Saturated and stabilized white electroluminescence with simultaneous three-color emission from a six-armed star-shaped single-polymer system

Abstract

A six-armed star-shaped single-polymer system consisting of three simultaneous emission species, 7,7′,7′′-(5,5,10,10,15,15-hexahexyl-10,15-dihydro-5H-diindeno[1,2-a:1′,2′-c]fluorene-2,7,12-triyl)tris(4-(4-(9H-carbazol-9-yl)phenyl)benzo[c] [1,2,5]thiadiazole) (TRCZ) as the green emissive core, polyfluorene (PF) as blue emissive arms and 4,7-bis(4-hexylthiophen-2-yl) benzo[c][1,2,5] thiadiazole (BBT) as red emissive dopants, has been designed and synthesized. The resulting polymers (P1–P3) exhibited excellent thermal stability and solution processability. A combination of UV-absorption, photoluminescence (PL) and electroluminescence (EL) measurements has provided an opportunity to elucidate the various properties of the polymers, which is favorable to understand the mechanism of the origin of white emission in this system. The incomplete energy transfer can be realized by adjusting the doping concentration of different components in the single-polymer system, facilitating white emission in a facile way. Thus, by means of modulating the doping concentration of TRCZ and BBT, P1–P3 are found to exhibit high-color-quality and stable white EL. For P2 with suitable doping contents, a single-emissive-layer device exhibited a promising white EL performance with a luminous efficiency (LE) of 1.45 cd A⁻¹, an external quantum efficiency (EQE) of 1.37% and saturated Commission Internationale d'Eclairage (CIE) coordinates of (0.33, 0.33), which matches exactly with the standard white color. The comparative experimental results further confirm that the performance of this six-armed single-polymer system is superior to that of the linear single-polymer system and the polymer-blend system in terms of color purity and stability. The achievement of pure and stable white emission is mainly attributed to the multi-armed hyperbranched structures, which play an important role in fine controlling the incomplete energy transfer as well as preventing intermolecular interactions and phase separation in a star-shaped single-polymer system.