Disorder-Effect on The Photophysics of Solution Processed Polymeric Photovoltaic Materials through Sequential QM/CMD Approach

The development of new organic photovoltaic materials based on non-fullerene acceptors (NFAs) has led to a significant increase in the power conversion efficiency of organic photovoltaics (OPV) in the last years [1]. However, the fundamental understanding of the charge photogeneration mechanism at the molecular level is still lacking, a scientific challenge whose solution could be the watershed in the discovery of novel OPV materials. To contribute to this end, we have developed a multi-scale method that combines Quantum Mechanics (QM) calculations and Classical Molecular Dynamics (CMD) simulations within the scope of a sequential QM/CMD approach [2] to assess the photophysics of organic-polymeric photovoltaic materials. Despite being a well-established method to study small and medium-sized molecules in solution, it has not yet been applied to investigate polymer films cast from solution. Our methodology starts with the simulation of film formation through solvent molecules evaporation procedure using CMD simulations. Afterwards, additional CMD simulations are carried out on the obtained film to generate uncorrelated configurations to be subsequently used on the properties’ calculations. The latter is assessed through an electronic embedding scheme where a pre-defined molecular region of the generated configuration is treated at the QM level, incorporating explicit effects of the environment. The quality of the force field parameters adopted in the CMD simulations has also been carefully analyzed. For the QM calculations, density functional theory (DFT) and its time-dependent version (TD-DFT) have been employed along with the wavefunction-based method ADC(2) [3]. The latter has been widely used as reference method to assess the accuracy of TD-DFT method on the description of vertical excitation energies and excited-state potential energy surfaces. We have focused the study on the PF5-Y5 polymer [4,5], which can be seen as a model system to study covalently bound donor-acceptor interfaces. First, we have analyzed the structure of the films, focusing, for instance, on the tendency to stabilize π-π stacking conformations. Then, the dynamics and molecular environment effects on the electronic transitions have been quantified with an improved description of the optical absorption. The recently developed double-hybrid functionals [6], which include a MP2-like perturbation in the correlation part, were found to be the best theory-level within TDDFT framework to describe the singlet-triplet energy gaps in this complex donor-acceptor polymer. The comparisons with experimental results confirm the suitability of the developed s-QM/CMD approach, highlighting the importance of properly describing the disorder, dynamics and molecular environment effects in the modeling of the electronic properties of OPV materials.<br/><br/>References<br/>1. A. Armin et al. <i>Advanced Energy Materials</i> <b>11</b>, 2003570 (2021).<br/>2. K. Coutinho et al. Springer, Dordrecht, 2008. 159-189.<br/>3. A. Tajti et al. <i>J. Chem. </i><i>Theory Comput.</i> <b>16</b>, 468 (2020).<br/>4. Q Fan et al. <i>Energy & Environmental Science</i> <b>13</b>, 5017 (2020).<br/>5. I. Jalan et al. <i>Journal of Materials Chemistry C</i> <b>11</b>, 9316 (2023).<br/>6. D. Mester et al. <i>J. Chem. </i><i>Theory Comput. </i><b>18</b>, 1646 (2022).