppz)3/Bphen/MoOx”, respectively. Compared to the single-unit OLED, both n/p-type and p/n-type CGLs based TOLEDs exhibited about two folds enhancement in current efficiency. The influence of both MoOx and LiF as carrier extraction layers on the charge generation process of CGLs was conscientiously investigated. The working mechanism of two CGLs was discussed in details. It was also found that the novel design concept, a buffer-modified p/n-type planar OHJ could generate enough charges under a forward applied voltage and the carriers extraction was a tunneling process. Our results provide the great practicality of implementing planar OHJs as CGLs in high performance TOLEDs.

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