Ionic liquids with variable cations as cathode interlayer for conventional polymer solar cells

Abstract

Interfacial layer materials have been demonstrated to be crucial for high-efficiency polymer solar cells (PSCs). In this work, we use ionic liquid (IL) as cathode interfacial layer (CIL) for highly efficient conventional PSCs (c-PSCs) and investigate functions of ILs with different cations and substituents. Employing IL as the CIL, PBDTTT-C:PC71BM-based c-PSC affords a power conversion efficiency (PCE) of 7.29%, much higher than that without the CIL (2.78%) and that with Ca/Al electrode (6.18%). When the photoactive layer is a PTB7-Th:PC71BM blend, a higher PCE of 8.67% can be obtained. The ILs reduce the energy barrier due to the existence of interfacial dipole in c-PSCs, leading to increased electron and hole mobilities, reduced series resistance and enhanced contact at the cathode interface. Meanwhile, alkyl chain-substituted ILs offer higher fill factor and PCE than aromatic groups-substituted analogue, which is mainly contributed to more balanced electron and hole mobilities. This work suggests that the ILs are qualified candidates as the CIL for c-PSCs and that one should take the substitution effect into account when choosing a CIL from a large library of materials.