Improved color purity and electroluminescent efficiency obtained by modulating thicknesses and evaporation rates of hole block and electron transport layers

In this work, a series of electroluminescent (EL) devices based on trivalent europium (Eu ³⁺ ) complex Eu(TTA) ₃ phen (TTA = thenoyltrifluoroacetone, phen = 1,10-phenanthroline) were fabricated by selecting 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) and tris(8-hydroxyquinoline) aluminum (Alq ₃ ) as hole block and electron transport materials, respectively. Interestingly, we found the transport of electrons decreases gradually with increasing thicknesses and evaporation rates of BCP and Alq ₃ layers. Analyzing carrier distribution and EL spectra, we conclude that appropriately modulating the thicknesses and evaporation rates is an efficient way to decrease the accumulation of electrons in HBL, thus suppressing the EL of hole block material. On the other hand, decreasing the transport of electrons can also facilitate the balance of holes and electrons on Eu(TTA) ₃ phen molecules, thus further enhancing the EL efficiency. As a result, pure Eu ³⁺ emission with the efficiency as high as 8.49 cd/A was realized by controlling the thicknesses and evaporation rates of BCP and Alq ₃ layers to be 30 nm and 0.10 nm/s, 40 nm and 0.10 nm/s, respectively.