Doping-induced dielectric catastrophe prompts free-carrier release in organic semiconductors

TL;DR

This paper reveals that a dielectric catastrophe caused by collective screening phenomena in doped organic semiconductors leads to a significant increase in dielectric constant, facilitating free-carrier release and improving conductivity at lower doping levels.

Contribution

It introduces a multiscale model linking dielectric properties and doping levels, explaining the dielectric catastrophe and free-carrier release mechanism in organic semiconductors.

Findings

Dielectric constant increases ten-fold at 8% doping load.

Enhanced screening reduces energy barriers for charge separation.

Mechanism aligns with conductivity improvements at lower doping levels.

Abstract

The control over material properties attainable through molecular doping is essential to many technological applications of organic semiconductors, such as OLED or thermoelectrics. These excitonic semiconductors typically reach the degenerate limit only at impurity concentrations of 5-10\%, a phenomenon that has been put in relation to the strong Coulomb binding between charge carriers and ionized dopants, and whose comprehension remained elusive so far. This study proposes a general mechanism for the release of carriers at finite doping in terms of collective screening phenomena. A multiscale model for the dielectric properties of doped organic semiconductor is set up by combining first principles and microelectrostatic calculations. Our results predict a large nonlinear enhancement of the dielectric constant (ten-fold at 8\% load) as the system approaches a dielectric instability (catastrophe) upon increasing doping. This can be attributed to the presence of highly polarizable host-dopant complexes, plus a nontrivial leading contribution from dipolar interactions in the disordered and heterogeneous system. The enhanced screening in the material drastically reduces the (free) energy barriers for electron-hole separation, rationalizing the possibility for thermal charge release. The proposed mechanism is consistent with conductivity data and sets the basis for achieving higher conductivities at lower doping loads.