Precursor Chemistry Enables the Surface Ligand Control of PbS Quantum Dots for Efficient Photovoltaics

The surface ligand environment plays a dominant role in determining the physicochemical, optical, and electronic properties of colloidal quantum dots (CQDs). Specifically, the ligand-related electronic traps are the main reason for the carrier nonradiative recombination and the energetic losses in colloidal quantum dot solar cells (CQDSCs), which are usually solved with numerous advanced ligand exchange reactions. However, the synthesis process, as the essential initial step to control the surface ligand environment of CQDs, has lagged behind these post-synthesis ligand exchange reactions. The current PbS CQDs synthesis tactic generally uses lead oxide (PbO) as lead precursor, and thus suffers from the water byproducts issue increasing the surface-hydroxyl ligands and aggravating trap-induced recombination in the PbS CQDSCs. Herein, an organic-Pb precursor, lead (II) acetylacetonate (Pb(acac)₂ ), is used instead of a PbO precursor to avoid the adverse impact of water byproducts. Consequently, the Pb(acac)₂ precursor successfully optimizes the surface ligands of PbS CQDs by reducing the hydroxyl ligands and increasing the iodine ligands with trap-passivation ability. Finally, the Pb(acac)₂ -based CQDSCs possess remarkably reduced trap states and suppressed nonradiative recombination, generating a certified record V_oc of 0.652 V and a champion power conversion efficiency (PCE) of 11.48% with long-term stability in planar heterojunction-structure CQDSCs.