Role of charge transfer, dipole-dipole interactions, and electrostatics in Fermi-level pinning at a molecular heterojunction on a metal surface

Abstract

Recently, Niederhausen et al. [Phys. Rev. B 86, 081411(R) (2012)] have reported on the energy level alignment of C 60 adsorbed on a bilayer α-sexithiophene (6T) film on Ag(111). The possibility of charge transfer from the metal to the C 60 through the bilayer 6T as discussed by the authors may have a strong impact on understanding the energy level alignment (ELA) at organic-organic (O-O) heterojunctions grown on electrodes. In this paper, we aim at a comprehensive picture of the ELA at O-O interfaces on a metal. We carry out a detailed investigation of the same pair of materials on Ag(111) as employed previously, however, with varying 6T interlayer thickness. The results allow unambiguous identification of integer charge transfer towards a fraction of the C 60 molecules as the mechanism leading to the formation of interface dipoles. Varying the 6T interlayer thickness also reveals the dependence of the observed features on the C 60 -metal distance. This dependence is quantitatively addressed by electrostatic considerations involving a metal-to-overlayer charge transfer. From this, we demonstrate the important role of dipole-dipole interaction potentials in the molecular layer and electric fields resulting from interface dipole formation for the energy level alignment. These findings provide a deeper understanding of the fundamental mechanisms that establish electronic equilibrium at molecular heterojunctions and will aid the prediction of an accurate energy level alignment at device relevant heterojunctions, e.g. in organic opto-electronic devices.

Overcoming Electrode‐Induced Losses in Organic Solar Cells by Tailoring a Quasi‐Ohmic Contact to Fullerenes via Solution‐Processed Alkali Hydroxide Layers

“…[ 20 ] Koch and co-workers have recently shown that there is clear electron injection from alkali halideand sexithiophene-modifi ed Ag electrodes that are in contact with C 60 . [ 3,21,22 ] Here, we demonstrate for the fi rst time that these anionic hybridized interfacial gap states that are formed at alkali metal hydroxide-modifi ed AZO contacts result in dramatically enhanced performance for bulk heterojunction solar cells composed of a pDPP5T-2:PC 61 BM blend.…”

From Chiral Islands to Smectic Layers: A Computational Journey Across Sexithiophene Morphologies on C60

“…[ 31 ] Conversely C 60 fullerene is a standard choice as n-type semiconductor either used as such in its crystalline form or, more often, functionalized to make it soluble and polar as the widely used PCBM which is most often amorphous. The T6-C 60 interface has been studied with ultraviolet and X-ray photoelectron spectroscopy to assess Fermi level alignment and the extent of charge transfer, [32][33][34] as it constitutes a prototypical system for solar cells. [35][36][37] In addition, it has been shown that the relative weak intermolecular interactions at this interface can be exploited and controlled in order to achieve a variety complex architectures.…”

Templating Effects of α-Sexithiophene in Donor–Acceptor Organic Thin Films

“…25−28 For the same configuration on Ag substrates, a charge transfer from the substrate directly to the C 60 through the neutral 6T has been reported. 29,30 For 6T OPV cells using the PHJ geometry, an extraordinarily high open-circuit voltage, utilizing DIP (Figure 1 right) as electron-acceptor material, is reported. 20 In contrast with the sphere-like C 60 , the perylene derivate DIP is rod-like and hence sterically more compatible to 6T.…”