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

Poly(9,9-dioctylfluorene-alt-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 1H and 13C 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 (10 521 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.