FULL PAPER Hybrid Materials www.advtheorysimul.com Electronic and Optical Excitations at the Pyridine/ZnO(10 10) Hybrid Interface Olga Turkina,* Dmitrii Nabok, Andris Gulans, Caterina Cocchi, and Claudia Draxl* By combining all-electron density-functional theory with many-body perturbation theory, a prototypical inorganic/organic hybrid system, composed of pyridine molecules that are chemisorbed on the nonpolar ZnO(10 10) surface is investigated. The G 0 W 0 approximation is employed to describe its one-particle excitations in terms of the quasiparticle band structure, and the Bethe–Salpeter equation is solved to obtain the absorption spectrum. The different character of the constituents leads to very diverse self-energy corrections of individual Kohn–Sham states, and thus the G 0 W 0 band structure is distinctively different from its DFT counterpart, that is, many-body effects cannot be regarded as a rigid shift of the conduction bands. The nature of the optical excitations at the interface over a wide energy range is explored and it is shown that various kinds of electron-hole pairs are formed, comprising hybrid excitons and (hybrid) charge-transfer excitations. The absorption onset is characterized by a strongly bound bright ZnO-dominated hybrid exciton. For the selected examples of either exciton type, the individual contributions from the valence and conduction bands are analyzed and the binding strength and extension of the electron-hole wavefunctions are discussed. 1. Introduction Combining the best of two worlds is the main aim behind in- tensive investigations of hybrid materials. In organic/inorganic hybrid systems, for instance, their major advantages for optoelec- tronic applications are seen in the strong light–matter coupling of the organic components, while the inorganic counterparts excel in large carrier mobilities and thus efficient charge-carrier trans- port as well as small exciton binding energies. [1–3] These proper- ties together with a type-I level alignment at the hybrid interface would be ideally suited for light-emitting applications. [4,5] Type- II alignment, in turn, is favored for a hybrid solar cell where optical absorption of sunlight is expected to directly lead to electron-hole separation. [6–10] Prerequisite for that is the light- induced creation of hybrid or charge-transfer excitons, which ex- hibit the hole to a large extent on one side of the interface while the excited electron would reside on the other side. Investiga- tions along these lines on hybrid inorganic/organic materials O. Turkina, Dr. D. Nabok, Dr. A. Gulans, Prof. C. Cocchi, Prof. C. Draxl Institut f¨ ur Physik and IRIS Adlershof Humboldt-Universit¨ at zu Berlin 12489 Berlin, Germany E-mail: olga.turkina@physik.hu-berlin.de; claudia.draxl@physik.hu- berlin.de DOI: 10.1002/adts.201800108 composed of wide-gap semiconductors like ZnO and organic chromophores have cre- ated an active field of research. [11–27] The adsorption of a molecular layer on inorganic surfaces has also been exploited to tune the work function of the inor- ganic component. [14,16,17,28] A special proto- typical hybrid system, consisting of a mono- layer of pyridine molecules chemisorbed on the nonpolar ZnO(10 10) surface (labeled Py/ZnO in the following) has been stud- ied experimentally by photoelectron spec- troscopy to determine the reduction of the electron injection barrier at the interface. [29] The binding mechanism and the inter- face morphology of this system has been studied theoretically by density functional theory (DFT), including van der Waals interactions. [29] Insight from ab initio theory is essen- tial in order to gain understanding of the optoelectronic properties of such complex materials. However, even when applying state-of-the-art methods, a fully quantita- tive description of hybrid materials remains extremely challenging. [30] Approximations most suitable for the organic part may not be applicable to the inorganic one, and vice versa. [31,32] This situation may hamper obtaining the correct level alignment at the interface which, in turn, is the base for optical excitations. Further challenges are presented by the sen- sible interplay of geometry, bonding, hybridization, and electron- transfer processes. Ultimately, calculations of hybrid materials are computationally very demanding due to the large number of atoms per unit cell. Indeed, so far the electronic and optical properties of only a few prototypical hybrid systems have been successfully investigated by means of first-principles many-body approaches. [33–36] In this work, we study Py/ZnO making use of the structure determined by Hofmann et al. [29] The G 0 W 0 approximation of many-body perturbation theory (MBPT) is employed on top of DFT to obtain the level alignment at the interface, and the Bethe– Salpeter equation (BSE) is solved on the search for hybrid and charge-transfer excitations. We demonstrate the role of many- body effects in both approaches, discussing the impact of the electron self-energy on the quasiparticle band structure and the existence of pronounced excitonic effects in the optical absorp- tion spectra. We provide a detailed analysis of the absorption fea- tures, determining their character in terms of spatial extension and electron-hole binding strength. Finally, we critically evaluate the used methodology. Adv. Theory Simul. 2018, 1800108 C  2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1800108 (1 of 8)