Origins of the Stokes Shift in PbS Quantum Dots: Impact of Polydispersity, Ligands, and Defects

Abstract

Understanding the origins of the excessive Stokes shift in the lead chalcogenides family of colloidal quantum dots (CQDs) is of great importance at both the fundamental and applied levels; however, our current understanding is far from satisfactory. Here, utilizing a combination of ab initio computations and UV-vis and photoluminescence measurements, we investigated the contributions to the Stokes shift from polydispersity, ligands, and defects in PbS CQDs. The key results are as follows: (1) The size and energetic disorder of a polydisperse CQD film increase the Stokes shift by 20 to 50 meV compared to that of an isolated CQD; (2) Franck-Condon (FC) shifts increase as the electronegativities of the ligands increase, but the variations are small (<15 meV). (3) Unlike the aforementioned two minor factors, the presence of certain intrinsic defects such as V (in Cl-passivated CQDs) can cause substantial electron density localization of the band edge states and consequent large FC shifts (100s of meV). This effect arising from defects can explain the excessive Stokes shifts in PbS CQDs and improve our understanding of the optical properties of PbS CQDs.

Highly Stable CsPbX3/PbSO4 Core/Shell Nanocrystals Synthesized by a Simple Post‐Treatment Strategy

“…The smaller Stokes shift (8 nm) of CsPbBr 3 /PbSO 4 NCs compared to naked CsPbBr 3 NCs (24 nm) indicates the lower defect density on the surface resulting from the effective passivation of lead sulfate. [ 37–39 ] Importantly, the PL intensity of as‐prepared CsPbBr 3 /PbSO 4 NCs is enhanced by about threefold in comparison with the pristine one, which can be ascribed to the significant increment in the direct radiative recombination. [ 40,41 ] From their 3D PL spectra (Figure S4, Supporting Information), CsPbBr 3 NCs could be excited when the wavelength range is 330–390 nm, comparatively, CsPbBr 3 /PbSO 4 NCs exhibited higher PL intensity when excited by the wavelength from 335 to 415 nm, confirming the effective conversion of absorbed light.…”

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.…”