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Abstract: The success of optoelectronic devices based on p-conjugated organic semiconductors relies on the possibility to carefully control charge transport levels (e.g. ionization potential, electron affinity) and optical excitations. The description of such excitations in complex supramolecular systems, such as those of interest for organic electronics, represents a severe challenge for theoretical modelling. In this talk, I will present our multiscale approach to the modelling of structure-property relationships in functional molecular. Special emphasis will be given to the description of long-range intermolecular interactions as described by classical atomistic electrostatic models, and to the recent merging of these approaches with many-body formalisms for charged (GW) and optical (Bethe-Salpeter equation) excitations. [1,2]

Applications of these techniques will be then discussed, starting from the photoelectron spectra of molecular solids. Our embedded GW calculations, achieve quantitative accuracy in describing the ionization energies and electron affinities of prototypical molecular crystals, shedding light on the interplay between dielectric screening, molecular electrostatics and band dispersion. [2]

I will then discuss molecular doping in organic semiconductors. Our calculations reveal that for typical molecular p-type semiconductors (e.g, pentacene or NPD) the dopant acceptor levels lie very deep into the gap, also for strong electron-withdrawing dopants such as F4TCNQ or F6TCNNQ. This is the result of a pronounced dependence of the dopant acceptor levels (up to 1 eV) on the host semiconductor, arising from intermolecular electrostatic interactions in the solid state. The ionization of dopant impurities is rationalized upon including the Coulomb electron-hole (excitonic) interaction that, together with structural relaxation (polaronic) effects, strongly stabilizes dopant-semiconductor charge transfer states. [3,4] The implications of our findings for the ionization of impurities and for the release of doping-induced charges will be finally discussed, suggesting rational design rules for performances optimization.

References : [1] J. Phys. Chem. Lett. 7, 2814 (2016); [2] Phys. Rev. B 97, 035108 (2018); [3] Phys. Rev. Materials, 1, 025602 (2017); [4] Mater. Horiz. 6, 107 (2019)