Computational investigation of charge injection and transport properties of a series of thiophene-pyrrole based oligo-azomethines

The present study explores the structural, charge carrier injection and transport properties of a series of thiophene-pyrrole based oligo-azomethines using density functional theory (DFT) methods. Our findings show that the presence of a bulky substituent adversely affects these properties. However, the electronic effect of substituents may be utilized to tune these properties by substitutions at suitable positions. Values of frontier orbitals, ionization energies, and electron affinities are calculated for each compound to predict the ease of charge injection from metal electrodes to these azomethines and the stabilities of their ionic forms. In addition to having large injection barriers, lack of stability of the anions may hinder the electron injection. However, most of the compounds have excellent hole injection capability. Computation of reorganization energies and electronic couplings followed by charge transfer rates and mobilities show large carrier mobilities for some of the studied compounds. Considering both the injection capability and carrier mobilities, it is found that a thiophene-pyrrole azomethine without any substituent and substituted azomethines with a methyl, methoxy or amine group at the 3 position of the pyrrole ring may act as efficient materials for the hole transport layer.