Role of Polymer Segregation on the Mechanical Behavior of All-Polymer Solar Cell Active Layers

• Published in: ACS applied materials & interfaces Vol. 9; no. 50; pp. 43886 - 43892
• Main Authors: Balar, Nrup, Xiong, Yuan, Ye, Long, Li, Sunsun, Nevola, Daniel, Dougherty, Daniel B, Hou, Jianhui, Ade, Harald, O’Connor, Brendan T
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 20.12.2017
• Subjects: chemical structure; energy; mechanical properties; modulus of elasticity; molecular weight; polymers; solar cells; solvents; all-polymer solar cell; crack onset strain; multilength scale morphology; polymer blend film; fracture energy
• ISSN: 1944-8244; 1944-8252; 1944-8252
• DOI: 10.1021/acsami.7b13719

An all-polymer bulk heterojunction (BHJ) active layer that removes the use of commonly used small molecule electron acceptors is a promising approach to improve the thermomechanical behavior of organic solar cells. However, there has been limited research on their mechanical properties. Here, we report on the mechanical behavior of high-performance blade-coated all-polymer BHJ films cast using eco-friendly solvents. The mechanical properties considered include the elastic modulus, crack onset strain, and cohesive fracture energy. We show that the mechanical behavior of the blend is largely unaffected by significant changes in the segregation characteristics of the polymers, which was varied systematically through solvent formulation. In comparison to a polymer:fullerene BHJ counterpart, the all-polymer films were found to have lower stiffness and increased ductility. Yet, the fracture energy of the all-polymer films is not significantly improved compared to that of the polymer:fullerene films. This study highlights that improved mechanical behavior of all-polymer systems cannot be assumed, and that details of the molecular structure, molecular weight, and film morphology play an important role in both the optoelectronic and mechanical properties. Furthermore, we show that simple composite modeling provides a predictive tool for the mechanical properties of the polymer blend films, providing a framework to guide future optimization of the mechanical behavior.