Charge trapping dynamics in PbS colloidal quantum dot photovoltaic devices

Area of Science:

• Materials Science
• Photovoltaics
• Nanotechnology

Background:

• Solution-processed colloidal quantum dot (QD) solar cells face efficiency limitations due to poor charge transport.
• Surface defects on QDs create trapping sites, hindering charge carrier movement within the active layer.

Purpose of the Study:

• To investigate charge trapping dynamics in lead sulfide (PbS) colloidal-QD photovoltaic devices under working conditions.
• To elucidate the role of QD surface ligands and electron-accepting layers in charge trapping processes.

Main Methods:

• Application of ultrafast electro-optical pump-push photocurrent spectroscopy.
• Probing infrared photoinduced absorption in the 0.2-0.5 eV region to monitor charge trapping.
• Optical detrapping of immobile charges to understand their nature.

Main Results:

• Identified IR photoinduced absorption linked to optically detrappable, immobile charges.
• Demonstrated that early charge trapping dynamics are highly dependent on QD surface ligand type, but minimally on the electron-accepting layer.
• Observed instantaneous population of weakly bound states (0.2 eV activation energy), suggesting intrinsic bound-state character.
• Detected sequential population of deeper traps (0.3-0.5 eV activation energy) on nanosecond timescales, indicating trapping occurs post-charge relaxation.

Conclusions:

• The study disentangles distinct contributions to charge trapping dynamics in nanocrystal-based optoelectronics.
• Findings provide insights into the fundamental mechanisms of charge carrier behavior in QD solar cells.
• The developed technique offers a valuable tool for advancing QD solar cell development and optimization.