Solvent Additive-Assisted Kinetic Stabilization of Nonfullerene Bulk Heterojunction Active Layers under Thermal Stress

The thermal stability of bulk heterojunction (BHJ) active layers critically determines the lifetime of nonfullerene organic solar cells. In this work, we track the thermally driven kinetic evolution of PffBT4T-2OD:ITIC BHJs over a range of annealing temperatures and times. Neat PffBT4T-2OD films remain stable under the investigated conditions, whereas neat ITIC films develop progressively diffuse interfaces and show the onset of structural evolution upon annealing at 120 °C. In pristine PffBT4T-2OD:ITIC BHJs, annealing above 120 °C triggers vertical phase stratification, forming an ITIC-enriched layer near the air interface and a PffBT4T-2OD-rich region adjacent to the substrate. Time-resolved neutron reflectometry provides quantitative kinetic evidence that incorporating a small amount of a solvent additive (1,8-diiodooctane, DIO, 0.25 vol %) delays and mitigates thermally induced vertical redistribution. Specifically, DIO incorporation extends the extrapolated onset time for redistribution by ∼2.3-fold, accompanied by an ∼38% reduction in the redistribution rate. Complementary X-ray scattering further shows that the development of ITIC crystalline ordering is delayed in the presence of DIO. These results demonstrate an additive-assisted kinetic stabilization pathway in nonfullerene BHJs, where improved donor–acceptor intermixing slows early-stage acceptor ordering and mitigates progression toward larger-scale phase separation under thermal stress.