Enhancement of spectral stability and efficiency on blue light-emitters via introducing dibenzothiophene- S, S-dioxide isomers into polyfluorene backbone

Abstract

Dibenzothiophene- S, S-dioxide isomer (FSO) monomers were copolymerized with extensively employed 9,9-dioctyl-2,7-fluorene monomer by Suzuki polycondensation. By introducing FSO unit into polyfluorene backbone, the spectral stability and efficiency of the blue-emitting polyfluorenes are significantly improved. All of the obtained copolymers and their devices exhibit stable photoluminescence (PL) and electroluminescence (EL) spectra, respectively, upon change of thermal annealing temperature and current densities. Furthermore green emission which is usually associated with excimer/aggregation or defects is absent in PL and EL spectra. In fact, the FSO unit acts as a deep trap for electron in the polyfluorene backbone, which could suppress the long wavelength emission. Moreover, the FSO unit lowers LUMO energy levels of the polyfluorenes, thus can maintain a balanced charge carrier injection and transport in the device based on the polymers, and therefore improves the device efficiencies. The device based on PF-2,8FSO1 and PF-3,7FSO5 show an external quantum efficiency (EQE) of 3.6% and a luminance efficiency (LE) of 3.7 cd/A with CIE coordinate of (0.16, 0.07), and an EQE of 3.8% and a LE of 4.6 cd/A with CIE coordinate of (0.15, 0.12), respectively. The results indicate that poly (dibenzothiophene- S, S-dioxide -co-9,9-dioctyl-2,7-fluorene)s could be a promising candidate for blue-emitting polymers with spectral stability and high efficiency.