Excitonic Engineering in 2D-SiC through Organic Donor–Acceptor Adsorption

Controlling electronic excitations at molecule-semiconductor interfaces is key to designing hybrid optoelectronics. Using many-body perturbation theory, we study monolayer and bilayer SiC and their interfaces with four organic molecules bearing donor and/or acceptor substituents. Pristine monolayer SiC hosts strongly bound excitons with optical activity in the ultraviolet. Upon stable physisorption, new molecule-derived excitons emerge in the visible range. Through a detailed quantitative analysis of these excitons and the corresponding free-standing molecular excitations, we show that donor–acceptor molecules with predominant intramolecular charge-transfer excitation largely retain their character upon adsorption and undergo only modest optical redshifts in response to the substrate-induced screening. In contrast, donor-only or acceptor-only molecules with predominantly localized excitations exhibit significantly larger redshifts after adsorption. Bilayer SiC with stronger screening amplifies these shifts. The magnitude of the optical redshift reflects the competition between quasiparticle gap renormalization and the simultaneous reduction in exciton binding energy. This balance is controlled by the spatial distribution of the frontier orbitals at the interface and by the extent of electron-hole overlap within the exciton. Our results highlight the interplay between intrinsic molecular polarizability, excitonic characteristics, and substrate-induced screening effects as complementary knobs for tuning interfacial level alignment and optical functionality in molecule-semiconductor heterostructures.