Electron transport in the organic small-molecule material BAlq - the role of correlated disorder and traps

An analysis is presented of the layer-thickness dependent and temperature-dependent current density in sandwich-type electron-only devices based on the amorphous small-molecule organic semiconductor BAlq, which is frequently used in organic light-emitting diodes. The electron transport can be consistently described by assuming a density of states (DOS) which is a superposition of a Gaussian DOS and an exponential trap DOS, with 85 and 100 meV widths, respectively, using a mobility model which includes the carrier density dependence of the mobility in the Gaussian DOS and assuming either random or spatially correlated site energies. From a comparison of the density of hopping sites obtained from both models and the density of molecules as obtained from chemical analysis, evidence for the presence of correlated disorder is found.