(575e) Towards Ordered Layer-by-Layer Growth of Organic Semiconductors: Calculating the Ehlich-Schwöebel Barrier for Step Edge Descent

Semiconducting organic thin films are under intense scrutiny as materials for large area, mechanically flexible electronic displays, for ?smart tags? (RFID), for OLEDs (organic light emitting diodes) and for solar cells. The most promising candidate materials are π-conjugated molecules with high field effect mobility if they can be grown in an ordered 2D layer-by-layer thin film. It remains a challenge to determine effective nucleation and growth strategies to grow such films.

Our most recent computer simulation work has studied one phenomenon that affects the ability to grow smooth films, namely, the tendency of a molecule diffusing over the surface of a thin film to encounter a significant uphill energy barrier (the so-called Ehrich-Schwöebel barrier) near the edge of an island (the step edge). If this energy barrier is high, the molecule will tend to stay on the higher terrace rather than dropping down to the incomplete layer below, and thereby promote unwanted 3D growth.

We use a combination of energy minimization techniques and Molecular Dynamics simulations with semi-empirical models (MM3-π ) whose accuracy has been tested successfully against a variety of Density Functional Theory models. We will show results for the way that a group of organic semiconductor molecules (the acenes, DIP, rubrene, and C60) fall over a step edge and determine the factors that affect the energy barrier that they encounter.

We unravel the mystery why results in a 2008 Science paper show a very high barrier for step-edge descent by sexiphenyl molecules, highlighting the importance of molecular rotational degrees of freedom and the unexpected influence that the tendency of a molecule to twist and bend can have on the approach to the step edge. Finally, we offer a way to predict the Schwöebel barrier without doing any computer simulations.