Effect of guest concentration on carrier transportation in blends of conjugated polymers

Abstract

In this paper, the charge transport in pure poly (9,9-dioctylfluorene-2, 7-diyl) (PFO) and its blend with poly (5-methoxy-2-2-ethyl-hexylthio)-p-phenylenevinylene (MEH-PPV) is studied. The mobility (μeff) and diffusivity (D) of the pure and blend thin films have been calculated using electroluminescence transient (ELT) technique. MEH-PPV concentration was varied from 0.8 to 15 wt%. It is found that at relatively low concentration of MEH-PPV, less than 1.2 wt%, the mobility of PFO:MEH-PPV blend increases with the concentration of MEH-PPV, and after that, it starts decreasing. The field dependence of effective hole mobility follows the Poole–Frenkel (P–F) plot of mobility (μ) as a function applied electric field with positive slope (βPF > 0) up to 5.0 wt% concentration of MEH-PPV. A negative P-F type dependence is then observed for 8.0–15 wt% MEH-PPV concentration. Diffusivity (D) of blends is following the same trends as mobility i.e. blend with 1.2 wt% MEH-PPV shows the highest diffusivity. These results have been correlated with the morphology of pure and blend thin films. Gaussian Disorder Model (GDM) alone is not able to explain the change in βPF with the MEH-PPV concentration, guest induced crystallization of PFO plays an important role at low concentrations of MEH-PPV. At higher concentrations of MEH-PPV (≥8 wt%), crystallization is suppressed, and position disorder induced behavior of polymer determines the charge transport in the blends.