Publications by authors named "Ahra Yi"

4 Publications

  • Page 1 of 1

Resolving Atomic-Scale Interactions in Non-Fullerene Acceptor Organic Solar Cells with Solid-State NMR Spectroscopy, Crystallographic Modelling, and Molecular Dynamics Simulations.

Adv Mater 2021 Nov 24:e2105943. Epub 2021 Nov 24.

University of Lille, CNRS, Centrale Lille Institut, Univ. Artois, UMR 8181, Unité de Catalyse et Chimie du Solide, Lille, F-59000, France.

Fused-ring core non-fullerene 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, e.g., in the case of PM6:Y6 OSCs. Here, we introduce a combined solid-state NMR, crystallography, and molecular modelling approach to elucidate the atomic-scale interactions in Y6 crystals, thin films, and PM6:Y6 bulk heterojunction (BHJ) blends. We show 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. This article is protected by copyright. All rights reserved.
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http://dx.doi.org/10.1002/adma.202105943DOI Listing
November 2021

Alkyl Conformation and π-π Interaction Dependent on Polymorphism in the 1,8-Naphthalimide (NI) Derivative.

ACS Omega 2019 Nov 5;4(22):19705-19709. Epub 2019 Nov 5.

Department of Organic Material Science and Engineering, Pusan National University, Busan 46241, Republic of Korea.

The 1,8-naphthalimide (NI) derivative Lumogen F Violet 570 exhibits different photoluminescence (PL) and aggregation-caused quenching properties due to its crystal polymorphism, which depends on the solvent evaporation process in tetrahydrofuran solution. In the slow drying process, molecules aggregated into an energetically more stable form (time-dependent density functional theory calculation), of which the PL peak maximum was 453 nm, corresponding to blue emission at the 365 nm excitation. However, the fast evaporation process induces an energetically less stable form, with a PL peak maximum of 508 nm, corresponding to green emission. The main difference between the two crystal structures is the alkyl conformation, as confirmed by X-ray single-crystal analysis. Due to the different alkyl conformations, NI groups aggregated into more obliquely aligned structures that emit blue PL, which plays a role in weakening the π-π interactions between molecules relative to green PL crystals. We found that the conformational stable molecular stacking induced instability in the electronic energy levels of the blue crystal compared to the green crystal.
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http://dx.doi.org/10.1021/acsomega.9b02377DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6881847PMC
November 2019

Manipulating the crystal structure of a conjugated polymer for efficient sequentially processed organic solar cells.

Nanoscale 2018 Dec 14;10(45):21052-21061. Epub 2018 Sep 14.

Department of Organic Material Science and Engineering, Pusan National University, Busan 46241, South Korea.

Recently, the sequential (Sq) process, which forms nanoscale network structures from quasi-solid-state inter-diffusion through swelling and annealing, is considered to be one of the most efficient methods for fabricating organic solar cells and blend films. Here, we examined the effect of the crystallinity and orientation of poly(3-hexylthiophene) (P3HT) molecules on the formation of the nanostructure by carrying out a Sq process using various solvents with different boiling points. We showed that the moderate crystallinity promoted suitable inter-diffusion between the donor (P3HT) and acceptor ([6,6]-pentadeuterophenyl C61 butyric acid methyl ester, PCBM), and hence was important for achieving high-performance solar cells using Sq processing. Nanostructure formation by inter-diffusion was investigated and visualized by taking a combination of grazing-incidence wide-angle X-ray scattering (GIWAXS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) measurements. In addition, our Sq-processed solar cell yielded a device efficiency as high as 3.25%, and was also impressive because it was made with an eco-friendly solvent and using a short-duration annealing process, in contrast to the conventional BHJ process. The present findings provided advanced insight into the Sq process, and we anticipate this efficacious sequential process to contribute not only to the development of higher-efficiency organic solar cells but also to the fabrication of functional blend films.
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http://dx.doi.org/10.1039/c8nr05407jDOI Listing
December 2018

Using Femtosecond Laser Irradiation to Enhance the Vertical Electrical Properties and Tailor the Morphology of a Conducting Polymer Blend Film.

ACS Appl Mater Interfaces 2017 Jul 12;9(29):24422-24427. Epub 2017 Jul 12.

Department of Organic Material Science and Engineering, Pusan National University , Busan 46241, South Korea.

We report femtosecond infrared laser-induced selective tailoring of carrier transport as well as surface morphology on a conducting polymer blend thin film. Maximal 2.4 times enhancement on vertical current transport in poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester, was achieved by this irradiation. The laser irradiation induced a photo expansion without deteriorating its molecular structure and the film morphology could be customized in the micron scale by adjusting the laser writing parameters. In the photoexpanded region, the face-on populations were about 2.2 times larger in comparison with the pristine region, which was a major contributor to the enhanced carrier transport.
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http://dx.doi.org/10.1021/acsami.7b05937DOI Listing
July 2017
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