A comprehensive approach for the instability of PbTe quantum dots and design of a combinatorial passivation strategy

Limited air stability circumvents the outstanding properties of the PbTe quantum dots (QDs) and hinders its utilization in the field of optoelectronics. In this respect, we present a detailed understanding on the instability of the PbTe QDs and develop a combinatorial passivation protocol based on engineering the QD surface during the growth phase and solid state ligand exchange process. The dual passivation approach yields thin films and solar cells with outstanding stabilities under ambient conditions. Prioritization of the effects of the synthetic conditions with a systematic approach discloses the crucial roles of the size, shape and stabilizing ligand/catalyst concentration on the stability. Our findings show that the stability diminishes beyond a critical size (3.6-3.9 nm) where the shape changes from octahedron to truncated octahedron. We discovered that the catalyst is a major source of the instability and optimization of its concentration leads to the smallest, extremely air stable PbTe QDs with a size less than 2 nm. We illustrate that the mid gap state formation (MGS), indispensable for any ligand exchange processes, can only be inhibited by using QDs whose capacity to withstand the severe conditions of the exchange process is improved by passivating in-situ. In-situ growth phase passivation not only control the shape by dictating the {111}/{200} facet ratio but also controls the ligand exchange rate, packing direction and MGS formation. The combinatorial passivation strategy yields air stable solar cells with a 100% increase in the efficiency and up to a fivefold increase in the short circuit current.