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NRL Achieves Highest Open-circuit Voltage For QD Solar Cells

It is possible to improve the passivation of PbS QDs by using an optimised lithium fluoride thickness
U.S. Naval Research Laboratory (NRL) research scientists and engineers in the Electronics Science and Technology Division have demonstrated what they say is the highest recorded open-circuit voltages for quantum dot solar cells to date.

Using colloidal lead sulphide (PbS) nanocrystal quantum dot (QD) substances, researchers achieved an open-circuit voltage (VOC) of 692 millivolts (mV) using the QD bandgap of a 1.4 electron volt (eV) in QD solar cell under one-sun illumination.



Schematic of metal-lead sulphide quantum dot Schottky junction solar cells (glass/ITO/PbS QDs/LiF/Al). Novel Schottky junction solar cells developed at NRL are capable of achieving the highest open-circuit voltages ever reported for colloidal QD based solar cell. (Photo: U.S. Naval Research Laboratory)

"These results clearly demonstrate that there is a tremendous opportunity for improvement of open-circuit voltages greater than one volt by using smaller QDs in QD solar cells," says Woojun Yoon, NRC postdoctoral researcher, NRL Solid State Devices Branch.

"Solution processability coupled with the potential for multiple exciton generation processes make nanocrystal quantum dots promising candidates for third generation low-cost and high-efficiency photovoltaics," he continues.

Despite this remarkable potential for high photocurrent generation, the achievable open-circuit voltage is fundamentally limited due to non-radiative recombination processes in QD solar cells.

To overcome this boundary, NRL researchers have reengineered molecular passivation in metal-QD Schottky junction (unidirectional metal to semiconductor junction) solar cells capable of achieving the highest open-circuit voltages ever reported for colloidal QD based solar cells.

Experimental results have improved the passivation of the PbS QD surface through tailored annealing of QD and metal-QD interface using lithium fluoride (LiF) passivation with an optimised LiF thickness.

This proves critical for reducing dark current densities by passivating localised traps in the PbS QD surface and metal-QD interface close to the junction, therefore minimising non-radiative recombination processes in the cells.





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