All Roads Lead to Rome:\nIsomers with Divergent Cathode\nModification Mechanisms toward Ohmic Contact

Cathode\ninterfacial layers (CILs) hold utmost importance\nfor achieving\nohmic contact at the organic semiconductor–cathode interface\nof organic photovoltaic devices. Delving deep into diverse design\nprinciples and working mechanisms is of great significance for designing\nnovel CILs with high performance. Herein, two novel nonamine-based\nCILs are designed: one featuring a cyclopentadiene unit, designated\nas CIL-cp; while the other, lacking cyclopentadiene, is referred to\nas CIL-ph, which is an isomer of CIL-cp. The subtle changes in chemical\nstructures result in distinct modification mechanisms toward ohmic\ncontact. On one hand, the robust electron-withdrawing characteristic\nof cyclopentadiene endows CIL-cp with lower energy levels, resulting\nin an interfacial dipole at the active layer–CIL-cp interface\ndue to electron transfer from D18 to CIL-cp. On the other hand, CIL-ph\nexhibits a strong interfacial dipole at the CIL–Ag interface,\nwhich significantly reduces the work function (W<sub>F</sub>) of the\nsilver electrode. Both CIL-cp and CIL-ph demonstrate excellent interfacial\nmodification capability, whereas CIL-cp possesses a stronger electron\nextraction ability, thus leading to a high power conversion efficiency\nof 19.31% in the D18:L8-BO system. Our results reveal the distinctive\noperational mechanism of cyclopentadiene-based CILs, thus offering\ninnovative design ideas for CIL materials.