Mapping Energy Levels for Organic Heterojunctions

Abstract

An organic semiconductor thin film is a solid-state matter comprising one or more molecules. For applications in electronics and photonics, several distinct functional organic thin films are stacked together to create a variety of devices such as organic light-emitting diodes and organic solar cells. The energy levels at these thin-film junctions dictate various electronic processes such as the charge transport across these junctions, the exciton dissociation rates at donor-acceptor molecular interfaces, and the charge trapping during exciton formation in a host-dopant system. These electronic processes are vital to a device's performance and functionality. To uncover a general scientific principle in governing the interface energy levels, highest occupied molecular orbitals, and vacuum level dipoles, herein a comprehensive experimental research is conducted on several dozens of organic-organic heterojunctions representative of various device applications. It is found that the experimental data map on interface energy levels, after correcting variables such as molecular orientation-dependent ionization energies, consists of three distinct regions depending on interface fundamental physical parameters such as Fermi energy, work function, highest occupied molecular orbitals, and lowest unoccupied molecular orbitals. This general energy map provides a master guide in selection of new materials for fabricating future generations of organic semiconductor devices.

Failure of Fermi Level in Referencing Chemical Shift of Molecules on Solid Surfaces

“…For characterizing organic thin films (i.e., molecules on solid surfaces), PES is useful for investigating the composition, thickness, electronic states (e.g., frontier molecular-orbital energies), and molecular orientations of the self-assembled thin films. [25,28,31] Interestingly, we note that such impact is restricted not only to frontier orbitals but also on all electronic states including core shell electronic energies. [25][26][27][28] One distinct advantage of this technique (X-ray photoemission spectroscopy) is its ability to discriminate between different oxidation states and chemical environments.…”

“…Studies on the frontier molecular orbital energies of molecules on solid surfaces have been extensively conducted particularly for organic electronic device applications as the relative alignment of these valence orbital energies plays a critical role to the functionality of the devices. [28,32] Figure 2a inset shows that the schematic energy diagram indicates all key energy level parameters (see Supporting Information for photoemission spectra). [28] This rule describes three distinct regions of the frontier molecular orbital energy-level alignment, which is governed by electron chemicalpotential equilibration as the driven force: the lowest unoccupied molecular orbital (LUMO) is pinned to the underlayer Fermi level at extreme low end of substrate work function (region I); the linear elastic region where HOMO shifts with substrate work function proportionally (region II); the HOMO is pinned to the underlayer Fermi level at extreme high end of substrate work function (region III).…”

“…Figure 5a presents the master plot of interface dipole against underlayer work function (ionization energy is taken into consideration to adjust the difference between different types of molecules [28] ) for C 60 (red star), Alq 3 (blue star), and literature [28] (gray cross). In order to investigate if a true charge transfer occurs at an interface, one needs to measure interface dipoles.…”

“…

Photoemission spectroscopy (PES) is perhaps the most important analytical tool for characterizing the chemical properties of thin film layers and interfaces. [25][26][27][28] One distinct advantage of this technique (X-ray photoemission spectroscopy) is its ability to discriminate between different oxidation states and chemical environments. For characterizing organic thin films (i.e., molecules on solid surfaces), PES is useful for investigating the composition, thickness, electronic states (e.g., frontier molecular-orbital energies), and molecular orientations of the self-assembled thin films.

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“…Data shown in gray open circles are taken from refs [28]. Plot of XPS C 1s core-level (filled circles, corresponds to left vertical axis) and UPS HOMO binding energies (open circles, corresponds to right vertical axis) for C 60 on various substrates versus substrate work function.…”