Trap states in lead chalcogenide colloidal quantum dots - Origin, impact, and remedies

Colloidal quantum dots (CQDs) based on lead chalcogenides (PbX), i.e., lead sulfide, selenide, or telluride, constitute a class of materials with many intriguing properties and potential applications in (opto-)electronics. These nanosized crystals are employed successfully in a broad variety of devices including field-effect transistors, solar cells, and light emitting diodes, and their performance has increased significantly over the last 20 years. Often, such improvements have been associated with the suppression of detrimental recombination of charge carriers via trap states. Historically, traps have been attributed to dangling bonds on the surface of CQDs that needed to be passivated for proper electronic behavior. More recent understanding goes beyond such simplified views. Surfaces can be bare without necessarily evoking traps. On the other hand, imperfect separation of CQDs and their subsequent agglomeration can generate trapping sites without the need of chemical defects. Experimental and computational approaches that have led to a more accurate understanding are here discussed, and rivaling concepts and ideas are highlighted. Although the community established a much improved understanding of carrier trapping, there is still room to further the knowledge about the precise mechanisms, especially with respect to impacts from the environment. With these limitations notwithstanding, PbX CQDs exhibit large potential that we expect to be unlocked through future improvements in control of the surface chemistry and strategies of thin film assembly.