Impact of additive-induced nonfullerene acceptor aggregation on photovoltaic performance in organic photovoltaics

The emergence of nonfullerene acceptors (NFAs), particularly Y6 derivatives, has propelled organic photovoltaics (OPVs) to power conversion efficiency (PCE) exceeding 20 %. However, these highly efficient NFAs exhibit strong aggregation in the solid state, often leading to suboptimal morphology and restricted charge transport. To address this issue, liquid or solid additives are commonly introduced during film fabrication; however, the mechanisms by which different additives regulate NFA aggregation remain elusive. Herein, the relationship between photovoltaic performance and NFA aggregation in the state-of-the-art PM6/L8-BO blend systems is investigated using 1,8-diiodooctane (DIO, liquid additive) and 1,4-diiodobenzene (DIB, solid additive) as representative additives. As a result, DIO is found to promote excessive L8-BO aggregation, leading to reduced photoluminescence quenching efficiency and charge mobility, which deteriorates photovoltaic performance. In contrast, DIB does not directly promote aggregation but acts as a plasticizer for PM6, lowering its glass transition temperature, and thereby enabling controlled L8-BO aggregation during thermal annealing. In-situ absorption spectroscopy during spin coating suggests that DIO facilitates liquid–liquid phase separation, whereas DIB regulates aggregation indirectly through polymer softening. These findings clarify the distinct roles of liquid and solid additives in morphology regulation, providing new insights for designing highly efficient OPVs via precise control of active layer aggregation.