Charge Carrier Transport in Disordered Polymers

TL;DR

This paper analyzes charge carrier transport in disordered organic materials, emphasizing the influence of spatial correlations, material polarity, and charge traps on mobility and transient photocurrent measurements.

Contribution

It introduces improved methods for calculating mobility from photocurrent transients and challenges the role of charged traps in observed transport properties.

Findings

Mobility depends on spatial energy correlations and material polarity.

Certain photocurrent transient analysis methods are unreliable.

Charged traps have limited impact on transport properties.

Abstract

General properties of charge carrier transport in disordered organic materials are discussed. Spatial correlation between energies of transport sites determines the form of the drift mobility field dependence. Particular kind of spatial correlation in a disordered material depends on its nature. Mobility field dependences have to be different in polar and nonpolar materials. Different methods of mobility calculation from the shape of photocurrent transient are analyzed. A widely used method is very sensitive to the variation of the shape of the transient and sometimes produces results that effectively masquerade the true dependence of the mobility on electric field or trap concentration. Arguments in favor of the better, more reliable method are suggested. Charge transport in materials containing charged traps is considered without using the isolated trap approximation and this leads to qualitatively different results. They indicate that the effect of charged traps can hardly be responsible for experimentally observed transport properties of disordered organic materials.