Study on energy relation between blue and red emissive layer of organic light-emitting diodes by inserting spacer layer

In this paper, energy relation between blue emissive layer (blue-EML) and red emissive layer (red-EML) in organic light-emitting diodes based on blue-emitting and red-emitting phosphorescent dopants, bis(3,5-difluoro-2-(2-pyridyl)phenyl-(2-carboxypyridyl) iridium III (Firpic) and bis(2-(2′-benzo[4,5-α]thienyl)pyridinato-N,C3′)iridium(acetylacetonate) (Btp₂Ir(acac)), was studied. Two phosphorescent dopants, Firpic and Btp₂Ir(acac), were co-doped in the single emissive layer, and the results exhibit complete energy transfer from Firpic to Btp₂Ir(acac). Then, Firpic and Btp₂Ir(acac) were doped into blue-EML and red-EML, separately. By inserting 4,4′-bis(N-carbazolyl)biphenyl (CBP) spacer between blue- and red-EML, energy relation between blue- and red-EML was researched. The results of this work reveal that, CBP spacer may strengthen energy transfer between blue- and red-EML. The reason is that CBP triplets at blue-/red-EML interface can transfer their energies to both CBP molecules of red-EML and Firpic molecules of blue-EML in spacer-without devices, while CBP triplets in the spacer can transfer their energies only to CBP molecules of red-EML. Therefore, energy flow from blue- to red-EML is strengthened because of the avoidance of energy transfer from CBP triplets in the spacer to Firpic molecules of blue-EML, leading to the relative enhancement of red emission in CBP-spacer devices.