The cis- and trans-orientation of benzo[1,2-b:4,5-b′]dithiophene-based isomers in organic solar cells

Abstract

Significant breakthroughs have been achieved in bulk-heterojunction (BHJ)-based organic solar cells (OSCs) with power conversion efficiencies (PCE) of over 17% upon the development of new cores of non-fullerene acceptors and the isomers that are often produced in the synthesis of the cores. Here, two benzo[1,2-b:4,5-b′]dithiophene (BDT)-based acceptors, which are isomers with different orientations of the thiophene rings on the backbone, are reported. The different molecular configurations and electron cloud distributions of the isomers were studied using theoretical calculations, which indicate that the isomers have completely different optical properties and various electrochemical energy levels. Grazing-incidence wide-angle X-ray scattering (GIWAXS) tests showed that the two isomers have different crystalline structures. OSC devices developed from each of the two isomers exhibit significant differences in electron/hole mobilities and packing modes, leading to clearly distinct fill factors (FFs) and short-circuit currents, affecting the PCEs of the final devices. These studies of core isomerism provide some new insights into the configurations of backbones and could assist in the molecular design of new n-type electron acceptors.