Luminescence properties of poly(p-phenylenevinylene): Role of the conversion temperature on the photoluminescence and electroluminescence efficiencies

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Abstract
We have investigated the luminescence properties of poly(p-phenylenevinylene) (PPV) prepared via the standard precursor route as a function of the conversion temperature in the range 170–270 °C. In particular, we have determined the absolute photoluminescence (PL) efficiencies of PPV thin films prepared on quartz or indium–tin oxide (ITO) coated glass substrates and found that the dependence on conversion temperature is different, depending on the type of substrate. The optical data show that heating at 170 °C for 10 h is sufficient to achieve full conversion. For PPV on quartz, a further increase of the temperature induces a decrease of the PL efficiency, whereas for PPV on ITO the PL efficiency shows a nonmonotonic dependence on the conversion temperature, with a maximum for conversion at about 205 °C. We discuss this behavior with reference to the interplay between the decrease in concentration of PL-quenching impurities (formed by reaction of the conversion byproducts and ITO) and an increase of exciton quenching efficiency due to polymer oxidation and/or crystallization, with increasing conversion temperature. We have also investigated the dependence on conversion temperature of the electroluminescence (EL) efficiency of single-layer ITO/PPV/Ca light-emitting diodes (LEDs) and of two-layer LEDs where an electron-transport/hole-blocking layer [namely 2-(4-biphenylyl)-5-butylphenyl-1,3,4-oxadiazole blended with polystyrene] is inserted between the PPV and the calcium cathode. The EL efficiency for the single-layer devices increases monotonically with increasing conversion temperature, whereas it decreases for the two-layer diodes. This complex behavior is due to the combined effects of the conversion temperature on the luminescence and injection/transport processes in PPV and is consistent with an interpretation based on a hole mobility which decreases with increasing conversion temperature.