Controlling charge separation and recombination rates in CdSe/ZnS type I core-shell quantum dots by shell thicknesses

Area of Science:

• Materials Science
• Photovoltaics
• Nanotechnology

Background:

• Type I core/shell quantum dots (QDs) offer enhanced stability and efficiency in QD-sensitized solar cells over core-only QDs.
• Understanding the role of shell thickness is crucial for optimizing QD solar cell performance.

Purpose of the Study:

• To investigate the effect of shell thickness on interfacial charge separation and recombination kinetics in CdSe/ZnS core/shell QDs.
• To correlate shell thickness with electron and hole densities at the QD surface and their impact on solar cell performance.

Main Methods:

• Utilized time-resolved transient absorption spectroscopy to measure charge separation and recombination kinetics.
• Employed CdSe/ZnS type I core/shell QDs with anthroquinone as the electron acceptor.
• Performed model calculations to analyze electron and hole densities at the QD surface.

Main Results:

• Charge separation and recombination rates decrease exponentially with increasing shell thickness (d), following k(d) = k(0)e(-βd).
• Observed exponential decay factors (β) of 0.35 ± 0.03 per Å for charge separation and 0.91 ± 0.14 per Å for recombination.
• Model calculations indicated an exponential decrease in surface electron and hole densities with shell thickness, with a steeper decline in recombination due to higher hole effective mass in the ZnS shell.

Conclusions:

• Shell thickness is a critical parameter for optimizing charge separation yield and lifetime in QD solar cells.
• Controlling shell thickness and material properties provides a pathway to enhance solar cell efficiency.
• The findings offer insights into tailoring QD structures for improved photovoltaic applications.