Organic Photovoltaics: Relating Chemical Structure, Local Morphology, and Electronic Properties

Substantial enhancements in the efficiencies of bulk-heterojunction (BHJ) organic solar cells (OSCs) have come from largely trial-and-error-based optimizations of the morphology of the active layers. Further improvements, however, require a detailed understanding of the relationships among chemical structure, morphology, electronic properties, and device performance. On the experimental side, characterization of the local (i.e., nanoscale) morphology remains challenging, which has called for the development of robust computational methodologies that can reliably address those aspects. In this review, we describe how a methodology that combines all-atom molecular dynamics (AA-MD) simulations with density functional theory (DFT) calculations allows the establishment of chemical structure–local morphology–electronic properties relationships. We also provide a brief overview of coarse-graining methods in an effort to bridge local to global (i.e., mesoscale to microscale) morphology. Finally, we give a few examples of machine learning (ML) applications that can assist in the discovery of these relationships.Office of Naval Research12 month embargo; published: April 25, 2020This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Molecular Packing of Non-Fullerene Acceptors for Organic Solar Cells: Distinctive Local Morphology in Y6 Versus ITIC Derivatives

Since a couple of years ago, Y6 has emerged as one of the main non-fullerene acceptors for organic solar cells as its use leads to superior power conversion efficiencies. It is thus of major interest to investigate the multi-scale phenomena that are responsible for Y6’s efficacy. Here, we modeled neat films of Y6 and earlier non-fullerene acceptors, IT-4F and ITIC, using a combination of density functional theory calculations and molecular dynamics simulations, to investigate the various factors that control their charge and exciton transport rates. We find that the molecular packing in Y6 is drastically different from that in IT-4F and ITIC. At the nano-scale, the local morphology of Y6 consists of a large number of directional face-on stackings and well-connected transport networks. Y6 also consistently shows higher electronic couplings for LUMOs, HOMOs, and local excitations than ITIC-type acceptors, which results in fast transport rates for electron, holes, and excitons. Importantly, when considering dimers, their configurations in Y6 are more diverse than in ITIC-type acceptors, with many of those similar to the configurations observed in the Y6 crystal structure reported recently. Most Y6 dimer configurations exhibit strong binding interactions, large electronic couplings, and high transport rates, which when taken together rationalize the better performance of OSCs based on Y6

NLDFT Pore Size Distribution in Amorphous Microporous Materials

The pore size distribution (PSD) is one of the most important properties when characterizing and designing materials for gas storage and separation applications. Experimentally, one of the current standards for determining microscopic PSD is using indirect molecular adsorption methods such as nonlocal density functional theory (NLDFT) and N₂ isotherms at 77 K. Because determining the PSD from NLDFT is an indirect method, the validation can be a nontrivial task for amorphous microporous materials. This is especially crucial since this method is known to produce artifacts. In this work, the accuracy of NLDFT PSD was compared against the exact geometric PSD for 11 different simulated amorphous microporous materials. The geometric surface area and micropore volumes of these materials were between 5 and 1698 m²/g and 0.039 and 0.55 cm³/g, respectively. N₂ isotherms at 77 K were constructed using Gibbs ensemble Monte Carlo (GEMC) simulations. Our results show that the discrepancies between NLDFT and geometric PSD are significant. NLDFT PSD produced several artificial gaps and peaks that were further confirmed by the coordinates of inserted particles of a specific size. We found that dominant peaks from NLDFT typically reported in the literature do not necessarily represent the truly dominant pore size within the system. The confirmation provides concrete evidence for artifacts that arise from the NLDFT method. Furthermore, a sensitivity analysis was performed to show the high dependency of PSD as a function of the regularization parameter, λ. A higher value of λ produced a broader and smoother PSD that closely resembles geometric PSD. As an alternative, a new criterion for choosing λ, called here the smooth-shift method (SSNLDFT), is proposed that tuned the NLDFT PSD to better match the true geometric PSD. Using the geometric pore size distribution as our reference, the smooth-shift method reduced the root-mean-square deviation by ∼70% when the geometric surface area of the material is greater than 100 m²/g

Resolving atomic-scale interactions in non-fullerene acceptor organic solar cells by high-field NMR crystallography

International audienc

Resolving Atomic‐Scale Interactions in Nonfullerene Acceptor Organic Solar Cells with Solid‐State NMR Spectroscopy, Crystallographic Modelling, and Molecular Dynamics Simulations

Fused-ring core nonfullerene acceptors (NFAs), designated “Y-series,” have enabled high-performance organic solar cells (OSCs) achieving over 18% power conversion efficiency (PCE). Since the introduction of these NFAs, much effort has been expended to understand the reasons for their exceptional performance. While several studies have identified key optoelectronic properties that govern high PCEs, little is known about the molecular level origins of large variations in performance, spanning from 5% to 18% PCE, for example, in the case of PM6:Y6 OSCs. Here, a combined solid-state NMR, crystallography, and molecular modeling approach to elucidate the atomic-scale interactions in Y6 crystals, thin films, and PM6:Y6 bulk heterojunction (BHJ) blends is introduced. It is shown that the Y6 morphologies in BHJ blends are not governed by the morphology in neat films or single crystals. Notably, PM6:Y6 blends processed from different solvents self-assemble into different structures and morphologies, whereby the relative orientations of the sidechains and end groups of the Y6 molecules to their fused-ring cores play a crucial role in determining the resulting morphology and overall performance of the solar cells. The molecular-level understanding of BHJs enabled by this approach will guide the engineering of next-generation NFAs for stable and efficient OSCs.Office of Naval Research12 month embargo; first published: 24 November 2021This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Delocalization of exciton and electron wavefunction in non-fullerene acceptor molecules enables efficient organic solar cells

A major challenge for organic solar cell (OSC) research is how to minimize the tradeoffbetween voltage loss and charge generation. In early 2019, we reported a non-fullereneacceptor (named Y6) that can simultaneously achieve high external quantum efficiency andlow voltage loss for OSC. Here, we use a combination of experimental and theoreticalmodeling to reveal the structure-property-performance relationships of this state-of-the-artOSC system. We find that the distinctive π–π molecular packing of Y6 not only exists inmolecular single crystals but also in thin films. Importantly, such molecular packing leads to(i) the formation of delocalized and emissive excitons that enable small non-radiative voltageloss, and (ii) delocalization of electron wavefunctions at donor/acceptor interfaces thatsignificantly reduces the Coulomb attraction between interfacial electron-hole pairs. Theseproperties are critical in enabling highly efficient charge generation in OSC systems withnegligible donor-acceptor energy offset