Abstract
Abstract By means of chemical modification, flexible branched alkoxy chains were successfully attached to the para- site of the side phenyl groups of DP-PPV to obtain a new soluble fully conjugated PPV, named poly(2,3-di-[p-(2’-ethylhexoxy)phenyl]- 1,4-phenylenevinylene) (p-EHDP-PPV). Spectroscopic characterizations using UV, PL and PLE showed only the emission of single chromophore of this polymer in the solution state, indicating that the solubility of DP-PPV was significantly improved by attaching the alkoxy side chains. In the spin-coated film, the emission of the single chromophore with longer conjugating length was observed. In addition, the spectral feature showed the absence of the emission from the aggregate, which was usually observed in the alkoxy-modified PPV, revealing that the polymer chains were well-separated by the side chains. The performance of the single-layer ITO/PEDOT/ p-EHDP-PPV/Ca/Al LED device was also reported. The maximum luminescence and efficiency at 11 V was 3735 Cd/m2 and 0.57 Cd/A, respectively. The structure of p-EHDP-PPV in the solution with THF and toluene and the morphology of the films cast from these solutions were further investigated by spectroscopic and X-ray scattering techniques. The emission of the single chromophore with slightly different conjugation length on varying the polymer concentration was observed for both solutions. However, the photoluminescent properties of the drop-cast films were strongly dependent on the solvent used. A new ground-state emitting species with lower energy level formed in the film as cast from THF solution. On basis of the SAXS results, the lower energy species was attributed to a lamellar structure (L1) with the interlamellar distance of 19.6 Å. Through high-temperature annealing, another lamellar mesophase (L2) with the interlamellar distance of 26.2 Å developed from the disordered phase and coexisted with the solvent-induced L1 phase. Analysis of the WAXS patterns revealed that these two lamellar mesophase had similar microstructure and were considered to form independently in the polymer film via different pathways.