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

Significance 

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.

New look at Stokes shifts in PbS quantum dots might lead to improved optoelectronic performance - Advances in Engineering

 

About the author

Jeffrey C. Grossman is the Morton and Claire Goulder and Family Professor in Environmental Systems and a Professor in the Department of Materials Science and Engineering at the Massachusetts Institute of Technology. He received his Ph.D. in theoretical physics from the University of Illinois, performed postdoctoral work at U.C. Berkeley, and was a Lawrence Fellow at the Lawrence Livermore National Laboratory. He returned to Berkeley as Director of a Nanoscience Center and Head of the Computational Nanoscience research group with focus on energy applications, prior to joining MIT in fall, 2009.

Dr. Grossman’s group uses a combination of modeling and experiment to gain fundamental understanding, develop new insights based on this understanding, then use these insights to develop new materials with improved properties for energy conversion, energy storage, and clean water technologies. Prof. Grossman has been named a MacVicar Fellow of MIT, recognizing his contributions to engineering education, and he has been named a Fellow of the American Physical Society.

He has published more than 150 scientific papers, holds 17 current or pending U.S. patents, and has recently co-founded a company to commercialize graphene-oxide membranes. He has appeared on a number of television/radio shows to discuss new materials for energy and water including the Fred Friendly PBS series, the Ecopolis program on the Discovery Channel, and NPR’s OnPoint Radio.

About the author

Yun Liu is a graduate student at the Department of Materials Science and Engineering at Massachusetts Institute of Technology. His thesis advisor is Professor Jeffrey C. Grossman and he is currently investigating the electronic, optical and transport properties of colloidal quantum dots for optoelectronic applications using a combination of first-principles calculations, classical molecular dynamics and machine learning techniques.

He received his B.A. and M.A. in Natural Sciences from University of Cambridge and was a research engineer at the Institute of High Performance Computing in Singapore.

About the author

Dr. Donghun Kim is a senior research scientist in Computational Science Research Center at Korea Institute of Science and Technology (KIST). He received his B.S. in Materials Science from Pohang University of Science and Tech. (POSTECH) in 2010, Ph. D. in Materials Science from Massachusetts Institute of Tech. (MIT) in 2015, under the supervision of Professor Jeffrey C. Grossman.

Dr. Kim and his group develop or apply first-principles quantum mechanical calculations and machine learning techniques in order to discover and understand improved materials, primarily for solar cells and catalysis.

About the author

Owen Morris is a doctoral candidate in the Department of Materials Science and Engineering at Massachusetts Institute of Technology. He received an MSci in Physics and Mathematics from the University of Glasgow in 2014, following a Masters project in the Institute of Gravitational Research.

Now working in the research group of Prof. Jeffrey C. Grossman, his research focuses on materials for renewable energy, including quantum dot photovoltaics and electronics from natural carbon feedstocks, doing device fabrication and characterisation using a variety of experimental techniques.

Reference

Yun Liu, Donghun Kim, Owen P. Morris, David Zhitomirsky, Jeffrey C. Grossman. Origins of the Stokes Shift in PbS Quantum Dots Impact of Polydispersity, Ligands, and Defects. ACS Nano 2018, volume 12, page 2838−2845

Go To ACS Nano

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