Optimizing surface chemistry of PbS colloidal quantum dot for highly efficient and stable solar cells via chemical binding

The surface chemistry of colloidal quantum dots (CQD) play a crucial role in fabricating highly efficient and stable solar cells. However, as-synthesized PbS CQDs are significantly off-stoichiometric and contain inhomogeneously distributed S and Pb atoms at the surface, which results in undercharged Pb atoms, dangling bonds of S atoms and uncapped sites, thus causing surface trap states. Moreover, conventional ligand exchange processes cannot efficiently eliminate these undesired atom configurations and defect sites. Here, potassium triiodide (KI₃) additives are combined with conventional PbX₂ matrix ligands to simultaneously eliminate the undercharged Pb species and dangling S sites via reacting with molecular I₂ generated from the reversible reaction KI₃ ⇌ I₂ + KI. Meanwhile, high surface coverage shells on PbS CQDs are built via PbX₂ and KI ligands. The implementation of KI₃ additives remarkably suppresses the surface trap states and enhances the device stability due to the surface chemistry optimization. The resultant solar cells achieve the best power convention efficiency of 12.1% and retain 94% of its initial efficiency under 20 h continuous operation in air, while the control devices with KI additive deliver an efficiency of 11.0% and retains 87% of their initial efficiency under the same conditions.