Assessing the optical properties of the non-fullerene acceptor Y5 in a chlorobenzene solution through the s-QM/MM simulation approach
L. R. Franco a, C. Marchiori a, E. Moons a, C. M. Araujo a b
a Department of Engineering and Physics, Karlstad University, Sweden
b Materials Theory Division, Department of Physics and Astronomy, Uppsala University, Sweden, 752 36 Uppsala, Suecia, Sweden
International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV22)
València, Spain, 2022 May 19th - 25th
Organizers: Pablo Docampo, Eva Unger and Elizabeth Gibson
Poster, L. R. Franco, 283
Publication date: 20th April 2022

In recent years, fullerene-free electron acceptors have been brought to the forefront of organic solar cells (OSCs) development and have been established as a more efficient, low-cost, and versatile alternative to fullerenes.[1] In the OSC fabrication process, the blending of the electron donor and acceptor organic materials in a randomized network to form thin polymer films is a solvent-dependent strategy.[2] The understanding of the solvent effects on the structural and electronic properties of both acceptor and donor is of crucial importance to the development of more efficient and stable devices. In this work, the optical absorption spectrum of a model of fullerene-free acceptor, known in the literature as Y5,[3] is simulated with the inclusion of explicit solvent effects in a dilute chlorobenzene solution. The study was carried out under the scope of a sequential molecular mechanics/quantum mechanics (s-QM/MM) approach.[4] The solute/solvent system was simulated at room temperature and pressure conditions using Molecular Dynamics (MD) simulations. The Quantum Mechanics (QM) calculations were performed for a few hundred MD frames. The initial geometries of Y5 and chlorobenzene were obtained from DFT geometry optimizations in vacuum and the OPLS-AA force field was adopted for both solute and solvent molecules. Comparisons between classical and ab-initio MD simulations of Y5 in vacuum validated our force field choices. From the MD simulations, two hundred statistically uncorrelated solute/solvent configurations were extracted for the subsequent electronic structure calculations. The Time-Dependent Density Functional Theory (TD-DFT) was used for the calculation of the first 60 electronic excitations of Y5. The solvent was treated under two-level of approximations: (i) as the electrostatic embedding composed of all atoms of solvent molecules surrounding the solute and (ii) as before but including the closest solvent molecules composing the first solvation shell around Y5 in the QM part with the remaining ones treated like point charges. The calculated electronic excitations and the gaussian convolution of them were compared to the experimental UV-Vis spectrum of Y5 in chlorobenzene, explaining the structure and characteristics of all absorption bands in the visible and near ultra-violet. The present methodology allows us to assess details of the electronic structure of a relevant fullerene-free small-molecule acceptor unveiling fundamental aspects related to the light absorption process. This methodology highlights the importance of properly describing the dynamics and environment effects in the modeling of the electronic properties of such systems.

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