Black Silicon for Solar Cells: What does it mean?

Silicon is the “material of choice” for solar voltaic cells because it takes relatively little energy to excite the electrons in a silicon crystal into the conduction band, where they are free to conduct electricity that can be used to generate electric power. Scientists and engineers have spent many years trying to modify silicon crystals by doping them with other materials that would make the photovoltaic process even more efficient. In the last few years, scientists at Harvard University found that if they blasted the surface of a silicon wafer (in the presence of sulfur hexafluoride gas) with femtosecond laser pulses, the silicon received a heavy doping of sulfur and the surface of the silicon developed deep, microscopic cones. The result was that this newly formed, thin silicon layer, called “black silicon” is 200-500 times more sensitive to light than untreated silicon. How is this possible?

With its new structure, the band gap in the thin silicon layer - the difference between the valence band and the conduction band - is smaller. This means that longer wavelengths of sunlight (infrared) are also able to excite electrons into the conduction band - contributing energy to the solar-electric conversion. Furthermore, by applying a small voltage (a bias) to black silicon creates conditions in which each incoming photon can excite still more electrons. So, not only is the material responsive to wavelengths that silicon-based devices couldn’t detect in the past - it also produces a much stronger signal in response to a weak stimulus. The increased sensitivity makes black silicon good for detection applications; and the increase in absorption wavelengths improves the efficiency of silicon for solar cells.

The Harvard Scientists (Stephen Saylor and James Carey) formed a spinoff company, SiOnyx, to commercialize the process for solar energy and highly-sensitive, imaging applications, such as night vision, surveillance, digital cameras and medical imaging. For over three years, they’ve been pretty quiet about this, but in the last few months, they are starting to talk. Maybe that’s a sign that some of the applications of black silicon are about to “come to market”.

This is another example of photonics (femtosecond lasers) as an enabling technology - this time to enhance the efficiencies of solar voltaic cells.

To read the complete article about black silicon, go to:

www.xconomy.com/boston/2008/10/12/sionyx

What do you think about the potential of “black silicon” for solar cell improvement? What other breakthroughs do you see in solar electric energy?

Solid State Lighting: Energy Conservation through Improved Photonic Efficiencies

Energy resources and energy consumption aren’t just hot topics for the news media: they’re major, world-wide issues that we must confront for the next few decades. Last week I talked about recent developments in solar electric (voltaics) that may lead to its widespread growth as a renewable energy source. This week, I want to look at how photonics will contribute to energy conservation.

Lighting consumes over 20% of all the electrical energy produced in the world. The incandescent bulb is only about 25% efficient. We are not only losing 75% of the energy supplied to incandescent, in many cases this wasted energy is producing unwanted heat that will require more energy to remove it. Several states and other countries have already moved to ban the sale of incandescent bulbs this year and through 2012. It’s likely that this ban will grow to most or all states soon.

Presently, compact fluorescent lamps (CFL) and fluorescent tubes are the choice for room lighting in most commercial and residential buildings. But fluorescent lighting is only 40% efficient, its color tone does not match sunlight (an aesthetic quality we hold dear) and when they are no longer useable, we have to dispose of devices that contain mercury, which is harmful to the environment.

Solid state lighting (light-emitting diodes, or LEDs) shows the greatest promise for improved lighting efficiency and energy conservation. LED’s have the potential of being over 90% efficient and lasting for 50,000 hours (as compared with a few thousand hours with incandescents and CFLs.) Typical LEDs emit a single color of light from a small source (semiconductor junction.) If we want to use them as white light sources, we have to combine at least three LEDs with different emitting color characteristics. (i.e., red, blue and green) Some optical elements are usually placed in front of the LED’s to spread or diffuse the “point source”.

Presently, LEDs are in use in the following niche applications:

• Colored Light Sources—Traffic lights, exit signs, candles and holiday lighting
• Indoor White Light—Recessed down lights and refrigerated display cases
• Outdoor White Lights— Street and area lights & step, path and porch lights

Of these, the greatest energy savings are coming from recessed down lights and street/area lights (luminaries). For room and hall lighting in commercial and home buildings, special power supplies are being developed to control dimming and color temperatures. These power supplies are the limiting factor on lifetime (~6-10,000 hours).

In May, 2008, General Electric announced the demonstration of roll-to-roll manufactured organic light-emitting diode (OLED) lighting devices. This product will be available in thin sheets which, presumably, could be applied to walls or ceilings of rooms. The OLED offers the potential of having entire walls or the ceiling emit light at controlled colors and levels of intensity.

So, what new or renewed jobs will solid state lighting produce as it inevitably will emerge in the next several years? Will it require retraining for electricians, architects and room designers? Or, will there also be a need for SSL technicians with comprehensive knowledge and skills to rebuild our lighting infrastructure for more energy-efficient consumption?

OP-TEC staff will continue to follow this emerging application of photonics to anticipate new education and training needs.

To learn more about solid state lighting visit www.netl.doe.gov/ssl.

Solar Electric Power: Using Holographic Concentrators to Improve Efficiency

The use of solar electric power has grown significantly in recent years, particularly in Europe, China and the U.S. The worldwide output of solar installations remains relatively small (~7 GW), primarily because of the high cost of refined silicon and low solar module (PV) efficiency. However, recent breakthroughs have occurred in optical devices to improve the efficiency of the solar-collecting cells and to concentrate greater amounts of solar energy on the cells.

In the December 2008 issue of Laser Focus World, the article, "Holographic planar concentrator increases solar-panel efficiency" by Rosenberg, Kostuk & Zecchino, describes the holographic planar concentrator (HPC) as an approach that achieves both these goals. A holographic film is used in a planar concentrator that:

• Collects more indirect and diffuse sunlight throughout the day, and concentrates it on the solar cell. This “passive tracking” allows the collector to produce electrical power for more hours during the day, and during overcast conditions.


• Filters the sunlight, allowing only useful wavelengths to strike the photovoltaic (PV). The unwanted wavelengths are blocked, thus preventing a temperature rise which decreases the efficiency of the PV process.


• Solar modules with HPC cells are currently being developed to incorporate “bifacial PV cells”, which means that the solar light can strike both sides of the cells, improving the efficiency and using less pure silicon. The reduction of silicon, and other improvements have lowered the cost of a solar module below a dollar per watt.

So it’s quite possible that solar electric collectors are becoming sufficiently efficient and competitive that the rapid expansion of solar energy may be realized in the next few years. Some U.S. plants that will manufacture solar PV systems are scheduled to open in 2009.

It appears that optical devices, such as the HPC, will provide the critical technology that is needed for solar PV’s—another field where photonics is an “enabling technology.

OP-TEC staff and Partner Colleges will continue to track the solar PV development to identify new careers for photonics technicians.

What are your thoughts on the possible emergence of solar electric energy? Do you know of educational courses or programs that are addressing technician needs in this area?

Reference:
Kostuk, Raymond K., Rosenberg, Glenn & Zecchino, Mike (December 2008). Holographic planar concentrator increases solar-panel efficiency [Electronic version]. Laser Focus World, (29)12, pp. 41-44.

Precision Optics Technicians: A Critical Need for Our Country

Precision Optics Technicians (POTs) create, test and handle optical (infrared, visible and ultraviolet) components that are used in lasers and sophisticated electro-optical systems for defense, homeland security, aerospace, biomedical equipment, digital displays, controlled thermonuclear fusion and nanotechnology. POT’s also integrate precision optical components into these electro-optical systems and maintain them.

There is a perceived shortage of POT’s that could require our country to outsource this work to foreign nations - a situation that would compromise our nation’s security and sacrifice a vital sector of future economic development.

Several factors have contributed to this shortage. Many experienced POT’s have retired, and more are expected to retire in the near future. The two community colleges that have offered education/training in precision optics have discontinued the programs due to faculty retirements and poor support. One of these colleges has committed to update and reinstate its POT program.

Under a supplemental grant from NSF, OP-TEC and the Photonics Industry Clusters are conducting studies and preparing to support the development of additional POT programs at community colleges.

• In January, 2009, OP-TEC will complete the National Skill Standards for POT’s - the employer’s specifications for new technicians in precision optics (and the basis for a new curriculum design.)


• OP-TEC is designing the new curriculum model for preparing POT’s.


• The Rochester Photonics Cluster is preparing a design and an equipment list for college labs to train POT’s.


• OP-TEC, through the University of North Texas, is conducting an employer needs assessment to determine the number of POT jobs that will need to be filled over the next five years.


• OP-TEC will host a meeting in Waco TX, February 19, 2009, for regional teams of colleges and employers that have an interest in initiating education and training for POT’s.

Hopefully, several teams will be identified and committed to begin planning POT education/training programs. OP-TEC will then move forward to secure funding to support them through their start-up process. We will also search for support to equip the laboratories.

We will be building a network of employers and educators for POT education/training. Are you interested in participating? Please contact Christine Dossey cdossey@cord.org at the OP-TEC office.