A Facile Method for Thermally, Light, and Mechanically Stable Organic Solar Cells Using Ultraviolet‐Initiated Crosslinkable Additive

Abstract Crosslinkable additive in organic solar cells has great potential to fix metastable bulk heterojunction, supplying solutions for burn‐in loss and long‐term instability. New roles of crosslinkable additive as a solvent additive, radical scavenger, and stretchability enhancer beyond its well‐known role as a crosslinker with morphology frozen effect are reported. According to the main chain structure between Br in end groups, crosslinkable additive is divided into two types of alkyl and siloxane main chain, expecting different roles. First, the alkyl moiety molecule, 1,8‐dibromooctane (DBr‐octane), provides the effects of improved thermal and light stability without any loss in efficiency. The DBr‐octane device shows remarkable enhanced results in burn‐in‐free, maintaining 90% of its initial efficiency after 600 min at 85 °C, and light stability, maintaining 90% of its initial efficiency after 360 min under 1 SUN, in a fullerene system. Second, crosslinkable additive is treated with siloxane main chain molecule, bis(3‐bromopropyl)‐terminated poly(dimethyl siloxane) (DBr‐siloxane), in an active layer to secure stretchability. As a result, the DBr‐siloxane‐treated film stretches up to 70% and the flexible device of the fullerene system maintains an initial efficiency of over 60% after 1000 bending cycles at a bending radius of 1.8 mm.