Morphology evolution in nanoscale light-emitting domains in MEH-PPV/PMMA blends

The compositional evolution of the morphology of blends of a light-emitting conjugated polymer MEH-PPV [MEH-PPV = poly(2-methoxy-5-(2'-ethylhexyloxy)-p-phenylenevinylene)] with poly(methyl methacrylate) (PMMA) on a PEDOT/PSS [PEDOT/PSS = poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate)] coated glass substrate is reported. Results of AFM (topographic and phase,images), optical microscopy, fluorescence microscopy, and photographs of the emission from light-emitting devices (LEDs) illustrate this morphological evolution. The 5 wt % compositional study demonstrates the stability of morphology for the blend films dried in air, vacuum-dried at room temperature, and vacuum-dried at 80 degreesC. Annealing above the T-g (106 degreesC) of PMMA caused significant dewetting between the blend and the PEDOT/PSS. At low MEH-PPV compositions, isolated light-emitting domains ranging in diameter from 300 to 900 nm and similar to100 nm in thickness were observed. At approximately 50 wt % MEH-PPV, a phase inversion was observed and a bicontinuous morphology evolved. The highest efficiency LEDs were obtained at 75 wt % MEH-PPV where a continuous light-emitting polymer structure exists surrounding isolated domains of PMMA.