Examinando por Autor "Alvarez, Angel Luis"

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Influence of electrical operating conditions and active layer thickness on electroluminescence degradation in polyfluorene-phenylene based light emitting diodes
(Elsevier, 2009) Romero, Beatriz; Arredondo, Belén; Alvarez, Angel Luis; Mallavia, Ricardo; Salinas, A; Quintana, Xabier; O, José Manuel
We have studied the influence of the electrical working conditions (voltage or current biased). and the active layer thickness on electroluminescence (EL) properties of polymeric light emitting diodes based on poly-[9,9-bis(6'-cyanohexyl)-2,7-fluorene-alt-co-1,4-phenylene], [PFP:(CN)(2)]. Diodes with different active layer thicknesses (55-140 nm) have been fabricated and characterized. Temporal evolution of the spectra at constant bias and current, as well as the spectral evolution with the current, has been performed. Excitation photoluminescence has been used to discriminate between intrinsic and defect-related transitions. The relative spectral area arising from defects has been quantified by means of Gaussian deconvolution for different device excitations. Active layer thickness has been observed to play an important role on the emissive spectral shape. In thick samples EL tends to resemble fluorescence from the pristine material. In contrast, thinner samples clearly show two additional bands related to defects: the first one is structured in the range 470-510 nm, which is proposed to be due to electron accumulation in the active layer, and a second band at 535 nm. arising from on-chain keto defects due to the presence of oxygen. The role of the electron blocking character of the PEDOT:PSS on the spectral shape, as well as the influence of the active layer thickness on the oxygen concentration, are discussed
On the electrical degradation and green band formation in alpha- and beta-phase poly(9,9-dioctyfluorene) polymer light-emitting diodes
(Elsevier, 2011) Arredondo, Belén; Romero, Beatriz; Gutiérrez, Araceli; Martínez, Armando; Alvarez, Angel Luis; Quintana, Xabier; Otón, José Manuel
In this work we report a detailed comparison of optical and electrical degradation between alpha- and beta-phase poly(9,9-dioctyfluorene) (PFO) based diodes. Analysis of the EL spectra along continuous operation time in alpha- and beta-PFO based diodes reveals that the unwanted green emission traditionally associated to fluorenone is more likely to occur in a-phase PFO. The relative spectral areas arising from excitonic and vibronic transitions as well as fluorenone defects have been quantified by means of Gaussian deconvolution along the operation time. The relative spectral area associated to the formation of the fluorenone increases 13% for the beta-PFO diode and up to 21% for the alpha-PFO diode only after 35 min of continuous operation. Analysis of the I-V curve before and after electrical stressing has lead to hole mobilities in pristine diodes of 1.4 x 10(-4) cm(2)/Vs and 1.6 x 10(-5) cm(2)/Vs for beta-PFO and alpha-PFO respectively. Both beta-PFO and alpha-PFO degraded samples show a reduction in the hole mobility, as well as an increase in the width of the Gaussian density of states.