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The doped organic/organic p-n heterojunctions have been applied as charge generation structure (CGS) in tandem organic light emitting diodes (TOLEDs). It is found that the field-induced charge generation takes place more efficiently at the interface between Li2CO3 n-doped bathocuproine (BCP:Li2CO3) and MoO3 p-doped 4,4′- N, N′-dicarbazole-biphenyl (CBP:MoO3) than at the interface between BCP:Li2CO3 and MoO3 p-doped 4,4-bis[ N-1-naphthyl- N-phenylamino]biphenyl (NPB:MoO3). It is because the process of electrons tunneling through the depletion zone from the highest occupied molecular orbit (HOMO) of CBP:MoO3 to the lowest unoccupied molecular orbit (LUMO) of BCP:Li2CO3 is more efficient than that from the HOMO of NPB:MoO3 to the LUMO of BCP:Li2CO3. Compared to the TOLED using the conventional CGS of 10-nm BCP:Li2CO3/20-nm NPB:MoO3, the one using the CGS of 10-nm BCP:Li2CO3/10-nm CBP:MoO3/10-nm NPB:MoO3 shows increased device performance. In addition, the interconnecting property of CGS of 10-nm BCP:Li2CO3/ x nm CBP:MoO3/20 − x nm NPB:MoO3 shows a strong dependence on the thickness of CBP:MoO3. We provide a new insight on optimizing Ohmic loss in the CGSs, useful for improving the performance of TOLEDs. [ABSTRACT FROM AUTHOR]