Nondoped blue fluorescent OLED based on cyanophenanthrimidazole-styryl-triphenylamine/carbazole materials

Abstract

New 2-(4′-9H-carbazole-9-yl)-styryl-1H-phenathro[9,10-d]imidazole-1-yl)benzonitrile (SPICN-Cz) and 4-(2-(4-(diphenylamino)phenyl-styryl-1H-phenathro[9,10-d]imidazole-1-yl)benzonitrile (SPICN-TPA) have been synthesised, and their photophysical, electrochemical, and electroluminescent properties were analysed in comparison with their cyano-free parent compounds, SPI-Cz, and SPI-TPA. Solvatochromic effects show the transformation of an excited state character from locally excited (LE) state to charge transfer (CT) state. Using time-dependent density functional theory calculation, the excited state properties of these donor-acceptor blue emissive materials have been analysed. Their excited state properties have been tuned by replacing the strong donor triphenylamine to weak donor carbazole to achieve the combination of high photoluminance efficiency locally excited (LE) component and high exciton-utilizing CT component in one excited state. Hybridization processes between LE and CT components of SPICN-Cz and SPICN-TPA in the emissive state have been discussed. The nondoped organic light emitting diode device based on SPICN-Cz exhibit better electroluminescent performances than those of SPICN-TPA–based device: high external quantum efficiency of 2.58 %, current efficiency of 2.90 cd A-1, and power efficiency of 2.26 lm W-1 with Commission Internationale de l'Eclairage (CIE) coordinates of (0.15, 0.12). The excited state modulation and the composition of LE and CT states in the donor-acceptor system could be useful to design low-cost, high-efficiency fluorescent organic light emitting diode materials.