Making Inorganic Solar Cells with an Airbrush Spray


 

Nano Particles for Steel 324x182(Nanowerk Spotlight) There is currently a tremendous  amount of interest in the solution processing of inorganic materials. Low cost,  large area deposition of inorganic materials could revolutionize the fabrication  of solar cells, LEDs, and photodetectors. The use of inorganic nanocrystals to  form these structures is an attractive route as the ligand shell that surrounds  the inorganic core allows them to be manipulated and deposited using organic  solvents.

The most common methods currently used for film formation are  spin coating and dip coating, which provide uniform thin films but limit the  geometry of the substrate used in the process. The same nanocrystal solutions  used in these procedures can also be sprayed using an airbrush, enabling larger  areas and multiple substrates to be covered much more rapidly.

The trade-off is  the roughness and uniformity of the film, both of which can be substantially  higher.    Reporting their findings in a recent online edition of ACS  Applied Materials & Interfaces (“Inorganic Photovoltaic Devices Fabricated Using  Nanocrystal Spray Deposition”), researchers have now attempted to quantify  these differences for a single-layer solar cell structure, and found the main  difference to be a reduction in the open circuit voltage of the device.            deposited films of CdTe nanocrystals SEM  images of the top surface of the deposited films following deposition and  sintering, showing (a) CdTe spin coated and (b) CdTe spray coated. The scale bar  in both images represents 200 nm. (Reprinted with permission from American  Chemical Society)

“Our work was motivated by a desire to coat larger substrate  areas more efficiently,” Edward Foos, a research scientists in the Materials  Synthesis and Processing Section of the Chemistry Division at the Naval  Research Laboratory, and first author of the paper, tells Nanowerk. “Our initial  work indicated that if the layers were thick enough to cover the substrate  completely and avoid pinhole formation that would lead to shorting of the  device, then the increased surface roughness might be tolerable.”

He adds that this is the first time the impact of this surface  roughness on the performance characteristics has been directly compared for  these types of devices.

The team prepared single-layer Schottky-barrier solar cells  using spray deposition of inorganic (CdTe) nanocrystals with an airbrush. The  spray deposition results in a rougher film morphology that manifests itself as a  2 orders of magnitude higher saturation current density compared to spin  coating.   “We’re currently working to improve the spray coating process to  improve the layer uniformity,” says Foos. “If the surface roughness can be  reduced, then the overall device performance should increase.”   The team is confident that further optimization of the spray  process to reduce this surface roughness and limit the Voc suppression should be possible and eventually lead  to comparable performances between the two deposition techniques.   “Importantly” Foos points out, “the spray-coating process  enables larger areas to be covered more efficiently, reducing waste of the  active layer components, while enabling deposition on asymmetric substrates.

These advantages should be of substantial interest as inorganic  nanocrystal-based solar cells become increasingly competitive as  third-generation devices.”   The team’s next step will be the fabrication of more complex  device architectures that incorporate multiple solution processed layers. These  structures will have an even smaller tolerance for variation. In addition, the  deposition chemistry used must not interfere with the material applied in the  previous step.

By Michael Berger. Copyright © Nanowerk

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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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