Individual Pristine Research Articles

Poly(2-methoxy-5-(2′-ethyl-hexyloxy)-p-phenylenevinylene) (MEH-PPV) and poly(2,3-diphenyl-5-octyl-p-phenylenevinylene) (DPO-PPV) are basically immiscible as verified by fluorescence spectroscopy and differential scanning calorimetry. This immiscibility results in insufficient energy transfer from DPO-PPV to MEH-PPV in photoluminescence. A vertically segregated structure with DPO-PPV green domains dispersing on the MEH-PPV-rich matrix was observed in a spin-cast film by two-photon excitation microscopy. The turn-on voltages for electroluminescence (EL) of polyblends were lower than those of their individual pristine polymers, while their EL quantum efficiencies were higher. Because both the highest occupied molecular orbital and the lowest unoccupied molecular orbital levels of MEH-PPV are higher than those of the DPO-PPV, the energy gap of the vertical heterojunction (1.98 eV) in polyblends is lower than that of the pristine MEH-PPV (2.14 eV), which is believed to result in the improved EL properties.