Abstract
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 (KI(3)) additives are combined with conventional PbX(2) matrix ligands to simultaneously eliminate the undercharged Pb species and dangling S sites via reacting with molecular I(2) generated from the reversible reaction KI(3) ⇌ I(2) + KI. Meanwhile, high surface coverage shells on PbS CQDs are built via PbX(2) and KI ligands. The implementation of KI(3) 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.