Charge mobility determination by current extraction under linear increasing voltages: the case of non-equilibrium charges and field-dependent mobilities

TL;DR

This paper critically examines the CELIV method for measuring charge mobility in organic photovoltaic films, highlighting potential errors caused by charge recombination and field-dependent mobilities, and urging careful interpretation of previous results.

Contribution

The study provides analytical and numerical insights into the limitations of CELIV analysis, emphasizing the need to revisit prior mobility measurements in OPV materials.

Findings

CELIV analysis can produce erroneous mobility values when charge recombination occurs.

Field-dependent mobilities may be misinterpreted as time-dependent due to analysis errors.

Previous reports of mobility relaxation in OPV materials using CELIV should be re-evaluated.

Abstract

The method of current extraction under linear increasing voltages (CELIV) allows for the simultaneous determination of charge mobilities and charge densities directly in thin films as used in organic photovoltaic cells (OPV). In the past, it has been specifically applied to investigate the interrelation of microstructure and charge transport properties in such systems. Numerical and analytical calculations presented in this work show that the evaluation of CELIV transients with the commonly used analysis scheme is error prone once charge recombination and, possibly, field dependent charge mobilities are taken into account. The most important effects are an apparent time-dependence of charge mobilities and errors in the determined field dependencies. Our results implicate that reports on time-dependent mobility relaxation in OPV materials obtained by the CELIV technique should be carefully revisited and confirmed by other measurement methods.