Aggregate-State Effects in the Atomistic Modeling of Organic Materials for Electrochemical Energy Conversion and Storage Devices: A Perspective.

Sergei Manzhos

doi:10.3390/molecules25092233

Abstract

Development of new functional materials for novel energy conversion and storage technologies is often assisted by ab initio modeling. Specifically, for organic materials, such as electron and hole transport materials for perovskite solar cells, LED (light emitting diodes) emitters for organic LEDs (OLEDs), and active electrode materials for organic batteries, such modeling is often done at the molecular level. Modeling of aggregate-state effects is onerous, as packing may not be known or large simulation cells may be required for amorphous materials. Yet aggregate-state effects are essential to estimate charge transport rates, and they may also have substantial effects on redox potentials (voltages) and optical properties. This paper summarizes recent studies by the author’s group of aggregation effects on the electronic properties of organic materials used in optoelectronic devices and in organic batteries. We show that in some cases it is possible to understand the mechanism and predict specific performance characteristics based on simple molecular models, while in other cases the inclusion of effects of aggregation is essential. For example, it is possible to understand the mechanism and predict the overall shape of the voltage-capacity curve for insertion-type organic battery materials, but not the absolute voltage. On the other hand, oligomeric models of p-type organic electrode materials can allow for relatively reliable estimates of voltages. Inclusion of aggregate state modeling is critically important for estimating charge transport rates in materials and interfaces used in optoelectronic devices or when intermolecular charge transfer bands are important. We highlight the use of the semi-empirical DFTB (density functional tight binding) method to simplify such calculations.

Highlights

The story of the development of novel energy conversion and storage technologies, including novel types of solar cells, such as organic solar cells (OSC) [1] and perovskite solar cells (PSC) [2], electrochemical batteries [3], organic [4] and perovskite [5] light-emitting diodes (OLED and PLED, respectively), etc., is largely a story of developing the related functional materials
We highlight the use of alternative to (TD-)density functional theory (DFT) approaches, such as (TD-)density functional tight binding (DFTB) which significantly cuts the CPU cost of the modeling, and an alternative approach we proposed for computing absorption spectra which may be advantageous for aggregates [43,44]
We considered the effects of molecular aggregation on optoelectronic properties important We considered the effects of molecular aggregation on optoelectronic properties important for for solar cells and LEDs and solar cells and LEDs and properties properties important for use of organic materials as active electrode materials in metal ion batteries important for use of organic materials as active electrode materials in metal ion batteries

Summary

Introduction

The story of the development of novel energy conversion and storage technologies, including novel types of solar cells, such as organic solar cells (OSC) [1] and perovskite solar cells (PSC) [2], electrochemical batteries [3], organic [4] and perovskite [5] light-emitting diodes (OLED and PLED, respectively), etc., is largely a story of developing the related functional materials. Charge transfer calculations and ionic insertion and transport (which are critical phenomena in organic batteries) require going beyond single molecules and including aggregate-state effects explicitly Another interesting phenomenon often observed in experimental literature is the effect on electronic properties (such as the oxidation potential or visible absorption peak) of alkyl chain length [21,22,23]. Inclusion of aggregate state modeling is critically important for estimating charge transport rates, the effects of alkyl chain length, or when intermolecular charge transfer bands are important In these applications, we highlight the use of alternative to (TD-)DFT approaches, such as (TD-)DFTB which significantly cuts the CPU cost of the modeling, and an alternative approach we proposed for computing absorption spectra which may be advantageous for aggregates [43,44]

Absorption Spectra and Band Alignment

HOMO and phenomenon

Charge Transport

Insertion-Type Materials

Conclusions