Time-Dependent Mechanical Properties of Bulk Heterojunction Films in Organic Solar Cells

The mechanical properties of bulk heterojunction (BHJ) films play critical roles in the operational stability of organic solar cells (OSCs) in real-world applications. While existing literature extensively explores the static mechanical characteristics of BHJ films, the time-dependent mechanical properties are largely overlooked despite their significance in addressing the temporal stresses encountered during practical use. To address this gap, representative BHJ films are prepared, and their time-dependent mechanical properties are reported. It is demonstrated that regardless of whether they are subjected to tensile or compressive stress, the mechanical stability of BHJ films based on PC71BM, Y6, and PYF-T-o decreases gradually, and that the all-polymer-based BHJ films have lower durability than the BHJ films based on small molecule and fullerene acceptors. Furthermore, the impacts of continuous tension on acceptor molecule aggregation and stacking are meticulously studied, evidencing that mechanical stress modifies the degree of aggregation through a relative displacement of the molecules within the BHJ films. Additionally, carrier dynamics studies show that the lifetimes of carriers are extended following the application of tension. This work raises awareness of the time-dependent mechanical stability of OSCs and provides important insights into the development of novel flexible OSC materials and device fabrication processes.