High-efficiency multilayer polymeric blue light-emitting diodes using boronate esters as cross-linking linkages

Abstract

A novel approach for the production of cross-linked and robust hole transport layers for use in multilayer polymeric light-emitting diodes (PLEDs) has been developed. Two alternating triphenylamine–fluorene copolymers (TPAFn, n = 2, 3) with hydroxyl groups on the side chains and tris(4-dihydroxyboranylphenyl)amine (TBPA) as a cross-linker have been designed and synthesized. The mixture of TPAFn and TBPA when subjected to mild reaction conditions (2 h baking at 130 °C under vacuum) undergoes cross-linking reactions to produce cross-linked films (X-TPAFn), due to the formation of boronate ester linkages. The resulting X-TPAFn films have excellent solvent resistance to common organic solvents, such as THF and CHCl₃, thereby facilitating the fabrication of multilayer PLEDs. Studies have shown that the cross-linking reaction had no detrimental effects on the photophysical properties of the resulting X-TPAFn films. In addition, the cross-linked X-TPAFn networks have been shown to have much better electron-blocking properties than the widely used PEDOT–PSS. Using X-TPAFn as a hole transport layer, we have investigated the electroluminescent (EL) properties of alternating fluorene–oxadiazole copolymers OxFn (n = 2, 3) and the poly(9,9-dioctylfluorene) (POF) homopolymer. The experimental data indicated that an increase in the oxadiazole content of the polymer lowered the LUMO energy level while decreasing the photoluminescence (PL) quantum yield. Consequently the best device performance was obtained with OxF3 which was found to have a maximum luminance of 2010 cd m⁻² at 11.5 V and a maximum luminous efficiency of 1.0 cd A⁻¹ at 820 cd m⁻² when used with X-TPAF2 as the hole transport layer and calcium as the cathode.