Effect of alkyl side chain length on the electroluminescent performance of blue light-emitting poly(fluorene-co-dibenzothiophene-S,S-dioxide)

Abstract

In this manuscript, we report a simple strategy of tuning the alkyl side chain length of emissive polymer to realize blue polymer light-emitting diodes (PLED) with high efficiency and enhanced device lifetime. Two poly(fluorene-co-dibenzothiophene-S,S-dioxide) derivatives, PF16SO and PF8SO, comprising the same backbone but different side chains lengths (hexadecyl only or hexadecyl mixed with octyl), were synthesized, respectively. Attributing to the stronger rigidity caused by its shorter side chains, PF8SO exhibits high glass-transition temperature of 218 °C and photoluminescence quantum yield of 78%. Furthermore, the grazing incidence X-ray diffraction measurements reveal closer π-π packing for PF8SO, resulting in improved and more balanced charge carrier transport. As a result, an enhanced electroluminescence performance with a low turn-on voltage of 2.8 V, a high luminous efficiency of 8.36 cd A−1 (corresponding to an external quantum efficiency of 6.19%) with the Commission Internationale de L’E'clairage coordinates of (0.15, 0.18) was achieved based on a single-layer device structure of ITO/PEDOT:PSS/Emissive Layer/Ba/Al, which is among the best performance reported so far for pure blue light-emitting polymers based on this simple device structure. In addition, PF8SO based device exhibits longer lifetime than that of PF16SO. The result indicates the great potential of tuning the alkyl side chain length in enhancement of electroluminescent performance for blue light-emitting poly (fluorene-co-dibenzothiophene-S,S-dioxide).