The Role of Dipole Interactions on Intermolecular Charge Transfer in Y6 Non-Fullerene Acceptor Studied by Electroabsorption Spectroscopy

Organic photovoltaic cells (OPVs) with Y6 as electron acceptor have recently been reported with record high efficiency and negligible photovoltage loss. It is generally believed that intermolecular electrostatic interactions between Y6 molecules lead to strong charge transfer (CT) characteristics which results in efficient exciton dissociation and reduced non-radiative recombination. However, it is still unclear about the contribution of intermolecular dipoles and its effect on different dimer formations and enhanced CT characteristics. Here, we investigate the contribution of anisotropic dipoles to excimer formation and its impact on CT properties using electroabsorption (EA) spectroscopy. The EA spectra of Y6 are strongly dependent on the molecular packing as we observe strong second derivative characteristics around the S0 to S1 excitation while increasing the loading ratio of Y6 in the polymer matrix. We also observe dipole orientation in packed Y6 film while increasing the molecular concentration by quantitative analysis of the first harmonic EA spectra of pristine Y6. These preferred oriented dipoles are found responsible for the distinctive molecular packing of Y6 which leads to strong intermolecular charge transfer. Quantitatively, fitting the EA spectrum highlights the significance of polarizability (p) between ground and excited states in electron delocalization and intermolecular charge transfer property of Y6. The extracted Δ<i>u </i>values of monomer and dimer configurations show excellent consistency with theoretical values from quantum calculation results. Our work mainly highlights the application of electroabsorption spectroscopy as an powerful technique to probe charge transfer characteristics, intermolecular interactions, binding energy, and delocalization of excitons etc. in pristine Y6. This research work provides insight into better molecular design of novel photovoltaic materials by tuning the intermolecular electrostatic interactions and delocalization of electron wavefunctions.