A Quantum Mechanics/Molecular Mechanics Simulation Approach to Access the Charge Photo-Generation Properties in Organic Photovoltaic Materials

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 are developing a multi-scale method that combines Quantum Mechanics (QM) calculations and Molecular Dynamics (MD) simulations within the scope of a sequential-QM/MD approach [2] to assess the charge photo-generation properties in organic photovoltaic materials. Despite being a well-established method to study small molecules in solution, it has not yet been developed to investigate polymer (and small organic molecule) films cast from solution. Our methodology starts with the simulation of film formation through solvent molecules evaporation using classical molecular dynamics simulations. Then additional MD simulations are carried out on the obtained film to generate uncorrelated configurations to be 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. For the QM calculations, density functional theory (DFT) and time-dependent DFT have been employed. The quality of the force field parameters adopted in the MD simulations have been carefully analyzed. A set of representative NFAs small molecules and polymers have been studied, e.g. Y5, Y6, and PF5-Y5 polymer. Given the flexibility of the computational approach, we have been able to study these systems both in solution (chlorobenzene) and film. First, we have analyzed the structure of the films with focus, for instance, on the tendency to stabilize pi-pi stacking conformations. Then, the dynamics and molecular environment effects on the electronic transitions have been quantified with an improved description of the optical absorption. Finally, through the calculation of the fundamental and optical gaps the exciton binding energies have been estimated. Here, we have considered both the singlet and triplet excitons. The comparisons with experimental results confirm the suitability of the developed s-QM/MD approach, highlighting the importance of properly describing the dynamics and molecular environment effects in the modeling of the electronic properties of OPV materials. <br/> <br/>References:<br/>1. Armin, Ardalan, <i>et al. </i>Advanced Energy Materials <b>11</b>, 2003570 (2021).<br/>2. Coutinho, Kaline, <i>et al.</i> Springer, Dordrecht, 2008. 159-189.