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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Chlorine and Graphene Combine


Nanotubes imagesResearchers at the Massachusetts Institute of Technology in the US are reporting on a new way to p-dope graphene that does not sacrifice its excellent electronic properties too much – something that has proved to be somewhat of a challenge until now. The resulting material could be ideal for making all-graphene integrated circuits on a chip, radio-frequency transistors and nanoelectronic circuit interconnects to name a few examples.

In the lab

Graphene is a flat sheet of carbon just one atom thick – with the carbon atoms arranged in a honeycombed lattice. Since the material was first isolated in 2004, its unique electronic and mechanical properties, which include extremely high mobility, and high strength, have amazed researchers who say that it could be used in a host of device applications. Indeed, graphene might even replace silicon as the electronic material of choice in the future according to some. This is because electrons whiz through graphene at extremely high speeds, behaving like “Dirac” particles with no rest mass, a property that could allow for transistors that are faster than any existing today.

However, unlike the semiconductor silicon, graphene has no gap between its valence and conduction bands. Such a bandgap is essential for electronics applications because it allows a material to switch the flow of electrons on and off. One way of introducing a bandgap into graphene is to chemically dope it, but this has to be done carefully so as not to destroy graphene’s unique electronic properties too much.

Plasma-based surface functionalization technique

A team led by Mildred Dresselhaus and Tomas Palacios has now succeeded in p-doping graphene with chlorine using a plasma-based surface functionalization technique. “Compared with other chemical doping methods, the advantages of our approach are very significant,” says team member Xu Zhang. “First and foremost, the chlorine-doped graphene keeps a high charge mobility of around 1500 cm2/Vs after the hole doping. This value is impressively high compared to those obtained with other chemical species previously.”

The chlorine can also cover over 45% of the graphene sample surface, he adds. This is the highest surface coverage area reported for any graphene doping material until now, according to the researchers.

Density functional theory predicts that a bandgap of up to 1.2 eV can be opened up in graphene if both sides of the sample are chlorinated, and if the amount of chlorine on each side covers 50% of the total sample area. “The 45.3% coverage in single-sided chlorinated graphene observed in our work is thus important and paves the way to ultimately opening up a sizeable bandgap in the material while maintaining a reasonably high mobility,” Zhang told nanotechweb.org.

In their work, the researchers studied both “exfoliated” graphene and that obtained using chemical vapour deposition (CVD). They performed the chlorine plasma treatments in an Electron Cyclotron Resonance Reactive Ion Etcher (ECR/RIE) in which chlorine gas was excited into the plasma state by absorbing energy from an in-phase electromagnetic field at a certain frequency. The chlorine plasma was accelerated by applying a DC bias relative to the sample stage. “We carefully optimized both the ECR power and DC bias to control the reaction conditions,” explained Zhang, “and the experiments were performed at room temperature.”

The p-doped material produced could be used to make all-graphene integrated circuits on a chip and RF transistors, he added. Doping the graphene with chlorine also reduces its sheet resistance, making it suitable for use in electronic circuit interconnects.

The team now plans to dope suspended samples of graphene with chlorine – to access both sides of a sample – and so open up an even bigger electronic band gap.

The present work is detailed in ACS Nano DOI: 10.1021/nn4026756.

NRL Designs Multi-Junction Solar Cell to Break Efficiency Barrier


QDOTS imagesCAKXSY1K 8U.S. Naval Research Laboratory scientists in the Electronics Technology and Science Division, in collaboration with the Imperial College London and MicroLink Devices, Inc., Niles, Ill., have proposed a novel triple-junction solar cell with the potential to break the 50 percent conversion efficiency barrier, which is the current goal in multi-junction photovoltaic development.

 

04-13R_multijunction_solar_cell_372x328Schematic diagram of a multi-junction (MJ) solar cell formed from materials lattice-matched to InP and achieving the bandgaps for maximum efficiency. (Photo: U.S. Naval Research Laboratory)

“This research has produced a novel, realistically achievable, lattice-matched, multi-junction solar cell design with the potential to break the 50 percent power conversion efficiency mark under concentrated illumination,” said Robert Walters, Ph.D., NRL research physicist. “At present, the world record triple-junction solar cell efficiency is 44 percent under concentration and it is generally accepted that a major technology breakthrough will be required for the efficiency of these cells to increase much further.”

In multi-junction (MJ) solar cells, each junction is ‘tuned’ to different wavelength bands in the solar spectrum to increase efficiency. High bandgap semiconductor material is used to absorb the short wavelength radiation with longer wavelength parts transmitted to subsequent semiconductors. In theory, an infinite-junction cell could obtain a maximum power conversion percentage of nearly 87 percent. The challenge is to develop a semiconductor material system that can attain a wide range of bandgaps and be grown with high crystalline quality.

By exploring novel semiconductor materials and applying band structure engineering, via strain-balanced quantum wells, the NRL research team has produced a design for a MJ solar cell that can achieve direct band gaps from 0.7 to 1.8 electron volts (eV) with materials that are all lattice-matched to an indium phosphide (InP) substrate.

“Having all lattice-matched materials with this wide range of band gaps is the key to breaking the current world record” adds Walters. “It is well known that materials lattice-matched to InP can achieve band gaps of about 1.4 eV and below, but no ternary alloy semiconductors exist with a higher direct band-gap.”

The primary innovation enabling this new path to high efficiency is the identification of InAlAsSb quaternary alloys as a high band gap material layer that can be grown lattice-matched to InP. Drawing from their experience with Sb-based compounds for detector and laser applications, NRL scientists modeled the band structure of InAlAsSb and showed that this material could potentially achieve a direct band-gap as high as 1.8eV. With this result, and using a model that includes both radiative and non-radiative recombination, the NRL scientists created a solar cell design that is a potential route to over 50 percent power conversion efficiency under concentrated solar illumination.

Recently awarded a U.S. Department of Energy (DoE), Advanced Research Projects Agency-Energy (ARPA-E) project, NRL scientists, working with MicroLink and Rochester Institute of Technology, Rochester, N.Y., will execute a three year materials and device development program to realize this new solar cell technology.

Through a highly competitive, peer-reviewed proposal process, ARPA-E seeks out transformational, breakthrough technologies that show fundamental technical promise but are too early for private-sector investment. These projects have the potential to produce game-changing breakthroughs in energy technology, form the foundation for entirely new industries, and to have large commercial impacts.