All Roads Lead to Rome: Isomers with Divergent Cathode Modification Mechanisms toward Ohmic Contact

Abstract

Cathode interfacial layers (CILs) hold utmost importance for achieving ohmic contact at the organic semiconductor–cathode interface of organic photovoltaic devices. Delving deep into diverse design principles and working mechanisms is of great significance for designing novel CILs with high performance. Herein, two novel nonamine-based CILs are designed: one featuring a cyclopentadiene unit, designated as CIL-cp; while the other, lacking cyclopentadiene, is referred to as CIL-ph, which is an isomer of CIL-cp. The subtle changes in chemical structures result in distinct modification mechanisms toward ohmic contact. On one hand, the robust electron-withdrawing characteristic of cyclopentadiene endows CIL-cp with lower energy levels, resulting in an interfacial dipole at the active layer–CIL-cp interface due to electron transfer from D18 to CIL-cp. On the other hand, CIL-ph exhibits a strong interfacial dipole at the CIL–Ag interface, which significantly reduces the work function (W_F) of the silver electrode. Both CIL-cp and CIL-ph demonstrate excellent interfacial modification capability, whereas CIL-cp possesses a stronger electron extraction ability, thus leading to a high power conversion efficiency of 19.31% in the D18:L8-BO system. Our results reveal the distinctive operational mechanism of cyclopentadiene-based CILs, thus offering innovative design ideas for CIL materials.