N-type small molecule electron transport materials with D-A-D conjugated core for non-fullerene organic solar cells

Two D (donor)-A (acceptor)-D-type small-molecular conjugated electrolytes (SMCEs) electron transport materials (ETMs) with self-doping effects, namely DPPN2TPA and DPPM2TPA, were successfully synthesized by introducing diketopyrrolopyrrole (DPP) as the electron acceptor, triphenylamine (TPA) as the electron donor and quaternary ammonium or imidazolium salts as polar pendants. Impressively, the electron-deficient group DPP is double-doped by the amine-based groups and the electron-rich groups TPA, thus DPPN2TPA and DPPM2TPA exhibit prominent intramolecular charge transfer (ICT) and distinct n-type self-doping property. These characters can increase the electron mobility and ameliorate the work function (WF) of the stable metal Ag to create ohmic contact, making for significant elevated photovoltaic performance of the SMCEs. Especially, DPPM2TPA emerges more prominent optoelectronic property than DPPN2TPA due to the extra interfacial dipoles generated by the tertiary amine of imidazole moieties. The PM6:Y6-based non-fullerene organic solar cells (NOSCs) with DPPN2TPA and DPPM2TPA as ETMs achieve excellent power conversion efficiency (PCE) of 15.93 % and 16.40 %, respectively. It is noticeably superior to the NOSC with classical ETM PDINO (15.35 %). Delightfully, the PCE remained at 14.64 % (≈90 %) when the thickness of DPPM2TPA film reached 37 nm, which manifests that DPPM2TPA has satisfactory thickness insensitivity and can be used to manufacture large-scale devices. Furthermore, DPPM2TPA has conspicuous universality, when DPPM2TPA ETM was applied to the device with PM6: BTP-eC9, a remarkable PCE of 17.31 % was realized. These results demonstrate the potential of DPPM2TPA for industrialized large-scale manufacturing and afford an effective approach for exploring high-performance D-A-D SMCEs NOSCs.