Increasing the Strength, Hardness, and Survivability of Semiconducting Polymers by Crosslinking

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 pi-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).