Engineering Surface Tension of Active Layer Solutions to form Uniform Films on Water Surface for Large-Area Flexible Organic Photovoltaic Modules.

Fabricating large-area uniform thin (about 100 nm) active layer films via solution processing is still challenging to realize efficient scalable organic photovoltaic (OPV) modules. In this work, we report a method to fabricate large-area active layer films with the help of Marangoni force via engineering the surface tension of their solutions. Silicone oil was first adopted as an additive to substantially reduce surface tension of the active layer solutions from 34.8 to 20.6 mN/m. Large-area (up to 700 cm) thin active layer films formed spontaneously on water by Marangoni force due to the increased surface tension difference between the active layer solution and water. The films were then transferred onto charge transporting layer to fabricate devices. The active layer films fabricated by Marangoni force-assisted coating (MAC) displayed power conversion efficiencies (PCE), 17.4 ±0.3 % for PM6:BTP-eC9, 17.9±0.7 % for D18:N3 and 16.4±0.3 % for PM6:QM-1. Furthermore, large-area (32.5 cm) OPV modules were fabricated based on the MAC method with a PCE of 14.3 %. This is the first example that MAC method is used to successfully fabricate efficient OPV modules via the surface tension engineering of active layer films with silicone oil used as a low surface tension additive.