Structure-morphology-electroluminescence relationship for hybrid conjugated polymers

A comparative study of a series of conjugated polymers is described. The solution absorption spectra of polymers 1-3 show unusual red shifts relative to their film spectra, which is due to strong aggregate formation in the concentrated solution. The polymer with octadecyloxy side chains shows improved LED performance because of increased solubility, film-forming capability, and optimized film morphology. The long side chains are also capable of forming small insulating domains which inhibit exciton interchain migration thereby enhancing exciton confinement and EL efficiency. 2-Ethylhexyloxy side chains also favor exciton confinement due to their increased steric hindrance to interaction between neighboring conjugated main chains. AFM and contact angle measurements indicate that polymer film surface properties are controlled by polymer backbone structure and by the length, geometry, and polarity of the side group. The polymer film surface morphology affects the local electrical field distribution within the LED, electron injection/transport at the polymer/Ca interface, and overall it influences the device performance.