(Invited) Toward a Molecular Understanding of Photoinduced Charge Separation Process in Organic Solar Cells with Computational Approaches

Abstract

Photoinduced charge separation (CS) process plays important roles in various fields such as natural and artificial photosynthesis, solar cell, and light-emitting diode. To investigate the photoinduced CS process theoretically, the CS state is often regarded as “static exciton” and described by several parameters such as free energy difference and electronic coupling between initial and final states according to Marcus theory. However, dynamical effects such as nuclear motions and spin-orbit coupling (SOC) are recently being recognized as important factors for controlling the CS efficiency. Therefore, the CS state should be considered as “dynamic exciton.” To analyze the dynamic exciton theoretically, we need to treat the photoexcited reaction dynamics quantum mechanically, where the locally excited and CS states, electronic coupling and SOC dynamically fluctuates. In addition, surrounding environment such as solvent are also important and required to be explicitly considered because the charge distribution of CS state is much different from that of initial state. It is impossible to treat all the systems including surroundings quantum mechanically. Therefore, combined quantum mechanical and molecular mechanical (QM/MM) method, in which an important subsystem is described quantum mechanically whereas the other surrounding is treated by molecular mechanics force field, is an essential tool. However, even the QM/MM method still requires large computational cost for the analysis of dynamic exciton because the calculation time of excited state is much longer than that of ground state. To overcome this difficulty, we have been developing several methods combining the QM/MM method and molecular dynamics simulation efficiently. In this presentation, we will show several applications toward a molecular understanding of photoinduced CS process in organic solar cells.