Comparative Study of the Mechanical Properties of All-Polymer and Fullerene-Polymer Solar Cells: The Importance of Polymer Acceptors for High Fracture Resistance

Abstract

High fracture resistance of polymer solar cells (PSCs) is of great importance to ensure long-term mechanical reliability, especially considering their potential in roll-to-roll printing processes and flexible devices. In this paper, we compare mechanical properties, such as the cohesive fracture energy, elastic modulus, and crack-onset strain, of all polymer solar cells (all-PSCs) and fullerene-based solar cells (PCBM-PSCs) based on the same, representative low-bandgap polymer donor (PTB7-Th) as a function of acceptor content. The all-PSCs exhibit higher fracture energy (2.45 J m(-2)) than PCBM-PSCs (0.29 J m(-2)) at optimized device conditions. Additionally, a 15-fold higher crack-onset strain is observed in all-PSCs than in PCBM-PSCs. Dramatically different mechanical compliances observed for all-PSCs and PCBM-PSCs are investigated in detail by analysis of the blend morphologies as a function of acceptor content (either P(NDI2HD-T) or PCBM acceptors). The superior fracture resistance of all-PSCs is attributed to the more ductile characteristics of the polymer acceptor and the large degree of plastic deformation during crack growth, in contrast to the brittle nature of PCBM and the weak interaction between the polymer-rich phase and highly aggregated PCBM-rich domains. Therefore, this work demonstrates that replacing a small-molecule acceptor (i.e., PCBM) with polymeric materials can be an effective strategy toward mechanically robust PSCs.