Semicrystalline Cathode Interlayer Based on Morphology Control Additives Using Nonconjugated Small‐Molecule Zwitterions for Efficient Organic Solar Cells

The cathode interlayer (CIL) has significant impact on the performance of organic solar cells (OSCs), including the abilities to align energy levels, form ohmic contacts between the active layer and the electrode, and promote the electron extraction and transportation. However, the developments of CILs are still far behind the rapid explosion of active layer materials, especially for these nonfullerene acceptors (NFAs). This research provides a brand‐new CIL optimization approach by applying the nonconjugated small‐molecule zwitterion (NSMZ) as the morphology control additives (MCAs). Two novel NSMZs, 4‐(1‐methyl piperidin‐1‐ium‐1‐yl)butane‐1‐sulfonate (MPBS) and 3‐(1‐methylpiperidin‐1‐ium‐1‐yl)‐propane‐1‐sulfonate, are developed as MCAs for the improvements of short‐circuit current, fill factor, and power conversion efficiency (PCE) in various solar cell systems of the classic active layers and CILs. The interaction mechanism is systematically investigated. With the reduced energy barrier and suppressed electron recombination, a champion PCE of 18.65% on binary NFA–OSCs is achieved by incorporating the zwitterion of MPBS into the cathode interlayer as the additive, which is among the highest efficiency of the reported binary OSCs. The application of the zwitterion as MCAs for the cathode interlayer opens a novel avenue for highly efficient OSCs.