Infrared spectra of the light-emitting diodes fabricated with poly(2-methoxy-5-(2′-ethylhexyloxy)-p-phenylenevinylene) have been measured in situ with an external reflection apparatus on a Fourier-transform infrared spectrophotometer. Voltage-induced infrared absorption spectra have been measured using the difference-spectrum method. The observed bands have been attributed to injected positive carriers (polarons) on the basis of the spectra of the carriers generated by iodine-doping, Na-doping, and photo-irradiation. The observation of positive carriers is probably related to the predominance of injected positive carriers, which reduces the electroluminescence efficiency of the light-emitting diodes.
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