Morphology-Driven Exciton Diffusion Enhances the Charge Generation in Bilayer Organic Solar Cells.

Bilayer organic solar cells (b-OSCs) based on non-fullerene acceptors (NFAs) have reached the efficiencies of bulk heterojunction (BHJ) devices, owing to their long exciton diffusion lengths and improved interfacial control. However, further efficiency gains hinge on overcoming persistent charge-generation losses in these excitonic systems. In this work, we show a simple strategy- adding a small amount of the solvent additive 1-chloronaphthalene (CN)-that can improve the charge generation rate and thereby the power conversion efficiency (PCE) in the widely studied acceptor Y6. We show that incorporation of trace CN induces stable morphology features that are largely independent of the processing solvents chloroform (CF) and dichloromethane (DCM). The optimized acceptor morphology significantly extends the exciton diffusion length, with CN-assisted DCM yielding the longest diffusion length and thereby reducing the charge generation loss in b-OSCs. Time-resolved absorption spectroscopy corroborates these findings, directly linking morphology-driven exciton transport with improved charge generation. The kinetic Monte Carlo (KMC) simulation reveals that the compact molecular packing mainly contributes to the long exciton diffusion lengths in the optimized morphology. Our results establish a rational design strategy for exploiting long-range exciton diffusion as a key lever to minimize energy losses and advance the performance of b-OSCs.