Ehrlich-Schwoebel Effect for Organic Molecules: Direct Calculation of the Step Edge Barrier using Empirical Potentials

The step edge barrier of a prototypical organic semiconductor molecule, 3,4,9,10-perylene-tetracaboxylic-dianhydride (PTCDA) has been analysed by means of calculations based on emperical potentials. The minimum energy path (MEP) has been calculated for a single molecule on a substrate of three molecular layers between equivalent minimum energy positions within two neighboring unit cells. To determine the step edge barrier, we have calculated the MEP over a step to a fourth layer of molecules. We found energy barriers of E_D= 80 meV for in-layer diffusion and E_S = 750 meV for step crossing, indicating a strong Ehrlich-Schwoebel effect for PTCDA.