Orientation-Dependent Work-Function Modification Using Substituted Pyrene-Based Acceptors

Abstract

The adsorption of molecular acceptors is a viable method for tuning the work function of metal electrodes. This, in turn, enables adjusting charge injection barriers between the electrode and organic semiconductors. Here, we demonstrate the potential of pyrene-tetraone (PyT) and its derivatives dibromopyrene-tetraone (Br-PyT) and dinitropyrene-tetraone (NO2-PyT) for modifying the electronic properties of Au(111) and Ag(111) surfaces. The systems are investigated by complementary theoretical and experimental approaches, including photoelectron spectroscopy, the X-ray standing wave technique, and density functional theory simulations. For some of the investigated interfaces the trends expected for Fermi-level pinning are observed, i.e., an increase of the metal work function along with increasing molecular electron affinity and the same work function for Au and Ag with monolayer acceptor coverage. Substantial deviations are, however, found for Br-PyT/Ag(111) and NO2-PyT/Ag(111), where in the latter case an adsorption-induced work function increase of as much as 1.6 eV is observed. This behavior is explained as arising from a face-on to edge-on reorientation of molecules in the monolayer. Our calculations show that for an edge-on orientation much larger work-function changes can be expected despite the prevalence of Fermi-level pinning. This is primarily ascribed to a change of the electron affinity of the adsorbate layer that results from a change of the molecular orientation. This work provides a comprehensive understanding of how changing the molecular electron affinity as well as the adsorbate structure impacts the electronic properties of electrodes.

Affecting an Ultra‐High Work Function of Silver

“…Recent computational work predicted that the WF change of Ag or Au by a chemical modification can be as much as 4 eV and above . Yet, to the best of our knowledge, the highest experimental WF increase of a metal did not exceed 2 eV, as reported, for instance, by de Boer, Hofmann, and Frisbie …”

“…[5][6][7] Beyond fine-tuning,along standing challenge has been to increase the WF as high as possible,ino rder to further expand the range of possibilities of tailoring the desired WF value.Recent computational work predicted that the WF change of Ag or Au by ac hemical modification can be as much as 4eVand above. [8,9] Yet, to the best of our knowledge,the highest experimental WF increase of am etal did not exceed 2eV, as reported, for instance,b y de Boer, [5] Hofmann, [10] and Frisbie. [7] Recently we have introduced an entirely new methodology for affecting the WF of metals,namely the doping of the metal with WF-modifying agents;t hat is,w em ove from derivatizing the 2D interface to the 3D volume of the metal.…”

“…4 eV) of silver has been predicted in a triple organic—silicone–silver system, quite close to the ultrahigh WF increase (>3 eV) we achieved in our system. We also note that recent investigations reveal that the WF alternation by chemical modification is highly dependent on the molecules orientation (bending and rotation of substituents) at the metal–dielectric interface. And indeed, as revealed by the INS spectra, the molecular state of cysteine in the entrapped case has been changed significantly.…”

“…[11] Being anew approach and following the above cited theoretical predictions,weset to explore the possibility to utilize the doping methodology in order to push the WF of silver into the ultra-high domain, which in the literature is defined as an increase of 2-3 eV. [9][10][11] Them aterials methodology of molecular doping of metals employed herein involves av ariety of reducing processes of metal cations in the presence of the molecule or nanoparticle (NP) to be entrapped (see refs. [12,13] for reviews and [11,14,18] for recent examples).…”

The Impact of Dipolar Layers on the Electronic Properties of Organic/Inorganic Hybrid Interfaces

“…There, for upright-standing molecular layers, work functions in excess of 6.1 eV have been measured. [272] A particularly interesting situation is encountered for 1,2,5,6,9,10-coronenehexone (COHON) on coinage metals (see Figure 15c): [271] While the work functions of the pristine substrates and at very low coverages of COHON on Au, Cu, and Ag are significantly different, at a nominal coverage of ≈6 Å (which is attributed to roughly a closed monolayer of lying molecules) the work functions of COHON/Ag and COHON/Cu become virtually identical. This suggests Fermi-level pinning at those two interfaces, while on Au pinning apparently does not occur yet and the electronic structure of the interface is dominated by Pauli pushback.…”