Highly efficient deep-blue light-emitting copolymers containing phenoxazine: enhanced device efficiency and lifetime by blending a hole transport molecule

Abstract

A series of deep-blue light-emitting copolymers with an emission peak at 455 nm and narrow full width at half-maximum (FWHM) of about 40 nm were synthesized by incorporating a small amount of 10-(4-hexylphenyl)-10H-phenoxazine (PO) unit into the backbone of poly(9,9-dioctylfluorene). All the copolymers exhibit good thermal stability, high photoluminescent quantum yield and appropriate electrochemical energy levels. The copolymer PF-TD2PO1 which contains 1 mol% PO unit shows a maximum luminous efficiency (LE_max) of 3.07 cd A⁻¹ and 5.18 cd A⁻¹ based on single-layer and double-layer devices, respectively, with deep-blue Commission Internationale de L’E’clairage (CIE) coordinates of (0.15, 0.11). To improve the hole transport of the emissive layer, a hole transport molecule N-([1,1′-biphenyl]-4-yl)-9,9-dimethyl-N-(4-(9-phenyl-9H-carbazo-yl)-phenyl)-9H-fluorene-2-amine (BCFN) was blended into PF-TD2PO1. As a result, the best electroluminescence performance with a low turn-on voltage of 3.0 V, a superior LE_max of 6.20 cd A⁻¹ (corresponding to a maximum external quantum efficiency of 6.22%), an unprecedented luminance of 44 685 cd m⁻² and deep-blue CIE coordinates of (0.15, 0.11) were obtained from the blended PF-TD2PO1:BCFN light emitting device. It is particularly important that PF-TD2PO1:BCFN based devices exhibit very slow efficiency roll-off and a more than 14 times enhanced lifetime compared with the devices without BCFN. The results indicate the great potential of this class of copolymers for application in solution processed deep-blue light-emitting diodes with high efficiency and long lifetime.