Cost-Effective Schiff-Base Nickel Complexes as Cathode Interlayers for High-Efficiency Organic Solar Cells

Efficient cathode interlayers (CILs) materials are crucial for high performance bulk-heterojunction organic solar cells (BHJ-OSCs). Herein, we report three excellent and low-cost CILs Ni-Me, Ni-iPr, and Ni-tBu based on Schiff-base nickel complexes for the first time. The target molecules are obtained by highly efficient reactions and simple purification methods, affording a high overall yield of over 73%. These CILs show great work function tuning capability and film-forming property. Among them, Ni-Me demonstrates the highest electron conductivity (6.5 × 10^-4 S cm^-1) and electron mobility (5.1 × 10^-4 cm² V^-1 S^-1), surpassing those of the widely used high-performance CILs PDINN (5.2 × 10^-4 S cm^-1, 1.76 × 10^-4 cm² V^-1 S^-1), and PNDIT-F3N (3.23 × 10^-5 S cm^-1, 2.64 × 10^-4 cm² V^-1 S^-1). Grazing incidence wide-angle x-ray scattering measurements indicate that Ni-Me has higher crystallinity than Ni-iPr and Ni-tBu. When employed as the CIL in PM6:L8-BO OSCs, Ni-Me delivers a remarkable power conversion efficiency of 19.11%, outperforming PDINN (18.52%) and PNDIT-F3N (18.5%). The Ni-Me CIL also shows excellent universality (19.06% for PM6:BTP-eC9 and 19.62% for D18:L8-BO). This work demonstrates that Schiff-base nickel complexes have great potential as CILs in OSCs.