Fluorene-Benzothiadiazole Copolymer for Single Component Green Light-Emitting Electrochemical Cells

Solid-state, single-component green light-emitting electrochemical cells (LECs) based on a fluorene-benzothiadiazole copolymer has been successfully demonstrated. The solubilizing side groups are octyl and 2-(2-(2-methoxyethoxy)ethoxy)ethyl, with the latter introduced to promote ionic conductivity necessary for the operation of the LECs. The copolymer was synthesized by Suzuki coupling reactions with number-average molecular weight between 5,600 and 13,200 and polydispersity between 2.06 and 3.21. The LECs were fabricated by spin-coating the copolymer admixed with lithium trifluoromethanesulfonate onto indium-tin oxide (ITO) substrate coated with polyethylenedioxythiophene-polystyrene sulfonic acid (PEDOT:PSS). The opposite electrode was vapor evaporated aluminum. The LEC devices shows green electroluminescence with CIE coordinates of (0.37, 0.59). The devices exhibit comparable performance with corresponding light emitting diodes using evaporated barium as cathode. The copolymer with higher benzothiadiazole (BT) content shows relatively higher LEC device performance. Cyclic voltammetry and chemical doping measurements indicate that the polymer with higher BT content in the backbone can be more readily n-doped, which is consistent with the higher LEC performance.