Invited Paper SS1-ThA6
Interface Formation and Nanocrystallization in Molecular Semiconductor Films

Thursday, October 31, 2013, 3:40 pm, Room 201 A

Molecular materials have a natural tendency toward polymorphism due to the highly anisotropic nature of intermolecular interactions. Because crystal structure dictates key material properties, such as solubility and charge transport, crystallographic phase control is of intense interest to applications ranging from pharmaceuticals to organic electronics. In this talk we describe the kinetic assembly of molecular semiconductors into controlled arrangements. We show how crystalline monolayer films of pentacene (P) and titanyl phthalocyanine (TiOPc) can be produced with varied packings and orientations through controlled deposition. We subject metastable film structures to chemical (fullerene adsorption) tests to create charge-separating interfaces expected in OPV devices. Low-density molecular films rearrange spontaneously under C₆₀ (and C₇₀) deposition, yielding new co-crystalline phases with distinctive electronic features; whereas dense films generally yield abrupt semiconductor interfaces. We further show how highly polarizable fullerenes respond to the electrostatic contours of oriented molecular semiconductor films to form unexpected fullerene arrangements. Structure evolution in functionalized fullerenes further demonstrate how electrostatics impact phase selection. C₆₀- and C₇₀-PCBM molecular ordering is monitored from a glassy monolayer phase, produced by pulsed delivery of solute PCBM from a liquid microaerosol, to neat hcp crystalline arrangements inaccessible through homogeneous PCBM crystallization. We relate kinetic barriers for molecular ordering to solvent expulsion and PCBM reorientation, determining the critical PCBM surface density needed to induce nanocrystallization.

Acknowledgement: Portions of this work have been supported by the National Science Foundation Division of Chemistry-MSN and Division of Materials Research DMR-05-20471.