Spontaneous-Symmetry-Breaking Charge Separation Induced by Pseudo-Jahn–Teller Distortion in Organic Photovoltaic Material

The driving force of charge separation in the initial photovoltaic conversion process was theoretically investigated using ITIC, non-fullerene acceptor material for organic photovoltaic (OPV) devices, as an example. The density functional theory (DFT) calculations show that the pseudo-Jahn–Teller (PJT) distortion of the S1 excimer state induces spontaneous-symmetry-breaking charge separation (SSB-CS) between the identical ITIC molecules to give an intermolecular charge-transfer (ICT) excited state, even without the asymmetry of the surrounding environment. The strong PJT effect arises from the vibronic coupling between the pseudo-degenerate S1 and S2 excited states with different irreducible representations (irreps), i.e., Au for S1 and Ag for S2, via the asymmetric vibrational mode with Au irrep. The vibrational mode responsible for the spontaneous polarization, opposite in one ITIC monomer and the other, is the intramolecular C-C stretching vibration between the core IT and terminal IC units at the stacked region. These results suggest that controlling the PJT effect of the excited states can influence the charge separation efficiency in the initial photovoltaic conversion process.