Correlation between electroluminescence, charge transport and photophysical properties of polymer blends

Abstract

A comprehensive study of the electrical and optical performance of white polymer light-emitting diodes (PLEDs) based on blends of polyfluorene (PFO) and poly (2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV) is reported in this paper. MEH-PPV concentration was varied from 0 to 15wt.%. Maximum luminance and current efficiency was obtained at 2.0wt.% of MEH-PPV. To understand the origin of this enhancement, electrical and emission properties of the blends were investigated over the entire range of MEH-PPV concentration. Charge carrier mobility was estimated using an electroluminescent transient technique (ELT) which was found to be increased at the lower concentrations of MEH-PPV, however, at higher concentration it was lower than that of pristine PFO.The highest carrier mobility of 2.6×10−5cm2v−1s−1 was observed at 1.2wt.% MEH-PPV in PFO. Fowler–Nordheim (F-N) model was used to estimate the potential barrier height at anode interface. It was found that the blending with MEH-PPV reduced the hole injection barrier in the PLEDs up to 5.0wt.% of MEH-PPV. At concentrations greater than 5.0wt.%, barrier values started increasing again. Correlating these properties with the current density-voltage curves of PLED, it was observed that current density in the device increased up to 5.0wt.% of MEH-PPV, this was in good agreement with the behavior of barrier height with MEH-PPV concentration. Therefore, it was inferred that reduction in the barrier height is the dominant effect in improving the luminance of blend based PLEDs. However, it was also observed that maximum current efficiency (1.92cd/A) occurred at 2.0wt.% of MEH-PPV blend in PFO with Commission Internationale de l’Enclairage (CIE) coordinates close to white light (0.33, 0.33). This meant that emissive characteristics of the blends were also changing with wt.% of MEH-PPV. To investigate this, photophysical properties, photoluminescence (PL) and photoluminescence quantum efficiency (PLQE) of the binary blends were also studied. It was observed that blending lead to lower PLQE and hence would lead to decrease in the current efficiency with MEH-PPV wt.%, if energy transfer from host to guest was the only mechanism for exciton formation in blend based PLEDs. However, comparing the electroluminescence (EL) and PL behavior of blends, enhancement in current efficiency was observed due to direct excitation of MEH-PPV in PLEDs. These two opposite effects, reduction in PLQE of the blend and direct excitation of MEH-PPV, resulted in best current efficiency of PLED at 2.0wt.%.