Enhancing phase separation with a conformation-locked nonfullerene acceptor for over 14.4% efficiency solar cells

To fine-tune the morphology and miscibility of the active layer of organic solar cells (OSCs), the molecular backbone design and sidechain control are necessary but complex and challenging for acceptor-donor-acceptor type nonfullerene acceptors. In this work, both fluorination on accepting ends and sidechain modification on the central donating core were optimized for the design of fused-ring electron acceptors (FREAs). The sidechain-locked conformation finely modulates the molecular energy levels and improves the miscibility with weakened structural order. Fluorination effectively enhances the crystallinity to afford an enlarged phase separation and higher domain purity. Benefiting from their synergistic effects, the as-designed ITC6-4F when blended with the PM7 polymer donor enables an impressive power conversion efficiency of 14.47% with a high V-OC of 0.90 V and improved FF of 74.31%, which is much higher than those of the devices based on reference FREAs without fluorination (8.21%) or conformation lock (12.48%). Our results demonstrate that enhancing phase separation with a conformation-locked nonfullerene acceptor could be an effective way for further improving the related performance of OSCs.