Hopping-tunneling model to describe electric charge injection at metal/organic semiconductor heterojunctions

Abstract

This paper describes a theoretical model which takes into account charge injection by thermally stimulated hopping (HP), as proposed by Arkhipov, and Fowler-Nordheim tunneling (FNT), for steady-state response of a metal/organic semiconductor hetero-junction in a polymeric light-emitting diode (PLED). The reason for combining these two models is that both injection processes are usually observed in metal/organic semiconductor hetero-junctions. The present model takes into account the material disorder and the escape probability introduced by Arkhipov et al. for both injection processes. The proposed model was tested with experimental results obtained from a PLED ITO/MEH-PPV/Al. Analysis of the experimental results took into account the well-known characteristic of the MEH-PPV in which the charge transport is dominated by holes. Because of this, the electronic current component is assumed to be negligible compared to that for holes. Considering that the device is composed of two different metal/semiconductor junctions, that is, ITO/MEH-PPV and MEH-PPV/Al, the heights of the different potential barriers could be determined. The injection potential barrier, for holes, was determined as 0.4 eV for direct bias (ITO as anode), and as 0.8 eV for reverse bias (aluminum as anode). It was verified that the FNT process is preferable to the HP process in the following situations: (i) high interface potential barrier, (ii) high electric field, and (iii) low temperatures. On the other hand, that is, for a low interface potential barrier, low electric field and high temperatures, the HP process is dominant. With the application of the proposed model, which combines HP and FNT, it was possible to fit the current versus voltage curves for the studied PLED in a wide range of temperatures and electric fields, for direct and reverse bias. Additionally, it was shown that the condition where the HP injection probability is neglected, the non-linearity in FNT plots, ln(J(FNT)/F-2) versus F-1, obtained experimentally by several authors, may be a consequence of the material disorder. When the HP process is associated with the FNT a more significant non-linearity in the FNT plot was verified. (C) 2014 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim