Hyperbranched fluorene-alt-carbazole copolymers with spiro[3.3]heptane-2,6-dispirofluorene as the core and their application in white polymer light-emitting devices

A series of hyperbranched copolymers with fluorene-alt-carbazole as the branches and three-dimensional-structured spiro[3.3]heptane-2,6-dispirofluorene (SDF) as the core were synthesized by one-pot Suzuki polycondensation. 4,7-Dithienyl-2,1,3-benzothiadiazole (DBT) as the orange-light emitting unit was introduced into the backbones to obtain white-light emission. The thermal, photoluminescent (PL), electrochemical and electroluminescent (EL) properties of the copolymers were investigated. The copolymers show great thermal stabilities by the introduction of a carbazole moiety. Besides, the HOMO energy levels of the copolymers were enhanced and the hole injection was improved because of the hole-transporting ability of the carbazole unit. The hyperbranched structures suppress the interchain interactions efficiently, and help to form amorphous films. The copolymers exhibit efficient EL performance as a result of the hyperbranched structure with the incorporation of the carbazole moiety. A quite low turn-on voltage of 5.3 V, a maximal luminance of 7409.5 cd m^-2 and a luminous efficiency of 4.27 cd A^-1 were achieved with a CIE coordinate of (0.32, 0.26) for the PFCzSDF10DBT10 (10 mol% of SDF and 0.1 mol% of DBT) device. The hyperbranched framework based on fluorene-alt-carbazole branches and SDF core are attractive candidates for solution-processable white polymer light-emitting device (WPLED) applications.