Influence of the aggregate state on band structure and optical properties of C60 computed with different methods

TL;DR

This study investigates how solid state packing influences the electronic and optical properties of C60 molecules, revealing that certain computational methods better match experimental optical absorption data.

Contribution

It demonstrates that frequency-dependent polarizability calculations more accurately capture aggregation effects on C60's optical properties than traditional TD-DFT methods.

Findings

Valence and conduction band edges differ by ~0.1 eV from single molecule energies.

Linear response TD-DFT can overestimate redshift in optical spectra.

Frequency-dependent polarizability aligns better with experimental absorption data.

Abstract

C60 and C60 based molecules are efficient acceptor and electron transport layers for planar perovskite solar cells. While properties of these molecules are well studied by ab initiomethods, those of solid C60, specifically its optical absorption properties, are not. We present a combined Density Functional Theory - Density Functional Tight Binding study of the effect of solid state packing on bandstructure and optical absorption of C60. The valence and conduction band edge energies of solid C60 differ on the order of 0.1 eV from single molecule frontier orbital energies. We show that calculations of optical properties using linear response TD-DFT(B) or the imaginary part of the dielectric constant (dipole approximation) can result in unrealistically large redshift in the presence of intermolecular interactions compared to available experimental data. We show that optical spectra computed from the frequency-dependent real polarizability better reproduce the effect of C60 aggregation on optical absorption and may be more suited to study effects of molecular aggregation.