Molecular co-self-assembly of technological-relevant organic electron-donor and electron-acceptor molecules into ordered heterostructures on surfaces provides a fundamental molecular-level insight into how these two kinds of molecules would interact in a model organic photovoltaic cell with appropriate electrodes. I will present a study of the self-assembly of functionalized pentacenes (electron donors) and fullerenes (electron acceptors) into a layer-by-layer heterojunction on stepped gold surfaces by scanning tunneling microscopy (STM) characterization and density functional theory (DFT) calculations . The pentacene derivative -- 6,13-dichloropentacene (DCP) -- forms a striking long-range ordered brick-wall self-assembled monolayer (SAM) on a stepped Au(788) vicinal surface, with the long-axis parallel to the step edges [1]. Subsequently deposited fullerene ( C60) molecules form parallel triple, double, and single long molecular chains on top of the intact DCP SAM on Au(788) [2]. The novel organic-metal and organic-organic interfacial interactions, as well as the adsorption geometry, have been explored by DFT. The adsorbed C60’s form parallel commensurable (3:2) molecular chains on top of the DCP lattice -- three fullerenes line up with two DCP molecules along the long axis. The initial C60 chains grow along the upper step edges of the DCP/gold terraces. The observed adsorption position for the second C60 chain is the trough between two DCP molecular rows in accordance with the DFT calculations. The calculated C60-chain spacing of 1.1 nm agrees well with the experimental result. This subtle chain formation is attributed to the delicate balance of intermolecular interactions, interfacial dipolar interactions, and stepped-substrate interactions. The electronic properties for this model 3-component organic/metallic system, such as charge transfer between donors and acceptors, between the gold substrate and the molecular layers, will be discussed.

 

[1] J. Wang, I. Kaur, B. Diaconescu, J.-M. Tang, G. P. Miller, and K. Pohl, ACS Nano 5 (2011) 1792.

[2] J. Wang, J.-M. Tang, G. P. Miller, and K. Pohl, in preparation, (2012).