Determination of Traps in Poly( p -phenylene vinylene) Light Emitting Diodes by Chargebased Deep Level Transient Spectro
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Determination of Traps in Poly(p-phenylene vinylene) Light Emitting Diodes by Chargebased Deep Level Transient Spectroscopy Olivier Gaudin, Richard B. Jackman, Thien-Phap Nguyen1, Philippe Le Rendu1 Department of Electronic and Electrical Engineering, University College London, Torrington Place, London WC1E 7JE, United Kingdom 1 Laboratoire de Physique Cristalline, Institut des Matériaux Jean Rouxel, Université de Nantes, 2 rue de la Houssinière, 44 322 Nantes Cedex 03, France
ABSTRACT Charge-based deep level transient spectroscopy (Q-DLTS) has been used to study the defect states that exist within poly(p-phenylene vinylene) (PPV), a semiconducting polymer with a band gap of about 2.4 eV. The technique allows the determination of activation energies, capture cross-sections and trap concentrations. In some circumstances, it is also possible to distinguish between minority and majority carrier traps. The structures investigated here consisted of ITO/PPV/MgAg light emitting diode (LED) devices. Two types of trapping centres were found. The first type has activation energies in the range 0.49 – 0.53 eV and capture cross-sections of the order of 10-16 – 10-18 cm2. It shows a Poole-Frenkel, field assisted-emission process. This level has been identified as a bulk acceptor-like majority carrier (i.e., hole) trap. The second type has activation energies in the range 0.40 – 0.42 eV and capture cross-sections of the order of 10-19 cm2. This level has been identified as a minority carrier (i.e., electron) trap. This second trap type is therefore expected to limit minority carrier injection into the PPV layer within the LED, and hence reduce electroluminescence under forward bias conditions.
INTRODUCTION Defect states in semiconductors and insulators are known to affect the device performance by the introduction of energy levels in the band gap or at the metal/semiconductor interface. In the specific case of light emitting diodes (LEDs) traps in the emitter reduce the efficiency of the device either by acting as non-radiative recombination centers or by trapping the charge carriers, thus producing an unbalanced charge state within the emitter and eventually limiting the radiative recombination process. A number of techniques like thermally stimulated current (TSC) [1-3], broadband impedance spectroscopy [4] and capacitance transients measurements [5] have so far been applied to the study of defect states within PPV films. In this article we report on the study of traps within PPV-based LEDs consisting of ITO/PPV/MgAg structures using chargebased deep level transient spectroscopy (Q-DLTS). The technique allows the determination of the activation energy, capture cross-section and density of the traps. Q-DLTS, which is based on the measurement of charge transients, has been shown to present a number of advantages over the more widely used capacitance-based DLTS (C-DLTS) technique [6] in a number of specific situations where the small signal capacitance of the device is independent from the charge state of the sample [7-11]. This
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