Mechanical Properties of Polymer-Fullerene Bulk Heterojunction Films: Role of Nanomorphology of Composite Films

This paper reports the tensile properties and fracture mechanism of PTB7:PC71BM bulk heterojunction (BHJ) films as a function of composition mixing ratio. An increased concentration of fullerene makes the BHJ films stiffer and more brittle, and fracture occurs along aggregated fullerene domain boundaries. The tensile strength is maximized at a polymer fullerene content ratio of 1:1. Furthermore, an additive, 1,8-diiodoctane (DIO), in the films induces fine nanomorphology, which increases the stiffness and strength and reduces the ductility of the films further. This is especially true under a high PC71BM load due to the expanded interfacial surface areas between the PC71BM and PTB7 polymer domains. The photovoltaic performance of the BHJ films on polydimethylsiloxane (PDMS) substrates after tensile stretching cycles is also examined in detail.