Shifting Perspective: New look at Stokes shifts in PbS quantum dots might lead to improved optoelectronic performance

Lead sulphide (PbS) is one of the best performing quantum dot materials for photovoltaic applications. However, it suffers from a particularly large Stokes shift that limits its efficiency. Stokes shift, defined as the redshift of an emission spectrum with respect to the corresponding absorption spectrum, is a critical attribute in a variety of applications based on semiconductor colloidal quantum dots (CQDs). To effectively control the Stokes shifts, it is first necessary to understand its origins. From the plethora of published literature, two different mechanisms responsible for the Stokes shifts have been identified: the dark exciton arising from the exciton fine structure, and the Franck−Condon (FC) relaxation. The dark exciton contribution has been more extensively studied in lead chalcogenide family of CQDs, but it is not satisfactory to explain the experimental observations. Compounding this problem, FC relaxations in PbS CQDs, as well as the effects of different passivating ligands and intrinsic defects, have not been thoroughly investigated.

Professor Jeffrey Grossman at Massachusetts Institute of Technology and his research team: Yun Liu and Owen Morris (both PhD candidates) and Dr. David Zhitomirsky (Post-doctoral fellow), in collaboration with Dr. Donghun Kim at Korea Institute of Science and Technology, investigated the origins of the Stokes shift in PbS colloidal quantum dots using both density functional theory (DFT) calculations and UV−vis and photoluminescence measurements in order to quantify the contributions of polydispersity, ligands and defects. Their work is currently published in the research journal ACS Nano.

The research team began their investigation by performing DFT calculations, where the energies of model PbS CQDs with various ligands at different electronic and structural configurations were calculated. Next, they synthesized PbS colloidal quantum dots where ligand exchanges were achieved by adapting various two-phase solution exchange methods. Lastly, Stokes shifts were measured by performing absorption and photoluminescence experiments.

The authors observed that the size and energetic disorder of a polydisperse colloidal quantum dot film can redshift the photoluminescence spectra by 20 to 50 meV compared to that of an isolated CQD. Secondly, they noted that the Franck−Condon shifts increased as the electronegativities of the ligands increased, although the variations were small. Finally, the researchers noticed that unlike the aforementioned two minor factors, the presence of certain intrinsic defects, which are energetically favourable to form during the synthesis processes, caused substantial wavefunction localization of the band edge states and consequent large FC shifts.

To sum up, Professor Jeffrey Grossman and his research team successfully combined experiments with ab initio calculations to investigate the Stokes shift of PbS CQDs to understand the origins of the excessive Stokes shift in PbS nanocrystals. The results suggested that the FC shift was a crucial source of the large Stokes shift in the lead chalcogenide family of colloidal quantum dots, reconciling the previous discrepancies between theory and experiments. Altogether, this improved understanding of the optical properties of PbS colloidal quantum dots is of great importance when designing the next generation of photovoltaic applications.