Toughening of semiconducting polymers for mechanically-robust, flexible electroactive thin films

Abstract

With the rapid advancements in flexible/wearable electronic devices, the development of mechanically-robust, flexible semiconducting polymers is becoming increasingly important. However, previous studies on the semiconducting polymers have mostly focused on the optoelectrical properties, and the study on the mechanical properties has been relatively overlooked. In particular, there have been almost no reports on the mechanical properties of semicrystalline n-type polymer thin films and the low-bandgap polymer-based blend thin films. Therefore, herein, important factors affecting the mechanical properties and the fracture mechanisms were investigated for the semicrystalline n-type polymer thin films and the low-bandgap polymer based blend thin films to suggest guidelines to improve the mechanical properties. The mechanical properties of semiconducting polymers have been investigated in two aspects

the cohesive fracture energy measured with double cantilever beam fracture mechanics testing and the tensile properties obtained with pseudo free-standing tensile testing on water surface. Moreover, based on the results of mechanical testing and morphology analysis, the correlation between the thin film structure and mechanical properties were unveiled. This dissertation is divided into three parts. In the first part, the fracture behavior of polymer acceptor based all-polymer solar cells is compared with the fullerene derivative based-polymer solar cells, and the fracture mechanisms are proposed. In the second part, the impact of molecular weight on the mechanical properties of semicrystalline n-type polymer thin films is investigated, and the presence of critical molecular weight is demonstrated. Based on the comprehensive understanding of mechanical properties for the pristine n-type polymer thin films depending on the molecular weight, impact of the molecular weight on the mechanical behaviors of all-polymer solar cells is investigated. Finally, the third part presents the role of film morphology on the mechanical behavior of all-polymer blend thin films. These results provide important guidelines to produce mechanically-robust and flexible electroactive thin films for the commercialization of flexible/wearable organic electronics.