Influence of electrical operating conditions and active layer thickness on electroluminescence degradation in polyfluorene–phenylene based light emitting diodes

Abstract

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.