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Article | Center for Nanoscale Materials

Large rectification in molecular heterojunctions

The outstanding challenge in using molecules in optoelectronics devices is to create electrical functionality through molecular design and to go beyond the use of molecules as mere light absorbers and/or resistive elements. The earliest proposal for such non-linear electrical behavior is the Aviram-Ratner molecular diode model, proposed in 1974. However, more than forty years later, the electrical performance of such molecular devices remains several orders of magnitude below the one of their inorganic counterparts. A primary reason is that, despite their high tunability, molecules are very sensitive to their immediate environment, so that much of their desirable intrinsic electrical properties are lost when integrated into actual devices. Minimizing such effects leads to an apparent paradox, as it implies to physically decouple the electrodes from the active region of the device, which dramatically degrades its electrical performance.

In their work published in Nano Letters, CNM user Joseph Smerdon (University of Central Lancashire) and researchers in CNM’s Theory and Modeling and QEM Groups show this paradox can be entirely overcome by using metallized molecules as a buffer layer between the active device region and the metallic electrodes. Using the inherently weak interactions between two prototypical organic materials (C60 --or Buckminsterfullerene--  and Pentacene) and the strong coupling between C60 and the (111) surface of Copper, they show that electrical rectification can be enhanced by more than two orders of magnitude in the presence of the metallized C60 layer. Using CNM’s ultrahigh vacuum system with surface preparation and scanning tunneling microscopy capabilities, they demonstrate that this first-of-its-kind system, reminiscent of a Schottky diode, already has a diodic behavior comparable with the best performers in the field of molecular diodes, with average rectification ratios in excess of 1000 at 1V. These findings open the possibility of engineering non-linear electrical behavior on a nanometer length-scale in organic optoelectronics and photovoltaics.

These findings were further corroborated by using density functional theory and electronic transport calculations, performed using CNM’s high-performance computing cluster, Carbon.

J. A. Smerdon, N. C. Giebink, N. P. Guisinger, P. Darancet, and J. R. Guest, Nano Letters, 16, 2603, 2016