3 years ago

Origins of Stokes Shift in PbS Nanocrystals

Origins of Stokes Shift in PbS Nanocrystals
Alexander H. Ip, Larissa Levina, Susanna M. Thon, Amirreza Kiani, Fengjia Fan, Grant Walters, Oleksandr Voznyy, James Z. Fan, Edward H. Sargent, Mengxia Liu, Andrew H. Proppe
Stokes shift, an energy difference between the excitonic absorption and emission, is a property of colloidal quantum dots (CQDs) typically ascribed to splitting between dark and bright excitons. In some materials, e.g., PbS, CuInS2, and CdHgTe, a Stokes shift of up to 200 meV is observed, substantially larger than the estimates of dark–bright state splitting or vibronic relaxations. The shift origin remains highly debated because contradictory signatures of both surface and bulk character were reported for the Stokes-shifted electronic state. Here, we show that the energy transfer among CQDs in a polydispersed ensemble in solution suffices to explain the excess Stokes shift. This energy transfer is primarily due to CQD aggregation and can be substantially eliminated by extreme dilution, higher-viscosity solvent, or better-dispersed colloids. Our findings highlight that ensemble polydispersity remains the primary source of the Stokes shift in CQDs in solution, propagating into the Stokes shift in films and the open-circuit voltage deficit in CQD solar cells. Improved synthetic control can bring notable advancements in CQD photovoltaics, and the Stokes shift continues to provide a sensitive and significant metric to monitor ensemble size distribution.

Publisher URL: http://dx.doi.org/10.1021/acs.nanolett.7b01843

DOI: 10.1021/acs.nanolett.7b01843

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