Bis(N-naphthyl-N-phenylamino)benzophenones as exciton-modulating materials for white TADF OLEDs with separated charge and exciton recombination zones

Abstract

Organic semiconductors were employed as exciton modulators, blue emitters, hole-transporting materials and hosts with resonant-appropriate singlet and triplet energies for efficient and stable white organic light emitting diodes (OLEDs). Two 4,4'-bis(N-naphthyl-N-phenylamino)benzophenones were synthesized using isomeric N-naphthyl-N-phenylamines as the donors and benzophenone as the acceptor moiety. Molecular design of new compounds allowed to obtain required combination of properties, i.e. blue prompt fluorescence in solid state with singlet energies close to those of the selected blue emitter exhibiting thermally activated delayed fluorescence (TADF), low triplet energies of 2.32 and 2.45 eV which are close to those of orange TADF emitter, good charge injecting properties (ionization potentials of 5.68 and 5.79 eV), good charge transporting properties with hole mobilities exceeding 10−4 cm2 (V s)−1 and high thermal stability with five percent weight loss temperatures up to 428 °C. The blue-emitting compounds were used as exciton modulators between the known blue and orange TADF emitters for fabrication of white OLEDs exploiting spatial exciton allocation strategy. In the frame of this strategy, resonant energy transfers: NPABP emitters → blue TADF and NPABP emitters → orange TADF emitters were investigated using different device structures towards efficient white electroluminescence. About twice higher external quantum efficiency was obtained for devices with two resonant energy transfers in comparison to that of the reference devices with one resonant energy transfer proving efficiency of spatial exciton allocation strategy for white TADF OLEDs. The best quality of white electroluminescence is characterized by CIE coordinates of (0.32, 0.31), colour temperature of 4490 K and colour rendering index of 80. Similar stability of blue and orange emission bands in white electroluminescence spectra was achieved due to the separation of charge and exciton recombination zones in the device structure.