The Role of Processing Solvent on Morphology Optimization for Slot-Die Printed Organic Photovoltaics

The morphology manipulation of the active layers is important for improving the performance of organic photovoltaics (OPVs). The choice of processing solvent has great impact on the crystallization and phase separation during film formation, since solvent properties, including solvent effect on molecular crystallization, boiling point, and interaction parameters, can directly change the evolution pathways associated with thermodynamics and kinetics. Therefore, revealing the underlying solvent-regulated morphology mechanism is potential to provide guiding strategies for device optimization. In this study, chloroform, chlorobenzene, and toluene are used to process PM6:Y6 blends by slot-die printing to fabricate OPV devices. The chloroform printed film forms a fibrillar network morphology with enhanced crystallization, facilitating exciton dissociation, charge transport and extraction, resulting in an optimal power conversion efficiency of 16.22%. However, the addition of the additive chloronaphthalene in chloroform solution leads to over-crystallization of Y6, and thus, increasing domain size that exceeds the exciton diffusion length, resulting in lower device efficiency. In addition, both the chlorobenzene and toluene suppress the crystallization of Y6, which drastically decreased short-circuit current and fill factor. These results demonstrate the important role of processing solvent in dictating film morphology, which critically connects with the resultant printed OPV performance.