Electric Field-Dependent Conductivity as Probe for Charge Carrier Delocalization and Morphology in Organic Semiconductors

Abstract

The charge carrier localization length α 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 α, we use the fact 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 (Teff) 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 > 106V/m), Teff 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.