Molecular orientation and charge transfer dynamics of terpolymer films for optoelectronic applications

The integration of donor-acceptor (D-A) functional units has been widely used as a strategy for designing more efficient copolymers to optoelectronic devices. In the present work, four terpolymers – three statistical and one block – films composed by 9,9-dioctylfluorene (A), 2,1,3-benzothiadiazole (B) and 9-(9-heptadecanyl)-9H-carbazole (C) monomers in addition to two others terpolymers – one statistical and one block – composed by A and C along with 4,7-dithien-2-yl-2,1,3-benzothiadiazole (D) monomer were evaluated by X-ray photoabsorption (NEXAFS) and resonant Auger electron (RAES) spectroscopies. According to transitions intensities variation with the incident angle of the photon beam at the S K-edge photoabsorption spectra, all films present a well-defined molecular orientation in which all the units with a sulphur atom in the copolymer has a preferred "lie down" geometry. By employing the core-hole clock (CHC) method from RAES results, ultrafast charge transfer times were estimated for each film. The polymers with the D monomer showed a low charge transfer time for most of their resonances while the ones with B monomer yield longer times, possibly reflecting the acceptor character of this unit. (author)