Highly efficient deep-blue light-emitting copolymers containing phenoxazine: enhanced device efficiency and lifetime by blending a hole transport molecule
A series of deep-blue light-emitting copolymers with 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 level. The copolymer PF-TD2PO1 which contains 1 mol% PO unit shows maximum luminous efficiency (LEmax) 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 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 champion electroluminescence performance with a low turn-on voltage of 3.0 V, a superior LEmax of 6.20 cd A-1 (corresponding to a maximum external quantum efficiency of 6.22%), an unprecedented luminance of 44685 cd m-2 and deep-blue CIE coordinates of (0.15, 0.11) was obtained from blended PF-TD2PO1:BCFN light emitting device. Of particular importance is that PF-TD2PO1:BCFN based device exhibits very slow efficiency roll-off and 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.
- This article is part of the themed collection: 2019 Journal of Materials Chemistry C HOT Papers