Nano: the next big thing in solar

Imagine this, a photovoltaic (PV) panel as thin as a piece of foil and produced by printing it like a newspaper. Now imagine using a spray to convert your window into a transparent solar panel. It’s all happening now as companies and researchers adopt nanotechnology production methods to the world of PV. This new class of photovoltaics, called nanophotovoltaics, are starting to enter the market in 2010. As the technology continues to advance, it may help speed the adoption of PV.

Nanophotovoltaics are the third generation of PV and the latest in the quest to develop less-expensive solar panels that are cheaper to produce, and lighter than their predecessors. Right now scientists are creating photovoltaic panels using technologies including carbon nanotubes, multiple quantum wells, nanowires, nanoantennas, and quantum dots. These nanophotovoltaics can consist of PV components that are 1/1,000th the thickness of a human hair, and are printed directly onto sheets of metal or other substrate.

By using these mighty micro PV conductors, companies are greatly reducing the amount of materials needed to make a PV cell, and are able to cut production costs. While some are market-ready and are already on their way to PV installations, other technologies are still being researched to determine their efficiency and how inexpensive they are to produce.

Nanoparticle ink panels

Nanosolar is one company that’s now making partnerships to market its panels, which consist of a nanoparticle ink printed on a thin foil. You can see a video of the printing process here. The National Renewable Energy Laboratory (NREL) certified that the PV foil is capable of capturing 15.3 percent of the sun’s energy. That puts the technology in the range of most photovoltaics including thin-film and silicon. At this point, most installed systems convert between 10 to 11 percent of the sun's energy into electricity, NREL said. However, commercially available photovoltaics convert between 8 percent and 20 percent of sunlight into energy.

Nanosolar has had two plants making photovoltaic panels. The first is in San Jose, CA, and a newer plant is in Germany. The company’s long-term goal, when the production process is fully optimized, is to produce photovoltaic panels at 60 cents per watt and retail them for about $1.00 a watt, according to company spokesperson Joey Marquart. When that efficiency is reached, a fully installed Nanosolar panel system would cost about $2.50 a watt.

Nanosolar is field-testing its solar panels now and plans to increase deployment of the panels “sevenfold” in 2010, Marquart said.

Thinner still, Nanowires are on the Horizon

Another promising PV technology being developed at the Lawrence Berkeley National Laboratory is silicon nanowires, which are about 1/1,000th the thickness of a human hair. Each nanowire is a complete photovoltaic cell with a p (positive) and n (negative) junction, according to the lab. By incorporating a p-n junction into each wire, it greatly reduces the distance of electric transmission to electrodes and allows electricity produced by the wire to move much faster across the panel’s surface.

Cells manufactured with nanowire technology use minute amounts of silicon and can use lower-grade silicon, making production much less expensive than crystalline silicon cells, which need expensive, high-grade silicon. The lab said that with improvements, PV panels made with nanowire technology should be suitable for large-scale deployment.

At this point, panels made of nanowires are 5 percent to 6 percent efficient at converting sunlight to electricity. However, chemist Peidong Yang, who led the research, asserted that this efficiency was reached with little effort in efficiency-increasing modifications.

“With further improvements, most importantly in surface passivation, we think it is possible to push the efficiency to above 10 percent,” he explained. “The fabrication technique behind this extraordinary light-trapping enhancement is a relatively simple and scalable aqueous chemistry process.”

He added, “We believe our approach represents an economically viable path toward high-efficiency, low-cost thin-film solar cells.”

Microscopic antennas: 80 percent efficient and produce PV power after dark

Some PV nanotechnology promises to be the most efficient PV technology yet. Nanoantenna arrays have captured up to 80 percent of the sun’s rays mid-infrared rays at the U.S. Department of Energy's Idaho National Laboratory. The nanoantennas are 1/25th the width of a human hair.

The lab said the material, which looks like gold on a sheet of plastic, could “cost pennies a yard, be imprinted on flexible materials, and still draw energy after the sun has set.” Yeah, photovoltaic power produced after dark. These tiny antennas are able to produce energy from infrared light reradiated from the earth after dark. They also are able to absorb infrared heat from other things, like industrial processes. And researchers think the material could produce electricity from waste heat, allowing the nanoantennas to cool buildings and computers without air conditioning.

There’s just one teensy problem. The electricity produced by the nanoantennas moves too fast. The alternating-current (AC) electricity they produce oscillates trillions of times every second. To compare, the AC power used throughout the U.S. oscillates at 60 times a second. Electric lines, transformers, as well as appliances and motors powered by conventional AC electricity can’t handle the higher oscillations.

"We need to design nanorectifiers that go with our nanoantennas," researcher Dale Kotter explained. A nanoscale rectifier will require new manufacturing methods since it is about 1,000 times smaller than available commercial rectifiers. Another possibility is developing electrical circuitry to slow the current to usable frequencies.

Originally etched into a gold silicon wafer, researchers at the lab have achieved the same efficiency results by depositing the antennas on a plastic, like those used for food-storage bags.

The lab partnered with Microcontinuum Inc. and Patrick Pinhero of the University of Missouri, to develop the material.

Nanospray? Is that like sunscreen?

Not quite. New Energy Technologies, Inc. said it developed a method of spraying windows with a transparent nano-thin PV material. Its PV cells are 1/4 the size of a grain of rice and 1/1,000th the thickness of a human hair. The tiny cells produce electricity both from natural and artificial light. The techonology was developed by the company in partnership with the Nanostructure Optoelectronics Lab in USF.

The company is developing a product called SolarWindows, which will use the spray process to create electricity-producing windows. However, the company did not disclose how efficient the PV windows are.