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

Abstract

Organic molecular crystals are appealing for next-generation optoelectronic applications due to their multiexciton generation processes that can increase the efficiency of photovoltaic devices. However, a general understanding of how crystal structures affect these processes is lacking, requiring computationally demanding calculations for each material. Here we present an approach to understand and classify organic 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 bulk polymorph supports fast Coulomb-mediated singlet fission with a transition rate about 2 orders of magnitude faster than that of the thin-film polymorph, a result confirmed with many-body perturbation theory calculations. Our approach is based on density-functional theory calculations and provides design principles for the experimental and computational discovery of new materials with tailored excitonic properties.

Modeling the electronic structure of organic materials: a solid-state physicist’s perspective

“…In such a context, molecular semiconductor modeling was devoted to addressing more specific questions related, among others, to doping mechanisms [41][42][43][44][45][46][47], polymorphism [48][49][50][51][52], and singlet fission [53][54][55][56][57][58][59]. Moreover, increasing attention was dedicated to disclosing the excitonic properties of organic materials [52,[60][61][62], including quantifying resonance energies and clarifying the character of electron-hole pairs.…”

Computational Discovery of Intermolecular Singlet Fission Materials Using Many-Body Perturbation Theory

“…TDDFT is formally exact, but in practice its performance depends to a great extent on the choice of approximation for the exchange-correlation (xc) functional. − TDDFT offers an appealing balance between accuracy and efficiency, if a judicious choice of approximation for the exchange-correlation functional is used. , Hence, TDDFT has been widely used to search for SF, − TTA, ,, and TADF chromophores. ,, However, because TDDFT does not easily lend itself to periodic implementations, for the most part its use is limited to isolated molecules and clusters. To some extent, it may be possible to mimic some of the effects of periodicity by conducting calculations for dimers or embedded clusters of molecules. ,− Although these approaches may be able to capture the effect of the local environment of a molecule in a crystal by explicitly considering its nearest neighbors, and to approximate the presence of the surrounding medium by embedding, they cannot fully reproduce the evolution of molecular orbital energies into a band structure and the delocalization of excitons in molecular crystals (discussed in more detail below). ,,,,,− …”

“… 123 , 144 − 146 Although these approaches may be able to capture the effect of the local environment of a molecule in a crystal by explicitly considering its nearest neighbors, and to approximate the presence of the surrounding medium by embedding, they cannot fully reproduce the evolution of molecular orbital energies into a band structure and the delocalization of excitons in molecular crystals (discussed in more detail below). 71 , 73 , 91 , 98 , 116 , 147 − 150 …”

“…Although unsubstituted acenes are not promising for commercial applications due to their instability and poor solubility, they have served as benchmark systems for studying SF mechanisms, developing new SF materials, and optimizing device architectures. Within the acene family, the SF driving force, S 1 – 2 T 1 , increases with the backbone length. , SF in crystalline pentacene is slightly exoergic, whereas SF in tetracene is slightly endoergic. ,, In hexacene, the overly high driving force leads to a large energy loss . Anthracene and its derivatives are more likely to undergo TTA than SF, owing to too negative S 1 – 2 T 1 .…”

“… 73 , 90 SF in crystalline pentacene is slightly exoergic, whereas SF in tetracene is slightly endoergic. 72 , 73 , 91 In hexacene, the overly high driving force leads to a large energy loss. 92 Anthracene and its derivatives are more likely to undergo TTA than SF, owing to too negative S 1 – 2 T 1 .…”