Evaluation of propylene-, meta-, and para-linked triazine and tert-butyltriphenylamine as bipolar hosts for phosphorescent organic light-emitting diodes

Abstract

Three representative bipolar hybrids – tBu-TPA-p-TRZ, tBu-TPA-m-TRZ, and tBu-TPA--TRZ with triplet energies, E_T = 2.5, 2.7 and 3.0 eV, respectively – were synthesized and characterized for a comprehensive evaluation of their potential as a host for phosphorescent organic light-emitting diodes (PhOLEDs) using red-emitting Ir(piq)₃ as the dopant with an E_T value of 2.1 eV. Formation of charge transfer complexes, CTCs, was diagnosed by fluorescence bathochromism in increasingly polar solvents. Both intra- and inter-molecular charge transfer processes are invoked to explain CTC formation in all three hybrids. The ppp-hybrid is by far the most susceptible to CTC formation both in solution and neat solid film, resulting in PhOLEDs with reduced external quantum efficiency, EQE, despite the best balance between charge fluxes across the emitting layer, EML, as revealed by the electron- and hole-only devices in addition to PhOLEDs containing a sensing layer. The highest EQE is achieved with the mm-hybrid thanks to the compromise between balanced charge fluxes and CTC formation. The -hybrid is the least prone to CTC formation while suffering charge flux imbalance to yield an EQE intermediate between those of the mm- and ppp-hybrids. The least CTC formation involving the -hybrid is advantageous in accommodating the most singlets and triplets readily transferrable to both red and blue phosphors on account of its relatively high E_T value. Furthermore, the -hybrid offers the best morphological stability of the desired glassy EML, thus holding promise for the fabrication of superior PhOLEDs overall.