Origins of Stokes Shift in PbS Nanocrystals

Abstract

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, CuInS, 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.

Efficient Blue and White Perovskite Light-Emitting Diodes via Manganese Doping

“…Indeed, by monitoring the emission spectrum as a function of time with a streak camera, we can clearly see a red shift in emission peak in the first few nanoseconds in the undoped samples, which disappears in the Mn-doped nanocrystals (see Figure S4), correlating with the reduced FWHM. This red shift in undoped nanocrystals is greatly reduced in solution, indicating that it is likely inter-nanocrystal energy transfer from polydispersity, 32 although further study of this phenomenon is required. See Table 1 for the FWHM of all synthesized samples.…”

Spatial Collection in Colloidal Quantum Dot Solar Cells

“…Despite these recent strides in performance, there remains room to improve CQD solar cells in short circuit current ( J SC ), fill factor (FF), and open‐circuit voltage ( V OC ) . Understanding of the origins of present‐day limits to performance has progressed thanks to research focused on the energetic distribution of imperfections—bandedge and trap states . Some losses have been ascribed to specific regions and interfaces within the active layer; however, the spatial distribution of these imperfections and their impact on performance has yet to be measured in CQD solar cells operando.…”

Semiconductor quantum dots reveal dipolar coupling from exciton to ligand vibration

“…Whether the size-dependent electronic structure of NCs is well understood and described [17][18][19] , a significant number of their fundamental properties is still subject to discussions. For instance, the origin of fluorescence in QDs and the nature of the associated Stokes shift are not well established [20][21][22][23] , especially since NCs are noteworthy for blinking 24,25 . In essence, the different models developed to account for the processes leading to electron-hole recombination, mainly based on dark excitons 26,27 , electron-phonon coupling 28 , trap states and charge transfer 29 , are not unified.…”