LAUSR

Conventional processes for depositing thin films of conjugated polymers are restricted to those based on vapor, liquid, and solution‐phase precursors. Each of these methods bear some limitations. For example, low‐bandgap polymers with alternating donor–acceptor structures cannot be deposited from the vapor phase, and solution‐phase deposition is always subject to issues related to the incompatibility of the substrate with the solvent. Here, a technique to enable deposition of large‐area, ultra‐thin films (≈20 nm or more), which are transferred from the surface of water, is demonstrated. From the water, these pre‐solidified films can then be transferred to a desired substrate, circumventing limitations such as solvent orthogonality. The quality of these films is characterized by a variety of imaging and electrochemical measurements. Mechanical toughness is identified as a limiting property of polymer compatibility, along with some strategies to address this limitation. As a demonstration, the films are used as the hole‐transport layer in perovskite solar cells, in which their performance is shown to be comparable to controls formed by spin‐coating.