Tuning Mechanical Properties of High-Performance Organic Solar Cells with the Addition of a Thermoplastic Elastomer

Mechanical durability of the organic solar cell (OSC) active layer has been a major point of interest to achieve high-performance flexible OSCs. One major challenge is that in polymer: small molecular acceptor (SMA) bulk heterojunction (BHJ) films, the SMA embrittles the active layer. This results in poor fracture toughness in many high-performance OSCs that employ non-fullerene SMAs making them prone to failure. There remains a need to develop strategies to maintain high power conversion efficiency (PCE), maintain morphological stability, and improve mechanical stability. In this work, we have introduced a thermoplastic elastomer, styrene-ethylene-butylene-styrene (SEBS) into a PM6:Y6 BHJ to improve the film’s mechanical properties. This approach was inspired by the rubber toughening of commodity polymers. SEBS is a solution-processable elastomer that can be effectively integrated into the BHJ. We perform an optimization of the SEBS loading and processing conditions to maximize the PCE and toughness of the active layer. We find that SEBS loading as high as 10% by weight retains the PCE values as neat PM6:Y6 while improving film toughness. Additional SEBS improves toughness further but comes with a small decrease in PCE. For example, the fracture strain increases from 3% for the neat PM6:Y6 film to over 11% for the film with 20 wt.% SEBS. At this SEBS loading, the PCE of the solar cell was found to decrease from approximately 14% to 11%. To understand this behavior and optimize this ternary system, a detailed thermomechanical and morphological analysis is completed. We find that processing has a clear impact on the segregation behavior of the SEBS that impacts both device performance and mechanical behavior. Through this work, we show that active layers with elastomer additives is an effective strategy to improve the mechanical stability of organic solar cells.