Carbazole dendrimers containing oligoarylfluorene cores as solution-processed hole-transporting non-doped emitters for efficient pure red, green, blue and white organic light-emitting diodes

Abstract

A series of oligoarylfluorene-cored dendrimers containing bifluorene, bis(fluoren-2-yl)benzothiadiazole and bis[5-(fluoren-2-yl)thiophen-2-yl]benzothiadiazole as fluorescent cores, and third generation carbazole dendrons as end-caps were synthesized and characterized as RGB light-emitting materials. Their photophysical, thermal, electrochemical, and electroluminescence properties as non-doped solution processed emitters and hole-transporters for OLEDs were investigated. They displayed a bright RGB fluorescence and could form morphologically stable amorphous thin films with glass transition temperatures as high as 338 °C. Simple structured solution-processed OLEDs using these dendrimers as hole-transporting non-doped emitters emitted stable RGB colors (λ^EL_max = 415, 521 and 622 nm) with high luminance efficiencies (up to 9.12 cd A⁻¹ at 3.9 mA cm⁻²). The CIE coordinates of emitted colors were (0.63, 0.36) for red, (0.27, 0.62) for green and (0.17, 0.11) for blue, which are close to the pure RGB colors. The bifluorene-cored dendrimer as a bifunctional material showed great ability as a hole-transporting material for Alq3-based green OLEDs with a high luminance efficiency (5.63 cd A⁻¹). Solution-processed WOLEDs based on these three dendrimers exhibited a high luminance efficiency (3.06 cd A⁻¹ at 17.4 mA cm⁻²) with a high color rendering index (CRI) of 97.