Effect of Nuclear Motion on Charge Transport in Fullerenes: A Combined Density Functional Tight Binding—Density Functional Theory Investigation

Saeid Arabnejad,Sergei Manzhos,Amrita Pal,Koichi Yamashita

doi:10.3389/fenrg.2019.00003

Abstract

Fullerene-based materials are widely used as acceptor and electron transport layer materials in organic and planar perovskite solar cells. Modeling of electronic properties such as band alignment and charge transport for these applications is typically done using optimized geometries. Here, we estimate the effects of nuclear motions on band structure and electron and hole transport in two prototypical fullerenes, C60 and C70. We model the dynamics in solid fullerenes using Density Functional Tight Binding and we use the Density Functional Theory based Projection of Monomer Orbitals on Dimer Orbitals (DIPRO) approach to estimate effects on charge transfer integral in the Marcus approximation. We show that room-temperature molecular dynamics cause a shift and spread of frontier orbital energies on the order of 0.1 eV which leads to an increase by more than a factor of two in the Marcus exponent, and can cause a decrease by up to orders of magnitude in the overlap integral, leading in most cases to an overall decrease in the charge transport rate.

Highlights

Fullerene-based materials remain the preferred acceptor materials in bulk heterojunction (BHJ) organic solar cells (OSC) (Lee et al, 2008; Park et al, 2009; Cowan et al, 2010; Ganesamoorthy et al, 2017), and fullerenes are efficient electron transport layer materials in planar perovskite solar cells (PSC) (Liang et al, 2015; Nie et al, 2015; Gil-Escrig et al, 2016)
We model the dynamics in solid fullerenes using Density Functional Tight Binding and we use the Density Functional Theory based Projection of Monomer Orbitals on Dimer Orbitals (DIPRO) approach to estimate the effects on the charge transfer integral in the Marcus approximation
We show that room-temperature molecular dynamics cause a shift and spread of frontier orbital energies on the order of 0.1 eV, which leads to an increase by more than a factor of two in the Marcus exponent, and can cause a decrease by up to orders of magnitude in the overlap integral, leading, in most cases, to an overall decrease in the charge transport rate

Summary

Introduction

Fullerene-based materials remain the preferred acceptor materials in bulk heterojunction (BHJ) organic solar cells (OSC) (Lee et al, 2008; Park et al, 2009; Cowan et al, 2010; Ganesamoorthy et al, 2017), and fullerenes are efficient electron transport layer materials in planar perovskite solar cells (PSC) (Liang et al, 2015; Nie et al, 2015; Gil-Escrig et al, 2016). Fullerenes with addends can be used in PSCs to control film morphology and band alignment with the perovskite and they have a significant effect on the charge transport (Pal et al, 2017). Critical electronic properties of fullerenes in these applications include band alignment with the organic donor or the perovskite, which determines charge separation rate at the interface, and the Dynamics Effects on Charge Transport electron transport rate in bulk and across grain boundaries.

Methods

Results

Conclusion