High-efficiency red light-emitting diodes based on polyfluorene copolymers with extremely low content of 4,7-di-2-thienyl-2,1,3-benzothiadiazole-comparative studies of intrachain and interchain interaction

High-efficiency red light-emitting diodes based on poly(9,9-dioctylfluorene) (PFO) doped chemically into the polymer main chain with extremely low dopant content of 4,7-di-2-thienyl-2,1,3-benzothiadiazole (DBT) (comonomer ratio:99.9∕0.1) are realized. External quantum efficiency of PFO-DBT0.1 reached 2.66% (photon/electron) with a luminance efficiency of 2.5cd∕A, significantly higher than that of the device with high DBT content. The copolymer exclusively emits red light peaked at 615nm due to strong intrachain energy transfer. Comparing photophysical properties and device performance between single copolymer PFO-DBT0.1 and blends with the same DBT composition by adding neat PFO into a PFO-DBT15, PFO-DBT25, or PFO-DBT35 copolymers, we concluded that energy transfer via intrachain interaction along a conjugated polymer main chain is more efficient than via interchain interaction in such copolymers and that chemically doped copolymers show higher device performance than polymer blend.