Electric field-dependent conductivity as probe for charge carrier delocalization and morphology in organic semiconductors

TL;DR

This study demonstrates that electric field-dependent conductivity measurements can effectively probe charge carrier delocalization and morphology in organic semiconductors by analyzing the effective localization length through temperature and field effects.

Contribution

It introduces a novel method to determine charge carrier localization lengths in organic semiconductors using electric field-dependent conductivity measurements, linking electronic properties to morphology.

Findings

Localization length varies from ~1nm in amorphous to ~10nm in ordered polymers.

High-field effective temperature correlates with structural order and doping level.

Simulations support the experimental extraction of localization lengths.

Abstract

The charge carrier localization length {\alpha} is a crucial, yet often ignored parameter of conjugated polymers that exponentially influences electronic conductivity. Here, we argue it is a unique proxy of the energy landscape as determined by sample morphology and experienced by mobile charges. To determine {\alpha}, we use that in disordered organic semiconductors, slow thermalization of charge carriers after excitation, e.g. by hopping in a finite electric field, can lead to an effective electronic temperature (T_eff) exceeding the lattice temperature, thereby enhancing conductivity. We experimentally probe this effect by combining temperature- and field-dependent conductivity measurements for a range of representative conjugated polymers, using different dopants, doping protocols and doping concentrations. We find that in the high-field regime (F>1E6V/m), T_eff exhibits distinct trends vs. structural order and doping level, which can be used to extract (effective) localization lengths ranging from ~1nm in fully amorphous systems to over ~10nm in highly ordered polymers. Tight-binding and kinetic Monte Carlo simulations are used to connect measured values to morphological properties and to rule out alternative explanations. Our results demonstrate that finite-field conductivity measurements provide a powerful probe of a characteristic length scale of charge transport that is complementary to conventional structural characterization.