Importance of Molecular Weight of Conjugated Polymer Thin Films on Thermomechanical Reliability of Stretchable Organic Semiconductors

The molecular weight ( M W ) of conjugated polymers (CPs) plays a fundamental role in the formation of polymer chain networks, which determines the mechanical, electrical, and chemical properties of the polymers. For stretchable electronics, superior mechanical elongation and strength of CPs in a thin-film state can be obtained when polymer chains exceed their critical M W ( M c ), resulting in polymer chain entanglement and tie molecules. However, the organic stretchable devices are inevitably subjected to thermal stress as temperature changes in actual operation and processing. Despite the high thermal expansivity of stretchable polymer substrates, few studies have been conducted on thermomechanical reliability of the stretchable devices. Herein, we investigated thermomechanical reliability issues of stretchable organic semiconductors with CP thin films. Stretchable devices consisting of the CP films with different MWs on the poly(dimethylsiloxane) (PDMS) substrate were subjected to a thermal stress test. After the test, thermal cracking occurred in the CP film having M W below M c due to its brittleness, resulting in a dramatic electrical mobility drop (more than 1 order of magnitude). In contrast, the high- M W CP films ( M W > M c ) formed randomly oriented wrinkles after the test, and the decrease in the electrical mobility of the stretchable device was lower than that of the films with lower M W . To analyze the thermomechanical stabilities of stretchable devices, both the tensile tests of CP thin films with different M W and finite-element simulation of the stretchable device deformation as the temperature changes were performed. It was confirmed that although the high- M W CP films are subjected to higher thermal stress than that of the films with lower M W, the high- M W films can maintain electrical pathways without mechanical fracture by the formation of wrinkles due to the plastic deformation. Our study demonstrates that employing CPs with high M W above its M c enables the fabrication of thermomechanically robust stretchable devices.