Identifying Hidden Intracell Symmetries in Molecular Crystals and their Impact for Multiexciton Generation

TL;DR

This paper introduces a novel approach to classify organic molecular crystals based on hidden intracell symmetries, revealing their impact on multiexciton generation and enabling the design of more efficient optoelectronic materials.

Contribution

The study develops a framework to understand and predict multiexciton processes in organic crystals using density-functional theory, uncovering hidden symmetries and selection rules that influence exciton dynamics.

Findings

Bulk polymorph of pentacene exhibits faster singlet fission than thin-film form.

Effective higher symmetry models accurately describe two-sublattice crystals.

The approach guides the discovery of materials with improved exciton decay rates.

Abstract

Organic molecular crystals are appealing for next-generation optoelectronic applications, most notably due to their multiexciton generation process that can increase the efficiency of photovoltaic devices. However, a general understanding of how crystal structures affects multiexciton generation processes is lacking, requiring computationally demanding calculations for each material. Here we present an approach to understand and classify such crystals and elucidate multiexciton processes. We show that organic crystals that are composed of two sublattices are well-approximated by effective fictitious systems of higher translational symmetry. Within this framework, we derive hidden selection rules in crystal pentacene and predict that the common bulk polymorph supports fast Coulomb-mediated singlet fission about one order of magnitude more than the thin-film polymorph, a result confirmed with many-body perturbation theory calculations. Our approach is fully based on density-functional theory calculations, and provides design principles for the experimental and computational discovery of new materials with efficient non-radiative exciton decay rates.