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

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. © 1999 American Institute of Physics.