Dead zones in colloidal quantum dot photovoltaics: evidence and implications

Area of Science:

• Materials Science
• Photovoltaics
• Nanotechnology

Background:

• Fabricating advanced photovoltaic (PV) cells requires efficient light management and charge extraction.
• Illumination through the top contact is desirable for specific PV designs, including those with light-trapping electrodes or multiple junctions.

Purpose of the Study:

• To investigate the relative charge collection efficiency of lead sulfide (PbS) colloidal quantum dot (CQD) photovoltaic devices when illuminated from the top versus the bottom.
• To identify performance limitations related to illumination direction in CQD PV devices.

Main Methods:

• Fabrication of FTO/TiO₂/PbS CQD/ITO photovoltaic devices with varying PbS layer thicknesses.
• Measurement of external quantum efficiency (EQE) spectra under top (ITO) and bottom (FTO) illumination.
• Analysis of EQE spectra in conjunction with carrier generation profiles and internal quantum efficiency (IQE) estimations.

Main Results:

• A substantial "dead zone" with significantly reduced carrier extraction efficiency was identified near the top ITO contact.
• The location and impact of this dead zone were correlated with PbS layer thickness and illumination direction.
• Differences in EQE spectra between top and bottom illumination indicate directional charge collection disparities.

Conclusions:

• The presence of a top contact dead zone critically impacts the performance of PbS CQD PV devices.
• Understanding and mitigating this dead zone is crucial for optimizing device design and improving overall power conversion efficiency.
• Strategies to avoid dead zone formation are essential for advancing CQD solar cell technology.