Bright Deep Blue TADF OLEDs: The Role of Triphenylphosphine Oxide in NPB/TPBi:PPh₃O Exciplex Emission

Abstract Very bright (≈14 000 cd m⁻²) deep blue exciplex organic light emitting diodes (OLEDs) peaking at ≈435 nm, where the photopic response is ≈0.033, and with CIE color coordinates (0.1525, 0.0820), are described. The OLED properties are interestingly linked to PPh₃O (triphenylphosphine oxide) and attributes of the emitting layer (EML) comprising NPB interfacing host:guest TPBi:PPh₃O 5:1 weight ratio. A neat PPh₃O layer that is central for device performance follows the EML (NPB/TPBi:PPh₃O 5:1/PPh₃O). The bright electroluminescence originates from NPB/TPBi:PPh₃O exciplexes involving triplets via thermally activated delayed fluorescence, as evident from the strong quenching of the photoluminescence (PL) by oxygen and interestingly, the monomolecular emission process. The transient PL decay times of a NPB/TPBi:PPh₃O 5:1/PPh₃O film are 43 ns in air versus 136, 610, and weak ≈2000 ns in N₂. For comparison, the respective PL decay times of films of NPB:TPBi are 16 ns in air versus 131 and 600 ns in N₂, and of NPB:PPh₃O they are 29 ns in air versus 56, 483, and weak ≈2000 ns in N₂. It is suspected that slow emitting states are associated with a PPh₃O aggregate interacting with NPB.