Transmission Electron Microscopy Analysis of the Microstructure of Nanocrystalline Small-Molecule Semiconductors in Functional Organic Thin-Film Transistors.

Transmission electron microscopy has been employed to investigate the microstructure of nanocrystalline vacuum-deposited thin films of the small-molecule organic semiconductor 2,9-diphenyl-dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (DPh-DNTT) in functional bottom-gate organic thin-film transistors (TFTs) using both cross-sectional and plan-view specimens. Since the charge transport in organic TFTs is confined to the first molecular layer adjacent to the interface with the gate dielectric, the microstructure of this first molecular layer is of critical importance for the TFT performance, and transmission electron microscopy (TEM) is the most powerful technique to analyze this buried molecular layer. Direct imaging reveals that the DPh-DNTT molecules are oriented edge-on with their long axis in the vertical configuration with respect to the gate dielectric surface. An additional phase in which the DPh-DNTT molecules are oriented in the face-on configuration is observed in the protrusions that typically form in vacuum-deposited DPh-DNTT thin films; however, this face-on configuration is found only on top of layers in which the DPh-DNTT molecules are in the edge-on configuration. This suggests that the edge-on configuration serves as a template for the growth of the face-on configuration.