Cross-Linking-Integrated Sequential Deposition: A Method for Efficient and Reproducible Bulk Heterojunctions in Organic Solar Cells

The formation of bulk heterojunctions (BHJs) through sequential deposition (SqD) of polymer donor and nonfullerene acceptor (NFA) solutions offers advantages over the widely used single-step deposition of polymer:NFA blend solutions (BSD). To enhance the application of SqD in organic solar cell production, it is crucial to improve reproducibility and stability while maintaining a high efficiency. This study introduces a novel method termed cross-linking-integrated sequential deposition (XSqD) for fabricating efficient and reproducible BHJs. In this method, polymers are cross-linked using efficient 2Bx-4EO or 2Bx-8EO cross-linkers, which enhance the solvent resistance of the polymer donor layer against the solvents used for NFAs. This approach addresses the challenge of selecting a suitable solvent for NFAs, a major obstacle in SqD-processed OSCs. The utilization of 2Bx-4EO in XSqD leads to a significant increase in reproducibility compared to that of conventional SqD, coupled with a high-power conversion efficiency (PCE) of 14.1%. Furthermore, XSqD devices exhibit superior stability, showing only 1% and 6% reductions in their initial PCE after thermal stress at 80 and 120 °C for 50 h, respectively.