Picosecond Charge Transfer and Long Carrier Diffusion Lengths in Colloidal Quantum Dot Solids

Area of Science:

• Materials Science
• Nanotechnology
• Photovoltaics

Background:

• Quantum dots (QDs) are key for solution-processed optoelectronics.
• Charge carrier diffusion length and mobility are critical for solar cell performance.
• Existing measurement techniques yield inconsistent parameter values for QD films.

Purpose of the Study:

• To accurately measure diffusion lengths and interdot charge transfer rates in QD solids.
• To investigate the impact of QD size on charge transport properties.
• To establish a reliable method for evaluating QD film performance.

Main Methods:

• Employed a 3D donor/acceptor technique to monitor photoexcitation transfer to QD inclusions.
• Utilized ultrafast transient absorption spectroscopy to overcome limitations of photoluminescence.
• Fabricated and characterized metal halide-exchanged PbS QD solids with varying QD sizes.

Main Results:

• Achieved record diffusion lengths of approximately 300 nm in PbS QD solids.
• Observed 8 ps interdot hopping of carriers, one of the fastest rates for PbS QDs.
• Demonstrated 12% power conversion efficiency in QD solar cells.
• Found smaller QDs (3.2 nm) exhibit 5x faster charge transfer and 10x lower trap densities than larger QDs (5.5 nm).

Conclusions:

• The 3D donor/acceptor technique provides accurate charge transport metrics for QD solids.
• Record diffusion lengths and fast charge transfer rates in PbS QDs enable high solar cell efficiencies.
• Smaller quantum dots offer superior charge transport properties and reduced defect densities for improved optoelectronic device performance.