The mechanical properties of π-conjugated (semiconducting) polymers are a key determinant of the stability and manufacturability of devices envisioned for applications in energy and healthcare. These propertiesincluding modulus, extensibility, toughness, and strengthare influenced by the morphology of the solid film, which depends on the method of processing. To date, the majority of work done on the mechanical properties of semiconducting polymers has been performed on films deposited by spin coating, a process not amenable to the manufacturing of largearea films. Here, we compare the mechanical properties of thin films of regioregular poly(3-heptylthiophene) (P3HpT) produced by three scalable deposition processesinterfacial spreading, solution shearing, and spray coatingand spin coating (as a reference). Our results lead to four principal conclusions. (1) Spray-coated films have poor mechanical robustness due to defects and inhomogeneous thickness. (2) Sheared films show the highest modulus, strength, and toughness, likely resulting from a decrease in free volume. (3) Interfacially spread films show a lower modulus but greater fracture strain than spin-coated films. (4) The trends observed in the tensile behavior of films cast using different deposition processes held true for both P3HpT and poly(3butylthiophene) (P3BT), an analogue with a higher glass transition temperature. Grazing incidence X-ray diffraction and ultraviolet−visible spectroscopy reveal many notable differences in the solid structures of P3HpT films generated by all four processes. While these morphological differences provide possible explanations for differences in the electronic properties (hole mobility), we find that the mechanical properties of the film are dominated by the free volume and surface topography. In field-effect transistors, spread films had mobilities more than 1 magnitude greater than any other films, likely due to a relatively high proportion of edge-on texturing and long coherence length in the crystalline domains. Overall, spread films offer the best combination of deformability and charge-transport properties.

Crosslinking is a ubiquitous strategy in polymer engineering to increase the thermomechanical robustness of solid polymers but has been relatively unexplored in the context of π‐conjugated (semiconducting) polymers. Notwithstanding, mechanical stability is key to many envisioned applications of organic electronic devices. For example, the wide‐scale distribution of photovoltaic devices incorporating conjugated polymers may depend on integration with substrates subject to mechanical insult—for example, road surfaces, flooring tiles, and vehicle paint. Here, a four‐armed azide‐based crosslinker (“4Bx”) is used to modify the mechanical properties of a library of semiconducting polymers. Three polymers used in bulk heterojunction solar cells (donors J51 and PTB7‐Th, and acceptor N2200) are selected for detailed investigation. In doing so, it is shown that low loadings of 4Bx can be used to increase the strength (up to 30%), toughness (up to 75%), hardness (up to 25%), and cohesion of crosslinked films. Likewise, crosslinked films show greater physical stability in comparison to non‐crosslinked counterparts (20% vs 90% volume lost after sonication). Finally, the locked‐in morphologies and increased mechanical robustness enable crosslinked solar cells to have greater survivability to four degradation tests: abrasion (using a sponge), direct exposure to chloroform, thermal aging, and accelerated degradation (heat, moisture, and oxygen).

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.

The functionality and usability of π-conjugated (semiconducting) polymers is dependent on the adhesive and interfacial properties of the solid film. Such properties are critical in devices incorporating semiconducting polymers because these layers serve both an active and structural role. They are load bearing in the sense that bending, stretching, scratching, and impact places stress within the semiconducting film at the interfaces with other layers in the device stack. Thus, these organic semiconductors must have good cohesive and adhesive properties despite being designed primarily for optoelectronic function (as opposed to mechanical stability). Here, we measure the effect of the alkyl side chain length on the mechanical and adhesive properties of poly­(3-alkylthiophene) (P3AT) using three different measurement techniques not often applied to conjugated polymers: nanoindentation (quasi-static and dynamic), a lap-joint shear test, and adhesive peel tests (90 and 180°). We performed these measurements alongside pseudo-free-standing (“film-on-water”) tensile tests commonly reported in the literature. We find a monotonic relationship between the length of the side chain and parameters associated with the storage of energy: decreased elastic modulus, strength, and resilience and increased elastic range, from the shortest to the longest side chain. However, we observed a maximum in toughness, fracture strain, and adhesive energy dissipation at A = heptyl or octyl, as well as differences in debonding behavior when P3AT films were deposited on top of a poly­(3,4-ethylenedioxythiophene):poly­(styrene sulfonate) (PEDOT:PSS) film. Notably, our findings suggest that an increase in the alkyl side chain length (beyond n = 8 for P3ATs) may be detrimental to adhesion and thus mechanical robustness.