[Paper Review] The influence of impurities on the charge carrier mobility of small molecule organic semiconductors

Pascal Friederich, Artem Fediai+11 more|arXiv (Cornell University)|Aug 30, 2019

Organic Electronics and Photovoltaics42 references4 citations

TL;DR

This study uses a multiscale computational model to investigate how impurities—particularly water and water-oxygen complexes—affect charge carrier mobility in amorphous small-molecule organic semiconductors. It reveals that these impurities introduce deep trap states within the band gap, significantly reducing electron and hole mobilities, with molecular oxygen creating traps deep enough to limit performance in widely used materials.

ABSTRACT

Amorphous organic semiconductors based on small molecules and polymers are used in many applications, most prominently organic light emitting diodes (OLEDs) and organic solar cells. Impurities and charge traps are omnipresent in most currently available organic semiconductors and limit charge transport and thus device efficiency. The microscopic cause as well as the chemical nature of these traps are presently not well understood. Using a multiscale model we characterize the influence of impurities on the density of states and charge transport in small-molecule amorphous organic semiconductors. We use the model to quantitatively describe the influence of water molecules and water-oxygen complexes on the electron and hole mobilities. These species are seen to impact the shape of the density of states and to act as explicit charge traps within the energy gap. Our results show that trap states introduced by molecular oxygen can be deep enough to limit the electron mobility in widely used materials.

Motivation & Objective

• To understand the microscopic origins of charge traps caused by impurities in amorphous small-molecule organic semiconductors.
• To identify the chemical nature and electronic impact of common impurities such as water and water-oxygen complexes.
• To quantify how these impurities alter the density of states and degrade charge transport properties.
• To explain why electron mobility is particularly limited in high-performance organic semiconductors due to impurity-induced traps.

Proposed method

• A multiscale modeling approach combining quantum chemistry calculations and kinetic Monte Carlo simulations.
• Density functional theory (DFT) calculations to determine the electronic structure of impurity-doped organic semiconductors.
• Modeling of water and water-oxygen complexes in amorphous organic semiconductor matrices to assess their energetic and structural effects.
• Calculation of the density of states (DOS) in the presence of impurities to analyze trap formation.
• Simulation of charge transport using kinetic Monte Carlo methods to extract electron and hole mobilities.
• Systematic variation of impurity concentration and type to assess their influence on mobility and DOS shape.

Experimental results

Research questions

• RQ1 How do water molecules and water-oxygen complexes alter the density of states in amorphous small-molecule organic semiconductors?
• RQ2 What is the energetic depth of trap states introduced by molecular oxygen in common organic semiconductors?
• RQ3 Why is electron mobility more severely affected by impurities than hole mobility in these materials?
• RQ4 To what extent do impurities with deep trap states limit charge transport in practical organic semiconductor devices?
• RQ5 How does the presence of impurities modify the distribution and localization of charge carriers in disordered organic systems?

Key findings

• Water molecules and water-oxygen complexes act as explicit charge traps within the band gap of organic semiconductors, reducing both electron and hole mobilities.
• Molecular oxygen introduces trap states that are deep enough to significantly limit electron mobility in widely used small-molecule organic semiconductors.
• The shape of the density of states is substantially altered by impurities, with a broadening and tailing of the distribution toward the band edges.
• Impurity-induced traps are more detrimental to electron transport than hole transport, explaining the observed asymmetry in mobility degradation.
• The model quantitatively reproduces experimental trends in mobility reduction, validating the role of specific impurities as key performance-limiting factors.
• Even low concentrations of oxygen-related impurities can lead to substantial reductions in electron mobility due to deep trap formation.