Grain Orientation Mapping of Polycrystalline Organic Semiconductor Films by Transverse Shear Microscopy

Polycrystalline organic semiconductor films play a central role in organic electronics because their inherent order, relative to amorphous films, facilitates more efficient charge transport. Carrier mobilities in crystalline organic semiconductors are generally at least a factor of one hundred greater than in their amorphous counterparts, which is attractive for certain device applications, such as organic field effect transistors (OFETs), where higher charge mobilities result in better performance. In analogy with conventional semiconductors (e.g., poly-Si), the electrical performance of polycrystalline organic semiconductor layers is sensitive to grain morphology and alignment, as well as to defects. Indeed, recognition of the importance of microstructure has lead to extensive structural characterization of organic semiconductor films by X-ray diffraction, and optical, electron, and scanning probe microscopy. Yet there are still many aspects of organic semiconductor microstructure that are not well understood and detailed correlationswith transport are rare. One surprising bottleneck to understanding microstructureproperty relationships has been the difficulty of producing clear images of grains in extremely thin, coalesced layers of organic semiconductors on technologically relevant substrates, such as gate dielectrics, which are critical components of OFETs.