New sulfone-based electron-transport materials with high triplet energy for highly efficient blue phosphorescent organic light-emitting diodes†
A series of new electron transport materials, which combines a diphenylsulfone core with different electron withdrawing end groups, has been synthesized, characterized, and found to exhibit high triplet energy (ET > 2.8 eV) for use in phosphorescent organic light emitting diodes (PhOLEDs). The new materials, including 3,3′-(4,4′-sulfonylbis(4,1-phenylene))dipyridine (SPDP), 5,5′-(4,4′-sulfonylbis(4,1-phenylene))bis(3-phenylpyridine) (SPPP), and 3,3′-(4,4′-sulfonylbis(4,1-phenylene))diquinoline (SPDQ) had wide band gaps (3.6–3.8 eV) and LUMO levels of −2.4 to −2.7 eV. The triplet energy measured from phosphorescence spectra at 77 K varied from 2.53 eV for SPDQ and 2.81 eV for SPPP to 2.90 eV for SPDP, which are in good agreement with density functional theory calculated values. High performance blue PhOLEDs using the sulfone-based materials are exemplified by devices containing a poly(N-vinylcarbazole) host and SPDP electron transport layer, which had a high quantum efficiency (19.6%) and a high current efficiency (33.6 cd A−1) even at very high luminances (4500 cd m−2). These results demonstrate that sulfone-based molecules are promising electron transport materials for application in developing highly efficient phosphorescent OLEDs.