Regulating energy levels of near-infrared PbS quantum dots via zinc-ion interfacial dipole effect

Lead sulfide quantum dots (PbS QDs) are promising for near-infrared photovoltaics due to their large exciton Bohr radius and size-tunable bandgap. However, extending absorption into the near-infrared (bandgap < 1.13 eV) necessitates larger QDs, which weakens quantum confinement and lowers the conduction band (CB) energy towards bulk-like levels. This CB shift induces severe energy-level misalignment at the ZnO/QDs heterojunction interface, impeding charge extraction efficiency. Conventional dipole-based energy-level tuning strategies rely on ligands coordinated to Pb sites. However, large PbS QDs expose more non-polar (100) facets with a stoichiometric 1:1 Pb:S ratio, where traditional ligands fail to bind S sites, presenting a fundamental barrier to precise energy-level control. To address this issue, we introduce a novel approach: modulating PbS QD energy levels by inducing interfacial dipoles through direct metal cation coordination to the S sites. Systematic screening of metal salts revealed that Zn²⁺ coordination induces the most prominent dipole effect, reducing the work function (WF) of PbS QDs from 4.38 to 4.28 eV. This optimization aligns the band arrangement at the ZnO/QDs interface and facilitates efficient extraction of photogenerated electrons from the PbS absorber layer to the ZnO electron transport layer (ETL). Solar cell devices fabricated using this strategy achieved a power conversion efficiency (PCE) of 11.0%, representing a 12% relative enhancement over the control group (9.8%).