Conjugated Copolymers Based on Fluorene-Thieno[3,2-b]thiophene for Light-Emitting Diodes and Photovoltaic Cells

摘要

Alternating conjugated copolymers comprised of 9,9-dihexylfluorene and unsubstituted/substituted thieno[3,2-b]thiophene moieties were synthesized using the Suzuki coupling reaction.The structures of these polymers were characterized,and their thermal,photophysical,electrochemical,and electroluminescent properties were investigated.By beta-substitution of thieno[3,2-b]thiophene moiety with alkyl chain,the thermal,electrochemical,and electroluminescence properties of the resulted polymers were tuned.The polymers exhibited good thermal stability with decomposition temperature(T_d)in the region of 308-431 °C and their glass transition temperatures(T_g)ranging from 126 to 146 °C.Incorporation of beta-alkyl chain onto thieno[3,2-b]thiophene of the alternating copolymers led to lowered T_d and T_g and blue-shifted maximum wavelengths in both absorption and emission spectra.The influence of beta-alkyl substitution on the conformation of the polymer chains was evaluated by simulating the optimized geometries of three model oligomers using density functional theory.Light-emitting diode devices were fabricated with a configuration of ITO/PEDOT:PSS/polymer/Ca/Ag for all the polymers.A device based on fluorene and unsubstituted thieno[3,2-b]thiophene polymer(P2)exhibited pure green emission with a maximum brightness of 6000 cd/m~2 and current efficiency of 2.7 cd/A.More importantly,this device demonstrated very good spectral stability and robust emission,with only 21% decay in luminance after 64 h driving at 1 mA of constant current.In addition,the potential application of the copolymers for photovoltaic solar cells was discussed.