Custom Molecular Design of Ligands for Perovskite Photovoltaics

ConspectusPerovskite photovoltaics have witnessed overwhelming success owing to their high power conversion efficiency, low voltage deficit, sensitive photoelectric response and good operational stability. However, solution-processed, polycrystalline perovskite films inevitably contain a high density of crystallographic defects, such as uncoordinated ions and dangling bonds at the surfaces and grain boundaries, which can result in charge recombination, thus causing energy loss and impaired device performance. These intrinsic imperfections can be remedied through a chemically induced intermarriage between halide perovskites of soft crystallographic nature and judiciously designed exotic ligand molecules. Utilizing rational molecular design of the component moieties, i.e., the core and tail functional groups, the ligand molecules can be endowed with both more comprehensive and salient advantages to further boost device performance, thus setting perovskite photovoltaics on course for a more prosperous future.In this Account, we present our narrative of designing a series of favorable ligand molecules with multifunctionalities in terms of crystal growth modulation, grains cross-linking, defect passivation, surface functionalization, phase stabilization, and moisture invasion inhibition of perovskite films and summarize the advances made in designing molecular level custom ligands that have progressively lifted the record performances of perovskite photovoltaics. We first identify the origins of imperfections in perovskites and their detrimental impacts on device efficiency and stability. We then review feasible rules for ligand design, including tuning the length of the linear or branched alkyl chain core, the substitution of one or more carbon atoms with O, P, S, or NH, and the number of functional groups (double, triple, quadruple, or even multiple ones), to design "all-in-one" ligands with ideal molecular structures and multifunctionalities. The chemical interactions between ligands and perovskites (e.g., coordination bond, ionic bond, hydrogen bond, electrostatic interaction and chelation, etc.) are discussed, aiming to elucidate their impact on the necessary trade-off between defect passivation efficacy and charge transport property in ligand-modified perovskite films, and establish a triangular structure–property–performance relationship for perovskite-based optoelectronic devices with ligands modulation and passivation. We also insightfully explore novel ligand management strategies in perovskite quantum dots, especially the development of ligand-assisted cation exchange and surface ligand density optimization. Finally, we present an astute perspective of future trends in the rational design of innovative ligand molecules with desired structures and properties for more efficient, stable, perovskite-based optoelectronic devices.