Temperature dependent broadband impedance spectroscopy on poly-(p-phenylene-vinylene) light-emitting diodes

Abstract

Articles you may be interested inTemperature-dependent electroluminescence spectra of poly(phenylene-vinylene) derivatives-based polymer light-emitting diodes J. Appl. Phys. 97, 034505 (2005); 10.1063/1.1845580Determination of traps in poly(p-phenylene vinylene) light emitting diodes by charge-based deep level transient spectroscopy Using temperature dependent impedance spectroscopy in a broad frequency range (10 Ϫ1 -10 7 Hz͒, we have found that the ac behavior of indium-tin oxide ͑ITO͒/poly-͑p-phenylene-vinylene͒ ͑PPV͒/aluminum light-emitting diodes shows several features which cannot be described by the usual simple double RC circuit representing a depleted junction region and an undepleted bulk. Instead, our measurements in combination with a theoretical modeling suggest that the PPV bulk is composed of a highly doped region at the ITO interface and a region with lower doping at a higher distance to the ITO. Moreover, the boundary between these two regions is not sharp but there is a gradual change in dopant concentration. The large frequency range allowed us to identify two distinct processes corresponding to the PPV bulk and a third one to the junction. The bulk relaxation frequencies correspond to the characteristic dielectric relaxation frequencies of charge carriers in the high and low conducting sublayers and are proportional to the respective conductivities. The magnitude and activation energy of the relaxation time correlates well with results obtained from temperature dependent DC conductivity measurements. For ITO substrates we obtain activation energies of 0.4 eV and room temperature conductivity of about 10 Ϫ7 and 10 Ϫ9 S/cm for the high and low conducting sublayers, respectively. On gold substrates only one bulk process and no junction process with an activation energy of about 0.6 eV and a corresponding conductivity of 3ϫ10 Ϫ11 S/cm at room temperature is observed. The Schottky junction has been studied by temperature dependent capacitance-voltage spectroscopy at a low frequency of 0.16 Hz. The obtained acceptor dopant concentration from 1/C 2 plots varies from 1.4ϫ10 17 at room temperature to 6.9ϫ10 16 cm Ϫ3 at 200 K. Assuming a density of states between 5ϫ10 20 and 5ϫ10 21 cm Ϫ3 for the valence band the temperature dependent acceptor dopant density can be described with an acceptor ionization energy between 0.16 and 0.2 eV.

Analysis of deep levels in a phenylenevinylene polymer by transient capacitance methods

“…The found activation energies agree with the values ͑0.6-1.0 eV͒ reported by others using thermally stimulated currents in PPV prepared by the precursor route. 7,8 The results provide evidence for more levels for which the activation energies could not be determined. Subsidiary experiments using admittance measurements gave a bulk activation energy for conduction of 0.12 eV, suggesting the presence of a shallow acceptor state.…”

“…Thermally stimulated currents in ITO/PPV/Al devices 7,8 indicate the presence of several distinct gap states with energies in the range 0.05-0.2 and 0.6-1.0 eV. It was also reported 9 that the current-voltage characteristics of ITO/ PPV/Al devices can be fitted using an exponential distribution of traps with a characteristic energy of 0.15 eV.…”

Effect of dielectric barrier on rectification, injection and transport properties of printed organic diodes

“…One of the most important effects in determining the electrical structure of the contact is Fermi-level pinning of the semiconductor. Here, we verify the different behaviour of two diode structures by using impedance spectroscopy, which has been used by several groups to study the behaviour of metal/organic interfaces and interfacial layers in organic (light-emitting) diode structures [10][11][12][13][14]. X-ray photoelectron spectroscopy (XPS) is a well-known characterization method for determining the chemical composition and depth distribution of elements on the surface of various materials.…”

Small‐signal characteristics of organic semiconductors with continuous energy distribution of traps

“…Within this technique, a small ac component of frequency ƒ is superimposed to the steady state applied voltage V0, yielding electrical characteristics presented as frequency-dependent real or imaginary parts of the admittance, or equivalently, of the impedance or the dielectric function. The as-obtained results are frequently interpreted in terms of equivalent electrical circuits [9][10][11][12][13][14][15][16]. Transient capacitance measurements have also been used to determine trap concentrations and trap activation energies [17,18].…”