Stress Engineering of Active Layer Films Coated on Water Surface for Efficient Large‐Area Organic Photovoltaic Modules

ABSTRACT Water transfer printing is a promising method for fabricating large‐area organic photovoltaic (OPV) devices. However, the films formed on the water surface (liquid) differ from those formed on glass or plastic substrates (solid). Specifically, the films on the water surface exhibit higher sensitivity to interior stress due to the lack of constrains from bottom substrates. In this work, we report that interior stress drives the formation of wrinkling in the films due to generated capillary force during drying process. This wrinkling makes the subsequent film transfer process challenging, and the resulting devices often exhibit poor performance. To address this issue, drying‐induced stress was mitigated from two aspects: reducing film thickness, and adding liquid additive (1,8‐diiodooctane) to delay its drying. Therefore, uniform, flat, and smooth films on the water surface were obtained on water surface and then transferred onto target substrates for fabricating OPV device. OPV devices based on the water transfer printed film achieved a power conversion efficiency (PCE) of 17.23%. Large‐area flexible OPV modules (70 cm ² ) fabricated via this method were successfully fabricated and delivered a PCE of 13.84%.