ITIC derivative acceptors for ternary organic solar cells: fine-tuning of absorption bands, LUMO energy levels, and cascade charge transfer

For effective supplementary acceptor molecules (A2) in ternary organic solar cell (TOSC) devices, a series of ITIC derivatives were designed and synthesized by incorporating symmetrically or asymmetrically functional termini of various electron-accepting 1,3-indanedione (IND) groups having distinct different electron affinities to the fused core unit, indacenodithieno[3,2-b]thiophene (IDTT). The absorption, thermal properties and LUMO energy level of the prepared ITIC derivatives were precisely controlled according to the terminal group as well as the open-circuit voltage (VOC) in binary organic solar cells (BOSCs) employing PBDB-T as a host donor polymer and ITIC derivatives as a host acceptor material. In the PBDB-T:ITIC:A2-based ternary system, simply adding ITIC derivatives, which can form a cascade energy alignment between the LUMO levels of the host PBDB-T donor polymer and ITIC acceptor molecule, as a small amount (approximately 5%) of A2 acceptor molecules improved the photoelectric efficiency by up to 10% more than that of a PBDB-T:ITIC binary system of the same structure through enhancing VOC, JSC, and FF, synchronously. These results offer a route to find the selection criteria for A2 acceptor molecules suitable for effective use in TOSC devices and develop high-performance TOSC devices.