Optimizing interface properties of perylene-diimide-based cathode interlayer material by reducing 2-hydroxyethyl groups to achieve organic solar cells with efficiency over 19 %

Polar functional side group plays an important role in regulating interface properties of diimide-based cathode interlayer materials (CIMs). Herein, we designed and synthesized two perylenediimide (PDI)- and naphthodiimide (NDI)-based CIMs, PDINHOH and NDINHOH, through properly reducing the number of polar 2hydroxyethyl groups to optimize interface properties. Two CIMs were easily obtained by readily available raw materials and one-step reaction, exhibiting low product cost and excellent solubility. Results demonstrate that these 2-hydroxyethyl amino functionalized CIMs not only remain many outstanding advantages of bis(2hydroxyethyl) amino, including reduced work function of Ag electrode, obvious self-doping effect, wellmatched molecular energy levels and good contact with metal electrode, but also can improve thermal stability and dipole moment, reduce surface energy, optimize the contact with active layer, and increase electron conductivity and mobility. Benefitting from these comprehensive characteristics, organic solar cells (OSCs) based on PM6:L8-BO with PDINHOH as the CIM achieved an outstanding PCE up to 19.25 %, which is obviously higher than that of the counterpart NDINHOH (18.60 %) and that with the commonly used CIMs of PFN-Br (17.37 %) and PDINN (18.09 %) as well as the bis(2-hydroxyethyl) amino functionalized analogue. Importantly, these CIMs show good universal application in different active layer systems, metal electrodes and device structures. Additionally, PDINHOH-modified devices exhibit excellent storage stability, thermal and photo stability. This finding suggests that the rational modification of the polar functional side chains of diimide-based CIMs is a simple and effective strategy to optimize interface properties, and thereby obtaining thickness-insensitive, highperformance and stable OSCs.