Study of direct current charge transport mechanism in a series of iridium metal complexes based on varying ancillary and cyclometalated ligands

Abstract

The mechanism of charge transport in a series of iridium metal complexes prepared with varying ancillary and cyclometalated ligands is reported here. The structural conformation of the synthesized complexes is acquired via NMR spectroscopy and elemental analysis. The photophysical properties of the samples are investigated through UV–visible and photoluminescence spectroscopy. The investigation of the temperature-dependent dc electrical conductivity of the samples is conducted within the temperature range of 280–473 K. The room temperature conductivity of all samples is determined to be of the order ∼10⁻⁸ S/cm. All the complexes exhibit semiconducting behavior in a certain temperature range above room temperature. A continued elevation in temperature leads to reduction in the conductivity of all the samples resulting in metal-like conduction behavior above a certain temperature. The transition temperature of the iridium metal complexes is attributed to the removal of the coordinated water molecule from the samples as suggested by thermal analysis of the complexes. The removal of water molecule from the sample is expected to localize the electron densities which consequently results in a fall of the conductivity in the samples. The conduction mechanism in the samples is analyzed in the light of band conduction mechanism and Mott’s variable range hopping model. All the samples are found to obey 1-D VRH mechanism. The activation energy and the corresponding Mott’s parameters are derived from the ln σ vs 1/T curves.