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  1. Abstract

    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).

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  2. Abstract

    An overlooked factor affecting stability: the residual stresses in perovskite films, which are tensile and can exceed 50 MPa in magnitude, a value high enough to deform copper, is reported. These stresses provide a significant driving force for fracture. Films are shown to be more unstable under tensile stress—and conversely more stable under compressive stress—when exposed to heat or humidity. Increasing the formation temperature of perovskite films directly correlates with larger residual stresses, a result of the high thermal expansion coefficient of perovskites. Specifically, this tensile stress forms upon cooling to room temperature, as the substrate constrains the perovskite from shrinking. No evidence of stress relaxation is observed, with the purely elastic film stress attributed to the thermal expansion mismatch between the perovskite and substrate. Additionally, the authors demonstrate that using a bath conversion method to form the perovskite film at room temperature leads to low stress values that are unaffected by further annealing, indicating complete perovskite formation prior to annealing. It is concluded that reducing the film stress is a novel method for improving perovskite stability, which can be accomplished by lower formation temperatures, flexible substrates with high thermal expansion coefficients, and externally applied compressive stress after fabrication.

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  3. Abstract

    Photoactive perovskite semiconductors are highly tunable, with numerous inorganic and organic cations readily incorporated to modify optoelectronic properties. However, despite the importance of device reliability and long service lifetimes, the effects of various cations on the mechanical properties of perovskites are largely overlooked. In this study, the cohesion energy of perovskites containing various cation combinations of methylammonium, formamidinium, cesium, butylammonium, and 5‐aminovaleric acid is reported. A trade‐off is observed between the mechanical integrity and the efficiency of perovskite devices. High efficiency devices exhibit decreased cohesion, which is attributed to reduced grain sizes with the inclusion of additional cations and PbI2additives. Microindentation hardness testing is performed to estimate the fracture toughness of single‐crystal perovskite, and the results indicated perovskites are inherently fragile, even in the absence of grain boundaries and defects. The devices found to have the highest fracture energies are perovskites infiltrated into a porous TiO2/ZrO2/C triple layer, which provide extrinsic reinforcement and shielding for enhanced mechanical and chemical stability.

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