Naphthalene-based host materials for highly efficient red phosphorescent OLEDs at low doping ratios

Abstract

Abstract Four naphthalene-based host materials, 4′-[8-(4-phenoxazin-10-yl-phenyl)-naphthalen-1-yl]-biphenyl-4-carbonitrile (POZ), 4′-[8-(4-phenothiazin-10-yl-phenyl)-naphthalen-1-yl]-biphenyl-4-carbonitrile (POT), 4′-[8-(9-phenyl-9H-carbazol-3-yl)-naphthalen-1-yl]-biphenyl-4-carbonitrile (3CZ), and 4′-[8-(4-carbazol-9-yl-phenyl)-naphthalen-1-yl]-biphenyl-4-carbonitrile (9CZ), were designed and synthesized, where one electron-donating group and one electron-withdrawing group were covalently linked to the 1,8-positions of each naphthalene unit, respectively. Single crystal X-ray diffraction analyses of the four hosts revealed that the host molecules are held together by intensive non-covalent interactions, such as π⋯π and hydrogen bonding interaction at solid state, which can facilitate carrier transport and migration of excitons and improve the ability of thin film formation. The photophysical properties, thermal stability, and electrochemical behaviors of these hosts were systematically investigated. Red phosphorescent organic light-emitting diodes (PHOLED) based on these host materials with Ir(MDQ)2(acac) as dopant were fabricated and exhibited excellent device performance at low doping concentrations of 1–2 wt% due to the efficient energy transfer and reduced concentration quenching. Remarkably, POT based OLED at the doping ratio of 1 wt% achieved a maximum efficiency of 39.5 cd A−1, 37.6 lm W−1 and 20.3% with Commission Internationale de l'Eclairage (CIE) coordinates of (0.60, 0.40) without an external out-coupling structure. This result is comparable to the best red PHOLEDs with such a low doping ratio, indicating their potential utility as a host for cost-effective and highly efficient red PHOLEDs.