Researchers Demonstrate Highest Open-Circuit Voltages for Quantum Dot Solar Cells

Nanotubes images(Nanowerk News) U.S. Naval Research Laboratory (NRL)  research scientists and engineers in the Electronics Science and Technology  Division have demonstrated the highest recorded open-circuit voltages for  quantum dot solar cells to date. Using colloidal lead sulfide (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.
metal-lead sulfide quantum dot Schottky junction solar cell
Schematic of metal-lead sulfide 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 cells.
“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,” said Woojun Yoon, Ph.D., 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.”
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 demonstrate that by improving the  passivation of the PbS QD surface through tailored annealing of QD and metal-QD  interface using lithium fluoride (LiF) passivation with an optimized LiF  thickness. This proves critical for reducing dark current densities by  passivating localized traps in the PbS QD surface and metal-QD interface close  to the junction, therefore minimizing non-radiative recombination processes in  the cells.
Over the last decade, Department of Defense (DoD) analyses and  the department’s recent FY12 Strategic Sustainability Performance Plan, has  cited the military’s fossil fuel dependence as a strategic risk and identified  renewable energy and energy efficiency investments as key mitigation measures.  Research at NRL is committed to supporting the goals and mission of the DoD by  providing basic and applied research toward mission-ready renewable and  sustainable energy technologies that include hybrid fuels and fuel cells,  photovoltaics, and carbon-neutral biological microorganisms.
Source: U.S. Naval Research Laboratory

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