Asymmetric Surface Potential Energy Distributions in Organic Electronic Materials via Kelvin Probe Force Microscopy

Abstract

Organic electronic devices promise cheaper solution processability than their inorganic counterparts and allow for the vast tailorability of synthetic chemistry to tune properties and efficiency. A critical fundamental challenge is to understand the dynamics and mechanisms of charge transport, particularly the role of defects and traps. We use Kelvin probe force microscopy to compare potential energy distributions of organic, semiconducting thin films to comparable histograms calculated via our dynamic Monte Carlo simulation. A combination of theoretical and experimental investigations indicates that the common assumption of a Gaussian disorder model (GDM) is not always a good approximation for these materials. Instead we find asymmetric distributions due to nanoscale heterogeneity of charge and a resulting mix of Lorentzian and Gaussian disorder fit through a Voigt profile. Determining a more accurate model for disorder in these commonly used materials has the potential to influence the future of organic electronics design.

Direct Imaging of Space‐Charge Accumulation and Work Function Characteristics of Functional Organic Interfaces

“…This may suggest nonhomogeneous carrier distributions and charge densities at the surface and materials interface. When observing such subtle variations and asymmetric distributions is not unreasonable to consider that such asymmetry can be derived from electrostatic disorder in the films like nanoscale charge heterogeneity and effects of charge traps in the organic films and the interface with the substrates . For example, if charged traps of an specific polarization (positive or negative) are concentrated in nanoscale locations, while the charged traps of the opposite polarization are more homogeneously distributed through the film.…”

“…When observing such subtle variations and asymmetric distributions is not unreasonable to consider that such asymmetry can be derived from electrostatic disorder in the films like nanoscale charge heterogeneity and effects of charge traps in the organic films and the interface with the substrates. [20] For example, if charged traps of an specific polarization (positive or negative) are concentrated in nanoscale locations, while the charged traps of the opposite polarization are more homogeneously distributed through the film. This would increase the probability of charge interaction of the same polarization and such imbalance would drive the distribution of chemical potential and surface potential in an asymmetric fashion.…”

“…This is of great relevance for organic solar cells (OSCs), once this drives the open circuit voltage . Similarly, the tuning of the energy‐level alignment at the metal/organic interface is paramount in order to optimize charge extraction in OSCs as well as the charge injection in organic light‐emitting diodes and organic field‐effect transistors . Due to the charge characteristics at the organic interface, diverse electrical processes, such as charge transfer and charge recombination, can take place.…”

“…[18] Similarly, the tuning of the energy-level alignment at the metal/organic interface is paramount in order to optimize charge extraction in OSCs as well as the charge injection in organic light-emitting diodes and organic field-effect transistors. [17,[19][20][21] Due to the charge characteristics at the organic interface, diverse electrical processes, such as charge transfer and charge recombination, can take place. Therefore, the quantification of the spatial dimensions of the interface dipole or charge accumulation layer becomes highly relevant to determine the electrical performance of functional organic interfaces.…”

The effect of TiO2 nanotube morphological engineering and ZnS quantum dots on the water splitting reaction: A theoretical and experimental study

“…Therefore, the amorphous and crystalline TiO 2 structures exhibited differences in tail states. Hutchison et al [53] studied potential energy distributions of organic molecules on several substrates, and observed asymmetric Gaussian distributions by Kelvin probe force microscopy (KPFM) and dynamic Monte Carlo simulations. According to the authors [53], with a distribution of spatially heterogeneous traps (e.g., positive charges) and more homogeneous opposing charges, the simulated histograms of surface potentials show a clear asymmetric skew, much like the experimental KPFM results.…”