Impact of ligand modification on the photovoltaic performance of cyclometalated Ir(III)-complexes based organic solar cells

In this work, systematical comparisons are performed between two Ir-complex based electron donor materials. Compared to the control TBzIr with alkylterthiophene coupled benzo[d]thiazole as cyclometalated ligand, the new-developed TBzRIr is designed by attaching additional strong electron-accepting dicyanorhodanine endcaps to the main ligands. The enlarged ligand conjugation length and amplified intraligand donor-acceptor interactions induces TBzRIr to show less efficient spin-orbital coupling with obviously weakened triplet emission and shorter triplet lifetimes than TBzIr. Notably, TBzRIr demonstrates beneficial physical parameters for organic solar cells with deeper energy levels, remarkably enhanced light-harvest capacity and visibly increased neat-film hole mobility than TBzIr. However, TBzRIr conversely yields inferior device efficiency than TBzIr in the corresponding organic solar cells, with sharply decreased power conversion efficiency from 5.36% for TBzIr:Y6 to 3.05% for TBzRIr:Y6. Besides on the influence of improved film morphologies for TBzIr:Y6, the benefits of stronger triplet feature of TBzIr is also expected to facilitate exciton dissociation, suppress charge recombination, and thus to enhance photovoltaic performance.