Performance enhancement of PbS quantum dot solar cells employing a hybrid solid-state ligand exchange protocol

In this work, a hybrid (HB) solid-state ligand exchange strategy was developed for PbS quantum dots (QDs) films, incorporating a synergistic utilization of the popular inorganic ligand tetrabutylammonium iodide (TBAI) and the organic short bidentate linker 1, 2-Ethanedithiol (EDT) in a two-stage process. The two-stage process consisted of a primary solid-state ligand treatment with TBAI in a controlled manner followed by a second-stage ligand treatment with EDT. We could achieve an optimal iodine/EDT ligand passivation ratio for the QDs films by controlling the TBAI ligand treatment time. The method integrates the benefits of both the ligands and resolves the intrinsic incompleteness of the solid-state TBAI ligand exchange. The QDs films thus created have an optimal surface coverage and reduced inter-dot spacing due to an improved QDs close packing. This in turn led to lesser recombination routes due to reduced mid-bandgap trap states and an improved charge transport property through an enhanced electronic coupling between the QDs. A power conversion efficiency of 8.2 % has been achieved in the device with an optimized TBAI /EDT ligand passivation ratio for the QDs films with an EDT hole transport layer.