Effects of Molecular Stresses on Energy Transfer Pathways in Opto- and Electro-Excited Conjugated Polymers for High-Efficiency Optoelectronic Devices.

摘要

The role of mechanical stresses in the optoelectronic behavior and exciton formation of conjugated polymers was explored in order to understand and to exploit the massive efficiency enhancements produced by mechanical stretching. The molecular chains of conjugated polymer MEH-PPV were mechanically stretched by capillary forces via the dewetting process induced by solvent annealing rather than thermal annealing in order to rid of the potential effects of thermal degradation and to open the possibility for industrial applications. Similar results of photoluminescence enhancements, about two orders of magnitude increases, were obtained as those by thermal dewetting, indicating that capillary force indeed can be used to stretch the polymer chains for the purpose of efficiency enhancements. These blue shifts were proved to be due to molecular kinks that resulted from molecular flows of the entangled macromolecules. Poor solvents produced even greater chain stretching effects than with good solvents, a result attributed to reduced stress relaxation during the dewetting process. Stretching enacted by capillary forces required full penetration of the solvent into the whole film, indicating that interfacial adhesion to the substrate can effectively constrain movement of confined polymer chains. As revealed by temperature effects stretching-induced efficiency enhancements were found to accompany a decrease in the Huang-Rhays parameter, which is a key identifier of electron-phonon coupling. In addition, as revealed by the up-conversion confocal experiments, energy decay in the ultrafast regime (< 10 ps) was found to slow remarkably in stretched polymer chains residing in the dewetting droplets. A separate experiment of stretching graphene-containing MEH-PPV films was performed to check the effect of mechanical stretching on heterojunction quenching. It was found that mechanical stretching indeed can reduce the heterojunction quenching.