Ultra-fast charge transfer in organic electronic materials and at hybrid interfaces studied using the core-hole clock technique

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Abstract

The focus of this brief review is the use of resonant photoemission in its “core-hole clock” expression for the study of two important problems relevant for the field of organic electronics: the dynamical charge transfer across hybrid organic–inorganic interfaces, and the intermolecular charge transfer in the bulk of organic thin films. Following an outline of the technique, a discussion of its applicability and a short overview of experimental results obtained thus far, two examples are used to illustrate particular results relevant for the understanding of the charge transport in organic electronic devices. First, for Fe(II)-tetraphenylporphyrin molecules on semi-metallic molybdenum disulfide substrates, the electronic coupling to the substrate and the efficiency of charge transport across the interface different for the individual molecular electronic subsystems is discussed. And second, a discotic liquid crystalline material forming columnar assemblies is used to illustrate ultra-fast intermolecular charge transfer on the order of a few femtoseconds indicating an electronic coupling between the phthalocyanine units stronger than expected from the macroscopic charge transport characteristics of the material.

Research highlights

▶ The use of resonant photoemission in its “core-hole clock” expression for the study of the dynamical charge transfer across hybrid organic–inorganic interfaces and for the intermolecular charge transfer in the bulk of organic thin films is reviewed. ▶ The electronic coupling to the substrate and the efficiency of charge transport across hybrid interfaces is different for individual electronic subsystems of the molecular adsorbate. ▶ The intermolecular charge transfer in the bulk of discotic liquid crystals occurs on the order of a few femtoseconds and is faster than expected from the macroscopic charge transport characteristics of the material.

Introduction

Static charge transfer has been shown to control interfacial electronic properties at hybrid organic–inorganic and organic–organic interfaces in organic electronics [1], [2], [3], [4], [5]. Knowledge of the dynamics of electron and hole transfer processes between π-conjugated molecules and metal or semiconductor surfaces, however, is key to the understanding of molecular charge-injection or charge-extraction devices.

Studies of the interfacial electronic structure, as a static property, are often carried out using the rather uncomplicated technique of photoelectron spectroscopy, in particular, ultraviolet photoelectron spectroscopy, or UPS [6], [7], [8], [9]. Charge transfer (CT) dynamics, on the other hand, are more difficult to access. Pump-and-probe techniques [10], [11] that study the behavior of optically excited electrons as a function of time after the excitation allow an independent tuning of pump and probe processes. In a rather flexible way, this technique can therefore be employed to study charge transport processes occurring at different time scales. With no suitable high-energy lasers available thus far, they are, however, restricted to low-energy excitations that relate, in the case of organic materials and interfaces, to molecular valence orbitals and their hybrid interfacial states.

On the other hand, in order to obtain atomic-site specific information related to different elements and chemically distinguishable electronic environments, techniques sensitive to the chemical shift of core-levels shall be employed. These chemical shifts are discerned in X-ray photoelectron (XPS) spectra, derived from the original name “Electron Spectroscopy for Chemical Analysis”, or ESCA, by Professor Kai Siegbahn and his colleagues at Uppsala University [12]. The availability of tunable X-ray light sources, namely synchrotrons, and advances in the theoretical description of scattering processes involving core-excited states [13] have lead to the development of powerful experimental techniques related to X-ray absorption and X-ray resonant scattering processes. Today, near-edge X-ray absorption fine-structure spectroscopy (NEXAFS) is a technique well established for both the study of the orientation of molecular adsorbates and of the properties of core-excited states [14]. Inclusion of the decay channels of the core-excited state leads to the development of resonant X-ray emission (XES) and resonant Auger spectroscopies, as well as of resonant photoemission (RPES), depending on which radiative or non-radiative decay channels are exploited. These different techniques and their applications have been described in excellent reviews [13], [15], [16], [17]. We therefore summarize only details needed to discuss the RPES technique as applied to organic materials and hybrid interfaces.

Note that the application of X-ray resonant spectroscopic techniques for the study of thin solid films and surface problems related to the electrical engineering of (nanoscale) devices is not and will never be as straightforward as XPS and UPS. In this regard, while the general principle is quite intriguing and relatively easy to understand, it is always necessary to critically validate assumptions made in the interpretation of “core-hole clock” experiments.

Section snippets

Resonant photoemission

Core-excited states created upon X-ray absorption (Fig. 1, case A) decay either through radiative processes or by emitting one or even more electrons. In the non-radiative case, participator (case B) and spectator decay (case C) processes are distinguished [17], [18], [19]. Sharp features in the low-binding energy region, with strongly varying intensities as a function of the photon energy, are usually related to participator one-hole final states that are electronically similar to those

“Core-hole clock” experiments

As mentioned above, one of the major advantages of the use of resonant spectroscopic techniques is the sensitivity to chemical elements and the chemical environment provided by shifts in the core-hole binding energies. For organic molecules, chemical shifts are often large enough such that excitations can be chosen to occur on particular functional groups of the molecule leading to a high selectivity of individual molecular valence orbitals [19].

In a correct description, X-ray radiative and

Dynamic charge transfer at hybrid interfaces

Because of its complexity, on one hand, and the limited time range accessible, on the other, results of experiments applying the “core-hole clock” technique for the study of hybrid interfaces of relatively large organic molecules and metals, semi-metals or semiconductors have so far been reported only rarely. A (certainly incomplete) overview of some of those results is provided in Table 1.

In one of the first experiments, Maxwell et al. derived a CT time of about 6 fs for fullerene molecules

Dynamic intermolecular charge transfer

Some of the most interesting topics in the field of organic electronics are the mechanisms of charge transport within the bulk of organic materials or thin films. Depending upon the degree of static and dynamic disorder, and the size of various intrinsic parameters, e.g., the intermolecular overlap integral and electronic and geometric relaxation (re-organization) energies, the transport of charge carriers can be described using models for either band-like or hopping mechanisms [39]. In

Summary

The basic principle underlying the “core-hole-clock” technique, its applicability to organic materials and hybrid interfaces, and the limitations of this technique have been outlined and discussed. Photoelectron spectroscopy involving X-rays, initiated by and with important contributions from Professor Kai Siegbahn and his colleagues at Uppsala University, has progressed to sophisticated levels. As shown here, on examples from the applied field of organic electronics, useful information

Acknowledgements

We thank S.L. Sorensen (Lund University, Sweden), S. Kera (Chiba University, Japan), H. Yamane (Institute for Molecular Science, Okazaki, Japan), K. Akaike (Okayama University, Japan), S.W. Han (formerly JAIST) for participation in experiments related to yet unpublished results, and a referee for important comments. For JAIST, this work has been supported by Special Coordination Funds for Promoting Science and Technology, commissioned by MEXT, Japan. In Linköping, this work was carried out

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