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

• Published in: Macromolecules Vol. 40; no. 17; pp. 6164 - 6171
• Main Authors: Tang, Weihua, Ke, Lin, Tan, Liwei, Lin, Tingting, Kietzke, Thomas, Chen, Zhi-Kuan
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 21.08.2007
• Subjects: Applied sciences; Electronics; Energy; Exact sciences and technology; Natural energy; Optoelectronic devices; Organic polymers; Photovoltaic conversion; Physicochemistry of polymers; Polymers with particular properties; Preparation, kinetics, thermodynamics, mechanism and catalysts; Semiconductor electronics. Microelectronics. Optoelectronics. Solid state devices; Solar cells. Photoelectrochemical cells; Solar energy; Substituent effect; Spectral stability; Voltage current curve; Solution polycondensation; Brightness; Photoluminescence; Electroluminescence; Thiophene copolymer; Experimental study; Thermal stability; Photovoltaic cell; Suzuki coupling; Organic solution; Electrochemical properties; Conjugated copolymer; Preparation; Aromatic copolymer; Organic light emitting diodes; Current efficiency; Alternating copolymer; Thiophene derivative copolymer; Fluorene derivative copolymer; Green light
• ISSN: 0024-9297; 1520-5835
• DOI: 10.1021/ma070575h

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 β-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 β-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 β-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 (P 2) exhibited pure green emission with a maximum brightness of 6000 cd/m2 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.