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 is theoretically investigated using ITIC, a nonfullerene acceptor material for organic photovoltaic devices. The density functional theory calculations show that the pseudo-Jahn-Teller (PJT) distortion of the S 1 excimer state induces spontaneous symmetry-breaking charge separation between the identical ITIC molecules even without the asymmetry of the surrounding environment. The strong PJT effect arises from the vibronic coupling between the pseudodegenerate S 1 and S 2 excited states with different irreducible representations (irreps), i.e., A u for S 1 and A g for S 2 , via the asymmetric vibrational mode with the A u irrep. The vibrational mode responsible for the spontaneous polarization, which is opposite in one ITIC monomer and the other, is the intramolecular C-C stretching vibration between the core IT and terminal IC units. These results suggest that controlling the PJT effect can improve the charge separation efficiency of the initial photovoltaic conversion process.