Microcrystal Electron Diffraction for Molecular Design of Functional Non-Fullerene Acceptor Structures
Steve Halaby a, Michael Martynowycz a, Ziyue Zhu b, Sergei Tretiak c, Andriy Zhugayevych d, Tamir Gonen a, Martin Seifrid b
a Howard Hughes Medical Institute, David Geffen School of Medicine, Departments of Biological Chemistry and Physiology, University of California, Los Angeles, California 90095, United States
b Center for Polymers and Organic Solids, Department of Chemistry and Biochemistry, University of California, Santa Barbara, California 93106, United States
c Physics and Chemistry of Materials, Theoretical Division and Center for Integrated Nanotechnologies, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States
d Skolkovo Institute of Science and Technology, 143026, Moscow, Russia
Online Conference
Proceedings of NFA-Based Organic Solar Cells: Materials, Morphology and Fundamentals (NFASC)
Online, Spain, 2021 February 3rd - 4th
Organizers: Natalie Banerji and Feng Gao
Oral, Martin Seifrid, presentation 006
DOI: https://doi.org/10.29363/nanoge.nfasc.2021.006
Publication date: 25th January 2021

The molecular structure, processing conditions and solid-state arrangement of organic semiconducting (OSC) materials dictate their optoelectronic properties. Understanding the delicate balance between these different factors can be used to design better OSC materials. However, determining their solid-state arrangement, especially  the crystal structure, remains a significant challenge. Recently, a new experimental technique, microcrystal electron diffraction (MicroED),[1] has been used to determine the structures of organic compounds from crystals that are too small for X-ray crystallography. In this presentation, we report the MicroED structures of two well-known non-fullerene acceptors (NFAs). The structure of o-IDTBR was determined from a commercially available powder without additional crystallization, and a new polymorph of ITIC-Th was identified with the most distorted backbone of any NFA reported to date. Density functional theory (DFT) calculations are employed to study the relationships between molecular structures, lattice arrangement and charge transport properties for these and a number of other NFA lattices.

The 3D wire mesh lattice topology is believed to be an important factor in NFAs surpassing fullerenes as the electron acceptor of choice in organic photovoltaics. However, molecular design and processing condition rules for self-assembly of NFAs into the desired crystal lattice have yet to be developed. The results of our MicroED and DFT studies can be used to extract a number of important lessons. The 3D wire mesh may be the best crystal lattice for charge transport in multiple dimensions within NFA crystals. However, many of the NFAs that are reported to organize into this lattice suffer from uneven electronic couplings, which may impede charge transport. Analysis of intermolecular interactions suggests that segregating the alkyl chains and aromatic backbones, and favoring face-to-face stacking of the conjugated backbones, could facilitate formation of the 3D wire mesh lattice.

Another important challenge in understanding the structure-processing-property relationship in NFAs is the possibility of polymorphism, highlighted by our determination of a very contorted polymorph of ITIC-Th. MicroED could become an important tool for understanding the relationships between crystallization or processing conditions and OSC solid-state structure, as demonstrated by our determination of the crystal structure of o-IDTBR from a powder sample. It may enable researchers to determine the crystal structure of OSCs without going through the laborious crystal growth process by allowing them to study crystals that are too small for single crystal X-ray crystallography.

© Fundació Scito
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