Quantitative Understanding of Charge-Transfer Exciton Diffusion in Y-Type Acceptors for Efficient Organic Solar Cells

Exciton diffusion length (L_D) is a critical parameter for organic solar cells (OSCs). State-of-the-art OSCs are based on Y-type non-fullerene acceptor (NFA) materials (including Y6 and its derivatives). Recent studies have revealed that intermolecular charge-transfer (ICT) excitons are created in Y-type NFAs, but the precise role of ICT exciton formation on L_D has not been discussed. Here, it is reported that, due to the spectral evolution near the optical gap associated with ICT exciton formation on the picosecond timescale, overlooking this phenomenon may lead to significant overestimation of L_D from transient absorption data analysis for Y-type NFA films. Moreover, when performing numerical fitting using the exciton-exciton annihilation model, taking the intrinsic relaxation lifetime of the ICT exciton is essential for the reliable extraction of diffusion coefficient and L_D. Finally, besides showing increasing L_D with reducing π–π stacking spacing, these results show that L_D is defined by the crystalline domain size of the solution-processed Y-type NFA films, suggesting that the reported L_D may yet to have reached its fundamental limit. These results provide new insights for achieving long-range exciton diffusion in OSC materials, paving the way for the realization of highly efficient OSCs with increased domain sizes and film thicknesses.