Cracking in polymer substrates for flexible devices and its mitigation
Mechanical reliability plays an outsized role in determining the durability of flexible electronic devices because of the significant mechanical stresses they can experience during manufacturing and operation. These devices are typically built on sheets comprising stiff thin-film electrodes on compliant polymer substrates, and it is generally assumed that the high-toughness substrates do not crack easily. Contrary to this widespread assumption, here we reveal severe, pervasive, and extensive cracking in the polymer substrates during bending of electrode/substrate sheets, which compromises the overall mechanical integrity of the entire device. The substrate-cracking phenomenon appears to be general, and it is driven by the amplified stress intensity factor caused by the elastic mismatch at the film/substrate interface. To mitigate this substrate cracking, an interlayer-engineering approach is designed and experimentally demonstrated. This approach is generic, and it is potentially applicable to myriad flexible electronic devices that utilize stiff films on compliant substrates, for improving their durability and reliability.