Terminal groups play an important role in enhancing the performance of organic solar cells based on non-fused electron acceptors

Abstract

Non-fused-ring electron acceptors (NFRAs) have drawn strong interest in organic solar cells (OSCs) resulting from their cheapness and efficacy. In order to solve the disadvantage of relatively low device performances caused by their poor planarity, non-covalent bonds are introduced to form a conformational lock in the molecule, thereby reducing the molecular twist and improving planarity. Considering that the terminal groups are the key to providing the main electron transport channel, and the large electrostatic potential (ESP) difference between the donor and acceptor can contribute to effective exciton dissociation, we designed and synthesized the NFREA molecules, O-IC2F and O-IC for OSCs. When paired with D18 as the donor to form OSCs, it was found that the PCE of D18/O-IC based devices was close to 0, while the D18/O-IC2F based device had a power conversion efficiency (PCE) of 7.20%, with a short-circuit current (J_sc) of 14.70 mA cm⁻², open-circuit voltage (V_oc) of 0.96 V and FF of 51.03%. The photoelectric processes demonstrate that these two devices have similar exciton generation, exciton transfer and charge collection processes. However, the D18/O-IC2F based device exhibits more efficient exciton dissociation, a lower degree of trap-assisted recombination and higher electron mobility in the blend films than the D18/O-IC based one. This demonstrates that F atoms in the terminal group of NFRAs promote exciton dissociation, elevate the electron transfer rate and mobility, increase the charge separation rate, and decrease the charge recombination.