Green polyfluorene-conducting polymer interfaces: Energy level alignment and device performance

Photoemission studies on interfaces between an electroluminescent green polyfluorene (GPF) and two conducting polymers comprising ethylenedioxythiophene and sulfonate moieties are reported. Despite the chemical similarity of both conducting polymers, the hole injection barriers to GPF were found to differ by $0.35eV$ and the interface dipoles by up to $−0.4eV$⁠. Polymer light emitting devices fabricated with the two conducting polymers exhibit nearly identical current-voltage characteristics, but the luminance-voltage characteristic is enhanced by a factor of 20 by the presence of near-surface poly(styrenesulfonate). We attribute the enhancement to the electron- blocking behavior of the sulfonate moieties at the GPF interface and the concomitant modification of the device built-in voltage.