Charge carriers at organic heterojunction interface: Exciplex emission or electroplex emission?

We report the electroluminescence (EL) of organic heterojunction devices based on N, N′ -diphenyl- N, N′ -bis(3-methylphenyl)- (1, 1′ -biphenyl)- 4, 4′, -diamine (TPD) and 2- (4′ -biphenyl)-5- (4″ -tert-butylphenyl)-1,3,4-oxadiazole (PBD). Besides monomolecular emissions from TPD, there are two additional EL peaks at around 460 and 480 nm from the bilayer device indium tin oxide (ITO)/ TPD (100 nm) PBD (45 nm) Al. Our experimental data confirmed that the EL emission maximized at around 460 nm is from electroplex as the result of charge carriers cross recombination at the TPD/PBD interface and the EL emission maximized at around 480 nm originates from (TPD* PBD) -type exciplex.