A Green Polymeric Light-Emitting Diode Material: Poly(9,9-dioctylfluorene-alt-thiophene) End-Capped with Gold Nanoparticles†

Poly(9,9-dioctylfluorene-alr-thiophene copolymer (PDOFT) is functionalized with thiol and end-capped with in-situ-reduced gold nanoparticles (AuNPs). The molecular structure of the resulting material (PDOFT-Au) is corroborated by 'H and 13 C NMR spectroscopy, and direct evidence for the binding between the PDOFT-bis-4-thiol and gold nanoparticles is provided from X-ray photoelectron spectroscopy: PDOFT-Au is not only soluble in common organic solvents, but also has a broad range of thermal stability, up to 414 °C. The photoluminescence and electroluminescence spectra show that excitation of PDOFT is virtually unaffected by the end-capping with gold nanoparticles. However, atomic force microscopy shows that the root-mean-square roughness of the PDOFT-Au film is nearly ten times higher than that of the PDOFT film, resulting in an increased interfacial area between the film and the deposited cathode in a PDOFT-Au device. This increased interfacial area, together with the photo-oxidation-suppressing and hole-blocking characteristics of AuNPs, significantly enhances the electron injection, lowers the threshold voltage, and increases the electroluminescence (10521 cd m -2 ) and photometric efficiency (1.986 cd A -1 ) of the PDOFT-Au device by nearly an order of magnitude. These increases in electroluminescence and photometric efficiency would be much lower if AuNPs were blended into-rather than capped onto-the copolymer. The Commission International de L'Eclairage color coordinates of PDOFT-Au (0.237,0.655) are very close to the standard green demanded by the National Television System Committee, making PDOFT-Au an excellent candidate for a green-light-emitting material.