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Recent work aiming at a deeper understanding of the electronic properties of classical and novel charge‐transfer (CT) complexes is reviewed herein. From a variety of studied CT salts, the prototypical examples TTF‐TCNQ, (TMTTF)2SbF6, (TMTSF)2PF6, and TMP/HMP‐TCNQ have been selected. Three different types of electron and X‐ray spectroscopies were applied, namely ultraviolet photoelectron spectroscopy (UPS), hard X‐ray photoelectron spectroscopy (HAXPES), and near‐edge X‐ray absorption fine structure (NEXAFS) spectroscopy. Using UPS, the development of the valence band structure during co‐deposition of the donor and acceptor moieties TMP/HMP and TCNQ has been studied. The spectra reveal characteristic level shifts and a strong reduction of the HOMO–LUMO gap upon formation of the complex. HAXPES probes the core levels and reveals characteristic shifts of the binding energies in (TMTTF)2SbF6 at the phase transition to the low‐temperature charge‐ordered state. NEXAFS is a local, element‐specific probe for the unoccupied bands above the Fermi level and reveals changes in the population of specific orbitals when the CT complex is formed. As an outlook, the novel method of photoelectron momentum microscopy is presented, giving access to the valence bands and core‐levels and in particular to high‐resolution X‐ray photoelectron diffraction patterns. [ABSTRACT FROM AUTHOR]

This article reviews recent progress in the understanding of many‐body effects in the quasi‐two‐dimensional electronic system of organic charge‐transfer salts using element‐selective X‐ray absorption and scanning tunneling spectroscopy techniques. A few previously unpublished topographical scanning tunneling microscopy images illustrate structural consequences of the charge transfer mechanism. The first part of the review focuses on the understanding of the charge transfer between acceptor and donor molecules in general. Results for dithiophene‐based organic semiconductors as a representative model system is reviewed that is relevant for electronic applications, too. Experimental results are used to develop appropriate calculation schemes with large predictive power. The combination of experiment and theory reveals a charge transfer mechanism that is more complex than just the interaction of single HOMO and LUMO states in donor and acceptor molecules. The second part of the review highlights the emergence of superconductivity as a result of the charge transfer leaving strongly correlated and partly occupied electronic states at the Fermi level. The authors review recent experimental results for the κ‐phase organic superconductors obtained with scanning tunneling spectroscopy and compare the temperature‐dependent local density of states function with theoretical approaches. The superconducting order parameter for κ‐(ET)2Cu[N(CN)2]Br reveals an eight‐node gap structure resulting from a strongly anisotropic mixed‐symmetry superconducting gap. [ABSTRACT FROM AUTHOR]