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.


ARAPA-E Backs 66 New Projects

The Department of Energy’s high-risk early stage grant program, ARPA-E, has announced 66 new energy-related projects that will get small amounts of funding and mentorship from the DOE. ARPA-E said that it will give 66 groups — from universities, to startups, to government labs to large companies — a combined $130 million through its Open 2012 program to help them with cutting edge innovation around cleaner and more efficient transportation as well as energy generation and consumption.

The ARPA-E program is one of the DOE’s lauded programs, which has managed to gain bipartisan support and avoid controversy. In contrast, the DOE’s loan guarantee program and battery grant programs allocated large funds to single companies, and when a few of those companies went bankrupt, the DOE received significant criticism.

The ARPA-E program, on the other hand, only gives grants of small — hundreds of thousands to single digit millions — amounts and doesn’t expect to get a return back. It’s funding for basic scientific research. The program also backs so-called “moonshots,” which are innovations that could be transformational, but are at a very early stage — a very small amount of these technologies will probably ever be commercialized. The folks at ARPA-E now say they’ve backed 285 projects for a total of about $770 million in funding.

There were fewer startups in the mix than I’ve seen in recent years. It’s a lot harder to be an entrepreneur in this space these days. Some of the more interesting sounding projects in this crop include:

  • Energy beets: Say wha? A company called Plant Sensory Systems, has received a $1.8 million grant to engineer a beet with enhanced energy density that can be turned into biofuels, and which can also be grown with less water and fertilizer.
  • Waste natural gas to fuel: A company called Ceramatec was granted $1.7 million to build a reactor that can convert natural gas unearthed at remote oil field sites into fuel in one step. This natural gas is usually flared off and wasted.
  • Smart window coatings: Lawrence Berkeley National Labs will use a $3 million grant to low cost coatings for windows that will control light and heat.
  • Portable building mapping tech: LBNL received another grant, this one for $1.9 million, to make a device that senses and maps the internal and thermal characteristics for a building. Using this technology, you can see where heat loss is occurring. Sounds like Essess.
  • Cool roofs: Stanford University is looking to develop a low cost coating for roofs, buildings and cars that reflects sunlight and enables passive cooling. ARPA-E gave Stanford $400,000 to build the tech.
  • Smart grid security modelling: The University of Illinois at Urbana-Champaign received a $1.5 million grant to create a modelling and analysis tools to make the smart grid more secure.
  • Gas-based tech for high voltage power lines: The traditional way to control electricity over high voltage transmission lines is using silicon-based switches. GE’s Global Research division received a $4.1 million grant to work on a gas-based switch that can lower the cost of transmission lines, improve grid reliability, and help with clean power deployment.
  • Super wires: A startup called Grid Logic is working on low cost and high temperature superconducting wires. ARPA-E gave the company a $3.8 million grant.
  • Transmission line analytics: Pacific Northwest National Labs received a $1.6 million grant to develop analytics to find unused space on transmission lines and increase efficiency of the use of transmission lines by 30 percent.
  • Big data grid collection: The University of California, Berkeley, along with the California Institute for Energy and Environment, have received $4 million to develop “micro” synchrophasors to collect real time grid data. Are these even smaller versions of the synchrophasors out there? Not sure, I’ll do some research on it.
  • Water wing: Brown University will work on an “oscillating underwater wing” that can capture energy from flowing water in rivers and tides. They’ll control it with software. I feel like a lot of companies who make these design really nice ones, but the problem is in making sure it lasts years while being battered by water and the elements. Brown received $750,000 for this project.
  • Fabric wind blades: GE has quite a few projects in here. Another one is a project to create wind blades made out of fabric stretched across a frame. GE says such blades could enable wind turbines to be “manufactured in sections and assembled on-site, enabling the construction of much larger wind turbines with higher efficiency and lower cost.”
  • Energy from dust devils: Here’s a weird one (for @go2cleanbreak’s book). The Georgia Institute for Technology wants to use a $3.7 million grant to capture energy from wind vortices by harvesting a thin layer of hot air along the ground created by the sun. Like a manufactured, controlled dust devil. I don’t know what to say about that one.
  • Mini mirror solar field: San Francisco’s own Otherlab is working on developing solar projects with small mirrors that will focus light onto towers. Usually these types of fields (like Ivanpah) use large mirrors.
  • New Valley battery startup?: A startup called Alveo Energy won a $4 million grant for a battery for grid storage that will use Prussian Blue dye as the active material in the battery. They were founded in 2012, based in Palo Alto and their CEO is Colin Wessells, according to Google searches (they don’t have a website). If anyone knows more about this company, ping me.
  • Magnetic energy storage: Here’s a new one. The Tai Yang Research Company wants to create a device that stores energy in superconducting cables, by increasing magnetic field strength of the cable.
  • Solar fuel: The Georgia Institute of Technology received $3.6 million to build a solar reactor to produce solar fuel. Sounds like what Joule has been working on … by the way, whatever happened to them?
  • Printed batteries: The Palo Alto Research Center got close to a million dollars to develop printing technology for lithium ion batteries

Image courtesy of Peyri, and rosmary.