Admittance Spectroscopy on Polymer Light-Emitting Diodes
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Admittance spectroscopy on polymer light-emitting diodes P.W.M. Blom1, H.C.F. Martens2, H.B. Brom2, J.N. Huiberts3 1 Materials Science Centre, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands 2 Kamerlingh Onnes Laboratory, Leiden University, P.O. Box 9504, 2300 RA Leiden, The Netherlands 3 Philips Research Laboratories, Prof. Holstlaan 4, 5656 AA Eindhoven, The Netherlands
ABSTRACT From admittance spectroscopy measurements on poly(p-phenylene vinylene) based light-emitting diodes various relaxation processes can be observed. At low bias inductive contributions due to the transit of charge carriers dominate, at high bias capacitive contributions as a result of charge redistribution appears. These processes reveal a transition from space-charge limited to recombination limited behavior, as is also indicated by the current-voltage characteristics.
INTRODUCTION Due to their favorable processing and mechanical properties polymer lightemitting diodes (PLEDs) have been considered as promising candidates for large area displays [1]. Attention has been focused on PLEDs of poly(p-phenylene vinylene) and its derivatives as a result of their promising conversion efficiencies, larger than 1% photon/carrier. Understanding of the device operation of PPV-based PLEDs is important for the further improvement of the performance of future devices. An intriguing question is how far existing device models, as developed for inorganic LEDS, are applicable to PLEDS. First we review the basic model for a double carrier (electron and hole) device with two ohmic contacts [2]. In such a device the currentvoltage characteristics are governed by a combination of space-charge effects and recombination efficiency of electrons and holes. Depending on the relative strength of the recombination process the current is either space-charge limited, recombination limited, or a combination of both [2]. For bimolecular recombination an analytical solution for the current density J as a function of voltage V has been obtained by Pamenter and Ruppel, described by 9 V2 J = ε 0ε r µeff 3 8 L
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with ε0εr the dielectric constant, L the device thickness and µeff an effective mobility. The relative strength of the recombination process is expressed in a so-called recombination mobility µr = ε0εrB/2q, with q the electronic charge and B the bimolecular recombination constant. Comparing this µr with the charge carrier mobilities µn and µp for electrons and holes then reveals whether space-charge effects or recombination dominate the current. For strong recombination, with µr >> µn , µp , µeff equals µn + µp [2]; the total current in the device equals the sum of the two singleC3.3.1
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Figure 1. Experimental and calculated J-V characteristics for a hole-only (ITO/PPV/Au) and PLED (ITO/PPV/Ca) device with thickness L=300 nm.
carrier currents. In this case injected electrons and holes an
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