Events

The success of electronic and optoelectronic technologies relies on the possibility to control the charge carrier concentration in materials and modulate their electric properties by introduction of dopant impurities. The physics of doping is well understood in the context of inorganic semiconductors, in which the advent of shallow donor or acceptor impurity levels is correctly predicted within the Hydrogenic model. Conversely, the mechanism for molecular doping in organic semiconductors is believed to be qualitatively different. These excitonic semiconductors typically reach the degenerate limit only at impurity concentrations of 5-10%, which are orders of magnitude larger than those needed in their inorganic analogues. This has been related to the Coulomb binding between ionized dopants and charge carriers, which is particularly strong in organic materials featuring low dielectric constants. The mechanisms determining the charge release upon doping and the ensuing conductivity enhancements remain elusive so far.

Doping in organic semiconductors has been depicted as a two-step process, namely the ionization of dopant impurities and the subsequent release of free charges available for conduction. The present Thesis investigates these two aspects by means of a multiscale framework encompassing many-body ab initio electronic structure approaches, parameterized Hamiltonian models and classical polarizable models.

By taking the technologically relevant case of a doped polymer as a case study, our calculations target the ground and excited state properties of host-dopant complexes, drawing a coherent picture of the different factors at play in the ionization process such as the Coulomb electron-hole (excitonic) binding, environmental electrostatic interactions and the crucial role of the position of the dopant in the polymer structure. By combining many-body perturbation theory with the Micro-Electrostatic framework, our results explain the striking differences in conductivity arising from samples with different morphologies, and confirm the appearance of low-lying charge-transfer excitations from the dopant to the host semiconductor, as the first step to dopant ionization.

We have then focused on the release of carriers at finite doping loads, for which we propose a general mechanism in terms of collective screening phenomena. A multiscale model for the dielectric properties of doped organic semiconductors is set up by combining first principles and Micro-Electrostatic calculations. Our results predict a large nonlinear enhancement of the dielectric constant (tenfold at 8% load) at doping concentrations comparable to those determining orders-of-magnitude conductivity enhancements in experiments. The system approaches a dielectric catastrophe upon increasing doping, which is attributed to the presence of highly polarizable host–dopant complexes. The leading contribution, as compared to the Clausius-Mossotti relation applied on an effective homogeneous polarizability, arises from the formation of soft and eventually unstable polarization modes. The enhanced screening in the material drastically reduces the (free) energy barriers for electron–hole separation, rationalizing the possibility for thermal charge release. Our results suggest that such a doping-induced dielectric catastrophe represents a driving factor for the insulator-to-metal transition in doped organic semiconductors.