Naphthalene diimide-based cathode interlayer material enables 20.2% efficiency in organic photovoltaic cells

Cathode interlayer (CIL) materials play an important role in improving the power conversion efficiency (PCE) of organic photovoltaic (OPV) cells. However, the current understanding of the structure-property relationship in CIL materials is limited, and systematic studies in this regard are scarce. Here, two new CIL materials, NDI-PhC4 and NDI-PhC6 were synthesized by varying the alkylamine chain length on the NDI-Ph core. Our investigation reveals a systematic variation in the physical and chemical properties of these materials with increasing alkylamine chain length. Specifically, we observe a sequential decrease in melting point and self-doping effect, accompanied by an enhancement in crystallinity. Among these CIL materials, NDI-PhC4 has a notable balance across various performance metrics. It also exhibits excellent surface modification capabilities, leading to a low surface roughness. Consequently, OPV cells based on NDI-PhC4 achieve a PCE of 20.2%, which is one of the highest reported efficiencies for OPV cells. In addition, the appropriate melting point of NDI-PhC4 contributes to the excellent stability of OPV cells.