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Molecular doping provides a route toward designing new organic compounds with improved performance for optoelectronics. Here, we investigate the p-type doping of crystalline diindenoperylene (DIP) with two recently proposed electron-accepting molecular dopants using many-body perturbation theory. For the pristine DIP crystal, the quasiparticle band structure and the optical absorption spectra are found in agreement with the experimental data. Using the same methodology, we then characterize the optical and electronic properties of the two doped DIP crystals. The bandgap of both doped crystals is narrowed considerably due to the formation of hybridized states at the valence band edge. Moreover, a hybrid unoccupied mid-gap band is created with a host–dopant charge-transfer characteristic, giving rise to broader absorption spectra and a much lower absorption onset as compared to pristine DIP. Our results highlight that the interaction and hybridization with the host environment, including many-body effects, must be carefully considered in order to identify appropriate molecular dopants for a given organic crystal.

Spin-flip excitations of iron porphyrin molecules on Au(111) are investigated with a low-temperature scanning tunneling microscope. The molecules adopt two distinct adsorption configurations on the surface that exhibit different magnetic anisotropy energies. Density functional theory calculations show that the different structures and excitation energies reflect unlike occupations of the Fe 3d levels. We demonstrate that the magnetic anisotropy energy can be controlled by changing the adsorption site, the orientation, or the tip–molecule distance.

180 p., The theory describing the interaction between light and matter at nanoscale is nearly as old as quantum mechanics. Over the years, it has been shown that such theoretical models not only enable materials scientists to deepen their physical understanding of the underlying microscopic mechanisms but also provide the possibility to develop novel materials and devise advanced mechanisms to use within emerging technological applications. With the steady increase in computational power, the combination of experiments with theoretical and computational modeling is currently perceived as a promising approach to significantly reduce the time and effort to optimize the functionality of a material for a given application. This usually involves simulating materials at different scales, making use of the so-called ab initio electronic structure methods to describe the behavior of materials at the atomic scale. In this thesis, we particularly focus on the ab initio many-body perturbation theory (MBPT) providing powerful tools to describe the electronic excitations of materials. Within the MBPT, the GW approximation is a Green's function-based framework which is extensively employed to investigate the electronic structure of diverse materials in both finite and extended phases at the same level of reasonable accuracy. However, the computational complexity associated with the canonical implementation of the method often hinders its application in large systems with more than a hundred atoms. In the present dissertation, after introducing the underlying methodology, we discuss a new implementation of the one-shot GW wherein the computation of the quasiparticle energies requires neither the explicit calculation of the response function nor the inversion of dielectric matrices. In doing so, we ultimately benefit from the sparsity associated with the use of a basis set of atomic orbital, and design iterative algorithms dealing with matrix-vector products instead of memory-demanding mat