Investigation on the Effect of Molecular Packing on Charge Transfer Characteristics of Y6 Non-Fullerene Acceptor Using Electroabsorption Spectroscopy

Organic solar cells (OSC) with Y6 as acceptor have been reported with very low voltage loss. It is generally believed that the large dipole moment and distinctive molecular stacking of Y6 molecules lead to strong charge transfer (CT) characteristics which results in low driving force for exciton dissociation and reduced non-radiative recombination. However, it is still not clear how the dipoles and molecular stacking separately contribute to the CT characteristics. Here, we use a solution processing approach to isolate Y6 molecules and study the packing effect on the excitonic properties in Y6. Electroabsorption (EA) spectroscopy is used to investigate the CT characteristics of Y6 thin films with different degrees of molecular packing. We find that the EA spectra are strongly dependent on the molecular packing. We observe a transition from Frenkel to CT excitons around the S0 to S1 excitation while increasing the packing of the molecules. The EA results in packed Y6 are also consistent with the generation of quasi-free charge observed in quantum efficiency measurement. Quantitatively, by fitting the EA spectra, it brings insight into the packing effect on exciton delocalization radius and the probability of back charge transfer at organic bulk heterojunctions. This work not only provides direct experimental evidence of the contribution of molecular packing on charge transfer characteristics in Y6, but also emphasizes the role of molecular packing in reducing energy losses in organic solar cells.