Conjugated copolymers based on fluorene-thieno[3,2-b]thiophene for light-emitting diodes and photovoltaic cells

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 degrees C and their glass transition temperatures (T-g) ranging from 126 to 146 degrees 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.